Growth Kinetics and Characterization of Chromia Scales Formed on Ni–30Cr Alloy in Impure Argon at 700 °C

Oxidation of Metals - The oxidation of a Ni–30Cr alloy at 700 °C in impure argon was studied in order to provide new elements of understanding on chromia scale growth in low...     

Is UO2 irradiation resistance due to its unusual high temperature behaviour?

Explaining and predicting the radiation resistance of structural and functional materials is a primary goal for engineering materials able to withstand severe radiation environments. Szenes has developed an empirical criterion based on the thermal behaviour of a compound at high temperature. Though the specific heat at high temperature of most materials obeys the classic Dulong–Petit law, this is not true for uranium dioxide, perhaps the most important ceramic compound in a nuclear power plant. An original analysis of the different contributions to the heat capacity of UO₂ is presented showing that the large increase of UO₂ heat capacity at high temperature (T > 1300 K) is microscopically connected to a high concentration of polarons that are responsible for the departure from the Dulong–Petit law. This is in particular related to the contribution of the uranium sublattice. At the microscopic scale, this thermodynamic anomaly can be related to the thermally activated charge disproportionation of U atoms that is experimentally observed by electrical conductivity measurements. This singular behaviour of the polaron concentration has a direct impact on the uranium sublattice partial molar heat capacity and an indirect effect on the energy interactions between the electronic and ionic structure of the target mediated by these polarons. This could explain, at least partially, the irradiation resistance to amorphisation of UO₂.     

On the origin of the sigmoid shape in the UO2 oxidation weight gain curves

Cracking and spalling are known to occur during the oxidation of UO₂. However, these phenomena are not considered by the existing kinetic models of the oxidation of UO₂ into U₃O₈. In this study the oxidation of UO₂ samples of various sizes from the single crystal to nanopowders, has been followed by isothermal and isobaric thermogravimetry, environmental scanning electron microscopy and in situ X-ray diffraction at temperatures ranging from 250 to 370 °C in air. It has been shown that cracking occurs once a critical layer thickness of intermediate oxide has been reached, which corresponds to the beginning of the sigmoid kinetic curve. Cracking contribution to the sigmoid kinetic curve is then discussed as a function of temperature, and on the basis of nucleation and growth processes.     

Transient global amnesia: implicit/explicit memory dissociation and PET assessment of brain perfusion and oxygen metabolism in the acute stage

OBJECTIVES: To assess explicit memory and two components of implicit memory--that is, perceptual-verbal skill learning and lexical-semantic priming effects--as well as resting cerebral blood flow (CBF) and oxygen metabolism (CMRO2) during the acute phase of transient global amnesia. METHODS: In a 59 year old woman, whose amnestic episode fulfilled all current criteria for transient global amnesia, a neuropsychological protocol was administered, including word learning, story recall, categorical fluency, mirror reading, and word stem completion tasks. PET was performed using the (15)O steady state inhalation method, while the patient still exhibited severe anterograde amnesia and was interleaved with the cognitive tests. RESULTS: There was a clear cut dissociation between impaired long term episodic memory and preserved implicit memory for its two components. Categorical fluency was significantly altered, suggesting word retrieval strategy--rather than semantic memory--impairment. The PET study disclosed a reduced CMRO2 with relatively or fully preserved CBF in the left prefrontotemporal cortex and lentiform nucleus, and the reverse pattern over the left occipital cortex. CONCLUSIONS: The PET alterations with patchy CBF-CMRO2 uncoupling would be compatible with a migraine-like phenomenon and indicate that the isolated assessment of perfusion in transient global amnesia may be misleading. The pattern of metabolic depression, with sparing of the hippocampal area, is one among the distinct patterns of brain dysfunction that underlie the (apparently) uniform clinical presentation of transient global amnesia. The finding of a left prefrontal hypometabolism in the face of impaired episodic memory and altered verbal fluency would fit present day concepts from PET activation studies about the role of this area in episodic and semantic memory encoding/retrieval. Likewise, the changes affecting the lenticular nucleus but sparing the caudate would be consistent with the normal performance in perceptual-verbal skill learning. Finally, unaltered lexical-semantic priming effects, despite left temporal cortex hypometabolism, suggest that these processes are subserved by a more distributed neocortical network.     

Influence of Hydrogen and Water Vapour on the Kinetics of Chromium Oxide Growth at High Temperature

In support of the selection of structural materials for heat exchangers in helium-cooled high temperature reactors, the oxidation behaviour of the Ni-base chromia-former alloy 230 was investigated at 850 °C in diluted helium atmosphere with a low water vapour content. In such a media, the equivalent partial pressure of oxygen (imposed by the $ P_{{{\text{H}}_{2} {\text{O}}}} $ / $ P_{{{\text{H}}_{2}}} $ ratio) is very low ( $ P_{{{\text{O}}_{ 2} }}^{\text{eq}} $ around 10^?16 Pa). The equivalent partial pressure of oxygen has no straight influence on the parabolic rate constant (k _p); on the other hand, $ P_{{{\text{H}}_{2} }} $ and $ P_{{{\text{H}}_{2} {\text{O}}}} $ demonstrate a complex influence on k _p. Photoelectrochemistry analyses revealed that this oxide could simultaneously contain two types of cationic defects. Specific oxidation tests with D₂O showed that the oxide scale also contains hydrogen. A mechanist model is proposed in order to describe the scale growth using both cationic defects. Those theoretical results show, at least qualitatively, how $ P_{{{\text{H}}_{2} }} $ and $ P_{{{\text{H}}_{2} {\text{O}}}} $ may concurrently influence the oxidation rate.     

Hypogastric arterial aneurysms associated with abdominal aortic aneurysms

The purpose of this study was to compare the physiological responses of professional and elite road cyclists during an incremental cycle ergometer test. Twenty-five elite cyclists (EC; 23+/-1 yr) and 25 professional cyclists (PC; 25+/-2yr) performed a ramp protocol (increases of 25 W x min(-1)) during which the following parameters were measured: oxygen consumption (VO2), pulmonary ventilation (VE), ventilatory equivalents for oxygen and carbon dioxide (VE x VO2(-1) and VE x VCO2(-1), respectively), respiratory exchange ratio (RER), ventilatory thresholds 1 and 2 (VT1 and VT2, respectively), blood lactate, and electromyographic activity (EMG) of the vastus lateralis. Significant differences existed between the two groups mainly at submaximal intensities, since both VT1 and VT2 occurred at a higher exercise intensity (p<0.001) in PC than in EC (VT2: 80.4+/-6.6 vs 87.0+/- 5.9% VO2max in EC and PC, respectively). Lactate levels showed a similar response in both groups at low-to-moderate intensities (< 300 W), and thereafter blood lactate was significantly higher in EC. Finally, the "electromyographic threshold" (EMGT) occurred at a significantly higher intensity (p < 0.05) in PC when compared to EC (64.7+/-14.2 vs 56.0+/-14.9% VO2max, respectively). It was concluded that, in comparison with EC, PC exhibit some remarkable physiological characteristics such as a high VT2, an important reliance on fat metabolism even at high power outputs, and several neuromuscular adaptations.     

Study of Conductivity of K41X Chromia Forming Alloy in High Temperature Electrolysis Environment

Alloy K41X has been proposed as interconnect material for high temperature vapor electrolysis (HTVE) devices. This chromia forming alloy (alloy K41X) was oxidized at 800 °C in a thermobalance in oxidizing (synthetic air) and reducing (Ar–1 %H₂–9 %H₂O) environments for 250 h. The evolution of the contact resistance was evaluated using a dedicated device under the same conditions. There were higher oxidation kinetics rate in air than in Ar–1 %H₂–9 %H₂O but surprisingly, the corresponding area specific resistance (ASR) values were 20 times higher in Ar–1 %H₂–9 %H₂O mixture than in air. Additional tests and analyses (exposure in Ar–D₂–H₂O environment, GD-OES and SIMS analyses) clearly showed that the higher ASR value can be attributed to the presence of hydrogen in the oxide scale when exposed in Ar–H₂–H₂O mixture. In situ changes of atmosphere during ASR measurement showed the rapid kinetics for hydrogen desorption.     

Contribution to Modeling of Hydrogen Effect on Oxygen Diffusion in Zy-4 Alloy During High Temperature Steam Oxidation

Previous studies have shown that the numerical model EKINOX-Zr was able to simulate with accuracy oxide growth and oxygen diffusion into the matrix during high-temperature oxidation of Zy-4. In this study, the aim of the development was to evaluate if the observed effect of hydrogen cladding content on the increase of oxygen solubility in the high-temperature β_Zr was only a thermodynamic effect. Previous experimental studies have shown that hydrogen induces an evolution of equilibrium oxygen concentration at the α_Zr/β_Zr interface. The present work showed that EKINOX-Zr linked with the thermodynamic database Zircobase reproduced the evolution induced by hydrogen during the high-temperature steam oxidation. However, the results showed also that additional studies are necessary to better understand hydrogen behavior during high-temperature oxidation of Zr.     

Evaluation of a new Cr-free alloy as interconnect material for hydrogen production by high temperature water vapour electrolysis: Study in cathode atmosphere

For economic and ecological reasons, hydrogen is considered as a major energetic vector for the future. Hydrogen production via high temperature water vapour electrolysis (HTE) is a promising technology. A major technical difficulty related to high temperature water vapour electrolysis is the development of interconnects working efficiently for a long period. Working temperature of 800 °C enables the use of metallic materials as interconnects. High temperature corrosion behaviour and electrical conductivity of a new Cr-free Fe–Ni–Co alloy were tested in cathode atmosphere (H₂/H₂O) at 800 °C. The alloy exhibits a poor oxidation resistance but an excellent ASR parameter, as a result of the formation of a highly-conductive Cr-free surface spinel layer. Moreover, the role of water vapour and hydrogen was discussed and a diffusion mechanism in cathode atmosphere could be suggested.     

Role of adenosine kinase in Saccharomyces cerevisiae: identification of the ADO1 gene and study of the mutant phenotypes

Sequencing of the Saccharomyces cerevisiae genome revealed an open reading frame (YJR105w) encoding a putative protein highly similar to adenosine kinases from other species. Disruption of this gene (renamed ADO1) affected utilization of S-adenosyl methionine (AdoMet) as a purine source and resulted in a severe reduction of adenosine kinase activity in crude extracts. Furthermore, knock-out of ADO1 led to adenosine excretion in the medium and resistance to the toxic adenosine analogue cordycepin. From these data we conclude that ADO1 encodes yeast adenosine kinase. We also show that ADO1 does not play a major role in adenine utilization in yeast and we propose that the physiological role of adenosine kinase in S. cerevisiae could primarily be to recycle adenosine produced by the methyl cycle.