Monika Backhaus-Ricoult

Internal solid state reactions

Abstract Internal solid state reactions in nonmetallic systems (in particular in oxide systems) are reviewed. Firstly, internal oxidations are discussed, both in respect of theory and experiment. Secondly, internal reductions are treated. Thirdly, reactions of type A + B = AB which occur in a solid matrix (γ) are introduced and some pertinent examples are given. Finally, electrochemical internal reactions are analyzed, the characteristics of which are junctions of the type electronic-ionic.

Effect of yttrium on the corrosion of AB5-type alloys for nickel–metal hydride batteries

Abstract The influence of Y 2 O 3 powder addition (as well as of rare earth oxides of Gd, Ho, Dy, Nd, Sm or Yb) to the metal hydride electrode on AB 5 alloy corrosion rates and on the chemical composition, morphology and density of the corrosion products was studied by X-ray, SEM, TEM and HRTEM. Adding Y 2 O 3 reduces the corrosion rate. This is particularly observed in the long-term behavior of the batteries and is more pronounced at higher temperature. When yttrium is added as an oxide to the electrode or electrolyte, it is dissolved in the electrolyte and is further incorporated as hydroxide in the corrosion scale. The decrease in the global alloy corrosion rate can be explained by a decrease of the driving force for diffusion of Mm 3+ and OH − . Yb is the rare earth element which is expected to produce the largest decrease in corrosion rate, since its solubility in the electrolyte and its content in the corrosion scale are higher than for any other rare earth element.

Internal reduction of (Mg,Cu)O

Abstract (Mg, Cu)O single crystals are internally reduced at temperatures ranging from 1273 to 1673 K in the presence of either a C/CO buffer or a CO/CO2 gas flow. As a result, a reduction scale containing discrete precipitates develops. At low reaction temperature, this scale is divided in an outer part where discrete copper precipitates are present in the MgO matrix, and an inner part where both metallic copper and cuprite Cu2O precipitates coexist in the MgO matrix. At high temperatures, the inner scale is very narrow. Transmission electron microscopy (TEM) investigations of the reduction scale reveal special orientation relationships between the MgO lattice (m) and that of the precipitates (p):

Modelling of the Gibbs adsorption at transition-metal-oxide interfaces: effect of the oxygen chemical potential on interfacial bonding, interfacial energy and equilibrium precipitate shape

Abstract Non-reactive ternary metal–oxide interfaces are thermodynamically stable over extended ranges of oxygen activities and temperatures. At each condition within this range, the interface adopts a different equilibrium structure and chemistry. A continuum model of the Gibbs adsorption–desorption at transition-metal–oxide interfaces is developed, which predicts interfacial chemistry and modifications in the specific free interfacial energy as a function of oxygen activity. Three oxygen activity domains can be distinguished according to this model: the upper part of the metal–oxide coexistence range characterized by an enrichment in interfacial oxygen, established by adsorption of oxygen at structural vacancies in the case of polar interfaces or by desorption of the less noble metal in the case of mixed interfaces; an intermediate-oxygen-activity range with the interface remaining free of adsorption; a lower-oxygen-activity range, where the interface is enriched in less noble metal by adsorption of excess less noble metal at structural vacancies or by desorption of oxygen. Largest excess concentrations are reached at the limits of the coexistence range; absolute values depend on the ability of the transition metal to undergo partial charge transfer. Any charge transfer across the interface imposes a formal charge in the terminating oxide plane and leads to the formation of a space charge layer in the oxide. In the present work, defect concentration profiles in the space charge layer are calculated for different oxygen activities and their influence on the cohesive energy is evaluated. The general model is applied to the MgO–Cu system. Computed interfacial occupancies are compared with experimental observations of the chemical bonding at polar and mixed topotactical MgO–Cu interfaces at different oxygen activities (electron-energy-loss near-edge structure studies). The evolution of relative specific free interfacial energy ratios, inferred from the equilibrium shape of MgO precipitates within a copper matrix and of liquid copper inclusions in a MgO matrix, is compared with model predictions of the interfacial energy of different facets and their evolution with oxygen chemical potential. Qualitative and quantitative agreement between model and experimental results is found.

Interfacial chemistry in internally oxidized (Cu,Mg)-alloys

Abstract (Cu,Mg) alloys are internally oxidized at different oxygen chemical potential at 900°C. Oxidation scale microstructure is studied by SEM and TEM. MgO forms as large magnesia agglomerates without any special orientation relationship and isolated cubo-octahedral topotaxial MgO precipitates, the shape of which varies with decreasing oxygen activity from octahedral to cubic. The interfaces of the cubo-octahedral precipitates are studied in detail by CTEM, HREM and EELS. At the highest oxygen activity, important rigid-body contraction/expansion across the interface is found together with a strong modification in the interfacial electronic structure (compared to the adjacent bulk phases) indicating important hybridization of O 2 p and Cu 3 d states. Both suggest oxide bonding. At lower oxygen activity, interfaces show increasing structural disorder in the copper phase and microfaceting or terracing of the interfacial plane; the intensity of interfacial ELNES features associated to the O 2 p and Cu 3 d hybridization diminishes and finally disappears with decreasing oxygen activity. Changes with oxygen chemical potential in precipitate morphology, interface atomic and electronic structure are explained by Gibbs’ adsorption/desorption of excess oxygen to the interface. Adsorption isotherms are modeled for various configurations and compared to the experimental results.

Creep properties of an alumina-zirconia composite reinforced with silicon carbide whiskers

Abstract The compressive creep behaviour of a ceramic composite consisting of a zirconia-toughened alumina matrix reinforced with silicon carbide whiskers is studied between 1550 and 1673 K in different atmospheres and at stresses ranging from 10 to 250 M Pa. Macroscopic parameters, like the deformation rate, the stress exponent and the activation energy, are determined. At 1573 K, steady-state creep rates between 10 −8 s −1 (30 M Pa) and 10 −6 s −1 (200 M Pa) are measured with only minor differences for reducing neutral and oxidizing atmospheres. At 1573 K and above a given stress threshold, a stress exponent of 2 and an activation energy of 800 kJ mol −1 are measured. At higher temperatures and above this stress threshold, the stress exponent is 2·5. Electron microscopy observations of the deformed material reveal cavities at triple junctions and stress whirls at grain boundaries. The dislocation density is not found to be increased during creep. Therefore, grain boundary sliding, compensated by diffusion along the interfaces and grain boundaries, and cavitation are considered to be the main mechanisms contributing to the deformation of the composite. At high temperature, microcracking also contributes to the deformation.

Diffusion-induced grain-boundary migration during internal reduction of chromium-doped Al2O3

Abstract Diffusion-induced grain-boundary migration is observed during internal reduction of chromium-doped A12O3. Chemical diffusion of oxygen along grain boundaries is fast compared with volume diffusion; therefore the internal reduction front penetrates from the grain boundaries into the grains, leading to chromium precipitation and formation of a thin, coherently strained chromium-depleted oxide layer. Migration of the grain boundary to either side of the boundary occurs, while the global energy in the overswept area decreases owing to oxygen potential relaxation, precipitate coarsening and elastic strain relaxation. The role of multiple factors, namely the chemical composition of the starting mixed oxide, chemical composition inhomogeneities, crystallographic orientation of adjacent grains and of the boundary plane, kinetics and mechanism of the internal reduction itself, on the direction and velocity of boundary migration is experimentally studied and interpreted.

Microstructural characterisation of surface layers of ZrM2 powders (Laves phases) obtained by various corrosion treatments

Abstract C14 and C15 Laves phase alloys containing Zr, Ni, Mn, V and Ti as major alloy constituents are exposed to air and subsequently treated in various aqueous solutions, such as diluted hydrofluoric acid and concentrated alkaline solution. Corrosion products at the surface of the alloy powders are analyzed by transmission electron microscopy (TEM) and Auger spectroscopy to identify their chemical composition and crystal structure.

Accommodation of Volume Changes During Diffusion-Controlled Metal Precipitation Inside an Oxide Matrix

Under certain conditions, partial reduction of mixed oxides leads to diffusion- controlled formation of metal precipitates inside the oxide matrix. Local volume changes related to this precipitation process are important. Depending on the mechanism of metal formation, it can be either negative, as in the case of substitutional mechanisms, or positive, as in the case of interstitial mechanisms. In both cases it induces large stresses during precipitate growth. At high temperatures these stresses may be relaxed by plastic deformation of the matrix. In the present work, internal reduction of doped magnesia and alumina single crystals is investigated. For magnesia, metal precipitation is always accompanied by formation of free matrix dislocations and stresses due to volume changes are fully compensated by dislocation climb. For doped alumina, no formation of matrix dislocations is observed in the temperature range investigated; stresses due to precipitation and precipitate growth are relaxed by diffusional creep. Only when the metal phase precipitates out of the supersaturated mixed oxide during cooling pore formation at the precipitate interfaces is observed.

Role of Interfacial and Strain Energy for the Formation of Native Metal-Oxide Interfaces

The equilibrium shape of semi-coherent precipitates in the MgO-Cu system is determined at various oxygen chemical potentials by a TEM study of small inclusions obtained by internal reaction. Solid or liquid copper inclusions in MgO are formed via internal reduction of the corresponding mixed oxides, whereas MgO precipitates inside copper are produced by internal oxidation of the alloys. Precipitate equilibrium shape is achieved by long time annealing. The shape of soft Cu precipitates in the hard MgO matrix is compared to that of hard MgO precipitates in the soft Cu matrix. Contributions of strain and interfacial energy are evaluated and their role for the formation of the final precipitate shape is discussed. Influence of crystal anisotropy and bond character on the interfacial energy are also examined and used to explain the precipitate shape dependency on oxygen chemical potential.