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Waste
heat is a byproduct of nearly all electrical devices and industrial processes.
Engineering researchers at Rensselaer Polytechnic Institute have developed new
nanomaterials that could lead to techniques for better capturing and putting
this waste heat to work. The key ingredients for making marble-sized pellets of
the new material are aluminum and a common, everyday microwave oven.
Harvesting
electricity from waste heat requires a material that is good at conducting
electricity but poor at conducting heat. One of the most promising candidates
for this job is zinc oxide, a nontoxic, inexpensive material with a high
melting point. While nanoengineering techniques exist for boosting the
electrical conductivity of zinc oxide, the material’s high thermal conductivity
is a roadblock to its effectiveness in collecting and converting waste heat.
Because thermal and electrical conductivity are related properties, it’s very
difficult to decrease one without also diminishing the other.
However,
a team of researchers led by Ganpati Ramanath, professor in the Materials
Science and Engineering Department at Rensselaer,
in collaboration with the University of Wollongong, Australia, have
demonstrated a new way to decrease zinc oxide’s thermal conductivity without
reducing its electrical conductivity. The innovation involves adding minute
amounts of aluminum to zinc oxide, and processing the materials in a microwave
oven. The process is adapted from a technique invented at Rensselaer
by Ramanath, graduate student Rutvik Mehta, and Theo Borca-Tasciuc, associate
professor in the Department of Mechanical, Aerospace, and Nuclear Engineering
(MANE). This work could open the door to new technologies for harvesting waste
heat and creating highly energy efficient cars, aircraft, power plants, and
other systems.
“Harvesting
waste heat is a very attractive proposition, since we can convert the heat into
electricity and use it to power devices—like in a car or a jet—that is creating
the heat in the first place. This would lead to greater efficiency in nearly
everything we do and, ultimately, reduce our dependence on fossil fuels,”
Ramanath says. “We are the first to demonstrate such favorable thermoelectric
properties in bulk-sized high-temperature materials, and we feel that our
discovery will pave the way to new power harvesting devices from waste heat.”
Results
of the study are detailed in the paper “Al-Doped Zinc Oxide Nanocomposites with
Enhanced Thermoelectric Properties,” published in Nano Letters.
To
create the new nanomaterial, researchers added minute quantities of aluminum to
shape-controlled zinc oxide nanocrystals, and heated them in a $40 microwave
oven. Ramanath’s team is able to produce several grams of the nanomaterial in a
matter of few minutes, which is enough to make a device measuring a few
centimeters long. The process is less expensive and more scalable than
conventional methods and is environmentally friendly, Ramanath says. Unlike
many nanomaterials that are fabricated directly onto a substrate or surface,
this new microwave method can produce pellets of nanomaterials that can be
applied to different surfaces. These attributes, together with low thermal
conductivity and high electrical conductivity, are highly suitable for heat
harvesting applications.
“Our discovery could be key to
overcoming major fundamental challenges related to working with thermoelectric
materials,” says project collaborator Borca-Tasciuc. “Moreover, our process is
amenable to scaling for large-scale production. It’s really amazing that a few
atoms of aluminum can conspire to give us thermoelectric properties we’re
interested in.”
