Effect of partial filling of the structural vacant sites on the thermoelectric properties of Zr 0.25 Hf 0.75 NiSn half-H
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1267-DD05-12
Effect of partial filling of the structural vacant sites on the thermoelectric properties of Zr0.25Hf0.75NiSn half-Heusler alloy Julien P. A. Makongo1; Dinesh K. Misra1; Nathan Takas1; Kevin Stokes1,2; Heike Gabrisch1,3; Pierre F. P. Poudeu1,3*` 1) The Advanced Materials Research Institute, University of New Orleans, LA, 70148 2) Department of Physics, University of New Orleans, New Orleans, LA, 70148 3) Department of Chemistry, University of New Orleans, New Orleans, LA, 70148 *Corresponding author:[email protected]
Abstract Composites containing mainly-half-Heusler MNiSn (HH) and full-Heusler MNi2Sn (FH) were prepared by solid state reaction of a mixture of polycrystalline bulk HH alloy with various concentrations of Ni up to 10 wt.%. Electrical conductivities, thermal conductivities and thermopowers of spark plasma sintered specimens of the as synthesized composite materials were measured in the temperature range from 300 K to 750 K. The conduction type of the composite changes from semiconductor to semimetal for Ni concentrations up to 2 wt.% and from semimetal to metal for higher Ni concentrations above 5 wt.%. A strong reduction in lattice thermal conductivity was observed for the composite containing 10 wt. % Ni inclusions.
Introduction Half-Heusler alloys (HH) with the general formula ZrxHf1-xNiSn1-ySby are currently being investigated for their potential use as n-type thermoelectric materials. They crystallize in the cubic MgAgAs-type structure with the unit cell consisting of three filled and one vacant sublattices. Previous investigations have shown that MNiSn-based HH are semiconductor with a narrow band gap of about 0.1-0.4 eV [1-4] and these compounds can exhibit semiconducting (i.e. the electrical resistivity decreases with rising temperature, ∂ρ/∂T < 0), semimetallic (∂ρ/∂T = 0) or metallic (∂ρ/∂T > 0) behavior depending on the carrier type (donor or acceptor) and the carrier concentration in the system [5-7]. The temperature dependence of the electrical resistivity and the thermopower of ZrNiSn-based HH alloys doped with Cu, donor impurity at the Ni sublattice have been investigated [7]. The conduction type of these solid solution alloys was revealed to change from semiconducting to semimetallic for smaller Cu concentrations and from semimetal to metal for larger Cu concentrations. However, not all ZrNiSn-based HH alloys exhibit such a change in the electrical resistivity via doping. Doping the atoms at the filled sublattices can
improve the power factor or reduce the total thermal conductivity. The thermoelectric performance of a material at a given temperature is evaluated by the dimensionless figure of merit ZT = [(S2σ)/ (κe + κl)]T where S is the Seebeck coefficient, σ is the electrical conductivity, (S2σ) the power factor and κ = κe + κl is the total thermal conductivity which is equal to the sum of two components: κe (the electronic contribution depends both on the temperature and the carrier concentration and is related to the conductivity of the material) and κl (the lattice
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