Wonoh Lee

Partially reduced graphene oxide as a multi-functional sizing agent for carbon fiber composites by electrophoretic deposition

Partially reduced graphene oxide (pRGO) was coated on carbon fibers by an anodic electrophoretic deposition (EPD). Coated-pRGO increased surface energy and wettability of carbon fiber. As a multi-functional sizing agent, pRGO enhanced both mechanical and electrical properties of the carbon fiber composite in spite of introducing a very small amount of less than 0.001 wt%.

Highly Tunable Interfacial Adhesion of Glass Fiber by Hybrid Multilayers of Graphene Oxide and Aramid Nanofiber

The performance of fiber-reinforced composites is governed not only by the nature of each individual component comprising the composite but also by the interfacial properties between the fiber and the matrix. We present a novel layer-by-layer (LbL) assembly for the surface modification of a glass fiber to enhance the interfacial properties between the glass fiber and the epoxy matrix. Solution-processable graphene oxide (GO) and an aramid nanofiber (ANF) were employed as active components for the LbL assembly onto the glass fiber, owing to their abundant functional groups and mechanical properties. We found that the interfacial properties of the glass fibers uniformly coated with GO and ANF multilayers, such as surface free energy and interfacial shear strength, were improved by 23.6% and 39.2%, respectively, compared with those of the bare glass fiber. In addition, the interfacial adhesion interactions between the glass fiber and the epoxy matrix were highly tunable simply by changing the composition and the architecture of layers, taking advantage of the versatility of the LbL assembly.

Highly Conductive Graphene/Ag Hybrid Fibers for Flexible Fiber-Type Transistors

Mechanically robust, flexible, and electrically conductive textiles are highly suitable for use in wearable electronic applications. In this study, highly conductive and flexible graphene/Ag hybrid fibers were prepared and used as electrodes for planar and fiber-type transistors. The graphene/Ag hybrid fibers were fabricated by the wet-spinning/drawing of giant graphene oxide and subsequent functionalization with Ag nanoparticles. The graphene/Ag hybrid fibers exhibited record-high electrical conductivity of up to 15,800 S cm−1. As the graphene/Ag hybrid fibers can be easily cut and placed onto flexible substrates by simply gluing or stitching, ion gel-gated planar transistors were fabricated by using the hybrid fibers as source, drain, and gate electrodes. Finally, fiber-type transistors were constructed by embedding the graphene/Ag hybrid fiber electrodes onto conventional polyurethane monofilaments, which exhibited excellent flexibility (highly bendable and rollable properties), high electrical performance (μh = 15.6 cm2 V−1 s−1, Ion/Ioff > 104), and outstanding device performance stability (stable after 1,000 cycles of bending tests and being exposed for 30 days to ambient conditions). We believe that our simple methods for the fabrication of graphene/Ag hybrid fiber electrodes for use in fiber-type transistors can potentially be applied to the development all-organic wearable devices.

Preparation of highly stacked graphene papers via site-selective functionalization of graphene oxide

Simple and effective preparation methods for highly stacked graphene papers are presented by site-selective functionalization of graphene oxides with di-functional small molecules, 1,4-butandiol and ethylenediamine. It was found that hydroxyl groups of 1,4-butandiol react mostly with edges of graphene oxides, while more reactive amine groups of ethylenediamine can react with carboxylic acid and epoxy groups on both edges and basal planes, which results in the interconnected graphene morphology in aqueous media. The graphene papers fabricated from the functionalized graphene oxide suspensions show a well stacked layer structure with a smaller interlayer distance than that of unmodified graphene oxide papers. Furthermore, high temperature annealing enhances the electrical conductivities of graphene papers fabricated from graphene sheets linked by ethylenediamine as high as 1716 S cm−1, due to the better stacked structure, further graphitization, and N-doping effects.

Effect of hardening laws and yield function types on spring-back simulations of dual-phase steel automotive sheets

In order to simulate the spring-back of DP-steel automotive sheets, the effect of hardening laws and yield function types has been studied based on finite element simulations performed for 2-D draw bending and S-rail tests. Specifically, the performance of the combined isotropic-kinematic hardening based on the modified Chaboche model was compared with those of the pure isotropic hardening and kinematic hardening laws, along with the non-quadratic anisotropic yield (Yld2000-2d) and Mises yield functions. As for the 2-D draw bending test, numerical results were compared with experimental results for verification purposes.

Constitutive Equations Based on Cell Modeling Method for 3D Circular Braided Glass Fiber Reinforced Composites

The cell modeling homogenization method to derive the constitutive equation considering the microstructures of the fiber reinforced composites has been previously developed for composites with simple microstructures such as 2D plane composites and 3D rectangular shaped composites. Here, the method has been further extended for 3D circular braided composites, utilizing B-spline curves to properly describe the more complex geometry of 3D braided composites. For verification purposes, the method has been applied for orthotropic elastic properties of the 3D circular braided glass fiber reinforced composite, in particular for the tensile property. Prepregs of the specimen have been fabricated using the 3D braiding machine through RTM (resin transfer molding) with epoxy as a matrix. Experimentally measured uniaxial tensile properties agreed well with predicted values obtained for two volume fractions.

Fabrication of Carbon Nanotube/Copper Hybrid Nanoplatelets Coated Carbon Fiber Composites by Thermal Vapor and Electrophoretic Depositions

Carbon nanotube reinforced copper (CNT/Cu) nanoplatelets were developed by a simple two-step deposition method; thermal vapor deposition of Cu and anodic eletrophoretic deposition of CNT. Due to the fast corrosion during the electrophoresis, the Cu layer is broken into pieces and exfoliated to form the platelet structures. Consequently, highly conductive and strong CNT/Cu nanoplatelets are effectively built-up. When applied in carbon fiber composites, the CNT/Cu nanoplatelets significantly enhanced both interlaminar shear strength and electrical conductivity comparing to baseline composites.

Kinematics of the nonsteady axi-symmetric ideal plastic flow process

A nonsteady axi-symmetric ideal flow solution is obtained here. It is based on the rigid perfect-plastic constitutive law with the Tresca yield condition and its associated flow rule. The process is to deform a circular solid disk into a spherical shell of prescribed geometry. It is assumed that there are no rigid zones and friction stresses. The solution obtained provides the distribution of kinematic variables and involves one undetermined function of the time. This function can be in general found by superimposing an optimality criterion.

Regrowth of diluted magnetic semiconductor GaMnAs on InGaP (001) surfaces to realize freestanding micromechanical structures

Low temperature molecular beam epitaxy regrowths of Ga1−xMnxAs (x≈0.04) diluted magnetic semiconductors on GaAs∕In1−yGayP∕GaAs(001) and In1−yGayP∕GaAs(001) (y≈0.51) heterostructures prepared by metal-organic chemical vapor deposition are described. The resulting Ga1−xMnxAs properties are comparable to epitaxial films grown directly on GaAs (001) substrates from in situ reflection high-energy electron diffraction, x-ray diffraction, magnetometry, and transport measurements with magnetic ordering temperature of as-grown films to range between ∼50 and ∼60K. Postgrowth low temperature annealing enhances both magnetic and transport properties. Perfect etch selectivity between Ga1−xMnxAs∕GaAs and In1−yGayP is utilized to realize suspended Ga1−xMnxAs∕GaAs doubly clamped beam micromechanical freestanding structures.

Nonsteady plane-strain ideal forming without elastic dead-zone

Ever since the ideal forming theory has been developed for process design purposes, application has been limited to sheet forming and, for bulk forming, to two-dimensional steady flow. Here, application for the non-steady case was made under the plane-strain condition. In the ideal flow, material elements deform following the minimum plastic work path (or mostly proportional true strain path) so that the ideal plane-strain flow can be effectively described using the two-dimensional orthogonal convective coordinate system. Besides kinematics, schemes to optimize preform shapes for a prescribed final part shape and also to define the evolution of shapes and frictionless boundary tractions were developed. Discussions include numerical calculations made for a real automotive part under forging.