A cutting-edge development in spacecraft power systems is a class of materials with an unfamiliar name: skutterudites (skut-ta-RU-dites). Researchers are studying the use of these advanced materials in a proposed next-generation power system called an eMMRTG, which stands for Enhanced Multi-Mission Radioisotope Thermoelectric Generator.
NASA’s Mars Curiosity rover and the New Horizons mission, which flew by Pluto in 2015 both used Radioisotope Thermoelectric Generators.
The new eMMRTG would provide 25 percent more power than Curiosity’s generator at the start of a mission, according to current analyses. Additionally, since skutterudites naturally degrade more slowly that the current materials in the MMRTG, a spacecraft outfitted with an eMMRTG would have at least 50 percent more power at the end of a 17-year design life than it does today.
“Having a more efficient thermoelectric system means we’d need to use less plutonium. We could go farther, for longer and do more,” Bux said.
Samad Firdosy, a materials engineer at JPL, holds a thermoelectric module made of four thermocouples, which are devices that help turn heat into electricity. Thermocouples are used in household heating applications, as well as power systems for spacecraft. Credit: NASA/JPL-Caltech
What are skutterudites?
The defining new ingredients in the proposed eMMRTG are materials called skutterudites, which have unique properties that make them especially useful for power systems. These materials conduct electricity like metal, but heat up like glass, and can generate sizable electrical voltages.
Materials with all of these characteristics are hard to come by. A copper pot, for example, is an excellent conductor of electricity, but gets very hot quickly. Glass, on the other hand, insulates against heat well, but it can’t conduct electricity. Neither of these properties are appropriate in a thermoelectric material, which converts heat into electricity.
“We needed to design high temperature compounds with the best mix of electrical and heat transfer properties,” said Sabah Bux, a technologist at JPL who works on thermoelectric materials. “Skutterudites, with their complex structures composed of heavy atoms like antimony, allow us to do that.”
RTGs in space
A team at JPL is working on turning skutterudites into thermocouples. A thermocouple is a device that generates an electrical voltage from the temperature difference in its components. Compared to other materials, thermocouples made of skutterudites need a smaller temperature difference to produce the same amount of useful power, making them more efficient.
There are many potential applications for these advanced thermoelectric materials here on Earth.
“In situations where waste heat is emitted, skutterudite materials could be used to improve efficiency and convert that heat into useful electricity,” said Thierry Caillat, project leader for the technology maturation project at JPL.
For example, exhaust heat from a car could be converted into electricity and fed back into the vehicle, which could be used to charge batteries and reduce fuel use. Industrial processes that require high temperatures, such as ceramic and glass processing, could also use skutterudite materials to make use of waste heat. In 2015, JPL licensed patents on these high-temperature thermoelectric materials to a company called Evident Technologies, Troy, New York.
SOURCES NASA, JPL