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Abstract: Laser powder bed fusion (LPBF) can produce high-value metallic components for many industries; however, its adoption for safety-critical applications is hampered by the presence of imperfections. The interdependency between imperfections and processing parameters remains unclear. Here, the evolution of porosity and humps during LPBF using X-ray and electron imaging, and a high-fidelity multiphase process simulation, is quantified. The pore and keyhole formation mechanisms are driven by the mixing of high temperatures and high metal vapor concentrations in the keyhole is revealed. The irregular pores are formed via keyhole collapse, pore coalescence, and then pore entrapment by the solidification front. The mixing of the fast-moving vapor plume and molten pool induces a Kelvin–Helmholtz instability at the melt track surface, forming humps. X-ray imaging and a high-fidelity model are used to quantify the pore evolution kinetics, pore size distribution, waviness, surface roughness, and melt volume under single layer conditions. This work provides insights on key criteria that govern the formation of imperfections in LPBF and suggest ways to improve process reliability.

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Abstract: Methane-oxidizing bacteria play a central role in greenhouse gas mitigation and have potential applications in biomanufacturing. Their primary metabolic enzyme, particulate methane monooxygenase (pMMO), is housed in copper-induced intracytoplasmic membranes (ICMs), of which the function and biogenesis are not known. We show by serial cryo-focused ion beam (cryoFIB) milling/scanning electron microscope (SEM) volume imaging and lamellae-based cellular cryo-electron tomography (cryoET) that these ICMs are derived from the inner cell membrane. The pMMO trimer, resolved by cryoET and subtomogram averaging to 4.8 Å in the ICM, forms higher-order hexagonal arrays in intact cells. Array formation correlates with increased enzymatic activity, highlighting the importance of studying the enzyme in its native environment. These findings also demonstrate the power of cryoET to structurally characterize native membrane enzymes in the cellular context.

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Abstract: Age-related changes in bone microstructure can inform our understanding the biology of both extant and fossil birds, but to date, histological work in birds, and particularly work using high-resolution 3D imaging, has largely been restricted to limited growth stages. We used minimally destructive synchrotron radiation-based X-ray computed tomography to visualise and measure key morphological and histological traits in 3D across development in the domestic duck and ring-necked pheasant. We use these measurements to build on the database of key reference material for interpreting bone histology. We found that growth patterns differed between the two species, with the ducks showing rapid growth in their lower limbs and early lower limb maturation, while pheasants grew more slowly, reflecting their later age at maturity. In the pheasant, both walking and flight occur early and their upper and lower limbs grew at similar rates. In the duck, flight and wing development are delayed until the bird is almost at full body mass. Through juvenile development, the second moment of area for the duck wing was low but increased rapidly towards the age of flight, at which point it became significantly greater than that of the lower limb, or the pheasant. On a microstructural level, both cortical porosity and canal diameter were related to cortical bone deposition rate. In terms of orientation, vascular canals in the bone cortex were more laminar in the humerus and femur compared with the tibiotarsus, and laminarity increased through juvenile development in the humerus, but not the tibiotarsus, possibly reflecting torsional vs compressive loading. These age-related changes in cortical bone vascular microstructure of the domestic duck and pheasant will help understanding the biology of both extant and fossil birds, including age estimation, growth rate and growth patterns, and limb function.

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Abstract: In this work, a characterization and optimization of phase-sensitive X-ray imaging techniques with a focus on the field of laboratory astrophysics is given. Here, the advent of hard X-ray free electron lasers offers novel opportunities as single-pulse imaging with sub-picosecond temporal resolution becomes possible. The use of phase-sensitive techniques is often mandatory as micro- and nanoscopic samples show little or no attenuation contrast. In order to fully benefit from the short pulse lengths at X-ray free electron lasers, these methods should be reconcilable with single-exposure acquisition schemes. This task is complicated by zeroes in the respective transfer functions of the imaging systems. Therefore, direct inversions are typically not possible and sophisticated algorithms are required for the image reconstruction. Overall, this thesis mainly focuses upon the grating-based X-ray imaging technique, also known as Talbot interferometry. For comparison, propagation-based phase contrast imaging will also be considered. The investigations are divided into analytical considerations, numerical simulations, and experimental implementations of the respective imaging techniques. An analytical examination of the image formation within a Talbot interferometer is presented. This process can become complicated, especially for applications in X-ray microscopes. Here, transverse shifts of the interference pattern in general depend nonlinearly on the phase differences across the X-ray wave field. Existing reconstruction methods on the basis of deconvolutions then rely on idealized conditions, thus limiting the experimental applicability of the method. In addition, the achievable spatial resolution of Talbot interferometry in single-exposure applications is typically limited to the demagnified fringe period of the interference pattern. In order to resolve the limitations regarding the applicability, three novel reconstruction methods for Talbot interferometry are conceptualized and implemented: the design of a beam-splitting diffraction grating featuring only two diffraction orders, a two-stage deconvolution approach, and a statistical image reconstruction method based on an analytical forward model of the imaging process and a regularized maximum likelihood approach. The three schemes are validated on the basis of simulated data. They all prove advantageous when the premises for standard deconvolution-based reconstructions are not met. The statistical image reconstruction technique seems most promising as it achieves the best reconstruction quality at low photon numbers and also circumvents the abovementioned limitations regarding the spatial resolution. Building up on the simulative studies, two experimental realizations of Talbot interferometry at synchrotron light sources are presented. In the first experiment, the single-exposure phase imaging capabilities of Talbot interferometry in conjunction with the statistical image reconstruction method are investigated and characterized on the basis of simple test samples. The broadened experimental applicability is demonstrated through the retrieval of Fresnel diffraction images in an X-ray projection microscope. While a comparative implementation of propagation-based phase contrast imaging at the same instrument still yields a superior spatial resolution, the mitigation of limitations due to the fringe period is also verified experimentally. In the second experiment, single-exposure phase imaging with both the grating-based and the propagation-based approach is employed in order to monitor the moistening process of wood on the level of single wood cells...

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Abstract: Purpose: Freshwater is an increasingly scarce natural resource, essential for agricultural production. As plants consume 70% of the world’s freshwater, a reduction in their water use would greatly reduce global water scarcity. Plants with improved Water Use Efficiency (WUE) such as those with altered expression of the Epidermal Patterning Factor (EPF) family of genes regulating stomatal density, could help reduce plant water footprint. Little however, is known about how this modification in Arabidopsis thaliana. L. affects root architectural development in soil, thus we aim to improve our understanding of root growth when stomatal density is altered. Methods: We used X-Ray synchrotron and neutron imaging to measure in three dimensions, the root system architecture (RSA) of Arabidopsis thaliana. L. plants of three different genotypes, namely that of the wild type Columbia (Col 0) and two different EPF mutants, EPF2OE and epf2-1 (which show reduced and increased stomatal density, respectively). We also used the total biomass and carbon isotope discrimination (Δ) methods to determine how WUE varies in these genotypes when grown in a sandy loam soil under controlled conditions. Results: Our results confirm that the EPF2OE line had superior WUE as compared to the wild type using both the Δ and total biomass method. The epf2-1 mutant, on the other hand, had significantly reduced WUE using the Δ but not with the biomass method. In terms of root growth, the RSAs of the different genotypes had no significant difference between each other. There was also no significant difference in rhizosphere porosity around their roots as compared to bulk soil for all genotypes. Conclusion: Our results indicate that the EPF mutation altering stomatal density in Arabidopsis thaliana. L. plants did not have an adverse effect on root characteristics thus their wide adoption to reduce the global freshwater footprint is unlikely to compromise their soil foraging ability.