Copyright 2016 Robert Clark
(patents pending)
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Note: this is the technical background to an announced crowdfunding campaign now live, as of February 5, 2016:
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For the twenty
years since their discovery, the "Holy Grail" of nanotube research
has been to produce them in arbitrarily long lengths. They initially were only
produced at micron-scale lengths. After intense research, so far they have still
only been made in millimeter to centimeter lengths.
Still, in the
micron-scale samples tested, their tensile strength has been measured to be a
maximum of 150 gigapascals (GPa) at a density of only 1.6 g/cm3, 200
times stronger than steel at only 1/5th the weight, for an improvement of 1,000
times in strength-to-weight ratio.
The big question
is can we make or combine the nanotubes to macroscale sizes while maintaining the strength of the individual nanotubes? Individual
carbon nanotubes and the 2-dimensional form monolayer graphene have been measured at micron-scale lengths to have tensile
strengths in the range of 100 to 150 GPa, [1], [2], [3]. Carbon nanotubes have
been combined, intermingled into bundles and threads for awhile now. These
always have significantly lower strength than the individual nanotubes, [4].
However, this is because there were many single nanotubes connected together by
weaker van der Waals forces rather than the stronger carbon-carbon molecular
bonds that prevail in individual nanotubes. In these cases, with separate
nanotubes weakly connected end-to-end, they can just peal apart under tensile load.
This is explained here, [5].
However, some
tests of aligned, arrays of nanotubes
at millimeter length scales also showed significantly lower strength than individual
micro-scale nanotubes, [6], [7], [8]. This may be because of the predicted
effect of longer nanotubes having more defects and therefore becoming weaker. If this is the case, then rather than
pursuing arbitrarily long nanotubes it may be better to pursue methods of
bonding the micro-scale nanotubes at their ends so that their ultra high
strength is maintained. Some possibilities
will be suggested in the following pages.
Joining nanotubes to arbitrary lengths.
Tying ropes
together has been known to create longer ropes whose strength can be 80% to 90%
as strong as the component ropes, [9], [10]. Quite key then is that the capability
exists to manipulate individual nanotubes at the nanoscale:
The
stress test: One experiment repeatedly bent a nanotube through contortions to
see if it would break. All these modifications were performed by the
NanoManipulator, with the user guiding the AFM tip by moving the Phantom
force-feedback pen. [11], [12], [13]
See also, [14],
[15], [16]. Then
the suggestion is to tie the nanotubes together using some of the knots known
to maintain near the strength of the original ropes. (patent pending.)
To prevent slipping
of the nanotubes under high tensile load we might use them with “nanobuds”
along their lengths, [17].
Also, since the
nanotubes are quite thin they might be expected to cut into each other when
knotted thus weakening the strength of the knot. One possibility might be to
fill the portion of nanotube that is to be knotted with water or other fluid to
make the nanotube more spongy there, [18].
This method of tying nanotubes together to produce greater
lengths has already been proven to work to preserve at least one characteristic
of nanotubes, high conductivity:
Energy
Nanotube Cables Hit a Milestone: As Good as Copper.
Researchers
achieve a goal they've been after since the 1980s—the advance could
make cars and airplanes lighter, and renewable energy more practical.
Monday, September 19, 2011 By Katherine Bourzac
The article describes
research by scientists at Rice University who created lightweight electrical cables by mechanically tying together
nanotubes.
An alternative method
for linking the nanotubes together would be to connect them with nanotube
rings:
Ring Closure of Carbon Nanotubes.
Science, Vol. 293, No. 5533, p. 1299-1301, 17 August 2001
Lightly
etched single-walled carbon nanotubes are chemically reacted to form rings. The
rings appear to be fully closed as opposed to open coils, as ring-opening
reactions did not change the structure of the observed rings. The average
diameter of the rings was 540 nanometers with a narrow size distribution. [19]
These are closed
rings formed from one or more nanotubes. They are about 540 nm across so
several of the aligned nanotubes would have to be fitted into the rings. One
question would be how to tighten the rings around the nanotubes once they were
fitted into the rings. One possibility might be to apply heating to the rings
so that they lengthen then insert the nanotubes inside. Then as the rings
cooled they would shrink back to their normal size forming a tight stricture
around the nanotubes. As before with the knotting we may have to fill the rings
with a fluid so that they are spongy and don’t cut into the nanotubes.
Another method for fitting the carbon
nanotubes into the rings would be by using ring-shaped nanotubes of materials
that are piezoelectric. Carbon nanotubes are not piezoelectric but nanotubes of
many different types of materials have been made, such as boron nitride
nanotubes and zinc oxide nanotubes. Nanotubes of both these types are
piezoelectric, and they can also be made in the form of nanorings, [20], [21].
Then we could apply electric current to these nanorings to get them to expand,
insert the carbon nanotubes, then remove the current to get the nanorings to
shrink back to their regular size.
Note that using the rings as a means of
binding the ropes together means you are using frictional effects to get the
nanotubes to hold together. Then is this any better than the van der Waals
forces holding just intermingled nanotubes together? I believe it can be as
long as you make the rings stricture tight enough. But if it is made too tight,
this would cut into the nanotube ropes reducing their strength. Then the
optimal degree of tightening would have to be found to maintain the greatest
strength.
Interestingly, the
method of knotting the nanotubes together or binding them by rings might also
be applied to the intermingled bundles, that is, to the case where the
nanotubes are of different lengths held together by van der Waals forces. You
would note the shortest length of the nanotubes composing a bundle and tie
knots around the bundle or bind it with rings at short enough intervals to
insure that every nanotube is held tightly with a tie or knot at least once all
along the length of the bundle.
Another question that would need to be
answered is how binding together a group of equally long nanotubes effects the
strength of the nanotubes when the binding is only going around the outer
nanotubes. That is, suppose you created a string made from single
nanotubes bound end-to-end and measured the string’s strength.
Then you composed a
string by using aligned nanotube arrays that all contained the same number of nanotubes,
say 100, and bound these ropes end-to-end with the rings. Would the string
composed of the aligned ropes be able to hold 100 times as much as the string
composed of individual nanotubes? This is asking a somewhat different question
than how knotting weakens the nanotubes. It's asking how strong a composed
string will be when a binding can only go around the outer nanotubes composing
the string.
Yet another mechanical method for joining the
ends together might be to use some nanotubes bent into shapes as clamps. Since
nanotubes have such high stiffness they should as clamps be able to hold the
ends of aligned arrays of nanotubes together. Again so the clamps don’t cut
into the nanotubes you might want to have the clamping nanotubes and/or the
nanotubes that are being tied to be fluid filled.
A different method of
joining individual nanotubes or aligned nanotube arrays end-to-end is suggested
by the recent research that created diamond-nanotube composites, [22], [23]. To
form the strongest bonds for our purposes, I suggest that the method of
creating the strongest nanotubes be used first to create the nanotubes, the
arc-discharge method by which the 150 GPa tensile strength nanotubes were made,
as in [1]. Then the ends of separate nanotubes or nanotube arrays should be
placed on the same diamond seed particle and the high strength microwave CVD
method of [4] be used to grow diamond around the ends of both, encasing the
tips of each of them inside the diamond thus grown. In order to keep the weight
low, you only use a small seed particle and you only grow the diamond large
enough to maintain the strong bonds that prevail in individual nanotubes.
Another highly
promising method for joining the nanotube ends arises from the surprising
effects found by irradiating nanotubes by electron beam:
Reinforcement of single-walled carbon nanotube bundles by
intertube bridging.
Nature Materials, 3, p. 153 – 157, March 2004
During
their production, single-walled carbon nanotubes form bundles. Owing to the
weak van der Waals interaction that holds them together in the bundle, the
tubes can easily slide on each other, resulting in a shear modulus comparable
to that of graphite. This low shear modulus is also a major obstacle in the
fabrication of macroscopic fibres composed of carbon nanotubes. Here, we have
introduced stable links between neighbouring carbon nanotubes within bundles,
using moderate electron-beam irradiation inside a transmission electron
microscope. [24]
NEWS & VIEWS
Strong bundles.
Nature Materials, 3,
135-136, March 2004.
The mechanical properties of nanotube bundles are
limited by the sliding of individual nanotubes across each other.
Introducing crosslinks between the nanotubes by electron
irradiation prevents sliding, and leads to dramatic improvements in strength. [25]
The researchers noted
as had others that intermingled bundles of nanotubes were relatively weak
compared to the strength of individual tubes, in this case their measurements
being of bending strength. However, after electron beam irradiation the bundles
achieved almost 70% of the bending modulus strength of individual nanotubes. A
similar effect was seen in [26], [27],
[28]. The irradiation produced interconnections between the nanotubes that
prevented slipping. Then quite likely this can also be used to combine
nanotubes at their ends.
Electron beam irradiation has also been used
to attach nanotubes to sensors in scanning electron microscopes for strength
testing. One method used was to direct a small amount of hydrocarbons by
focused e-beam to weld the nanotubes to the SEM sensor tip. Then this may also
work to weld nanotube ends together, [29]. Note that e-beam irradiation can
also be used in concert with the tying or ring binding methods to insure no
slipping of the nanotubes.
Additionally laser irradiation
has been used to connect double-walled nanotubes strands together, [30]. This
resulted in longer nanotube strands as strong as the original ones. However,
the starting strength of these was low at 335.6 MPa. It needs to be tested if
this method can maintain the strength of the original nanotubes at the highest
measured strengths of 150 GPa.
Note that these
e-beam or laser irradiation methods may also work to produce graphene sheets of
large size as well. Currently the 2-dimensional graphene has only been produced
in micron-scale sizes, though its strength has been shown to be comparable to
that of the highest measured strengths of the nanotubes at 130 GPa, [3].
However, irradiating overlapping graphene sheets on their edges may also allow
these to be bonded together.
Friction-stir
welding of nanotube arrays.
Another method for joining the aligned arrays of nanotubes
might be the method friction-stir welding. This method is used to weld metals
while maintaining relatively low temperatures. This reduces the damage to the
metals and helps to maintain strength. Since this uses relatively low
temperatures it may also work to combine the ends of the aligned arrays of
nanotubes.
The Space Elevator.
Such high
strengths in the 100 to 150 GPa range if they can be maintained in the bonded
nanotubes are within the range to make the “space elevator” possible.
However, even at such
high strengths it is expected the space elevator ribbon would require tapering.
Then you would need a means of connecting nanotubes ropes to each other of ever
increasing diameter. One possibility for accomplishing this might be by using
the “y-shaped” nanotubes, [31]. These are nanotubes that branch off into a
Y-shape. If each branch is as strong as the base column then we could attach a
base column of one to a branch of another, thereby creating larger and larger
diameters.
Using “y-shaped” nanotubes might also be a way
to maintain the high strength across connections in general, assuming each
branch is as strong as the base, if multiple branches of one are attached to
multiple branches of another. To continue this indefinitely, you would need the
y-branches to be on both ends of each nanotube.
If in general, the connections weakened the
strength by some factor we would just use enough branches so that the total
strength would be the same as the individual nanotubes. Then if the branches
are quite short compared to the base column, the total mass would be just a
small fraction larger than that of just the base columns alone, so the strength
to weight ratio would be about the same.
In regards to the
space elevator, NASA and the SpaceWard Foundation had sponsored a competition
with a $1 million prize for a team that can produce a cable material at about
double the strength to weight ratio of the strongest commonly used materials
now:
Tether Strength Competition.
By the numbers:
Tether Length: 2 m (closed loop)
Tether Weight: 2 g
Breaking Force: 1 ton, 1.5 ton (approx)
Prize Purse: $900k, $1.1M
Best performance to date: 0.72 Ton
Number of Teams: None Yet
Competition Date: February-March, 2009. [32]
I believe both a carbon nanotube cable joined
by one the methods described above and a cable made of the new synthetic
diamond could each win this competition.
Bob Clark
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32.) Tether Strength Competition.
