The Research and Development of Silicon-Germanium Thermoelements for Power Generation
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THE RESEARCH AND DEVELOPMENT OF SILICON-GERMANIUM THERMOELEMENTS FOR POWER GENERATION Fred D. Rosi, University of Virginia, Department of Materials Science, Charlottesville, Virginia Abstract On the 25th of August 1989, radio signals from the unmanned Voyager II
spacecraft produced high resolution radio displays of the planet
Neptune and its moon, Triton, on the monitors of Caltech's Jet Propulsion Laboratory. Thus ended the 12-year NASA Voyager I and II missions to the giant outer planets of our solar systems. Somewhat overlooked in the excitement over the impressive scientific findings from Voyager's epic planetary encounters was the power system aboard the spacecraft for data collection and signal transmission. This was the multi-hundred watt, radioisotope fueled thermoelectric generator using thermocouple modules made of Si-Ge alloys. The first section of this paper presents a historical review of the research program at the RCA Laboratories in Princeton, N.J. during the 4-year period required for the conception and development of Si-Ge thermoelements for power generation*. This includes RCA's strategy in thermoelectric materials research from the viewpoint of device principles and requirements, scientific issues relating to the growth and thermoelectric characterization of Si-Ce alloys, and device feasibility studies. The performance of Si-Ge thermoelements in the power source of several space missions is also discussed. Finally, a perspective is presented on the research management of this remarkably successful program. The second section of the paper describes the results of more recent research on the thermoelectric properties of hot-pressed, sintered Si-Ge alloys and, in particular, the effects of grain size and additions of GaP. In the final section, consideration is given to future areas of research based, in part, on the results of the earlier studies of Si-Ge technology for thermoelectric power generation. I.
Introduction
The development of the Si-Ge thermoelements used in the power system of the Voyager I and II spacecraft was an exercise in serendipity occasioned by an unusual environment in an industrial laboratory. Selected research groups at RCA's David Sarnoff Research Center in the early 1950's enjoyed a corporate climate which supported and encouraged investigation of basic scientific phenomena within broad parameters of long-term corporate interests. The luxury of broad investigation was often stimulated by specific challenges to the research staff by the technological foresight of an unusual Chairman of the Board. The study of thermoelectricity was initially stimulated by General David Sarnoff's specific request about 1952 for an electronic refrigerator using the Peltier effect. The Laboratories responded with pioneering research on improved thermoelectric materials. This led to the construction of a 4 1 cu. ft. working refrigerator and freezer( ) and a thermoelectric airconditioned room on the occasion of Ceneral Sarnoff's 50 yrs. in the field of communications and entertainment.
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