Showing papers by "Patrick S. Nicholson published in 2000"

Electrophoretic deposition of MgO from organic suspensions

Abstract: 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.

Mimicking Nanometer Atomic Processes on a Micrometer Scale via Electrophoretic Deposition

Abstract: The process of submonolayer formation during the electrophoretic deposition (EPD) of colloidal films of micrometer-sized (diameter ∼ 0.5 μm) silica particles on a silicon wafer has been observed as a function of deposition time. The process of nucleation and growth of the silica monolayer is compared with that of atomic film growth (10000 times smaller scale) via molecular-beam epitaxy (MBE), and for the first time, a striking similarity between the two growth processes is observed. Likewise in the atomic growth process via MBE, the entire nucleation, growth, and aggregation process during EPD of silica particles can be broadly classified into two regions. At low surface coverage when silica particles are deposited outside of clusters, diffuse randomly, and stick to a cluster on touching them, the mechanism of growth in this region follows diffusion-limited aggregation (DLA) and the fractal dimension of the two-dimensional clusters is found to be close to 1.65. Later on, as the clusters grow in size, deposition of particles inside the clusters become important and clusters become more and more compact, resulting in a dense, close-packed, and homogeneous monolayer. This region is termed a consolidation region, and a change in fractal dimension from 1.65 toward 2 with increasing surface coverage has been observed.

Fabrication of Silicon Nitride Ceramics by Elecrophoretic Deposition

Abstract: Surface coating of Si3N4 powder with an alumina precursor was achieved by a sol-gel method using aluminum isopropoxide. Electrophoretic data showed the surface-coated Si3N4 powder behaved like alumina. Consolidation of the surface-coated powder was attempted by an electrophoretic deposition (EPD) technique. EPD rate was improved for the surface-coated powder. Sintering characteristics and fracture toughness were investigated for EPDed compacts. It was demonstrated that the surface coating of Si3N4 powder with alumina precursor had the advantage of not only the improvement of suspension stability, but also the addition of sintering aid.

β-Al2O3 single-crystal films and their optical properties

Abstract: The development of large-area, Na-β″-Al2O3 single-crystal films on sapphire substrates via vapor reaction is reported. The films form at the expense of the sapphire which reacts with the alkali-vapors generated from mixed (Na,Li)-β″-Al2O3 powders occupying the same space as the sapphire. A number of β″-Al2O3 single-crystal, large area isomorphs have been synthesized via ion exchange and their refraction and luminescence properties are reported. Patterned luminescence has been achieved in Cu+-doped single-crystal films using an electrochemical cell with + electrolyte separator between aqueous-solution electrodes. Ag+ ions are injected into the β″-Al2O3 electrolyte and Na+ expelled. The distribution of luminescent and environment ions is controlled by electrical charge and the chemical potential of the ions in the solution electrodes.