A Self-Healing Conductive Ink - Paul Braun Research Group ...

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A Self-healing Conductive Ink
Susan A. Odom , Sarut Chayanupatkul , Benjamin J. Blaiszik , Ou Zhao , Aaron C.
Paul V. Braun , Nancy R. Sottos , Scott R. White , * and Jeffrey S. Moore *
The mechanical durability of conductive materials affects the
performance and lifetimes of devices ranging from electrical
circuits to battery electrode materials. Mismatches in thermal
expansion coeffi cient, Young’s modulus, and Poisson’s ratio
between the conductive materials and packaging materials in
integrated circuits lead to delaminations and fractures both
within conductive pathways and at interconnects.
Dr. S. A. Odom , O. Zhao , Prof. J. S. Moore
Department of Chemistry
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
E-mail: jsmoore@illinois.edu
S. Chayanupatkul , B. J. Blaiszik , A. C. Jackson ,
Prof. P. V. Braun , Prof. N. R. Sottos
Department of Materials Science & Engineering
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
Prof. S. R. White
Department of Aerospace Engineering
Beckman Institute for Advanced Science & Technology
University of Illinois at Urbana-Champaign
405 N. Mathews Ave. Urbana, IL 61801, USA
E-mail: swhite@illinois.edu
DOI: 10.1002/adma.201200196
[ 1 , 2 ]
Fatigue
causes microcracks that can lead to channeling, debonding, and
failure. [ 3–5 ] The repeated lithiation and delithiation of battery
electrode materials upon charging and discharging contribute
to decreased capacitance due to a loss of interparticle connectivity
and conductivity. [ 6–9 ] Numerous approaches have been
used to design circuits and materials to prevent mechanical
failure, but only a few have focused on restoring conductivity
after mechanical damage.
[ 10–15 ]
Autonomic restoration of electrical conductivity may greatly
extend the lifetime of electronic materials. The greatest opportunity
for signifi cant short-term impact may be in devices
in which human intervention is diffi cult and/or costly. For
example, fault-tolerant computer chips are of interest in space
applications in which fi eld-programmable gate arrays
used to self-diagnose and reroute damaged circuits. Redundant
circuitry and integrated sensing add complexity, weight, and
cost to fault-tolerant designs. In battery materials, longevity is
an important concern limiting applications. Improving battery
longevity by repairing mechanical failures in electrode materials
would require complete disassembly of the battery cell to
achieve repair. We are interested in developing general concepts
to restore conductivity in mechanically damaged electronic
materials without external intervention or relying on back-up
circuits.
Adv. Mater. 2012,
DOI: 10.1002/adma.201200196
[ 16 ]
are
www.advmat.de
Jackson ,
Recent efforts towards the autonomic restoration of conductivity
have focused on delivering conductive materials to the
site of damage from core–shell microcapsules. [ 10–12 ] Release
of conductive materials has been demonstrated using microcapsules
containing a suspension of conductive carbon nanotubes,
[ 10 ] solutions of precursors to a conductive charge transfer
salt, [ 11 ] and liquid metal alloys. [ 12 ] Conductivity restoration was
demonstrated by manual delivery of core solutions to simulated
cracks [ 11 ] or autonomic delivery to a cracked circuit. [ 12 ] In
this paper, we report a new approach for self-healing: instead
of releasing conductive materials from microcapsules, we utilize
the conductive particles from the conductive ink itself to
heal damage through subsequent particle redistribution. Upon
mechanical damage, solvent released from microcapsules locally
dissolves the polymer binder of the conductive ink, allowing for
particle redistribution and restoration of conductivity upon solvent
evaporation ( Figure 1 a–c).
Conductive inks are a mixture of conductive particles,
poly mer binders, and dispersing solvents, and they have been
used in the metallization of microcircuits, [ 17 ] solar cells, [ 18 ] large
area electronic structures, [ 19 ] and solder for microelectronics
packages. [ 20 ] More recently, conductive inks have been used to
print fl exible silver microelectrodes, [ 21 ] circuits on curvilinear
surfaces, [ 22 ] and conductive text, electronic art, and 3D antennas
on paper. [ 23 ] With direct screen printing, [ 24 ] ink-jet printing, [ 25 ]
dip-pen nanolithography, [ 26 ] e-jet printing, [ 27 ] and direct
writing, [ 28 ] many traditional processing steps can be eliminated,
including the use of photoresists and etching. Metallic particles
and polymer binder are usually mixed with one or more solvents
that dissolve the polymer binder. We envisioned a healing
concept where the polymer binder remains soluble after circuit
deposition and the conductive components could subsequently
redistribute upon contact with appropriate solvents. Here we
demonstrate that encapsulated solvent delivered to particle/
binder circuits autonomically restores conductivity to mechanically
damaged conductive inks, and that circuits with lines
spaced 200–500 μ m apart do not short circuit during the restoration
process.
We fi rst explored the response of mechanically damaged
conductive inks to a variety of solvents. Inks consisted of silver
particles in a acrylic binder, deposited as lines, on a glass substrate.
Scratching these lines resulted in a complete loss of electrical
conductivity. Subsequently, a drop of solvent was applied
to the scratched region. After solvent evaporation, conductivity
was restored for a subset of the solvents screened. Conductivity
restoration occurrs with a variety of organic solvents that
dissolve the polymer binder of the conductive ink including
xylenes, chlorobenzene, ethyl phenylacetate, and hexyl acetate.
Optical microscopy images of a line of conductive ink before
and after solvent healing ( Figure 2 a and b) demonstrate the
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