Electron tomography of defects

Tomography of crystal defects in the electron microscope was rst attempted in 2005 by the author and colleagues. This thesis further develops the technique, using a variety of samples and methods. Use of a more optimised, commercial tomographic reconstruction program on the original GaN weakbeam dark- eld (WBDF) tilt series gave a ner reconstruction with lower background, line width 10-20 nm. Four WBDF tilt series were obtained of a microcrack surrounded by dislocations in a sample of indented silicon, tilt axes parallel to g = 220, 220, 400 and 040. Moir e fringes in the defect impaired alignment and reconstruction. The e ect on reconstruction of moir e fringe motion with tilt was simulated, resulting in an array of rods, not a at plane. Dislocations in a TiAl alloy were reconstructed from WBDF images with no thickness contours, giving an exceptionally clear reconstruction. The e ect of misalignment of the tilt axis with systematic row g(ng) was assessed by simulating tilt series with di raction condition variation across the tilt range of n = 0, 1 and 2. Misalignment changed the inclination of the reconstructed dislocation with the foil surfaces, and elongated the reconstruction in the foil normal direction; this may explain elongation additional to the missing wedge e ect in experiments. Tomography from annular dark- eld (ADF) STEM dislocation images was also attempted. A tilt series was obtained from the GaN sample; the reconstructed dislocations had a core of bright intensity of comparable width to WBDF reconstructions, with a surrounding region of low intensity to 60 nm width. An ADF STEM reconstruction was obtained from the Si sample at the same microcrack as for WBDF; here automatic specimen drift correction in tomography acquisition software succeeded, a signi cant improvement. The microcrack surfaces in Si reconstructed as faint planes and dislocations were recovered as less fragmented lines than from the WBDF reconstruction. ADF STEM tomography was also carried out on the TiAl sample, using an detector inner angle ( in) that included the rst order Bragg spots (in other series in had been 4-6 B). Extinctions occurred which were dependent on tilt; this produced only weak lines in the reconstruction. Bragg scattering in the ADF STEM image was estimated by summing simulated dark- eld dislocation images from all Bragg beams at a zone axis; a double line was produced. It was hypothised that choosing the inner detector angle to omit these rst Bragg peaks may preclude most dynamical image features. Additional thermal di use scattering (TDS) intensity due to dilatation around an edge dislocation was estimated and found to be insigni cant. The Huang scattering cross section was estimated and found to be 9 A, ten times thinner than experimental ADF STEM dislocation images. The remaining intensity may be from changes to TDS from Bloch wave transitions at the dislocation; assessing this as a function of tilt is for further work. On simple assessment, only three possible axial channeling orientations were found over the tilt range for GaN; if this is typical, dechanneling contrast probably does not apply to defect tomography.This work was supported by an EPSRC Doctoral Training Account

Driving improvements in emerging disease surveillance through locally-relevant capacity strengthening

Emerging infectious diseases (EIDs) threaten the health of people, animals, and crops globally, but our ability to predict their occurrence is limited. Current public health capacity and ability to detect and respond to EIDs is typically weakest in low- and middle-income countries (LMICs). Many known drivers of EID emergence also converge in LMICs. Strengthening capacity for surveillance of diseases of relevance to local populations can provide a mechanism for building the cross-cutting and flexible capacities needed to tackle both the burden of existing diseases and EID threats. A focus on locally relevant diseases in LMICs and the economic, social, and cultural contexts of surveillance can help address existing inequalities in health systems, improve the capacity to detect and contain EIDs, and contribute to broader global goals for development

A subaltern critical geopolitics of the war on terror: postcolonial security in Tanzania

Currently, hegemonic geographical imaginations are dominated by the affective geopolitics of the War on Terror, and related security practice is universalised into what has been called ‘‘globalized fear’’ (Pain, 2009). Critical approaches to geopolitics have been attentive to the Westerncentric nature of this imaginary, however, studies of non-Western perceptions of current geopolitics and the nature of fear will help to further displace dominant geopolitical imaginations. Africa, for example, is a continent that is often captured in Western geopolitics – as a site of failed states, the coming anarchy, passive recipient of aid, and so on – but geopolitical representations originating in Africa rarely make much of an impact on political theory. This paper aims to add to critical work on the so-called War on Terror from a perspective emerging from the margins of the dominant geopolitical imagination. It considers the geopolitical imagination of the War on Terror from a non-Western source, newspapers in Tanzania

Liquid chromatography–flame ionisation detection using a nebuliser/spray chamber interface. Part 1. Design and testing

AbstractA nebuliser and spray chamber have been used to link a flow injection analyser to a flame ionisation detector, with the potential for the combination to be used as a universal detector for liquid chromatography. The hydrogen and air flows were adjusted to achieve a stable system. The detector responded to both volatile and involatile analytes and to compounds with and without chromophores, including alkanes, alkanols, aromatic amides and acids, phenols, amino-acids and carbohydrates and gave a linear response for many analytes. However, for involatile polar analytes it was necessary to add traces of acid or salt to the carrier stream to obtain a linear response

Applications of a novel flame ionisation detector for liquid chromatography

Over the last twenty years liquid chromatography has come to dominate analytical chemistry because of its ability to analyse a wide range of products from pharmaceuticals to environmental and forensic samples but some groups of analytes still cause practical difficulties in detection. The most common detector for HPLC is the UV-Visible spectroscopic detector, which is both sensitive and linear. However, detection is limited to analytes containing chromophores. For other analytes the analyst has to either rely on derivatisation or employ a “universal detector”, such as the less sensitive refractive index detector or the evaporative light scattering detector, which cannot easily detect small, volatile compounds. The universal flame ionisation detector when interfaced to LC has had problems in the past because of the signal from the organic component of the mobile phase, however, the use of superheated water as the eluent overcomes this problem and enables reversed-phase separations with the ability to detect analytes with and without chromophores. A revised design of interface (patent pending) enables a wide range of columns to be employed with differing flow rates

The detectability of Wolf-Rayet Stars in M33-like spirals up to 30 Mpc

Abstract We analyse the impact that spatial resolution has on the inferred numbers and types of Wolf-Rayet (WR) and other massive stars in external galaxies. Continuum and line images of the nearby galaxy M33 are increasingly blurred to mimic effects of different distances from 8.4 Mpc to 30 Mpc, for a constant level of seeing. We use differences in magnitudes between continuum and Helium II line images, plus visual inspection of images, to identify WR candidates via their ionized helium excess. The result is a surprisingly large decrease in the numbers of WR detections, with only 15% of the known WR stars predicted to be detected at 30Mpc. The mixture of WR subtypes is also shown to vary significantly with increasing distance (poorer resolution), with cooler WN stars more easily detectable than other subtypes. We discuss how spatial clustering of different subtypes and line dilution could cause these differences and the implications for their ages, this will be useful for calibrating numbers of massive stars detected in current surveys. We investigate the ability of ELT/HARMONI to undertake WR surveys and show that by using adaptive optics at visible wavelengths even the faintest (MV = –3 mag) WR stars will be detectable out to 30 Mpc

Geopolitics at the margins? Reconsidering genealogies of critical geopolitics

Critical geopolitics has become one of the most vibrant parts of political geography. However it remains a particularly western way of knowing which has been much less attentive to other traditions of thinking. This paper engages with Pan-Africanism, and specifically the vision of the architect of post-colonial Tanzania, Julius Nyerere, to explore this overlooked contribution to critical engagements with geopoli- tics. Pan-Africanism sought to forge alternative post-colonial worlds to the binary geopolitics of the Cold War and the geopolitical economy of neo-colonialism. The academic division of labour has meant that these ideas have been consigned to African studies rather than being drawn into wider debates around the definitions of key disciplinary concepts. However Nyerere’s continental thinking can be seen as a form of geopolitical imagination that challenges dominant neo-realist projections, and which still has much to offer contemporary political geography