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Structure and energeticsof carbon nanotube ropes
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Introduction
Carbon nanotube is the strongest man made material which has
excellent mechanical properties. It s stronger than Kevlar which has ever
been the best material for producing bullet-proof armor. Carbon nanotube
structure consists of whole carbon atom bonding in a sheet form (in nano
scale) that be rolled up like a tube, so Its called Carbon nanotube
Carbon nanotubeKevlar
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Even It consists of whole carbon atomEven It consists of whole carbon atom
like a charcoal and Graphite (includinglike a charcoal and Graphite (including
diamond). But because of its structure, It isdiamond). But because of its structure, It ismuch stronger than its component-likemuch stronger than its component-like
materials .Some forms are even strongermaterials .Some forms are even stronger
than diamond!!than diamond!!
Diamond s face center
cubic structure
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Hence, the structure and the orientation of molecules play a big
role on material s mechanical properties. Carbon nanotubes are present
mainly in three configurations:
single-walled carbon nanotubes
Multiwalled carbon nanotubes
carbon nanotube bundles
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As the researchers study about the stabilityof carbon nanotubes for using as high strengthmaterial , the carbon nanotubes in a bundle form
are the best for mechanical applications. Theraft-like form of carbon nanotube bundles arewidely produced while there is some weakpoints of this form. Individual carbon nanotubesin the bundles interact with each other through th
e weak van der Waals forces. The van derWaals force which hold nanotubes together areweak in the raft-like structure
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However, The carbon nanotube in a
rope formed from twisted individual tubestogether tend to have higher strength and
provide more stability also.
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Purpose and scope
This Project will focus on the structure and energetics ofcarbon nanotube ropes made from twisted carbon nanotubes by us
ing high-resolution transmission electron microscopy (HRTEM) and
some other simulations of twisted carbon nanotube.
electron microscopic will offers image features of carbonnanotube ropes in a double-strand nanotube rope, a triple-strand n
anotube rope and a larger rope, respectively. The energetic stabilit
y and formation of these carbon nanotube ropes were analyzed thr
ough molecular mechanics calculations.
HRTEM image simulations of a septuple-strand carbon
nanotube rope were also performed in order to compare with the e
xperimental results.
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Design of experiment
The carbon nanotubes in this
experiment were prepared by
electric-arc technique and then
dispersed in methanol. The
carbon nanotubes will be
agitated in ultrasonic bath.
A small drop of the suspension was
then put onto a holey carbon grid and al
lowed to dry in air. Twisted carbon
nanotubes in various cases were
provided for the study.
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HRTEM imaging of the sample was
performed in the transmission electron microscope operated at 200 kV and the images were coll
ected on a Gatan CCD camera.
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Result & DiscussionResult & Discussion
The twisting rate of two carbonThe twisting rate of two carbon
nanotubes are determined bynanotubes are determined by measuring
the axial length and the twisting angles byby
HRTEM image which show the Thetwisted carbon nanotubes rope twist with r
espect to each other along the roping axis
by 90 degree in an axial length of 18 nm,so this two twisted carbon tubes rope are
twisted at a rate of 5 degree/nm.
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Fig. 1 (a) Transmission electron microscopy image of a double-tubule
carbon nanorope. The two tubules twist around their roping axis by about
180 degree
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Fig. 1 (b) Transmission electron microscopy image of a triple-tubule
carbon
nanorope.
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Twisting of individual nanotubes to
form a thicker rope was also observed, alt
hough the twisting features are much mor
e complicated than the small nanotube ropes
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A twisted carbon nanorope composed of
more than 40 individual single-walled carbon
nanotubes.
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The energetic analysis
The structural stability of carbon nanotube ropes was analyzedby carrying out molecular mechanics simulations, which can handle a muc
h larger system of atoms
The energetics stability of nanoropes should be observed
experimentally on only straight carbon nanotube ropes with respect to suc
h parameters as the twisting angle and helicity of individual nanotubes. A
classical force field method was used to account for the interactions between carbon atoms in the carbon nanotube ropes.
To cope with the large model structures for carbon nanotube
ropes that are composed of thousands of atoms, they have only included t
he diagonal, harmonic terms in the force field as follow :
Note that no electrostatic
interactions were considered in
the simulations.
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Twisted energy that was calculatedTwisted energy that was calculatedfrom that equation for simulation of variousfrom that equation for simulation of varioussizes of carbon nanotubes rope aresizes of carbon nanotubes rope arecomparable with result from other morecomparable with result from other moreaccurate method includingaccurate method including the result fromthe result from
the experimentthe experimentTherefore , the simulations of twistedTherefore , the simulations of twisted
energy of carbon nanotubes rope could beenergy of carbon nanotubes rope could beimagined as follow slideimagined as follow slide
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Fig. 3 (a) shows the schematic illustration of a pair of twisting single-walled
carbon nanotubes of the same atomic structure (10, 10) (the indices define
the perimeter vector on graphene [4]) to form a double-strand rope. Each single-walled carbon nanotube was twisted about its axis by an angle and t
hen the twisted nanotubes twine around each other to form a nanotube rop
e of roping angle
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The torsion energy increases harmonically with the distorsion initially
and then increases faster as the nanotube is further twisted.
From the energetic considerations, due to the increase of strainenergy, the distorted structure will not be stable without constraints.
However, if the two twisted carbon nanotubes join together, due to the
tendency of energy minimization,the structure can be stable energetically while
enhancingstructural stability.
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shows the total formation energy as a function of the twist angle b of the
nanotube rope formed by two identical (10,10) nanotubes, each of
which is twisted about its own tubule axis by an angle = 1 = 2 and t
hen the two twisted nanotubes twine around each other by a roping angl
e .
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Fig. 3(c) shows calculated results for the case 1 = 0 and 2 = 5, 10,15, and 20, respectively. As in the first case, the plied carbonnanorope is a meta-stable structure when the roping angle 0 assumes an appropriate value.
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On the other hand, since the structure
of carbon nanotubes is often helical like the DNA molecule, They have also
examined the role of helicity in the formati
on of a plied carbon nanorope.
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Fig. 4. Twisting energy varies with roping angle for combinations of
carbon nanotubes of different helicity(10,10) and (14, 5), (10,10)
and (10,10), (11, 9) and (12, 8) and (10,10) and (13, 7). The length of
all individual carbon nanotubes is about 8.8 nm.
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HRTEM image simulations
The additional simulation of larger carbonThe additional simulation of larger carbon
nanotubes rope was also examine. They foundnanotubes rope was also examine. They found
that thethat the Molecular mechanics simulations of the
septuple-strand carbon nanotube rope show thatthe energy variation relative to the roping angle
is the same as that for the double-strand
nanotube rope.Seven nanotubes of the same
(10,10) were used to form a septuple-strandcarbon nanotube rope, which is shown as a
picture in next slide
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Fig. 5.
(a) Cross-sectional view of a model of a rope-like bundle ofseven carbon nanotubes with diameter of 11 nm. Six carbonnanotubes twist around the central tubule by approximately10, which is comparable to the experimental roping angle of60 in 70nm length. (b) Simulated HRTEM image with electron beam incident in the B direction indicated in (a). White arrows D and E highlight the characteristic image features of pl
ied carbon nanoropes.
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ConclusionConclusion
observing of single-walled carbon nanotubes invarious condition of forming ropes. A double-strand,triple-strand and a larger carbon nanotube rope have been investigated using high-resolution electron microscopy.
Molecular mechanics simulations show that, by twisting asingle model (10, 10) nanotube about its tubule axis, the strain energy increases non-linearly with the twist angle. When two twisted carbon nanotubes join together to form a rope, the nanotube rope is a metastable structure energeti
cally. It has also been demonstrated that the helicity of nanotubes plays an insignificant role in the formation of carbon nanotube ropes.
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