Researchers recently invented a new type of compounds, called thermoelectric materials, which could make internal combustion engines and other devices that lose most of the energy they produce through heat more efficient by directly converting the extra amount of thermal energy into electricity.

Thermoelectric materials such as sodium-doped lead telluride alloys have been commercially available for some time, but the efficiency rate was only 0.71. The newly invented material however, thallium-doped lead telluride, has a rating of 1.5, at temperatures between 232 and 510 degrees Celsius, which, as it turns out, is also the temperature range reached by internal combustion engines during operation.

"The material does all the work. It produces electrical power just like conventional heat engines - steam 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. Thermoelectrics are also very small. I like to say that TE converters compare to other heat engines like the transistor compares to the vacuum tube," said Joseph Heremans of the Ohio Eminent Scholar in Nanotechnology at the Ohio State University, leader of the project.

Usually when designing thermoelectric materials, researchers approach a strategy in which the compound's energy conversion capability is achieved by minimizing the heat loss through the material. However, the Ohio team, which had previously worked with nanomaterials in similar directions, decided to abandon the quest for low thermal conductivity and concentrate on how to efficiently convert the heat delivered to the material.

In order to do so, Heremans' team had to back up on some of their early work that suggested that electrons of thallium and tellurium experience quantum mechanical interactions, allowing the electrons of thallium to resonate with those of the lead telluride.

"It comes down to a peculiar behavior of an electron in a thallium atom when it has tellurium neighbors. We'd been working for 10 years to engineer this kind of behavior using different kinds of nanostructured materials, but with limited success. Then I saw this paper, and I knew we could do the same thing we'd been trying to do with nanostructures, but with this bulk semiconductor instead," he said.

With the help of Vladimir Jovovic of the Department of Mechanical Engineering at Ohio State University, Heremans designed the new material which was then created at the Osaka University and put through a series of test to see whether or not predictions were confirmed by experiment. The results of the tests showed that indeed the predicted physical mechanisms were present, giving the materials a rate of efficiency of 0.75 at 232 degrees Celsius and a rating of 1.5 at 510 Celsius.

"We hope to go much further. I think it should be quite possible to apply other lessons learned from thermoelectric nanotechnology to boost the rating by another factor of two - that's what we're shooting for now," Heremans said.