Production of Si3N4 porous beads via carbothermal reduction and nitridation technique

Abstract

ABSTRACT Si3N4 porous beads were produced from direct carbothermal reduction and nitridation of spheres composed of carbon+SiO2 mixtures. A simple one–step sol–gel templating technique was used to prepare carbon+SiO2 (molar ratio of carbon/SiO2 was set as 4) containing spheres with a diameter of 2 mm. A natural and abundant biopolymer, alginate, is used as a sacrificial template to produce porous Si3N4 ceramic beads. Carbothermal reduction and nitridation process was conducted at two different temperatures (1500 and 1550°C) and atmospheres (pure nitrogen and 5% hydrogen in nitrogen) to observe the effect of process parameters on the yield of Si3N4. Porous beads with a smooth shape and without any deformation were successfully obtained after the carbothermal reduction and nitridation process. X–ray powder diffraction studies showed that the use of H2 increased the amount of Si3N4 formed by accelerating the reduction and nitridation reactions. Microstructural investigations revealed oxide addition changed grain morphology from fiber–like to short, angular geometry. This microstructural development showed that the addition of oxide powder increased the amount of liquid phase formed during the heat treatment process and changed carbothermal reduction and nitridation mechanism from vapor–solid to vapor–liquid solid.