Morphologies and growth mechanisms of aluminum nitride whiskers synthesized by carbothermal reduction

The growth mechanism and morphologies of AlN whiskers prepared by the carbo thermal reduction method are studied as well as the technological condition . All experiments were carried out in a vertical, controlled-atmosphere, gr aphite clement resistance furnace. The reactor chamber volume is (Phi 250 x 210 mm, and the reactant weight is about 20 g each times tit contains abou t 70 wt.% Al2O3). Nitrogen pressure in the furnace is 0.1 MPa (1 arm). The optimum growth condition is at 1700 degrees C for 5 h with flowing nitrogen (0.6 1 min(-1)), when the raw materials are alpha-Al2O3 and graphite (Al2O 3:C = 4:1 in weight). The yield of AlN whiskers is about 3-5 g each time, a nd 50 wt.% or more of Al2O3 can convert to AlN whiskers. Generally, whisker s with hexagonal cross section grow along [0001] and plate-like whiskers ha ve a growth orientation parallel to [01(1) over bar 1] et al other than (00 01). During the growth of whiskers, the VLS mechanism and VS mechanism serv e the function together. In the early stage, alpha-Al2O3 will react with Ca O to form Ca-aluminates such as CaO . 6Al(2)O(3)(CA(6)), CaO . Al2O3(CA(2)) , CaO . Al2O3 (CA). These Ca-aluminates can provide liquid catalysts for th e nucleation of AlN whiskers. After that, the AlN whiskers will grow out fr om the substrate surface by the VS mechanism. As proximity to substrate sur face, there is less growth space and higher supersaturation, and far from s ubstrate surface, there is large growth space and low supersaturation. So n ear the substrate surface AlN whiskers are small and with irregular shape. Far from the substrate surface, AlN whiskers are sensibly straight and with regular shape. (C) 1999 Elsevier Science S.A. All rights reserved.