Spinodal Decomposition in Off-stoichiometric Zr 0.5 Hf 0.5 Co 1-y Ir y Sb 1-z Sn z half-Heusler Phases
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1267-DD07-07
Spinodal decomposition in off-stoichiometric Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler phases Nathan J. Takas, Dinesh K. Misra, Heike Gabrisch and P. Ferdinand P. Poudeu* Advanced Materials Research Institute and Department of Chemistry, University of New Orleans, New Orleans, LA 70148 Corresponding author: [email protected] ABSTRACT The formation of nanostructures within the matrix of half-Heusler thermoelectric materials can be produced by spinodal decomposition of off-stoichiometric compositions. CoSb is insoluble at high temperatures in Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler phases. This phase can be solubilized into the half-Heusler matrix by the use of high energy ball milling at room temperature as the synthetic method of choice. The metastable half-Heusler material decomposes in-situ while hot-pressing the powder sample into a compact pellet. Despite the fact that the thermal conductivity of the inclusion material, CoSb, is very large, (>35W/m·K), we observed reduction in the lattice thermal conductivity of the composite material. Furthermore, the electrical resistivity of the specimen was also reduced due to the metallic nature of the CoSb inclusion phase. Addition of a large fraction of the metallic inclusion leads to a percolation network of the metallic phase, thus reducing the Seebeck coefficient of the composites. Changes in the thermoelectric properties of Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler matrix with increasing mass percent of CoSb inclusion are discussed. INTRODUCTION Growing interest in renewable energies such as solar and thermal energy conversion technologies has been triggered by the prospects of climate change and eventual fossil fuel depletion. In order for these technologies to become practical for application, modern concentrating solar power technologies (CSP), which concentrate direct sunlight several times to reach higher energy densities and thus higher temperatures, must be combined with highly efficient thermal to electricity (thermoelectric) conversion devices. The conversion efficiency of a thermoelectric device strongly depends on the performance of the materials used to build the device, which is a function of the macroscopic transport parameters of the materials and is defined by the dimensionless figure of merit, ZT = S2σT/κ (where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity and T is the absolute temperature) [1]. Half-Heusler phases are known as promising thermoelectric materials with large Seebeck coefficients, and moderately high electrical conductivities [2-5]. However, their thermoelectric conversion efficiency is severely limited by their extremely high total thermal conductivity (7 – 17 W/m·K) [2-4]. The introduction of nano-inclusions has been shown to be effective in reducing the thermal conductivity of other semiconducting phases [6]. In this paper, attempts are made to reduce the thermal conductivity of the half-Heusler material Zr0.5Hf0.5Co0.5Ir0.5Sb0.99Sn0.01 by first solubilizing an excess of CoSb i
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