Low-temperature synthesis of dense Si-based nitride ceramics for solar thermal storage: Role of cordierite as sintering additive

Abstract

Direct nitridation method was used to prepare dense Si-based ceramics for solar thermal storage, and the effect of cordierite as sintering additive on the sintering behaviors of the ceramics was studied. The influence priority between densification and nitride content on the properties of thermal storage ceramics was discussed. Cordierite exhibited a good capability of promoting densification, and the densification temperature could reach 1500 °C. The liquid formed by cordierite helped to construct an interlocking structure of well-grown Si2N2O flakes. Results of hot stage microscopy and DTA indicated that the endothermic silicothermic reaction between Si and cordierite proceeded at 1440–1460 °C, and released SiO gas. The SiO gas inhibited the densification, but resulted in the low-temperature synthesis of nano-sized Si3N4 whiskers. Samples added with more cordierite showed the cost-effective merits as the smaller costs of raw materials, lower densification temperature, higher thermal conductivity (1.72 W m−1 K−1 deviation), and higher bending strength (34.3–68.1 MPa deviations), compared with the samples having the larger nitride content. Densification by adding cordierite as sintering additive showed the priority on the properties of thermal storage ceramics than increasing the nitride content.