Half-Heusler phases and nanocomposites as emerging high-ZT thermoelectric materials
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Song Zhu, Wenjie Xie, and Terry M. Trittb) Department of Physics & Astronomy, Clemson University, Clemson, South Carolina 29634
Peter Thomas and Rama Venkatasubramanian Center for Solid State Energetics, RTI International, Research Triangle Park, North Carolina 27709 (Received 2 August 2011; accepted 20 September 2011)
Half-Heusler (HH) phases, a versatile class of alloys with promising functional properties, have recently gained attention as emerging thermoelectric materials. These materials are investigated from the perspective of thermal and electronic transport properties for enhancing the dimensionless figure of merit (ZT) at 800–1000 K. The electronic origin of thermopower enhancement is reviewed. Grain refinement and embedment of nanoparticles in HH alloy hosts were used to produce fine-grained as well as nanocomposites and monolithic nanostructured materials. Present experiments indicated that n-type Hf0.6Zr0.4NiSn0.995Sb0.005 HH alloys and p-type Hf0.3Zr0.7CoSn0.3Sb0.7/nano-ZrO2 composites can attain ZT 5 1.05 and 0.8 near 900–1000 K, respectively. The observed ZT enhancements could be attributed to multiple origins; in particular, the electronic origin was identified. The prospect for higher ZT was investigated in light of a recently developed nanostructure model of lattice thermal conductivity. Tests performed on p–n couple devices from the newly developed HH materials showed good power generation efficiencies—achieving 8.7% efficiency for hot-side temperatures of about 700 °C.
I. INTRODUCTION
Thermoelectric technology represents a key approach that can directly tap the currently under-utilized thermal energy from renewable and waste heat sources and convert it directly to electrical power in an environmentally friendly manner. For thermoelectric (TE) technology to become competitive, there is a need to develop high efficiency bulk TE materials with a dimensionless figure of merit, ZT, higher than 1. The efficiency of a TE material is gauged by the dimensionless figure of merit (ZT), which is defined as (S2T/jq), where S is the Seebeck coefficient or thermopower, T is the average temperature of the sample, and q is the dc electrical resistivity. In this article, the power factor PF, conventionally known as S2/q, will be defined as S2T/q so that ZT is simply given by the PF/j. The total thermal conductivity j is given by the sum je+jL, the electronic and lattice components of j, respectively. The potential of the class of intermetallic compounds known as half-Heusler (HH) phases for a)
Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/. This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.329 J. Mater. Res., Vol. 26, No. 22, Nov 28, 2011
high-temperature TE power generation has recently received much attention. This is largely due to the facts tha
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