Dang-Hyok Yoon

Segregation-induced grain boundary premelting in nickel-doped tungsten

High-resolution transmission electron microscopy and Auger spectroscopy have revealed the formation of nanometer-thick, Ni-enriched, disordered, grain boundary layers in Ni-doped W specimens at 95 °C below the bulk eutectic temperature. The stabilization of subeutectic liquid-like grain boundary cores in this model two-component metallic alloy is phenomenologically analogous to the long-sought phenomenon of grain boundary premelting. The existence of such disordered nanostructures at metallic grain boundaries provides insights to resolve several long-standing controversies in interpreting the unique grain boundary diffusion∕migration kinetics and mechanical properties for this system, and can have technological importance for a broader range of materials.

Dielectric constant and mixing model of BaTiO3 composite thick films

Abstract BaTiO 3 (BT) composite thick films of X7R BT particles with different BT gel fractions were prepared by using an aqueous BT sol. The dielectric constant versus different BT gel volume fractions showed a sigmoidal behavior as the BT gel phase filled the interstitials of the X7R BT particle compact. To explain and predict the effect of the BT gel as a second phase based on the experimental results, various models such as series, parallel, cubic, Lichtenecker’s model, and Hashin–Shtrikman bounds were considered. None of the existing theoretical models fit the experimental results. An empirical sigmoidal fitting function was proposed to fit the experimental data.

Processing of barium titanate tapes with different binders for MLCC applications—Part I: Optimization using design of experiments

Abstract Twenty-four kinds of BaTiO 3 slips for MLCC application were investigated using three different binder systems: one solvent-based, and two water-based with water-soluble acrylic binder and aqueous emulsion binder systems. The half-fractional factorial design method was used for each system with four input factors with two levels for each factor. Tape casting, K-square preparation, sintering and characterization were conducted. Slip viscosity, mechanical properties of the green tapes, green and sintered density of K-squares, and dielectric permittivity were analyzed as output responses using statistical analysis methods. Most of the green body properties from solvent-based system such as tensile strength, tape morphology and bulk density depended on the ceramic powder. While, dispersant was the most significant factor for the two water-based systems. The sintered properties such as microstructure and dielectric permittivity for the three systems depended significantly on the type of ceramic powder. Finally, an optimization was performed for each system by means of a scorecard which was used to prioritize all samples to important output responses through the numerical ranking method.

Processing of barium titanate tapes with different binders for MLCC applications—Part II: Comparison of the properties

Abstract Most of multilayer ceramic capacitor (MLCC) industries are currently using solvent-based slip system for tape casting. On the other hand, water-based slip formulation has received special attention recently due to reduced health and environmental hazards coupled with a lower cost compared to solvent-based process. Nevertheless, there are no publications on the direct comparison of the two systems using BaTiO 3 slip for MLCC applications. Therefore, solvent- and water-based systems were compared systematically in this paper using three different kinds of binders: polyvinyl butyral (PVB) for solvent-based system, and water-soluble acrylic binder and acrylic emulsion for water-based systems. By following several important MLCC production steps, responses such as slip viscosity, mechanical properties of the green tapes, green and sintered density, and dielectric permittivity were compared. Slip viscosity and green tape strength depended significantly on the binder type, while other responses such as green, sintered, and dielectric properties depended on dispersant, ceramic powder and other processing additives.

Barium ion leaching from barium titanate powder in water

A submicrometer-sized commercial BaTiO3 (BT) powder was aged in water under three different conditions: pH, aging times, and pre-heat treatments of the powder. The amount of Ba2+ ions leaching from the BT particles was determined by the EDTA titration method. As predicted by thermodynamic calculation, the greater extent and the faster rate of Ba2+ leaching were found at the lower solution pH, leveling off at pH 8. The pre-heat treatments of the BT powder increased the amount of Ba2+ leaching when compared to the as-received one. This result was shown by the formation of soluble surface BaCO3, which was detected using FT-IR spectroscopy. It was also shown that organic passivation agents were effective reducing the Ba2+ leaching but at high solution pH.