JPH0446889B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a novel method for producing aluminum nitride, and more particularly, to a method for producing aluminum nitride powder with extremely low unreacted alumina content. BACKGROUND ART Conventional methods for producing aluminum nitride powder include, for example, a method of firing a mixed composition of alumina powder and carbon powder in an atmosphere containing nitrogen, as disclosed in JP-A No. 59-50008; For example, a method is known in which a nitriding reaction is carried out by bringing aluminum into contact with nitrogen gas, as disclosed in JP-A-60-161314. Problems to be Solved by the Invention However, in the method as cited above, unreacted alumina tends to remain in the produced aluminum nitride, and unreacted aluminum tends to remain in the latter, and the resulting nitride Aluminum powder has purity problems when used as a raw material for products that require high purity, such as heat-dissipating substrates. In order to solve the above problems, the present inventors have developed a method for producing aluminum nitride powder by reacting a mixture of alumina and carbon with nitrogen gas, in which the unreacted alumina content of the produced aluminum nitride powder is reduced. The present invention was completed after intensive research into a method that would reduce the amount of waste to an extremely low level. Means for Solving the Problems That is, the present invention provides a method for producing aluminum nitride powder by reacting a mixture of alumina and carbon with nitrogen gas, in which a nitrogen-containing inert gas is added to the mixture at a temperature of 1250°C or higher. and then holding the thus treated mixture at a temperature above 1250°C in a nitrogen-containing inert atmosphere at a pressure below normal pressure. The present invention provides a method for producing aluminum nitride powder, which is characterized by a two-stage treatment. The alumina and carbon used in the present invention may be powders of each or a mixture of the two powders formed into granules, but usually fine powders of each are used. The nitrogen-containing inert gas used in the present invention is a gas that contains nitrogen gas and does not contain gases that are oxidizing at high temperatures, such as oxygen, carbon dioxide, and water vapor as much as possible. However, it may inevitably contain carbon monoxide, which is a reaction product. Now, it is known that when a mixture of alumina and carbon is heated to a high temperature in a nitrogen-containing inert gas atmosphere, it converts into aluminum nitride and carbon monoxide according to equation (1). Al ₂ O ₃ +3C+N ₂ =2AlN+3CO (1) However, in the initial stage of the reaction, unreacted alumina particles grow in parallel with the reaction of formula (1). If such alumina particles grow significantly, the conversion according to formula (1) will not proceed sufficiently even during the subsequent long-term reaction, and unreacted alumina will usually remain in the produced aluminum nitride.・
It will remain in the form of alumina. Furthermore, if alumina particle growth occurs, the particle size of the produced aluminum nitride powder becomes large, which is undesirable. First, the present inventors researched a method to reduce the growth rate of alumina particles in the initial stage of the reaction in equation (1) above, and found that in a temperature range of 1250°C or higher, the pressure of the nitrogen-containing inert gas is It was found that the higher the value, the lower the growth rate of alumina particles, and that the growth of alumina particles can be effectively suppressed. In the present invention, pressure higher than normal pressure means 1
The pressure may be greater than atmospheric pressure, but preferably 1.1
The pressure range is ~10 atmospheres, most preferably 1.2-2.0 atmospheres. If the pressure exceeds 10 atmospheres, problems may arise in maintaining the airtightness of the reactor and in pressure resistance. In addition,
In the temperature range below 1250° C., alumina particle growth is not noticeable in any case, so the pressure of the nitrogen-containing inert gas has little effect on the quality of the aluminum nitride produced. In addition, the time for holding the nitrogen-containing inert gas higher than normal pressure is sufficient as long as the surface of the alumina particles is substantially transformed into aluminum nitride, and depends on the holding temperature, for example, at a temperature of 1550°C. When holding at a temperature of about 1 to 5 hours, it is suitable. However, at the end of the step of bringing the nitrogen-containing inert gas into contact with the mixture of alumina and carbon under a pressure higher than normal pressure as described above, the surface of the alumina grains becomes aluminum nitride,
Since a large amount of unreacted alumina still remains within the grains, it is necessary to continue the reaction of formula (1) efficiently. Therefore, the present inventors researched a method to minimize the amount of unreacted alumina remaining in the final stage of the reaction of formula (1) above, and found that the unreacted alumina in the produced aluminum nitride powder was substantially reduced. It has been found that the pressure of the atmosphere needs to be lower than normal pressure in order for it to disappear or become extremely low, and that 0.2 to 0.4 atmospheres is particularly preferable. If the pressure is less than 0.2 atm, the particle size of the produced aluminum nitride powder may become slightly larger.
Further, the time period for which the nitrogen-containing inert gas is maintained at a pressure lower than normal pressure depends on other reaction conditions such as the holding temperature, but is usually preferably about 5 to 100 hours. It is not clear why the above favorable effects are obtained when the pressure of the atmosphere is lower than normal pressure, but the free energy of the reaction of the above formula (1) is expressed by the following formula (2).
As can be seen from the equation (1), it is thought that when the total pressure of the atmosphere is lowered, the chemical equilibrium shifts to a direction in which the reaction of the above formula (1) tends to proceed. ΔG=ΔG ⁰ +RTln(Pco) ³ /PP _N2 (2) As mentioned above, the characteristics of the present invention are (a) contacting the mixture of alumina and carbon with a nitrogen-containing inert gas under a pressure higher than normal pressure; and (b) the step of holding the thus treated mixture in a nitrogen-containing inert gas atmosphere at a pressure lower than normal pressure. The temperature throughout the process is above 1250℃, preferably 1250~
It is in the range of 1700℃. If the temperature is less than 1250°C, the reaction of formula (1) is too slow and unsuitable, while if it exceeds 1700°C, the particle size of the aluminum nitride powder produced may increase. In addition, when reacting a mixture of alumina and carbon with a nitrogen-containing inert gas at a temperature of 1250°C or higher, the pressure of the gas must be kept above normal pressure from beginning to end, or below normal pressure throughout. Although it is possible to produce aluminum nitride powder even if the reaction conditions are adopted, for the reasons mentioned above, under such reaction conditions, the amount of unreacted alumina remaining in the produced aluminum nitride is less than the amount by the production method of the present invention. cannot be decreased. EXAMPLES The present invention will be specifically explained below using examples. Examples 1 to 3 100 g of alumina powder with an average particle size of 1.0 μm and carbon powder with the same average particle size of 0.05 μm measured with an electron microscope
After mixing and pulverizing 40 g with a ball mill, the mixture was filled into a carbon tray (length 210 mm, width 210 mm, height 40 mm). The thickness of the raw material mixture at this time was 30 mm. The effective dimensions of this tray are 230mm long and 250mm wide.
It was placed in an electric furnace with a diameter of 220 mm and a height of 220 mm, and a reductive nitriding reaction was carried out while nitrogen gas was flowing through it. The temperature conditions at this time are a heating rate of 100℃/hr.
It was heated to 1550°C and then maintained at 1550°C for 24 hours. During that time, the pressure inside the electric furnace varies from room temperature to
Normal pressure is used up to 1250℃, and (a) the temperature rise process from 1250℃ to 1550℃ and the first two stages of maintaining the temperature at 1550℃
maintained at various pressures higher than normal pressure for a period of time,
(b) The remaining time of maintaining the temperature at 1550°C was maintained at various pressures lower than normal pressure. After the reaction was completed, the amount of α-Al ₂ O ₃ (alpha alumina) remaining in the produced aluminum nitride powder was determined by X-ray diffraction. The results are shown in Table 1. Comparative Example 1 The same procedure as in Example was carried out except that the pressure in the electric furnace was always maintained at normal pressure. The results are also shown in Table 1.

[Table] Effects of the Invention As can be seen from the above examples, the production method of the present invention produces aluminum nitride that does not substantially contain unreacted alumina or contains only a very small amount of unreacted alumina, compared to the conventional technology. A powder can be produced. Such aluminum nitride powder is suitable as a raw material for products requiring high purity such as heat-dissipating substrates, so the present invention is extremely useful for industrial development.

Claims (1)

[Claims] 1 In a method for producing aluminum nitride powder by reacting a mixture of alumina and carbon with nitrogen gas, a nitrogen-containing inert gas is added to the mixture.
contacting at a temperature above 1250°C and a pressure above normal pressure, and then holding the so-treated mixture in a nitrogen-containing inert atmosphere at a temperature above 1250°C and a pressure below normal pressure. A method for producing aluminum nitride powder, which comprises a two-stage process comprising steps of: