A Phenomenological Model of Unconventional Heat Transport Induced by Phase Transition in Cu2−xSe

Abstract

Research in copper selenide thermoelectric (TE) alloys has raised the possibility of a significant enhancement of the TE figure-of-merit ZT when the Seebeck coefficient is affected by a concurrent phase transformation. This ZT increase has also been related to a radical reduction of the thermal conductivity evaluated by transient laser flash thermal diffusivity measurements. In contrast, steady-state Harman-based measurements do not support a significant ZT increase only a modest one, because the thermal conductivity instead of decreasing goes through a sharp maximum as it approaches the critical phase transformation temperature of 407 K. The nature of this sharp increase of heat transfer has not been related to the well-known electronic or phononic contributions. Below the critical temperature, when the alloy is exposed to a steady-state temperature gradient, an additional heat transfer phenomenon takes place, induced by the ongoing gradual phase transition. We show that the enthalpy associated to the α to β and β to α phase transformations can lead to heat flow in the direction of the temperature gradient above and beyond conventional heat conduction. This unconventional heat transfer mechanism disappears when the temperature rises above the critical temperature where only a stable β phase remains. We propose a model of such a heat transport which leads to the sharp maximum of the related thermal conductivity. Numerical results obtained from the model compare favorably to the experimentally measured thermal conductivity.