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October 2012

The Difference One Extra Selenium Makes

Complex computations show why less is more when moving heat around

Wenbo Yan

The chemical and stoichiometric composition of Cu3SbSe3 and Cu3SbSe4 are quite similar, but the crystal structures, used to carry out the complex calculations, are different.

To develop materials that can turn heat into electricity, scientists at the Center for Revolutionary Materials for Solid State Energy Conversions (RMSSEC) used powerful theories and computers to explain why seemingly similar compounds have completely different abilities to conduct heat. Previous experimental results show that the copper-antimony-selenium material, shorthanded Cu3SbSe3, is a good candidate, while its counterpart, Cu3SbSe4, is not. Addressing this abnormal phenomenon, team members specialize in complex calculations − density functional theory (DFT) calculations − and demonstrate that how the atoms move in their crystal structure is strikingly different.

Thermoelectrics Are Power Engines: Thermoelectric devices convert waste heat into electricity or convert electrical power to thermal energy for cooling or heating, and are increasingly seen as having the potential for reducing greenhouse gas emissions. Scientists in the center also believe it will provide cleaner forms of energy and become a substitute for fossil fuels. At the heart of a thermoelectric device are two types of semiconductors arranged in series. The efficiency of the device can be improved either by increasing the electrical conductivity or by decreasing thermal conductivity. However, in classical semiconductors, the electrical and thermal conductivity are linked such that increasing one also increases the other. Thermal conductivity is the property of a material’s ability to transmit heat.

Using Theory to Learn Why the Materials Are Different: The chemical and stoichiometric composition of Cu3SbSe3 and Cu3SbSe4 are quite similar, but the crystal structures, used to carry out DFT calculations, are different: orthorhombic and zinc-blende-based tetragonal, respectively, with different lattice parameters. The difference in crystal structures creates different chemical environments for each atom, and as a result the thermal conductivity is affected. The DFT results showed that the structure of Cu3SbSe3 is more complicated than that of Cu3SbSe4, which would suggest a lower thermal conductivity.

To calculate the thermal conductivity of Cu-Sb-Se compounds, the lattice vibrations, also known as phonons, inside the lattice must be understood, because the heat is carried out by phonons. By applying DFT calculations to the two Cu-Sb-Se compounds, researchers observed opposite phonon behaviors. The reason is that the chemical state of Sb in Cu3SbSe3 and Cu3SbSe4 is different, +3 and +5, respectively, which means that there is a lone pair of electrons around the Sb site in Cu3SbSe3. The electrostatic repulsion between the lone pair of electrons significantly weakens the atomic vibrations in Cu3SbSe3, lowering its thermal conductivity, relative to Cu3SbSe4.

With the help of DFT calculations, scientists now have a proven model and better insights to design new types of thermoelectric semiconductors with intrinsically low thermal conductivity.

Acknowledgments

The RMSSEC EFRC: Center for Revolutionary Materials for Solid State Energy Conversions, an Energy Frontier Research Center, funded this work. The center is funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Zhang Y, E Skoug, J Cain, V Ozolins, D Morelli, and C Wolverton. 2012. "First-Principles Description of Anomalously Low Lattice Thermal Conductivity in Thermoelectric Cu-Sb-Se Ternary Semiconductors." Physical Review B 85:054306-1-054306-6. DOI: 10.1103/PhysRevB.85.054306

About the author(s):

  • Wenbo Yan is a Ph.D. student at University of California-Irvine, a member of the Center for Nanostructures for Electrical Energy Storage based in University of Maryland at College Park. She is interested in making mesoporous manganese oxide nanowires for supercapacitors and lithium batteries.

Understanding Why Materials Behave Differently

Computer study shows adding selenium rearranges antimony’s electrons

Creating mass-market devices that turn wasted heat into needed electricity could lower the nation’s need for imported oil. Increased efficiency demands tailored materials that transmit electrons, but not heat. To design such materials, scientists need to know how rearranging atoms influences the conducting heat. So, scientists used theoretical calculations to show why two materials with similar mixtures of selenium and antimony, along with copper, conducted heat differently. With three selenium atoms present, the material conducts heat poorly. When another selenium atom is added, the material conducts heat. The three-selenium material’s extra pair of electrons quiets the metal’s vibrations, lowering its thermal conductivity compared to the four-selenium material. This study validated the conductivity model for selenium-based materials aiding in the design of poorly conducting materials. The Center for Revolutionary Materials for Solid State Energy Conversions, led by Michigan State University, conducted this work.

More Information

Zhang Y, E Skoug, J Cain, V Ozolins, D Morelli, and C Wolverton. 2012. "First-Principles Description of Anomalously Low Lattice Thermal Conductivity in Thermoelectric Cu-Sb-Se Ternary Semiconductors." Physical Review B 85:054306-1-054306-6. DOI: 10.1103/PhysRevB.85.054306

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.