JP2001354479A - Aluminum nitride sintered body and method for producing the same - Google Patents

Abstract

(57) [Problem] 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. The aluminum nitride sintered body according to the present invention comprises:
YAG, YAM and YA in aluminum nitride crystal structure
A grain boundary phase containing at least one of L is formed, the maximum diameter of the grain boundary phase is 1 μm or less, and the thermal conductivity is 188 W /
m · K or more.

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 producing the same, and more particularly, to an aluminum nitride sintered body having high strength, high thermal conductivity and excellent heat radiation characteristics, and a method for producing the same.

[0002]

2. Description of the Related Art Compared with conventional metal materials, ceramic sintered bodies having excellent properties such as strength, heat resistance, corrosion resistance, abrasion resistance, light weight, etc. are used for semiconductors, electronic equipment materials, engine members, high-speed materials. It is widely used as mechanical parts, structural materials, and decorative materials used under severe temperature, stress, and wear conditions that are inferior to conventional metal materials, such as cutting tool materials, nozzles, and bearings.

In particular, an aluminum nitride (AlN) sintered body is an insulator having a high thermal conductivity, and has a thermal expansion coefficient close to that of silicon (Si). Therefore, it is used as a heat sink or a substrate of a highly integrated semiconductor device. Applications are expanding.

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

In the sintering operation, as shown in FIG. 3, a box-shaped sintering vessel 3 is placed on a hearth 2 of a sintering furnace 1 on which a furnace material made of, for example, graphite (graphite) is attached. After heating one or a plurality of degreased aluminum nitride compacts 4 in a firing vessel 3 and maintaining the same at a predetermined sintering temperature of 1700 to 2000 ° C. for 4 to 8 hours, the heating furnace 1 The power was turned off and the sintered body was cooled in a furnace.

The firing vessel 3 and the hearth 2 for holding and holding the aluminum nitride compact 4 are used to prevent the properties of the sintered compact from deteriorating by reacting with the compact during high-temperature sintering.
In order to prevent contamination of the sintered body by impurities, high-purity aluminum nitride (Al
N) It is formed of a sintered body or a boron nitride (BN) sintered body.

As described above, since the compacts 4 are accommodated in the firing vessel having a large heat capacity without reacting with the aluminum nitride compacts 4 and sintered, the heat is uniformly applied to the entire aluminum nitride compacts 4. It works, and a homogeneous sintered body with less unevenness is obtained. Further, impurities such as carbon contained in the furnace material of the sintering furnace 1 are blocked by the sintering vessel and do not directly act on the compact 4 during sintering, and these impurities cause unevenness or deformation of the sintered body. And the thermal conductivity of the sintered body is prevented from lowering. Without using the firing container 3,
When the aluminum nitride compact 4 is directly placed in the firing furnace 1 and fired, the surface of the sintered body is significantly damaged by excessive amounts of carbon vapor and impurities released from the furnace material of the firing furnace 1, and The amount of deformation of the whole body also increases, and the product yield decreases sharply.

In the above production method, when an ultrafine raw material powder having an average particle diameter 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 or the like dissolve in the AlN crystal lattice during sintering or Al-O-
As a result of forming a composite oxide such as an N compound, the thermal conductivity of the AlN sintered body without using the sintering aid was relatively low.

On the other hand, the raw material powder has an average particle size of 0.5 μm.
When the above AlN powder is used, since the raw material powder alone has poor sinterability, it is difficult to obtain a dense sintered body without adding an auxiliary agent other than the hot pressing method,
There was a disadvantage of low mass productivity. Therefore, in order to efficiently mass-produce the sintered body by the normal pressure sintering method, it is necessary to densify the sintered body and prevent the impurity oxygen in the AlN raw material powder from forming a solid solution in the AlN crystal particles. It is common practice to add a rare earth oxide such as yttria oxide (Y ₂ O ₃ ) or an alkaline earth metal oxide such as calcium oxide as a sintering aid.

[0010] These sintering aids, AlN raw material powder reacts with oxygen impurity yttrium contained - 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 densification of the sintered body, It is considered that oxygen is fixed as a grain boundary phase to achieve high thermal conductivity. After sintering, these liquid phases solidify as vitreous or crystalline at the grain boundaries of the AlN crystal grains to form a grain boundary phase, and the grain boundary phase strongly binds the AlN crystal grains to each other to form AlN crystal grains. It is believed that the strength of the entire sintered body is increased.

[0011]

However, in the conventional production method, even when the average particle size of the raw material powder, the type and amount of the sintering aid, the degreasing sintering conditions, and the like are strictly controlled, the sintering is not performed. In many cases, the product strength was reduced due to insufficient strength of the body, or a predetermined thermal conductivity was not obtained, and excellent heat radiation characteristics, which is the maximum utilization characteristic of AlN, were often impaired.

The present invention has been made to solve the above problems, and has as its object 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]

Means for Solving the Problems In order to achieve the above object, the present inventors first attempted to elucidate the cause of the decrease in the strength and thermal conductivity of a sintered body by experiments. In other words, the type and amount of sintering aids and additives to be added to the raw aluminum nitride powder, the residual amount of impurities, molding sintering conditions, the composition of the sintered body, etc. are variously changed, and they affect the properties of the sintered body. The effects and relationships were studied experimentally, and the following findings were obtained.

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

That is, in the conventional manufacturing method, after the compact is heated and held at a predetermined sintering temperature for a certain period of time to perform densification sintering, the heating power supply of the firing furnace is turned off and the sintered body is placed in the furnace. Because it was cooled, there is a difference depending on the type of firing furnace, but the cooling rate of the sintered body is an extremely large value of about 400 to 800 ° C. per hour, and the aluminum nitride crystal structure of the sintered body becomes coarse. The cause was confirmed. When the fracture surface of the sintered body was observed by an X-ray diffraction diagram, a scanning electron microscope photograph, or the like, as shown in FIG. 2, a coarse oxide grain boundary phase 5 having a diameter Db of about 5 to 10 μm or a diameter Dp was obtained. It was found that a large number of coarse pores 6 having a size of about 50 to 100 μm were formed at the grain boundaries of the AlN crystal grains 7.

The grain boundary phase 5 is composed of a liquid phase (component: a rare earth compound such as Y ₂ O ₃ added as a sintering aid) formed by the reaction 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. Pores 6 in which the liquid phase has disappeared are formed around the coarse grain boundary phase 5, and both the grain boundary phase 5 and the pores 6 act as a resistance to hinder the heat conduction of the aluminum nitride sintered body.
It has been found that the pores 6 become unsintered portions, and the bonding strength between the AlN crystal grains 7 decreases, and the strength of the entire sintered body decreases.

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

The present invention has been completed based on the above findings. That is, in the method for producing an aluminum nitride sintered body according to the present invention, a raw material mixture obtained by adding a rare earth element as a grain boundary phase forming component to aluminum nitride powder is formed, and the obtained molded body is heated to a temperature of 400 to 500. After sintering the obtained degreased body at a sintering temperature of 1700 to 2000 ° C., a temperature at which the liquid phase formed at the time of sintering by the rare earth element solidifies is obtained from the sintering temperature. The cooling rate of the sintered body is controlled 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 YAG, YAM and YAL is formed in the aluminum nitride crystal structure, the maximum diameter of the grain boundary phase is 1 μm or less, and Thermal conductivity 18
8 W / m · K or more.

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

Further, the content of the rare earth element constituting the grain boundary phase with respect to 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 and serving as a main component of the sintered body includes:
Considering sinterability and thermal conductivity, the impurity oxygen content is 7
% Or less, preferably 3% by weight or less, and has an average particle size of about 0.05 to 5 μm, preferably 3 μm or less.

The rare earth element is 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, D
y, Gd, etc.) oxides, nitrides, or substances (carbonates, etc.) which become these compounds by sintering operation alone,
Alternatively, two or more kinds are used in combination, and yttrium oxide (Y ₂ O ₃ ) is particularly preferable. These sintering aids react with the aluminum oxide phase on the surface of the raw material powder of aluminum nitride to form composite oxides (Al ₅ Y ₃ O ₁₂ , AlYO ₃ ,
A liquid phase of Al ₂ Y ₄ O ₉ or the like is formed, and this liquid phase brings about densification (densification) of the sintered body. For example, when Y ₂ O ₃ is used as a sintering aid, it is considered that yttrium aluminate is generated and liquid phase sintering proceeds. When the sintering aid is added and normal pressure sintering is performed, not only the sinterability is improved (densification), but also the thermal conductivity can be improved. That is, impurity oxygen dissolved in AlN at the time of sintering reacts with Y ₂ O ₃ and segregates as an oxide phase at a crystal grain boundary, so that a sintered body with few lattice defects is obtained and the thermal conductivity is reduced. improves.

The amount of the sintering aid is 1 to 7.5% by weight.
Is adjusted within the range. If the amount added is less than 1% by weight,
The effect of improving the sinterability is not sufficiently exhibited, the sintered body is not densified, a low-strength sintered body is formed, or oxygen is dissolved in the AlN crystal, and the sintered body has a high thermal conductivity. The body cannot be formed. On the other hand, if the addition 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 the pores are formed in the sintered body. (Pores) remain and the shrinkage rate increases, and deformation tends to occur.

The size of the grain boundary phase and pores formed in the aluminum nitride crystal structure greatly affects the heat transfer characteristics and strength characteristics of the sintered body. The diameter is 1 μm or less, while the maximum diameter of the pores is 1 μm or less. Maximum diameter of grain boundary phase is 1μm
When the aggregation and segregation of the liquid phase becomes remarkable so that
At the same time, the bonding action of the crystal particles by the liquid phase is reduced, and the strength of the sintered body as a whole is likely to be reduced.

On the other hand, if the pores are coarse so that the maximum diameter exceeds 1 μm, the heat transfer resistance is increased, the thermal conductivity of the sintered body is reduced, and the strength of the sintered body is significantly reduced.

In order to reduce 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 as described above, the sintered body immediately after the sintering operation is completed. It is necessary to control the cooling rate to 100 ° C. or less per hour. When the cooling rate is set to be higher than 100 ° C./hour, the liquid phase generated in the sintered body is liable to be aggregated and segregated at the grain boundary, and a coarse grain boundary phase and pores are formed.

The temperature range for adjusting the cooling rate ranges from a predetermined sintering temperature (1700 to 2000 ° C.) to a temperature (liquid phase freezing point) at which the liquid phase produced by the reaction of the sintering aid solidifies. Is enough. The liquidus freezing point when using the sintering aid as described above is approximately 1650 to 150.
It is about 0 ° C. By controlling the cooling rate of the sintered body at least from the sintering temperature to the liquidus freezing point to 100 ° C. or less per hour in this manner, fine grain boundary phases are uniformly distributed around the AlN crystal grains, and pores are formed. A small 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 necessary additive such as a sintering aid and an organic binder is added to aluminum nitride to prepare a raw material mixture, and then the obtained raw material mixture is molded to obtain a molded body having a predetermined shape. As a forming method of the raw material mixture, a general-purpose mold pressing method, an isostatic 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 to a temperature of 400 to 400 ° C.
By heating at 500 ° C. for 1 to 2 hours, the organic binder added in advance is sufficiently removed.

Next, the degreased compacts are housed in a firing vessel and stacked in a sintering furnace in multiple stages, and in this arrangement, the plurality of compacts are sintered together at a predetermined temperature.
The sintering operation is performed by heating the molded body to a temperature of 1700 to 2000 ° C. for about 2 to 10 hours in a non-oxidizing atmosphere such as nitrogen gas. The sintering is performed in a nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas. As the reducing gas, H ₂ 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. Sintering temperature is 1750
When calcined at a low temperature of less than ℃, it depends on the particle size and oxygen content of the raw material powder, but it is difficult to obtain a dense sintered body. On the other hand, when calcined at a temperature higher than 2000 ℃, the vapor pressure of AlN itself in the calcination furnace Therefore, the sintering temperature should be controlled in the above range since the density may increase and densification may be difficult.

In the above sintering operation, a dense sintered body is obtained.
To improve the thermal conductivity of the sintered body
However, it is necessary to add some sintering aid. But
However, the sintering aid reacts with AlN and impurity oxygen₅
Y₃O₁₂, AlYO₃, Al ₂Y₄O₉Oxides such as
And precipitate in the grain boundary phase. These grain boundary phase oxides
It has been confirmed that it has an effect of preventing heat conduction. I
Therefore, a sintering aid should be used to prevent the formation of excessive grain boundary phases.
Must be strictly controlled.

Each of the aluminum nitride sintered bodies manufactured by the above-described method has a very high polycrystalline structure.
It has a thermal conductivity close to 0 w / m · k (25 ° C.) and is excellent in mechanical properties such as bending strength.

[0034]

According to the aluminum nitride sintered body and the method of manufacturing the same according to the above construction, the cooling rate of the sintered body immediately after the completion of the sintering process is set to a low rate of 100 ° C./hour or less, so that the cooling rate of the furnace can be reduced. Unlike rapid cooling,
The liquid phase generated during sintering has less aggregation and 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 reduced as well as miniaturized. Therefore, the heat transfer and densification are not hindered by the coarse grain boundary phase and pores, and an aluminum nitride sintered body having high strength and high thermal conductivity can be obtained.

[0035]

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

Examples 1 to 3 Contains 1.0% by weight of oxygen as an impurity and has an average particle diameter 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,
The mixture was wet-mixed in ethyl alcohol for 30 hours and then dried to prepare a raw material powder mixture. 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 disc-shaped heat-dissipating compacts were prepared by compression molding under a pressure of 00 kg / cm ² , and each compact was subsequently heated in air at 375 ° C. for 2 hours to be degreased.

[0037] 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 arranged in a firing furnace filled with gas. After maintaining the temperature in the firing furnace 1 at 1815 ° C. for 4 hours and performing densification sintering, the amount of electricity supplied to the heating device attached to the firing furnace was reduced to increase the temperature in the firing furnace to 1500.
The cooling rate of the sintered body until the temperature drops to 100 ° C. is 100 ° C./hr (Example 1), 50 ° C./hr (Example 2), and 25 ° C./hr (Example 3), respectively. After adjustment, the sintered body was cooled. As a result, each has a diameter of 10
Al according to Examples 1 to 3 having a thickness of 0 mm and a thickness of 3.0 mm
Eight N ceramic sintered bodies were prepared.

Comparative Example 1 On the other hand, immediately after completion of the densification sintering, the power of the heating device was turned off and the sintered body was cooled at a conventional cooling rate of furnace cooling (about 500 ° C./hr). Sintering was performed under the same conditions as in Example 1 to prepare an AlN sintered body according to Comparative Example 1 having the same dimensions.

Comparative Example 2 A sintering process was performed under the same conditions as in Example 1 except that the cooling rate of the sintered body immediately after completion of the densification sintering was set to an excessively high value of 250 ° C./hr. A according to Example 2
An 1N sintered body was prepared.

In order to evaluate the properties of the obtained aluminum nitride sintered bodies according to Examples 1 to 3 and Comparative Examples 1 and 2, the surface of each sintered body was analyzed by an X-ray diffraction method.
Further, 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 were measured, and the presence or absence of aggregation of the grain boundary phase in the aluminum nitride crystal structure and the pore size were measured. The presence or absence of aggregation was observed.
Furthermore, 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.

[0041]

[Table 1]

As is clear from the results shown in Table 1, in the aluminum nitride sintered bodies according to Examples 1 to 3, the cooling of the sintered bodies immediately after the completion of the densification sintering was compared with Comparative Examples 1 and 2. Since the rate was set lower than in the conventional method, there was little aggregation and segregation of the liquid phase in the crystal structure, and there was no aggregation of pores. When the sintered body was observed with a microscope, the crystal structures were all as shown in FIG. 1, and the maximum diameter Db of the grain boundary phase 5a was as small as less than 1 μm and the maximum diameter Dp of the pores 6a.
Was as small as less than 1 μm. And since it has a crystal structure in which fine grain boundary phases are uniformly distributed, a sintered body having high heat dissipation and high density (high strength) and high thermal conductivity was obtained.

On the other hand, when the cooling rate of the sintered body was set to a high value as in Comparative Examples 1 and 2, and the material was rapidly cooled, coarse particles having a maximum diameter Db of 15 μm were obtained 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 reduced, and the strength and thermal conductivity were also reduced.

[0044]

As described above, according to the aluminum nitride sintered body and the method of 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 low value of 100 ° C./hour or less. Unlike the case where rapid cooling such as furnace cooling is performed, a liquid crystal formed during sintering has less aggregation and 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 reduced as well as miniaturized. Therefore, heat transfer and densification are hardly hindered by the coarse grain boundary phase and pores, and an aluminum nitride sintered body having both high strength and high thermal conductivity can be obtained.

[Brief description of the 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 cross-sectional view of a firing furnace showing a state in which a plurality of compacts are accommodated in a firing vessel and fired simultaneously.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Firing furnace 2 Furnace floor 3 Firing container 4 Molded body 5, 5a Grain boundary phase 6, 6a Voids (pores) 7, 7a AlN crystal grains

Claims (3)

[Claims] 1. An aluminum nitride crystal structure having YAG, Y
An aluminum nitride sintered body characterized in that a grain boundary phase containing at least one of AM and YAL is formed, the maximum diameter of the grain boundary phase is 1 μm or less, and the thermal conductivity is 188 W / m · K or more. . 2. A raw material mixture obtained by adding a rare earth element as a grain boundary phase forming component to aluminum nitride powder, degreasing the obtained molded body at a temperature of 400 to 500 ° C., and degreasing the resultant. After heating and sintering the molded body at a sintering temperature of 1700 to 2000 ° C., the cooling rate of the sintered body from the sintering temperature to a temperature at which a liquid phase formed at the time of sintering by the rare earth element solidifies is reduced. A method for producing an aluminum nitride sintered body, wherein the temperature is controlled to 100 ° C. or less per hour. 3. The aluminum nitride powder has an impurity oxygen content of 7% by weight or less and an average particle size of 0.05 to 5 μm.
The method for producing an aluminum nitride sintered body according to claim 2, wherein: