While long valued for high-temperature applications, the bulk alloy semiconductor SiGe hasn’t lent itself to broader adoption because of its low thermoelectric performance and the high cost of germanium. A novel nanotechnology design created by researchers from Boston College and MIT has shown a 30 to 40% increase in thermoelectric performance and reduced the amount of costly germanium. Credit: Nano Letters

The
intense interest in harvesting energy from heat sources has led to a
renewed push to discover materials that can more efficiently convert
heat into electricity. Some researchers are finding those gains by
re-designing materials scientists have been working with for years.

   

A
team of Boston College and Massachusetts Institute of Technology
researchers report developing a novel, nanotech design that boosts the
thermoelectric performance of a bulk alloy semiconductor by 30 to 40%
above its previously achieved figure of merit, the measuring stick of
conversion efficiency in thermoelectrics.

   

The
alloy in question, silicon germanium, has been valued for its
performance in high-temperature thermoelectric applications, including
its use in radioisotope thermoelectric generators on NASA flight
missions. But broader applications have been limited because of its low
thermoelectric performance and the high cost of germanium.

   

Boston
College Professor of Physics Zhifeng Ren and graduate researcher Bo Yu,
and MIT Professors Gang Chen and Mildred S. Dresselhause and
post-doctoral researcher Mona Zebarjadi, report in the journal Nano Letters that
altering the design of bulk SiGe with a process borrowed from the
thin-film semiconductor industry helped produce a more than 50% increase
in electrical conductivity.

   

The
process, known as a 3D modulation-doping strategy, succeeded in
creating a solid-state device that achieved a simultaneous reduction in
the thermal conductivity, which combined with conductivity gains to
provide a high figure of merit value of ~1.3 at 900 C.

   

“To
improve a material’s figure of merit is extremely challenging because
all the internal parameters are closely related to each other,” said Yu.
“Once you change one factor, the others may most likely change, leading
to no net improvement. As a result, a more popular trend in this field
of study is to look into new opportunities, or new material systems. Our
study proved that opportunities are still there for the existing
materials, if one could work smartly enough to find some alternative
material designs.”

   

Ren
pointed out that the performance gains the team reported compete with
the state-of-the-art n-type SiGe alloy materials, with a crucial
difference that the team’s design requires the use of 30% less
germanium, which poses a challenge to energy research because of its
high cost. Lowering costs is crucial to new clean energy technologies,
he noted.

   

“Using
30% less germanium is a significant advantage to cut down the
fabrication costs,” said Ren. “We want all the materials we are studying
in the group to help remove cost barriers. This is one of our goals for
everyday research.”

   

The
collaboration between Ren and MIT’s Chen has produced several
breakthroughs in thermoelectric science, particularly in controlling
phonon transport in bulk thermoelectric composite materials. The team’s
research is funded by the Solid State Solar Thermal Energy Conversion
Center.

   

The
3STEC Center is part of the U.S. Department of Energy’s Energy Frontier
Research Center program, which is aimed at advancing fundamental
science and developing materials to harness heat from the sun and
convert the heat into electricity via solid-state thermoelectric and
thermophotovoltaic technologies.

Source: Boston College