The Power of Plastic

Winter 2016

Howard KatzCars and computers spew a lot of heat into the atmosphere. A family of materials known as thermoelectrics can turn that waste heat into electricity. But thermoelectric materials developed to date have been expensive and contain rare or toxic elements.

So Howard Katz, professor of materials science and engineering, is developing plastic thermoelectrics instead. Because they are flexible, nontoxic, and printable, plastics would solve the cost barriers, opening the door to cheap thermoelectric devices that could power gadgets and medical implants.

Plastic thermoelectrics so far have been inefficient at converting heat into electricity, partly because they need to be assembled in positive and negative current pairs. But now, Katz and his research team have come up with a new plastic formulation that is 10 times more efficient than reported options for the negative plastics. The finding, reported in the June 2015 issue of Advanced Science, brings plastic thermoelectrics a step closer to practical application.

Thermoelectrics generate electric current because of a temperature difference across the material, and thus rely on the flow of electrons from a warmer area to a cooler one. The better the material conducts electrons, the higher its thermoelectric efficiency.

Katz’s team made the new material by adding tin chloride into a polymer solution that is deposited on glass. As the solution dries and hardens into plastic, the tin chloride forms a network of tiny, thin, long crystals spread out through the material. The overlapping crystals offer an easy path for electrons to flow, upping the new material’s conductivity and voltage—increasing its efficiency.

With the improvement, the plastic’s efficiency is about one-tenth that of the leading commercial thermoelectric, Katz says. By adding other elements, such as sulfur and aluminum, and by growing precisely shaped nanostructures, he says it should be possible to further increase efficiency.

Plastic thermoelectrics couldn’t be used at very high temperatures because they would melt, Katz says. But there are many places they could harness waste. “Our body temperature is slightly higher than that of ambient air,” he says. “From that temperature difference, you could make a very small amount of electricity to run a medical device so that it wouldn’t need a battery.”