Tokyo Institute of Technology has designed a material for thermo-electric generators without toxic materials such as lead (Pb) or tellurium.

TiTech inverse thermoelectric function
To be a good thermo-electric material (TEM), something has to have low thermal conductivity (to prevent heat leaking through) as well as converting heat to electricity effectively.

The university cites heavy metal chalcogenides Bi2Te3 and PbTe as high-performance TEMs, while more benign oxide TEMs, such as SrTiO3, are limited in effectiveness by high thermal conductivity.

TiTech inverse perovskite thermoelectric structureSeeking a better non-toxic TEM, Tokyo Tech scientist Takayoshi Katase started with a crystal structure that has naturally poor thermal conductivity – the ‘inverse-perovskite’ (image left)- building it from either Ba3SiO or Ba3GeO.

“Unlike normal perovskites, such as SrTiO3, the positions of cation and anion sites are inverted in inverse-perovskites, so they contain a large amount of the heavy element [Ba],” said Katase. “Their crystal structure is formed by a soft framework made up of weak O-Ba bonds. These characteristics realise the low thermal conductivity.”

Synthesised bulk polycrystals of the new TEMs conducted at 0.4 to 1W/mK at 300 – 600K, which is less thermally conductive than bulk Bi2Te3 or PbTe, according to the university.

Electrical generation is also good, it said, as the Si or Ge ion, which usually behaves as a positively charged cation, is a  negatively charged anion in these inverse-perovskites, and these anions are effective at transporting carriers.

TiTech inverse perovskite thermoelectric performanceTokyo Tech’s view of thermo-electric material effectiveness

Bulk Ba3SiO or Ba3GeO scored 0.16 – 0.84 (at 300-623K) in ‘ZT’, the figure-of-merit used for thermo-electric materials .

“Theoretical calculations predicted a potential maximum ZT of 2.14 for Ba3SiO and 1.21 for Ba3GeO at 600K, by optimising hole concentration,” said Tokyo Tech. “The maximum ZT of these non-toxic TEMs is much higher than that of other eco-friendly TEMs, and comparable to the toxic heavy element ones in the same temperature range.”

“We believe that our insight into designing high ZT materials without using toxic elements would have a strong impact on the materials science and chemistry communities as well as among innovators looking to expand the horizon of thermoelectric material applications,” said Katase.

Tokyo Tech worked with Japan’s National Institute for Materials Science, its National Institute of Advanced Industrial Science and Technology and Tohoku University