Progress of Fabricating Heritage Silicon Germanium Unicouples

NASA has successfully demonstrated Radioisotope Thermoelectric Generators (RTGs) as an effective power system to support numerous scientific deep space explorations. RTGs have consistently demonstrated their extraordinary reliability and longevity (40 years of continuous operation for Multi-Hundred Watt Radioisotope Thermoelectric Generator (MHW-RTG) on Voyager 1 and 2) while relying on thermoelectric materials and device technologies that were developed in the 1960s and 1970s. Recently, NASA announced plans to reestablish the General Purposes Heat Source-Radioisotope Thermoelectric Generator (GPHS-RTG) with Silicon-Germanium Unicouple(s) for potential future deep-space missions. However, the Silicon-Germanium unicouple production line in industry was discontinued in the late 1990s, resulting in loss of facilities, a combination of legacy equipment obsolescence and scattering of remaining legacy equipment to various government entities, bulk storage of legacy build-to-print procedures, loss of trained and knowledgeable personnel, and a likely loss in qualified vendors. To reduce potential risks due to this obsolescence, NASA’s Radioisotope Power Systems program established a SiGe Task Force (NASA RPS DIR-006) in support of the Next-Generation RTG Project (Mod 1). The focus of task force support at NASA’s Jet Propulsion Laboratory (JPL) has been on identifying and assessing initial key risks and exploring options relative to reestablishing the GPHS-RTG SiGe unicouple production line. To this end, JPL utilized its existing capabilities and heritage procedures of legacy SiGe unicouple production to conduct “heritage build-to-print” SiGe unicouple fabrication. Performing this “heritage” unicouple assembly and pellets/segments fabrication steps enabled JPL to identify critical processes and evaluate their robustness to variations from available legacy documentation and processes. Here, we report on the results of JPL SiGe unicouple assembly pathfinder practices using Cassini RTG production line ready-to-use parts.