Thermal properties of porous ceramics manufactured by direct foaming using silicon sludge and silica fume

In this study, waste silica fume was used as the raw material for manufacturing porous ceramics, and its mechanical and thermal properties required for application as thermal insulators were studied. For pore formation with direct foaming, Si sludge (0.01, 0.05, and 0.1 wt%) and NaOH aqueous solution (14 N) were used as the foaming agents. The porous ceramics sintered at 900°C showed an apparent porosity of 85.9–89.8%, and the specimen with 0.1 wt% Si sludge showed the highest apparent porosity of 89.8% and a bulk density of 0.24 g/cm3. Scanning electron microscopy results showed that with increasing Si sludge substitution, an open pore structure was formed because of the increase in the pore size from several tens to several hundreds of micrometers. The compressive strength decreased from 3.82 to 2.11 MPa as the amount of added Si sludge increased from 0.01 to 0.1%, respectively. The thermal conductivity was less than 1.0 W·m−1·K−1 in the temperature range of 25–800°C, and the specimen with 0.1 wt% Si sludge showed a low thermal conductivity of 0.2–0.6 W·m−1·K−1. The thermal expansion coefficients were 17.1–22.4 × 10−6/K in the temperature range of 25–800°C