Denis T. Keane

Microstructure and fracture in three dimensions

Abstract A high resolution three dimensional (3D) scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders under compressive loading. Tomographic scans were made at different load increments in the same specimen. 3D image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach using the measured surface area of the internal cracks created. The measured fracture energy was of the same magnitude that is typically measured in concrete tensile fracture. A nominally bilinear incremental fracture energy curve was measured. Separate components for crack formation energy and secondary toughening mechanisms are proposed. The secondary toughening mechanisms were found to be about three times the initial crack formation energy.

Small angle x-ray scattering metrology for sidewall angle and cross section of nanometer scale line gratings

High-volume fabrication of nanostructures requires nondestructive metrologies capable of measuring not only the pattern size but also the pattern shape profile. Measurement tool requirements will become more stringent as the feature size approaches 50nm and tolerances of pattern shape will reach a few nanometers. A small angle x-ray scattering (SAXS) based technique has been demonstrated to have the capability of characterizing the average pitch size and pattern width to subnanometer precision. In this study, we report a simple, modeling-free protocol to extract cross-section information such as the average sidewall angle and the pattern height of line grating patterns from the SAXS data. Diffraction peak intensities and reciprocal space positions are measured while the sample is rotated around the axis perpendicular to the grating direction. Linear extrapolations of peak positions in reciprocal space allow a precise determination of both the sidewall angle and the pattern height.

Identification of a Ni0.5(Al0.5−xMnx) B2 phase at the heterophase interfaces of Cu-rich precipitates in an α-Fe matrix

A phase with the stoichiometry Ni0.5(Al0.5−xMnx) is observed at heterophase interfaces of Cu-rich precipitates in an α-Fe matrix, utilizing atom-probe tomography. First-principles calculations are utilized to determine the substitutional energies, yielding EMn→Ni=0.916eVatom−1 and EMn→Al=−0.016eVatom−1 indicating that the manganese atoms prefer substituting at Al sublattice sites instead of Ni sites. A synchrotron radiation experiment demonstrates that the identified phase possesses the B2 structure.

Measuring three-dimensional damage in concrete under compression

To study the relationship between surface and internal cracking, 2 high-resolution, non-destructive evaluation techniques were used to measure crack growth caused by compressive loading in the fracture process of cement-based materials. Digital image correlation (DIC) was used to characterize the 2-D, surface fracture pattern, while 3-D, internal behavior was measured with x-ray microtomography (XMT). Rectangular mortar specimens containing both sand and graphite aggregates were examined. These techniques gave complementary information about crack geometry and development: DIC was more effective at determining crack width and location of small cracks, while XMT depicted the shape of larger cracks more successfully and showed the influence of internal features on the fracture process. The effect of aggregate shape and strength and the subsequent influence of increased crack distribution on ductility are discussed.

Structure of a passivated Ge surface prepared from aqueous solution

The structure of a passivating sulfide layer on Ge(001) was studied using X-ray standing waves and X-ray fluorescence. The sulfide layer was formed by reacting clean Ge substrates in (NH 4 ) 2 S solutions of various concentrations at 80°C. For each treatment, a sulfide layer containing approximately two to three monolayers (ML) of S was formed on the surface, and an ordered structure was found at the interface that contained approximately 0.4 ML of S. Our results suggest the rapid formation of a glassy GeS x layer containing 1.5‐2.5 ML S residing atop a partially ordered interfacial layer of bridge-bonded S. The passivating reaction appears to be self-limited to 2‐3 ML at this reaction temperature.

Key Factors Limiting Carbon Nanotube Yarn Strength: Exploring Processing-Structure-Property Relationships

Studies of carbon nanotube (CNT) based composites have been unable to translate the extraordinary load-bearing capabilities of individual CNTs to macroscale composites such as yarns. A key challenge lies in the lack of understanding of how properties of filaments and interfaces across yarn hierarchical levels govern the properties of macroscale yarns. To provide insight required to enable the development of superior CNT yarns, we investigate the fabrication-structure-mechanical property relationships among CNT yarns prepared by different techniques and employ a Monte Carlo based model to predict upper bounds on their mechanical properties. We study the correlations between different levels of alignment and porosity and yarn strengths up to 2.4 GPa. The uniqueness of this experimentally informed modeling approach is the models ability to predict when filament rupture or interface sliding dominates yarn failure based on constituent mechanical properties and structural organization observed experimentally. By capturing this transition and predicting the yarn strengths that could be obtained under ideal fabrication conditions, the model provides critical insights to guide future efforts to improve the mechanical performance of CNT yarn systems. This multifaceted study provides a new perspective on CNT yarn design that can serve as a foundation for the development of future composites that effectively exploit the superior mechanical performance of CNTs.

Phase diagram of the PrFeAsO1 xFx superconductor

The electronic phase diagram of

Fracture of a textured anisotropic ceramic

{\text{PrFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}

Phase diagram of thePrFeAsO1−xFxsuperconductor

(0\ensuremath{\le}x\ensuremath{\le}0.225)