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{at}unsw.edu.au) and Linke's proposed circuit would do
(see figure at 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 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
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 im
provements should lower the base temperature to near 100 mK.  According to
NIST physicist Joel Ullom (ullom{at}boulder.nist.gov), 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)

---