Billinge Helps Make Thermoelectric Discovery
01/05/2011
The following was originally published on the Columbia Engineering web site.
Prof. Simon Billinge
is part of a team scientists to discover that a class of materials
known to convert heat to electricity and vice versa behaves quite
unexpectedly at the nanoscale in response to changes in temperature. The
discovery — described in the Dec. 17, 2010, issue of Science —
is a new “opposite-direction” phase transition that helps explain the
strong thermoelectric response of these materials. It may also help
scientists identify other useful thermoelectrics, and could further
their application in capturing energy lost as heat, for example, in
automotive and factory exhaust.
The scientists — from Columbia University’s School of Engineering
and Applied Science, the U.S. Department of Energy’s (DOE) Brookhaven
National Laboratory, Argonne National Laboratory, Los Alamos National
Laboratory, Northwestern University, and the Swiss Federal Institute of
Technology — were studying lead chalcogenides (lead paired with
tellurium, selenium, or sulfur) using newly available experimental
techniques and theoretical approaches that allow them to “see” and model
behavior of individual atoms at the nanoscale, or on the order of
billionths of a meter. With those tools they were able to observe subtle
changes in atomic arrangements invisible to conventional probes of
structure.
To understand the phase transition the scientists observed, think
of the everyday response of a gas like steam cooling to form liquid
water, and then freezing to form solid ice. In each case, the atoms
undergo some form of structural rearrangement, explains Billinge, a
physicist at Columbia Engineering and Brookhaven Lab and a lead author
on the Science paper. He is a professor of Materials Science and of
Applied Physics and Applied Mathematics.
“Sometimes, further cooling will lead to further structural
transitions: Atoms in the crystal rearrange or become displaced to lower
the overall symmetry,” Billinge says. The development of such localized
atomic distortions upon cooling is normal, he says. “What we discovered
in lead chalcogenides is the opposite behavior: At the very lowest
temperature, there were no atomic displacements, nothing — but on
warming, displacements appear!”
The techniques the scientists used to observe this nanoscale
atomic action were high-tech versions of x-ray vision, aided by
mathematical and computer analysis of the results. First the lead
materials were made in a purified powder form at Northwestern
University. Then the scientists bombarded the samples with two kinds of
beams — x-rays at the Advanced Photon Source at Argonne and neutrons at
the Lujan Neutron Scattering Center at Los Alamos. Detectors gather
information about how these beams scatter off the sample to produce
diffraction patterns that indicate positions and arrangements of the
atoms. Further mathematical and computational analysis of the data using
computer programs developed at Brookhaven and Columbia allowed the
scientists to model and interpret what was happening at the atomic level
over a range of temperatures.
Brookhaven physicist Emil Bozin, first author on the paper, was
the first to notice the odd behavior in the data, and he worked
tenaciously to prove it was something new and not a data artifact. “If
we had just looked at the average structure, we never would have
observed this effect. Our analysis of atomic pair distribution functions
gives us a much more local view — the distance from one particular atom
to its nearest neighbors — rather than just the average,” Bozin says.
The detailed analysis revealed that, as the material got warmer, these
distances were changing on a tiny scale — about 0.025 nanometers —
indicating that individual atoms were becoming displaced.
The scientists have made an animation to illustrate the emergence
of these displacements upon heating. In it, the displacements are
represented by arrows to indicate the changing orientations of the atoms
as they flip back and forth, or fluctuate, like tiny dipoles. According
to the scientists, it is this random flipping behavior that is key to
the materials’ ability to convert heat into electricity.
“The randomly flipping dipoles impede the movement of heat
through the material in much the same way that it is more difficult to
move through a disorderly wood than an orderly apple orchard where the
trees are lined up in rows,” Billinge says. “This low thermal
conductivity allows a large temperature gradient to be maintained across
the sample, which is crucial to the thermoelectric properties.”
When one side of the material comes in contact with heat — say,
in the exhaust system of a car — the gradient will cause charge carriers
in the thermoelectric material (e.g., electrons) to diffuse from the
hot side to the cold side. Capturing this thermally induced electric
current could put the “waste” heat to use. This research may help
scientists search for other thermoelectric materials with exceptional
properties, since it links the good thermoelectric response to the
existence of fluctuating dipoles.
“Our next step will be searching for new materials that show this
novel phase transition, and finding other structural signatures for
this behavior,” Billinge said. “The new tools that allow us to probe
nanoscale structures are essential to this research.
“Such studies of complex materials at the nanoscale hold the key
to many of the transformative technological breakthroughs we seek to
solve problems in energy, health, and the environment.” This research
was funded by the DOE Office of Science, the Office of Naval Research,
and the National Science Foundation.