First-principles study of the anisotropic thermal expansion and thermal transport properties in h-BN

The thermal expansion coefficient (TEC) and thermal conductivity (k) of thermal fillers are key factors for designing thermal management and thermal protection composite materials. Due to its unique advantages, hexagonal boron nitride (h-BN) is one of the most commonly used thermal fillers. However, its TEC and k values are still unclear due to the inconsistency of characterization techniques and sample preparations. In this work, these disputes were addressed using the quasi-harmonic approximation (QHA) method and phonon Boltzmann transport equation (BTE) theory based on the density functional theory (DFT), respectively. The accuracy of our calculated TEC and k values was confirmed by previously reported experimental results, and the underlying physical principles were analyzed from the phonon behaviors. Our TEC results show that the h-BN has small in-plane negative value and large cross-plane positive value, which are −2.4×10^−6 and 36.4×10^−6 K^−1 at 300 K, respectively. And the anisotropic TEC is mainly determined by the anisotropic isothermal bulk modulus and the low-frequency out-of-plane longitudinal phonon modes. We found that the convergence of cutoff radius and q-grid size have significant effect on the accuracy of k of h-BN. Our results show that the in-plane k is much higher than the cross-plane k, and the values at 300 K are 286.6 and 2.7 W m^−1 K^−1, respectively. The anisotropic phonon group velocity arising from the vibration behaviors of acoustic phonon modes should be primarily responsible for the anisotropic k. Our calculated TEC and k values will provide important references for the design of h-BN composite materials.