Number 691, July 7, 2004
by Phil Schewe and Ben Stein
Switchable Nanotube Diodes
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@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.)
Weak Localization of Seismic Waves
A group of scientists at the University Joseph Fourier of Grenoble
and at the Centre National de la Recherche Scientifique, France believes
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 Update
356.)
Would such localization of waves be observed at the much larger terrestrial
scale and under conditions where 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 Physical
Review Focus article). Now they are reporting that 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@grenoble.cnrs.fr,
33-4-76-88-12-76.)
Turning Passenger Trains Into Rail-Crack Detectors
Turning passenger trains into rail-crack detectors is possible with
a new ultrasonic device developed by physicists at the University of
Warwick in England (Steve Dixon, s.m.Dixon@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.)
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