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Carbon nanotubes have attracted great interest since they were first
synthesized in 1991. The tubes have substantial promise in a variety of
applications due to their unique mechanical and electronic properties.
Efforts have been made to characterize the mechanical properties of carbon
nanotubes, experimentally and theoretically. However,
previous work has concentrated on the tubes’ longitudinal properties, and
studies of their radial properties lag behind. We used a
scanning probe microscope with an indentation/scratch function to
investigate the radial compression of multiwalled carbon nanotubes under an
asymmetric stress. In particular, we have determined the
radial compressive elastic modulus at different compression levels, and
estimated the compressive strength to be well beyond 5.3GPa.
Fig. 1.
The AFM image of CNT taken in
the tapping mode (size: 2mm ´ 2mm).
Fig. 2.
Manipulation of the tubes using the indentation/scratching function
of the NanoScopeTM IIIa.
All images are 500nm by 500nm.
(a) Tapping mode image of tubes
before manipulation.
(b) After knocking the
overlapping tubes at point A, one end of the
upper
tube slipped off the lower one.
(c) Making a scratch at the
surface of the substrate from point B along
the
direction indicated in (b) completely separated the two tubes.
(d) Making a scratching from C
to D as shown in (c) pulled the upper
tube
further away from the lower one.
Fig. 3. The AFM image
u
To estimate the compression strength of the tube, we used excessive
forces, in spite of substrate indentation. We
have tried to break the tubes with a force as large as 20µN for more
than two dozen times, but did not succeed. It is a further evidence
that the carbon nanotubes are remarkably resilient due to its
hexagonal network structure. The tubes can sustain extreme
compression with no signs of breaking, plasticity or atomic
rearrangements.
u
This figure shows a restored nanotube sitting on top of an indent
between the pile-ups caused by indenting on silicon wafer substrate,
which was made by a 20µN force exerted by the tip through the
compressed and flattened tube. Based on the dimension of the indent,
the tube was bent to an angle of about 50° during indenting, in
addition to the extreme compression. According
to the forces we used in the trials and the size of the indents, the
compressive strength of the nanotubes should be beyond 5.3Gpa.
Fig. 4. The AFM
images
Using the scratching function to stretch the tubes in their
middle section along the direction perpendicular to their
axis with the two ends more or less fixed or blocked, we
have broken the tubes for several times. This suggests that
longitudinal stretch strength of the tubes may not be as
strong as the radial compressive strength.
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