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.\u

Study of phase transitions and properties of tetragonal (Pb, La) (Zr, Ti)O3 ceramics—III : Transitions induced by electric fields

The phase diagram of electrically poled, tetragonal PLZT materials has been investigated. In the higher La concentration range, a tetragonal, so-called β-phase occurs, which has no clear FE properties. The transition to the FEt phase has first order character and takes place only after a strong electric field has been applied to the material. The energetics of theβt ah FEt transition were investigated in several ways. The heat-effects associated with this transition have a small value of 6–20cal/mol, dependent on composition. The transition βt ah FEt is accompanied by a relatively large activation energy ΔG2. The ΔG2(T) curve shows a minimum for materials, which do not show a spontaneous transition to the FEt phase. This minimum value of ΔG2 (1–2.5 cal/mol) is large in comparison to the average thermal energy kT and prevents the spontaneous transition from the βt phase to the FEt phase

Relaxational polarization and diffuse phase transitions of LA-substituted Pb (Zr,Ti)O3-ceramics

Dielectric properties of (Pb,La)Zr0.55Ti0.45O3 ceramics have been investigated as functions of temperature and frequency. The dielectric constant as a function of temperature can be represented over a considerable temperature interval by a quadratic law of the type: ε−1=ε−1max + C(T−TC')2. The permittivity versus temperature curves are strongly broadened around TC' (where is at a maximum) and the dielectric behaviour is at least partly of a relaxational nature. Interpretation of the dielectric behaviour (broadening, frequency dependence, quadratic law) seems possible by assuming a distribution of local Curie temperatures; the standard deviation of which can be correlated with the La concentration

The preparation and characterization of alumina membranes with ultrafine pores 2. The formation of supported membranes

Supported γ-A12O3 membranes are prepared by a dipping procedure. According to this method, a capillary pressure drop is created by bringing a microporous ceramic support into contact with a boehmite (γ-A1OOH) sol. This pressure drop forces the dispersion medium of the sol to flow into the dry part of the support. The sol particles are concentrated at the entrance of the pores and a gel is formed. This gel can be dried and calcined to form a crack-free alumina membrane with pores measuring only a few nanometers, if the membrane thickness after calcination is about 5 μm or less. The gel formation process can be quantitatively well described with the slibcasting model. The observations that the membrane thickness increases linearly with the square root of the dipping time and that the casting rate increases if the sol concentration increases, the pore size of the support decreases or the gelling concentration decreases, are all in accordance with this slibcasting model

Oxygen Transfer on Substituted ZrO2, Bi2O3, and CeO2 Electrolytes with Platinum Electrodes II. A-C Impedance Study

An equivalent electrical circuit that describes the electrode processes on different electrolytes, using porous Pt electrodes,is given. Diffusional processes are important and have to be presented by Warburg components in the circuit. Theoverall electrode process is rate limited by diffusion of atomic oxygen on the electrode surface for stabilized zirconia andsubstituted ceria (low PO2). On stabilized bismuth sesquioxide diffusion of atomic oxygen on the electrolyte surface is ratelimiting at high PO2 while at low PO2 another process, probably diffusion of electronic species in the electrolyte, is dominant.One of these processes plays a role too on substituted ceria at high PO2, where a charge transfer process is dominant. Theseresults are consistent with the mechanisms developed in part I of this paper

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

Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes

Sol-gel modification of mesoporous alumina membranes is a very successful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and molecular sieve-like separation factors. This paper concentrates on the mechanism of activated transport (also often referred to as micropore diffusion or molecular sieving). Based on a theoretical analysis, results from permeation and separation experiments with H2, CO2, O2, N2, CH4 and iso-C4H10 on microporous sol-gel modified supported ceramic membranes are integrated with sorption data.\ud \ud Gas permeation through these membranes is activated, and for defect-free membranes the activation energies are in the order of 13¿15 kJ.mol¿1 and 5¿6 kJ.mol¿1 for H2 and CO2 respectively. Representative permeation values are in the order of 6×10¿7 mol.m¿2.s¿1.Pa¿1 and 20×10¿7 mol.m¿2.s¿1.Pa¿1 for H2 at 25°C and 200°C, respectively. Separation factors for H2/CH4 and H2/iso-butane are in the order of 30 and 200 at 200°C, respectively, for high quality membranes.\ud \ud Processes which strongly determine gas transport through microporous materials are sorption and micropore diffusion. Consequently, the activation energy for permeation is an apparent one, consisting of a contribution from the isosteric heat of adsorption and the activation energy for micropore diffusion. An extensive model is given to analyse these contributions.\ud \ud For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rate determining. The calculated activation energies for micropore diffusion are 21 kJ.mol¿1 and 32 kJ.mol¿1 for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpore=4Å), indicating that the average pore size of the sol-gel derived membranes is probably smaller

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

A hydrothermal route for production of dense, nanostructured Y-TZP

Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal conditions of a hydrous gel, obtained by coprecipitation. Differences in powder properties, green compact structure and sinterability were examined. Crystallization under hydrothermal conditions occurs at temperatures as low as 190°C in the presence of ammonia. The hydrothermally treated powders are composed of soft agglomerates, that collapse under very low pressures, resulting in green bodies with high densities and small pore radii. The sinterability is greatly improved by the hydrothermal treatment and allowed the production of dense, nanostructured Y-TZP by free sintering at 1050°C