| Aluminum nitride is an excellent ceramic material. It has a series of excellent features. Such as good thermal conductivity, reliable electrical insulation, low dielectric constant and dielectric loss, coefficient of thermal expansion match with silicon.The primary techniques of synthesis AlN powder include direct nitridation of aluminum, carbothermal nitridation reaction and Self-Propagating High-temperature Synthesis. Compared with SHS and carbothermal nitridation reaction, direct nitridation of aluminum has the advantages of simple process and low cost. In this paper, traditional direct nitridatong process has been improved. First, prepare for aluminum precursor containing additives. And then put the precursor powder into a alumina crucible and put them into tube furnace. The nitrogen flow is0.1L/min, heating rate controlled in10℃/min, heating to800-1200℃and preservation time1-4h. And the-n cooled to room temperature under flowing nitrogen. The synthesis products of AlN block presents a porous loose.The DSC analysis of pure aluminum powder and mixed with50wt%of NH4Cl, the precursor powder and found that the nitriding temperature of aluminum fell by195℃by adding NH4C1. This is mainly due to lower the temperature of the NH4Cl decomposition of the aluminum powder was porous fluffy state, conducive to the spread of the nitrogen penetration. Second, the HCl generated after the decomposition of NH4C1damage the aluminum surface of the oxide film, exposing fresh aluminum matrix, so that it can access to the N2at lower temperatures, and thus at a lower temperature nitride. Simply by adding NH4Cl as an additive does not promote aluminum fully nitriding, there are a large number of elemental aluminum products exist. The main reason for this result:of NH4Cl the decomposition temperature is far below the nitriding temperature of the aluminum powder, aluminum nitride when, of NH4Cl has been basically decomposed completely. After the beginning of the nitridation reaction is first generated in the molten aluminum particle surface a thin layer of dense nitride aluminum layer of nitride aluminum will hinder the N2to the particles inside spread, and ultimately affect the nitrogen content in the product.After a certain amount of KCl in the additives that can further improve the nitrogen content of synthetic products, indicating that KCl at high temperatures of N2to the spread of molten aluminum particles inside played a catalytic role.This is a body powder start nitride, steam evaporation of KCl melt aluminum particles showing porous, increasing its contact area with N2, while the KCl evaporation will destroy the aluminum particle surface nitride aluminum,N2to diffuse into the aluminum particles within the nitride.The study showed that the use of NH4Cl/KCl as an additive can be prepared by a higher nitrogen content of aluminum nitride.However, with the increase of additive content, the oxygen content in the synthesized product will increase, considering the synthetic products in the nitrogen, oxygen content, the content of additives in the precursor powder be more appropriate to control between40and50%, the additive composition of NH4C1:KC1=0.7:0.3(weight ratio).The ICP determination of the content of the synthetic product in K, and results show that the NH4CI/KCl low residual additives, with the reaction temperature is increased, its residues to decline further. The residues in the final product of Cl less than0.3%, K less than0.1%of residues.From SEM observation, the synthesized product and found that there are some1-2microns, the length of tens to hundreds of microns in diameter wool-like objects in the synthetic product. From EDX analysis, the results show that the wool-like objects of AlN whiskers. This shows that the additive aluminum powder direct nitridation can be prepared AlN whiskers and AlN whiskers is not the primary form of the synthetic product, the product of a small amount of fine and uniform AlN whiskers, there are a lot of granularity AlN particles.This paper also studied the influence of reaction temperature and holding time on the nitride product, results show that the increase of reaction temperature, holding time can increase the nitrogen content of the synthetic products. Considering energy consumption, the product of nitrogen content, oxygen content, additives, residues and other factors, the reasonable preparation of aluminum nitriding process parameters are:aluminum+additives(40-60wt%) after24hours milling mixed precursor powder additive formula NH4Cl:KC1=0.7:0.3(mass ratio). The precursor powder in a flowing nitrogen atmosphere1000℃for3hours followed by furnace cooling porous loose block product.Porous product of the aluminum nitride powder obtained after24hours of ball milling, were related to the performance of aluminum nitride powder, the results are as follows: powder color pale gray, the average particle size D50is2.2μm, the specific surface area of1.3m2/g, bulk density0.44g/cm3. Powder, the nitrogen content is greater than32%, the oxygen content of less than1.5%. The experiment preparation of aluminum nitride powder key performance indicators to meet the military standards of the People’s Republic of China (SJ20637-97) the relevant indicators of electronic ceramics with a nitride aluminum nitride aluminum specification.Aluminum nitride powder can easily absorb moisture in the air hydrolysis, the resulting powder oxygen content increased, the nitrogen content decreased. In this thesis, a preliminary analysis of the hydrolysis mechanism of aluminum nitride powder, and related thermodynamic calculations. The thermodynamic calculation results show that the aluminum nitride powder can easily hydrolytic reaction with H2O in the air, hydrolyzed AlOOH and Al (OH)3, while the release of NH3. In this thesis, using the heat treatment of aluminum nitride powder surface modification, although the method can inhibit the hydrolysis of aluminum nitride powder to a certain extent, but after heat treatment of aluminum nitride powder oxygen content has also been significantly improved. The same anti-hydrolysis in the cooling phase of the direct nitridation of aluminum pass into the air for cooling, can play and ALN powder at900℃, and this approach reduces energy consumption and increase efficiency. |
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