Text 64, 75 rader
Skriven 2004-08-06 17:09:01 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 695
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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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 * Origin: Big Bang (1:106/2000.7)