P-type doping of Hf 0.6 Zr 0.4 NiSn half-Heusler thermoelectric materials prepared by levitation melting and spark plasm
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The Y-doped (Hf0.6Zr0.4)1-xYxNiSn (x 5 0, 0.01, 0.02, 0.04, 0.06, 0.1, and 0.2) half-Heusler alloys have been prepared by levitation melting and spark plasma sintering. The effect of Y doping on thermoelectric properties of the alloys was investigated in the temperature range of 300–900 K. Y-doped samples had the lower electrical conductivity compared with the parent compound without Y doping. The thermal conductivity had weak dependence on Y doping content. The absolute values of Seebeck coefficient decreased significantly when x , 0.04. The sign of Seebeck coefficient turned from negative to positive at room temperature for x 5 0.04 and 0.1, which means that the hole carriers became dominant in these alloys. However, the alloys changed to n-type conduction again at high temperatures. The maximum figure of merit value of about 0.45 was obtained at 780 K for the undoped sample.
I. INTRODUCTION
The past decades have witnessed an alarming trend of decreasing fossil fuel supplies and an increase in greenhouse emissions, which imposes a pressing need for sustainable and eco-friendly energy conversion technology such as the thermoelectric (TE) energy conversion for waste heat harvesting. TEs study is a materialsoriented endeavor in which the central theme is to develop higher performance material so as to achieve higher energy-conversion efficiency. The performance of TE materials is gauged by the dimensionless figure of merit ZT 5 a2rT/j, where a is the Seebeck coefficient, r the electrical conductivity, j the thermal conductivity, and T the absolute temperature.1,2 As such a good TE material should possess large Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Half-Heusler (HH) alloys have been identified as promising TE materials for high temperature power generation. HH alloys adopt a MgAgAs-type crystal structure formed by four interpenetrating face-centered cubic sublattices, and each occupied sublattice can be substituted or doped individually to optimize certain aspects of TE properties. Therefore, HH alloys have a wide range of tunability in crystal chemistry by isoelectron, acceptor, or donor doping. In general, HH alloys with valence electron count of 18 exhibit a small bandgap and a fairly large effective mass, giving rise to large Seebeck coefficients and moderate electrical a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.82 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
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conductivities.3,4 It is also noteworthy that the constituent elements of these HH alloys are eco-friendly. The improvement in TE properties of HH alloys is tied primarily to reducing the thermal conductivity that is relatively high compared with other state-of-the-art TE materials. It has been demonstrated that nanostructuring and site substitution are two effective ways to reduce the lattice thermal conductivity. The thermal conductivity reduction by nanostructuring is mainly due to the enhanced phonon scattering a
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