By Josh Perry, Editor
Researchers from Duke University (Durham, N.C.) are using cold neutron scattering techniques from the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tenn. to study the vibrational motion of phonons to better understand how the atoms are scattered in thermoelectric materials and ultimately how to better control them to improve electrical conductivity while minimizing heat transfer.
Tyson Lanigan-Atkins, a Ph. D student at Duke University, uses the cold (lower-energy) neutron triple-axis spectrometer at ORNL’s High Flux Isotope Reactor to study thermoelectric materials. (ORNL/Genevieve Martin)
According to a report from ORNL, the Duke engineers are using neutrons because their energy levels can be tuned to match the lower energy of phonons and give a clearer picture of the movement. Researchers also said that the neutrons enable them to study phonons in more complex environments.
Researchers used a single crystal of lead selenide and investigated its structural phase transition at high temperatures and how this transition impacts its thermal conductivity.
“While conducting their research at the cold neutron triple-axis spectrometer (CTAX) neutron beamline at ORNL’s High Flux Isotope Reactor (HFIR), the scientists needed to align large crystals to within a degree or two of each other,” the article said. “They encountered several engineering challenges in designing their experiment, including developing a sample holder to correctly position the encapsulated crystals within the neutron beam.”
To avoid the sample evaporating under the high-temperature, vacuum conditions in which it was studied, scientists encased the samples in quartz to control the atmosphere around the material.
“Previous efforts to resolve acoustic phonon linewidths below 1.0 milli-electron volt (meV) were not successful due to the resolution limits of the neutron instruments employed,” the article explained. “However, the cold neutrons delivered by the CTAX beamline are well-suited for high-resolution measurement of lattice dynamics in crystalline solids that have a high signal-to-noise ratio, such as thermoelectric materials.”
This process enabled the Duke researchers to get clear data about the heat transport phenomena in thermoelectric materials, which they will use to enhance those properties.