Low Oxygen Activity Research Articles

Determination of electronic conductivity limits of mullite and ZrO2-9 mol% MgO solid electrolytes

A coulometric titration technique was used for the determination of the parameterPϑ, defined as the oxygen partial pressure at which the ionic conductivity and n-type electronic conductivity of the electrolyte are equal, for mullite and 9 mol% MgO stabilized zirconia electrolytes in the temperature range 1674–1921 K. The results ofPϑ for mullite and ZrO2-9 mol% MgO were log(Pϑ/101 325 Pa)=54.32−137000/T and log (Pϑ/101 325 Pa)=27.16–75500/T, respectively. The results for low oxygen activities in liquid iron are discussed on the basis of the present results forPϑ values.

The influence of molybdenum on the degradation resistance of aluminized iron-nickel-base alloys in sulphidizing environments

Since Al2O3−forming nickel-and iron-nickel-base alloys are known to be more effective than Cr2O3−forming alloys in sulphur-containing gases, several commercial Cr2O3−forming alloys have been pack aluminized to establish aluminide coatings for use in mixed-gas environments of high sulphur and low oxygen activity. Results are given for long-term exposures (up to 2500 h) to H2/1.6%H2O/1.1%H2S at 1000°C under thermal-cycling conditions. Uncoated specimens of such alloys are sulphidized completely in very short periods in this environment. Early work indicated that an aluminized molybdenum-containing nickel-base alloy is the most resistant system, so a series of alloys based on nickel and nickel-iron and incorporating various amounts of molybdenum was included in the tests. Most of the aluminized systems showed reasonable resistance for periods of 500 h or more. Subsequently, the protective Al2O3 scale broke down following back-diffusion of aluminium into the alloy substrate and depletion of the surface in thi...

Zirconia — a non-inert material reacting with platinum and oxygen containing gases

Recently, the oxygen contents of high purity zirconia, ZrO 2+ σ , undoped and doped with calcium oxide, and of commercially available, calcium (partially) stabilized zirconia have been investigated by using thermogravimetry as a function of oxygen activity and temperature. In this study, some additional measurements of the oxygen content in commercial, CaO-doped zirconia were made. Moreover, based on the knowledge of the oxygen activity dependent oxygen contents of different zirconia materials, the oxygen activity dependent dissolution of zirconium from zirconia into platinum has been studied thermogravimetrically at temperatures between 1100°C and 1400°C. The results obtained indicate that at temperatures below 1300°C the solubility of zirconium in Pt at equilibrium with zirconia is negligibly small at all oxygen activities experimentally accessible in this study. However at temperatures above 1300°C and at oxygen activities below 10 −8, zirconium dissolves to a measurable extent into platinum. The experimental results obtained on this subject indicate that the equilibrium constant for the dissolution of zirconium from zirconia into platinum follows an Arrhenius-type temperature dependence. Furthermore, some qualitative observations on the formation of Pt-Zr intermetallics at very low oxygen activity are briefly discussed.

Electrochemically-induced reactions at Ni/ZrO2 interfaces

A symmetrical solid state galvanic cell, Ni/ZrO2 + 9.5 mol% Y2O3/Ni, was used as a model system to study and modify chemical reactions at Ni—ZrO2 interfaces. The cell was produced by diffusion bonding Ni on either side of a single crystal of yttria-doped cubic zirconia. Different oxygen activities were established at the interfaces by applying an electrical potential across the galvanic cell. When the electrochemical potential was greater than a critical value, of the order of V, the intermetallic compound Ni5Zr formed at the interface with low oxygen activity and NiO formed at the interface with high oxygen activity. Under these experimental conditions, the ionic transference, number of the electrolyte was ∼0.03. In order to avoid internal, electrical short circuiting of the cell, a voltage had to be applied during cooling. In a different experiment, after applying the electrical current, the opn circuit voltage of the cell was measured. During this period, the cell was short circuited internally, which caused the oxidation of the Ni5Zr layer to Ni and monoclinic ZrO2 and the reduction of the NiO layer to Ni. The microstructure, chemistry and morphology of the phases, grown under different conditions, were investigated using scanning and transmission electron microscopy. In addition, the measured and calculated thickness of the reaction products were compared.

An ionic model of the crystal chemistry in the superconducting copper oxides of stoichiometry (RE) 2CuO 4

The mixed rare earth systems La 2− x RE x CuO 4 (RE = NdY) have been investigated in an effort to understand the crystal chemistry and phase stability of the related K 2NiF 4 ( T)-, Nd 2CuO 4 ( T′)-, and hybrid T∗-type structures. The crystal chemistry of these (RE) 2CuO 4 phases is then discussed on the basis of a simple ionic model. A definitive correlation between the size of the RE cation and structure is found, and the stability limits of the T, T′, and T∗ phases are defined in terms of a perovskite-like tolerance factor ( t). The T structure is found to exist for 0.87 ≤ t ≤ 0.99, while the T′ structure occurs for 0.83 ≤ t ≤ 0.86. The smallest rare earths, or t ≤ 0.83, do not form stable (RE) 2CuO 4 compounds. The T∗ structure occurs in a very narrow region adjacent to the boundary of the T T′ stability field and is seen as resulting from a thermodynamic competition between the T and T′ structures, due to a tendency toward T T′ - site ordering. Metastable T∗ phases are observed for the larger rare earths Nd, Eu, and Gd, but the structure becomes quite stable for RE = Tb, Dy. Where both T′ and T∗ are of comparable thermodynamic stability, high oxygen activity is found to stabilize T∗, whereas low oxygen activity favors T′. The role of RE coordination preferences and the influence of the REO framework upon the structure and properties of these materials are discussed.

ChemInform Abstract: Oxidation of CoO Studied with the Perturbed Angular Correlation Method (PCA).

AbstractThe oxidation of CoO to Co3O4 is studied at low oxygen activity (10‐9 atm) with emphasis on the initial steps.

Point Defects and Surface Diffusion in Iron Oxide and Nickel Oxide

Diffusion of the 57Co isotope on Fe3O4 (110) and NiO (100) surfaces was investigated by the edge‐source method. The surface diffusion parameter, αDsδ, where α is the segregation factor, Ds the surface diffusion coefficient, and δ the thickness of the high‐diffusivity layer, was determined at 750°C for different partial pressures of oxygen. In both cases point defects strongly influenced surface diffusion. For Fe3O4 the vacancy mechanism is dominant at high oxygen activities and interstitial (or interstitialcy) mechanism dominates at low oxygen activities. For NiO the surface diffusion at low oxygen activities is influenced by aliovalent impurities and at high oxygen activities the strong influence of the intrinsic defects, nickel vacancies, on surface diffusion was observed. It was concluded that similar mechanisms operate during surface diffusion in the near surface layer and during diffusion in the lattice.

Oxidation of CoO Studied with the Perturbed Angular Correlation Method

AbstractThe substages of the oxidation of CoO have been investigated with the PAC method during isochronal and isothermal annealings at low oxygen activity (10−9 atm). The successive trapping of cation vacancies at the 111In probe atoms up to the formation of spinel‐like microclusters has been observed. The results of the PAC experiments are compared with those of X‐ray diffraction and RBS analyses and confirmed by a PAC experiment for 111In in Co3O4. A quantitative analysis of the oxidation kinetics in terms of first order rate equations yields an activation enthalpy of ΔH = 0.27(4) eV. It is found that the absorption of oxygen and the formation of vacancies at the surface are the rate controlling processes in this experiment.

The surface reactivity of NbNi 3 and Nb 2Ni 3 alloys toward oxygen and its role in modeling catalysts

The surface reactivities of NbNi 3 and Nb 2Ni 3 toward oxygen have been investigated using X-ray photoelectron spectroscopy and in situ reaction. At low temperature and/or low oxygen activities, Nb oxidizes preferentially. At high temperature (greater than 250° C) and high oxygen activity Ni preferentially oxidizes with the product, NiO, overgrowing the niobium oxide. NiO reduces easily to Ni° with loss of surface coverage. It has been found that Ni does not wet Nb 2O 5 as well as it wets TiO 2 and ZrO 2. The role of alloy oxidation, in particular the Ni-Nb alloy, in modeling catalysts is discussed.

Hydrogen‐Transparent Metal Surfaces Produced by Use of Molten Salts with Very Low Oxygen and Water Activities

One can employ simple thermodynamic principles to predict conditions under which metals will not form hydrogen‐blocking surface oxide layers in electrolyte environments. These principles are used to show that very low oxygen and water activities can be produced in some halide‐based molten salt systems by the incorporation of alkali hydrides. Under these conditions, some normally very reactive metals will not form oxide surface layers. As a result, rapid interfacial and surface/bulk equilibria with hydrogen can be attained. Experiments are presented that verify these principles in the titanium‐hydrogen and vanadium‐hydrogen systems by using electrochemical methods to measure the thermodynamic and kinetic aspects of the solution of hydrogen and the formation of metal hydrides. In addition, it is shown that interfacial equilibrium is obtained very rapidly upon iron and aluminum in such metals.

Controlled electrolyte environments and their use for studying and modifying materials properties: potentials for employment in practical devices

Abstract Molten salt electrolytes containing hydride ions, and with very low oxygen and water activities can produce hydrogen-transparent metal surfaces, as the hydrogen-blocking surface metal oxides are thermodynamically unstable and are removed. Their potential use in novel high conductivity solid electrolyte-like configurations for practical applications such as elevated temperature hydrogen sensors, as well as for a variety of other purposes will be discussed.

The calcium sulfate-induced corrosion of iron-chromium alloys

The calcium sulfate-induced corrosion behavior of binary FeCr alloys has been studied at a temperature of 1223 K under an oxygen partial pressure of 10−11.2 atm, generated using a CO/CO2 gas mixture. These conditions are intended to simulate the oxygen activities in the low oxygen partial pressure locations in the fluidized-bed combustor. Under this low oxygen activity, the presence of calcium sulfate induces sulfidation/oxidation corrosion, and breakaway corrosion was observed for an Fe-20Cr alloy. The formation of internal Cr-rich sulfide/oxide particles in the underlying alloy below an inner Cr-rich oxide layer was generally observed. The reaction products between the oxide and the calcium salts formed at the external surface of scale on iron-chromium alloys, such as a eutectic CaO-Cr2O3 oxide mixture, appear to be insignificant in controlling the overall degradation process. The corrosion mechanism appears to be a sulfidation/oxidation process consistent with the low oxygen partial pressure and the relatively high sulfur activity calculated from the CaOS equilibria.

Factors influencing the performance of a Cr‐Ni‐Fe alloy exposed to sulfidising/oxidising/carburising environments at 800°C

AbstractStudies concerned with the corrosive degradation of engineering alloys exposed to multi‐reactant gaseous environments containing sulphur, oxygen and carbon‐bearing species have intensified during the past decade. In particular, the attack experienced by materials exposed in coal conversion plants has become one of the major areas of concern. These process atmospheres are generally characterised by low oxygen activities and high sulphur and carbon activities and sulphidation can therefore pose significant problems in the design and operation of such plant.This paper presents the results of studies carried out on a laboratory cast austenitic 25 Cr‐35 Ni‐Fe “model” alloy exposed to a range of H2‐CO‐H2O gas mixtures containing H2S additions such that the oxygen and carbon activities remain constant (pO2 = 10−21 bar, ac = 0.3) whilst the sulphur activity is varied systematically within the range, pS2 = 10−9 to 10−7 bar at 800°C. Tests have also been carried out in an equivalent sulphur‐free atmosphere for reference purposes. The influence of surface condition upon corrosion behaviour has received some attention by comparing the results obtained on ground samples with those obtained on work‐free electropolished specimens.The kinetics and mechanisms by which this alloy corrodes have been established for exposure periods ranging from 5 min to 5000 h. These studies provide a clear understanding of the factors which can lead to catastrophic sulphidation attack in these types of environment and they also establish the critical dependence of long‐term corrosion resistance upon the sulphur activity of the atmosphere.

Defect Structure and Transport Properties of Manganese Oxides: (III) Relaxation Kinetics of Manganosite (Mn1‐ΔO) in CO/CO2 Gas Mixtures

AbstractThe kinetics of point defect relaxation in manganosite after sudden changes of the oxygen activity in a surrounding CO/CO2 gas atmosphere have been studied by means of electrical conductivity measurements on single crystalline needle‐like samples at 1000°C. The results show that at very low oxygen activity the relaxation process is essentially controlled by diffusion processes in the bulk, but that at high oxygen activity the oxygen transfer at the manganosite/gas interface determines the kinetics.

Oxygen and carbon sensing in Fe—O—C and Fe—O—C—Xn melts at elevated carbon contents

In the present study on solid electrolyte probes, the attempt was made to measure accurate and reproducible EMF values representing the extremely low oxygen activities in Fe—O—C melts at varying C contents. Various types of sensors were designed and successfully tested in laboratory experiments. Reliable oxygen activities were measured in Fe—O—C melts up to 4 wt. % C under pure CO gas, and fo and fc values were derived as a function of C content at 1 400 to 1 600°C. Further measurements were made in Fe—O—4 wt. % C—Xn melts at various contents of Xn (Sn = Si, Mn, Cr). Moreover, a solid oxide electrolyte probe with a CO gas channel to measure oxygen and carbon activities was developed and successfully tested in high‐carbon iron melts.

Electrical Conductivity of Y2O3 as a Function of Oxygen Partial Pressure in Wet and Dry Atmospheres

The electrical conductivity of polycrystalline Y2O3 has been studied as a function of the partial pressure of oxygen (10–14 to 105 Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P2O5. Yttria is a p‐conductor at high oxygen activities. The p‐conductivity increases with increasing PO2 and decreases with increasing PH2O. At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of PO2 and increases with increasing PH2O. In the Y2O3 samples, excesses of lower‐valent cation impurities (in the 10 to 100 mol‐ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high PH2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high PO2 and low PH2O, the protons are no longer dominant, and the lower‐valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis‐like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm2. s–1. V–1

Defect Structure and Transport Properties of Manganese Oxides: (II) The Nonstoichiometry of Hausmannite (Mn3δO4)

AbstractThe deviation from stoichiometry δ in hausmannite (Mn3δO4) has been thermogravimetrically studied as a function of oxygen activity between 1000 and 1130°C for the low temperature phase (α‐Mn3δO4) and between 1200 and 1350°C for the high temperature phase (ß‐Mn3δO4). A cation deficit is observed at high oxygen activities and a cation excess at low oxygen activities. In the temperature range investigated, it is concluded that the dominant defect species in both phases are cation vacancies at high oxygen activities and manganese interstitials at low oxygen activities. In addition, based on the present nonstoichiometry data, the phase diagram of the maganese‐oxygen

Oxygen activity dependence of the defect structure of La-doped BaTiO3

A model for the behaviour of donor-doped BaTiO3 as a function of oxygen activity is proposed. Thermogravimetric measurements show good agreement with the proposed model. The results show that donor-doped BaTiO3 is stoichiometric with electronic compensation of the donor at low oxygen activity, but with increasing oxygen activity, compensation becomes ionic as excess oxygen is absorbed. The possible compensation mechanisms are discussed.

The transport of sulphur through preformed Al 2O 3 scales on iron-base alloys in environments of high sulphur and low oxygen activities pertinent to coal conversion processes at high temperature

Preoxidation of Fe-27% Cr-4% Al and similar alloys containing yttrium additions in H 2/H 2O gas mixtures to develop external Al 2O 3-rich scales improves considerably the subsequent sulphidation resistance on exposure to gaseous environments of high sulphur and low oxygen activities at high temperatures. However, breakdown of these oxide scales can occur eventually, particularly in the more aggressive environments, following localized transport of sulphur-containing species to the scale/alloy interface. The rate of such transport is determined by the mechanical integrity of the scale and the relative sulphur and oxygen activities in the environment. Sulphur penetration is relatively rapid through cracks in the scale but is much slower through other localized short-circuit diffusion paths. Scale breakdown then results from the progressive development of less-protective sulphide ducts from the alloy/scale interface to the scale/gas interface which provide easy diffusion paths for metal ions to the scale/gas interface, leading to formation and relatively rapid growth of sulphides at the surface.

Compound formation in the rubidium-chromium-oxygen system

Results from a study of the Rb-Cr-O system in the presence of liquid rubidium metal are presented. Compound formation in the system was studied by means of bulk reactions between various chromium oxides and rubidium metal at 500° C followed by vacuum distillation of the excess metal and X-ray powder diffraction analysis of the residues. Equilibrium oxygen potential measurements of Rb-O solutions in contact with chromium metal or chromium compounds using an electrochemical oxygen meter (ThO 2/7.5 wt%Y 2O 3 electrolyte, Sn/SnO 2 reference electrode) gave information on the stability and conditions of formation of these compounds in the liquid. The investigations showed that Cr metal getters oxygen from rubidium to form a compound (possibly RbCrO 2) in equilibrium with low oxygen activities ( a 0 = 5.43 × 10 −7, 500°C ). At higher oxygen levels, however, the formation of Rb 4CrO 4 takes place. A metastable phase Rb 3CrO 4 was also observed in some bulk reactions.