Key Engineering Materials Vol. 507

Abstract: The recent development of the alternating current electrophoretic deposition (AC-EPD) technique has rendered it possible to deposit material from aqueous suspensions while preventing the electrochemical reactions associated with the application of high voltages on such systems. This does not only allow for more economical and ecological processes but also opens up electrophoretic deposition as a processing technique to a whole range of materials sensitive to either electrochemical reactions or non-aqueous solvents. Living cells can be considered as one class of such materials. In this paper the deposition of two types of bacteria, the Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli, and one type of yeast cells, Saccharomyces cerevisiae, is demonstrated.

Abstract: Electrophoretic deposition (EPD) has been successfully used to deposit composite coatings composed of polyetheretherketone (PEEK) and titanium dioxide (TiO₂) nanoparticles on 316L stainless steel substrates. The suspensions of TiO₂ nanoparticles and PEEK microparticles for EPD were prepared in ethanol. PEEK-TiO₂ composite coatings were optimized using suspensions containing 6wt% PEEK-TiO₂ in ethanol with a 3:1 ratio of PEEK to TiO₂ in weight and by applying a potential difference of 30 V for 1 minute. A heat-treatment process of the optimized PEEK-TiO₂ composite coatings was performed at 335°C for 30 minutes with a heating rate of 10°Cminto densify the deposits. The EPD coatings were microstructurally evaluated by scanning electron microscopy (SEM). It was demonstrated that EPD is a convenient and rapid method to fabricate PEEK/TiO₂ coatings on stainless steel which are interesting for biomedical applications.

Abstract: Titanium alloys used in orthopedic surgery are usually coated with hydroxyapatite to improve their biocompatibility and osseointegration. Bioactive glasses (BGs) are an interesting alternative to hydroxyapatite for the production of prosthetic coatings due to their osteoproductive property (Class A bioactivity) and to their resorbability. However the classical techniques used to obtain prosthetic coatings are not suitable in the case of BGs. In this study bioactive glass coatings are obtained by electrophoretic deposition on a Ti12Mo5Ta alloy. These coatings were obtained from ethanol suspensions of two different bioglass powders: a Sol-Gel derived 58S and a Melting-Quenching derived 46S. Scanning electron microscopy observations were used to characterize the coatings (morphology and thickness) and the coating/substrate interfaces. The chemical composition of the coatings was studied by X-ray microanalysis and X-ray maps were performed to characterize the spatial distributions of all elements composing the coatings.

Abstract: Stainless steel (SS) is often used for orthopaedic and dental implants because of its excellent mechanical characteristics. However, from an electrochemical perspective, SS can be susceptible to corrosion-related problems. Inorganic bioactive coatings on SS surfaces are reported to impart corrosion resistance and enhance biocompatibility. In this paper, hydroxyapatite (HA) coatings were developed on SS 316L by an electrophoretic deposition (EPD) technique at applied deposition voltages from 10 to 60 V in an acidic aqueous solution. The present study was performed to optimise the applied voltage required to produce stable HA coatings on SS 316L. Their corrosion resistance in simulated body conditions were investigated using the potentiodynamic polarisation curves. The results of the electrochemical studies revealed that the optimal applied voltage for EPD of HA on SS 316L was 40 V. The polarisation parameters, such as the corrosion potential, breakdown potential and repassivation potential of HA coated materials demonstrated nobler behaviours than the uncoated SS 316L. These results validated the successful formation of stable and protective HA coatings on SS 316L.

Abstract: Electrophoretic deposition (EPD) has been used for phosphor screening for a variety of emissive information displays and more recently, for solid state lighting. EPD is well suited to deposit the fine (nanometer to micrometer diameter) phosphor particles needed for high resolution displays. The fundamentals of the EPD process in an isopropanol (IPA) bath have been characterized by the dissociation behavior of nitrate salts in IPA, measurement of the effects of pH and nitrate salt concentration on the zeta potential of the particles, studying of the processing conditions and modeling of the deposition rates. The electrochemical precipitation reactions form an adhesive agent for the particles and the adhesion strength can be enhanced by various methods to meet the requirements of these technologies.

Abstract: Local electrophoretic deposition of alumina nanoparticles under external DC electric field conditions with submerged impinging jet type capillaries arranged at ion exchange membrane substrates is presented. In order to evaluate particle deposition mechanisms a mathematical model is derived describing electroosmotic pumping of electrolyte through a micrometre scaled channel. The system is governed by surficial charge discontinuities and modeled by coupled mass balances, Ohmic law, Navier Stokes, and Nernst-Planck equations. Based on the boundary conditions of bulk convective electrodiffusion the effect of the imposed surface potential on the fluid flow behaviour and on particle tracing characteristics is studied by means of numerical analysis. The following findings have been obtained. At the corner edges of the charged surficial boundaries micro-vortices are generated to build up local stagnation points onto the modeled membrane surface. Particle tracing analysis reveal that the particle movement is caused by mass transport within the membrane directed velocity field to the stagnation point. The complex electrokinetics and electrohydro-dynamics suggest further investigations at membrane pore sizes in the range of the Debye-length to model the non-linear current-voltage characteristic that has already been experimentally proven for these kind of membrane EPD systems.

Abstract: A polymer wall was developed between interdigitated comb-type current collectors on a SiO₂/Si substrate as the support for three-dimensionally (3D) accurate deposition of electrode materials by electrophoretic deposition (EPD) method. As anode and cathode materials, Li₄Ti₅O₁₂ and LiCoO₂ with a particle size of ~ 200 nm were synthesized, respectively, and their dispersibility in the suspensions including Ketjen Black and poly (vinylidene fluoride) was investigated. Consequently, N-methylpyrrolidone provided their good suspensions, and an electrode aspect ratio of 1:1, which can provide a high capacity and good rate performance to micro lithium-ion batteries, was successfully achieved in both Li₄Ti₅O₁₂ composite anode and LiCoO₂ composite cathode.

Abstract: We investigated properties of nanolayers electrophoretically deposited (EPD) onto semiconductor indium phosphide (InP) or gallium nitride (GaN) single crystals from colloid solutions of metal palladium (Pd), platinum (Pt) or bimetallic Pd/Pt nanoparticles (NPs) in isooctane. Colloids with metal NPs were prepared by reaction of metal compounds with the reducing agent hydrazine in water confined to reverse micelles of surfactant AOT.. Chopped DC electric voltage was applied for the time period to deposit metal NPs, only partly covering surface of the wafer. The deposits were image-observed by scanning electron microscopy (SEM)..Diodes with porous Schottky contacts were made by printing colloidal graphite on the NPs deposited surface and making ohmic contact on the blank side of the wafer. The diodes showed current-voltage characteristics of excellent rectification ratio and barrier height values close to Schottky-Mott limit, which was an evidence of negligible Fermi level pinning. Large increase of current was observed after switching on a flow of gas blend hydrogen in nitrogen (H₂/N₂). The diodes were measured with various H₂/N₂ in the range from 1000 ppm to 1 ppm of H₂. Current change ratios about 10⁶ and about 10 were achieved with 1000 ppm and 1 ppm H₂/N₂.

Abstract: nvestigating the electrodeposition properties of PEEK from solvents such as ethanol and acetone presents difficulties due to the higher density of the polymer particles relative to that of the solvent. The settling rate in unagitated baths is too rapid to obtain consistent deposits. Whole-bath or bulk agitation aimed at maintaining particle suspension can disturb electrodeposits due to eddies and other turbulent flow effects. To eliminate settling, mixtures of two solvents have been employed, the individual solvents having respectively lower and higher densities than PEEK. The proportions of the mixture are adjusted to be the same density as PEEK so that a colloid-like suspension of PEEK particles is possible. This enables the electrodeposition of PEEK to be studied without any significant gravitation or whole-bath agitation effects. Eliminating mechanical agitation of the suspension enables a study of the influence of target electrode movement alone on the rate and quality of electrodeposition. Note: The origin of this paper is in a project to establish if PEEK could be applied as a controllable conformal thin film onto a specific non-planar substrate geometry by using electrophoretic deposition. The paper describes only the development of a reliable methodology to investigate the feasibility of this project. The details of the project as a whole are outside the scope of the paper.

Abstract: Examples of electrophoretic deposition applications taken from the electronics industry illustrate the versatility of EPD. Determined by substrate dimensions and layer requirements, each application often requires its own unique system configuration. In the following examples EPD will be shown to have distinctive advantages over other layer application technologies like spray-, dip-or spin coating in for example layer conformity.