Neutron scattering experiments performed at ORNL show that lead telluride exhibits a strong anharmonic coupling between its optical and acoustic lattice vibrations, with a drop in thermal conductivity resembling a waterfall in this data image. This newly discovered coupling helps explain the low thermal conductivity that makes lead telluride a promising material for thermoelectric devices. Image: Oak Ridge National Laboratory |
Neutron analysis of the atomic dynamics
behind thermal conductivity is helping scientists at the Department of Energy’s
Oak Ridge National Laboratory gain a deeper understanding of how thermoelectric
materials work. The analysis could spur the development of a broader range of
products with the capability to transform heat to electricity.
Researchers performed experiments at both of
ORNL’s neutron facilities—the Spallation Neutron Source and the High Flux
Isotope Reactor—to learn why the material lead telluride, which has a similar
molecular structure to common table salt, has very low thermal conductivity, or
heat loss—a property that makes lead telluride a compelling thermoelectric
material.
“The microscopic origin of the low
thermal conductivity is not well understood. Once we do understand it better we
can design materials that perform better at converting heat to
electricity,” said Olivier Delaire, a researcher and Clifford Shull Fellow
in ORNL’s Neutron Sciences Directorate.
Delaire’s research, reported in Nature Materials, shows that an unusual
coupling of phonons is responsible for the disruption of the dynamics that
transport the thermal energy in lead telluride.
In typical crystalline materials, which have
a lattice-like atomic structure, the conduction of heat is enhanced by the
propagation of phonons along the lattice. The atoms conduct heat by vibrating
in a chain, similar to vibrations propagating along a spring.
Delaire’s team determined through analysis
at the SNS that lead telluride, although having the same crystal lattice as
rock salt, exhibits a strong coupling of phonons, which results in a disruption
of the lattice effect and cancels the ability to conduct heat.
“The resolution of the SNS’s Cold
Neutron Chopper Spectrometer, along with the high flux, have been quite
important to making these time of flight measurements,” Delaire said.
By controlling thermal conductivity in
thermoelectrics, less energy is dispersed and more heat can be directed to
power generation. Today, thermoelectric materials are used to power the
deep-space probes that explore the outer planets and solar system. Cruising beyond
the range of solar collectors, the crafts’ reactor thermoelectric generators
use heat from decaying radioisotopes to generate power.
New, advanced thermoelectric
materials could be used to develop more earthly applications, such as vehicle
exhaust systems that convert exhaust heat into electricity, reducing the need
for alternators. New thermoelectric materials could also help concentrate solar
energy for power generation and recover waste heat for industrial processes.
