Seongpil An

Self-junctioned copper nanofiber transparent flexible conducting film via electrospinning and electroplating

Self-junctioned copper nanofiber transparent flexible films are produced using electrospinning and electroplating processes that provide high performances of T = 97% and Rs = 0.42 Ω sq(-1) by eliminating junction resistance at wire intersections. The film remains conductive after being stretched by up to 770% (films with T = 76%) and after 1000 cycles of bending to a 5 mm radius.

Flexible, Freestanding, and Binder-free SnOx–ZnO/Carbon Nanofiber Composites for Lithium Ion Battery Anodes

Here, we demonstrate the production of electrospun SnO(x)-ZnO polyacrylonitrile (PAN) nanofibers (NFs) that are flexible, freestanding, and binder-free. This NF fabric is flexible and thus can be readily tailored into a coin for further cell fabrication. These properties allow volume expansion of the oxide materials and provide shortened diffusion pathways for Li ions than those achieved using the nanoparticle approach. Amorphous SnO(x)-ZnO particles were uniformly dispersed in the carbon NF (CNF). The SnO(x)-ZnO CNFs with a Sn:Zn ratio of 3:1 exhibited a superior reversible capacity of 963 mA·h·g(-1) after 55 cycles at a current density of 100 mA·g(-1), which is three times higher than the capacity of graphite-based anodes. The amorphous NFs facilitated Li2O decomposition, thereby enhancing the reversible capacity. ZnO prevented the aggregation of Sn, which, in turn, conferred stable and high discharge capacity to the cell. Overall, the SnO(x)-ZnO CNFs were shown to exhibit remarkably high capacity retention and high reversible and rate capacities as Li ion battery anodes.

Self-Healing Nanofiber-Reinforced Polymer Composites. 1. Tensile Testing and Recovery of Mechanical Properties

The present work aims at development of self-healing materials capable of partially restoring their mechanical properties under the conditions of prolonged periodic loading and unloading, which is characteristic, for example, of aerospace applications. Composite materials used in these and many other applications frequently reveal multiple defects stemming from their original inhomogeneity, which facilitates microcracking and delamination at ply interfaces. Self-healing nanofiber mats may effectively prevent such damage without compromising material integrity. Two types of core-shell nanofibers were simultaneously electrospun onto the same substrate in order to form a mutually entangled mat. The first type of core-shell fibers consisted of resin monomer (dimethylsiloxane) within the core and polyacrylonitrile within the shell. The second type of core-shell nanofibers consisted of cure (dimethyl-methyl hydrogen-siloxane) within the core and polyacrylonitrile within the shell. These mutually entangled nanofiber mats were used for tensile testing, and they were also encased in polydimethylsiloxane to form composites that were also subsequently subjected to tensile testing. During tensile tests, the nanofibers can be damaged in stretching up to the plastic regime of deformation. Then, the resin monomer and cure was released from the cores and the polydimethylsiloxane resin was polymerized, which might be expected to result in the self-healing properties of these materials. To reveal and evaluate the self-healing properties of the polyacrylonitrile-resin-cure nanofiber mats and their composites, the results were compared to the tensile test results of the monolithic polyacrylonitrile nanofiber mats or composites formed by encasing polyacrylonitrile nanofibers in a polydimethylsiloxane matrix. The latter do not possess self-healing properties, and indeed, do not recover their mechanical characteristics, in contrast to the polyacrylonitrile-resin-cure nanofiber mats and the composites reinforced by such mats. This is the first work, to the best of our knowledge, where self-healing nanofibers and composites based on them were developed, tested, and revealed restoration of mechanical properties (stiffness) in a 24 h rest period at room temperature.

Hybrid self-healing matrix using core-shell nanofibers and capsuleless microdroplets.

In this work, we developed novel self-healing anticorrosive hierarchical coatings that consist of several components. Namely, as a skeleton we prepared a core-shell nanofiber mat electrospun from emulsions of cure material (dimethyl methylhydrogen siloxane) in a poly(acrylonitrile) (PAN) solution in dimethylformamide. In these nanofibers, cure is in the core, while PAN is in the shell. The skeleton deposited on a protected surface is encased in an epoxy-based matrix, which contains emulsified liquid droplets of dimethylvinyl-terminated dimethylsiloxane resin monomer. When such hierarchical coatings are damaged, cure is released from the nanofiber cores and the resin monomer, released from the damaged matrix, is polymerized in the presence of cure. This polymerization and solidification process takes about 1-2 days and eventually heals the damaged material when solid poly(dimethylsiloxane) resin is formed. The self-healing effect was demonstrated using an electrochemical analogue of the scanning vibrating electrode technique. Damaged samples were left for 2 days. After that, the electric current through a damaged coating was found to be negligibly small for the samples with self-healing properties. On the other hand, for the samples without self-healing properties, the electric current was significant.

Self-healing Nanofiber-Reinforced Polymer Composites. 2. Delamination/Debonding and Adhesive and Cohesive Properties

The capacity for core-shell nanofiber mats containing healing agents (resin monomer and cure) in their cores to adhere to a substrate was studied using blister testing. After extended periodic bending, the adhesion energy was measured, and the effect of self-healing on the composites delamination from the substrate was considered. In addition, the cohesion of two layers of the self-healing nanofibers was examined using blister testing and compared to that of ordinary nanofiber mats. The damage inflicted by prolonged periodic bending to the interface of the two nanofiber mats was demonstrated to have self-healed, and the cohesion energy was measured.

Highly efficient wettability control via three-dimensional (3D) suspension of titania nanoparticles in polystyrene nanofibers.

Electrospinning is a simple and highly versatile method for the large-scale fabrication of polymeric nanofibers. Additives or fillers can also be used to functionalize the nanofibers for use in specific applications. Herein, we demonstrate a novel and efficient way to fabricate superhydrophobic to hydrophilic tunable mats with the combined use of electrospinning and electrospraying that may be suitable for mass production on the merits of rapid deposition. The tunable nanocomposite mats were comprised of hydrophobic polystyrene nanofibers and hydrophilic titania nanoparticles. When the electrical conductivity of the electrospinning solution was increased, the surface morphology of the mats changed noticeably from a bead-on-string structure to an almost bead-free structure. Polystyrene (PS)-titania nanocomposite mats initially yielded a static water contact angle as high as 140° ± 3°. Subsequently exposing these mats with relatively weak ultraviolet irradiation (λ = 365 nm, I = 0.6 mW/cm²) for 2 h, the unique 3D suspension of the photoactive titania nanoparticles maximized the hydrophilic performance of the mats, reducing the static water contact angle to as low as 26° ± 2°. The tunable mats were characterized by scanning electron microscopy (SEM), static water contact angle (WCA) measurements, and energy-dispersive X-ray spectroscopy (EDX). Our findings confirmed that the tunable mats fabricated by the simultaneous implementation of electrospraying and electrospinning had the most efficient ultraviolet (UV)-driven wettability control in terms of cost-effectiveness. Well-controlled tunable hydrophobic and hydrophilic mats find potential applications in functional textiles, environmental membranes, biological sensors, scaffolds, and transport media.

Highly flexible transparent self-healing composite based on electrospun core-shell nanofibers produced by coaxial electrospinning for anti-corrosion and electrical insulation

Coaxial electrospinning was used to fabricate two types of core-shell fibers: the first type with liquid resin monomer in the core and polyacrylonitrile in the shell, and the second type with liquid curing agent in the core and polyacrylonitrile in the shell. These two types of core-shell fibers were mutually entangled and embedded into two flexible transparent matrices thus forming transparent flexible self-healing composite materials. Such materials could be formed before only using emulsion electrospinning, rather than coaxial electrospinning. The self-healing properties of such materials are associated with release of healing agents (resin monomer and cure) from nanofiber cores in damaged locations with the subsequent polymerization reaction filing the micro-crack with polydimethylsiloxane. Transparency of these materials is measured and the anti-corrosive protection provided by them is demonstrated in electrochemical experiments.

Highly flexible, stretchable, wearable, patternable and transparent heaters on complex 3D surfaces formed from supersonically sprayed silver nanowires

Here we demonstrate a scalable production process for a highly transparent, flexible, patternable, and wearable heater using a single-step supersonic kinetic spraying technique that deposits silver nanowires (AgNWs) on rollable substrates, facilitating a roll-to-roll process. AgNWs were suspended in an aqueous solution and supersonically sprayed onto a rolling substrate to produce a flexible heater film without use of any binders or additional post-process treatments. Because of the high-speed impact, the intersections of the nanowires were fused, thus creating a junction-free network of nanowires, which significantly reduced the contact and thus the sheet resistance. Cyclic temperature testing confirmed the thermal stability of the AgNW heater. A heater bent to a radius of less than 2 mm was tested for 600000 cycles; the heater exhibited little change in the sheet resistance. Moreover, it does not experience significant thermal expansion, which would manifest itself in buckling, and thus such heaters do not buckle during operation. AgNWs were sprayed onto a complex surface of a replica of Venus de Milo and Jejus Dol Hareubang statues, demonstrating the deposition capability onto a 3D surface. Defogging and defrosting tests showed potential applications of this heater in smart mirrors or windows. The highest heating temperature of 160 °C was achieved in a transparent fibrous film having 95% transparency and 15 Ω sq−1 sheet resistance at a supplied voltage of 8 V. Because the film fabrication method is rapid and scalable with the installation of multiple nozzles, the method is commercially viable.

Supersonically blown ultrathin thorny devil nanofibers for efficient air cooling

The effect of the supersonically blown below-74 nm nanofibers on cooling of high-temperature surfaces is studied experimentally and theoretically. The ultrathin supersonically blown nanofibers were deposited and then copper-plated, while their surfaces resembled those of the thorny-devil nanofibers. Here, we study for the first time the enhancement of surface cooling in gas in the cases of the forced and natural convection with the help of ultrathin thorny-devil nanofibers. These polymer core-metal shell nanofibers in nanometric scale possess a relatively high thickness of the metal shell and a high effective thermal conductivity, which facilitates heat transfer. The additional surface temperature reduction close to 5 °C in the case of the forced convection in the impinging air jet and close to 17 °C in the case of the natural convection was achieved. Correspondingly, an increase in the value of the heat transfer coefficient of about 41% in the forced convection, and about 20% in the natural convection was achieved due to the presence of the thorny devil electrospun and/or supersonically blown nanofibers.

Novel Composite Layer Based on Electrospun Polymer Nanofibers for Efficient Light Scattering

We fabricated a PAN (polyacrylonitrile) NF (nanofiber)-embedded composite layer to adjust the light-control layer in light-emitting-diode (LED) and organic-light-emitting-diode (OLED) lighting systems with unique optical characteristics, for effective light scattering. The newly designed light-control composite layers with a composition of PAN NF/SU-8 exhibited a change in the optical properties, which was identified by the diameter control of the NF using a simple process. The change in the optical properties was largely dependent on the embedded NFs features. Therefore, the NF can be applied in different types of lighting systems, depending on each lighting devices purpose.