Since the 1960s, the National Aeronautics and Space Administration (NASA) has successfully utilized radioisotope thermoelectric generators (RTGs) as the primary power for many deep space probes, landers and orbiters for many missions including Voyagers 1 and 2, Cassini and Curiosity. RTGs are commonly used when other forms of on board power are not practical due to low solar flux, dayight cycles and settling of dust, or when they would significantly enable or enhance a mission's ability to meet its objectives. RTGs are a proven technology and have been demonstrated to have high reliability, redundancy and long life. Historically, the thermoelectric (TE) materials used in the RTGs by NASA have either been based upon Si-Ge alloys or PbTe/TAGS (tellurium, silver, germanium, antimony), with system level conversion efficiencies of ~6.5%. The goal of the Advanced ThermOelectric Materials (ATOM) project in the Thermoelectric Technology Development Project at the Jet Propulsion Laboratory (JPL) is to develop TE materials that are capable of providing 20% conversion efficiency across a wide temperature range 400-1275 K. In this paper we'll highlight the technical progress of the program and pathways to 20% efficiency. Specifically we will discuss the investigation of f-orbital chemistry on the electronic properties of La)_(3-x)Te_4 and their impact on the thermoelectric figure of merit.
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