Thermoelectric materials can be used to convert waste heat from vehicles, factories, or power plants into electricity. Over the past two decades, researchers have worked to improve the conversion efficiency of these materials by reducing their thermal conductivity. Now new research shows that improving a different operating parameter—power output—can also improve the performance of these materials.

Thermoelectric materials convert phonons, an atomic lattice vibration that carries thermal energy, into electrical charge carriers—electrons or holes depending on the material. Thermal energy lost to conduction lowers the conversion efficiency of the material. As one way to reduce thermal conductivity, researchers have used nanostructured materials—phonon scattering around the edges of the nanostructures reduces the thermal conductivity.

Along with conversion efficiency, power output is also an important parameter for thermoelectric materials. If a thermoelectric material has a high output power density, then less of that material would be needed to provide the same amount of power as compared to a material with a lower output power density.

Zhifeng Ren, at the University of Houston, and his colleagues wondered if they could improve the power output of a p-type NbFeSb thermoelectric material known to have a high power factor, a physical parameter that relates to the amount of power produced per unit area of material, or the output power density. As reported recently in the Proceedings of the National Academy of Sciences, the researchers replaced some of the niobium with titanium, creating Nb_0.95Ti_0.05FeSb, and prepared the material using high pressing temperatures of 1,123 K; 1,173 K; 1,273 K; and 1,373 K. Hot pressing is a processing method where metal powders, when heated under pressure, consolidate to form a solid.

Measuring the physical properties of these hot pressed materials, the researchers noticed that the power factor of the materials increased with the pressing temperature. The material prepared at 1,373 K had a power factor of about 106 µW cm^-1 K^-2, about three times higher than typical thermoelectric materials, Ren says. Two metal thermoelectric materials also have similar power factors to Nb_0.95Ti_0.05FeSb, but the metallic nature of these materials also increases their thermal conductivity, making their conversion efficiency lower than that of Nb_0.95Ti_0.05FeSb, which is based on a semiconducting material.

Ren thinks the high power factor of Nb_0.95Ti_0.05FeSb is due to increased carrier mobility made possible by high pressing temperatures. Using scanning electron microscopy, the researchers noticed that the grain size of each material increased from about 0.3 μm to about 4.5 μm as the pressing temperature increased. The carrier mobility, of the holes in the case of this p-type material, also increased with pressing temperature. Just as nanostructure edges scatter phonons, Ren suspects fewer boundaries with larger grains improved hole mobility in the material.

As a result of the high power factor, Nb_0.95Ti_0.05FeSb, hot pressed at 1,373 K, had a record high output power density of about 22 W cm^-2. Materials with such a high power output could be used to generate kilowatts or megawatts of power, Ren says.

Bao Yang, of the University of Maryland, agrees that this is a record output power density. He appreciates the work of the researchers, adding that materials research on thermoelectrics is challenging because it involves tailoring electrical transport, thermal transport, and the conversion of electrical energy to thermal energy.

Read the abstract in Proceedings of the National Academy of Sciences.