Target Design in SEM-Based Nano-CT and Its Influence on X-ray Imaging.

Nano-computed tomography (nano-CT) based on scanning electron microscopy (SEM) is utilized for multimodal material characterization in one instrument. Since SEM-based CT uses geometrical magnification, X-ray targets can be adapted without any further changes to the system. This allows for designing targets with varying geometry and chemical composition to influence the X-ray focal spot, intensity and energy distribution with the aim to enhance the image quality. In this paper, three different target geometries with a varying volume are presented: bulk, foil and needle target. Based on the analyzed electron beam properties and X-ray beam path, the influence of the different target designs on X-ray imaging is investigated. With the obtained information, three targets for different applications are recommended. A platinum (Pt) bulk target tilted by 25° as an optimal combination of high photon flux and spatial resolution is used for fast CT scans and the investigation of high-absorbing or large sample volumes. To image low-absorbing materials, e.g., polymers or organic materials, a target material with a characteristic line energy right above the detector energy threshold is recommended. In the case of the observed system, we used a 30° tilted chromium (Cr) target, leading to a higher image contrast. To reach a maximum spatial resolution of about 100 nm, we recommend a tungsten (W) needle target with a tip diameter of about 100 nm.

Characterization of Filigree Additively Manufactured NiTi Structures Using Micro Tomography and Micromechanical Testing for Metamaterial Material Models.

This study focuses on the influence of additive manufacturing process strategies on the specimen geometry, porosity, microstructure and mechanical properties as well as their impacts on the design of metamaterials. Filigree additively manufactured NiTi specimens with diameters between 180 and 350 µm and a nominal composition of Ni50.9Ti49.1 (at %) were processed by laser powder bed fusion in a first step. Secondly, they structures were characterized by optical and electron microscopy as well as micro tomography to investigate the interrelations between the process parameters, specimen diameters and microstructure. Each specimen was finally tested in a micro tensile machine to acquire the mechanical performance. The process strategy had, besides the resulting specimen diameter, an impact on the microstructure (grain size) without negatively influencing its quality (porosity). All specimens revealed a superelastic response while the critical martensitic phase transition stress decreased with the applied vector length. As a conclusion, and since the design of programmable metamaterials relies on the accuracy of FEM simulations, precise and resource-efficient testing of filigree and complex structures remains an important part of creating a new type of metamaterials with locally adjusted material behavior.

A FIB-SEM Based Correlative Methodology for X-Ray Nanotomography and Secondary Ion Mass Spectrometry: An Application Example in Lithium Batteries Research.

Correlative microscopy approaches are attracting considerable interest in several research fields such as materials and battery research. Recent developments regarding X-ray computer tomography have made this technique available in a compact module for scanning electron microscopes (SEMs). Nano-computed tomography (nanoCT) allows morphological analysis of samples in a nondestructive way and to generate 2D and 3D overviews. However, morphological analysis alone is not sufficient for advanced studies, and to draw conclusions beyond morphology, chemical analysis is needed. While conventional SEM-based chemical analysis techniques such as energy-dispersive X-ray spectroscopy (EDS) are adequate in many cases, they are not well suited for the analysis of trace elements and low-Z elements such as hydrogen or lithium. Furthermore, the large information depth in typical SEM-EDS imaging conditions limits the lateral resolution to micrometer length scales. In contrast, secondary ion mass spectrometry (SIMS) can perform elemental mapping with good surface sensitivity, nanoscale lateral resolution, and the possibility to analyze even low-Z elements and isotopes. In this study, we demonstrate the feasibility and compatibility of a novel FIB-SEM-based correlative nanoCT-SIMS imaging approach to correlate morphological and chemical data of the exact same sample volume, using a cathode material of a commercial lithium battery as an example.