Reidar Haugsrud

On the high-temperature oxidation of nickel

Abstract This paper summarizes on some of the extensive experimental data and corresponding models suggested to account for the oxidation mechanism of Ni in the temperature range 500–1400 °C. In addition it reports on in-house experimental data from investigations related to the oxidation of high-purity Ni from 500 to 1300 °C in the oxygen pressure range 1×10 −4 –1 atm based on TG, measurements of surface kinetics, two-stage oxidation, scanning electron microscopy, atomic force microscopy, secondary ion mass spectroscopy etc. The main part of this paper focuses on the more complex models suggested to account for experimental observations of the oxidation kinetics and the oxide morphology below 1000 °C.

Complete structural model for lanthanum tungstate: a chemically stable high temperature proton conductor by means of intrinsic defects

This is the first paper reporting that lanthanum tungstate, earlier believed to be La6WO12, is in fact La28−xW4+xO54+δ , where tungsten dissolves in lanthanum sites to form a stable solid-state electrolyte, exhibiting proton conduction by hydration at intermediate temperatures.

Proton mobility through a second order phase transition: theoretical and experimental study of LaNbO4

The gradual change in the crystal structure of the high temperature proton conductor LaNbO(4) through a second order phase transition and its relation to the activation enthalpy of mobility of protons have been studied by means of first principles calculations and conductivity measurements. The computations have revealed that protons diffuse by an inter-tetrahedral mechanism where the activation enthalpies of mobility are 39 and 60 kJ mol(-1) in tetragonal and monoclinic LaNbO(4), respectively. The activation enthalpy of mobility of protons for tetragonal LaNbO(4), determined from the conductivity curve, is 35 kJ mol(-1). Below the transition temperature the conductivity curve bends; initially dropping off steeply, followed by a less steep decrease towards lower temperatures. The bend in the conductivity curve at the onset of the phase transition in LaNbO(4) should not be given the traditional interpretation as an abrupt change in the activation enthalpy of mobility. After application of the proper analysis of the conductivity data, which takes the second order transition into account, the activation enthalpy of mobility of protons is found to continuously increase with increasing monoclinic angle at decreasing temperature, reaching approximately 57 kJ mol(-1) at 205 degrees C for the end monoclinic phase.

Effects of the La/W ratio and doping on the structure, defect structure, stability and functional properties of proton-conducting lanthanum tungstate La28−xW4+xO54+δ. A review

The present review focuses on characteristics of lanthanum tungstate in the compositional region 25–30 mol% La2O3 in the La2O3–WO3 phase diagram. These tungstates represent an interesting family of materials for technological applications being proton or mixed proton–electron conductors depending on conditions. The material family was traditionally identified as La6WO12. However, recent efforts have shown that lanthanum tungstate can more correctly be represented by the formula La28−xW4+xO54+δ, where nearly one tungsten (x ∼ 1) dissolves in lanthanum sites to form a stable composition. In the present contribution, the importance of the crystal structure and the effect of the La/W ratio on stability, defect chemistry and, accordingly, transport properties of this material are reviewed. A revisited phase diagram in this compositional region is presented. Reported doping strategies on both the A- and B-sites are discussed in view of the applicability of these materials as dense ceramic H2 separation membranes.

Self-repairing metal oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O2 gas near 20 mbar. Information on the position of oxide growth has been gained from the 18O/SIMS technique. Rates of O2 dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O2. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O2. An increased rate of O2 dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.

Mixed Ionic and Electronic Conductivity of Undoped and Acceptor-Doped Er6WO12

The ac conductivities of Er 6 WO 12 and 1% Ca-doped Er 6 WO 12 have been characterized as a function of oxygen and water vapor partial pressures in the temperature range 300-1200°C. Partial conductivities have been determined from the open-circuit voltage of p(O 2 ), p(H 2 ), and p(H 2 O) concentration cells. The materials exhibit n- and p-type electronic conductivity and oxygen ion and proton conductivity, depending on the conditions. Protons are the major ionic charge carriers in wet atmospheres below 1150°C. Hydration thermodynamics and transport parameters have been determined under conditions where the acceptor doping is predominantly compensated by oxygen vacancies and protons.

On the influence of sol–gel derived CeO2 coatings on high-temperature oxidation of Co, Ni and Cu

The effects of CeO2 coatings on high-temperature oxidation of Co, Ni and Cu have been investigated as a function of temperature at oxygen pressures from 1×10−4 to 1 atm. The oxidation mechanisms for Co and Cu are essentially unaffected by CeO2 coatings, whereas the oxidation rate of Ni decreases by approximately one order of magnitude. The oxygen pressure dependence does not change markedly with CeO2 coatings for any of the metals studied. For oxidation of Ni plus CeO2 coatings, the temperature dependence is less marked at lower temperatures, whereas essentially the same behavior is observed for Co and Cu with and without the coating. Differences in the effects of CeO2 coatings for the three metal systems have been attributed to the relative influence of grain boundary transport on the overall rates of oxidation.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2 (x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28-xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD(2)O and pO(2) at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fm (3) over barm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 degrees C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (similar to 11 x 10(-6) K-1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

HT Corrosion of a Cr-5 wt % Fe-1 wt % Y 2 O 3 Alloy and Conductivity of the Oxide Scale Effects of Water Vapor

Corrosion rates of Cr-5 wt % Fe-1 wt % Y 2 O 3 (Plansee Ducrolloy alloy, PD) were measured by thermogravimetry in air or argon with different water vapor partial pressures at 800, 900, and 950°C. The rates of oxidation and evaporation increased with increasing oxygen and water vapor pressures. A reduction of both rates was achieved by Ce-treatment of the alloy surface. The conductivity measured in the oxide scale after the oxidation increased with increasing water vapor pressure and decreasing temperature used in the oxidation. The oxide scale on the Ce-treated samples had a lower conductivity than similarly grown scales on untreated samples. It is suggested that preoxidation in wet inert gas can help develop adherent, protective, and well-conducting scales on PD interconnect components prior to assembly in fuel cells.

The Influence of Water Vapor on the Oxidation of Copper at Intermediate Temperatures

The oxidation rate of high-purity Cu has been measured by means of thermogravimetry in oxygen in the pressure range 1 x 10 -4 to I atm and water vapor in the pressure range 3 X 10 -5 to 0.022 atm at 500°C. respectively, and as a function of temperature under wet (0.022 atm H 2 O) and dry (3 X 10 -5 atm H 2 O conditions from 400 to 1000°C. The oxidation is parabolic and the rate increases with increasing water vapor pressure below 700°C. The oxide scale consists of two phases, an outer CuO and an inner Cu 2 O phase. To distinguish between effects of the reaction conditions on the two Cu oxides, the Wagnerian rate constants of the individual Cu oxides have been deconvoluted from the gravimetric rate constant. The functional pressure dependences of the parabolic rate constants are discussed in terms of derived relations between the Wagnerian rate constants and potential prevailing point defects.