Distribution Of 18O Research Articles

Large, Negative Atmospheric Carbon Monoxide Clumped Isotope Values Result From Kinetic Isotope Fractionation, Tracing OH• Reactivity

AbstractBecause of its global abundance and reactivity with hydroxyl radicals (OH•), tropospheric carbon monoxide indirectly impacts the lifetimes of other OH•‐reactive gases, in particular methane and reactive hydrocarbons. The origin and chemistry of atmospheric CO have been studied using stable isotopes. Both 13CO and C18O undergo isotopic fractionation during its main chemical loss reaction, CO + OH•. The kinetic isotope effect (KIE) for 13CO is mass dependent, with a value of ∼5‰; 12CO reacts faster than 13CO with OH. Whereas C18O + OH• exhibits an inversely mass dependent KIE ∼−10‰. We hypothesize these KIEs result in a relative depletion of 13C18O, a CO clumped isotope. To test this, we collected CO from air samples on Long Island, NY, and discovered a −3 to −8‰ difference in the clumped isotope ratio, Δ31, relative to a random distribution of 13C and 18O in CO. A clear negative trend between [CO] and Δ31 is driven by two factors: (a) the atmospheric addition of CO from either a primary or secondary source with a Δ31 of ∼0‰ and (b) the continuing reaction of CO with OH•, leaving the remaining CO pool relatively depleted in 13C18O. This is analogous to the mechanism that determines CO Δ17O values. This study is among the first to show clumped isotope fractionation resulting from atmospheric chemistry and not thermal equilibration, which may inform the identification of clumped isotope KIEs in other atmospheric trace gases. These first Δ31 observations motivate future experimental and observational studies targeted at characterizing the clumped isotopes of CO sources, background CO, and experimentally fractionated CO.

Isotopic Tracer Study of Initiation of Porosity in Anodic Alumina Formed in Chromic Acid.

In this paper, we focused on the initiation of porosity in the anodic alumina under galvanostatic conditions in chromic acid, using an 18O isotope tracer. The general concept of the initiation and growth of porous anodic oxide films on metals has undergone constant development over many years. A mechanism of viscous flow of the oxide from the barrier layer to the pore walls has recently been proposed. In this work, two types of pre-formed oxide films were analysed: pure Al2O3 formed in chromic acid, and a film containing As ions formed in a sodium arsenate solution. Both were anodized in chromic acid for several different time durations. Both pre-formed films contained the oxygen isotope 18O. The locations and quantities of 18O and As were analysed by means of ion accelerator-based methods supported by transmission electron microscopy. The significant difference observed between the two oxide films is in the 18O distribution following the second step of anodization, when compared with galvanostatic anodization in phosphoric or sulfuric acid reported in previous works. From the current experiment, it is evident that a small amount of As in the pre-formed barrier layer appears to alter the ionic conductivity of the film; thus, somehow, it inhibits the movement of oxygen ions ahead of advancing pores during anodization in chromic acid. However, anodising pure alumina film under these conditions does not enhance oxygen movement within the oxide layer. In addition, the tracer stays in the outer part of the growing porous oxide film. A lower-than-expected value for pure alumina enrichment in 18O in the pre-formed films suggests, indirectly, that the pre-formed film may contain hydrogen species, as well as trapped electrons, since no Cr is detected. This may lead to the presence of space charge distribution, which has a dual effect: it both retards the ejection of Al3+ ions and prevents O2- ions from migrating inward. Thus, the negative- and positive-charge distributions might play a role in the initiation of pores via a flow mechanism.

High Temperature Oxidation of Zircaloy-4 Under Conditions Simulating a Loss of Cooling Accident in Spent Fuel Pools Examined with Raman Imaging and 18o Tracer Techniques

Since the Fukushima accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, for different reasons, degradation of the fuel rods induced by oxidation in air-steam mixtures after dewatering is a major safety concern: (i) the cladding material is the only barrier left against fission product dissemination, (ii) the heat released by the oxidation reactions can overcome the residual decay heat of the fuel and may become the main driving force to the accident escalation, (iii) hydrogen production is a strong concern because of the presence of steam, (iv) presence of air in the atmosphere is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation mechanism.In this study, high temperature oxidation tests in oxygen, air and air - steam atmospheres were performed with Zircaloy-4 cladding tubes and plates specimens. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was also considered. The tests described in this study were restricted to the 700 – 950°C range, and mostly performed at 850°C. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations.We first used new opportunities given by Raman imaging to analyze, at least qualitatively, the structural and mechanical phenomena involved in this particular corrosion phenomena. Indeed, this method can provide information on the nature of phases that are present in the scale and on the stress level with sub-micron spatial resolution. The different methods used to extract this information that is present in the Raman data will be described and discussed.Moreover, isotopic substitution was also considered. Actually, Raman spectroscopy has also the ability to quantitatively extract the 18O distribution in a zirconia oxide scale after a two stage experiment with a fair accuracy. Raman imaging gave clear evidence for different characteristic distributions of 18O in the scales. Some of them have been correlated with the development of cracks and porosity in the oxide which allows the corroding medium to penetrate locally in the scales. Nitrogen appears to have no or limited influence on the oxygen diffusion, but is observed to reach the metal/oxide interface much faster than oxygen. Thus, we also emphasize the potential of this method to investigate the 18O distribution in such complex corrosion scales.

Seeking the Oxidation Mechanism of Debris in the Fretting Wear of Titanium Functionalized by Surface Laser Treatments

Surface laser treatment (SLT) using nanosecond IR lasers has been shown to improve the tribological behaviour of titanium. Here, we studied the fretting wear of SLT-functionalized pure titanium in a mixture of reactive gases O2 (20 vol.%) + N2 (80 vol.%). The contact geometry was a ball on a plane and the ball was made of bearing steel. The very small amplitude of relative displacement between reciprocating parts in fretting wear makes the evacuation of wear particles difficult. Moreover, the oxidation mechanism of the debris depends on the accessibility of the surrounding atmosphere to the tribological contact. This work focused in the analysis of debris generation and oxidation mechanisms, and sought to differentiate the role of oxygen forming part of the ambient O2 + N2 gas mixture from oxygen present in the surface layer of the SL-treated titanium. Before the fretting test, the surface of the commercially pure titanium plates was treated with a laser under a mixture of O2 + N2 gases with oxygen enriched in the 18O isotope. Then, the fretting tests were performed in regular air containing natural oxygen. Micro-Raman spectroscopy and ion beam analysis (IBA) techniques were used to analyse the TiO2 surface layers and fretting scars. Iron oxide particles were identified by Raman spectroscopy and IBA as the third body in the tribological contact. The spatial distribution of 18O, Ti, 16O and Fe in the fretting scars was studied by IBA. The analysis showed that the areas containing high concentrations of Fe displayed also high concentrations of 16O, but smaller concentrations of 18O and Ti. Therefore, it was concluded that tribological contact allows the oxidation of iron debris by its reaction with ambient air.

Visualization of oxygen storage process in Pd/CeO2-ZrO2 three-way catalyst based on isotope quenching technique

Elucidation of oxygen dynamics and transport in oxygen storage component of three-way catalyst (TWC) is a crucial issue for clarifying the reaction mechanism and precise kinetic modeling. Herein, we report a direct visualization of the oxygen storage process in a Pd/CeO2-ZrO2 (CZ) via a combined method of oxygen isotope labeling and reaction quenching. 18O2 is stored in a model TWC comprising a dense CZ substrate with Pd at 600 °C, the catalyst is quenched, and the 18O distribution in the surface and bulk is visualized by secondary-ion mass spectrometry with a 100-nm-scale spatial resolution. The results show that the presence of Pd enhances both the surface 18O concentration and the 18O diffusion depth into the CZ bulk. 18O concentration on the Pd surface is higher near the Pd/CZ interface, indicating that oxygen adsorption near the Pd/CZ interface is preferred to that close to the Pd center. The promoting effect of Pd on oxygen vacancy formation in CZ is supported by density functional theory calculations. In addition, the comparison between the 18O distribution and oxygen release/storage simulation based on diffusion equation provides the local oxygen release/storage rates. These are of the same order of magnitude and qualitatively consistent with the trend of conventional oxygen storage capacity measurements. The visualization technique reported here is a promising tool for directly investigating the oxygen dynamics and transport at the metal/oxygen storage material system from both qualitative and quantitative aspects.

Oxygen Isotope Tracing Study to Directly Reveal the Role of O2 and H2O in the Photocatalytic Oxidation Mechanism of Gaseous Monoaromatics.

O2 and H2O influence the photocatalytic oxidation mechanism of gaseous monoaromatics, but still in an unclear manner, due to the lack of direct evidence. Tracing an oxygen atom from 16O2 and H218O to intermediates can clarify their roles. The low H218O content suppressed the formation of benzenedicarboxaldehydes during the oxidation of xylenes and 16O2 greatly affected the yield of total intermediates, while neither of them altered the percentage order of the products. Methylbenzaldehydes, methylbenzyl alcohols, and benzenedicarboxaldehydes possessed greater 16O percentage (≥69.49%), while higher 18O distribution was observed in methylbenzoic acids and phthalide (≥59.51%). Together with the interconversion results of the products revealed, 16O2 determined the transformation of xylenes initially to methylbenzaldehydes and then to methylbenzyl alcohols or benzenedicarboxaldehydes, while H218O mainly contributed to conversion of methylbenzaldehydes to methylbenzoic acids or phthalide. Further interaction sites of xylene and its products with H2O and O2 were confirmed by molecular dynamics calculations. The same roles of 16O2 and H218O in the degradation of toluene, ethylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene were also verified. This is the first report that provides direct evidence for the roles of O2 and H2O in the photocatalytic oxidation mechanism of gaseous monoaromatics. These findings are helpful to achieve controllable product formation from the oxidation of monoaromatics and predict their migration process in the atmospheric environment.

Atomic-level insight into effect of substrate concentration and relative humidity on photocatalytic degradation mechanism of gaseous styrene

Substrate concentration and relative humidity (RH) impact the photocatalytic efficiency of industrial aromatic hydrocarbons, but how they influence intermediate formation and degradation pathway remains unclear. With the help of oxygen isotope tracing method, the effects of these two environmental parameters on degradation mechanism of styrene were revealed at atomic level. Increasing styrene concentration favored product formation, which was however inhibited by RH elevation. Gaseous products were not directly formed in gaseous phase, but originated from desorption of interfacial intermediates. The volatile aldehydes and furans further exchanged their 16O with 18O in H218O. Increase of RH showed higher enhancement on 18O distribution in all products and pathways than that of substrate concentration. Low RH preferred high generation of 16O2•- and (16)1O2, dominating reaction to form 1-phenyl-1,2-ethandiol, 2-hydroxy-1-phenyl-ethanon and phenylglyoxal monohydrate in sequence. Successive production of benzyl alcohol, benzaldehyde and benzoic acid through the reaction of styrene with promoted •18OH by increasing RH became predominant. Hydration was firstly observed and confirmed as an important gaseous transformation step of aldehyde and furan products. Our findings provide a deep insight into photocatalytic degradation mechanism of aromatic hydrocarbons regulated by environmental parameters to further improve their industrial purification efficiency, and are helpful predicting environmental geochemistry fate of organics and preventing their negative impact on natural environment.

Electrochemical Reaction Mechanism of LSM-YSZ Composite Cathode Based on 3D Simulation of Oxygen Diffusion and Oxygen Labeling Experiment

Electrochemical reaction mechanism of an LSM-YSZ composite cathode is investigated using a combined measurement of oxygen labeling experiment and 3D microstructural reconstruction. First, oxygen isotope gas 18O2 was exposed to the LSM-YSZ cathode at 750 °C and a current density of 0.052 A/cm2. After 2 minutes of 18O2 supply, the gas was switched to water-cooled He to quench the cell and fix the incorporated isotope at the operating condition. After the oxygen labeling experiment, the cell piece was filled with an epoxy resin and polished to prepare the sample. Three-dimensional microstructure was obtained using a focused ion beam-scanning electron microscopy (FIB-SEM). Furthermore, between the measurement of the FIB-SEM, secondary ion mass spectroscopy (SIMS) analysis was conducted at the middle plane of the observed volume to correlate 18O distribution with the 3D microstructure. Based on percolation analysis of the microstructure, several isolated LSMs showed quite low 18O concentration, whereas some isolated LSMs showed 18O incorporation. For a further understanding of the correlation between the 18O distribution and microstructure, 3D numerical simulation using the reconstructed microstructure was carried out. First, potential distribution was obtained by solving gas diffusion and charge transportations which are coupled with the reaction at the reaction sites using the lattice Boltzmann method. Then, 18O distribution was predicted by solving oxygen diffusion along the calculated potential field. The electrochemical reaction mechanism was discussed by comparing the results of numerical simulation and experiment.

Electrochemical Reaction Mechanism of LSM-YSZ Composite Cathode Based on 3D Simulation of Oxygen Diffusion and Oxygen Labeling Experiment

Electrochemical reaction mechanism of an LSM-YSZ composite cathode is investigated using a combined measurement of oxygen labeling experiment and 3D numerical simulation using a reconstructed microstructure. First, an oxygen labeling experiment was conducted using 18O2 at 750°C under polarized condition. Then, a three-dimensional microstructure was obtained using a focused ion beam-scanning electron microscopy (FIB-SEM). Secondary ion mass spectroscopy (SIMS) analysis was conducted at the middle plane of the measured volume to correlate 18O distribution with the 3D microstructure. Using the 3D microstructure, electrochemical simulation was carried out considering the oxygen potential dependence of LSM ionic conductivity. Then, 18O transport was solved considering the balance between diffusion by concentration gradient, ionic current transport, and production by reaction current. The electrochemical reaction mechanism was discussed by comparing the numerically solved 18O concentration with that obtained from oxygen labeling experiment.

Identifying hydrological conditions of the Pihe River catchment in the Chengdu Plain based on spatio-temporal distribution of 2H and 18O

Stable isotope composition in precipitation, river water, and groundwater was used to investigate the hydrological conditions in the Pihe River catchment of Southwest China. Significant seasonal isotopic variations exist in precipitation and river water, and there is 1-month delay in the river water response to precipitation. The river water is gradually enriched under the influence of evaporation. However, due to the influence of recharge from external water source, the evaporation ratios do not strictly increase with the flow distance. The seasonal isotopic values in groundwater fluctuate the least. River water supplies groundwater more than precipitation in dry season.

Raman study of oxide layers on zirconium alloys using 18O tracers

ABSTRACT In this work, samples of zirconium alloys Zr1Nb and Zircaloy-4W were subjected to two-stage oxidation experiments – at first in H2 16O solution and then in H2 18O steam. Raman spectroscopy was used to analyse distribution of 18O isotope in corrosion layers of the alloys. Layered distribution of 18O was observed in case of Zr1Nb samples; Zircaloy-4W had 18O present only at metal/oxide interface. This observation is attributed to differing kinetic transitions, which influence transport of oxygen through oxide layer.

High Temperature Oxidation of Zircaloy-4 Under Conditions Simulating a Loss of Cooling Accident in Spent Fuel Pools Examined with Raman Imaging and 18O Tracer Techniques

Since the Fukushima accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, for different reasons, degradation of the fuel rods induced by oxidation in air-steam mixtures after dewatering is a major safety concern: (i) the cladding material is the only barrier left against fission product dissemination, (ii) the heat released by the oxidation reactions can overcome the residual decay heat of the fuel and may become the main driving force to the accident escalation, (iii) hydrogen production is a strong concern because of the presence of steam, (iv) presence of air in the atmosphere is known to be an aggravating factor because of the “catalytic” role of nitrogen on the oxidation mechanism. In this study, high temperature oxidation tests in oxygen, air and air - steam atmospheres were performed with Zircaloy-4 cladding tubes and plates specimens. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was also considered. The tests described in this study were restricted to the 700 – 950°C range, and mostly performed at 850°C. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations. We first used new opportunities given by Raman imaging to analyze, at least qualitatively, the structural and mechanical phenomena involved in this particular corrosion phenomena. Indeed, this method can provide information on the nature of phases that are present in the scale and on the stress level with sub-micron spatial resolution. The different methods used to extract this information that is present in the Raman data will be described and discussed. Moreover, isotopic substitution was also considered. Actually, Raman spectroscopy has also the ability to quantitatively extract the 18O distribution in a zirconia oxide scale after a two stage experiment with a fair accuracy. Raman imaging gave clear evidence for different characteristic distributions of 18O in the scales. Some of them have been correlated with the development of cracks and porosity in the oxide which allows the corroding medium to penetrate locally in the scales. Nitrogen appears to have no or limited influence on the oxygen diffusion, but is observed to reach the metal/oxide interface much faster than oxygen. Thus, we also emphasize the potential of this method to investigate the 18O distribution in such complex corrosion scales.

Direct Visualization of Oxygen Incorporation for Oxygen Reduction on Porous Mixed-Conducting LaSrCoO3 Cathode

Oxygen incorporation for the oxygen reduction reaction at a porous mixed-conducting LaSrCoO3 (LSC) electrode was experimentally investigated using technique of oxygen isotope (18O) labelling coupled with subsequent quenching and secondary ion mass spectrometer (SIMS) imaging. The isotopic oxygen exchange experiments of porous LSC cathode were conducted under current density conditions of 0 (OCV), 0.05 and 0.24 A cm−2. Distribution images of incorporated 18O inside solid oxide microstructures and analyzed concentration profiles along the LSC cathode thickness are presented. The 18O distribution image and concentration profile under an applied current density clearly reflect extension of active reaction zone over surface area of LSC, as well as distribution of oxygen chemical potential in the porous LSC cathode. The non-uniform profiles under influence of applied currents revealed that oxygen chemical potential distribution in porous mixed-conducting LSC cathode strongly depends on an applied current density. The analyzed profiles also showed a significant interfacial barrier to the oxygen diffusion across the interface between the GDC interlayer and the YSZ electrolyte due to the unfavorable diffusion of Sr from the LSC to the GDC/YSZ interface. The present results provide the first experimental evidence of oxygen chemical potential distribution in the porous mixed-conducting LSC cathode.

Water vapor interaction with borosilicate glass

Hydration of borosilicate glass (Corning Pyrex glass) in isotopic water H218O vapor was performed at different temperatures. The depth distributions of 1H and 18O in the borosilicate glass measured by the Time-of-Flight Secondary Ion Mass-Spectrometry (TOF-SIMS) were found to be drastically different. The activation energies for the 18O diffusion and for the glass hydrogenation were measured experimentally. The depth distribution of the 18O isotope was described well by the complementary error function, whereas the depth distribution of hydrogen represented the so-called “S-curve”. A similar hydrogen distribution was observed earlier in hydrated natural aluminosilicate glasses (obsidians). This led us to the conclusion that the hydrogenated layer was formed due to a solid state chemical reaction between protons and the glass network. The protons were the result of another solid state chemical reaction between the absorbed water molecules and glass network occurring in a top surface layer (<30 nm).

Particle-Scaled Visualization of Active Sites in LSM/ScSZ Composite Cathode of SOFC through Oxygen Isotope Labeling

Introduction Composite electrodes such as LSM/ScSZ or LSM/YSZ are widely used as a cathode material of SOFC. In these electrodes, both triple phase boundary (TPB) consisting of LSM, ScSZ, and gas phases and double phase boundary (DPB) between LSM and gas phases are considered to work as an active site for oxygen reduction.1 Length of TPB, surface area of DPB, and spatial distribution of those have a great impact on electrochemical performance of cathode. Such kind of geometrical information can be investigated by focused-ion beam scanning electron microscope (FIB-SEM) while almost no information about active reaction site distribution or oxide ion path in composite cathode has been obtained so far. Atomic labeling using isotope oxygen is a powerful tool to investigate oxide ion flow in SOFC materials, and has been mainly applied to pattern electrode experiments to investigate reaction paths at anode or cathode.2 Here, we first conduct particle-scaled visualization of active site in composite LSM/ScSZ cathode by oxygen isotope labeling. Methods, Results, and Discussions A YSZ electrolyte-supported cell with Ni/YSZ anode and LSM/ScSZ cathode was fabricated for the experiment. Wight ratio of LSM and ScSZ is 1:1. In order to quench a reaction in a SOFC cell within a short time during operation at high temperature, a power generation equipment with a nozzle for a helium impinging jet, which is covered by a water cooling jacket, was prepared. At first, the cell was operated in 50% H2-50% Ar for the anode side and in pure 18O2 for the cathode side at 1073 K for 3 minutes. After that, the cell was cooled to below 423 K within 1.5 sec. by the helium impinging jet. The 18O distribution in cross section of the cathode was obtained by secondary ion mass spectroscopy (SIMS) with a spatial resolution of 50 nm (NanoSIMS 50L, CAMECA). In this work, 18O concentration mappings of an interface between LSM/ScSZ cathode and YSZ electrolyte under OCV and 0.11 A/cm2 were obtained. The mappings of LSM (red) and ScSZ or YSZ (green) and of 18O concentration denoted by the ratio of 18O-/(16O- + 18O-) under 0.11 A/cm2 are shown in Figures 1(a) and 1(b), respectively. At cathode part, 18O diffuses to only ScSZ phase uniformly under OCV condition, which is driven by the isotopic oxygen concentration difference. On the other hand, relatively high 18O intensity (0.2-0.25) is observed at LSM particles near the cathode/electrolyte interface under 0.11 A/cm2 as shown in Fig. 1(b). This means that 18O diffuses into LSM particles only near the cathode/electrolyte interface. Isotopic exchange experiment of LSM-patterned cathode on YSZ was conducted by Horita et al.2 Their work disclosed that diffusion of oxide ion into LSM bulk under the higher cathodic polarization is promoted than that under the lower. This is because under the higher cathodic polarization, charge transfer currents increase and active sites are expanded from TPB to DPB. From Fig. 1(b), more charge transfer currents are generated at LSM particles in the region within about 3µm from the cathode/electrolyte interface than at other regions. This result gives the first direct evidence that reaction sites near the cathode/electrolyte interface are more electrochemically active than those at other regions, which has been predicted by electrochemical measurements and numerical simulations.3 Acknowledgements SIMS analysis was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Study of Scale Formation on Al2O3-Scale Forming Alloys by Time-of-Flight Secondary Ion Mass Spectrometry (TOF–SIMS)

Abstract Al2O3-scale formation on high temperature Ni-base and Fe-base structural alloys is a subject of significant technological and scientific interest. Such scale formation involves either (or both) inward oxygen grain boundary diffusion and new oxide growth at the scale/metal interface, or outward aluminum grain boundary diffusion and new oxide growth at the scale/gas interface. The details of scale growth have been studied using a commercial Kanthal A1 alloy and so-called double oxidation experiments—growth of an Al2O3 scale in 16O2, followed by further oxidation for twice the 16O2 duration in 18O2—at temperatures between 1000 and 1400°C. A time-of-flight secondary ion mass spectrometer (TOF-SIMS) with isotopic imaging resolution of ≈100 nm was used for determination of the 16O and 18O distributions in the scale. A focused primary Ga+ ion beam was used for imaging the oxygen isotopes in the negative polarity mode, and a de-focused Xe+ beam was used for sputtering. One novel aspect of this work was the use of taper sections to compensate for the sub-micrometer scale thickness of this oxidation-resistant alloy.

High temperature oxidation kinetics of amorphous silicoboron carbonitride monoliths and silica scale growth mechanisms determined by SIMS

The highly dense, amorphous silicoboron carbonitride (Si-B-C-N) monoliths provide superior oxidation resistance above 1600°C, but the in-depth oxidation kinetics are unknown. Here oxidation kinetics of amorphous Si-B-C-N monoliths was investigated by one-step oxidation (in flowing 16O-rich air) over 1500–1700°C. Coincidently, the growth mechanisms of oxide scales on ceramic surfaces were studied by oxidizing ceramics in 18O-rich air followed by 16O where 16O was used as an equivalent isotopic tracer and the distributions of 16O and 18O in oxide scales were determined directly by secondary ion mass spectrometry (SIMS). The thickness changes of silica scales fit well to parabolic rules at 1500°C (32.5μm2h−1) and 1600°C (86.1μm2h−1) within 16h, but are irregular at 1700°C. The calculated Arrhenius activation energy is ca. 116kJmol−1 for 1500–1600°C/16h oxidation. The relative concentration profiles of 16O from SIMS analysis indicate growth of oxide scales is mainly by oxygen lattice diffusion inside scales during oxidation of the amorphous Si-B-C-N ceramics.

High-Temperature Oxidation of Zircaloy-4 in Air Studied with Labeled Oxygen and Raman Imaging

High-temperature oxidation of zirconium-based cladding materials is expected to be the primary cause of the fuel assemblies’ degradation in spent fuel storage pool loss-of-cooling accidents. Here, high-temperature Zircaloy-4 oxidation has been studied through two-stage oxidation experiments using 18O isotope. Particular attention is paid to the effect of a low-temperature pre-oxidation scale which aims to simulate the corrosion scale existing on spent fuel cladding. Raman imaging was used to investigate the 18O distribution in the scales exposed either to 18O2 or to 18O2 + N2 atmosphere at high temperature. Results were assessed against more conventional SIMS mapping. The use of the 18O isotope tracer technique associated with micro-Raman mapping of the scales is demonstrated to be a powerful method to investigate the transport properties in the scales and will help to gain understanding of the kinetic differences between different pre-oxides.

Natural Isotope Abundance in Metabolites: Techniques and Kinetic Isotope Effect Measurement in Plant, Animal, and Human Tissues.

The natural isotope abundance in bulk organic matter or tissues is not a sufficient base to investigate physiological properties, biosynthetic mechanisms, and nutrition sources of biological systems. In fact, isotope effects in metabolism lead to a heterogeneous distribution of 2H, 18O, 13C, and 15N isotopes in metabolites. Therefore, compound-specific isotopic analysis (CSIA) is crucial to biological and medical applications of stable isotopes. Here, we review methods to implement CSIA for 15N and 13C from plant, animal, and human samples and discuss technical solutions that have been used for the conversion to CO2 and N2 for IRMS analysis, derivatization and isotope effect measurements. It appears that despite the flexibility of instruments used for CSIA, there is no universal method simply because the chemical nature of metabolites of interest varies considerably. Also, CSIA methods are often limited by isotope effects in sample preparation or the addition of atoms from the derivatizing reagents, and this implies that corrections must be made to calculate a proper δ-value. Therefore, CSIA has an enormous potential for biomedical applications, but its utilization requires precautions for its successful application.

Oxygen diffusion and exchange in dolomite rock at 700 °C, 100 MPa

In contact-metamorphic environments dolomite commonly breaks down to calcite + periclase + CO2 as a result of the infiltration of H2O. The transport and exchange of oxygen in dolomite rock during the breakdown reaction were examined experimentally by reacting a cylindrical core of Reed Dolomite with isotopically enriched water having the composition HD18O0.516 O0.5 at 700 °C and 100 MPa for 29 days. Reaction products formed along grain boundaries, fractures, and on the outside surface of the core. Some dolomite grains became enriched in Fe as a result of replacement of the host dolomite. Extensive voids are found in the grain boundaries as a result of the ~25% loss in solid volume during reaction. There are also pores, ~1 μm in diameter, in the dolomite, notably in the vicinity of the replaced dolomite. The distribution of 18O in the dolomite and reaction products was used as a tracer of the transport and exchange of O during reaction. Electron probe microanalysis (EPMA) and secondary ion mass spectrometry (SIMS) analyses showed pervasive infiltration of fluid along grain boundaries and fractures, growth and isotopic exchange with products of reaction, and diffusion of 18O into dolomite grains. The fluid infiltrated efficiently along grain boundaries to the dolomite grain surface. The host dolomite shows an enrichment in 18O along grain boundaries, indicating a diffusive exchange with the fluid. An estimate of the diffusion coefficient of oxygen in dolomite, determined from ion probe spot analyses, gives D ≈ 1 × 10−12 mm2/s. This value is comparable to the oxygen diffusion coefficient for calcite in an H2O-rich fluid. Mass balance of O in the experiment (including dolomite–fluid exchange, the amounts of neomorphic reaction products, and the fluid components) indicates that the reaction products have a 18O concentration only about half that of the fluid. Ion probe spot analyses of calcite from both the center and the edge of the core have the fraction F = 18O/(18O+16O) of 0.14 ± 0.1, whereas the value calculated for the fluid is 0.31. The measured F values of calcite are intermediate between the initial F values of starting water and dolomite, indicating that the reaction products record a mix of both dolomite- and fluid-derived oxygen. The products reached about 45% of isotopic equilibrium, similar to the extent of the mineral–fluid reaction. The Fe-rich, replacement dolomite near the core edge has an elevated value of F = 0.02 ± 0.002, 10 times the value of F ≈ 0.002 for unreacted dolomite, but less than the value for the calcite. The distribution of 18O in the minerals indicates that the breakdown and replacement reactions were faster than O diffusion in dolomite.