A major boost in the effectiveness of a material that transforms waste heat into electricity could significantly boost energy efficiency in anything from air conditioners to car engines. It is the first major improvement in such “thermoelectric” materials in 50 years, say researchers.

Thermoelectric materials can also work in reverse to convert electricity into differences in temperature, allowing cooling without pipes, pumps or coolants.

Since the 1950s, engineers have used a semiconductor alloy called bismuth antimony telluride in niche applications, such as solid state cooling for precision medical equipment. But although it is the best material around for the job, the alloy is far from efficient. The new efficiency boost could see thermoelectric materials used in many more areas.

Rip it up and start again

The dramatic 40% boost is relatively simple to achieve. Grinding bismuth antimony telluride into fine particles and then pressing it back together again using heat transforms its thermoelectric properties, according to researchers from Massachusetts Institute of Technology (MIT) and Boston College, both Boston, US.

Sticking the nanoscale particles back together increased the alloy’s peak figure of merit, a term used to measure metals’ relative thermodynamic performance, by 40% from 1.0 to 1.4.

The researchers say the jump happens because the reincarnated alloy has a finer-grained crystalline structure. The new structure offers greater resistance to the quantum vibrations called phonons that transport heat within solids, making it a better thermal insulator.

This is crucial because thermoelectric materials work by maintaining differences in temperature while letting electricity flow freely. If less of the incoming heat can escape through heat conduction, more will be used to drive electrons, and the material will be more efficient.

Heat hurdles

For phonons carrying heat, having more crystal grains to cross “is like the difference between running the 100-metre dash and running the same distance with hurdles every 10 metres,” says study author Zhifeng Ren.

Prior, unsuccessful, attempts to shrink the crystal structure of thermoelectric alloys tried to build the new materials from scratch, layer by layer, in an expensive method called thin-film deposition.

“That’s more like artists making fine art,” Ren says. “Our [process] is like a copy machine, making much larger quantities much faster.”