Among techniques used to probe the structure and dynamics of materials, neutrons have clear advantages when it comes to characterizing thermal transport and phase stability. Thermal neutrons have energies comparable to thermal excitations while simultaneously having wavelengths comparable to the spacing between atoms. This allows researchers to fully characterize the detailed structure of the quantized atomic vibrations (phonons), which transport heat, contain most of the entropy, and play a mechanistic role in some important phase transformations in materials.
Characterization of the lattice vibrations provides critical guidance for understanding a wide range of problems of practical importance. For example, understanding the phase stability of nuclear energy materials is important for making long-term predictions for waste storage, and understanding the thermal transport in nuclear fuels is essential to predicting the operational efficiency and safety of nuclear reactors. Another example is the direct conversion of heat into electrical energy in thermoelectric materials where minimizing the thermal conductivity contributed by the phonons is often the limiting factor in improving efficiency. Finally, the direct conversion between electrical and mechanical forms of energy is facilitated by a large electromechanical coupling in relaxor ferroelectric materials – a property derived from a poorly understood interplay between a soft-phonon instability and disorder. Hence, lattice vibrations play a central role in electromechanical, thermoelectric, or nuclear energy conversion in materials. ORNL actively works on these and related areas employing advanced neutron scattering and modeling techniques to establish a solid scientific understanding of energy materials.
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