Researchers at Ohio State University have invented a new material designed to make cars more efficient by converting heat wasted through engine exhausts into electricity.

Scientists rate the efficiency of thermoelectric (TE) materials based on how much heat they can convert into electricity at a given temperature. To maximise the amount of electricity produced by a TE material, engineers would normally try to limit the amount of heat that can pass through it without being captures and converted to electricity. So the typical strategy for making a good TE material is to lower its thermal conductivity.

Project leader, Joesph Heremans took a different approach, focussing on how to convert the maximum amount of heat that was naturally trapped in the TE material. To do this he took embraced some new ideas in quantum mechanics.

Heremans referenced a 2006 paper published by other researchers in the journal, ‘Physical Review Letters’. The paper suggested that elements such as thallium and tellurium could interact on a quantum mechanical level to create a resonance between the thallium electrodes, and those host lead telluride TE material, depending on the bonds between the atoms.

Heremans explained: “It comes down to a peculiar behaviour of an electron in a thallium atom when it has tellurium neighbours. We’d been working for 10 years to engineer this kind of behaviour using different kinds of nanostructured materials, but with limited success. Then I saw this paper, and I knew could do the same thing we’d been trying to do with nanostructures, but with this bulk semiconductor instead.”

When Heremans and his team tested the new material, they found that near 450 degrees Fahrenheit, the material converted heat to electricity with an efficiency rating of about 0.75 – close to that of sodium doped telluride -  the previous most efficient material. But as the temperature rose, so did the efficiency of the new material, known as thallium-doped lead telluride. According to the researchers, it peaked at 950 degrees Fahrenheit, with a rating of 1.5.

The high efficiency of this material at between 450 and 950 degrees Celsius is important to the engineers because this is the typical temperature range of power systems such as automobile engines.

Some experts argue that only around 25 per cent of the energy produced by a typical gasoline engine is used to move a car or power its accessories, and nearly 60 per cent is lost through waste heat – with much of this escaping through the exhaust. According to Heremans, a TE device could capture some of that heat and turn it into electricity.

He said: “The material does all the hard work. It produces electrical power just like conventional heat engines, gas or diesel engines that are coupled to electrical generators, but it uses electrons as the working fluids instead of water or gases, and makes electricity directly.”