PHYSICS NEWS UPDATE -- Number 727 April 14, 2005 by Phillip F. Schewe, Ben Stein
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 727 April 14, 2005 by Phillip F. Schewe, Ben Stein
A NEW KIND OF EQUILIBRIUM. Normally heat will flow from a hot place
to a neighboring cold place. In a new form of thermoelectric
refrigerator, proposed by Tammy Humphrey (University of Wollongong,
Australia) and Heiner Linke (University of Oregon), temperature
imbalances can be held at bay by electrochemical imbalances. The
implications? Possibly much more efficient forms of no-moving-parts
electric refrigerators.
Heat and electricity are two forms of energy, and in a special
circuit, made from thermoelectric materials, a temperature
difference can generate electricity and, conversely, a voltage
difference can bring about a temperature difference. A
thermoelectric circuit usually consists of two semiconductors joined
at two junctions. One of the semiconductors is of the p type with a
surplus of holes, the other of the n type with a surplus of
electrons. Here's how you can generate heat or electricity in
contrary phenomena. In the Peltier effect, a voltage imbalance will
pull electrons and holes out of one of the junctions, thus cooling
that junction and warming the other junction. In the Seebeck
effect, things work in reverse: a temperature imbalance between the
junctions will set electrons and holes in motion, thus constituting
an electric current. The Peltier effect is at work, for example, in
on-chip cooling of critical microcircuitry. The Seebeck effect is
used in powering spacecraft (too far from the sun for photocells to
be of use), where the heat from a radioactive source is used to make
electricity. What keeps thermoelectric devices from greater
applicability is the poor efficiency, typically 10%. One of the
main problems is that some of the heat (applied at one junction)
used to drive a current through the circuit is carried by electrons
to the other junction, reducing the thermal gradient and therefore
sapping the process of generating electricity. What one needs is a
circuit good for electric conduction but poor for thermal conduction
by electrons. And this is what Humphrey (tammy.humphrey@xxxxxxxxxxx
) and Linke's proposed circuit would do (see figure at
http://www.aip.org/png ). The p-leg and n-leg parts of the circuits would
consist not of bulk matter but of quantum dots, nanoscopic pieces of
matter in which only select electron energies are allowed. Engineer
the dots to discourage the higher-energy electrons carrying thermal
energy, heat leakage will drop, and the overall efficiency will go
up. The best thermoelectric efficiencies are about 10%. If
efficiencies could be pushed to 50%, the thermoelectric approach
(silent, less bulky, no refrigerant, long lived) would compete to
take over even bulk household refrigeration, Humphrey says.
(Physical Review Letters, 11 March 2005; lab website
http://www.humphrey.id.au,
http://darkwing.uoregon.edu/~linke/ )
COOLING OF BULK MATERIAL has been achieved with a solid-state
refrigerator. At the heart of the NIST-Boulder device is a tiny
sandwich-shaped diode whose layers are successively a normal metal,
an insulator, and a superconductor. The stack has the effect of
pulling the hottest electrons out of the normal-metal layer. This
no-moving-parts refrigerator is not the first to achieve 100 mK
temperatures but it is the first to do so with technologically
useful cooling powers. The NIST micro-fridge chilled a cube of
germanium about 250 microns on a side and with a mass of 80
micrograms. This sounds like a small speck of matter, but it was
enormous compared to the size of the refrigerating junctions (see
figure at http://www.aip.org/png ). Indeed, the ratio of the volume of the
cube to the volume of the junctions is 11,000. This is equivalent
to a refrigerator the size of a person chilling something the size
of the Statue of Liberty. In preliminary tests, the cube was cooled
from 320 mK down to 240 mK. Future improvements should lower the
base temperature to near 100 mK. According to NIST physicist Joel
Ullom (ullom@xxxxxxxxxxxxxxxx), their refrigerator works best at
temperatures below 1 K, so it won't be used to cool foods. But it
will be very useful for chilling circuitry on chips and maybe
samples as large as the centimeter size. (Clark et al., Applied
Physics Letters, upcoming article)
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