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Abstract: Soil structure is one of the most important environmental factors affecting root architectural development and consequently plant yield. Understanding how plant roots respond to soils with variable soil structure is important as it enables soil management practices that promote optimal root growth. Many contemporary, non-invasive experiments investigating how plant root architecture responds to soil structural variations have often focused on compaction, often neglecting the role of soil aggregate size in determining root configuration. To better understand this, in this study, we used non-invasive neutron and X-Ray imaging to investigate how variable aggregate size affects the early root architectural establishment in wheat plants. Sandy loam soil derived macro-aggregates of two distinct sizes (0.25−0.5 and 2−4 mm) were used to infer the suitability of each aggregate size for use in wheat seedbeds. We also grew wheat seedlings in partitioned containers with the two different aggregate size classes filled side by side to establish whether there would be preferential growth of roots in either aggregate size class. Our results showed significantly increased root growth in the smaller 0.25−0.5 mm aggregates as compared to the larger 2−4 mm aggregates. This was mainly as a result of enhanced lateral root growth when the wheat plants were grown in the finer aggregates. On the other hand, coarser aggregates induced significantly increased seminal root axes which partially offset the differences in total root length between the two aggregate sizes. Plants growing in partitioned containers similarly indicated preferential root growth in smaller aggregate with an even more pronounced difference in root growth in the smaller aggregates. As inferred from our results, seedbeds dominated by smaller macro-aggregates (finer soil tilth) may be optimal to enhance wheat seedling root growth in sandy loam soils.

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Abstract: Crystallisation is a complex process that significantly affects the rheology of magma, and thus the flow dynamics during a volcanic eruption. For example, the evolution of crystal fraction, size and shape has a strong impact on the surface crust formation of a lava flow, and accessing such information is essential for accurate modelling of lava flow dynamics. To investigate the role of crystallisation kinetics on lava flow behaviour, we performed real-time, in situ synchrotron X-ray microtomography, studying the influence of temperature-time paths on the nucleation and growth of clinopyroxene and plagioclase in an oxidised, nominally anhydrous basaltic magma. Crystallisation experiments were performed at atmospheric pressure in air and temperatures from 1250 °C to 1100 °C, using a bespoke high-temperature resistance furnace. Depending on the cooling regime (single step versus continuous), two different crystal phases (either clinopyroxene or plagioclase) were produced, and we quantified their growth from both global and individual 3D texture analyses. The textural evolution of charges suggests that suppression of crystal nucleation is due to changes in the melt composition with increasing undercooling and time. Using existing viscosity models, we inferred the effect of crystals on the viscosity evolution of our crystal-bearing samples to trace changes in rheological behaviour during lava emplacement. We observe that under continuous cooling, both the onsets of the pāhoehoe-‘a‘ā transition and of non-Newtonian behaviour occur within a shorter time frame. With varying both temperature and time, we also either reproduced or approached the clinopyroxene and plagioclase phenocryst abundances and compositions of the Etna lava used as starting material, demonstrating that real-time synchrotron X-ray tomography is an ideal approach to unravel the final solidification history of basaltic lavas. This imaging technology has indeed the potential to provide input into lava flow models and hence our ability to forecast volcanic hazards.

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Abstract: The addition of nano-fillers has been widely proposed as a method to enhance the dielectric properties of high voltage polymeric insulation, though there are mixed reports in the literature. Here the potential of silica nano-particles to extend the time to failure specifically through resistance to electrical tree growth in epoxy resin is determined. The benefit of silane treating the nano-particles before compounding is clearly established with regard to slowing tree growth and subsequent time to failure. The growth of trees in needle-plane samples is measured in the laboratory with loadings of 1, 3 and 5 wt% nano-filler. In all cases the average times to failure are extended, but silane treatment of the nano-particles prior to compounding yields much superior results. The emergence of a pronounced inception time before tree growth is also noted for the higher-filled, silane-treated cases. The average time to failure of silane-treated 5 wt% filled material was 28 times that of the unfilled resin. The improvement in performance between the nanocomposites with untreated and treated fillers is attributed to fewer agglomerations and improved dispersion of the filler in the treated cases. Measurements of Partial Discharge (PD) indicated significant differences in PD patterns during the growth of trees in the treated and untreated cases. This distinction may provide a quality control method for monitoring materials. In particular, long periods in which PDs were not measured were observed in the silane-treated cases. Visual imaging of tree growth in the unfilled material allowed the changing nature of the tree from fine to tree to dark tree to be observed as it grew. Corresponding PD measurements suggest the dark tree is gradually becoming conductive, and that growth of maximum PD measured is dependent on the relative rates of the growth of the tree and its carbonization. X-ray computer tomography identified significant differences in average tree channel diameters (a reduction from 2.8 µm to 2.0 µm for 1 wt% and 3 wt% cases). This implies that in addition to tree length changes, evaporated tree volumes also change and may explain the change in partial discharge characteristics observed.

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Abstract: Neutron or proton irradiation induced dislocation loops in Zr and its alloys are usually characterized by electron microscopy methods. In plastically deformed materials X-ray line profile analysis (XLPA) is now a well-established tool to determine dislocation densities (DDs) with excellent agreement between XLPA and TEM analysis. In irradiated zirconium alloys, however, XLPA determined DDs are often considerably larger than those obtained by TEM. In ion irradiated Cu and W it was shown that X-ray diffraction or MD simulations give significantly larger dislocation loop densities than conventional TEM analysis, which suggests that the smallest loops remain undetected by TEM. Based on these results we developed a new methodology to determine power-law size-distributions of irradiation-induced dislocation loops. We assume that only loops larger than a certain threshold are fully counted in TEM micrographs, whereas XLPA detects all the loops in the entire size range. This new analysis procedure shows that in neutron irradiated Zircaloy-2 in the channel-box materials the total DD is larger than in the cladding materials, even though TEM counting shows the opposite. We also found that there is a correlation between the reciprocal square-root of loop DDs and the diameter of loops. Our work shows that irradiation induced loop-formation and irradiation-damage in general can be better determined when we combine TEM investigations with XLPA.

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Abstract: The investigation of the properties of electron correlated materials under pressure remains a fertile arena for the discovery of novel electronic ground states, often beyond conventional wisdom. However, direct microscopic insight into pressure-driven quantum phenomena is certainly limited to a small number of techniques and usually hampered by the lack of suitable instrumentation. In this regard, resonant elastic x-ray scattering (REXS) is one of the best techniques that combines the elements of diffraction and spectroscopy, offering information about the atomic species, their positions in the crystal lattice and their electronic orbital configuration. Combining REXS with high-pressure (HP) presents an invaluable potential since pressure tuning of the interatomic electron-electron interactions in a crystal can be probed directly via the distortion of the lattice. Thus, HP-REXS experiments allow simultaneous observation of the crystallographic, magnetic and electronic degrees of freedom within the same experiment. This is extremely important in high-pressure studies where often inconsistencies stem from the use of different pressure devices or due to sample-dependent effects. Nevertheless, non-trivial technological challenges need to be overcome when developing the hardware and the methodology for HP-REXS experiments, mainly dictated by two factors. Firstly, the observation of electron-electron interatomic interactions frequently requires low-temperature, where the electronic ground state is free of thermal motion and, therefore, other energy scales such as the on-site Coulomb repulsion, the crystal field splitting or the spin-orbit coupling prevail over the electronic fluctuations induced by the temperature. Secondly, the absorption cross-section of materials is severe in the range of energies demanded to excite the elementary resonant processes of interest (typically below 15 keV), making the detection of weak magnetic reflections challenging. Moreover, the signal arising from these weak interactions gets further screened by the high-pressure device. The aim of this work is to provide a set of instrumentation for HP-REXS experiments on I16, the beamline for materials and magnetism at the Diamond Light Source (DLS). Likewise, to establish the working methodology and to collect REXS data at HP. The thesis begins with the introduction to synchrotron radiation and the fundamentals of REXS before describing the state-of-the-art of the HP instrumentation dedicated to x-ray studies under cryogenic conditions. Then, the new setup for HP-REXS experiments is described. It consists of a membrane-driven diamond anvil cell, a panoramic dome and an optical system for in situ pressure measurement using the ruby fluorescence method. The membrane cell presents an asymmetric layout for operating in back-scattering geometry, with a panoramic aperture of 100 degrees. This system allows the observation of resonant signals using excitation energies at least as low as ∼ 8 keV, within a temperature range of 30-300 K and up to 20 GPa for anvils of 500 µm in culet diameter. Finally, the thesis presents the results obtained from investigating the evolution of magnetic correlations in Sr3Ir2O7 and the lanthanum doped counterpart (Sr1-xLax)3Ir2O7 [x = 0.007(1)] upon application of hydrostatic pressure. The experimental evidence reveals the presence of long-range 3D magnetic order at up to at least 11 GPa of pressure. Combining the HP-REXS results with additional resonant inelastic x-ray scattering data and theoretical modelling a conclusion can be made about the presence of a spin-flop transition at the critical pressure of Pc ∼ 14 - 15 GPa, with putative short-range in-plane magnetic order above Pc. In summary, this thesis presents a set of instrumentation and detailed methodology for conducting REXS experiments under high-pressure.