JPH0519486B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a novel method for producing aluminum nitride powder, and more particularly, to a method for producing aluminum nitride powder suitable as a raw material for a high-density and highly thermally conductive aluminum nitride sintered body. It is something. BACKGROUND OF THE INVENTION Aluminum nitride has excellent thermal conductivity, so it is attracting attention as a material for use in highly thermally conductive substrates, heat dissipation components, and the like. Such aluminum nitride is used as a sintered body, but since aluminum nitride powder with high purity and low oxygen content has poor sintering properties, it has been difficult to obtain a dense sintered body. On the other hand, the fact that oxygen present in aluminum nitride sintered bodies has an adverse effect on thermal conductivity has been reported, for example, by G., I., Slack, Journal of Physics and
Chemistry of Solitude (Slack, GA,
J.Phys.Chem.Solids), Vol.34, pp.321−35
(1973), pages 328-329, or Toshikazu Sakai,
Others are well known, as described in Figure 4, page 177 of Ceramics Association Journal, Vol. 86, pp. 174-179 (1978). For this reason, it has been extremely difficult to produce an aluminum nitride sintered body that has high density and high thermal conductivity at the same time. In order to solve the above-mentioned problems, JP-A-60-127267 detoxifies the oxygen present in aluminum nitride powder, which is an impediment to heat conduction, without itself becoming an impediment to heat conduction. The present invention also proposes a highly thermally conductive aluminum nitride sintered body obtained by adding a rare earth element or a rare earth element-containing substance that acts as a sintering aid to aluminum nitride powder, and sintering this mixed powder. In addition, Japanese Patent Application Laid-Open No. 60-65768 discloses that lanthanum group metals,
One of the methods for producing an aluminum nitride composition containing a metal or a metal compound such as yttrium is that when alumina powder and carbon powder are mixed, the metal or metal compound is simultaneously mixed and then the mixture is heated under a nitrogen or ammonia atmosphere. We suggest firing it below. Problems to be Solved by the Invention However, in order to obtain a highly thermally conductive aluminum nitride sintered body containing rare earth elements, it is difficult to nitride rare earth elements or rare earth element-containing substances according to the conventional techniques such as those cited above. A process of adding and mixing aluminum powder or a mixture of alumina powder and carbon powder is essential, but there is a limit to the extent to which rare earth elements can be uniformly dispersed by methods such as ball mill mixing for the former and wet mixing and drying for the latter. Therefore, in the process of manufacturing a sintered body, there was a fear that the desirable effects of rare earth elements could not be fully exhibited. Therefore, in order to solve the above-mentioned problems, the present inventors first developed a nitriding method that consists of aluminum nitride as a main component and a compound of a rare earth element as a solid solution or ultrafinely uniformly dispersed in aluminum nitride powder particles. In addition to proposing an aluminum powder, as a method for manufacturing such aluminum nitride powder, an alumina powder in which a compound of a rare earth element is mainly dissolved in alumina powder particles or extremely finely uniformly dispersed therein, and a carbon powder are combined. We proposed a method in which the mixed composition is fired in an atmosphere containing nitrogen. As an example of the method for producing the alumina powder described above, a hydroxide precipitate obtained by neutralizing a mixed aqueous solution of an aluminum-containing salt and a rare earth element-containing salt with an ammonium hydroxide aqueous solution was thoroughly washed. After that, he invented a method of heating it to form an oxide and pulverizing it to a predetermined particle size, and described it in the patent application specification related to the above proposal. The present inventors have conducted further research on a method for producing aluminum nitride powder, which has the above-mentioned aluminum nitride as its main component and has a compound of a rare earth element as a solid solution or extremely finely uniformly dispersed within the aluminum nitride powder particles. The present inventors have come up with a method for producing such aluminum nitride powder more efficiently than the method previously proposed by the present inventors, and have completed the present invention. Means for Solving the Problems The present invention provides a method for producing aluminum nitride powder containing aluminum nitride as a main component and containing a compound of a rare earth element as a solid solution or extremely finely uniformly dispersed in aluminum nitride particles. (a)
Alumina obtained by contacting activated alumina with a BET specific surface area of 10 m ² /g or more with an aqueous solution of a rare earth element-containing substance, drying it, and dry-pulverizing it in the coexistence of higher fatty acids or higher fatty acid salts of alkaline earth metals. The present invention provides a method for producing aluminum nitride powder, which comprises mixing the powder and (b) carbon powder, and firing the thus obtained mixed composition in an atmosphere containing nitrogen. The rare earth elements mentioned above include yttrium (V),
Lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), etc. are preferably used. The number of such rare earth elements may be one type, or two or more types may be used. Further, the content ratio of such rare earth elements in the aluminum nitride powder is preferably in the range of 0.01 to 7% by weight in terms of rare earth elements. If it is less than 0.01% by weight, a high-density sintered body cannot be obtained, and if it exceeds 7% by weight, the rate of increase in the density and thermal conductivity of the sintered body becomes small, which is not economical. In the present invention, a state in which a compound of a rare earth element is mainly dissolved in solid solution or extremely finely uniformly dispersed in aluminum nitride powder particles means that the main portion of the rare earth element in the form of a compound is a unit of one atom or a plurality of units. This refers to the state in which atomic units are dispersed within the aluminum nitride particles. The presence or absence of such a state depends on the content of rare earth elements, but it can be determined by using a scanning/transmission electron microscope with X-ray diffraction and analysis functions.
This can often be confirmed by comparative analysis of the aluminum nitride powder of the present invention and a mixture of simple aluminum nitride powder and a rare earth element compound. In other words, in the case of a simple mixture, a peak unique to rare earth element compounds is often detected by X-ray diffraction, and rare earth elements are not detected on aluminum nitride particles by electron microscopy, and rare earth elements are not detected on aluminum nitride particles. However, in the case of the aluminum nitride powder used in the present invention, peaks characteristic of rare earth element compounds are hardly detected by X-ray diffraction, and rare earth element compounds are detected by electron microscopy. The element is often detected on the aluminum nitride particles, and is not detected or only in small amounts outside of the aluminum nitride particles. The average particle diameter of the aluminum nitride powder in the present invention is preferably 3 μm or less. If the average particle diameter exceeds 3 μm, there is a possibility that a sintered body having high density and high thermal conductivity, which is a desirable effect of the present invention, cannot be obtained. In addition, the average particle size in the present invention refers to the sieve analysis using a JIS standard sieve for the particle size range of 44 μm or more, and the particle size distribution measurement using the light transmission sedimentation method for the particle size range of less than 44 μm, and the results of both measurements. required overall
This is the particle size corresponding to 50% by weight. The reason why the aluminum nitride powder of the present invention exhibits the above-mentioned favorable effects is not clear at present, but since the rare earth element compound is uniformly dispersed within the aluminum nitride powder particles, the aluminum nitride powder of the present invention exhibits the above favorable effects. No matter where the particles are in contact with each other, the rare earth element compound immediately migrates to the contact point and acts as a sintering agent, forming a dense sintered body, resulting in improved thermal conductivity. This is thought to be due to the higher prices. Below, the method for producing the aluminum nitride powder of the present invention will be explained in more detail. First, the specific surface area of activated alumina used as a raw material in the present invention is 10 m ² /g or more, preferably 100 m ² /g or more. Activated alumina as used in the present invention refers to alumina at least partially having a crystal form other than alpha alumina (these are often collectively referred to as "transition alumina" or "intermediate alumina") or amorphous alumina. Alumina is composed of alumina. Examples of transition alumina include x (chi) alumina, k (katsupa) alumina, γ (gamma) alumina, δ (delta) alumina, θ (theta) alumina, η (eta) alumina, and the like. Furthermore, amorphous alumina refers to alumina that essentially does not exhibit any diffraction peaks by X-ray diffraction.
These single particles of transition alumina are generally much smaller than single particles of α-alumina, and the transition alumina particles observed by ordinary electron microscopy are secondary particles formed by aggregation of these single particles. be. There are fine gaps (hereinafter referred to as "pores") between these single particles, so various gases,
Liquids or substances that can be dispersed on the atomic order, such as low molecules or ions dissolved in the liquid, easily enter these pores and can be uniformly dispersed within the secondary particles of transition alumina. As a measure of the fineness of the single particles forming the above-mentioned secondary particles, when gas molecules such as nitrogen gas are allowed to enter the above-mentioned pores, the above-mentioned single particles are calculated from the amount of gas adsorbed. The specific surface area of
BET specific surface area is preferably used. If the specific surface area of the activated alumina is less than 10 m ² /g, there is a risk that the rare earth element compound will not be sufficiently dispersed within the particles of the aluminum nitride powder produced using the activated alumina as a raw material. In particular, in order to fully exhibit the favorable effects of rare earth elements in the process of producing an aluminum nitride sintered body from aluminum nitride powder by the production method of the present invention, it is necessary to
The BET specific surface area is preferably 100 m ² /g or more. The average particle size of activated alumina used in the present invention is not particularly limited, but is usually 1 to 80 μm.
Those are preferably used. In addition, the purity of the above activated alumina is 99.5% by weight or more (purity obtained by subtracting cations in terms of oxides)
It is preferable that When the purity is less than 99.5% by weight, the content of cationic impurities in the sintered body produced from the aluminum nitride powder in the present invention increases, making it difficult to obtain a sintered body with sufficiently high density and high thermal conductivity. There is a risk that you will not be able to do so. Next, in order to cause the activated alumina to adsorb rare earth elements, the activated alumina is brought into contact with an aqueous solution of a rare earth element-containing substance. In the present invention, the method of bringing activated alumina into contact with the aqueous solution of a substance containing a rare earth element includes a method of immersing the activated alumina in the aqueous solution, a method of spraying the aqueous solution onto the activated alumina, etc. Any method that can be maintained over time can be used. For convenience, only the immersion method will be described below, but this is not intended to limit the method of the invention. As the rare earth element-containing substance, for example, rare earth element-containing salts such as yttrium chloride and samarium nitrate are preferably used. Such a rare earth element-containing substance is dissolved in water to a concentration of, for example, about 0.1 to 1 N (in terms of rare earth element cations) to form an aqueous solution, and activated alumina is added to this to form a slurry. At this time, the addition ratio of activated alumina to the aqueous solution is determined by characteristic values such as the target adsorption amount of rare earth elements, the type and concentration of rare earth elements in the aqueous solution, the chemical properties of the rare earth element-containing substance, and the BET specific surface area of activated alumina. It goes without saying that it is desirable to set the value in consideration of the above, but in order to facilitate the dipping operation, it is most preferable to set the ratio to 100 to 500 s/(number of grams of activated alumina to 1 part of the aqueous solution). Such immersion of activated alumina may be carried out by leaving the slurry still, but in order to increase the rate of adsorption of rare earth elements to activated alumina, it is preferable to stir the slurry. The activated alumina may be immersed at room temperature, but in order to increase the rate of adsorption of rare earth elements, the temperature of the slurry may be raised. The time for immersing the activated alumina varies depending on other conditions, but is usually preferably 5 minutes or more. Next, the activated alumina soaked in the aqueous solution of the rare earth element-containing substance as described above is dried. During drying, the slurry containing activated alumina is processed by known solid-liquid separation methods such as vacuum filtration, pressure filtration, centrifugal deliquidation, and gravity sedimentation to reduce the adhering liquid content of activated alumina that has adsorbed rare earth elements. Drying is preferred. The aqueous solution separated by solid-liquid separation still has rare earth element-containing substances dissolved in it, so the rare earth element-containing substances are further added and dissolved to adjust the concentration of rare earth elements, and then reused for subsequent immersion of activated alumina. can be used. As a drying method, known methods such as reduced pressure drying, normal pressure heating drying, and flash drying can be used as appropriate. The drying temperature may be any temperature as long as the physically adsorbed moisture of the activated alumina is substantially removed by the drying method used, but it is preferably 500° C. or lower in order to avoid significant deterioration of the activated alumina that has adsorbed rare earth elements. The dried activated alumina with adsorbed rare earth elements obtained as above is a lump or granular aggregate, although its shape and size vary depending on the drying method. To use it as a raw material, it is ground to an appropriate particle size. As for the grinding method, wet grinding in an aqueous solvent is not suitable because there is a risk that rare earth elements adsorbed on activated alumina may be desorbed, and if the activated alumina is dried again after grinding, it will form aggregates again. However, wet milling in a non-aqueous solvent has a problem in economic efficiency, and therefore dry milling is required. However, when such dried activated alumina is dry-pulverized using known equipment such as a rotary ball mill or a vibrating ball mill, activated alumina itself forms aggregates due to its unique cohesive force, and also forms particles such as balls. As a result of adhesion to the grinding media and the inner wall of the grinding device such as a mill, the grinding effect cannot be achieved beyond a certain limit, and carbon powder is mixed with the thus obtained ground material and fired in an atmosphere containing nitrogen. However, it is very difficult to obtain sufficiently fine aluminum nitride powder. In view of the above circumstances, the present inventors conducted intensive research on a dry pulverization method that can effectively pulverize the dried product of activated alumina, and found that higher fatty acids or alkaline earth When higher fatty acid salts of similar metals are added and allowed to coexist, neither the aggregation of the activated alumina itself nor the adhesion of the activated alumina to the grinding media or the inner wall of the grinding device occurs, and carbon powder is mixed into the pulverized product thus obtained. It has been found that sufficiently fine aluminum nitride powder can be obtained by firing in an atmosphere containing nitrogen. Such favorable effects cannot be obtained by using grinding additives other than those mentioned above, such as other grinding aids or surfactants. Examples of the above-mentioned higher fatty acids include valmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid, and examples of the above-mentioned alkaline earth metal higher fatty acid salts include magnesium salts and calcium salts of each of the above-mentioned fatty acids. , strontium salts and barium salts are preferably used. When such alkaline earth metal salts are used, these metal elements remain as compounds in the aluminum nitride powder of the present invention, but this does not affect the quality of the sintered body obtained from the aluminum nitride powder. It does not cause any deterioration of physical properties such as thermal conductivity, but rather has the favorable effect of promoting higher density of the sintered body. In the present invention, the proportion of higher fatty acids or higher fatty acid salts of alkaline earth metals added to the dried product of activated alumina is not particularly limited, but it is preferably in the range of 0.01 to 10% by weight, considering the effect and economy at the same time. , particularly a range of 0.05 to 3% by weight is most preferred. Next, the activated alumina adsorbed with rare earth elements obtained as described above is mixed with carbon powder. This mixing operation in the present invention may be performed on the activated alumina pulverized as described above, that is, the alumina powder in the present invention, or carbon powder may be added during the pulverization of the dried activated alumina to perform pulverization and mixing. may be performed at the same time. When mixing the alumina powder and carbon powder in the present invention, any known method may be used, such as placing these powders together with balls in a rotary ball mill and mixing them. The average particle diameter of the carb powder used in the present invention is preferably 1 μm or less. If the average particle diameter exceeds 1 μm, there is a risk that mixing with the alumina powder will be insufficient. Further, it is preferable that the ash content of the carbon powder is 0.2% by weight or less,
If it exceeds 0.2% by weight, the physical properties of the sintered body produced from the aluminum nitride powder in the present invention may deteriorate. The mixing ratio of alumina powder and carbon powder is preferably in the range of 1:0.4 to 1:4 by weight. If the mixing ratio of carbon powder is less than 0.4, there is a risk that the reductive nitriding reaction will not proceed sufficiently, and if it exceeds 4, there is a risk that carbon will not be sufficiently removed even if unreacted carbon is removed by oxidation after the reductive nitriding reaction. There is. Further, the mixed composition of alumina powder and carbon powder obtained as described above is fired in an atmosphere containing nitrogen to proceed with a reductive nitridation reaction to obtain aluminum nitride powder. Firing temperature is 1400
~1700°C is preferred. If the firing temperature is less than 1400°C, it will take an extremely long time for the reduction nitriding reaction to proceed sufficiently, and if it exceeds 1700°C, the volatilization loss of aluminum nitride will increase and the average particle size of the aluminum nitride powder will decrease. There is a possibility that the aluminum nitride powder may increase, making it impossible to obtain the aluminum nitride powder preferred in the present invention. Note that since the aluminum nitride powder obtained as described above usually contains unreacted carbon, it is preferable to remove this by oxidation.
This oxidation removal is performed, for example, by removing aluminum nitride powder after a reductive nitriding reaction in an oxidizing gas such as air.
This can be done by heating at a temperature of 500 to 1000°C. Examples Hereinafter, the present invention will be specifically explained using examples. Example 1 The average particle diameter is 53 μm, the purity is 99.81% by weight, the BET specific surface area is 45 m ² /g, and the crystal form is k (katsupa).
100 g of activated alumina containing alumina, γ (gamma) alumina and δ (delta) alumina,
The slurry was added to 500 ml of a 0.1N aqueous solution of samarium chloride to form a slurry, and the slurry was stirred for 2 hours while maintaining the temperature at 40°C, followed by vacuum filtration to obtain an activated alumina cake adsorbing samarium chloride.
After drying this cake at 150°C for 5 hours to obtain a dry product, 1.0 parts of oleic acid was added to 100 parts by weight of this dry product.
Parts by weight were added, placed in an alumina pot mill together with an alumina ball, and pulverized for 15 hours to obtain an alumina powder. The particle size of the alumina powder determined by electron microscopy was approximately less than 1 μm. Next, 10g of this alumina powder and ash content 0.10
5g of carbon black with an average particle size of 0.4μm in weight%
were placed in a nylon pot mill with a nylon ball and mixed. The mixture thus obtained was transferred to a high-purity graphite flat plate, placed in a tubular firing furnace using a graphite furnace tube, and nitrogen gas was heated at 5/min.
It was baked at 1550°C for 6 hours while feeding at a rate of .
The fired product was heated in air at 700°C for 4 hours to oxidize and remove unreacted carbon. The aluminum nitride powder obtained in this way showed no clear peaks other than aluminum nitride in X-ray diffraction, summarium was detected on the entire surface by an X-ray microanalyzer, and a scanning electron microscope In the backscattered electron image, uniformly dispersed bright spots, thought to be caused by clusters of samarium compounds, were observed on the aluminum nitride powder. Furthermore, 2 g of the above aluminum nitride powder was filled into a carbon mold with a diameter of 10 mm, and the pressure was 300 Kgf/cm ² .
An aluminum nitride sintered body was obtained by hot pressing in a nitrogen atmosphere for 0.5 hours at a temperature of 1800°C. Comparative Example 1 100 parts by weight of aluminum nitride powder with an oxygen content of 1.0% by weight, a cationic impurity content (total of Fe, Si, Ca, and Na) of 0.005% by weight, and an average particle size of 1.5 μm and oxidation with an average particle size of 1.0 μm. 2.0 parts by weight of samarium powder was placed in a nylon pot mill together with a nylon ball and mixed. In the thus obtained mixed powder, a samarium oxide peak was detected by X-ray diffraction, and samarium was detected on the entire surface by an X-ray microanalyzer, but a backscattered electron image by a scanning electron microscope. No bright spots indicating uniform dispersion of samarium compound clusters were observed on the aluminum nitride powder. Next, using this mixed powder, hot pressing was performed in the same manner as in Example 1 to obtain an aluminum nitride sintered body. After each of the sintered bodies obtained in Example 1 and Comparative Example 1 was polished to a thickness of 3 mm, the density and thermal conductivity were measured by a laser flash method. The results of Example 1 and Comparative Example 1 are shown in Table 1. Example 2 The average particle diameter is 12 μm, the purity is 99.85% by weight, the BET specific surface area is 127 m ² /g, and the crystal form is x (chi).
100 g of activated alumina containing alumina and γ (gamma) alumina was added to 300 ml of a mixed aqueous solution of 0.1 N salimaum chloride and 0.1 N gadolinium chloride to form a slurry, and the slurry was stirred for 1 hour while maintaining the temperature at 40°C. Thereafter, an activated alumina cake adsorbing samarium chloride and gadolinium chloride was obtained by vacuum filtration. 150 for this cake
After drying at ℃ for 5 hours to make a dry product, this dry product
2.0 parts by weight of stearic acid and 50 parts by weight of the same carbon black as in Example 1 were added to 100 parts by weight, and the mixture was placed in an alumina pole mill together with alumina balls and pulverized for 15 hours to form alumina powder and carbon black. A mixture was obtained. 15 g of this mixture was calcined in a nitrogen gas stream in the same manner as in Example 1, and then unreacted carbon was removed by oxidation to obtain aluminum nitride powder. Furthermore, the same procedure as in Example 1 was carried out to obtain an aluminum nitride sintered body. Comparative Example 2 100 parts by weight of aluminum nitride powder similar to Comparative Example 1, samarium oxide powder with an average particle size of 1.0 μm
Using a mixed powder of 0.8 parts by weight and 1.2 parts by weight of gadolinium oxide powder having an average particle size of 1.0 μm, hot pressing was performed in the same manner as in Example 1 to obtain an aluminum nitride sintered body. The density and thermal conductivity of the sintered bodies obtained in Example 2 and Comparative Example 2 were measured in the same manner as in Example 1. The results of the above Example 2 and Comparative Example 2 are shown in Table 1. Example Average particle diameter 2.9μm, purity 99.96% by weight, BET specific surface area 264m ² /g, crystal form x (chi)
100% activated alumina containing alumina was added to 500 ml of a 0.5N aqueous solution of yttrium nitrate to form a slurry. After stirring the slurry for 1 hour while keeping the temperature of the slurry at 40°C, activated alumina containing yttrium nitrate was filtered under reduced pressure. got a cake.
After drying this cake at 150°C for 5 hours to obtain a dry product, 0.2 parts by weight of calcium stearate was added to 100 parts by weight of this dry product, and the mixture was placed in an alumina pot mill together with an alumina ball, and pulverized for 15 hours to form alumina. A fine powder was obtained. Next, using 20 g of this alumina powder and 10 g of the same carbon black as in Example 1, Example 1 was prepared.
In the same manner as above, aluminum nitride powder was obtained. Further, 5 parts by weight of paraffin was added to 100 parts by weight of this aluminum nitride powder, granulated, and then cold-formed at a pressure of 300 kgf/cm ² to a size of 20 mm x 20 mm.
A plate-shaped molded product with a size of 10 mm was obtained. This molded body is heated to 300℃
After gradually heating until 1 hour and degreasing,
It was placed in an aluminum nitride container and sintered under normal pressure at a temperature of 1800° C. for 1 hour in a nitrogen gas atmosphere to obtain an aluminum nitride sintered body. Comparative Example 3 100 parts by weight of aluminum nitride powder and yttrium oxide powder with an average particle size of 1.0 μm as in Comparative Example 1
Using a mixture with 8.0 parts by weight, granulation, molding and sintering were performed in the same manner as in Example 3 to obtain an aluminum nitride sintered body. The density and thermal conductivity of the sintered bodies obtained in Example 3 and Comparative Example 3 were measured in the same manner as in Example 1. The results of the above Example 3 and Comparative Example 3 are shown in Table 1.

[Table] * Weight of aluminum nitride powder produced by the production method of the present invention or mixed powder of comparative example
** Relative to the theoretical density of aluminum nitride
Effects of the Invention As is clear from the above examples, the aluminum nitride sintered body obtained by the production method of the present invention using aluminum nitride powder as a raw material has a higher density than the sintered body produced by the conventional technology. In addition, since the aluminum nitride powder has high thermal conductivity, a significant improvement in performance can be expected if highly thermally conductive substrates, heat dissipation parts, etc. are manufactured from the aluminum nitride powder by the manufacturing method of the present invention. Therefore, the present invention is extremely useful industrially.

Claims (1)

[Claims] 1. A method for producing aluminum nitride powder containing aluminum nitride as the main component and containing a rare earth element compound mainly as a solid solution or ultrafinely uniformly dispersed in aluminum nitride particles, (a) having a BET specific surface area of 10 m ² /g. An alumina powder obtained by contacting the above activated alumina with an aqueous solution of a rare earth element-containing substance, drying it, and dry-pulverizing it in the coexistence of a higher fatty acid or a higher fatty acid salt of an alkaline earth metal; and (b) carbon powder. 1. A method for producing aluminum nitride powder, which comprises mixing the above-mentioned mixed composition and firing the resulting mixed composition in a nitrogen-containing atmosphere.