Laser Flash Analysis Determination of the Thermal Diffusivity of Si/SiGe Superlattices
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Laser Flash Analysis Determination of the Thermal Diffusivity of Si/SiGe Superlattices Anthony L. Davidson III1, James P. Thomas1, Terrance Worchesky2, Mark E. Twigg1, and Phillip E. Thompson2 1 Naval Research Laboratory, 4555 Overlook Ave, Washington DC 2 Retiree Naval Research Laboratory, 4555 Overlook Ave, Washington DC 3 University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore MD ABSTRACT Applications that produce a large amount of heat, such as combustion engines, can benefit from high temperature thermoelectrics to reduce the amount of energy lost. Superlattice (SL) structures have shown reduced thermal conductivity at room temperature and below, suggesting applicability at high temperatures may be possible. This reduction could greatly increase the thermoelectric figure of merit. The Si/SiGe material system is studied here for high temperature application. Two growth temperatures of 300 C and 500 C are examined. Two superlattice periods were studied (8 nm and 20 nm) to determine the effects of lattice spacing on thermal conductivity. Laser Flash Analysis is applied to determine the thermal diffusivity, hence thermal conductivity, from 100 C to 500 C. Thermal diffusivity was found to be an order of magnitude lower than the constituent alloy at 100 C. Superlattice spacing and growth temperature showed little effect on the diffusivity within the error of this measurement. INTRODUCTION Converting heat into electrical energy through thermoelectrics has the promise of scavenging waste heat in any combustion process. Automotive combustion engines lose 63% of the energy in the fuel to heat. Equation 1 gives the efficiency (η) of a thermoelectric device. ܶଵ െ ܶଶ ܯെ ͳ ܶଵ ܯ ܶଶ ܶଵ ܯൌ ሺͳ ܼܶ ሻଵΤଶ
ߟൌ
(1)
T1 is the heat source temperature, T2 is the heat sink temperature, Tm is the average of sink and source temperatures, and Z is the figure of merit of the device.1 The dimensionless figure of merit is normally quoted and is defined as ZT. If the heat source is set at 600 C and the heat sink at 200 C a ZT of 3 gives an efficiency of ~28.57%. This value of ZT is generally considered desirable in thermoelectric generation. SiGe alloy has been used in applications at this temperature range and has a figure of merit of approximately 0.4 giving an efficiency of ~4.75%. The figure of merit is controlled by the total thermal conductivity (ߢ ߢ ), electrical conductivity (σ), and the Seebeck coefficient (S) as shown in equation 2. ܵଶߪ (2) ܼൌ ߢ ߢ
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Superlattice (SL) structures combine the nanostructuring and alloying techniques to reduce the thermal conductivity of the lattice. Silicon-germanium (Si/Ge)2 and silicon-alloy (Si/Si1-xGex)3 superlattices have shown that this technique results in equal or lower thermal conductivity in the temperature range of -200 C to 127 C compared to the constituent bulk alloy. In this study Si/SiGe SL structures are studied at high temperatures (100 C to 500 C) for application to power harvesting. EXPERIMENT The superlattices in this study are grown using the
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