Polar Effects on the Thermal Conductivity of Cubic Boron Nitride under Pressure

Abstract

We report the lattice thermal conductivity (κ) of cubic boron nitride (c-BN) under pressure calculated using density functional theory. Pressure was used to manipulate the c-BN phonon dispersion and study its effect on thermal conductivity. These results were compared to c-BN's mass-equivalent, nonpolar counterpart, diamond, in order to isolate the effect of polar bonds on thermal conductivity. Unlike diamond, the variation of κ at room temperature (κ RT ) with applied pressure in c-BN is nonlinear in the lowpressure regime followed by a transition to a linear regime with a distinct change in the slope at P > 114 GPa. We find that the change in κ with pressure cannot be described with power law expressions commonly used for Earth mantle materials. The nonlinearity in the low-pressure regime can be related to the nonlinear change in LO-TO gap, group velocities, and specific heat with increasing pressure. In addition, we find that, although optical branch contributions to thermal conductivity are small (∼2% at RT), the rise in κ RT for P > 114 GPa is due to (1) the decoupling of the longitudinal acoustic branch from the optical branches and (2) depopulation of the optical branches. These lead to a sharp reduction in acousticacoustic-optic (a-a-o) scattering and a discrete change in the acoustic phonon mean free paths. This study illustrates the importance of optical branches and their interactions with acoustic branches in determining the total thermal conductivity of polar materials. This finding is also relevant for current research in geologic minerals under pressure and the design of thermoelectrics. The last decade has seen a renaissance in the study and understanding of thermal conductivity (κ) in materials and nanostructures, driven largely by new ab initio Boltzmann transport approaches [1-6], large-scale molecular dynamics simulations [7][8][9][10], and novel κ measurement techniques [11][12][13]. These approaches offer an unprecedented window into the secret life of phonons, providing, for the first time, accurate estimates for (1) normal and umklapp scattering rates in crystals [2,14], (2) thermal conductivity accumulation function [11,15,16], and (3) the often underestimated role of optical phonons [17,18]. These insights have led to the prediction of new high κ materials (e.g., BAs [19]), techniques for improving thermoelectrics [20,21], and κ estimates for minerals deep in the Earth [6,22].While the thermal conductivities of numerous materials have been calculated from first principles recently, a fundamental question of how thermal transport in polar materials differs from their nonpolar counterparts is still unresolved. The answer to this question also has practical implications for fields ranging from heat flow in Earth mantle materials to thermoelectrics design. It is well known that long-range Coulomb interactions between ions in a polar system result in a nonanalytic term in the dynamical matrix that depends on the Born effective charge Z Ã and dielectric constant ε ∞ . This term leads to Lydan...

Anomalous pressure dependence of thermal conductivities of large mass ratio compounds

“…It is interesting that the value (dκ/dP)/κ 0 =0.009 GPa -1 given in Table I for BN (also obtained independently from the ShengBTE code [47]) is significantly larger than that given in Ref. 29 Table I). In such cases, λ τ and κ depend on aaa scattering.…”

“…The first principles approach for determining κ has been described previously [16][17][18][19][20][21][22][23][24][25][26][27][28][29], including examination of κ as a function of P [27][28][29]. This approach has no adjustable parameters and has demonstrated very good agreement with measured κ for a variety of systems, which has validated its predictive capability.…”

The curious case of cuprous chloride: Giant thermal resistance and anharmonic quasiparticle spectra driven by dispersion nesting

“…Calculated κ(P) of CuCl at RT is shown in Figure 5 with measured data 12 . For P>0.74 GPa, both calculated and measured κ(P) decrease with increasing P, an unusual behavior as κ typically increases 57,58 due to increasing acoustic velocities and reduced acousticoptic coupling as optic frequencies shift higher. Previously, decreasing κ(P) was attributed to increasingly negative TA Grüneisen parameters 12 , however, TA contributions to the calculated κ are minimal (inset to Fig.…”

Effect of exchange–correlation on first-principles-driven lattice thermal conductivity predictions of crystalline silicon

“…DFT-driven calculations have been successfully used to predict the experimentally-measured thermal conductivities of materials ranging from simple semiconductors such as silicon [2] and diamond [5] to compound semiconductors [4], graphene [6], and SiGe alloys [3]. DFT-driven calculations have also been used to study the effects of strain and isotopes on the thermal conductivity of semiconductors [7][8][9] and to predict the thermal conductivity of novel two-dimensional materials [6,10,11].…”