Negative Thermal Expansion Materials under Pressure: a New Route for High Performance Thermoelectrics

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Thermoelectrics offer an attractive pathway for addressing an important niche in the globally growing landscape of energy demand, since they can convert heat and, in particular waste heat, to electricity. In the past decades, searching for high efficiency thermoelectrics has been guided by the concept of phonon glass electron crystal. Despite remarkable progress have been made under this concept, the energy conversion performance is still well below what is needed for thermoelectrics to compete with traditional electricity producing methods, primarily due to the nature of strong coupling between electronic and phononic transport. This proposal aims at exploring a new area where the electronic and phononic transport can be decoupled, when negative thermal expansion material, pressure, and thermoelectrics meet together.Negative thermal expansion materials have inherent nature that a large number of low frequency acoustic phonons possess strong phonon anharmonicity compared with less flexible materials, leading to very low intrinsic thermal conductivity, which is beneficial for thermoelectrics. However, they have not been extensively studied for thermoelectrics so far, due to the low thermopower or extremely low temperature range that negative thermal expansion normally occurs. Herewith we propose a new concept of applying pressure to enhance the thermoelectric coefficient by taking advantage of the unique feature that the electrical and phononic transport properties can be largely tuned in an opposite way.The overall goal of this project is to advance the fundamental science underlying the electrical and phononic transport properties of some representative negative thermal expansion materials under mechanical compression and rationally performed doping and alloying as novel strategy for high efficiency thermoelectrics. Combined anharmonic lattice dynamics based on first principles and Boltzmann transport equation are proposed as approaches to this end. The result of these investigations is likely to provide a major advancement to rational optimizing of thermoelectrics based on negative thermal expansion materials, with the potential to make a clear contribution to the energy needs of the future.The novelty of this project manifests itself in that, applying pressure to negative thermal expansion materials not only reduces the lattice thermal conductivity by augmenting phonon anharmonicity, but also largely enhances their thermopower and pushes the negative thermal expansion range upward to higher temperatures, which in turn achieves decoupling the electrical and phononic transport in thermoelectrics and results in significant improvement in thermoelectric performance. Such opposing effects have never been realized in traditional methods for optimizing thermoelectric materials, where the electrical and phononic transport is always strongly correlated and acts in unison.