Direct nitridation is the earliest method used to synthesize aluminum nitride powder. It is also one of the few technologies that have achieved large-scale industrial production of aluminum nitride powder. For example, Japan's Toyo Aluminum Company and Germany's Starck Company use this technology to produce aluminum nitride powder. On the surface, this process is very simple, which is to directly react aluminum powder with nitrogen at high temperature (usually above 1200°C): 2Al+N2=2AlN. If you synthesize a small amount of aluminum nitride powder in the laboratory for your use, it doesn't matter whether it is good or bad then it may be true. However, suppose you want to synthesize powders with industrial application value, such as fine particles with narrow distribution, less agglomeration, high sintering activity, and relatively cheap price. In that case, you will find that this seemingly simple process is not as simple as imagined. There are at least three problems:
Aluminum Nitride AlN powderof Electron microscope image
First, the reaction temperature is above the melting point of metal aluminum (660℃). After the aluminum powder is melted, it is easy to form large aggregations or "droplets," reducing the contact surface between nitrogen and aluminum powder, slowing down the reaction rate; second, in the early stage of the reaction (when the temperature is low), an aluminum nitride passivation film is easily formed on the surface of the aluminum powder, which hinders the further diffusion of nitrogen into the interior, affecting the nitridation reaction and the purity of the final aluminum nitride; third, the nitridation reaction is exothermic, and a large amount of heat will be released during the process, which can easily cause the self-sintering of the aluminum nitride powder, forming agglomerates and coarsening the powder particles. The existence of these problems brings a lot of trouble to the direct nitridation process!
Of course, in actual production, the above problems can be solved by crushing the obtained powder and then continuing to nitriding, but this will undoubtedly increase the production cost, inevitably introduce other impurities and reduce the performance of the powder.
So, the best way is to find a way to accelerate the nitriding reaction while reducing the reaction temperature and avoiding the formation of agglomerates. This is also the main direction of various research today, and the targeted measures taken mainly include the following aspects:
Electron microscope image of Aluminum Nitride AlN powder
First, choose a suitable aluminum source.
In addition to purity, the most important thing is the particle size of aluminum powder. Generally, aluminum powder with a smaller particle size has a large specific surface area, high porosity, and a faster nitriding reaction rate, which can quickly reach the maximum nitriding degree in a short time. Interestingly, it has been found that when a mixture of aluminum powders with different particle size gradations is used as a raw material, its nitriding degree is often higher than that of a single particle size aluminum powder - even higher than the smallest particle size aluminum powder in the composition.
Second, change the reaction gas.
A typical example is to use ammonia, which is more active than nitrogen. Some people have found that when a mixture of nitrogen and ammonia is used instead of pure nitrogen, the temperature of the nitriding reaction can be effectively reduced. For example, at 1250 °C, the nitriding rate is less than 1% when pure nitrogen is used, while when 5 vol% ammonia is added, the nitriding rate is directly increased to 100%. Other studies have shown that for the same metal aluminum source, it takes 1400 °C to complete nitride when nitrogen is used, while when ammonia is used instead of nitrogen, it will be completely converted into aluminum nitride at 1100 °C. Studies have found that in the presence of ammonia, the aluminum nitride film on the surface of metal aluminum will break at high temperatures, the passivation effect disappears, the diffusion of nitrogen atoms to the aluminum nucleus is accelerated, and the speed of the entire nitriding reaction is increased. Some people also introduce argon before the rapid nitriding reaction of the aluminum powder mixture and use argon to control the progress of the nitriding reaction. Finally, the aluminum powder can reach a nitriding rate of 98.7% at a low temperature of 600 °C.
Third, Introduce additives.
In order to solve the problem of nitridation reaction, different additives can be introduced into the raw materials to play different roles. For example, after aluminum powder comes into contact with air, an aluminum oxide film will form on the surface. If this part of oxygen is not removed in the subsequent process, it will enter the product, making the oxygen content in the aluminum nitride powder high. In order to solve this problem, some people add a small amount of carbon powder to the aluminum powder, which can effectively reduce the oxygen content in the synthesized aluminum nitride powder. At the same time, carbon can also act as a dispersant to prevent aluminum powder from agglomerating. For another example, in order to solve the problem of aluminum nitride passivation film, some people introduce metal zinc into the raw aluminum powder. Zinc forms a zinc-aluminum alloy with aluminum during heating. Since zinc evaporates easily, it can destroy the aluminum nitride film on the surface, making it incomplete and accelerating the diffusion of nitrogen to the inside, thereby accelerating the reaction. Zinc itself will basically evaporate and be removed in this process, and the residual amount in the final product is small. Some people also mix NH4Cl and KCl into aluminum powder. These additives will decompose and vaporize when the temperature rises. On the one hand, they make the aluminum powder porous and loose, and on the other hand, they can destroy the nitride film on the surface of the aluminum particles, thereby achieving complete nitridation of the aluminum powder.
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