Influences of impurities and mineralogical structure of different kaolin minerals on thermal properties of cordierite ceramics for high-temperature thermal storage

Abstract Cordierite ceramics were prepared for high-temperature thermal storage by reaction sintering method using Longyan kaolin, Lincang kaolin and Xingzi kaolin, respectively. Effects of impurities and mineralogical structure of three different kaolin minerals on the phase compositions, microstructure, physical properties and thermal properties of cordierite ceramics for concentrated solar power were studied. Results indicated that illite in Longyan kaolin provided Na2O and K2O to promote the microstructure densification of cordierite ceramics and endowed samples sintered at 1280 °C with the highest bending strength of 94.4 MPa and thermal conductivity of 3.71 W·(m·K)−1, but increasing thermal expansion coefficients to 2.61 × 10−6 (room temperature–800 °C). Fe2O3 and TiO2 impurities in Xingzi kaolin could lower the crystallization temperature of cordierite to 1183.2 °C and the usage of Xingzi kaolin increased the softening temperature of cordierite ceramics to 1496 °C. Cordierite ceramics synthesized using Lincang kaolin with the minimum contents of impurities obtained the smallest thermal expansion coefficient of 1.72 × 10−6 (room temperature–800 °C). Tubular structure of Lincang kaolin particles was detrimental for microstructure densification and thermal conductivity. According to the requirements on thermal properties of high-temperature thermal storage ceramics, kaolin minerals for preparing cordierite thermal storage ceramics by reaction sintering method should be chosen in terms of the chemical composition and mineralogical structure.