Crystal Lattice Controlled SiGe Thermoelectric Materials with High Figure of Merit
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Crystal Lattice Controlled SiGe Thermoelectric Materials with High Figure of Merit Hyun Jung Kim1 Yeonjoon Park1 Glen C. King2 Kunik Lee3 and Sang H. Choi2 National Institute of Aerospace (NIA), Hampton, VA 23666, U.S.A. 2 NASA Langley Research Center, Hampton, VA 23669, U.S.A. 3 Federal Highway Administration of Department of Transportation, McLean, VA 22101, USA
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ABSTRACT Direct energy conversion between thermal and electrical energy, based on thermoelectric (TE) effect, has the potential to recover waste heat and convert it to provide clean electric power. The energy conversion efficiency is related to the thermoelectric figure of merit ZT expressed as ZT=S2σT/κ, T is temperature, S is the Seebeck coefficient, σ is conductance and κ is thermal conductivity. For a lower thermal conductivity κ and high power factor (S2σ), our current strategy is the development of rhombohedrally strained single crystalline SiGe materials that are highly [111]oriented twinned. The development of a SiGe “twin lattice structure (TLS)” plays a key role in phonon scattering. The TLS increases the electrical conductivity and decreases thermal conductivity due to phonon scattering at stacking faults generated from the 60° rotated primary twin structure. To develop high performance materials, the substrate temperature, chamber working pressure, and DC sputtering power are controlled for the aligned growth production of SiGe layer and TLS on a cplane sapphire. Additionally, a new elevated temperature thermoelectric characterization system, that measures the thermal diffusivity and Seebeck effect nondestructively, was developed. The material properties were characterized at various temperatures and optimized process conditions were experimentally determined. The present paper encompasses the technical discussions toward the development of thermoelectric materials and the measurement techniques.
INTRODUCTION Increasing global interest in energy and environmental concerns has drawn attention to research in renewable and sustainable energy systems. Thermoelectric (TE) devices are promising because of their potential application as replacements or supplements to conventional energy conversion systems, such as waste heat recovery and environmentally-friendly refrigeration.1,2 Significant enhancement in the energy conversion efficiency of TE materials has been possible with morphologically nanostructured materials with low thermal conductivities.3-5,9-12 The efficiency of TE materials is determined by the dimensionless Figure of Merit, ZT, expressed as ZT=S2σT/κ, wherein the Seebeck coefficient S (µV/K), electrical conductivity σ (S/m), thermal conductivity κ (W/m·Κ), absolute temperature T (K) are the characteristic parameters and Z is multiplied by T to afford the Figure of Merit (FoM).2,13 Therefore, good TE materials require a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Such materials are difficult to find in nature, and harder to engineer the individual properties without affecting other character
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