Frank Ko


Relevant Degree Programs

Affiliations to Research Centres, Institutes & Clusters


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - April 2022)
A multiscale analysis of forming induced wrinkles in woven composite preforms (2018)

A multi-scale analysis of forming induced wrinkles in woven composite preforms is conducted. The main objective of the present thesis is to propose an analytical model for the prediction of a class of forming induced wrinkles. In this research, wrinkling phenomenon in woven fabrics is divided into two distinct classes based on the nature and mechanism of formation, namely shear wrinkling (i.e., developed as a result of large shear deformation) and non-shear wrinkling (i.e., developed due to longitudinal contraction of the length of the fabric strip). Shear wrinkling as one of the most critical defects in the draping of woven fabrics is investigated in this study. To develop an analytical model for the prediction of shear wrinkling, an in-depth investigation is conducted to scrutinize the mesoscale mechanisms of woven fabrics during shear deformation. It is revealed that shear of woven fabrics occurs through two distinct deformation modes (i.e., trellising mode and pure multi-scale shear mode) depending on the boundary conditions applied to the fabric. In accordance with the deformation mode of woven fabrics, analytical equations are derived to predict the onset of shear wrinkling of the fabrics. Experimental investigations are conducted to assess the accuracy of the analytical model. Afterward, the analytical model for the prediction of shear wrinkling is implemented into ABAQUS Finite Element (FE) package as wrinkling indicator/predictor to improve the accuracy of the FE results. Furthermore, a parametric study is carried out to determine the effect of fabric and process parameters on the resistance of woven fabrics against wrinkling.

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Design and fabrication of electrospun nanofiber catalyst support for polymer electrolyte membrane fuel cells (2018)

Progress in fuel cell technology is impeded by the lack of understanding of the fundamental design characteristics necessary to improve the performance. Current catalyst design suffers from challenges related to platinum utilization, triple phase boundary, mass transport, and durability. Little improvement has been made in novel fuel cell catalyst designs, in particularly, there is a lack of microstructural optimization. As a result, a relationship between the catalyst layer microstructure and fuel cell performance has not been established. This study addresses this crucial problem by finding connections between controlled microstructure and key fuel cell performance factors. Specifically, electrospun nanofibers as a catalyst support offered a number of controllable structural parameters including: porosity, fiber diameter, fiber alignment, and layer thickness. The material and structural properties of carbonized nanofibers were optimized by factorial design for incorporation into a fuel cell membrane electrode assembly. Validation of the structural and material properties of the carbon nanofiber catalyst support was analyzed by electrochemical, physiochemical, and microscopy methods. Carbon nanofiber were integrated into membrane electrode assemblies and tested in situ to develop structure-property-performance relationships pertaining to Pt loading, ionomer loading, substrate electrical properties, and fiber mesh geometry. Performance was characterized by cyclic voltammetry, polarization, and electrochemical impedance spectroscopy. Results confirm not only ionomer thickness and loading, but more importantly ionomer distribution influenced polarization losses. 150 µg cm-² and 250 µg cm-² Pt loading achieved the same maximum current density, suggesting reduced loading maintained satisfactory Pt utilization, decreasing the amount of Pt. Although the influence of fiber orientation on fuel cell performance was inconclusive, fiber electrical conductivity, ionomer thickness, and Pt distribution were found to be essential for the development of efficient low cost catalyst layers. In conclusion, carbon nanofiber catalyst support revealed enhanced surface area, durability, Pt utilization, and efficiencies due to the porous mesh structure.

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Electrospinning of lignin based composite nanofibres with nanocrystalline cellulose (2018)

In this study, composite nanofibres were fabricated from solvent fractionated softwood kraft lignin (SKL), NCC and polyethylene oxide (PEO) by electrospinning. The molecular organization of lignin was investigated in the form of spun fibres and films with and without NCC. Subsequently, the as-spun composite nanofibre mats were thermally stabilized in the air under controlled conditions. The chemical and mechanical properties were studied as a function of the processing conditions. The oxidized nanofibres were then carbonized at 1000 ºC in inert nitrogen atmospheres. The responses investigated include changes in yield, diameter/distribution of nanofibres, thermal stability, elemental composition, the molecular structure and mechanical properties. Lastly, effects of NCCs on lignin structure in the fibres at different stages of heat treatments were determined. Lignin molecules demonstrated organization within aligned electrospun fibres and within solvent cast lignin films. The nanofibres and films of lignin with and without NCC had birefringence as revealed with polarized optical microscope. Also, through heat treatment, the lignin-based nanofibres mats with or without NCC, showed improved mechanical and thermal-chemical properties after thermal stabilization and carbonization processes. Specifically the properties of thermally stabilized samples were more variable than carbonized samples. Furthermore, NCC loadings gave a significant reduction in mobility of lignin molecules during heat treatment allowing for direct carbonization for lignin carbon fibres production with NCC loadings for 5 wt.%. NCC overall did not enhance the mechanical properties of the electrospun fibre. However significant interactions between the NCC and lignin were revealed with FTIR spectroscopy and thermal rheological analysis. In summary, the work investigated how thermal treatments can enhance the performance of lignin-based materials and further enhanced by the presence of nanofillers. This study investigated extensively the effect of NCC in lignin-based composite nanofibres through fundamental understanding of the interaction between lignin and NCC during the different heat treatment stages for carbon fibre production.

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Flexible film and breathable textile electrodes for electrodermal activity monitoring (2018)

The field of research on wearable systems that monitor human biological responses for healthcare applications is constantly advancing. Electrodermal activity (EDA) is related to the neurological system and is a result of the autonomic nervous system being stimulated, which produces sweat on the surface of the skin, thereby changing its electrical characteristics. The current clinical devices used to monitor EDA utilize rigid and non-breathable silver/silver chloride (Ag/AgCl) electrodes, possibly in combination with gels and irritating adhesives. The research detailed in this dissertation is focused on advancing our understanding of the design and development of comfortable, flexible and breathable EDA electrodes. Flexible dry Ag/AgCl electrodes were fabricated on a compliant substrate with various surface areas, distances between and geometries. The flexible electrodes were systematically characterized to determine their ability to detect EDA stimulus responses and these were compared to the responses simultaneously collected by rigid dry Ag/AgCl electrodes. The data demonstrated that surface area, spacing and geometry of electrodes affected the detection of the EDA stimulus response. The minimum number of sweat glands to be covered by flexible EDA electrodes has been estimated at 140 to maintain functionality. The optimal design of flexible electrodes is a serpentine geometry (0.15 cm² surface area, 0.20 cm distance).Ag/AgCl electronic yarns were developed through a novel roll-to-roll system and integrated into textile substrates of cotton, nylon and polyester. The EDA stimulus responses detected by dry electronic textile (e-textile) electrodes at various locations on the hand were compared to the EDA signals collected by dry solid Ag/AgCl electrodes. The cotton textile substrate with e-textile electrodes (0.12 cm² surface area, 0.40 cm distance) was the optimal material to detect the EDA stimulus responses. Also, differences with EDA waveforms recorded on various fingers were observed. Trends of long-term measurements showed that skin surface temperature affected EDA signals recorded by non-breathable electrodes more than when e-textile electrodes were used. The effects of electrode design and material for flexible and breathable EDA electrodes detailed in this dissertation can promote the development of effective and wearable EDA monitoring systems, which can help improve our knowledge of the human neurological system.

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Electrospun nanofibrous membranes for water vapour transport applications (2017)

This study discusses the development of selective water vapour permeable membranes for the separation of water vapour from air. These membranes are useful as separation media for membrane-based energy recovery ventilation devices. Current generation composite membranes for these devices consist of a polymer film layer which is permeable to water vapour but selective for water vapour over gases. In these composite membranes, this film layer is attached to one surface of a microporous polymeric support substrate. However, as it is demonstrated in this work, the microporous support contributes 30 to 50% of the resistance to water vapour transport in the composite membranes.In an attempt to eliminate this microporous substrate and its associated resistance, a membrane was developed using an electrospun nanofibrous layer as a support structure for the selective coating layer. In these membranes, the electrospun nanofibers are deposited on a non-woven micro-fibrous carrier and then the nanofibers are impregnated with a selectively permeable polymer to make impregnated electrospun nanofibrous membranes (IENM). The nanofibers are found to contribute a resistance to vapour transport in these membranes and their effect on water vapour permeability is quantified through a fiber-filled-film model. An optimization study of the IENMs demonstrated that fiber diameter and fiber volume in the film layer effect the water vapour permeance of these membranes and reducing these variables improved water vapour permeance. IEMNs were demonstrated to have water vapour permeance (>10000 GPU) while still having sufficient selectivity for the application, exceeding the performance of current generation composite membranes.The membranes were demonstrated to be particularly useful in the fabrication of ‘formable membranes’ which could be thermally-formed into exchanger plates for energy recovery ventilator exchangers. Thermal and mechanical properties of the membrane components were reported individually and as complete membranes. A membrane composition was demonstrated to fabricate exchanger plates from formable IENMs.This work contributes to the development of membranes for air-to-air exchangers specifically for energy recovery ventilation. A new class of membrane based on electrospun nanofibers for these devices was successfully demonstrated to have improved performance properties and a novel formable membrane was developed.

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Structure and properties of lignin-based composite carbon nanofibres (2017)

This research investigates the feasibility of developing value-added products from lignin in nanofibre form for structural and functional applications. Specifically, softwood Kraft lignin (SKL) was used as the precursor to fabricate nanofibres via the electrospinning process and then converted into carbon nanofibres (CNF). The mechanical properties of SKL nanofibres were characterized for structural applications at the nanofibre mat level and the single nanofibre level. The electrochemical performance of SKL CNFs was characterized for functional applications at the nanofibre mat level.This research harnessed different processing methods through hierarchical improvements of the mechanical properties of SKL CNF. The mechanical properties of SKL nanofibre mats were improved by the reduction of nanofibre diameter through the optimization of electrospinning process. The mechanical properties of SKL nanofibres gained an order of magnitude improvement through heat treatment processes of thermo-stabilization and carbonization. Aligned nanofibre mats were successfully fabricated via the rotating drum method resulting in further enhancement of the mechanical properties of SKL CNF. Single-walled carbon nanotube (SWNT)-SKL core-shell composite nanofibres were successfully fabricated via the emulsion electrospinning process. The mechanical properties of the SWNT/SKL composite nanofibres were found to increase as the percent of SWNT increases.This research also investigated the mechanical properties translation between SKL single nanofibres and their fibre assemblies. The mechanical properties of the SKL single nanofibres were characterized and then analyzed by the statistical Weibull distribution. The experimental results were in good agreement with the analytical values. A prototype supercapacitor (SC) cell using SKL-based CNF as binder-free electrodes was constructed to demonstrate the feasibility of the SKL CNF for functional applications. The electrochemical performance of the SC cell was characterized and the energy density and power density of the SC cell were found to meet the requirement for commercial products. In summary, this research sheds light on our understanding of the mechanism of the mechanical properties improvement of SKL CNF, which helps guide the tailoring of the mechanical performance of SKL CNF. Moreover, the electrochemical performance of SKL-based CNFs demonstrated that they are promising candidates as electrode materials for SC and a wide range of energy storage devices.

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Engineering Design of Nanofibre Wound Dressings (2015)

This research aimed to develop a nanofibrous carrier design process for hydrophilic, small-molecule drugs for controlled wound healing. Kynurenine was used as representative example, as it presented challenges with its size and structure necessitating significant optimization to reach release target. The objective of the design is thus to facilitate controlled healing via addressing hypotheses on carrier material compatibility, release control through process or material modification, and fabrication of continuous structures. The design process began with material selection, which identified poly(vinyl alcohol) (PVA) as the candidate carrier. Experimental verification via drug-polymer interaction characterization suggested that kynurenine formed a solution with PVA, and was encapsulated within PVA nanofibres, implying drug release is diffusion-controlled. The characterization process provided more insightful understanding of drug release mechanism compared to data fitting to empirical models performed in existing literatures.Release assays showed complete kynurenine release from PVA within five hours. In subsequent optimization studies, three methods to control release from nanofibres were proposed. First, material parameters such as molecular weight, electrospinning concentration and drug dosage were shown to be a suitable fine-control mechanism. The second method was matrix modification via heat treatment, which changed the burst release behavior, although drug entrapment was observed. The third method was a composite approach in which the drug-polymer system was encased in the more hydrophobic poly(lactic-co-glycolic acid) (PLGA), which significantly reduced burst release, and extended the release period to over 120 hours. Applicability of the PVA kynurenine carrier, planar dressings and braided sutures were explored, which could become useful for a variety of wounds. For planar dressings, the proposed design showed tensile properties within the range of various commercial dressing products and thus was considered robust for handling and application to open acute and chronic wounds. For sutures, process modification in 3D braiding was introduced to significantly increase tensile strength, which could help create robust wound closure devices for patients prone to scarring.The outcomes of this study demonstrated customization of drug release and structural properties of wound dressing materials to suit various open wounds, to provide a platform for supporting the expanding therapeutic functionalities in next generation wound dressings.

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Structures, properties and applications of multifunctional lignin nanofibres (2015)

This study explored the feasibility of creating multifunctional lignin materials in nanofibre form to establish a material platform for the development of value-added products. Specifically, softwood kraft lignin was electrospun, thermostabilized and carbonized into carbon nanofibres. Subsequently, functionalization of lignin based carbon nanofibres were conducted by (1) designing lignin-based composite carbon nanofibre; (2) preparing architecturally-designed lignin-based nanofibres and (3) preparing architecturally-designed lignin-based composite nanofibres. Examples of the advanced applications of the functionalized lignin based nanofibres were demonstrated such as electromagnetic interference shielding, energy storage and actuator. Flexible composite carbon nanofibres were embedded with functional fillers e.g. flexible electromagnetic lignin carbon nanofibres embedded with magnetic nanoparticles was developed. The amorphous structure of lignin and the addition of functional fillers impart the mechanical flexibility to lignin carbon nanofibre mats. By combining the magnetic permeability of magnetic nanoparticles and the electrical conductivity of lignin carbon nanofibre, flexible multifunctional lignin composite carbon nanofibres were created. Electromagnetic shielding effectiveness (SE) of lignin-based carbon nanofibres was comparable to that of the petroleum-based (such as polyacrylonitrile (PAN)-based) nanofibres. The feasibility of using above flexible composite carbon nanofibres from lignin as the lithium ion battery anode was demonstrated. This anode is free-standing, binder-free and mechanically flexible mats. Using lignin nanofibres electrodes and solid electrolytes, flexible solid-state lithium ion batteries were successfully assembled and characterized. Moreover, functions were added to electrospun lignin nanofibres by developing architecturally-designed lignin based thermostabilized nanofibres. A unique actuating phenomenon in thermostabilized lignin nanofibres was observed. It exhibits fast, reversible and dramatic mechanical deformation and recovery in response to environmental moisture gradient at milliseconds level. The actuation mechanism was investigated at the molecular level, and fibre assembly level. In summary, this study demonstrated that renewable biomaterials such as lignin has the potential for adding value through multifunctionalization in nanofibres form, thus creating a promising material platform for petroleum free feedstock for advanced applications.

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Polymer grafted carbon nanotube reinforced ultra high molecular weight polyethylene fibre (2014)

In this research, a series of experiments have been conducted to develop a high performance ultra-high molecular weight polyethylene (UHMWPE) fibre with improved mechanical properties. A novel process was developed whereby polyethylene grafted multi-walled carbon nanotubes (PE-g-MWCNTs) were used to reinforce UHMWPE fibre. PE-g-MWCNT/UHMWPE fibres with remarkable modulus and tensile strength of 125.5 GPa and 4.0 GPa, respectively, were successfully fabricated and showed definite potential for reducing the weight of body armour.A systematic study was carried out to investigate the effects of gel spinning conditions on tensile properties and morphologies of UHMWPE fibre. Spinning parameters, including polymer concentration, spinning temperature and winding-up speed, were selected and studied systematically and the spinning condition of UHMWPE fibre was optimized by design of experiment. Intensive experiments were conducted to investigate the feasibility of reinforcing UHMWPE fibre with pristine multi-walled carbon nanotubes (MWCNTs). Various mechanical methods include ultra-sonication, ball milling, microfluidizing, etc. were applied for dispersing pristine MWCNTs. Studies on tensile properties and morphologies of formed MWCNT/ UHMWPE fibre demonstrated that pristine MWCNTs tend to exist in micro-meter size agglomerations and no improvement in tensile properties of the MWCNT/UHMWPE fibres was found. Finally, chemical functionalization of MWCNTs using a coupling agent and polymer grafting technology was studied. The effective modulus and strength of MWCNTs were calculated based on the ‘rule of mixture’. Compared to coupling agent functionalization, polymer grafting has been found to be more effective in improving reinforcement of MWCNTs in UHMWPE fibre due to a stronger load transfer on the interface. The reinforcement mechanism of polymer grafted MWCNTs was analyzed based on experimental observations.

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Master's Student Supervision (2010 - 2021)
Development of a conductive lignin-based current collector for wearable batteries (2021)

The continuing demand for wearable electronics requires the development of lightweight and low-volume energy storage systems as the source of power. We consider the integration of a solar cell and a battery as a continuous power source for wearables. This thesis focuses only on the battery. The current collector of batteries currently used in wearables are heavy and bulky; thus, a metalized nanofibrous network is proposed to retain functionality (high electrical conductivity and support) of the current collector, while possessing lower weight and volume. Furthermore, compared to state-of-the-art lithium-ion batteries, zinc – manganese dioxide (Zn-MnO₂) batteries are of interest due to their lower cost, environmentally-friendliness, and abundance of materials. To fabricate the metalized nanofibrous network, lignin, a natural polymer, is used as the precursor for electrospinning. The produced fibers are thermally stabilized and electroless plated with copper to meet the conductivity requirements. Finally, a layer of MnO₂ paste is brush-coated on the copper plated fibers and the electrochemical performance of the assembled Zn-MnO₂ battery is analyzed.This work harnesses the optimization of the electrospinning solution, thermal stabilization, and electroless copper plating solution. The electrospinning solution is optimized by varying its viscosity. In general, higher viscosity results in larger diameters and fewer beads. Thermal stabilization optimization involves changing the final temperature of the process. Higher final temperature allows for more cross-linking and cleavage of bonds; however, above thermal degradation, fiber fusion is observed. These two optimizations allow for achievement of smallest fiber diameter for lightweight and low-volume applications. The plating solution is optimized for attainment of highest conductivity, by adjusting the reducing agent amount, sonication time, and plating time. Generally, conductivity increases by increasing these parameters. However, above a certain threshold, higher formaldehyde amount reduces the reaction rate and longer sonication can break down the sample. This lignin current collector is assessed with respect to the currently used carbon paper current collector. It is evident that lignin current collector has a higher conductivity and longer cycle life, while possessing smaller initial capacity. For wearable batteries, lifespan (cycle life) is a significant factor. Hence, the lignin current collector shows promise for wearable batteries.

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Development of nanofibrous target materials for short-lived isotope production (2018)

For over a decade, refractory metals and carbide powder materials have been used to produce radioactive ion beams (RIB) using the isotope separation online method at the ISAC facility at TRIUMF. A continuous 500 MeV proton beam generated from the world’s largest cyclotron is used to bombard the target materials to produce exotic isotopes. Particularly, short-lived isotopes are of high degree of interest in many disciplines including medicine, but only limited quantities are available due to the nuclear decay during the time associated with the diffusion and effusion of the species to migrate from the target materials to the ion source. The target materials are required to operate at high temperature to promote the release of the species; therefore, sintering of the grains is promoted, which results in lengthening of the diffusion paths for the isotopes causing the reduction of RIB yields. A possible way to improve the intensities of RIBs is by incorporating nanoparticle materials into nanofibres to increase the release efficiencies. In this study, nanometric SiC fibre target materials are fabricated by electrospinning. Upon a high-temperature heat treatment process, the organic carrier converts into a carbon fibre backbone that immobilizes the SiC target material with controlled nanometric grain size and reduced sintering dynamics. The weight composition of the final product is determined to be 60% SiC and 40% carbon nanofibre. The nanofibres were pressed into discs to achieve a combination of highly dense (1.1-1.2 g/cm³) and porous (56-54% total porosity) target materials, preserving the fibre morphology. The nano-SiC fibrous target material will be tested in September 2018 to evaluate the RIB yields explicitly for the short-lived isotopes at several milliseconds such as ²²Al (91.1 ms), ²⁰Mg (90.8 ms), ²¹Mg (122 ms), and ²⁰Na (447.9 ms).

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Single Wall Carbon Nanotube Composite Nanofibres from Electrospun Polyacrylonitrile Copolymer as a Potential Transparent Conductor (2015)

Diversifying and securing sources of energy is considered one of the greatest challenges that humanity faces over the next 50 years. Of all the potential energy sources, solar energy is one of the most promising, with costs dropping and capacity increasing at an exponential rate. As new photovoltaic technologies become available, the need to develop new transparent conductor technologies with a range of functionality increases. The potential for using electrospinning as a method to develop a transparent conductor based on carbon nanofibres is explored in this study. Electrospinning has great potential for this type of application due to its ability to create fibres with high aspect ratios and the ability of the process to easily scale up. A copolymer of polyacrylonitrile (PAN), polyacrylonitrile-co-methyl acrylate (PAN-co-MA) is characterized and explored as a precursor for creating carbon nanofibres. By exploring and specifying the solution properties, future work using PAN-co-MA can be optimized more efficiently. In addition to PAN-co-MA, varying amounts of single wall carbon nanotubes (SWNT) were added to the spinning solution to determine how composite SWNT/carbon nanofibres perform compared to the original carbon nanofibres. Varying carbonization temperatures from 700˚C to 1000˚C were explored and samples containing SWNT showed up to two orders of magnitude better conductivity compared to the benchmark condition for some scenarios. In all conditions the samples with SWNT outperformed those without. A method to coat the nanofibre membranes with PEDOT:PSS was developed, which has uses both for thin film and bulk functionalized nanofibre uses where conductivity is important. Thin film samples of composite SWNT/carbon nanofibres were created and characterized with respect to their transparency and sheet resistance. Transparencies over 96% were achieved. Once coated with PEDOT:PSS, the sheet resistance dropped to 414 ohm/sq while maintaining over 93% transparency for some conditions.

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Magnetic-actuated electrospun shape memory mats (2014)

Shape memory materials are the materials that can be deformed and fixed to a temporary shape and recover their original shape on exposure to external stimulus. Shape memory properties have been widely used for various applications such as actuators, adaptive mateirals and so on. This study firstly imparts this special property to nanofibers in light of various advantages including nanoporous structure, excellent recoverability and good biocompatibility. Then magnetite nanoparticles are incorporated into this system to realize the magnetic actuation of shape memory effect. In the first stage, poly (ɛ-caprolactone)-based polyurethane nanofibers were successfully fabricated by means of electrospinning. Through cyclic tensile testing, it was shown that in comparison with shape memory bulk films, the polyurethane shape memory nanofiber nonwovens had better shape recovery ability ascribed to molecular orientation. In the second stage, magnetite was successfully incorporated into polyurethane shape memory mats through the sonic mixing and subsequently the electrospinning process. SEM images showed that the fibers became more uniform and the diameter increased after magnetite incorporation. Differential scanning calorimetry and dynamic mechanical analysis revealed that the magnetite-incorporated mats still featured the melting transition which was similar with the pure mats, providing the magnetite-incorporated mats large possibilities to have shape memory effect. Through cyclic tensile testing, the recovery ratio decreased slightly because incorporated magnetite nanoparticles damaged the polyurethane matrix continuity. Moreover, electrospun mats with 5 wt%, 7.5 wt%, and 10 wt% magnetite were able to be heated to above their transition temperatures under a magnetic field with strength of 0.03T and frequency of 410 kHz. Helix recovery directly demonstrated that shape memory effect of magnetite-incorporated electrospun mats could be triggered under a specific magnetic field. Magnetic-actuated electrospun shape memory mats hold great potential for biomedical applications such as scaffolds in tissue engineering and drug delivery matrix in drug releasing system.

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Electrospun carbon model system for a nanofiber transparent conductor (2013)

Though absolute quantities of fossil fuels are abundant, the costs of drilling additional oil wells have increased more than 500% since 2000. Building a future that is less reliant on increasingly costly non-renewable energy resources will require the exploration of electricity generation from readily available solar power. Installed photovoltaic (PV) capacity has increased from 700 MW in 1996 to more than 69 GW in 2011. PV is contributing a rapidly growing share of electricity from a renewable source to the global energy picture but the manufacturing process of solar cells require many non-renewable resources and large amounts of energy. Performance of photovoltaic modules and cells are highly dependent on the properties of the transparent conductors. Indium tin oxide (ITO) is widely used in thin film and organic solar cell structures but recent projections for indium supplies show that only a few decades of this material may remain. Materials used in the manufacturing process of photovoltaics will need to use readily abundant resources if they are to become a significant portion of the electricity generation profile over the next century. This study explores the process of electrospinning and its ability to produce a transparent conductor layer for solar cells by using carbon instead of the scarce materials that constitute many transparent conductors. Thin layers of multi-walled carbon nanotube composites are electrospun, carbonized at temperatures of 700-1000˚C and characterized as a transparent conductor (TC) model system. This first iteration of the nanofiber TC system achieves transparencies of greater than 85% and a sheet resistance as low as 700 Ω/☐. Polyacrylonitrile (PAN) is evaluated as a carbon fiber precursor against PAN-co-methyl acrylate (PAN-co-MA) for electrical applications. PAN-co-MA is found to have as much as a 700% increase in conductivity compared to homopolymer PAN. The nanofiber transparent conductor model is evaluated in the context of an ITO replacement for solar cells and touch screen devices.

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Electromagnetic composite nanofibers (2012)

Multifunctional composite nanofibers containing magnetite (Fe₃O₄) nanoparticles are developedin this work. The multifunctional composite nanofibers are proved to be electrically conductiveand magnetically permeable. Polyacrylonitrile (PAN) is used as an appropriate polymer which iscapable of being pyrolized to produce electrically conductive carbon nanofiber matrix. In orderto develop magnetic nanofibers, various amounts of Fe₃O₄ nanoparticles ranging from 3 to10wt.% are embedded in the PAN nanofiber matrix. In addition, the electromagnetic behaviourof nanocomposites made of two different sizes (GA:20-30nm and GB:10-20nm) of Fe₃O₄nanoparticles is examined. Electrospun composite nanofibers are thermally treated at both 700°Cand 900°C to produce electromagnetic carbon nanofiber composites. The composite nanofibersare characterized using scanning electron microscopy (SEM), transmission electron microscopy(TEM), X-ray diffractometry (XRD), Raman spectroscopy, four-point probe andSuperconducting Quantum Interference Device (SQUID). Electromagnetic InterferenceShielding Effectiveness (EMI SE) of the pristine carbon nanofibers as well as electromagneticcomposite nanofibers is examined using Vector Network Analyzer with Thru-Reflect-Line(TRL) calibration. Uniform nanofibers are obtained for all samples with choosing 10wt.% PANconcentration in Dimethylformamide (DMF) with larger fiber diameters for compositenanofibers as compared with pristine carbon nanofiber. The magnetic properties of Fe₃O₄nanoparticles are successfully transferred into the as-spun Fe₃O₄/PAN composite nanofibrousstructure. The electromagnetic properties of the composite materials are tuned by adjusting theamount and size of Fe₃O₄ nanoparticles in the matrix and carbonization process. By embedding10wt.% of GA:20-30nm Fe₃O₄ nanoparticle, saturation magnetization (Ms) of 16emu/g is obtained with electrical conductivity of 9.2S/cm for composite nanofiber carbonized at 900°C. However, the Ms and electrical conductivity values respectively decrease to 9.0emu/g and 1.96S/cm for composite made of 10wt.% GB:10-20nm Fe₃O₄ nanoparticle carbonized at 900°C. The high surface area provided by the ultrafine fibrous structures, the flexibility and tuneable electromagnetic properties are expected to enable the expansion of the design options for a wide range of electronic devices such as sensors and actuators as well as Electromagnetic Interference Shielding Effectiveness (EMI SE). The electromagnetic composite nanofibers are demonstrated to act as strong electromagnetic interference shield of up to 70-80dB.

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Application of electrospun carbon nanofibres for batteries and supercapacitors (2011)

With the great demand for energy storage devices with much higher energy density, better power performance and longer cycle life, researchers are looking into nano-structured battery and supercapacitor electrodes due to the higher accessibility of ions to electrodes, improved specific capacitance, and reduced chance of mechanical degradation compared to bulk materials. In this study, a composite system of conductive carbon nanofibres with active materials (polypyrrole and silicon nanoparticles) are fabricated for supercapacitor and lithium ion battery electrode applications. The aim is to use carbon as a strong mechanical and electrical support and the high energy storage capability of active materials to develop new generation electrode systems.Carbon nanofibres with 3.9±0.5×10² nm in diameter are fabricated from a copolymer precursor, poly(acrylonitrile-co-acrylamide), through electrospinning and carbonization. The mild exothermic heat reaction of this copolymer and the enhanced heat flow into nano-scaled fibres during stabilization permits fabrication of high quality and large-scale carbon nanofibre mats with reasonable conductivity (18±1 S/cm), high porosity, and high accessible surface area. Carbon nanofibres are subsequently deposited on electrochemically with polypyrrole for 4 and 8 hours for use as a supercapacitor electrode. Capacity is compared with that of a bulk polypyrrole film. Both systems possess a gravimetric capacity of ca. 150 F/g, but an enhanced volumetric capacity in the nanofibrous electrode (9±1×10⁷ F/m³ for nanofibres vs. 6.0±0.5×10⁷ F/m³ for film) at greater amount of polypyrole deposition (8 hour deposition) is observed. The porous nanofibrous system also reduces the ionic resistance from that of the pure polypyrrole film, which is at least 1.9±0.8×10²Ω, to just 2 – 3 Ω at the highly reduced state.In lithium ion battery applications, a core-shell electrospinning method is used to fabricate carbon nanofibres containing silicon nanoparticles in the core. The core-shell structural advantage over non-core-shell structure observed to be in the prevention of silicon particle fusion and reduced breakage after interacting with lithium ions. At 30 wt% nanoparticle loading, both systems can reach over 1000 mAh/g initial capacity at 100 – 600 mA/g cycle rates. After 20 cycles, the capacity retention of Si in most core-shell systems are significantly higher than that of the non-core-shell system by ca. 10 – 20 %. Further optimization is required to improve the cycle life and electrode stability.

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Nonlinear mechanical behavior of automotive airbag fabrics: an experimental and numerical investigation (2011)

Over the past two decades, the airbag has become an essential safety device in automobiles. The airbag cushion is composed of a woven fabric which is rapidly inflated during a car crash. The airbag dissipates the passenger’s kinetic energy thereby reducing injury through biaxial stretching of the fabric bag and escaping gas through vents. Therefore, the performance of the airbag is greatly influenced by the mechanical properties of the fabric. Unlike traditional engineering materials, airbag fabrics are composed of discrete constituents and have highly nonlinear mechanical behavior that arises from both geometric deformations and material nonlinearity. Henceforth, airbag designers are forced to make simplified assumptions regarding the mechanical behavior of the fabric cushion. This incontrovertibly limits designers in taking advantage of the full potential of the fabric system. In order to optimize the airbag design, improve deployment simulations and overall dependability, a more sophisticated approach is needed. In this study, a simple unit cell model representing a single crossover of two orthogonal woven yarns is developed to simulate the in-plane mechanical behavior of both coated and uncoated plain weave airbag fabrics under multiple states of stress. Since the structural analysis of the deployment of the airbag is performed using the finite element method, the proposed mechanistic model is implemented as a User-Material-Model in the commercial code LS-DYNA. Here, the unit cell model represents the constitutive behavior of a continuum membrane. The approach results in capturing, in detail, the discrete nature of the fabric while retaining the computational efficiency of simple membrane formulation compared to explicitly modeling each yarn within the fabric. The procedure to calibrate the model inputs, namely the yarn geometric and mechanical properties for a given fabric is detailed. The sensitivity of the unit cell model and verification of the finite element implementation is discussed. A series of experiments were performed to validate the in-plane behavior of the model. The proposed model can be adopted by designers to better represent the nonlinear mechanical behavior of the fabric. It can also be used as a tool to design novel fabrics that are optimized for a particular application.

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