The hybrid organic-inorganic perovskites (HOIPs), e.g. methylammonium and formamidinium lead halide (MA/FAPbX3, X = I, Br or Cl), are a class of materials that has recently achieved remarkable performances in photovoltaic applications. This thesis describes the synthesis, structure and properties of this class of perovskites, with particular focus on their crystal chemistry, mechanical responses and structural diversity. Understanding the unique crystal chemistry of HOIPs is crucial for device design. While MA-based perovskites have been widely studied, there are still many open questions on the crystal chemistry of FA-based perovskites. In this work, FAPbX3 (X= Br or I) was shown to undergo a cubic (Pm3 ̅m) to tetragonal (P4/mbm) transition on cooling. Studies on the high-pressure crystallography of FAPbI3 exhibited a similar trend and further illustrated band gap tuning via external stimuli. In addition, the cubic lattice of FAPbBr3 was found to be more strained than its MA counterpart. The observed intrinsic strain was modelled with anisotropic line broadening and < 100 > was found to be the least strained direction. To explore potential applications in flexible devices, crystals of single (Pb-based) and double (Bi-based) perovskites were probed by nanoindentation and their mechanical properties, such as Young’s moduli (E) (10 – 20 GPa) and hardnesses (H) (0.2 -0.5 GPa), were determined. The mechanical responses of MA- and FA-based hybrid perovskites correlated well with the chemical and structural variations in these analogues, showing a general trend of ECl > EBr > EI and EPb > EBi. By analogy with classical inorganic perovskites, the hybrid phases can crystallise in both three-dimensional (3D) and low dimensional perovskite-like forms. To improve the stability and remove the toxicity in the current prototypical hybrid perovskites, compositional engineering was applied, focusing on non-toxic bismuth (Bi) as a viable alternative to lead (Pb) in future photovoltaic materials. We report a new layered perovskite, (NH4)3Bi2I9, which exhibits a band gap of 2.0 eV, comparable to MAPbBr3 and FAPbBr3. This work contributes to the materials design goal of more stable and eco-friendly perovskite devices.

Interruption of DC is more challenging than that of AC because of the absence of DC zero-crossings. This thesis proposes an alternative counter-voltage (ACV) DC hybrid circuit breaker (DC-HCB). This DC-HCB can interrupt the DC up to 5 kA with the source voltage of up to 1 kV and it can meet the general requirements i.e., interrupting the DC within 5ms, and limiting over-voltages due to the CB operation. The existing/investigated DC circuit breakers, i.e., active mode DC-HCB and the traditional counter-voltage (TCV) DC-HCB are also investigated and compared with the proposed DC-HCB. The investigations are based on the time-domain simulation studies in the PSCAD/EMTDC. The studies show that the ACV DC-HCB is the preferred option to both the existing DC-HCBs. The main features of the proposed ACV DC-HCB include lower capacitance in the commutation path and the need for a smaller dielectric strength for the primary-path circuit breaker.

The ever-increasing demand for better mobile experiences is propelling the research communities to look ahead at how future networks can be geared up to meet such demands. It is likely that the next-generation of wireless communications will be revolutionary, outpacing the current systems capabilities in terms connectivity, reliability and intelligence. These trends and predictions will cause a revolutionary change in the wireless communications. In this context, the concept of Ultra-Dense Network (UDN) is poised to be the cornerstone of the development of fifth generation(5G) systems, whereby a massive number of base stations (BSs) are deployed for enhancing the network performance metrics. Though such densification might be economically viable in urban areas, it is mostly unfavorable in rural ones due to the sheer complexity and the various factors involved the planning and installation processes; all of which trigger the need for cost-effective, flexible and easily-implementable solutions. As a result, unmanned aerial vehicles (UAVs) emerge as a promising alternative solution for enhancing wireless coverage. Due to their mobility capabilities, UAVs are of particular importance in events of (i) terrestrial-based cellular systems dilapidation, (ii) infrastructure absence in remote and suburban areas, or (iii) limited-duration events or activities wherein there is a short-term need for supplementary network resources to handle the overload. While a growing body literature works towards characterizing and providing insights into the performance of UAVs-only networks (serving the first two purposes), understanding the performance of such networks when coupled with existing terrestrial BSs remains a challenging, yet interesting, open research venue. Towards this direction, this thesis provides a rigorous analysis of the downlink coverage probability of hybrid aerial-terrestrial networks using tools from Stochastic Geometry. The thesis presents a mathematical model that characterizes the coverage probability metric under different network environments. The proposed model is validated against intensive simulations so as to substantiate the analytical results. The developed work is essential to understanding the premises of one possible solution to the UDNs of tomorrow, capture its key performance metrics and, most importantly, to uncover key design insights and reveal new directions for the wireless communication industry.

This research investigates the Neo-Baroque aesthetic of excess in contemporary South African art, and explores reasons for the emergence of this style. It investigates artists who use their bodies as a site of resistance, to contest or reconstruct the dominant social values which establish differences between bodies to place them within the marginal position of ‘Other’. This investigation relates to post-colonial concerns. The artists’ exploitation of the Neo-Baroque aesthetic of excess as a comment on social concerns reveals a sense of crisis within South African society, similar to the conditions from which the seventeenth century Baroque style evolved. Neo-Baroque aesthetics of excess manifest in a variety of ways, and are particularly evident when artists transgress social boundaries placed on the body through abject and erotic associations. Excess ultimately arises from complexity, as hybrid art forms are created from the combination of media and content found within the art work. / Dissertation (MA)--University of Pretoria, 2012. / Visual Arts / unrestricted