11,426 research outputs found

    Antenna Arrangement in UWB Helmet Brain Applicators for Deep Microwave Hyperthermia

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    Deep microwave hyperthermia applicators are typically designed as narrow-band conformal antenna arrays with equally spaced elements, arranged in one or more rings. This solution, while adequate for most body regions, might be sub-optimal for brain treatments. The introduction of ultra-wide-band semi-spherical applicators, with elements arranged around the head and not necessarily aligned, has the potential to enhance the selective thermal dose delivery in this challenging anatomical region. However, the additional degrees of freedom in this design make the problem non-trivial. We address this by treating the antenna arrangement as a global SAR-based optimization process aiming at maximizing target coverage and hot-spot suppression in a given patient. To enable the quick evaluation of a certain arrangement, we propose a novel E-field interpolation technique which calculates the field generated by an antenna at any location around the scalp from a limited number of initial simulations. We evaluate the approximation error against full array simulations. We demonstrate the design technique in the optimization of a helmet applicator for the treatment of a medulloblastoma in a paediatric patient. The optimized applicator achieves 0.3\ua0 (Formula presented.) C higher T90 than a conventional ring applicator with the same number of elements

    Quantum dots based superluminescent diodes and photonic crystal surface emitting lasers

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    This thesis reports the design, fabrication, and electrical and optical characterisations of GaAs-based quantum dot (QD) photonic devices, specifically focusing on superluminescent diodes (SLDs) and photonic crystal surface-emitting lasers (PCSELs). The integration of QD active regions in these devices is advantageous due to their characteristics such as temperature insensitivity, feedback insensitivity, and ability to utilise the ground state (GS) and excited state (ES) of the dots. In an initial study concerning the fabrication of QD-SLDs, the influence of ridge waveguide etch depth on the electrical and optical properties of the devices are investigated. It is shown that the output power and modal gain from shallow etched ridge waveguide is higher than those of deep etched waveguides. Subsequently, the thermal performance of the devices is analysed. With increased temperature over 170 ÂșC, the spectral bandwidth is dramatically increased by thermally excited carrier transition in excited states of the dots. Following this, an investigation of a high dot density hybrid quantum well/ quantum dot (QW/QD) active structure for broadband, high-modal gain SLDs is presented. The influence of the number of QD layers on the modal gain of hybrid QW/QD structures is analysed. It is shown that higher number of dot layer provides higher modal gain value, however, there is lack of emission from QW due to the requirement of large number of carriers to saturate the QD. Additionally, a comparison is made between “unchirped QD” and “ chirped QD” of hybrid QW/QD structure in terms of modal gain and spectral bandwidth. It is showed that “chirped” of the QD can improve the “flatness” of the spectral bandwidth. Lastly, the use of self-assembled InAs QD as the active material in epitaxially regrown GaAs-based PCSELs is explored for the first time. Initially, it is shown that both GS and ES lasing can be achieved for QD-PCSELs by changing the grating period of the photonic crystal (PC). The careful design of these grating periods allows lasing from neighbouring devices at GS ( ~1230 nm) and ES (~1140 nm), 90 nm apart in wavelength. Following this, the effect of device area, PC etch depth, PC atom shape (circle or triangle or orientation) on lasing performance is presented. It is shown that lower threshold current density and higher slope efficiencies is achieved with increasing the device size. The deeper PC height device has higher output power due to more suitable height and minimal distance to active region. The triangular atom shape has slightly higher slope efficiency compared to triangular atom shape which is attributed to breaking in-plane symmetry and increase out-of-plane emission

    Perspectives on high-frequency nanomechanics, nanoacoustics, and nanophononics

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    Nanomechanics, nanoacoustics, and nanophononics refer to the engineering of acoustic phonons and elastic waves at the nanoscale and their interactions with other excitations such as magnons, electrons, and photons. This engineering enables the manipulation and control of solid-state properties that depend on the relative positions of atoms in a lattice. The access to advanced nanofabrication and novel characterization techniques enabled a fast development of the fields over the last decade. The applications of nanophononics include thermal management, ultrafast data processing, simulation, sensing, and the development of quantum technologies. In this review, we cover some of the milestones and breakthroughs, and identify promising pathways of these emerging fields.Comment: 19 pages, 3 figure

    Machine Learning Research Trends in Africa: A 30 Years Overview with Bibliometric Analysis Review

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    In this paper, a critical bibliometric analysis study is conducted, coupled with an extensive literature survey on recent developments and associated applications in machine learning research with a perspective on Africa. The presented bibliometric analysis study consists of 2761 machine learning-related documents, of which 98% were articles with at least 482 citations published in 903 journals during the past 30 years. Furthermore, the collated documents were retrieved from the Science Citation Index EXPANDED, comprising research publications from 54 African countries between 1993 and 2021. The bibliometric study shows the visualization of the current landscape and future trends in machine learning research and its application to facilitate future collaborative research and knowledge exchange among authors from different research institutions scattered across the African continent

    GENDERED EMBODIMENT, STABILITY AND CHANGE: WOMEN’S WEIGHTLIFTING AS A TOOL FOR RECOVERY FROM EATING DISORDERS

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    This thesis explores the everyday embodied experiences of women who use amateur weightlifting as a vehicle for recovery from eating disorders. Within online spaces and on social media, women frequently share their experiences of using weightlifting to overcome issues relating to disordered eating, body image, and mental health. In particular, women with a history of eating disorders credit weightlifting to be integral to their recovery journey. However, there is a dearth of research on women’s experiences with exercise during eating disorder recovery and no research that identifies weightlifting as beneficial to this process. To the contrary, discursive links are drawn between the practices of self-surveillance exercised by both eating disorder sufferers and weightlifters alike. In this regard, engagement with weightlifting during eating disorder recovery may signal the transferal of pathology from one set of behaviours to another. That is, from disordered eating to rigid and self-regulatory exercise routines. This thesis examines how women subjectively navigate and make sense of this pathologisation. The data for this research comes from longitudinal semi-structured interviews and photo elicitation with 19 women, living in the United Kingdom, who engaged in weightlifting during their eating disorder recovery. In addition, to build up a holistic picture and to explore how this phenomenon also ‘takes place’ online, I conducted a netnography of the overlapping subcultures of female weightlifting and eating disorder recovery on Instagram. Women’s standpoint theory and interpretative phenomenological analysis are combined to form the underpinning theoretical and analytical tools used to engage with these three rich data sets. Moreover, throughout I draw on an eclectic range of disciplinary perspectives, in order to bring together multiple fields of research and develop novel theoretical frameworks. In the findings, I argue that women’s experiences using weightlifting as a tool for recovery from eating disorders manifests in an embodied sense of multiplicity. In this sense, understandings of the body that are often viewed as ontologically distinct (muscularity/thinness/fatness) hang-together at once in the lived experience of a single individual. I argue that women, particularly those who have previously struggled with an eating disorder, are too readily positioned as vulnerable to media and representation. To theoretically combat these ideas regarding women’s assumed passivity, I develop the concept of ‘digital pruning’ to account for women’s agency in relation to new media. I contend that weightlifting offers women in recovery from eating disorders a new framework for approaching eating and exercise. Specifically, weightlifting’s norms and values legitimate occupying a larger body, which gives women in recovery permission to eat and gain-weight in a way that is both culturally sanctioned and health-promoting. Finally, I explore identity transformation as a specific tenet of recovery from eating disorders. I argue that, on social media, recovery identities are characterised by personal empowerment, resilience, and independence. While offline, quieter and less culturally glorified aspects of recovery (such as relationships of care) are central to women’s accounts of developing a new sense of self as they transition away from an eating disorder identity. In summary, this thesis is an examination of the ways in which women strategically navigate pathology in relation to their bodies, social media, food/exercise practices, and identity. I argue that women develop a set of ‘DIY’ recovery practices that allow them to consciously channel and draw on their negative experiences with eating disorders, to develop new ways of living that serve their overall wellbeing. Weightlifting is integral to this process, as it provides women transitioning out of this difficult phase in their lives with new ways of relating to their bodies and of being in the world. I situate this phenomenon within a neoliberal socio-political climate in which individuals are required to take personal responsibility for their mental health and wellbeing, despite living within conditions which are not conducive to recovery

    Expanding the scope of single-molecule energy transfer with gold nanoparticles and graphene

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    Förster resonance energy transfer (FRET) is a common tool to measure the distances between a donor and an acceptor fluorophore and is employed as a spectroscopic ruler. This non-radiative energy transfer is utilized to not only measure distances but also to observe dynamics in the field of biophysics and medicine. However, main limitations of FRET are the limited time resolution and working range between donor and acceptor molecules of 10 nm. To increase the application of FRET, this limitation can be circumvented by the introduction of different ma-terials in the close proximity. For characterization of the altered distance dependence, a precise distance control between the dyes and the applied material is required, which here is provided by the DNA origami technique. In DNA origami, DNA self-assembles into programmable, complex, and robust structures, which can be easily modified with dyes and other entities with nanometric control. DNA origami nanoantennas constructed of a pair of gold nanoparticles have recently been introduced to substantially increase the obtainable fluorescence signal that yields a higher time resolution in biophysical single-molecule FRET experiments. In this context, it is crucial to understand the influence of the gold nanoparticles on the FRET process itself. In this work, gold nanoparticles are placed next to FRET pairs using the DNA origami technique (see publication I). A measurement procedure to accurately determine energy transfer efficiencies is estab-lished and reveals that in the intermediate coupling regime, the energy transfer efficiency drops in the presence of nanoparticles whereas the energy transfer rate constant from the donor to the acceptor is not significantly altered. Next, graphene is introduced to increase the range of energy transfer. Graphene is a 2D carbon lattice, which can also be employed as an unbleachable broadband acceptor without labeling. To understand the principles of the energy transfer between a fluorophore and the graphene surface, the distance dependence of the energy transfer from a fluorophore to graphene is investigated (see publication II). As such experiments require high quality graphene surfac-es, a cleaning and transferring procedure to generate reproducible graphene-on-glass-coverslips is established (see publication III) and characterized by different spectroscopic methods. Finally, the full potential of graphene-on-glass coverslips as microscopy platforms are highlighted by adopting graphene in the fields of biosensing, biophysics and super-resolution microscopy (see publication IV). The designed biosensors are capable to detect a DNA target, a viscosity change, or the binding of a biomolecule. In addition, FRET between two dyes is expanded by additional graphene energy transfer (GET) that reveals the relative orientation of the FRET pairs to the graphene surface. Finally, GET is used in super-resolution experiments to reach isotopic nanometric 3D-resolution and track a single fluorophore that undergoes 6-nm jumps. The developed techniques and assays have the potential to become the basis for numerous new applications in single-molecule sensing, biophysics, and super-resolution microscopy

    Optical coherence tomography methods using 2-D detector arrays

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    Optical coherence tomography (OCT) is a non-invasive, non-contact optical technique that allows cross-section imaging of biological tissues with high spatial resolution, high sensitivity and high dynamic range. Standard OCT uses a focused beam to illuminate a point on the target and detects the signal using a single photodetector. To acquire transverse information, transversal scanning of the illumination point is required. Alternatively, multiple OCT channels can be operated in parallel simultaneously; parallel OCT signals are recorded by a two-dimensional (2D) detector array. This approach is known as Parallel-detection OCT. In this thesis, methods, experiments and results using three parallel OCT techniques, including full -field (time-domain) OCT (FF-OCT), full-field swept-source OCT (FF-SS-OCT) and line-field Fourier-domain OCT (LF-FD-OCT), are presented. Several 2D digital cameras of different formats have been used and evaluated in the experiments of different methods. With the LF-FD-OCT method, photography equipment, such as flashtubes and commercial DSLR cameras have been equipped and tested for OCT imaging. The techniques used in FF-OCT and FF-SS-OCT are employed in a novel wavefront sensing technique, which combines OCT methods with a Shack-Hartmann wavefront sensor (SH-WFS). This combination technique is demonstrated capable of measuring depth-resolved wavefront aberrations, which has the potential to extend the applications of SH-WFS in wavefront-guided biomedical imaging techniques

    REDESIGNING THE COUNTER UNMANNED SYSTEMS ARCHITECTURE

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    Includes supplementary material. Please contact [email protected] for access.When the Islamic State used Unmanned Aerial Vehicles (UAV) to target coalition forces in 2014, the use of UAVs rapidly expanded, giving weak states and non-state actors an asymmetric advantage over their technologically superior foes. This asymmetry led the Department of Defense (DOD) and the Department of Homeland Security (DHS) to spend vast sums of money on counter-unmanned aircraft systems (C-UAS). Despite the market density, many C-UAS technologies use expensive, bulky, and high-power-consuming electronic attack methods for ground-to-air interdiction. This thesis outlines the current technology used for C-UAS and proposes a defense-in-depth framework using airborne C-UAS patrols outfitted with cyber-attack capabilities. Using aerial interdiction, this thesis develops a novel C-UAS device called the Detachable Drone Hijacker—a low-size, weight, and power C-UAS device designed to deliver cyber-attacks against commercial UAVs using the IEEE 802.11 wireless communication specification. The experimentation results show that the Detachable Drone Hijacker, which weighs 400 grams, consumes one Watt of power, and costs $250, can interdict adversarial UAVs with no unintended collateral damage. This thesis recommends that the DOD and DHS incorporates aerial interdiction to support its C-UAS defense-in-depth, using technologies similar to the Detachable Drone Hijacker.DASN-OE, Washington DC, 20310Captain, United States Marine CorpsApproved for public release. Distribution is unlimited

    A comprehensive review on laser powder bed fusion of steels : processing, microstructure, defects and control methods, mechanical properties, current challenges and future trends

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    Laser Powder Bed Fusion process is regarded as the most versatile metal additive manufacturing process, which has been proven to manufacture near net shape up to 99.9% relative density, with geometrically complex and high-performance metallic parts at reduced time. Steels and iron-based alloys are the most predominant engi-neering materials used for structural and sub-structural applications. Availability of steels in more than 3500 grades with their wide range of properties including high strength, corrosion resistance, good ductility, low cost, recyclability etc., have put them in forefront of other metallic materials. However, LPBF process of steels and iron-based alloys have not been completely established in industrial applications due to: (i) limited insight available in regards to the processing conditions, (ii) lack of specific materials standards, and (iii) inadequate knowledge to correlate the process parameters and other technical obstacles such as dimensional accuracy from a design model to actual component, part variability, limited feedstock materials, manual post-processing and etc. Continued efforts have been made to address these issues. This review aims to provide an overview of steels and iron-based alloys used in LPBF process by summarizing their key process parameters, describing thermophysical phenomena that is strongly linked to the phase transformation and microstructure evolution during solidifica-tion, highlighting metallurgical defects and their potential control methods, along with the impact of various post-process treatments; all of this have a direct impact on the mechanical performance. Finally, a summary of LPBF processed steels and iron-based alloys with functional properties and their application perspectives are presented. This review can provide a foundation of knowledge on LPBF process of steels by identifying missing information from the existing literature

    Two techniques for determining F-region ion velocities at meso-scales: Differences and impacts on Joule heating

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    We have investigated the difference between two standard techniques for deriving the ionospheric ion velocity using data taken with the EISCAT incoherent scatter radar between 1987 and 2007. For large-scale convection flows, there is little difference between the tristatic and monostatic techniques, though the biggest relative difference occurs during periods when the interplanetary magnetic field (IMF) is strongly northward. At small scales the difference between the two techniques is correlated with a measure of the variability of the tristatic measurement. This suggests that small-scale flow bursts, such as those associated with enhanced auroral arcs, could explain the local time variation in the velocity difference distributions. The difference in velocities obtained from the monostatic and tristatic techniques can make a significant difference in the estimate of the magnitude of Joule heating in the thermosphere. Considering only the electric field dominated component of Joule heating, Q, the difference in the two techniques can be as much as 52% of the tristatic measurement (Qm = 0.48Qt) in the morning sector (0 – 6 MLT), during a moderate to large geomagnetic storm. This reduces to a difference of 36% at non-storm times in the same MLT period. Careful averaging of the velocity field with the future EISCAT_3D radar system will allow us to establish the impact of both spatial and temporal scales on the magnitude of the observations
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