H.W. Brinkman

Permporometry study on the size distribution of active pores in porous ceramic membranes

Permporometry as well as nitrogen adsorption-desorption techniques have been applied to study the pore size distribution in γ-alumina membranes with a pore radius ranging from about 2 nm to 10 nm. The permporometry technique measures the active pores only, while nitrogen adsorption-desorption measures the active as well as the passive pores. The pore size distribution in the porous ceramic membrane supported with an -alumina substrate (supported membrane) was determined by using permporometry, while the membrane without a supporting substrate (unsupported membrane) was characterized by nitrogen adsorption-desorption. The experimental results of permporometry and nitrogen adsorption-desorption indicate that the size distribution of the active pores in the supported membranes is similar to that of the active and passive pores in the unsupported membranes. The -alumina supporting substrate seems to have no detectable influence on the size distribution of the active pores of the ceramic membranes studied.

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.

Mixed conducting yttrium-barium-cobalt-oxide for high oxygen permeation

Yttrium-barium-cobalt-oxide (YBC), especially with low Y content, has been prepared. Oxygen permeation in these materials is very high at moderate temperature. The materials Y0.05BaCo0.95O3-δ and Y0.10Ba0.90CoO3-δ consisted of a BaCoO3-δ like main phase and some minor phases. For the Y0.05BaCo0.95O3-δ material these minor phases were not exceeding 10 vol%. Y0.05BaCo0.95O3-δ had the highest oxygen permeation value of 3.9 × 10-7 mol/cm2s at 900°C; the surface exchange reaction may be the rate limiting step here. The material Y0.33Ba0.67CoO3-δ consisted mainly of two unknown cubic phases.

Modelling and analysis of CVD processes for ceramic membrane preparation

A mathematical model is presented that describes the modified chemical vapour deposition (CVD) process (which takes place in advance of the electrochemical vapour deposition (EVD) process) to deposit ZrO2 inside porous media for the preparation and modification of ceramic membranes. The isobaric model takes into account intrapore Knudsen diffusion of ZrCl4 and H2O, which enter the membrane from opposite sides, and Langmuir-Hinshelwood reaction of the solid product ZrO2 on the internal pore wall. The processes occurring in one single pore are investigated, and the change in pore geometry during deposition is taken into account. Based upon this model, the deposition profile is studied. The model fits reasonably well with experimental results.

Ceramic Membranes by Electrochemical Vapor Deposition of Zirconia‐Yttria‐Terbia Layers on Porous Substrates

By means of electrochemical vapor deposition (EVD), it is possible to grow thin, dense layers of zirconia/yttria/terbiasolid solution (ZYT) on porous ceramic substrates. These layers can be used as ceramic membranes for oxygen separation.The kinetics of the EVD process, the morphology of the grown layers and their oxygen permeation properties are investigated.At a deposition temperature of 800°C, the EVD layer growth is limited by bulk electrochemical transport. At 1000°Cthe layer growth is limited by pore diffusion of the oxygen source reactant. The EVD-grown ZYT layers show columnarstructures with prismatic grains on top; the size of the grains (1 to 2 µm) increases slightly with temperature and depositiontime. ZYT is deposited mainly in the cubic doped zirconia phase. Oxygen permeation experiments show that the oxygenpermeation flux through the ZYT layers is limited by an electrochemical process. Permeation values in the order of10–8 mol/cm2 s have been observed (900–1000°C, air vs. CO/CO2).

Kinetic study of the modified chemical vapour deposition process in porous media

A kinetic study of the modified CVD process in porous ceramic substrates is reported by both theoretical analysis and an experimental approach. The theoretical analysis indicates that with a heterogeneous reaction the decrease in the pore size is proportional to the deposition time and the pore closure time is proportional to the original pore size. With a homogeneous reaction the pore size decreases exponentially with the deposition time and it is very difficult to achieve pore closure. The deposition width should decrease with increasing deposition temperature in a homogeneous reaction, while in a heterogeneous reaction the deposition width could either increase or decrease with increasing deposition temperature. The experimental results indicate that the deposition of yttria-stabilized zirconia inside pores of porous ceramic membranes is a heterogeneous reaction.

Growth of thin dense gas-tight (Tb,Y)-ZrO2 films by electrochemical vapour deposition

Electrochemical vapour deposition has been applied to depositing thin, dense, gas-tight terbia- and yttria-stabilized zirconia films on porous ceramics using metal chlorides as precursors. (Tb,Y)-ZrO2 solid solutions have a fluorite-type structure and have a high mixed conductivity and oxygen semipermeability.

Kinetics and morphology of electrochemical vapour deposited thin zirconia/yttria layers on porous substrates

By means of electrochemical vapour deposition (EVD), it is possible to grow thin (0.5-5 µm), dense zirconia/yttria layers on porous ceramic substrates. Kinetics of the EVD process, morphology and oxygen permeation properties of the grown layers are investigated. Very thin (~ 0.5 µm) layers are grown at relatively low temperatures (700-800 °C). Water vapour as reactant enhances the surface reaction rate at the solid oxide/oxygen source reactant interface. A transition occurs from pore diffusion (above 1000 °C) to bulk electrochemical diffusion (below 900 °C) as rate-limiting step for layer growth. The zirconia/yttria solid solution is mainly deposited in the cubic phase; the layers grow in a typical columnar way and are polycrystalline. Oxygen permeation measurements show that the oxygen permeation flux through the zirconia/yttria layers is influenced by the layer thickness, morphology, presence of water vapour and the oxygen pressure gradient over the layer.