New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.     

Emerging X-ray imaging technologies for energy materials

With the growing need for sustainable energy technologies, advanced characterization methods become more and more critical for optimizing energy materials and understanding their operation mechanisms. In this review, we focus on the synchrotron-based X-ray imaging technologies and the associated applications in gaining fundamental insights into the physical/chemical properties and reaction mechanisms of energy materials. We will discuss a few major X-ray imaging technologies, including X-ray projection imaging, transmission X-ray microscopy, scanning transmission X-ray microscopy, tender and soft X-ray imaging, and coherent diffraction imaging. Researchers can choose from various X-ray imaging techniques with different working principles based on research goals and sample specifications. With the X-ray imaging techniques, we can obtain the morphology, phase, lattice and strain information of energy materials in both 2D and 3D in an intuitive way. In addition, with the high-penetration X-rays and the high-brilliance synchrotron sources, operando/in-situ experiments can be designed to track the qualitative and quantitative changes of the samples during operation. We expect this review can broaden readers’ view on X-ray imaging techniques and inspire new ideas and possibilities in energy materials research.     

Neutron laminography—a novel approach to three-dimensional imaging of flat objects with neutrons

Computed tomography (CT) is a three-dimensional (3D) imaging method which, for compact or prolate (i.e. rather isotropically extended around the rotation axis) specimens, can yield artefact-free reconstructed cross-sections. Laterally extended specimens like plate-like objects, however, are much less amenable to CT since reliable projection data cannot be acquired from angles where the plate is oriented parallel to the irradiation direction.To overcome this drawback, computed laminography (CL) was introduced recently to imaging set-ups at synchrotron storage rings. Here, we report on the first implementation of computed laminography with neutron radiation, showing measurements that were performed at the ANTARES neutron imaging facility at the FRM II research reactor of Technische Universität München. In general, neutrons are highly interesting probes for imaging since they provide a sensitivity to chemical elements very different from X-rays, yielding complementary information about the specimens investigated. Like for X-ray laminography, we avoid the projection directions where the beam is parallel to the long extensions of the specimen. We accomplish this by tilting of the rotation axis with respect to the transmitted-beam to an angle smaller than 90° (which would be the limiting case of CT) and roughly aligning the specimen's surface normal parallel to this rotation axis. The principles of neutron laminography are introduced and first test experiments are described.     

Accurate micro-computed tomography imaging of pore spaces in collagen-based scaffold

In this work we have used X-ray micro-computed tomography (μCT) as a method to observe the morphology of 3D porous pure collagen and collagen-composite scaffolds useful in tissue engineering. Two aspects of visualizations were taken into consideration: improvement of the scan and investigation of its sensitivity to the scan parameters. Due to the low material density some parts of collagen scaffolds are invisible in a μCT scan. Therefore, here we present different contrast agents, which increase the contrast of the scanned biopolymeric sample for μCT visualization. The increase of contrast of collagenous scaffolds was performed with ceramic hydroxyapatite microparticles (HAp), silver ions (Ag⁺) and silver nanoparticles (Ag-NPs). Since a relatively small change in imaging parameters (e.g. in 3D volume rendering, threshold value and μCT acquisition conditions) leads to a completely different visualized pattern, we have optimized these parameters to obtain the most realistic picture for visual and qualitative evaluation of the biopolymeric scaffold. Moreover, scaffold images were stereoscopically visualized in order to better see the 3D biopolymer composite scaffold morphology. However, the optimized visualization has some discontinuities in zoomed view, which can be problematic for further analysis of interconnected pores by commonly used numerical methods. Therefore, we applied the locally adaptive method to solve discontinuities issue. The combination of contrast agent and imaging techniques presented in this paper help us to better understand the structure and morphology of the biopolymeric scaffold that is crucial in the design of new biomaterials useful in tissue engineering.     

Progress in electron tomography to assess the 3D nanostructure of catalysts

The activity, selectivity and stability of solid catalysts depend critically on the details of their structure at all relevant length scales. Electron tomography (or 3D-TEM) has emerged as a powerful technique for nanostructural characterization. In this review we highlight recent advances in the field of electron tomography for the analysis of solid catalyst. Several examples demonstrate how unique quantitative information can be derived on relevant structural properties such as pore connectivity and corrugation, particle size distributions, and the 3D location of metal nanoparticles in porous oxide or carbon supports. The development of high-resolution imaging and novel reconstruction algorithms is promising to obtain atomically resolved electron tomograms of single catalyst nanoparticles. New reconstruction algorithms allow reconstruction from only a few projections, and hold potential for analyzing beam sensitive samples, as well as for time resolved electron tomography. Element specific or ‘chemical’ electron tomography, using electron energy-loss (EELS) or energy-dispersive X-ray spectroscopy (EDX), is an emerging tool for obtaining both chemical and structural information at nanoscale resolution. The rapid progress in electron tomography over the past few years holds great promise for detailed and quantitative insight into relevant nanostructural properties, thus allowing us to further develop our understanding of the relation between nanostructure and performance for catalysts and related materials.     

Toward atomic-scale bright-field electron tomography for the study of fullerene-like nanostructures

We present the advancement of electron tomography for three-dimensional structure reconstruction of fullerene-like particles toward atomic-scale resolution. The three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is achieved by the combination of low voltage operation of the electron microscope with aberration-corrected phase contrast imaging. The method enables the study of defects and irregularities in the three-dimensional structure of individual fullerene-like particles on the scale of 2-3 A. Control over shape, size, and atomic architecture is a key issue in synthesis and design of functional nanoparticles. Transmission electron microscopy (TEM) is the primary technique to characterize materials down to the atomic level, albeit the images are two-dimensional projections of the studied objects. Recent advancements in aberration-corrected TEM have demonstrated single atom sensitivity for light elements at sub?ngstr?m resolution. Yet, the resolution of tomographic schemes for three-dimensional structure reconstruction has not surpassed 1 nm3, preventing it from becoming a powerful tool for characterization in the physical sciences on the atomic scale. Here we demonstrate that negative spherical aberration imaging at low acceleration voltage enables tomography down to the atomic scale at reduced radiation damage. First experimental data on the three-dimensional reconstruction of nested molybdenum disulfide nanooctahedra is presented. The method is applicable to the analysis of the atomic architecture of a wide range of nanostructures where strong electron channeling is absent, in particular to carbon fullerenes and inorganic fullerenes.     

X-ray Raman spectroscopy of carbon in asphaltene: light element characterization with bulk sensitivity

X-ray Raman spectra of the carbon K-edge have been recorded using 6.461 keV radiation for a petroleum asphaltene. By comparison with coronene, graphite, and paraffin standards, the asphaltene spectrum is seen to be composed of contributions from saturated and aromatic carbon species. The information contained in the carbon K-edge was extracted with bulk (approximately 1 mm) sensitivity, because the Raman method used hard X-rays. This helps alleviate concerns about surface artifacts that frequently occur with soft X-ray spectroscopy of light elements. X-ray Raman spectroscopy shows great potential for characterization of light elements in fuels, catalysts, and other complex materials under chemically relevant conditions.     

Micro X-ray fluorescence imaging without scans: toward an element-selective movie

Micro X-ray fluorescence imaging is a promising method for obtaining positional distribution on specific elements in a nondestructive manner. So far, the technique has usually been performed by a 2D positional scan of a sample against a collimated beam. However, the total measuring time can become quite long, since a number of scanning points are needed in order to obtain a high-quality image. The present report discusses a completely different way of performing imaging of elements much more quickly. A combination of grazing-incidence geometry using a rather wide beam and parallel optics for detecting X-rays can produce an X-ray fluorescence image with approximately 1 M pixels and with approximately 20-microm resolution in 1-2 min or less. The technique has the potential to open up new frontiers in X-ray imaging, particularly in element-selective movie applications.     

High-Resolution and Low-Voltage FE-SEM Imaging and Microanalysis in Materials Characterization

Nanometer-resolution imaging in field-emission SEM (FE-SEM) instruments is now widely used in materials characterization. The use of a high-brightness field-emission gun and a high-resolution lens system makes it possible to acquire nanometer-resolution surface images at low voltages (<5kV). The advantages of low-voltage FE-SEM include enhanced surface sensitivity, reduced sample charging for nonconducting materials, reduced damage of delicate samples, and significantly reduced electron range and interaction volume in bulk samples. For microanalysis using characteristic X-ray signals, the spatial resolution is significantly improved and the surface sensitivity is enhanced because of the fall of electron range at low voltages. With further development of high energy resolution X-ray detectors and probe-forming lenses, low-voltage imaging and microanalysis in FE-SEM instruments will be competitive both in spatial resolution and in chemical sensitivity to those now achievable in analytical TEM instruments. Applications of low-voltage SE imaging and microanalysis techniques to the study of various types of bulk materials are discussed.     

Local Tomography Study of the Fracture of an ERG Metal Foam

As one of the most important analysis techniques for non‐destructive imaging, X‐ray tomography has been widely used in materials science, medical science, and industry to evaluate the behavior of porous materials. By using this method, a three dimensional volume can be inspected in order to visualize in situ the progress of damage in materials and this can be analyzed qualitatively and quantitatively. In the present study, we have used X‐ray tomography to investigate the fracture behavior of an ERG open cell aluminum foam. The process of damage development of a sample undergoing tension and the relation between the inter‐metallics and the cracks can be observed totally by the X‐ray tomography set‐up. Local tomography has in particular been used to image the microstructure at high resolution. A finite element model has also been developed in order to simulate this process of the damage using the 3D data obtained by the tomography.     

Reconstruction procedure for 3D micro X-ray absorption fine structure

A new approach for chemical speciation in stratified systems using 3D Micro-XAFS spectroscopy is developed by combining 3D Micro X-ray Fluorescence Spectroscopy (3D Micro-XRF) and conventional X-ray Absorption Fine Structure Spectroscopy (XAFS). A prominent field of application is stratified materials within which depth-resolved chemical speciation is required. Measurements are collected in fluorescence mode which in general lead to distorted spectra due to absorption effects. Developing a reliable reconstruction algorithm for obtaining undistorted spectra for superficial and in-depth layers is proposed and validated. The developed algorithm calculates the attenuation coefficients of the analyte for the successive layers facilitating a new spectroscopic tool for three-dimensionally resolved nondestructive chemical speciation.     

Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.     

X-ray tomography applied to the characterization of cellular materials. Related finite element modeling problems

The analyses of several materials exhibiting a cellular structure have been carried out using X-ray tomography. This new technique allows the three dimensional and non destructive visualisation of the studied materials at the scale of their cellular microstructure. Qualitative examples are given for metal foams, bread and cellular concrete. The similarity between these materials is striking. It has been measured by quantitative 3D image processing. The different Finite Element Methods available today to produce meshes from these images are presented and discussed in the final part of this paper.     

A Review of X-Ray Imaging at the BAMline (BESSY II)

The hard X-ray beamline BAMline at BESSY II (Berlin, Germany) has now been in service for 20 years. Several improvements have been implemented in this time, and this review provides an overview of the imaging methods available at the BAMline. Besides classic full-field synchrotron X-ray computed tomography (SXCT), also absorption edge CT, synchrotron X-ray refraction radiography (SXRR), and synchrotron X-ray refraction tomography (SXRCT) are used for imaging. Moreover, virtually any of those techniques are currently coupled in situ or operando with ancillary equipment such as load rigs, furnaces, or potentiostats. Each of the available techniques is explained and both the current and the potential usage are described with corresponding examples. The potential use is manifold, the examples cover organic materials, composite materials, energy-related materials, biological samples, and materials related to additive manufacturing. The article includes published examples as well as some unpublished applications.     

X-ray texture tomography of near-surface areas

The author’s aim was to develop a research tool for non-destructive investigation of space arrangement of crystallites in the near-surface areas of structural elements and materials of gradient or laminar structure. Such composites are as a rule structurally inhomogeneous and are numbered among modern class so called functionally graded materials. Their physical properties depend to a great extent on crystallographic texture, the effective analysis of which can be carried out only in a way not disturbing the subtle arrangement of the layers, formed most often deliberately in a technologically advanced process.The X-ray texture tomography, described in the study and verified experimentally, represents a non-invasive method of investigating the texture of the near-surface areas on X-ray penetration depth, usually up to 100 μm. It allows to localise the texture changes occurring under the sample surface to a certain definite depth.X-ray texture tomography consists in the registration of the diffraction effects according to a definite method and their transformation according to the developed procedure utilising the available calculation methods. As a result an information set is obtained helpful at the interpretation of various structural effects, quality control of deposited coatings and in designing the production technology of new materials of specific properties. Using texture tomography it is possible to investigate such problems as; anisotropy of physical properties, inhomogeneity and heredity of texture, distribution of residual stresses, fatigue wear of surface.In spite of some limitations, the described method adds the missing element to make the set of the research tools of the microstructure and supplements the electron microscopy wherever the scale of the examined phenomenon is beyond the nano-metric area. An essential of this research procedure is the possibility of its automatisation which facilitates its application not only in scientific laboratories but also in industry.Introduced classification of texture inhomogeneity made on the basis of the authoritative components, the methods of its evaluation and a precise measure of this effect, defined as the degree of inhomogeneity, makes complete the developed method of texture tomography.Experience gained by the author in the course of elaborating and practical application of the described method at the research laboratory of the Aleksander Krupkowski Institute of Metallurgy and Materials Science of the Polish Academy of Sciences (IMMS PAS) in Krakow has made him believe that texture inhomogeneity and its heredity are still not adequately studied areas of structure analysis considering their importance for the present-day materials engineering. It is expected that X-ray texture tomography will contribute to the knowledge in this field.     

Ultrabroadband Photodetectors up to 10.6 µm Based on 2D Fe3O4 Nanosheets

The ultrabroadband spectrum detection from ultraviolet (UV) to long-wavelength infrared (LWIR) is promising for diversified optoelectronic applications of imaging, sensing, and communication. However, the current LWIR-detecting devices suffer from low photoresponsivity, high cost, and cryogenic environment. Herein, a high-performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air-stable nonlayered ultrathin Fe₃O₄ nanosheets synthesized via a space-confined chemical vapor deposition (CVD) method. Ultrahigh photoresponsivity (R) of 561.2 A W⁻¹, external quantum efficiency (EQE) of 6.6 × 10³%, and detectivity (D*) of 7.42 × 10⁸ Jones are achieved at the wavelength of 10.6 µm. The multimechanism synergistic effect of photoconductive effect and bolometric effect demonstrates the high sensitivity for light with any light intensities. The outstanding device performance and complementary mixing photoresponse mechanisms open up new potential applications of nonlayered 2D materials for future infrared optoelectronic devices.     

3D Strain Field Measurement by Correlation of Volume Images Using Scattered Light: Recording of Images and Choice of Marks

Abstract:  We have extended the Digital Image Correlation technique to the case in three dimensions. This new technique, allowing the full three-dimensional (3D) strain measurement in the bulk of a solid, needs volume images containing a 3D variation of the grey levels. Generally, volume images are obtained by X-ray computed tomography. In this paper, we present a procedure that is easier to implement and enables to generate volume image in transparent materials. The principle consists in the optical slicing of the specimen. To obtain a random distribution of grey levels within the volume image, we use the scattered light phenomenon induced by particles included in the specimen. The recording of 3D images by optical slicing is presented and the influence of different kinds of particles on the scattered light and on the accuracy of measurement is described. Through several tests involving rigid body displacements and a tensile test we show the performance of this technique and we evaluate the measurement error of displacement and strain components.     

Computational methods for materials characterization by electron tomography

Electron tomography (ET) is a powerful imaging technique that enables thorough three-dimensional (3D) analysis of materials at the nanometre and even atomic level. The recent technical advances have established ET as an invaluable tool to carry out detailed 3D morphological studies and derive quantitative structural information. Originally from life sciences, ET was rapidly adapted to this field and has already provided new and unique insights into a variety of materials. The principles of ET are based on the acquisition of a series of images from the sample at different views, which are subsequently processed and combined to yield the 3D volume or tomogram. Thereafter, the tomogram is subjected to 3D visualization and post-processing for proper interpretation. Computation is of utmost importance throughout the process and the development of advanced specific methods is proving to be essential to fully take advantage of ET in materials science. This article aims to comprehensively review the computational methods involved in these ET studies, from image acquisition to tomogram interpretation, with special focus on the emerging methods.     

High-resolution 3D imaging of microvascular architecture in human glioma tissues using X-ray phase-contrast computed tomography as a potential adjunct to histopathology

Gliomas are rich in blood vessels, and the generation of tumor-associated vessels plays an important role in glioma growth and transfer. Histology can directly depict microvascular architecture in the tumor, but it just provides two-dimensional (2D) images obtained by destroying three-dimensional (3D) tissue specimens. There is a lack of high-resolution 3D imaging methods for observing the microvasculature throughout the entire specimens. X-ray phase-contrast computed tomography (PCCT) which is an emerging imaging method has demonstrated its outstanding potential in imaging soft tissues. Thus, this study aims to evaluate the potential of PCCT as an adjunct to histopathology in nondestructive and 3D visualization of the microvascular architecture in human glioma tissues. In this study, seven resected glioma tissues were scanned via PCCT and then processed histologically. The obtained PCCT data was analyzed and compared with corresponding histological results. Significant anatomical structures of the glioma such as microvessels, thrombi inside the microvessels, and areas of vascular proliferation could be clearly presented via PCCT, confirmed by the histological findings. Moreover, PCCT data also provided additional 3D information such as morphological alterations of the microvasculature, 3D distribution of the thrombi and stenosis severity of the vessels in glioma tissues, which cannot be fully analyzed in 2D histological slices. In conclusion, this study demonstrated that PCCT can offer excellent images at a near-histological level and additional valuable information in screening gliomas, without impeding further histological investigations. Thus, this technique could be potentially used as an adjunct to conventional histopathology in 3D nondestructive characterization of glioma vasculature.