Non-equilibrium Green's function predictions of band tails and band gap narrowing in III-V semiconductors and nanodevices

High-doping induced Urbach tails and band gap narrowing play a significant role in determining the performance of tunneling devices and optoelectronic devices such as tunnel field-effect transistors (TFETs), Esaki diodes and light-emitting diodes. In this work, Urbach tails and band gap narrowing values are calculated explicitly for GaAs, InAs, GaSb and GaN as well as ultra-thin bodies and nanowires of the same. Electrons are solved in the non-equilibrium Green's function method in multi-band atomistic tight binding. Scattering on polar optical phonons and charged impurities is solved in the self-consistent Born approximation. The corresponding nonlocal scattering self-energies as well as their numerically efficient formulations are introduced for ultra-thin bodies and nanowires. Predicted Urbach band tails and conduction band gap narrowing agree well with experimental literature for a range of temperatures and doping concentrations. Polynomial fits of the Urbach tail and band gap narrowing as a function of doping are tabulated for quick reference

Experimental evaluation of active-member control of precision structures

The results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure are described. These experiments are directed toward the development of high-performance structural systems as part of the Control/Structure Interaction (CSI) program at JPL. The focus of CSI activity at JPL is to develop the technology necessary to accurately control both the shape and vibration levels in the precision structures from which proposed large space-based observatories will be built. Structural error budgets for these types of structures will likely be in the sub-micron regime; optical tolerances will be even tighter. In order to achieve system level stability and local positioning at this level, it is generally expected that some form of active control will be required

Optimal realizations of floating-point implemented digital controllers with finite word length considerations.

The closed-loop stability issue of finite word length (FWL) realizations is investigated for digital controllers implemented in floating-point arithmetic. Unlike the existing methods which only address the effect of the mantissa bits in floating-point implementation to the sensitivity of closed-loop stability, the sensitivity of closed-loop stability is analysed with respect to both the mantissa and exponent bits of floating-point implementation. A computationally tractable FWL closed-loop stability measure is then defined, and the method of computing the value of this measure is given. The optimal controller realization problem is posed as searching for a floating-point realization that maximizes the proposed FWL closed-loop stability measure, and a numerical optimization technique is adopted to solve for the resulting optimization problem. Simulation results show that the proposed design procedure yields computationally efficient controller realizations with enhanced FWL closed-loop stability performance

Determination of the phase diagram of the electron doped superconductor Ba(Fe1−x_{1-x}Cox_x)2_2As2_2

Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. These data delineate an $x-T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$_2$As$_2$ at 134 K apparently splits into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $x \sim 0.025$, and appears to coexist with the broken symmetry state for an appreciable range of doping, up to $x \sim 0.06$. The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed, at least within the resolution provided by the finite step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for $x \sim 0.018$ and above. The form of this $x-T$ phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions

Synthesis and experimental validation of a new probabilistic strategy to minimize heat transfers used in conditioning of dry air in buildings with fluctuating ambient and room occupancy

Steady-state unit-operations are globally used in chemical engineering. Advantages include ease of control and a uniform product quality. Nonetheless there will be naturally occurring, random (stochastic) fluctuations about any steady-state ‘set’ value of a process parameter. Traditional chemical engineering does not explicitly take account of these. This is because, generally, fluctuation in one parameter appears to be off-set by change in another – with the process outcome remaining apparently steady. However Davey and co-workers (e.g. Davey et al., 2015; Davey, 2015 a; Zou and Davey, 2016; Abdul-Halim and Davey, 2016; Chandrakash and Davey, 2017 a) have shown these naturally occurring fluctuations can accumulate and combine unexpectedly to leverage significant impact and thereby make apparently well-running processes vulnerable to sudden and surprise failure. They developed a probabilistic and quantitative risk framework they titled Fr 13 (Friday 13th) to underscore the nature of these events. Significantly, the framework can be used in ‘second-tier’ studies for re-design to reduce vulnerability to failure. Here, this framework is applied for the first time to show how naturally occurring fluctuations in peak ambient temperature (T₀) and occupancy (room traffic flows) (Lᴛ) can impact heat transfers for conditioning of room air. The conditioning of air in large buildings, including hotels and hospitals, is globally important (Anon., 2012 a). The overarching aim is to quantitatively ‘use’ these fluctuations to develop a strategy for minimum energy. A justification is that methods that permit quantitative determination of reliable strategies for conditioning of air can lead to better energy use, with potential savings, together with reductions in greenhouse gases (GHG). Oddly many buildings do not appear to have a quantitative strategy to minimize conditioning heat transfers. Wide-spread default practice is to simply use an on-off strategy i.e. conditioning-on when the room is occupied and conditioning-off, when un-occupied. One alternative is an on-only strategy i.e. leave the conditioner run continuously. A logical and stepwise combined theoretical-and-experimental, approach was used as a research strategy. A search of the literature showed that work had generally focused on discrete, deterministic aspects and not on mathematically rigorous developments to minimise overall whole-of-building conditioning heat transfers. A preliminary steady-state convective model was therefore synthesized for conditioning air in a (hotel) room (4.5 x 5.0 x 2.5, m) in dry, S-E Australia during summer (20 ≤ T₀ ≤ 40, °C) to an auto-set room bulk temperature of 22 °C for the first time. This was solved using traditional, deterministic methods to show the alternative on-only strategy would use less electrical energy than that of the default on-off for Lᴛ > 36 % (Chu et al., 2016). Findings underscored the importance of the thermal capacitance of a building. The model was again solved using the probabilistic Fr 13 framework in which distributions to mimic fluctuations in T₀ and Lᴛ were (reasonably) assumed and a new energy risk factor (p) was synthesized such that all p > 0 characterized a failure in applied energy strategy (Chu and Davey, 2015). Predictions showed on-only would use less energy on 86.6 % of summer days. Practically, this meant that a continuous on-only strategy would be expected to fail in only 12 of the 90 days of summer, averaged over the long term. It was concluded the Fr 13 framework was an advance over the traditional, deterministic method because all conditioning scenarios that can practically exist are simulated. It was acknowledged however that: 1) a more realistic model was needed to account for radiative heat transfers, and; 2) to improve predictive accuracy, local distributions for T₀ and Lᴛ were needed. To address these: 1) the model was extended mathematically to account for radiative transfers from ambient to the room-interior, and; 2) distributions were carefully-defined based on extensive historical data for S-E Australia from, respectively, Bureau of Meteorology (BoM) (Essendon Airport) and Clarion Suites Gateway Hotel (CSGH) (Melbourne) – a large (85 x 2-room suites) commercial hotel (latitude -37.819708, longitude 144.959936) – for T₀ and Lᴛ for 541 summer days (Dec. 2009 to Feb. 2015) (Chu and Davey, 2017 a). Predictions showed that radiative heat transfers were significant and highlighted that for Lᴛ ≥ 70 %, that is, all commercially viable occupancies, the on-only conditioning strategy would be expected to use less energy. Because findings predicted meaningful savings with the on-only strategy, ‘proof-of-concept’ experiments were carried out for the first time in a controlled-trial in-situ in CSGH over 10 (2 x 5 contiguous) days of summer with 24.2 ≤ T₀ ≤ 40.5, °C and 13.3 ≤ Lᴛ ≤ 100, %. Independent invoices (Origin Energy Ltd, or Simply Energy Ltd, Australia) (at 30 min intervals from nationally registered ‘smart’ power meters) for geometrically identical control and treated suites showed a mean saving of 18.9 % (AUD $2.23 per suite per day) with the on-only strategy, with a concomitant 20.7 It was concluded that because findings supported model predictions, and because robust experimental SOPs had been established and agreed by CSGH, a large-scale validation test of energy strategies should be undertaken in the hotel. Commercial-scale testing over 77 contiguous days of summer (Jan. to Mar., 2016) was carried out in two, dimensionally-identical 2-room suites, with the same fit-out and (S-E) aspect, together with identical air-conditioner (8.1 kW) and nationally registered meters to automatically transmit contiguous (24-7) electrical use (at 30 min intervals) (n = 3,696) for the first time. Each suite (10.164 x 9.675, m floor plan) was auto-set to a bulk air temperature of 22 °C (Chu and Davey, 2017 b). In the treated suite the air-conditioner was operated on-only, whilst in the control it was left to wide-spread industry practice of on-off. The suites had (standard) single-glazed pane windows with heat-attenuating (fabric) internal curtains. Peak ambient ranged from 17.8 ≤ T₀ ≤ 39.1, °C. There were 32 days with recorded rainfall. The overall occupancy Lᴛ of both suites was almost identical at 69.7 and 71.2, An actual reduction in electrical energy costs of AUD$0.75 per day (9 %) averaged over the period was demonstrated for the treated suite. It was concluded therefore that experimental findings directly confirmed the strategy hypothesis that continuous on-only conditioning will use less energy. Although the hypothesis appeared generalizable, and adaptable to a range of room geometries, it was acknowledged that a drawback was that extrapolation of results could not be reliably done because actual energy used would be impacted by seasons. The in-situ commercial-scale experimental study was therefore extended to encompass four consecutive seasons. The research aim was to provide sufficient experimental evidence (n = 13,008) to reliably test the generalizability of the on-only hypothesis (Chu and Davey, 2017 c). Ambient peak ranged from 9.8 ≤ T₀ ≤ 40.5, °C, with rainfall on 169 days (62 %). Overall, Lᴛ was almost identical at 71.9 and 71.7, % respectively, for the treated and control suite. Results based on independent electrical energy invoices showed the on-only strategy used less energy on 147 days (54 %) than the on-off. An overall mean energy saving of 2.68 kWh per suite per day (9.2 %) (i.e. AUD $0.58 or 8.0$18,006 per annum - plus credit certificates that could be used to increase savings. Overall, it was concluded therefore the on-only conditioning hypothesis is generalizable to all seasons, and that there appears no barrier to adaption to a range of room geometries. Highly significantly, the methodology could be readily applied to existing buildings without capital outlays or increases in maintenance. A total of five (5) summative research presentations of results and findings were made to the General Manager and support staff of CSGH over the period to July 2017 inclusive (see Appendix I) that maintained active industry-engagement for the study. To apply these new findings, the synthesis of a computational algorithm in the form of a novel App (Anon., 2012 b; Davey, 2015 b) was carried out for the first time (Chu and Davey, 2017 d). The aim was to demonstrate an App that could be used practically to minimize energy in conditioning of dry air in buildings that must maintain an auto-set temperature despite the impact of fluctuations in T₀ and Lᴛ . The App was synthesized from the extensive experimental commercial-scale data and was applied to compute energy for both strategies from independently forecast T₀ and Lᴛ . Practical performance of the App was shown to be dependent on the accuracy of locally forecast T₀ and Lᴛ . Overall results predicted a saving of 2.62 kWh per 2-room suite per day ($47,870 per annum for CSGH) where accuracy of forecast T₀ is 77 % and Lᴛ is 99 %, averaged over the long term. A concomitant benefit was a predicted reduction greenhouse emissions of 3.1 kg CO₂-e per day. The App appears generalizable – and importantly it is not limited by any underlying heat-model. Its predictive accuracy can be refined with accumulation of experimental data for a range of geo-locations and building-types to make it globally applicable. It was concluded that the App is a useful new tool to minimize energy transfers in conditioning of room dry air in large buildings – and could be readily developed commercially 6. Importantly, it can be applied without capital outlays or additional maintenance cost and at both design and analysis stages. This research is original and not incremental work. Results of this research will be of immediate benefit to risk analysts, heat-design engineers, and owners and operators of large buildings.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 201