J. Meijerink

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.

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.