Effect of substitutional doping on the thermal conductivity of Ti-based Half-Heusler compounds
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Effect of substitutional doping on the thermal conductivity of Ti-based HalfHeusler compounds 1
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S.Bhattacharya , V. Ponnambalam , A.L.Pope , Y.Xia , S.J.Poon , R.T.Littleton IV , T.M.Tritt 1. Department of Physics and Astronomy, Clemson University, Clemson, SC, USA 2. Department of Physics, University of Virginia, Charlottesville, VA, USA
ABSTRACT Half-Heusler alloys with the general formula TiNiSn1-XSbX are currently being investigated for their potential as thermoelectric (TE) materials. 1, 2, 3, 4 These materials exhibit high thermopower (40-250µV/K) and low electrical resistivity values (0.1 - 8mΩ-cm) which 2 yields a relatively large power factor (α σT) of (0.2 - 1.0) W/msK at room temperature. The challenge is to reduce the relatively high thermal conductivity (≈ 10 W/msK) that is evident in these materials. The focus of this research is to investigate the effect of Sb-doping on the Sn site and Zr doping on the Ti site on the thermal conductivity of TiNiSn. Highly doped half-Heusler alloys have shown marked reduction in thermal conductivity to values on the order of 3.5 - 4.5 W/msK. Systematic determination of thermal conductivity in a variety of these doped materials as well as Sb and Zr doped TiNiSn are presented and discussed. INTRODUCTION There has been a renewed interest in thermoelectricity with the possibility of optimizing the electronic and transport properties of both new and existing novel materials. New thermoelectric (TE) materials for applications such as refrigeration and power generation are heavily being investigated. Applications for power generation are of interest to the automotive industry for waste heat recovery for power conversion to enhance fuel efficiency utilizing an environmentally friendly energy source. Refrigeration is used in beverage coolers, cooling of electronics and opto-electronics, as well as temperature stabilization of biological samples. The TE materials and devices are of interest not only because of their reliability and durability, but also because of the technology being environmentally friendly. The efficiency of a TE material is given by the dimensionless parameter ZT, the figure of merit, of the material and is given by5: ZT =
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T
where, α is the Seebeck coefficient or thermopower, σ is the electrical conductivity which is the reciprocal of electrical resistivity (ρ), λ is the thermal conductivity (comprised of the lattice (λL) and electrical contributions (λE). It is very desirable to have a high figure of merit (ZT ≈ 3 - 4). However current state-of-the-art materials have ZT ≈ 1 (Bi2Te3 has ZT ~ 1 at 400 K)6. A “good TE” material should have a large Seebeck coefficient, high electrical conductivity and a low thermal conductivity in order to have a high ZT. Generally, semiconductors or semimetals are chosen for research since they tend to fulfill the above requirements of having high thermopower and a favorable electrical conductivity. Apart from the current state-of–the-art materials Bi2Te3 and SiGe, several new materials like skutterudit
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