Anisotropic Thermal Expansion of Zirconium Diboride: An Energy-Dispersive X-Ray Diffraction Study

Abstract

Zirconium diboride (ZrB 2 ) is an attractive material due to its thermal and electrical properties. In recent years, ZrB 2 has been investigated as a superior replacement for sapphire when used as a substrate for gallium nitride devices. Like sapphire, ZrB 2 has an anisotropic hexagonal structure which defines its directionally dependent properties. However, the anisotropic behavior of ZrB 2 is not well understood. In this paper, we use energy-dispersive synchrotron X-ray diffraction to measure the thermal expansion of polycrystalline ZrB 2 powder from 300 to 1150 K. Nine Bragg reflections are fit using Pseudo-Voigt peak profiles and used to compute the a and c lattice parameters using a nonlinear least-squares approximation. The temperature-dependent instantaneous thermal expansion coefficients are determined for each a -axis and c -axis direction and are described by the following equations: α a = ( 4.1507 × 10 - 6 + 5.1086 × 10 - 9 ( T - 293.15 ) )/( 1 + 4.1507 × 10 - 6 ( T - 293.15 ) + 2.5543 × 10 - 9 ( T - 293.15 ) 2 ) and α c = ( 4.5374 × 10 - 6 + 4.3004 × 10 - 9 ( T - 293.15 ) )/( 1 + 4.5374 × 10 - 6 ( T - 293.15 ) + 2.1502 × 10 - 9 ( T - 293.15 ) 2 ). Our results are within range of previously reported values but describe the temperature anisotropy in more detail. We show that anisotropic expansion coefficients converge to the same value at about 780 K and diverge at higher temperatures. Results are compared with other reported values.