Effect of additives on self-propagating high-temperature synthesis of AlN

Effect of additives on the oxygen content and the growth mechanism of self-propagating high-temperature synthesis (SHS) of AlN were investigated in detail. Although the addition of NH4Cl made the product loosen, it also increased the water vapor concentration of the system at the same time, which brought a great increase of the oxygen content in the product. Carbon decreased the oxygen content of AlN by two kinds of mechanisms: carbothermal reduction of Al2O3, and decrease of the water vapor concentration that served as the catalyst of AlN oxidation at elevated temperature. The latter was found to be the dominant mechanism. Partly carbonized sucrose decreased the oxygen content of AlN more significantly than black carbon due to its ever-increasing surface area, arising from its carbonization during combustion. At the lower oxygen content, AlN grew from the vapor in the form of “platelet growth”. With increasing oxygen content, the growing steps were blocked up by the impurities containing oxygen. The “platelet growth” was gradually inhibited, and AlN inclined to grow into whiskers by VLS mechanism.