The dimensionless zT for thermoelectric materials is calculated by the formula zT= T*(S²σ)/κ), where S is the thermoelectric power or Seebeck coefficient of the TE material, σ and κ are the electrical and thermal conductivities, respectively, and T is the absolute temperature.

Recent progress in increasing the efficiency of thermoelectric materials has primarily involved decreasing κ by using nanomaterials to lower the thermal conductivity by scattering phonons.

Quantum-dot superlattices have reported values of zT >2, and silicon nanowires have such a reduced κ that zT approaches that of commercial materials. Although this certainly provides the evidence that high-zT material can be prepared, the results were obtained on thin films or nanowires that are challenging for high-volume applications that normally rely on bulk materials. Structural complexity on various length scales has successfully reduced κ in bulk TE materials, also yielding zT >1.

Unfortunately, in bulk material at least, there is a lower limit to the lattice thermal conductivity imposed by wave mechanics: The phonon mean free path cannot become shorter than the interatomic distance. The minimum thermal conductivity of PbTe is about 0.35 W/mK at 300 K, a value measured on quantum-dot superlattices. Although lower values have been seen for interfacial heat transfer, progress beyond this point in bulk materials must come from the numerator [of the equation] and in particular the Seebeck coefficient; we describe here a successful approach in this direction for bulk materials.

—Heremans 2008

For the new material, the researchers left out the nanostructures, and instead focused on how to convert the maximum amount of heat that was trapped in the material naturally.

A 2006 paper (S. Ahmad et al.) published in the journal Physical Review Letters suggested that elements such as thallium and tellurium could interact on a quantum-mechanical level to create a resonance between the thallium electrons and those in the host lead telluride thermoelectric material, depending on the bonds between the atoms.

It comes down to a peculiar behavior of an electron in a thallium atom when it has tellurium neighbors. We’d been working for 10 years to engineer this kind of behavior using different kinds of nanostructured materials, but with limited success. Then I saw this paper, and I knew we could do the same thing we’d been trying to do with nanostructures, but with this bulk semiconductor instead.

—Joseph Heremans

Heremans designed the new material with Vladimir Jovovic, who did this work for his doctoral thesis in the Department of Mechanical Engineering at Ohio State. Researchers at Osaka University—Ken Kurosaki, Anek Charoenphakdee, and Shinsuke Yamanaka—created samples of the material for testing. Then researchers at the California Institute of Technology—G. Jeffrey Snyder, Eric S. Toberer, and Ali Saramat—tested the material at high temperatures. Heremans and Jovovic tested it at low temperatures and provided experimental proof that the physical mechanism they postulated was indeed at work.

The team found that near 450° F, the material converted heat to electricity with a zT of about 0.75&madsh;close to that of sodium doped telluride—but as the temperature rose, so did the efficiency of the new material. It peaked at 950° F with a zT of 1.5. Heremans’ team is continuing to work on this patent-pending technology, and is targeting boosting zT by another factor of two.

We anticipate that deliberately engineered impurity-induced band-structure distortions will be a generally applicable route to enhanced S and zT in all TE materials. We are optimistic about the commercial use of such PbTe-based materials because there is an extensive knowledge base among the manufacturers of thermoelectric generators about the assembly of PbTe-based devices, in particular the ability to make stable metallic contacts with low thermal and electrical resistance.

—Heremans 2008

The research was funded by the BSST Corporation; the State of Ohio Department of Development’s Center for Photovoltaic Innovation and Commercialization at Ohio State University; the Beckman Institute; the Swedish Bengt Lundqvist Minne Foundation; and NASA’s Jet Propulsion Laboratory.

Resources

• Joseph P. Heremans, Vladimir Jovovic, Eric S. Toberer, Ali Saramat, Ken Kurosaki, Anek Charoenphakdee, Shinsuke Yamanaka, G. Jeffrey Snyder (2008) Enhancement of Thermoelectric Efficiency in PbTe by Distortion of the Electronic Density of States. Science 25 July 2008: Vol. 321. no. 5888, pp. 554 - 557 doi: 10.1126/science.1159725

• S. Ahmad et al. (2006) Ab Initio Study of Deep Defect States in Narrow Band-Gap Semiconductors: Group III Impurities in PbTe. Phys. Rev. Lett. 96, 056403 doi: 10.1103/PhysRevLett.96.056403