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

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

Luminometer for the future International Linear Collider - simulation and beam test results

LumiCal will be the luminosity calorimeter for the proposed International Large Detector of the International Linear Collider (ILC). The ILC physics program requires the integrated luminosity to be measured with a relative precision on the order of 10e-3, or 10e-4 when running in GigaZ mode. Luminosity will be determined by counting Bhabha scattering events coincident in the two calorimeter modules placed symmetrically on opposite sides of the interaction point. To meet these goals, the energy resolution of the calorimeter must be better than 1.5% at high energies. LumiCal has been designed as a 30-layer sampling calorimeter with tungsten as the passive material and silicon as the active material. Monte Carlo simulation using the Geant4 software framework has been used to identify design elements which adversely impact energy resolution and correct for them without loss of statistics. BeamCal, covering polar angles smaller than LumiCal, will serve for beam tuning, luminosity optimisation and high energy electron detection. Secondly, prototypes of the sensors and electronics for both detectors have been evaluated during beam tests, the results of which are also presented here.Comment: Technology and Instrumentation in Particle Physics 2011, Chicago, IL, USA. Presented June 11, 2011, and submitted to Physics Procedi

ECFA Detector R&D Panel, Review Report

Two special calorimeters are foreseen for the instrumentation of the very forward region of an ILC or CLIC detector; a luminometer (LumiCal) designed to measure the rate of low angle Bhabha scattering events with a precision better than 10$^{-3}$ at the ILC and 10$^{-2}$ at CLIC, and a low polar-angle calorimeter (BeamCal). The latter will be hit by a large amount of beamstrahlung remnants. The intensity and the spatial shape of these depositions will provide a fast luminosity estimate, as well as determination of beam parameters. The sensors of this calorimeter must be radiation-hard. Both devices will improve the e.m. hermeticity of the detector in the search for new particles. Finely segmented and very compact electromagnetic calorimeters will match these requirements. Due to the high occupancy, fast front-end electronics will be needed. Monte Carlo studies were performed to investigate the impact of beam-beam interactions and physics background processes on the luminosity measurement, and of beamstrahlung on the performance of BeamCal, as well as to optimise the design of both calorimeters. Dedicated sensors, front-end and ADC ASICs have been designed for the ILC and prototypes are available. Prototypes of sensor planes fully assembled with readout electronics have been studied in electron beams.Comment: 61 pages, 51 figure