Researchers from Hokkaido University and their colleagues in Japan and Taiwan have more than doubled the ability of a material to transform wasted heat into usable electricity by significantly narrowing the space through which spread electrons move, according to a new open-access study published in the journal Nature Communications.

It has been theoretically predicted that thermoelectric power factor can be largely enhanced when the two-dimensional electron layer is far narrower than the de Broglie wavelength. Although many studies have been made, the effectiveness has not been experimentally clarified thus far. Here we experimentally clarify that an enhanced two-dimensionality is efficient to enhance thermoelectric power factor.

We fabricated superlattices of [N unit cell SrTi_1−xNb_xO₃|11 unit cell SrTiO₃]₁₀—there are two different de Broglie wavelength in the SrTi_1−xNb_xO₃ system. The maximum power factor of the superlattice composed of the longer de Broglie wavelength SrTi_1−xNb_xO₃ exceeded ∼5 mW m^-1K^-2, which doubles the value of optimized bulk SrTi_1−xNb_xO₃. The present approach—use of longer de Broglie wavelength—is epoch-making and is fruitful to design good thermoelectric materials showing high power factor.

—Zhang et al.

Conceptual drawing of the superlattice in which spread electrons are confined to a narrow space to enhance thermoelectric conversion. Copyright : Hokkaido University

More than 60% of energy produced by fossil fuels is lost as waste heat. One way to address this problem is to convert the wasted heat into electricity, known as thermoelectric energy conversion. However, improving the conversion rate has been difficult because of a trade-off relationship between the required properties within the material.

Thermoelectric materials convert heat into electricity when there is a temperature difference, a phenomenon known as the Seebeck effect. Scientists have been investigating ways to confine electrons to a narrow space as a way to enhance conversion rates.

In 2007, researchers built an artificial superlattice composed of conducting ultrathin layers sandwiched by thick insulating layers. This method yielded higher voltage but did not improve conversion rates. Researchers have predicted that performance can be significantly improved if electrons with longer de Broglie wavelength, which means they are more spread, are confined into a narrow conducting layer, but it had not yet been proven experimentally.

The research team, led by Hiromichi Ohta of Hokkaido University, designed a superlattice in which electrons are spread by 30% wider as compared to previous experiments. This resulted in much higher voltage and doubled the thermoelectric conversion rate recorded from previous methods.

This is a significant step forward towards reducing the amount of heat wasted by power plants, factories, automobiles, computers, and even human bodies.

—Hiromichi Ohta

This research was supported by Grants-in-Aid for Scientific Research on Innovative Areas “Nano Informatics” (25106003, 25106005, and 25106007), Grants-in-Aid for Scientific Research A (17H01314) and B (26287064) from the Japan Society for the Promotion of Science (JSPS), and the grants from Taiwan Ministry of Science and Technology (MOST 104-2112-M-009-023-MY3 and MOST 104-2738-M-009-006).

Resources

• Zhang Y. et al. (2018) “Double thermoelectric power factor of a 2D electron system.” Nature Communications, doi: 10.1038/s41467-018-04660-4