Sung-Churl Choi

Crystallization behavior of nano-ceria powders by hydrothermal synthesis using a mixture of H2O2 and NH4OH

Abstract Homogeneous and well-crystallized nano-ceria (CeO 2 ) powders were produced by hydrothermal synthesis using a mixture of H 2 O 2 as the oxidizer and NH 4 OH as the mineralizer. The precipitates were prepared by mixture ligands of OH and OOH derived from NH 4 OH and H 2 O 2 . The hydrothermal synthesis was conducted at 200 °C for 6 h. The synthesized ceria powders were characterized as the crystalline phase identification by X-ray diffractometry, the chemical bonding by infrared spectrophotometer (FT-IR) and the morphology and particle size by transmission electron microscopy (TEM). Crystallization behavior of ceria powders using a mixture of oxidizer and mineralizer was more influenced by OOH ligands than by OH ligands to form Ce(OH) x (OOH) 4− x as precursor with increasing concentrations of H 2 O 2 with 10–20 times of the cerium(III) ion. In all samples, synthesized particle size was decreased with increasing concentration of oxidizer ranging from 6 to 12 nm in diameter. Morphology and size distribution of synthesized particles were relatively spherical and uniform, respectively.

Gas sensing properties of WO3 thick film for NO2 gas dependent on process condition

Abstract The thick film sensor of tungsten oxide for NO2 gas detection was fabricated by changing the process conditions, which were firing temperature and annealing atmosphere. The origin of sensor property variation was interpreted by the characterization of crystallite size, specific surface area and microstructure. The degree of oxygen deficiency in the WO3 sensor also affected sensor properties and the optimum oxygen content of WO3 was necessary to get high sensitivity for NO2. The WO3 thick film fired at 700°C and operated at 100°C showed excellent sensor properties, and its operation temperature is lower than that from other reports.

The effect of electrostatic repulsive forces on the stability of BaTiO3 particles suspended in non-aqueous media

Abstract The effects of physicochemical properties of organic solvents and dispersants on the dispersion of barium titanate (BaTiO 3 ) was investigated in a system where organic solvents, dispersants, binders, and plasticizers are used as processing additives in multilayer ceramic capacitor fabrication processes. In this study, phosphate esters and menhaden fish oil were used as dispersants. The dispersion properties of BaTiO 3 suspensions were characterized by electrokinetic sonic amplitude, polymer adsorption and viscosity measurements. The steric and electrostatic stabilization mechanisms affecting the suspension properties of BaTiO 3 were evaluated. The adsorption characteristics and electrokinetic behavior of BaTiO 3 suspensions were correlated with the stability of BaTiO 3 suspended in non-aqueous media. The stability of BaTiO 3 achieved by steric stabilization was dependent on the fraction of surface coverage resulting from dispersant adsorption on the BaTiO 3 particle. The electrostatic repulsive forces acting between BaTiO 3 particles dispersed in organic media was found to be significantly greater than predicted and dependent mainly on the physicochemical properties of the organic solvent media.

Zirconia-stainless steel functionally graded material by tape casting

Abstract Ceramic/metal functionally graded material (FGM) was fabricated by tape casting. Zirconia (ZrO 2 ) and stainless steel (SUS) were stably dispersed in deionized water (DI-water). An optimal dispersion condition of ZrO 2 was obtained from electrokinetic sonic amplitude (ESA) data, and ZrO 2 particles could be dispersed by electrostatic repulsion. Conversely, a stable SUS slurry was prepared by increasing solution viscosity and using steric hindrance. Monophase and binary slurries were cast at uniform thickness through a doctor blade. ZrO 2 / SUS FGM was sintered at 1350 °C in Ar/H 2 atmosphere. The sintering defects could be controlled by the adjustment of the particle size and phase-type of ZrO 2 . As a consequence, the microstructure and interface showed a compositional gradient continuously.

Design and microstructure of ZrO2/SUS316 functionally graded materials by tape casting

Abstract Zirconia (ZrO 2 ) and stainless steel 316 (SUS316) functionally graded materials (FGM) were fabricated by tape casting from an aqueous system. For the stable dispersion of ZrO 2 and SUS316, we observed the zeta-potential ( ζ ) of each phase with pH variation and investigated the effect of organic additives on dispersion. Tetragonal zirconia polycrystals (TZP) and monoclinic zirconia polycrystals (MZP), which was used to control the shrinkage and sintering behaviors of TZP and SUS316, could be dispersed with addition of polymethacrylic acid (PMAA) as dispersant and SUS316 could be dispersed with carboxymethylcellulose sodium salt (Na–CMC) as suspending agent. Monophase and composite slurries were cast when the optimized conditions were established from the electrokinetic sonic amplitude (ESA) and viscosity data. ZrO 2 /SUS316 FGM was fabricated by sintering at 1350°C in Ar/H 2 atmosphere. As a consequence, the microstructure and the interface of the FGM showed the continuous compositional gradient. It was verified that the sintering defects, such as delamination, cracking and warping, can be reduced or eliminated by the adjustment of the particle size and the phase-type of ZrO 2 .

Fabrication of 3Y-TZP/SUS304 functionally graded materials by slip casting; application of porous alumina molds

Abstract Three mole percent Y2O3 doped tetragonal zirconia polycrystals (TZP) and stainless steel 304 (SUS) functionally graded materials (FGM) were fabricated by slip casting. To overcome the problems of the gypsum mold (GM) in the slip casting process, an alumina mold (AM) was prepared and its properties investigated after controlling the microstructure and the sintering conditions. The optimal conditions for the dispersion of monophase and binary-phase slurries were investigated. According to these results, eleven layer TZP/SUS-FGM were fabricated with a GM and an AM. The specimens, using an AM, showed no contamination on the surface, easier thickness control of each layer and resulted in higher productivity. Especially, no symptoms of degradation were observed in the SUS made by AM. Therefore, it is more desirable to use a porous AM than a GM in the fabrication of TZP/SUS-FGM by the slip casting process.

Effect of CH4 and H2 on CVD of SiC and TiC for possible fabrication of SiC/TiC/C FGM

Abstract SiC and TiC coatings were deposited by CVD on graphite substrates and the effect of the variation of the methane (CH4) and hydrogen (H2) ratio on deposition was investigated. SiCl4, TiCl4 and CH4 were used as sources of Si, Ti and C. In case of the SiC coatings, stoichiometric SiC was obtained when the ratios of CH 4 (SiCl 4 + CH 4 ) and H 2 (SiCl 4 + CH 4 ) are 0.4 and 10, respectively. Stoichiometric TiC was also obtained under similar conditions. In order to obtain non-stoichiometric materials for possible fabrication of functionally gradient materials (FGM), a change of microstructure and composition was observed with changes of the CH4 and H2 ratio. As a result, SiC, TiC and C contents were more easily controlled by a change of the H2 ratio compared to the CH4 ratio for SiC and TiC deposition. It has been verified that the change of the H2 ratio is more desirable for possible manufacturing of SiC/TiC/C FGM.

Synthesis and microstructure of silica-doped alumina composite membrane by sol-gel process

The preparation and application of ceramic membranes has received much attention in the past few years [1]. Ceramic membranes are technically important in separation and filtration as well as in catalytic reactions, because of their high thermal and chemical stability, long life time and good defouling properties in comparison with polymeric membranes [2, 3]. Many researchers have reported on the synthesis and microstructure development of ceramic membranes, and their application [2, 4–8]. Nowadays, commercial applications of alumina membranes can be found in wine and beer clarification as well as the pharmaceutical industry [4]. Of the various methods used for inorganic membrane preparation, sol-gel process is considered the most practical for ceramic membrane synthesis because of the advantages of being able to make micro-scale thin membrane top layers with nanoscale pore diameter and narrow pore size distribution [2]. In sol-gel membrane synthesis, the pore size is determined by the primary particle size in the sol [4]. The sol is deposited on the porous body by a dipping or a slip casting method. After drying and heat treatment, the thin film forms the smallest pore size at a relatively low temperature. In most cases, the pore size of thin film increases with the heat treatment temperature. There are many reports referring to the preparation of alumina membrane by the sol-gel process [2, 4, 5]. Alumina membranes are usually synthesized with their transition forms (e.g. γ -, δ-, θ -Al2O3). Because of their fine particle size, high surface area, and catalytic activity of their surfaces, the transition alumina (especially theγ form) find application in industry as absorbents, catalysts or catalyst carriers, coatings and soft abrasives [9]. But they are transformed at high temperatures (usually about 1000–1100 ◦C) and develop abnormal grain growth (vermicular structure). At above 800◦C, the BET surface area of pure alumina membranes start to decrease and at above 1000 ◦C the pore structure which is preferable for membrane application, is destroyed [4]. So their high temperature application is limited. Lin and Burggraff [2] investigated the effects lanthanum’s presence has on the thermal stability of alumina membranes. According to this report, up to 1200◦C, they were thermally stabilized, e.g. the pore characteristics were maintained up to 1200 ◦C. In this study, silica doped alumina membranes were prepared by the sol-gel method for gas separation application. Transition alumina shows relatively weak chemical durability than other oxide, and silica has excellent chemical stability except against hydrogen fluoride (HF). It is expected that silica-doping could improve the chemical stability of alumina membrane and delay the surface diffusion in the sintering process, resulting in pore characteristics improvement. The microstructure change of silica-doped alumina membrane as a function of heat treatment temperature was examined. The alumina sol was prepared by the Yoldas process [10] and the ceramic membranes were coated by the sealed dipping method. The alumina sol (0.2 mol l −1) was prepared by adding aluminum-tri-sec-butoxide (ATSB, Aldrich) to distilled water. The sol was vigorously stirred for complete alkoxide hydrolysis and was peptized with hydrogen chloride (HCl) at 75–85 ◦C. This solution was stirred on a hot plate for over 24 h to ensure complete mixing and hydrolysis. HCl was added at 75–80◦C for peptization of the sol (molar ratio; 0.07). A polyvinyl alcohol (PVA) solution, prepared by dissolving 10 g of PVA (Aldrich, MW=8000–10000) in 90 g of distilled water, was used as a drying control chemical additive (DCCA) for making supported ceramic membrane. A silica sol (5 wt % sol, particle size; several nm, Seok-Keyng Chemical Co.) was purchased. The Al2O3 sol and the SiO2 sol were mixed in a molar ratio of 5 to 15 mol % per alumina. For complete mixing, the mixed sol was stirred vigorously on a magnetic plate for over 10 min. The support used was tubular type α-Al2O3 which was fabricated at Dong-Su Co. The mean pore size of the support is about 0.1 μm and the porosity is about 30%. The porosity of the support was determined by the Archimedes method. The support has desirable properties for inorganic composite membrane as reported earlier [3]. The support was coated with the mixed sol by the sealed dipping method for 45 s and dried in the atmosphere at room temperature. The dried supports were calcined from 700 to 1300 ◦C (heating rate; 1◦C min−1) for 1 h. This sealed dip coating method is the one that controls the coating thickness with ease. The surface of support that is not to be coated is sealed, and the sealed

The role of 2-methyl-2,4-pentanediol modifier and its interaction with poly(vinyl butyral) binder in BaTiO3 and Li2O–B2O3–BaO–SiO2 glass suspensions

Abstract The role of the modifier, 2-methyl-2,4-pentanediol (MPD), and its interaction with poly(vinyl butyral) (PVB) binder with BaTiO 3 -based dielectric particles and Li 2 O–B 2 O 3 –BaO–SiO 2 glass suspensions were investigated using viscosity and adsorption isotherm measurements, Fourier transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM). Commercial MPD and PVB commonly serve as a modifier and binder, respectively, in organic suspension media in low-temperature cofired ceramic (LTCC) fabrication processes. In the absence of a modifier, a suspension prepared with PVB behaves as a highly viscous fluid that is readily flocculated to form a gel. This is due to the reaction of PVB hydroxyl groups with boron in the glass, as shown by FT-IR spectra. In suspensions prepared with MPD and PVB, the suspensions did not form a gel, and the BOH peak in FT-IR spectra disappeared. This is because PVB binder interacts with MPD modifier, and this interaction between the binder and the modifier affects the rheology of the suspension and combined adsorption of PVB and MPD. Sequential studies of viscosity and adsorption show that the flow behavior of the suspension was independent of the addition sequence, and was primarily dependent on the presence of the modifier with high affinity and the reaction of the modifier with the binder species.

Effects of residual stress on fracture strength of Si3N4/stainless steel joints with a Cu-interlayer

The variation in fracture strength of a brazed Si3N4/Cu/steel joint was compared with the change in residual stress as a function of the Cu-interlayer thickness that was used. The higher residual stress and the lower measured fracture strength for the joint, using a 0.1 mm thick Cu-interlayer, were ascribed to the entire dissolution of the Cu-interlayer into the brazing alloy. The finite element analysis of residual stress, which considered the microstructure at the interface region, could explain the fracture behavior for the brazed joints, which is dependent on the thickness of the Cu-interlayer.