Low valence cation doping of bulk Cr2O3: Charge compensation and oxygen vacancy formation

The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO2, and CeO2, the effect of substitutional metal dopants in bulk Cr2O3 and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard + U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal-doped bulk Cr2O3 to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr2O3 is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and, dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed "the active" oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr2O3 by lowering the energy cost.

Investigation of numerical atomic orbitals for first-principles calculations of the electronic and transport properties of silicon nanowire structures

This thesis is focused on the application of numerical atomic basis sets in studies of the structural, electronic and transport properties of silicon nanowire structures from first-principles within the framework of Density Functional Theory. First we critically examine the applied methodology and then offer predictions regarding the transport properties and realisation of silicon nanowire devices. The performance of numerical atomic orbitals is benchmarked against calculations performed with plane waves basis sets. After establishing the convergence of total energy and electronic structure calculations with increasing basis size we have shown that their quality greatly improves with the optimisation of the contraction for a fixed basis size. The double zeta polarised basis offers a reasonable approximation to study structural and electronic properties and transferability exists between various nanowire structures. This is most important to reduce the computational cost. The impact of basis sets on transport properties in silicon nanowires with oxygen and dopant impurities have also been studied. It is found that whilst transmission features quantitatively converge with increasing contraction there is a weaker dependence on basis set for the mean free path; the double zeta polarised basis offers a good compromise whereas the single zeta basis set yields qualitatively reasonable results. Studying the transport properties of nanowire-based transistor setups with p+-n-p+ and p+-i-p+ doping profiles it is shown that charge self-consistency affects the I-V characteristics more significantly than the basis set choice. It is predicted that such ultrascaled (3 nm length) transistors would show degraded performance due to relatively high source-drain tunnelling currents. Finally, it is shown the hole mobility of Si nanowires nominally doped with boron decreases monotonically with decreasing width at fixed doping density and increasing dopant concentration. Significant mobility variations are identified which can explain experimental observations.

Development of miniaturized wireless sensor nodes suitable for building energy management and modelling

Buildings consume 40% of Ireland's total annual energy translating to 3.5 billion (2004). The EPBD directive (effective January 2003) places an onus on all member states to rate the energy performance of all buildings in excess of 50m2. Energy and environmental performance management systems for residential buildings do not exist and consist of an ad-hoc integration of wired building management systems and Monitoring & Targeting systems for non-residential buildings. These systems are unsophisticated and do not easily lend themselves to cost effective retrofit or integration with other enterprise management systems. It is commonly agreed that a 15-40% reduction of building energy consumption is achievable by efficiently operating buildings when compared with typical practice. Existing research has identified that the level of information available to Building Managers with existing Building Management Systems and Environmental Monitoring Systems (BMS/EMS) is insufficient to perform the required performance based building assessment. The cost of installing additional sensors and meters is extremely high, primarily due to the estimated cost of wiring and the needed labour. From this perspective wireless sensor technology provides the capability to provide reliable sensor data at the required temporal and spatial granularity associated with building energy management. In this paper, a wireless sensor network mote hardware design and implementation is presented for a building energy management application. Appropriate sensors were selected and interfaced with the developed system based on user requirements to meet both the building monitoring and metering requirements. Beside the sensing capability, actuation and interfacing to external meters/sensors are provided to perform different management control and data recording tasks associated with minimisation of energy consumption in the built environment and the development of appropriate Building information models(BIM)to enable the design and development of energy efficient spaces.

Experimental study of quantum dot and dash lasers

Quantum dashes are elongated quantum dots. Polarized edge-photovoltage and spontaneous emission spectroscopy are used to study the anisotropy of optical properties in 1.5μm InGaAsP and AlGaInAs-based quantum dash lasers. Strain, which causes TM-polarized transitions to be suppressed at the band edge, coupled with carrier confinement and dash shape leads to an enhancement of the optical properties for light polarized along the dash long axis, in excellent agreement with theoretical results. An analysis of the integrated facet and spontaneous emission rate with total current and temperature reveals that, in both undoped and p-doped InGaAsP-based quantum dash lasers at room temperature, the threshold current and its temperature dependence remain dominated by Auger recombination. We also identify two processes which can limit the output power and propose that the effects of the dopant in p-doped InGaAsP-based lasers dominate at low temperature but decrease with increasing temperature. A high threshold current density in undoped AlGaInAs-based quantum dash laser samples studied, which degrade rapidly at low temperature, is not due to intrinsic carrier recombination processes. 1.3μm GaAs-based quantum dots lasers have been widely studied, but there remains issues as to the nature of the electronic structure. Polarized edge-photovoltage spectroscopy is used to investigate the energy distribution and nature of the energy states in InAs/GaAs quantum dot material. A non-negligible TM-polarized transition, which is often neglected in calculations and analyses, is measured close to the main TE-polarized ground state transition. Theory is in very good agreement with the experimental results and indicates that the measured low-energy TM-polarized transition is due to the strong spatial overlap between the ground state electron and the light-hole component of a low-lying excited hole state. Further calculations suggest that the TM-polarized transition reduces at the band edge as the quantum dot aspect ratio decreases.

The use of fault detection and diagnostics to reduce air handling unit energy consumption

The contribution of buildings towards total worldwide energy consumption in developed countries is between 20% and 40%. Heating Ventilation and Air Conditioning (HVAC), and more specifically Air Handling Units (AHUs) energy consumption accounts on average for 40% of a typical medical device manufacturing or pharmaceutical facility’s energy consumption. Studies have indicated that 20 – 30% energy savings are achievable by recommissioning HVAC systems, and more specifically AHU operations, to rectify faulty operation. Automated Fault Detection and Diagnosis (AFDD) is a process concerned with potentially partially or fully automating the commissioning process through the detection of faults. An expert system is a knowledge-based system, which employs Artificial Intelligence (AI) methods to replicate the knowledge of a human subject matter expert, in a particular field, such as engineering, medicine, finance and marketing, to name a few. This thesis details the research and development work undertaken in the development and testing of a new AFDD expert system for AHUs which can be installed in minimal set up time on a large cross section of AHU types in a building management system vendor neutral manner. Both simulated and extensive field testing was undertaken against a widely available and industry known expert set of rules known as the Air Handling Unit Performance Assessment Rules (APAR) (and a later more developed version known as APAR_extended) in order to prove its effectiveness. Specifically, in tests against a dataset of 52 simulated faults, this new AFDD expert system identified all 52 derived issues whereas the APAR ruleset identified just 10. In tests using actual field data from 5 operating AHUs in 4 manufacturing facilities, the newly developed AFDD expert system for AHUs was shown to identify four individual fault case categories that the APAR method did not, as well as showing improvements made in the area of fault diagnosis.

Quantifying the value of improved wind energy forecasts in a pool-based electricity market

This work illustrates the influence of wind forecast errors on system costs, wind curtailment and generator dispatch in a system with high wind penetration. Realistic wind forecasts of different specified accuracy levels are created using an auto-regressive moving average model and these are then used in the creation of day-ahead unit commitment schedules. The schedules are generated for a model of the 2020 Irish electricity system with 33% wind penetration using both stochastic and deterministic approaches. Improvements in wind forecast accuracy are demonstrated to deliver: (i) clear savings in total system costs for deterministic and, to a lesser extent, stochastic scheduling; (ii) a decrease in the level of wind curtailment, with close agreement between stochastic and deterministic scheduling; and (iii) a decrease in the dispatch of open cycle gas turbine generation, evident with deterministic, and to a lesser extent, with stochastic scheduling.