Progress in the Search for Advanced Thermoelectric Materials for Terrestrial Apf’lications at the Jet Propulsion Laboratory

The thermoelectric properties of several materials with the skutterudite structure have been investigated at the Jet Propulsion Laboratory (JPL). Based on literature data and experimental findings at JPL, semiconductors with the skutterudite structure have interesting transport properties and show a good potential for high ZT values. A large number of binary and ternary compounds, solid solutions with the skutterudite crystal structure are known. The composition, band gap and doping level can bc tailored to optimize the thermoelectric properties for a specific range of temperature. The investigation of the potential of skutterudite materials as advanced thermoelectric materials was initially focused on antimonide compounds. This paper describes the progress made in the preparation and characterimtion of the arsenide compounds COASJ, RhAss and IrAsj. Preliminary results obtained on hot-pressed samples are presented and discussed. INTRODUCTION The search for new thermoelectric materials has recently focused at JPL on compounds with the skutterudite structure (Caillat et al., 1994a). Experimental data obtained on the binary slnrtterudite compounds CoSbj (Caillat et al., 1994b), IrSbJ (Caillat e t a l . , 1994c) and their solid solutions @orshchevsky et al., 1994) have indicated these materials combine interesting electrical and thermal properties: high carrier mobility, low electrical resistivity, moderate Seebeck coefficient and thermal conductivity. Binary skutterudite compounds are formed with all “nine possible combinations of Co, Rh, Ir with P, As and Sb. We prepared and characterized of the antimonide compounds first. Little information is available in the literature on the prope~ties of binary phosphides and riraenides compounds. Ile investigation of the properties of these binary compounds has been recently started at JPL. We report in this paper on the preliminary results obtained on the preparation and character-htion of the binary arsenides compounds COASJ, IUIAS3 and IrAsj. SAMPLE PREPARATION San,plcs of t20As3 and IWASJ were successfully prepared b y annealing stoichiometric mixtures of the elements in sealed quartz ampoules. The arsenic (99.9995%), cobalt (99.998%) and rhodium (99.9 S0/0) powders were loaded in an argon atmosphere inside a glove-box to prevent any oxidation. The quartz arnpoules containing the mixtures were transferred to a vacuum station, evacuated and scaled under 105 Torr vacuum. The ampoules were introduced in an isothermal fimace and annealed for 8 days at 600°C. After annealing the products were analyzed by X-ray diffractometry (XRD) which was conducted on a Siemens D-5(KI diffractometer with the O’-Ka radiation. Silicon powder was used as a standard and mixed to the samples. The products were found to be single phase materials with the skutterudite structure. The materials were then hot-pressed at 800°C for 2 hours in graphite dies of 6.35 mm II) at a pressure of 15,000 psi. An “Astro” hot-pressing furnace with a constant flow of argon was used. Compact cylindrical pellets from 5 to 8 mm long were obtained. The density of the samples was measured by the immersion technique using toluene as the liquid. The measured densities were found to be between 98 Md Q$)yo of the theoretical density. Selected samples were polished and their microstructure was investigated under an optical microscope. Microprobe analysis (MPA) was also performed on the same