George Xomeritakis

Fabrication of thin metallic membranes by MOCVD and sputtering

Abstract Thin (0.1–1.5 μm) Pd and Pd Ag alloy membranes have been prepared on porous ceramic substrates consisting of a macroporous α-Al2O3 disk coated with a sol-gel derived, mesoporous γ-Al2O3 top layer. Metallorganic chemical vapor deposition (MOCVD) and magnetron sputtering were used to coat the thin metallic membranes employing Pd(II) acetylacetonate and a 75% Pd-25% Ag alloy target, respectively. The gas transport properties of the thin metallic membranes were determined by multicomponent permeation experiments with He, H2 and Ar at 25–300°C and 1 atm total pressure. The H2 permeance and H2: He selectivity were in the range 1.0–2.0 × 10−7 mol m−2 s−1 Pa−1 and 30–200 at 300°C, respectively. The dependence of H2 permeation rates on membrane thickness and temperature suggest that surface reaction steps are rate-limiting for H2 transport through the thin, ceramic-supported metallic membranes made by MOCVD and sputtering.

Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl2) vapor and H2 as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore γ-Al2O3 top layer and a coarse-pore α-Al2O3 substrate was used as Pd membrane support. A 0.5–1 μm thick metallic membrane was deposited in the γ-Al2O3 top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the γ-Al2O3 top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H2 permeation flux of these membranes was in the range 0.5–1.0 × 10−6 mol m−2 s−1 Pa−1 at 350–450°C. The H2 permeation data suggest that surface reaction steps are rate-limiting for H2 transport through such thin membranes in the temperature range studied.

Oxygen permeation through thin zirconia/yttria membranes prepared by EVD

Abstract Ultra-thin dense zirconia/yttria membrane composites were fabricated by EVD technique on porous α-alumina substrates. The deposited membranes consisted primarily of cubic/tetragonal phases as determined by XRD. The EVD film growth rate estimated by SEM photographs was 3–4 μm/h. Oxygen permeation data of thin dense zirconia/yttria membranes with different thickness (2–15 μm) were measured at 900 °C and 1000 °C. From the oxygen permeation data, the surface reaction and bulk diffusion rate parameters, α and β, were calculated. At 900 °C, α = 3.0 × 10 −8 mol/cm 2 · s · atm 1 2 and β = 3.0 × 10 −8 mol/cm · s · atm 1 4 with activation energy 53 KJ/mol and 72 KJ/mol for α and β, respectively. Presence of steam in the high oxygen partial pressure side promoted electrochemical surface reaction.

Electrochemical vapor deposition synthesis and oxygen permeation properties of dense zirconia-yttria-ceria membranes

Abstract Thin ( 2 –Y 2 O 3 –CeO 2 membranes with different CeO 2 composition and membrane thickness were deposited on porous α-alumina substrates by the EVD process using corresponding metal chlorides as the precursors. CeO 2 composition in the membrane is the same as CeCl 3 concentration in the gas phase. The film growth rate for the EVD process is about 7 μm/h. The EVD prepared ZrO 2 –Y 2 O 3 –CeO 2 membranes do not show single phase, but consist of two separate phases: a ZrO 2 -dominant cubic phase and a CeO 2 -dominant cubic phase. Under present conditions (950–1050°C, P O 2 (high) =0.21 atm, P O 2 (low) =0.01 atm), oxygen permeation through the EVD prepared ZrO 2 –Y 2 O 3 –CeO 2 membranes is limited by the bulk diffusion in the membrane. The oxygen permeability of the ZrO 2 –Y 2 O 3 –CeO 2 membranes is in the range of 1–4×10 −11 mol/cm s, about an order of magnitude higher than that of ZrO 2 –Y 2 O 3 membranes. The oxygen permeation flux through the EVD prepared ZrO 2 –Y 2 O 3 –CeO 2 membrane increases with CeO 2 composition in the film.

CVD of solid oxides in porous media for ceramic membrane preparation or modification. Explicit solutions for deposition characteristics

Abstract A theoretical analysis of modified chemical vapor deposition (MCVD) of solid oxides in porous media for ceramic membrane preparation and/or modification is presented in this paper. Semi-analytical solutions have been obtained to describe the evolution of solid deposit profiles inside porous substrates, using a phenomenological model that takes into account intrapore precursor diffusion, chemical reaction and pore geometry change. Explicit equations have been derived which correlate the three main deposition characteristics, deposit location, deposit zone thickness and pore closure time to the CVD experimental parameters and substrate pore structure. Such explicit equations provide better insight into this unique CVD process for advanced materials synthesis and are very useful in practical applications. A comparison of the semi-analytical solutions with a numerical solution based on the finite element method is also presented. The semi-analytical solutions are in good agreement with the numerical solutions, especially under the conditions used for ceramic membrane preparation and/or modification.

Evolution of pore size distribution and average pore size of porous ceramic membranes during modification by counter-diffusion chemical vapor deposition

Abstract The modification of porous ceramic membranes by counter-diffusion chemical vapor deposition (CVD) is studied theoretically and experimentally. Numerical simulations of the evolution of the membrane permeance, average pore size and pore size distribution as a function of extent of modification are presented and compared with experimetal data. It is found that the change of the average pore size of the membranes after modification strongly depends on the initial pore size distribution of the membrane, CVD reaction kinetics and characterization method. Experimental data suggest that CVD of zirconia (and yttria) inside porous ceramic membranes by reaction of zirconium (and yttrium) chlorides with steam/air at elevated temperatures proceeds by quasi-zero reaction kinetics with respect to the oxidant, typical of non-stoichiometric supply of the reactants from opposite sides of the membrane. Under such conditions, CVD modification may result in a modest increase of the average pore size of coarse-pore ceramic membranes as suggested by numerical calculations and experimental data.

Analysis of mass transfer in composite materials with irregular interfaces

We present a theoretical analysis of permeation through composite bi-layer materials with an irregular interface. The conservation of mass equation for the permeating species is solved using both approximate analytic and numerical techniques. The approximate analytic solution is valid for small deviations of the interface from planarity. To assess the accuracy of the analytic solution, the analytic solution is compared to numerical approximations for a wide range of composite geometries and relative material permeabilities. The analytic solution proves to be accurate and easy to use. The analysis demonstrates that a composite material with an irregular interface possesses a higher permeability and slightly lower selectivity than an equivalent composite with a flat interface. Furthermore, the changes in permeability depend strongly on the magnitude of the interface irregularity but are relatively insensitive to the relative thickness of the two layers.