Papers by Author: Linda Stappers

Abstract: Deposition experiments in a Hull cell showed that high conductivity suspensions yield uniform deposits while low conductivity suspensions result in non-uniform deposits. This difference in deposition behavior is related to the resistance increase of the deposit during EPD. Impedance measurements during EPD showed that the ratio of the deposit resistance to the suspension resistance increases much more for high than for the low conductivity suspensions. They also showed that the total resistance of the EPD cell dropped almost to the suspension resistance after the electric field was turned off. This means that the deposit has no inherent resistance, but that its resistance during polarization is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. The change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.

Abstract: Metal coatings with embedded phase change material (PCM) particles were made by electrolytic deposition. These composite coatings have useful thermo-mechanical properties for thermal actuators. The PCM particles which are homogeneously dispersed in the metal matrix provide a large thermal expansion of the composite at the phase change temperature. Since the metal matrix has a good thermal conductivity, it allows fast heating of the embedded PCM particles and hence fast actuation. In this study, paraffin and water were used as PCM in copper, zinc and nickel coatings. To embed PCM material in electrolytic metal coatings, the PCM has to be encapsulated first. This was done by emulsion polymerization for paraffin and by solvent evaporation of a double emulsion for water. PCM-metal composite coatings are made by adding the PCM particles to the electrolyte used for metal plating. The properties of the metal-PCM composite coatings were examined by differential scanning calorimetry (DSC) and vertical dilatometry. The thermal expansion of the paraffin composite coatings showed a sharp increase in a small temperature range above the melting point of the paraffin and a total expansion of 1 % was found. Although a sharp expansion increase in a small temperature range is ideal for thermal actuators, the effect decreased by thermal cycling. A thermoelastoplastic model was developed to describe the thermal expansion of the composites. For water containing composite coatings, very large expansions of up to 15 % were obtained, but the temperature range over which this expansion occurs is large.

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.