Our paper – Duda et al., “Influence of crystallographic orientation and anisotropy on Kapitza conductance via classical molecular dynamics simulations” – was recently published in Journal of Applied Physics (J. Appl. Phys. 112, 093515 (2012)). In this work, we studied the influence of crystallographic orientation on thermal boundary conductance between two solids with molecular dynamics simulations and wave packet simulations.  We found that anisotropy in a solid has a greater effect on thermal boundary conductance than on thermal conductivity.  We also conclude that the Debye temperatures of two materials comprising an interface does not serve an accurate gauge of the efficiency of interfacial thermal transport when those materials have different crystal structures.

Abstract

We investigate the influence of crystallographic orientation and anisotropy on local phonon density of states, phonon transmissivity, and Kapitza conductance at interfaces between Lennard-Jones solids via classical molecular dynamics simulations. In agreement with prior works, we find that the Kapitza conductance at an interface between two face-centered cubic materials is independent of crystallographic orientation. On the other hand, at an interface between a face-centered cubic material and a tetragonal material, the Kapitza conductance is strongly dependent on the relative orientation of the tetragonal material, albeit this dependence is subject to the overlap in vibrational spectra of the cubic and tetragonal materials. Furthermore, we show that interactions between acoustic phonons in the cubic material and optical phonons in the tetragonal material can lead to the interface exhibiting greater “thermal anisotropy” as compared to that of the constituent materials. Finally, it is noted that the relative match or mismatch between the Debye temperatures of two materials comprising an interface does not serve an accurate gauge of the efficiency of interfacial thermal transport when those materials have different crystal structures.

This work was funded by NSF (CBET Award #1134311) and Sandia National Laboratories through the LDRD Program Office.

In collaboration with Prof. Joshua Zide’s group at University of Delaware, our paper “Enhanced room temperature electronic and thermoelectric properties of the dilute bismuthide InGaBiAs” was recently published in Journal of Applied Physics (J. Appl. Phys. 112, 093710 (2012)).  In this work, the ExSiTE lab measured the thermal conductivity of Si doped InGaBiAs films with varying Bi concentration to support the electrical resistivity and Seebeck coefficients measurements conducted at the University of Delaware.  We found that the bismuth scattering suppressed the thermal conductivity of these bismuthides, that, along with enhanced electrical conductivity and carrier concentrations, could make these bismuthides an exceptional thermoelectric material.

Abstract

We report room temperature electronic and thermoelectric properties of Si-doped In0.52Ga0.48 BiyAs1-y with varying Bi concentrations. These films were grown epitaxially on a semi-insulating InP substrate by molecular beam epitaxy. We show that low Bi concentrations are optimal in improving the conductivity, Seebeck coefficient, and thermoelectric power factor, possibly due to the surfactant effects of bismuth. We observed a reduction in thermal conductivity with increasing Bi concentration, which is expected because of alloy scattering. We report a peak ZT of 0.23 at 300 K.