Grain Boundary Engineering of Thermoelectrics for RTGs

All future NASA missions require reliable power generation capabilities. However, there are many missions that cannot rely on the conventional solar cell or battery technologies because of extended duration, distance from the sun, or harsh conditions. Radioisotope thermoelectric generators (RTGs) offer a consistent power supply regardless of solar irradiance and independent of exterior conditions. RTGs consist of a radioactive heat source surrounded by thermoelectric n and p type couples. These couples convert the heat into usable power without any moving parts or possible wear. The power output of an RTG is governed by two factors: the amount of heat that the heat source can generate and the efficiency the thermoelectric n and p type materials. High temperature thermoelectrics are needed to enable high temperature heat sources, but these materials often have poor thermoelectric properties at low temperatures. Grain boundaries have been shown to severely impair the electrical conductivity of thermoelectric materials at low temperatures paired with smaller reductions at high temperature. By removing or controlling the grain boundaries in high temperature thermoelectric materials, their conductivity and hence efficiency can be greatly improved, resulting in higher power outputs for RTGs. This proposed research plans to analyze and exploit the influence of grain boundaries on the thermal and electrical conductivity of the high temperature thermoelectric materials Yb14MnSb11 and La3Te4 to boost RTG efficiency. Recent work on the thermoelectric material Mg2Sb3 has shown a more than two-fold improvement in thermoelectric figure of merit at low temperatures while still improving high temperature properties through grain boundary engineering and the tailoring of processing conditions. This approach will be applied to these high temperature RTG materials in conjunction with long term stability testing to ensure reliable performance in typical RTG operating conditions.

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