Assessing the Thermal Conductivity of Cu2−xSe Alloys Undergoing a Phase Transition via the Simultaneous Measurement of Thermoelectric Parameters by a Harman-Based Setup.

Some materials such as Cu_2−xSe, Cu_1.97Ag_0.03Se, and SnSe have attracted attention by demonstrating a significant enhancement of their thermoelectric performance, which is associated with a phase transition. This phenomenon, observed in a limited temperature (<italic>T</italic>) interval, results in sharp changes of the Seebeck coefficient (<italic>S</italic>), the electrical resistivity (<italic>ρ</italic>), and the thermal conductivity (<italic>κ</italic>), which may render the correct evaluation of the dimensionless figure of merit (ZT) difficult. We report the thermoelectric properties of a polycrystalline Cu_2−xSe sample which is known to undergo a phase transition near 410 K, containing a mixture of <italic>α</italic>- and <italic>β</italic>-phases at room temperature, as determined by x-ray diffraction measurements. We have used a Harman-based setup (TEMTE Inc.), which assures the direct measurement of ZT at all temperatures, including the phase transition region. This approach ensures that <italic>κ</italic>(<italic>T</italic>) is determined under steady-state conditions at any given temperature, including points arbitrarily close to the transition temperature which cannot be guaranteed by previously used techniques such as laser flash. We have observed a sharp maximum for <italic>κ</italic>(<italic>T</italic>) near 410 K, similar to the reported specific heat variation, with a ZT peak value of 0.2 at 400 K. The expected gain in ZT related to the phase transition is reduced because the increase in <italic>S</italic> is counterbalanced by the increase in <italic>κ</italic>(<italic>T</italic>). Thus, our detailed assessment of the temperature variation of the individual thermoelectric properties accurately evaluates the performance enhancement associated to a structural phase transition and helps to elucidate this complex phenomenon. [ABSTRACT FROM AUTHOR]