In a world of materials that normally expand on heating, one that contracts along one 3D axis while expanding along another stands out. This is especially true when the unusual shrinkage relates to a property important to thermoelectric devices, which convert heat to electricity or electricity to heat.
In an article just published in the journal Advanced materialsa team of scientists from Northwestern University and the U.S. Department of Energy’s Brookhaven National Laboratory describe the previously hidden sub-nano-scale origins of the unusual shrinkage and exceptional thermoelectric properties of this material, iron telluride silver and gallium (AgGaTe2). The discovery reveals a quantum mechanical twist on what drives the emergence of these properties – and opens up a whole new direction for the search for new high-performance thermoelectrics.
“Thermoelectric materials will transform green and sustainable energy technologies for thermal energy harvesting and cooling, but only if their performance can be improved,” said Hongyao Xie, postdoctoral researcher at Northwestern and first author of the paper. “We want to find the underlying design principles that will allow us to optimize the performance of these materials,” Xie said.
Thermoelectric devices are currently used in limited niche applications, including NASA’s Mars rover, where heat from the radioactive decay of plutonium is converted into electricity. Future applications could include voltage-controlled materials to achieve very stable temperatures critical for the operation of optical detectors and high-tech lasers.
The main obstacle to wider adoption is the need for materials with just the right cocktail of properties, including good electrical conductivity but resistance to heat flow.
“The problem is that these desirable properties tend to compete with each other,” said Mercouri Kanadzidis, the North West professor who initiated the study. “In most materials, electronic conductivity and thermal conductivity are coupled and both are high or low. Very few materials have the special high-low combination.”
Under certain conditions, silver gallium telluride appears to have what it takes: highly mobile conductive electrons and ultra-low thermal conductivity. In fact, its thermal conductivity is significantly lower than suggested by theoretical calculations and comparisons with similar materials such as copper gallium telluride.
Northwestern scientists turned to colleagues and tools at Brookhaven Lab to find out why.
“It took meticulous X-ray examination at Brookhaven’s National Synchrotron Light Source II (NSLS-II) to reveal a previously hidden sub-nanoscopic distortion in the positions of the silver atoms in this material,” said Emil. Bozin, a physicist from the Brookhaven laboratory. structural analysis.
Computer modeling has revealed how these distortions trigger the crystal to shrink along one axis – and how this structural change disperses atomic vibrations, thereby blocking the propagation of heat through the material.
But even with this understanding, there was no clear explanation of what was causing the sub-nano scale distortions. Complementary computer modeling by Northwestern professor Christopher Wolverton indicated a new and subtle quantum mechanical origin for the effect.
Together, the results indicate a new mechanism for reducing thermal conductivity and a new guiding principle in the search for better thermoelectric materials.
Mapping of atomic positions
The team used X-rays on the Pair Distribution Function (PDF) beamline of NSLS-II to map the “large” scale arrangement of atoms in copper gallium telluride and telluride of silver gallium over a range of temperatures to see if they could figure out why these two materials behave differently.
“A stream of hot air heats the sample with degree-by-degree precision,” said PDF beamline lead scientist Milinda Abeykoon. “At each temperature, when X-rays bounce off atoms, they produce patterns that can be translated into high spatial resolution measurements of the distances between each atom and its neighbors (each pair). Computers then stitch the measurements together in the most probable 3D arrangements of atoms.”
The team also made additional measurements over a wider temperature range but at a lower resolution using the light source at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. And they extrapolated their results to a temperature of absolute zero, the coldest that can happen.
The data shows that both materials have a diamond-like tetragonal structure of corner-connected tetrahedra, one with a single copper atom and the other with silver in the center of the tetrahedral cavity of the 3D object . Describing what happened when these diamond-like crystals were heated, Bozin said, “We immediately saw a big difference between the silver and copper versions of the material.”
The crystal with copper at its center expanded in all directions, but the one containing silver expanded along one axis while contraction along another.
“This strange behavior turned out to have its origin in the silver atoms of this material having a very large amplitude and disordered vibrations in the structural layers,” said Simon Billinge, a professor at Columbia University with a nomination. joint as a physicist at Brookhaven. “These vibrations cause the bound tetrahedrons to shake and jump with great amplitude,” he said.
It was a hint that symmetry – the regular arrangement of atoms – could be “broken” or disturbed on a more “local” (smaller) scale.
The team turned to computer modeling to see how the various local symmetry distortions of the silver atoms would fit their data.
“The one that worked best showed that the silver atom is off-center in the tetrahedron in one of four directions, toward the edge of the crystal formed by two of the tellurium atoms,” Bozin said. On average, the random and off-center shifts cancel each other out, so the overall tetragonal symmetry is retained.
“But we know that the larger-scale structure also changes, shrinking in one direction,” he noted. “It turns out that local and larger-scale distortions are linked.”
“Local distortions are not completely random,” Bozin explained. “They are correlated between adjacent silver atoms – those connected to the same tellurium atom. These local distortions cause adjacent tetrahedra to rotate relative to each other, and this twist causes the crystal lattice to shrink in one direction.”
As the moving silver atoms twist the crystal, they also scatter certain wave-like vibrations, called phonons, which allow heat to travel through the lattice. AgGate diffusing2The energy-carrying phonons prevent the propagation of heat, greatly reducing the thermal conductivity of the material.
But why do the silver atoms move in the first place?
Brookhaven scientists had observed similar behavior a decade earlier, in a material similar to lead telluride. In this case, when the material was heated, “lone pairs” of electrons were formed, generating tiny areas of split electrical charge, called dipoles. These dipoles moved the centrally located lead atoms and scattered the phonons.
“But in silver gallium telluride there are no lone pairs. So there must be something else in this material – and probably other ‘diamondoid’ structures,” said Bozin.
Bonding behavior in bending
Calculations by Christopher Wolverton at Northwestern revealed that “something else” was the bonding characteristics of electrons orbiting silver atoms.
“These calculations compared silver and copper atoms and revealed that there is a difference in the arrangement of electrons in the orbitals, such that silver tends to form weaker bonds than copper,” said Northwestern’s Xie. “Silver wants to bond with fewer neighboring tellurium atoms; it wants a simpler binding environment.”
Thus, instead of bonding equally to the surrounding four tellurium atoms, as copper does, silver tends to preferentially (but randomly) bond to two of the four. These bonding electrons are what pull the silver atom from the center, triggering the twisting, shrinking, and vibrational changes that eventually reduce the thermal conductivity in AgGaTe2.
“We have stumbled upon a new mechanism by which the network’s thermal conductivity can be reduced,” said Mercouri Kanadzidis of Northwestern. “Perhaps this mechanism can be used to design or research other new materials that have this type of behavior for future high-performance thermoelectrics.”
This research was primarily supported by the DOE Office of Science. NSLS-II is a user facility of the DOE Office of Science.