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Suppose
you read a science fiction story in which a new material had been
discovered. It was supposed to conduct
electricity better than copper or silver, but it could also be made into a
semiconductor for transistor fabrication. It was far stronger than steel, strong enough to support much more than
its own weight as a fiber hanging down to the surface of the Earth from
geosynchronous orbit, yet so compact that such a fiber long enough to reach
from Earth to Moon could be rolled up and stored inside a poppy seed. The material could absorb and store vast
quantities of hydrogen, thereby storing fuel for powering fuel cells and
pollution-free automobile engines. It
could also change its length by 10% in an electric field, forming an artificial
muscle, and could be used in reverse to produce electrical energy when flexed,
for example by wind or ocean waves. It
could be used to emit electrons to make wall-size ultra-thin video displays. It could be used to cut or strengthen other
materials. And it was made from one of
the commonest of materials of all, ordinary soot.
I
think you will agree that a story line involving such a material would strain
your suspension of disbelief to the breaking point. How could the author ask us to swallow such a ridiculous
fabrication? And yet, there is an
actual material that, even as you read this, is emerging from the physics
laboratories of the world, and it has all of these properties. It is the carbon nanotube, a new form of
carbon discovered in 1991.
The
nanotube is a molecular cousin of a form of carbon that is perhaps more
familiar, the C60 molecule or buckyball, an array of 60 carbon atoms
forming a soccer-ball-like icosahedron. If the buckyball could be sliced in half, with a “belt” made of a
connected loop of 10 carbon atoms inserted in the cut, it would form another
stable configuration, a C70 molecule that is a larger and somewhat
elongated buckyball. If instead one inserted
two or three 10-carbon loops, this would make even more elongated C80
or C90. The process could be
repeated to produce an arbitrarily long tube, with end caps made of
half-buckyball hemispheres. The object
thereby produced is a species of carbon nanotube.
Alternatively,
a nanotube might be produced by starting with a planar one atom thick sheet of
graphite. Graphite is an array of
linked carbon atoms lying in a plane, with the carbons forming the vertices of
linked hexagons, like the cross section of a honeycomb. In principle, a skilled nano-tailor could
cut a strip out of such a graphite sheet, join the cut edges to form a tube,
and then cap the open tube ends with half buckyballs. Again, the result is a carbon nanotube.
However,
there are several ways that the nano-tailor could have cut the strip from the
graphite sheet. A graphite sheet of
hexagons can be oriented so that one set of hexagon sides is vertical. In this case, a horizontal cut along a row
of hexagons goes alternately up and down in a zig-zag, following the hexagon
edges. This is called a “zig-zag”
cut. If the strip used to form the
nanotube is made such that the open end of the rolled tube forms such a
zig-zag, it is called a zig-zag nanotube. If the length of the strip along this edge spans, say 6 hexagons,
then the resulting nanotube is identified as having the index (6,0). The first number is related to the
circumference of the nanotube and the following 0 tell us it as the zig-zag configuration.
On
the other hand, the graphite sheet could also have been oriented so that one
set of hexagon sides was horizontal. In
this case, a horizontal cut along a row of hexagons goes up, across, down, and
across, following the edges. This is
called an “armchair” cut because the cut line resembles a row or armchairs
arranged side-by-side. If the strip used to form the nanotube is
made such that the open end of the rolled tube forms such a armchair pattern,
it is called an armchair nanotube. If
the length of the strip along this open edge spans, say 6 ups and downs, then
the resulting nanotube is identified as having the index (6,6). For all armchair nanotubes, the second index
number must match the first, and is related to the circumference of the
nanotube.
There
are also “chiral” nanotubes that, if viewed end-on, show a clockwise or
counter-clockwise spiral pattern. These
are intermediate between the zig-zag and armchair configurations and are
characterized by the index (m,n), where 0<n<m for counter-clockwise
spirals and 0<m<n for clockwise spirals.
Some,
but not all, nanotubes are good electrical conductors. This is because nanotubes are small enough
that quantum mechanical effects dominate their electrical conductivity. In particular, the quantum wave functions of
the conduction electrons are either reinforced or destroyed by the symmetries
of the nanotube. For example, nanotubes
with indices (6,0), (6,6), (9,0), and (9,9) are all excellent electrical
conductors. On the other hand,
nanotubes with indices (7,0), (8,0), (6,2), and (7,5) are all semiconductors,
materials in which there are no free conduction electrons. If a nanotube is an electrical conductor at
all, it is a very good conductor indeed, because the electrical conduction is the
result of coherent participation by all the carbon atoms in the nanotube. Consequently, nanotubes have a theoretical
maximum current density that is 1000 times higher than that of excellent
metallic conductors like silver and copper.
We
find that nanotubes produced in a strong electric current often form concentric
multi-layer nanotubes. For example,
published electron microscope images show nanotubes with two, five, and seven
concentric walls. Theoretical calculations
show that it should be fairly easy to produce such concentric nanotubes with
layers that are alternately insulators and conductors. Therefore, a nanotube “coaxial cable” with a
conducting core and conducting jacket separated by one or more insulating
layers should be possible. Such a
nanotube should have interesting applications, not only for signal transmission
at the nanoscale, but also for creating linear capacitors with a very high
capacitance for storage of electrical energy.
The
semiconductor nanotubes also have their uses. Scientists at IBM have deposited nanotubes randomly on a microchip
pattern of conducting copper, then applied a large current pulse to burn away
the conducting nanotubes. They have
then demonstrated field-effect transistor behavior in surviving semiconductor nanotubes.
Nanotubes
are among the sharpest objects in nature, leading to other interesting
electrical applications. If a sharp
object is placed at a high electrical potential, an extremely large electric
field develops near the pointed end. If the pointed tip is made electrically
negative, it will emit electrons that are accelerated in the strong field. This phenomenon is called field emission. Samsung Electronics has produced a prototype
ultra-thin very high resolution flat-panel display that uses the field emission
from nanotubes to fluoresce phosphor pixels placed opposite in planar
geometry. This is a big step towards
the “video wallpaper” that is a common techno-prop of science fiction.
The
mechanical strength and low mass density of nanotubes is also impressive. They share many of the strength properties
of their molecular cousin, the diamond. For example, nanotubes are expected to have a tensile strength equal to
that of diamond, about 130 GPa (18,900,000 pounds per square inch). For comparison, the best steel has a tensile
strength of about 5 GPa (725,000 pounds per square inch). On the other hand, nanotubes should have
about the mass density of graphite, about 1.9 times that of water or about 1/3
the density of steel.
In
his novel The Fountains of Paradise, Arthur C. Clarke described
an orbital elevator or "skyhook" that used a 22,000 mile long cable
to raise and lower payloads from the Earth’s surface to geosynchronous orbit
and beyond. A cable that hangs from
geosynchronous orbit to Earth and has a density that is 1.9 times that of water
will produce a tension on the highest part of the cable of 92 GPa (13,300,000
pounds per square inch). This is only
about 2/3 of the estimated tensile strength of nanotubes, so a uniform
cylindrical cable of any diameter that is fabricated with nanotubes should be
able to function as a skyhook. The 22:1
tapering techniques that would be needed with weaker materials are unnecessary
with nanotubes fabrication.
To
put it another way, the relevant quantity for judging construction materials
for skyhook construction is the ratio of tensile strength to density. Under this criterion, carbon nanotubes
should be about 80 times better than steel and about 16 times better than the
best plastic materials so far considered for this purpose. Moreover, nanotubes have already shown the
tendency to form themselves into bundles or “nano-ropes” that should be an
ideal skyhook construction material.
Unfortunately,
present fabrication techniques can make carbon nanotubes only a few tenths of a
millimeter long. This falls far short
of the lengths (a few meters) required for skyhook cable fabrication, or even
the centimeter lengths required for strengthening composite materials.
It
has been discovered that shining laser light of the right wavelength on a
nanotubes can make it vibrate in a “radial breathing mode”, in which the entire
nanotube alternately increases and decreases its diameter. The light wavelength that excites this
vibration mode depends directly on the diameter of the nanotube. Therefore, the technique can be used to
measure diameters of nanotubes and perhaps, by applying a pulse of high
laser-light power, to destroy unwanted nanotubes of a selected diameter.
Vibration
techniques have also been used to estimate the spring-related properties of
nanotubes,. Vibrations of the free end
of a nanotube that is clamped at the other end are measured. This work indicates that nanotubes can make
exceptionally stiff springs, with a Young’s modulus that is 5 times higher than
that of steel (1.0 TPa as compared to 0.2 TPa).
Under
compression, nanotubes appear to be more forgiving than other materials,
because they tend to fold rather than fracture. They are seen in electron microscope images to form bends and
kink-like ridges under compression and torsion, and to relax elastically when
the distorting forces are removed. It
has been suggested that mechanical compression of nanotubes can achieve such a
large mechanical energy density that stressed nanotubes should be considered as
candidates for explosives as well as for mechanical energy storage devices.
Because
of their tubular geometry and variable diameter, nanotubes can also be
considered as pipes for transporting other atoms or as test tubes for
containing them. Techniques have been
developed for opening and closing nanotube ends. Nanotubes encasing silver nitrate molecules have been used to
deposit nano-beads of silver. It has
also been discovered that nanotubes holes provide excellent high-density
storage for hydrogen gas.
This
brings us to the problem of how to make nanotubes. For long nanotubes we would like to be able to pull single
nanotube fibers (or even complete nano-ropes) out of some hot carbon source
medium. Unfortunately, the only
fabrication method presently available is to vaporize carbon in an arc
discharge (20 volts at 100 amps), with a small amount of iron, cobalt, or
nickel added to the arc. This creates
nanotubes in at best sub-millimeter lengths.
However,
the nanotube fabrication technology is rapidly evolving, and there are already
at least nine commercial nanotube suppliers. I predict that this is a growth industry. Watch this column for further developments.
John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.
Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .
References:
Nanotubes:
Fullerene Nanotubes: C1,000,000 and Beyond, Boris I. Yakobson and Richard E. Smalley, American Scientist, July-August issue (1997);
URL: http://www.sigmaxi.org/amsci/articles/97articles/yakobson.html .Carbon Nanotubes, Mildred Dresselhaus, Gene Dresselhaus, Peter Eklund, and Richiro Sato. Physics World 11, Issue 1, (January, 1998);
URL: http://physicsweb.org/article/world/11/1/9 .Carbon Nanotubes and Related Structures, Peter J. Harris, Cambridge University Press (1999), ISBN: 0521554462; URL: http://www.rdg.ac.uk/~scsharip/tubes.htm .
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