Thermoelectric Properties of Bi 2 SrCo 2 O 9 Tellurium-Doped Single Crystalline Whiskers
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Thermoelectric Properties of Bi2SrCo2O9 Tellurium-Doped Single Crystalline Whiskers Dwayne Bourne1, Xiaofeng Tang2, Kelvin Aaron2, Julius Barnes1, James Payne1 and Terry M. Tritt2; 1 Department of Biological & Physical Sciences, South Carolina State University, Orangeburg, South Carolina 29117, U.S.A. 2 Department of Physics and Astronomy, Clemson University, Clemson, South Carolina, 29634, U.S.A ABSTRACT Long single crystalline whiskers (10-200 µm diameter) were synthesized using telluriumdoped precursors. The length of these whiskers varies from less than 1 mm up to 9 mm. The thermopower and resistivity were approximately 150 µV/K and 5 mΩ-cm respectively at 325K. The thermopower was measured using a differential technique, while the resistivity was measured using a standard four-probe method. The thermal conductivity of these small samples was measured using our parallel thermal conductance technique. The total thermal conductivity was on the order of 2 Wm-1K-1. INTRODUCTION Thermoelectric materials have already been employed in space to power the Apollo, Viking, Pioneer, and Voyager space missions and most recently the Cassini mission.. Materials used in these missions have lasted for more than 30 years attesting to their longevity. Thermoelectric materials are viewed as a potential part of the solution to our depleting fuel resources for future generations. Thermoelectric oxide materials are attractive options for power generation applications from waste heat because of their high reliability, stability at high temperatures (T > 600K), vibration-free operation, and lack of toxic or scarce chemicals. Inter-metallic compounds were once viewed as the best high temperature power generation thermoelectric materials because of their high thermoelectric performance, however in 1997, Terasaki published results on NaCo2O4 single crystals as having high thermoelectric performance. [1, 2] This discovery introduced the world to transition metal oxides as thermoelectric materials. The dimensionless figure of merit, ZT, is used to gauge the conversion efficiency of thermoelectric materials. The dimensionless figure of merit is a quantity given by ZT=α2T / ρκ, where α is the Seebeck coefficient, T is temperature in degrees Kelvin, ρ is the resistivity, and κ the total thermal conductivity. [2] EXPERIMENTAL DETAILS The thermoelectric samples were cobalt oxide single crystalline whiskers prepared by the sintered precursor method. They were prepared by first mixing SrCO3, Bi2O3, CaCO3, Co3O4, and TeO2 powders in the cationic ratios of 1:1:1:2:0.25. [3] This mixture was calcined for 10 hours at
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1073K. The resulting mixture was then ground and pelletized. The pellets were sintered for 20 hours at 1123K. After sintering, the pellets were heated in a stream of oxygen for 100 hours at 1148K. Single crystal whiskers grew from the surface of the pellets and ranged in length from less than 1 mm up to 9 mm. EDX analysis was performed in order to obtain the elemental composition of the whiskers. The ratios
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