Compressed sensing electron tomography using adaptive dictionaries: a simulation study

Electron tomography (ET) is an increasingly important technique for examining the three-dimensional morphologies of nanostructures. ET involves the acquisition of a set of 2D projection images to be reconstructed into a volumetric image by solving an inverse problem. However, due to limitations in the acquisition process this inverse problem is considered ill-posed (i.e., no unique solution exists). Furthermore reconstruction usually suffers from missing wedge artifacts (e.g., star, fan, blurring, and elongation artifacts). Compressed sensing (CS) has recently been applied to ET and showed promising results for reducing missing wedge artifacts caused by limited angle sampling. CS uses a nonlinear reconstruction algorithm that employs image sparsity as a priori knowledge to improve the accuracy of density reconstruction from a relatively small number of projections compared to other reconstruction techniques. However, The performance of CS recovery depends heavily on the degree of sparsity of the reconstructed image in the selected transform domain. Prespecified transformations such as spatial gradients provide sparse image representation, while synthesising the sparsifying transform based on the properties of the particular specimen may give even sparser results and can extend the application of CS to specimens that can not be sparsely represented with other transforms such as Total variation (TV). In this work, we show that CS reconstruction in ET can be significantly improved by tailoring the sparsity representation using a sparse dictionary learning principle

Thermoelectric performance of multiphase XNiSn (X = Ti, Zr, Hf) half-Heusler alloys

Quantitative X-ray powder diffraction analysis demonstrates that mixing Ti, Zr and Hf on the ionic site in the half-Heusler structure, which is a common strategy to lower the lattice thermal conductivity in this important class of thermoelectric materials, leads to multiphase behaviour. For example, nominal Ti0.5Zr0.5NiSn has a distribution of Ti1−xZrxNiSn compositions between 0.24 ≤ x ≤ 0.70. Similar variations are observed for Zr0.50Hf0.5NiSn and Ti0.5Hf0.5NiSn. Electron microscopy and elemental mapping demonstrate that the main compositional variations occur over micrometre length scales. The thermoelectric power factors of the mixed phase samples are improved compared to the single phase end-members (e.g. S2/ρ = 1.8 mW m−1 K−2 for Ti0.5Zr0.5NiSn, compared to S2/ρ = 1.5 mW m−1 K−2 for TiNiSn), demonstrating that the multiphase behaviour is not detrimental to electronic transport. Thermal conductivity measurements for Ti0.5Zr0.5NiSn0.95 suggest that the dominant reduction comes from Ti/Zr mass and size difference phonon scattering with the multiphase behaviour a secondary effect

HOW COMPETITIVE IS THE WORLD WHEAT MARKET?

Japan is one of the largest importers of wheat in the world, with imports originating from three countries, Australia, Canada, and the United States. Australia, Canada, and Japan all use a government single-desk agency to control wheat trade. Many previous studies on competition in the world grain trade have argued the market is imperfectly competitive, and they often point to the Japanese market. We study the Japanese wheat import market for this reason, but find no compelling evidence of imperfect competition.wheat trade, competition, Crop Production/Industries, International Relations/Trade, F14, L10, Q17,

Simulation and analysis of solenoidal ion sources

We present a detailed analysis and simulation of solenoidal, magnetically confined electron bombardment ion sources, aimed at molecular beam detection. The aim is to achieve high efficiency for singly ionized species while minimizing multiple ionization. Electron space charge plays a major role and we apply combined ray tracing and finite element simulations to determine the properties of a realistic geometry. The factors controlling electron injection and ion extraction are discussed. The results from simulations are benchmarked against experimental measurements on a prototype source

The microstructure and microtexture of zirconium oxide films studied by transmission electron backscatter diffraction and automated crystal orientation mapping with transmission electron microscopy

A detailed characterization of nanostructured thin zirconium oxide films formed during aqueous corrosion of a nuclear-grade zirconium alloy (Zircaloy-4) has been carried out by means of two novel, ultra-high-spatial-resolution grain mapping techniques, namely automated crystal orientation mapping in the transmission electron microscope (TEM) and transmission electron backscatter diffraction (t-EBSD). While the former provided excellent spatial resolution with the ability to identify tetragonal ZrO<sub>2</sub> grains as small as ∼5 nm, the superior angular resolution and unambiguous indexing with t-EBSD enabled verification of the TEM observations. Both techniques revealed that in a stress-free condition (TEM foil prepared by focused ion beam milling), the oxide consists mainly of well-oriented columnar monoclinic grains with a high fraction of transformation twin boundaries, which indicates that the transformation from tetragonal to monoclinic ZrO<sub>2</sub> is a continuous process, and that a significant fraction of the columnar grains transformed from stress-stabilized tetragonal grains with (0 0 1) planes parallel to the metal–oxide interface. The TEM analysis also revealed a small fraction of size-stabilized, equiaxed tetragonal grains throughout the oxide. Those grains were found to show significant misalignment from the expected (0 0 1) growth direction, which explains the limited growth of those grains. The observations are discussed in the context of providing new insights into corrosion mechanisms of zirconium alloys, which is of particular importance for improving service life of fuel assemblies used in water-cooled reactors

A discussion of the concepts reason, revelation and nature in Bishop Butler’s moral philosophy

This thesis is a study of the three concepts Reason, Revelation and Nature in the moral philosophy of Bishop Butler. Butler was a Christian apologist who wanted to provide an empirical theory of morals in keeping with the secular tone of his age, yet which did not exclude the divine. His method of study was to investigate the constitution of human nature, and from this he concluded that to follow nature was to follow the path of virtue. Although his method of study appears consistently empirical, Butler's use of speculative reason and admittance of revealed knowledge indicates that he was not a strict empiricist in conviction. Butler's naturalism is firmly grounded in religion, by his belief that man is the work of God, is naturally virtuous, and that this, together with the commands of conscience, leads man to act a just and good role in life. It is in Butler's conception of conscience that we most clearly see how Reason, Revelation and Nature are related to each other in his philosophy. I interpret Butler's view of conscience as a moral faculty with two operational levels, the rational part which takes note of all the facts in a moral situation, and the intuitional part by which a moral pronouncement is made. This latter process is easier to understand when it is realized that conscience for Butler is a medium of communication between God and man. Butler's contribution to moral philosophy stems not only from his interesting analysis of conscience, but also from the way he reconciles self-interest and duty. According to Butler, men act virtuously, because they are secure in the knowledge that they are obeying God and so will be ultimately rewarded. There is an additional sanction in acting virtuously, we have a better chance of happiness because it is obeying our own nature to so do. Thus a man knows his obligations not only through the revelatory medium of the Scriptures, but through the nature of man, and specifically through the divinely implanted moral faculty

Exploring the ecological intensification of weed management in cropping systems of South Africa’s winter rainfall region

Taking an ecological intensification approach to weed management could increase agriculture’s ability to meet human needs for food, fuel and fibre whilst avoiding impacts on the integrity of Earth’s life-supporting ecosystems. Ecological intensification would achieve this by replacing anthropogenic inputs with ecosystem function. In this thesis, ecosystem processes are sought that could replace conventional weed control inputs and actions such as herbicides and tillage, and so reduce the environmental impacts associated with these. First, there is a need to shift thinking away from isolated mechanisms that seek to remove weeds, and to instead aim for ‘agroecosystem resilience to weeds’ in which farming systems would be designed to incorporate ecological processes and properties that inherently limit the negative impacts and promote the positive roles of weeds. To understand how this resilience could be achieved, recent advances in weed ecology are reviewed to identify relevant processes and properties, and to consider how these might be implemented in farm design and management. In sum, practices that could increase the diversity of filters applied to weed communities while decreasing filter strength, and that could reduce resource availability, would be expected to confer agroecosystem resilience to weeds. This thesis explores the practical implementation of some of these practices in South Africa’s winter rainfall region through field studies and trials. The practices were chosen for their suitability for conservation agriculture systems (the dominant farm management style in the region) and for their potential to harness or enhance ecological processes for weed management. The first practical study, a field survey of weeds in 15 vineyards with differing weed management practices, confirmed that the use of management techniques imposing lower disturbance leads to more diverse weed communities composed of less competitive species, and this effect can be enhanced by using specific management techniques to select for specific weed traits. The second practical study was linked to a long-term crop rotation experiment, and explored the effects of increasing crop diversity and integrating livestock as methods to increase the variability experienced by weeds in these rotation systems. Combining these two practices substantially reduced weed abundance and conserved weed diversity over the twelve years investigated. They also reduced herbicide and fertiliser requirements, and sustained cash crop yields, thus contributing to both profitability and sustainability. The final practical study applied theories of biotic resistance from invasion ecology to investigate how best to design cover crop mixes for weed suppression, that could be used in field crop systems or vineyards. Mixes composed of highly productive species were most effective at resource capture, and thus most effective at reducing resource availability to suppress weeds. In sum, reducing herbicide use in favour of grazing or mowing, increased crop and management diversity, and competitive crops (in mixes or monoculture) are viable practices that constitute first steps toward the ecological intensification of weed management in South Africa’s winter rainfall region. They offer improvements to agricultural sustainability through sustaining yields and farm incomes whilst reducing the environmental impacts and health risks associated with conventional weed control such as herbicides and tillage. Overall, the findings of this thesis suggest that ecological intensification offers a promising direction for future weed management to achieve agricultural sustainability, both in South Africa’s winter rainfall region and around the world. Weed researchers can assist farmers in this challenge by drawing on global advances in weed ecology to design and test locally appropriate weed management techniques and strategies

HREM studies of intergrowths in Sr2[Srn-1TinO3n+1] Ruddlesden-Popper phases synthesized by mechanochemical activation

A mechanochemical activation route has been applied in order to obtain the <i>n</i>=1–4 and ∞ members of the Sr<sub>2</sub>[Sr<sub>n</sub><sub>−1</sub>Ti<sub>n</sub>O<sub>3n+1</sub>] Ruddlesden– Popper series from different (<i>n</i>+1)SrO:nTiO<sub>2</sub> mixtures. The mechanosynthesis of SrTiO<sub>3</sub> and Sr<sub>2</sub>TiO<sub>4</sub> was observed during the milling process from the initial stoichiometric mixture, but in the cases of the <i>n</i>=2–4 members, a subsequent thermal treatment was needed. The synthesis protocol of Sr<sub>3</sub>Ti<sub>2</sub>O<sub>7</sub> has been greatly improved and this compound can be isolated as a single, crystalline phase after annealing at 800°C. In the case of Sr<sub>4</sub>Ti<sub>3</sub>O<sub>10</sub> and Sr<sub>5</sub>Ti<sub>4</sub>O<sub>13</sub>, the formation temperature was also decreased, but members with <i>n</i>=3 and 4 could not be isolated. Detailed investigations using electron microscopy methods (TEM, HREM and SAED) were carried out in the samples corresponding to <i>n</i>=2–4. Although a single ordered Sr<sub>3</sub>Ti<sub>2</sub>O<sub>7</sub> structure is dominant in the sample corresponding to <i>n</i>=2, a few intergrowths of other Ruddlesden–Popper phases were observed. In the cases of <i>n</i>=3 and 4, the intergrowths of Ruddlesden–Popper phases are more frequent than in the <i>n</i>=2 composition and are randomly distributed in the sample. The more frequent occurrence of such stacking faults, with increasing <i>n</i> value, leads to a somewhat disordered layer stacking sequence

Investigating the effect of a stress-based uniaxial anisotropy on the magnetic behaviour of La_0.7Sr_0.3MnO₃ elements

We investigate the interplay between shape anisotropy and a stress-based uniaxial anisotropy on the magnetic domain structure of La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> nanoelements as a function of aspect ratio, using micromagnetic simulations. We show that a direct competition between the anisotropies gives rise to high energy multi-domain flux closure configurations, whilst an alignment of the anisotropies can modify the effective element dimensions and act to stabilise a single domain configuration. Our results demonstrate the ability to control the spin state of La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> elements in addition to tailoring the domain wall width by controlling the anisotropy of the material, which is key for spintronic applications that require a high spin-polarization and stable magnetic configurations

Metal Distributions, Efficient n-Type Doping, and Evidence for in-Gap States in TiNiM_<i>y</i>Sn (M = Co, Ni, Cu) half-Heusler Nanocomposites

XNi1+ySn nanocomposites consisting of a XNiSn half-Heusler (HH) matrix with segregated XNi2Sn Full Heusler (FH) inclusions promise improvements in thermoelectric efficiencies. We extend recent research by reporting on TiNiMySn (0 ≤ y ≤ 1) nanocomposites with M = Co (3d9), Ni (3d10) and Cu (3d104s1). Neutron powder diffraction reveals that the Ni and Cu series produce a matrix of TiNiSn with nanosegregated TiNi2Sn and TiNi1+dCu1–dSn, respectively. For the Co series, the Co inserts into both phases to obtain a TiNi1–yCoySn matrix with nanosegregated TiNi2–yCoySn. Systematic changes in Seebeck coefficient (S) and electrical resistivity (ρ) are observed in all three series. For M = Ni, changes in S and ρ are attributed to in-gap states arising from the nanosegregation. The M = Co composites show a complex interplay between the hole doped TiNi1–yCoySn matrix and similar in-gap states, where the p- to n-type transition temperature increases but the maximum S remains unchanged at +30 μV K–1. The 4s1 electron for M = Cu is delocalized in the HH matrix, leading to metal-like ρ(T) and up to 100% improved thermoelectric power factors compared to TiNiSn (S2/ρ = 2 mW m–1 K–2 at 600–700 K for y = 0.025). These results broaden the range of segregated FH phases that could be used to enhance HH thermoelectric performance