24 July 2008--Engineers in the United States and Japan have found a new technique to improve the efficiency of a thermoelectric energy converter. Instead of decreasing the thermal conductivity, which is normally how efficiency is improved, they increased the electrical conductivity by doping it with the toxin thallium.
”It is for sure a significant development,” says Zhifeng Ren, a physics professor at Boston College who studies thermoelectrics but was not involved in the research. Thermoelectric efficiency, or ZT, is proportional to electrical conductivity, temperature, and thermoelectric power--the voltage produced in a material per degree. ZT is inversely proportional to thermal conductivity. Ren says that until now, no one has been able to produce a significant increase in the thermoelectric power (also called the Seebeck coefficient), and all improvements to ZT are the result of decreasing the thermal conductivity.
The scientists involved in the research--from Ohio State University, Caltech, and Osaka University--sought to increase electrical conductivity by adding an impurity to an existing thermoelectric converter that allows its electrons to pack in more tightly, creating a steeper electrical gradient. The researchers raised the ZT from .71 to 1.5 and say that when combined with techniques to reduce thermal conductivity, the efficiency could increase to over 2. The results were published this week in Science .
One problem with the technique, admits one of its inventors, Joseph Heremans of Ohio State University, is that the materials used are toxic. The converter was made from lead telluride, the standard compound for high-temperature thermoelectrics, and doped with the famously poisonous thallium. So researchers are working on applying the same technique with less-toxic materials.
”Now that we understand the principle, we can apply it to other known classic thermoelectric materials,” says Heremans, a professor of mechanical engineering and physics.
A thermoelectric converter is like a heat engine, Heremans explains. If you have a block of gas and apply a temperature gradient, the gas will become more dense on the cold side. The same holds true for electrons. So a block of electrons will form an electrical gradient because the electrons will become denser on the cold side. This creates a negatively charged cold side, leaving a positive charge on the warmer side. The denser the electrons, the larger the voltage difference between the two sides.
Typically, scientists can increase efficiency by reducing thermal conductivity. Recent research shows that a technique called ”ball milling,” in which the material is ground up into nanosized pieces, boosts ZT by 40 percent. Nanostructures have more surface interactions, which obstruct the movement of heat.
But in this case, Heremans says, they didn't change the thermal conductivity and focused instead on increasing the Seebeck coefficient. The Seebeck coefficient is essentially the voltage difference across a temperature gradient. Heremans says they were able to do this by doping the material with thallium. The thallium increases the number of available energy states in telluride. More energy levels means that more electrons can squeeze in, which increases the voltage. They found the converter worked at a peak ZT of 1.5 at a temperature of around 773 K (500 C).
Gang Chen, a professor in the department of mechanical engineering at MIT, says the thallium work is a good step that will stimulate more work in that direction. But he is concerned about the use of such poisonous materials. ”There is a concern with the lead,” he says. ”We need to look further and see if it can be done with other materials.”
Boston College's Ren is also concerned with the lead, and he thinks that the same technique could be used with another common material called skutterudites, an environmentally friendly material made from cobalt antimony.
Harald Böttner, of Fraunhofer Gesellschaft, in Munich, notes that when combined with telluride, lead is very stable and has been used for years in thermoelectrics. The bigger problem, he says, is the thallium. ”Thallium is one of the most poisoning elements,” he says.
Aside from experimenting with other less-toxic materials, one of the thallium technique's researchers, Jeffrey Snyder, faculty associate in the department of materials science at Caltech, says the thermoelectric efficiency has to be increased even further for it to be economically viable. He says a ZT of 2 is a good goal. ”With that value, I think we can start seeing some real applications,” he says.
He predicts the first application will be in the automotive industry, converting the heat lost from the tailpipe into electricity. Cars lose around two-thirds of their energy in heat. ”If a significant amount of that waste energy can be converted to electricity, we can use that to charge the system,” Snyder says, which will reduce fuel consumption. Snyder says the auto industry is interested in developing the technology.
BMW has built a research vehicle with a thermoelectric generator. Its system, which is integrated within the exhaust pipe, generates around 200 watts of power, but the company hopes to increase that to 1000 W. BMW says the generator could reduce fuel consumption by about 5 percent. The technology is not yet ready for commercial development, nor could BMW predict when a car with a thermoelectric generator would come on the market, but spokesperson Katharina Bölsterl did say it would be at least five years.