Effect of Heat-Treatment Temperature on Properties of SiO_2f/SiO₂ Composites

It is of great practical significance to develop thermal insulation materials with integrated load bearing and thermal insulation for energy saving and emission reduction in high-temperature engineering. In this work, SiO_2f/SiO₂ composites were prepared by a precursor impregnation heat-treatment method with quartz fiber needled felt as a reinforcement and silica sol as a precursor. The effect of heat-treatment temperature on the density, porosity, mechanical properties and thermal properties of SiO_2f/SiO₂ composites was investigated. The composition and microstructure of the composites were characterized by X-ray diffraction and scanning electron microscopy. The coefficient of thermal expansion and thermal conductivity of composites were measured by a push rod method and a transient hot-wire method. The results indicate that the apparent porosity and mechanical strength firstly increase and then decrease with the increase of heat-treatment temperature. The sample after heat-treatment at 450 ℃ has the optimum overall performance (i.e., the flexural strength of 23.5 MPa, and the compressive strength of 61.9 MPa). The fracture mechanism is a ductile fracture. At 300–700 ℃, the thermal expansion coefficient of the sample is 0.564×10^-6/℃. The thermal conductivity of the samples decreases with the increase of temperature, and it is as low as 0.096 W/m·K at 700 ℃. The SiO_2f/SiO₂ composite prepared has some advantages in the integrated application of load bearing and thermal insulation, and meets the harsh requirements of hot pressing and sintering, non-ferrous metallurgy and other industries.