Mechanical response of carbon nanotubes turfs
under lateral strains
Melinda C. Lopez, David F. Bahr
Intro to Multiscale Engineering
School of Mechanical and Materials Engineering
Introduction
Carbon nanotubes have a broad array of applications due to
their remarkable mechanical, electrical, and thermal properties.
In particular, they have been observed to be among the
strongest materials tested in tensile loading and possessing one
of the highest elastic moduli. Much of the research though has
been conducted on the mechanical properties of single carbon
nanotubes under stress, with less performed on large
collections of carbon nanotubes, or “turfs”, and their response
under different strains. In this experiment the mechanical
response of carbon nanotube (CNT) turfs are examined by
nanoindentation under different strains.
(a)
(b)
Figure 1. Array of CNT turfs at (a) 0% and (b) 3.5% strain
Materials and Methods
The CNT turfs are mechanically transferred to a substrate
using the method developed by Ryan Johnson [1]. The CNT turf
is placed on top of a Kapton coated in 300 nm of gold. The two
are then placed for two hours in a furnace 150° C under a stress
equal to that of the buckling stress of the CNT.
Figure 2. An illustration of how the carbon nanotubes were
transferred to the gold coated Kapton.
The CNT turfs were tested using nanoindentation at 0%
strain and at 3.5 % strain, where the gold coated Kapton
containing the transferred CNT turf is placed in a vice and
fastened. The CNT turf is then manually stretched in this
machine reaching a strain of 3.5% prior to the gold film
cracking. Larger strains can be achieved, but 3.5% is the upper
limit used for this system.
(a)
Figure 3. The vice used to manually stretch the CNT turf
Once the transfer and strain had been completed, the
mechanical response of the CNT turf is observed using
nanoindention using the Hysitron Triboscope using dynamic
mechanical analyzer, the same method as McCarter [2]. With
this method, an oscillating force is applied to the carbon
nanotubes allowing for multiple measurements to be taken on
one location. In this particular experiment a frequency 30 Hz
and a maximum force of 500 µN was used.
Figure 4. An illustration of the
nanoindenter tip used. In this
particular test, the tip was a
spherical in shape, and the
diamond was gold coated to
prevent adhesion from the carbon nanotubes
Results
At 0% strain, the nanoindenter reached an average depth of
1,313 nm, a minimum of 951 nm, and a maximum of 1,946 nm
in the carbon nanotube turfs. The elastic modulus of the carbon
nanotube, no matter where the indentation was made, levels
out at 61.9 MPa.
At 3.5% strain, the nanoindenter reached an average depth
of 2,436 nm, a minimum of 1895 nm, and a maximum of 2,866
nm in the carbon nanotube turfs. The elastic modulus varies at
this strain but is consistently lower than the 0 % strain.
The CNT turf at 0% has a higher elastic moduli then the turf
at 3.5 %, but does not indent as far. One reason the VACNT turf
at 3.5 % strain appears to be stiffer is that as it is stretched, the
area becomes less dense and exposes more of the gold coated
Kapton.
(b)
Figure 5. Elastic modulus of carbon nanotubes at (a) 0 % and (b)
3.5% strain
Figure 6. The moduli of a CNT turf
at 0% strain in red versus the
modulus of a CNT turf at 3.5%
strain in black.
Conclusions
The objective of this work was to observe if there was a
change in the effective elastic modulus of a carbon nanotube
turf as it is induced under different strains. Nanoindentation
showed that a carbon nanotube turf at 0% strain has a higher
elastic modulus than when stretched to a 3.5 % strain, whereby
the CNT turf appears to be a stiffer material.
Literature Cited
[1] Johnson R D, Bahr D F, Richards C D, Richards R F, McClain D,
Green J, and Jiao J. Thermocompression boding of vertically
aligned carbon nanotube turfs to metalized substrates.
Nanotechnology 20, 6 (2009)
[2] McCarter C M, Richards R F, Mesarovic S Dj, Richards C D,
Bahr D F, McClain D, Jiao J, Mechanical compliance of
photolithographically defined vertically aligned carbon
nanotube turf. Journal of Material Science 41, 7872-7878 (2006)
Acknowledgements
I would like to give special thanks to Katerina Bellou and Ryan
Johnson at WSU for their help in my research.
This work was supported by the National Science
Foundation's Research Experience for Undergraduates program
under grant number EEC-0754370.