Ankita Katre

almaBTE : A solver of the space–time dependent Boltzmann transport equation for phonons in structured materials

Abstract almaBTE is a software package that solves the space- and time-dependent Boltzmann transport equation for phonons, using only ab-initio calculated quantities as inputs. The program can predictively tackle phonon transport in bulk crystals and alloys, thin films, superlattices, and multiscale structures with size features in the nm – μ m range. Among many other quantities, the program can output thermal conductances and effective thermal conductivities, space-resolved average temperature profiles, and heat-current distributions resolved in frequency and space. Its first-principles character makes almaBTE especially well suited to investigate novel materials and structures. This article gives an overview of the program structure and presents illustrative examples for some of its uses. PROGRAM SUMMARY Program Title: almaBTE Program Files doi: http://dx.doi.org/10.17632/8tfzwgtp73.1 Licensing provisions: Apache License, version 2.0 Programming language: C++ External routines/libraries: BOOST, MPI, Eigen, HDF5, spglib Nature of problem: Calculation of temperature profiles, thermal flux distributions and effective thermal conductivities in structured systems where heat is carried by phonons Solution method: Solution of linearized phonon Boltzmann transport equation, Variance-reduced Monte Carlo

First principles study of thermal conductivity cross-over in nanostructured zinc-chalcogenides

Systematic first principles studies of zinc-chalcogenides have been performed to understand their thermal transport behaviour. We have applied the Boltzmann transport equation in the relaxation time approximation to calculate the thermal conductivity of ZnS, ZnSe, and ZnTe. We find a thermal conductivity cross-over between ZnS and ZnSe at nanostructure sizes around 0.1–0.2 μm and explain this in terms of the different contributions of phonon modes in these materials. We study the effect of nanostructuring using both the diffusive boundary scattering and confined mean free path limit and discuss the variations in the results. Furthermore, we show the strong influence of isotope scattering on the thermal conductivity. The calculated thermal conductivity is found to be strongly dependent on the volume and we explain the observed differences between local density and generalized gradient approximation calculations. We compare further calculated thermal properties, such as the thermal expansion coefficient, to experiment to validate our approach.

Modelling the lattice dynamics in SixGe1−x alloys

The development of simplified models for the simulation of thermodynamic and thermal transport properties in random alloys is of great importance. In this paper we show how a simple second nearest neighbour model can reliably capture the lattice dynamics of Si(x)Ge(1-x) alloys. The model parameters are extracted from DFT-calculated force constant matrices for pure Si, pure Ge and the Si0.5Ge0.5 ordered alloy. We extract the nearest neighbour contributions directly from density functional theory, whereas effective interactions are obtained for the second nearest neighbour contributions. We demonstrate how the thermal properties, including the expansion coefficient, can be reliably reproduced and that the model is transferable to random Si(x)Ge(1-x) alloys.

Resonant phonon scattering in semiconductors

Boron impurities have recently been shown to induce resonant phonon scattering in 3C-SiC, dramatically lowering its thermal conductivity. The B-doped 3C-SiC is associated with an off-center relaxation of the B atom, inducing a local transition from Td to C3v symmetry. Similar relaxations in B and N-doped diamond, with a similarly large effect on the interatomic force constants (IFCs), fail to produce resonances. Here we develop an intuitive understanding of such dopant-induced resonant phonon scattering in semiconductors with the help of a 1D monoatomic chain model. We find that the phenomenon is connected to a slight asymmetry in the relaxed position of the defect, with its origin in two or more minima of the potential energy surface in close proximity. The large perturbation they introduce in the IFCs is the essential ingredient of a resonance.

Influence of Antisite Defects on the Thermoelectric Properties of Fe2VAl

ABSTRACT Fe2VAl is well known as a promising candidate for thermoelectric applications due to its sharp pseudogap at the Fermi level. However, its energy conversion performance is compromised by its high thermal conductivity. Our previous studies revealed that antisite defects like AlV, AlFe, and VAl are the most likely imperfections in Fe2VAl [1]. It is thus important to understand the electron and phonon transport properties in these defective crystals to estimate their thermoelectric efficiency. Here we analyze the electronic transport properties of Fe2VAl solid solutions based on Boltzmann transport theory within the constant relaxation time approximation. We then calculate the lattice thermal conductivity of Fe2VAl containing AlV antisite defects by solving the linearized Boltzmann transport equation based on an ab initio model for defects. We find a significant increase of around an order of magnitude in ZT at 300 K compared to the stoichiometric compound.

The lattice thermal conductivity of Ti

In spite of their relatively high lattice thermal conductivity

_x

\kappa_{\ell}

Zr

, the XNiSn (X=Ti, Zr or Hf) half-Heusler compounds are good thermoelectric materials. Previous studies have shown that

_y

\kappa_{\ell}

Hf

can be reduced by sublattice-alloying on the X-site. To cast light on how the alloy composition affects