Ppm O2 Research Articles

EFFECTS OF OXYGEN SUPER-SATURATED NUTRIENT SOLUTIONS ON GREENHOUSE TOMATO

The effects of varying levels of nutrient solution oxygenation on greenhouse tomato (Lycopersicon esculentum L. 'Macarena') was investigated in 2006 and 2007 using rockwool, coir and perlite growing media. In 2006, early marketable and total yields were higher with 75 ppm O2 in the nutrient solution (14.5 kg m-2), than the control (12.8 kg m-2) and compressed air (12.9 kg m-2) treatments, while no significant difference was observed between 20, 30, 35 and 75 ppm. The seasonal yield followed the same trend, i.e. higher values were obtained with 75 ppm O2 (28.9 kg m-2) than with the control (27.1 kg m-2) and compressed air treatments (26.2 kg m-2). The seasonal average weight of marketable fruit was significantly higher with 35 ppm O2 (206 g) than with the control (196 g) and compressed air treatments (194 g). In 2007, in general, the late-season and seasonal marketable yield with all oxygen treatments was either equal to or higher than the control. Except for the 30 ppm treatment, the usual decline in marketable yield across the season was slower with oxygen super-saturated nutrient solution (24-29% between the mid and the last season) than with the control (i.e. a corresponding 40-45% reduction), indicating that oxygen can sustain productivity under ageing stress condition. The 60 ppm O2 treatment resulted in higher total seasonal yield than the control on the rockwool medium. These results suggest that application of nutrient solution oxygenation under certain conditions can improve greenhouse tomato productivity. Differences in climatic conditions may explain partly the differences in crop performance between the two seasons.

Oxygen-Induced Barrier Failure in Ti-Based Self-Formed and Ta/TaN Barriers for Cu Interconnects

To understand the electromigration degradation in Cu interconnects that utilize the TiOx self-formed barrier (SFB) probably due to Cu oxidation at the Cu/barrier interface, Cu films deposited on TiOx SFB and conventional Ta/TaN barriers were annealed in atmospheres of various oxygen concentrations. The Ta layer was preferentially oxidized to give Ta2O5, and contained a large amount of oxygen. The barrier layer, which consisted of Ta2O5 and Ta(O), could not suppress Cu diffusion. The TaN layer seemed to remain even after annealing at 400 °C in 10 ppm O2, and still suppressed Cu diffusion. This suggests that the TaN layer plays a key role to suppress barrier failure induced by oxygen originating from pores in dielectrics. On the other hand, the oxygen-induced barrier failure was observed in the TiOx SFB after annealing at 500 °C in 5 ppm O2 and more. Oxygen facilitated Cu2O formation above the TiOx SFB, and the Cu2O formation caused discontinuity of the TiOx SFB, leading to the barrier failure. The less oxidized Ti2O3 and TiO in the TiOx SFB were not further oxidized to TiO2 by oxygen in atmospheres, and thus they would not be oxygen absorbers suppressing the Cu2O formation above the barrier. Thus, for suppressing the Cu2O formation, it is essential to increase oxygen barrier ability of the TiOx SFB (probably increasing Ti concentration of the TiOx SFB).

Investigation of Galvanic Corrosion Characteristics between Tantalum Nitride and Poly Silicon in Dilute HF Solutions

Galvanic corrosion characteristics between poly-Si and tantalum nitride (TaN) (area ratio 1:1 and 1:4) have been investigated in very dilute HF solutions (0.01 to 0.05%) containing controlled levels of dissolved oxygen. The increase in exposed cathode (metal) area as well as aeration results in higher corrosion of poly-Si. In de-aerated HF solutions (less than 0.5 ppm of O2), irrespective of the area ratio, there appears to be no silicon loss. Morphological changes on poly-Si due to galvanic corrosion have been characterized using Scanning Electron Microscopy.

Oxygen-enhanced wet thermal oxidation of GaAs

An oxygen-enhanced wet thermal oxidation process is used to grow smooth, uniform, insulating native oxides of GaAs. At 420 °C, a maximum linear growth rate of 4.8 nm/min is observed for oxidation in water vapor with 2000 ppm O2 added relative to the N2 carrier gas, with growth ceasing by 7000 ppm. Films as thick as 800 nm with surface roughness as low as 0.2 nm are demonstrated. In fabricated metal-oxide-semiconductor capacitors, a 412 nm thick native oxide film exhibits a factor of ∼2700 reduction in leakage current density at 1 V relative to a direct metal (Au:Ti) to GaAs contact.

Thermodynamics of (Ga, In)‐Sb‐O‐Si and impact on dewetting process

AbstractA thermodynamic study is performed for the systems (Ga or In)‐Sb‐O‐Si in order to better understand the difference observed during dewetting experiments of GaSb and InSb in silica ampoules. Results show that the melts can be considered as non reactive toward silica. When the atmosphere is clean (≤ 1 ppm O2), no oxide is formed, while, under oxidising atmosphere, oxides exist above the melting point of the antimonide and are known to increase the wetting angle of the melt on the crucible. However the temperature range for oxide stability is smaller in the case of InSb and this may explain why dewetting is easy for GaSb in presence of oxygen, while it is difficult for InSb. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of the Thiol/Disulfide Redox Systems of the Anaerobe Desulfovibrio vulgaris Points Out Pyruvate:Ferredoxin Oxidoreductase as a New Target for Thioredoxin 1

Sulfate reducers have developed a multifaceted adaptative strategy to survive against oxidative stresses. Along with this oxidative stress response, we recently characterized an elegant reversible disulfide bond-dependent protective mechanism in the pyruvate:ferredoxin oxidoreductase (PFOR) of various Desulfovibrio species. Here, we searched for thiol redox systems involved in this mechanism. Using thiol fluorescent labeling, we show that glutathione is not the major thiol/disulfide balance-controlling compound in four different Desulfovibrio species and that no other plentiful low molecular weight thiol can be detected. Enzymatic analyses of two thioredoxins (Trxs) and three thioredoxin reductases allow us to propose the existence of two independent Trx systems in Desulfovibrio vulgaris Hildenborough (DvH). The TR1/Trx1 system corresponds to the typical bacterial Trx system. We measured a TR1 apparent K(m) value for Trx1 of 8.9 μM. Moreover, our results showed that activity of TR1 was NADPH-dependent. The second system named TR3/Trx3 corresponds to an unconventional Trx system as TR3 used preferentially NADH (K(m) for NADPH, 743 μM; K(m) for NADH, 5.6 μM), and Trx3 was unable to reduce insulin. The K(m) value of TR3 for Trx3 was 1.12 μM. In vitro experiments demonstrated that the TR1/Trx1 system was the only one able to reactivate the oxygen-protected form of Desulfovibrio africanus PFOR. Moreover, ex vivo pulldown assays using the mutant Trx1(C33S) as bait allowed us to capture PFOR from the DvH extract. Altogether, these data demonstrate that PFOR is a new target for Trx1, which is probably involved in the protective switch mechanism of the enzyme.

Co-precipitation of Phosphate and Iron Limits Mitochondrial Phosphate Availability in Saccharomyces cerevisiae Lacking the Yeast Frataxin Homologue (YFH1)

Saccharomyces cerevisiae cells lacking the yeast frataxin homologue (Δyfh1) accumulate iron in the mitochondria in the form of nanoparticles of ferric phosphate. The phosphate content of Δyfh1 mitochondria was higher than that of wild-type mitochondria, but the proportion of mitochondrial phosphate that was soluble was much lower in Δyfh1 cells. The rates of phosphate and iron uptake in vitro by isolated mitochondria were higher for Δyfh1 than wild-type mitochondria, and a significant proportion of the phosphate and iron rapidly became insoluble in the mitochondrial matrix, suggesting co-precipitation of these species after oxidation of iron by oxygen. Increasing the amount of phosphate in the medium decreased the amount of iron accumulated by Δyfh1 cells and improved their growth in an iron-dependent manner, and this effect was mostly transcriptional. Overexpressing the major mitochondrial phosphate carrier, MIR1, slightly increased the concentration of soluble mitochondrial phosphate and significantly improved various mitochondrial functions (cytochromes, [Fe-S] clusters, and respiration) in Δyfh1 cells. We conclude that in Δyfh1 cells, soluble phosphate is limiting, due to its co-precipitation with iron.

Oxygen, pH, and oral-aboral axis specification in the sea urchin embryo

The oral-aboral axis of the sea urchin embryo is specified via the redox-regulated activation of nodal (Coffman et al., 2009, and references therein). Embryos cultured at high density under cover glass develop a radialized phenotype owing to inhibition of nodal expression (Coffman et al. 2004), an effect that we attributed to hypoxia. Recently Agca et al. (2009) reported that controlled hypoxia does inhibit nodal expression as well as ectoderm differentiation, but only when it is maintained until pluteus stage. However, we found that embryos developed under cover glass become radialized even when the cover glass is removed at early blastula stage (Supplemental Figure 1). We therefore asked whether other factors might contribute to this teratogenic effect. Since embryos cultured under otherwise identical conditions develop normally, it is clear that the relevant effect of cover glass is to limit gas exchange, which in addition to hypoxia will promote acidification by preventing escape of CO2. To determine which of these factors causes radialized development, embryos were developed to hatching blastula stage under normoxic (∼10 ppm O2) or hypoxic (≤1 ppm O2) conditions in culture medium buffered to pH 8, 7.5, 7, or 6.5. While acidic pH was mildly radializing, hypoxia alone produced significant radialization, which was enhanced by acidification (Fig. 1 A, Supplemental Table 1 and Supplemental Fig. 2). Since the embryos were released from hypoxia at blastula stage, these results differ from those of Agca et al. (2009), for reasons that remain unclear. One difference that may be relevant is that we determined the level of hypoxia by measuring dissolved oxygen, whereas Agca et al. (2009) monitored atmospheric oxygen. In addition, we scored morphology at prism stage, whereas Agca et al. did so at pluteus stage, which may have allowed for recovery by regulative development. Finally, Agca et al. did not quantify the phenotypes they reported. Figure 1 Hypoxia induces radialized development and reduces mitochondrial H2O2 levels in sea urchin embryos. (A) Differential effects of controlled hypoxia and pH on development of oral-aboral polarity. Embryos were developed under the indicated conditions (see ... Ratiometric imaging was used to measure the effect of limiting gas exchange on intracellular H2O2 and pH (Fig. 1 B, Supplemental Fig. 3). Both mitochondrial and cytoplasmic H2O2 were reduced by this treatment, with a decrease in mitochondrial H2O2 comparable in magnitude to that produced by a mitochondrially-targeted catalase (Mt-Cat) shown previously to both inhibit nodal activation and entrain oral-aboral polarity (Coffman et al. 2009) (Fig. 1B). This decrease in H2O2, a major reactive oxygen species (ROS), contradicts the untested assumption of Agca et al. (2009) that hypoxia elevates ROS. As would be expected, limiting gas exchange also lowers intracellular pH, but less so than the decrease produced by pH buffering that did not induce radialization (Supplemental Fig. 3), suggesting that the relevant pH effects are extracellular. We conclude that hypoxia is the major cause of the radialization caused by development under cover glass. This is probably attributable at least in part to decreased H2O2 levels, as the window of vulnerability corresponds to the initial H2O2-dependent activation phase of nodal expression (Supplemental Figure 1; Coffman et al., 2009). Extracellular acidification appears to play an ancillary role, perhaps weakening ligand-receptor interactions and thus by dampening the feedback amplification of nodal expression.

Probing photo-ionization: simulations of positive streamers in varying N2 : O2-mixtures

Photo-ionization is the accepted mechanism for the propagation of positive streamers in air though the parameters are not very well known; the efficiency of this mechanism largely depends on the presence of both nitrogen and oxygen. But experiments show that streamer propagation is amazingly robust against changes of the gas composition; even for pure nitrogen with impurity levels below 1 ppm streamers propagate essentially with the same velocity as in air, but their minimal diameter is smaller, and they branch more frequently. Additionally, they move more in a zigzag fashion and sometimes exhibit a feathery structure. In our simulations, we test the relative importance of photo-ionization and of the background ionization from pulsed repetitive discharges, in air as well as in nitrogen with 1 ppm O2. We also test reasonable parameter changes of the photo-ionization model. We find that photo-ionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition frequency has to be included above 1 Hz. Finally, we explain the feather-like structures around streamer channels that are observed in experiments in high purity nitrogen, but not in air.

Identification of Eukaryotic and Prokaryotic Methylthiotransferase for Biosynthesis of 2-Methylthio-N6-threonylcarbamoyladenosine in tRNA

Bacterial and eukaryotic transfer RNAs have been shown to contain hypermodified adenosine, 2-methylthio-N(6)-threonylcarbamoyladenosine, at position 37 (A(37)) adjacent to the 3'-end of the anticodon, which is essential for efficient and highly accurate protein translation by the ribosome. Using a combination of bioinformatic sequence analysis and in vivo assay coupled to HPLC/MS technique, we have identified, from distinct sequence signatures, two methylthiotransferase (MTTase) subfamilies, designated as MtaB in bacterial cells and e-MtaB in eukaryotic and archaeal cells. Both subfamilies are responsible for the transformation of N(6)-threonylcarbamoyladenosine into 2-methylthio-N(6)-threonylcarbamoyladenosine. Recently, a variant within the human CDKAL1 gene belonging to the e-MtaB subfamily was shown to predispose for type 2 diabetes. CDKAL1 is thus the first eukaryotic MTTase identified so far. Using purified preparations of Bacillus subtilis MtaB (YqeV), a CDKAL1 bacterial homolog, we demonstrate that YqeV/CDKAL1 enzymes, as the previously studied MTTases MiaB and RimO, contain two [4Fe-4S] clusters. This work lays the foundation for elucidating the function of CDKAL1.

HCN and NH3Formation during Coal/Char Gasification in the Presence of NO

Understanding the conversion of coal-N during gasification is an important part of the development of gasification-based power generation technologies to reduce NO(x) emissions from coal utilization. This study investigated the conversion of coal-N in the presence of NO during the gasification of three rank-ordered coals and their chars in steam and low-concentration O(2). Our results show that NO can be incorporated into the char structure during gasification. The inherent char-N and the N incorporated into the char from NO-char reactions behave very similarly during gasification. During the gasification in steam, significant amounts of HCN and NH(3) can be formed from the incorporated N structure in char, especially for the relatively "aged" chars, mainly due to the availability of abundant H radicals on the char surface during the gasification in steam. During the gasification in 2000 ppm O(2), the formation of HCN or NH(3) from the N structures in char, including those incorporated into the char from the NO-char reactions, was not a favored route of reaction mainly due to the lack of H on char surface in the presence of O(2).

Selective Formation of a SnO2 Cap Layer, Its Growth Behavior, and Oxidation Resistance

In this paper, we propose a new method of cap-layer formation. The cap layer of SnO2 is formed by the displacement plating of Sn, followed by the oxidation of the plated Sn. An excellent plating selectivity was found between Cu and SiO2, but not with SiOCH. The SnO2 layer of 90 nm thickness showed a good oxidation resistance of the underlying Cu after heat treatment at 480 °C in air (PO2=105 Pa). In contrast, the SnO2 layers of 6 and 9 nm thickness showed oxidation resistance at 400 °C in Ar+10 ppm O2 (PO2=1 Pa), but not in Ar+1000 ppm O2 (PO2=100 Pa).

Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production

Efficient production of solar fuels is an imperative for meeting future fossil-fuel-free energy demands. Hydrogen that is derived from the splitting of water by solar energy is clearly attractive as a clean energy vector, and there have been many attempts to construct viable molecular and biomolecular devices for photohydrogen production. A common approach in the construction of such devices is the utilization of tris(bipyridine)ruthenium, zinc porphyrin, or related molecular materials as photosensitizers in conjunction with a tethered or free electrocatalyst or enzymic system. Apart from cost, such systems suffer from having limited lifetimes, which may be attributed at least in part to the intrinsic reactivity of the organic N-donor ligands in the radical anion form of the photoexcited state and photodegradation pathways.

Products and Mechanism of the Reaction of Chlorine Atoms with 3-Pentanone in 700−950 Torr of N2/O2 Diluent at 297−515 K

The products, kinetics, and mechanism of the reaction Cl + 3-pentanone have been measured by UV irradiation of Cl(2)/3-pentanone/N(2) (O(2)) mixtures using primarily GC analysis with selected cross checks by FTIR. In the absence of O(2), the products are 1- and 2-chloro-3-pentanone with yields of 21 and 78%, respectively. As the temperature is increased, the yield of 1-chloro-3-pentanone increases modestly relative to the 2-chloro-3-pentanone yield. On the basis of this increase, the activation energy for hydrogen abstraction at the 1 position is determined to be 500 (+/-500) cal mole(-1) relative to abstraction at the 2 position. In the presence of 500 ppm of O(2) with 900-1000 ppm of Cl(2) at 297 K, the yield of 2-chloro-3-pentanone decreases dramatically from 78 to 2.5%, while the 1-chloro-3-pentanone decreases only modestly from 21 to 17%. The observed oxygenated species are acetaldehyde (59%), 2,3-pentanedione (9%), and propionyl chloride (56%). Increasing the temperature to 420 K (O(2) = 500 ppm) suppresses these oxygenated products, and 2-chloro-3-pentanone again becomes the primary product, indicating that the O(2) addition reaction to the 2-pentanonyl radical has become reversible. At 500 K and 10 000 ppm O(2), a new product channel opens which forms a small yield ( approximately 4%) of ethylvinylketone. Computer modeling of the product yields has been performed to gain an understanding of the overall reaction mechanism in the presence and absence of O(2). The reaction of chlorine atoms with 3-pentanone proceeds with a rate constant of 8.1 (+/-0.8) x 10(-11) cm(3) molecule(-1) s(-1) independent of temperature over the range of 297-490 K (E(a) = 0 +/- 200 cal mole(-1)). Rate constant ratios of k(C(2)H(5)C(O)CHCH(3) + Cl(2))/k(C(2)H(5)C(O)CHCH(3) + O(2)) = 0.0185 +/- 0.0037 and k(C(2)H(5)C(O)CH(2)CH(2) + Cl(2))/k(C(2)H(5)CH(2)C(O)CH(2)CH(2) + O(2)) = 2.7 +/- 0.4 were determined at 297 K in 800-950 Torr of N(2)/O(2) diluent. In 800-950 Torr of N(2)/O(2) diluent, the major fate of the alkoxy radical CH(3)CH(O)C(O)C(2)H(5) is decomposition to give C(2)H(5)C(O) radicals and CH(3)CHO. These results show that the chemical mechanisms of the 3-pentanone reactions are very similar to those observed for butanone. In addition, the rate constants of the reactions of chlorine atoms with 1-chloro-3-pentanone [3 (+/-0.6) x 10(-11) over the range of 297-460 K], 2,3-pentanedione [1.4 (+/-0.3) x 10(-11) at 297 K], and ethylvinylketone [1.9 (+/-0.4) x 10(-10) over the range of 297-400 K, decreasing rapidly above 400 K] were measured at ambient pressure.

${\rm SmBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Coated Conductors on the IBAD-MgO Substrate Fabricated by the Ex-situ Conversion Process

We investigated the ex-situ conversion process of amorphous precursor films for SmBa2Cu3O7-delta (SmBCO) coated conductors (CC). Amorphous precursor films were first deposited on the LaMnO3/homoepi-MgO/ IBAD-MgO/Y2O3/Al2O3/Hastelloy substrate by e-beam co-evaporation and subsequently crystallized to the superconducting phase at various post-annealing temperatures in two different reduced oxygen pressures. In 100 ppm of O2 in Ar, c-axis oriented SmBCO grains with minor a-axis oriented grains were grown at 825degC whereas polycrystalline SmBCO film with some a -axis oriented grains was formed at 800degC. In the temperature region of 825 ~ 850degC, protruded spots were observed on the sample surface. In 6 ppm of O2 in Ar, SmBCO film composed of a-axis oriented SmBCO grains with partial c-axis oriented grains were obtained at 700degC but other phases were formed above 725degC. The SmBCO sample grown at 700degC exhibited Tc ,zero value of 90.2 K. However, its Jc value at 77 K was negligibly small due to the formation of a-axis oriented grains.

Low-temperature gas from marine shales

Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas (thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200°C in the subsurface and generally above 300°C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100°C, robust gas generation below 100°C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300° below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50°C, six percent of the hydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1–C5). The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gases were compositionally distinct. In sequential isothermal heating cycles (~1 hour), nearly five times more gas was generated at 50°C (57.4 μg C1–C5/g rock) than at 350°C by thermal cracking (12 μg C1–C5/g rock).The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300° below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible.

Hydrolysis constants of tetravalent neptunium by using solvent extraction method

Abstract The hydrolysis constants of tetravalent neptunium (Np(IV)) were determined by solvent extraction method using thenoyltrifluoroacetone(TTA). In order to avoid colloid formation, a stock solution of carrier-free 239Np(V) was from 243Am milked. The valence of Np in the solution was then reduced to Np(IV) by using zinc amalgam. The hydrolysis constants (β m ) of the reactions, Np4++mOH−=Np(OH) m (4−m)+ was evaluated by using distribution ratios at ionic strengths (I)=0.1, 0.5 and 1.0. All experiments were performed in oxygen- free 0.5% H2−N2 atmosphere (below 1.0 ppm of O2) in a glovebox at room temperature (23±2 °C) to avoid oxidation of Np(IV). The β m values were extrapolated to the standard state (I=0) by using the specific ion interaction theory (SIT), and the formation constants at I=0 were determined to be logβ 1°=13.91±0.23, logβ 2°=27.13±0.15, logβ 3°=37.70±0.30 and logβ 4°=46.16±0.30. The ion interaction coefficients were also evaluated to be ε(NpOH3+, ClO4 −)=0.49±0.15, ε(Np(OH)2 2+, ClO4 −)=0.35±0.11, and ε(Np(OH)3 +, ClO4 −)=0.29±0.15.

A CO-Tolerant Hydrogen Fuel Cell System Designed by Combining with an Extremely Active Pt/CNT Catalyst

Complete oxidation of 2,000 ppm CO in H2 is attained over 10 wt.% Pt/CNT (carbon nano-tube) catalyst (0.019 g) at room temperature by adding 2,000 ppm O2 to hydrogen flowing at 100 mL/min. By insetting 5 wt.% Pt/CNT catalyst (0.8 g) in front of a PEFC (polymer electrolyte hydrogen fuel cell), the PEFC generated maximum wattage of 0.45 V × 0.75 A/cm2 with no suppression for more than 7 h in a flow of H2 (400 mL/min) containing 1,000 ppm CO. This result suggests a possibility of the hydrogen fuel cell operating in the shift gas.

Changes in Char Structure during the Gasification of a Victorian Brown Coal in Steam and Oxygen at 800 °C

Char structure is an important factor influencing its reactivity during gasification. This study aims to investigate the changes in char structure during the gasification of brown coal. A Victorian brown coal was gasified in a fluidized-bed/fixed-bed reactor at 800 °C in atmospheres containing 15% H2O, 2000 ppm O2, or 15% H2O and 2000 ppm O2, respectively. Although the char gasification in 2000 ppm O2 was mainly rate-limited by the external diffusion of O2, the char−H2O reaction was mainly rate-limited by the chemical reactions. The structural features of char at different levels of char gasification conversion were examined with FT-Raman spectroscopy. Our results show that the chars from the gasification in the mixture of 2000 ppm O2 and 15% H2O had almost the same features as the chars from the gasification in 15% H2O alone when the same levels of char conversion were achieved. Both the thermal decomposition of char and the char gasification reactions could result in changes in char structure during gas...

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10−4 atm (argon + 100 ppm O2). A nickel film of ∼35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.