First- principles study of semiconducting heusler alloys for high temperature thermoelectric applications (PhD)

Abstract

The aim of the thesis is to study, understand and prediction of thermoelectric properties of semiconducting Heusler compounds by using first-principles methods for high temperature thermoelectric applications. In this direction the electronic structure, phonon spectrum, the derived transport properties and heat and charge carrier lifetimes are studied. The figure of merit zT and efficiency are predicted for the few Heusler thermoelectric materials. The density functional theory (DFT), semiclassical transport theory, DFT based harmonic and anharmonic phonon calculations are mainly applied. The present thesis is divided into seven chapters. In chapter one, we give an introduction to the thermoelectric energy conversion, the efficiency of thermoelectric generator (TEG), and Heusler family of materials. The formula for efficiency of TEG and its relation to material’s properties connected through zT are described. Next, the cause of limitation in conversion efficiency, need for high zT materials, desired material’s features for good thermoelectric material, conflicting relation among transport coefficients and criteria for high zT are discussed. We provide some of the general properties of Heusler family of compounds, crystal structure, possible disorders and the features of Heusler materials that attract them to explore for thermoelectric applications. After this, we discuss a brief theoretical background of DFT, phonon calculations, semiclassical transport theory and a short overview of carrier lifetime calculations approaches in chapter two. Chapter three explains the effect of exchange-correlation (XC) functionals on the electronic and phonon properties of Fe2VAl and Fe2TiSn. Using different functionals electronic dispersion and density of states (DOS) are calculated to see the effect on electronic structure.Mainly, the effect on Eg and band features and effective mass are checked. Further, the effect on the phonon properties is studied by comparing the phonon dispersion and thermal properties. The mBJ functional is found to be suitable to obtain proper Eg values. However, the band features described by mBJ shows reasonable differences compared to other functionals which is also observed through effective mass. Comparison of phonon energy in dispersion and DOS from various functionals shows the lowest phonon energies from PBE while LDA and SCAN are giving higher energy values. The change in XC functionals has negligible effect on thermal properties. The dynamical stability and thermoelectric properties of Fe2ScX (X=P, As, Sb) Heuser compounds are predicted in chapter four. We use the two functionals approach i.e., mBJ for the Eg and SCAN functional to describe band features in reliable prediction of properties of these new compounds. The total energy calculations suggested the ground state structure of these compounds is ordered L21 phase compared to other possible Heusler structure. The phonon calculations showed the dynamical stability in the L21 phase. The zT predicted for the n- and p-type compounds suggested Fe2ScX compounds are worth considering for high temperature thermoelectric applications on successful synthesis. In chapter five, we try to explain the experimental Seebeck coefficient S of a ZrNiSn sample using combined DFT and semiclassical transport calculations. The ZrNiSn is a promising thermoelectric material with high S and moderately high electrical conductivity. The thermal expansion behaviour is calculated which can be helpful in TEG design. The analysis of S suggested the Eg in sample could be ∼0.18 eV with the possibility of disorder or defect. Based on this result, we further predict the zT and efficiency obtainable by doping the pure (stoichiometric and ordered, Eg of ∼0.54 eV) ZrNiSn. The highest zT predicted for the n-type and p-type compounds are ∼0.5 and ∼0.6 at 1200 K, respectively. The study of electronic structure, phonon properties, thermal expansion, prediction of zT and efficiency, as well as calculation of carrier lifetimes of FeVSb are carried out in chapter six. We try to understand the experimental S of two samples using combined DFT and transport calculations. The best possible explanation to experimental S is found for Eg of 0.7 eV which is also in agreement to Eg from mBJ calculation. From ab-initio anharmonic lattice dynamics calculations the lattice thermal conductivity of FeVSb is calculated by considering phonon-phonon interaction under single mode relaxation time approximation. Further, we extend our study on FeVSb by calculating the charge and heat carrier lifetimes from first-principles considering the three intrinsic scattering mechanisms viz. electron-electron interaction, electron-phonon interaction and phonon-phonon interaction. Using the calculated lifetime values thermoelectric properties are predicted. At last in chapter seven, we summarize the thesis, with a brief overview of the significant conclusions drawn and give direction for future work.