Fast Oxygen Diffusion Research Articles

Microstructure and electrical properties of aluminium-substituted La(Sr)Ga(Mg)O3–δ-based solid electrolytes

A study of the influence of the substitution of Al for Ga in the ceramic processing and electrical properties of La0.95Sr0.05Ga0.90–x Al x Mg0.10O3–δ (0 ≤ x ≤ 0.3) solid electrolytes is presented. The materials retained orthorhombic symmetry over the entire substitution range, whereas a deviation from Vegard’s law for x > 0.20 suggested a maximum Al solubility of x = 0.20. Scanning electron microscopy analysis of ceramic samples revealed that grain growth was inhibited for x ≥ 0.2. This microstructural change was related to an apparent deterioration of mechanical properties, as suggested by room-temperature Vickers hardness measurements. Impedance spectroscopy revealed a significant degradation of the grain-boundary electrical properties for x ≥ 0.20, whereas the bulk conductivity was enhanced for 0.10 ≤ x ≤ 0.15. Oxygen-permeability measurements confirmed that the studied materials remain essentially pure ionic conductors. An ionic conductivity maximum of 0.047 S/cm at 700 °C was obtained for x = 0.10. The effect of aluminium in the grain-bulk ionic conductivity is discussed in terms of defect cluster models and assuming fast oxygen diffusion along domain walls.

Abnormal oxidation behavior of YBa2Cu3O7−δ ceramics

The influence of sample thickness and cooling rate on the oxidation kinetics of dense small grain YBa2Cu3O7−δ ceramics is presented. The oxidation behavior is strongly dependent on the phase type exhibited by the material. At high temperature, in the tetragonal phase, the oxygen stoichiometry of the ceramic core is dependent on the diffusion length and isothermal treatments improve the overall. Value increase in the sample thickness decreases the oxygen content. At low temperature, in the orthorhombic phase, the oxidation during cooling becomes very fast and the sample thickness is not observed to be a limiting parameter. An ultrasonic determination of Young’s modulus as a function of temperature shows that this behavior cannot be related to the formation of microcracks. The improvement in oxidation observed in this temperature range is considered as characteristic of a very fast diffusion of oxygen along the grain boundaries, which is enhanced below the tetragonal to orthorhombic transition by the stresses resulting from anisotropic thermal contraction.

The Influence of Oxygen on the Electrical Properties of Bulk and Thin Films of PbTe Semiconductors

PbTe based semiconductors are characterized by a narrow energy gap and can be used for IR detectors, light emission diodes, lasers and thermoelectric devices. The objective of the present work was to study the effect of oxidation on the properties of n- and p-type PbTe samples prepared by powder metallurgy (bulk materials) and physical vapor deposition (thin films with thickness ∼1 μm). The samples were characterized by SEM, AES and XRD. The Hall effect and electrical conductivity of PbTe samples have been examined over the 80 – 300 K temperature range. The experimental results are accounted for in the framework of a model that is based on: 1- the fast diffusion of oxygen along grain boundaries (GB); 2 - oxygen absorption that generates acceptor states at GB (short time annealing) and the growth of PbTe oxides on GB with properties corresponding to wide band semiconductor (lengthy annealing); 3 - the creation of potential barriers on GB due to oxidation with a thermally activated dependence of the conductivity.

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO 3 have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10 − 15 cm 2 s − 1 with an activation energy of 3.9 ± 0.7 eV.

Oxidation of Zr-based metallic glasses and nanocrystalline alloys

Oxidation kinetics of bulk Zr-based metallic glasses seems to be controlled by oxygen diffusion through the scale towards the ZrO 2/glass interface. Two modes of oxidation were observed: depending on the alloy either: (1) relative slow growth of homogeneous scales consisting of nodules or lamellae of tetragonal ZrO 2 embedding nanocrystals of the late transition metals or (2) formation of fast growing oxide cones exhibiting a lamellar structure of the tetragonal and the less dense monoclinic ZrO 2 thus allowing very fast oxygen diffusion along the interfaces between the lamellae. For both modes an influence of surface preparation on the oxidation reactions has to be mentioned. In addition, the oxidation behavior of these metallic glasses was compared with that of nanocrystalline or coarse crystalline alloys of similar composition. Whereas nanocrystallization improves the oxidation resistance of many Zr-based metallic glasses, coarse crystalline structures promote fast oxidation along grain boundaries, at least in Zr 2Pd.

Mechanical properties and oxidation behavior of the Ti–24Al–14Nb–3V–0.5Mo alloy sheet

Tensile properties of the Ti–24Al–14Nb–3V–0.5Mo alloy (at.%) solution treated at 1000 °C after hot-rolling was studied in the temperature range between room temperature and 700 °C. The tensile strength was sensitive to the test temperature and was decreased as the temperature increased from 200 to 500 °C. Tensile strength of the alloy at room temperature initially increased gradually with an increase in the strain rate from 8.3 × 10 −4 to 0.33 s −1 followed by a slow hardening with increasing the strain rate to 8.3 × 10 −1 s −1. The alloy exposed at 700 °C showed the best oxidation resistance due to the formation of a continuous α-Al 2O 3 layer in the scale. However, the beneficial doping effect of Nb addition on the oxidation resistance above 700 °C was probably overlaid by fast internal diffusion of oxygen, which was proved by a thick transitional layer beneath the scale in the specimen exposed at 800 °C.

Activation of O 2 and CH 4 on yttrium-stabilized zirconia for the partial oxidation of methane to synthesis gas

The isotopic exchange reaction on ZrO 2 and yttrium-stabilized ZrO 2 (YSZ) during catalytic partial oxidation of methane to synthesis gas (CPOM) was studied with transient pulse experiments. The results reveal, surprisingly, that CPOM over both oxides proceeds via a Mars–van Krevelen mechanism. Despite the presence of adsorbed oxygen species, as confirmed by isotopic exchange experiments under reaction conditions, methane is selectively oxidized by lattice oxygen ions on the surfaces of YSZ and ZrO 2. At 900 °C, about 8 and 14% of lattice oxygen in the outermost surface layer of ZrO 2 and YSZ, respectively, can be extracted by methane. Extraction of lattice oxygen results in the formation of surface oxygen vacancies. However, the routes for replenishing oxygen differ for the two oxides. For ZrO 2, the extracted lattice oxygen ions are replenished by direct activation of molecular oxygen at the site of the surface vacancy. The presence of a high concentration of surface oxygen vacancies on YSZ, generated by doping of ZrO 2 with Y 2O 3, permits fast activation of oxygen molecules and fast lattice diffusion of oxygen. The two effects together lead to a rapid replenishment of the surface lattice oxygen extracted by methane. The proposed mechanism explains both the comparatively high activity of YSZ in CPOM and the observation that, in contrast to ZrO 2, lattice oxygen is found exclusively in oxidation products of methane over YSZ during pulse experiments.

High-temperature oxidation of CrN/AlN multilayer coatings

Experiments are reported on sputter depth profiling of CrN/AlN multilayer abrasive coatings by secondary ion mass spectrometry (SIMS) coupled with sample current measurements (SCM). The coatings were deposited by a closed-field unbalanced magnetron sputtering. It is shown that after oxidation tests, performed in air at 900 °C for 2 h and at 1100 °C for 4 h, the layered structure begins to degrade but is not destroyed completely. Oxidation at 1100 °C for 20 h causes total destruction of the coatings that can be attributed to a fast diffusion of oxygen, nickel, manganese and other elements along defect paths (grain boundaries, dislocations, etc.) in the coating. There are practically no nitrides in the near-surface layer after such a treatment and all the metallic components are in the oxidized form as follows from the data obtained by X-ray photoelectron spectroscopy (XPS). According to XPS and mass-resolved ion scattering spectrometry (MARISS), the surface content of Al in the heat-treated coatings has decreased in comparison with the as-received sample and that of Cr increased. Both XPS and MARISS data exhibit real increase in superficial concentration of the substrate materials (Mn and Ni) that is controversial if using SIMS alone. SCM turned out to be an informative depth profiling method complementary to more expensive and complicated SIMS, being particularly useful for structures with different secondary electron emission properties of the layers. SCM with predetermined SIMS calibration allows a routine characterization of coatings and other multilayer structures, particularly, in situations where the expenses of analysis can be justified.

Achieving fast oxygen diffusion in perovskites by cation ordering

The oxygen-exchange behavior has been studied in half-doped manganese and cobalt perovskite oxides. We have found that the oxygen diffusivity in Gd0.5Ba0.5MnO3−δ can be enhanced by orders of magnitude by inducing crystallographic ordering among lanthanide and alkali-earth ions in the A-site sublattice. Transformation of a simple cubic perovskite, with randomly occupied A sites, into a layered crystal GdBaMn2O5+x (or isostructural GdBaCo2O5+x for cobalt oxide) with alternating lanthanide and alkali-earth planes reduces the oxygen bonding strength and provides disorder-free channels for ion motion, pointing to an efficient way to design new ionic conductors.

Oxidation of amorphous Zr70Pd30 and coarse crystalline Zr2Pd

ABSTRACTZr-based bulk metallic glasses are of increasing interest due to their excellent properties, e.g. high elastic limit or catalytic activity. For some applications (golf clubs, hydrogen storage) a good oxidation resistance is necessary; in other cases (catalysis) fast oxidation is required.Zr-Pd melt-spun glasses are known to exhibit “catastrophic” oxidation even at temperatures far below their glass transition. However, the surface of as-cast amorphous Zr70Pd30 was found to be protected by a thin native ZrO2 layer, thus allowing only localized nucleation of the oxidation reaction at preferred nucleation sites activated by destruction of the native oxide layer.Detailed cross-sectional SEM and TEM revealed a rather complicated microstructure of the oxide islands consisting of a lamellar structure of tetragonal and moniclinic ZrO2 with different Pd contents and embedded nanocrystalline Pd. There is also evidence for a Pd enriched reaction zone between the oxide cone and the glassy matrix.In order to understand the influence of the structure on the oxidation, the behavior of the metallic glass was compared with that of a crystallized alloy or even coarse crystalline Zr2Pd. Coarse crystalline Zr2Pd was found to be rather stable against oxidation, but exhibit very fast oxygen diffusion along the grain boundaries followed by oxide formation deep inside the material. Grain boundaries in the crystallized Zr70Pd30 does not exhibit such a behavior.

Ultra-thin titanium oxide film with a rutile-type structure

A titanium oxide film was deposited on a silicon substrate by RF-magnetron sputtering using a sintered oxide target in an argon-gas atmosphere. Phase transition and crystallization during the post-heat treatment were investigated. The thermodynamic stability of phase, oxygen diffusion through the film to form an interfacial SiO 2 layer, and the reaction process in which the rutile-type TiO 2 is crystallized were investigated. Rock-salt-type TiO with large deficiency of titanium ions is quenched in the as-deposited film in non-equilibrium. The following post-heat treatment in nitrogen gas transforms the film to Ti 3O 5, while post-oxidation transforms the film into rutile-type TiO 2, even in an ultra-thin film of 5 nm, at the expense of the growth of interfacial SiO 2 with a thickness of 2.5 nm. The growth of the interfacial oxide is explained in terms of three origins: oxygen defects incorporated into the rutile-type TiO 2, fast oxygen diffusion via the defects, and the fact that non-stoichiometric TiO 2 − x is less stable than SiO 2. We also discuss the main criteria that must be satisfied in order to apply the high-permittivity oxide to a gate insulator.

Bulk Structural Investigation of the Reduction of MoO 3 with Propene and the Oxidation of MoO 2 with Oxygen

Reduction of MoO 3 in propene and oxidation of MoO 2 in oxygen are investigated by in situ X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Temperature-programmed and isothermal experiments (573–773 K) are performed to elucidate the structural evolution of phases present during the reactions and, in addition, to reveal the solid-state kinetics of the processes involved. During the reduction of MoO 3 in propene and the oxidation of MoO 2, only crystalline MoO 3 and MoO 2 were detected by in situ XRD. The formation of a “Mo 18O 52”-type shear structure as intermediate during reduction of MoO 3 in propene and during oxidation of MoO 2 in oxygen was observed by in situ XAS. The solid-state kinetics of the reduction of MoO 3 in propene exhibits a change in the rate-limiting step as a function of both temperature and extent of reduction. The solid-state kinetics of the oxidation of MoO 2 is governed by three-dimensional diffusion. A schematic reaction mechanism for the reduction of MoO 3 in propene and reoxidation in oxygen is proposed that consists of (i) generation of oxygen vacancies at the (100) or (001) facets by reaction with propene, (ii) vacancy diffusion in the MoO 3 bulk, (iii) formation of Mo 18O 52-type shear structures in the lattice, and (iv) formation and growth of MoO 2 nuclei. With respect to a redox mechanism for the partial oxidation of propene on MoO 3, three stages are distinguished. (i) at temperatures below ∼600 K the participation of oxygen from the MoO 3 bulk is negligible. (ii) At temperatures between ∼600 and ∼700 K oxygen vacancy diffusion in the bulk is sufficient to make a redox mechanism feasible, affording a partially reduced MoO 3 under reaction conditions. (iii) At temperatures above ∼700 K sufficiently fast oxygen diffusion in the lattice combined with rapid formation and annihilation of crystallographic-shear planes permits the participation of a considerable amount of the lattice oxygen of MoO 3 in the partial oxidation of propene.

Metal Oxide Composites for Lithium-Ion Battery Anodes Synthesized by the Partial Reduction Process

A thermochemical process based on the partial reduction of mixed oxides is used to create ultrafine metal-ceramic composites for Li-ion battery electrodes. Mixed oxides containing a more noble metal selected to be capable of alloying with lithium at potentials useful as a Li-ion battery anode are partially reduced to form electrochemically active metal-ceramic composites. Experiments show the differences in microstructure obtained in systems with slow oxygen diffusion and fast oxygen diffusion distorted fluorite), and microphase separation rutile). Materials are characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and scanning transmission electron microscopy, and electrochemical tests are presented. Reversible charge capacities of 200-350 mAh/g (1100-2200 mAh/cm3) have been obtained in experiments to date. © 2002 The Electrochemical Society. All rights reserved.

Formation and oxidation properties of (Ti1−xAlx)N thin films prepared by dc reactive sputtering

We prepared (Ti1−xAlx)N thin films by dc reactive sputtering using alloy targets and investigated their oxidation properties. The oxidation rate of the (Ti1−xAlx)N film decreases with increasing aluminum content because Al2O3, with a low oxygen diffusion coefficient, is segregated at the film surface. In particular, the phase separation into a double layer of Al2O3 and TiO2 is more noticeable at high temperature. On the other hand, at an initial stage of oxidation, a mixed oxide of titanium and aluminum (>5 nm thick) is formed after oxidation at 550 °C for 1 min. This mixed-oxide formation is a result of insufficient growth of the Al2O3 layer, which causes fast oxygen diffusion and hence high oxidation rate. We thus conclude that (Ti1−xAlx)N is difficult to use as a barrier metal for metal-insulator-metal capacitors as long as the oxidation resistance is based on the growth of a protective Al2O3 layer on the film surface.

Vibrations of the Interstitial Oxygen Pairs in Silicon

First-principles methods are used to calculate the structures and local vibrational modes of interstitial oxygen pairs in silicon. The staggered ${\mathrm{O}}_{i}\ensuremath{-}\mathrm{Si}\ensuremath{-}{\mathrm{O}}_{i}$ and skewed ${\mathrm{O}}_{i}\ensuremath{-}\mathrm{Si}\ensuremath{-}\mathrm{Si}\ensuremath{-}{\mathrm{O}}_{i}$ structures are nearly degenerate in energy. The calculated local vibration frequencies and their pure and mixed ${}^{18}\mathrm{O}{\ensuremath{\rightarrow}}^{16}\mathrm{O}$ isotopic shifts agree closely with experiments: the highest frequency is assigned to the skewed and the four lower ones to the staggered structure. This result may clear up the controversy of oxygen dimers in silicon, and also suggests a mechanism for fast oxygen diffusion.

Modelling the performance of the cathode catalyst layer of polymer electrolyte fuel cells

Abstract A known macrohomogeneous model for the cathode catalyst layer of a low temperature fuel cell, which includes the kinetics of oxygen reduction at the catalyst ∣ electrolyte interface, proton transport through the polymer electrolyte network, and oxygen diffusion through hydrophobized voids, is considered. Analytical expressions in the relevant ranges of parameter values are obtained. These are the limits of (i) small currents, (ii) fast oxygen diffusion, (iii) fast proton transport, (iv) high current densities (when the layer is depleted in oxygen). They help to trace the main system properties and operation modes, and the routes for the structure-function optimization. Namely, they rationalize the explicit effect of the kinetic parameters of the oxygen reduction on the performance, the effect of the catalyst layer thickness, the width and the position of the active fraction of the layer, and the dependence of the performance on the oxygen partial pressure. In current–voltage plots, three regions are distinguished, each dominated by one of the three main physico-chemical processes in the cathode. Ranges of the optimum cathode thickness are revealed depending on the current density regimes. A bridge between the structural composition of the cathode and its performance is built with the help of an effective medium theory; the model reproduces the experimentally observed trends.

Characterisation of high temperature oxidation of NdFeB magnets

The long term high temperature oxidation properties of a Nd 16.4Fe 75.7B 7.9 commercial sintered magnet were investigated in pure oxygen atmosphere up to 500°C and in air between 350 and 600°C. In pure oxygen atmosphere, three exothermic reactions occur, corresponding to the oxidation of the Nd-rich intergranular regions, the Nd 2Fe 14B matrix phase, and the α-Fe phase that forms during the oxidation of Nd 2Fe 14B. In air, the oxidation of Nd 2Fe 14B was investigated further. Instead of the oxidation proceeding along the grain boundaries, the Nd 2Fe 14B matrix dissociates to form an adherent grey surface layer which grows transgranularly into the magnet. This is probably due to a reaction occurring in the Nd-rich regions which prevents fast path oxygen diffusion along the grain boundaries. The main reaction is the dissociation of the Nd 2Fe 14B matrix into α-Fe nanocrystals which contain small precipitates of oxides of Nd. The products of this reaction form the adherent grey layer which grows transgranularly into the magnet. The activation energy and the diffusivity pre-exponential factor for this reaction were found to be 114 kJ mol- −1 and 0.7 mm 2 s −1, respectively. After further oxidation of the dissociated grey layer, α-Fe is oxidised to form αFe 2O 3 and finally, from about 600°C, some of the α-Fe 2O 3 reacts with the small precipitates of oxides of Nd to form FeNdO 3.

Post-irradiation fission gas release from high burn-up UO 2 fuel annealed under oxidising conditions

In post-irradiation fission gas release experiments on UO 2 fuel, large increases in effective diffusion coefficients have been reported when anneals have been carried out under oxidising conditions. These conditions affect in some way the processes involved in the diffusive phase of gas release associated with the important step of intragranular fission gas transfer to the grain boundaries. The purpose of this paper is to extend a recent model involving fission gas bubble movement to propose a possible explanation. A central part of the model concerns the importance of grain boundaries as a dominant vacancy source. Under normal conditions, boundary concentrations are due to thermal vacancy production but under oxidising conditions, it is suggested that if fast oxygen diffusion takes place along grain boundaries then the subsequent oxidation reaction at the boundary has the potential to cause a huge enhancement of the local vacancy level. As will be demonstrated, an acceleration in the rate of fission gas bubble movement to the grain boundaries will result. A comparison of model calculations with a few selected literature results suggests that the mechanism has more than sufficient strength to give the large effects observed.

Electrochemical studies on the cathodic reaction of marine atmospheric corrosion

Electrochemical studies on the cathodic reaction of marine atmospheric corrosion using Kelvin probe as reference electrode showed that the rate of cathodic reaction-oxygen reduction first increases then decreases, with the reaction maximizing at a certain thickness as the electrolyte film decreases during evaporation. It was indicated that with decreasing electrolyte thickness by drying, the oxygen reduction rate was accelerated by the faster oxygen diffusion due to the thinner electrolyte layer on the metal surface. The results also revealed that although the oxygen salting out effect has great influence on the rate of oxygen reduction, it is not the main causative factor for the decrease in cathodic limiting current in the case of a very thin electrolyte layer.

Ball Milling-Induced Combustion in Powder Mixtures Containing Titanium, Zirconium, or Hafnium

Ball milling induces self propagating high temperature reactions in many highly exothermic powder mixtures. This phenomenon has been studied in a variety of reactions with titanium, zirconium, and hafnium. Several oxides (CuO, Cu2O, NiO, Fe3O4, and ZnO) were reduced with Ti, Zr, and Hf and the borides, carbides, silicides, and sulfides of these metals were prepared from elemental mixtures. The ignition time is much shorter with Zr than with either Ti or Hf whenever oxygen or sulfur is involved in the reaction, but no similar variation is observed for the formation of borides, carbides, and silicides. It is suggested that the fast diffusion of oxygen in ZrO2and very likely of sulfur in ZrS2are responsible for this behavior.