FORMATION OF REACTION-BONDED SILICON-NITRIDE USING MICROWAVE-HEATING

Rod-shaped silicon compacts were nitrided in a microwave applicator us ing a minimum of insulation in order to maximize the temperature gradi ents to effect an inside-out reaction. These specimens exhibited very nonuniform conversion to Si3N4, and fully nitrided areas had poor micr ostructures consisting of alternating regions of high and low density. The key factor was found to be sintering of the silicon powder during initial heating which caused the specimens to be electrically conduct ing during the early stages of the reaction and led to large changes i n the microwave heating behavior of the specimens as they nitrided, Th e temperature and composition profiles in the specimens were simulated using a numerical model, The results of the model indicated that the observed microstructures were caused by high temperatures and temperat ure gradients in the areas of maximum nitridation rate which caused si licon vapor to diffuse within the specimens. Some compacts were made f rom a mixture of silicon powder and silicon nitride powder to avoid si ntering of the silicon particles. These specimens nitrided uniformly w ith inside-out composition profiles, indicating that microwave heating would be beneficial for the nitridation of pure silicon powder compac ts if sintering could be avoided.