Synthesis and Thermoelectric Properties of CoP 3
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Synthesis and Thermoelectric Properties of CoP3 Virgil B. Shields and Thierry Caillat Jet Propulsion Laboratory California Institute of Technology Pasadena, CA 91109, USA ABSTRACT In an effort to expand the range of operation for highly efficient, segmented thermoelectric unicouples currently being developed at the Jet Propulsion Laboratory (JPL), skutterudite phosphides are being investigated as potential high temperature segments to supplement antimonide segments that limit the use of these unicouples at a hot-side temperature of about 873-973 K. We report here on the synthesis and transport properties of one of these phosphide skutterudite materials, CoP3. Large amounts of cobalt triphosphide have been prepared by direct reaction from stochiometric powders at synthesis temperatures ranging from 873 to 1223 K. The synthesized powders were analyzed by x-ray diffractometry and hot pressed at 1273 K. The samples were analyzed by microprobe analysis and electrical conductivity, Seebeck voltage and thermal conductivity measurements were performed. The thermoelectric properties are presented and discussed as a function of temperature up to 1000 K. Initial thermal stability results are presented to assess the potential of this material for high temperature operation.
INTRODUCTION Skutterudite compounds have shown significant promise for use in high efficiency thermoelectric devices [1-4]. Significant interest has been devoted over the past few years to study their electrical and thermal properties to improve the state of the art materials [5]. Phosphorus skutterudite compounds have been successfully synthesized in small amounts using several techniques [6,7]. The synthesis of such compounds is challenging due to the high phosphorus vapor pressures, the incongruent melting of the materials and the potential for dissociating at the high synthesis temperatures [8]. However large amounts of CoP3 have been synthesized using direct reaction of stochiometric amounts of pure elemental mixtures. Direct synthesis was the selected process due to the potential for producing large amounts of product. This work discusses the process used to produce bulk CoP3 and presents initial electrical and thermal stability characteristics measurements.
EXPERIMENTAL DETAILS Stochiometric amounts of cobalt and phosphorus were reacted using a direct synthesis technique at temperatures ranging from 873 to 1223 K. High purity cobalt (99.995% at –22 mesh) was premixed with phosphorus (99.999% at -200 mesh) for 5 minutes in a polystyrene container using a high-speed shaker. A uniform mixture of small grain size was used to allow the cobalt to take up the free phosphorus at a low enough temperature during the reaction to prevent the buildup of excessive pressures within the ampoule resulting in an explosion. A 3 g G2.5.1
mixture was then sealed under vacuum (< 1 x 10-5 torr) in thick wall (1.5 mm) quartz ampoules. The ampoules were inserted into a furnace under nearly isothermal conditions ( 100 K/hr
allowing the samples to cool naturally. Electrica
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