X-ray Dark-Field Imaging for Building Materials Characterization

Abstract

Material scientists optimize materials and develop new ones to improve their durability, sustainability, and functionality. An important part of this goal is the characterization of the internal structure on different length scales, as changes in the nano- and micrometer ranges can influence the properties of the material on macroscopic scales. A better understanding of the structure therefore enables the production of new, higher-quality materials and the efficient use of existing materials. Although there are several standardized techniques for structural characterization at different length scales, such as mercury porosity intrusion or x-ray micro-computed tomography (µCT), the analysis of nanostructure remains a challenge. To date, the nanoscale can be assessed using high-precision quantitative methods, but without visual information about the structure. High-resolution imaging techniques can image the surface of the nanostructure but are limited to small, non-representative areas of up to a few micrometers. The most advanced techniques for material structural characterization at high resolution, such as ptychography or high-resolution $\mu$CT, require high-brilliance radiation sources such as synchrotrons, which are expensive and not easily accessible. Consequently, there is a need for novel characterization techniques that, on one side, allow analysis and visualization in the nano-scale range within representative material volumes and, on the other side, are easily accessible to the community. This dissertation investigates the potential of table-top x-ray dark-field imaging to locally characterize nanoscale features of building materials with sample sizes in the millimeter range and fields of view of a few centimeters. The work is divided into three stages. In the first part, a dual-phase grating interferometer for dark-field imaging was developed that is sensitive to the signal of structures on the nanometer scale. A simulation framework based on Fresnel wave propagation was developed to optimize the parameters of the interferometer, such as the grating parameters. Based on the optimization results, two sets of gratings were produced: one set made of silicon for a design energy of 22 keV, and one made of gold for a design energy of 40.8 keV. The challenging production of the gratings was successfully carried out at the Paul Scherrer Institute (PSI) in Switzerland. In the second stage, within a transfer of knowledge scheme, the gratings were integrated into a conventional µCT system at the Center of X-ray Tomography of the Ghent University (UGCT). The commissioned interferometer is sensitive to structures in the range of a few to hundreds of nanometers with fields of view of a few centimeters. The ability of dual-phase X-ray grating interferometry (DP-XGI) to identify the nanostructure of samples with complex internal structures was demonstrated with natural wood and Ketton limestones, which are relevant materials to the construction industry. Structural features in a range of hundreds of nanometers were identified by dark-field contrast retrieval. Due to the use of a conventional x-ray source and its compact configuration, this method promises to be more accessible and less expensive than synchrotron imaging. Finally, using the link between small-angle scattering and dark-field contrast, the correlation function in the spatial domain of two alumina samples with different grain sizes was measured and modeled to derive quantitative information about the nanostructure. The results were validated using small-angle scattering and Talbot interferometry measurements at the Swiss Light Source (SLS). These results demonstrate that the dual-phase interferometer is capable of quantifying grain sizes of hundreds of nanometres, showcasing the potential of the interferometer to distinguish structural features in the nanometre range. In this thesis, a dual-phase interferometer has been successfully developed, implemented in a conventional µCT, and used to quantify the nanometer structure of building materials. Thereby, it demonstrates the potential of x-ray dark-field imaging with a DP-XGI as a highly sensitive and useful imaging method for nanoscale material characterization. The method can easily be applied to other areas of science and industry due to the use of conventional X-ray sources and its compact design. This project was developed within the framework of the Moccha CT project, a collaboration with The Laboratory of Wood Technology (UGent-Woodlab) and The Pore-scale Processes in Geomaterials Research group (UGent-PProGRess) at the University of Ghent in Belgium.