In recent years, aluminum nitride has been extensively researched in a variety of application fields as ceramic material with excellent performance and third-generation semiconductor material. To obtain AlN ceramic materials with excellent performance, AlN powder with high purity, small particle size, narrow particle size distribution, and stable performance must first be prepared.
Synthetic Methods of Aluminum Nitride Ceramics
Currently, the most common techniques for producing AlN powder include direct nitriding, Al2O3 carbothermal reduction, self-propagating combustion, plasma synthesis, aerosol methods, etc. The first two methods have been used on a large scale in industrial production.
The direct nitriding method is one that can be carried out at a lower temperature and features a straightforward procedure. The conversion rate of Al powder, on the other hand, is relatively low when AlN is synthesized using this method. Besides, agglomeration is easy to form, the product quality is poor, and the reaction process is challenging to control.
The carbothermal reduction method has a wide source of raw materials, and bauxite, which is cheaper than alumina, can be used as raw material. It is a straightforward process, and the product that it generates has desirable qualities such as high purity, uniform particle size distribution, resistance to agglomeration, ease of sintering, and the ability to be produced in large quantities; as a result, it is an excellent choice for the industrial production of AlN powder. However, after the reaction is finished, the extra carbon powder needs to be decarburized in dry air at 600–900°C, which increases the cost of production.
To some extent, the plasma synthesis method and the aerosol method can produce powder with excellent performance, but they are only suitable for small-scale production in the laboratory.
When it comes to refractory material synthesis, the self-propagating combustion method not only has the benefits of a shorter synthesis time, lower energy consumption, and higher product purity, but it also has a noticeable effect in increasing the nitriding speed and the conversion rate of Al powder.
Self-Propagating Combustion Synthesis of Aluminum Nitride Ceramics
The self-propagating combustion method is a new type of material synthesis method that uses the heat released by the combustion of chemical reaction raw materials to cause the chemical reaction process to continue spontaneously to produce products with specific components and structures.
To prepare AlN, for instance, the heat generated by the nitriding reaction of aluminum powder is used to keep the reaction going on its own, resulting in a powder of extremely high purity. The reaction process is: 2Al+N2=2AlN.
The essence of this method is identical to that of the direct nitriding method of Al powder. However, in contrast to the direct nitriding method, this method does not require nitriding the aluminum powder at a temperature higher than 1000 °C for an extended period of time, nor does it require an external heat source with the exception of ignition.
Advantages and Disadvantages of Self-Propagating Combustion Method
Advantages:
(1) It can be produced quickly and in large quantities at a low cost;
(2) The preparation process and equipment are simple, energy consumption is low, and is environmentally friendly.
(3) The combustion's high temperature can cause the reactants' volatile impurities to vaporize and remove from the reactants;
(4) Products with various shapes and structures, such as porous blocks, dense powders, etc., can be created;
(5) The preparation process can be properly designed to meet various requirements, and when combined with other preparation processes, the design space is quite large.
Disadvantages:
(1) The reaction is fast and the process is difficult to regulate;
(2) The prepared AlN powder has a large particle size and a low purity;
(3) Since the produced AlN ceramics have a low thermal conductivity, they are frequently used as additives in the steel, rubber, and plastics industries.
Optimization of Self-Propagating Combustion Method
The main issue in the synthesis of aluminum nitride by the self-propagating combustion method is how to reduce the nitrogen pressure and combustion temperature during the preparation process, as well as how to suppress the agglomeration caused by an extremely fast reaction. In order to prevent the introduction of impurities and a rise in cost, the main solution is to add suitable sintering aids to obtain a uniform and dispersed loose powder recently.
Sintering Aids for Aluminum Nitride Ceramics
The sintering aids for aluminum nitride ceramics should follow the principles below:
(1) It can react with Al2O3 on the AlN particle's surface; the resulting compound's liquefaction temperature should be lower than 1600°C;
(2) It can avoid causing AlN to oxidize and decompose during sintering;
(3) It can lower the oxygen content of the AlN lattice by precipitating Al2O3 at the grain boundaries;
(4) Either the amount of the formed Al2O3 compound at the grain boundary is low enough, or the thermal conductivity of the compound should not lower than that of AlN.
Conclusion
Aluminum nitride has become a strategic new material with the arrival of the 5G era. However, the current high-performance aluminum nitride powder in our nation mainly comes from imports, which is not only costly but also runs the risk of severing the raw material supply. Therefore, it is of the utmost importance to master the technology for the preparation of aluminum nitride powder and to actualize the localization of high-performance aluminum nitride manufacturing technology.