Thermoelectric devices convert heat directly into electricity without any moving parts. Increasing the efficiency of thermoelectric devices is important for their widespread use. The efficiency of a thermoelectric material can be abstracted from the overall efficiency of a thermoelectric device through the thermoelectric figure of merit, ZT. ZT is a dimensionless quantity composed of several materials properties, specifically: the electronic conductivity, the Seebeck coefficient, the electronic thermal conductivity, and the lattice thermal conductivity. Improving the efficiency of a thermoelectric material can be done by increasing the electronic conductivity or Seebeck coefficient, or by reducing the lattice thermal conductivity of a material.
Our group uses various state-of-the-art computational tools such as density functional theory calculations, Monte Carlo simulations, molecular dynamics simulations, and high-throughput database analysis to study (i) the thermodynamic aspects of the processing-structure relationships of thermoelectric materials, (ii) the physics underlying the structure-property relationships of these materials, and (iii) various mechanisms for improving the thermoelectric properties by altering the structure of these materials. Our work is funded by the Revolutionary Materials for Solid State Energy Conversion (RMSSEC), an Energy Frontier Research Center (EFRC) based out of Michigan State University.
Coherent and Incoherent Phase Stability of IV-VI Rocksalt Thermoelectrics
Nanostructures formed by phase separation can improve the ZT of lead chalcogenide semiconductor alloys, with coherent nanostructures giving larger improvements than incoherent nanostructures. However, large coherency strains in these alloys drastically alter the thermodynamics of phase stability. We use density functional theory calculations to investigate the coherent and incoherent phase stability of the IV–VI rocksalt semiconductor alloy systems Pb(S,Te), Pb(Te,Se), Pb(Se,S), (Pb,Sn)Te, (Sn,Ge)Te, and (Ge,Pb)Te. We find that strain energy dominates the thermodynamics in these systems: strain both drives incoherent phase separation and prevents coherent phase separation.
High Thermoelectric Performance via Hierarchical Compositionally Alloyed Nanostructures
Previous efforts to enhance thermoelectric performance have primarily focused on reduction in lattice thermal conductivity caused by broad-based phonon scattering across multiple length scales. Herein, we demonstrate a design strategy which provides for simultaneous improvement of electrical and thermal properties of p-type PbSe and leads to ZT ∼ 1.6 at 923 K, the highest ever reported for a tellurium-free chalcogenide. Our strategy goes beyond the recent ideas of reducing thermal conductivity by adding two key new theory-guided concepts in engineering, both electronic structure and band alignment across nanostructure−matrix interface. Utilizing density functional theory for calculations of valence band energy levels of nanoscale precipitates of CdS, CdSe, ZnS, and ZnSe, we infer favorable valence band alignments between PbSe and compositionally alloyed nanostructures of CdS1−xSex/ZnS1−xSex. Then by alloying Cd on the cation sublattice of PbSe, we tailor the electronic structure of its two valence bands (lighthole L and heavy hole Σ) to move closer in energy, thereby enabling the enhancement of the Seebeck coefficients and the power
Intrinsically Low Thermal Conductivity
Anomalously low lattice thermal conductivity in thermoelectric Cu-Sb-Se ternary semiconductors
Many methodologies have been developed to improve ZT, for example
enhancing Seebeck coefficients by introducing quantum confinement
effects and electron energy filtering, obtaining a high thermoelectric
power factor by producing unusual electron density of states effects,
achieving a low lattice thermal conductivity in phonon-glass electron
crystal compounds and creating nanostructured materials. Although
nanostructured materials reduce thermal conductivity by enhancing phonon
scattering, they also can scatter electrons, which decreases the
electrical conductivity as well. A solution is to seek materials with
ordered crystal structures having low thermal conductivity due to strong
lattice anharmonicity, such as the ternary semiconductors Cu3SbSe3. The
strong lattice anharmonicity in Cu3SbSe3 arise from the electrostatic
repulsion between the lone s2 pair at Sb sites and the
bonding charge in Sb-Se bonds. Using our first-principles determined
longitudinal and transverse acoustic mode Gruneisen parameters,
zone-boundary frequencies, and phonon group velocities, we calculate the
lattice thermal conductivity using the Debye-Callaway model. The
theoretical thermal conductivity is good agreement with the experimental
measurements. This work has been selected as the scientific highlight topic for EFRC (the Energy Frontier Research Center).
Further details: Y. Zhang, E. Skoug, J. Cain, V. Ozolins, D. Morelli and C. Wolverton,
Phys. Rev. B 2012 85, 054306
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