CVD of solid oxides in porous substrates for ceramic membrane modification

The deposition of yttria-doped zirconia has been experimented systematically in various types of porous ceramic substrates by a modified chemical vapor deposition (CVD) process operating in an opposing reactant geometry using water vapor and corresponding metal chloride vapors as reactants. The effects of substrate pore dimension and structure, bulk-phase reactant concentration, reactant diffusivity in substrate pores and deposition temperature are experimentally studied and explained qualitatively by a theoretical modeling analysis. The experimental and theoretical results suggest a reaction mechanism which depends on water vapor and chloride vapor concentrations. Consequently, the diffusivity, bulk-phase reactant concentration, and substrate pore dimension are important in the CVD process. Effects of deposition temperature on the deposition results and narrow deposition zone compared to the substrate thickness also suggest a Langmuir-Hinshelwood reaction mechanism involved in the CVD process with a very fast CVD reaction rate. Gas permeation data indicate that whether deposition of solid in substrate pores could result in the pore-size reduction depends strongly on the initial pore-size distribution of the substrate

Experimental studies on pore size change of porous ceramic membranes after modification

Experimental results on pore size change of a microfiltration (MF) -alumina membrane and an ultrafiltration (UF) γ-alumina membrane after modification by chemical vapor deposition (CVD) of solid oxides in the membrane pores are presented and explained using the results of a theoretical analysis. With an approx. 10-fold reduction in permeability, the average pore size of the MF membrane is found to increase after CVD modification, due to its relatively broader initial pore size distribution with a small amount of large pores and due to the particular CVD conditions (heterogeneous deposition mechanism) which give a pore narrowing rate independent of pore size. The effective pore size of the UF membrane appears to remain unchanged after modification with an approx. 50-fold reduction in permeability, as a result of the slit-shaped pores of the γ-alumina film and the particular modification conditions. The experimental and theoretical results suggest that, in order to reduce effectively the average pore size of a membrane by a modification process, the membrane should have a rather uniform pore size distribution, or the modification process should be conducted under conditions which give a pore narrowing rate proportional to the pore size

Preparation and Characterization of High-Temperature Thermally Stable Alumina Composite Membrane

A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C

Oxygen semipermeable solid oxide membrane composites prepared by electrochemical vapor deposition

Ceramic membrane composites consisting of a coarse porous -alumina or two-layer porous alumina membrane support and an oxygen semipermeable gas tight thin (0.2–5 μm) yttria stabilized zirconia (YSZ) film are prepared by the electrochemical vapor deposition (EVD) method. The minimum gas-tight thickness of the YSZ films depends strongly on the average pore size of the support on which the films are deposited by the EVD process. The oxygen permeation fluxes through such gas tight YSZ membrane composites, measured in situ on the EVD apparatus, are in the range of 3 × 10−9 to 6 × 10−8 mol/cm2-sec with an oxygen partial pressures of P′O2 (high) ≈ 3 × 10−2 atm and P″O2 (low) ≈ 10−5 atm, much larger than the literature data for thicker YSZ pellets. During the oxygen permeation experiments the rate-limiting step is found to be the bulk electrochemical transport in the grown YSZ films with a thickness smaller than 10 μm.\u

On the kinetic study of electrochemical vapour deposition

A theoretical analysis is presented which quantitatively describes the transition behavior of the kinetics of the electrochemical vapour deposition of yttria-stabilized zirconia on porous substrates. It is shown that up to a certain deposition time and corresponding film thickness the rate limiting step is oxygen diffusion through the substrate pores, giving a linear dependence of the film thickness on the deposition time. For longer deposition times, i.e. thicker films, a transition of the rate limiting step to bulk electrochemical diffusion in the film occurs, resulting in a parabolic dependence of the film thickness on the deposition time. Simulation results are presented to show the effects of the experimental conditions on this transition time

Modelling and analysis of CVD processes in porous media for ceramic composite preparation

A continuum phenomenological model is presented to describe chemical vapour deposition (CVD) of solid product inside porous substrate media for the preparation of reinforced ceramic-matrix composites [by the chemical vapour infiltration (CVI) process] and ceramic membrane composites (by a modified CVD process). The chemical reaction, intrapore diffusion, non-isobaric viscous flow and variation of substrate pore geometry during deposition are considered in the model which is readily solved by the orthogonal collocation numerical technique. Simulated deposition profiles across substrate are given to examine the effects of the reaction mechanism, reaction and diffusion rate, substrate pore dimension, deposition temperature, bulk phase reactant concentration, intrapore diffusivity of reactants and pressure drop on the deposition results of a one-dimensional isothermal forced-flow CVI process and a modified non-isobaric CVD process for ceramic composite preparation. The theoretical analysis provides a better insight of the CVD processes in porous media and is useful in explaining experimental findings and guiding the selection of optimum process conditions for the CVD preparation of ceramic composites

Modified CVD of nanoscale structures in and EVD of thin layers on porous ceramic membranes

Experiments on the modified chemical vapour deposition (CVD) and the electrochemical vapour deposition (EVD) of yttria-stabilized zirconia on porous substrates are reported. It is shown that, in the CVD stage, deposition occurs in a small (<20 um) region at the edge of the substrate, very likely leading to pore narrowing. This result illustrates the feasibility of the CVD technique for the modification of ceramic membranes to the (sub)nanometer scale. Film growth in the EVD stage is shown to be controlled by the inpore diffusion of the oxygen source reactant for short (<5 h) deposition times. The yttria to zirconia ratio in the deposited film is determined by the ratio present in the vapour phase. Very thin (<2 um) films can be deposited, which have a potential application in solid oxide fuel cells

Bound states of neutral particles in external electric fields

Neutral fermions of spin $\frac 12$ with magnetic moment can interact with electromagnetic fields through nonminimal coupling. The Dirac--Pauli equation for such a fermion coupled to a spherically symmetric or central electric field can be reduced to two simultaneous ordinary differential equations by separation of variables in spherical coordinates. For a wide variety of central electric fields, bound-state solutions of critical energy values can be found analytically. The degeneracy of these energy levels turns out to be numerably infinite. This reveals the possibility of condensing infinitely many fermions into a single energy level. For radially constant and radially linear electric fields, the system of ordinary differential equations can be completely solved, and all bound-state solutions are obtained in closed forms. The radially constant field supports scattering solutions as well. For radially linear fields, more energy levels (in addition to the critical one) are infinitely degenerate. The simultaneous presence of central magnetic and electric fields is discussed.Comment: REVTeX, 14 pages, no figur

Exercise Induced Non-Sustained Ventricular Tachycardia and Indication for Invasive Management

Diagnostic stress echo testing is commonly performed in patients with known or suspected cardiovascular disease. There has been considerable debate in management of exercise induced non-sustained ventricular tachycardia (NSVT). In this case report, we present our experience with a case of exercise induced NSVT, and subsequent angiographically significant left anterior descending (LAD) coronary artery lesion