The anisotropic thermal expansion of boron nitride

Abstract

Interferometric measurements of the linear thermal expansion coefficients αc and αa in the c and a axial directions of well-orientated hexagonal pyrolytic boron nitride are reported for temperatures between approximately 80 and 780 K. The primary data for the direction parallel to the c-crystallographic axis yield smoothed results which are believed to be accurate in an absolute sense to within 3% at 780 K, falling to 5% at 80 K. The corresponding figures for the direction perpendicular to the c-crystallographic axis are 9% at 780 K, passing smoothly through 3% at 230 K to 6% at 80 K. In association with earlier specific heat capacity results and provisional values for the elastic constants c ₁₃ and c ₃₃, a Grüneisen parameter corresponding to the direction parallel to the c-crystallographic axis has been calculated as a function of temperature. This parameter has been applied to the determination of the temperature dependence of averaged reduced vibrational frequencies associated with this axial direction. The quasiharmonic approximation has been applied to calculate the characteristic temperatures θ(n) corresponding to the maximum frequencies of the Debye distributions having the same nth order moments as the frequency spectrum in the actual solid for − 3 ≤ n ≤ 6. The geometric mean frequency of the vibrational spectrum correlates satisfactorily with infra-red absorption data and all the thermodynamic quantities measured and calculated resemble the corresponding properties of pyrolytic graphite, with which hexagonal boron nitride shares a number of structural features.