Papers by Author: Kwon Koo Cho

Abstract: Silicon oxide (SiOx) nanowires may have many applications due to their electrical, mechanical and optical properties. Many methods have been reported for the synthesis of SiOx nanowires, including laser ablation, sol–gel, thermal evaporation, carbothermal reduction, physical evaporation, rapid thermal annealing, chemical vapor deposition and thermal oxidation route, oxide assisted growth and thermal sublimation. In this paper, we reported SiOx nanowires fabricated by simple thermal heating process of catalyst thin film-coated Si substrates with various parameters, such as synthesis temperature, synthesis gases, catalysts, and buffer layer (SiO₂ layer). Synthesized silicon oxide nanowires were amorphous crystalline. The best synthesis condition of prepared SiOx nanowire is slightly varied with catalysts and buffer layer. The flow rate of synthesis gas affected diameter of silicon oxide nanowires.

Abstract: Ni-3at.%W alloy tapes for YBCO coated conductor were fabricated by powder metallurgy process including powder compaction, cold isostatic pressing(CIP), cold rolling and annealing for recrystallization. The Ni-3at.%W tapes annealed in an atmosphere of 96%Ar and 4%H2 mixing gas were characterized by X-ray pole figures, orientation distribution function(ODF) and optical microscopy(OM). The Ni-W alloy tapes were heat-treated at 700~1000°C for the development of cube texture {001}<100>. The texture analysis indicated that as fabricated tapes have a sharp cube texture after annealing at 900 and 1000°C. Also, thoroughly recrystallization takes place for the annealed tapes at the both temperatures and the average grain size of the two tapes is about the same, which is about 35㎛.

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: High purity nanowires are successfully synthesized by chemical vapor deposition. In this work, we have tried synthesis of GaP nanowires with copper oxide catalyst using chemical vapor deposition method involving a metal oxide-assisted vapor-liquid-solid (VLS) growth mechanism. The synthesis process is the same as that described in existing work except for a catalyst. The mixture of GaP and Ga powder was used as GaP source for synthesis of GaP nanowires. And the mixture powder was directly vaporized in the range of 700~1000°C under argon ambient in a furnace. The wire-like products was observed in the range of 800~950°C. The diameter of nanowires increases with increasing synthesis temperature, but reversely, the length of nanowires decreases steadily. The nanowires prepared at 850°C possess perfect wire-like shape and uniform distribution of diameter. The average diameter and length of nanowires are about 50
 and 150, respectively. HRTEM and EDX analysis were carried out to obtain more detailed information of its microstructure. Nevertheless, all condition of processing was set for making the high purity GaP nanowires as existing reported method, the nanowires were identified as well-crystallized gallium oxide nanowires with an amorphous outer layer. It does not accord with existing reported results. This result means that the catalysts play a key role in the growth of nanowires.

Abstract: LiNiO2 thin films for the application of cathode of the rechargeable battery were fabricated by Li ion diffusion on the surface oxidized NiO layer. Bi-axially textured Ni-tapes with 50 ~ 80 μm thickness were fabricated using cold rolling and annealing of Ni-rod prepared by cold isostatic pressing of Ni powder. Surface oxidation of Ni-tapes were conducted using tube furnace or line-focused infrared heater at 700 °C for 150 sec in flowing oxygen atmosphere, resulted in NiO layer with thickness of 400 and 800 μm, respectively. After Li was deposited on the NiO layer by thermal evaporation, LiNiO2 was formed by Li diffusion through the NiO layer during subsequent heat treatment using IR heater with various heat treatment conditions. IR-heating resulted in the smoother surface and finer grain size of NiO and LiNiO2 layer compared to the tube-furnace heating. The average grain size of LiNiO2 layer was 0.5~1 μm, which is much smaller than that of sol-gel processed LiNiO2. The reacted LiNiO2 region showed homogeneous composition throughout the thickness and did not show any noticeable defects frequently found in the solid state reacted LiNiO2, but crack and delamination between the reacted LiNiO2 and Ni occurred as the reaction time increased above 4hrs.

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: FeSx nanoparticles were synthesized by the chemical vapor condensation (CVC) process using the pyrolysis of iron pentacarbonyl (Fe(CO)5) and sulfur (S). The influence of CVC parameter on the formation of nanoparticle and size distribution was studied. The synthesized nanoparticles consisting of FeS, FeS2 and Fe2O3 were nearly spherical shape and 5~40 nm in mean diameter. Obtained particles were studied by applying the field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) methods. Electrochemical properties of the electrode fabricated with synthesized nanoparticles were evaluated.

Abstract: The microstructure of Ni sulfides prepared by thermal sulfidation of pure Ni and their dependence of fabrication parameters were investigated by means of scanning electron microscopy and X-ray diffractions. Sulfidation was made by isothermally annealing Ni with the sulfur in vacuum sealed glass ampoules at 673 K for 120 – 600s under the sulfur pressure of 100 and 220 kPa. The sulfide layers formed in the early stage were found to consist of spherical particles smaller than 0.5um, which were grown and agglomerated with increasing annealing temperature. Thickness of sulfides developed on Ni substrate was found to increase with increasing annealing time and sulfur pressure. It was also found that compositions of dominant Ni sulfides changed with varying annealing time. At the initial stage, only Ni3S2 sulfide was formed on pure Ni, which was tightly bonded to Ni substrate. On increasing annealing time, NiS sulfide was formed. On further increasing annealing time, NiS1.97 sulfide was formed, which always coexisted with NiS sulfide. A mechanism for sulfidation of Ni is proposed as follows: 3Ni + 2S
Ni3S2, Ni3S2 +S
NiS, NiS + S
NiS1.97