Auger recombination in low-dimensional semiconductor structures

In this thesis, calculations of Auger recombination rates in semiconductor quantum wells are presented. Chapter One introduces Auger recombination, and the reasons for studying the Auger process are explained. Basically, Auger recombination is a non- radiative recombination mechanism that becomes more important as the carrier density increases and the bandgap decreases. In direct gap semiconductors, the Auger process has an activation energy, and the resulting highly temperature dependent Auger process is thought to be a possible cause of the high temperature sensitivity of long wavelength semiconductor lasers that are being considered for use as sources in optical fibre communications systems. In Chapter Two, an expression is derived for the CHSH Auger recombination rate in a quantum well (QW) heterostructure. The possible Auger processes in a QW are discussed as are the differences between Auger recombination in a QW and in bulk semiconductors, and the magnitudes of QW and bulk Auger rates are compared. In Chapter Three, the theory of Auger recombination is extended to the case of a quantum well wire (QWW), a semiconductor structure in which carriers are free to move in one direction only. It is found that there are no significant physical differences between Auger recombination in a QW and in a QWW. The ratio of QW and QWW Auger rates is evaluated. Numerical results for Auger transition rates in 1.3µm and 1.55µm In- GaAsP/InP QWs and QWWs are presented in Chapter Four, and comparison with experimental values is made. In particular, the result found in Chapter Two, that, under certain conditions, the Auger rates in the QW and the bulk are approximately the same is found to agree with experimental results from the literature. The derivation of the CHSH Auger transition rates in QWs and QWWs that was presented in Chapters Two and Three required a number of approximations concerning the carrier statistics and the semiconductor bandstructure. In Chapter Five, these approximations are examined, and, although it is found that the use of non-degenerate carrier statistics is reasonably accurate, the assumption of parabolic energy bands can lead to overestimates of .the Auger transition rates. The first five chapters constitute the first part of the thesis, concerning Auger recombination in low-dimensional semiconductor structures. In the second part of the thesis, the realistic bandstructure of low-dimensional semi conductor structures, such as superlattices, is examined. The method used is described in Chapter Six, and is based on an empirical pseudopotential method. Results for the GaAs/AlAs superlattice are presented in Chapter Seven

Lubricant degradation, transport and the effect of extended oil drain intervals on piston assembly tribology

There are ever increasing demands on lubricant manufacturers to meet governmental legislation and customer needs by improving fuel economy, engine durability and exhaust system compatibility as shown by the introduction of GF4 and move towards GF5 specification oils. This has created an ever increasing need to understand how oil degrades in an engine and how this degraded oil affects piston assembly tribology. This review conference paper will give an overview of a collaborative project that has been undertaken to further enhance the understanding of how lubricant degrades in an operating engine, its transport through the engine and effect upon piston assembly tribology

Extraction and tribological investigation of top piston ring zone oil from a gasoline engine

With tightening emission regulations, increased expected fuel economy, and longer drain intervals impacting on lubricant formulation, greater understanding of how oil degrades in an automotive engine is becoming ever more important. Equally significant is the effect that this degraded lubricant has on the tribological operation of the engine, particularly its overall internal friction and component wear. In a previous paper, four tests to degrade oil in a single cylinder engine were reported [1]. These tests were set up such that the lubricating oil was degraded in the ring pack before returning to the sump, where it was sampled and chemical and rheological analysis undertaken. This paper reports the extension of this work using the same Hydra engine and describes how oil has additionally been extracted from the rear of the top piston ring during engine operation. This extracted oil has then been subjected to similar analysis as the sump oil samples in the previous tests, along with additional analysis to look at the tribological properties of the oil using tribometers. The results clearly show significant differences in the rheological, tribological, and chemical properties of the fresh oil and used sump oil samples when compared with the top ring zone (TRZ) oil samples, particularly the effect of load on the levels of volatiles present in the TRZ samples and their effect on traction and friction coefficient values during tribological testing

Engine oil acidity detection using solid state ion selective electrodes

Initial results from oil acidity measurements using thick film electrodes are presented. The results suggest that as the oil degrades, its pH/acidity follows a specific trend. Furthermore, an investigation into the feasibility of detecting changes in oil acidity (i.e. TAN value) using ion selective electrodes fabricated utilising thick film technology is presented. The thick-film (screen printing) technique is a decent means for the mass production of rugged, compact and disposable sensors as many such devices can be printed at the same time making them very cost effective to manufacture. Thick-Film ion selective and reference electrodes were fabricated, calibrated and tested in different oil samples varying its acidity. Ruthenium oxide (RuO2) pH sensitive electrodes were screen printed and were used against silver/silver chloride (Ag/AgCl) reference electrodes as well as a commercial glass Ag/AgCl reference electrode. The potentiometric sets of electrodes were calibrated in pH 4, 7 and 10 buffers in a cyclic manner and the voltage was recorded using a high input impedance voltmete

Hard Thermal Loops, Gauged WZNW Action and the Energy of Hot Quark-Gluon Plasma

The generating functional for hard thermal loops in QCD is rewritten in terms of a gauged WZNW action by introducing an auxiliary field. This shows in a simple way that the contribution of hard thermal loops to the energy of the quark-gluon plasma is positive.Comment: 9 pages, CU-TP 60

Search for Possible Variation of the Fine Structure Constant

Determination of the fine structure constant alpha and search for its possible variation are considered. We focus on a role of the fine structure constant in modern physics and discuss precision tests of quantum electrodynamics. Different methods of a search for possible variations of fundamental constants are compared and those related to optical measurements are considered in detail.Comment: An invited talk at HYPER symposium (Paris, 2002

Application of the dual attribute time-intensity (DATI) sensory method to the temporal measurement of bitterness and astringency in sorghum

Bitterness and astringency are attributes generally cited as the cause of tannin sorghums being unpalatable. The objective here was to determine the effect of sorghums with varying tannin content on bitterness and astringency simultaneously using dual attribute time-intensity (DATI). A trained panel assessed bitterness and astringency of bran infusions of tannin and tannin-free sorghums. In both sorghum types, bitterness developed and reached maximum intensity faster (Tmax 22.5 s; P < 0.001) than astringency (27.9 s). The duration of the astringent sensation (Dtot 69.9 s) lasted longer than bitterness (66.3 s). Overall, the temporal parameters for bitterness distinguished infusions from tannin and tannin-free sorghums slightly more clearly than those for astringency. The research established that, for sorghum, bitterness and astringency are related. The DATI method also revealed that panellists’ perceptions of the time course of bitterness and astringency differ.The South African National Research Foundationhttp://onlinelibrary.wiley.co

OPTIma:a tracking solution for proton computed tomography in high proton flux environments

Currently there is a large discrepancy between the currents that are used for treatments in proton beam therapy facilities and the ultra low beam currents required for many proton CT imaging systems. Here we provide details of the OPTIma silicon strip based tracking system, which has been designed for performing proton CT imaging in conditions closer to the high proton flux environments of modern spot scanning treatment facilities. Details on the physical design, sensor testing, modelling, and track reconstruction are provided along with Monte-Carlo simulation studies of the expected performance for proton beam currents of up to 50 pA at the nozzle when using a σ= ∼10 mm spot scanning cyclotron system. Using a detailed simulation of the proposed OPTIma system, a discrepancy of less than 1% on the Relative Stopping Power is found for various tissues when embedded within a 150 mm diameter Perspex sphere. It is found that by accepting up to 7 protons per bunch it is possible to operate at cyclotron beam currents up to 5 times higher than would be possible with a single proton based readout, significantly reducing the total beam time required to produce an image, while also reducing the discrepancy between the beam currents required for treatment and those used for proton CT