Modified embedded atom method study of the mechanical properties of carbon nanotube reinforced nickel composites Page: 10
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PHYSICAL REVIEW B 81, 104103 (2010)
implicates a strong Ni/CNT interfacial interaction. The RoM
Ell increases linearly with volume fractions and also shows
dependence with CNT's radii.
Comparison of the RoM estimated E1l with simulated val-
ues in the case of SWCNTs shows an opposite trend. Accord-
ing to the RoM, we should expect SWCNT(15,0) to give the
largest value of E11 compared to that of SWCNT(5,0). How-
ever, simulated results predict the opposite. SWCNT(10,0)
shows a more remarkable agreement between calculated and
simulated values than any other CNTs. Note that the plots in
Fig. 5 are very useful as these can be used to predict the
modulus for CNT with any diameter and volume fraction.
To reasonably compare the different CNT composites, one
can use the rate of increase in Young's modulus with volume
fraction (dEll /dVf) as a convenient yardstick for assessing
the effect of reinforcement (CNT effectiveness). This useful
parameter shows the magnitude of the increase in stiffness
and the CNT content required achieving that increase. For all
four composites, the simulated E1l increases approximately
linearly with volume fraction of CNT. The dEl l/dVf are 707
GPa, 629 GPa, 474 GPa, and 951 GPa for Ni/SWCNT(5,0),
Ni/SWCNT(10,0), Ni/SWCNT(15,0), and Ni/MWCNT, re-
spectively. The highest value of dEll/dVf (951 GPa) was
found with the MWCNT. Using the RoM data, the enhance-
ment dEll/dVf is found to be 415 GPa, 695 GPa, 755 GPa,
and 603 GPa, respectively, for Ni/SWCNT(5,0), Ni/
SWCNT(10,0), Ni/SWCNT(15,0), and Ni/MWCNT. This
opposite trend between the RoM and simulated results sug-
gests that the reinforcement mechanism is not only from
mixing alone. This deviation likely arises from the large
surface-area-to-volume ratios of CNTs that affect matrix
properties beyond what would be predicted from RoM cal-
culations.
It is known that the performance of a reinforced compos-
ite depends on the interfacial characteristics of the reinforce-
ment fillers and the matrix materials.60 For example, strong
adhesion forces at the interface lead to superior mechanical
properties. It also critically depends on the effectiveness of
the interfacial stress transfer, which in turn depends on the
nature and strength of the interface. Chemical bonding, na-
nomechanical interlocking, and nonbonded (van der Waals
and electrostatic) interactions yield strong interaction be-
tween a polymer matrix and CNTs. Furthermore, chemical
functionalization of the filler surface and increasing non-
bonded attractive interactions between the filler and the ma-
trix increases such interfacial adhesion and improve the me-
chanical properties of composites. For most polymers, the
polymer/CNT interfacial stress transfer is low with values
typically less than 50 MPa.61'
For metal/CNT interfaces, above-discussed information is
not available. The large increase in Ell for Ni/CNT compos-
ites indicates that there exists a surprisingly strong adhesive
force between the CNT and the Ni matrix. An applied load is
largely transferred to the reinforcing CNTs, indicating an ef-
fective stress transfer across the Ni-CNT interfaces that in
turn enhance the mechanical response. We would like to em-
phasize that a thorough investigation using high-level theo-
ries is necessary to fully understand the chemistry at the
Ni-CNT interfaces. Recent first-principles calculations ondifferent structural models (e.g., top fcc, bridge top, etc.) of
graphene on Ni(l11) show very weak or no binding interac-
tion between graphene and Ni surface,62,63 which may be due
to shortcomings of current levels of density-functional theory
utilized in solid-state calculations. Simulations using novel
and adequate theory to rationalize the interfacial behavior in
Ni/CNT composites are beyond reach at this time due to the
sheer size of these systems.
IV. CONCLUSIONS
A MEAM formalism has been used to investigate the me-
chanical properties of Ni/CNT nanocomposites made by in-
corporating SWCNTs with three different diameters and a
MWCNT into fcc Ni matrices. Two different volume frac-
tions for each CNT were used to assess the CNT concentra-
tion effects upon mechanical properties of the composites.
Using quasistatic energy-minimization methods and periodic
boundary conditions, the single-crystal elastic constants,
Young's and shear moduli, Poisson's ratios for pure nickel
matrix, and Ni/CNT composites were calculated. The elastic
anisotropy in the composites was also systematically inves-
tigated. The volume fraction dependence of the Young's
modulus was analyzed using rule-of-mixture and compared
that with the simulated values.
Calculations show that pristine SWCNTs have high
Young's modulus values of 0.55 TPa, 0.83 TPa, and 0.89 TPa
for the SWCNT(5,0), SWCNT(10,0) and SWCNT(15,0), re-
spectively. The MWCNT made from the above three
SWCNTs show a Young's modulus of 0.74 TPa. These val-
ues are in reasonable agreement with the observed and theo-
retically predicted values published in the literature.
The results for Ni/CNT composites are primarily com-
pared with the results for a pure fcc nickel matrix. Our cal-
culations predict that all Ni/CNTs satisfy the conditions for
mechanical stability. In the cases of single-crystal Ni/CNT
composites, the Young's modulus shows a significant in-
crease in the longitudinal direction (the direction of CNT
alignment) and a moderate decrease in the transverse direc-
tions. It is found that CNT volume fractions have large ef-
fects on the single-crystal Young's modulus. For pure fcc
nickel, our calculated Young's modulus is 137 GPa. In the
composites with high CNT volume fraction, the single-
crystal Young's modulus along the longitudinal direction is
found to be 160 GPa, 183 GPa, and 201 GPa for Ni/
SWCNT(5,0), Ni/SWCNT(10,0), and Ni/SWNCT(15,0), re-
spectively. Ni/MWCNT shows the highest value of 266 GPa
for Young's modulus in same direction when high CNT vol-
ume fractions are used. In the transverse directions, the
Young's modulus decreases by 1-7 GPa for Ni/SWCNTs de-
pending on the CNT volume fraction and. For Ni/MWCNT
however, 3-12 GPa increase is seen.
For the composites with SWCNTs, the polycrystalline
Young's modulus decreases with increasing CNT diameters
and CNT volume fractions. In the Ni/MWCNT nanocompos-
ite, however, a slight increase is seen at high CNT volume
fractions, indicating possibility of fabricating composites
with enhanced overall mechanical properties when
MWCNTs with sufficiently larger size and volume fractionswere used. The polycrystalline shear modulus remain similar
104103-10
UDDIN et al.
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Uddin, Jamal; Baskes, Michael I.; Srivilliputhur, Srinivasan; Cundari, Thomas R., 1964- & Wilson, Angela K. Modified embedded atom method study of the mechanical properties of carbon nanotube reinforced nickel composites, article, March 11, 2010; [College Park, Maryland]. (https://digital.library.unt.edu/ark:/67531/metadc107769/m1/10/?rotate=270: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.