Oxygen Self-diffusion Coefficients Research Articles

Volume self-diffusion of oxygen in titanite.

Oxygen self-diffusion coefficients in natural titanite (sphene) single crystals were experimentally determined under hydrothermal conditions at 700° to 900°C and 100 MPa water pressure using an ion microprobe (SIMS). Oxygen self-diffusion parallel to the c-axis is described by an Arrhenius relation with pre-exponential factor D0 = 1–1+5 × 10–8 m2/sec and activation energy Q = 254 ± 28 kJ/mol. Oxygen diffusion is isotropic within the measurement reproducibility of approximately a factor of 2. The relatively low diffusion coefficients for oxygen yield closure temperatures of 450° to 750°C for small crystals of titanite at typical cooling rates, which suggests that useful data on cooling rates of igneous rocks, or of metamorphic episodes, can be obtained because the size of titanite crystals in rocks is normally small.

Study of oxygen ion transport in acceptor doped samarium cobalt oxide

Dense samples of the perovskite Sm1−xSrxCoO3 (where x=0, 0.2, 0.4, 0.5 and 0.6) have been prepared. The samples were18O/16O isotopically exchanged at a variety of temperatures between 500°C and 900°C. Subsequent analysis of the diffusion profile using Secondary Ion Mass Spectrometry (SIMS) allowed the oxygen self-diffusion coefficient, D*, and surface exchange co-efficient, k, to be determined. From the temperature dependence of these parameters the activation energies for both processes were evaluated. The surface coverage of samarium, cobalt and oxygen in SmCoO3 was determined from calibration samples using Low Energy Ion Scattering (LEIS). LEIS was used to determine the effect of strontium doping on the surface composition and the18O-concentration at the surface of a number of the18O-exchanged samples.

Interfacial reactions associated with ceramic ion transport membranes

Assuming ohmic behaviour for the relevant interfacial kinetics a simple equivalent circuit has been used to identify experimentally accessible parameters which may control the oxygen flux through a variety of technological devices. In particular the oxygen surface exchange coefficient (k cm s−1), which can be determined by isotopic exchange measurements is proportional to a characteristic electrode current density (jEA cm−2) which determines the electrode resistance (RE Ωcm2) in solid-state electrochemical systems. For ceramic ion-conducting membranes a characteristic membrane thickness (Lc) at which the change-over from bulk to surface control occurs is shown to be equal to D*/k where D* (cm2 s−1) is the oxygen self-diffusion coefficient in the oxide material. Attention is also drawn to correlations between D and k. It is noted, for example, that the ratio D/k often has a value around 10−2 cm (100 μm) for most AO2 fluorite and ABO3 perovskite oxide materials, which implies that fabricating membranes less than 100 μm thick will not be advantageous unless the value of k can be specifically increased. Mechanisms responsible for correlations between D and k remain obscure and should be a fruitful area for further investigations. Finally, specific examples of materials selection for ceramic fuel cell operation over a wide range of temperatures (450–;1000 °C) are briefly surveyed.

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation: effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (18O) and aluminium (26Al) self-diffusion coefficients in Al2O3 were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped α-alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr2O3.

Raman Spectrometric Determination of the Oxygen Self‐Diffusion Coefficients in Oxides

New methods of determining the oxygen self‐diffusion coefficients (D*o) in oxides have been developed using Raman spectroscopy combined with the 16O–18O exchange technique. From the depth‐profiles of the 18O concentration in the 16O–18O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D*o was determined for 2.8 mol% Y2O3‐containing tetragonal zirconia polycrystall (the depth‐profile method). Thus‐obtained results are expressed as D*O,D‐P= 1.82(+0.41−0.40) × 10−1·exp{−(139.3 ± 0.2) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–950°C. We also determined D*o for the same sample from the Raman spectrometric monitoring of the ambient gas during the 16O–18O exchange reaction (the gas‐monitoring method). Thus‐obtained results are expressed as D*O,G‐M= 1.14(+0.05−0.04) × 10−2 exp{−(117.5 ± 0.4) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D*o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D*o in oxides.

Determination of oxygen self-diffusion in åkermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions

Oxygen self-diffusion coefficients have been measured for three natural diopsidic clinopyroxenes, a natural anorthite, a synthetic magnesium aluminate spinel, and a synthetic åkermanite for oxygen fugacities ranging from the NNO to IW buffers. The experiments employed a gas-solid isotopic exchange technique utilizing 99% 18O-enriched CO-CO2 gas mixtures to control both the oxygen fugacity and the isotopic composition of the exchange reservoir. Diffusion profiles of the 18O tracer were obtained by in-depth analysis with an ion microprobe. The experimental results, fit to the Arrhenius relation D = D0e(−QRT), yield the following: Do (m2 s−1)Q (kJ mol−1)diopside4.3− 3.8+32.6× 10 − 4457 ± 26åkermanite4.7− 4.4+83.5× 10 − 7457 ± 26278 ± 33spinel2.2− 1.8+8.7× 10 − 7404 ± 21anorthite8.4− 8.0+174× 10 − 13162 ± 36At a given temperature, oxygen diffuses about 100 times more slowly in diopside than indicated by previous bulk-exchange experiments (Connolly and Muehlenbachs, 1988). Our data for anorthite, spinel, and åkermanite agree well with prior results obtained by gas-solid exchange and depth profiling methods (Elphick et al., 1988; Reddy and Cooper, 1981; Yurimoto et al., 1989, respectively). Since these other experiments were conducted at different oxygen fugacities, this agreement indicates that diffusion of oxygen in these nominally Fe-free minerals is not greatly affected by fO2 in the range between pure oxygen and the iron-wüstite buffer. However, our diffusion coefficients for anorthite, melilite, and spinel are also uniformly lower than those obtained by bulk analysis of crushed powders at similar temperatures (Muehlenbachs and Kushiro, 1974; Hayashi and Muehlenbachs, 1986; Ando and Oishi, 1974).The oxygen diffusion data are used to evaluate the effects of three different types of thermal histories upon the oxygen isotopic compositions of minerals found in Type B Ca-Al-rich inclusions (CAIBs) in carbonaceous chondrites: 1.(1) gas-solid exchange during isothermal heating,2.(2) gas-solid exchange as a function of cooling rate subsequent to instantaneous heating, and3.(3) isotopic exchange with a gaseous reservoir during partial melting and recrystallization. With the assumptions that the mineral compositions within a CAIB were uniformly enriched in 16O prior to any thermal processing, that effective diffusion dimensions may be estimated from observed grain sizes, and that diffusion in diopside is similar to that in fassaitic clinopyroxene, none of the above scenarios can reproduce the relative oxygen isotopic anomalies observed in CAIBs without improbably long or unrealistically intense thermal histories relative to current theoretical models of nebular evolution. The failure of these simple models, coupled with recent observations of “disturbed” magnesium isotopic abundances and correlated petrographic features in anorthite and melilite indicative of alteration and recrystallization, suggests that the oxygen isotopic compositions of these phases may have also been modified by alteration and recrystallization possibly interspersed with multiple melting events. Because the modal abundance of spinel remains relatively constant for plausible melting scenarios, and its relatively sluggish diffusion kinetics prevent substantial equilibration, Mg-Al spinel is the most reliable indicator of the oxygen isotopic composition of precursor material which formed Type B CAIs.

Is the splitting of the superconducting transition in YBa 2Cu 3O 7− x (x

The oxygen self-diffusion coefficient in the strongly anisotropic process of the isotope exchange of chain sites in YBa 2Cu 3O 7− x (x<0.02) has been measured. The chemical diffusion coefficient was estimated for the process of the oxidation of the nearly stoichiometric 123. It is shown that the oxygen-diffusion rate is large enough to assure a homogeneous oxygen distribution after annealing at about 320°C, 200–300 h in oxygen. Therefore, the splitting of the superconducting transition in 123 cannot be explained by the oxygen inhomogeneity in the grains of this material.

Self Diffusion of Oxygen in PLZT Ceramics

Self diffusion coefficients of oxygen in PLZT ceramics were evaluated to estimate the vacancy structure by the oxygen-18 isotope exchange reaction and the isotope analysis by the secondary ion mass spectroscopy (SIMS). In the piezoelectric composition of PLZT (10/50/50), the oxygen vacancy concentration was estimated as about one ppm/mol by means of oxygen diffusion measurements and it did not change by doping impurities except for manganese. It was considered that point defects existed at all sites of the perovskite structure of PLZT. On the other hand, the oxygen vacancy concentration increased to 100ppm/mol for manganese doped samples, and the reason was considered that manganese ions were reduced during sintering and that the oxygen vacancies were created to compensate the charge imbalance. In the transparent PLZT (9/65/35), the oxygen vacancy was ten times as much as that in PLZT (10/50/50). The increase in oxygen vacancy was ascribed to volatilization of lead or variation of solubility of lanthanum in B site. All samples showed extraordinary oxygen diffusion at high temperatures near the surface, which was ascribed to the volatilization of PbO.

Mixed electronic/ionic conductivity of the solid solution La(1 − x)SrxCo(1 − y)NiyO3 − δ (x: 0·4, 0·5, 0·6 and y: 0·2, 0·4, 0·6)

The effect of Sr and Ni substitution on the mixed electronic/ionic conductivity was investigated in a series of the perovskite-type oxides mLa(1 − x)SrxCo(1 − y)NiyO3 − δ (x: 0·4, 0·5, 0·6 and y: 0·2, 0·4, 0·6) prepared. Electrical conductivity was measured, in air, by a four-point probe technique, within the temperature range 500–1000°C. Oxygen self-diffusion coefficient (D∗), activation energy (Ea), and surface exchange (k) data were obtained, by combining the isotopic 18O/16O exchange diffusion profile (IEDP) technique, at 700, 800 and 900°C, with the secondary ion mass spectroscopy (SIMS) technique for the 18O depth profile measurement. The results derived suggest that the present perovskite-type oxides are excellent mixed conductors, having high electronic conductivity in addition to their high O2− self-diffusivity. Oxygen flux through the material was principally controlled by the surface exchange kinetics. As far as the effect of Sr and Ni substitution on the electrical properties is concerned, a strong interdependence was found between the dopant's content.

Oxygen self-diffusion in rutile under hydrothermal conditions

Oxygen self-diffusion coefficients have been determined for synthetic and natural rutile single crystals under hydrothermal conditions at 100 MPa total pressure and in the temperature range 873–1373 K. The diffusion coefficients are lower than the results from dry gas exchange studies would predict. Between 973 and 1373 K the results can be characterized by two linear Arrhenius relationships. D=1.14×10−11 exp(−168.8 kJ mol−1/RT) m−2s−1 for the natural rutile, and D=2.41×10−12 exp(−172.5 kJ mol−1/RT) m2s−1 for the synthetic crystal. The results have been interpreted in terms of a defect model involving the dissolution of water in rutile as substitutional hydroxyl defects on oxygen lattice sites, with a solution enthalpy in the range 81–106 kJmol−1.

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

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 −6cm 2/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 O 18 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.

Defect structure and oxygen diffusion in UO 2+δ

Coherent diffuse neutron scattering techniques have been used to study disorder in single crystal samples of hyperstoichiometric UO 2+δ (δ = 0.13, 0.14). Above 850 K the crystals have a single phase and the excess anions give rise to diffuse scattering intensity. At 858 K a pattern of elastic diffuse scattering is observed, which is interpreted in terms of uncorrelated simple clusters of defective anions of the type first postulated by Willis. At 1573 K quasielastic energy broadening of the diffuse scattering is observed, indicating the dynamic nature of the disorder. The characteristic distribution of intensity at this temperature resembles that observed in other fluorite compounds in the fast-ion phase. It is possible to estimate the oxygen self-diffusion coefficient at 1573 K, and it is found to be comparable to that in the pure compound some 1000 K higher.

Relation between the oxidation growth rate of chromia scales and self-diffusion in Cr2O3

In most cases, chromia scales are assumed to grow by predominant chromium diffusion. However, results of Atkinson and Taylor indicated that chromium bulk diffusion could not account for the growth rate of chromia scales. Moreover, recent results of Parket al. showed that oxygen diffusion in chromia was faster than chromium diffusion. So, at this date, the controlling process of the growth of chromia scales is not elucidated. To interpret such a phenomenon, oxygen and chromium self-diffusion coefficients in Cr2O3 single crystals and polycrystals were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. Tracers were introduced by ion implantation, thick film methods, and isotopic exchange, using the54Cr,50Cr and18O isotopes. Depth profiling was made by secondary ion mass spectroscopy (SIMS). The bulk diffusion coefficients were computed by using a general solution of the Fick's law taking into account evaporation and exchange at the surface. Grain-boundary diffusion coefficients were computed by using the Whipple-Le Claire equation for type B intergranular diffusion. Lattice and grain boundary self-diffusien coefficients were determined as a function of temperature and oxygen pressure. The diffusion coefficients are lower than results given in the literature and do not depend on the oxygen pressure. Moreover, it is found that oxygen diffusion is faster than chromium diffusion. These results are compared to the oxidation constants of chromia-forming alloys and it is shown that neither lattice self-diffusion, nor grain-boundary self-diffusion can justify the growth rate of chromia scales. Such a situation is compared to NiO case, for which authors found important differences in grain-boundary diffusivity, according to the elaboration mode of NiO (thermal oxidation or growth from the melt).

Anisotropy of oxygen self-diffusion in YBa 2Cu 3O 7−δ single crystals

The oxygen self-diffusion coefficient in YBa 2Cu 3O 7−δ single crystals has been measured along different crystallographic directions at two temperatures. The role of different crystallographic positions of oxygen has been established. A mechanism has been proposed explaining the strong anisotropy of the oxygen diffusion.

Diffusion coefficients of oxygen in Nd 2CuO 4−δ

The chemical diffusion in Nd 2CuO 4−δ was measured by means of a thermomicrobalance in a temperature range of 650 to 900°C under oxygen pressure of 1 to 0.01 atm, using dense polycrystalline specimens. It was found that the relaxation after a step change in oxygen pressure from 0.3 to 0.01 atm was exponential, and the reduction rate coincided with that of oxidation. From this result, the relaxation was concluded to obey diffusion-controlled kinetics, and the chemical diffusion coefficient was determined. The self-diffusion coefficient of oxygen and the oxygen vacancy diffusion coefficient were calculated from the chemical diffusion coefficient. The self-diffusion coefficient of oxygen in Nd 2CuO 4−δ was slightly larger than that in perovskites, and lower than that of YBa 2Cu 3O 6.4. The vacancy diffusion coefficient of Nd 2CuO 4−δ is almost the same as that of perovskite-type oxides.

Oxygen diffusion in quartz: Dependence on temperature and water fugacity

Oxygen self-diffusion coefficients ( D) were determined in single crystals of Brazilian quartz by measuring 18O concentration profiles with an ion microprobe. In the α-quartz field at 450–590°C and 100-MPa confining pressure, for diffusion parallel to c, the Arrhenius relation is described by the parameters D 0 = 2.9 · 10 −5 m 2 s −1 and Q = 243 ± 17 kJ (g-at. O) −1. The diffusion rate was also measured in β-quartz from 5- to 350-MPa confining pressure and at 700°C as a function of water, hydrogen and oxygen fugacities as well as for different proton activities. There is a good correlation of D with water fugacity, no correlation with oxygen nor hydrogen fugacities, and no correlation with proton activity over the range for which this parameter could be fixed independent of the water fugacity, The available evidence suggests that the oxygen-bearing species responsible for the rapid transport of oxygen under hydrothermal conditions is molecular water. The rate controlling step for oxygen diffusion may be either the rate of migration of water molecules, or the rate of oxygen exchange between the water molecules and the quartz structure.

Oxygen transport in superconducting ceramic YBa 2Cu 3O 7 and (BiO 2)Sr 2Ca n − 1 Cu nO 2 n + 2 samples

Electrical d.c. measurements using electronic blocking electrodes of stabilized δ-Bi 2O 3 have been made to find oxygen self-diffusion coefficients ( D ★) in YBa 2Cu 3O 7 and Bi 0.7Pb 0.3SrCa 1.8Cu 2O 8+ x . Experiments on 18O- 16O isotopic exchange at various temperatures have been carried out in YBa 2Cu 3O 7 followed by secondary ion mass spectroscopy to determine the penetration profile and hence the self-diffusion coefficient. A survey of the oxygen diffusion data for YBa 2Cu 3O 7 obtained by other authors using different experimental techniques is also provided.

Oxygen self-diffusion in diopside with application to cooling rate determinations

The kinetics of oxygen self-diffusion in a natural diopside have been measured over the temperature range 700–1250°C. Experiments were run under hydrothermal conditions using 18O-enriched water. Profiles of 18O/( 16O+ 18O) versus depth into the crystal were obtained using an ion microprobe. At 1000 bars (100 MPa) confining pressure, the Arrhenius relation for diffusion parallel to the c crystallographic direction yields a pre-exponential factor ( D 0) = 1.5 × 10 −6 cm 2/s and an activation energy ( Q) = 54 ± 5 kcal/g-atom O (226 kJ/g-atom O) over the temperature range of the experiments. Diffusion coefficients parallel to the c crystallographic direction are ≈ 100 times greater than perpendicular to c. The oxygen self-diffusion coefficient obtained for diopside is ≈ 1000 times less than that for diffusion in feldspars, and ≈ 100 times less than that for quartz at 800°C, transport parallel to the c axis. Closure temperatures calculated for oxygen diffusional exchange in natural diopside are significantly higher than for quartz or feldspars. Measurable oxygen isotope exchange in diopside by diffusion would require geological settings with very high temperatures maintained for very long durations. The oxygen diffusional exchange kinetics in diopside presented in this paper find important applications in studies of meteoric hydrothermal circulation systems and the time-temperature history of high-grade regionally metamorphosed terrains. Examples considered include the Outer Unlayered Gabbro, Cuillins Gabbro Complex, Isle of Skye, Scotland, and the granulite-grade Turpentine Hill Metamorphics near Einasleigh, Queensland, Australia.

Oxygen self-diffusion in non-stoichiometric rutile TiO2$minus;x at high temperature

Abstract Oxygen self-diffusion coefficients, D 0 ∗ , were determined in TiO 2− x at 1373 K (8.4 × 10 −3 ⩾ mx ⩾ 5 × 10 −4 ). D 0 ∗ increase with increasing non-stoichiometry. It is concluded that oxygen vacancies contribute 20% to the non-stoichiometry at x=8.4×10 −3 and it is suggested that both singly and doubly ionized vacancies are present in presetn in the aproximate the ration of 1–2. The interpretation of the results also indicate that the complete description of defect structure includes interstitial titanium ions with three positive effective charges as major defects for the more reduced oxide whereas interstitial titanium ions with four effective charges may become important in almost stoichiometric TiO 2 .

Contrasting oxygen diffusion in nepheline, diopside and other silicates and their relevance to isotopic systematics in meteorites

Oxygen self-diffusion coefficients have been determined in natural nepheline Na 3(Na, K)[Al 4Si 4O 16] and synthetic diopside CaMg(SiO 3) 2, at one atmosphere, by monitoring the rate of 18O exchange between CO 2 gas and minerals of pre-determined grain size (10, 12.5, and 15 μm radius). Oxygen self-diffusion in nepheline between 1000° and 1300°C fits a linear Arrhenius relation of the form: D = 5.9 × 10 −9 exp[(−25.0 ± 2.5) × 10 3 cal RT ] ; and for diopside between 1150° and 1350°C: D = 6.3 exp[(−96.7 ± 5.8) × 10 3 cal RT ] . The activation energy for oxygen diffusion in nepheline is similar to that of melilite (31.9 ± 0.1 kcal/mol), but the oxygen diffusivity is lower by two orders of magnitude. The diffusion of oxygen in diopside is similar to that of nepheline at 1375°C; however, the activation energy is almost four times higher. This contrast suggests a difference in the crystal chemical and structural controls of the minerals. Both nepheline and melilite are characterized by strongly distorted alkali sites within their frameworks, which could account for their relatively low activation energies and high diffusivities. More importantly, nepheline and melilite have sheet-like structures and high anionic porosities which may provide ready migration paths for oxygen. The concentration of open space between the silicate sheets in melilite may explain the greater diffusivity of oxygen in melilite than in nepheline. Diopside, forsterite, anorthite, spinel are characterized by high activation energies (80 to 100 kcal) and are refractory with very stable bonds and low anion porosities. Furthermore, these minerals are much denser and closer packed than nepheline or melilite making it difficult for oxygen to migrate within the crystalline structure and have access to the cation-oxygen bonds. Oxygen diffusion studies constrain interpretations of post-solidus oxygen exchange among minerals and may be used to explain the stable isotope systematics observed in Ca-Al rich inclusions of carbonaceous chondrites. The slow exchange and diffusion of oxygen in diopside suggests that the isotopic composition of clinopyroxenes in Ca-Al rich inclusions of carbonaceous chondrites has not changed appreciably since their incorporation into the meteorite. However, minerals such as nepheline and melilite, which readily exchange oxygen, record the isotopic composition of the last nebular gas to which they were exposed.