JPH09268069A - Highly heat conductive material and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a material having both a high strength at high temperatures and a high thermal conductivity. SOLUTION: This highly heat conductive material is obtained by baking a formed compact, consisting essentially of silicon nitride and containing a group IIIa element (RE) of the periodic table in an amount of 3-5mol% expressed in terms of RE2 O3 at 0.9-3.0molar ratio SiO2 /RE2 O3 of the amount of impure oxygen in the sintered compact expressed in terms of SiO2 to the amount of the RE2 O3 and having <=0.2wt.% Al content expressed in terms of an oxide at 1,500-1,800 deg.C under atmospheric pressure, subsequently baking the compact at >=2,000 deg.C temperature under >=1.5atom of nitrogen pressure and further carrying out the crystallizing treatment at 1,000-1,400 deg.C. The resultant material has >=5μm average crystal grain diameter of the silicon nitride, the grain boundary composed of the crystal phase containing at least RE, Si and O, further >=70W/m.K thermal conductivity and >=400 MPa strength at 1,300 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly heat-conductive material composed of a sintered body containing silicon nitride as a main component and a method for producing the same, and more particularly to a gas turbine, an engine part, a ceramic heater, and a nucleus. Heat-resistant members used at high temperatures, such as fusion reactor parts and microwave heating device members,
The present invention relates to various heat dissipation substrates for semiconductors and electronic parts, as well as high thermal conductivity materials most suitable as cutting tools.

[0002]

2. Description of the Related Art Conventionally, silicon nitride has excellent mechanical properties such as high strength characteristics from room temperature to high temperature, wear resistance, high fracture toughness, and high hardness, as well as low thermal expansion and thermal shock resistance. Since it is lightweight, it has been used in various applications up to now, such as automobile engines, high-temperature structural parts such as gas turbines, bearings, crushers, surface plates, guides, wear-resistant parts such as cutting tools, etc.
Furthermore, due to its corrosion resistance to molten metal, it has been applied to various applications such as molten metal parts.

In recent years, in high temperature structural parts such as automobile engine parts and gas turbine parts, increasing the operating temperature has been studied from the viewpoint of improving the output and thermal efficiency of the heat engine. High thermal conductivity is desired. Further, in the cutting tool made of ceramics, the cutting edge has become higher in temperature due to high-speed cutting, and a ceramic cutting tool having high thermal conductivity is also desired.

Also, in the case of semiconductor packages, electronic parts, etc., along with the recent rapid increase in the density and performance of semiconductor elements, the high heat conductivity has been demanded as the material used for them. .

Hitherto, aluminum nitride, silicon carbide, beryllium oxide and the like have been known as high thermal conductive ceramic materials, and in particular, aluminum nitride sintered as a heat radiating member for semiconductor packages and electronic parts. The body is the most used.

[0006]

However, conventional high thermal conductivity materials such as aluminum nitride and silicon carbide are
Although it has some properties from the viewpoint of thermal conductivity,
There is a problem that mechanical strength is insufficient, especially engine parts whose operating temperature exceeds 1000 ° C,
It could not be used at all for members used at high temperatures such as parts for gas turbines, and its applications were very limited.

The silicon nitride sintered material, which is known as a high temperature structural material, has very excellent characteristics in high temperature strength, but its thermal conductivity is at most 30 to 60 W.
/ M · K, which is not satisfactory as a member requiring high thermal conductivity.

[0008]

As a result of repeated studies on the above problems, the present inventors have found that in a silicon nitride sintered body containing a Group 3a element oxide of the periodic table as a sintering aid. , Increasing the average crystal grain size of the silicon nitride crystal increases the thermal conductivity, Al (aluminum) forms a solid solution in the silicon nitride crystal to reduce the thermal conductivity, and at least the grain boundary of the silicon nitride crystal The present inventors have found that the presence of a crystal phase containing a Group 3a element of the periodic table, silicon, oxygen and nitrogen can improve the thermal conductivity, and have reached the present invention.

That is, the highly heat-conductive material of the present invention contains silicon nitride as a main component, and the group 3a element (RE) of the periodic table is 3 to 5 mol% in terms of oxide (RE ₂ O ₃ ) in the above-mentioned period. The molar ratio represented by SiO ₂ / RE ₂ O ₃ between the oxide equivalent of Group 3a element in the table and the SiO ₂ equivalent of impurity oxygen in the sintered body is 0.9 to 3.3, and aluminum. The composition has a content of 0.2 wt% or less in terms of oxide, the silicon nitride crystal has an average grain size of 5 μm or more, and the grain boundaries include at least a Group 3a element of the periodic table, silicon, and oxygen. It is composed of a crystalline phase and has a thermal conductivity of 70 W / m
It is characterized in that the bending strength at K or higher and 1300 ° C. is 400 MPa or higher.

Further, as a method for producing a high thermal conductive material,
Silicon nitride as a main component, 3 to 5 mol% of Group 3a element oxide of the periodic table, oxide conversion amount of the Group 3a element of the periodic table, and SiO ₂ conversion amount of impurity oxygen in the sintered body. And S
The molar ratio represented by iO ₂ / RE ₂ O ₃ is 0.9 to 3.
3. A step of producing a molded body having a composition in which the aluminum content is 0.2% by weight or less in terms of oxide, and a first firing step of firing the molded body at 1500 to 1800 ° C. under normal pressure containing nitrogen. , Nitrogen pressure of 1.5 after the first firing step
A second firing step of firing at a temperature of 2000 ° C. or more, which is equal to or higher than atmospheric pressure, to grow the average grain size of silicon nitride crystals to 5 μm or more, and 1000 sintered bodies obtained by the second firing step are performed.
And a step of crystallizing the grain boundaries by performing a heat treatment in a non-oxidizing atmosphere at a temperature of ℃ to 1400 ℃.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The high thermal conductive material of the present invention is composed of silicon nitride as a main component in terms of composition, and as another component, a Group 3a element of the periodic table in an amount of 3 to 5 in terms of oxide. It is contained at a ratio of mol%. As a Group 3a element of the periodic table,
Y, Sc, Sm, Gd, Nd, Dy, Ho, Er, T
m, Yb, Lu, etc., but among these, D
Heavy rare earth elements such as y, Ho, Er, Tm, Yb, and Lu are preferable in order to improve high temperature strength and thermal conductivity. The amount of the Group 3a element of the periodic table is limited to the above amount by 3
If it is less than mol%, the compactness will be insufficient and the thermal conductivity and mechanical properties will be significantly deteriorated. Further, if the content exceeds 5 mol%, the amount of grain boundary phase increases and the thermal conductivity becomes 70 W /
This is because mK or less. The density of the product of the present invention needs to be 99% or more in terms of theoretical density ratio, and particularly 99.5.
% Or more is desirable.

Further, in the present invention, the grain boundaries of the silicon nitride crystal grains are at least a Group 3a element (RE) of the periodic table,
It is important to be composed of a crystalline phase containing silicon, oxygen and nitrogen as constituent elements. Specific crystal phases include oxide crystals such as RE ₂ Si ₂ O ₇ and RE ₂ SiO ₅ ,
RE ₄ Si ₂ N ₂ O ₇ , RE ₁₀ Si ₆ N ₂ O ₉ , RE ₂
Examples include oxynitride crystals such as Si ₃ N ₄ O ₃ and RE ₂ Si ₃ N ₂ O ₅ . Among these, RE ₄ Si ₂ N ₂ O ₇ ,
Precipitation of at least one of RE ₁₀ Si ₆ N ₂ O ₉ and RE ₂ Si ₂ O ₇ is desirable from the viewpoint of high temperature strength and high thermal conductivity.

By crystallizing the grain boundaries in this manner, the melting point of the grain boundaries becomes higher than that of the glass phase, so that the strength at high temperature becomes high, and at the same time, from the viewpoint of heat transfer, the crystal phase is higher than that of the glass phase. This is because the phase has a higher heat transfer property, and thus the high thermal conductivity of the entire sintered body can be achieved.

In precipitating the above crystals at the above grain boundaries,
S of the oxide conversion amount of the Group 3a element of the periodic table contained in the sintered body and the SiO ₂ conversion amount of the impurity oxygen in the sintered body
The molar ratio represented by iO ₂ / RE ₂ O ₃ is 0.9 to 3.
3, especially in the range of 1.5 to 2.5 is important. Here, the impurity oxygen is the remaining oxygen amount obtained by subtracting the oxygen component mixed as a sintering aid such as the oxide of the Group 3a element of the periodic table from the total oxygen amount contained in the sintered body, For example, it is presumed that oxygen as unavoidable impurities in the silicon nitride powder and oxygen components adhering due to the oxidative action during the manufacturing process, and these impurity oxygens are bonded to Si. Note that the above-mentioned molar ratio is limited to the above range. When it deviates from the above range, a glass phase is likely to be generated in addition to the grain boundary crystal phase during crystallization of the grain boundary, and these glass phases have thermal conductivity. And to deteriorate the high temperature characteristics.

According to the present invention, it is also important that the content of aluminum in the sintered body is 0.2% by weight or less in terms of oxide. This is because aluminum is dissolved as an oxide or a nitride in a silicon nitride crystal to form a sialon crystal (SiAlON).

This sialon has a strong ionic bond and scatters phonons due to covalent bonds, so that the thermal conductivity of the silicon nitride crystal itself is reduced, and as a result, the thermal conductivity of the sintered body is reduced. . Moreover, the presence of aluminum becomes a factor that hinders the crystallization of grain boundaries. For this reason, when the aluminum content exceeds 0.2% by weight, the thermal conductivity of 70 W / m · K or more cannot be achieved, and for this reason, the thermal conductivity is preferably as low as possible. It is preferably 1% by weight or less.

Further, in relation to the crystallization of grain boundaries, the amount of metal elements other than silicon nitride and Group 3a elements of the periodic table in the sintered body is 1000 ppm or less, particularly 50 ppm in terms of oxide.
It is preferably 0 ppm or less, and when the amount of these metal elements exceeds 1000 ppm, crystallization of grain boundaries may be hindered.

Further, according to the present invention, the average crystal grain size of the silicon nitride crystal grains constituting the main crystal phase of the sintered body is 5 μm.
It is also important that this is done. The average crystal grain size here means the diameter when the area of the crystal calculated from the structure of the two-dimensional cross section of the crystal is converted into a circular shape, even if the crystal shape is acicular or equiaxed. As a specific measuring method, it can be calculated by using an image analyzer based on a photograph of a scanning electron microscope or the like. This is because the phonon diffusion increases and the heat transfer efficiency decreases as the number of interfaces between the crystal grains increases, so that the phonon scattering due to the grain boundary phase is reduced by increasing the crystal grains. Is. Therefore, if the average crystal grain size is smaller than 5 μm, the desired high thermal conductivity cannot be obtained. Desirably 5
It is desirable that the thickness is -30 μm, and more desirably 10-20 μm. Next, a method for manufacturing the high thermal conductive material of the present invention will be described. The Group 3a element oxide of the periodic table is added to the silicon nitride raw material powder in a proportion of 3 to 5 mol%. The silicon nitride raw material powder used here may be either α type or β type, but β type is preferable from the viewpoint of cost.
The raw material may be produced by either the imide decomposition method or the silicon direct nitriding method, but a raw material having a purity of 99% or more is desirable. The primary particle size of the silicon nitride raw material is not particularly required to be a fine powder, but a fine powder having an average particle size of about 1 μm is preferable from the viewpoint of sinterability. Further, it is desirable that the amount of impurity oxygen on the surface of silicon nitride is 1.5% by weight or less. Moreover, 2 to 5 mol% of a Group 3a element oxide of the periodic table is used as a sintering aid.
At the same rate. As a Group 3a element of the periodic table,
Y, Sc, Sm, Gd, Nd, Dy, Ho, Er, T
m, Yb, Lu, etc., among them, Dy, H
O, Er, Tm, Yb and Lu are preferably used.

For the silicon nitride raw material, the periodic table 3a is used.
After adding the group element oxide in the above range and thoroughly mixing it with a ball mill or the like, the mixture is formed into a desired shape by a desired molding means, for example, a die press, a cold isostatic press, an extrusion molding or the like. To mold. According to the present invention,
In the composition of the molded body produced in this manner, it is represented by SiO ₂ / RE ₂ O ₃ which is the oxide equivalent of the Group 3a element of the periodic table and the SiO ₂ equivalent of the impurity oxygen in the sintered body. It is important that the molar ratio is 0.9 to 3.3 and the aluminum content is 0.2% by weight or less in terms of oxide.

The amount of impurity oxygen can be easily controlled by adding silicon oxide separately from the amount of impurity oxygen in the silicon nitride raw material. Further, it is necessary to adjust the amount of aluminum in consideration of the amount of impurities in the raw material and the amount mixed from balls and the like during mixing.

Next, the molded body obtained as described above is fired in a non-oxidizing atmosphere such as nitrogen.

At this time, in the present invention, the first firing step is firing at 1500 to 1800 ° C. under normal pressure containing nitrogen. This first firing step is a step for completing the phase transition of α-type to β-type of silicon nitride and precipitating a large number of β-silicon nitride particles serving as nuclei of crystal grains.
It is desirable to hold it for about 10 hours.

Next, following the first firing step, as the second firing step, firing is performed in nitrogen at 2000 ° C. or higher. At this time, the atmosphere has a nitrogen pressure of 1. so that the silicon nitride is not decomposed.
It is preferable to perform firing in high-pressure nitrogen of 5 atm or more, particularly 10 atm or more. This second firing step is a step necessary for producing a dense body having a theoretical density ratio of 99% or more, and at the same time, crystal growth of silicon nitride crystals and grain growth into a structure having an average crystal grain size of 5 μm or more. . For that purpose, it is suitable to maintain the maximum temperature for about 5 to 10 hours.

Further, at the time of cooling after the second firing step or after cooling once, a crystallization heat treatment of the grain boundary phase is performed at 1000 to 1400 ° C. in a non-oxidizing atmosphere such as nitrogen or in the atmosphere. Although the temperature slightly changes depending on the kind of the grain boundary crystal phase to be precipitated, it is usually more effective to carry out the higher temperature for the grain boundary phase having a higher melting point. The heat treatment time is sufficient for the crystallinity to reach equilibrium, but it is usually required to be 5 hours or longer. Cooling after the crystallization treatment does not particularly affect the properties.

[0025]

Example A β-type silicon nitride raw material produced by the silicon direct nitriding method (A raw material; oxygen content 1.2% by weight, average particle diameter 1 μm) and an α-type silicon nitride raw material produced by imide decomposition method (B raw material) ; Oxygen content 1.0% by weight, average particle size 0.7μ
m) is added with an oxide powder of a Group 3a element of the periodic table having a purity of 99.9% or more and, if desired, a silicon oxide powder, and the prepared raw material is added to a 500 ml polyethylene pot with a silicon nitride grinding ball and ethanol. Put the binder,
The mixture was pulverized by a rotary mill and then spray-dried to obtain a granulated powder. Molding was performed by die press molding under a molding pressure of 1 ton / cm ² so as to form a shape having a diameter of 60 mm and a thickness of 10 mm. The molding composition is shown in Tables 1 and 2. For samples Nos. 12 to 14, Al ₂ O ₃ was added and adjusted in order to investigate the relationship between the amount of Al and the characteristics.

Next, the molded body was held at 500 ° C. in the atmosphere for 5 hours to be degreased and then fired under the conditions shown in Table 1.
The first baking step was carried out at a maximum temperature of 5 hours under normal pressure and the second baking step was carried out at 50 atmospheres of nitrogen at a maximum temperature of 5 hours. The sintered body after firing was further treated for 5 hours in nitrogen at atmospheric pressure at the heat treatment temperatures shown in Tables 1 and 2.

The resulting sintered body is cut and processed into 3 × 4 × 4
A 0 mm test piece was subjected to a 4-point bending strength test at room temperature and 1300 ° C. according to JIS R1601, and five points each were subjected to a 4-point bending strength test, and an average value thereof was determined as a strength value. The density was measured by the Archimedes method, and the theoretical density ratio with the theoretical density at the time of preparation was calculated. In addition, the sample was crushed, the crystal phase was determined by X-ray diffraction, and processed into a shape with a diameter of 10 mm and a thickness of 2 mm,
The thermal conductivity at room temperature was measured by the laser flash method. Further, the mirror surface of the test piece was image-analyzed by a Luzex apparatus from a structure photograph of 1000 times with a scanning electron microscope to determine the average crystal grain size of the silicon nitride crystal. The results are also shown in Tables 1 and 2.

[0028]

[Table 1]

[0029]

[Table 2]

As is clear from the results of Tables 1 and 2, the sample No. 8 in which the oxide conversion amount of the Group 3a element of the periodic table is less than 3 mol% has low strength and low thermal conductivity due to insufficient densification. Sample No. 7 exceeding 5 mol%
, The high temperature strength is high, but the thermal conductivity is low.

Sample N whose second firing temperature is lower than 2000 ° C.
o.17, sample No. 19 without the first firing step is Si ₃ N
_{4 The} average crystal grain size was smaller than 5 μm, and all had excellent high temperature strength but low thermal conductivity.

Regarding the amount of Al, samples No. 10, 13,
From the comparison of Nos. 14 and 15, it can be seen that when the amount of Al is more than 0.2% by weight, a glass phase is likely to be formed in the grain boundary, the high temperature strength is lowered, and at the same time the thermal conductivity is lowered.

Further, in the sample No. 27 having a SiO ₂ / RE ₂ O ₃ ratio of lower than 0.9, and the sample No. 28 having a ratio of more than 3.3, formation of a glass phase was observed at the grain boundaries, and as a result, , The thermal conductivity and the high temperature strength both decreased.

In contrast to these comparative examples, the products of the present invention each have a room temperature strength of 600 MPa or more and a 1300 ° C. strength of 40 MPa.
A thermal conductivity of 0 MPa or more and a thermal conductivity of 70 W / m · K or more are achieved. In particular, as a Group 3a element of the periodic table, Dy, H
o, Er, Tm, Yb, Lu, and Samples Nos. 5, 6, 9 to 12 and 1 having an Al content of 0.1 wt% or less.
6, 18, 20 to 24, room temperature strength 700 MPa or more, 1300 ° C. strength 500 MPa or more, thermal conductivity 80 W
/ M · K or higher was achieved.

[0035]

As described in detail above, the high thermal conductivity material according to the present invention has high strength at 1300 ° C. and high thermal conductivity, and therefore, it is of course suitable for heat resistant materials such as various gas turbines and engine parts. Therefore, it can be applied to various fields such as ceramic heaters, cutting tools, and heat dissipation boards for semiconductors.

Claims (2)

[Claims] 1. A periodic table 3a containing silicon nitride as a main component.
₃ to 5 mol% of the group element (RE) in terms of oxide (RE ₂ O ₃ ), the amount of oxide of the group 3a element of the periodic table and the amount of impurity oxygen in the sintered body, converted to SiO ₂ SiO ₂ / R
When the molar ratio represented by E ₂ O ₃ is 0.9 to 3.3, the aluminum content is 0.2% by weight or less in terms of oxide, and the average grain size of the silicon nitride crystals is 5 μm or more. And its grain boundary is at least an element of Group 3a of the periodic table,
A high thermal conductivity material which is composed of a crystal phase containing silicon and oxygen and has a thermal conductivity of 70 W / m · K or more and a bending strength at 1300 ° C. of 400 MPa or more. 2. A periodic table 3a containing silicon nitride as a main component.
SiO ₂ of 3 to 5 mol% of group element oxide, the oxide conversion amount of the group 3a element of the periodic table, and impurity oxygen in the sintered body.
A step of producing a molded product having a composition in which a molar ratio represented by SiO ₂ / RE ₂ O ₃ with a converted amount is 0.9 to 3.3 and an aluminum content is 0.2% by weight or less in terms of oxide. And the molded body under a normal pressure containing nitrogen at 1500 to 1800
The first firing step of firing at ℃, and the second firing that follows the first firing step, firing at a temperature of 2000 ° C. or higher with a nitrogen pressure of 1.5 atm or higher to grow the average grain size of silicon nitride crystals to 5 μm or higher. And a step of crystallizing grain boundaries by heat-treating the sintered body obtained by the second firing step in a non-oxidizing atmosphere at 1000 ° C to 1400 ° C. Production method.