Oxygen diffusion and surface exchange studies on (La0.75Sr0.25)0.95Cr0.5Mn0.5O3−δ

Oxygen tracer diffusion coefficient (D^⁎) and surface exchange coefficient (k^⁎) have been measured for (La_0.75Sr_0.25)_0.95Cr_0.5Mn_0.5O_3−δ using isotopic exchange and depth profiling by secondary ion mass spectrometry technique as a function of temperature (700–1000 °C) in dry oxygen and in a water vapour-forming gas mixture. The typical values of D^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 4 × 10^− 10 cm² s^− 1 and 3 × 10^− 8 cm² s^− 1 respectively, whereas the values of k^⁎ under oxidising and reducing conditions at ∼ 1000 °C are 5 × 10^− 8 cm s^− 1 and 4 × 10^− 8 cm s^− 1 respectively. The apparent activation energies for D^⁎ in oxidising and reducing conditions are 0.8 eV and 1.9 eV respectively.     

Oxygen diffusion in SrZrO3

Oxygen diffusion coefficients in SrZrO₃ polycrystals were determined using the isotopic exchange method with ¹⁸O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion (D_vol) and surface exchange (k) coefficients of oxygen in SrZrO₃ polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.     

Transport properties and structural stability of tetragonal CeNbO4+δ

The electrical properties of CeNbO_4+δ have been investigated at 1073–1223 K in the oxygen partial pressure range 10^− 17 to 0.36 atm. The conductivity and Seebeck coefficient behaviour indicates that, at oxygen chemical potentials close to atmospheric, tetragonal CeNbO_4+δ possesses a mixed ionic and p-type electronic conductivity. The ion transference numbers under the p(O₂) gradient of 0.93/0.21 atm, measured by the modified e.m.f. technique, are close to 0.4 decreasing in more reducing environments. The variations of partial ionic and electronic conductivities can be described in terms of the oxygen intercalation into the scheelite-type lattice, which results in increasing concentrations of both dominant charge carriers, oxygen interstitials and holes, when p(O₂) increases. Reduction leads to p(O₂)-independent electrical properties, followed by a drastic decrease in the conductivity at oxygen pressures below 10^− 15–10^− 9 atm due to a reversible transition into the monoclinic phase. Contrary to the zircon-type CeVO_4±δ, no traces of the parent binary oxides were detected in the reduced cerium niobate.     

Enhancement of oxygen surface kinetics of SrTiO3 by alkaline earth metal oxides

The kinetics of oxygen incorporation into semiconducting metal oxides such as strontium titanate are of particular interest for an application as a fast resistive-type oxygen sensor operating at high temperatures (T > 600 °C), e.g. in automotive exhaust gas monitoring.Based on a frequency-domain analysis of the response signal (resistance R) obtained from an electrically contacted sample exposed to a modulated oxygen partial pressure pO₂ in a fast kinetic measurement setup, the surface transfer controlled response behaviour of undoped and Fe-doped SrTiO₃ single crystals with thicknesses in the order of d = 45–125 μm was investigated with regard to the effect of thin PVD layers with alkaline earth (Ca, Sr, Ba) metal oxide compounds.The results of this electrical characterisation are compared with results obtained from ¹⁸O tracer exchange experiments and subsequent depth profile analysis by secondary ion mass spectrometry (SIMS). Both the electrical measurements and the ¹⁸O self-diffusion experiments revealed an enhancement of oxygen surface exchange kinetics in the case of all three alkaline earth metal oxides investigated.     

Study of the oxygen incorporation and diffusion in Sr(Ti0.65Fe0.35)O3 ceramics

For applications in high temperature fast conductometric gas sensors and oxygen membranes, several mixed conductors show promising features. In particular, acceptor-doped strontium titanate (STO) has been widely investigated for an application as a fast conductometric oxygen sensor. By a B-site substitution with 35% iron, the resulting ceramic solid solution SrTi_0.65Fe_0.35O₃ (STF35) exhibits a temperature-independent conductivity, an ideal prerequisite for a gas sensor.In the presented study, the oxygen tracer exchange behavior and the tracer diffusion of dense ceramic STF35 bulks have been investigated in the temperature range between 600 and 900 °C by means of ¹⁸O₂ tracer exchange experiments and subsequent secondary ion mass spectrometry (SIMS), resulting in the determination of k* and D* values, respectively.Furthermore, by coating the samples with a thin alkaline earth metal oxide layer (CaO), a significantly enhanced oxygen surface exchange reaction was observed. These findings are in good agreement with previous results on STO single crystals.     

A static SIMS/TPD study of the kinetics of methoxy formation and decomposition on O/Pt(111)

The kinetics and mechanism of the decomposition of methanol (CH₃OD) on oxygen-covered Pt(111) were studied using static secondary ion mass spectrometry (SIMS) and temperature programmed desorption (TPD). The initial sticking coefficient and the saturation first layer coverage of CH₃OD are unity and 0.36 ML, respectively. The maximum amounts decomposed in TPD on O/Pt(111) and clean Pt(111) are 0.19 and 0.047 ML, respectively. At low methanol coverages (< 0.05 ML) on O/Pt(111) the only reaction products were CO₂, H₂O and D₂O, whereas at saturation CO, H₂O, D₂O and H₂ were observed. The decomposed amount did not saturate at or before the onset of molecular methanol desorption, but increeased monotonically until saturation of the first layer. No oxygen exchange was observed between CH₃OD and preadsorbed ¹⁸O. A decomposition mechanism involving methoxy and hydroxyl type species is proposed. Methanol coverages as low as 0.002 ML could be detected with SIMS. The CH₃⁺ ion was the most intense ion in the positive SIMS spectrum of both methanol and methoxy. Larger ion clusters such as (CH₃OD)_n⁺ (n = 2, 3) developed successively at specific multilayer coverages. At low coverages on O/Pt(111), methoxy formation occurs above 125 K and its decomposition becomes detectable above 150 K during temperature programming. In the isothermal mode, the SIMS CH₃⁺ ion was used to follow the kinetics. Over the temperature range 120–145 K, the second order Arrhenius rate parameters for methoxy formation are E = 5.5±0.7 kcal/mol and A = 1.5×10^−7±0.6 cm²/s·molecule for an initial methanol coverage of 0.05 ML. Methoxy decomposition was studied in the temperature range 150–165 K and at an initial coverage of 0.015 ML. The first order kinetic parameters, E = 11.4±0.5 kcal/mol and A = 5.3×10^13±1 s⁻¹ were derived. Advantages and limitations of using SIMS as a tool for kinetic studies are discussed.     

The interaction of O2 with the surface of polycrystalline gadolinium at the temperature range 300–670 K

Auger-Electron-Spectroscopy (AES) and Direct-Recoils-Spectrometry (DRS) were applied to study the interaction of O₂ with a polycrystalline gadolinium surface, in the temperature range 300–670 K and oxygen pressure up to 2 × 10^? 6 Torr. It has been found that initial uptake of oxygen, at coverage measurable by the techniques used here, results in rapid oxide island formation. The subsurface is believed to be a mixture of oxide particles and oxygen dissolved in the Gd metal, the latter being the mobile species, even at relatively low temperatures.Enhanced inward diffusion of oxygen starts as early as 420 K and dictates the surface oxygen concentration and effective thickness of the forming oxide. The oxygen accumulation rate at the near-surface region, as measured by the O(KLL) AES signal intensity, goes through a maximum as a function of temperature at 420 K. This is a result of the combination of still efficient oxygen chemisorption that increases surface occupation and slow inward diffusion. The thickest oxide, ~ 1.7 nm, is formed at 300 K and its effective thickness was found to decrease with increasing temperature (due to oxygen dissolution into the metal bulk).Diffusion coefficients of the oxygen dissolution into the bulk were evaluated for various temperatures utilizing models for infinitely thin oxide layer and thick oxide layer, respectively. The best fit under our experimental procedure was obtained by the thick layer model, and the coefficients that were calculated are D₀ = 2.2 × 10^? 16m²s^? 1 and E_a = 46kJ/mol.     

Transport properties and phase stability of mixed conducting oxide membranes

At high strontium doping levels, perovskite oxides containing iron have suitable stability and transport properties for use as oxide ion transport membranes. In our studies of these materials, we have investigated the pO₂ and temperature dependence of the conductivity and non-stoichiometry of La_1−xSr_xFe_1−yM_yO_3−δ (M = Cr, Ti) by using electrochemical cells and the thermal expansion by dilatometry. Non-equilibrium behavior is observed in both the chemical expansion data and also in the conductivity and stoichiometry and suggests the occurrence of microscopic phase segregation on reduction. Analysis of the microstructure of quenched samples confirms the occurrence of local phase separation. Bulk diffusion and surface exchange coefficients under near-gradientless conditions have been determined by the electrical conductivity relaxation (ECR) technique and by isotope exchange depth profiling (IEDP). Evaluation of transport under a chemical gradient was accomplished by transient isotopic tracing of operating membranes. The isotope transients (¹⁶O₂–¹⁸O₂) were performed on tubular membranes operating at steady state at temperatures between 1023 K and 1173 K and allow an unambiguous separation of surface and bulk resistances to oxygen permeation under steady state conditions, a separation not possible by permeation measurements alone.     

Oxygen transport model for layered MIEC composite membranes

Surface exchange resistance can reduce the oxygen transport through dense mixed ionic-electronic conducting (MIEC) membranes. Addition of an MIEC surface layer to a base substrate can reduce the surface exchange resistance. Existing oxygen transport relations that consider bulk diffusion and surface exchange resistance are extended to treat coated membranes formed by depositing a highly conductive, thin layer of MIEC on the surface of a dissimilar MIEC substrate and accounting for the solid/solid interfacial resistance. The oxygen flux through the coated membrane may exceed that through the bare membrane only if: 1) the surface exchange coefficient of the added layer is larger than the surface exchange coefficient of the bare membrane; and 2) the solid/solid interfacial resistance is sufficiently small. In general, deposition of the surface layer on the membrane tube surface exposed to lean gas leads to a larger oxygen flux than deposition of the layer on the oxygen rich side. A La_0.5Sr_0.5Fe_0.8Ga_0.2O_3-δ/SrCo_0.8Fe_0.2O_3-δ membrane achieved an oxygen outwards flux of 0.45 mL/min^⁎cm² at 1000 °C from an air/helium gradient. This was a ∼ 50% increase over that obtained using an uncoated LSFG tube.     

Oxygen diffusion studies on (Y2O3)2(Sc2O3)9(ZrO2)89

The oxygen tracer diffusion coefficient (D?) has been measured for 9 mol% scandia 2 mol% yttria co-doped zirconia solid solution, (Y₂O₃)₂(Sc₂O₃)₉(ZrO₂)₈₉, using isotopic exchange and line scanning by Secondary Ion Mass Spectrometry, as a function of temperature. The values of the tracer diffusion coefficient are in the range of 10^? 8–10^? 7 cm² s^? 1 and the Arrhenius activation energy was calculated to be 0.9 eV; both valid in the temperature range of 600–900 °C. Electrical conductivity measurements were carried out using 2-probe and 4-probe AC impedance spectroscopy, and a 4-point DC method at various temperatures. There is a good agreement between the measured tracer diffusion coefficients (D?, Ea = 0.9 eV) and the diffusion coefficients calculated from the DC total conductivity data (D_σ, Ea = 1.0 eV), the latter calculated using the Nernst–Einstein relationship.     

Oxidation of nickel and transport properties of nickel oxide

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at the highest temperature (1673 K), when coarse-grained scale was formed, and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and the chemical diffusion coefficient in Ni_1−yO have also been determined on such coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found, that under such conditions oxidation of nickel follows strictly the parabolic rate law, and the parabolic rate constant of this reaction is the following function of temperature and oxygen pressure: The results of nonstoichiometry measurements, in turn, may be described by the following relationship Finally, chemical diffusion coefficient in Ni_1−yO has been found to be independent on oxygen activity, indicating that the mobility of point defects in this oxide does not depend on their concentration, being the following function of temperature: It has been shown, that the parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data are in excellent agreement with experimentally determined k_p values. All these results clearly indicate that the predominant defects in nonstoichiometric nickel oxide (Ni_1−yO) are double ionized cation vacancies and electron holes and the oxide scale on nickel growths by the outward volume diffusion of cations.     

The influence of chromium on the defect structure and their mobility in nonstoichiometric cobaltous oxide

Defect structure and the mobility of point defects in pure metal deficient cobalt oxide (Co_1−yO) and in Co_1−yO-Cr₂O₃ solid solutions have been studied as a function of temperature (1223-1573 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. It has been shown that the predominant defects in pure and Cr-doped cobaltous oxide are singly ionized cation vacancies, and 3% at of dopant is high enough to fix the concentration of predominant defects in such solid solutions on a constant level being much higher than in pure Co_1−yO. Re-equilibration rate measurements have demonstrated that the chemical diffusion coefficient and thereby the mobility of point defects in pure Co_1−yO is concentration independent, strongly suggesting that in spite of rather high their concentration no interactions and clustering of defects is to be expected. On the other hand, in Cr-doped cobaltous oxide, re-equilibration rate measurements have shown, that in this case the defect structure is more complicated, although singly ionized cation vacancies seem to be still predominant defects.     

Diffusion study of 18O implanted into α-Hf using the nuclear resonance technique

The diffusion of ¹⁸O in α-Hf was studied in the 723–923 K temperature range by using ion implantation and nuclear resonance techniques. The measurements show that the diffusion coefficients follows an Arrhenius behavior D(T)=D₀e^(-Q/RT), where D₀=(5±3)×10^-9 m²s^-1 and Q=162±5 kJ/mol. A comparison of the present results with previous ones is also done. PACS 66:30 Jt; 85:40 Ry     

Synthesis and characterization of CaLaFe11O19

A new ferrimagnetic compound with the chemical formula CaLaFe₁₁O₁₉ has been synthesized by solid state reaction between the respective oxides and their structural, electrical and magnetic properties have been studied. One magnetic Fe³⁺ ion in CaLaFe₁₂O₁₉ is replaced by La³⁺ ion. The crystallographic results show the compound is hexagonal magnetoplumbite. The electrical conductivity has been measured from 300 to 800 K. The activation energy changes at Curie temperature (653 K). The compound is ferrimagnetic from 300 to 653 K and above T_c it acts as a paramagnetic. Variation of inverse molar susceptibility has been measured at various temperature in paramagnetic region and Curie molar constant (C_m) is calculated. AC susceptibility measurements are made at room temperature. The Seebeck coefficient (S) measurements show that the compound is n-type.     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

Absorption spectrum of deuterated water vapor enriched by 18O between 6000 and 9200 cm−1

The absorption spectrum of water vapor enriched by deuterium and oxygen-18 is analyzed in the 6000–9200 cm^?1 region. The spectrum has been recorded at room temperature with a Bruker IFS 120 h Fourier transform spectrometer. More than 14,000 absorption lines were measured in the recorded spectrum. The vibration–rotation assignments were performed on the basis of previously published experimental energies and of variational calculations. Nine water species (H₂¹⁶O, HD¹⁶O, D₂¹⁶O, H₂¹⁸O, HD¹⁸O, D₂¹⁸O, H₂¹⁷O, HD¹⁷O and D₂¹⁷O) were found to contribute to the observed absorption. More than 3600 lines of 19 vibrational bands of D₂¹⁸O and about 4700 lines of 16 bands of HD¹⁸O with J as high as 19 and K_a as high as 11 were assigned. The main part of the HD¹⁸O and D₂¹⁸O lines and all lines of HD¹⁷O and D₂¹⁷O were observed in the laboratory for the first time. The obtained vibration–rotation energy levels are compared with previous experimental studies and the results of variational calculations.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.     

Oxygen permeability and stability of Ba0.5Sr0.5Co0.8Fe0.2O3−δ as an oxygen-permeable membrane at high pressures

Oxygen permeation fluxes across the dense Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) membrane disks were measured under an air/helium oxygen partial pressure gradient at high pressures (up to 10 atm) and various temperatures (973–1123 K). The fabricated BSCFO membrane exhibited good oxygen permeability with a high oxygen permeation flux of 2.01 ml min^− 1cm^− 2 (thickness: 1.37 mm) at 1123 K and 10 atm. Oxygen permeation results were analyzed theoretically using the surface exchange current model. The dependences of the oxygen permeation fluxes on the oxygen partial pressure gradient, suggested that the bulk oxygen ionic diffusion was the rate-limiting step for the overall oxygen permeation process across the BSCFO membrane. The ambipolar diffusion coefficients (D_a), the oxygen vacancy diffusion coefficients (D_v) and the oxygen ionic conductivities (σ_i) of the BSCFO material at different temperatures (973–1123 K) were calculated. It was found that BSCFO possessed high oxygen diffusion coefficients and ionic conductivities, which resulted in the good oxygen permeability of BSCFO. In addition, the BSCFO membrane exhibited good stability of oxygen permeation at 1123 K, while the deterioration of oxygen permeation stability was observed at 1098 K due to structural changes occurring at the surface of the BSCFO membrane disk as demonstrated by XRD.     

The reaction of CH3 + O2: experimental determination of the rate coefficients for the product channels at high temperatures

The reaction of methyl radicals (CH₃) with molecular oxygen (O₂) has been investigated in high-temperature shock tube experiments. The overall rate coefficient, k₁ = k_1a + k_1b, and individual rate coefficients for the two high-temperature product channels, (1a) producing CH₃O + O and (1b) producing CH₂O + OH, were determined using ultra-lean mixtures of CH₃I and O₂ in Ar/He. Narrow-linewidth UV laser absorption at 306.7 nm was used to measure OH concentrations, for which the normalized rise time is sensitive to the overall rate coefficient k₁ but relatively insensitive to the branching ratio of the individual channels and to secondary reactions. Atomic resonance absorption spectroscopy measurements of O-atoms were used for a direct measurement of channel (1a). Through the combination of measurements using the two different diagnostics, rate coefficient expressions for both channels were determined. Over the temperature range 1590–2430 K, k_1a = 6.08 × 10⁷T^1.54 exp (−14005/T) cm³ mol⁻¹ s⁻¹ and k_1b = 68.6 T^2.86 exp (−4916/T) cm³ mol⁻¹ s⁻¹. The overall rate coefficient is in close agreement with a recent ab initio calculation and one other shock tube study, while comparison of k_1a and k_1b to these and other experimental studies yields mixed results. In contrast to one recent experimental study, reaction (1b) is found to be the dominant channel over the entire experimental temperature range.     

Ionic conductivity of brownmillerite-type calcium ferrite under oxidizing conditions

The thermogravimetric and Mössbauer spectroscopy studies showed that, at atmospheric oxygen pressure, the oxygen content in Ca₂Fe₂O₅ brownmillerite is very close to stoichiometric at 300–1270 K. The orthorhombic lattice of calcium ferrite undergoes a transition from primitive (space group Pnma) to body-centered (I2mb) at 950–1000 K, which is accompanied with decreasing thermal expansion coefficient (TEC) and increasing activation energy for the total conductivity, predominantly p-type electronic. The steady-state oxygen permeation through dense Ca₂Fe₂O₅ ceramics is limited by the bulk ionic conduction. The ion transference numbers in air vary in the range 0.002–0.007 at 1123–1273 K, increasing with temperature. Analysis of stereological factors, which may affect oxygen diffusivity, suggests a dominant role of the ion jumps along octahedral and, possibly, tetrahedral layers of the brownmillerite structure. The ionic conductivity of calcium ferrite is higher than that of Ca₂FeAlO_5+δ, but lower compared to the oxygen-deficient perovskite phases based on SrFeO_3−δ where the diffusion pathways form a three-dimensional network. The average TECs of Ca₂Fe₂O₅ ceramics, calculated from dilatometric data in air, are 13.1 × 10^− 6 K^− 1 at 370–950 K and 11.3 × 10^− 6 K^− 1 at 970–1270 K.