Text 403, 66 rader
Skriven 2005-04-14 20:37:52 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 727
===============
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@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@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)

---
 * Origin: Big Bang (1:106/2000.7)