1.1 Direct nitriding
Aluminum nitride does not exist in nature. It is now prepared by direct nitriding of metallic aluminum powder or direct nitriding method after carbon thermal reduction of Al2O3. Its chemical reaction formula is:
2 al (s) + N2 (g) – > 2 aln (s)
Direct nitriding has several advantages: 1) low cost; 2) Rich raw materials; 3) The reaction system is simple and there is no side reaction; 4) The reaction temperature is lower than that of carbothermic reduction; 5) Suitable for mass production.
However, this method still has some disadvantages: 1) Nitriding requires a reaction temperature of up to 1500℃ (but lower than the reaction temperature of carbothermic reduction) and a long reaction time; 2) The formation of aluminum particles on the surface of aluminum nitride inhibits the diffusion of nitrogen reactants to unreacted aluminum; 3) The large amount of heat generated by the reaction of aluminum and nitrogen makes the reactant aluminum particles coalesce, thus increasing the difficulty of nitrogen diffusion.
Therefore, high reaction temperature and extended reaction time are needed for complete nitriding of aluminum powder. On top of this, a grinding process is required to inhibit densification of the sintering process. Because the reactant aluminum and high reaction temperature will lead to sintered aluminum nitride powder coalescence. The grinding of agglomerated aluminum nitride powder to reduce its size is a necessary condition to produce good sintering. However, this treatment also increases impurities, leading to degradation of aluminum nitride products, including thermal conductivity. These processes also make aluminum nitride powders more expensive than other materials.
1.2 carbothermic reduction
Carbon thermal reduction method is to heat uniformly mixed Al2O3 and C in N2 atmosphere, and Al2O3 is first heated by C
Reduction, the resulting product Al reacts with N2 to form AlN, and its chemical reaction formula is
Al2O3 + 3 c + n2 – > 2 aln + 3 co
Carbothermic reduction has several advantages: 1) abundant raw materials; 2) The process is simple; 3) The synthesized powder has high purity; 4) Small particle size and uniform distribution. Its main disadvantages are: 1) the synthesis time is long; 2) High nitriding temperature; 3) Excessive carbon needs to be removed after the reaction, resulting in high production cost. [5]
To under the preparation of low cost to improve the yield of the carbon thermal reduction process, and get a nitride aluminum nitride body completely, the early researchers mainly by discussing different calcination temperature and holding time on the influence law of carbon thermal reduction product, in an effort to as low as possible under the calcination temperature and short time of heat preservation of high purity aluminum nitride body, so as to achieve the aim of reducing costs [6]. In addition to the calcination temperature and holding time, the particle size of the raw material, the mixing uniformity of Al2O3 and C powder, different aluminum sources and carbon sources, and different nitriding atmosphere and heating methods have important effects on the carbon thermal reduction products.
1.3 High-energy ball milling method
High energy ball grinding method is in nitrogen or ammonia atmosphere, using the rotation or vibration of the ball mill, so that the hard ball of alumina or aluminum powder and other raw materials for strong impact, grinding and mixing, so as to directly nitride into aluminum nitride powder body method.
The advantages of this method are: 1) the equipment required is simple; 2) Short process; 3) High production efficiency. However, there are also some disadvantages: 1) the impurity content of the product is high; 2) Nitriding is not sufficient to complete.
The improvement ideas are as follows: 1) The mechanism of mechanical force induced nitriding reaction is deeply studied, so as to guide the process; 2) The effects of different technological conditions on high-energy ball milling products were studied; 3) Optimize the material of high-energy ball milling equipment to reduce the introduction of impurities in the ball milling process.
High temperature self-propagating synthesis
High temperature self-propagating synthesis is a new preparation method of aluminum nitride powder powder developed in recent years. Its essence is the direct nitriding of aluminum powder, and the chemical formula of the reaction is
2 al (s) + N2 (g) – > 2 aln (s)
The method makes full use of the strong exothermic reaction characteristic of direct nitriding of aluminum powder, ignites aluminum powder in nitrogen, and uses the high chemical reaction heat between aluminum and nitrogen to maintain the reaction, so as to synthesize aluminum nitride powder. The aluminum nitride powder prepared by this method is not heated to high temperature and nitrided for a long time like the direct nitriding method and carbothermic reduction method. This method does not need external heat source except ignition. [7]
The advantages of this method are: 1) low energy consumption and low cost; 2) High production efficiency. However, it also has some disadvantages: 1) The pressure of nitrogen is high; 2) It is difficult to achieve industrial production.
