http://journals.cambridge.org Downloaded: 19 Jul 2012 IP address: 152.88.84.51 INVITED FEATURE PAPERS Half-Heusler phases and nanocomposites as emerging high-ZT thermoelectric materials S. Joseph Poon a) and Di Wu Department of Physics, University of Virginia, Charlottesville, Virginia 22904-4714 Song Zhu, Wenjie Xie, and Terry M. Tritt b) 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 Hf 0.6 Zr 0.4 NiSn 0.995 Sb 0.005 HH alloys and p-type Hf 0.3 Zr 0.7 CoSn 0.3 Sb 0.7 /nano-ZrO 2 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 environmen- tally 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 (S 2 T/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 S 2 /q, will be defined as S 2 T/q so that ZT is simply given by the PF/j. The total thermal conductivity j is given by the sum j e +j L , the electronic and lattice components of j, re- spectively. The potential of the class of intermetallic compounds known as half-Heusler (HH) phases for high-temperature TE power generation has recently received much attention. This is largely due to the facts that ZT ; 1 was reported at high temperatures (;1000 K) 1–3 and the materials are relatively easy to synthesize as 100% dense samples and are more environmentally benign than other PbTe-based materials. The crystal structure of HH phases is of the MgAgAs type consisting of three interpenetrating filled face-centered cubic (fcc) sublattices, and a fourth open fcc sublattice of vacancies. Each filled sublattice can be tuned independently, through substitutions, to optimize the TE material system. HH phases are known to be semi- conductors when the valence electron count, per unit cell, is 8 (e.g., LiMgP with band gap of ;3.7 eV) or 18 (e.g., MNiSn, where M 5 Ti, Zr, Hf, with band gaps ;0.2–0.4 eV). For the MNiSn phases, by doping the Sn site with Sb, the semi- conducting MNiSn system is transitioned into semimetallic material with a high power factor. 4 Opportunities also exist for exploring other HH compositions. 5 In recent years, new approaches for controlling electron and phonon transports through nanostructuring have been developed, leading to a significant enhance- ment in bulk and thin-film TE performance. 6,7 Indeed, thin-film and bulk nanocomposites have emerged as promising high-ZT materials. These bulk materials, produced by either compacting nanostructured pow- ders 3,8–12 or embedding nanoparticles in a host, 13 were reported to attain ZT values significantly higher than those of their prenanostructured states. Indeed, ZT as high as a) Address all correspondence to this author. e-mail: sjp9x@virginia.edu 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 Ó Materials Research Society 2011 2795