Richard J. Chater

Oxygen transport in selected nonstoichiometric perovskite-structure oxides

New results on oxygen self-diffusion (D∗), surface exchange coefficient (k) and activation energy (Ea) of oxygen self-diffusion for chosen compositions of manganite and cobaltite perovskites illustrate the effect of doping in the A and B sites of the ABO3 structure. The electrical conductivity (σT) was measured for the manganite group and permeability (J) was determined for the cobaltite perovskites. D∗ increases with increasing x in La1−x(Sr,Ca)x(Mn,Co)O3−δ due to formation of new oxyge vacancies by introducing metals of lower valency (M2+) into the A3+ sites. A substitution of M2+ for B3+ or a reduction of the metal in the B3+ site to a lower positive valency also increases D∗ . D∗ cobaltites is significantly higher than that of the manganites (by 4–6 orders of magnitude), however, the potentially high oxygen fluxes that would be allowed through the materials by the high D∗ values seem to be limited by the surface exchange kinetics. Ea-values of the manganites are considerably higher than those the cobaltites. In general, the electrical conductivity, σT, decreased on doping the B site of the manganites with Co and Ni. However, whilst the pure manganite material exhibits a metallic type of conduction (i.e. σT decreased with increasing T), the conduction mechanism in the Co-doped and Ni-doped manganites changed to a localized hopping of charge carriers between the Mn3+ and Mn4+ sites (σT increases with increasing T).

Layered perovskites as promising cathodes for intermediate temperature solid oxide fuel cells

The suitability of GdBaCo2O5+δ as a cathode material for intermediate temperature solid oxide fuel cells has been evaluated. The 18O/16O isotope exchange depth profile (IEDP) method has been used to obtain the oxygen surface exchange and oxygen tracer diffusion coefficients yielding optimum values for applicability in fuel cells (k* = 2.8 × 10−7 cm s−1 and D* = 4.8 × 10−10 cm2 s−1 at 575 °C) especially in terms of low activation energies (EAk = 0.81(4) and EAD = 0.60(4) eV). The same material has been characterized electrically as a part of a symmetrical electrochemical system (GdBaCo2O5+δ/Ce0.9Gd0.1O2−x/GdBaCo2O5+δ), by means of impedance spectroscopy measurements, corroborating an excellent performance in the classical intermediate temperature range for solid oxide fuel cells (500–700 °C). An area specific resistance (electrode–electrolyte interface) of 0.25 Ω cm2 at 625 °C was achieved for a cell processing temperature of 975 °C. Finally, layered perovskites are presented as a promising new family of materials for cathode use in solid oxide fuel cells at low temperatures.

Development of a novel SIMS technique for oxygen self-diffusion and surface exchange coefficient measurements in oxides of high diffusivity

Abstract The determination of D∗ and k provide key information about technologically important oxides in applications such as electrode and electrolyte materials for high-temperature electrochemical devices, e.g., the solid oxide fuel cell. A high oxygen self-diffusion coefficient, D∗, of approximately 10−6cm2/s and surface exchange coefficient, k, of approximately 10−4 cm/s are typical requirements for these applications. Measurement of D∗ and k may be performed by the isotopic exchange/diffusion profile technique with secondary ion mass spectrometry (SIMS) used to determine the O18 stable isotope depth distribution. In the case of oxides of high diffusivity the penetration depth at the chosen anneal temperature, approximately (D∗t)0.5, is of the order of hundreds of micrometers from the surface into the bulk of the sample for the shortest practicable anneal times, t. SIMS depth profiles are generally limited to tens of micrometers due to various considerations including the time required for sputtering and roughening at the base of the SIMS crater. Thus the sputter depth profiling approach must be abandoned in favour of a new SIMS technique described in this paper. Crater base roughening is particularly severe for polycrystalline bulk samples which also have a high defect density. Results from polycrystalline cobaltite perovskite solid solutions and YBCO single crystals are used to demonstrate the technique and precautions required for its successful application.

Anisotropic oxygen diffusion properties in epitaxial thin films of La2NiO4+δ

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La2NiO4+δ films grown on SrTiO3 (100) and NdGaO3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film–substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.

How cracks in SiOx-coated polyester films affect gas permeation

In this paper theoretical models have been established that can account for the gas transmission through nanocomposite laminates, consisting of an oxide layer of finite permeability containing defects, on a polymer sheet of finite thickness. The defect shapes can either be in the form of long cracks or rectangular holes. The models offer a choice of exact numerical calculations or fast and intuitive analytical approximations. The experimental measurements of oxygen permeation through four different SiOx/poly (ethylene terephthalate) samples that were strained to produce distributions or cracks showed good agreement when compared with predicted results from the approximate analytic model. As a consequence of this observation, a key practical conclusion is that, because of the logarithmic dependence of transmission on the width of a crack, for a given strain it is better to have a small number of large cracks rather than a large number of small cracks

An investigation of as‐implanted material formed by high dose 40 keV oxygen implantation into silicon at 550 °C

Device grade 〈100〉 single crystal silicon wafers have been implanted with 40 keV oxygen ions (16O+) over the dose range of 1×1017–8×1017/cm2 at a temperature of 550±10 °C. Transmission electron microscopy, ion channeling, and secondary ion mass spectroscopy studies show that during implantation the critical dose required to form a buried oxygen‐rich amorphous (SiOx, x<2) layer is lower than 1×1017 O+/cm2. As the dose increases from 1×1017 to 4×1017/cm2 the thickness of the buried SiOx layer increases and there is a corresponding decrease in the thickness of the single crystal silicon top layer, with the oxygen concentration and residual radiation damage playing important roles in determining its position and thickness. A dose of 5×1017/cm2 results in a continuous surface amorphous layer, with a buried SiO2 sublayer being formed in the region corresponding to the implanted oxygen peak. With further increasing dose, the buried SiO2 sublayer grows primarily towards the surface. The results for the sample imp...

The role of implantation temperature and dose in the control of the microstructure of SIMOX structures

Abstract Single-crystal ⇇100↩ silicon wafers have been implanted with 200 keV oxygen ions over a dose range of 0.1×10 18 O + cm -2 to 1.4×10 18 O + cm -2 and a temperature range of ≈250°C to 550°C. The specimens have been analyzed, both before and after high-temperature annealing, using a variety of techniques, such as cross-sectional and planar Transmission Electron Microscopy (TEM), Rutherford backscattering (RBS), and ion channelling, Secondary Ion Mass Spectroscopy (SIMS), Infra-red Spectroscopy (IR), and Raman Spectroscopy. This has enabled us to evaluate the development of the SIMOX structure both with respect to implantation temperature and dose and also with respect to annealing temperature and time.

Ion beam synthesis of thin buried layers of SiO2 in silicon

New experiments are reported which explore the possibility of using ion implantation to form thin (<2000 A) buried layers of stoichiometric SiO2 in single crystal silicon, Silicon (100) wafers have been implanted with O+ ions within the dose range 0.1×1018–1.8×1018O+ cm−2 at a particle energy of 200 keV and a substrate temperature of 500°C. Both (100) channelled and non-channelled implants have been carried out. Samples were subsequently annealed at temperatures of up to 1405°C, which causes the oxygen to segregate near the peak of the implanted oxygen distribution. The high dose samples have a continuous buried oxide layer whose thickness scales with the implanted dose, whilst in samples implanted with 0.1×1018O cm−2, discrete, strain free polyhedral precipitates, of diameter 500–1600 A, grow within the single crystal silicon matrix, by a mechanism which is qualitatively similar to oxygen precipitation in C-Z bulk silicon.

Analytical scanning and transmission electron microscopy of laboratory impacts on Stardust aluminum foils: Interpreting impact crater morphology and the composition of impact residues

The known encounter velocity (6.1 kms(-1)) and particle incidence angle (perpendicular) between the Starchist spacecraft and the dust emanating from the nucleus of comet Wild-2 fall within a range that allows simulation in laboratory light-gas gun (LGG) experiments designed to validate analytical methods for the interpretation of dust impacts on the aluminum foil components of the Stardust collector. Buckshot of a wide size, shape, and density range of mineral, glass, polymer, and metal grains, have been fired to impact perpendicularly on samples of Stardust Al 1100 foil, tightly wrapped onto aluminum alloy plate as an analogue of foil on the spacecraft collector. We have not yet been able to produce laboratory impacts by projectiles with weak and porous aggregate structure, as may occur in some cometary dust grains. In this report we present information on crater gross morphology and its dependence on particle size and density, the pre-existing major- and trace-element composition of the foil, geometrical issues for energy dispersive X-ray analysis of the impact residues in scanning electron microscopes, and the modification of dust chemical composition during creation of impact craters as revealed by analytical transmission electron microscopy. Together, these observations help to underpin the interpretation of size, density, and composition for particles impacted on the Stardust aluminum foils.

Oxygen transfer in BIMEVOX materials

Two steps govern the oxygen transport in ceramic oxide ion conductors: (i) the oxygen exchange at the surface of the material and (ii) the oxygen diffusion through the material. The 18 O/ 16 O Isotope Exchange Depth Profile technique (IEDP) was applied to BIMEVOX materials to characterise the oxygen transfer in these ceramics. The isotope concentration profiles, obtained by secondary ion mass spectrometry (SIMS), revealed that the equilibrium exchange kinetics in BIMEVOXes under nominally dry oxygen are dominated by a relatively slow surface exchange step. This produces deep penetration profiles with very low isotopic concentrations close to the natural isotopic background. The oxygen surface exchange coefficient in these materials is of the same orderofmagnitudeasintheclassicaloxideelectrolytes,ceriagadolinia(CGO)andyttria-stabilisedzirconia(YSZ).Thetransferof oxygen from water is far easier, which makes the accurate determination of the true coefficient of exchange of the molecular oxygen for these materials more complicated and is probably dominated by the presence of residual water. D 2003 Published by Elsevier Science B.V.