JPH05238830A - Sintered aluminum nitride and its production - Google Patents

Abstract

PURPOSE:To obtain sintered aluminum nitride having high strength, thermal conductivity and heat-radiation performance by heating a formed material composed of AlN powder and a grain boundary forming component under prescribed condition and solidifying the product by cooling at a prescribed cooling rate. CONSTITUTION:A sintered AlN having high strength and thermal conductivity and a grain boundary size and maximum pore diameter of <=1mum can be produced by adding 1-7.5wt.% of a grain boundary forming component (rare earth element and/or alkaline earth metal element) to AlN powder, forming and degreasing the obtained raw material mixture, sintering at 1700-2000 deg.C and solidifying the formed liquid phase formed from the grain boundary component by cooling the sintered compact at a cooling rate of <100 deg.C/h until the temperature is lowered to the freezing temperature of the liquid phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aluminum nitride sintered body and a method for manufacturing the same, and more particularly to an aluminum nitride sintered body having high strength, high thermal conductivity and excellent heat dissipation characteristics, and a method for manufacturing the same.

[0002]

2. Description of the Related Art Sintered ceramics, which are superior in properties such as strength, heat resistance, corrosion resistance, wear resistance, and lightness to conventional metal materials, are used in semiconductors, electronic equipment materials, engine parts, high speed It is widely used as a material for cutting tools, nozzles, bearings, and other mechanical parts, structural materials, and ornamental materials used under severe temperature, stress, and wear conditions that conventional metal materials do not have.

Particularly, an aluminum nitride (AlN) sintered body is an insulator having a high thermal conductivity and has a coefficient of thermal expansion close to that of silicon (Si), so that it is used as a heat sink or a substrate of a highly integrated semiconductor device. Expanding applications.

Conventionally, the aluminum nitride sintered body is generally mass-produced by the following manufacturing method. That is, a sintering aid, an organic binder, and if necessary, various additives, solvents, and dispersants are added to aluminum nitride powder as a ceramic raw material to prepare a raw material mixture, and the obtained raw material mixture is obtained. Is molded by a roll molding method or a doctor blade method to form a thin plate-shaped or sheet-shaped molded body, or the raw material mixture is press-molded to form a thick plate-shaped or large molded body. Next, the obtained molded body is heated to 400 to 500 ° C. in an air or nitrogen gas atmosphere to be degreased, and the hydrocarbon component and the like added as an organic binder is removed and degreased from the molded body. The degreased compact is heated to a high temperature in a nitrogen gas atmosphere or the like and densified and sintered to form an aluminum nitride sintered compact.

In the above-mentioned sintering operation, generally, as shown in FIG. 3, a box-shaped firing container 3 is arranged on a hearth 2 of a firing furnace 1 on which a furnace material made of graphite is attached. For heating the firing furnace 1 after heating in a state where one or more degreased aluminum nitride compacts 4 are accommodated in the firing container 3 and holding at a predetermined sintering temperature of 1700 to 2000 ° C. for 4 to 8 hours. The power was turned off and the sintered body was cooled in the furnace to carry out.

The firing container 3 and the hearth 2 for holding and holding the above-mentioned aluminum nitride compact 4 prevent the reaction of the compact with the compact during high-temperature sintering to deteriorate the characteristics of the sintered compact.
In order to prevent the contamination of the sintered body by impurities, high-purity aluminum nitride (Al
N) Sintered body or boron nitride (BN) sintered body.

As described above, since the compact 4 is housed and sintered in the firing container having a large heat capacity without reacting with the compact aluminum nitride 4, the heat is uniformly distributed over the entire compacts 4 of aluminum nitride. It works, and a homogeneous sintered body with less unevenness is obtained. Further, impurities such as carbon contained in the furnace material of the firing furnace 1 are not blocked by the firing container and do not directly act on the molded body 4 during firing, and these impurities cause uneven coloring and deformation of the sintered body. And the thermal conductivity of the sintered body is prevented from decreasing. When the aluminum nitride compact 4 is placed directly in the firing furnace 1 without using the firing container 3 and fired, sintering is performed by an excessive amount of carbon vapor or impurities released from the furnace material of the firing furnace 1. The body surface is significantly damaged,
Moreover, the amount of deformation of the entire sintered body also increases, resulting in a rapid decrease in product yield.

In the above manufacturing method, when an ultrafine raw material powder having an average particle size of about 0.3 μm or less is used as the raw material AlN powder, a considerably dense sintered body can be obtained by using the AlN powder alone. However, a large amount of impurities such as oxygen adhering to the surface of the raw material powder form a solid solution in the AlN crystal lattice at the time of sintering, or Al-O- which hinders the propagation of lattice vibration.
As a result of producing a complex oxide such as an N compound, the thermal conductivity of the AlN sintered body that did not use a sintering aid was relatively low.

On the other hand, when AlN powder having an average particle size of 0.5 μm or more is used as the raw material powder, the raw material powder alone does not have good sinterability. It was difficult to obtain a bound body, and there was a drawback that mass productivity was low. Therefore, in order to efficiently mass-produce the sintered body by the atmospheric pressure sintering method, in order to prevent the densification of the sintered body and the impurity oxygen in the AlN raw material powder from forming a solid solution in the AlN crystal grains. In addition, rare earth oxides such as yttrium oxide (Y ₂ O ₃ ) and alkaline earth metal oxides such as calcium oxide are generally added as sintering aids.

These sintering aids react with impurity oxygen contained in the AlN raw material powder and react with yttrium-aluminum-garnet (YAG, 3Y ₂ O ₃ .5Al).
₂ O ₃ ), yttria-alumina compound (YAL, Y ₂
O ₃ · Al ₂ O ₃ ), a liquid phase composed of yttria-alumina-metal compound (YAM, 2Y ₂ O ₃ · Al ₂ O ₃ ) or the like is formed to achieve the densification of the sintered body and the impurities. It is believed that oxygen is fixed as a grain boundary phase to achieve high thermal conductivity. Further, these liquid phases are solidified as glassy or crystalline at the grain boundary portion of AlN crystal grains after sintering to form a grain boundary phase, and this grain boundary phase firmly bonds the AlN crystal grains to each other. It is believed to increase the strength of the sintered body as a whole.

[0011]

However, in the conventional manufacturing method, even when the average particle size of the raw material powder, the type and addition amount of the sintering aid, the degreasing and sintering conditions, etc. are strictly controlled, the sintering is performed. In many cases, the strength of the body was insufficient, the product yield was lowered, or a predetermined thermal conductivity was not obtained, and the excellent heat dissipation property which was the maximum utilization property of AlN was impaired.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an aluminum nitride sintered body having high strength, high thermal conductivity, and excellent heat dissipation characteristics, and a method for producing the same. ..

[0013]

In order to achieve the above object, the inventors of the present application first tried to elucidate the cause of the decrease in strength and thermal conductivity of the sintered body by an experiment. That is, the type and amount of sintering aids and additives to be added to the raw material aluminum nitride powder, the residual amount of impurities, the forming and sintering conditions, the composition of the sintered body, etc. are variously changed to affect the characteristics of the sintered body. We conducted experimental studies on the effects and relationships and obtained the following findings.

That is, the present inventors have found that the magnitude of the cooling rate of the sintered body immediately after the completion of the heating and sintering operation has a great influence on the quality characteristics of the finally produced sintered body.

That is, in the conventional manufacturing method, the compact is heated and held at a predetermined sintering temperature for a certain period of time for densification and sintering, and then the power source for heating the firing furnace is turned off. Since it was cooled, there was a difference depending on the type of firing furnace, but the cooling rate of the sintered body was an extremely large value of 400 to 800 ° C./hour, and the aluminum nitride crystal structure of the sintered body became coarse. It was confirmed to be the cause. When the fracture surface of the sintered body was observed by an X-ray diffraction diagram or a scanning electron microscope photograph, as shown in FIG. 2, a coarse oxide grain boundary phase 5 having a diameter Db of about 5 to 10 μm and a diameter Dp were found. Coarse pores 6 of about 50-100 μm are AlN crystal grains 7
It was found that a large number of grains were formed at the grain boundary part of the. The grain boundary phase 5 is a liquid phase (component: component of the rare earth compound such as Y ₂ O ₃ added as a sintering aid, which reacts with the aluminum oxide on the surface of the aluminum nitride raw material powder during sintering.
(YAG, YAL, YAM, etc.) are formed by aggregation and segregation during cooling. Further, pores 6 having no liquid phase are formed around the coarse grain boundary phase 5, and the grain boundary phase 5 and the pores 6 both act as a resistance to prevent heat conduction of the aluminum nitride sintered body, and the pores 6 It was found that the non-sintered portion becomes a non-sintered portion, the joint strength between the AlN crystal grains 7 is reduced, and the strength of the sintered body as a whole is reduced.

On the other hand, the crystal structure of the sintered body obtained by controlling the amount of electricity to the heating device of the firing furnace to set the cooling rate of the sintered body immediately after sintering lower than the cooling rate by the conventional furnace cooling. Was observed and various characteristics of the sintered body were measured. As a result, as shown in FIG. 1, the grain boundary phase 5a of the aluminum nitride crystal structure has a small diameter Db, no liquid phase aggregation segregation, and a fine grain boundary phase uniformly distributed crystal structure. .. Further, regarding the pores 6a as well, a structure having a small diameter Dp and a uniform distribution with little aggregation was obtained, and an AlN sintered body having high thermal conductivity and high strength was obtained.

The present invention has been completed based on the above findings. That is, the method for producing a ceramics sintered body according to the present invention, the powder to aluminum nitride, the raw material mixture obtained by adding at least one of a rare earth element and an alkaline earth metal element as a grain boundary phase forming component, degreasing,
After the obtained molded body is heated and sintered at a sintering temperature of 1700 to 2000 ° C. for a predetermined time, a liquid phase formed at the time of sintering by the rare earth element and / or the alkaline earth metal element is changed from the above sintering temperature. The feature is that the cooling rate of the sintered body up to the solidifying temperature is set to 100 ° C. or less per hour.

Further, in the aluminum nitride sintered body according to the present invention, a grain boundary phase containing at least one of a rare earth element and an alkaline earth metal is formed in the aluminum nitride crystal structure, and the maximum diameter of the grain boundary phase is 1 μm or less. It is characterized by

The maximum diameter of the pores existing in the aluminum nitride crystal structure is preferably set to 1 μm or less.

Further, the content of at least one of the rare earth element and the alkaline earth metal forming the grain boundary phase in the sintered body is preferably set to 1 to 7.5% by weight.

The aluminum nitride (AlN) powder used in the method of the present invention, which is the main component of the sintered body, has an impurity oxygen content of 7% by weight or less, preferably in consideration of sinterability and thermal conductivity. The average particle size is suppressed to 3% by weight or less and the average particle size is about 0.05 to 5 μm, preferably 3 μm or less.

The rare earth element and the alkaline earth metal are added to the aluminum nitride raw material powder as a sintering aid. Specific examples of the sintering aid include rare earth elements (Y, La, Sc,
Pr, Ce, Nd, Dy, Gd, etc.) oxides, nitrides, oxides of alkaline earth metals (Ca, Ba, Sr, etc.), or substances (carbonates, etc.) that become these compounds by the sintering operation. Are used alone or in combination of two or more, and yttrium oxide (Y ₂ O ₃ ) and calcium oxide (CaO) are particularly preferable. These sintering aids react with the aluminum oxide phase on the surface of the raw material powder of aluminum nitride to react with the complex oxides (Al ₅ Y ₃ O ₁₂ , AlYO ₃ , Al
₂ Y ₄ O _{9 and the} like), and this liquid phase causes the sintered body to have a higher density (densification). For example, when Y ₂ O ₃ is used as a sintering aid, it is considered that yttrium aluminate is produced and liquid phase sintering proceeds. When these sintering aids are added and pressureless sintering is performed, not only the sinterability is improved (densification), but also the thermal conductivity can be improved. That is, since the impurity oxygen dissolved in AlN at the time of sintering reacts with Y ₂ O ₃ and segregates as an oxide phase of crystal grain boundaries, a sintered body with few lattice defects can be obtained and the thermal conductivity can be improved. improves.

The amount of the sintering aid added is 1 to 7.5% by weight.
Adjusted in the range of. If the addition amount is less than 1% by weight,
Sintering with high thermal conductivity due to insufficient effect of improving sinterability, densification of sintered body to form low-strength sintered body, and solid solution of oxygen in AlN crystal. The body cannot be formed. On the other hand, if the added amount exceeds 7.5 wt%, an excessive amount of the grain boundary phase remains in the sintered body, or the volume of the grain boundary phase removed by the heat treatment is large, so that voids are formed in the sintered body. (Porosity) remains and the contraction rate increases, and deformation easily occurs.

Since the grain boundary phase and the size of the pores formed in the aluminum nitride crystal structure have a great influence on the heat transfer characteristics and strength characteristics of the sintered body, the maximum grain boundary phase of the sintered body according to the present invention. The diameter is set to 1 μm or less, while the maximum diameter of the pores is set to 1 μm or less. If the cohesive segregation of the liquid phase becomes remarkable such that the maximum diameter of the grain boundary phase exceeds 1 μm, the binding action of the AlN crystal grains due to the liquid phase decreases, and the strength of the sintered body as a whole tends to decrease, and at the same time, the coarse particles The boundary phase hinders heat conduction and reduces the heat conductivity of the sintered body.

On the other hand, similarly, when the pores become coarse so that the maximum diameter exceeds 1 μm, the heat transfer resistance is increased, the thermal conductivity of the sintered body is lowered, and the strength of the sintered body is remarkably lowered.

As described above, in order to set the maximum diameter of the grain boundary phase formed in the aluminum nitride crystal structure to 1 μm or less and the maximum diameter of the pores to 1 μm or less, the sintered body immediately after the completion of the sintering operation is It is necessary to adjust the cooling rate to 100 ° C. or less per hour. When the cooling rate is set to exceed 100 ° C. per hour, the liquid phase generated in the sintered body is likely to aggregate and segregate at the grain boundary portion, and a coarse grain boundary phase and pores are formed.

The temperature range for adjusting the cooling rate is from a predetermined sintering temperature (1700 to 2000 ° C.) to a temperature (liquid phase freezing point) until the liquid phase produced by the reaction of the sintering aid solidifies. Is enough. The liquidus freezing point when the above-mentioned sintering aid is used is approximately 1650 to 150.
It is about 0 ° C. In this way, by controlling the cooling rate of the sintered body from at least the sintering temperature to the liquidus solidification point at 100 ° C. or less per hour, the fine grain boundary phase is uniformly distributed around the AlN crystal grains, and pores are formed. A small amount of sintered body can be obtained.

Next, a schematic process for producing the aluminum nitride sintered body will be described. That is, a predetermined amount of a sintering aid, a necessary additive such as an organic binder, and the like are added to aluminum nitride to prepare a raw material mixture, and then the obtained raw material mixture is molded to obtain a molded product having a predetermined shape. As a forming method of the raw material mixture, a general-purpose die pressing method, a hydrostatic pressing method, or a sheet forming method such as a doctor blade method or a roll forming method can be applied.

Subsequent to the above molding operation, the molded body is heated in a non-oxidizing atmosphere, for example, in a nitrogen gas atmosphere at a temperature of 400 to 400 ° C.
By heating at 500 ° C. for 1 to 2 hours, the previously added organic binder is sufficiently removed.

Next, the degreased compacts are housed in a firing container and stacked in multiple layers in a firing furnace. In this arrangement, a plurality of compacts are sintered together at a predetermined temperature.
The sintering operation is performed by heating the compact at a temperature of 1700 to 2000 ° C. for about 2 to 10 hours in a non-oxidizing atmosphere such as nitrogen gas. The sintering atmosphere is nitrogen gas or a reducing atmosphere containing nitrogen gas. H ₂ as reducing gas
Gas or CO gas may be used. The sintering may be performed in an atmosphere including vacuum (including a slight reducing atmosphere), reduced pressure, increased pressure and normal pressure. When firing at a low sintering temperature of less than 1750 ° C., it is difficult to obtain a dense sintered body, although it depends on the particle size of the raw material powder and the oxygen content.
When firing at a temperature higher than 000 ° C, A in the firing furnace
Since the vapor pressure of 1N itself becomes high and densification may become difficult, the sintering temperature is set within the above range.

In order to obtain a dense sintered body in the above-mentioned sintering operation and to improve the thermal conductivity of the sintered body, it is necessary to add a sintering aid to some extent. However, the sintering aid reacts with AlN and impurity oxygen to cause Al ₅
Oxides such as Y ₃ O ₁₂ , AlYO ₃ and Al ₂ Y ₄ O ₉ are formed and precipitated in the grain boundary phase. It has been confirmed that these oxides in the grain boundary phase have a function of hindering heat conduction. Therefore, it is necessary to strictly control the addition amount of the sintering aid so that an excessive amount of grain boundary phase is not formed.

The aluminum nitride sintered bodies manufactured by the above-mentioned manufacturing method are all very high as a polycrystalline body.
It has a thermal conductivity close to 0 w / m · k (25 ° C) and is also excellent in mechanical properties such as bending strength.

[0033]

According to the aluminum nitride sintered body and the method of manufacturing the same having the above-described structure, the cooling rate of the sintered body immediately after the completion of the sintering process is set to a low value of 100 ° C. or less per hour, so that it is possible to cool the furnace like a furnace. Unlike the case of performing rapid cooling,
Agglomeration and segregation of the liquid phase generated during sintering are small, and a crystal structure in which fine grain boundary phases are uniformly distributed can be obtained. Further, the pores formed in the crystal structure can be made small and at the same time reduced. Therefore, it is possible to obtain a high-strength and high-thermal-conductivity aluminum nitride sintered body in which heat transfer and densification are not impeded by the coarse grain boundary phase and pores.

[0034]

EXAMPLES Next, the effects of the aluminum nitride sintered body and the method for producing the same according to the present invention will be described more specifically with reference to the following examples.

Examples 1 to 3 contain 1.0% by weight of oxygen as an impurity and have an average particle size of 1.
5% by weight of Y ₂ O ₃ (yttrium oxide) as a sintering aid was added to 5 μm of aluminum nitride powder,
A raw material powder mixture was prepared by wet-mixing in ethyl alcohol for 30 hours and then drying. Next, the raw material powder mixture obtained by drying is filled in a molding die of a press molding machine and
A large number of disk-shaped radiator plate molded bodies were prepared by compression molding under a pressure of 00 kg / cm ² , and subsequently each molded body was heated in air at a temperature of 375 ° C. for 2 hours for degreasing treatment.

[0036] The next several moldings degreased in the step, together two by two, as shown in FIG. 3 is accommodated in a high-purity AlN made firing vessel 3, the four firing container 3 N ₂ Two layers were stacked in a firing furnace filled with gas. Then, the temperature in the firing furnace 1 is maintained at 1815 ° C. for 4 hours, and densification and sintering are performed. Then, the energization amount to the heating device attached to the firing furnace is reduced to 1500 ° C.
The cooling rates of the sintered body until the temperature falls to 100 ° C. are 100 ° C./hr (Example 1), 50 ° C./hr (Example 2), and 25 ° C./hr (Example 3), respectively. The sintered body was adjusted and cooled. As a result, each has a diameter of 100 mm,
Eight AlN ceramics sintered bodies according to Examples 1 to 3 each having a thickness of 3.0 mm were prepared.

Comparative Example 1 On the other hand, immediately after the completion of the densification sintering, the heating apparatus power supply was turned off, and the sintered body was cooled at the conventional cooling rate by the furnace cooling (about 500 ° C./hr). An AlN sintered body according to Comparative Example 1 having the same dimensions was prepared by performing a sintering process under the same conditions as described above.

Comparative Example 2 Further , a comparison was made by sintering under the same conditions as in Example 1 except that the cooling rate of the sintered body immediately after the completion of the densification sintering was set to an excessively high 250 ° C./hr. Al according to Example 2
An N sintered body was prepared.

Then, in order to evaluate the characteristics of the respective aluminum nitride sintered bodies according to the obtained Examples 1 to 3 and Comparative Examples 1 and 2, the surface portions of the respective sintered bodies were analyzed by the X-ray analysis method,
Furthermore, by observing the fracture surface of each sintered body with a scanning electron microscope, the maximum diameter of the grain boundary phase and the maximum diameter of the pores are measured, and the presence or absence of aggregation of the grain boundary phase in the aluminum nitride crystal structure and the pores The presence or absence of aggregation was observed.
Further, the average values of the thermal conductivity and the bending strength of each sintered body were measured, and the results shown in the right column of Table 1 below were obtained.

[0040]

[Table 1]

As is clear from the results shown in Table 1, in the aluminum nitride sintered bodies according to Examples 1 to 3, cooling of the sintered bodies immediately after the completion of densification sintering was compared with Comparative Examples 1 and 2. Since the speed was set lower than that of the conventional method, there was little aggregation and segregation of the liquid phase within the crystal structure and no aggregation of pores. As a result of microscopic observation, as shown in FIG. 1, the crystal structure shows that the maximum diameter Db of the grain boundary phase 5a is 1 μm.
The maximum diameter Dp of the pores 6a was as small as less than 1 μm. Further, because of the crystal structure in which the fine grain boundary phases are uniformly distributed, a sintered body having a high density (high strength) and high thermal conductivity and high heat dissipation was obtained.

On the other hand, when the cooling rate of the sintered body is set high as in Comparative Examples 1 and 2 and is rapidly cooled, coarse particles having a maximum diameter Db of 15 μm as shown in FIG. Boundary phase 5 was formed, and large pores 6 having a maximum diameter Dp of 4 μm were observed in various places, and the sinterability was lowered and the strength and thermal conductivity were also lowered.

[0043]

As described above, according to the aluminum nitride sintered body and the method for manufacturing the same according to the present invention, the cooling rate of the sintered body immediately after the completion of the sintering process is set to a small value of 100 ° C. or less per hour. Unlike the case of performing rapid cooling such as furnace cooling, the liquid phase generated during sintering has less aggregate segregation and a crystal structure in which fine grain boundary phases are uniformly distributed can be obtained. Further, the pores formed in the crystal structure can be made small and at the same time reduced. Therefore, it is possible to obtain a high-strength and high-thermal-conductivity aluminum nitride sintered body in which heat transfer and densification are not impeded by the coarse grain boundary phase and pores.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a crystal structure of an aluminum nitride sintered body according to the present invention.

FIG. 2 is a diagram schematically showing a crystal structure of a conventional aluminum nitride sintered body.

FIG. 3 is a sectional view of a firing furnace showing a state in which a plurality of molded bodies are housed in a firing container and fired simultaneously.

[Explanation of symbols]

 1 Firing Furnace 2 Hearth 3 Firing Container 4 Formed Body 5,5a Grain Boundary Phase 6,6a Void (Pore) 7,7a AlN Crystal Grain

Claims (4)

[Claims] 1. An aluminum nitride sintered body characterized in that a grain boundary phase containing at least one of a rare earth element and an alkaline earth metal is formed in an aluminum nitride crystal structure, and the maximum diameter of the grain boundary phase is 1 μm or less. .. 2. The aluminum nitride sintered body according to claim 1, wherein the pores present in the aluminum nitride crystal structure have a maximum diameter of 1 μm or less. 3. The aluminum nitride according to claim 1, wherein the content of at least one of the rare earth element and the alkaline earth metal forming the grain boundary phase in the sintered body is 1 to 7.5% by weight. Sintered body. 4. A raw material mixture obtained by adding at least one of a rare earth element and an alkaline earth metal element to aluminum powder as a grain boundary phase forming component is molded and degreased to obtain a molded body of 1700 to 2000. A sintered body which reaches a temperature from the above sintering temperature to a temperature at which a liquid phase formed at the time of sintering is solidified by the above rare earth element and / or alkaline earth metal element after being heated and sintered at a sintering temperature of ° C for a predetermined time. Is set to 100 ° C. or less per hour, and a method for manufacturing an aluminum nitride sintered body.