Limits of Lorenz number estimation from the temperature-dependent Seebeck coefficient

Abstract

Separating electronic and lattice thermal conductivity often relies on a Lorenz number estimated from the Seebeck coefficient using an assumed band and scattering model. We ask how much information about the Lorenz number comes from temperature-dependent Seebeck coefficients rather than the assumed model. We formulate compatible minimum and maximum Lorenz numbers as linear programs in a fixed finite spectral representation, assuming known chemical potentials and a common energy dependence across observation temperatures for each candidate spectrum. Reference Seebeck coefficients are generated by single-band and same-sign two-band models. Increasing the prescribed observations from one to five temperatures reduces the Lorenz interval widths by approximately 97.6% and 97.8%, respectively. Nevertheless, the single-band example retains an attained interval of approximately $[1.46453,2.37037]\times10^{-8}$ W $Ω$ K$^{-2}$ at 400 K. Exact electrical conductivity values do not narrow these ranges when spectral amplitudes can vary independently with temperature. Thus, even exact Seebeck coefficients and electrical conductivities at all five temperatures need not uniquely determine electronic thermal conductivity at 400 K. For the same single-band example, known ratios of spectral amplitudes across temperatures allow electrical conductivity values to confine all compatible Lorenz numbers to an enclosing interval less than 1% as wide as the five-temperature free-amplitude interval. Temperature-dependent Seebeck data strongly narrow the compatible range, while conductivity provides further spectral constraints when the temperature dependence of spectral amplitudes is independently restricted.