Key Engineering Materials Vol. 314

Abstract: Colloidal probe atomic force microscopy is a very useful tool in the study of colloidal interactions. Although this technique has been applied to study interactions between a particle and a polarized electrode during electrodeposition, it has never been used to study interactions in high electric fields as encountered in electrophoretic deposition. In this work, a preliminary study was undertaken to verify whether colloidal probe AFM could be used to measure the electrophoretic force on a particle. It was found that the electrophoretic force could be detected by colloidal probe AFM under certain circumstances. In order to prevent that the contribution of the cantilever on the measurement of the electrophoretic force becomes large, the charge on the cantilever should be small compared to the charge of the particle, which is attached to the cantilever. Moreover, the area of cantilever surface which is oriented parallel to the electric field should be small to minimize the contribution of the cantilever.

Abstract: A novel micro-fabrication technique for particle assembly has been performed by an electrophoretic deposition (EPD) method using a local electric field in a colloidal suspension generated by a microelectrode. This unique EPD technique was called a “μ-EPD process”. Monodispersed polystyrene microspheres with diameters of 204, 290, and 320 nm were used in this study. A 50 μm Pt wire embedded into a polytetrafluoroethylene tube and an ITO glass slide were employed as the micro-counter electrode and the substrate, respectively. A slow deposition rate in the μ-EPD process was preferable to form a high quality micro-deposit consisting of a three-dimensional periodic polystyrene array. Under the optimized μ-EPD conditions, three-dimensionally ordered polystyrene particles were deposited in front of the micro-counter electrode. This micro-deposit constructed from polystyrene particles with a close-packed structure showed a characteristic optical absorption peak due to Bragg’s law.

Abstract: The electrical field drop over a deposit during electrophoretic deposition (EPD) determines the deposition rate and the uniformity of the deposit when a non-uniform electrical field is present. Due to the large practical consequences of a potential drop over the deposit, a procedure was developed to calculate the electrical field strength at the deposition front from currentconductivity measurements during EPD. The evolution of the electrical field strength during EPD was calculated for MEK and ethanol based suspensions. It was found that the suspension composition determines whether a potential drop over the deposit is present or not. EPD experiments on a membrane revealed that the extra potential drop is over the deposit and not caused by electrode polarization for the ethanol-acid based suspensions.

Abstract: The process of electrophoretic deposition depends strongly on the electrokinetic properties and with it the surface properties of the material that will be processed. Different additives, conditioners but also the suspending liquid influence the surface of the applied material by adsorption. Electrokinetic investigations reflect changes in properties at the outermost solid surface very sensitive. Streaming potential measurements are especially suited for studying such changes of surface chemistry at solids with different shapes. Two approaches are applicable: 1. The adsorption process was done before measuring. The result of this process should be shown. In this case it will be interesting to see differences in the functionality of the solid surface. The zeta potential will be measured versus different pH value. 2. The adsorption process will be studied directly. The zeta potential will be determined versus the concentration of the adsorptive. The second approach can be used for investigation of adsorption of multicomponent mixtures. Competing adsorption processes are detectable.

Abstract: Highly crystalline-textured alumina ceramics were fabricated by electrophoretic deposition (EPD) in a strong magnetic field of 12 T. Preferred orientation of the bulk was controlled by changing the direction of the applied electric field E relative to the magnetic field B during the EPD. Average orientation angle of the prepared monoliths as a function of the angle between the vectors E and B, ϕ B-E was estimated from the X-ray diffraction analysis. Alumina/alumina laminar composites with crystalline- oriented layers were also fabricated by alternately changing the ϕ B-E layer by layer during EPD in a magnetic field of 12 T.

Abstract: In the present contribution, the electrophoretic deposition (EPD) of nanocrystalline SiC powders will be discussed. In order to avoid the electrolysis of water during deposition as well as oxygen uptake of the nanocrystalline SiC by hydrolysis reactions, nonaqueous solvents were tested including ethanol and diethyl formamide. The solvents were compared regarding their effect on particle size distributions and sedimentation tests. Auxiliary information on the surface conditions as a function of the acidity of the suspension was drawn from aqueous zeta-potential measurements. The influence of dispersants, binders and the type of powder on the suspension properties was studied, and EPD was performed. For promising solvent-dispersant systems, the film thickness, current and deposition time at a given voltage were monitored during EPD.

Abstract: Electrophoretic deposition was investigated as a procedure for preparing supported membranes of zeolite 5A for use in gas separations. In particular, the addition of polyethylene imine (PEI) to the non-aqueous bath was explored as means of improving gas separation selectivity, deposit morphology, and adhesion. It was found that the addition of PEI improved all of these qualities. Post-deposition treatments such as baking and coating with polystyrene were also studied.

Abstract: Electrophoretic deposition of silicon carbide in ethanol suspensions was studied with the aim to fabricate liquid-phase sintered (LPS) SiC-based ceramics densified at temperatures below 1500°C. The effect of suspensions composition (solids content, pH, sintering additives) on the deposition rate and the properties of the deposits was analysed. The consistency of deposits and their solids loading was found to be significantly dependent on composition of the starting suspension: the highest solids loading (54.5 vol. %) for pure SiC was obtained with the addition of citric acid, while for the suspension containing aluminum phosphate the deposition was more successful from alkaline suspensions.

Abstract: The electrophoretic deposition (EPD) of AlN, Al2O3 and SiC, respectively, from ethanolic suspensions stabilised by suitable organic additives has been investigated. A main topic of this paper is the EPD in the direction of the gravitational force. Using an AlN suspension, the deposit weight depending on the experimental parameters was determined. The deposition was carried out under constant voltage or constant current conditions. Several factors influencing the effective electric field strength will be discussed. It could be shown that the conductivity of the suspension results mainly from the free ionic species. By EPD from the different suspensions, green bodies with monomodal pore size distributions were produced. The electrophoretic deposition of alumina from an ethanolic and from an aqueous suspension of the same composition led to similar results regarding the homogeneity of the particle packing in the deposits. The SiC suspension was also used for infiltrating woven carbon fibre mats by EPD in order to fabricate ceramic matrix composites.