Current Issue : October - December Volume : 2015 Issue Number : 4 Articles : 4 Articles
Bimodal grain structures are common in many\nalloys, arising from a number of different causes including\nincomplete recrystallization and abnormal grain growth.\nThese bimodal grain structures have important technological\nimplications, such as the well-known Goss texture\nwhich is now a cornerstone for electrical steels. Yet our\nability to detect bimodal grain distributions is largely\nconfined to brute force cross-sectional metallography. The\npresent study presents a new method for rapid detection of\nunusually large grains embedded in a sea of much finer\ngrains. Traditional X-ray diffraction-based grain size\nmeasurement techniques such as Scherrer, Williamsonââ?¬â??\nHall, or Warrenââ?¬â??Averbach rely on peak breadth and shape\nto extract information regarding the average crystallite\nsize. However, these line broadening techniques are not\nwell suited to identify a very small fraction of abnormally\nlarge grains. The present method utilizes statistically\nanomalous intensity spikes in the Bragg peak to identify\nregions where abnormally large grains are contributing to\ndiffraction. This needle-in-a-haystack technique is\ndemonstrated on a nanocrystalline Niââ?¬â??Fe alloy which has\nundergone fatigue-induced abnormal grain growth. In this\ndemonstration, the technique readily identifies a few large\ngrains that occupy \\0.00001 % of the interrogation volume.\nWhile the technique is demonstrated in the current\nstudy on nanocrystalline metal, it would likely apply to any\nbimodal polycrystal including ultrafine grained and fine\nmicrocrystalline materials with sufficiently distinct bimodal\ngrain statistics....
Multiscale nondestructive characterization of coal microscopic physical structure can provide important information for coal\nconversion and coal-bed methane extraction. In this study, the physical structure of a coal sample was investigated by synchrotronbasedmultiple-\nenergy X-ray CT at three beamenergies and two different spatial resolutions. A data-constrained modeling (DCM)\napproach was used to quantitatively characterize the multiscale compositional distributions at the two resolutions. The volume\nfractions of each voxel for four different composition groups were obtained at the two resolutions. Between the two resolutions,\nthe difference for DCM computed volume fractions of coal matrix and pores is less than 0.3%, and the difference for mineral\ncomposition groups is less than 0.17%. This demonstrates that the DCM approach can account for compositions beyond the Xray\nCT imaging resolution with adequate accuracy. By using DCM, it is possible to characterize a relatively large coal sample at a\nrelatively low spatial resolution with minimal loss of the effect due to subpixel fine length scale structures....
We demonstrate a new analytical X-ray computed tomography technique for \nempirical linear relationship between the X-ray mass attenuation coefficient of the materials\nand X-ray energy was found for X-ray energies between 8 keV and 30 keV. The mass densityvisualizing\nand quantifying the mass density of materials comprised of low atomic number elements\nwith unknown atomic ratios. The mass density was obtained from the experimentally\nobserved ratio of the imaginary and real parts of the complex X-ray refractive index. An\nimage of two polymer fibers was quantified using the proposed technique using a scanning-\ntype X-ray micro beam computed tomography system equipped with a wedge\nabsorber. The reconstructed mass density agrees well with the calculated one....
Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have\nremained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication,\nremain central to almost all X-ray applications, there have been few fundamental technological advances. However, with\nthe emergence of ever more demanding medical and inspection techniques, including computed tomography\nand tomosynthesis, security inspection, high throughput manufacturing and radiotherapy, has resulted in a\nconsiderable level of interest in the development of new fabrication methods. The use of conventional thermionic\nsources is limited by their slow temporal response and large physical size. In response, field electron emission has\nemerged as a promising alternative means of deriving a highly controllable electron beam of a well-defined\ndistribution. When coupled to the burgeoning field of nanomaterials, and in particular, carbon nanotubes, such\nsystems present a unique technological opportunity. This review provides a summary of the current state-of-the-art in\ncarbon nanotube-based field emission X-ray sources. We detail the various fabrication techniques and functional\nadvantages associated with their use, including the ability to produce ever smaller electron beam assembles, shaped\ncathodes, enhanced temporal stability and emergent fast-switching pulsed sources. We conclude with an overview of\nsome of the commercial progress made towards the realisation of an innovative and disruptive technology...
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