Jang-Kyo Kim

Highly Aligned Graphene/Polymer Nanocomposites with Excellent Dielectric Properties for High-Performance Electromagnetic Interference Shielding

Nanocomposites that contain reinforcements with preferred orientation have attracted significant attention because of their promising applications in a wide range of multifunctional fields. Many efforts have recently been focused on developing facile methods for preparing aligned graphene sheets in solvents and polymers because of their fascinating properties including liquid crystallinity and highly anisotropic characteristics. Self-aligned in situ reduced graphene oxide (rGO)/polymer nanocomposites are prepared using an all aqueous casting method. A remarkably low percolation threshold of 0.12 vol% is achieved in the rGO/epoxy system owing to the uniformly dispersed, monolayer graphene sheets with extremely high aspect ratios (>30000). The self-alignment into a layered structure at above a critical filler content induces a unique anisotropy in electrical and mechanical properties due to the preferential formation of conductive and reinforcing networks along the alignment direction. Accompanied by the anisotropic electrical conductivities are exceptionally high dielectric constants of over 14000 with 3 wt% of rGO at 1 kHz due to the charge accumulation at the highly-aligned conductive filler/insulating polymer interface according to the Maxwell-Wagner-Sillars polarization principle. The highly dielectric rGO/epoxy nanocomposites with the engineered structure and properties present high performance electromagnetic interference shielding with a remarkable shilding efficiency of 38 dB.

High strength, high fracture toughness fibre composites with interface control—A review

Abstract The subject of improving the fracture toughness of fibre composites is receiving significant attention because a critical design criterion in damage tolerant fibre composites is the possession of a sufficiently high fracture energy absorption capability, particularly under impact loading conditions. For a given brittle-fibre/brittle-matrix composite, high strength requires a strong interfacial bond, but this may lead to a low fracture energy absorption. However, by proper control of the physical and mechanical properties of the fibre-matrix interface high strength characteristics can be combined with high toughness. In order to fully utilise the potential of such composites without introducing a reduction in strength, it is necessary to understand the failure mechanisms leading to eventual fracture. This paper reviews the existing theories of fracture toughness of fibre composites and the various methods for improving the fracture toughness by means of interface control. Conclusions and generalisations which can be drawn from the literature are presented with discussions of areas in which further research is required.

Transparent conductive films consisting of ultralarge graphene sheets produced by Langmuir-Blodgett assembly.

Monolayer graphene oxide (GO) sheets with sizes ranging from a few to ∼200 μm are synthesized based on a chemical method and are sorted out to obtain four different grades having uniform sizes. Transparent conductive films are produced using the ultralarge graphene oxide (UL-GO) sheets that are deposited layer-by-layer on a substrate using the Langmuir-Blodgett (LB) assembly technique. The density and degree of wrinkling of the UL-GO monolayers are turned from dilute, close-packed flat UL-GO to graphene oxide wrinkles (GOWs) and concentrated graphene oxide wrinkles (CGOWs) by varying the LB processing conditions. The method demonstrated here opens up a new avenue for high-yield fabrication of GOWs or CGOWs that are considered promising materials for hydrogen storage, supercapacitors, and nanomechanical devices. The films produced from UL-GO sheets with a close-packed flat structure exhibit exceptionally high electrical conductivity and transparency after thermal reduction and chemical doping treatments. A remarkable sheet resistance of ∼500 Ω/sq at 90% transparency is obtained, which outperforms the graphene films grown on a Ni substrate by chemical vapor deposition. The technique used in this work to produce transparent conductive UL-GO thin films is facile, inexpensive, and tunable for mass production.

Impact and delamination failure of woven-fabric composites

An overview is presented of the fracture behaviour and failure mechanisms of composite laminates containing woven fabrics in mode I and mode II delamination and under impact loading. Potential advantages of using woven fabrics as opposed to cross-ply unidirectional prepreg tapes are specifically discussed from the viewpoint of the microstructure/property relationship. Correlations are established between resistance to and tolerance of impact damage and the delamination resistance of composites. Salient differences are identified between composites made from different fibre configurations in terms of the process of damage development, damage modes/states and residual performance after impact damage. The effects of fibre/matrix interface properties influenced by different silane coupling agents on interlaminar fracture and the impact performance of woven-glass-fabric composites are evaluated. The implications of these findings are discussed.

Enhanced Electrical Conductivity of Nanocomposites Containing Hybrid Fillers of Carbon Nanotubes and Carbon Black

Nanocomposites reinforced with hybrid fillers of carbon nanotubes (CNTs) and carbon black (CB) are developed, aiming at enhancing the electrical conductivity of composites with balanced mechanical properties while lowering the cost of the final product. Epoxy-based nanocomposites were prepared with varying combinations of CNTs and CB as conducting fillers, and their electrical and mechanical properties were evaluated. It was shown that the addition of CNTs in CB composites enhanced the electrical conductivity of composites: a low percolation threshold was achieved with 0.2 wt % CNTs and 0.2 wt % CB particles. The CB particles also enhanced the ductility and fracture toughness of nanocomposites, confirming the synergistic effect of CB as a multifunctional filler. The novelty of this work lies in the synergy arising from the combination of two conducting fillers with unique geometric shapes and aspect ratios as well as different dispersion characteristics, which have not been specifically considered previously.

Gassing in Li 4 Ti 5 O 12 -based batteries and its remedy

Destructive gas generation with associated swelling has been a major challenge to the large-scale application of lithium ion batteries (LIBs) made from Li4Ti5O12 (LTO) anodes. Here we report root causes of the gassing behavior, and suggest remedy to suppress it. The generated gases mainly contain H2, CO2 and CO, which originate from interfacial reactions between LTO and surrounding alkyl carbonate solvents. The reactions occur at the very thin outermost surface of LTO (111) plane, which result in transformation from (111) to (222) plane and formation of (101) plane of anatase TiO2. A nanoscale carbon coating along with a stable solid electrolyte interface (SEI) film around LTO is seen most effective as a barrier layer in suppressing the interfacial reaction and resulting gassing from the LTO surface. Such an ability to tune the interface nanostructure of electrodes has practical implications in the design of next-generation high power LIBs.

Effect of fiber pretreatment condition on the interfacial strength and mechanical properties of wood fiber/PP composites

The effect of fiber surface pretreatment on the interfacial strength and mechanical properties of wood fiber/polypropylene (WF/PP) composites are investigated. The results demonstrate that fiber surface conditions significantly influence the fiber–matrix interfacial bond, which, in turn, determines the mechanical properties of the composites. The WF/PP composite containing fibers pretreated with an acid–silane aqueous solution exhibits the highest tensile properties among the materials studied. This observation is a direct result of the strong interfacial bond caused by the acid/water condition used in the fiber pretreatment. Evidence from coupling chemistry, rheological and electron microscopic studies support the above conclusion. When SEBS-g-MA copolymer is used, a synergistic toughening effect between the wood fiber and the copolymer is observed. The V-notch Charpy impact strength of the WF/PP/SEBS-g-MA composite is substantially higher than that of the WF/PP composite. The synergistic toughening mechanisms are discussed with respect to the interfacial bond strength, fiber-matrix debonding, and matrix plastic deformation.

Fabrication of Highly-aligned, Conductive, and Strong Graphene Papers Using Ultralarge Graphene Oxide Sheets

This study demonstrates that large-size graphene oxide (GO) sheets can impart a tremendous positive impact on self-alignment, electrical conductivity, and mechanical properties of graphene papers. There is a remarkable, more than 3-fold improvement in electrical conductivity of the papers made from ultralarge GO sheets (with an average area of 272.2 μm(2)) compared to that of the small GO counterpart (with an average area of 1.1 μm(2)). The corresponding improvements in Youngs modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multiscale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular-scale, and the degree of dispersion and stacking behavior of GO sheets on the microscale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties. The molecular dynamics simulation further elucidates that the enhanced intersheet interactions between large GO sheets play a key role in improving the Youngs modulus of GO papers. The implication is that the said properties can be further improved by enhancing the intersheet stress transfer and electrical conduction especially through the thickness direction.

Self-alignment and high electrical conductivity of ultralarge graphene oxide–polyurethane nanocomposites

Polyurethane (PU)-based composite films containing highly aligned graphene sheets are produced through an environmentally benign process. An aqueous liquid crystalline dispersion of graphene oxide (GO) is in situ reduced in PU, resulting in a fine dispersion and a high degree of orientation of graphene sheets. The PU particles are adsorbed onto the surface of the reduced graphene oxide (rGO), and the rGO sheets with a large aspect ratio of over 10 000 tend to self-align during the film formation when the graphene content is high enough, say more than 2 wt%. The resulting composites show excellent electrical conductivity with an extremely low percolation threshold of 0.078 vol%, which is considered one of the lowest values ever reported for polymer composites containing graphene. The electrical conductivity of the composites with high graphene contents presents significant anisotropy due to the preferential formation of conductive networks along the in-plane direction, another proof of the existence of the self-aligned, layered structure.

Impact response of woven glass-fabric composites—I.: Effect of fibre surface treatment

Abstract Instrumented impact tests have been employed to study the impact response of vinyl-ester-matrix composites reinforced with woven E-glass fabric. Special emphasis has been placed on an evaluation of the extent of damage and the residual mechanical properties as affected by five different fibre surface treatments. Substantial differences are noted in the shape, mode and area of damage between the front and back surfaces of impact and between the laminates with different fibre surface treatments. Compression-after-impact (CAI) tests were performed to measure the residual compressive strength. A simple model is adopted to predict the threshold impact energy and the threshold damage below which no degradation in residual compressive strength occurs. The major conclusion of the work is that an increase in the γ-MPS silane concentration improves the damage resistance and damage tolerance of the laminates in terms of incipient energy, threshold energy and threshold damage width.