Crystallization behaviour of sol-gel derived cordierite precursors

Abstract

Cordierite (Mg2Al4Si5O18) ceramics is technologically important material due to its low thermal expansion, small dielectric constant, and high chemical and thermal stability [1]. It occurs in two stable forms: in the high-temperature hexagonal α form, stable above 1450 °C, [2] and in the orthorhombic beta form, stable below 1450 °C [3]. Preparation roots of cordierite involve solid-state reactions, liquid phase sintering, glass-ceramic route or sol-gel synthesis. The interest in sol-gel synthesis lately increased because it enables excellent control of chemical composition and reduces the temperature of ceramic processing. Two sol-gel derived cordierite precursors were prepared using tetraetoxysilane, Mg(NO3)2• 6H2O and Al-sec-butoxide (for precursor designated as BC) or Al(NO3)3• 9H2O (for precursor designated as NC). Detailed studies of crystallization behaviour of those precursors were performed by DSC and in-situ high temperature XRD from RT up to 1165 °C. The Rietveld method was performed for quantitative phase analysis of the crystallization products and for the refinement of their crystal structures. Crystallization started at ~800 °C for both precursors, but their crystallization sequences were rather different. For precursor BC crystallization involved the appearance of stuffed derivative of beta-quartz, α -cordierite, mullite and enstatite, while for precursor NC it involved the formation of stuffed derivative of beta-quartz, sapphirine, mullite, α -cordierite and beta-cristobalite.