Nanostructured materials for Lithium-ion battery technology

The Li-ion battery has for several years been at the forefront of powering an ever-increasing number of modem consumer electronic devices such as laptops, tablet PCs, cell phones, portable music players etc., while in more recent times, has also been sought to power a range of emerging electric and hybrid-electric vehicle classes. Given their extreme popularity, a number of features which define the performance of the Li-ion battery have become a target of improvement and have garnered tremendous research effort over the past two decades. Features such as battery capacity, voltage, lifetime, rate performance, together with important implications such as safety, environmental benignity and cost have all attracted attention. Although properties such as cell voltage and theoretical capacity are bound by the selection of electrode materials which constitute its interior, other performance makers of the Li-ion battery such as actual capacity, lifetime and rate performance may be improved by tailoring such materials with characteristics favourable to Li+ intercalation. One such tailoring route involves shrinking of the constituent electrode materials to that of the nanoscale, where the ultra-small diameters may bestow favourable Li+ intercalation properties while providing a necessary mechanical robustness during routine electrochemical operation. The work detailed in this thesis describes a range of synthetic routes taken in nanostructuring a selection of choice Li-ion positive electrode candidates, together with a review of their respective Li-ion performances. Chapter one of this thesis serves to highlight a number of key advancements which have been made and detailed in the literature over recent years pertaining to the use of nanostructured materials in Li-ion technology. Chapter two provides an overview of the experimental conditions and techniques employed in the synthesis and electrochemical characterisation of the as-prepared electrode materials constituting this doctoral thesis. Chapter three details the synthesis of small-diameter V2O5 and V2O5/TiO2 nanocomposite structures prepared by a novel carbon nanocage templating method using liquid precursors. Chapter four details a hydrothermal synthesis and characterisation of nanostructured β-LiVOPO4 powders together with an overview of their Li+ insertion properties while chapter five focuses on supercritical fluid synthesis as one technique in the tailoring of FeF2 and CoF2 powders having potentially appealing Li-ion 'conversion' properties. Finally, chapter six summarises the overall conclusions drawn from the results presented in this thesis, coupled with an indication of potential future work which may be explored upon the materials described in this work.

Low power predictable memory and processing architectures

Great demand in power optimized devices shows promising economic potential and draws lots of attention in industry and research area. Due to the continuously shrinking CMOS process, not only dynamic power but also static power has emerged as a big concern in power reduction. Other than power optimization, average-case power estimation is quite significant for power budget allocation but also challenging in terms of time and effort. In this thesis, we will introduce a methodology to support modular quantitative analysis in order to estimate average power of circuits, on the basis of two concepts named Random Bag Preserving and Linear Compositionality. It can shorten simulation time and sustain high accuracy, resulting in increasing the feasibility of power estimation of big systems. For power saving, firstly, we take advantages of the low power characteristic of adiabatic logic and asynchronous logic to achieve ultra-low dynamic and static power. We will propose two memory cells, which could run in adiabatic and non-adiabatic mode. About 90% dynamic power can be saved in adiabatic mode when compared to other up-to-date designs. About 90% leakage power is saved. Secondly, a novel logic, named Asynchronous Charge Sharing Logic (ACSL), will be introduced. The realization of completion detection is simplified considerably. Not just the power reduction improvement, ACSL brings another promising feature in average power estimation called data-independency where this characteristic would make power estimation effortless and be meaningful for modular quantitative average case analysis. Finally, a new asynchronous Arithmetic Logic Unit (ALU) with a ripple carry adder implemented using the logically reversible/bidirectional characteristic exhibiting ultra-low power dissipation with sub-threshold region operating point will be presented. The proposed adder is able to operate multi-functionally.

Modifying germanium nanowires for future devices: an in situ TEM study

Germanium was of great interest in the 1950’s when it was used for the first transistor device. However, due to the water soluble and unstable oxide it was surpassed by silicon. Today, as device dimensions are shrinking the silicon oxide is no longer suitable due to gate leakage and other low-κ dielectrics such as Al2O3 and HfO2 are being used. Germanium (Ge) is a promising material to replace or integrate with silicon (Si) to continue the trend of Moore’s law. Germanium has better intrinsic mobilities than silicon and is also silicon fab compatible so it would be an ideal material choice to integrate into silicon-based technologies. The progression towards nanoelectronics requires a lot of in depth studies. Dynamic TEM studies allow observations of reactions to allow a better understanding of mechanisms and how an external stimulus may affect a material/structure. This thesis details in situ TEM experiments to investigate some essential processes for germanium nanowire (NW) integration into nanoelectronic devices; i.e. doping and Ohmic contact formation. Chapter 1 reviews recent advances in dynamic TEM studies on semiconductor (namely silicon and germanium) nanostructures. The areas included are nanowire/crystal growth, germanide/silicide formation, irradiation, electrical biasing, batteries and strain. Chapter 2 details the study of ion irradiation and the damage incurred in germanium nanowires. An experimental set-up is described to allow for concurrent observation in the TEM of a nanowire following sequential ion implantation steps. Grown nanowires were deposited on a FIB labelled SiN membrane grid which facilitated HRTEM imaging and facile navigation to a specific nanowire. Cross sections of irradiated nanowires were also performed to evaluate the damage across the nanowire diameter. Experiments were conducted at 30 kV and 5 kV ion energies to study the effect of beam energy on nanowires of varied diameters. The results on nanowires were also compared to the damage profile in bulk germanium with both 30 kV and 5 kV ion beam energies. Chapter 3 extends the work from chapter 2 whereby nanowires are annealed post ion irradiation. In situ thermal annealing experiments were conducted to observe the recrystallization of the nanowires. A method to promote solid phase epitaxial growth is investigated by irradiating only small areas of a nanowire to maintain a seed from which the epitaxial growth can initiate. It was also found that strain in the nanowire greatly effects defect formation and random nucleation and growth. To obtain full recovery of the crystal structure of a nanowire, a stable support which reduces strain in the nanowire is essential as well as containing a seed from which solid phase epitaxial growth can initiate. Chapter 4 details the study of nickel germanide formation in germanium nanostructures. Rows of EBL (electron beam lithography) defined Ni-capped germanium nanopillars were extracted in FIB cross sections and annealed in situ to observe the germanide formation. Chapter 5 summarizes the key conclusions of each chapter and discusses an outlook on the future of germanium nanowire studies to facilitate their future incorporation into nanodevices.

Leadership in the community sector: promoting collaboration and social change in Belfast

Community development is centrally concerned with people in communities working together to achieve a common goal, that is, to collaborate, whether within local geographical communities, in communities of shared interests or among groups sharing a common identity. Its overarching goal is one of progressive transformational social change. As Belfast transitions from a conflict to a post-conflict society, there is a need for greater, more effective work at local community level in order to address a range of ongoing social and economic issues facing communities, including high levels of disadvantage and division. Given the significance of leadership in building effective collaboration and the centrality of collaboration for community development, it is important to understand how leadership is currently enacted and what kinds of leadership are required to support communities to collaborate effectively to bring about social change. This thesis thus centers on the kind of leadership practised and required to support collaboration for social change within the community sector in Belfast, a city that contains an estimated 28% of the total number of community and voluntary sector (CVS) organisations in Northern Ireland (Northern Ireland Council for Voluntary Action, 2012). Through a series of qualitative, in-depth interviews with people playing leadership roles in local communities, the study critically explores and analyses their experiences and perceptions in relation to leadership and collaboration. Community development in Belfast today is practised within a wider context of neoliberal policies, characterised by austerity and public spending cuts. Whilst not the only influencing factor, this context has had a particular and profound impact on the nature and role of community development practised, and on the kind of leadership enacted within it. The space for reflection and transformative action appears to be shrinking as the contraction of resources to support community development in local communities continues unabated. Those playing leadership roles increasingly find themselves compelled to spend time seeking resources and managing complex funding arrangements rather than focusing on the social change dimensions of their work. Collaboration as promoted by the state seems to have become an instrumental tactic used to implement its austerity measures and curtail the potential of the community sector. Despite this, local leaders are driving initiatives that attempt to push back, helping the sector refocus on its transformational goals of social change. To do this requires support. Those playing leadership roles require resources, including time, to encourage and enable communities to reconnect with the purpose and underpinning values of community development. Leaders also need support to develop and promote new, progressive narratives and visions and pursue these through building collaboration and solidarity.