Abstract: In this paper, the feasibility of obtaining well-defined compact EPD deposits in a short time was studied. EPD from organic and aqueous suspensions were tested. The formation of bubbles coming from solvent electrolysis damages the deposits. 4 different EPD geometric configurations were developed and tested with different experimental conditions. One of them, consisting of both electrodes tilted 30° helped to avoid bubbles getting trapped in the deposit. Our preliminary results showed that there is a very strong influence of the voltage applied, and the separation distance between the electrodes, over the width and height of the structures deposited. Working with aqueous suspensions is more suitable than with organic solvents from the environmental point of view. More work needs to be done in this direction to improve the quality of the deposits and to fine-tune the experimental conditions.
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: The spatially distributed local electric field strength during the electrophoretic deposition (EPD) of alumina suspensions on a membrane is within the scope of the present article. The water-based alumina suspensions were optimized in order to achieve stable suspensions, high deposition rates and maximum green densities of the deposited bodies. In-situ measurements of the local potential drop inside the EPD-cell were carried out using a computer assisted process control set-up with electronic data acquisition. The behaviour of the spatial- and time-dependent electric field distribution was calculated from measured potential drops. The influence of varrying input voltages on the output parameters, such as deposition rate and green density for EPD on membranes was investigated and is discussed. It is shown that the distribution of the local electric field depends on the applied voltage and on time for the case of higher voltages. A dependance of the local electrical fields on the deposition rate is suggested as well.
Abstract: Silica coatings were fabricated on stainless steel substrates by electrophoretic deposition (EPD) of aqueous slurries. An cationic surfactant, polyethylenimine (PEI) was used for obtaining positively charged silica particles and coatings on the cathode in order to prevent the anode elution. In addition, in order to prevent formation of cracks in coatings on metal substrates after drying, polyvinyl alcohol (PVA) was added in slurries as a binder. Furthermore, we could obtain crack free coatings after drying by aqueous EPD assisted by sweeping flow.
Abstract: Ionic Liquids (IL) are novel materials with a wide range of applications. The variable combination of organic cation and inorganic anion offers the possibility of tuning their properties.
In the present investigation we test the application of different ionic liquids as charging agents for the preparation of stable oxide suspensions for direct use in electrophoretic deposition to produce oxide layers on different substrates. Main objective of these investigations was to prove that this substance class constitutes suitable additives for successful electrophoretic deposition (EPD).
As one key parameter for the EPD process is the particle charge, a big part of our investigations involved the measurement of the zeta-potential of in alcoholic suspension under the influence of phosphonium and imidazolium based ionic liquids using a micro-electrophoresis measurement
Abstract: From an environmental, safety and economic perspective water should be the solvent of choice for electrophoretic deposition under industrial circumstances. However, because of the electrolytic decomposition of water under the influence of direct current, the majority of EPD is carried out in non-aqueous solvents.
In this work, experiments prove that deposits can be obtained from aqueous alumina suspensions while avoiding electrolysis of the medium by using unbalanced alternating current fields . In addition it is shown that the formed deposits have a green density which is intrinsically higher than those formed by traditional DC EPD from ethanol based suspensions. A theoretical basis for both electrophoretic deposition by means of unbalanced alternating fields and the higher density of deposits formed by application of such fields is provided.
Abstract: Electrophoretic deposition (EPD) from aqueous suspension generally forms deposit containing incorporated bubbles because of evolution of gases at electrodes due to electrolysis of water. We have demonstrated here that application of pulsed voltage /current instead of continuous DC enables controlling the amount of bubble incorporation and obtain bubble-free deposits during EPD of aqueous suspension. The yield and bubble incorporation decreased progressively with decrease in size of the applied pulse. A characteristic band of deposition window was found in the plot of voltage/current vs. pulse width within which smooth and bubble-free deposits are obtained. The window is wider at low applied voltages/currents than at higher voltages/currents implying that it is more easier to control the pulsed EPD at lower applied voltages and/currents. No deposition occurred below the window whereas deposits with incorporated bubbles formed above the window. Suppression of bubbles with decreasing pulse size was attributed to decrease in the amount of hydrogen evolved per pulse and verified by monitoring the gain in weight of palladium (Pd) electrode used as cathode during electrolysis of water.
Abstract: The electrophoretic deposition (EPD) is a very promising process for shaping compacts, especially for nanopowders. Up till now EPD is not used on a large-scale production of ceramic products. Additionally, the higher cost of nanopowders was also a barrier for the development of new products. As the deposition rate at EPD is independent of particle size, it is an ideal process for shaping compacts of powder mixtures with nanopowders. Silica and zirconia powders were used as model systems to demonstrate the advantages of EPD. Applications might be the preparation of high-purity silica glass for processing of semiconductors and zirconia ceramics for near net-shaped dental crowns. In both cases the optimum green density was achieved for approx. 10 % addition of nanopowders to a coarser matrix powder. The compacts were homogenous. Alternatively, compacts of different powders can be prepared in a two-step process. Examples are shown for the electrophoretic impregnation (EPI) and a reactive forming of nanoparticles inside the pore volume of a compact consisting of coarser particles.
Abstract: This study was triggered by our experience on electrophoretic deposition (EPD) with different suspension systems showing evidence of a particle concentration threshold, below which no deposit was formed. In this study, the role of particle concentration in the mechanism of EPD was investigated with a model system, consisting of isopropanol suspensions with TiO2 nanosized particles (d50 = 130 nm). The investigated concentration range was 0.01 - 0.4 vol% TiO2. Constant voltage EPD tests with variable particle concentration were performed for 1 min under different applied voltages (25 - 300 V corresponding to 62.5 - 750 V/cm). A longer deposition time (30 min) was tested for a lower concentration value (0.003 vol% TiO2). The deposition process was evaluated in situ by means of the current measured during EPD. The deposits obtained were characterized by weight and profile measurements and scanning electron microscope (SEM).
The results confirmed the finding of a lower limit value of particle concentration, determining a threshold in the formation of an EPD coating. Above this threshold, proportionality between deposited mass and particle concentration was observed, in agreement with the equation of Hamaker. Below this threshold, the proportionality was lost with evidence of a lack of coating formation. A possible interpretation for this experimental finding was provided.