Papers by Author: Gyu Bong Cho

Abstract: Si and Si/Ni thin film electrodes less than 1
m in total thickness were fabricated on the roughened Cu substrate by rf sputtering. Their surface morphology and crystalline structure were carefully investigated by means of FESEM and XRD. The morphology of films is dependent on the surface feature of substrate, and the grown films were amorphous. The initial capacity and the irreversible capacity loss of a Li/Si film cell were improved with insertion of a Ni buffer layer. The effect of the film morphology on the electrochemical properties of cells was demonstrated based on the observations of film electrodes.

Abstract: Si electrode with different amount of polymer binder was fabricated by ball milling. With increasing of polymer content, not only porosity but also the first charge capacity of Si electrode was increased. The Si electrode enhanced electrical conductivity showed high first capacity of 3644mAh/g and improved cycle performance. Si electrode having good electrochemical property could be fabricated by controlling amount of polymer.

Abstract: The nickel sulfide (Ni3S2) thin film could be prepared from Ni/S double layer, which was deposited on nickel foil using evaporation and sputtering. The nickel sulfide electrode was discharged and charged between 0.6V and 2.6V versus Li/Li+ at room temperature. The nickel sulfide film had the first discharge capacity of 270mAh/g, and two plateaus at 1.3V and 1.8V.

Abstract: Si film electrodes for Li micro-film batteries were fabricated on a Cu substrate and a Ci/Cu film. In the structural properties, FE-SEM observation demonstrated difference in surface morphologies of Si films with different under layers. Surface of the Si film deposited consists of clusters and average size of the cluster was 165 nm for Si/Cu film and 80 nm for Si/C/Cu film. Si film has amorphous phase in spite of different under layers; Cu substrate and C/Cu film. In the electrochemical properties, the carbon-inserted film showed a good cycleability compared with Si/Cu electrode. It is believed that the insertion of carbon film as a buffer film absorbed the stress generated during charge-discharge process and improved cycle performance of Si anode electrode.

Abstract: We investigated on the additive effect of carbon nanotube in the sulfur electrode on the first discharge curve and cycling property of lithium/sulfur cell. The sulfur electrode with carbon nanotube had two discharge plateau potentials and the first discharge capacity about 1200 mAh/g sulfur. The addition carbon nanotube into the sulfur electrode did not affect the first discharge behavior, but improved the cycling property of lithium/sulfur cell. The optimum content of carbon nanotube was 6 wt% of sulfur electrode.

Abstract: Various physical, chemical and mechanical methods, such as inert gas condensation, chemical vapor condensation, sol-gel, pulsed wire evaporation, evaporation technique, and mechanical alloying have been used to synthesize nanoparticles. Among them, chemical vapor condensation(CVC) represents the benefit for its applicability to almost materials because a wide range of precursors are available for large-scale production with a non-agglomerated state. In this work, iron nanoparticles and nanowires have synthesized by chemical vapor condensation(CVC) process, using iron pentacarbonyl(Fe(CO)5) as precursor. The effects of processing parameters on the morphology, microstructure and size of iron nanoparticles and nanowires were studied. Iron nanoparticles and nanowires having various diameters were obtained by controlling the inflow of metallic organic precursor. Both nanoparticles and nanowires were crystallized. Characterization of obtained nanoparticles and nanowires were investigated by using a field emission scanning electron microscopy, transmission microscopy and X-ray diffraction.

Abstract: Gallium phosphide nanowires were successfully synthesized by the catalytic chemical vapor deposition (CVD) method using MgO powder-impregnated nickel oxide as catalyst and gallium phosphide and gallium powders as GaP source. The synthesis of GaP nanowires were carried out at 900°C for 30min under argon ambient and directly vaporized Ga and GaP powder. The diameter of GaP nanowires is about 25~70nm and the length is up to several tens of micrometers. The GaP NWs was core-shell structure, which consists of the GaP core and the Ga oxide outer layers. The GaP nanowires have a single-crystalline zinc blend structured crystals with the [111] growth direction. Nanowires larger than around 50nm in diameter exhibited twinning faults, which appears in the TEM images as discrete dark lines and alternating wire contrast. We demonstrate that MgO powder-impregnated nickel oxide catalyst exhibited a large catalytic effect on the growth of high-purity and -quantity gallium phosphide(GaP).

Abstract: Si film electrodes for Li micro-film batteries were fabricated on a Cu substrate and a Ni/Cu film. In the structural properties, Si film had an amorphous structure, while the crystallinity of Ni buffer film was improved after annealing. FE-SEM observation demonstrated differences in surface morphologies of Si films fabricated with different methods. In the electrochemical properties, the Ni-inserted film showed the largest initial charge and discharge capacity, and the cell with annealed Si/Ni/Cu film showed good cycleability. It is believed that the insertion of Ni film as a buffer film improved the charge and discharge capacity owing to the enhancement of adhesion between Si film and Cu substrate.

Abstract: Shape memory characteristics and superelasticity of an equiatomic Ti-Ni alloy with surface sulfide layers were investigated by means of thermal cycling tests under constant load, tensile tests, and scanning electron microscopy. Martensitic transformation start temperature (Ms) increased by sulfidation, which is ascribed to a compressive stress imposed by surface sulfide layers. Surface sulfide layers were found to make transformation elongation be small when their thickness was less than 5 ㎛. This is ascribed to the fact that the surface sulfide layers were not detached from substrates and constrained martensitic transformation. When thickness of the surface sulfide layers was 35 ㎛, transformation elongation was not affected by them because they were detached during transformation. Superelastic recovery decreased by the sulfide layers when their thickness was about 5 ㎛, while it did not change when the thickness was about 35 ㎛.

Abstract: The lithium ionic conductivity of Poly (acrylonitrile) (PAN) gel polymer electrolyte with PC/EC was found to be about 1.3 x 10-3S/cm at room temperature. The discharge curve of Li/ PAN (PC+EC)/S battery showed only one plateau region, which is different from that using PVdF(TEGDME) gel polymer electrolyte. Also, the first discharge capacity was 556mAh/g-sulfur in Li/S battery using PAN (PC+EC) gel electrolyte at room temperature.