1998/12/04 by W. E. Bies, Bies, W. E., R. J. Radtke +4
Engineering · Materials Science · Physics and Astronomy · #Advanced Thermodynamics and Statistical Mechanics #Advanced Thermoelectric Materials and Devices #FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #Thermography and Photoacoustic Techniques #cond-mat.mtrl-sci
paper · pdf · doi:10.48550/arxiv.cond-mat/9812084
17 pages, 6 figures, submitted to Phys. Rev. B
arxiv created 1998/12/04 · openalex publication_date 1998/12/04 · arxiv updated 2009/11/30 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
The effective transport coefficients and figure of merit ZT for anisotropic systems are derived from a macroscopic formalism. The full tensorial structure of the transport coefficients and the effect of the sample boundaries are included. Induced transverse fields develop which can be larger than the applied fields and which reduce the effective transport coefficients. A microscopic model relevant for multi-valleyed materials is introduced which utilizes the effective-mass and relaxation-time approximations. The thermopower and Lorentz number are independent of the tensorial structure of the transport coefficients in this case and are therefore isotropic. ZT is also isotropic for vanishing lattice thermal conductivity κ_ℓ. For non-vanishing but sufficiently isotropic κ_ℓ, ZT is maximal along the direction of highest electrical conductivity σ. Numerical calculations suggest that maximal ZT generally occurs along the principal direction with the largest σ/κ_ℓ. An explicit bound on ZT is derived. Several results for specific systems are obtained: (1) Bulk n-type Bi2Te3 exhibits easily observable induced transverse fields and anisotropic ZTs. (2) Increased anisotropy in HgTe/ Hg1-xCdxTe superlattices (SLs) is associated with larger induced fields. (3) The valley degeneracy is split and the bulk masses modified in isolated Bi2Te3 quantum wells, resulting in optimal ZTs for wells grown along the trigonal direction. (4) Non-parabolic dispersion in SLs has little effect on the thermopower at the carrier concentrations which maximize ZT.