X-Ray Diffraction Imaging: Technology And Appli... [HOT]

Coherent x-ray diffraction imaging (CXDI or CXD) uses x-rays (typically .5-4keV)[5] to form a diffraction pattern which may be more attractive for 3D applications than electron diffraction since x-rays typically have better penetration. For imaging surfaces, the penetration of X-rays may be undesirable, in which case a glancing angle geometry may be used such as GISAXS.[2] A typical x-ray CCD is used to record the diffraction pattern. If the sample is rotated about an axis perpendicular to the beam a 3-Dimensional image may be reconstructed.[11]

X-Ray Diffraction Imaging: Technology and Appli...

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Coherent electron diffraction imaging works the same as CXDI in principle only electrons are the diffracted waves and an imaging plate is used to detect electrons rather than a CCD. In one published report[1] a double walled carbon nanotube (DWCNT) was imaged using nano area electron diffraction (NAED) with atomic resolution. In principle, electron diffraction imaging should yield a higher resolution image because the wavelength of electrons can be much smaller than photons without going to very high energies. Electrons also have much weaker penetration so they are more surface sensitive than X-rays. However, typically electron beams are more damaging than x-rays so this technique may be limited to inorganic materials.

Incomplete measurements have been a problem observed across all algorithms in CDI. Since the detector is too sensitive to absorb a particle beam directly, a beamstop or hole must be placed at its center to prevent direct contact (Pham 2020). Furthermore, detectors are often constructed with multiple panels with gaps between them where data again cannot be collected (Pham 2020). Ultimately, these qualities of the detector result in missing data within the diffraction patterns. In situ CDI is a new method of this imaging technology that could increase resistance to incomplete measurements. In situ CDI images a static region and a dynamic region that changes over time as a result of external stimuli (Hung Lo 2018). A series of diffraction patterns are collected over time with interference from the static and dynamic regions (Hung Lo 2018). Because of this interference, the static region acts as a time invariant constraint that phases patterns together in fewer iterations (Hung Lo 2018). Enforcing this static region as a constraint makes in situ CDI more robust to incomplete data and noise interference in the diffraction patterns (Hung Lo 2018). Overall, in situ CDI provides clearer data collection in fewer iterations than other CDI techniques.

Conventional methods for examination of single crystal specimens rely on the Laue technique of x-ray diffraction whereby patterns of individual spots (reflections) are obtained from individual planes. In the Laue method, a continuous (white radiation) x-ray source is utilized. The x-ray source is collimated (usually with a pin-hole collimator) and then incident upon the sample when the transmission configuration is used, or through a hole in a film followed by the sample when the back-reflection configuration is used. Transmission Laue diffraction analysis is limited to thin specimens (in general, mt EXPERIMENTAL PROCEDURE In the topographic set-up used in this investigation, called asymmetric crystal topography (ACT)[12], a slit collimated white radiation x-ray source was incident upon a high quality asymmetrically cut silicon crystal. The silicon crystal served as both a monochromator of Ka and possibly Kb wavelengths, and a beam expander,resulting in an x-ray beam of approximate dimensions 6 cm high by 2 cm wide. A crystal specimen of interest was mounted on a Newport rotation stage and placed in the path of the monochromated and expanded x-ray beam. Thesample was rotated in the monochromated and expanded x-ray beam until a Bragg condition was Fig 1: Schematic of the asymmetric crystal topography (ACT) system.satisfied for some plane reflection as detected using an image intensifier with a fluorescent screen faceplate placed near the specimen.The beam diffracted from the specimen crystal and captured by the image intensifier was ensured to be monochromatic when the stringent simultaneous diffraction conditions of the first silicon crystal and the sample crystal was satisfied. Diffraction images of interest were permanently recorded on x-ray film (Kodak occlusal DF-50 dental film) with exposure times ranging from 1/2 hour up to 6 hours depending upon the sample investigated and the intensity of the diffracted reflection observed. Due to the relatively weak x-ray source available in thelaboratory (copper target tube operated at 50 kV and 32 mA), as well as the thickness of the majority of the samples examined, the back-reflection configuration was utilized in this research.In the ACT technique each individual topographic image is essentially a large Laue "spot" generated by diffraction from a particular set of "parallel" lattice planes covering a large area of the crystal. The x-ray beam incident on the specimen illuminated a large area unlike conventional Laue pin-hole techniques, and because of the special beamexpanding monochromizing silicon crystal, this large incident beam experienced minimal divergence. A schematic of the ACT system showing positions of the highly perfect, asymmetrically cut silicon first crystal (monochromator and beam expander), the second crystal (specimen under investigation), and the x-ray image intensifier (for direct real-time viewing of topographic images) is shown in Figure 1.EXPERIMENTAL RESULTS Turbine Blades The efficiency of modern gas turbine engines, used for both aircraft and ground based electric power generation, increases with increasing combustion temperature. The physical requirements which limit the choice of turbine blade materials for high temperature operation are low density, thermal stability, thermal fatigue resistance, toughness, resistance to high-temperature oxidation, and resistance to creep. Creep caused by dislocation motion is resisted by addition of alloying elements in solid solution and formation of stable hard precipitates, both of which serve as dislocation pinning points. Diffusional creep is resisted by increasing the grain size, directional solidification to produce long grains with boundaries parallel to the applied stress, and most optimally by eliminating the grain boundaries completely, i.e. using single crystal blades. The use of metallic single crystals for structural engineering applications places new requirements on nondestructive evaluation techniques for quality control.One of the greatest obstacles in the quality assurance process for single crystal nickel based alloy turbine blades is the determination of the overall crystalline perfection of the final blades. The existing method relies upon chemical etching and visual inspection. While the problems associated with visual inspection are self-evident, there areintricacies associated with the etch process. The present research focuses on nondestructive inspection techniques, based on x-ray diffraction, which would eliminate the need for chemical etching and visual examination. Fig 2: Schematic diagram of the ACT experimental arrangement used for inspection of a"single" crystal, nickel based alloy turbine blade.

Quartz Resonators While quartz resonators have been the mainstay of the ultrasonics industry for some time, intracacies exist in the production of quality resonators and therefore fabrication remains somewhat of an art form. Recently the Johns Hopkins University Center for Nondestructive Evaluation has initiated a research program to investigate methodsto analyze quartz single crystals for the purpose of assessing the relative crystal quality of the raw material as well as material which has been manufactured into resonators.One application for which the ACT system was applied to quartz crystals was to determine if the x-ray topographicsystem could distinguish between a "good" resonator versus a "bad" resonator and futhermore, the possible cause(s) of failure of the "bad" resonator. Two quartz resonators each of 1 1/2 cm diameter and approximately 1 mm thickness were examined in the ACT system. A feature which was present on each sample, a major flat (for crystallographic purposes), was used as a fiduciary marker to determine whether a sample was in the up or down position. Since there was no way to distinguish the front from the back of a given sample, an arbitrary designationwas made.Each sample was examined individually using the back-reflection configuration of the ACT system. In order that direct comparisons could be made between the front and back of a given sample and between the two different samples, it was necessary to ensure that the same Bragg reflection was selected. This was done by recording a selected diffraction pattern from one sample and then, without changing the Bragg angle, removing the first sample and either flipping the sample over to examine the other side or replacing it with the second, making sure that the position of the fiduciary flat coincided. Minor adjustments in the Bragg angle ( 041b061a72