The Hard X-ray Nanoprobe (HXN) at NSLS-II provides a nanoscale 3D multi-modality imaging capability, useful for investigating diverse material systems. The multi-modality scanning-probe imaging utilizes a variety of imaging contrasts such as fluorescence, transmission, scattering, and diffraction. Images taken simultaneously using different contrast mechanisms can provide 3D visualization of a sample, producing complementary information about the sample. Such comprehensive 3D characterizations are extremely useful in studying materials with multiple phases or complex internal structures. An important scientific problem is to investigate phase or grain boundaries of multi-component materials during or after material processing such as sintering, since re-organization of these boundaries due to annealing or phase-separation often result in profound impact on material property or functionality. However, accurate quantification of 3D elemental concentration is hampered by a well-known self-absorption problem, particularly severe for the low energy fluorescence x-rays. Correcting absorption is non-trivial and requires an iterative and three-dimensional solution. In this presentation, we will describe our approach using experimental data taken from mixed ionic ceramic membrane samples and elaborate on how accurate absorption correction led to discovery of a new material phase in this material system.
The stresses in a microstructured optical fiber are determined in tension and in bending. A model for the failure process in porous structures with applications to weak microstructured optical fibers with large flaws is proposed. The model assumes that a continuous crack front splits on contact with a hole and then propagates such that the direction of the front in contact with the boundary of the hole is normal to it. Three-dimensional finite perturbation analysis of Bower and Ortiz1 is extended to model the interaction of cracks with a periodic array of holes. Crack propagation in the cross-section is examined using the modified finite perturbation method. Test cases are presented for the stress intensity factor distribution in elliptical cracks, and periodic perturbations in straight cracks. During crack front interaction, the front can split upon contact with the hole or an array of holes, and then travel around them.
KEYWORDS: Carbon, Optical fibers, Fiber lasers, Temperature metrology, Optical coatings, Pyrometry, Chemical vapor deposition, Signal attenuation, Gas lasers, Laser irradiation
Using a CO2 laser as a heat source, carbon coatings have been successfully deposited on moving optical fibers in an open-air laser-induced chemical vapor deposition (LCVD) reactor. Applications include fusion splice recoat and in-line coating of optical fibers. The relationship between operating parameters and the carbon deposition temperature and rate was investigated. Results indicate that they are strongly dependent on the laser power density and the optical fiber's traverse velocity. In order to provide a deeper understanding of the fundamental principles that govern laser heating and the carbon LCVD processes, a heat transport model was developed to predict the fiber surface temperature during deposition. The surface temperatures obtained from experiments compared well with the predicted temperature. Based on the temperature calculations, various kinetic parameters, including the activation energy EA and pre-exponential factor k0 for the carbon deposition process, are determined. The optical fiber signal loss at 1550 nm induced by the LCVD process is observed to be less than 10–3 dB at low laser power density.
The inert strength of carbon-coated optical fibers (hermetic
fibers) has been observed to be less than that of standard polymer
coated optical fibers. A scaled version of the of the
carbon-coated optical fiber was developed in the laboratory and
used in an experimental investigation of the mechanical properties
of this system. The microstructure and phase similarity between
the scaled system and the optical fiber was established using
Raman spectroscopy. The mechanical properties, residual stress in
the film and the fracture toughness of the scaled system were
determined using nano-indentation. A fracture mechanics model was
developed to explain the mechanism of this strength reduction. The
model, based on the cracking of thin films in residual tension,
will be used to predict growth of flaws from the carbon film and
penetrating into the substrate. The model can be applied to all
brittle coatings where delamination of the coating is not
observed. Conditions under which cracks in the carbon film
propagate into the substrate were investigated using a recently
developed superposition scheme. Possible methods of crack arrest
will be discussed.
Pyrolytic carbon coatings show excellent hermetic properties and can be applied to optical fibers during the draw process by thermally activated chemical vapor deposition (CVD). This study investigates the relationship between carbon growth rates, microstructure, and deposition conditions at 600 torr chamber pressure. The films are grown from methane, propane and butane precursors on stationary 3 mm quartz rods. Hydrocarbon precursor gas concentrations are varied between 10% and 100% in N2, and surface temperatures are varied between 800 and 1500°C. X-Ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), and Secondary Ion Mass Spectroscopy (SIMS) are used to determine the film composition, and to inspect for a SiC transition layer between the carbon and the glass. The carbon films are analyzed for thickness, carbon phase, and surface roughness using ESEM and Raman Spectroscopy.
Coatings on optical sensors provide protection from mechanical damage and chemical attack. In high pressure, high temperature environments, conventional polymer coatings allow hydrogen and moisture to diffuse into the fiber core, causing failure in sensors that operate in harsh environments. A hermetic carbon coating can be used in these applications to improve reliability in sensing and signal transmission, and to extend fiber life. An experimental study of thin carbon films applied by chemical vapor deposition (CVD) was conducted to determine the effects of deposition parameters on film hermeticity. The goal is to control carbon disposition in a cold walled CVD reactor by controlling the gas flow, temperature, and concentration of reaction gases within the chamber. The experiments grow carbon films by CVD of hydrocarbon precursors on stationary quartz glass rods. Smooth, pinhole free graphite films have been obtained and are desired because they prevent the diffusion of hydrogen. The carbon layers are analyzed for thickness, carbon phase, and surface roughness using ESEM and Raman Spectroscopy. The information will be combined with a numerical study to optimize the growth of graphite films on optical fibers.
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