High-current inductors for high-power automotive DC-DC converters

This thesis is focused on the investigation of magnetic materials for high-power dcdc converters in hybrid and fuel cell vehicles and the development of an optimized high-power inductor for a multi-phase converter. The thesis introduces the power system architectures for hybrid and fuel cell vehicles. The requirements for power electronic converters are established and the dc-dc converter topologies of interest are introduced. A compact and efficient inductor is critical to reduce the overall cost, weight and volume of the dc-dc converter and optimize vehicle driving range and traction power. Firstly, materials suitable for a gapped CC-core inductor are analyzed and investigated. A novel inductor-design algorithm is developed and automated in order to compare and contrast the various magnetic materials over a range of frequencies and ripple ratios. The algorithm is developed for foil-wound inductors with gapped CC-cores in the low (10 kHz) to medium (30 kHz) frequency range and investigates the materials in a natural-convection-cooled environment. The practical effects of frequency, ripple, air-gap fringing, and thermal configuration are investigated next for the iron-based amorphous metal and 6.5 % silicon steel materials. A 2.5 kW converter is built to verify the optimum material selection and thermal configuration over the frequency range and ripple ratios of interest. Inductor size can increase in both of these laminated materials due to increased airgap fringing losses. Distributing the airgap is demonstrated to reduce the inductor losses and size but has practical limitations for iron-based amorphous metal cores. The effects of the manufacturing process are shown to degrade the iron-based amorphous metal multi-cut core loss. The experimental results also suggest that gap loss is not a significant consideration in these experiments. The predicted losses by the equation developed by Reuben Lee and cited by Colonel McLyman are significantly higher than the experimental results suggest. Iron-based amorphous metal has better preformance than 6.5 % silicon steel when a single cut core and natural-convection-cooling are used. Conduction cooling, rather than natural convection, can result in the highest power density inductor. The cooling for these laminated materials is very dependent on the direction of the lamination and the component mounting. Experimental results are produced showing the effects of lamination direction on the cooling path. A significant temperature reduction is demonstrated for conduction cooling versus natural-convection cooling. Iron-based amorphous metal and 6.5% silicon steel are competitive materials when conduction cooled. A novel inductor design algorithm is developed for foil-wound inductors with gapped CC-cores for conduction cooling of core and copper. Again, conduction cooling, rather than natural convection, is shown to reduce the size and weight of the inductor. The weight of the 6.5 % silicon steel inductor is reduced by around a factor of ten compared to natural-convection cooling due to the high thermal conductivity of the material. The conduction cooling algorithm is used to develop high-power custom inductors for use in a high power multi-phase boost converter. Finally, a high power digitally-controlled multi-phase boost converter system is designed and constructed to test the high-power inductors. The performance of the inductors is compared to the predictions used in the design process and very good correlation is achieved. The thesis results have been documented at IEEE APEC, PESC and IAS conferences in 2007 and at the IEEE EPE conference in 2008.

The human papillomavirus and cancer: a multiphase study investigating preventative behaviours in young males

Background: The Human Papillomavirus (HPV) is one of the world’s most common sexually transmitted infections, and a causative factor of oropharyngeal, anal and penile cancers in males. Worldwide, an estimated 39,000 HPV-associated cancers occur each year in men. The highest rates of HPV infection are found in adults aged 18 to 28 years. Clinical evidence indicates that use of a condom in addition to obtaining the HPV vaccine provides the greatest protection from HPV infections. Aim: To explore young men’s attitudes, beliefs, and behavioural intention in relation to receiving the HPV vaccine and using a condom correctly and consistently. Collectively, both behaviours are linked to the prevention of HPV transmission and associated infections with HPV. Method: A multi- phase study, underpinned by the Theory of Planned Behaviour, involving a qualitative belief elicitation, pilot, and quantitative cross-sectional study was conducted. A belief elicitation (n=12) phase was used to generate items to include in a newly developed research instrument. Post pilot the research instrument was utilised in a cross sectional online survey to explore the attitudes, beliefs, and behavioural intention of young men (n= 359) with regard to receiving the HPV vaccine, and using a condom correctly and consistently. Data Collection: Data collection took place over a three month time frame. Male participants were recruited from a university in Southern Ireland via a student email system, as well as posting advertisements on numerous health, social and sports websites. Sample: Three hundred and fifty nine male participants aged 18- 28 years completed the online questionnaire. Data Analysis: Data were analysed using SPSS. Descriptive, correlational, multiple and hierarchical regression analysis were performed on the indirect and direct variables of the Theory of Planned Behaviour i.e. attitude, subjective norm, perceived behavioural control, and intention. Status variables were also included in descriptive analysis and hierarchical regressions. Findings are presented through text and graphical representation. Results: Alarming sexual health statistics identified that only 44.3% of participants always used a condom, and 78.6% never used a condom for oral sex. Furthermore, findings reveal that the constructs of the Theory of Planned Behaviour adequately measure male’s attitudes, beliefs and behavioural intention with regard to both behaviours. The Theory of Planned Behaviour has assisted in identifying how social pressures play an influential role in relation to males receiving the HPV vaccine. Attitudes presented as the most significant predictor of male’s intentions to use a condom correctly and consistently. Intention to perform both behaviours was identified as moderate to high. Conclusion: This study has contributed to the field of HPV research, as it is the first piece of research to explore preventative HPV behaviours i.e. receiving the HPV vaccine, and condom use, amongst young males, utilising the Theory of Planned Behaviour. A deeper understanding of young male’s attitudes, beliefs, and behavioural intention on this topic has been achieved. Additionally, a new robust research instrument has been constructed. Findings from this study will undoubtedly help in the implementation of the HPV vaccine in Ireland, as well as influence health promotion campaigns aimed at young males, addressing the topic of condom use.

Accommodating curvature in a highly ordered functionalized metal oxide nanofiber: synthesis, characterization and multi-scale modeling of layered nanosheets

A key element in the rational design of hybrid organic-inorganic nanostructures, is control of surfactant packing and adsorption onto the inorganic phase in crystal growth and assembly. In layered single crystal nanofibers and bilayered 2D nanosheets of vanadium oxide, we show how the chemisorption of preferred densities of surfactant molecules can direct formation of ordered, curved layers. The atom-scale features of the structures are described using molecular dynamics simulations that quantify surfactant packing effects and confirm the preference for a density of 5 dodecanethiol molecules per 8 vanadium attachment sites in the synthesised structures. This assembly maintains a remarkably well ordered interlayer spacing, even when curved. The assemblies of interdigitated organic bilayers on V2O5 are shown to be sufficiently flexible to tolerate curvature while maintaining a constant interlayer distance without rupture, delamination or cleavage. The accommodation of curvature and invariant structural integrity points to a beneficial role for oxide-directed organic film packing effects in layered architectures such as stacked nanofibers and hybrid 2D nanosheet systems.

Multi-scale modelling of atomic layer deposition

High-permittivity ("high-k") dielectric materials are used in the transistor gate stack in integrated circuits. As the thickness of silicon oxide dielectric reduces below 2 nm with continued downscaling, the leakage current because of tunnelling increases, leading to high power consumption and reduced device reliability. Hence, research concentrates on finding materials with high dielectric constant that can be easily integrated into a manufacturing process and show the desired properties as a thin film. Atomic layer deposition (ALD) is used practically to deposit high-k materials like HfO2, ZrO2, and Al2O3 as gate oxides. ALD is a technique for producing conformal layers of material with nanometer-scale thickness, used commercially in non-planar electronics and increasingly in other areas of science and technology. ALD is a type of chemical vapor deposition that depends on self-limiting surface chemistry. In ALD, gaseous precursors are allowed individually into the reactor chamber in alternating pulses. Between each pulse, inert gas is admitted to prevent gas phase reactions. This thesis provides a profound understanding of the ALD of oxides such as HfO2, showing how the chemistry affects the properties of the deposited film. Using multi-scale modelling of ALD, the kinetics of reactions at the growing surface is connected to experimental data. In this thesis, we use density functional theory (DFT) method to simulate more realistic models for the growth of HfO2 from Hf(N(CH3)2)4/H2O and HfCl4/H2O and for Al2O3 from Al(CH3)3/H2O.Three major breakthroughs are discovered. First, a new reaction pathway, ’multiple proton diffusion’, is proposed for the growth of HfO2 from Hf(N(CH3)2)4/H2O.1 As a second major breakthrough, a ’cooperative’ action between adsorbed precursors is shown to play an important role in ALD. By this we mean that previously-inert fragments can become reactive once sufficient molecules adsorb in their neighbourhood during either precursor pulse. As a third breakthrough, the ALD of HfO2 from Hf(N(CH3)2)4 and H2O is implemented for the first time into 3D on-lattice kinetic Monte-Carlo (KMC).2 In this integrated approach (DFT+KMC), retaining the accuracy of the atomistic model in the higher-scale model leads to remarkable breakthroughs in our understanding. The resulting atomistic model allows direct comparison with experimental techniques such as X-ray photoelectron spectroscopy and quartz crystal microbalance.

Photonic integrated circuits for the generation of coherent optical signals

The demand for optical bandwidth continues to increase year on year and is being driven primarily by entertainment services and video streaming to the home. Current photonic systems are coping with this demand by increasing data rates through faster modulation techniques, spectrally efficient transmission systems and by increasing the number of modulated optical channels per fibre strand. Such photonic systems are large and power hungry due to the high number of discrete components required in their operation. Photonic integration offers excellent potential for combining otherwise discrete system components together on a single device to provide robust, power efficient and cost effective solutions. In particular, the design of optical modulators has been an area of immense interest in recent times. Not only has research been aimed at developing modulators with faster data rates, but there has also a push towards making modulators as compact as possible. Mach-Zehnder modulators (MZM) have proven to be highly successful in many optical communication applications. However, due to the relatively weak electro-optic effect on which they are based, they remain large with typical device lengths of 4 to 7 mm while requiring a travelling wave structure for high-speed operation. Nested MZMs have been extensively used in the generation of advanced modulation formats, where multi-symbol transmission can be used to increase data rates at a given modulation frequency. Such nested structures have high losses and require both complex fabrication and packaging. In recent times, it has been shown that Electro-absorption modulators (EAMs) can be used in a specific arrangement to generate Quadrature Phase Shift Keying (QPSK) modulation. EAM based QPSK modulators have increased potential for integration and can be made significantly more compact than MZM based modulators. Such modulator designs suffer from losses in excess of 40 dB, which limits their use in practical applications. The work in this thesis has focused on how these losses can be reduced by using photonic integration. In particular, the integration of multiple lasers with the modulator structure was considered as an excellent means of reducing fibre coupling losses while maximising the optical power on chip. A significant difficultly when using multiple integrated lasers in such an arrangement was to ensure coherence between the integrated lasers. The work investigated in this thesis demonstrates for the first time how optical injection locking between discrete lasers on a single photonic integrated circuit (PIC) can be used in the generation of coherent optical signals. This was done by first considering the monolithic integration of lasers and optical couplers to form an on chip optical power splitter, before then examining the behaviour of a mutually coupled system of integrated lasers. By operating the system in a highly asymmetric coupling regime, a stable phase locking region was found between the integrated lasers. It was then shown that in this stable phase locked region the optical outputs of each laser were coherent with each other and phase locked to a common master laser.

Novel processes, test structures and characterisation for future germanium technologies

In order to widely use Ge and III-V materials instead of Si in advanced CMOS technology, the process and integration of these materials has to be well established so that their high mobility benefit is not swamped by imperfect manufacturing procedures. In this dissertation number of key bottlenecks in realization of Ge devices are investigated; We address the challenge of the formation of low resistivity contacts on n-type Ge, comparing conventional and advanced rapid thermal annealing (RTA) and laser thermal annealing (LTA) techniques respectively. LTA appears to be a feasible approach for realization of low resistivity contacts with an incredibly sharp germanide-substrate interface and contact resistivity in the order of 10 -7 Ω.cm2. Furthermore the influence of RTA and LTA on dopant activation and leakage current suppression in n+/p Ge junction were compared. Providing very high active carrier concentration > 1020 cm-3, LTA resulted in higher leakage current compared to RTA which provided lower carrier concentration ~1019 cm-3. This is an indication of a trade-off between high activation level and junction leakage current. High ION/IOFF ratio ~ 107 was obtained, which to the best of our knowledge is the best reported value for n-type Ge so far. Simulations were carried out to investigate how target sputtering, dose retention, and damage formation is generated in thin-body semiconductors by means of energetic ion impacts and how they are dependent on the target physical material properties. Solid phase epitaxy studies in wide and thin Ge fins confirmed the formation of twin boundary defects and random nucleation growth, like in Si, but here 600 °C annealing temperature was found to be effective to reduce these defects. Finally, a non-destructive doping technique was successfully implemented to dope Ge nanowires, where nanowire resistivity was reduced by 5 orders of magnitude using PH3 based in-diffusion process.