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MICRO AND NANO-STRUCTURED COMPOSITES FOR SENSING
AND ACTUATION
AFOSR Grantees’ / Contractor’s Meeting in Mechanics of Multifunctional Materials and Microsystems
Arlington, VA · August 2, 2012
Erik T. Thostenson
Department of Mechanical Engineering & Center for Composite Materials
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Erik T. Thostenson
OUTLINE
• Introduction
– Background and Motivation
– Key Objectives
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
• Structured Nanocomposites for Strain Sensing
• In Situ Thermoresistive Distributed Sensing of
Thermochemical and Thermomechanical
Transitions
• Conclusions
• Acknowledgments
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Erik T. Thostenson
MICRO / NANO STRUCTURED COMPOSITES
• To exploit both the unique properties of nanomaterials
along with their nanoscale size, recent attention has
focused on development of “hierarchal nanocomposites”
where the nanotubes or nanoparticles are phase-
segregated at the microscopic scales.
• Controlled dispersion enables selective reinforcement /
local property changes that can alter the bulk structural
and functional material properties.
“These materials are a paradigm of exploiting the hierarchical
nanocomposite structure, rather than the inherent physical properties or
the reinforcement ability of the nanofillers, to give rise to novel
properties.” - E. Manias, Nature Materials, 6, 9-11 (2007)
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BRIDGING THE MICRO AND NANO SCALES
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KEY OBJECTIVES AND APPROACH
• The objective of the research is to explore the
development of micro/nano structured composites,
where the nanoscale reinforcement is locally
segregated at the microscopic scales.
• Explore processing techniques applicable to a wide
range of material systems and holds potential for future
scale-up of micro/nano structured composites.
• The research is investigating the micro and nano-scale
structures and relate the morphology to the structural
and functional properties of the as-processed
composites based on electroactive polymer systems
and piezoresistive sensing.
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Erik T. Thostenson
OUTLINE
• Introduction
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
– Approach and Previous Results
– Recent Advances in Processing and
Simulations
• Structured Nanocomposites for Strain Sensing
• In Situ Thermoresistive Distributed Sensing of
Thermochemical Transitions
• Conclusions
• Acknowledgments
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ELECTROACTIVE NANOCOMPOSITES
• Nanocomposite electroactive polymer actuators and sensors
have shown substantial promise in overcoming high field and
low output in sensing.
• Zhang et al. reported drastically enhanced electromechanical
coupling coefficients. Laxminarayana and Jalili reported
substantial increases in strain sensing ability.
• Li established that nanoscale exchange coupling between the
low dielectric constant polymer and high-dielectric constant
regions leads to drastically enhanced polarization in the
polymer matrix. The micro-scale size of the conducting
region plays a large role in the response.
• Carbon nanotubes can result in enhancements in dielectric
properties (coupling) and also stiffen the polymer matrix.
Li, Physical Review Letters (2003)
Zhang et al, Nature (2002)
Laxminarayana and Jalili, Textile Research Journal (2005)
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ELECTROSPINNING AS A PLATFORM FOR MULTI-FUNCTIONAL, HIERARCHICALLY ORGANIZED COMPOSITES
Li and Xia, Advanced Materials (2004)
Teo and Ramakrishna, Composites Science and Technology (2009)
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Erik T. Thostenson
PARALLEL ELECTROSPINNING FOR CO-MINGLED NANOFIBROUS FILMS
• Entanglement of the electrospun jets in the instability
region can result in a uniformly co-mingled film of fibers
where nanotubes are confined within individual filaments.
• The parallel electrospinning approach is also broadly
applicable to combinations of polymer fiber networks.
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Erik T. Thostenson
CO-MINGLED NANOCOMPOSITE FILMS FROM ELECTROSPINNING
• Secondary-processing of co-
mingled electrospun micro /
nanofiber assemblies in a
dense film results in a self-
reinforced hierarchical
composite.
• Consolidation is in the solid-
state, analogous to sintering of
particles.
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MODEL NANOCOMPOSITE SYSTEM
Hanwha Nanotech (CM95)
Acid-Treated MWCNT
– Acid reflux
Ozone-Treated MWCNT
– Continuous circulative sonication
• Functionalization verified using XPS
Carbon Nanotubes Poly(vinylidene fluoride)
PVDF - Scientific
Polymer Products, Inc.
MW – 530,000
PVDF – KYNAR 761A
Arkema
MW – 625,000
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MODEL NANOCOMPOSITE SYSTEM
Solution
Crystal
Formation
Temp
< 70 deg C
> 160 deg C
𝛽-crystals
𝛼+𝛽-crystals
> 70 deg C 𝛼-crystals
Mech.
Drawing
(x5)
𝛽-crystals
• The processing history is very important for PVDF
• 𝜷 crystal formation only exists through certain process routes
• Electrospinning: (1) generates 𝜷 phase PVDF, (2) allows for mutli-
phase control, (3) segregates carbon nanotubes to increase
percolation threshold.
Melt
Processing
Crystal
Formation
Temp
< 160 deg C
> 160 deg C
𝛼-crystals
𝛼+𝛽-crystals
Mech.
Drawing
(x5)
𝛽-crystals
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Erik T. Thostenson
CO-MINGLED NANOCOMPOSITE FILMS FROM ELECTROSPINNING
Final Consolidation (Solid State)
Secondary Processing
Axial Stretching Pre-Consolidation
Co-Electrospun Films
Nanotube Dispersion Co-Mingled Films
• Varying molecular weights
enable a lower softening
temperature for one fiber phase
• Melting is not desirable as it
alters the desired crystal
structure.
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PARALLEL ELECTROSPINNING FOR CO-MINGLED NANOFIBROUS FILMS
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PARALLEL ELECTROSPINNING FOR CO-MINGLED NANOFIBROUS FILMS
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PARALLEL ELECTROSPINNING FOR CO-MINGLED NANOFIBROUS FILMS
Fluorescein
Film Separation
Fully Co-Mingled Film
• The addition of fluorescein in
small concentrations enables
the observation of fiber co-
mingling at the macroscopic
and microscopic levels.
• The film uniformity can be
visually observed during the
spinning process.
• Confocal laser scanning
microscopy (CLSM) can
obtain high-resolution images
of as-spun and consolidated
films.
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PARALLEL ELECTROSPINNING FOR CO-MINGLED NANOFIBROUS FILMS
• The addition of fluorescein in
small concentrations enables
the observation of fiber co-
mingling at the macroscopic
and microscopic levels.
• The film uniformity can be
visually observed during the
spinning process.
• Confocal laser scanning
microscopy (CLSM) can
obtain high-resolution images
of as-spun and consolidated
films.
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PROCESSING OF SELF – REINFORCED CO-MINGLED NANOCOMPOSITE FILMS
• Consolidation accomplished using
different methods.
• Beading of the fibers strongly affects
consolidation of the films and results in
large variability in the final
microstructure.
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ELECTROMECHANICAL CHARACTERIZATION
Digital Image Correlation – Anisotropic Strains
Sensing or actuation. Actuation provides load
feedback and sensing provides voltage feedback
Load or strain controlled tension profiles allow for
analysis of phase-shift
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ADVANCEMENT IN PROCESSING AND SIMULATION
• Improved processing of electrospun fibers
– Second generation electrospinning system with full
environmental and multi-point high voltage control
• Greater control over nanotube morphology
• Improved numerical models of sensing/actuation system
– Algorithmically controlled FEA (COMSOL LiveLink with
MATLAB)
– Much larger models with precise variable control
– Stochastic classification of structures
• Characterization of sensor/actuator systems
– Different compliant electrodes for accurate actuation
force measurement
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CONTROLLED ELECTROSPINNING
• Laminar air flow control
• Full environmental control
• Fully articulated modular
positioning system
• 4-channel voltage control
• Constant high-accuracy
digital temperature/humidity
measurement
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CONTROLLED ELECTROSPINNING
• Humidity and temperature control
allows for greater batch-to-batch
consistency.
• The laminar air flow rate combined
with humidity allows for tight
control over evaporation rate
• Four channel + ground voltage
control allows for precise electric
field control and modulation
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FUNCTIONALIZATION AND NANOTUBE MORPHOLOGY CONTROL
Chilled Water
Bath
MWCNT
Solution
Peristaltic
Pump
Ozone
Generator
Moisture
Trap
Flow-
meter
Sonicator
Cell
O2
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FUNCTIONALIZATION AND NANOTUBE MORPHOLOGY CONTROL
Acid Treated Ozone Treated
• Nanotube Treatments
– Acid Treated (<1 m)
– Ozone Treated, High-Energy Pulse (2-5 m)
– Ozone Treated, Medium-Energy Pulse (>5 m)
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ELECTROACTIVE COMPOSITES MODELING
Model
• COMSOL© Multiphysics simulation of smart composite system
• Matlab LIVELINK providing input for simulation parameters –continuously varying microstructures and stochastic distributions
Motivation
• Improve understanding of target system
• Explore fundamental physics
• Rapidly test geometries and ideas
Study
• Generate geometric targets for produced thin films
• Run parametric study to evaluate optimal composite structure for electroactive reinforcement
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EARLIER SIMULATION RESULTS
• There is strong dependence on
volume fraction
• Agglomeration creates “short-
circuits” nullifying the
piezoelectric contribution of
regions of polymer
• High degree of dependence on
CNT-CNT interactions (clusters)
• Results highly variable due
to small simulation size,
magnifying the influence of
clusters
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SIMULATION RESULTS
ORIGINAL STUDY
• Original study was limited to 20
carbon nanotubes and 10 micron unit
cell
• Small variable parameter set, fixed
reinforcement geometry
RECENT STUDY
• Over 600 nanotubes and 52 micron unit cell and expandable
• Programmatic parameter set
• Variable reinforcement geometry
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SINGLE NANOTUBE UNIT CELL
23
25
27
29
31
33
35
37
0 20 40 60 80
/E
(n
m/V
)
Rotation Angle (°)
• Electric potential visualizes the electric field in the medium. The
field is dictated by the material permittivity.
• Deformation is directly proportional to the electric field, so local
increases in field strength (caused by CNTs) increase the strain.
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INFLUENCE OF VOLUME FRACTION
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INFLUENCE OF ORIENTATION
• Degree of alignment is controlled
by defining a sweep angle
• Sweep angle provides the largest
angle that the CNT can deviate
from the principal direction
• Sweep angle ranges from 0
(aligned) to 90° (fully random)
• Model uses a fully random seed
to determine CNT rotations
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INFLUENCE OF ORIENTATION
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Erik T. Thostenson
INFLUENCE OF ORIENTATION
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Erik T. Thostenson
OUTLINE
• Introduction
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
• Structured Nanocomposites for Strain Sensing
– Negative piezoresistivity in carbon nanotube
electrodes
– Network modification for enhanced strain
sensitivity
• In Situ Thermoresistive Distributed Sensing
• Conclusions
• Acknowledgments
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Erik T. Thostenson
OBSERVATION OF NEGATIVE PIEZORESISTIVTY
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Erik T. Thostenson
OUTLINE
• Introduction
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
• Structured Nanocomposites for Strain Sensing
– Negative piezoresistivity in carbon nanotube
electrodes
– Network modification for enhanced strain
sensitivity
• In Situ Thermoresistive Distributed Sensing
• Conclusions
• Acknowledgments
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HIGH FREQUENCY ELECTRICAL RESPONSE
• Impedance measurement related to
dielectric properties based on
geometry using time-domain
reflectometry (TDR) approach.
• Picosecond rise time at GHz
frequencies enables probing of
smaller length scales.
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GLASS MICROSPHERES FOR ALTERING THE CARBON NANOTUBE NETWORK
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Erik T. Thostenson
SENSING OF EXTREMELY SMALL STRAINS
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Erik T. Thostenson
OUTLINE
• Introduction
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
• Structured Nanocomposites for Strain Sensing
• In Situ Thermoresistive Distributed Sensing of
Thermochemical Transitions
– Motivation and approach
– In situ sensing of late stages of transient
cure (diffusion limited) and glass transition
• Conclusions
• Acknowledgments
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TUNNELING – BASED SENSING
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Erik T. Thostenson
MOTIVATION
• Applications for multifunctional
materials experience extreme
temperature variations
• Knowledge of the
thermoresistive behavior of
sensing materials critical for
future applications
• Methods and findings reported
in the literature vary widely
• Parallel Plate Method
– For insulating materials
– For conductive particle systems?
• Rectangular Bar
– Well-defined cross section and length
– Reliable bulk electrical measurement
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INSTRUMENTATION
• Mettler Toledo TMA/SDTA841e
– Parameters
· Time
· Temperature (°C)
· Normal force (1 N)
· Ramping rate (3 °C/min)
– Output
· Height (μm)
• Electrical measurement details
– Specimen wired in series with reference resistor
– Silver paint/conductive epoxy electrodes
Volume resistivity: 𝝆𝒗 = 𝑹𝒘𝒐
𝒍𝒐𝒉
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TRANSIENT ELECTRICAL BEHAVIOR WELL ABOVE PERCOLATION
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TRANSIENT TRENDS NEAR PERCOLATION
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TRANSIENT RESISTANCE CHANGE
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SENSING OF POST – CURE
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THERMORESISTIVE ANALYSIS
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RESISTIVITY AND THERMAL EXPANSION
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Erik T. Thostenson
OUTLINE
• Introduction
• Self-Reinforced Electroactive Nanocomposites:
Experiment and Simulation
• Structured Nanocomposites for Strain Sensing
• In Situ Thermoresistive Distributed Sensing of
Thermochemical Transitions
• Conclusions
• Acknowledgments
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Erik T. Thostenson
CONCLUSIONS
• The co-electrospinning process has been established to
structure dense nanocomposites through solid-state
processing.
• Process variability in the form of fiber beading results in
challenges in consolidation and controlled-atmosphere
processing can eliminate beading.
• Results demonstrate clustering and CNT-CNT interaction
effects have a strong influence on the overall response.
• Alteration of the percolating network can result in
significant changes to the resistance-strain behavior in
nanotube films and structured nanotube foams.
• Thermoresistive characterization is able to detect transient
changes in the cure behavior and thermal transitions.
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Erik T. Thostenson
ACKNOWLEDGMENTS
Collaborators
• Andrew Rider (DSTO)
• Woong-Ryeol Yu (Seoul National University)
Graduate Students
• Cedric Jacob (Ph.D. MEEG)
• Qi An (Ph.D. MSEG)
• Kalon Laster (MS MEEG)
• Gaurav Pandey (Ph.D. MEEG)
Undergraduate Interns
• Sarah Friedrich (Delaware / Johns Hopkins)
• Nicholas Neal (Winona State / UW Madison)
• Britania Vondrashek (Virginia Tech)
This work is funded by the Air Force Office of Scientific Research Young Investigator Grant (Dr. Byung-Lip “Les” Lee, Program Director)