From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Jul 08 2004 - 06:03:44 PDT
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From: physnews_at_aip.org<mailto:physnews_at_aip.org>
To: ljk4_at_MSN.COM<mailto:ljk4_at_MSN.COM>
Sent: Wednesday, July 07, 2004 2:41 PM
Subject: Physics News Update 691
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 691 July 7, 2004 by Phillip F. Schewe, Ben Stein
SWITCHABLE NANOTUBE DIODES made by scientists at the research arm of
General Electric combine the practical electrical properties of
carbon nanotubes (ability to carry high currents; ability to emit
light) with the flexibility of being changed over from a p-n type of
diode (allowing current to flow in one direction only) to an n-p
diode type (allowing current only in the opposite direction). Most
solid state transistors are three-terminal devices: current comes in
at one terminal (the source) and exits at a second terminal (the
drain) if a third terminal (the gate) carries a certain voltage,
which has the effect of electrostatically clearing out a realm for
charge carriers to flow through. In the GE device, the "realm" is a
single-walled carbon nanotube (NT), while the "gate" is actually two
separate gates located beneath the NT. These split gates can
electrostatically dope the two ends of the NT in such a way that
current
will flow in only one direction or only in the other depending on
the gate voltages. If you count the source, drain, two gate
electrodes, and another electrode attached to an underlying silicon
substrate, the device overall has five terminals. Diodes are
intrinsically simpler than transistors, but up till now more work
has gone into developing NT transistors than for NT diodes. The GE
researchers (contact Ji-Ung Lee, leeji_at_research.ge.com<mailto:leeji_at_research.ge.com>) expect their
device to function as both a field effect transistor (FET) or as a
light emitting diode (LED). Because of its ability to carry high
currents, and because the company in
question is GE, it might also find applications in power
electronics, where huge currents and voltages are to be found. (Lee
et al., Applied Physics Letters, 5 July 2004, cover story; text at
www.aip.org/physnews/select<http://www.aip.org/physnews/select>)
WEAK LOCALIZATION OF SEISMIC WAVES. A group of scientists at the
TURNING PASSENGER TRAINS INTO RAIL-CRACK DETECTORS is possible with
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: Thu Jul 08 2004 - 06:12:43 PDT
University Joseph Fourier of Grenoble and at the Centre National de
la Recherche Scientifique, France believe they have observed the
temporary trapping of seismic waves in a natural environment. Years
ago the localization of waves was observed under laboratory
conditions for electron waves (electrons, acting like waves as they
move through a material) and light waves; the waves, traveling in a
diffuse medium such as milk or powder, were repeatedly scattered but
not absorbed and were, in effect, bottled up or "localized." (For a
report on the localization of light waves see
http://www.aip.org/pnu/1998/split/pnu356-1.htm
localization of waves be observed at the much larger terrestrial
scale and under conditions with very little control could be
exercised? The Grenoble scientists sought and found an example of
what could be the first step towards a "seismic insulator," a
strongly heterogeneous geographic environment which would scatter
but not absorb waves in the earth. Previously the same researchers
had found evidence for seismic waves rattling around underground in
the wake of some earthquakes (see
http://focus.aps.org/story/v7/st17
interference of the seismic waves can be detected and that this
method can be used to determine the mean wavelengths of "randomly
walking" seismic waves. The waves in this case were propagating
inside a volcano located in the French Auvergne and tracked with an
array of detectors. (Larose et al., Physical Review Letters,
upcoming article; contact Bart van Tiggelen,
bart.van-tiggelen_at_grenoble.cnrs.fr<mailto:bart.van-tiggelen_at_grenoble.cnrsfr>, 33-4-76-88-12-76; text of
article at www.aip.org/physnews/select<http://www.aip.org/physnews/select>)
a new ultrasonic device developed by physicists at the University of
Warwick in England (Steve Dixon, s.m.Dixon_at_warwick.ac.uk<mailto:s.m.Dixon_at_warwick.ac.uk>). Current
ultrasonic track-inspection equipment must be operated on special
work trains running 20-30 miles per hour. With the new device, the
idea is to enable an ordinary fleet of passenger-carrying trains,
traveling as fast as 200 miles per hour, to continuously and
routinely check for early signs of track failure. The new ultrasonic
technique can detect track defects within 15 mm of the rail
surface. Furthermore, it can detect "gauge-corner" cracks, those
that occur from rolling wheels making contact with the inside of a
rail head (the wide stubby top part of a rail). Track failure from
gauge-corner cracking is believed responsible for numerous
accidents, including a UK train derailment in October 2000 that
killed four people. Mounted on a train, the device generates
"low-frequency, wide-band Rayleigh waves," multiple-frequency sound
waves that travel swiftly along the length of the surface skin of
the rail. Different frequencies penetrate to different depths in
the rail, with the lower frequencies having a deeper penetration of
around 15 mm. If the waves encounter a crack, they get partially
blocked or reflected in a way that can be detected by the device,
which can then record its exact location and depth, by determining
which frequencies are able to pass underneath the crack.
Preliminary results suggest that this technique can even detect
changes in microscopic structure and stress levels within the rail
that could identify crack-susceptible stretches of track. However,
more testing is necessary to confirm this capability, and further
development is required to bring the device from the lab to
real-world passenger trains. The work, published in the June 2004
issue of Insight, the Journal of the British Institute of
Non-Destructive Testing, was presented at this week's 7th
International Railway Engineering conference in London. (University
of Warwick press release, 5 July.)
PHYSICS NEWS UPDATE is a digest of physics news items arising
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