Carbon nanotubes are cylindrical all-carbon molecules composed of concentric graphitic shells with extremely strong covalent bonding of atoms within the shells but very weak van der Waals type interaction between them. Due to the unique atomic structure nanotubes have exceptional electronic and mechanical properties which imply a broad range of possible applications as constituents of nanometer-scale devices and novel composite materials.
In this work we are using computational methods to study the atomistic structure of twist grain boundaries of different rotation angles. We are interested in knowing if the interface of all grain boundaries is comprised of a thin amorphous layer, or if there are certain angles for which the interface is crystalline. We are also interested in the grain boundary energy, since it can be compared to experimental results.
The properties of a carbon nanotube depend on the local atomic configuration and defects. For composite and device development it is essential to understand how structural changes affect the properties and our work strives after shedding some more light on the occurring phenomena. Current projects concentrate on evaluating irradiation and irradiation induced defects as a means to improve carbon nanotube strength, load transfer and inter-shell friction. The tools employed are both classical molecular dynamics and dynamical tight binding methods. Fig. 1 shows an example of a defect typical to irradiation and how such defects can link tubes which efficiently prevents tube-tube slippage
Figure 1. A defect typical to irradiation, a vacancy.
Figure 2.Example of a nanotube bundle in which the nanotubes are linked together by the presence of vacancies
It is becoming clear from recent experiments3-9 that carbon nanotubes (CNTs) are
fulfilling their promise to be the ultimate high strength fibres for use in materials applications.
There are many outstanding problems to be overcome before composite materials, which
reflect the exceptional mechanical properties of the individual nanotubes, can be fabricated.
Arc-discharge methods are unlikely to produce sufficient quantities of nanotubes for such
applications. Therefore, catalytically grown tubes are preferred, but these generally contain
more disorder in the graphene walls and consequently they have lower moduli than the arc-
grown ones. Catalytic nanotubes, however, have the advantage that the amount of disorder
(and therefore their material properties) can be controlled through the catalysis conditions, as
mentioned before. As well as optimizing the material properties of the individual tubes for
any given application, the tubes must be bonded to a surrounding matrix in an efficient way to
enable load transfer from the matrix to the tubes. In addition, efficient load bearing within the
tubes themselves needs to be accomplished, since, for multi-walled nanotubes (MWNTs),
experiments have indicated that only the outer graphitic shell can support stress when the
tubes are dispersed in an epoxy matrix10, and for single wall nanotube (SWNT) bundles (also
known as ropes), it has been demonstrated that shearing effects due to the weak intertube
cohesion gives significantly reduced moduli compared to individual SWNTs6. The reduced
bending modulus of these SWNT bundles is a function of their diameter. An individual tube
has an elastic modulus of about 1 TPa, but this falls to around 100 GPa for bundles 15 to 20
nm in diameter. In summary, there are two main challenges to address: to enable strong
bonding between the CNTs and the surrounding matrix; to create crosslinks between the
shells of MWNTs and also between the individual SWNTs in SWNT bundles, so that loads
can be homogeneously distributed throughout the CNTs. Ideally, both these goals should be
achieved without compromising the mechanical properties of the CNTs too drastically. Efforts
within this group have begun to address these problems using post production modification of
CNTs via chemical means and controlled irradiation.
High resolution transmission electron microscopy (HRTEM) can be used to give
invaluable information about the structure of CNTs, in particular, the amount of
order/disorder within the walls of MWNTs. Atomic force microscopy (AFM) can be used to
measure the mechanical properties of individual CNTs4-7. Use of both techniques has allowed
us to make a correlation between the strength of MWNTs, grown in different ways, with the
amount of disorder within the graphene walls (see below).
The AFM technique developed in our laboratories has already enabled characterisation
of the moduli of SWNT bundles6 and MWNTs, both arc-grown and catalytically grown7. The
method has been described in detail previously6,7. Briefly, it involves depositing CNTs from a
suspension in liquid onto well-polished alumina ultrafiltration membranes with a pore size of
about 200 nm (Whatman anodisc). By chance, CNTs occasionally span the pores and these
can be subjected to mechanical testing on the nanometer length scale. Contact mode AFM
(M5 Park Scientific Instruments) under ambient conditions is used to collect images of the
suspended CNTs at various loading forces. Fig. 4 shows an AFM image of a SWNT bundle
suspended across a pore and a schematic representation of the mechanical test. The maximum
deflection of the CNT into the pore as a function of the loading force can be used to ascertain
whether the behavior is elastic. If the expected linear behavior is observed, the YoungÕs
modulus can be extracted using a continuum mechanics model for a clamped beam
configuration6,7. The suspended length of the CNT, its deflection as a function of load and its
diameter can all be determined from the images, enabling the modulus to be deduced. The
diameter is taken as the height of the tube above the membrane surface at the clamped ends.
Although tip convolution can be a problem in measuring lateral dimensions using AFM, the
height is a reliable measure because the CNTs are essentially incompressible at these loads
(nominal loading forces are in the range 1 to 5 nN). The suspended length can be determined
from line profiles taken either side of the tube. A minimum suspended length is measured
because of the convolution of the tip with the edge of the pore. This means that all the
determined moduli quoted here are minimum values.
The powerful advantage of the AFM technique employed in our laboratory is its
simplicity. There is no need for complex lithographic techniques for suspending and clamping
tubes. The surface forces between the CNTs and the alumina membrane are sufficiently high
to maintain the clamped beam condition in the majority of cases. In addition, the nanotubes
are never exposed to electron radiation during measurement, which is the case for TEM
studies. Radiation will induce defects, if the energy of the electrons is high enough, and
thereby alter the material properties. This is one effect that we are currently utilising in a
positive way in an effort to modulate CNTsÕ mechanical properties. The relative ease of
sample preparation in our AFM method enables a high measurement throughput allowing us
to measure a variety of CNTs synthesized in different ways and to compare the results.
Described below are new data on catalytic MWNTs, and arc-grown SWNT bundles that have
been hydrogenated and exposed to a low-level of radiation. Catalytic MWNTs were produced
through decomposition of acetylene over a cobalt/silica catalyst. The previously measured
catalytic MWNTs were produced at a temperature of 900oC 11, whereas the new data
presented here were obtained on MWNTs fabricated at 720oC. The microstructure of the
catalytic MWNTs has a strong dependence on the synthesis temperature which can be readily
seen via HRTEM.
Figure 4. (a) 3D rendering of an AFM image of a SWNT bundle adhered to the alumina ultrafiltration membrane, leading to a clamped beam configuration for mechanical testing. (b) Schematic representation of the measurement technique. The AFM applies a load, F, to the portion of nanotube with a suspended length of L and the maximum deflection d at the center of the beam is directly measured from the topographic image, along with L and the diameter of the tube (measured as the height of the tube above the membrane).
To produce crosslinks between the shells of MWNTs and between the SWNTs of
SWNT bundles, the sp2 carbon bonding must be disrupted to sp3 bonding so that dangling
bonds are available for crosslinking. Since the sp2 bonding is the essence of the CNTs
strength, this must not be disrupted to such a degree that the properties of the individual shells
in MWNTs or individual SWNTs in bundles are degraded. Hydrogenating CNTs is a first step
towards producing them with internal crosslinking. The MWNTs and SWNT bundles were
hydrogenated using a modified Birch reduction using Lithium and methanol in liquid
ammonia12. The SWNT bundles were subsequently exposed to 2.5 MeV electrons with a total
radiation dose of 11 C/cm2. A theoretical estimation of the number of displacements that this
dose produces suggests that it will create about 1 defect per 360 carbon atoms13.
AFM measurements on the hydrogenated, irradiated SWNT bundles are shown in Fig.
5, along with the previous measurements of untreated SWNT bundles6. As before, the bundles
were dispersed in ethanol using an ultrasonic probe and deposited on the alumina membranes
for measurement by AFM in air. Within the errors of our measurement technique a
strengthening of the bundles was not observed: the YoungÕs modulus still decreases in a
similar trend to the as-grown bundles. However, the treatment does not appear to have
compromised the strength of the individual SWNTs either, since the lower diameter bundles
have comparable moduli. In addition, it was noticed that the treated bundles were more
difficult to disperse in ethanol and the morphology of the sample in the AFM showed that the
bundles exhibited a higher degree of aggregation. Taken together these data suggest that the
radiation treatment produced bonding between the tubes but was not sufficient to produce
enough crosslinks within the bundles to reduce shearing effects and produce bundles with
higher YoungÕs moduli. Future efforts will concentrate on optimising the chemistry and
irradiation doses to improve their mechanical properties.