JP7062230B2 - Plate-shaped silicon nitride sintered body and its manufacturing method - Google Patents

Description

The present invention relates to a plate-shaped silicon nitride sintered body containing β-type silicon nitride as a main component, which has particularly high thermal conductivity, high mechanical strength and toughness, and is suitable for use as an insulating substrate and a circuit substrate. The present invention relates to a silicon nitride sintered body in the form of a silicon nitride and a method for producing the same.

Silicon nitride sintered body is used for various mechanical parts and wear-resistant parts because it has excellent mechanical strength, toughness, thermal shock resistance, etc., and it is also electrically insulated by utilizing high electrical insulation and excellent thermal conductivity. It is also applied to materials. As conventional electrically insulating ceramics, aluminum oxide, aluminum nitride and the like are known. Since aluminum oxide has a low thermal conductivity, there is a problem that heat dissipation is insufficient for application to power semiconductors and the like. On the other hand, aluminum nitride has high thermal conductivity and excellent heat dissipation, but has low mechanical strength and fracture toughness, so that there is a problem that cracks occur in the module assembly process. Further, in the circuit board on which the semiconductor element is mounted, there is a problem that cracks and cracks occur due to the thermal cycle due to the difference in thermal expansion from the semiconductor element, and the mounting reliability is lowered. In applications such as circuit boards, there is a demand for a plate-shaped silicon nitride sintered body that has both high thermal conductivity and excellent mechanical properties (strength and toughness) at a particularly high level.

Therefore, there are various proposals using silicon nitride, which has excellent strength and toughness, as an electrically insulating ceramic. For example, Patent Document 1 describes a silicon nitride sintered body having a fracture toughness of 6 MPa√m or more and a thermal conductivity of 60 W / (m · K), but Al ₂ O ₃ is used as a sintering aid. The fracture toughness value is 7.4 MPa√m or less, and the thermal conductivity is 78 W / (m · K) or less, probably because 0.1 wt% or more is added.

For example, in Patent Document 2, D ₁₀ , D ₅₀ and D ₉₀ have particle size distributions of 0.5 to 0.8 μm, 2.5 to 4.5 μm and 7.5 to 10.0 μm, respectively, and contain oxygen. Silicon nitride powder having an amount of 0.01 to 0.5 wt% and having a proportion of β-type silicon nitride particles present in particles having an average particle size (D ₅₀ ) or more of 1 to 50% has good sheet formability. It is described to provide a sintered body having excellent, high strength, high toughness, and excellent heat dissipation. However, probably because the MgO / Y2O3 weight ratio is _3.0 , the bending strength is 850 MPa or less and the fracture toughness value is 7.5 _MPa√m or less.

Further, in Patent Document 3, when the cut surface of the silicon nitride sintered body is observed, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is 20,000 per 1 mm ² . Silicon nitride having a thermal conductivity of 75 W / (m · K) or more at room temperature, 45 W / (m · K) or more from room temperature to 200 ° C., and a three-point bending strength of 800 MPa or more at room temperature. It is described to provide a sintered body.

Further, in Patent Document 4, the β fraction is 30 to 100%, the amount of oxygen is less than 0.5 wt%, the average particle size is 0.2 to 10 μm, the aspect ratio is 10 or less, and the particles. Silicon nitride powder containing columnar particles having grooves formed in the long axis direction and having Fe content and Al content of 100 ppm or less, respectively, does not require a high-cost firing method such as high-temperature and high-pressure firing. It is described that it is possible to provide a silicon nitride sintered body having high thermal conductivity and high strength. However, the bending strength is 850 MPa or less probably because the oxygen content of the raw material Si ₃ N ₄ powder is extremely low, the average particle size is large, the amount of impurity Fe is high, and the MgO / RExOy weight ratio is 1.5 or more. And the fracture toughness value has not been measured.

Further, Patent Document 5 contains MgO _, Y2O3 and SiO ₂ in 100 parts by mass of silicon nitride powder, and the ratio thereof is (1) MgO / (MgO _{+ SiO 2 )} = 34 to 59 mol%, and ( 2) Silicon nitride sintering obtained by sintering silicon nitride powder in the presence of 5 to 15 parts by mass of a sintering aid having Y ₂ O ₃ / (Y ₂ O ₃ + SiO ₂ ) = 50 to 66 mol%. A silicon nitride substrate made of a body is disclosed. However _, the MgO / Y2O3 weight ratio calculated from the sintering aid composition shown in Table ₁ showing the examples and comparative examples is 0.055 to 0.194 (wt / wt). The blending ratio of MgO is small. Probably because of this, although the obtained silicon nitride substrate has excellent electrical characteristics, the bending strength remains as low as 750 MPa or less.

Further, Patent Document 6 describes an orientation degree fa indicating an orientation ratio in a plane perpendicular to the thickness direction, which is determined by the following formula (1) from the ratio of the X-ray diffraction line intensity of each predetermined lattice surface of silicon nitride particles. The surface is 0.33 or less, the surface obtained by grinding from the surface to the inside of 20% or more of the substrate thickness is 0.16 to 0.33, and the degree of orientation fa on the surface is the above. A silicon nitride substrate is disclosed, which is larger than the degree of orientation fa on the surface obtained by grinding from the surface to the inside by 20% or more of the substrate thickness, and has a warp of 2.0 μm / mm or less. According to the same publication, the degree of orientation fa is defined as follows.
fa = (P-P ₀ ) / (1-P ₀ ) ... (1)
In this formula (1), P is represented by the following formula (2), and the silicon nitride particles in the silicon nitride substrate have (110) planes, (200) planes, (210) planes, (310) planes, and (320) planes. The total of the X-ray diffraction line intensities I of the planes, the (110) plane, the (200) plane, the (101) plane, the (210) plane, the (201) plane, the (310) plane, the (320) plane, and the (002) plane. The ratio with the sum of the X-ray diffraction line intensities I of the surface is shown. Further, P ₀ is represented by the following equation (3'), I ₀ (110), I ₀ (200), I ₀ (101), I ₀ (210), I ₀ (201), I ₀ (310). ), I ₀ (320), and I ₀ (002) are the (110), (200), (210), (310), and (320) planes X of the silicon nitride particles in the silicon nitride powder. The total of the line diffraction line strength I'and the (110) plane, (200) plane, (101) plane, (210) plane, (201) plane, (310) plane, (320) plane and (002) plane. The ratio with the sum of the X-ray diffraction line intensity I'is shown.
P = (I (110) + I (200) + I (210) + I (310) + I (320)) / (I (110) + I (200) + I (101) + I (210) + I (201) + I (310) + I (320) + I (002)) ... (2)
P ₀ = (I'(110) + I'(200) + I'(210) + I'(310) + I'(320)) / (I'(110) + I'(200) + I'(101) + I'( 210) + I'(201) + I'(310) + I'(320) + I'(002)) ... (3')

However, probably because the weight ratio and the amount of the sintering aid added are different, the three-point bending strength of the obtained silicon nitride sintered body is 864 MPa or less, and the fracture toughness value is 6.8 MPa√m or less.

Further, Patent Document 7 discloses a silicon nitride substrate having an amorphous phase and a _MgSiN2 crystal phase, and having an improved thermal conductivity by not containing a crystal phase containing a rare earth element (RE). Has been done. However, probably because the _MgSiN2 crystal phase grows at the grain boundaries, the three-point bending strength of the obtained silicon nitride sintered body remains at 862 MPa or less, and the fracture toughness value has not been measured.

In the examples of Patent Document 8, characteristic values of a silicon nitride sintered body using a silicon nitride powder having a specific surface area of 5 to 30 m ² / g produced by rotary kiln firing as a raw material are disclosed. Tables 3 and 4 show the bending of the silicon nitride sintered body obtained by adding yttrium oxide and aluminum oxide as sintering aids and sintering at 1780 ° C. for 2 hours in a nitrogen gas atmosphere, respectively. The bending strength and thermal conductivity of the silicon nitride sintered body obtained by adding yttrium oxide and magnesium oxide as sintering aids and sintering at 1900 ° C. under pressurized nitrogen gas for 22 hours can be obtained. It is posted. According to Table 3, the bending strength of the silicon nitride sintered body obtained by sintering at 1780 ° C. for 2 hours under a nitrogen gas atmosphere is 1020 to 1220 MPa. It is common knowledge of those skilled in the art that a silicon nitride sintered body to which aluminum oxide is added exhibits a remarkably low thermal conductivity. On the other hand, according to Table 4, the silicon nitride sintered body to which yttrium oxide and magnesium oxide are added shows a high thermal conductivity of 130 to 142 W / mK, but at a high temperature of 1900 ° C. to 22 hours for a long time. In sintering, grain growth progresses remarkably, so that only a low bending strength of 605 to 660 MPa is obtained. That is, a silicon nitride sintered body having both high thermal conductivity and excellent mechanical strength has not been obtained, and it shows that it is difficult to achieve both high thermal conductivity and excellent mechanical strength.

Japanese Unexamined Patent Publication No. 11-100276 Japanese Unexamined Patent Publication No. 2002-265276 Japanese Unexamined Patent Publication No. 2002-293641 Japanese Unexamined Patent Publication No. 2004-262756 International Publication No. 2007/018050 Japanese Unexamined Patent Publication No. 2009-218322 International Publication No. 2010/002001 International Publication No. 2013/146713 Japanese Unexamined Patent Publication No. 2015-63440

These conventional silicon nitride sintered bodies tend to lack thermal conductivity or mechanical properties for semiconductor modules whose calorific value is increasing in recent years, and particularly stabilize heat dissipation even in the high temperature range during operation. In the present situation where it is more desired to secure it, the performance is insufficient in terms of both thermal conductivity and mechanical properties. Sintering at a high temperature of 1900 ° C or higher to increase thermal conductivity causes excessive grain growth and lowers mechanical properties, and conversely, sintering at a temperature of less than 1790 ° C to improve mechanical properties. It is very difficult to obtain a plate-shaped silicon nitride sintered body having both high thermal conductivity and excellent mechanical properties (strength and fracture toughness) because the grain growth is significantly insufficient and the thermal conductivity is lowered. Further, in Patent Document 3, in order to achieve both high thermal conductivity and high mechanical strength, a high atmospheric pressure of 40 atm (4 MPa) or more is required, so a sintering furnace that can be used under high pressure is required. .. As can be seen from the embodiment, at 9 atm (0.9 MPa), the characteristics are significantly insufficient in terms of both thermal conductivity and mechanical strength. In view of this situation, the present invention does not increase the atmospheric pressure during sintering as in Patent Document 3, but at a lower pressure, plate-shaped silicon nitride baked with high thermal conductivity and excellent mechanical properties. The purpose is to provide a unity.

As a result of intensive research on a method for obtaining a plate-shaped silicon nitride sintered body having both high thermal conductivity and excellent mechanical properties (strength and fracture toughness), the present inventors have conducted intensive studies on a specific specific surface area and oxygen. By using silicon nitride powder with a content as a raw material and highly controlling the grain growth in the sintering process in combination with the sheet forming conditions, high thermal conductivity can be achieved without increasing the atmospheric pressure during sintering. We have found that it is possible to produce a silicon nitride sintered body having excellent mechanical properties (strength and fracture toughness), and have completed the present invention. That is, the present invention relates to the following matters.

In the plate-shaped silicon nitride sintered body of the present invention, the ratio of the measured alkaline earth metal content to the measured rare earth metal content as a sintered body is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal. When the cut surface perpendicular to the plate surface of the silicon nitride sintered body is observed, the content is ≤1.30, the measured aluminum content is less than 50 ppm, the relative density is 98% or more, and the columnar β is observed. Among the type silicon nitride particles, the number of those having a major axis length of more than 10 μm is 500 or more and 10,000 or less per 1 mm ² , and the arithmetic average roughness Ra is 0.05 μm or more and 0.5 μm or less. The orientation degree fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the polished surface is 0.08 or more and 0.25 or less. Here, the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles is the degree of orientation fa represented by the formula (1) described in <Method for calculating the degree of orientation fa> described later. This plate-shaped silicon nitride sintered body can be manufactured by sintering a plate-shaped molded body (green sheet) produced by a sheet forming process at an atmospheric gas pressure of 3 MPa or less.

The ratio of the measured alkaline earth metal content to the measured rare earth metal content as a sintered body is the weight of the alkaline earth metal oxide and the rare earth metal oxide in the sintered body based on the oxide. In terms of ratio, 0.34 ≤ alkaline earth metal oxide / rare earth metal oxide ≤ 1.95.

In one aspect of the present invention, the plate-shaped silicon nitride sintered body is characterized in that the thickness is 1.5 mm or less and the thickness / area ratio is 0.015 (1 / mm) or less. do. In this plate-shaped silicon nitride sintered body, the amount of removal of the plate surface surface layer portion perpendicular to the thickness direction by grinding or polishing is preferably 0.03 mm or less per surface.

In one aspect of the present invention, the columnar β-type silicon nitride inside the surface obtained by grinding from a surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less to the inside by 0.08 mm or more. The orientation degree fa indicating the orientation ratio of the silicon particles is 0.01 or more and less than 0.16.

The degree of orientation fa on the inner surface obtained by grinding from the surface to the inside by 0.08 mm or more is preferably smaller than the degree of orientation fa on the surface, and the difference is 0.03 or more and 0.08 or less. Is more preferable.

One aspect of the present invention is characterized by the above-mentioned plate-shaped silicon nitride sintered body having an actually measured oxygen content of 1.4% by weight or more and 2.9% by weight or less as a sintered body. ..

In one aspect of the present invention, the alkaline earth metal oxide is magnesium oxide, and the rare earth metal oxide is at least one oxide selected from yttrium oxide, erbium oxide, scandium oxide and lutetium oxide. do.

In one aspect of the present invention, the metal element content derived from the auxiliary agent, which is the sum of the measured magnesium content as the sintered body and the measured rare earth metal content, is 1.8% by weight to 5.0% by weight. It is characterized by being the above-mentioned plate-shaped silicon nitride sintered body.

Here, the measured content of the magnesium as a sintered body and the rare earth metal is converted into the total content of magnesium oxide and the rare earth metal oxide in the sintered body based on the oxide. It is 2.7% by weight to 6.8% by weight.

In one aspect of the present invention, the open porosity on the surface polished to an arithmetic average roughness Ra of 0.06 μm or more and 0.4 μm or less is 1.0% or less, and the maximum opening diameter of the open pores is 1.0 μm. It is characterized by the following.

One aspect of the present invention is characterized in that the silicon nitride sintered body is a plate-shaped silicon nitride sintered body in which color tone unevenness is suppressed.

One aspect of the present invention is characterized in that the grain boundaries of the silicon nitride sintered body do not substantially contain the crystal phase of the Mg compound made of _MgSiN2 or the like.

In one aspect of the present invention, the thermal conductivity is 90 W / (m · K) or more at room temperature, the four-point bending strength is 900 MPa or more at room temperature, and the fracture toughness value measured by the IF method (indentation method). The K _IC is 7.6 MPa√m or more.

In one aspect of the present invention, the ratio of the measured magnesium content as the sintered body to the measured rare earth metal content is 0.26 ≤ measured magnesium content / measured rare earth metal content ≤ 1.05. The content of the metal element derived from the auxiliary agent, which is the sum of the measured magnesium content and the measured rare earth metal content, is 2.4% by weight to 4.0% by weight.

When the measured magnesium content and the measured rare earth metal content in the sintered body are converted into magnesium oxide content and rare earth metal oxide content on an oxide basis, the ratio is 0.34 ≤ magnesium oxide. Content / rare earth metal oxide content ≦ 1.37, and the total content of magnesium oxide and rare earth metal oxide is 3.4% by weight to 5.8% by weight.

One aspect of the present invention is characterized in that the measured oxygen content of the sintered body is 1.75% by weight or more and 2.10% by weight or less.

In one aspect of the present invention, when the cut surface perpendicular to the plate surface of the silicon nitride sintered body is observed, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is determined. It is characterized in that the number is 1000 or more and 5000 or less per 1 mm ² .

In one aspect of the present invention, the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface where the arithmetic average roughness Ra is polished to 0.05 μm or more and 0.5 μm or less is 0. It is characterized by being 10 to 0.20.

In one aspect of the present invention, columnar β-type silicon nitride on a surface obtained by grinding from a surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less to the inside by 0.08 mm or more. The orientation degree fa, which indicates the orientation ratio of the particles, is 0.01 or more and 0.13 or less.

The degree of orientation fa on the inner surface obtained by grinding from the surface to the inside by 0.08 mm or more is preferably smaller than the degree of orientation fa on the surface, and the difference is 0.03 or more and 0.08 or less. Is more preferable.

In one aspect of the present invention, the thermal conductivity is 100 W / (m · K) or more at room temperature, the four-point bending strength is 1000 MPa or more at room temperature, and the fracture toughness value measured by the IF method (indentation method). The K _IC is 9.0 MPa √ m or more.

In one aspect of the present invention, as a raw material for silicon nitride, the specific surface area is 13.0 m ² / g or more, the oxygen content is 1.2 wt% or more and 2.3 wt% or less, and the surface oxygen content ratio FSO is 0.76 to 0.76. Using silicon nitride powder having 1.10% by weight and an aluminum content of less than 50 ppm, the weight ratio of alkaline earth metal oxide to rare earth metal oxide is 0.40 ≤ alkali as a sintering aid. Starting composition by adding 3.2 to 7.0 wt% of alkaline earth metal oxide and rare earth metal oxide in a compounding ratio satisfying earth metal oxide / rare earth metal oxide ≤ 2.0. (Green sheet manufacturing raw material for manufacturing silicon nitride sintered body) is adjusted, and a plate-shaped molded body (green sheet) is prepared from the starting composition by a sheet molding process, and a plate-shaped molded body (green sheet) is prepared. By sintering in a pressurized atmosphere with a nitrogen-containing gas pressure of 0.15 to 3 MPa and a maximum holding temperature of 1790 ° C or higher and 1880 ° C or lower, the measured alkaline earth metal content and the measured rare earth metal content The ratio with is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal content ≤ 1.30, the measured aluminum content is less than 50 ppm, and the relative density is 98% or more. It is characterized by producing a siliconic sintered body.

One aspect of the present invention is characterized in that the plate-shaped silicon nitride sintered body having an actually measured oxygen content of 1.4% by weight or more and 2.9% by weight or less is produced.

In one embodiment of the present invention, the plate-like plate-like oxide in which the alkaline earth metal oxide is magnesium oxide and the rare earth metal oxide is at least one oxide selected from yttrium oxide, erbium oxide, scandium oxide and lutetium oxide. It is characterized by producing a silicon nitride sintered body of the above.

In one aspect of the present invention, as the sintering aid, the blending ratio is such that the weight ratio of magnesium oxide and the rare earth metal oxide satisfies 0.40 ≤ magnesium oxide / rare earth metal oxide ≤ 1.4. Add 4.0 to 6.0 wt% of magnesium oxide and rare earth metal oxide based on the total mass of silicon nitride powder and sintering aid, and plate-shaped molded body (green sheet) produced by the sheet forming process. Sintered by holding at the maximum holding temperature for 6 to 20 hours in a temperature range of 1790 ° C or higher and 1880 ° C or lower under a pressurized atmosphere with a nitrogen-containing gas pressure of 0.15 to 0.9 MPa. The ratio of the measured magnesium content to the measured rare earth metal content is 0.26 ≤ measured magnesium content / measured rare earth metal content ≤ 1.05, the measured aluminum content is less than 50 ppm, and the relative density is It is characterized by producing a plate-shaped silicon nitride sintered body having a content of 98% or more.

In one aspect of the present invention, an insulating substrate or a circuit board using the plate-shaped silicon nitride sintered body described in the above paragraph is provided.

According to the present invention, a plate-shaped silicon nitride sintered body having both high thermal conductivity and excellent mechanical properties (strength and fracture toughness) is provided, and the plate-shaped silicon nitride sintered body is still available. It can be manufactured without increasing the atmospheric pressure at the time of sintering.

In the silicon nitride sintered body, heat is transferred by lattice vibration (phonon). Therefore, phonon scattering by different ions causes a decrease in thermal conductivity. Further, the silicon nitride sintered body is composed of a silicon nitride particle phase and a grain boundary phase thereof. Since the thermal conductivity of the grain boundary phase is low, the thermal conductivity decreases as the amount of grain boundary phase increases. Further, since the pores remaining in the silicon nitride sintered body significantly reduce the thermal conductivity, it is necessary to use a dense sintered body.

Therefore, in order to obtain a silicon nitride sintered body having high thermal conductivity, the weight ratio of the alkaline earth metal oxide to the rare earth metal oxide is 0.40 ≤ alkali as a sintering aid in the silicon nitride powder. Alkaline earth metal oxide and rare earth metal oxide are prepared based on the total weight of silicon nitride powder and sintering aid in a compounding ratio that satisfies earth metal oxide / rare earth metal oxide ≤ 2.0. By adding 3.2 to 7.0 wt%, the plate-shaped molded body produced by the sheet molding process is sintered at an atmospheric gas pressure of 3 MPa or less, and the measured alkaline earth metal content and measured rare earth as the sintered body. The metal content ratio is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal content ≤ 1.30, the measured aluminum content is less than 50 ppm, and the relative density is 98% or more. The body.

Here, the ratio of the measured alkaline earth metal content to the measured rare earth metal content as a sintered body is calculated as the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide in the sintered body based on the oxide. In terms of conversion, 0.34 ≤ alkaline earth metal oxide / rare earth metal oxide ≤ 1.95.

Furthermore, when observing the cut surface perpendicular to the plate surface of the silicon nitride sintered body, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm was 500 per 1 mm ² . The degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface polished to 10,000 or less and the arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less is 0.08 or more and 0.25. The properties of the silicon nitride raw material, sheet forming conditions, and baking so that the degree of orientation fa on the inner surface obtained by further grinding from the surface to the inside by 0.08 mm or more is smaller than the degree of orientation fa on the above surface. By highly controlling the forming conditions, a plate-shaped silicon nitride sintered body having desired characteristics can be obtained. The degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles is the degree of orientation fa represented by the formula (1) described in the following <Method for calculating the degree of orientation fa>.

<Calculation method of orientation degree fa>
The degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface and inside of the plate-shaped silicon nitride sintered body, is obtained as follows.

The degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface, is obtained by performing X-ray diffraction measurement of the surface polished to an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less. If the arithmetic average roughness Ra of the surface is not within this range, the degree of orientation fa cannot be accurately measured. When the arithmetic mean roughness Ra of the surface of the silicon nitride sintered body is 0.05 μm or more and 0.5 μm or less, X-ray diffraction measurement may be performed on the surface of the sintered body. When the arithmetic average roughness Ra of the surface of the silicon nitride sintered body is not within this range, the surface of the sintered body is polished so that the arithmetic average roughness Ra is 0.05 μm or more and 0.5 μm or less. , X-ray diffraction measurement is performed on the polished surface. The polishing method for reducing the arithmetic average roughness Ra of the surface to 0.05 μm or more and 0.5 μm or less is not particularly limited, and the polishing amount may be the minimum necessary to realize the above arithmetic average roughness Ra. Generally, for example, about 10 μm in the depth direction is sufficient. In the X-ray diffraction measurement, the (110) plane, the (200) plane, the (101) plane, the (210) plane, the (201) plane, the (310) plane, the (320) plane, and the (002) plane X-ray. Measure the diffraction pattern intensity. The degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles inside, is obtained from the surface whose surface roughness Ra is polished to 0.05 μm or more and 0.5 μm or less. After grinding to the inside of the sintered body by 0.08 mm or more, X-ray diffraction measurement of the obtained surface was performed, and the (110) surface, the (200) surface, the (101) surface, the (210) surface, and the (201) surface were measured. , (310) plane, (320) plane, and (002) plane X-ray diffraction pattern intensity is measured. The surface obtained by grinding to the inside of 0.08 mm or more is polished so that the arithmetic average roughness Ra is 0.05 μm or more and 0.5 μm or less in order to make the measurement accurate.

The degree of orientation of the hexagonal columnar particles is F.I. K. It is expressed by the following equation (1) proposed by Lotgerling (see F.K. Lotgerling, J. Inorg. Nucl. Chem., 9 (1959), pp. 113-123). Therefore, based on the results of the X-ray diffraction measurement of the surface and the inner surface, the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles was calculated from the formula represented by the following formula (1).
fa = (P-P ₀ ) / (1-P ₀ ) ... (1)
In this formula (1), P is represented by the following formula (2), and I (110), I (200), I (210), I (310), I (320), I (101), I. (201) and I (002) are the (110) plane, (200) plane, (210) plane, (310) plane, (320) plane, (101) plane, (201) plane, and (. 002) It means the X-ray diffraction peak intensity of each surface.
Further, P ₀ is represented by the following equation (3), I ₀ (110), I ₀ (200), I ₀ (101), I ₀ (210), I ₀ (201), I ₀ (310). , I ₀ (320), and I ₀ (002) are the (110), (200), (101), and (210) planes of the β-type silicon nitride in the isotropic β-type silicon nitride powder. It is calculated from the X-ray diffraction pattern intensity of the (201) plane, the (310) plane, the (320) plane, and the (002) plane.
P = (I (110) + I (200) + I (210) + I (310) + I (320)) / (I (110) + I (200) + I (101) + I (210) + I (201) + I (310) + I (320) + I (002)) ... (2)
P ₀ = (I ₀ (110) + I ₀ (200) + I ₀ (210) + I ₀ (310) + I ₀ (320)) / (I ₀ (110) + I ₀ (200) + I ₀ (101) + I ₀ ( 210) + I ₀ (201) + I ₀ (310) + I ₀ (320) + I ₀ (002)) ... (3)

The degree of orientation fa of the silicon nitride sintered body is measured on a surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less, and the surface of the silicon nitride sintered body of the present invention is measured. The arithmetic mean roughness Ra does not have to be 0.05 μm or more and 0.5 μm or less, and may or may not be polished.

If the weight ratio of the alkaline earth metal oxide to the rare earth metal oxide (alkali earth metal oxide / rare earth metal oxide) in the composition is less than 0.40, the ratio of the rare earth metal oxide increases. The melting temperature of the grain boundary phase rises in the sintering process. Therefore, unless a large amount of silica (SiO ₂ ) is added, the relative density of the sintered body decreases, and a dense sintered body cannot be obtained. Further, even if the alkaline earth metal oxide / rare earth metal oxide representing the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide is less than 0.40 or more than 2.0. It is not preferable because it reduces mechanical properties (strength and breaking toughness). Furthermore, the weight ratio of alkaline earth metal oxide / rare earth metal oxide in the compounding composition is 0.43 or more, 0.45 or more, 0.50 or more, and 1.40 or less, 1.00 or less, 0.66. It may be as follows.

If the amount of alkaline earth metal oxide and rare earth metal oxide added is less than 3.2 wt%, a high-density sintered body cannot be obtained, so that the thermal conductivity is lowered and the mechanical properties (strength and fracture toughness) are also improved. descend. Even if the amount of the alkaline earth metal oxide and the rare earth metal oxide added exceeds 7.0 wt%, the mechanical properties (strength and fracture toughness) hardly decrease, but the thermal conductivity decreases, which is not preferable. The amount of the alkaline earth metal oxide and the rare earth metal oxide added is preferably 4.0 wt% or more and 6.0 wt% or less. The amount of the alkaline earth metal oxide added is more preferably 2.9 wt% or less.

Since the silicon nitride molded product is sintered in a nitrogen atmosphere or a nitrogen-containing inert atmosphere, it is one of the alkaline earth metal oxides and rare earth metal oxides added as sintering aids in the sintering process. The portion volatilizes due to evaporation together with the silica component in the silicon nitride raw material. Therefore, the content of the sintering aid mainly contained in the grain boundaries of the silicon nitride sintered body differs from the compounding composition of the starting material. In the present invention, the ratio of the measured alkaline earth metal content to the measured rare earth metal content as a sintered body is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal content ≤ 1.30. , The measured aluminum content is less than 50 ppm. When the measured alkaline earth metal content / measured rare earth metal content is less than 0.26, the relative density of the sintered body decreases. Further, even if the measured alkaline earth metal content / measured rare earth metal content is less than 0.26 or more than 1.30, the mechanical properties (strength and fracture toughness) are deteriorated. Not preferred. Further, the weight ratio of the measured alkaline earth metal content / the measured rare earth metal content of the sintered body may be 0.30 or more, 0.37 or more, and 0.80 or less, 0.55 or less.

The measured oxygen content of the silicon nitride sintered body is 1.4% by weight or more and 2.9% by weight or less, preferably 1.4% by weight or more and 2.4% by weight or less, and more preferably 1.75% by weight. It is 2% by weight or more and 2.10% by weight or less. Under the compounding composition and sintering conditions such that the measured oxygen content is less than 1.4% by weight, the relative density of the sintered body becomes less than 98%. Further, the crystal phase is precipitated at the grain boundaries of the plate-shaped silicon nitride sintered body, which causes uneven color tone, which is not preferable. On the other hand, a plate-shaped silicon nitride sintered body having an actually measured oxygen content of more than 2.9% by weight is not preferable because the thermal conductivity is lowered. Further, when directly bonded to a metal plate such as copper or aluminum, voids are generated at the bonding interface and the bonding strength is lowered, which is not preferable.

Magnesium oxide is preferably used as the alkaline earth metal oxide, and at least one oxide selected from yttrium oxide, erbium oxide, scandium oxide and lutetium oxide is preferably used as the rare earth metal oxide. In addition, magnesium oxynitride or magnesium nitride may be used instead of magnesium oxide.

The weight ratio of magnesium oxide to the rare earth metal oxide in the compounding composition is preferably 0.40 ≤ magnesium oxide / rare earth metal oxide ≤ 1.4, and further 0.40 ≤ magnesium oxide / rare earth metal oxide <1. It is more preferably 0.0. Further, it is particularly preferable that 0.40 ≦ magnesium oxide / rare earth metal oxide ≦ 0.66. Alternatively, 0.45 ≦ magnesium oxide / rare earth metal oxide ≦ 0.66 may be used.

Due to the effects of both setting the weight ratio of magnesium oxide and rare earth metal oxide and selecting the sintering conditions, the ratio of the measured magnesium content and the measured rare earth metal content as a sintered body is 0.26. ≤ Measured magnesium content / Measured rare earth metal content ≤ 1.05, and more preferably 0.26 ≤ Measured magnesium content / Measured rare earth metal content ≤ 0.75. Further, it is particularly preferable that 0.26 ≦ actually measured magnesium content / actually measured rare earth metal content ≦ 0.49.

As described above, in the sintering process of the silicon nitride molded product, a part of magnesium oxide and rare earth metal oxide added as a sintering aid is volatilized by evaporation together with the silica component in the silicon nitride raw material. Further, nitrogen dissolves in the grain boundary phase which is in a molten state at high temperature. Therefore, in the temperature lowering process after sintering, Y ₂ Si ₃ O ₃ N ₄ (N-merylite), Y ₁₀ Si ₇ O ₂₃ N ₄ (H phase), Y ₄ Si ₂ O ₇ N ₂ (J phase). , _YSiO 2N (K phase) or the like precipitates, and the plate-shaped silicon nitride sintered body taken out causes uneven color tone due to the precipitation of the crystal phase. Since the precipitated crystal phase generally has a higher true density than the amorphous phase, shrinkage causes a dense region of micropores in the peripheral portion of the precipitated crystal phase. The dense region of micropores becomes the starting point of crack growth due to repeated stress and load due to thermal cycle, and causes fatigue fracture and thermal cycle fracture. In addition, the orientation of the growth surface of the precipitated crystal phase and the presence of the micropore dense region cause uneven color tone on the surface of the sintered body. The plate-shaped silicon nitride sintered body of the present invention is characterized in that uneven color tone is suppressed. Suppressing color unevenness means that the material is highly reliable because it is less likely to deteriorate due to the application of stress cycle or thermal cycle.

Further, the grain boundaries of the plate-shaped silicon nitride sintered body of the present invention do not substantially contain the crystal phase of the Mg compound made of _MgSiN2 or the like. Here, the fact that the crystal phase composed of MgSiN ₂ is not substantially contained means that the X-ray diffraction peak intensity of (121) of the MgSiN ₂ crystal phase constitutes a silicon nitride sintered body. Less than 0.0005 times the sum of the X-ray diffraction peak intensities of the (110), (200), (101), (210), (201), (310), (320) and (002) planes of the crystal particles of. Means that

In the present invention, when a crystal phase of an Mg compound composed of MgSiN ₂ or the like is generated at the grain boundaries of the silicon nitride sintered body, the mechanical properties (bending strength and fracture toughness) of the silicon nitride sintered body tend to decrease. It is in. Specifically, the four-point bending strength in the present invention is 900 MPa or more at room temperature, and the fracture toughness value KIC measured by the IF method (indentation method) is 7.6 _MPa√m or more. It is not preferable because a sintered body cannot be obtained.

In the production of the plate-shaped silicon nitride sintered body of the present invention, magnesium oxide and yttrium oxide are preferably added as sintering aids, and the amount thereof is 4.0 wt% or more and 6.0 wt% or less, and the weight thereof. After adding so as to satisfy the ratio of 0.40 ≤ magnesium oxide / yttrium oxide ≤ 1.4, the plate-shaped molded body (green sheet) produced by the sheet forming process is sintered at an atmospheric gas pressure of 3 MPa or less. Therefore, a sintered body having a relative density of 98% or more is obtained. In particular, it is sintered by holding for 6 to 20 hours in a temperature range where the maximum holding temperature is 1790 ° C. or higher and 1880 ° C. or lower under a pressurized atmosphere where the nitrogen-containing gas pressure is 0.15 to 0.9 MPa, and the relative density is 98. % Or more, preferably 99.0% or more of sintered body is obtained. At the time of sintering, it is more preferable to raise the temperature range from 1520 ° C. to the maximum holding temperature at a rate of less than 150 ° C./hr. Further, holding at a constant temperature for a certain period of time in the temperature range from 1520 ° C. to the maximum holding temperature is also effective in reducing residual pores. For example, it is held at a predetermined temperature in the range of 1520 ° C to 1670 ° C for 1 to 3 hours. It is one of the essential requirements of the present invention to produce a plate-shaped silicon nitride sintered body that is dense and has few residual pores.

The sheet molding method is also called a tape molding method, and a slurry containing, for example, 8 parts by mass or more of an organic binder or a resin binder is applied to 100 parts by mass of raw material powder using a device such as a doctor blade or a die coater to form a carrier film. A green sheet is prepared by casting it on top to a predetermined thickness. Green sheet production by an extrusion molding method or an injection molding method is also included in the sheet molding method, but in the present invention, the CIP molding method and the mold press molding method are not included in the sheet molding method. In particular, the test piece obtained by sintering a bulk CIP molded body having a thickness of 3 mm or more without adding an organic binder or a resin binder, and then cutting and polishing the obtained silicon nitride sintered body. The bending strength cannot be compared with the bending strength of the plate-shaped silicon nitride sintered body of the present invention.

The sheet forming method itself is known, and a known sheet forming method may be used in the present invention. Silicon nitride powder, sintering aid, organic binder such as polyvinyl butyral (PVB), dispersant such as alkylpolyamine-based composition, plasticizer such as dimethylphthalate, toluene-isopropanol-xylene, if necessary. A green sheet forming slurry containing a solvent such as a mixed solvent is prepared and cast on a carrier film to a predetermined thickness using a device such as a doctor blade or a die coater to prepare a green sheet. A correlation is observed between the coating speed in sheet molding and the orientation of β-type silicon nitride particles after sintering. The coating speed of the green sheet is related to other production conditions such as slurry composition and sheet thickness, but generally, for example, 0.02 to 0.5 m / min and further 0.05 to 0.3 m. It may be 0.14 to 0.3 m / min or 0.1 to 0.2 m / min. However, the conditions for sheet forming and sintering in producing the plate-shaped silicon nitride sintered body of the present invention are the degree of orientation of the β-type silicon nitride particles and the number of columnar β-type silicon nitride particles exceeding 10 μm. Since it is selected so as to be within the predetermined range of the present invention, the specific coating speed of the green sheet is selected in relation to it. The green sheet can be a laminated green sheet in consideration of the thickness after sintering. A green sheet or a laminated green sheet (hereinafter, simply referred to as a green sheet) produced by a sheet molding method is usually cut into a molded product having a predetermined shape.

When sintering a green sheet molded product, especially when producing a thin plate-shaped silicon nitride sintered body having a thickness of 1.5 mm or less and further 1.0 mm or less, warpage of the thin plate is suppressed and damage is prevented. In consideration of handleability, production efficiency, etc., a plurality of green sheet compacts are laminated with a separating material (typically boron nitride powder having a particle size of about 4 to 20 μm) interposed therebetween. Degreased and sintered. A plurality of green sheet molded bodies are stacked and placed in a container such as boron nitride, heated to 400 to 600 ° C. in air at a heating rate of about 100 ° C./hour, and heated at the same temperature for 2 to 5 hours. Therefore, the organic binder component and the like added in advance can be sufficiently degreased (removed). Next, this degreased body is heat-treated as described later to produce a sintered body. After that, it is cooled to room temperature, and the obtained silicon nitride sintered body is peeled off with a separating material layer to obtain a plate-shaped silicon nitride sintered body. The obtained plate-shaped silicon nitride sintered body is usually blast-polished to obtain a silicon nitride sintered body for a substrate having a desired surface roughness. The thickness removed by the blast polishing process may be, for example, about 20 μm or less on average. Wrap polishing may be performed after blast polishing or without blast polishing.

In a molded product (green sheet) using an organic binder or a resin binder, not only is it easy to generate coarse pores in the molded body due to the aggregation of the binder, but also a small amount of carbon remains in the molded body even after degreasing, and residual carbon remains. Affects grain growth in the sintering process, which deteriorates the mechanical properties (bending strength and fracture toughness) of the obtained silicon nitride sintered body. The effect is particularly remarkable in the plate-shaped silicon nitride sintered body. Further, it is known that the silicon nitride sintered body has different microstructures (particle size and aspect ratio, grain boundary phase composition and crystal phase) between the surface and the inside of the sintered body. Therefore, the bending strength of the test piece obtained by grinding and removing the surface layer portion where defects such as pores and cracks are likely to occur by 0.2 mm or more is higher than the bending strength of the test piece in which the surface layer portion remains. As described above, it is known that the bending strength of the silicon nitride sintered body changes depending on the amount of the organic binder or the resin binder used and the cutting / polishing process at the time of preparing the test piece. Even if the bending strength of the obtained test piece is already disclosed, it cannot be said that it is equivalent to the bending strength of the plate-shaped silicon nitride sintered body in the present invention, and the equivalent bending strength value is already obtained. It does not mean that it was disclosed.

The plate-shaped silicon nitride sintered body in the present invention can be produced by a sheet forming process, but has a thickness of 1.5 mm or less, preferably 1.0 mm or less, and a thickness / area ratio of 0. It means the one which is 015 (1 / mm) or less. The amount of the surface layer portion of the plate surface perpendicular to the thickness direction by grinding or polishing is preferably 0.02 mm or less per surface. When the plate-shaped molded body produced by the sheet forming process is laminated and sintered via the separating material layer, the thickness obtained by peeling with the separating material layer is preferably 1.5 mm or less, preferably. A plate-shaped silicon nitride sintered body of 1.0 mm or less, having a thickness / area ratio of 0.015 (1 / mm) or less, and a plate surface surface layer perpendicular to the thickness direction by grinding or polishing. The amount of the portion removed may be 0.02 mm or less per side. For example, a high thermal conductive silicon nitride substrate for a power module is required to have a thickness of 0.32 ± 0.05 mm.

In the sintering process, when the molded body (green sheet) shrinks and becomes densified, the open pores in the molded body (green sheet) gradually decrease, and only a few percent of the closed pores remain. As the densification progresses, the closed pores disappear, but if the atmospheric gas pressure is higher than 3 MPa, high-pressure nitrogen gas is taken into the closed pores. Since the high-pressure nitrogen gas once taken in cannot go out of the sintered body, residual stress is generated around the pores remaining after sintering, and the mechanical properties and heat of the silicon nitride sintered body at high temperature are generated. It adversely affects the cycle characteristics. Further, since the atmospheric gas pressure acts isotropically, a sintered body in which columnar β-type silicon nitride particles are oriented as in the present invention cannot be obtained. Specifically, the degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less, becomes a small value near zero. , Not preferable due to the balance between thermal conductivity and mechanical properties. Further, in order to raise the atmospheric gas pressure to more than 3 MPa, a special sintering furnace that can be used under high pressure is required, which is not preferable because the equipment cost is remarkably high.

Patent Document 3 realizes high thermal conductivity and high bending strength at atmospheric gas pressures of 40, 60, 100 and 2000 atm, but the data shown in Table 1 shows bulk CIPs with a thickness of 3 mm or more. After sintering the molded body, the characteristics of the test piece obtained by cutting and polishing the obtained silicon nitride sintered body are measured, and it is a sheet molding process in which a large amount of organic binder is added. It is not the characteristic value of the obtained plate-shaped silicon nitride sintered body. Further, the amount of MgO added is 0.9 to 1.0% by weight, and the amount of Y2O3 added is _3.1 to _3.0 % by weight ₍ MgO / _Y2O3 weight ratio is 0.29 to 0.33). Because of the strict sintering conditions for densification, the temperature at which densification progresses becomes high, and the sintering temperature actually shown in Table 2 is also high, so that the length of the major axis exceeds 10 μm, and the columnar β-type silicon nitride The number of silicon particles is a large value of 15223 to 19022 per 1 mm ² . When the number of columnar β-type silicon nitride particles is increased in this way, not only the fracture toughness is lowered because the coarse particles act as the starting point of fracture, but also the surface of the plate-shaped silicon nitride sintered body is roughened. It is difficult to realize a surface condition in which the arithmetic average roughness Ra is 0.06 μm or more and 0.4 μm or less by ordinary blast polishing. If the arithmetic average roughness Ra exceeds 0.4 μm, it becomes difficult to join a copper plate or an aluminum plate by a direct joining method (DBC method) that does not use an active metal brazing material. Further, even if the bonding is possible, it is not preferable because peeling and cracking of the substrate occur in the repeated heat cycle in the heat resistance cycle test. It is preferable that the bonded body with the metal has a durability of 2000 cycles or more when the temperature raising / lowering cycle from −40 ° C. to 180 ° C. is repeated. Further, even if the desired surface roughness can be achieved by lap polishing or the like, which increases the cost, the open porosity on the surface polished to an arithmetic average roughness Ra of 0.06 μm or more and 0.4 μm or less is large, and the surface is opened. It is not preferable because the maximum opening diameter of the pores exceeds 1.0 μm.

On the other hand, if the nitrogen-containing gas pressure is less than 0.15 MPa, the maximum holding temperature at the time of sintering cannot be raised to 1790 ° C. or higher. If the maximum holding temperature is less than 1790 ° C., the progress rate of sintering is slow, and it is difficult to obtain a dense plate-shaped silicon nitride sintered body having a relative density of 98% or more. Alternatively, even if a dense silicon nitride sintered body is obtained at a maximum holding temperature of less than 1790 ° C., the growth of columnar β-type silicon nitride particles is insufficient, and the silicon nitride sintered body has a low thermal conductivity. Since only the body can be obtained, it is difficult to raise the thermal conductivity of the plate-shaped silicon nitride sintered body to 90 W / (m · K) or more. When the maximum holding temperature exceeds 1880 ° C., the growth of columnar β-type silicon nitride particles becomes remarkably fast, and the number of particles having a major axis length of more than 10 μm exceeds 10,000 per 1 mm ² . not. Further, the maximum holding temperature may be 1800 ° C. or higher, or 1850 ° C. or lower.

When the holding time in the temperature range of 1790 ° C. or higher and 1880 ° C. or lower is less than 6 hours, it is difficult to obtain a plate-shaped silicon nitride sintered body having a desired relative density and desired columnar β-type silicon nitride particles. If the holding time in the temperature range of 1790 ° C. or higher and 1880 ° C. or lower exceeds 20 hours, not only the growth of the columnar β-type silicon nitride particles proceeds excessively, but also a long time is required to manufacture the plate-shaped silicon nitride sintered body. However, it is not preferable because it leads to an increase in cost. In particular, the plate-shaped silicon nitride sintered body obtained under the sintering conditions of a maximum holding temperature of over 1880 ° C. and a holding time of over 20 hours, in which the growth of columnar β-type silicon nitride particles is remarkably fast, has a long axis. The number of β-type silicon nitride particles having a length of more than 10 μm is remarkably large, and although the thermal conductivity is high, the mechanical properties are remarkably inferior. For example, the bending strength is reduced to less than 700 MPa. Further, the holding time in the above temperature range may be 8 hours or more or 14 hours or less.

In the cooling process after the above sintering, it is preferable to lower the temperature up to 1500 ° C. at a rate of 350 ° C./hr or more. On the contrary, within the range in which the formation of the _MgSiN2 crystal phase at the grain boundaries can be suppressed, the temperature is gradually cooled down to 1000 ° C. at a temperature lowering rate of 200 ° C./hr or less, or in the range of 1450 ° C. to 1650 ° C. It is also possible to further improve the thermal conductivity and mechanical properties by keeping at temperature for a certain period of time.

By optimizing the properties of the β-type silicon nitride particles in the silicon nitride sintered body, the thermal conductivity and bending strength can be improved. In the production of the plate-shaped silicon nitride sintered body of the present invention, a correlation was observed between the coating speed in sheet forming and the orientation of β-type silicon nitride particles after sintering. In the present invention, the orientation of β-type silicon nitride particles after sintering is controlled by adjusting the coating speed. That is, the plate-shaped silicon nitride sintered body of the present invention has an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less, further 0.40 μm or less, and further on a surface polished to 0.30 μm or less. The degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles is 0.08 or more and 0.25 or less, and the degree of orientation on the inner surface obtained by grinding from the surface to the inside by 0.08 mm or more. It is preferable that the fa is smaller than the degree of orientation fa on the surface. The degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles is the degree of orientation fa represented by the formula (1) described in the above-mentioned <method for calculating the degree of orientation fa>. The plate-shaped silicon nitride sintered body of the present invention is polished to an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less, further 0.05 μm or more, 0.40 μm or less, and further 0.30 μm or less. The degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface can be 0.08 or more and 0.25 or less, and further 0.10 or more and 0.20 or less.

The silicon nitride raw material inevitably contains a small amount of fine β-type silicon nitride particles. Since these fine β-type silicon nitride particles are columnar, they tend to tilt in a direction perpendicular to the thickness direction of the substrate when the coating speed during sheet molding is increased. In the sintering process, columnar β-type silicon nitride particles grow around the fine β-type silicon nitride particles oriented in this way as nuclei. Therefore, by changing the coating speed, columnar β-type silicon nitride obtained after sintering is obtained. It becomes possible to control the degree of orientation of particles. In the present invention, the orientation of β-type silicon nitride particles after sintering is controlled by adjusting the coating speed. Further, the plate-shaped silicon nitride sintered body of the present invention has an orientation degree fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface obtained by grinding 0.08 mm or more from the surface to the inside. It is 01 or more and less than 0.16, preferably smaller than the degree of orientation fa on the surface. The surface orientation fas described in Examples and Comparative Examples of Patent Document 6 are all 0.27 to 0.40, and the internal orientation fas described in Examples are all 0.18 to 0.29. .. The surface orientation fa of the plate-shaped silicon nitride sintered body of the present invention is smaller than the above-mentioned value disclosed in Patent Document 6. At the same time, the degree of internal orientation fa is also smaller than the above-mentioned value disclosed in Patent Document 6, and the plate-shaped silicon nitride sintered body of the present invention is described in Patent Document 6 over the entire area from the surface to the inside. Also has a small degree of orientation fa. It is more preferable that the difference between the surface orientation degree fa and the internal orientation degree fa is 0.03 or more and 0.08 or less.

Generally, a plate-shaped silicon nitride sintered body is composed mainly of coarse columnar particles and fine columnar particles, and the degree of orientation fa of the columnar particles is greatly affected by the coarse columnar particles. The degree of orientation fa can take a value from -1 to 1, and a degree of orientation fa of 0 means that the columnar particles are arranged in a disorderly manner. When the degree of orientation fa is greater than 0, the inclination of the long axis of the columnar particles with respect to the direction parallel to the surface of the plate-shaped silicon nitride sintered body (direction perpendicular to the thickness direction) is within 45 degrees. Contains more particles. Further, when the value of the degree of orientation fa approaches 1, it is shown that the inclination of the long axis of the columnar particles with respect to the direction parallel to the surface is close to 0 degrees. Reducing the inclination of the major axis of the columnar particles with respect to the direction parallel to the surface is advantageous for achieving high strength.

Therefore, in the plate-shaped silicon nitride sintered body of the present invention, the inclination of the long axis of the columnar particles with respect to the direction parallel to the surface of the sintered body (direction perpendicular to the thickness direction) from the surface to the inside is patented. The value is larger than that of the silicon nitride sintered body disclosed in Document 6. When the inclination of the major axis of the columnar particles with respect to the surface is large, the thermal conductivity in the thickness direction of the plate-shaped sintered body becomes high, which is suitable for an insulating substrate application. In particular, by controlling the internal orientation degree fa to 0.01 or more and less than 0.16, high thermal conductivity can be realized even if the coarsening of the columnar particles is suppressed. On the other hand, by controlling the surface orientation degree fa to 0.08 or more and 0.25 or less, both excellent mechanical properties (high strength and high fracture toughness) and high thermal conductivity can be satisfied.

Patent Document 9 discloses silicon nitride ceramics characterized in that the c-axis of columnar β-silicon nitride particles is oriented in the thickness direction of the substrate. In the same publication, 90% or more of the β-silicon nitride particles have a c-axis inclination of ± 20 degrees or less with respect to the thickness direction of the substrate, and 50% or more of the β-silicon nitride particles are the substrate. It is described that the thermal conductivity of the silicon nitride ceramics in which the inclination of the c-axis with respect to the thickness direction is within ± 5 degrees is high. However, contrary to the disclosure contents of the same publication, in the present invention, the β-type silicon nitride particles are not necessarily aligned and oriented in parallel with the thickness direction, and the columnar β-type nitride on the surface of the silicon nitride sintered body is formed. Even if the surface orientation fa represented by (1) above, which indicates the orientation ratio of the silicon particles, is 0.08 or more and 0.25 or less, the internal orientation fa is reduced to achieve high thermal conductivity. As will be described later, by controlling the grain growth of columnar particles and reducing the number of particles having a major axis length of more than 10 μm to 10,000 or less per 1 ^mm2 , mechanical properties (bending strength) can be achieved. And it was found that the breaking toughness value) can be increased.

On the other hand, if the surface orientation degree fa is less than 0.08, the mechanical properties (bending strength and fracture toughness value) are lowered, which is not preferable. A more preferable range of the degree of orientation fa of the columnar β-type silicon nitride particles on the surface is 0.10 to 0.20. Further, the degree of orientation fa may be 0.12 or more, 0.14 or more, and 0.18 or less.

Here, the polished surface is a surface obtained by, for example, barrel polishing, honing processing, lapping polishing, polishing polishing and buffing.

Further, the microstructure of the silicon nitride sintered body of the present invention contains columnar β-type silicon nitride particles having a major axis length of 10 μm or more, which is a good thermal conductor, in the matrix. The length of the major axis of the columnar β-type silicon nitride particles is the oxygen content and sintering conditions (heating rate, holding time at maximum holding temperature and holding time at maximum holding temperature) of the Si _{3N 4 powder} used as a raw material. Can be controlled by adjusting.

When observing the cut surface of the silicon nitride sintered body with a scanning electron microscope or the like, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is 500 or more per 1 mm ² . When the number is 10,000 or less, the bending strength and the fracture toughness value are significantly increased. The number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is preferably 800 or more and 9000 or less per 1 mm ² , and more preferably 1000 or more and 5000 or less per 1 mm ² . Is more preferable. On the other hand, when the maximum holding temperature at the time of sintering is too high and the number of particles having a major axis length exceeding 10 μm exceeds 10,000 particles per 1 mm ² , this coarseness is introduced into the structure. Since the particles act as the starting point of fracture, the fracture toughness is greatly reduced, and the four-point bending strength at room temperature is less than 900 MPa. Become. Conventionally, high thermal conductivity has been achieved by setting the number of long shafts exceeding 10 μm to a value exceeding 10,000 per 1 mm ² , but the presence of coarse columnar particles has mechanical properties (strength and fracture). It has an adverse effect on toughness). In the present invention, by controlling the surface orientation fa to 0.08 or more and 0.25 or less and the internal orientation fa to 0.01 or more and less than 0.16, the coarsening of columnar particles is suppressed and high. It was possible to achieve both thermal conductivity and excellent mechanical properties (strength and fracture toughness). On the other hand, if the maximum holding temperature at the time of sintering is too low and the number of long shafts exceeding 10 μm is less than 500 per 1 mm ² , not only the thermal conductivity is lowered, but also the fracture toughness value is lowered. It is not preferable because it decreases.

In the plate-shaped silicon nitride sintered body of the present invention, the surface may not be polished, but the surface is polished and the arithmetic mean roughness Ra of the surface is 0.06 μm or more and 0.4 μm or less. Further, it is preferably 0.30 μm or less and 0.20 μm or less. If the arithmetic average roughness Ra is less than 0.06 μm, the bending strength of the plate-shaped silicon nitride sintered body decreases due to residual stress during processing and the like. On the contrary, if the arithmetic average roughness Ra exceeds 0.4 μm, it becomes difficult to join the metal plate for forming a circuit, which is not preferable. In particular, it becomes difficult to join a copper plate or an aluminum plate by a direct joining method (DBC method) that does not use an active metal brazing material.

It is preferable that the open porosity on the surface polished to have an arithmetic average roughness Ra of 0.06 μm or more and 0.4 μm or less is 1.0% or less, and the maximum opening diameter of the open pores is 1.0 μm or less. Excellent electrical characteristics can be expected when the maximum opening diameter of the open pores on the surface is 1.0 μm or less. In particular, it is more preferable that the maximum opening diameter of the open pores on the surface is 0.5 μm or less. Since such a plate-shaped silicon nitride sintered body having a high density and few residual pores has excellent insulation resistance and withstand voltage, it is suitable for electronic substrate applications such as an insulating substrate and a circuit board.

The maximum opening diameter and open porosity on the polished surface were calculated as follows. First, using a scanning electron microscope (SEM), an image of 5 observation fields in a region set to 60 μm × 44 μm per observation field from the polished surface of the silicon nitride sintered body at an observation magnification of 2000 times. Was taken in. The maximum opening diameter was determined by measuring the diameter of the largest open pore in 5 observation fields / measurement total area 13200 μm ² with an image analyzer (Mac-View manufactured by Mountech Co., Ltd.). Next, using the same image analysis device, the measurement area of one field in the image was 400 μm ² , the number of measurement fields was 12, that is, the total measurement area was 4800 μm ² , and the area of the open pores in the total measurement area was obtained. The area of the open pores was divided by the total measured area, and the ratio of the area of the open pores to the total measured area was taken as the open pore ratio on the surface. This made it possible to calculate the open porosity on the surface.

As the silicon nitride raw material, silicon nitride powder having an oxygen content of 1.2% by weight or more and 2.3% by weight or less is used. Use silicon nitride powder having a specific surface area of 13.0 m ² / g or more. Preferably, as the silicon nitride raw material, a silicon nitride powder having a specific surface area of 13.0 m ² / g or more, an oxygen content of 1.2% by weight or more and 2.3% by weight or less, and an aluminum content of less than 50 ppm is used. use. A more preferable silicon nitride raw material has a specific surface area of 13.5 m ² / g to 25.0 m ² / g and an oxygen content of 1.25% by weight or more and 2.2% by weight or less. Particularly preferably, the specific surface area is 15.1 m ² / g to 25.0 m ² / g, and the oxygen content is 1.3% by weight or more and 2.0% by weight or less. Oxygen contained in the silicon nitride raw material is classified into surface oxygen existing from the particle surface to 3 nm directly below the particle surface and internal oxygen existing from 3 nm directly below the particle surface to the inside. The oxygen content is the sum of the surface oxygen content and the internal oxygen content. When the content ratio of surface oxygen is FSO (% by weight) and the content ratio of internal oxygen is FIO (% by weight), the FSO of the silicon nitride raw material is 0.76 to 1.10% by weight. More preferred. Further, it is particularly preferable that the FSO is 0.80 to 1.00% by weight.

The silicon nitride powder used in the present invention having a specific surface area of 13.0 m ² / g or more, an oxygen content of 1.2% by mass or more and 2.3% by mass or less, and an aluminum content of less than 50 ppm is used. For example, it can be produced by the method disclosed in Patent Document 8, and the content ratio of oxygen existing from the particle surface to 3 nm directly below the particle surface is defined as FSO (mass%), and the oxygen existing from 3 nm directly below the particle surface to the inside is defined as FSO (mass%). When the content ratio of is FIO (mass%) and the specific surface area is FS (m ² / g), the FSO / FS is 0.04 to 0.125 ((g · mass%) / m ² ). , FIO / FS is 0.045 ((g · mass%) / m ² ) or less, but is not limited to this. Here, mass% and weight% are the same value. The FSO / FS may be expressed as 0.4 to 1.25 (mg / m ² ), and the FIO / FS may be expressed as 0.45 (mg / m 2) or less. Reducing the aluminum content of the silicon nitride powder to less than 50 ppm reduces the aluminum content in the raw material for producing the silicon nitride powder, and at the same time, mixes aluminum oxide in the process of manufacturing the silicon nitride powder (for example, mixing from a pulverizing medium). ) Is possible.

If the specific surface area is 13.0 m ² / g or more and the oxygen content is 1.2% by weight or more and 2.3% by weight or less, two types of silicon nitride having different specific surface areas are used to control the particle size distribution. The powder may be mixed. For example, a silicon nitride powder having a specific surface area of 10.0 m ² / g or less and an oxygen content of less than 1.2% by weight and a silicon nitride powder having a specific surface area of 13.5 m ² / g or more and an oxygen content of 1.3% by weight or more. Even if a raw material mixed with silicon nitride powder is used, the specific surface area of the mixed silicon nitride raw material is 13.0 m ² / g or more, and the oxygen content is 1.2% by weight or more and 2.3% by weight or less. If the aluminum content is less than 50 ppm, the effect of the present invention can be obtained.

If the specific surface area of the silicon nitride powder is less than 13.0 m ² / g, the driving force for sintering decreases, so the density must be increased to an amount exceeding 7.0% by weight of the sintering aid. It is difficult to obtain a plate-shaped silicon nitride sintered body. Similarly, even if the oxygen content is less than 1.2% by weight, the progress of sintering is significantly slowed down, and the density is high unless the amount of the sintering aid added is increased to more than 7.0% by weight. It is difficult to obtain a plate-shaped silicon nitride sintered body. On the other hand, if the amount of the sintering aid added exceeds 7.0 wt%, the thermal conductivity decreases, which is not preferable.

Further, when the oxygen content is less than 1.2% by weight, the open porosity on the surface polished to an arithmetic average roughness Ra of 0.06 μm or more and 0.4 μm or less exceeds 1.0%, and the open pores are formed. It is not preferable because the maximum opening diameter of is a large value exceeding 1.0 μm. In particular, when the specific surface area is less than 13.0 m ² / g and the oxygen content is less than 1.2% by weight, the maximum opening diameter of the open pores becomes a larger value, which is not preferable. As the open porosity increases, the mechanical properties (strength and toughness) deteriorate. Further, when the maximum opening diameter becomes a large value exceeding 1.0 μm, the insulation resistance and the withstand voltage are deteriorated, and it becomes difficult to apply it to the electric insulating material application such as an insulating substrate and a circuit board. If the oxygen content exceeds 2.3% by weight, a high-density plate-shaped silicon nitride sintered body can be obtained, but thermal conductivity and mechanical properties (strength, fracture toughness) are deteriorated, which is not preferable. In particular, the decrease in thermal conductivity is remarkable.

When silicon nitride powder having an aluminum content of 50 ppm or more is used, the amount of aluminum that dissolves in the β-type silicon nitride particles after sintering increases. Phonon scattering by solid-dissolved aluminum ions causes a decrease in thermal conductivity, and the thermal conductivity of the obtained plate-shaped silicon nitride sintered body decreases to less than 90 W / (m · K), which is not preferable. A more preferable range of the measured aluminum content is 40 ppm or less, and within the range of the experimental conditions of the present invention, when the silicon nitride powder having an aluminum content of 40 ppm or less is used, the aluminum content is plate-like. The effect on the properties of the silicon nitride sintered body was not noticeable.

According to the present invention, a plate-shaped silicon nitride sintered body having both high thermal conductivity and excellent mechanical properties by a sheet forming process, which has conventionally lacked performance in terms of both thermal conductivity and mechanical properties. Is advantageous in terms of manufacturing cost because it can be manufactured. That is, according to the present invention, the thermal conductivity is 90 W / (m · K) or more at room temperature, the four-point bending strength is 900 MPa or more at room temperature, and the breaking toughness value measured by the IF method (indentation method). It is possible to manufacture a plate-shaped silicon nitride sintered body having both high thermal conductivity and excellent mechanical properties, which has a KIC of 7.6 _MPa√m or more, and has both thermal conductivity and mechanical properties. As a well-balanced plate-shaped silicon nitride sintered body, it can be used for electronic substrates such as insulating substrates and circuit substrates.

Further, according to the present invention, the thermal conductivity is 100 W / (m · K) or more at room temperature, the four-point bending strength is 1000 MPa or more at room temperature, and the fracture toughness value measured by the IF method (indentation method). It is possible to produce a plate-shaped silicon nitride sintered body having a KIC of 9.0 _MPa√m or more, which has both high thermal conductivity and excellent mechanical properties.

The present invention will be described in more detail with reference to specific examples below, but the present invention is not limited to those examples.

(Example 1)
Magnesium oxide (MgO) powder (specific surface area 3 m ² / g, manufactured by High Purity Chemical Laboratory) and yttrium oxide (Y ₂ O ₃ ) powder (specific surface area 3 m ² / g, manufactured by Shinetsu Chemical Industry Co., Ltd.) are used as sintering aids. I prepared it.

Silicon nitride balls, which are the crushing medium, usually contain a few percent of Al ₂ O ₃ , and the amount of wear during ball mill processing is large. Therefore, the compounded powder after adjusting the raw materials contains around 20 ppm of Al ₂ O ₃ . It is mixed. Therefore, in this embodiment, the Al ₂ O ₃ content is around 1.9 wt%, and a silicon nitride ball having particularly excellent wear resistance is used, and the amount of Al ₂ O ₃ mixed in when preparing the raw material. Was minimized.

As a sintering aid in 94.5 parts by mass of silicon nitride (Si ₃ N ₄ ) powder having a specific surface area of 18.5 m ² / g, an oxygen content of 1.77 wt%, and a β-type silicon nitride content of 3.5% by mass. A toluene-isopropanol-xylene solvent obtained by blending 3.5 parts by mass of the above-mentioned yttrium oxide and 2 parts by mass of the above-mentioned magnesium oxide and dissolving 2 parts by mass of a sorbitan ester-based dispersant in a powder and silicon nitride as a pulverizing medium. It was put into a resin pot for a ball mill together with a ball-making ball, and wet-mixed for 24 hours. After passing the obtained slurry through a sieve having an opening of 44 μm, 16 parts by mass of a PVB-based resin binder and 4 parts by mass of a plasticizer (dimethyl phthalate) were dissolved in 100 parts by mass of the mixed powder in the resin pot. -Isopropanol-xylene solvent was added and wet-mixed for another 24 hours to obtain a sheet-forming slurry. After vacuum defoaming to adjust the amount of solvent so that the viscosity of this molding slurry is about 50 poise, the obtained mixed powder slurry is cast on a carrier film to a predetermined thickness using a doctor blade device. , A sheet-molded green sheet was obtained.

Further, the obtained green sheet was laminated and crimped at a temperature of 120 ° C. and a predetermined pressure to prepare a laminated molded sheet having a baked size of about 0.35 mm. The produced laminated molded sheet was visually inspected to confirm the presence or absence of cracks. Then, this laminated molded body sheet was cut into 60 mm × 70 mm, and the dimensions, average thickness and weight were measured to calculate the molded body density. The laminated molded sheet sheet density in this example was 1.76 g / cm ³ . Further, for measuring the bulk density and the thermal conductivity of the sintered body, the number of laminated green sheets is increased, and the size of the green sheet is 10 mm in diameter and 1.0 mm in thickness for a disk-shaped test piece. A molded sheet was cut out.

Next, the laminated molded sheet was placed in a boron nitride container in layers via a separating material and heated in air at 400 to 600 ° C. for 2 to 5 hours to sufficiently add the organic binder component and the like added in advance. Degreased (removed). Next, this degreased body is heated to 1520 ° C. under a nitrogen atmosphere of 0.8 MPa, heated to 1800 ° C. at a heating rate of 120 ° C./hr from 1520 ° C. to 1800 ° C., and further heated to 1800 ° C. at 1800 ° C. It was held for a long time and sintered. Then, the cooling rate to 1500 ° C. was set to 350 ° C./hr, and then the mixture was cooled to room temperature, and the obtained silicon nitride sintered body was peeled off by a separating material layer to obtain a plate-shaped silicon nitride sintered body. rice field. The obtained plate-shaped silicon nitride sintered body was blast-polished to obtain a silicon nitride sintered body for a substrate having a desired surface roughness. The thickness removed by the blast polishing process was 10 μm or less on average.

The total oxygen content FTO and the surface oxygen content FSO of the silicon nitride powder used in the present invention were measured by the following methods. First, the silicon nitride powder is weighed, and the total oxygen content FTO, which is the total of the surface oxygen and the internal oxygen of the silicon nitride powder, is determined by the inert gas melting-carbon dioxide infrared absorption method based on the quantification method of JIS R1603-10 oxygen. It was measured with a TC-136 type manufactured by LECO. Next, the weighed silicon nitride powder was mixed with the silicon nitride powder and the hydrofluoric acid aqueous solution so that hydrogen fluoride was 5 parts by mass with respect to 1 part by mass of the silicon nitride powder, and the mixture was stirred at room temperature for 3 hours. This was suction-filtered, and the obtained solid matter was vacuum-dried at 120 ° C. for 1 hour, and then the weight and oxygen content of the hydrofluoric acid-treated powder were measured. This value was taken as FIO (mass% with respect to hydrofluoric acid-treated powder) before correction. The internal oxygen amount FIO (mass% with respect to the silicon nitride powder) was calculated from the following formula (4), and the surface oxygen amount FSO (mass% with respect to the silicon nitride powder) was calculated from the following formula (5). The fact that the amount of surface oxygen thus determined is due to the oxygen existing in the range of 3 nm below the particle surface from the particle surface is the depth profile of the X-ray photoelectron spectrum of the silicon nitride powder before and after the hydrofluoric acid treatment. It was confirmed from the change in powder weight before and after the treatment.
FIO (mass%) = ((hydrofluoric acid-treated powder weight) (g)) / (silicon nitride powder weight (g)) × pre-correction FIO (mass%) ... (4)
FSO (mass%) = FTO (mass%) -FIO (mass%) ... (5)

The appearance of the obtained plate-shaped silicon nitride sintered body was inspected, and the presence or absence of color tone unevenness was visually determined, and the presence or absence of patterns having different color tones was confirmed by a CCD camera.

The bulk density of the obtained plate-shaped silicon nitride sintered body was measured by the Archimedes method for measuring the weight and buoyancy of the test piece suspended from the thin wire. The relative density (ratio to the theoretical density based on the compounding composition) was obtained from the bulk density.

A RINT-TTRIII type wide-angle X-ray diffractometer manufactured by Rigaku Co., Ltd. was used for measuring the X-ray diffraction pattern of the obtained plate-shaped silicon nitride sintered body. The X-ray source is CuKα ray, and each diffraction peak ((110) plane, (200) plane, (101) plane, (210) plane, (201) plane, (310) plane, (320) plane of β-type silicon nitride. The peak intensities of the (plane) and (002) planes) were examined, and the presence or absence of a diffraction peak caused by MgSiN ₂ was examined. Further, it was confirmed whether or not the crystal phase caused by the sintering aid component was precipitated at the grain boundaries in addition to β-type silicon nitride and MgSiN ₂ .

The arithmetic average roughness Ra of the surface of the obtained plate-shaped silicon nitride sintered body was measured according to JIS B 0601-2001 (ISO4287-197). Using a stylus type surface roughness meter, a stylus with a stylus tip radius of 2 μm is applied to the polished surface of the silicon nitride sintered body, the measurement length is 5 mm, and the scanning speed of the stylus is 0. The surface roughness was measured by setting it to 5 mm / sec, and the average value of the five points obtained by this measurement was taken as the value of the arithmetic mean roughness Ra.

A bending test piece having a width of 4.0 mm, a thickness of 0.35 mm, and a length of 40 mm was used for measuring the bending strength of the obtained plate-shaped silicon nitride sintered body. Using a universal material tester manufactured by Instron, the method conforms to JIS R1601 except that the thickness (0.35 mmt) of the test piece is different. The four-point bending strength of was measured.

The fracture toughness value of the obtained plate-shaped silicon nitride sintered body was measured by the IF method based on JIS-R1607: 2015. A Vickers indenter is pushed into the mirror-polished surface of the plate-shaped silicon nitride sintered body with a predetermined indenter pressing load (5 kgf (49N)) for 15 seconds, and one diagonal of the Vickers indentation is a plate-shaped silicon nitride sintered body. The length of the diagonal line of the Vickers indentation and the length of the crack generated on the extension of the diagonal line were measured so as to be perpendicular to the thickness direction of the body. The fracture toughness value _KIC was calculated from the obtained measured length.

For the thermal conductivity measurement of the obtained plate-shaped silicon nitride sintered body, a disk-shaped test piece having a diameter of 10 mmφ and a thickness of 1 mmt was produced by the above method. Using this disk-shaped test piece, the thermal conductivity was measured at room temperature by the flash method conforming to JIS R1611.

Further, using a scanning electron microscope (SEM), an area of 0.01 mm ² (1/100 of 1 mm ² ) of the cut surface of the plate-shaped silicon nitride sintered body can be arbitrarily formed at an observation magnification of 1000 times. After observing three places, the number of columnar β-type silicon nitride particles having a major axis length exceeding 10 μm in the region was examined, converted into the number per 1 mm ² , and then the average value was obtained.

<Calculation method of orientation degree fa>
The degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface and inside of the plate-shaped silicon nitride sintered body, was determined as follows.

In order to obtain the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface, X-ray diffraction measurement of the surface polished to an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less was performed. When the arithmetic mean roughness Ra of the surface of the silicon nitride sintered body was not 0.05 μm or more and 0.5 μm or less, the surface was polished to adjust the arithmetic mean roughness Ra to 0.05 μm or more and 0.5 μm or less. The X-ray diffraction measurement is performed on the (110) plane, the (200) plane, the (101) plane, the (210) plane, the (201) plane, the (310) plane, the (320) plane, and the (002) plane. The pattern intensity was measured. In order to obtain the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles inside, the arithmetic average roughness Ra obtained by measuring the degree of orientation fa on the surface was polished to 0.05 μm or more and 0.5 μm or less. The surface is further ground to the inside of about 0.10 mm of the sintered body, and the X-ray diffraction measurement of the obtained surface is performed, and the (110) surface, the (200) surface, the (101) surface, and the (210) surface are measured. , (201) plane, (310) plane, (320) plane, and (002) plane X-ray diffraction pattern intensity was measured. For the above-mentioned grinding to the inside of about 0.10 mm, abrasive grains of about # 150 are used for rough polishing, and abrasive grains of about # 400 are used for finish polishing, and the arithmetic average roughness Ra is 0.05 μm or more and 0. Polished to 1.5 μm or less.

The degree of orientation of the hexagonal columnar particles is F.I. K. It is expressed by the following equation (1) proposed by Lotgerling (see F.K. Lotgerling, J. Inorg. Nucl. Chem., 9 (1959), pp. 113-123). Therefore, based on the results of the X-ray diffraction measurement of the surface and the inner surface, the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles was calculated from the formula represented by the following formula (1).
fa = (P-P ₀ ) / (1-P ₀ ) ... (1)
In this formula (1), P is represented by the following formula (2), and I (110), I (200), I (210), I (310), I (320), I (101), I. (201) and I (002) are the (110) plane, (200) plane, (210) plane, (310) plane, (320) plane, (101) plane, (201) plane, and (. 002) It means the X-ray diffraction peak intensity of each surface.
Further, P ₀ is represented by the following equation (3), I ₀ (110), I ₀ (200), I ₀ (101), I ₀ (210), I ₀ (201), I ₀ (310). , I ₀ (320), and I ₀ (002) are the (110), (200), (101), and (210) planes of the β-type silicon nitride in the isotropic β-type silicon nitride powder. It is calculated from the X-ray diffraction pattern intensity of the (201) plane, the (310) plane, the (320) plane, and the (002) plane. In the present invention, the measured value of _P0 of the β-type silicon nitride powder was 0.65.
P = (I (110) + I (200) + I (210) + I (310) + I (320)) / (I (110) + I (200) + I (101) + I (210) + I (201) + I (310) + I (320) + I (002)) ... (2)
P ₀ = (I ₀ (110) + I ₀ (200) + I ₀ (210) + I ₀ (310) + I ₀ (320)) / (I ₀ (110) + I ₀ (200) + I ₀ (101) + I ₀ ( 210) + I ₀ (201) + I ₀ (310) + I ₀ (320) + I ₀ (002)) ... (3)

The maximum opening diameter and open porosity on the polished surface were calculated as follows. First, using a scanning electron microscope (SEM), an image of 5 observation fields in a region set to 60 μm × 44 μm per observation field from the polished surface of the silicon nitride sintered body at an observation magnification of 2000 times. Was taken in. The maximum opening diameter was determined by measuring the diameter of the largest open pore in 5 observation fields / measurement total area 13200 μm ² with an image analyzer (Mac-View manufactured by Mountech Co., Ltd.). Next, using the same image analysis device, the measurement area of one field in the image was 400 μm ² , the number of measurement fields was 12, that is, the total measurement area was 4800 μm ² , and the area of the open pores in the total measurement area was obtained. The area of the open pores was divided by the total measured area, and the ratio of the area of the open pores to the total measured area was taken as the open pore ratio on the surface. This made it possible to calculate the open porosity on the surface.

The obtained plate-shaped silicon nitride sintered body was crushed and crushed, and passed through a sieve having an opening of 250 μm. The oxygen content of the crushed product sample was measured by an inert gas melting-carbon dioxide infrared absorption method (manufactured by LECO, TC-136 type) based on the JIS R1603-10 oxygen quantification method.

0.5 g of the above-mentioned crushed product sample is placed in a Teflon (registered trademark) pressure decomposition container for analysis together with nitric acid and hydrofluoric acid, and after being heated and decomposed by irradiating with microwaves, the volume is fixed with ultrapure water. It was used as a test solution. Next, quantitative analysis of each metal element (aluminum, yttrium, magnesium, scandium, erbium, lutetium) in the test solution was performed by an ICPE-9820 type inductively coupled plasma emission spectroscopic analysis (ICP-AES) apparatus manufactured by Shimadzu Corporation.

The composition and properties of the raw material powder used in the production of the silicon nitride sintered body, the outline of the coating conditions in sheet molding and the production conditions of the silicon nitride sintered body, and the obtained plate-shaped silicon nitride sintered body. The measurement results of the above evaluation items regarding the chemical composition and characteristics of the body are shown in Tables 1, 2 and 3. In Tables 1 to 3, Examples 1 to 52 are examples of the present invention, and Comparative Examples 1 to 21 are comparative examples to the present invention. The bending strength at room temperature is the 4-point bending strength, and the number of coarse β particles is the long axis of the β-type silicon nitride particles observed in the 1 mm ² region of the cut surface perpendicular to the plate surface of the silicon nitride sintered body. Represents the number of β-type silicon nitride particles having a length of more than 10 μm.

(Example 2)
A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 1 except that the sintering temperature was raised to 1850 ° C. Table 2 shows the sintering conditions and the chemical composition of the obtained plate-shaped silicon nitride sintered body, and Table 3 shows the characteristics of the obtained plate-shaped silicon nitride sintered body. By increasing the sintering temperature, the number of β-type silicon nitride particles having a major axis length of more than 10 μm increased, and the thermal conductivity increased.

(Examples 3 and 4)
A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 1 except that the holding time at the maximum temperature during sintering was changed. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 3, probably because the holding time at the maximum temperature was 6 hours, the oxygen content of the sintered body was slightly high, the number of coarse β particles decreased, and the thermal conductivity and fracture toughness value decreased slightly. bottom.

(Example 5)
Example 2 except that the silicon nitride raw material (specific surface area 16.9 m ² / g, oxygen content 1.50 wt%, β-type silicon nitride content ratio 3.0% by mass) and the holding time at the maximum temperature were changed. Similarly, a plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. High thermal conductivity and excellent mechanical properties (bending strength and breaking toughness value) even when the specific surface area of silicon nitride (Si ₃ N ₄ ) powder is 16.9 m ² / g and the oxygen content is 1.50 wt%. )showed that.

(Example 6)
Plate-shaped nitriding Kay under the conditions shown in Tables 1 and 2 in the same manner as in Example 2 except that the coating speed and the holding time at the maximum temperature in sheet forming using the doctor blade device were changed. A raw sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. By extending the holding time at the maximum temperature, the number of coarse β particles increased and the thermal conductivity increased.

(Examples 7 to 9)
Silicon Nitride Raw Materials (Examples 7 and 8: Specific Surface Area 13.7 m ² / g, Oxygen Content 1.25 wt%, β-Type Silicon Nitride Content 2.2 Mass%, Example 9: Specific Surface Area 14.0 m ² / g, oxygen content 1.30 wt%, β-type silicon nitride content ratio 2.3 mass%) and the coating speed in sheet forming using the doctor blade device were changed under the conditions shown in Tables 1 and 2. , A plate-shaped silicon nitride sintered body was obtained in the same manner as in Example 1. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. By changing the silicon nitride raw material, the laminated molded body sheet density of Examples 7 and 8 increased to 1.97 g / cm ³ , and the laminated molded body sheet density of Example 9 increased to 1.95 g / cm ³ . In Examples 7 and 9, the thermal conductivity and the fracture toughness value were slightly lowered probably because the oxygen content of the sintered body was slightly high.

(Examples 10 to 12)
A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 7 except that the sintering conditions were changed. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 12, the fracture toughness value KIC increased to 9.5 _MPa√m by holding at 1850 ° C. for 20 hours.

(Example 13)
Tables 1 and 2 are the same as in Example 3 except that the weight ratio of magnesium oxide to the rare earth oxide (magnesium oxide / rare earth oxide) and the holding time at the maximum holding temperature at the time of sintering are changed. A plate-shaped silicon nitride sintered body was obtained under the conditions described in 1. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. The oxygen content of the sintered body was rather high, the number of coarse β particles decreased, and the thermal conductivity decreased slightly.

(Examples 14 to 16)
Plate-like nitriding under the conditions described in Tables 1 and 2 in the same manner as in Example 2 except that the rare earth oxide was changed to Sc ₂ O ₃ , or Er ₂ O ₃ , or Lu ₃ O ₃ . A siliconic sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 16, the bending strength increased.

(Example 17)
The conditions shown in Tables 1 and 2 are the same as in Example 2 except that the weight ratio of magnesium oxide to the rare earth metal oxide and the coating speed in the sheet forming using the doctor blade device are changed. A plate-shaped silicon nitride sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Even when the weight ratio of magnesium oxide and rare earth metal oxide was changed, it showed high thermal conductivity and excellent mechanical properties (bending strength and fracture toughness value).

(Example 18)
Examples except that the weight ratio of magnesium oxide and rare earth metal oxide was maintained at 1550 ° C. for 2 hours in the heating process, and the heating rate from 1550 ° C. to the maximum holding temperature was changed to 140 ° C./hr. In the same manner as in 2, a plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. By holding at 1550 ° C. for 2 hours, the oxygen content of the sintered body was reduced as compared with the compounding composition, and high thermal conductivity and excellent mechanical properties (bending strength and fracture toughness value) were exhibited.

(Example 19)
Except for changing the weight ratio of the silicon nitride raw material (specific surface area 16.9 m ² / g, oxygen content 1.50 wt%, β-type silicon nitride content 3.0% by mass) and magnesium oxide to rare earth metal oxides. , A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Since the weight ratio of magnesium oxide to the rare earth oxide was changed to magnesium oxide / rare earth oxide = 1.4, the fracture toughness value was slightly lowered.

(Example 20)
A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 4 except that the weight ratio of magnesium oxide and the rare earth metal oxide was changed. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Even when the weight ratio of magnesium oxide to rare earth oxide was magnesium oxide / rare earth oxide = 0.62, high thermal conductivity and excellent mechanical properties (bending strength and breaking toughness value) were exhibited.

(Examples 21 and 22)
It is shown in Tables 1 and 2 in the same manner as in Example ₂ except that the addition amounts and weight ratios of magnesium oxide ₍ MgO) and yttrium oxide (Y2O3), which are sintering aids, were changed. Under the conditions, a plate-shaped silicon nitride sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 22, probably because the amount of the sintering aid added was large, the measured oxygen content of the sintered body was slightly high, and the thermal conductivity and the fracture toughness value were slightly lowered.

(Example 23)
A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 2 except that the gas pressure at the time of sintering was lowered to 0.4 MPa. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. At a gas pressure of 0.4 MPa, a silicon nitride sintered body having almost the same characteristics as that at 0.8 MPa was obtained.

(Examples 24 to 26)
The amount of the sintering aid added is 6.5% by weight, the weight ratio of magnesium oxide and yttrium oxide is 0.4, the coating speed in sheet molding using the doctor blade device, and the sintering conditions (maximum temperature). (Retention time at) was changed. Further, in Example 24, after holding at 1550 ° C. for 2 hours in the heating process, the heating rate from 1550 ° C. to the maximum holding temperature was 120 ° C./hr (in Examples 25 and 26, from 1520 ° C. to 1880 ° C. The temperature rise rate was 120 ° C./hr), and in Example 26, the silicon nitride raw material (specific surface area 13.7 m ² / g, oxygen content 1.25 wt%, β-type silicon nitride content ratio 2.2 mass%) was changed. rice field. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 24, since the number of coarse β particles was 3800, high thermal conductivity and excellent mechanical properties (strength and fracture toughness value) were exhibited. In Example 25, the mechanical properties (strength and fracture toughness value) were slightly lowered probably because the number of coarse β particles was increased. In Example 26, the ratio of the measured magnesium content as the sintered body to the measured rare earth metal content is lowered to the measured magnesium content / measured rare earth metal content = 0.27, and the number of coarse β particles. The mechanical properties (strength and fracture toughness value) were further reduced, although it was slightly increased.

(Example 27)
Except for changing the silicon nitride raw material (specific surface area 16.4 m ² / g, oxygen content 1.46 wt%, β-type silicon nitride content 2.7 mass%) and using silicon nitride powder having an aluminum content of 40 ppm. , A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Almost no deterioration in characteristics was observed up to the measured aluminum content of 43 ppm in the sintered body.

(Examples 28 and 29)
The conditions for surface polishing of the obtained plate-shaped silicon nitride sintered body were changed, and in Example 29, a silicon nitride raw material (specific surface area 16.9 m ² / g, oxygen content 1.50 wt%, β-type nitriding) was further changed. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2 in the same manner as in Example 2 except that the silicon content ratio (3.0% by mass) was changed. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Almost no deterioration in characteristics was observed up to the range of surface roughness shown in Table 3, and high thermal conductivity and high bending strength were exhibited.

(Examples 30 to 32)
The coating speed in sheet molding using the doctor blade device was changed, and the sintering conditions (holding time at the maximum holding temperature) were changed, and the conditions shown in Tables 1 and 2 were the same as in Example 4. A plate-shaped silicon nitride sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. In Example 30, the thermal conductivity was slightly lower than that in Example 31, probably because the oxygen content of the sintered body was slightly high. On the contrary, in Example 32, the mechanical properties (strength and fracture toughness value) were slightly lower than those in Example 31, probably because the oxygen content of the sintered body was slightly low.

(Example 33)
Example 33 is an example in which the gas pressure at the time of sintering is increased to 2.0 MPa. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. Even at a gas pressure of 2.0 MPa, the same characteristics as in the case of 0.8 MPa can be obtained, but the evaporation of magnesium oxide is suppressed by the high gas pressure, so that the oxygen content of the sintered body becomes slightly higher. No significant effect of improving the characteristics was observed by increasing the gas pressure to 2.0 MPa.

(Examples 34 and 35)
In Example 34, the lap polishing process was performed after the blast polishing. Table 1 and Table are the same as in Example 2 except that the coating speed in sheet forming using the doctor blade device and the conditions for surface polishing of the obtained plate-shaped silicon nitride sintered body are changed. A plate-shaped silicon nitride sintered body was obtained under the conditions described in 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. It was found that the mechanical properties (strength and fracture toughness value) tended to decrease slightly in the surface roughness shown in Table 3.

(Example 36)
A plate-shaped silicon nitride sintered body was prepared under the conditions shown in Tables 1 and 2 in the same manner as in Example 6 except that the weight ratio of magnesium oxide and yttrium oxide was changed to 0.60. Obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Even when the weight ratio of magnesium oxide to rare earth oxide was magnesium oxide / rare earth oxide = 0.60, high thermal conductivity and excellent mechanical properties (bending strength and breaking toughness value) were exhibited.

(Examples 37 and 38)
The table is the same as in Example 8 except that the weight ratio of magnesium oxide and yttrium oxide, the coating speed in sheet forming using the doctor blade device, and the sintering conditions (maximum holding temperature and holding time) are changed. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in 1 and Table 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Due to the influence of the holding time at the maximum holding temperature, the grain growth of the β-type silicon nitride particles was slightly insufficient, and the thermal conductivity was slightly lowered.

(Example 39)
Plate-shaped silicon nitride firing under the conditions shown in Tables 1 and 2 in the same manner as in Example 6 except that the coating speed in the sheet forming using the silicon nitride raw material and the doctor blade device was changed. I got a unity. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

Even if the material is changed to a silicon nitride raw material with a specific surface area of 15.5 m ² / g, an oxygen content of 1.4% by mass (surface oxygen content of 0.98% by mass), and a β-type silicon nitride content of 2.5% by mass. It showed high thermal conductivity and excellent mechanical properties (bending strength and breaking toughness value) comparable to Example 6.

In Examples 40 to 46, the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body were investigated in detail, paying particular attention to the influence of the weight ratio of magnesium oxide and yttrium oxide.

(Examples 40 to 42)
The amount of the sintering aid added was 5.9% by weight, the weight ratio of magnesium oxide and yttrium oxide was changed, the coating speed in sheet molding using the doctor blade device was changed, and in Example 42, the sintering conditions (maximum) were changed. The same as in Example 2 except that the holding time at the same temperature as the holding temperature was changed. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

In magnesium oxide / yttrium oxide = 0.97, 1.11 (Examples 40 and 41), the thermal conductivity was slightly lowered probably because the measured oxygen content of the sintered body was slightly high. On the other hand, magnesium oxide / yttrium oxide = 0.44 (Example 42) showed high thermal conductivity and excellent mechanical properties (bending strength and fracture toughness value). In Example 42, the mass ratio of the measured magnesium content and the measured yttrium content as the sintered body was the measured magnesium content / the measured yttrium content = 0.31.

(Examples 43 to 45)
The amount of the sintering aid added is 5.5% by weight, the weight ratio of magnesium oxide and yttrium oxide, the coating speed in sheet forming using the doctor blade device, and the sintering conditions (nitrogen gas pressure, maximum holding temperature). (Retention time at the same temperature) was changed. Except for these changes, the same as in Example 2. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

Magnesium oxide / yttrium oxide = 0.64 (Example 43) shows high thermal conductivity and excellent mechanical properties (bending strength and breaking toughness value), and magnesium oxide / yttrium oxide = 0.60, 0.57 (eg). Examples 44 and 45) also showed characteristics substantially comparable to those of Example 43. The mass ratios of the measured magnesium content and the measured yttrium content as the sintered body in Examples 43 to 45 are the measured magnesium content / measured yttrium content = 0.46, 0.43 and 0.43, respectively. there were.

It should be noted that high thermal conductivity and excellent mechanical properties (bending strength and fracture toughness value) can be realized with magnesium oxide / yttrium oxide = 0.57 in Examples 2, 4 to 6, 8, 10, 12 and 16. , 28, 31 and 39.

(Example 46)
The same as in Example 8 except that the amount of the sintering aid added was 5.5% by weight, the weight ratio of magnesium oxide and yttrium oxide, and the sintering conditions (maximum holding temperature) were changed. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

At magnesium oxide / yttrium oxide = 1.75 (the mass ratio between the measured magnesium content as a sintered body and the measured yttrium content is the measured magnesium content / the measured yttrium content = 1.23), the number of coarse β particles is The mechanical properties (strength and breaking toughness value) were slightly reduced, probably because it was a little high.

From the examination results of Examples 40 to 46, the weight ratio of magnesium oxide and yttrium oxide in the compounding composition was 0.40 ≤ magnesium oxide / rare earth metal oxide ≤ 0.66, and the measured magnesium content as a sintered body was obtained. It was confirmed that it is more preferable that the mass ratio of the measured yttrium content is 0.26 ≤ measured magnesium content / measured yttrium content ≤ 0.49.

(Examples 47 to 49)
The amount of the sintering aid added was 4.1% by weight, 3.5% by weight, and 6.5% by weight, respectively. The weight ratio of magnesium oxide to yttrium oxide, the coating speed in sheet forming using the doctor blade device, and the sintering conditions (holding time at the same temperature as the maximum holding temperature) were changed. Except for these changes, the same as in Example 6. A plate-shaped silicon nitride sintered body was obtained under the conditions shown in Tables 1 and 2. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

In Example 47, the amount of the sintering aid added is within a more preferable range (the content of the metal element derived from the auxiliary agent, which is the sum of the actually measured magnesium content and the actually measured yttrium content as the sintered body, is 2.52% by weight. ), The thermal conductivity and mechanical properties (strength and breaking toughness value) almost comparable to those of Example 6 were shown. On the other hand, in Example 48, since the amount of the sintering aid added was slightly less than 4.0% by weight, the mechanical time (strength and fracture toughness value) was slightly lowered, and in Example 49, the sintering aid was added. Since the amount of the addition was slightly larger than 6.0% by weight, the thermal conductivity was slightly lowered. Further, in Examples 48 and 49, the maximum opening diameter was also slightly large.

(Example 50)
Silicon nitride raw material (specific surface area 15.5 m ² / g, oxygen content 1.40 wt%, β-type silicon nitride content ratio 2.5% by mass), sintering conditions (holding time at maximum temperature), and obtained. Plate-shaped silicon nitride sintered body under the conditions shown in Tables 1 and 2 in the same manner as in Example 33, except that the conditions for surface polishing of the plate-shaped silicon nitride sintered body were changed. I got a body. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body. Since the degree of blast polishing in the surface polishing process was weakened, the arithmetic average surface roughness Ra was 0.46 μm, and the mechanical time properties (strength and fracture toughness value) were slightly lowered. The maximum opening diameter was also slightly large.

(Examples 51 and 52)
Examples 51 and 52 are examples in which the addition amounts of magnesium oxide ₍ MgO) and yttrium oxide ₍ Y2O3), which are sintering aids, are changed. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. The measured oxygen content was 2.82% by weight in Example 51 and 2.98% by weight in Example 52. As the amount of the sintering aid added increased, the oxygen content increased and the thermal conductivity increased. It dropped a little. Further, in Example 52, the mechanical properties (bending strength and fracture toughness value) were also slightly reduced.

When a silicon nitride raw material having a specific surface area of 13.7 to 14.0 m ² / g was used, in Examples 7, 9, 11, 26, 37, 38 and 46, the thermal conductivity or mechanical properties (bending strength and bending strength and) were used. Fracture toughness value) was slightly reduced.

When the addition amount of the sintering aid is 3.5% by weight or 6.5% by weight (the content of the metal element derived from the auxiliary agent as a sintered body is 2.01% by weight or 4.03 to 4.52). In the case of% by weight), in Examples 22, 25, 26, 48 and 49, the thermal conductivity or mechanical properties (bending strength and fracture toughness value) were slightly reduced. The weight ratio of magnesium oxide and yttrium oxide in the composition is magnesium oxide content / rare earth metal oxide content = 1.75 (measured magnesium content as a sintered body / measured rare earth metal content = 1.23). In Example 46, the thermal conductivity and mechanical properties (bending strength and breaking toughness value) were slightly reduced.

Further, when the addition amount of the sintering aid is 8.0% by weight or 8.1% by weight (the content of the metal element derived from the auxiliary agent as a sintered body is 5.18% by weight or 5.26% by weight). In the case of), in Examples 51 and 52, the thermal conductivity or mechanical properties (bending strength and fracture toughness value) were lower than those in the other examples.

When the measured oxygen content of the sintered body was less than 1.75, the mechanical properties (bending strength and fracture toughness value) were slightly lowered in Examples 15, 26, 32 and 48. When the measured oxygen content of the sintered body exceeds 2.10, Examples 3, 7, 9, 11, 13, 14, 19, 22, 23, 25, 27, 30, 33 to 35, 37, At 40, 41, 45, 46, 49 and 50, the thermal conductivity or mechanical properties (bending strength and fracture toughness value) were slightly reduced. Furthermore, in Examples 51 and 52 in which the measured oxygen content exceeded 2.8% by weight, the thermal conductivity or mechanical properties (bending strength and fracture toughness value) were lower than those in the other examples.

Examples of surface orientation fa indicating the orientation ratio of columnar β-type silicon nitride particles on a surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less is less than 0.10 or more than 0.20. At 7, 9, 11, 25, 26, 30, 32, 35, 37, 38, 44, 47, 48 and 52, the thermal conductivity or mechanical properties (bending strength and fracture toughness value) were slightly reduced. ..

In Examples 3, 7, 9, 11, 13, 30, 33, 37, 38, 45 and 52, where the number of coarse β-type silicon nitride particles is 800 to 990 particles / mm ² , the thermal conductivity or mechanical properties (Bending strength and fracture toughness value) were slightly reduced. On the other hand, in Examples 25, 26, 32, 46, 48 and 50 in which the number of coarse β-type silicon nitride particles is 5160 to 8800 particles / mm ² , the mechanical properties (bending strength and fracture toughness value) are slightly lowered. Was there.

In all the examples from Examples 1 to 52, the thickness of the plate-shaped silicon nitride sintered body excluding the disk-shaped test piece for measuring the thermal conductivity is 0.33 to 0.48 mm. The / area ratio was 1.0x10 ^-4 to 1.9x10 ^-4 (1 / mm), and the amount of the surface layer of the plate surface perpendicular to the thickness direction was 0.008 to 0.03 mm per side.

In the visual inspection, no color unevenness was observed in all the examples. The crystal phase of the Mg compound such as MgSiN ₂ was not detected by the X-ray diffraction measurement of the silicon nitride sintered body plate surface. Furthermore, the crystal phases of rare earth metal compounds such as N-merilite, H phase, J phase, and K phase were not detected.

(Comparative Examples 1 and 2)
Comparative Examples 1 and 2 are examples in which a powder having a low specific surface area or a powder having a low oxygen content is used as a raw material for silicon nitride. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. As the specific surface area or oxygen content decreases, the relative density of the obtained plate-shaped silicon nitride sintered body decreases, and as a result, properties such as thermal conductivity, bending strength, and fracture toughness value decrease. bottom.

(Comparative Example 3)
Comparative Example 3 is an example in which a powder having an excessively high oxygen content is used as a raw material for silicon nitride. The holding time at the maximum temperature was 8 hours, and the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2 are shown in Tables 2 and 3. The measured oxygen content of the sintered body was 2.55% by weight, and the grain growth of β-type silicon nitride particles was insufficient probably because the oxygen content of the silicon nitride raw material was too high (500 pieces / mm ² ). Less than). When the oxygen content was too high, the number of β-type silicon nitride particles having a major axis length of more than 10 μm decreased, and the thermal conductivity decreased.

(Comparative Example 4)
This is an example in which a silicon nitride raw material is changed and a silicon nitride powder having an aluminum content of 50 ppm is used. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. When the measured aluminum content of the sintered body increased to 55 ppm, the thermal conductivity decreased.

(Comparative Example 5)
Comparative Example 5 is an example in which the addition amounts of magnesium oxide ₍ MgO) and yttrium oxide ₍ Y2O3), which are sintering aids, are changed. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. When the amount of the sintering aid added was reduced, the relative density of the obtained plate-shaped silicon nitride sintered body decreased (the measured oxygen content of the sintered body of Comparative Example 5 was 1.34% by weight).

(Comparative Examples 6 and 7)
Comparative Examples 6 and 7 are examples in which the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide is changed. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. When the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide was lowered, the relative density of the obtained plate-shaped silicon nitride sintered body decreased (Comparative Example 6). The weight ratio (measured magnesium content / measured rare earth metal content) between the measured magnesium content and the measured rare earth metal content of the silicon nitride sintered body obtained in Comparative Examples 6 and 7 was 0.15, respectively. Since it was 2.33, whether the weight ratio of the alkaline earth metal oxide to the rare earth metal oxide in the compounding composition was too high or too low, the obtained plate-shaped silicon nitride sintered body could be obtained. The characteristics have deteriorated.

(Comparative Examples 8 to 10)
Comparative Examples 8 to 10 are examples in which the sintering conditions are inappropriate, such as the gas pressure at the time of sintering is too low, the maximum holding temperature is too low, or the maximum holding temperature is too high. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. The measured oxygen content of the sintered body of Comparative Example 9 was 2.50% by weight. Inappropriate sintering conditions reduced mechanical properties (bending strength and fracture toughness). Further, when the gas pressure at the time of sintering is too low or the maximum holding temperature is too low, the relative density of the sintered body is low and the thermal conductivity is also lowered.

(Comparative Examples 11 and 12)
Comparative Examples 11 and 12 are examples in which the holding time at the maximum temperature during sintering is too short or too long. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. The measured oxygen content of the sintered body of Comparative Example 11 was 2.48% by weight, and the grain growth of the β-type silicon nitride particles was insufficient (500 pieces / mm less than ² ). Inappropriate sintering conditions (holding time at maximum temperature too short or too long) reduced mechanical properties (bending strength and fracture toughness values). Moreover, when the holding time at the maximum temperature was too short, the thermal conductivity decreased.

(Comparative Example 13)
Comparative Example 13 is an example in which the degree of orientation fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface, becomes close to zero (slightly negative value) by changing the sheet forming conditions. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. When the β-type silicon nitride particles were randomly aligned and oriented in the thickness direction, the mechanical properties (bending strength and fracture toughness value) deteriorated.

(Comparative Example 14)
Comparative Example 14 is an example in which the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide is changed. The holding time at the maximum temperature was 25 hours, and Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. If the weight ratio of the alkaline earth metal oxide to the rare earth metal oxide is too high, the obtained silicon nitride sintering under the sintering conditions (holding time at the same temperature as the maximum temperature) shown in Table 2. The weight ratio (measured magnesium content / measured rare earth metal content) between the measured magnesium content and the measured rare earth metal content of the body was 1.40. Since the grain growth progressed too much and the number of coarse β-type silicon nitride particles exceeding 10 μm exceeded 10,000 / mm ² , the thermal conductivity increased, but the mechanical properties (bending strength and breaking toughness value). Has decreased.

(Comparative Example 15)
Comparative Example 15 is an example in which the weight ratio of the alkaline earth metal oxide and the rare earth metal oxide is small, the maximum temperature at the time of sintering is too high, and the holding time is too long. If the weight ratio of the alkaline earth metal oxide to the rare earth metal oxide is too small, the nitrogen gas pressure is raised to raise the maximum holding temperature, and if the holding time is not lengthened, a high-density silicon nitride sintered body can be obtained. I can't. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. The weight ratio (measured magnesium content / measured rare earth metal content) between the measured magnesium content and the measured rare earth metal content of the obtained silicon nitride sintered body was 0.23. Due to the improper composition of the auxiliary agent, when stricter sintering conditions are set, the number of β-type silicon nitride particles having a major axis length of more than 10 μm increases significantly (16000 / mm ² ), and the machine Specific properties (bending strength and fracture toughness value) were significantly reduced.
In addition, due to the high nitrogen gas pressure and the high maximum holding temperature, the plate-shaped silicon nitride sintered body taken out had remarkable color unevenness due to the growth of the precipitated crystal phase. ..

(Comparative Example 16)
Comparative Example 16 is an example in which the sheet forming conditions are changed to increase the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface and inside of the sintered body. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. Although the degree of orientation could be controlled, the grain growth was insufficient due to the slightly short holding time at 1800 ° C. and the slightly high measured oxygen content of the sintered body, and the columnar β-type silicon nitride particles. The thermal conductivity decreased due to the orientation of the particles toward the plate surface.

(Comparative Example 17)
Comparative Example 17 is an example in which the holding time at the maximum temperature at the time of sintering is too short. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. If the holding time at 1800 ° C is too short, not only the relative density of the silicon nitride sintered body decreases, but also the sintering aid (magnesium oxide and rare earth metal oxide) in the sintering process and silica in the silicon nitride raw material. Evaporation of the (SiO ₂ ) component was suppressed. Therefore, the measured oxygen content of the sintered body was 2.42% by weight. Since the holding time is too short, the number of β-type silicon nitride particles having a major axis length exceeding 10 μm is significantly reduced (460 / mm ² ), and the thermal conductivity and mechanical properties (bending strength and breaking toughness value) are significantly reduced. Both have declined.

(Comparative Example 18)
As a raw material for silicon nitride, the specific surface area is 8.9 m ² / g, the oxygen content is 0.94% by weight, the median diameter D ₅₀ obtained by volume-based particle size distribution measurement is 0.87 μm, and the minimum particle size is 0. The maximum particle size is .10 μm, the maximum particle size is 6.5 μm, the frequency distribution curve in the same particle size distribution measurement has two peaks, the peak top on the small particle size side of the peak is 0.45 μm, and the large particle of the peak top. The peak top on the diameter side is 1.5 μm, and the ratio of the frequency of the peak top on the small particle size side to the peak top frequency on the large particle size side (frequency of the peak top with a particle size of 0.45 μm / particle size). A silicon nitride powder having a peak top frequency of 1.5 μm) of 0.5 was used. It is shown in Tables 1 and 2 in the same manner as in Example 2 except that the coating speed in sheet forming using the doctor blade device and the sintering conditions (holding time at the same temperature as the maximum holding temperature) were changed. Under these conditions, a plate-shaped silicon nitride sintered body was obtained. Tables 2 and 3 show the chemical composition and characteristics of the obtained plate-shaped silicon nitride sintered body.

Since the specific surface area is low and the oxygen content is low, the densification rate during sintering is slow, and the maximum holding temperature is 1900 ° C and the holding time at the same temperature is 22 hours. A high-density sintered body could not be obtained. Therefore, the weight ratio (measured magnesium content / measured rare earth metal content) between the measured magnesium content and the measured rare earth metal content of the obtained silicon nitride sintered body is 0.25, and the measured oxygen content is 1. It was 0.9% by weight. Since the stricter sintering conditions were set, the number of β-type silicon nitride particles having a major axis length of more than 10 μm increased significantly (20,000 particles / mm ² ), and the fracture toughness value was low. Further, the open porosity on the polished surface was 1.8%, and the maximum open pore diameter was 2.5 μm, which made it difficult to apply to an insulating substrate or a circuit board.

(Comparative Examples 19 and 20)
Comparative Examples 19 and 20 are examples in which powders having a low specific surface area and a low oxygen content or a high specific surface area and a high oxygen content are used as the silicon nitride raw material. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. In the case of a silicon nitride raw material with a low specific surface area and low oxygen content, the relative density of the resulting silicon nitride sintered body is reduced, resulting in thermal conductivity and mechanical properties (bending strength and fracture toughness values). ) Both decreased. In the case of a silicon nitride raw material having a high specific surface area and a high oxygen content, the relative density of the obtained silicon nitride sintered body was high, but the measured oxygen content of the sintered body was 3.04% by weight. , The number of β-type silicon nitride particles having a major axis length of more than 10 μm is significantly reduced (475 / mm ² ). As a result, the thermal conductivity decreased. The mechanical properties (bending strength and fracture toughness value) were also low. The plate-shaped silicon nitride sintered body obtained in Comparative Example 20 was subjected to lap polishing after blast polishing to reduce the arithmetic average surface roughness Ra to 0.04 μm.

(Comparative Example 21)
Comparative Example 21 is an example in which the weight ratio of magnesium oxide and the rare earth metal oxide is changed to 2.20, and the amount of magnesium oxide added is increasing. The holding time at the maximum temperature at the time of sintering was extremely shortened and set to 3 hours. Tables 2 and 3 show the chemical composition and characteristics of the plate-shaped silicon nitride sintered body obtained under the conditions shown in Tables 1 and 2. Due to the large amount of magnesium oxide added, the weight ratio (measured magnesium content / measured rare earth metal content) between the measured magnesium content and the measured rare earth metal content of the obtained silicon nitride sintered body is 1.65. The measured oxygen content was 2.60% by weight. Although the relative density of the sintered body increased, the holding time was too short, so that the number of β-type silicon nitride particles having a major axis length of more than 10 μm was significantly reduced (300 / mm ² ). As a result, the thermal conductivity decreased. The mechanical properties (bending strength and fracture toughness value) were also low.

As described above, the thermal conductivity is remarkably lowered in the comparative examples other than Comparative Examples 7, 10, 12, 15 and 18, and the mechanical properties (bending strength and fracture toughness value) are lowered in the comparative examples other than Comparative Example 4. bottom.

In Comparative Example 16 in which the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less is 0.28, the thermal conductivity is remarkably high. In Comparative Examples 10, 13, and 15 in which the degree of orientation fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface is 0.04 or less, the mechanical properties (bending strength and breaking toughness value) are reduced. It was significantly reduced.

In Comparative Examples 3, 11, 17, 20 and 21 in which the number of coarse β-type silicon nitride particles was less than 500 / mm ² , the thermal conductivity was significantly reduced. On the other hand, in Comparative Examples 10, 12, 14, 15 and 18 in which the number of coarse β-type silicon nitride particles exceeds 10,000 / mm ² , the mechanical properties (bending strength and fracture toughness value) are high, although the thermal conductivity is high. It was declining.

Further, in Comparative Examples 1, 2, 5, 6, 8, 9 and 19 having a relative density of less than 98%, and Comparative Examples 15 and 18 in which the number of coarse particles exceeding 10 μm exceeds 10,000, the open porosity is 1. It exceeded 0.0% and the maximum opening diameter also exceeded 1.0 μm.

As is clear from the results in Tables 1, 2 and 3, the embodiments of the present invention include alkaline earth metal oxides (eg magnesium oxide) and rare earth metal oxides (eg yttrium oxide) which are sintering aids. The total amount added is 3.2 wt% or more and 7.0 wt% or less, and the weight ratio thereof satisfies 0.40 ≦ alkaline earth metal oxide / rare earth metal oxide ≦ 2.0. Further, columnar β-type silicon nitride particles having an actually measured aluminum content of less than 50 ppm, a relative density of 98.6% or more, and a length of the major axis in the silicon nitride sintered body of more than 10 μm per 1 mm ² . The surface orientation degree fa, which indicates the orientation ratio of the columnar β-type silicon nitride particles on the surface of the sintered body, is 0.08 or more and 0.25 or less, and 0.08 mm from the surface. Since the internal orientation degree fa indicating the orientation ratio of the columnar β-type silicon nitride particles on the surface obtained by grinding to the inside is 0.01 or more and less than 0.16, the thermal conductivity at room temperature is 90 W / (m. K) and above, it has excellent thermal and mechanical properties such as a 4-point bending strength of 900 MPa or more and a breaking toughness value of 7.6 _MPa√m or more, and exhibits stable heat dissipation and excellent durability. I found that I could do it. In particular, it has high thermal conductivity, high mechanical strength and toughness, and is therefore suitable for use as an insulating substrate and a circuit board.

Further, in the embodiment of the present invention, since the relative density is 99.0% or more, the thermal conductivity is 100 W / (m · K) or more at room temperature, ensuring high thermal conductivity and being stable. It can demonstrate the heat dissipation. Further, the plate-shaped silicon nitride sintered body of the present invention has a four-point bending strength of 1000 MPa or more and a fracture toughness value of 9.0 _MPa√m or more, and has a particularly high thermal conductivity, high mechanical strength and toughness. Therefore, it is suitable for use as an insulating substrate and a circuit substrate.

As described above, the plate-shaped silicon nitride sintered body of the present invention has a microstructure in which the length of the major axis of the columnar β-type silicon nitride particles constituting the sintered body and its orientation state are highly controlled. In addition to the mechanical properties of high strength / high toughness inherent in the silicon nitride sintered body, it has high thermal conductivity. Since it has high thermal conductivity and high mechanical strength and toughness, it not only can suppress the occurrence of cracks in the substrate when used as an insulating substrate or circuit board, but also has remarkable thermal shock resistance and cold heat cycle resistance. You can expect improvement.

Claims (18)

It is a plate-shaped silicon nitride sintered body,
The ratio of the measured alkaline earth metal content to the measured rare earth metal content as a sintered body is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal content ≤ 1.30.
The measured aluminum content is less than 50 ppm,
Relative density is 98% or more,
When observing the cut surface perpendicular to the plate surface of the silicon nitride sintered body, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is 500 or more per 1 mm ² 10,000. Less than or equal to
The degree of orientation fa represented by the following formula (1), which indicates the orientation ratio of columnar β-type silicon nitride particles on a surface polished to an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less, is 0.08. A plate-shaped silicon nitride sintered body having a value of 0.25 or less.
fa = (P-P ₀ ) / (1-P ₀ ) ... (1)
In this formula (1), P is represented by the following formula (2), and I (110), I (200), I (210), I (310), I (320), I (101), I. (201) and I (002) are the (110) plane, (200) plane, (210) plane, (310) plane, (320) plane, (101) plane, (201) plane, and (. 002) It means the X-ray diffraction peak intensity of each surface.
Further, P ₀ is represented by the following equation (3), I ₀ (110), I ₀ (200), I ₀ (101), I ₀ (210), I ₀ (201), I ₀ (310). , I ₀ (320), and I ₀ (002) are the (110), (200), (101), and (210) planes of the β-type silicon nitride in the isotropic β-type silicon nitride powder. It is calculated from the X-ray diffraction pattern intensity of the (201) plane, the (310) plane, the (320) plane, and the (002) plane.
P = (I (110) + I (200) + I (210) + I (310) + I (320)) / (I (110) + I (200) + I (101) + I (210) + I (201) + I (310) + I (320) + I (002)) ... (2)
P ₀ = (I ₀ (110) + I ₀ (200) + I ₀ (210) + I ₀ (310) + I ₀ (320)) / (I ₀ (110) + I ₀ (200) + I ₀ (101) + I ₀ ( 210) + I ₀ (201) + I ₀ (310) + I ₀ (320) + I ₀ (002)) ... (3) The plate-shaped silicon nitride sintered body according to claim 1, wherein the plate-shaped silicon nitride sintered body has a thickness of 1.5 mm or less and a thickness / area ratio of 0.015 (1 / mm) or less. Silicon nitride sintered body. The above shows the orientation ratio of the columnar β-type silicon nitride particles inside on the surface obtained by grinding from the surface polished to the arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less to the inside by 0.08 mm or more. The plate-shaped silicon nitride sintered body according to claim 1 or 2, wherein the degree of orientation fa of the above is 0.01 or more and less than 0.16. The plate-shaped silicon nitride sintered body according to any one of claims 1 to 3, wherein the measured oxygen content is 1.4% by weight or more and 2.9% by weight or less. Any of claims 1 to 4, wherein the alkaline earth metal oxide is magnesium oxide, and the rare earth metal oxide is at least one oxide selected from yttrium oxide, erbium oxide, scandium oxide and lutetium oxide. The plate-shaped silicon nitride sintered body according to item 1. It is characterized in that the metal element content derived from the sintering aid, which is the sum of the measured magnesium content as the sintered body and the measured rare earth metal content, is 1.8% by weight to 5.0% by weight. The plate-shaped silicon nitride sintered body according to claim 5. Arithmetic mean roughness Ra has a surface polished to 0.06 μm or more and 0.4 μm or less, the open porosity on the surface is 1.0% or less, and the maximum opening diameter of open pores is 1.0 μm or less. The plate-shaped silicon nitride sintered body according to claim 6, wherein the silicon nitride sintered body has a plate shape. The thermal conductivity is 90 W / (m · K) or more at room temperature, the four-point bending strength is 900 MPa or more at room temperature, and the fracture toughness value K _IC measured by the IF method (indentation method) is 7.6 MPa √ m. The plate-shaped silicon nitride sintered body according to claim 6 or 7, wherein the above is the above. The ratio of the measured magnesium content as the sintered body to the measured rare earth metal content is 0.26 ≤ measured magnesium content / measured rare earth metal content ≤ 1.05, and the measured magnesium content and the above According to any one of claims 6 to 8, the metal element content derived from the sintering aid, which is the sum of the actually measured rare earth metal contents, is 2.4% by weight to 4.0% by weight. The plate-shaped silicon nitride sintered body described. When observing the cut surface perpendicular to the plate surface of the silicon nitride sintered body, the number of columnar β-type silicon nitride particles having a major axis length of more than 10 μm is 1000 or more per 1 mm ² . The plate-shaped silicon nitride sintered body according to any one of claims 6 to 9, wherein the number is not more than one. The degree of orientation fa, which indicates the orientation ratio of columnar β-type silicon nitride particles on a surface polished to an arithmetic mean roughness Ra of 0.05 μm or more and 0.5 μm or less, is 0.10 or more and 0.20 or less. The plate-shaped silicon nitride sintered body according to any one of claims 6 to 10, wherein the silicon nitride sintered body has a plate shape. The degree of orientation fa on the surface obtained by grinding from a surface polished to an arithmetic average roughness Ra of 0.05 μm or more and 0.5 μm or less to the inside by 0.08 mm or more is 0.01 or more and 0.13 or less. The plate-shaped silicon nitride sintered body according to any one of claims 6 to 11, wherein the plate-shaped silicon nitride sintered body is characterized by the above. The thermal conductivity is 100 W / (m · K) or more at room temperature, the 4-point bending strength is 1000 MPa or more at room temperature, and the fracture toughness value K _IC measured by the IF method (indentation method) is 9.0 MPa√m. The plate-shaped silicon nitride sintered body according to any one of claims 9 to 12, wherein the plate-shaped silicon nitride sintered body is characterized by the above. As a raw material for silicon nitride, the specific surface area is 13.0 m ² / g or more, the oxygen content is 1.2% by weight or more and 2.3% by weight or less, and the surface oxygen content ratio FSO is 0.76 to 1.10% by weight. Yes, it contains silicon nitride powder with an aluminum content of less than 50 ppm, and as a sintering aid, the weight ratio of alkaline earth metal oxide to rare earth metal oxide is 0.40 ≤ alkaline earth metal oxide / rare earth. With a blending ratio that satisfies the metal oxide ≤ 2.0, the alkaline earth metal oxide and the rare earth metal oxide are 3.2 to 7.0 wt based on the total weight of the silicon nitride powder and the sintering aid. % To adjust the starting composition,
From the starting composition, a green sheet was prepared by a sheet forming process in which the coating speed was adjusted to 0.05 to 0.3 m / min .
The green sheet is sintered by holding it in a pressurized atmosphere with a nitrogen-containing gas pressure of 0.15 to 3 MPa in a temperature range of 1790 ° C. or higher and 1880 ° C. or lower for 6 to 20 hours to form a plate. To obtain a silicon nitride sintered body,
In the obtained plate-shaped silicon nitride sintered body, the ratio of the measured alkaline earth metal content to the measured rare earth metal content is 0.26 ≤ measured alkaline earth metal content / measured rare earth metal content ≤ 1. A method for producing a plate-shaped silicon nitride sintered body, which is 30 and has an actually measured aluminum content of less than 50 ppm and a relative density of 98% or more. The method for producing a plate-shaped silicon nitride sintered body according to claim 14, wherein the measured oxygen content of the silicon nitride sintered body is 1.4% by weight or more and 2.9% by weight or less. 13. Claim 14 or 15, wherein the alkaline earth metal oxide is magnesium oxide, and the rare earth metal oxide is at least one oxide selected from yttrium oxide, erbium oxide, scandium oxide and lutetium oxide. Method for manufacturing a plate-shaped silicon nitride sintered body. As a sintering aid, magnesium oxide and rare earth metal oxides are used in a blending ratio such that the weight ratio of magnesium oxide and rare earth metal oxides satisfies 0.40 ≤ magnesium oxide / rare earth metal oxides ≤ 1.4. Add 4.0 to 6.0 wt% based on the total mass of the magnesium oxide powder and the sintering aid.
The green sheet produced by the sheet forming process is subjected to a pressurized atmosphere having a nitrogen-containing gas pressure of 0.15 to 0.9 MPa, a maximum holding temperature of 1790 ° C. or higher and 1880 ° C. or lower, at the maximum holding temperature. Sintering by holding for 6 to 20 hours,
The ratio of the measured magnesium content to the measured rare earth metal content is 0.26 ≤ measured magnesium content / measured rare earth metal content ≤ 1.05, the measured aluminum content is less than 50 ppm, and the relative density is 98%. The method for producing a plate-shaped silicon nitride sintered body according to claim 16, wherein the plate-shaped silicon nitride sintered body is produced as described above. An insulating substrate or a circuit board, wherein the plate-shaped silicon nitride sintered body according to any one of claims 1 to 13 is used.