REACH out and feel the bonnet of your idling car, or the back of a running computer, refrigerator or television. You may not be using the warmth you feel, but you certainly are paying for it. Every time you fill up the car with fuel, or draw current from the mains in your house, as much as half of the energy will be lost as heat.
If only there were a reliable way to mop up that wasted heat and turn it into something useful. Well, now there is. Thanks to a new method of controlling the quantum particles known as phonons, power plants, generators and even your car exhaust could soon be turning waste heat into usable electric current.
The technology has taken nearly 200 years to arrive. In 1821, German physicist Thomas Seebeck noticed strange magnetic effects when he heated one side of a metal circuit. The explanation came later. Heat one part of a circuit and electrons in that side become agitated and bounce around. Some spill over into the colder side, making it more negative and creating a voltage difference that can drive a current. The effect is called thermoelectricity.
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The trouble is, metals conduct the heat as well as the electricity, so the cold end of a thermoelectric circuit quickly warms up, and the effect is lost. What is needed is a material that is capable of generating thermoelectricity, but does not conduct heat.
Just as light is made up of photons, heat is made up of phonons. As phonons skip from atom to atom within a solid material they spread heat. So the question is, how do you stop, or at least scatter, the phonons.
In 1996, physicists Mildred Dresselhaus and Lyndon Hicks of the Massachusetts Institute of Technology got a hint of an answer. They were studying a material made of layers of europium lead telluride sandwiched between pure lead telluride. Electrons passing across the layers were barely affected by the difference in materials. But for phonons it was a different story. Phonons are very sensitive to the structure of atomic bonds and, because lead is much heavier than europium, the atomic bonds at the interface of the two surfaces are strained. The interface has a similar effect on the phonons to that of the interface between water and air on photons, making it difficult for phonons to move across the join. The result was that the sandwiched material produced about four times as much thermoelectric power as pure lead telluride. 鈥淚t was not so large but we were showing proof of principle of the concept,鈥� Dresselhaus says.
Dresselhaus and Hicks鈥檚 findings triggered a huge interest in the idea of creating materials in which electrons and phonons travel at different speeds. The efficiency of a thermoelectric material in converting a temperature difference to a voltage is measured roughly by its 鈥渇igure of merit鈥�. In the 1950s, the best thermoelectrics were semiconductors like bismuth telluride with a figure of merit of about 1, which translates into a small percentage of heat energy being converted into electric current. This is enough to make the material useful for some specialist applications 鈥� there are camping lanterns that use thermoelectricity, and Marlow Industries in Dallas, Texas, uses thermoelectrics in reverse, by applying an electric field to produce a temperature difference that cools miniature refrigerators for space missions. But for years, no one could produce a thermoelectric material that scored better than 1. Some people even wondered if there was a limit to how much power you could get from thermoelectricity.
But earlier this year, Mercouri Kanatzidis of Michigan State University in East Lansing and colleagues blew that idea apart. Kanatzidis has created hundreds of alloys in his decade-long search for powerful thermoelectric materials, and now he has one that boasts a stunning score of 2.2 (see Figure). Given a temperature difference of about 600 掳C, less than the difference between a car exhaust pipe鈥檚 temperature and the surrounding air, his new material converts up to 18 per cent of waste heat into electricity. 鈥淭he 18 per cent efficiency is not small potatoes; it is an excellent amount of waste heat that can be recycled. People would kill for this,鈥� Kanatzidis says.
The new alloy, which Kanatzidis has patented, is remarkably simple to make. While Dresselhaus鈥檚 material, and others that have been manufactured since, are made by carefully depositing layers, Kanatzidis鈥檚 is made by melting the right proportions of antimony, lead, silver and tellurium together in a furnace. The ingredients are cooked at around 800 掳C for 4 hours, held at about 400 掳C for a further 40 hours, and then cooled to around room temperature.
But how does it work? Initially Kanatzidis himself was not sure. His team developed the new alloy expecting that mixing lighter and heavier atoms would create changes in bond structure that would affect phonons more than they did electrons. But the new material turned out to be more complicated than this. Pictures taken with an electron microscope showed that the silver and antimony in the alloy form nanoscale islands in a sea of atoms that is much richer in lead and tellurium (see Figure). 鈥淵ou find nanostructures that appear to scatter phonons,鈥� says Arun Majumdar, an expert on thermoelectrics at the University of California at Berkeley. Patches of a different material obstruct the path of phonons, just as happened in Dresselhaus and Hicks鈥檚 layered material.
Other teams are hot on the trail, trying to scatter phonons by creating similar patterns inside materials rather than by layering different materials. George Nolas of the University of South Florida is making crystals in which the lattice structure includes cage-like gaps, which can be filled with impurities that scatter phonons. And Terry Tritt and his colleagues at Clemson University in South Carolina are focusing on pentatellurides, a group of alloys that include five other metals as well as tellurium. Tritt says these appear to have thermoelectric power that goes beyond even Kanatzidis鈥檚 new alloy, although the science behind how they work is not well understood. 鈥淭hey have huge power,鈥� he says.
The US navy is developing Kanatzidis鈥檚 new alloys for use with steam turbines on ships. 鈥淎 module would produce a voltage directly from the temperature difference between heat from the energy source on one side and ambient temperature on the cold side,鈥� says Mihal Gross of the Office of Naval Research in Washington DC. The next step could be heat recovery in cars and power plants. A typical car exhaust might provide enough power to replace the car鈥檚 battery, with some to spare. 鈥淭here is enormous potential for fuel economy improvement,鈥� says a spokesman for General Motors in Detroit, Michigan, which is also developing thermoelectric materials.
It might have taken nearly two centuries, but waste heat could finally stop burning that hole in your pocket.
