Reducing high-temperature dielectric loss of h-BN ceramics by orienting grains

Abstract

The exponentially increasing dielectric loss with rising temperature is the biggest obstacle hindering the application of hexagonal boron nitride (h-BN) ceramics as radome materials. Herein, a novel strategy is proposed to reduce high-temperature dielectric loss of h-BN ceramics by orienting grains. The relationship between electrical conductivity and temperature of h-BN ceramics shows that conductance loss of intrinsic defect ions thermal-excited above 667 °C is main cause of dielectric loss. And ab initio molecular dynamics calculations indicate that thermal-excited defect ions show much higher self-diffusion coefficient along a/b-axis of layered h-BN than that along c-axis. Therefore, by orienting h-BN grains, high-temperature dielectric loss of bulk ceramics will be minimized due to the suppressed defect ions transport perpendicular to orientation direction. As validation, resulting h-BN ceramics with oriented grains shows low dielectric loss tangent of 6.1 × 10−3 at 1000 °C, which is 45.5% lower than that of typical h-BN ceramics with random grains.