The control of thermal energy (heat) flowing through a material, expressed as the thermal conductivity of the material, is critically important to many applications. A particular technology that is relevant to concerns over energy consumption and global warming is thermoelectric generators, which contain materials that allow the inevitable waste heat generated in any fuel-burning process to be further converted directly to electricity. Good thermoelectric materials (those that produce the most electricity for a given heat load) are semiconductors that must simultaneously have low thermal conductivity and high electrical conductivity. These two requirements are at odds with one another, and this tension has frustrated the development of highly efficient thermoelectric devices.
This proposal will investigate a new approach to making better thermoelectric materials. The primary concept is that alloys of Ge and Si will be grown into atomically ordered structures. This ordering takes place spontaneously, producing alternating chemical arrangements of Ge and Si atoms that cannot be formed by conventional synthetic approaches. Ordering should keep the electrical conductivity high, since it reduces the scattering of electrical charges compared to a disordered alloy, and it can reduce the thermal conductivity by exploiting a special kind of scattering mechanism, called Umklapp scattering. Simulations predict that Umklapp scattering will be enhanced by chemical ordering, especially at higher temperatures where thermoelectric generators typically operate. This project employs advanced growth and characterization techniques to produce these materials, ascertain their ordering and measure their transport properties. Molecular dynamics simulations of energy flow in ordered structures will be performed in support of the results. The research will lend new insights into how atomic-scale ordering affects both electrical and thermal conduction.
This research will investigate a new materials design strategy for ultimate use in thermoelectric generators. These devices convert waste heat into electricity, but much more efficient thermoelectric materials are needed in order to be cost effective. In addition to thermoelectrics, developing new approaches to controlling thermal conduction in materials is critical to many technologies. An important example is the computer chip, where high-power processors generate tremendous heat that must be efficiently conducted away from the device so that is does not destroy itself. Beyond the technical contributions of this project lies a strong commitment by the investigators to tightly integrate research and education through the experiential and formal training of two graduate students. Additionally, undergraduate students will be recruited from underrepresented groups in engineering. For these students, exposure to the research environment can be career-defining. Both the principal investigators direct and participate in extensive science outreach activities with K-12 schoolchildren and educators. Much of the focus of these effors is on bringing nano to the public, and for this new project a thermoelectric demonstrator module will be developed that will serve as an example of how materials nanostructuring can improve properties important in the drive to reduce fossil fuel consumption.