micro and nano-structured composites for sensing and … · and functional material properties....

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

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

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)

Erik T. Thostenson

BRIDGING THE MICRO AND NANO SCALES

Erik T. Thostenson

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.

Erik T. Thostenson

OUTLINE

• Introduction



– Approach and Previous Results

– Recent Advances in Processing and

Simulations



Thermochemical Transitions

• Conclusions

• Acknowledgments

Erik T. Thostenson

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)

Erik T. Thostenson

ELECTROSPINNING AS A PLATFORM FOR MULTI-FUNCTIONAL, HIERARCHICALLY ORGANIZED COMPOSITES

Li and Xia, Advanced Materials (2004)

Teo and Ramakrishna, Composites Science and Technology (2009)

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.

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.

Erik T. Thostenson

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

Erik T. Thostenson

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

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.

Erik T. Thostenson


Erik T. Thostenson


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.

Erik T. Thostenson


• 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.

Erik T. Thostenson

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.

Erik T. Thostenson

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

Erik T. Thostenson

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

Erik T. Thostenson

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

Erik T. Thostenson

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

Erik T. Thostenson

FUNCTIONALIZATION AND NANOTUBE MORPHOLOGY CONTROL

Chilled Water

Bath

MWCNT

Solution

Peristaltic

Pump

Ozone

Generator

Moisture

Trap

Flow-

meter

Sonicator

Cell

O2

Erik T. Thostenson

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)

Erik T. Thostenson

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

Erik T. Thostenson

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

Erik T. Thostenson

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

Erik T. Thostenson

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.

Erik T. Thostenson

INFLUENCE OF VOLUME FRACTION

Erik T. Thostenson

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

Erik T. Thostenson

INFLUENCE OF ORIENTATION

Erik T. Thostenson

OUTLINE

• Introduction




– Negative piezoresistivity in carbon nanotube

electrodes

– Network modification for enhanced strain

sensitivity

• In Situ Thermoresistive Distributed Sensing

• Conclusions

• Acknowledgments

Erik T. Thostenson

OBSERVATION OF NEGATIVE PIEZORESISTIVTY

Erik T. Thostenson

OUTLINE

• Introduction




– Negative piezoresistivity in carbon nanotube

electrodes

– Network modification for enhanced strain

sensitivity

• In Situ Thermoresistive Distributed Sensing

• Conclusions

• Acknowledgments

Erik T. Thostenson

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.

Erik T. Thostenson

GLASS MICROSPHERES FOR ALTERING THE CARBON NANOTUBE NETWORK

Erik T. Thostenson

SENSING OF EXTREMELY SMALL STRAINS

Erik T. Thostenson

OUTLINE

• Introduction






– Motivation and approach

– In situ sensing of late stages of transient

cure (diffusion limited) and glass transition

• Conclusions

• Acknowledgments

Erik T. Thostenson

TUNNELING – BASED SENSING

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

Erik T. Thostenson

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

Erik T. Thostenson

TRANSIENT ELECTRICAL BEHAVIOR WELL ABOVE PERCOLATION

Erik T. Thostenson

TRANSIENT TRENDS NEAR PERCOLATION

Erik T. Thostenson

TRANSIENT RESISTANCE CHANGE

Erik T. Thostenson

SENSING OF POST – CURE

Erik T. Thostenson

THERMORESISTIVE ANALYSIS

Erik T. Thostenson

RESISTIVITY AND THERMAL EXPANSION

Erik T. Thostenson

OUTLINE

• Introduction






• Conclusions

• Acknowledgments

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.

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)

micro and nano-structured composites for sensing and … · and functional material properties....

Documents