JPH0466805B2 - - 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 having an extremely low content of cationic impurities. <Conventional technology> A conventional method for producing aluminum nitride powder includes firing a mixed composition of alumina powder and carbon powder in an atmosphere containing nitrogen, as disclosed in JP-A No. 59-50008. A method of bringing aluminum into contact with nitrogen gas to cause a nitriding reaction, as disclosed in JP-A-60-161314, for example, is known. <Problems to be Solved by the Invention> However, in the method as cited above, the produced aluminum nitride contains sodium, iron, silicon, etc. in the former case, and silicon, iron, etc. in the latter case.
Cation impurities such as iron and magnesium tend to remain, and the resulting aluminum nitride powder has purity problems when used as a raw material for products that require high purity, such as heat-dissipating substrates. . The present inventors have already discovered that in a method for producing aluminum nitride by reacting a mixture of alumina and carbon with nitrogen gas, the reaction starts.
The amount of unreacted alumina contained in the aluminum nitride powder can be reduced by including a step of contacting the mixture under pressure higher than normal pressure in a temperature range of 1250°C or higher and a step of contacting it under pressure lower than normal pressure. A production method that suppresses the amount of water to zero or a very small amount, or similarly the mixture is heated to a pressure lower than normal pressure, preferably
He has invented a manufacturing method that brings the total oxygen content in aluminum nitride to an extremely low level by contacting the aluminum nitride at a pressure of 0.2 to 0.8 atmospheres, and has applied for a patent. The present inventors further found that the content of cationic impurities such as iron, silicon, sodium, magnesium, and calcium in the produced aluminum nitride powder was extremely low;
The present invention has been completed by intensive research into a method for obtaining high purity powder suitable for the production of high thermal conductivity sintered bodies. <Means and effects for solving the problems> That is, the method for producing aluminum nitride powder according to the present invention is 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 prior to the initiation of the aluminum nitride production reaction.
It is characterized by contacting at a temperature of 1000°C or more and 1400°C or less and a pressure of 0.1 atmosphere or less. 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, as much as possible, does not contain gases that are oxidizing at high temperatures, such as oxygen, carbon dioxide, and water vapor. 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 of 1400°C or higher 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) In parallel with the reaction of formula (1), cationic impurities contained in the raw material alumina or carbon, such as impurities such as sodium, silicon, iron, magnesium, and calcium, are also converted into nitrides, Alternatively, it is converted into carbide and remains in aluminum nitride. As a result of researching a method to minimize the content of cationic impurities in aluminum nitride powder, the present inventors found that in order to achieve this purpose, the temperature must be 1000 to 1400°C and the pressure must be lower than 0.1 atm. It has been found that it is necessary to include a step of bringing a nitrogen-containing inert gas into contact with the raw material mixture prior to the aluminum nitride production reaction. The ambient temperature during the pressure process lower than 0.1 atmosphere in the present invention is in the range of 1000 to 1400°C. The pressure of the nitrogen-containing inert gas may be kept lower than 0.1 atm for a period of time such that cationic impurities in the alumina or carbon mixture are substantially removed in the above temperature range, and may be maintained for 0.5 hours or more. Although it is not clear why the cation impurity content of the aluminum nitride powder becomes extremely low when the pressure of the nitrogen-containing inert gas is lowered to below 0.1 atm, as mentioned above, the cation impurities contained in the raw material alumina or carbon Cation impurities usually exist in the form of oxides, and are known to thermally dissociate when heated to high temperatures. For example, sodium, silicon, and magnesium are calculated from the following formula (2).
It dissociates according to the reaction formula shown in (4). Na ₂ O (s) = 2Na (g) + 1/2O ₂ (g) (2) SiO ₂ (s) = SiO (g) + 1/2O ₂ (g) (3) MgO (s) = Mg (g) +1/2O ₂ (g) (4) Although the equilibrium partial pressure of each vapor species is closely related to temperature, in comparison with alumina, the vapor pressure increases in the order of Na ₂ O > SiO ₂ > MgO > Al ₂ O ₃ . is getting lower. Iron exists in the form of Fe ₂ O ₃ , with a higher vapor pressure than SiO ₂ , and calcium exists in the form of CaO, lower than MgO, with a vapor pressure of Al ₂ O ₃
seems to be higher than that. The pressure of the nitrogen-containing inert gas is lowered to less than 0.1 atmosphere, and the cationic impurities contained in the alumina or carbon raw materials are evaporated and removed from the system according to the vapor pressure of each cationic impurity as described above. It can also be considered that In the present invention, in order to remove cationic impurities, the temperature when maintaining the pressure lower than 0.1 atm is in the range of 1000°C to 1400°C. Below 1000°C, the vapor pressure of the cationic impurities is extremely low and there is almost no effect. is not recognized. On the other hand, at temperatures exceeding 1400° C., although the vapor pressure of cationic impurities becomes high, a problem may arise in which the particle size of the produced aluminum nitride powder becomes large. In addition, the time to maintain the pressure of the nitrogen-containing inert gas lower than 0.1 atm and at 1000 to 1400°C is
It is necessary to do this before the reaction of formula (1) begins.
The time after the reaction of formula (1) starts does not contribute much to the effect of removing cationic impurities. This means that the cationic impurity in the alumina and carbon mixture, taking _SiO2 as an example, is expressed by the following formula:
As shown in (5) and (6), SiO ₂ +3C=SiC+2CO (5) 3SiO ₂ +6C+2N ₂ =Si ₃ N ₄ +6CO (6) Reacts with raw material carbon and nitrogen, and precedes or reacts with the reaction of formula (1). At the same time, the aluminum nitride powder transforms into a carbide or nitride, which is much more stable at high temperatures than when it is an oxide, making it extremely difficult to remove these cationic impurities by evaporation from the aluminum nitride powder. As mentioned above, the raw material mixture was heated to 0.1 atm or less,
After the heat treatment in the range of ~1400°C, the nitriding reaction treatment is carried out in a nitrogen-containing inert gas atmosphere, which may be carried out by a known method. However, heat treatment at 1250° C. or higher and 0.2 atmosphere or higher is preferable because the nitriding reaction progresses easily.
Note that the raw material mixture treated by the method of the present invention is
The process of contacting raw material alumina under pressure higher than normal pressure until the reaction rate of alumina reaches at least 5%, which is a method for which the present inventors have already applied for a patent, and the raw material mixture treated by the method of the present invention. By combining the steps of contacting with the nitrogen gas-containing inert gas under a pressure lower than normal pressure, preferably 0.2 to 0.4 atmospheres, cation impurities are extremely low, and each property is further added. It is also possible to obtain high-purity aluminum nitride powder with a small amount of total oxygen and unreacted alumina. <Examples> The present invention will be specifically explained below using examples. Examples 1 to 10 Alumina powder containing various impurities shown in Table 1
After mixing and pulverizing 100 g and 40 g of carbon powder in a ball mill, a carbon tray (height 210 mm,
210mm in width and 40mm in height). The thickness of the raw material mixture at this time was 30 mm. This tray
It was placed in an electric furnace with effective dimensions of 230 mm in length, 250 mm in width, and 220 mm in height, and a reductive nitriding reaction was carried out while nitrogen gas was flowing. At this time, heating was performed from room temperature to 1000°C at a temperature increase rate of 100°C/Hr while maintaining normal pressure. (a) The temperature was increased and heated while maintaining various pressures lower than 0.1 atm from when the temperature exceeded 1000°C until the predetermined temperature was reached (hereinafter referred to as step (a)). (b) Thereafter, the temperature was raised to a predetermined temperature while maintaining various pressures. At this time, when the temperature reached 1550°C, the temperature was kept constant from then on (hereinafter referred to as step (b)). (c) If the temperature does not reach 1550℃ in (b), maintain the inside of the furnace at various pressures and raise the temperature to 1550℃.
After reaching the temperature, the temperature was kept constant (hereinafter referred to as step (c)). The temperature increase rate for (a), (b), and (c) was all 100°C/Hr. After the reaction was completed, the cation impurities and total oxygen content of the produced aluminum nitride powder were analyzed using fluorescent X-ray analysis (System 3070, manufactured by Rika Denki Kogyo Co., Ltd.).
For Na only, atomic absorption spectrometry (AA− manufactured by Shimadzu Corporation)
646), respectively. The results are shown in Table 2 together with the pressure, time, and temperature conditions for each step (a), (b), and (c) above. In the table, the amount of unreacted alumina (α-Al ₂
O ₃ ), particle size (average particle size), and total oxygen amount are each listed. Amount of unreacted alumina (α
-Al ₂ O ₃ ) was measured by X-ray diffraction, and the average particle size was measured using a light transmission particle size distribution analyzer (manufactured by Seishin Enterprise Co., Ltd.).
SKN1000 model), and the total oxygen content was measured using fluorescent X-ray analysis, similar to the measurement of cationic impurities. The results are shown in Table 2. Comparative Example 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 2.

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【table】

[Table] <Effects of the Invention> As can be seen from the examples described above, according to the method of the present invention, aluminum nitride powder with an extremely low content of cationic impurities can be produced compared to the conventional technique. Such aluminum nitride powder is suitable as a raw material for products requiring high purity such as heat-dissipating substrates, and therefore 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 at 1000°C prior to the start of the aluminum nitride production reaction.
A method for producing aluminum nitride powder, characterized by contacting the powder at a temperature of 1400°C or less and a pressure of 0.1 atmosphere or less.