PHYSICS NEWS UPDATE -- Number 695 August 5, 2004 by Phillip F. Schewe, Ben Stein

From: Sam Wormley (swormley1_at_mchsi.com)
Date: 08/07/04 

Date: Sat, 07 Aug 2004 00:21:31 GMT

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 695 August 5, 2004 by Phillip F. Schewe, Ben Stein
        
GOLD AND DIAMONDS, accouterments at many weddings, have another
curious affinity. They have almost the same acoustic impedance, a
fact which two physicists are hoping to exploit in order to get
nanoparticles, embedded in a crystalline network, to ring with a
pure tone, which in turn should help in the development of various
nanotechnology devices. The acoustic impedance, the acoustic
analogue of a material's optical index of refraction, is defined as
the density times the velocity of sound in that material. Gold has
a high density but a moderate sound speed (3330 m/sec), while
diamond has a low density but a very high speed of sound; indeed, at
a speed of 18,190 m/sec, sound waves in diamond travel twice as fast
as the Space Shuttle in Earth orbit. Thus, these two materials are
very different in many respects but alike in their impedance to
sound, which is to say their propensity to take up or dissipate
sound energy. Now, one would expect that for two materials with
similar acoustic impedance sound would move all too easily from the
one to the other. (Optical analog: a piece of glass becomes almost
invisible in a bath of water since the indices of refraction for
glass and water are almost the same.) But the research turned this
expectation on its head. A gold nanoparticle, once set vibrating in
a diamond matrix, should actually keep vibrating, the new studies
show. In other words, the particle's sound energy, the energy of
its vibrating in place, does not leak out into the surrounding
crystal. According to Lucien Saviot at the Universite de Bourgogne
(Dijon, France) and Daniel Murray of Okanagan University College
(Kelowna, British Columbia, Canada), the resolution of this apparent
paradox is that people had for many years been using the wrong
formula for acoustic impedance. The correct formula, they argue, is
more complicated. It's not just density times speed of sound, but
involves also the radius of curvature of the interface and also the
sound frequency.
The authors of the new study have not yet implanted gold
nanoparticles inside diamonds but they have studied the case of how
gold particles ring while ensconced in silica and sapphire. Their
surprising result is that the particle keeps ringing. The particles
are set in motion by a pulse of laser light, shining in through the
crystal, and its ringing can also be monitored by laser light; the
vibrations show up as the amount of energy sapped from the probe
laser beam. (Saviot and Murray, Physical Review Letters, 30 July
2004; dbmurray@mail.silk.net;
lucien.saviot@u-bourgogne.fr)
                        
ACOUSTICALLY POWERED DEEP-SPACE ELECTRIC GENERATOR. Space is a new frontier for an acoustical
version of a 19th-century mechanical
device. For future deep space missions to the outer planets and
beyond, space agencies would like their probes to have a lighter,
smaller, and more efficient source of electricity. With this need in
mind, a Los Alamos-Northrop Grumman team (Scott Backhaus,
backhaus@lanl.gov) has built a device that uses sound waves to
produce 60 watts of electricity. The core of this device is called
TASHE, short for "thermoacoustic-Stirling heat engine." An
acoustical version of a 19th-century engine design (named after
Scottish minister Robert Stirling, who invented it), the TASHE is a
looped contraption made from pipes and heat-exchanging devices. In
the TASHE system, intense, spontaneously generated sound waves (in
the place of mechanical pistons in the 19th-century design) shuttle
parcels of helium gas between a cold end and hot end. The hot and
cold end temperatures are generated by connecting the engine to a
high-temperature heat source and an ambient-temperature heat sink
through the heat exchangers. Thermally driven expansion and
contraction of the gas, in concert with pressure oscillations
(induced by the temperature difference), intensify the power of the
initial sound waves which become strong enough to drive a piston
connected to the device. The motion of the piston vibrates a coil
of copper wire that produces electricity as it moves relative to a
permanent magnet. The acoustic device has 18% efficiency, compared
to 7% for thermoelectrics, the current electrical-generation
technology in spacecrafts in which a temperature difference across a
material is converted into electric power. (In both designs, small
amounts of radioactive material provide the high-temperature heat
needed for operation.) The new device can produce a projected 8.1
watts of electricity per kilogram, as opposed to 5.2 watts/kg for
thermoelectrics. These properties allow for a potential increase in
the size and power of science instruments in future space probes.
This is the latest application of the TASHE, which is also being
developed to liquefy remote reserves of natural gas for a more
economical transport of this fossil fuel resource to market than
previously possible. (Backhaus, Tward, and Petach, Applied Physics
Letters, 9 August 2004)

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