Nanowires convert heat to electricity
- From: Sam Wormley <swormley1@xxxxxxxxx>
- Date: Fri, 18 Jan 2008 03:17:02 GMT
Nanowires convert heat to electricity
Phonon effects boost thermoelectric efficiency
http://physicsworld.com/cws/article/news/32446
Silicon --a staple of modern electronics --could soon be used to make
low-cost devices that convert waste heat to electricity. That is the
bold future outlined by two independent teams of scientists in the
US, who have shown that arrays of tiny silicon wires have extremely
good "thermoelectric" properties. The findings could lead to the
development of cheap thermoelectric materials that boost the
efficiency of both massive coal-fired generators and tiny solar
cells.
Many methods of generating electricity --such as burning fossil fuels
and nuclear fission --create vast quantities of waste heat.
Thermoelectric materials that convert heat directly into electricity
could someday be used to recover some of this energy, thereby
boosting the efficiency of conventional power stations.
Thermoelectric materials could also improve the effectiveness of
solar cells and generate electricity from other sources of waste heat
such as computer chips and refrigerators.
Low efficiency
Unfortunately, today's thermoelectric materials are synthetic
nanostructures that are very expensive to make and are nowhere near
efficient enough to be used commercially.
Some researchers believe that silicon-based thermoelectric materials
could overcome these limitations because silicon is easy and
inexpensive to work with and has the right electrical properties to
be a thermoelectric material. However, silicon is also a good
conductor of heat. A thermoelectric material converts a temperature
difference across a material into a voltage and silicon\u2019s high
thermal conductivity means that a large amount of heat is required to
create a small temperature difference --making it a very inefficient
thermoelectric material.
Factor of 100
Now, two independent teams in California have worked out a way to
boost the thermoelectric efficiency of silicon by as much as a factor
of 100. Both results are reported in the journal Nature.
Allon Hochbaum and colleagues at the University of California,
Berkeley, created arrays of tiny silicon nanowires by dipping silicon
wafers into an aqueous solution of silver ions. The nanowires were
20\u2013300 nm in diameter and the team discovered that arrays have a
thermoelectric efficiency about 60 times greater than bulk silicon at
room temperature.
Rough surfaces
They believe that this rise in efficiency occurs because
heat-carrying sound waves called phonons have a very difficult time
moving along the very narrow nanowires, reducing their ability to
conduct heat. One important feature of Berkeley nanowires is that
their surfaces are rough, which the researchers believe contributes
to their high thermoelectric efficiency, although the physics behind
this remains unclear.
Meanwhile, at the California Institute of Technology, Akram Boukai
and colleagues have seen a similar effect in even smaller rectangular
nanowires with cross-sections of 10 × 20 nm and 20 × 20 nm. The team
measured a 40-fold boost in efficiency over bulk silicon at room
temperature. This increased to a 100-fold boost at -73 °C.
Intriguingly, their observations suggest that another phenomenon
--called "phonon drag" --also plays a role in boosting the
thermoelectric efficiency. Phonon drag occurs when phonons moving
through the silicon collide with charge carriers such as electrons
and drag them along.
.
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