A new approach to synchrotron energy-dispersive X-ray diffraction computed tomography

X-ray Diffraction Computed Nanotomography Applied to Solve the Structure of Hierarchically Phase-Separated Metallic Glass

The structure of matter at the nanoscale, in particular that of amorphous metallic alloys, is of vital importance for functionalization. With the availability of synchrotron radiation, it is now possible to visualize the internal features of metallic samples without physically destroying them. Methods based on computed tomography have recently been employed to explore the local features. Tomographic reconstruction, while it is relatively uncomplicated for crystalline materials, may generate undesired artifacts when applied to featureless amorphous or nanostructured metallic alloys. In this study we show that X-ray diffraction computed nanotomography can provide accurate details of the internal structure of a metallic glass. We demonstrate the power of the method by applying it to a hierarchically phase-separated amorphous sample with a small volume fraction of crystalline inclusions, focusing the X-ray beam to 500 nm and ensuring a sub-micrometer 2D resolution via the number of scans.

2D/3D Microanalysis by Energy Dispersive X-ray Absorption Spectroscopy Tomography

X-ray spectroscopic techniques have proven to be particularly useful in elucidating the molecular and electronic structural information of chemically heterogeneous and complex micro- and nano-structured materials. However, spatially resolved chemical characterization at the micrometre scale remains a challenge. Here, we report the novel hyperspectral technique of micro Energy Dispersive X-ray Absorption Spectroscopy (μED-XAS) tomography which can resolve in both 2D and 3D the spatial distribution of chemical species through the reconstruction of XANES spectra. To document the capability of the technique in resolving chemical species, we first analyse a sample containing 2-30 μm grains of various ferrous- and ferric-iron containing minerals, including hypersthene, magnetite and hematite, distributed in a light matrix of a resin. We accurately obtain the XANES spectra at the Fe K-edge of these four standards, with spatial resolution of 3 μm. Subsequently, a sample of ~1.9 billion-year-old microfossil from the Gunflint Formation in Canada is investigated, and for the first time ever, we are able to locally identify the oxidation state of iron compounds encrusting the 5 to 10 μm microfossils. Our results highlight the potential for attaining new insights into Precambrian ecosystems and the composition of Earth's earliest life forms.

Dark-field hyperspectral X-ray imaging

In recent times, there has been a drive to develop non-destructive X-ray imaging techniques that provide chemical or physical insight. To date, these methods have generally been limited; either requiring raster scanning of pencil beams, using narrow bandwidth radiation and/or limited to small samples. We have developed a novel full-field radiographic imaging technique that enables the entire physio-chemical state of an object to be imaged in a single snapshot. The method is sensitive to emitted and scattered radiation, using a spectral imaging detector and polychromatic hard X-radiation, making it particularly useful for studying large dense samples for materials science and engineering applications. The method and its extension to three-dimensional imaging is validated with a series of test objects and demonstrated to directly image the crystallographic preferred orientation and formed precipitates across an aluminium alloy friction stir weld section.

Pair distribution function computed tomography

An emerging theme of modern composites and devices is the coupling of nanostructural properties of materials with their targeted arrangement at the microscale. Of the imaging techniques developed that provide insight into such designer materials and devices, those based on diffraction are particularly useful. However, to date, these have been heavily restrictive, providing information only on materials that exhibit high crystallographic ordering. Here we describe a method that uses a combination of X-ray atomic pair distribution function analysis and computed tomography to overcome this limitation. It allows the structure of nanocrystalline and amorphous materials to be identified, quantified and mapped. We demonstrate the method with a phantom object and subsequently apply it to resolving, in situ, the physicochemical states of a heterogeneous catalyst system. The method may have potential impact across a range of disciplines from materials science, biomaterials, geology, environmental science, palaeontology and cultural heritage to health.

Energy-dispersive X-ray diffraction mapping on a benchtop X-ray fluorescence system

A method for energy-dispersive X-ray diffraction mapping is presented, using a conventional low-power benchtop X-ray fluorescence spectrometer, the Seiko Instruments SEA6000VX. Hyper spectral X-ray maps with a 10 µm step size were collected from polished metal surfaces, sectioned Bi, Pb and steel shot gun pellets. Candidate diffraction lines were identified by eliminating those that matched a characteristic line for an element and those predicted for escape peaks, sum peaks, and Rayleigh and Compton scattered primary X-rays. The maps showed that the crystallites in the Bi pellet were larger than those observed in the Pb and steel pellets. The application of benchtop spectrometers to energy-dispersive X-ray diffraction mapping is discussed, and the capability for lower atomic number and lower-symmetry materials is briefly explored using multi-crystalline Si and polycrystalline sucrose.

Understanding cement hydration at the microscale: new opportunities from `pencil-beam' synchrotron X-ray diffraction tomography

The present work describes some new improvements concerning the analysis of cement hydration processes using `pencil-beam' synchrotron X-ray diffraction tomography. (i) A new filtering procedure, applied to the diffraction images, has been developed to separate the powder-like contribution from that of the grains in the diffraction images. (ii) In addition to improving the quality of the diffraction images for the subsequent analysis and tomographic reconstruction, the filtering procedure can also be used to perform a qualitative analysis of the crystallite size distribution, whenever the more standard approaches cannot be applied. (iii) Given the importance of the calcium silicate hydrate phase (C–S–H) in cements, a procedure to obtain its spatial distribution using the diffraction signal has been successfully applied, even though C–S–H is a highly disordered phase, almost amorphous to X-ray diffraction. (iv) The main result of this study has been to show that, in spite of the long measurement times required, it is possible to usein situexperiments at different aging times of cement pastes to monitor the cement evolution. This allowed the evolution of the microstructure during the acceleration and deceleration periods of the hydration process to be checked with unprecedented detail, since the quantitative spatial distribution of each phase (including C–S–H) dissolved or precipitated in the sample has been obtained. The reported approach opens up a range of opportunities for the investigation of complex multiphase systems and processes, including hydration and microstructural development in cements.

Diffraction/scattering computed tomography for three-dimensional characterization of multi-phase crystalline and amorphous materials

The three-dimensional characterization method described herein is based on diffraction and scattering techniques combined with tomography and uses the variation of these signals to reconstruct a two-dimensional/three-dimensional structural image. To emphasize the capability of the method in discriminating between different poorly ordered phases, it is named diffraction/scattering computed tomography (DSCT). This combination not only allows structural imaging but also yields an enhancement of the weak signals coming from minor phases, thereby increasing the sensitivity of structural probes. This article reports the suitability of the method for discrimination of polycrystalline and amorphous phases and for extraction of their selective local patterns with a contrast sensitivity of about 0.1% in weight of minor phases relative to the matrix. The required background in tomography is given and then the selectivity of scattering signal, the efficiency of the method, reconstruction artefacts and limitations are addressed. The approach is illustrated through different examples covering a large range of applications based on recent literature, showing the potential of DSCT in crystallography and materials science, particularly when functional and/or precious samples with sub-micrometre features have to be investigated in a nondestructive way.