Thermoelectric properties of spark plasma sintered composites based on TiNiSn half-Heusler alloys
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Siham Ouardi, Benjamin Balke, and Claudia Felser Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University, Mainz, Germany
Martin Köhne Robert Bosch GmbH, 70049 Stuttgart, Germany (Received 15 December 2010; accepted 23 March 2011)
Half-Heusler (HH) and especially TiNiSn-based alloys have shown high potential as thermoelectric (TE) materials for power generation applications. The reported transport properties show, however, a significant spread of results, due mainly to the difficulty in fabricating single-phase HH samples in these multicomponent and multiphased systems. In particular, little attention has been paid to the influence of the various minority phases on the TE performance of these compounds. A clear understanding of these issues is mandatory for the design of improved and stable TE HH-based composites. This study examines the structural and compositional influence of the residual metallic (Sn) and intermetallic phases (mainly Ti6Sn5 and the Heusler compound TiNi2Sn) on the TE properties of the TiNiSn HH compounds processed by spark plasma sintering.
I. INTRODUCTION
Thermoelectric (TE) converters for power generation aim at reducing CO2 emission via the conversion of a part of the low-grade waste heat generated by engines, industrial furnaces, gas pipes, etc. to electricity. The recovery of waste heat from the exhaust of an automotive engine, in particular, is an attractive, albeit not very efficient way for reduction of fuel consumption. TE converters with high overall efficiency convert heat directly into electricity without moving parts and, thus, not only decrease our reliance on fossil fuels but also actively counteract global warming. State-of-the-art converters are simply too inefficient to be economic, partly due to expensive elementary constituents (Te, Ge, etc.). On this background, the half-Heusler (HH) compounds stand out on account of their relatively low cost components and, indeed, have been extensively studied as potential TE materials for high temperature power generation up to 1000 K.1–15 The HH compounds with a valence electron count of 18 crystallize in the cubic MgAgAs-type structure that consists of four interpenetrating face-centered cubic sublattices. The crystallographic sites (0, 0, 0) and (1/4, 1/4, 1/4) are occupied by two different transition metal elements, the (1/2, 1/2, 1/2) sites are occupied by sp elements such as Sn, Sb, and Bi, whereas the (3/4, 3/4, 3/4) site is vacant.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.107 J. Mater. Res., Vol. 26, No. 15, Aug 14, 2011
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Each occupied sublattice can be doped or substituted individually to optimize the TE properties. The most widely studied materials are the n-type ANiSn-based compounds1,2,5–9,15 and p-type ACoSb-based11,12 compounds (A 5 Ti, Zr, Hf ). The TE properties of the n-type HH compounds are higher to those of the p-type counterparts. Currently, the ANiSn-based compounds still display the best
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