Characterization of Lorenz number with Seebeck coefficient measurement

In analyzing zT improvements due to lattice thermal conductivity (kappa(L)) reduction, electrical conductivity (sigma) and total thermal conductivity (kappa(Total)) are often used to estimate the electronic component of the thermal conductivity (kappa(E)) and in turn kappa(L) from kappa(L) = similar to kappa(Total) - L sigma T. TheWiedemann-Franz law, kappa(E) = L sigma T, where L is Lorenz number, is widely used to estimate kappa(E) from sigma measurements. It is a common practice to treat L as a universal factor with 2.44 x 10(-8) W Omega K-2 (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 x 10(-8) W Omega K-2 for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, vertical bar S vertical bar, independent of temperature or doping. We propose the equation: L = 1.5 + exp [ -vertical bar S vertical bar/116] (where L is in 10(-8) W Omega K-2 and S in mu V/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si0.8Ge0.2 where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity. (C) 2015 Author(s).