Efficient cooling and power generation systems require thermoelectric materials that are good at conducting electricity but poor at conducting heat. However, to make a good thermoelectric device, a ZT of over 1.0 is required in the application temperature range. And, the problem is that the ZT of bismuth antimony telluride – one of the most commonly used thermoelectric materials – has remained at around 1.0 for more than half a century.

"ZT equalling 1.0 is not enough to make cooling, air conditioning and power generation systems that are competitive with mechanical systems already in use," team leaders Zhifeng Ren and Gang Chen told nanotechweb.org. "This is why we are improving ZT, to make it competitive, and our work is a significant step in this direction."

Improved ZT
Researchers may have come up with a new and simple way to significantly improve the performance of thermoelectric materials, which have remained pretty much the same for over 50 years. Bed Poudel and colleagues from Boston College and the Massachusetts Institute of Technology achieved their feat by grinding bismuth antimony telluride alloys into fine nanopowders and then pressing the powders into nanocrystalline ingots. The researchers found that the thermoelectric figure of merit (ZT) for the ingots increased to 1.2 at room temperature (from a value of 1.0 previously), which makes the materials useful for cooling and power generation.

Poudel and co-workers have increased the ZT value of bismuth antimony telluride to 1.2 at room temperature and 0.8 at 250 °C (from a previous value of 0.25 at this temperature). Moreover, the ZT peaks at 1.4 at 100 °C.

The researchers obtained their results by ball-milling the alloy into a fine powder that contained nanoparticles measuring about 20 nm across. Next, they hot-pressed the powder into nanocrystalline ingots.

Electrical transport measurements on the ingots, together with microstructure and modelling, showed that the ZT improves thanks to the low thermal conductivity caused by increased phonon scattering at grain boundaries and defects in the material. "In other words, the resistance to heat flow increases without a degradation in the material's electrical properties," said Ren and Chen. The researchers have also built a prototype cooling device to confirm the properties.

Promising for applications
The high ZT in the temperature range 25–250 °C makes these materials promising for cooling and waste heat recovery applications, say the researchers. Potential applications include converting the heat of car exhausts into electricity, for example. "Other applications include efficient thermoelectric cooling, such as air conditioning and refrigeration, and solar thermoelectricity," explained Ren and Chen.

The team now plans to make efficient coolers and power generators using the improved materials. "At the same time, we will apply the approach to other promising thermoelectric materials," they stated.

The company GMZ Energy Inc, a Newton-based start-up, is now mass-producing the materials.

The work was published in Science.