Mechanochemical effect on low temperature synthesis of AlN by direct nitridation method

Effect of mechanochemical treatment on low temperature synthesis of AlN from Al metal in N2 atmosphere was investigated. The Al powder, of 20-μm particle size, was dry milled using a planetary ball mill. Since Al powder is soft and malleable, the particle size tended to increase due to adhesion of the original particles during grinding. On the other hand, effective grinding was achieved by adding LiOH.H2O, which is thought to cover the surfaces of Al powder and suppress the adhesion. Samples containing 0.5 mass% Li in Li/(Li+Al) were ground for various times up to 6 h, then nitrided for 1 h at various temperatures. AlN formation in these samples began at 570–640 °C, much lower than without LiOH.H2O, indicating the accelerating effect of the added LiOH·H2O. The conversion temperature was lowest in the sample ground for 1 h. This sample (mechano-sample) also showed a higher AlN conversion rate than the other samples, being almost completely converted at 580 °C. The mechanochemical treatment is found to be very effective for synthesizing AlN powder at very low temperatures (below the melting point of Al metal). It is thought to facilitate the formation of lithium aluminate on the surface of the Al metal particles, enhancing the diffusion of nitrogen.