Open Access

Show Abstract ▼

Abstract: Magnesium is attractive for the application as a temporary bone implant due to its inherent biodegradability, non-toxicity and suitable mechanical properties. The degradation process of magnesium in physiological environments is complex and is thought to be a diffusion-limited transport problem. We use a multi-scale imaging approach using micro computed tomography and transmission X-ray microscopy (TXM) at resolutions below 40 nm. Thus, we are able to evaluate the nanoporosity of the degradation layer and infer its impact on the degradation process of pure magnesium in two physiological solutions. Magnesium samples were degraded in simulated body fluid (SBF) or Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum (FBS) for one to four weeks. TXM reveals the three-dimensional interconnected pore network within the degradation layer for both solutions. The pore network morphology and degradation layer composition are similar for all samples. By contrast, the degradation layer thickness in samples degraded in SBF was significantly higher and more inhomogeneous than in DMEM+10%FBS. Distinct features could be observed within the degradation layer of samples degraded in SBF, suggesting the formation of microgalvanic cells, which are not present in samples degraded in DMEM+10%FBS. The results suggest that the nanoporosity of the degradation layer and the resulting ion diffusion processes therein have a limited influence on the overall degradation process. This indicates that the influence of organic components on the dampening of the degradation rate by the suppression of microgalvanic degradation is much greater in the present study.

Show Abstract ▼

Abstract: Ceria (cerium oxide) based materials have been widely used for catalytic applications including; fluid catalytic cracking, water gas shift reactions, solid oxide fuel cells and automotive catalytic after-treatment. It has often been the material of choice for supporting precious metals in many of the applications previously mentioned, as it can provide a high surface area and has excellent redox properties. This enables ceria to absorb oxygen into its structure and release it again, to facilitate oxidation reactions. Cerium is also one of the most abundant rare earth metals, making it a relatively cost effective material to use in industrial applications. For these reasons, cerium oxide remains to be a material of high interest for the development of cost-effective industrial catalysts and is, therefore, the focal material for this work. Modern internal combustion engine technology is being directed towards lean burn operation, involving higher air-fuel ratios in order to improve thermal efficiency. In turn, the operating temperature of modern engines is being reduced, providing less of the heat energy required to activate the catalysts in the after-treatment system. Therefore, in order to mitigate the emission from efficient lean burn engines and comply with emission regulations, the requirement for low temperature automotive catalysts is apparent. The objective of this work is to develop effective ceria based materials for the application of modern automotive after-treatment catalysis. The first approach is the use of a novel method for the preparation of a mixed oxide catalysts, which show high activity for CO and HC oxidation at low temperature. The preparation of ceria/manganese mixed oxide catalysts using a novel synthesis method has been studied and the materials were characterised to gain insights of their structure and morphology. Temperature programmed reactions using a complex mixture of reactants, before and after hydrothermal aging, were carried out to investigate their application for automotive emission control. The results from these studies were compared with more conventional ceria based materials that are often used for automotive after-treatment. A second approach used in this work is a post synthesis technique for the surface modification of ceria catalysts using ion bombardment. This technique was carried out on a commercially sourced, model catalyst. A Pt-CeO2/ZrO2 was treated with nitrogen ion irradiation and the effects of the adjustable parameters of the ion beam were investigated by carrying out temperature programmed reactions. The samples were also extensively characterised using XAFS and XPS techniques to understand the effects of the ion beam treatment on the structure, morphology and Pt dispersion of the materials.

Show Abstract ▼

Abstract: Hematite nanoparticles were synthesized with U(VI) in circumneutral water through a coprecipitation and hydrothermal treatment process. XRD, TEM, and EXAFS analyses reveal that uranium may aggregate along grain boundaries and occupy Fe sites within hematite. The described synthesis method produces crystalline, single-phase iron oxide nanoparticles absent of surface-bound uranyl complexes. EXAFS data were comparable to spectra from existing studies whose syntheses were more representative of naturally occurring, extended aging processes. This work provides and validates an accelerated method of synthesizing uranium-immobilized iron oxide nanoparticles for further mechanistic studies. High temperature oxide melt solution calorimetry measurements were performed to calculate the thermodynamic stability of uranium-incorporated iron oxide nanoparticles. Increasing uranium content within hematite resulted in more positive formation enthalpies. Standard formation enthalpies of UxFe2–2xO3 were as high as 76.88 ± 2.83 kJ/mol relative to their binary oxides, or -764.04 ± 3.74 kJ/mol relative to their constituent elements, at x = 0.037. Data on the thermodynamic stability of uranium retention pathways may assist in predicting waste uranyl remobilization, as well as in developing more effective methods to retain uranium captured from aqueous environments.

Open Access

Show Abstract ▼

Abstract: X-ray ptychography and X-ray fluorescence are complementary nanoscale imaging techniques, providing structural and elemental information, respectively. Both methods acquire data by scanning a localized beam across the sample. X-ray ptychography processes the transmission signal of a coherent illumination interacting with the sample, to produce images with a resolution finer than the illumination spot and step size. By enlarging both the spot and the step size, the technique can cover extended regions efficiently. X-ray fluorescence records the emitted spectra as the sample is scanned through the localized beam and its spatial resolution is limited by the spot and step size. The requisites for fast ptychography and high-resolution fluorescence appear incompatible. Here, a novel scheme that mitigates the difference in requirements is proposed. The method makes use of two probes of different sizes at the sample, generated by using two different energies for the probes and chromatic focusing optics. The different probe sizes allow to reduce the number of acquisition steps for the joint fluorescence–ptychography scan compared with a standard single beam scan, while imaging the same field of view. The new method is demonstrated experimentally using two undulator harmonics, a Fresnel zone plate and an energy discriminating photon counting detector.

Open Access

Show Abstract ▼

Abstract: The complex flow within the molten metallic pool, during advanced manufacturing and joining processes such as welding and additive manufacturing, directly affects the performance of the final components; through the generation of various microstructures and strain gradients. Understanding of the flow within such melt pools can enhance the predictability of the final microstructures and therefore component performance. In situ synchrotron X-ray imaging is employed to observe and map the evolving flow patterns within gas tungsten arc weld pools using tracking particles. The experimentally observed flow patterns are compared with numerical simulations to gain additional insights on accurate predictability in relevance to the physical driving forces within the flow. The spatio-temporal distribution of the flow using the mapped data is analyzed by considering the evolution of driving forces acting throughout the weld pool. The results demonstrate quantitative benchmark flow mapping with confirmation of the overall mechanisms of weld pool flow.