B. Ferrari

Electrophoretic Deposition of Aqueous Alumina Slips

Abstract Electrophoretic deposition (EPD) of aqueous alumina slips on a graphite electrode is studied. The suspension is characterized in terms of viscosity, zeta potential and conductivity. The conductivity has proved to be a determining parameter for obtaining deposits. Well-dispersed slips having an adequate value of conductivity are used for constant current density experiments. Current densities of 1.6 to 12.8 mA cm −2 are applied for 10 min. After defining the optimal slip properties for the deposition, both the process development and the final properties of the materials obtained depend on the current/time relationship. Alumina deposits obtained at different current densities are characterized. The best conditions for obtaining uniform and dense coatings are 6.4 mA cm −2 for up to 10 min. The applied current allows not only to modify the properties and microstructure of the deposit, but also to increase the margin of slip conductivity assuring the success of the EPD process, that is, the working conditions.

The conductivity of aqueous Al2O3 slips for electrophoretic deposition

In electrophoretic deposition (EPD) experiments, the electrical conditions applied to the slip determine the deposit homogeneity, but the dispersing conditions (electrochemical and rheological) determine the reliability of this process. In this work, after a careful study, a critical slip parameter such as the conductivity is proposed as a key factor to take into account in EPD experiments. Keyw0rd.r: Conductivity; Al,O, slips; Electrophoretic; Deposition

Electrophoretic deposition of transparent ZnO thin films from highly stabilized colloidal suspensions.

The parameters that control the stability of ZnO-nanoparticles suspensions and their deposition by electrophoretic deposition were studied, so as to organize the assembly and compaction of nanoparticles. The addition of cationic polyelectrolyte - Polyethylenimine (PEI) - with different molecular weights was investigated, in order to study their effectiveness and the influence of the molecular weight of the organic chain on suspensions dispersion. It was found that PEI with the highest molecular weight provided better dispersion conditions. Cathodic EPD was performed under previously optimized suspensions conditions and over electropolished stainless steel substrates. Experimental results showed that the EPD process in these conditions allows obtaining dense transparent ZnO thin films. Deposition times and intensities were optimized by analyzing the resulting thin films characteristics. Finally, the deposits were characterized by FE-SEM, AFM, and different spectroscopic techniques.

Aqueous electrophoretic deposition of AL2O3/ZrO2 layered ceramics

Abstract Layered ceramics are being prepared by different forming methods such as slip or tape casting, dip coating, etc. Microlaminate materials have been also prepared by electrophoretic deposition (EPD), but usually from non-aqueous suspensions. In this work, the preparation of Al 2 O 3 /ZrO 2 layered ceramics from aqueous suspensions is described. The growth of the deposit thickness can be controlled in order to design laminar ceramics either as coatings or as self-supported deposits, with a few thick layers, reducing warping effects, and avoiding environmental problems.

Effect of the slurry properties on the homogeneity of alumina deposits obtained by aqueous electrophoretic deposition

Electrophoretic deposition (EPD) has been used to obtain self-supported alumina substrates from aqueous slurries. The uniformity of the final deposit is a function of the colloidal properties of the slurry. In this work, the conductivity and the solid content of the slurries are related to the deposition kinetics and the characteristics of the green compacts. The solid content is demonstrated to increase the experimental conductivity margins for suitable deposition. In addition, the more concentrated the slurry is, the lower the contamination of the deposit when metallic electrodes are used.

Electrophoretic deposition of MgO from organic suspensions

MgO is extensively used as a basic refractory material, but in the last decade it has also been considered as a suitable material for technological applications in fields such as catalysis or electronics. Such applications have imposed new requirements regarding the processing techniques. In this way, the electrophoretic deposition (EPD) has been demonstrated to be a useful and simple deposition method among the available coating technologies. A diAerence with other coating techniques is that the material to be processed by EPD does not require defined properties and hence, there are a wide number of materials that could be coated and deposited by EPD. Major advantages of EPD are versatility, low-cost and reproducibility. The aim of this work was to obtain controlled deposits of MgO onto metallic electrodes by EPD. For this purpose the stability of MgO suspensions in ethanolic media was studied by electrokinetic sonic amplitude (ESA) at diAerent pH conditions. The EPD kinetics was further studied at diAerent electrical conditions. The characteristics of the deposits were studied in relation to the suspension properties and the electrical conditions. # 2000 Elsevier Science Ltd. All rights reserved.

Electrophoretic Forming of Al2O3/Y-TZP Layered Ceramics from Aqueous Suspensions

Electrophoretic forming is emerging as an economical and reliable method for the fabrication of coatings and laminate ceramics. The method is very simple, but a major problem is the toxicity and environmental problems associated with the organic solvents usually employed as vehicle media. This work aims to obtain Al2O3/Y-TZP laminates by electrophoretic deposition using water as the liquid vehicle. The slip characteristics have been studied in terms of viscosity, zeta potential, and conductivity for both starting powders. Electrophoretic deposition was performed at constant values of current density to obtain the monolithics and layered materials. Very small variations in the deflocculating conditions allow control over the thickness of each layer in the laminate, providing a powerful method for obtaining tailored complex microstructures.

Electrophoretic Deposition of Flake-Shaped ZnO Nanoparticles

ZnO films have been grown on different substrates by using a variety of fabrication techniques. In the present work, we discuss the results on the preparation of ZnO films by electrophoretic deposition (EPD) starting from synthetic flake-shaped nanoparticles. The critical parameters related to the preparation of aqueous colloidal suspensions have been analyzed. Dispersing conditions have been evaluated in terms of particle/cluster morphology, electrokinetic properties, i.e., particle size distribution and cationic surface functionality, and zeta potential. EPD parameters for film assembly have been selected. Depending on both the particle morphology and the solids concentration in the colloidal system, different orientation effects have been observed in the built structure. For a low ZnO concentration, independent clusters arrive at the Ni substrate where the large dimension of the ZnO flakelike nanoparticles align with the surface of the substrate. When the solids concentration increases, such ordering is not observed. The electrical resistivity measured by the four-point probe method evidences the presence of a ZnO film with unusually high electrical resistivity for undoped ZnO.

A Resistivity Model for Electrophoretic Deposition

Electrophoretic deposition (EPD) is a powerful method for obtaining particulate layers in a broad range of thicknesses if an adequate control of the growing kinetics is reached. Existing models of EPD kinetics consider that the growth of the deposit increases linearly with deposition time and deviations are due to a reduction of powder concentration and/or a decrease of electric field when EPD is performed at constant voltage conditions. Experimental observations show that long time tests lead to a S-shaped growing kinetics. This work presents a resistivity model that predicts a S-shape variation of mass per unit area with deposition time, with a first step in which the deposition rate increases, as a consequence of resistivity changes, followed by a decreasing slope associated to the lose of powder concentration. Currently available EPD models, such as the Hamaker and Sarkar & Nicholson models are particular cases of the generalized resistivity model proposed in this work.

Silica Sol-Gel Coatings on Metals Produced by EPD

The objective of this work has been to combine the sol-gel method and the electrophoretic deposition (EPD) process to prepare thick coatings onto metallic substrates. Two different routes were used for preparing the sol-gel silica suspensions. On one hand, silica particulate sols were obtained by basic catalysis of alkoxides and alkylalkoxides. On the other, silica suspensions were prepared by adding a commercial colloidal silica sol to an organic-inorganic acid catalysed silica sol. The properties of the suspension and the physical parameters associated to EPD (current density, potential, electric field and deposition time) were studied. Crack-free deposits up to 20 μm were obtained after drying and crack-free glass-like coatings of 12 μm after sintering at 500°C for 30 minutes. The electrochemical behaviour of these coatings was evaluated by potentiodynamic methods, showing an excellent behaviour against corrosion.