Sang-Bok Lee

Effects of electrode degradation and solvent evaporation on the performance of ionic-polymer–metal composite sensors

An ionic-polymer–metal composite (IPMC) consists of an ionic polymer membrane and metallic electrodes plated on both surfaces. When it bends, a voltage is generated between the two electrodes across the membrane. Since it works not only in aqueous solution similar to in vivo but also in air, it can be used for embedded biomedical as well as surface-mounted sensors. The present study investigates the effect of solvent evaporation and mechanisms of electrode degradation of an IPMC when it is operated as a sensor. The output voltages and electrode resistances were measured with several cyclic bending motions applied on the sensor in both aqueous solution and air. There was a good correlation between the sensor voltage and the bending angle when the sensor was tested in aqueous solution. The sensor worked for a long time without attenuation in the output voltage in an aqueous solution. The output voltage, however, decreased rapidly when the sensor was operated in air. The results of resistance measurement showed that the electrode on the compressive side deformed more and generated more cracks than on the tensile side. Optical microscopic images taken on the electrode surfaces validated the results. The results provided very useful information needed to understand electrode degradation and solvent evaporation and to improve the performance of IPMC sensors.

Electro-magnetic properties of composites with aligned Fe-Co hollow fibers

A novel Fe-Co binary hollow fiber was synthesized by electroless plating using hydrolyzed polyester fiber and its anisotropy characteristic was investigated for electromagnetic wave absorbing materials. The hollow fibers in parallel with magnetic field show higher saturated magnetization of 202 emu/g at the applied magnetic field of 10 kOe and lower coercivity (27.658 Oe), compared with the random and vertical oriented hollow fibers. From complex permittivity measurement, the Fe-Co hollow fiber composites clearly display a single dielectric resonance, located at ∼14 GHz. The Fe-Co hollow fibers not only provide excellent EM properties in GHz frequency ranges, resulting mainly from the strong resonance, but also adjust the soft magnetic properties through fiber alignments. The cavitary structure of the Fe-Co hollow fibers, not only giving rise to a dielectric loss resonance and also adjusting its peak frequency, may be a pathway to useful EM wave absorptive devices in GHz frequency ranges.

Premartensitic behavior of the supersaturated beta phase in Ti alloys

The authors report on a study of the transformation behavior in Ti-15-3 and Ti{sub 3}Al-Nb alloys especially on the nature of the premartensitic phenomena by electron diffraction and microscopy. The premartensitic behavior of commercial beta Ti alloys has not been reported in other papers. The microstructure of premartensitic phase, called tweed or mottled structure composed of fine streak lamella, may have significant effect on mechanical properties of hardness, strength and toughness, since the structure of the phase provides many sites of nuclear in aging. The authors will consider the aging behavior of {beta} or B2 Ti alloys caused by fine precipitates from premartensitic phase, and the phase relationship between precipitates and matrix.

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.

Fabrication and electromagnetic characteristics of microwave absorbers containing carbon nanofibers and magnetic metals

The ultimate aim of this study is the development of microwave absorbers containing both dielectric and magnetic lossy materials. Carbon nanofibers (CNFs) were used as dielectric lossy materials and NiFe particles were used as magnetic lossy materials. Total twelve specimens for the three types such as dielectric, magnetic and mixed radar absorbing materials (RAMs) were fabricated. Their complex permittivities and permeabilities in the range of 2~18 GHz were measured using the transmission line technique. The parametric studies in the X-band (8.2~12.4 GHz) for reflection loss characteristics of each specimen to design the single-layered RAMs were performed. The mixed RAMs generally showed the improved absorbing characteristics with thinner matching thickness. One of the mixed RAMs, S09 with the thickness of 2.00 mm had the 10 dB absorbing bandwidth of 4.0 GHz in the X-band. The experimental results for selected specimens were in very good agreements with simulation ones in terms of the overall reflection loss characteristics and 10 dB absorbing bandwidth.

Fabrication of CNT dispersed Cu matrix composites by wet mixing and spark plasma sintering process

Multi-walled carbon nanotube (MWCNT)-copper (Cu) composites are successfully fabricated by a combination of a binder-free wet mixing and spark plasma sintering (SPS) process. The SPS is performed under various conditions to investigate optimized processing conditions for minimizing the structural de...

Enhancement of Magnetoelectric Conversion Achieved by Optimization of Interfacial Adhesion Layer in Laminate Composites

We report the effect of epoxy adhesion layers with different mechanical or physical property on a magnetoelectric (ME) composite laminate composed of FeBSi alloy (Metglas)/single-crystal Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3/Metglas to achieve an improved ME conversion performance. Through theoretical simulation, it was revealed that the Youngs modulus and the thickness of interfacial adhesives were major parameters that influence the conversion efficiency in ME composites. In the experimental evaluation, we utilized three epoxy materials with a distinct Youngs modulus and adjusted the average thickness of the adhesion layers to optimize the ME conversion. The experimental results show that a thin epoxy layer with a high Youngs modulus provided the best performance in the inorganic-based ME conversion process. By tailoring the interfacial adhesion property, the ME laminate generated a high conversion coefficient of 328.8 V/(cm Oe), with a mechanical quality factor of 132.0 at the resonance mode. Moreover, we demonstrated a highly sensitive alternating current magnetic field sensor that had a detection resolution below 10 pT. The optimization of the epoxy layers in the ME laminate composite provided significant enhancement of the ME response in a simple manner.

Quantitative Dispersion Evaluation of Carbon Nanotubes Reinforced Polymer Nano-composites

In order to maximize the performance of polymer nano-composites, it is essential to understand an effect of a dispersion state on material properties as well as to achieve highly dispersed composites. In this work, a simple quantitative approach to evaluate the degree of dispersion was suggested for carbon nanotube (CNT) embedded polymer nano-composites. Through UV-visible spectroscopy analysis, the transmittance of nano-composites was measured at various dispersion states and it was found that the transmittance reduced as the dispersion state of CNT improved. Based on the results, an effective con- centration factor for quantitative evaluation of dispersion state was introduced into the Beer-Lambert transmittance law. The proposed method and parameter to evaluate the degree of dispersion were verified by analyzing the transmittances at different dispersion states of CNT, concentrations of CNT and sample thicknesses.