Published: October 04, 2011 r2011 American Chemical Society 18843 dx.doi.org/10.1021/ja206491j | J. Am. Chem. Soc. 2011, 133, 18843–18852 ARTICLE pubs.acs.org/JACS Simultaneous Large Enhancements in Thermopower and Electrical Conductivity of Bulk Nanostructured Half-Heusler Alloys Julien P. A. Makongo, †,§ Dinesh K. Misra, § Xiaoyuan Zhou, ‡ Aditya Pant, § Michael R. Shabetai, § Xianli Su, ‡,# Ctirad Uher, ‡ Kevin L. Stokes, §,|| and Pierre F. P. Poudeu* ,†,§,^ † Laboratory for Emerging Energy and Electronic Materials, Materials Science and Engineering Department and ‡ Department of Physics, University of Michigan, Ann Arbor 48109, Michigan, United States § The Advanced Materials Research Institute, ) Department of Physics, and ^ Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148, United States # State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China b S Supporting Information ’ INTRODUCTION The realization of cost-effective and “clean” solid state energy conversion devices capable of competing with traditional me- chanical energy conversion systems requires the ability to fabricate high performance thermoelectric materials with figures of merit, ZT, g 3. 1 Despite substantial increases in research activities and innovative breakthroughs in this area 2À11 over the past six decades, the ability to design thermoelectric materials with ZT approaching the above targeted value has proven extremely difficult mainly because of the interdependence and coupling between electronic and thermal parameters (electrical conductivity, σ; thermopower, S; and thermal conductivity, k) that enter in the calculation of ZT = TσS 2 /k. Among various strategies employed to enhance the ZT of traditional and emerging thermoelectric materials, the concept of nanostructur- ing has drawn much attention primarily due to the prediction that quantum confinement of charge carriers in low dimensional structures within a given matrix could drastically increase ZT of the composite through a large enhancement of the power factor (PF = σS 2 ). 12,13 Although, the exploration of this concept led to significant increases in the ZT of bulk thermoelectric materials 2,5,6,9,14À21 and thin film superlattices, 11,22 these im- provements mainly originate from large reductions in k rather than an enhancement in the PF as originally predicted. There- fore, the concept of quantum confinement and the mechanism by which nanostructuring can be used to enhance the PF of thermoelectric materials is still poorly understood. ZT enhancement through large reductions in k using nano- structuring while minimizing reductions in the PF has therefore dominated thermoelectric research over the past 15 years, and the mechanism by which phonons are scattered at matrix/inclusion Received: July 12, 2011 ABSTRACT: Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demon- strated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these im- provements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and elec- trical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the “effective” carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T Àα law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 À x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 À x)/FH(x) nanocomposites.