Burtrand I. Lee

Preparation, structure evolution and dielectric properties of BaTiO3 thin films and powders by an aqueous sol–gel process

Abstract A highly stable, water-based BaTiO 3 sol system was developed by a chelate acetate sol–gel process. The BaTiO 3 thin films and powders were prepared by a spin coating and drying-firing method, respectively, from the aqueous BaTiO 3 sol. Structural evolution of the BaTiO 3 gel was investigated by X-ray diffraction, Fourier transform infrared spectroscopy, and thermal gravimetric analysis/differential thermal analysis. Scanning electron microscopy microstructure results showed that the BaTiO 3 thin films consisted of nano polycrystallites with a grain size of 30–60 nm, possessing a cubic perovskite structure. With ultraviolet light (UV) irradiation, the BaTiO 3 films could be crystallized to a cubic phase at 550°C. The dielectric measurement indicated no phase transition at the Curie temperature (125°C). It is believed that the small crystallites having a cubic perovskite structure are responsible for the low dielectric constant and the flat temperature coefficient of capacitance behavior is due to the lack of the phase transition from the tetragonal to cubic phase.

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.

Biopolymer composite films based on κ-carrageenan and chitosan

Abstract Bioploymer composite films of κ-carrageenan and chitosan were prepared by co-dissolving κ-carrageenan and chitosan in several different organic acids. Tensile strength (TS), elongation (E), and water vapor permeability (WVP) were determined as a function of ascorbic acid added. Ascorbic acid tends to increase the properties of the films from all acids. Malic acid exhibited largest increase in tensile strength and elongation at 2% ascorbic acid and lactic acid in water vapor permeability. The strongest acid, citric acid exhibits overall the worst properties regardless of the amount of ascorbic acid. The ascorbic acid generally showed increase in TS and %E but little effect on the WVP except that from acetic acid. The results of the properties of the composite films indicated that there are interactions among the organic solvents, the plasticizer, κ-carrageenan, and chitosan in a complex manner.

Dispersion of barium titanate with polyaspartic acid in aqueous media

The colloidal stability of barium titanate (BT) aqueous suspensions with the sodium salt of polyaspartic acid (PApA-Na) at different pH values has been investigated by means of adsorption isotherm, zeta potential, sedimentation density and rheology characterization. The adsorption of PApA-Na onto BT surfaces is found to be pH dependent. Results show that PApA adsorption follows Langmuir monolayer adsorption isotherms and the amount of PApA-Na required to stabilize aqueous BT suspensions decreased as pH increased. The mechanism of stabilization of BT is shown to be a combination of electrostatic repulsion and steric hindrance under the experiment conditions used. As for the passivating power of PApA-Na, about 7-times reduction in Ba2+ ion leaching in 24 h is observed.

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.

Experimental study on the engineering properties of carbon fiber reinforced cement composites

Abstract In order to reinforce ordinary cement matrix, carbon fibers (CF) derived from two different precursors are added to the cement mix containing two different silica aggregates. The main experimental variables applied in forming the carbon fiber reinforced cement composites (CFRC) are PAN-based CF vs. pitch-based CF, different fiber lengths and the aspect ratios, silica powder vs. silica fume, water to cement ratios, fiber volume loadings, and air or water curing vs. autoclave curing. Based on the test results, a fabrication procedure for lightweight CFRC, has been developed and the optimum proportions of the ingredients are successfully identified. It is concluded that the reinforcement of carbon fiber is very effective in improving the tensile strength, flexural strength, fracture toughness, and the reduction in drying shrinkage of CFRC compared with conventional mortar.

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.

Tyre rubber/cement matrix composites

Organic polymers or fibres in ordinary Portland cement matrices have been shown to improve the mechanical properties of these composites. Such industrial wastes as fly ash and silica fume have been successfully incorporated into cement. Based on these results and the disposal problem of waste tyres, granulated tyre rubbers were incorporated in Portland Type 1 cement pastes. Results from preliminary experiments in compressive, diametral tensile, and flexural strengths under freezing and thawing cycles are reported

Effects of processing and materials variations on mechanical properties of lightweight cement composites

Low-density/low-cost cement composites were fabricated. Carbon and alkali-resistant glass fibers were used to reinforce the matrix of industrial by-products; fly ash with silica fume, Portland cement, and calcium silicates were mixed in different proportions. The additional low density was obtained by adding perlite and foaming agents followed by hot water curing. The composites also were prepared by autoclave curing for comparison. The mechanical properties were improved by increasing the amount of silica fume, fly ash, and fibers. Both carbon fibers and alkali-resistant glass fibers were effective in reinforcing the matrices, but carbon fibers were superior to glass fibers. Fabrication techniques for producing lightweight cement composites that can substitute for autoclaved lightweight concrete was developed.

Chemical variations in barium titanate powders and dispersants

Commercial raw materials for multilayer ceramic capacitors—barium titanate (BT) and ammonium polyacrylate (APA) dispersant—were examined for lot-to-lot variations which cause poor reproducibility in BT slips and in capacitor chips. Two lots of BT supplied by a commercial source were different with respect to surface species examined by Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, electrokinetic sonic amplitude and total carbon analysis. It was found that there was more APA dispersant chemisorption on BT when the BT surface was more hydroxylated and less carbonated. For the dispersant, the two lots differed in pH and ammonium ion content. The amount of APA dispersant adsorbed on BT depended more on the ceramic powder surface species than on the chemical differences in APA lots.The electrokinetic sonic amplitude (ESA) and zeta potentials of the two lots of as-received BT were small but positive. The BT lot with more surface hydroxyls exhibited slightly higher zeta potential values. The BT with more carbonate was more stable in terms of aging rate in water. Aging in water increased the positive zeta potential by increasing Ba2+ dissolution and adsorption. With the APA dispersant, the BT surface became less positive, but more stable and exhibited an isoelectric point of∼10.6. It can be concluded that the dispersion of BT in the as-dispersed pH range is mainly by steric stabilization with little contribution from electrostatic charges.