Thermoelectric Figure of Merit, ZT, of Single Crystal Pentatellurides (MTe 5-X Se x : M = Hf, Zr and x = 0, 0.25)
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Thermoelectric Figure of Merit, ZT, of Single Crystal Pentatellurides (MTe5-XSex: M = Hf, Zr and x = 0, 0.25) R. T. Littleton IV1, Terry M. Tritt 1 ,2, B. Zawilski2, J. W. Kolis1,3, D. R. Ketchum3 and M. Brooks Derrick3 1.) Materials Science and Engineering Department 2.) Department of Physics and Astronomy 3.) Department of Chemistry Clemson University, Clemson, SC 29634 USA
ABSTRACT The thermoelectric figure of merit, ZT = α2σT/λ, has been measured for pentatelluride single crystals of HfTe5, ZrTe 5, as well as Se substituted pentatellurides. The parent materials, HfTe5 and ZrTe5, exhibit relatively large p- and n- type thermopower, |α| ≥ 125 µV/K, and low resistivity, ρ ≤ 1 mΩ•cm. These values lead to a large power factor (α2σT) which is substantially increased with proper Se substitution on the Te sites. The thermal conductivity of these needle-like crystals has also been measured as a function of temperature from 10 K ≤ T ≤ 300 K. The room temperature figure of merit for these materials varies from ZT ≈ 0.1 for the parent materials to ZT ≈ 0.25 for Se substituted samples. These results as well as experimental procedures will be presented and discussed. INTRODUCTION There have been extensive efforts recently to discover and optimize thermoelectric (TE) materials especially below room temperature.1,2 Efficient TE materials at lower temperatures could eventually lead to thermoelectric modules that would greatly aid in localized cooling of many electronic devices.3 The efficiency of a thermoelectric material is commonly determined by the dimensionless figure of merit, ZT (α2σT/λ or α2T/ρλ), where α is the Seebeck coefficient (or thermopower), σ is the electrical conductivity, ρ (1/σ) is the electrical resistivity, T is the absolute temperature, and λ is the thermal conductivity. The thermopower should be maximized so a large Peltier effect is observed. Electrical resistivity should be small to minimize the Joule heating (I2R) contribution. The thermal conductivity in a good thermoelectric material should be relatively low so that a temperature difference, ∆T, may be established and maintained across the material. An ideal thermoelectric material should exhibit a large amount of phonon scattering to minimize thermal conduction and a small amount of electron scattering to maximize electrical conduction. Low temperature thermoelectric materials are even more difficult to achieve, since the absolute value of the thermopower typically decreases with decreasing temperature.
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The numerator or power factor, α2σΤ, can typically be tuned through chemical doping and substitution. The total thermal conductivity is composed of a lattice and electronic part (λTOT = λL + λE, respectively). The electronic thermal conductivity, in many materials, may be related to σ through the Wie
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