Mechanics of Atomic Scale Interfaces in Carbon Nanotube Reinforced Composites

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Mechanics of Atomic Scale Interfaces in Carbon
Nanotube Reinforced Composites

• Namas Chandra and Sirish Namilae
• Department of Mechanical Engineering
• Florida State University

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Carbon Nanotubes (CNT)

• Carbon Nanotubes Graphite sheet rolled into a
  tube
• Single wall and Multiwall nanotubes
• Zigzag, armchair and chiral nanotubes
• Length 100 nm to few ?m Diameter 1 nm

Applications
E 1 TPa Strength 150 GPa Conductivity depends
on chirality
Nano sensors Medical applications
Do these properties extend to CNT reinforced
composites ?
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Answer Currently NO!!!
Parallel model Upper Bound
Series model Lower Bound
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Critical Issues

• Critical issues in nanotube composites
• Alignment
• Dispersion
• Load Transfer
• Load transfer and to some extant Dispersion
  affected by interfaces
• Interface ? Bounding surface with physical /
  chemical / mechanical discontinuity
• CNT-matrix interfaces
• Vanderwalls forces
• Mechanical interlocking
• Chemical bonding

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Functionalized Nanotubes

• Change in hybridization (SP2 to SP3)
• Experimental reports of different chemical
  attachments
• Application in composites, medicine, sensors
• Functionalized CNT are possibly fibers in
  composites

• How do fiber properties differ with chemical
  modification of surface?

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Functionalized nanotubes

• Increase in stiffness observed by functionalizing

Vinyl and Butyl Hydrocarbons T77K and
3000K Lutsko stress
Stiffness increase is more for higher number of
chemical attachments Stiffness increase higher
for longer chemical attachments
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Radius variation

• Increased radius of curvature at the attachment
  because of change in hybridization
• Radius of curvature lowered in adjoining area

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Evolution of defects in functionalized CNT

• Defects Evolve at much lower strain of 6.5 in
  CNT with chemical attachments
• Onset of plastic deformation at lower strain.
  Reduced fracture strain

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Different Fracture Mechanisms

• Fracture Behavior Different
• Fracture happens by formation of defects,
  coalescence of defects and final separation of
  damaged region in defect free CNT
• In Functionalized CNT it happens in a brittle
  manner by breaking of bonds

Chemical Physics Letters (2004)
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Multiscale approach to composite problem

• Need for multiscale approach ?
• Why hierarchical modeling ?
• Molecular dynamics of pull out tests
• Atomically informed cohesive zone model for
  interfaces
• Finite element model for composites with CZM
  interfaces

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Atomic simulation of CNT pullout test

• Simulation conditions
• Corner atoms of hydrocarbon attachments fixed
• Displacement applied as shown 0.02A/1500 steps
• T300K

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Debonding and Rebonding of Interfaces
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Debonding and Rebonding
Matrix
Matrix

• Energy for debonding of chemical attachment 3eV
• Strain energy in force-displacement plot 20 4
  eV
• Energy increase due to debonding-rebonding

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Variation in interface behavior
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Force distribution along the interface
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Force Distribution - continued

• Variation of reaction force with time as shown
• Black interface region fully loaded, white region
  is unloaded or fractured

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Cohesive zone model for interfaces

• Assumptions
• Nanotubes deform in linear elastic manner
• Interface character completely determined by
  traction-displacement plot

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Cohesive zone Models for nanoscale interfaces
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Finite element simulation Composite stiffness
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Parametric studies
Variation of fiber stiffness for different
interface strengths
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Summary and conclusions

• Functionalized CNT as fibers
• Marginal increase in stiffness
• Inelasticity sets in at lower strains
• Mechanism of fracture is different

• Nanoscale interfaces in CNT based composites
• Very high interface strength can be obtained by
  engineering chemically bonded interfaces
• Interface strength depends on various factors,
  primarily extant of bonding
• Unique features such as debonding-rebonding
  observed in nanoscale interfaces
• Multiscale model with interface described by
  atomically informed CZM developed
• Effect of interface strength on stiffness of
  composite studied using this multiscale model