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 The microwave oven - used to nuke leftovers and create breakthrough
thermoelectric materials. Wait… what?
Researchers at Rensselaer Polytechnic Institute (RPI) have
developed a process to create new thermoelectric materials using nanoscale
sulfur and an everyday microwave oven. Ganpati Ramanath, the RPI professor who
led the study, believes the new process could open the door to a completely new
age of cooling and heat capture technology.
The Science
 Thermoelectrics are materials that capture and convert heat
to electricity, and (vice versa). On an atomic level, this thermoelectric
effect occurs due to the diffusion of charge carriers in a substance induced by
a temperature gradient. In other words, charges move through a thermoelectric
material from the hot side to the cold side, generating an electric voltage.
The diagram to the right describes the layout of a
thermoelectric module acting as a generator. N-type and P-type thermoelectric
materials are sandwiched between layers of a ceramic substrate, arranged in a
way that determines the direction of current flow in the device. Applied heat
causes charges to flow through the materials, creating a current which flows
from the conductive materials into wires to generate electricity. Here is what
they look like on the outside:
Where Thermoelectrics
Stand Today
 What makes thermoelectrics so intriguing is that they
provide a means to capture heat generated from inefficient processes that would
otherwise go to waste. For example, thermoelectrics are used in the form of
automotive thermoelectric generators (ATEGs, see right image. Image Credit: thinkprogress.org) in cars
to increase fuel economy by capturing a portion of the energy sent to the car's
exhaust (40% of an internal combustion engine's total produced energy). But
they could be used in everything from power generation to refrigeration and
microprocessor cooling.
The problem with thermoelectrics is that it was impossible
for them have both low cost and high efficiency, making them suitable only for niche
applications. One of the reasons is that, up until now, researchers have only
been able to make large quantities of high efficiency p-type materials, but quality
materials of both types are needed to make an efficient thermoelectric device. RPI
says that its new process has overcome these shortcomings.
 A Bright Future
RPI's new process is driven by the idea of doping
nanostructured thermoelectric materials with tiny amounts of sulfur. Cooking
the doped solution in a commercial microwave for a few minutes results in the
formation of a powder of the desired material. The powder (made of hexagonal
bismuth telluride nanoplates, seen left - Image
Credit: Technology Review) can then be made into usable pellets by applying
heat and pressure.
This process produces about 15 grams of material in two to
three minutes using only one small microwave oven. The process can be easily
scaled to produce larger quantities with industrial-sized microwave ovens. Professor Ramanath gives his own explanation
of the process in
this video.
The ability to tweak the properties of the materials on a
nanoscale level is what has unlocked the ability to produce high efficiency
thermoelectrics of both n- and p-type. This has the potential to start a new
generation of thermoelectrics.
"This is not a one-off discovery. Rather, we have developed
and demonstrated a new way to create a whole new class of doped thermoelectric
materials with superior properties," says Ramanath. "Our findings truly hold
the potential to transform the technology landscape of refrigeration and make a
real impact on our lives."
I think the implications of this new development are really
exciting. Part of the problem with current technologies like IC engines,
generators, and refrigerators is that we are reaching the limits of efficiency
gains. But thermoelectric materials provide a means to make these devices
better. For cooling applications, thermoelectrics could both reduce footprint
and replace the need for potentially harmful refrigerant fluids. In terms of
heat transfer, they could transform the design of everything from cars to power
plants.
Perhaps it will even make oddball consumer products like
this a little more practical. Maybe…
Sources:
RPI
News
Technology
Review - New Process Makes Heat-Harvesting Materials Cheaply
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Re: Making Thermoelectrics in a Microwave
