Fabrication of nitrogen-containing cordierite ceramics

We have investigated the crystallization behavior of cordierite-based oxynitride glass prepared by melting a powder mixture of cordierite and silicon nitride. The results indicated that two polymorphs exist: a stable α-form, which has a low coefficient of thermal expansion, and a metastable μ-form. The time-temperature-transformation diagram of the crystallization of the α-form shows a typical C-shaped curve that has a nose around 1420 K. Above 1373 K, the α-form can be directly obtained by the isothermal crystallization of the oxynitride glass and the μ-form inverts to the α-form at temperatures below 1273 K. We also have fabricated an α-cordierite ceramic that has a nitrogen content of 1.55 mass%, which is much lower than the content of the batch composition (5 mass%); this is mainly owing to the decomposition reaction of nitrogen in the oxynitride melts and glasses during heat treatment. However, the fabricated nitrogen-containing α-cordierite ceramics showed the coefficient of thermal expansion of 10^-7 K^-1, which is comparable with that of the oxide α-cordierite ceramics. Moreover, Young's modulus of the α-cordierite ceramics improved with the incorporation of nitrogen.