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Synchronously-pumped prism-based optical parametric oscillators
Full text linkThis thesis presents the use of optical prisms as retroreflectors to create ring cavities
for ultrafast optical parametric oscillators (OPOs). Two main prism geometries are
used, the Pellin-Broca (PB) prism and the Brewster mirror (BM) prism.
Consideration is given as to how the base shape, carefully chosen internal angles
and constituent material can be used to manipulate the output characteristics.
Experimental chapters describing the use of PB prisms discuss two different cavity
layouts and the benefits of each. Throughout the experimental chapters modelling
is performed to further understand how the prism specifications affect the output
beam characteristics. Modelling was also used to create better-designed prisms and
cavity layouts, and to understand unexpected behaviours in the BM cavity.
The PBOPO chapters introduce two different cavity layouts using PB prisms with
three different glasses. These produce broadband spectra over a 250 nm range in
a dual-band regime. The BM prism chapter uses the familiar triangular prism in
a novel way, facilitated by a high refractive index material. The BMOPO has half
the total internal reflections (TIR) than the PBOPO producing signal spectra over a
450 nm range. The BMOPO can be wavelength-tuned in two different ways, offering
both coarse-tuning and fine-tuning, allowing sub-nm signal wavelength selection
Soundscape, engagement and spatial planning : an exploration of perceived control, annoyance, indirect health outcomes and wellbeing in the context of UK aviation expansion projects
Full text linkThe sound environment directly affects human health and wellbeing. Essential to soundscape
design, management and implementation in spatial planning are people’s perceptual responses
to the existing and/or imagined sound environment. Internationally standardised soundscape
practice places stakeholders as co-specifiers of projects from the planning inception stage, but
crucially challenging to assessing/predicting stakeholders’ response to sound is the impact of
non-acoustic factors, accounting for at least one-third of the human response to sound in
context. The non-acoustic factor of ‘perceived control’ critically influences person-environment spacetime interaction/s, making it essential to physical and mental health, while
perception of engagement in spatial planning substantially impacts stakeholders’ perceived
control. This qualitative study explores perceived control and engagement in the context of
spatial planning for UK aviation activities. Constructivist grounded theory methodology was
used while data collection comprised of a series of in-depth semi-structured interviews, focus
groups, stakeholder engagement activities and autoethnographic observations. Three project
outputs were delivered. First, the emergent Grounded Theory from the data conceptualising a
trauma informed response to contextually salient and relevant non-acoustic factors and the
impact on perceived control and engagement. Next, a Conceptual Framework applying the
Grounded Theory to environmental impact assessment. Third, recommendations and
implications of the outputs to inform salutogenic spatial planning were considered for aviation
and for large and small infrastructure projects. This project builds on existing soundscape,
transportation noise and health research adding a novel applied Grounded Theory to the corpus.
Finally, a dimensional evolvement of stress-related noise annoyance theory is posited regarding
perceived control and the impact on wellbeing.Engineering and Physical Sciences Research Council (EPSRC) grant EP/R003467/
Electrical and physical characterisation of modified NiO/Ysz fuel electrode material for solid oxide electrolysis cell
No full textHydrogen is touted as a primary future clean source of transportation fuel. However, current hydrogen (H2) production via natural gas steam reforming emits significant amounts
of CO2 compared to hydrogen production via electrolysis powered by renewable energy. One approach to produce H2 and minimize CO2 emissions involves CO2 utilization
through the co-electrolysis of CO2 and H2O using Solid Oxide Electrolysis Cells (SOEC)
to form CO and H2 at the fuel electrode and O2 at the oxygen electrode. However, the
main challenge for this technology is cell degradation due to high-temperature operation in the range of 700-900 oC, impacting the long-term stability of SOECs. Modifying
the material composition of fuel electrodes is one approach to improve material stability
and electrode electrochemical performance, such as enhancing conductivity and reducing electrode polarisation resistances. Moreover, it can lower operating temperatures and
prevent carbon formation. However, the materials synthesis processes and fabrication
methods proposed in the literature are complex and not economically sound. Accordingly, this work focuses on the stability of SOEC electrodes, where NiO-YSZ is modified
by doping metals such as barium (Ba) or cerium (Ce).
This work successfully doped NiO-YSZ electrodes with BaCO3, BaO, and CeO using a ball milling process (mechanical technique). It was found that BaCO3 doping influenced CO2 adsorption capacity, as indicated by Thermo Gravimetric Analysis (TGA),
showing a 0.6% reduction in CO2 adsorption fractions. Moreover, the addition of 20
wt% barium into the fuel electrode material enhanced its mechanical strength and reduced the nickel particle size, thus hindering particle swelling, as observed in SEM image
analysis. Electrochemical Impedance Spectroscopy (EIS) electrolysis and J-V curve tests
demonstrated improvements in the resistances of a single layer of 10 wt% BaCO3-doped
NiO-YSZ compared to undoped materials, with a reduction at 800 oC from 3.43 mΩ to
1.19 mΩ. However, full SOEC configurations tested under co-electrolysis showed that the
areic resistances of doped NiO-YSZ fuel electrodes, 10 wt% CeO/Ni–YSZ—YSZ—LSM
and 10 wt% BaCO3/Ni–YSZ—YSZ—LSM, were 0.28 mΩ.cm2
and 0.82 mΩ.cm2
, respectively, compared to the undoped Ni-YSZ resistance of 0.15 mΩ.cm2
. Importantly,
the study demonstrated the great potential of the fabrication method for SOECs and their lectrical performance, with produced fuel electrodes exhibiting enhanced overall conductivity, reduced areic resistances, and improved material durability
Antenna technology for satellite communication
Full text linkSatellite-based systems have become indispensable for modern life, serving as platforms
for global communication, navigation, and Earth monitoring. Antennas play a central role
in ensuring the efficient functioning of these systems, both in the space and ground
segments.
In this thesis, technology advancements on the ground segments are achieved with the
design of two different unit cells, suitable for phased array applications. Such antennas
are fully compatible with a Printed Circuit Board (PCB) process, without the need of
further assembly. Specifications such as dual circular polarization, steering capabilities
up to ±40° and the frequency range from 17.7 to 20.2 GHz, make them an ideal candidate
for Satellite Communication On the Move (SOTM) Receivers (RX) terminals. 2x2 Arrays
have been prototyped and tested.
Two antennas have been presented in relation to the Space segment. An end-fire antenna
operating at S-band has been integrated within the solar module of a CubeSat by the use
of miniaturization techniques. Characterizations of Photovoltaic (PV) cells and space
qualified materials have been included in the same framework. The measurements of the
antenna on the chassis show good agreement with simulated results. Simulated results of
a second antenna operating at S- / X- band, exhibiting circular polarization over a wide
bandwidth, is secondly presented. The antenna is based on a multilayer PCB, where the
two different frequency bands are enabled by two separate radiating elements sharing the
same aperture.
A Near Field measurement system for phased arrays has also been developed within the
scope of the thesis, allowing a quick characterization of antennas, being able to extract
information such as radiation pattern, along with the amplitude and phase at each sampled
point of the scanned area
Radio frequency impairments in transmitters : characterising power amplifiers for radio frequency fingerprinting identification
Full text linkThe explosive growth of wireless connectivity brings with it significant cybersecurity challenges. In response, there have been some research efforts on radio
frequency fingerprinting identification (RFFI) technology that has become one of
the promising physical (PHY) layer wireless security solutions. The radio frequency fingerprinting (RFF) refers to the unique fingerprint/characteristics inherently presented in analogue transmit RF chains. These distinctive RFF features
are extracted and used as device identities (IDs) to facilitate secure network access in the authentication processes. They introduce minor but unique distortions
to the transmitted signal waveforms, detectable by receivers. Compared to traditional authentication methods, RFF does not rely on shared secrets like passwords
or cryptographic keys, as they can be compromised through various attacks.
The RFF features in an RF transmitter chain can arise from various components, e.g., RF oscillator, digital-to-analogue converter (DAC), RF power amplifier
(PA), and antenna. Given that PAs are major contributors to signal non-linearity,
they are extensively studied for RFF applications. However, achieving high classification performance in low SNR regions remains a challenge. The main objective
of this research is the development of the RFFI schemes based on non-linear features of the PA.
In our proposed RFFI schemes, we first exploit the unique non-linear memory
effect of the transmitter RF chains, which consist of matched pulse shaping filters
and non-linear PAs. We also introduce a hybrid classification method that can significantly enhance the classification performance while at the low SNR region.
However, it remains a challenge to classify the device under test (DUT) with the
same model/ or with similar RFF features. This is expected as the differences
among the same models are more minute, compared to those of different models.
To overcome this limitation, we devise a strategy that involves deliberately
and random adjustments to the operating conditions of the PA, specifically using
the active load-pulling technique to modify the output impedance of the PAs.
Upon experimental observation, the non-linear characteristics of the PA have been
significantly altered. This strategy results in improved classification performances,
particularly in low SNR scenarios, when classifying the PA of the same model.
Furthermore, this thesis explores the RFF features in a RF multi-antenna array
transmitter chain. A key aspect of this exploration is the mutual coupling (MC)
effect, which is caused by electromagnetic interactions between adajacent antenna
elements in the antenna array, and this effect may impact the performance of
the PA. In the multi-antenna RF chain, a reconfigurable power divider (RPD)
is employed as a feeding network that can distribute the signal to each antenna
element, which, in this work, is also utilised to control the MC effect in the RF
chain. This approach results in varied behaviours of the non-linear RFF features
in the RF transmitter chain. Through these methodologies, the distinctions in
RFF features across different models and even the same model of wireless devices
are more pronounced
Diode laser assisted bonding using a silver nanoparticle material for assembly of power electronic devices
Full text linkWide bandgap (WBG) semiconductor power electronic devices based on silicon carbide
(SiC) and gallium nitride (GaN) possess high-power density and high-temperature
capabilities that can revolutionise the power electronics industry. Recently die-attach
materials based on silver nanoparticles have attracted significant interest for potential use
in the assembly of WBG power devices because of their high melting temperatures and
excellent electrical and thermal conductivities. The conventional method of sintering
silver nanoparticle materials for the assembly of power devices diminishes the
manufacturing efficiency due to long sintering time and increases the possibility of heat-and pressure-induced damage to electronic devices. This thesis presents the findings of a
novel study on the utilisation of a diode-laser-assisted sintering method for the bonding
of silicon (Si) dies with gold electrodes to direct bond copper (DBC) substrates, and the
attachment of silicon dies to Ag-metallised silicon substrates using silver nanoparticle
paste for potential applications in the assembly of power electronic devices. Although Si
dies have been used for initial testing and development, the research can be adapted for
WBG semiconductor devices. The research contributed to knowledge by delivering novel
insights that address the lack of studies on the high-temperature reliability performance
of laser-sintered Ag joints on Cu-, Au- and Ag-metallised substrates by conducting
thermal ageing test at 300 °C. Furthermore, the influence of various bonding process
parameters, including laser power, laser irradiation time, sintering pressure, and surface
metallisation on the quality of sintered nano-Ag joints was investigated. This study also
included a shear test, cross-sectional analysis, fracture surface examination, and 3D X-ray examination of the bonded assemblies. The results show that for silicon-to-DBC
bonding, an average shear strength of 35.2 MPa can be obtained under processing
conditions of 150 W of laser power for 5 minutes, and 1.2 MPa of bonding pressure. For
Si-to-Si bonding, an average shear strength of 48.9 MPa was achieved at 150 W for 5
minutes and 0.5 MPa. Strong Si-Ag-Si bonds were realised with a laser irradiation time
as short as 30 seconds. This study also investigated the use of infrared thermography to
develop a real-time temperature monitoring system in a laser-assisted bonding setup. The
simulation results of the 3D heat-transfer models of the setup provided valuable insights
into the heat distribution in the bonding assembly. The results of this study show that the
laser-based bonding method, with its rapid and selective heating capabilities, and sintered
nano-Ag joints with excellent high-temperature stability have potential applications in
the manufacturing of high temperature power electronic packages and modules.James-Watt Scholarshi
Exploring mental ill-health stigma in sport : a social identity approach
Full text linkThe phrase "we must challenge the stigma" echoes loudly in the world of sports when it comes
to mental health. Yet research on stigma in sport is limited and often lacks a theoretical basis,
with current interventions providing inconclusive evidence of mental health stigma reduction.
To address this gap, the present thesis is grounded in the Social Identity Approach, aiming to
bridge the conceptual foundations of stigma, rooted in identity-based differences that create a
distinction between an esteemed "us" and a devalued "them", with the current theoretical gaps
observed in sport literature. Three studies are presented to examine mental ill-health stigma at
the individual (micro), social (meso), and societal (macro) levels of sport. A comprehensive
overview of existing literature and core thesis arguments are presented in the foundational
chapters (1 and 2). Two studies are presented in Chapter 3: Upon reviewing 14,242 articles
against inclusion and exclusion criteria, Study 1 (n = 278 articles) details a mixed-method
content analysis examining stigmatising and anti-stigmatising narratives related to mental ill-health classifications in elite sport, as depicted in popular UK print media. Study 2 (n = 151
articles, 45,407 words) details a reflexive thematic analysis focused on the lived experiences
of elite athletes who have faced mental ill-health, as depicted in popular UK print media. The
final study, presented in Chapter 4, details empirical evidence (n = 388) on the influence of an
athlete's identity on their perceptions of mental ill-health stigma and their willingness to seek
help for mental ill-health concerns. To conclude, a discussion on the significance of these
studies in furthering our understanding of mental ill-health stigma within sport is presented in
Chapter 5. Three key theoretical implications are discussed: 1) stigma is maintained by
reinforcing an ‘us’ versus ‘them’ narrative; 2) what is important to my group is what is
important to me; and 3) role models are important, if they are relatable. Given the novelty of
this research area in sport, several future research directions are discussed
Resistance to change in UK universities : a Habermasian perspective
Full text linkThis thesis examines the potential for argumentation as a form of resistance management
during change within the context of UK Higher Education Institutions. Resistance to
change is an area that has received considerable attention, with many scholars attributing
it as a significant reason why change initiatives fail. However, resistance is seldom
studied in depth within the UK higher education context. The resistance to change
literature is also split regarding the most effective ways to manage resistance. Decades
of research have yielded mixed results for virtually all techniques (Huy, Corley and
Kraatz, 2014; Furst and Cable, 2008; Ford, Ford and D'Amelio, 2008). This suggests a
need for a more context-specific approach. Within the higher education context, many
scholars argue that there is a need for more constructive conflict at universities during
change (Hughes, 2007; Bland et al., 2005). Argumentation may be particularly useful in
this context as a result, as argumentation theory is designed to encourage rational debate
and resolve conflict.
In order to test this assumption, this study interviewed 37 academics from 12 institutions
from England and Scotland. The interviews were semi-structured and were analysed
using a thematic approach to data analysis with an interpretivist perspective. The findings
yielded both practical and theoretical contributions. The practical contributions included
guidance to management on how to encourage desirable resistance-type behaviours and
avoid undesirable ones. Desirable resistance-type behaviours can generate mutually
beneficial outcomes to both management and staff. One of the theoretical contributions
of this study is developed validity claims that help communicative action theory to
address some of its key criticisms, enhancing its practical value to this context.
Additionally, enhanced understanding of the resistance phenomena was achieved by
examining it through the lens of communicative action
Hardware optimizations for enhancing 5G ORAN networks : precoding, correlation detection, and channel estimation
Full text linkCurrent and future wireless communication networks need to support the demand for
ever-increasing data rates. Multiple solutions have been standardized by Open Radio
Access Network Alliance including the split of the network physical layer. This structure
allows the allocation of some baseband functions to the Radio Unit and others to the
Distributed Unit. On one hand, the inclusion of these functions in the Radio Unit reduces
the data rate of the fronthaul which is a critical bottleneck for the network. On the other
hand, it could significantly increase the complexity of the Radio Unit. Therefore, the
challenge is to develop valid hardware solutions in terms of complexity to benefit from a
higher-level functional split. For this reason, the first standardized split is the option
“7.2x” allowing variations in the precoding function location. Specifically, it defines two
cases: “Category A” which includes the precoding function in the Distributed Unit, and
“Category B” which includes it in the Radio Unit.
To support the latter, the present thesis discusses three main analyses addressing
computational cost, stability, and system throughput for several precoding algorithms.
Among six analysed algorithms, one was identified as the most promising, LDL, which
was subsequently refactored in the matrix form to be integrated in the channel inversion
provided by AMD Xilinx on Adaptable Intelligent Engine in Versal device. Additionally,
a possible improvement was investigated with the integration of the spatial correlation
knowledge.
Moreover, since precoding is highly connected to the channel estimation, the thesis also
focuses on the urgent need for high-accuracy and low-complex solutions. Indeed, the
proposed channel estimation based on a Convolutional Neural Network calculates the
channel matrix from DMRS avoiding the interpolation stage.
The integration of two of the most challenging functions on the Radio Unit would
facilitate communication with the Distributed Unit providing the possibility of larger
parallelized structures – thus, supporting much larger data rate in future networks
Deep learning applications for 3D imaging
No full textThis thesis investigates the application of Artificial Intelligence (AI) in combination with
Light Detection and Ranging (LiDAR) technology to enhance resolution and estimate
poses of humans across various sensors.
Initially, we create a specialized deep learning model to improve the resolution of a
research-grade Single-Photon Avalanche Diode (SPAD) array sensor. Employing a fusion
approach with intensity images and carefully chosen features from the SPAD raw data,
our method achieves fourfold super-resolution and maintains robustness in noisy environments. Extending this project to an extremely low spatial resolution commercial SPAD
sensor, we leverage its high depth resolution to generate detailed depth maps and estimate
poses for multiple individuals. Applying neural networks to this extremely low-resolution
sensor shows significant improvements in recovering high-resolution depth images compared to the sensor’s original spatial resolution, showing the ability of AI to automatically
identify patterns in data and reconstruct scenes from very low spatial resolution sensors.
We also extend our approach to radar data, which shares characteristics such as high-precision range information but low spatial resolution. Despite the radar’s limited resolution, our deep learning model successfully estimates poses in a restricted set of scenarios.
To enhance the interpretability of our models, we integrate an Explainable AI (XAI)
framework into the super-resolution work on SPAD sensors. This framework provides
insight into the relevant data features used by the model to work. We also adapt our
method to magnetoencephalography (MEG) data, to understand how a deep learning
model distinguishes various brain activities from MEG measurements in a simple case
study.
In summary, this research develops multiple deep-learning approaches for LiDAR imaging, complemented by XAI frameworks to provide intuition the decision-making processes
within neural networks. Beyond LiDAR, our research includes other data types like radar
and magnetoencephalography (MEG) data from the brain
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