Computational%20Nanotechnology

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Computational Nanotechnology
A preliminary proposal
N. Chandra Department of Mechanical
Engineering Florida AM and Florida State
University
Colloborators

• Proposed Areas
• Nanoscale composites
• Nanoscale interface Mechanics
• Defect Engineering in CNTs
• Hydrogen Storage
• Parallel computing, Time extension algorithms

Professor Ashok Srinivasan, Dept. of Computer
Science, FSU Professor Leon van Dommelen, Dept.
of Mechanical engineering, FAMU-FSU
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Application of Carbon Nanotubes (CNTs) to
Nanocomposites and Hydrogen Storage

• Significant Results
• Developed atomic level stress measures (3
  types), and strain measures (new) and validated
  the mechanical response of CNTs
• Determined mechanical properties of CNTs for
  various chiralities, with and without defects
  (Stone-Wales)
• Evaluated the effect of a single and multiple
  interacting and non-interacting defects on the
  mechanical properties using stress/strain
  measures
• Role of functionalization (attachment of radical,
  e.g. vinyl)on the in-situ mechanical properties
  of CNTs and their effect on evolution of defects
• Identification of the optimal set of parameters
  that maximizes the hydrogen storage? Parameters
  include chirality and diameter of
  single/multiwalled tubes, temperature, pressure
  and geometrical defects
• Parallelization of codes in IBM-p690-512
  processors

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Research Areas

• NanoScale Interfaces
• Nano-Composites
• Hydrogen Storage
• Parallel Algorithms

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Nano-Scale Interfaces

• Issues
• How does the thermo-mechanical load transfer take
  place at nano-scale?
• Can the macro theories (chemical bonding,
  mechanical serrations Kelly-Tyson) be applicable
  and if not how they should be modified?
• What methods enhance strength, stiffness and
  fracture behavior of nanocomposites ?
• Will any addition of short radicals to smooth
  surface enhance bonding? Will the nature of
  bonding be chemical, mechanical or electronic ?
  
• How will the application of mechanical loading
  (tension, compression, shear or combination
  thereof) and thermal loading affect the load
  transfer?
• What is the equivalence of thermal residual
  stresses at the nano-scale interfaces?

• Functionalized CNT have higher stiffness.
• High temperature deformation- defects are formed
  at lower strains (6 strain) .
• Fracture occurs at lower strains in
  functionalized tubes.

Functionalization a possible mechanism to
increase interface strength ? Different numbers
and groups of hydrocarbons were attached and
tested in tension
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Defect Engineering in CNTs
Background Defects (missing atoms, rotated
bonds, diameter/chiralty transition) arise during
processing and loading. They are either
deleterious or beneficial
Results

• Issues
• Effects of defects on elastic and also inelastic
  properties (strength, stiffness and elastic to
  plastic transition and failure)
• Role of chirality, diameters and location of
  single and multiple interacting/non-interacting
  defects and their effect on properties
• Aligned defects in single/multi-walled CNTs and
  their effect on mechanical properties and
  hydrogen storage
• Interaction of defects and functionalization in
  load transfer
• Origin of defects as a function of combined
  thermal and mechanical load application

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Hydrogen Storage
Back ground Interest in hydrogen as a fuel has
grown dramatically since 1990. However, hydrogen
storage technologies must be significantly
advanced if a hydrogen based energy system is to
be established. Nanotubes have been long heralded
as potentially useful for hydrogen storage to
meet energy densities at values of 6.5wt set by
DOE.

• Issues
• Mechanism of hydrogen adsorption is it a purely
  physical or chemical interaction or is it
  somewhere in between.
• Optimize a given carbon adsorbent system
  simulation of different parameters such as
  temperature, pressure, diameter and chirality.
• Simulation of adsorption considering nanotube
  with defects , disorder, diameter polydispersity,
  and functionlization.
• Simulation of adsorption of Li-doped nanotube
• Simulation of high energy hydrogen atoms
  implanting into nanoutbe.
• In theory, close ends nanotube can have a
  volumetric densities of 142kg/m3 storage since
  nanotube has a high tensile strength.

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Hydrogen Storage-MD Simulation
Preliminary Results
Results After 100ps simulation, about 3.18 wt
hydrogen absorbed within the intratube spacing.
Initial condition Carbon nanotube (10,10)
periods4 Pressure, 15atm. Temperature 77K
hydrogen atoms160 carbon
atoms480 Periodic box size60x78x9.84 A. Time
step0.25 fs.
Work in progress What forces are required to
separate the tubes (magnetic?) to store and
release hydrogen?
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Nanocomposites

• Process modeling of nanocomposite fabrication
  using multi-scale methods to enhance alignment
• Modification of nanoscale interfaces to improve
  load transfer through functionalization or/and
  defect engineering or/and surface modification
• Large-scale simulation of nanocomposites to
  determine thermo-mechanical properties
  optimization studies for strength, stiffness and
  fracture
• Enhance longitudinal and transverse stiffness by
  improved interfacial bonding
• Mechanics of defect formation- loss in strength
  and stiffness

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Computational Issues and Large scale parallel
computing
Issues

• Use FSUs 512 processor IBM 690p server
• Third fastest university owned supercomputer in
  the US
• Science-aware parallelization
• Predict regions likely to experience short
  time-scale phenomena and concentrate
  computational resources there
• Avoid fine granularity where possible
• Use Monte Carlo techniques for rare-event
  simulation when required, to avoid fine
  granularity
• Efficiently parallelizable through replication
• Faster versions of traditional parallelization
  techniques
• Stochastic versions of traditional domain
  decomposition techniques
• Trade computation for communication
• Mixed shared and distributed memory
  parallelization
• Optimize sequential component too
• Cache-aware computation

Solutions

• Large time scale
• Small system size
• Fine grained parallelization
• High communication cost
• Adaptive computations
• Regions experiencing short time-scale phenomena
  simulated with a finer resolution
• Spatial decomposition and granularity change
  dynamically, and quickly, with time
• Need fast and efficient load balancing strategies

Resources Third largest computer in U.S
universities (IBM-p690-512 processors)
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Nanocomposite simulation-prelim. results

• Model
• Matrix-nanotube interface modeled with springs
• An extra force term computed for atoms attached
  to springs
• Springs can break, requiring substantial increase
  in computations in that region

Polymer matrix
Spring

• Experimental parameters
• Nanotube with 1000 atoms
• Spring probability 0.05
• Probability of a spring breaking in an iteration
  0.01
• Load increase factor due to spring break 200
• Disturbance region depth 3
• Number of time steps 100

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Detailed information on each of the topics
Click on the arrow for any topic for direct link
Mechanics of Defects (presentation)
Interaction of Defects in nanotubes
Nanoscale interface mechanics