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Since their discovery in 1991, carbon nanotubes
have been the subject of intense research interest
based on early predictions of their unique mechanical,
electronic, and chemical properties. Materials
with the predicted unique properties of carbon
nanotubes are of great interest for use in future
generations of aerospace vehicles. For their structural
properties, carbon nanotubes could be used as
reinforcing fibers in ultralight multifunctional composites.
For their electronic properties, carbon
nanotubes offer the potential of very high-speed, low-power
computing elements, high-density data
storage, and unique sensors. In a continuing effort to
model and predict the properties of carbon
nanotubes, Ames accomplished three significant
results during FY99. First, accurate values of the
nanomechanics and plasticity of carbon nanotubes
based on quantum molecular dynamics simulations
were computed. Second, the concept of mechanical
deformation catalyzed - kinky - chemistry as a means
to control local chemistry of nanotubes was discovered.
Third, the ease of nano-indentation of silicon
surfaces with carbon nanotubes was established.
The elastic response and plastic failure mechanisms
of single-wall nanotubes were investigated by
means of quantum molecular dynamics simulations.
Working with researchers from Stanford University
and the University of Kentucky, it was found that the
elastic limit of thin carbon nanotubes under axial
compression is significantly lower than earlier
predictions based on classic molecular dynamics
investigations. A novel mechanism of nanoscale
plasticity is observed in which bonding geometry
collapses from a graphitic to a localized diamond-like
reconstruction. Figure 1 shows a compressed
nanotube collapsed near the two edges by plastic
deformation. The bonding geometry shown in
figure 1(b) reveals a diamond-like structure at the
location of the collapse. The computed critical stress
(approximately 153 gigapascals) for the collapse and
the shape of the resulting deformation are in good
agreement with recent experimental observations of
compressed nanotubes in polymer composites. These
results are a first step in the accurate characterization
of isolated nanotubes for their potential application in
ultralight structural composites for aerospace
applications.
The relationship between mechanical deforma-tion
and chemical reactivity (mechano- or kinky-chemistry)
of carbon nanotubes was investigated in a
collaborative effort between Ames, North Carolina
State University, and the University of Washington at
St. Louis. The sidewalls of pure nanotubes are
relatively inert, whereas the end-caps are reactive.
However, for many applications selective sidewall
functionalization or reactivity of carbon nanotubes is
highly desired. It is shown that such reactivity could
be enhanced and controlled by mechanical deformations.
When a mechanically twisted or kinked tube is
exposed to an environment of reactant, the reactant
specifically functionalizes (adsorbs) or etches the
twist or kink. Figure 2(a) shows a twisted nanotube
that has flattened into a ribbon-like structure with
sharp edges. Figure 2(b) shows the preferential
adsorption of atomic H at the strained edges of the
twisted nanotube. For the first time, computational
prediction of the kinky chemistry of nanotubes has
been experimentally verified in a proof-of-principle
experiment at University of Washington at St. Louis.
Indentation of diamond and silicon surfaces with
carbon nanotubes used as atomic force microscope
(AFM) tips was simulated. Indentation of a diamond
surface by a nanotube causes buckling and collapse
of the tube. However, a nanotube very easily indents
a silicon surface. Thus, this technique can be used for
making nanoscale holes on silicon surfaces with
potential applications in high-density data storage, or
nanolithography of silicon surfaces for electronics
applications.
Point of Contact: D. Srivastava
(650) 604-3486
deepak@nas.nasa.gov
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Fig. 1. (a) Compressed nanotube collapsed near the
two edges by plastic deformation. (b) A diamond-like
structure at the location of the collapse.
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Fig. 2(a). Twisted nanotube that has flattened into a
ribbon-like structure with sharp edges.
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Fig. 2(b). Preferential adsorption of atomic H at the
strained edges of the twisted nanotube.
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