Thermoelectric Properties of Hot-Pressed Ultra-Fine Particulate Sige Powder Alloys with Inert Additions
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THERMOELECTRIC PROPERTIES OF HOT-PRESSED ULTRA-FINE PARTICULATE SIGE POWDER ALLOYS WITH INERT ADDITIONS John S. Beaty*, Jonathan L Rolfe* and Jan W. Vandersande** *Thermo Electron Technologies Corporation, 85 First Avenue, Waltham, MA 02254-9046 **Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 ABSTRACT The objective of the work reported here is to reduce the thermal conductivity of thermoelectric materials in order to improve their figureof-merit and conversion efficiency. Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity [1]. To investigate this prediction, ultra-fine particulates (20A to 120A) of silicon nitride have been added to boron doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultra-fine powder have lower thermal conductivities, than that for standard SiGe, but high temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers, retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25% in thermal conductivity has been achieved in these samples. INTRODUCTION It has been postulated that the thermal conductivity of SiGe alloys can be lowered by adding small inert precipitates to them, which would serve as phonon scattering centers [1]. Lowering the thermal conductivity of thermoelectric materials would improve their figure-of-merit and conversion efficiency. For several years it has been possible to produce ultra-fine particulates of materials. The current program was initiated to take advantage of these two developments by pursuing the reduction of thermal conductivity through the addition of ultra-fine particles to boron doped, p-type, 80/20 silicon germanium, thus improving its figure-ofmerit. The theoretical model predicts that the thermal conductivity of SiGe alloys can be reduced by adding appropriately sized particles to act as scattering centers for those phonons, conducting most of the heat through a material. It estimates the appropriate size particle to be about 50A in diameter for silicon germanium alloys. The production of ultra-fine particulates by spark erosion produces particles of the appropriate size, 20A to 120A. In the work reported here, spark erosion has been used to produce ultra-fine particles of silicon germanium thermoelectric
Mat. Res. Soc. Symp. Proc. Vol. 234. @1991Materials Research Society
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material, and silicon nitride, an inert phonon-scattering material. The two powders were mixed to make a homogeneous powder. The mixture was then hot pressed to produce a thermoelectric material with uniformly dispersed, ultra-fine, inert, phonon-scattering centers. THEORY Lattice thermal conductivity is made up of contributions from the entire frequency spectrum of phonons, or lattice vibrations, and is proportional to the specific heat, the phonon wave velocity, and the phono
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