Preparation And Characterization Of AlN Powder By Three-Step Direct Nitriding Method

AlN is a ceramic materials with high integrated performance, and have been widely used in many industrial fields. In general, it need high quality Al N powder to prepare superior performance AlN ceramic. In this paper, a low-cost and high-quality Al N powder was prepared for the purpose. On the basis of the traditional direct nitriding method, add Mg powder and NH4 Cl as reaction aids, through a three-step process to obtain high purity, fine-grained and high hydrolysis resistance AlN powder.Experiments carried out in accordance with the following three-step process: the first step is prepare Al N powder and study nitride product under different reaction parameters;the second step is pickl product that have not completely nitrided; the third step is study the hydrolyzability of Al N powder and anti-hydrolysis process.In the experiment, aluminum powder and two additives were evenly mixed according to a certain proportion, then put the mixed power into a good-sealing tubular resistance furnace, throughing N2 and heating. Set N2 flow to 30 sccm and temperature rise rate to5℃/min in the heating phase. When the temperature raised to 800~1000℃, set the N2 flow to 60 sccm and holding time at 1~4h. After incubation, let the samples cooled to room temperature under flowing 20 sccm N2 atmosphere, then took it out and got AlN powder after grinding.Preliminary experiments found that while simultaneously add Mg powder and NH4 Cl as additives, the nitride product was easily crushed loose aggregate, and XRD analysis showed that aluminum power had been completly nitrided with no residual of additives.This is mainly because of the activity of Mg is higher than Al, so Mg react with N2 earlier than Al, forming Mg3N2 in the reaction. Mg3N2 has a poor stability, it will be sublimed at 700℃ and decomposed at 800℃, which will produce a large number of holes in the molten aluminum and improve the rate of nitriding. NH4 Cl also has a poor thermal stability, it will be decomposed into NH3 and HCl at 337.8℃. When these two gases escaped from the molten aluminum, multiple holes formed and the diffusion of N2 in the aluminum powder accelerated. The XRD analysis of the nitride product showed noresidual impurity peaks, indicated that Mg and NH4 Cl all are low residual additives. The SEM analysis of the nitride product found a small amount of whiskers. Regulate process parameters can obtaine different growth morphology of whiskers. EDS analysis showed whiskers are AlN.The results of adjustment parameters experiment showed that when Al/Mg/NH4 Cl mass ratio of 9:2:4, holding time of 3h and holding temperature of 1000℃, the aluminum powder was able to be completely nitrided, the product had a highest N content of 33.4%and AlN whiskers with uniform morphology. In the product-crush experiment, grind 2h by hand the average particle diameter up to 9.5um and grind 1h by mechanical mill the average particle diameter up to 3um, showed that the product has an easy grinding characteristic.Incomplete nitrided product(Nitriding temperature below 1000 ℃) in the experiment can be purified by pickl method. XRD analysis of purified product found no impurity peaks, indicated that pickl method can be used to purify the uncompletely nitrided product. Hydrolysis experiment found that AlN powder purified by phosphoric acid had more hydrolysis resistance than purified by sulfuric acid and hydrochloric acid.Soaked AlN powder in phosphoric acid for 24 h, then used for hydrolysis experiments,found that the solution remained acidic after 180 h, indicated that the powder has a very high resistance to hydrolysis. In summary, phosphoric acid not only can be used to purify nitride product, but also can be used to anti-hydrolysis process. Heat treatment of AlN powder found that the AlN powder heating at 900 ℃ for 1h had a certain degree of resistance to hydrolysis, but less than the phosphoric acid soaking treatment.