Per Kofstad

Crystal structure, electrical and magnetic properties of La1−xSrxCoO3−y

The crystal structure and properties of La1 − xSrxCoO3 − y with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO3 is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co4+ (cobalt ions with one positive effective charge, CoCo.) and oxygen vacancies (Vo..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.

Defects and transport properties of metal oxides

The paper gives an overview of different types of defects and defect structures in metal oxides and transport properties such as diffusion and electrical conductivity. Point defects (vacancies, interstitials, impurity and dopant ions, hydrogen ions, a.o.) and corresponding defect structures are described by defect equations and equilibria. Oxides of common use metals and which are of particular interest in many aspects of high-temperature oxidation of metals, e.g., cobalt and nickel oxides, chromia, alumina and silica, are used as examples in describing defect structures and transport properties.

High temperature corrosion in SOFC environments

Abstract The corrosive environment in solid oxide fuels cells (SOFCs) using natural gas (mainly methane) as fuel is considered. In the anode chamber the gas is highly carburizing at the gas inlet and after gradual oxidation it becomes highly humid, but weakly oxidizing (high CO 2 :CO ratio) at the gas outlet; in the cathode chamber the gas is highly oxidizing (air). In addition to noble metals, nickel and chromia-forming alloys are considered as potential metallic interconnects and their corrosion behavior in the SOFC environment is discussed.

On the formation of porosity and microchannels in growing scales

It is well known that oxide scales develop porosity and microchannels that permit inward transport of molecular species from the ambient gas even under conditions when there is no evidence of cracking of the scales. It is proposed that such porosity and microchannels develop as a result of grain growth and of plastic deformation (grain-boundary sliding, diffusion creep, etc.) under compressive stresses in the scales. The presence of small amounts of impurities enriched at grain boundaries in the scales may greatly affect deformation and mechanical and transport properties in scales.

Oxidation of titanium in the temperature range 800–1200°C

Abstract Oxidation of titanium in the temperature range 800–1200°C has been studied. In agreement with Wallwork and Jenkins 7 the initial approximate parabolic oxidation rate has been shown to be primarily associated with oxygen dissolution in the metal. When the oxygen concentration in the outer layer of the metal reaches a composition TiO0.35, heavy oxide formation begins to take place and the oxidation eventually follows a linear oxidation rate. It is suggested that the corresponding oxidation is governed by oxide nucleation and growth of oxide nuclei. Above 950°C recrystallization and sintering of the outer layers of the TiO2-scale take place.

Proton and native-ion conductivities in Y2O3 at high temperatures

Abstract The ionic conductivity of hot-pressed samples of undoped Y 2 O 3 has been studied by the emf-method in atmospheres of controlled oxygen and water-vapor pressures. The variation in the ionic conductivity was studied as a function of time (7 months at 1200°C), temperature (600–1300°C), water-vapor pressure (3–1400 Pa), and oxygen pressure (10 −10 −10 5 Pa). The overall conductivity can be divided into contributions from electronic carriers (mainly electron holes), native ionic defects, and hydrogen defects. The transport of charged hydrogen species is dominated by migration of “free” protons. The hydrogen-ion conductivity is detectable under all conditions and becomes the dominant ionic-conductivity contribution at high water-vapor pressures and low temperatures. The ionic contributions are discussed in terms of grain-boundary and bulk transport properties. Native-ion and proton-diffusion coefficients in yttria are estimated. Equations for the emf of oxide specimens containing charged hydrogen defects have been derived.

Electrical conductivity and defect structure of Cr2O3. I. High temperatures (>∼1000°C)

Abstract The electrical conductivity of Cr 2 O 3 has been studied by the four-point ac technique as a function of oxygen activity (O 2 +Ar, CO+CO 2 and H 2 +H 2 O mixtures) in the temperature range 440–1500°C. This paper focuses on the conductivity behaviour at high temperatures (>∼1000°C) and under these conditions it is concluded that the predominant defect structure situation involves the intrinsic electronic equilibrium. From the conductivity measurements it is estimated that the corresponding energy gap is 320 kJ/mol (3.3 eV). The results also suggest that hydrogen may dissolve in chromia in hydrogen-rich atmospheres at high temperatures but in amounts that do not significantly affect the electrical conductivity. The defect structure of chromia at high temperatures is discussed.

International Workshop on High-Temperature Corrosion

The results of a third international workshop on “New Knowledge and Open Questions of High-Temperature Corrosion” that took place in August 1994 in Gohrisch, Saxony, Germany, are presented. The workshop was sponsored by Stiftung Volkswagenwerk and the Electric Power Research Institute (EPRI). Twenty-eight leading corrosion scientists from Europe, North America, and Australia participated. The discussion of nine subject areas in the form of key questions and proposed answers is presented.

Electrical conductivity of Cr2O3 doped with TiO2

The electrical conductivity of Cr2O3 doped with TiO2 has been studied by the four point technique as a function of the oxygen activity (O2+Ar, CO+CO2 and H2+H2O) at 1000°C and hydrogen activity (H2+H2O) at temperatures from 400 to 1000°C. Ti-doped chromia is an n-conductor at reduced oxygen activities (ambient H2+H2O and CO+ CO2 gas mixtures) and a p-conductor at near atmospheric oxygen activities. It is concluded that the Ti-dopant is compensated by the formation of chromium vacancies at near-atmospheric oxygen activities and by electrons at highly reduced oxygen activities (near the Cr/Cr2O3 boundary). Chromia dissolves hydrogen as protons in ambient H2+H2O gas mixtures at reduced temperatures (<1000–800°C), but the solubility of hydrogen is decreased by the presence of higher-valent Ti-dopant.

Chromium transport through Cr2O3 scales I. On lattice diffusion of chromium

Cr specimens preoxidized at 1100–1300°C to give Cr2O3 scales with varying thicknesses and microstructures have been treated at temperature in high vacuum. During the high vacuum treatment the specimens lose weight due to outward Cr transport through the Cr2O3 scales. The initial rate of weight loss gradually diminishes, but eventually the weight loss reaches a linear rate. Concurrently the Cr2O3 scale exhibits grain growth and densifies. It is concluded that the mode of outward chromium transport gradually changes during the high vacuum treatment: from lattice and grain-boundary diffusion and possibly vapor transport along microcracks during the initial stage to lattice diffusion only for the densified scales. It is concluded that chromium diffuses by an interstitial type mechanism. Self-diffusion coefficients of Cr in Cr2O3 at the Cr-Cr2O3 phase boundary have been calculated from the linear rates of chromium transport for different defect structure situations.