Inorganic polymers (geopolymers) as precursors for carbothermal reduction and nitridation (CRN) synthesis of SiAlON ceramics

Inorganic polymers (geopolymers) containing carbon are shown to be viable precursors for CRN synthesis of SiAlONs at 1400 °C in nitrogen. The reaction mechanism and rate depends on the form and fineness of the carbon, the formation of β-SiAlON being faster and more complete with fine carbon black. The presence of the charge-compensating alkali cations in the geopolymer leads to the formation of a mixture of β-SiAlONs of low and high z-value, the former being formed via O-SiAlON resulting from the presence of silica-rich eutectics, and the latter forming by CRN reaction from an alumina-rich amorphous component. In ammonium-exchanged geopolymers lacking the alkali, SiAlON formation occurs primarily by conventional CRN of crystalline mullite and cristobalite intermediates. The potential advantages of geopolymer CRN precursors include their ability to be tailored to specific SiAlON compositions and their formation and curing at ambient temperatures to required shapes prior to the CRN reaction.