Tailoring the Nanostructure Anodically Formed in the Passive Oxide on Aluminum -Relevance to Localized Corrosion Initiation

Anodic processes at a passive metal surface like aluminum could be envisioned as being initiated or controlled by nano-scale structural changes within the metal's surface oxide. Examples include proposed roles for nanostructure formation in the pit initiation for Al corrosion (1,2), void formation on pit initiation of Al during anodic etching (3), and possibly deformationinduced nanostructure in pore formation in Al (4). Characterizing the nanostructure present in the passive oxide and learning how to control structure formation offers an opportunity to electrochemically explore deterministic relationships between known structure and anodic event initiation. Recent work has shown that voids can nucleate at the aluminum/aluminum oxide interface and grow into the passive oxide at standard slow rate polarization in moderate chloride solutions well below the onset for pit initiation (5). Analytical transmission electron microscopy argues that these voids are encapsulated absence of matter produced by cation and anion vacancy saturation and coalescence in the oxide. Atomic force microscopy coupled with scanning electron microscopy show that these voids can transition to pores at efficiencies up to 20% of the void population or 2x10 10 cm -2 prior to the initiation of pitting. These results demonstrate that interfacial voids are formed as a result of ion transport through the passive oxide and provide a foundation for exploring their role in pit initiation. Most of this work has focused on anhydrous oxides to model the initial passive oxide on Al. These oxides are formed on atomically clean (vacuum prepared) bulk Al surfaces (both single and poly-crystalline) and nanocrystalline evaporated Al films exposed to O 2 at room temperature and atmospheric pressure. The length of time for equilibration of the model oxide in a deaerated electrolyte is the factor that controls the extent to which voids and pores form. Time-of-flight secondary ion mass spectrometric (TOF-SIMS) measurements show that solution equilibration produces a slow growth of the initial 3 nm oxide up to an equilibration value of 4.5 nm over a period of 16 hours. Anodic polarization of the initial oxide produces a near-equivalent final thickness over a much faster time scale. It is this more rapidly formed film that exhibits a larger passive charge density (and larger void and pore densities) that exceeds values expected for uniform growth to its measured limiting thickness. The excess charge density has to be consumed either by vacancy generation (and subsequent coalescence) or dissolution. Dissolution alone appears an unlikely explanation given reported rates in a mixed chloride/borate electrolyte (6). An initial attempt can be made to identify whether these structures play a role in pit initiation by exploiting this equilibration time effect. In limited cases, we have been able to show that higher density void nucleation produced by using shorter equilibration times can lead to high densities of pores that maintain a minimum (< 20 nm) diameter. It is this high density of transitioning features that might be expected to increase the probability of pitting event when compared the absence of detectable pores for long equilibration times. We have observed as much as a 200 mV shift in the stable mean pitting potential to more active values with decreased equilibration time. Where these results provide an indication that voids and pores can contribute to pitting, they do not identify the necessity for these structures in a generalized pitting mechanism. More recent work is focused on exploiting how the passive oxide is formed in an effort to tailor its resulting ion transmission characteristics and range of nanostructure exhibited. Our eventual goal is to learn how to control this nanostructure to a level where statistical studies of current transient events (i.e. metastable pitting events) can be correlated to characteristics of the feature population. Hydrous oxides produced by exposure of atomically clean Al to pure water vapor with and without subsequent dehydration show even larger variation in passive charge density response with anodic polarization. These results indicate that the initial characteristics of the oxide prior to immersion in the electrolyte could provide a wider range of control of structure density and size. The use of alternate electrolytes containing borate produces unique differences in the nanostructure population. One quite interesting effect of the borate anion is that TOF-SIMS and x-ray photoelectron spectroscopy show it attenuates the equilibrium concentration of chloride within the outer layer and/or at the barrier oxide interface in the passive oxide. Electrochemical impedance measurements indicate a reduced defect concentration within the barrier layer when borate is present while electron microscopy shows the void population is suppressed when borate anion is present. These results suggest that electrolyte composition can be used to control the nanostructure population as well

Thermo-hydraulic modeling of the ITER radial neutron camera

The ITER Radial Neutron Camera (RNC) is a diagnostic system designed as a multichannel detection system to measure the uncollided neutron flux from the plasma, generated in the tokamak vacuum vessel, providing information on neutron emissivity profile. The RNC consists of array of cylindrical collimators located in two diagnostic structures: the ex-port system and the in-port system. The in-port system, contains the diamond detectors which need a temperature protection. Feasibility study of the efficiency of the cooling system for the In-port Detector Modules of the RNC during baking process was the main goal of thermo-hydraulic numerical modeling. The paper presents the concept of the cooling system layout and the original way of integration of numerical thermo-hydraulic analyses of the in-port detector cassette. Due to the large extent of the detector cassette it is impossible to include all relevant thermal and hydraulic effects in one global model with sufficient level of details. Thus the modelling strategy is based on the concept of three stage modelling from details to global model. The presented paper includes results of numerical calculations made with ANSYS Fluent software in order to provide the final answer, including calculation of heat loads in the detector cassette from adjacent walls during baking and normal operation conditions

Site occupancies in ternary C15 ordered Laves phases

Site occupancies in three C15-structured AB{sub 2}(X) Laves phases have been determined by Atom Location by CHanneling Enhanced MIcroanalysis (ALCHEMI). In NbCr{sub 2}(V), the results were consistent with exclusive site occupancies of Nb for the A sublattice and Cr and V for the B sublattice. The B-site occupancy of V is not expected from atom size effects alone. In NbCr{sub 2}(Ti), the results were consistent with Ti partitioning mostly to the A sites with some anti-site defects likely. In HfV{sub 2}(Nb), the results were consistent with Nb partitioning between the A and B sites. The results of the ALCHEMI analyses of these ternary C15 Laves phase materials will be discussed with respect to previously determined phase diagrams and first-principles total energy and electronic structure calculations

Mapping localized surface plasmons within silver nanocubes using cathodoluminescence hyperspectral imaging

Localized surface plasmons within silver nanocubes less than 50 nm in size are investigated using high resolution cathodoluminescence hyperspectral imaging. Multivariate statistical analysis of the multidimensional luminescence dataset allows both the identification of distinct spectral features in the emission and the mapping of their spatial distribution. These results show a 490 nm peak emitted from the cube faces, with shorter wavelength luminescence coming from the vertices and edges; this provides direct experimental confirmation of theoretical predictions

Loss of Wave1 gene defines a subtype of lethal prostate cancer

Genetic alterations involving TMPRSS2-ERG alterations and deletion of key tumor suppressor genes are associated with development and progression of prostate cancer (PCa). However, less defined are early events that may contribute to the development of high-risk metastatic prostate cancer. Bioinformatic analysis of existing tumor genomic data from PCa patients revealed that WAVE complex gene alterations are associated with a greater likelihood of prostate cancer recurrence. Further analysis of primary vs. castration resistant prostate cancer indicate that disruption of WAVE complex gene expression, and particularly WAVE1 gene (WASF1) loss, is also associated with castration resistance, where WASF1 is frequently co-deleted with PTEN and resists androgen deprivation therapy (ADT). Hence, we propose that WASF1 status defines a subtype of ADT-resistant patients. Better understanding of the effects of WAVE pathway disruption will lead to development of better diagnostic and treatment modalities

Crystal coherence length effects on the infrared optical response of MgO thin films

The role of crystal coherence length on the infrared optical response of MgO thin films was investigated with regard to Reststrahlen band photon-phonon coupling. Preferentially (001)-oriented sputtered and evaporated ion-beam assisted deposited thin films were prepared on silicon and annealed to vary film microstructure. Film crystalline coherence was characterized by x-ray diffraction line broadening and transmission electron microscopy. The infrared dielectric response revealed a strong dependence of dielectric resonance magnitude on crystalline coherence. Shifts to lower transverse optical phonon frequencies were observed with increased crystalline coherence. Increased optical phonon damping is attributed to increasing granularity and intergrain misorientation

Measurement of shower development and its Moli\`ere radius with a four-plane LumiCal test set-up

A prototype of a luminometer, designed for a future e+e- collider detector, and consisting at present of a four-plane module, was tested in the CERN PS accelerator T9 beam. The objective of this beam test was to demonstrate a multi-plane tungsten/silicon operation, to study the development of the electromagnetic shower and to compare it with MC simulations. The Moli\`ere radius has been determined to be 24.0 +/- 0.6 (stat.) +/- 1.5 (syst.) mm using a parametrization of the shower shape. Very good agreement was found between data and a detailed Geant4 simulation.Comment: Paper published in Eur. Phys. J., includes 25 figures and 3 Table