Skutterudites may sound like an alien race in some science-fiction B-rated movie, but this new class of materials being developed by NASA’s Jet Propulsion Laboratory (JPL) may bring humanity closer to actually finding alien races or at least to help study planets in our solar system.
Researchers at the JPL are using skutterudites in a proposed next-generation power system called Enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTG). The goal is to make spacecraft power systems more efficient, resilient and long-lasting on long trips to another planet.
“NASA needs reliable, long-term power systems to advance exploration of the solar system,” says Jean-Pierre Fleurial, supervisor for the thermal energy conversion research and advancement group at NASA's JPL. “This is particularly important for the outer planets, where the intensity of sunlight is only a few percent as strong as it is in Earth's orbit.”
In order to come up with a next-generation power source that can last longer than traditional power systems, JPL began testing using skutterudites as the defining ingredient in its proposed eMMRTG. Skutterudites have unique properties that make them especially useful in power systems by conducting electricity like metal but heating up like glass, and they can generate sizable electrical voltages.
Glass alone insulates well but it cannot conduct electricity, whereas copper is an excellent conductor of electricity but it gets really hot. NASA needed the best mix of these elements in order to create complex structures composed of heavy atoms.
Additionally NASA’s JPL found that skutterudite materials could be useful for those of us not traveling into space and planning to stay on our own planet. NASA gives the example of exhaust heat from a car that could be converted into electricity and then fed back into the vehicle to charge batteries or reduce fuel use. Another example is found in industrial processes that require high temperatures, such as ceramic and glass processing. These industries could use skutterudite materials to make use of the waste heat.
“In situations where waste heat is emitted, skutterudite materials could be used to improve efficiency and convert that heat into useful electricity,” says Thierry Caillat, project leader for the technology maturation project at JPL.
How It Works in Space
NASA has been using radioisotope thermoelectric generators (RTGs) in space since 1961. These RTGs powered the twin Voyager probes, and have powered NASA’s Mars Curiosity rover and the New Horizons mission, which flew by Pluto last year.
The RTGs naturally generate heat as they transform into other elements. Most of the RTGs in past missions harnessed heat from the natural radioactive decay of plutonium-238 oxide. The new eMMRTG would provide a 25% power boost over that of the Curiosity generator; and because skutterudites naturally degrade more slowly than current materials, this generator could have 50% more power at the end of a 17-year design life than those currently in use, JPL says.
Now JPL is working on turning skutterudites into thermocouples—devices that generate an electrical voltage from the temperature difference in its components. In the Curiosity’s power system, its MMRTG has 768 thermocouples that encircle a central can-like structure and all face the same direction—toward the heat source. The eMMRTG that JPL is working on would have the same number of thermocouples, but they would all be made from skutterudite material instead of the alloys of tulluride currently in use.
If the skutterudite-based thermocouples pass reviews planned by NASA in 2017 and 2018, the first eMMRTG could fly aboard NASA’s next New Frontiers-class mission, JPL says.