Melt-Textured Growth of Grain Aligned Bulk Oxide Thermoelectrics
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1166-N09-07
Melt-Textured Growth of Grain Aligned Bulk Oxide Thermoelectrics Venkat Selvamanickam and Bo Zhang Department of Mechanical Engineering 4800 Calhoun Road University of Houston Houston, Texas 77204, U.S.A. ABSTRACT A new process, based on melt processing has been investigated with oxide thermoelectrics to achieve long-range grain alignment for low resistivity as well as to embed secondary phase precipitates and associated crystal defects for low thermal conductivity. Melt-processing of Bi2Sr2Co2Ox and CaMnO3 has been studied. A high degree of (00l) grain alignment has been achieved by melt processing. Good values of Seebeck coefficient of nearly -250 μV/K were measured in the melt-processed CaMnO3. Secondary phases of Ca4Mn3O10 and CaMn2O4 are found to be trapped between the aligned grains which led to high electrical resistivity values and limited the figure of merit in our initial samples. INTRODUCTION The Figure of merit (ZT) values of thermoelectrics have typically hovered around one in bulk materials. Bulk materials are needed for large-scale generation of electric power from waste heat and in spite of significant progress [1], achieving ZT values higher than three is challenging. The challenge stems from conflicting performance requirements in thermoelectric materials in general, to achieve high electrical conductivity, but low thermal conductivity while maintaining a high Seebeck coefficient. Additional challenges have been faced with the deteriorating properties at high temperatures, instability of the structures at operating conditions, use of toxic materials such as Pb, Te, Se etc, high cost of Te, poor oxidation resistance and lack of mechanical robustness. Oxides are well suited for high-temperature operation since their ZT increases even at temperatures of 1000 K. Also, oxides exhibit excellent stability and oxidation resistance at high temperatures, that are key requirements for thermoelectric power generation from waste heat. Furthermore, thermoelectric oxides do not include Pb, Te, Se etc and can be processed in air instead of sealed quartz tube conditions. But, the ZT values of oxides have been low so far. So, we have recently begun investigation of a novel process technique based on melt-textured growth that deviates from the typical ceramic processing methods used so far. Since oxide thermoelectrics are highly anisotropic, electrical conductivity along the basal plane is about two orders of magnitude higher than that perpendicular to the plane [2, 3]. In another study, the resistivity along the basal plane of NaxCoO2-d single crystals was found to be an order of magnitude lower than that in polycrystalline bulk ceramic samples [4]. However, the thermal conductivity of single crystals has not been reduced due to the lack of phonon scattering defects such as grain boundaries, second-phase precipitates, and dislocations. The thermal conductivity of single crystal NaxCoO2-δ is 2.4 times higher than that of NaxCoO2-δ polycrystals [4]. If its thermal conductivity can be reduced to the level
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