214 research outputs found

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

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    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

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    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

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    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

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    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

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    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

    CVD of solid oxides in porous substrates for ceramic membrane modification

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    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

    The preparation and characterization of alumina membranes with ultra-fine pores : Part 4. Ultrafiltration and hyperfiltration experiments

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    The rejection characteristics of three types of alumina membranes for polyethylene glycol (PEG) polymers and dextrans soluted in water have been investigated. If the temperature at which the membranes are treated is increased, the pore size increases and as a result the cut-off value also increases. The Al2O3-400 membrane (i.e., the membrane treated at 400°C) exhibits a cut-off value of 2,000 for PEG while that of an Al2O3-800 membrane is about 20,000. Both Al2O3-500 and Al2O3-800 membranes show rejections higher than 80% for the dextrans investigated. The rejection behaviour is compared with literature data on Nuclepore membranes. The performance of the alumina membranes is compared with that of other inorganic membranes. Low molecular weight solutes show only small rejection values solar, although high values are reported in the literature for Vycor glass membranes with pores of similar size

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

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    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

    Characterization of Fe implanted yttria-stabilized zirconia by cyclic voltammetry

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    The technique of cyclic voltammetry has been applied to study reduction and oxidation phenomena which are observed at low oxygen partial pressures during steady state current-overpotential measurements of the Au, O2(g)/Fe implanted yttria-stabilized zirconia interface. The redox potential (EO) of the observed redox couple is in close agreement with the thermodynamic potential of coexistent Fe2O3 and Fe3O4 phases. Hence in the forward sweep of the cyclic voltammogram, defined for negatively swept potential, part of the Fe3+ is reduced to Fe2+. The peak currents in the voltammogram result from a redox reaction which is rate limited by the diffusion of electrons or electron holes in the Fe implanted YSZ surface to the implanted Fe ions rather than by the diffusion of the Fe ions themselves
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