Silicon-Germanium thermoelectric device
An APL-developed silicon-germanium thermoelectric device being tested under GPHS-operation-like conditions, simulating space-like vacuum with an electrical heat source at about 2,100 degrees Fahrenheit (1,150 degrees Celsius). (Credit: Johns Hopkins APL/Ed Whitman)

NASA’s Radioisotope Power Systems Program’s sponsored multidisciplinary team of scientists and engineers led by Rama Venkatasubramanian of the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, won the American Nuclear Society’s 2022 Best Radioisotope Power Paper award. The society made the announcement following the Nuclear and Emerging Technologies for Space (NETS) conference, held May 8-12 in Cleveland.

The paper, “GPHS Heritage-Like SiGe Unicouple Development with SiGe and SiMo Materials Prepared by Spark Plasma Sintering,” was praised for its quality, content and impact on the field.

Although once used as a key material for NASA’s radioisotope thermoelectric generators (RTGs) in its electricity-producing unicouples, silicon-germanium (SiGe) production halted with NASA’s termination of RTG development in the late 1990s, and the expertise to make the material went with it.

Renewed interest to include SiGe in NASA’s Next-Generation RTG design came after reexamining its huge success on the Voyager probes, Ulysses, Galileo, Cassini-Huygens and New Horizons spacecraft, the latter of which captured the first images of Pluto in 2015 on the power of a spare RTG made during the build of Cassini-Huygens. Starting in spring 2020, Venkatasubramanian and his APL colleague Paul Ostdiek designed a cross-sector, multi-institutional team that, within a few months, developed a way to recreate SiGe unicouples similar to those used before but with materials fabricated through the modern technique of spark plasma sintering.

“Silicon-germanium technology is key to building GPHS [general purpose heat source]-RTGs, which will be essential for outer-planetary missions and interstellar probes that have to last for as many as 50 years,” Venkatasubramanian said. “So, we’re ecstatic to have restarted the build of silicon-germanium technology, using the latest advances in semiconductor materials synthesis and manufacturing to a point where NASA could potentially use this pathway for its emerging mission needs.”

Applied Physics Laboratory Thermoelectrics Team
Members of the winning paper stand outside Johns Hopkins APL’s Building 201 in Laurel, Maryland. From left: Paul Ostdiek, Timothy Erickson, Priestly Shuler, Jonathan Pierce, Rama Venkatasubramanian, Richard Ung and Jake Ballard. (Credit: Johns Hopkins APL/Ed Whitman)

The team’s paper details its process for developing this robust and scalable technology as well as the broader implications for the Next-Generation RTG and the future of deep-space exploration. This technology has relevance to advanced energy conversion concepts in nuclear power sources that the U.S. Department of Defense and NASA and are developing as well as what commercial entities are looking at for mitigating climate-change concerns.

The project is a collaboration among researchers from APL, the University of Virginia and Alfred University in New York. Team members include Venkatasubramanian, Ostdiek, Jonathan Pierce, Richard Ung, Jake Ballard, Priestly Shuler and Timothy Erickson from APL; Joseph Poon and Mousumi Mitra from the University of Virginia; and Scott Misture from Alfred University.

The team collectively has more than 50 years of research and engineering experience, including expertise and leadership in the development of advanced thermoelectric materials, devices and mini-RTGs and conventional RTGs for NASA and other U.S. government agencies. Venkatasubramanian is a fellow of the Institute of Electrical and Electronic Engineers, and Poon is a fellow of the American Physical Society.

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