Preparation, characterization and properties of MgSiN2 ceramics

MgSiN2 ceramics with and without sintering additives were prepared by hot uni-axial pressing. For the sintered samples the lattice parameters, secondary phases, density, oxygen and nitrogen content, microstructure, oxidation resistance, hardness, elastic constants, linear thermal expansion coefficient and thermal diffusivity were determined. By suitable processing, fully dense MgSiN2 ceramics with an oxygen content <1.0 wt % could be obtained. The size of the MgSiN2 grains increased with increasing hot-pressing temperature and time. Transmission electron microscopy (TEM) showed the absence of grain-boundary phases and that secondary phases are present as separate grains in the MgSiN2 matrix. Scanning thermal microscopy (SThM) thermal imaging revealed a thermal barrier at the grain boundaries. The influence of the microstructure as well as the oxygen content and defect chemistry on the thermal diffusivity is limited. Supported by some theoretical considerations it is concluded that the thermal conductivity of the MgSiN2 ceramic samples is determined by intrinsic phonon-phonon scattering and will not exceed 35Wm(-1) K-1 at 300 K in agreement with the maximum observed value of 21-25Wm(-1) K-1. (C) 2002 Kluwer Academic Publishers.