Joo Sun Kim

Structural and electrochemical properties of ZrO2·Hx thin films deposited by reactive sputtering in hydrogen atmosphere as solid electrolytes

The feasibility of applying ZrO2Hx thin films as solid electrolytes in solid-state ionic energy systems, such as solid oxide fuel cells and supercapacitors was studied. ZrO2Hx thin films were deposited on Pt/Ti/SiO2/Si substrates by radio-frequency reactive sputtering with various hydrogen volume fractions in reactive gas. With a variation in hydrogen volume fraction, the surface roughness of the as-deposited films increased. In addition, the structure of the as-deposited films grew in the [111] direction with an increase in hydrogen volume fraction. By Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) studies, the Zr/O ratio and hydrogen distribution were evaluated. On the basis of a sample structure of Pt/ZrO2Hx/Pt/Ti/SiO2/Si for measuring an electrochemical property, an impedance measurement conducted at room temperature revealed an ionic conductivity of 1.67 ×10-6 S/cm, suggesting that ZrO2Hx thin films can possibly be used as solid oxide thin film electrolytes in all solid-state ionics power devices requiring a hydrogen conducting electrolyte.

Mechanochemical Synthesis of Nano-Sized CeO2 and its Application for CMP Slurry

Nano-crystalline CeO2 was synthesized by the mechanical milling and subsequent heat-treatment from the mixture of Ce(OH)4 as precursor, and NaCl as diluent. The diluent provided diffusion barrier during milling and heat-treatment, which was easily dissolved out by deionized water. The size of crystallite and the strain variance of CeO2 were depended on the temperature and heat-treatment time: increased with the temperature (400~700oC) and time (1~24 hours) increasing, and saturated near at 20nm in size owing to the densification of diluent. The synthesized nano-crystalline CeO2 powder was applied as an abrasive in CMP (Chemical Mechanical Planarization) slurry. When blanket-type SiO2 and Si3N4 wafers were polished with the slurries, the removal rates (RR) of SiO2 and Si3N4 wafers and selectivities (RRSiO2/RRSi3N4) were influenced by synthetic condition of abrasive, the suspension stability and the pHs of slurries.

Sealing Behavior of Visco-Elastic Composite Seals for SOFC Applications

Porous composites containing ceramic fiber have been developed for the fabrication of SOFC seals. They were fabricated using glass powder and alumino-silicate chopped fibers. Effect of mixing ratios of ceramic fiber and glass on the leak rates and strength of the composite seals was investigated. In addition, seal performance of commercial glasses was compared with that of SiO2-BaO-B2O3 glass synthesized in this work. The leak rate of the composite seals containing 55 vol% glass was seven times higher than the one containing 75 vol% glass. The flexural strength of the composite seals was reduced to one fourth of the initial value as the porosity increased from 1 to 29%. The incorporation of alumino-silicate chopped fibers into a sealing glass degraded room temperature strength and increased leak rates due to increase in porosity with increasing fiber content. The viscosity of glass at the seal test temperature is presumed to affect the leak rate of the glass seal.

Preparation and Properties of a Novel Anode of Interpenetrating Phase Composite Structure

A novel Ni-YSZ anode with interpenetrating phase composite (IPC) structure was developed using NiO-YSZ core-shell composite powder and evaluated in terms of microstructure, electrical conductivity, thermal expansion and flexural strength. In comparison to conventional anode, the anodic performance of IPC anode appeared to be more desirable for improving structural reliability of SOFC unit cells and stacks. This study reveals that the anodic performance of IPC anode can be readily tailored by controlling core-shell composite powder particles.

Mechanical Alloying Effect of Hematite and Graphite

The mechanochemical reaction of hematite with graphite by mechanical alloying (MA) has been investigated at room temperature. The solid state reduction of hematite to Fe3O4 and FeO has been observed after 120 hours of MA by a planetary ball mill. Saturation magnetization is gradually increased with milling time up to 80 h, and then deceased after 120 h of MA, indicating the transformation of Fe3O4 into nonmagnetic FeO through further reduction. Neither the solid state reduction of Fe2O3 by graphite nor a sizable grain refinement is observed in the MA process using a horizontal ball mill.

Effect of Carbon Black Addition on Reaction-Bonded Silicon Carbide Ceramics

High strength reaction-bonded silicon carbide ceramics was successfully produced by reducing the amount of residual silicon and the silicon pocket size with carbon black as an additional carbon source. A prototype of wafer carrier was also produced in near-net dimension by planar contact infiltration of molten silicon into a preform joined with six pieces of simple shape by eliminating process shrinkages. Forming shrinkages were decreased to a negligible level by compression molding, while sintering shrinkage was eliminated by reactive infiltration of molten silicon.

Fabrication of Large Ceramic Components with Controlled Microstructure by Powder Thermoset Molding

Thermoset molding in wet and dry state was successfully employed to fabricate high strength reaction bonded silicon carbide (RBSC) ceramics. Granule transfer molding (GCM) was developed to prevent segregation of component particles and binder phase in wet state, while granule compression molding was applied in dry state to fabricate green compact with significant variation of compaction ratio. Low-fill density granules with mixing homogeneity were critical for promoting lateral deformation of granules during consolidation. In addition, anodic performance of Ni-YSZ anode was significantly enhanced by replacing solid fugitive phase with viscoelastic fugitive phase used as binder in thermoset molding.

Fabrication of Nanoabrasive Grinding Wheels and Their Application to Grinding Silicon Wafers

In the surface machining of brittle materials, there exists a transition from brittle to ductile modes when the depth of cut is reduced below a critical size using ultrafine abrasive grains. Vitrified grinding wheels containing ultrafine abrasives in the sub-micrometer to nanometer range were fabricated by mechanochemically milling nanoabrasive particles and subsequent viscous sintering of abrasive-binder composites. The grinding characteristics of the nanoabrasive grinding wheels were evaluated for the fine grinding of silicon wafers in terms of a variety of variables. Preliminary wafer grinding results are presented on material removal rate and surface quality of silicon wafers.

A Feasibility Study on Advanced Methodology to Produce High-Performance Nano WC-Co Granule Feedstock for Thermal Spraying

A feasibility study was conducted, in order to fabricate WC-Co powder granule as high-performance feedstock for thermal spraying process, by spray drying weakly flocculated nano-particulate slurries under various conditions. The defects were evaluated with respect to the amount of additives to prepare spherical WC-Co feedstock powders with few morphological defects and homogeneous microstructures. The characteristics of feedstock powders heat-treated at 850~ 1200ı were analyzed using SEM, EPMA, and Hg-porosimetry. The microstructure investigation revealed that the relative portion of larger pores around 10 µm increased with increasing heattreatment temperature. On the other hand, the relative portion of smaller pores of 1 µm decreased due to the coalescence of WC particles, leading to dense feedstock powders with less defects such as hollow or craters. The hardness of thermal-sprayed coat developed with the feedstock in this study, which had exceptionally high feedstock strength, was superior to that of any other sprayed coat obtained with commercial feedstock powders.

Inductively coupled plasma reactive ion etching of ZrO2:H solid electrolyte film in BCl3-based plasmas

Inductively coupled plasma reactive ion etching (ICP-RIE) of ZrO 2 :H solid electrolyte films was investigated using BCl 3 -based plasma. ZrO 2 :H etch rates were studied as a function of the BCl 3 /Ar chemistry, ICP coil power, bias voltage, and working pressure. Scanning electron microscopy and atomic force microscopy were employed to characterize the etch rate and root-mean-square surface roughness of etched samples. It was found that in comparison with Cl 2 -based gas mixtures, pure BCl 3 plasma results in a high etch rate of ZrO 2 :H layer, suggesting an abundance of B and BCI radicals made up of a volatile compound such as B x O y , BCl-O, and Zr-Cl bond. In addition, Auger electron spectroscopy analysis exhibits that the BCl 3 -based etching process produces no change in surface stoichiometry of the ZrO 2 :H films.