A thermodynamic model for heat transport and thermal wave propagation in graded systems

We study the effects of a composition gradient and of a non-vanishing heat flux on the phase velocity of thermal waves along a graded system. We take into account non-local and non-linear effects by applying a generalized heat transport equation. We compare the results for high-frequency and low-frequency waves. For low frequency, we discuss the conditions in which thermal waves may propagate in SixGe1−xSixGe1−x and (Bi1−xSbx)2Te3(Bi1−xSbx)2Te3 systems. For high frequency, we discuss the influence of the relaxation of the flux of the heat flux on the heat wave propagation.