High-Temperature Properties of Needle-Like Mullite Obtained from New Zealand Kaolin

Needle-like mullite was prepared by leaching out the silica glass from New Zealand kaolin fired at 1550° to 1700°C. High-temperature properties of the needle-like mullite, such as thermal expansion and thermal shock were examined. Needle-like mullite 1-6μm long, 0.1-0.5μm thick was obtained at 1550° and 1600°C, and 10-20μm long, 1-2μm thick, at 1650° and 1700°C. The specific surface area of these crystals grown at 1550° and 1650°C were 8.2 and 4.1m2/g respectively, and did not decrease with reheating in the temperature range from 1000° to 1650°C for 20-30h. The axial thermal expansion coefficients of crystals at 25°-800°C were αa=2.2, αb=3.9 and αc=4.5×10-6/°C. No evidences of fracture and crack were detected on quenching needle-like mullite crystals from 1150°C to 0°C.