7 research outputs found
Roll-to-Roll Continuous Manufacturing Multifunctional Nanocomposites by Electric-Field-Assisted âZâ Direction Alignment of Graphite Flakes in Poly(dimethylsiloxane)
A roll-to-roll
continuous process was developed to manufacture
large-scale multifunctional polyÂ(dimethylsiloxane) (PDMS) films embedded
with thickness direction (âZâ direction) aligned graphite
nanoparticles by application of electric field. The kinetics of particle
âZâ alignment and chain formation was studied by tracking
the real-time change of optical light transmission through film thickness
direction. Benefiting from the anisotropic structure of aligned particle
chains, the electrical and thermal properties of the nanocomposites
were dramatically enhanced through the thickness direction as compared
to those of the nanocomposites containing the same particle loading
without electrical field alignment. With 5 vol % graphite loading,
250 times higher electrical conductivity, 43 times higher dielectric
permittivity, and 1.5 times higher thermal conductivity was achieved
in the film thickness direction after the particles were aligned under
electrical field. Moreover, the aligned nanocomposites with merely
2 vol % graphite particles exhibit even higher electric conductivity
and dielectric permittivity than those of the nonaligned nanocomposites
at random percolation threshold (10 vol % particles), as the âelectric-field-directedâ
percolation threshold concentration is substantially decreased using
this process. As the graphite loading increases to 20 vol %, the aligned
nanocomposites exhibit thermal conductivity as high as 6.05 W/m·K,
which is 35 times the thermal conductivity of pure matrix. This roll-to-roll
electric field continuous process provides a simple, low-cost, and
commercially viable method to manufacture multifunctional nanocomposites
for applications as embedded capacitor, electromagnetic (EM) shielding,
and thermal interface materials
Roll to Roll Electric Field âZâ Alignment of Nanoparticles from Polymer Solutions for Manufacturing Multifunctional Capacitor Films
A roll to roll continuous processing
method is developed for vertical alignment (âZâ alignment)
of barium titanate (BaTiO<sub>3</sub>) nanoparticle columns in polystyrene
(PS)/toluene solutions. This is accomplished by applying an electric
field to a two-layer solution film cast on a carrier: one is the top
sacrificial layer contacting the electrode and the second is the polymer
solution dispersed with BaTiO<sub>3</sub> particles. Flexible Teflon
coated mesh is utilized as the top electrode that allows the evaporation
of solvent through the openings. The kinetics of particle alignment
and chain buckling is studied by the custom-built instrument measuring
the real time optical light transmission during electric field application
and drying steps. The nanoparticles dispersed in the composite bottom
layer form chains due to dipoleâdipole interaction under an
applied electric field. In relatively weak electric fields, the particle
chain axis tilts away from electric field direction due to bending
caused by the shrinkage of the film during drying. The use of strong
electric fields leads to maintenance of alignment of particle chains
parallel to the electric field direction overcoming the compression
effect. At the end of the process, the surface features of the top
porous electrodes are imprinted at the top of the top sacrificial
layer. By removing this layer a smooth surface film is obtained. The
nanocomposite films with âZâ direction alignment of
BaTiO<sub>3</sub> particles show substantially increased dielectric
permittivity in the thickness direction for enhancing the performance
of capacitors
Enhanced Impact Resistance of Three-Dimensional-Printed Parts with Structured Filaments
Net-shape manufacture
of customizable objects through three-dimensional (3D) printing offers
tremendous promise for personalization to improve the fit, performance,
and comfort associated with devices and tools used in our daily lives.
However, the application of 3D printing in structural objects has
been limited by their poor mechanical performance that manifests from
the layer-by-layer process by which the part is produced. Here, this
interfacial weakness is overcome using a structured, coreâshell
polymer filament where a polycarbonate (PC) core solidifies quickly
to define the shape, whereas an olefin ionomer shell contains functionality
(crystallinity and ionic) that strengthen the interface between the
printed layers. This structured filament leads to improved dimensional
accuracy and impact resistance in comparison to the individual components.
The impact resistance from structured filaments containing 45 vol
% shell can exceed 800 J/m. The origins of this improved impact resistance
are probed using X-ray microcomputed tomography. Energy is dissipated
by delamination of the shell from PC near the crack tip, whereas PC
remains intact to provide stability to the part after impact. This
structured filament provides tremendous improvements in the critical
properties for manufacture and represents a major leap forward in
the impact properties obtainable for 3D-printed parts
Role of Hydrogen Bonding on Nonlinear Mechano-Optical Behavior of lâPhenylalanine-Based Poly(ester urea)s
The uniaxial mechano-optical behavior
of a series of amorphous l-phenylalanine-based polyÂ(ester
urea) (PEU) films was studied in the rubbery state. A custom, real-time
measurement system was used to capture the true stress, true strain,
and birefringence during deformation. When the materials were subjected
to deformation at temperatures near the glass transition temperature
(<i>T</i><sub>g</sub>), the photoelastic behavior was manifested
by a small increase in birefringence with a significant increase in
true stress. At temperatures above <i>T</i><sub>g</sub>,
PEUs with a shorter diol chain length exhibited a liquidâliquid
(<i>T</i><sub>ll</sub>) transition (rubberyâviscous
transition) at about 1.06<i>T</i><sub>g</sub> (K) under
the tested strain rate of 0.017 s<sup>â1</sup> (stretching
speed of 20 mm/min), above which the material transforms from a heterogeneous
âliquid of fixed structureâ to a âtrue liquidâ
state. The initial photoelastic behavior disappears with increasing
temperature, as the initial slope of the stress optical curves becomes
temperature independent. Fourier transform infrared spectroscopy (FTIR)
was used to study the effect of hydrogen bonding on the physical properties
of PEUs as a function of temperature. The average strength of hydrogen
bonding diminishes with increasing temperature. For PEUs with the
longest diol chain length, the area associated with NâH stretching
region exhibits a linear temperature dependence. However, a three-stage
temperature dependence was observed for PEUs with shorter diol chain
length. The presence of hydrogen bonding enhances the âstiffâ
segmental correlations between adjacent chains in the PEU structure.
As a result, the photoelastic constant decreases with increasing hydrogen
bonding strength
Large-Scale Roll-to-Roll Fabrication of Vertically Oriented Block Copolymer Thin Films
Large-scale roll-to-roll (R2R) fabrication of vertically oriented nanostructures <i>via</i> directed self-assembly of cylindrical block copolymer (c-BCP) thin films is reported. Nearly 100% vertical orientation of cylinders in sub-100 nm c-BCP films under optimized processing <i>via</i> a dynamic sharp temperature gradient field termed Cold Zone Annealing-Sharp or âCZA-Sâ is achieved, with successful scale-up on a prototype custom-built 70 ft Ă 1 ft R2R platform moving at 25 ÎŒm/s, with 9 consecutive CZA units. Static thermal annealing of identical films in a conventional vacuum oven fails to produce comparable results. As a potential for applications, we fabricate high-density silicon oxide nanodot arrays from the CZA-S annealed BCP thin film template
Large-Scale Roll-to-Roll Fabrication of Ordered Mesoporous Materials using Resol-Assisted Cooperative Assembly
Roll-to-roll (R2R) processing enables
the rapid fabrication of
large-area sheets of cooperatively assembled materials for production
of mesoporous materials. Evaporation induced self-assembly of a nonionic
surfactant (Pluronic F127) with solâgel precursors and phenolic
resin oligomers (resol) produce highly ordered mesostructures for
a variety of chemistries including silica, titania, and tin oxide.
The cast thick (>200 ÎŒm) film can be easily delaminated from
the carrier substrate (polyethylene terephthalate, PET) after cross-linking
the resol to produce meter-long self-assembled sheets. The surface
areas of these mesoporous materials range from 240 m<sup>2</sup>/g
to >1650 m<sup>2</sup>/g with these areas for each material comparing
favorably with prior reports in the literature. These R2R methods
provide a facile route to the scalable production of kilograms of
a wide variety of ordered mesoporous materials that have shown potential
for a wide variety of applications with small-batch syntheses
Hierarchical Electrospun and Cooperatively Assembled Nanoporous Ni/NiO/MnO<sub><i>x</i></sub>/Carbon Nanofiber Composites for Lithium Ion Battery Anodes
A facile
method to fabricate hierarchically structured fiber composites is
described based on the electrospinning of a dope containing nickel
and manganese nitrate salts, citric acid, phenolic resin, and an amphiphilic
block copolymer. Carbonization of these fiber mats at 800 °C
generates metallic Ni-encapsulated NiO/MnO<sub><i>x</i></sub>/carbon composite fibers with average BET surface area (150 m<sup>2</sup>/g) almost 3 times higher than those reported for nonporous
metal oxide nanofibers. The average diameter (âŒ900 nm) of these
fiber composites is nearly invariant of chemical composition and can
be easily tuned by the dope concentration and electrospinning conditions.
The metallic Ni nanoparticle encapsulation of NiO/MnO<sub><i>x</i></sub>/C fibers leads to enhanced electrical conductivity
of the fibers, while the block copolymers template an internal nanoporous
morphology and the carbon in these composite fibers helps to accommodate
volumetric changes during charging. These attributes can lead to lithium
ion battery anodes with decent rate performance and long-term cycle
stability, but performance strongly depends on the composition of
the composite fibers. The composite fibers produced from a dope where
the metal nitrate is 66% Ni generates the anode that exhibits the
highest reversible specific capacity at high rate for any composition,
even when including the mass of the nonactive carbon and Ni<sup>0</sup> in the calculation of the capacity. On the basis of the active oxides
alone, near-theoretical capacity and excellent cycling stability are
achieved for this composition. These cooperatively assembled hierarchical
composites provide a platform for fundamentally assessing compositional
dependencies for electrochemical performance. Moreover, this electrospinning
strategy is readily scalable for the fabrication of a wide variety
of nanoporous transition metal oxide fibers