Influence of kaolin and firing temperature on the mullite formation in porous mullite-corundum materials

Abstract

The refractory ceramics became very important in both the traditional and the advanced materials applications as it has outstanding thermal and mechanical properties. The refractoriness of ceramics can be achieved by getting the mullite-corundum. Refractory ceramics with high porosity serve as a heat insulator and constructional material. Three series of porous mullite-corundum ceramic samples were prepared from Al 2O 3 (Nabalox, Germany) and pure SiO 2 in 2.57:1 ratio that was conformed to mullite compositions (3Al 2O 3·2SiO 2). α-Al 2O 3 (d 50=4 μm) and γ-Al 2O 3 (d 50=80 μm) were in 1:3 ratio. Quantity of kaolin (MEKA, Germany) was 10, 20 and 30 wt.%. Porous materials were prepared by slip casting of suspension of raw materials, where the aluminium paste (0.18 wt.%) was used as a pore former. Water content in the suspensions was 38-40 wt.%. Pore formation occured in result of hydrogen formation in chemical reaction between aluminium paste and water. The samples were sintered at 1650, 1700 and 1750°C temperature for one hour. SiO 2 and γ-Al 2O 3 on the contrary reduced mechanical properties, but decreased shrinkage. Using of α-, γ-Al 2O 3, SiO 2 and kaolin in corresponding ratios the samples with open porosity of 30 to 54 vol% were acquired. The relative amounts of pores depended on the initial content of kaolin and on firing temperature.0.