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."

Cooler cars

One promising application for the improved material is transforming waste heat from car engines into electricity to help power the vehicles. The US Department of Energy (DOE) has set a goal of demonstrating a 10% increase in vehicle fuel economy through waste heat capture by 2014, according to John Fairbanks of the DOE.

No commercially available vehicle uses the technology today, but tests by car manufacturers including BMW suggest a 6-8% fuel efficiency increase is possible, Fairbanks says.

"Adding a 40% efficiency increase in thermoelectrics to that might meet that target," Jeff Snyder, of the California Institute of Technology in Pasadena, US, says.

The same approach could harvest human body heat to power medical implants although designing less power hungry devices is important too, Snyder notes.

Alternatively the improved alloy could be used in reverse for solid state cooling, without bulky pipes of gas or liquid coolant, in new areas. "We're not yet at the point where we will see air conditioning systems or large refrigerators with solid state cooling, but it's a significant advance," says Snyder.

Journal reference: Science, DOI: 10.1126/science.1156446

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