Northwestern University researchers have placed nanocrystals of rock salt into lead telluride, creating a thermoelectric material that can harness electricity from heat sources. The material exhibits a high thermoelectric figure of merit that is expected to enable 14 percent of heat waste converted to electricity, a new scientific record.  Chemists, physicists and material scientists at Northwestern collaborated to develop the material.

The results of the study titled “Strained endotaxial nanostructures with high thermoelectric figure of merit” has been published by the journal Nature Chemistry.

Thermoelectric devices offer to capture heat that would otherwise be lost.  The NU press release offers vehicle exhaust systems, industrial processes and equipment and even sunlight as probable sources.

Mercouri Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in The Weinberg College of Arts and Sciences said, “It has been known for 100 years that semiconductors have this property that can harness electricity. To make this an efficient process, all you need is the right material, and we have found a recipe or system to make this material.”

Kanatzidis, co-author of the study, and his team dispersed nanocrystals of rock salt (SrTe) into the material lead telluride (PbTe). Past attempts at this kind of nanoscale inclusion in bulk material have improved the energy conversion efficiency of lead telluride, but the nano inclusions also increased the scattering of electrons, which reduced overall conductivity. In this study, the Northwestern team offers the first example of using nanostructures in lead telluride to reduce electron scattering and increase the energy conversion efficiency of the material.

The study abstract offers a brief technical explanation.  “Nanostructuring in bulk materials dramatically reduces the thermal conductivity but simultaneously increases the charge carrier scattering, which has a detrimental effect on the carrier mobility. We have experimentally achieved concurrent phonon blocking and charge transmitting via the endotaxial placement of nanocrystals in a thermoelectric material host. Endotaxially arranged SrTe nanocrystals at concentrations as low as 2% were incorporated in a PbTe matrix doped with Na2Te. This effectively inhibits the heat flow in the system but does not affect the hole mobility, allowing a large power factor to be achieved. The crystallographic alignment of SrTe and PbTe lattices decouples phonon and electron transport and this allows the system to reach a thermoelectric figure of merit of 1.7 at ~800 K.

Northwestern's Thermoelectric Material with Salt Endotaxially Included. Click image for more information.

Endotaxial growth is a process during which a crystal is formed within a material as the result of deposition of new material.  Greek words endo, meaning ‘within’, and taxis, meaning ‘arrangement’, to conveys the notion of growing a new crystal whose orientation is determined by a crystalline substrate and to distinguish endotaxial growth from polycrystalline and amorphous growths.  Whew, got that?

Vinayak Dravid, professor of materials science and engineering at Northwestern’s McCormick School of Engineering and Applied Science and co-author of the paper explains more plainly, “We can put this material inside of an inexpensive device with a few electrical wires and attach it to something like a light bulb. The device can make the light bulb more efficient by taking the heat it generates and converting part of the heat, 10 to 15 percent, into a more useful energy like electricity.”

Kanatzidis, who also has a joint appointment at the Argonne National Laboratory, says any industry that uses heat to make products could make their system more efficient with the use of this scientific breakthrough.

Dravid looks further down the road saying, “The energy crisis and the environment are two major reasons to be excited about this discovery, but this could just be the beginning. These types of structures may have other implications in the scientific community that we haven’t thought of yet, in areas such as mechanical behavior and improving strength or toughness. Hopefully others will pick up this system and use it.”

Team members in the research and authoring the paper include Kanishka Biswas, postdoctoral researcher at Northwestern; Jiaqing He, research assistant professor of chemistry at Northwestern; Qichun Zhang, assistant professor of materials science and engineering at Nanyang Technical University; and Guoyu Wang and Ctirad Uher of the University of Michigan.

The money came from The Office of Naval Research, the National Science Foundation, the W. M. Keck Foundation and the State of Illinois. The work at the University of Michigan is supported as part of the Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U. S. Department of Energy.

We saw Ctirad Uher just last week in another thermoelectric report. Things are moving fast in the field’s effort to publish and get information out.  While not a major efficiency, these results are early and quite impressive.  A cost to benefit ratio in real applications will tell us what the value actually is.  This effort isn’t offering a need for wildly expensive raw materials. Let’s hope we see something soon.


1 Comment so far

  1. agilandeswari on April 28, 2011 7:26 AM

    i am also interested in this field it is more motivated in nanothermoelectric

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