JPH0583482B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a novel method for producing aluminum nitride, and more specifically, a method for producing a sintered body of high-purity aluminum nitride powder in large quantities or continuously, which provides properties such as high thermal conductivity, translucency, and corrosion resistance. The present invention provides a suitable method for manufacturing the same. (Prior art and its problems) Aluminum nitride is used as a substrate for high-density mounting, for example, because its sintered body generally has excellent industrial properties such as high thermal conductivity, high insulation, high corrosion resistance, and high strength.
It is attracting attention as a ceramic raw material for various industrial materials such as heat sinks for high-power semiconductor devices and high-power HIC substrates. Furthermore, Japanese Patent Application Laid-open No. 59-50008 discloses that a specified ultrafine powder with high purity aluminum nitride powder containing extremely low oxygen content has excellent sinterability and It has been shown that this results in a sintered body with high translucency. Conventionally, the following two typical methods are known as methods for producing aluminum nitride powder. Specifically, there is a direct nitriding method for aluminum in which metallic aluminum powder is nitrided at high temperature in a nitrogen-containing gas atmosphere, and a reductive nitriding method for alumina in which a mixture of alumina powder and carbon powder is fired at high temperature in a nitrogen-containing gas atmosphere. There is. In the former direct nitriding method, not only is it necessary to crush the raw material aluminum metal in order to increase the nitriding rate, but also to crush the produced aluminum nitride to provide it as a sintered body. Inevitable. On the other hand, in the latter alumina reduction nitriding method, it is possible to obtain high purity aluminum nitride powder with relatively fine and uniform particle size. However, when attempting to continuously produce large quantities of aluminum nitride powder using such alumina reduction nitriding method, it is difficult to maintain the raw material mixture of alumina powder and carbon powder at a uniform high temperature and nitrogen gas concentration. This method is difficult and the reduction nitriding reaction by firing cannot be achieved satisfactorily, so there is a problem that it is not possible to obtain aluminum nitride powder of good quality.
Therefore, the industrial method for producing aluminum nitride powder has generally been a small batch reaction in a small-scale kiln. (Means for Solving the Problems) In view of the above-mentioned problems, the present inventors have conducted extensive research on a method for continuously producing large quantities of high-quality aluminum nitride. As a result, when producing aluminum nitride by supplying a raw material mixture of alumina powder and carbon powder in a vertical firing furnace and firing it at high temperature in an atmosphere of nitrogen-containing gas, the raw material mixture is By pre-granulating aluminum nitride with a binder and using it as pellets so that the crushing strength falls within a specific range, the target high-quality aluminum nitride can be easily obtained, and the pellets after firing can be oxidized under specific conditions. The present invention was completed based on the discovery that the pellets could be processed into powder without being crushed and crushed. That is, according to the present invention, a mixture of alumina powder and carbon powder,
The crushing strength after firing in a nitrogen-containing gas atmosphere is 80
The pellets obtained by granulation and drying in the presence of a binder to a weight of ~4000 g are fired in a vertical kiln at a temperature of 1400 to 1800°C in an atmosphere of nitrogen-containing gas, and then in an atmosphere of oxygen-containing gas. 650 below
Provided is a method for producing aluminum nitride powder, characterized by firing at ~750°C. Specifically, in the manufacturing method of the present invention, pellets granulated from a mixture of alumina powder and carbon powder are fed into a vertical firing furnace from the top, filled with pellets, and fired at high temperature in an atmosphere of nitrogen-containing gas. , the aluminum nitride pellets produced are sequentially taken out from the lower part of the firing furnace. Therefore, in the present invention, it is extremely important to granulate the raw material mixture consisting of the above-mentioned alumina powder and carbon powder in advance and supply it to the vertical firing furnace in the form of pellets in order to obtain the desired aluminum nitride in a satisfactory manner. In particular, the crushing strength per granulated raw material pellet should be 2 to 2.
It is preferable to feed the aluminum nitride to the vertical firing furnace at 40 kg in order to obtain the desired aluminum nitride in a good manner. That is, the raw material pellets of the present invention are not destroyed by mechanical impact during feeding in a vertical kiln,
Since there is almost no crushing due to the filling load or collapse due to high-temperature firing, high-temperature firing can be successfully achieved while nitrogen-containing gas uniformly flows through the gaps between the pellets. As a result, according to the present invention, the alumina in the raw material pellets is reduced and nitrided homogeneously, and good aluminum nitride pellets can be obtained. In addition, in the present invention, aluminum nitride pellets produced by firing raw material pellets are
Since the crushing strength per piece is generally maintained at 80 to 4000 g, it does not collapse, and is then fired as it is in an oxygen-containing gas atmosphere to achieve a uniform oxidation reaction. In this way, the crushing strength of aluminum nitride pellets after oxidation and firing is reduced to 0 to several tens of grams per pellet, so they can be easily crushed and powdered without the need for unpulverized or special crushing means. I can do it. Therefore, in the present invention, the crushing strength per raw material pellet supplied to the vertical kiln is specified to be 2 to 40 kg,
It is also very important to sinter the resulting aluminum nitride pellets to such an extent that the crushing strength per pellet is generally 80 to 4000 g. On the other hand, if the crushing strength per raw material pellet is less than 2 kg, the raw material pellet may be broken due to mechanical shock when fed into the vertical firing furnace, crushed due to filling load, or caused by high temperature. Since collapse occurs due to firing, the uniform distribution of nitrogen-containing gas is inhibited, and the reduction nitriding reaction due to high temperature firing is not achieved homogeneously, resulting in the inability to obtain good aluminum nitride. Furthermore, if the raw material pellets collapse and become powder as described above, the vertical firing furnace will become clogged, and the flow of nitrogen-containing gas and firing at high temperatures will become insufficient, resulting in a long period of time for the reductive nitriding reaction. Not only does it require a lot of time, but it also ends up being impossible to react. On the other hand, the crushing strength per raw material pellet is 40
If it is larger than Kg, the raw material pellets are hardly destroyed, crushed, or disintegrated in the vertical firing furnace, so that high-temperature firing can be continuously achieved due to the uniform flow rate of the nitrogen-containing gas. however,
When such raw material pellets with a crushing strength of more than 40 kg per piece are used, it tends to be difficult to obtain homogeneous aluminum nitride, and long-term firing is required to obtain homogeneous aluminum nitride. be. The reason for this cannot be clearly determined, but if the crushing strength per raw pellet is greater than 40 kg, the pellets will not be fully sintered to the inside of the pellet due to the compaction, and the alumina will not be completely burned. It is assumed that the reduction-nitridation reaction takes a long time.
In addition, when raw material pellets having a crushing strength of more than 40 kg per piece are used, the aluminum nitride pellets obtained generally have a crushing strength of more than 4000 g. Even when fired in an atmosphere, the crushing strength does not easily decrease. Therefore, in order to obtain aluminum nitride powder, not only a separate pulverizing means is required, but also there is a problem that impurities are mixed in due to the pulverization. The alumina powder and carbon powder used as raw materials in the present invention are not particularly limited as long as they are powders that can be easily mixed and granulated into pellets, but as described in the above-mentioned Japanese Patent Application Laid-Open No. 59-50008, In order to obtain high purity aluminum nitride,
The following alumina powder and carbon powder are preferably used. That is, the purity of alumina powder is
Those having an average particle size of 99.0% by weight or more, preferably 99.9% by weight or more, and an average particle size of 2 μm or less, preferably 1 μm or less are used. In addition, carbon powder has an ash content of 0.2
Those with a purity of % by weight or less and an average particle size of 1 μm or less are preferably employed. When using alumina powder and carbon powder with a particle size outside the above range, aluminum nitride with an average particle size of 2 μm or less cannot be obtained, and unreacted alumina is usually used. Since it remains in an amount of 3% by weight or more, it tends not to become a fine powder with a low oxygen content. Furthermore, if the purity of the alumina powder and carbon powder is outside the above range, most of the cationic impurities contained in them will remain as impurities in the aluminum nitride powder, resulting in a powder with a small amount of cationic impurities. I can't. The above-mentioned alumina powder and carbon powder are generally mixed at a mixing ratio of 1:0.4 to 1:1 by weight.
In particular, in order to reduce impurities mixed in from carbon ash, it is preferable to mix the carbon ash in a ratio of 1:0.4 to 1:0.7. Further, the alumina powder and the carbon powder may be mixed by either a dry method or a wet method, but in order to uniformly react the alumina and increase the reaction rate, it is preferable to mix the alumina powder and the carbon powder sufficiently uniformly. The liquid dispersion medium used in wet mixing is not particularly limited, and generally hydrocarbons, aliphatic alcohols, or mixtures thereof are preferably used. For example, as the aliphatic alcohol, methyl alcohol, ethyl alcohol, isopropyl alcohol, etc. are generally preferably used, and as the hydrocarbon, ligroin, petroleum ether, hexane, benzene, toluene, etc. are generally suitably used. hydrocarbon,
Ethers, esters, etc. can also be used as needed, mixed with aliphatic alcohol or alone. Note that the mixing conditions and equipment described above are not particularly limited, as long as they can suppress impurities that inevitably mix into aluminum nitride. In the present invention, the method of producing raw material pellets by granulating and drying a mixture of alumina powder and carbon powder in the presence of a binder is not particularly limited;
It is preferable to manufacture the pellet to a weight of 40 kg, and it can be produced by generally known granulation methods and equipment. As a binder, it is possible to easily granulate raw material pellets with a crushing strength of 2 to 40 kg per piece, and the raw material pellets do not melt or sinter during firing, and aluminum nitride can be obtained. A binder that does not remain as an impurity is preferably used. Generally, organic binders are suitable as such binders, such as water-soluble polymers such as polyvinyl alcohol, hydroxyethyl cellulose, carboxymethyl cellulose, molasses, and hydrophobic binders such as petroleum resins and phenolic resins. However, there are no particular limitations. To granulate raw material pellets, a binder is added to a mixture of alumina powder and carbon powder, and pellets are generally formed into spherical or cylindrical shapes using a granulator such as a rolling type, extrusion type, or compression type. You can get it by doing this. In particular, in order to easily obtain raw material pellets having a crushing strength of 2 to 40 kg per piece, extrusion granulation is preferably employed. Alternatively, two-stage granulation may be used, in which cylindrical pellets are pre-granulated by extrusion granulation, and then the cylindrical pellets are sized into spherical pellets, and the method is not particularly limited. Note that the granulated raw material pellets are generally preferably dried at a temperature of 100 to 130°C. The raw material pellets of the present invention obtained in this way are uniformly grained pellets with an average particle diameter of generally 5 to 20 m/m, preferably 8 to 15 m/m, and a crushing strength of 2 to 40 kg per pellet. , preferably 10-30Kg, and the void volume is generally 0.1-0.45
cm ² /g. Next, in the present invention, the raw material pellets described above are fed to a vertical firing furnace and placed in an atmosphere of nitrogen-containing gas.
Fire at a temperature of 1400-1800℃. The vertical kiln may have a cylindrical shape with a feed port for raw material pellets and an outlet for the produced aluminum nitride. A structure that allows continuous extraction is preferable, and typically has a supply inlet (inlet) at the top, an outlet at the bottom, and a heating mechanism inside or outside. The nitrogen-containing gas may be pure nitrogen gas, a nitrogen-containing gas, or any gas that can be easily converted into nitrogen gas during high-temperature firing, such as ammonia.
Such nitrogen-containing gas is subjected to high-temperature firing while being brought into sufficient contact with raw material pellets introduced continuously or intermittently from the upper or lower part of the vertical firing furnace. In addition, the nitrogen-containing gas in the vertical firing furnace is
It is sufficient to maintain the nitrogen concentration at which the alumina in the raw material pellets is sufficiently reduced and nitrided, and generally the partial pressure of carbon monoxide in the generated exhaust gas may be controlled to 10% or less, particularly 3% or less. On the other hand, the firing conditions are such that the firing temperature is generally maintained at 1400 to 1800°C, preferably 1500 to 1700°C, for generally 1 to 10 hours, preferably 3 to 6 hours.
By firing for a long time, the alumina is sufficiently reduced and nitrided, and the desired aluminum nitride powder can be obtained. That is, if the firing temperature is lower than 1400°C, the reductive nitridation of alumina in the raw material pellets will not be achieved sufficiently, and not only will a long firing time be required, but it will also be difficult to obtain aluminum nitride with a particularly low oxygen content. do not have. In addition, when the firing temperature is higher than 1800°C, the reduction nitriding reaction of alumina is sufficiently achieved in a short time, but the particle size of the aluminum nitride produced increases, making it difficult to obtain the desired aluminum nitride powder. Can not. Since the aluminum nitride pellets obtained by firing the raw material pellets of the present invention in a nitrogen-containing gas atmosphere generally contain unreacted carbon, the pellets are then heated in the presence of air or oxygen to a It is preferable to oxidize and remove the carbon by firing at a temperature of 750°C. Note that if the above-mentioned oxidation firing temperature is higher than 750° C., the surface of aluminum nitride is excessively oxidized, so that the oxygen content of the target aluminum nitride powder tends to increase. (Effects of the Invention) In the present invention described above, the aluminum nitride pellets obtained by firing raw material pellets made of a mixture of alumina powder and carbon powder in an atmosphere of nitrogen-containing gas have a crushing strength per piece. Generally 80-4000g. Next, the above aluminum nitride pellets are easily crushed by firing in an oxygen-containing gas atmosphere to obtain aluminum nitride powder. The aluminum nitride powder thus obtained generally has an average particle size of 2 μm or less and an oxygen content of 1.5
% by weight or less, and cationic impurities are 0.3% by weight or less. With such aluminum nitride powder, a sintered body of high purity and high density can be obtained without using any auxiliary agent, and the sintered body has high translucency. Further, such a sintered body of aluminum nitride has excellent thermal properties and mechanical properties. Furthermore, the aluminum nitride powder of the present invention is suitably used as a raw material for a series of sialon compounds such as α-sialon and β-sialon. (Example) Examples of the present invention are shown below, but the present invention is not limited to these Examples. The crushing strength in the present invention was measured using a Kiya hardness tester and a granular hardness tester. The number of measurements was 20, and the arithmetic mean value was shown. In addition, the average particle diameter can be measured using Horiba's automatic particle size distribution analyzer CAPA.
It reached -500. Nitrogen analysis was based on the method of Ichinose et al. (Nitrogen Association Journal 83 465 (1975)). Example 1 Alumina with a purity of 99.99% by weight and an average particle size of 0.52 μm and carbon black with an ash content of 0.08% by weight and an average particle size of 0.45 μm were wet mixed in a ratio (weight ratio) of 1:0.5 using ethanol as a dispersion medium. Mixed raw materials. Next, a 7.0% by weight polyvinyl alcohol (PVA) aqueous solution was added to this blended raw material at a ratio (weight ratio) of solid/liquid = 1.14.
A wet material was prepared by kneading for 30 minutes. The above wet raw materials were processed using a screw-type pre-extrusion granulator with a die diameter of 8.0m/m.
After forming into mL cylindrical pellets, the cylindrical pellets were granulated by adding 10% by weight of the prepared raw materials using a ball adjusting machine to produce spherical pellets with an average particle diameter of 8 m/m. Next, this spherical pellet is dried in a dryer at 110%
Thoroughly dehydrated and dried at a temperature of ~120°C. The crushing strength of each spherical pellet obtained was 18 kg. After filling a vertical kiln with an internal volume of 3.0 mm with the above spherical pellets, nitrogen gas was passed from the bottom to the top of the kiln at a rate of 5/min.
Firing was carried out at a temperature of 1570°C for 120 minutes. The crushing strength per pellet after firing was 1100 g.
Next, in the same vertical kiln, the pellets were heated to 700°C while flowing air instead of nitrogen gas.
It was oxidized by firing at a temperature of 180 minutes. The aluminum nitride pellets obtained after the above oxidation treatment were easily crushed and powdered, and had an average particle diameter of 2 μm or less and a specific surface area of 4.5 m ² /g. Further, as a result of analyzing this aluminum nitride powder, powder X-ray diffraction showed that it was a single phase of aluminum nitride, and the nitrogen content was 33.8% by weight. Example 2 Spherical pellets were prepared in the same manner as in Example 1, except that the concentrations of polyvinyl alcohol and phenol resin added to the raw materials prepared in Example 1 were varied, and then dehydrated and dried. Table 1 shows the concentration of the above-mentioned binder added and the crushing strength per spherical pellet obtained by drying. The spherical pellets shown in Table 1 were fired in a nitrogen gas atmosphere in the same manner as in Example 1, and then further fired in the presence of air to obtain aluminum nitride powder. Table 1 shows the properties of the aluminum nitride powder obtained.

[Table] *Nos. 1 and 6 show comparative examples.

Claims (1)

[Claims] 1 A mixture of alumina powder and carbon powder,
The crushing strength after firing in a nitrogen-containing gas atmosphere is 80
The pellets obtained by granulation and drying in the presence of a binder to a weight of ~4000 g are fired in a vertical kiln at a temperature of 1400 to 1800°C in an atmosphere of nitrogen-containing gas, and then in an atmosphere of oxygen-containing gas. 650 below
A method for producing aluminum nitride powder, characterized by firing at ~750°C.