Tailoring microstructure and properties of porous mullite-based ceramics via sol-gel impregnation

Nanotechnology has been introduced into the refractory industry decades ago. However, there are still some peculiarities that hinder its routine application. Poor mechanical strength is one of them. Here we present the fabrication technique for mullite porous ceramics using a calcium-free colloidal binder system followed by secondary sol-gel impregnation intended for fabrication of large up-scaled monoliths. The primary foam was prepared via direct foaming of slurry containing colloidal silica, gelling agents, surfactant, silica fume, and different types of alumina. The foam mixture was cast, de-molded and dried using a cascade drying procedure. The investigation of mullitization that occurred during the high-temperature treatment was studied by XRD and TG-DTA analysis. The optimized sintering curve was then applied to obtain the highest mechanical properties. The resulting foam (with open porosity, volume density about 900 kg/m3 and mechanical strength about 10 MPa) was further processed by the impregnation step. The applied silica or combined silica/aluminum sol and unreacted alumina underwent secondary mullitization during following high-temperature treatment, which significantly enhanced the mechanical strength (up to 18 MPa). The secondary mullitization process was also investigated by HT-XRD and TG-DTA analysis. Foam microstructure was observed by an electron scanning microscope. Thermal conductivity of as-prepared ceramics foams varied from 0.3 to 0.4 W/m & sdot;K. The foam wall porosity evolution was studied by mercury intrusion porosimeter and estimated from image analysis.