Effect of alkaline oxides (CaO and MgO) on the mechanical properties of SiC-based foam ceramics

Abstract

Fly ash is potentially useful as a sintering additive in the synthesis of SiC-based foam ceramics (SFC). However, the influence of CaO and MgO, the main alkaline oxides in fly ash, on the performance of SFC is not well understood. This study synthesized SFC using SiC and fly ash as the primary raw materials (mass ratio of 7:3), and systematically investigating the effects of CaO and MgO on the properties of SFC, including porosity, density, strength, apparent morphology, and phase composition. The results indicated that addition of CaO and MgO promoted oxidation of SiC at relatively low temperatures. With a CaO content of 1.5 wt% or an MgO content of 2 wt%, SFC with uniformly distributed porous structures and exceptional properties was achieved at sintering temperatures as low as 1150 °C. Additionally, CaO and MgO facilitated formation of crystalline phases such as wollastonite, calcium feldspar, and cordierite, enhancing the thermophysical properties of SFC. However, excessive CaO or MgO led to over-sintering of the SFC samples. With a CaO content of 1.5 wt% and a sintering temperature of 1175 °C, SFC was completely deformed. This was also observed with an MgO content of 3 wt% and a sintering temperature of 1250 °C, indicating that CaO promotes SiC oxidation more effectively than MgO. It is suggested that addition of appropriate amounts of CaO and MgO is conducive to production of high-quality SFC at low sintering temperatures, significantly enhancing their commercial viability.