Employing some of the world's most powerful supercomputers, scientists at
Lawrence Berkeley National Laboratory
have shown that mismatched alloys are a good match for the future development
of high performance thermoelectric devices. Thermoelectrics hold enormous potential
for green energy production because of their ability to convert heat into electricity.
Contour plots showing electronic density of states in HMAs created from zinc selenide by the addition of (a) 3.125-percent oxygen atoms, and (b) 6.25 percent oxygen. The zinc and selenium atoms are shown in light blue and orange. Oxygen atoms (dark blue) are surrounded by high electronic density regions. (Image provided by Junqiao Wu)
Computations performed on “Franklin,” a Cray XT4 massively parallel
processing system operated by the National Energy Research Scientific Computing
Center (NERSC), showed that the introduction of oxygen impurities into a unique
class of semiconductors known as highly mismatched alloys (HMAs) can substantially
enhance the thermoelectric performance of these materials without the customary
degradation in electric conductivity.
“We are predicting a range of inexpensive, abundant, non-toxic materials
in which the band structure can be widely tuned for maximal thermoelectric efficiency,”
says Junqiao Wu, a physicist with Berkeley Lab’s Materials Sciences Division
and a professor with UC Berkeley’s Department of Materials Science and
Engineering who led this research.
“Specifically, we’ve shown that the hybridization of electronic
wave functions of alloy constituents in HMAs makes it possible to enhance thermopower
without much reduction of electric conductivity, which is not the case for conventional
thermoelectric materials,” he says.
Collaborating with Wu on this work were Joo-Hyoung Lee and Jeffrey Grossman,
both now at the Massachusetts Institute of Technology. The team published a
paper on these results in Physical Review Letters titled, “Enhancing the
Thermoelectric Power Factor with Highly Mismatched Isoelectronic Doping.”
Seebeck Effect and Green Energy
In 1821, the German-Estonian physicist Thomas Johann Seebeck observed that
a temperature difference between two ends of a metal bar created an electrical
current in between, with the voltage being directly proportional to the temperature
difference. This phenomenon became known as the Seebeck thermoelectric effect
and it holds great promise for capturing and converting into electricity some
of the vast amounts of heat now being lost in the turbine-driven production
of electrical power. For this lost heat to be reclaimed, however, thermoelectric
efficiency must be significantly improved.
“Good thermoelectric materials should have high thermopower, high electric
conductivity, and low thermal conductivity,” says Wu. “Enhancement
in thermoelectric performance can be achieved by reducing thermal conductivity
through nanostructuring. However, increasing performance by increasing thermopower
has proven difficult because an increase in thermopower has typically come at
the cost of a decrease in electric conductivity.”
To get around this conundrum, Wu and his colleagues turned to HMAs, an unusual
new class of materials whose development has been led by another physicist with
Berkeley Lab’s Materials Sciences Division, Wladyslaw Walukiewicz. HMAs
are formed from alloys that are highly mismatched in terms of electronegativity,
which is a measurement of their ability to attract electrons. The partial replacement
of anions with highly electronegative isoelectronic ions makes it possible to
fabricate HMAs whose properties can be dramatically altered with only a small
amount of doping. Anions are negatively charged atoms and isoelectronic ions
are different elements that have identical electronic configurations.
“In HMAs, the hybridization between extended states of the majority
component and localized states of the minority component results in a strong
band restructuring, leading to peaks in the electronic density of states and
new sub bands in the original band structure,” Wu says. “Owing to
the extended states hybridized into these sub bands, high electric conductivity
is largely maintained in spite of alloy scattering.”
In their theoretical work, Wu and his colleagues discovered that this type
of electronic structure engineering can be greatly beneficial for thermoelectricity.
Working with the semiconductor zinc selenide, they simulated the introduction
of two dilute concentrations of oxygen atoms (3.125 and 6.25 percent respectively)
to create model HMAs. In both cases, the oxygen impurities were shown to induce
peaks in the electronic density of states above the conduction band minimum.
It was also shown that charge densities near the density of state peaks were
substantially attracted toward the highly electronegative oxygen atoms.
Wu and his colleagues found that for each of the simulation scenarios, the
impurity-induced peaks in the electronic density of states resulted in a “sharp
increase” of both thermopower and electric conductivity compared to oxygen-free
zinc selenide. The increases were by factors of 30 and 180 respectively.
“Furthermore, this effect is found to be absent when the impurity electronegativity
matches the host that it substitutes,” Wu says. “These results suggest
that highly electronegativity-mismatched alloys can be designed for high performance
thermoelectric applications.”
Wu and his research group are now working to actually synthesize HMAs for physical
testing in the laboratory. In addition to capturing energy that is now being
wasted, Wu believes that HMA-based thermoelectrics can also be used for solid
state cooling, in which a thermoelectric device is used to cool other devices
or materials.
“Thermoelectric coolers have advantages over conventional refrigeration
technology in that they have no moving parts, need little maintenance, and work
at a much smaller spatial scale,” Wu says.
This project was supported under Berkeley Lab’s Laboratory Directed Research
and Development Program.