Cheol Jin Kim

Fabrication and optical characteristics of a novel optical fiber doped with the Au nanoparticles.

Optical fibers containing gold metal nanoparticles were developed by modified chemical vapor deposition, in which Au(OH)3 and tetraethyl-orthosilicate (TEOS) was used via sol-gel process to incorporate gold metals by providing the reduction atmosphere. The absorption peak appeared near 490 nm was found to be due to the surface plasmon resonance of the gold nanoparticles incorporated in the fiber core.

Effect of heat treatment of optical fiber incorporated with Au nano-particles on surface plasmon resonance

To improve the sensitivity of the surface plasmon resonance (SPR) sensor based on the specialty optical fiber incorporated with Au nano-particles (NPs) in the cladding region, the effect of heat treatment (800 °C - 1000 °C) of the fiber on sensing capability of refractive index (n = 1.418 - 1.448) was investigated. The SPR appeared at a particular wavelength around 390 nm for the corresponding refractive indices regardless of the heat treatment temperature and the SPR wavelength increased with the increase of the index. The SPR sensitivity was found to increase with the increase of heat treatment temperature, 178 nm/RIU, 299 nm/RIU, and 945 nm/RIU at 800 °C, 900 °C, and 1000 °C for an hour, respectively. On the other hand, the SPR absorption intensity decreased with the increase of heat treatment temperature due to the increase of the propagation loss of the incident light and the SPR band became spread due to the increase of the size distribution of the Au NPs at the various refractive indices.

MICROSTRUCTURAL ANALYSIS OF CORE-SHELL STRUCTURED MgB2 SUPERCONDUCTING FIBERS FABRICATED BY HYBRID PHYSICAL CHEMICAL VAPOR DEPOSITION

We have deposited MgB2 thin films on the carbon fiber by hybrid physical–chemical vapor deposition (HPCVD) method which is known as the most effective technique for depositing MgB2 thin films without pore formation. By adopting carbon fiber as substrate and gas phase reaction between diborane (B2H6) gas and Mg vapor for MgB2 formation, core-shell structured MgB2/C composite fiber could be synthesized. The high quality of the prepared MgB2 fibers was confirmed with scanning electron microscopy (SEM), and the critical temperature (Tc) of the sample deposited at 590°C and 12 sccm flow rate of B2H6 was measured at ~40 K. The microstructures of MgB2 layer was characterized using transmission electron microscopy (TEM). The bright-field TEM images showed MgB2 with the grain size of several nanometers. The MgB2 thin film showed highly dense microstructure without pore and well-connected to the carbon fiber interface. High-resolution TEM (HRTEM) images and scanning transmission electron microscopy (STEM) revealed the presence of interface phase between MgB2 thin films and carbon fiber.

CHARACTERIZATIONS OF THE CORE-SHELL STRUCTURED MgB2/CARBON FIBER SYNTHESIS BY RF-SPUTTERING AND THERMAL EVAPORATION

MgB2/carbon fibers have been synthesized by the combination of RF-sputtering of B and thermal evaporation of Mg, followed by co-evaporation. First, boron layer was deposited by RF-sputtering on the carbon fiber with average diameter of 7.1 μm. Later this coated layer of B was reacted with Mg vapor to transform into MgB2. Since the MgB2 reaction proceed with Mg diffusion into the boron layer, Mg vapor pressure and the diffusion time had to be controlled precisely to secure the complete reaction. Also the deposition rate of each element was controlled separately to obtain stoichiometric MgB2, since Mg was evaporated by thermal heating and B by sputtering system. The sintered B target was magnetron sputtered at the RF-power of ~200 W, which corresponded to the deposition rate of ~3.6 A/s. With the deposition rate of B fixed, the vapor pressure of Mg was controlled by varying the temperature of tungsten boat with heating element control unit between 100 and 900°C. The MgB2 layers with the thickness of 200–950 nm could be obtained and occasionally MgO appeared as a second phase. Superconducting transition temperatures were measured around ~38 K depending on the deposition condition.

Morphology control of a silicon nitride thick film derived from polysilazane precursor using UV curing and IR heat treatment

ABSTRACT Silicon nitride (Si3N4) has excellent thermo-mechanical properties, and can be used as heat dissipation substrate for various devices. Si3N4 thin films are generally synthesised by chemical vapour deposition (CVD) or plasma-enhanced CVD. The use of polysilazanes (PSZs) as a precursor to the synthesis of Si3N4 has attracted significant attention because of their high mouldability and processability. In this study, Si3N4 thick films were prepared on silicon wafers or aluminium substrates by a spin- or dip-coating liquid PSZ, followed by UV curing and IR heat treatment under various conditions. The effects of the heat treatment conditions on the Si3N4 thick film surface were analysed by optical microscopy, X-ray diffraction, and scanning electron microscopy. An almost single phase of Si3N4 was synthesised successfully on the single crystalline silicon with UV curing at 400°C for 30 min and IR heating at 800°C in N2 atmosphere.

OPTIMUM DESIGN OF CANTILEVERED MICROPROBES FOR INSPECTING LCD PANELS AND MEASUREMENT OF CONTACTING FORCES

Fine pitch microprobe arrays are microneedle-like probes for inspecting the pixels of LCD panels or IC. They are usually made of multi-layers of metallic, nonmetallic, or combination of the two. The design requirement for a contacting force is less than 2 gf and a deflection should be less than 100 µm. Many microprobe shapes satisfying the design requirements are possible. A cantilever-type microprobe having many needles was chosen and optimized in this study. Several candidate shapes were chosen using topology and shape optimization technique subjected to design requirements. Then, the microprobe arrays were fabricated using the process applied for MEMS fabrication and they were made of BeNi, BeCu, or Si. The contact probing forces and deflections were measured for checking the results from optimum design by newly developed measuring equipment in our laboratory. Numerical and experimental results were compared and both showed a good correlation.

Microstructure Analysis and Characterization of Li(NiCoMn)O2 Thin Film for Cathode Application Prepared by Li Diffusion on the Ni-Co-Mn Alloy Substrates

The currently used cathode material for secondary batteries such as LiCoO2 exhibited limit to further improve the functionality of the batteries, since the screen printing method cannot reduce the thickness of the battery further with the solid state reacted powder which has the size of several micrometer. In this study, we have synthesized Li(NiCoMn)O2 thin film to replace LiCoO2 thick film by employing Li-diffusion reaction on the surface of the textured Ni-Co-Mn alloy. The cube-textured Ni-Mn alloy was prepared by cold-isostatic pressing of mixed Ni-Mn powder, sintering, repeated rolling process, and annealing heat treatment for texture development. After thin layer of metallic Li was deposited on the surface of Ni-Co-Mn template using thermal evaporation method in the glove box or pulsed laser deposition, the Li/Ni-Co-Mn composite tape were heat treated at 800~900°C for 1~2hrs in oxidizing atmosphere to induce Li diffusion into the Ni-Co-Mn template and Li(NiCoMn)O2 phase formation. The Li(NiCoMn)O2 phase evolution was confirmed by XRD and microstructural characteristics such as grain size and surface morphology were analyzed by scanning electron microscopy and atomic force microscopy. Also the charge and discharge test was conducted to confirm the electrical characteristics of Li(NiCoMn)O2/Ni-Co-Mn thin film for the cathode application.

DIELECTRIC PROPERTIES OF ASYMMETRICALLY ANNEALED FERROELECTRIC (Ba,Sr)TiO3:MGO THICK FILMS

ABSTRACT A 20 wt.% of MgO added (Ba0.6Sr0.4)TiO3 (BST:MgO) thick film have been fabricated by a tape casting and firing method for tunable microwave applications. In order to improve ferroelectric properties, the composite BST:MgO thick films have been asymmetrically annealed by a focused beam method. Dielectric constants of BST:MgO films are changed from 1050 to 1300 at 100 kHz after 150 s annealing by the focused beam. Even though it was not observed any prominent changes from the thick films before-and after-annealing in terms of chemical composition and surface morphology, it is clear that the average particle size of the thick films calculated by Scherrers formula were increased by annealing. Furthermore, a strong correlation between particle size and dielectric constant of the BST:MgO has been observed; dielectric constant increases with increased particle size. This has been attributed to the increased volume of ferroelectric domain due to increased particle sizes.

Temperature and Vibration Dependence of the Faraday Effect of Gd2O3 NPs-Doped Alumino-Silicate Glass Optical Fiber

All-optical fiber magnetic field sensor based on the Gd2O3 nano-particles (NPs)-doped alumino-silicate glass optical fiber was developed, and its temperature and vibration dependence on the Faraday Effect were investigated. Uniformly embedded Gd2O3 NPs were identified to form in the core of the fiber, and the measured absorption peaks of the fiber appearing at 377 nm, 443 nm, and 551 nm were attributed to the Gd2O3 NPs incorporated in the fiber core. The Faraday rotation angle (FRA) of the linearly polarized light was measured at 650 nm with the induced magnetic field by the solenoid. The Faraday rotation angle was found to increase linearly with the magnetic field, and it was about 18.16° ± 0.048° at 0.142 Tesla (T) at temperatures of 25 °C–120 °C, by which the estimated Verdet constant was 3.19 rad/(T∙m) ± 0.01 rad/(T∙m). The variation of the FRA with time at 0.142 T and 120 °C was negligibly small (−9.78 × 10−4 °/min). The variation of the FRA under the mechanical vibration with the acceleration below 10 g and the frequency above 50 Hz was within 0.5°.

Reduced Pressure Curing on Polycarbosilane Precursor for Synthesis of Silicon Carbide Fiber

Low pressure curing method with iodine vapor was used on low softening temperature polycarbosilane (PCS) precursor for fabrication of continuous SiC fiber at relatively low temperature. The low curing temperature can provide with a wide range of softening temperature PCS precursors, especially with low softening PCSs, which have a good spinnability, but many difficulties with conventional oxidation curing method. The low pressure curing method having the presence of iodine vapor have shown the more positive effect on pyrolysis with early stage crystallization of β-SiC at 1300 °C. Crystal size of β-SiC, cured at 0.008 kPa is around 2–3 nm larger than cured at 101 kPa. In addition, the higher tensile strength of SiC fiber at elevated temperature can be obtained at 0.008 kPa with a value of 2.1 GPa, compare to 1.3 GPa at 101 kPa of curing pressure condition.