Researchers
from Boston College, MIT, Clemson University and the University of
Virginia have used nanotechnology to achieve a 60-90 percent increase in
the thermoelectric figure of merit of p-type half-Heusler, a common
bulk semiconductor compound, the team reported in the journal Nano
Letters.
The
dramatic increase in the figure of merit, used to measure a material’s
relative thermoelectric performance, could pave the way for a new
generation of products – from car exhaust systems and power plants to
solar power technology – that that runs cleaner, according to co-author
Xiao Yan, a researcher in the Department of Physics at Boston College.
The
team registered improvement in half-Heusler, which has been under study
for its thermal stability, mechanical sturdiness, non-toxicity and low
cost. However, the application of half-Heusler has been limited because
of its poor thermoelectric performance: it previously registered a peak
figure of merit of approximately 0.5 at 700 oC for bulk ingots.
Yan,
working with BC Professor of Physics Zhifeng Ren and MIT’s Soderberg
Professor of Power Engineering Gang Chen, have increased the figure of
merit value of p-type half-Heusler to 0.8 at 700 degrees C. Moreover,
the groups’ material preparation methods proved to save time and expense
compared with conventional methods.
“This
method is low cost and can be scaled for mass production,” Ren said.
“This represents an exciting opportunity to improve the performance of
thermoelectric materials in a cost-effective manner.”
The
researchers obtained their results by first forming alloyed ingots
using arc melting technique and then creating nanoscale powders by ball
milling the ingots and finally obtaining dense bulk by hot pressing.
Transport property measurements together with microstructure studies on
the nanostructured samples, in comparison with that of bulk ingots,
showed that the thermoelectric performance improves largely because of
low thermal conductivity produced by enhanced phonon scattering at grain
boundaries and defects in the material. The material was also found to
have a high Seebeck coefficient, a measure of thermoelectric power.
Researchers
in the BC and MIT labs are still trying to prevent grain growth during
press, which accounts for the still large thermal conductivity of
half-Heusler.
“Even
lower thermal conductivity and improved thermoelectric performance can
be expected when average grain sizes are made smaller than 100 nm,” said
Ren, who was joined on the team by fellow Boston College researchers
Giri Joshi, Weishu Liu, Yucheng Lan and Hui Wang, MIT’s Sangyeop Lee,
Virginia’s Rogers Professor of Physics Joe Poons and J.W. Simonson and
Clemson Professor of Physics Terry M. Tritt.
