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New Approach to Developing Thermoelectric Materials Doubles Efficiency

Heremans
The zT values for Tl0.02Pb0.98Te (black circles) and Tl0.01Pb0.99Te (blue circles) compared to that of a reference sample of Na-PbTe (purple diamonds).Click to enlarge. Source: Heremans 2008.

A team of researchers led by Dr. Joseph Heremans at Ohio State University has developed a new thermoelectric (TE) material with twice the efficiency of TE materials currently on the market. The most efficient commercial material in thermoelectric power generators is sodium-doped lead telluride (Na-PbTe), which has a thermoelectric figure of merit (zT) of 0.71. The new material, thallium-doped lead telluride (Tl-PbTe), has a zT of 1.5.

The new material is most effective between 450° and 950°F—a typical temperature range for power systems such as automobile engines. The application of TE material to automotive waste heat recovery systems is of interest to the research team, and to one of the project funders, BSST Corporation. (Earlier post.)

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

Comments

ToppaTom

This is good. The high electrical conductivity minimizes power dissipation within the TE module and low thermal conductivity minimizes the size of the “radiator” required to dump the heat (which is why it maximizes efficiency).
However I believe the ZT must get above 5 (maybe way above) to make thermoelectrics a main player. As long as the heat is being dumped, and the TE is cheap, it can increase efficiency. But stick some TE material on the back of some high $$ solar cells with high concentration and you loose, due to hotter solar cells.
OK, one step at a time. It should help the quixotic fight against climate change, or, if not, they work better when hot anyway.

GreenPlease

An easy application of this material would be in the exhaust of a vehicle after the catalyst. The waste heat could be used to charge a battery. You could do away with the alternator and, with Mazda's start/stop tech, you could do away with the starter as well.

This would reduce drag on the crank (to zero) and help improve fuel economy.

gr

What's interesting here is the physics of the materials. The quantum effect of electron resonance appears in other materials theory including Randy Mills' below ground state "hydrinos." There seems to be an ability to tune electron resonances to effect neighboring orbits dependent on bonds - used here for increasing TE conductivity and elsewhere to release energy.

Reality Czech
This would reduce drag on the crank (to zero) and help improve fuel economy.
Much more than that. If 10% of the engine exhaust heat (~40% of total fuel energy) can be converted to electricity, the total efficiency of the engine system could increase as much as 40%. The generated power could be thousands of watts, and the only good use for so much after equipment demand is met is to drive the crankshaft.
DaveD

Reality Czech.....I think you have finally asked the right question here (which I can never find in these announcements): What % of heat loss in a system can be converted into electricity? Is it 1%? Is it 10%?

This is an area that is totally new to me, can someone point me to some material that explains how to translate the zT numbers into some kind of real world usefulness?

Also, just out of curiousity....how large would it have to be to get X% or Y%? If it's a lead based material it can't be too light. So if we have to add 100kg to get a 1% engine efficiency payback, I'm not sure it's too interesting. If it's 20kg to get 10%, then it's a very different story.

Don't get me wrong, this is very exciting and I'm sure for some applications it will be wonderful. I'm just trying to understand how it will do in a vehicle.


Thanks.

Alan

See this page
http://nextbigfuture.com/2008/07/high-temperature-thermoelectric-at-zt.html
for a chart on how zT relates to actual thermal effiency. Basically, "The current commercial best ZT figure of 0.7 meant 5-10% recapture of energy from heat at 200-300 degrees temp difference. 1.5 means 12-18% recapture of energy from heat for 300-600K degree temperature differences."

William R. Riley

Thermoelectric material Thallium-doped lead Telluride. what is its maximum teperature and can it be cast or molded? wrrintexas@aol.com

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