ScienceDaily (Mar. 22, 2012) —
In the continual quest for better thermoelectric materials -- which
convert heat into electricity and vice versa -- researchers have
identified a liquid-like compound whose properties give it the potential
to be even more efficient than traditional thermoelectrics.
Thermoelectric materials have been used to power spacecraft ranging
from Apollo to the Curiosity rover now headed for Mars. Recently,
however, scientists and engineers have been turning to these materials
to use wasted heat -- released from automobiles or industrial machinery,
for instance -- as an efficient energy source. They have also proposed
using these materials to create more efficient heating systems in
electric cars or even as new ways to exploit solar power.
In identifying this new type of thermoelectric material, the
researchers studied a material made from copper and selenium. Although
it is physically a solid, it exhibits liquid-like behaviors due to the
way its copper atoms flow through the selenium's crystal lattice.
"It's like a wet sponge," explains Jeff Snyder, a faculty associate
in applied physics and materials science in the Division of Engineering
and Applied Science at the California Institute of Technology (Caltech)
and a member of the research team. "If you have a sponge with very fine
pores in it, it looks and acts like a solid. But inside, the water
molecules are diffusing just as fast as they would if they were a
regular liquid. That's how I imagine this material works. It has a solid
framework of selenium atoms, but the copper atoms are diffusing around
as fast as they would in a liquid."
The research, led by scientists from the Chinese Academy of Science's
Shanghai Institute of Ceramics in collaboration with researchers from
Brookhaven National Laboratory and the University of Michigan, as well
as from Caltech, is described in a paper recently published in the
journal Nature Materials.
A thermoelectric material generates electricity when there is a
temperature difference between one end of the material and the other.
For example, if you place a thermoelectric device right next to a heat
source -- say a laptop battery -- then the side closest to the battery
will be hotter. The electrons in the hot end will diffuse to the cool
end, producing an electric current.
A good thermoelectric material must be good at conducting electricity
but bad at conducting heat. If it were good at conducting heat, the
heat from the hot end would move to the cool end so fast that the whole
material would rapidly reach the same temperature. When that happens,
the electrons stop flowing.
One way to improve thermoelectric efficiency, then, is to decrease a
material's ability to conduct heat. To that end, researchers have been
developing thermoelectric materials with a mix of crystalline and
amorphous properties, Snyder says. A crystalline atomic structure allows
electrons to flow easily, while an amorphous material, such as glass,
has a more irregular atomic structure that hinders heat-carrying
vibrations from traveling.
These heat-carrying vibrations travel via two types of waves. The
first type is a longitudinal or pressure wave, in which the direction of
displacement -- in this case, the jiggling of atoms -- is the same as
the direction of the wave. The second type is a transverse wave, in
which the direction of displacement is perpendicular to the direction of
the wave, like when you shake a jump rope up and down, resulting in
waves that travel horizontally along the rope.
In a solid material, a transverse wave travels because there is
friction between the atoms, meaning that when one atom vibrates up and
down, an adjacent atom moves with it, and the wave propagates. But in a
liquid, there is minimal friction between the atoms, and a vibrating
atom just slides up and down next to its neighbor. As a result,
transverse waves cannot travel inside a liquid. Ocean waves are
different because they have an interface between the liquid and the air.
The team found that because heat-carrying vibrations in a liquid can
travel only via longitudinal waves, a material with liquid-like
properties is less thermally conductive. Therefore, a liquid-like
material that's also good at conducting electrically should be more
thermoelectrically efficient than traditional amorphous materials,
In the case of the copper-selenium material that the researchers
studied, the crystal structure of the selenium helps conduct
electricity, while the free-flowing copper atoms behave like a liquid,
damping down thermal conductivity. The efficiency of a thermoelectric
material is quantified using a number called a "thermoelectric figure of
merit." The copper-selenium material has a thermoelectric figure of
merit of 1.5 at 1000 degrees Kelvin, one of the highest values in any
bulk material, the researchers say.
NASA engineers first used this copper-selenium material roughly 40
years ago for spacecraft design, Snyder says. But its liquid-like
properties -- which were not understood at the time -- made it difficult
to work with. This new research, he says, has identified and explained
why this copper-selenium material has such efficient thermoelectric
properties, potentially opening up a whole new class of liquid-like
thermoelectric materials for investigation.
"Hopefully, the scientific community now has another strategy to work
with when looking for materials with a high thermoelectric figure of
merit," Snyder says.
In addition to Snyder, the research group includes Caltech graduate student Tristan Day. The other authors on the Nature Materials paper,
titled "Copper ion liquid-like thermoelectrics," are Huili Liu, Xun
Shi, Lidong Chen, Fangfang Xu, Linlin Zhang, and Wenqing Zhang of the
Chinese Academy of Science's Shanghai Institute of Ceramics; Qiang Li of
Brookhaven National Laboratory; and Citrad Uher of the University of
Note: Materials may be edited for content and length. For further information, please contact the source cited above.
- Huili Liu, Xun Shi, Fangfang Xu, Linlin Zhang, Wenqing Zhang, Lidong
Chen, Qiang Li, Ctirad Uher, Tristan Day, G. Jeffrey Snyder. Copper ion liquid-like thermoelectrics. Nature Materials, 2012; DOI: 10.1038/nmat3273