Rare earths-free thermoelectric converters now viable – research

Researchers at Japan’s Hokkaido University have synthesized a barium-cobalt oxide thermoelectric converter aimed at transforming waste heat into energy without having to use rare earths.

In a paper published in the journal ACS Applied Materials & Interfaces, the scientists state that their new converter is reproducibly stable and efficient at extremely high temperatures.

The article explains that thermoelectric conversion is driven by the Seebeck effect: when there is a temperature difference across a conducting material, an electric current is generated. However, the efficiency of thermoelectric conversion is dependent on a figure called the thermoelectric figure of merit ZT. Historically, oxide-based converters had a low ZT, but recent research has revealed many candidates with high ZT, but their stability at high temperatures was not well documented. Both these factors have led to these converters being of limited use.

Yet, the Hokkaido researchers note that one of the alternatives is oxide-based thermoelectric materials, but the primary drawback these suffer from is a lack of evidence of their stability at high temperatures.

This is where lead researcher Hiromichi Ohta and his team come in. Having worked on layered cobalt oxide films for over two decades, they sought to examine the thermal and chemical stability of these films, as well as measure their ZT values at high temperatures. They tested cobalt oxide films layered with sodium, calcium, strontium or barium, analyzing their structure, resistivity, and thermal conductivity.

They found that, of the four variants, the barium cobalt oxide layered film retained its stability in terms of structural integrity and electrical resistivity at temperatures as high as 600°C. In comparison, the sodium- and calcium cobalt oxide films were only stable until 350°C, and the strontium cobalt oxide film was stable up to 450°C. The ZT of the barium cobalt oxide film increased with the temperature, reaching ~0.55 at 600°C, comparable to some commercially available thermoelectric converters.

“Our study has shown that barium cobalt oxide films would be excellent candidates for high-temperature thermoelectric conversion devices,” Ohta said in a media statement.