Thermoelectric material converts heat waste to electricity
The Mars rover Curiosity is powered by lead telluride thermoelectrics (although it is system has a ZT of only 1, making it half as efficient as Northwestern’s system), and BMW is testing thermoelectrics in its cars by harvesting heat from the exhaust system.
“Now, having a material with a ZT greater than two, we are allowed to really think big, to think outside the box,” Dravid said. “This is an intellectual breakthrough.”
“Improving the ZT never stops — the higher the ZT, the better,” Kanatzidis said. “We would like to design even better materials and reach 2.5 or 3. We continue to have new ideas and are working to better understand the material we have.”
The efficiency of waste heat conversion in thermoelectrics is governed by its figure of merit, or ZT. This number represents a ratio of electrical conductivity and thermoelectric power in the numerator (which need to be high) and thermal conductivity in the denominator (which needs to be low).
“It is hard to increase one without compromising the other,” Dravid said. These contradictory requirements stalled the progress towards a higher ZT for many years, where it was stagnant at a nominal value of 1.
Kanatzidis and Dravid have pushed the ZT higher and higher in recent years by introducing nanostructures in bulk thermoelectrics. In January 2011 they published a report in Nature Chemistry of a thermoelectric material with a ZT of 1.7 at 800 degrees Kelvin. This was the first example of using nanostructures (nanocrystals of rock-salt structured strontium telluride) in lead telluride to reduce electron scattering and increase the energy conversion efficiency of the material.
The performance of the new material reported now in Nature is nearly 30 percent more efficient than its predecessor. The researchers achieved this by scattering a wider spectrum of phonons, across all wavelengths, which is important in reducing thermal conductivity.
“Every time a phonon is scattered the thermal conductivity gets lower, which is what we want for increased efficiency,” Kanatzidis said.
A phonon is a quantum of vibrational energy, and each has a different wavelength. When heat flows through a material, a spectrum of phonons needs to be scattered at different wavelengths (short, intermediate and long).
In this work, the researchers show that all length scales can be optimized for maximum phonon scattering with minor change in electrical conductivity. “We combined three techniques to scatter short, medium and long wavelengths all together in one material, and they all work simultaneously,” Kanatzidis said. “We are the first to scatter all three at once and at the widest spectrum known. We call this a panoscopic approach that goes beyond nanostructuring.”
“It’s a very elegant design,” Dravid said.
In particular, the researchers improved the long-wavelength scattering of phonons by controlling and tailoring the mesoscale architecture of the nanostructured thermoelectric materials. This resulted in the world record of a ZT of 2.2.
The successful approach of integrated all-length-scale scattering of phonons is applicable to all bulk thermoelectric materials, the researchers said.
— Read more in Kanishka Biswas et al., “High-performance bulk thermoelectrics with all-scale hierarchical architectures,” Nature 489 (20 September 2012): 414–18 (doi:10.1038/nature11439); and Kanishka Biswas et al., Strained endotaxial nanostructures with high thermoelectric figure of merit,” Nature Chemistry 3 (16 January 2011): 160–66 (doi:10.1038/nchem.955)
