Kay-Hyeok An

Comparative studies of porous carbon nanofibers by various activation methods

In this study, activated carbons nanofibers (ACNFs) were prepared from polyacrylonitrile-based nanofibers by physical (H2O and CO2) and chemical (KOH) activation. The surface and structural characteristics of the porous carbon were observed by scanning electron microscopy and X-ray diffraction, respectively. Pore characteristics were investigated by N2/77K adsorption isotherms. The specific surface area of the physically ACNFs was increased up to 2400 m2/g and the ACNFs were found to be mainly composed of micropore structures. Chemical activation using KOH produced ACNFs with high specific surface area (up to 2500 m2/g), and the micropores were mainly found in the ACNFs. The physically and chemically ACNFs showed both mainly type I from the International Union of Pure and Applied Chemistry classification.

Effects of pore structures on electrochemical behaviors of polyacrylonitrile-based activated carbon nanofibers by carbon dioxide activation

Activated carbon nanofibers (ACNF) were prepared from polyacrylonitrile (PAN)-based nanofibers using CO2 activation methods with varying activation process times. The surface and structural characteristics of the ACNF were observed by scanning electron microscopy and X-ray diffraction, respectively. N2 adsorption isotherm characteristics at 77 K were confirmed by Brunauer-Emmett-Teller and Dubinin-Radushkevich equations. As experimental results, many holes or cavernous structures were found on the fiber surfaces after the CO2 activation as confirmed by scanning electron microscopy analysis. Specific surface areas and pore volumes of the prepared ACNFs were enhanced within a range of 10 to 30 min of activation times. Performance of the porous PAN-based nanofibers as an electrode for electrical double layer capacitors was evaluated in terms of the activation conditions.

Facile synthesis of iron-ruthenium bimetallic oxide nanoparticles on carbon nanotube composites by liquid phase plasma method for supercapacitor

Iron-ruthenium bimetallic oxide nanoparticles were precipitated on carbon nanotubes by liquid-phase plasma method. We also evaluated the physicochemical and electrochemical properties of prepared composite for supercapacitor electrode. Polycrystalline about 10 to 25 nm-sized bimetallic nanoparticles were evenly precipitated on the carbon nanotube (CNT) and consisted of Fe3+ and Ru4+. Bimetallic oxide nanoparticles’ composition depended on the ratio of the metal precursor concentration and standard reduction potential. The C-V area and specific capacitance of iron-ruthenium oxide nanoparticle/carbon nanotube (IRCNT) composite electrodes was higher than that of untreated CNT electrode, and increased with increasing ruthenium content. The cycling stability of IRCNT composite electrode was higher than untreated CNT electrode, especially iron element was more stable.

HTS microstrip antenna array for circular polarization with cryostat

We report the comparison results of an HTS antenna array for circular polarization with a self cryostat and comparison with a gold counterpart. The HTS antenna array designed in this work was fabricated using a YBCO superconducting thin film on an MgO substrate, and the designed resonant frequency was 11.85 GHz. We used square patches with truncated corners for circular polarization and employed a corporate feed network using sequential rotation techniques for enhancement of axial ratio. A cryostat was constructed for the measurement of each antenna, and liquid nitrogen was used as a coolant. Measurements from HTS antennas, including return loss, bandwidth, gain, efficiency and axial ratio are presented and compared with those of gold counterparts. The measured results show that useful antennas can be made using the proposed architecture for satellite communication systems.

Preparation and thermal properties of polyethylene-based carbonized fibers

In this study, carbonized fibers were prepared by using acidically cross-linked LDPE fibers. The surface morphologies of the carbonized fibers were observed by SEM. The effects of cross-linking process temperatures were studied using thermal analyses such as DSC and TGA. The melting and heating enthalpy of the fibers decreased as the cross-linking temperature increased. The cross-linked fibers had a carbonization yield of over 50%. From SEM results the highest yield of carbonized LDPE-based fibers was obtained by cross-linking at a sulfate temperature (170oC). As a result, carbonation yield of the carbonized fibers was found to depend on the functions of the cross-linking ratio of the LDPE precursors.

Effects of cross-linking methods for polyethylene-based carbon fibers: review

In recent decades, there has been an increasing interest in the use of carbon fiber reinforced plastic (CFRP) in aerospace, renewable energy and other industries, due to its low weight and relatively good mechanical properties compared with traditional metals. However, due to the high cost of petroleum-based precursors and their associated processing costs, CF remains a specialty product and as such has been limited to use in high-end aerospace, sporting goods, automotive, and specialist industrial applications. The high cost of CF is a problem in various applications and the use of CFRP has been impeded by the high cost of CF in various applications. This paper presents an overview of research related to the fabrication of low cost CF using polyethylene (PE) control technology, and identifies areas requiring additional research and development. It critically reviews the results of cross-linked PE control technology studies, and the development of promising control technologies, including acid, peroxide, radiation and silane cross-linking methods.

Liquid phase plasma synthesis of iron oxide/carbon composite as dielectric material for capacitor

Iron oxide/carbon composite was synthesized using a liquid phase plasma process to be used as the electrode of supercapacitor. Spherical iron oxide nanoparticles with the size of 5-10 nm were dispersed uniformly on carbon powder surface. The specific capacitance of the composite increased with increasing quantity of iron oxide precipitate on the carbon powder up to a certain quantity. When the quantity of the iron oxide precipitate exceeds the threshold, however, the specific capacitance was rather reduced by the addition of precipitate. The iron oxide/carbon composite containing an optimum quantity (0.33 atomic %) of iron oxide precipitate exhibited the smallest resistance and the largest initial resistance slope.

Effects of carbonization temperature on pore development in polyacrylonitrile-based activated carbon nanofibers

In this work, activated carbon nanofiber(ACNF) electrodes with high double-layer capaci-tance and good rate capability were prepared from polyacrylonitrile nanofibersby optimiz-ing the carbonization temperature prior to H2O activation. The morphology of the ACNFs was observed by scanning electron microscopy. The elemental composition was determined by analysis of X-ray photoelectron spectroscopy. N2-adsorption-isotherm characteristics at 77 K were confirmedby Brunauer-Emmett-Teller and Dubinin-Radushkevich equations. ACNFs processed at different carbonization temperatures were applied as electrodes for electrical double-layer capacitors. The experimental results showed that the surface mor-phology of the CNFs was not significantlychanged after the carbonization process, although their diameters gradually decreased with increasing carbonization temperature. It was found that the carbon content in the CNFs could easily be tailored by controlling the carbonization temperature. The specificcapacitance of the prepared ACNFs was enhanced by increasing the carbonization temperature.

Effects of electrochemical oxidation of carbon fibers on interfacial shear strength using a micro-bond method

In this work, we studied the effects of electrochemical oxidation treatments of carbon fibers (CFs) on interfacial adhesion between CF and epoxy resin with various current densities. The surface morphologies and properties of the CFs before and after electrochemical-oxidation-treatment were characterized using field emission scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and single-fiber contact angle. The mechanical interfacial shear strength of the CFs/epoxy matrix composites was investigated by using a micro-bond method. From the results, electrochemical oxidation treatment introduced oxygen functional groups and increased roughness on the fiber surface. The mechanical interfacial adhesion strength also showed higher values than that of an untreated CF-reinforced composite.

Effects of maleic anhydride content on mechanical properties of carbon fibers-reinforced maleic anhydride-grafted-poly-propylene matrix composites

In this work, the effects of maleic anhydride (MA) content on mechanical properties of chopped carbon fibers (CFs)-reinforced MA-grafted-polypropylene (MAPP) matrix composites. A direct oxyfluorination on CF surfaces was applied to increase the interfacial strength between the CFs and MAPP matrix. The mechanical properties of the CFs/MAPP composites are likely to be different in terms of MA content. Surface characteristics were observed by scanning electron microscope, Fourier transform infrared spectroscopy, and single fiber contact angle method. The mechanical properties of the composites were also measured by a critical stress intensity factor (KIC). From the KIC test results, the KIC values were increased to a maximum value of 3.4 MPa with the 0.1 % of MA in the PP, and then decreased with higher MA content.