Mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation

The rate of oxygen permeation through La1−xSrxCo1−yFeyO3−δ was found to increase with an increase in Sr or Co content, showing that the permeability was mainly controlled by the amount of oxygen vacancies. The results obtained indicate that mixed conductive perovskite-type oxides are promising materials for oxygen permeation at elevated temperatures.

Oxygen permeation through perovskite-type oxides

The self‐diffusion coefficient of oxygen has been determined as a function of temperature in single crystal and polycrystalline aluminum oxide at temperatures up to 1780°C. The rate of exchange between a gas phase and solid particles was measured, utilizing the stable isotope, O18. In single crystals of aluminum oxide intrinsic diffusion occurs in a high temperature region, depending on temperature as D=1.9×103 exp (—152 000/RT). At temperatures below about 1600°C variable results were obtained depending on impurity content and previous heat treatment. For one set of samples experimental results could be represented as D=6.3×10—8 exp (—57 600/RT). The diffusion coefficient of oxygen in polycrystalline samples is about two orders of magnitude larger than that found for the single crystals, and has a somewhat smaller activation energy. With the polycrystalline oxide, variable results were also observed at lower temperatures.

Self-diffusion of oxygen in single crystal and polycrystalline aluminum oxide

This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

The formation of aluminum oxide scales on high-temperature alloys

The self‐diffusion coefficient of aluminum has been determined in polycrystalline aluminum oxide over the range 1670° to 1905°C. Diffusion couples were employed using aluminum‐26 in oxide form as a radioactive tracer. In the temperature range studied, experimental data can be represented by D = 28 exp {—[(114 000±15 000)/RT]}.

Aluminum Ion Diffusion in Aluminum Oxide

Oxygen ion mobility in the cubic fluorite-structure phase having the composition Zr0.85Ca0.15O1.85 has been directly determined by exchange measurements employing the stable isotope O18 and mass spectrometer analysis. Measurements of electrical conductivity have been carried out as a function of both temperature and oxygen pressure. Results indicate that the conductivity is wholly ionic with a transfer number near unity for the oxygen ion.

Oxygen Ion Mobility in Cubic Zr0.85Ca0.15O1.85

The electrical conductivity of single-crystal and polycrystalline aluminum oxide has been measured over the temperature range 1300° to 1750°C. and at oxygen partial pressures of 10° to 10−10 atmospheres. Alumina exhibits p- type conductivity at high oxygen pressures and n-type conductivity at low oxygen pressures. The variation of conductivity with temperature depends on the specimen purity, oxygen pressure, and temperature level. Activation energies varying between 2.6 and 5.8 e.v. were found. The conductivity of alumina does not result from any single simple process over wide ranges of temperature and oxygen partial pressure. Apparently intrinsic conduction is found for single crystals at temperatures above 1600°C.

Electrical Properties of Single-Crystal and Polycrystalline Alumina at High Temperatures

The failure of Al2O3, MgO, Pyrex-brand glass, soda-lime-silica soft glass, and a triaxial semi-vitreous whiteware body under impact loading has been observed in some cases from room temperature to 1600°C. The pendulum energy required to fracture these materials remains constant or decreases with temperature over the temperature region of possible use.

J. Pappis

Papers

Oxygen Ion Mobility in Cubic Zr0.85Ca0.15O1.85

Electrical Properties of Single-Crystal and Polycrystalline Alumina at High Temperatures

Note on Failure of Ceramic Materials at Elevated Temperatures Under Impact Loading