National Center for Computational Sciences

ORNL supercomputer shines light on thermoelectric material

As it turns out, pushing electrons to the side can turn the tailpipe of your car into an electric generator.

A research team led by General Motors physicist Jihui Yang has used Oak Ridge National Laboratory’s (ORNL’s) Jaguar supercomputer to nail down the arrangement of atoms within a promising thermoelectric material. In the process Yang and colleagues from the University of Las Vegas–Nevada, ORNL, and Brookhaven National Laboratory (BNL) have advanced the causes of both materials research and vehicle efficiency. They report their findings in the October 2, 2009, issue of the journal Physical Review Letters.

The material, known by the acronym LAST, is a mixture of lead and tellurium speckled with small clumps of silver and antimony atoms. These clumps—known as nanoprecipitates—subtly alter the flow of electrons and phonons (units of vibrational energy) through the material, allowing it to convert heat energy directly into electricity. A conventional car engine loses 70 percent of the energy it generates to waste heat. Thermoelectric materials used in the car’s exhaust system promise to capture and make use of that energy.

LAST is not new; in fact, it was discovered in the 1950s. But while researchers have long suspected its thermoelectric properties depended on the layout of silver and antimony atoms within the lead-telluride material, they only recently determined that the silver and antimony formed nanoprecipitates rather than blending evenly into the material.

Further, assumptions made for the atomic arrangement within the nanoprecipitates were inaccurate. Yang’s team used results from Jaguar to determine that one of the constituents—silver—sits off to the side from its expected position.

Silver to the side

“Imagine you have a cube,” Yang explained. “You have atoms populating the center, every corner, and the six surfaces. These atoms are alternating lead and tellurium. That’s your basic structure.

“Before our work, people assumed that when you introduce silver-antimony into the material, silver will replace lead, and antimony will replace lead. Our work shows that silver actually will not replace lead. Silver will sit somewhere in the middle of the two adjacent atomic positions. So we pinned down the exact atomic arrangement in the nanoprecipitates and found the results are totally different from what people assumed them to be in the past. We now understand the growth mechanisms of these nanoprecipitates.”

The team conducted its investigation with software known as the Vienna Ab-initio Simulation Package, or VASP. With VASP it was able to calculate the structure of the material directly from the principles of quantum mechanics, testing hundreds of possible structures until it found the one with the lowest energy (the structure it would adopt in nature). Yang and colleagues were able to validate their computational results using transmission electron microscopy (TEM) data from BNL on single-crystal samples. According to Yang instruments at BNL were able to resolve the structure of the material to less than an angstrom, or one ten-billionth of a meter.

Recycling energy saves gas

Thermoelectric materials promise and energy boon because vehicle engines are not very efficient at using the energy contained in gasoline or diesel fuel. The heat that goes out the tailpipe represents lost energy. But thermoelectric materials can move electrons from the hot side of the material to the cold side, where they are converted into electricity.

In a hybrid vehicle, this extra power would be routed back to the electric motor to power the vehicle, while in other vehicles it could be used to run components such as the electric water pump, lights, radio, and global positioning system. The result would be a vehicle with substantially improved fuel economy, potentially saving hundreds of million gallons of fuel each year.

“The material [LAST] is potentially a good thermoelectric material because it has very low thermal conductivity,” Yang said. “Our ultimate goal is to link this structural property to the phonon density of states, the heat-propagating property of the material. Hopefully this work will provide us with a better understanding of why the thermal conductivity of this material is so low and then, maybe, give us some predicting power.”

The project had to simulate an especially large system—more than 1,700 atoms—because the silver-antimony clumps collect in nanoprecipitates and these clumps are relatively few and far between.

“This is a grand challenge for material science,” Yang said, “to determine the material structure at the atomic level of nanostructured materials. On a computational level, for the structural calculations you need to have a huge supercell as part of the density functional theory calculation to be able to fully relax your nanoprecipitates, which are comparable in sizes to those observed under TEM in real samples, so that the energetic calculations are done accurately.”

While it was certainly an accomplishment to work out the atomic structure of this material, the work is not over by any means. Yang’s group is currently investigating the thermal and vibrational properties of LAST, a job that requires even more computing power. Eventually these simulations will help researchers design materials computationally, saving the expense of manufacturing for only those that show the most promise.

–by Leo Williams