Assessing the Thermal Conductivity of Cu2−xSe Alloys Undergoing a Phase Transition via the Simultaneous Measurement of Thermoelectric Parameters by a Harman-Based Setup
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Some materials such as Cu2−xSe, Cu1.97Ag0.03Se, 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 (T) interval, results in sharp changes of the Seebeck coefficient (S), the electrical resistivity (ρ), and the thermal conductivity (κ), which may render the correct evaluation of the dimensionless figure of merit (ZT) difficult. We report the thermoelectric properties of a polycrystalline Cu2−xSe sample which is known to undergo a phase transition near 410 K, containing a mixture of α- and β-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 κ(T) 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 κ(T) 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 S is counterbalanced by the increase in κ(T). 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.
KeywordsFigure of merit thermal conductivity thermoelectric measurements ZT-Scanner Harman method Cu2Se
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We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC, Discovery Program), and of the Fonds de Recherche du Québec—Nature et Technologies (FRQNT, Team Project). We are grateful to Mr. S. D. Kang of Northwestern University for providing the material analyzed in this work.