K.J. de Vries

Oxygen ion and mixed conductivity in compounds with the fluorite and pyrochlore structure

The effect of pyrochlore order in solid solutions with the defect fluorite structure was studied. The amount of order was changed by various heat treatments or by varying the cation composition. Examples of short range order and microdomain-formation have been observed in Tb2Zr2O7+x and Gd2Zr2O7. Pyrochlore order results in a decrease of the activation enthalpy and pre-exponential factor for oxygen ion conduction. The results are explained by a model based on the occurrence of a preferential diffusion path with a low strain energy component in ΔH. Optimal conductivity is achieved in materials which are not completely ordered. Mixed conductivity in Ln2Zr2O7 with P structure can be realized by partial replacement of Ln by Tb. This results in good ionic as well as electronic conductivities.

Thermal stability and its improvement of the alumina membrane top-layers prepared by sol-gel methods

The thermal stability of unsupported alumina membrane top-layers was studied by determining the pore structure (mainly pore size) change of alumina gels, prepared by sol-gel methods, after sintering at different temperatures ranging from 450 to 1200 °C. The average pore size of the pure alumina membranes and PVA-added membranes increased sharply after sintering at temperatures higher than 1000 °C. Addition of 3% lanthanum, either by mixing lanthanum nitrate in the alumina sol or impregnating lanthanum nitrate into calcined alumina gel, followed by a second heat treatment, can considerably stabilize the pore structure of the alumina membrane top-layers. The pore diameter for the lanthanum-doped membranes was stabilized within 25 nm after sintering at 1200 °C for 30 h, about one-sixth of that for the pure alumina membranes after sintering at 1200 °C for 30 h. The substantial increase in the pore size for the pure alumina membranes at the sintering temperature of 1000 to 1200 °C was accompanied by the phase transformation from γ-to α-alumina. The addition of lanthanum can raise this phase transformation temperature by about 200 °C.

Bulk and electrochemical properties of BiVO4

The mixed (oxygen ionic-electronic) conductor bismuth vanadate (BiVO4) was studied with respect to its electrochemical properties. The ionic transference numbers, measured by the concentration cell method in the temperature range of 800 to 1000 K, vary from 0.7 to 0.3. The total conductivity of this ceramic material isone order of magnitude lower than found for cubic yttria-stabilized zirconia (YSZ). The activation enthalpy for the electronic conductivity is high (193 kJ/mol) compared to the ionic conductivity (71 kJ/mol). The PO2 dependence of the conductivity data in combination with the Seebeck measurements showed electrons to be the majority charge carriers, indicating that BiVO4 is an n-type mixed conductor.

A Kinetic Study of the Electrochemical Vapor Deposition of Solid Oxide Electrolyte Films on Porous Substrates

The electrochemical vapor deposition (EVD) method is a very promising technique for making gas-tight dense solidelectrolyte films on porous substrates. In this paper, theoretical and experimental studies on the kinetics of the depositionof dense yttria-stabilized zirconia films on porous ceramic substrates by the EVD method are presented. The more systematictheoretical analysis is based on a model which takes into account pore diffusion, bulk electrochemical transport, andsurface charge-transfer reactions in the film growing process. The experimental work is focused on examining the effectsof the oxygen partial pressure and substrate pore dimension on the EVD film growth rates. In accordance with thetheoretical prediction, the pressure of oxygen source reactant (e.g., water vapor), the partial pressure of oxygen and substratepore dimension are very important in affecting the rate-limiting step and film growth rate of the EVD process. In thepresent experimental conditions (e.g., low pressure of oxygen source reactant and small substrate pore-size/thicknessratio), the diffusion of the oxygen source reactant in the substrate pore is found to be the rate-limiting step for the EVDprocess.

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.

The oxygen transfer process on solid oxide/noble metal electrodes, studied with impedance spectroscopy, dc polarization and isotope exchange

The electrochemical oxygen transfer process at the yttria stabilized zirconia (YSZ) and Fe-implanted YSZ, and at the erbia stabilized bismuth oxide (BE25) surface is studied with dc polarization and impedance spectroscopy using gold electrodes, and with 18O gas phase exchange. The surface modification by Fe-implantation increases the exchange current density up to a factor of 50, but analysis of the impedance spectra at different polarization levels indicates that the type of electrode reaction is not changed by the implantation. Inductive effects at cathodic polarizations are interpreted with a stepwise transfer of electrons. Isotope exchange experiments show an increase in adsorption/reaction sites at the surface after implantation. The high exchange current density, I0, for BE25 is independent of type of electrode but does depend on electrode morphology. I0 can be equated to the surface oxygen exchange rate, indicating that the entire electrolyte surface is active in the electrode exchange process. Qualitative interpretation of the impedance spectra measured at different levels of polarization results in a model where adsorbed oxygen species diffuse over the oxide surface, while charge transfer occurs across the surface.

Thermochemical stability and nonstoichiometry of yttria-stabilized bismuth oxide solid solutions

The thermochemical stability of fast oxygen ion conducting yttria stabilized bismuthoxide (YSB) solid solutions containing 22.0–32.5 mol% of yttria was investigated. It was shown that in the temperature range between 650–740 C the stabilized cubic δ-phase containing less than 31.8 mol% of yttria is not stable during long term annealing treatments (greater-or-equal, slanted 500 hours). During annealing at 650 C a sluggish transformation from the cubic to hexagonal phase appears, while above 740 C this hexagonal phase is converted very fast into the cubic phase again. It was shown, that the oxygen content of YSB solid solutions is a function of temperature and oxygen partial pressure

Deposition and Electrical Properties of Thin Porous Ceramic Electrode Layers for Solid Oxide Fuel Cell Application

The influence of the microstructure on the electrical properties and polarization behavior of thin porous ceramic electrodelayers used in solid oxide fuel cells has been investigated. Thin layers (2–10 µm) of the cathode materialSr0.15La0.85MnO3 (15SLM) were film coated on YSZ substrates from classified suspensions. Narrow particle-size distributionsin the suspension resulted in close-packed layers with a very homogeneous porosity and pore-size distribution. Thespecific conductivity of the layers decreased significantly with increasing porosity and mean pore size. A specific conductivityof 109 S · cm–1 was obtained at 1000°C for a 2.9 µm thick layer from a suspension with particles in the range 0.10–0.25 µm. The current-overvoltage behavior of the film-coated layers presented in this study did not, however, show anysignificant dependence on the thickness and the microstructure of the porous layers. Overvoltages (eta) at a current densityof 0.1 A/cm2 at 898°C were quite low, i.e., in the range 60–70 mV. In comparison with other studies it is shown that filmcoating improves the microstructure of the ceramic electrode layers, which in turn lowers the cathodic overvoltages forthe oxygen reduction reaction.

Electrical conductivity and defect chemistry of the system (TbxGd1−x)2Zr2O7+y (O≦x≦1;O≦y<0.25)

Structural and electrical properties of the homogeneous solid solution series (TbxGd1−x)2Zr2O7+y (Ox1; O y < 0.25) were investigated. The defect structure and electrical properties were studied by thermogravimetric analysis and electrical conductivity and ionic transport number measurements as a function of temperature and oxygen partial pressure. The (p-type) electronic conductivity was found to arise from a small polaron hopping mechanism on the Tb-sublattice. The charge carrier concentration can be correlated with the Tb4+ concentration. The maximum fraction of Tb4+ ions on the Tb-sublattice was found to be 0.25 and is hardly dependent on the composition (x). Charge carrier mobilities are in the range at 700°C. Ionic conductivities up to 7 × 10−1 ω−1m−1 at 700°C are found. The conductivity can well be described with a previous developed model, that relates the effects of pyrochlore order in the fluorite lattice with the conductivity parameters.

Thermochemical stability and nonstoichiometry of erbia-stabilized bismuth oxide

A phase study has been performed of high oxygen ion conducting erbia-stabilized bismutch oxide (1-x)Bi2O3·xEr2O3 (BE100X) using thermal analysis and X-ray powder diffraction. Investigation of the effect of a long-time (500 h) anneal of samples at 650°C in air revealed that the minimum amount of erbia needed to stabilize the high-temperature cubic ?-Bi2O3 phase is 27.5 at%. This boundary value is much larger than the one usually reported in literature where the sluggishenss of the transformation from cubic to hexagonal at high Bi contents is not taken into account. Changes in nonstoichiometry of solid solutions Bi2-2xEr2xO3+? between 550°C and 850°C upon varying the ambient oxygen partial pressure are minimal for samples with 27.5 at% erbia, increasing with increasing erbia content. The parameter ? in pure oxygen increases from 0. 0044 for BE27.5 to 0.022 for BE50 taking the composition in nitrogen (PO2 ? 10?4 atm) as stoichiometric reference (? = 0).