Alternative Refrigerants in Adiabatic Capillary Tubes

This study examines the mass flow rate of refrigerants in adiabatic capillary tubes, with an emphasis on the nature of the metastable region for both single component refrigerants and mixtures. The metastable region of a capillary tube was found to be much more predictable than previously thought. A new and revealing data taking technique, which takes the history of the system into account, allowed for the discovery of a hysteresis effect in the mass flow rate as the level of inlet subcooling is increased and decreased. This finding may have a profound impact on the future of capillary tube data acquisition and modeling.Air Conditioning and Refrigeration Project 6

Digital DLTS studies on radiation induced defects in Si, GaAs and GaN

Since the development of deep level transient spectroscopy (DLTS) in the 1970’s by Lang and others, the technique has become a powerful analytical tool to characterise the electrical properties of defects in semiconductors. With the development of more powerful computers and improved data acquisition systems, it has become possible to replace the original analogue boxcar analysers and lock-in amplifiers that were commonly used in early DLTS systems with digitisers and digital signal processing equipment. The use of a computer for signal processing allows for much more flexibility in the DLTS system. For instance, a digital DLTS system is capable of measuring a much wider range of emission rates than an analogue system. Furthermore, since the digital DLTS system does not rely on a repetitive signal, such a system can observe phenomena such as defect metastability that cannot be observed in an analogue system. In this thesis, the design and characterisation of a digital DLTS system is described. The results of a number of experiments that illustrated the capabilities of the system are reported. The extended range of emission rates that could be measured by the system were illustrated by the measuring of the EL2 defect in GaAs over the temperature range 270 – 380 K (corresponding to emission rates ranging from less than 10–3 s–1 to more than 103 s–1). The results compared well with previous results obtained by means of an analogue DLTS system. Further low temperature measurements on the E2 defect in GaAs showed that in the low temperature region, thermal radiation from the cryostat shroud influenced carrier emission. The field dependence of the emission rate of a number of defects, including defects in as-grown n-GaN, He-ion irradiated n-GaN and Si, was investigated as well. The ability of the digital DLTS system to measure single transients was used to investigate configurationally bistable defects in He-ion irradiated p-Si and a sputter-induced defect with negative-U properties in n-GaN. In both of these cases, the results proved far superior to those obtained by means of an analogue system.Thesis (PhD (Physics))--University of Pretoria, 2007.Physicsunrestricte

Height Systems and Vertical Datums: a Review in the Australian Context

This paper reviews (without equations) the various definitions of height systems and vertical geodetic datum surfaces, together with their practical realisation for users in Australia. Excluding geopotential numbers, a height system is a one-dimensional coordinate system used to express the metric distance (height) of a point from some reference surface. Its definition varies according to the reference surface chosen and the path along which the height is measured. A vertical geodetic datum is the practical realisation of a height system and its reference surface for users, nominally tied to mean sea level. In Australia, the normal-orthometric height system is used, which is embedded in the Australian Height Datum (AHD). The AHD was realised by the adjustment of ~195,000 km of spirit-levelling observations fixed to limited-term observations of mean sea level at multiple tide-gauges. The paper ends by giving some explanation of the problems with the AHD and of the differences between the AHD and the national geoid model, pointing out that it is preferable to recompute the AHD

A hybrid functional calculation of Tm3+ defects in germanium (Ge)

Please read abstract in the article.National Research foundation (NRF) of South Africa (Grantspecific unique reference number (UID) 78838).http://www.elsevier.com/locate/mssp2017-03-31hb201

The carbon-substitutional-carbon-interstitial (CsCi) defect pair in silicon from hybrid functional calculations

Using both the Perdew–Burke–Ernzerhof (PBE) functional and the hybrid functional of Heyd–Scuseria– Ernzerhof (HSE06), the metastability of the carbon-substitutional–carbon-interstitial (CsCi) defect in silicon has been investigated within density functional theory using the two experimentally proposed configurations of the defect. While the PBE functional predicted the defect complex to have both donor and acceptor levels, it did not predict any form of charge-state controlled metastability as was observed experimentally. In the case of HSE06 functional, the defect was found to exhibit charge-state controlled metastability in the 0 and 1 charge states with no metastability predicted for +1 charge state. The calculated binding energies for the neutral charge state indicate that the defect is a stable bound defect complex.National Research Foundation of South Africa.http://www.elsevier.com/locate/com/matsci2017-06-30hb2016Physic

Hybrid functional study of hydrogen passivation in carbon-oxygen related defect complexes in silicon

Please read abstract in the article.The National Research foundation (NRF) of South Africa (Grant specific unique reference number (UID) 98961).http://www.elsevier.com/locate/physbhj2020Physic

Ab initio study of metastability of Eu3+ defect complexes in GaN

Density functional theory (DFT) within the generalized gradient approximation (GGA) has been used to study the structural and electronic properties of Eu3þ defect complexes in GaN under Ga-rich conditions. Two distinct configurations of the EuGaVN defect complex, the axial and basal configuration, have been investigated. We report two forms of metastable defects namely; the Negative U defect in the lower half of the GaN band-gap and a metastable defect with two distinct configurations each with levels at EC 0.46 eV and 0.56 eV in the upper half of the GaN band-gap.http://www.elsevier.com/locate/physbhb201

Comparison of the Electronic Structures of Two Non-cuprate Layered Transition Metal Oxide Superconductors

Comparison is made of the electronic structure of the little-studied layered transition metal oxide LiNbO$_2$ with that of Na$_x$CoO$_2$, which has attracted tremendous interest since superconductivity was discovered in its hydrate. Although the active transition metal $d$ states are quite different due to different crystal fields and band filling, both systems show a strong change of electronic structure with changes in the distance between the transition metal ion layer and the oxygen layers. The niobate is unusual in having a large second-neighbor hopping amplitude, and a nearest neighbor hopping amplitude that is sensitive to the Nb-O separation. Li$_x$NbO$_2$ also presents the attractive simplicity of a single band triangular lattice system with variable carrier concentration that is superconducting.Comment: 5 pages, 3 embedded figures (Proceedings in third Hiroshima international workshop

Rare earth interstitials in Ge : a hybrid density functional theory study

In this work, the results of density functional theory calculations for rare earth (Ce, Pr, Eu, and Er) interstitials in Ge are presented. We employed the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) for all the calculations. We calculated the formation energies and charge state transition levels for the tetrahedral (T) and hexagonal (H) configurations of the Ce, Pr, Eu, and Er interstitials in Ge. While for the T configuration, the charge states of the Ce and Pr did not induce any thermodynamic accessible transition state level within the band gap of Ge, for both the T and H configurations the Eu and Er interstitials in Ge induce deep levels in the band gap. The H configuration of the Ce interstitial in Ge induces a shallow donor level at 0.03 eV below the conduction band. The Eu interstitial exhibits negative-U properties for the (+2/2) transition level and the Er interstitial displays characteristics of charge state controlled metastability.Supported partly by National Research Foundation (NRF) of South Africa (Grant speci c unique reference number (UID) 98961).http://link.springer.com/journal/116642018-02-28hb2017Physic

Ab initio study of aluminium impurity and interstitial-substitutional complexes in Ge using a hybrid functional (HSE)

The results of an ab initio modelling of aluminium substitutional impurity (AlGe), aluminium interstitial in Ge [IAl for the tetrahedral (T) and hexagonal (H) configurations] and aluminium interstitial-substitutional pairs in Ge (IAlAlGe) are presented. For all calculations, the hybrid functional of Heyd, Scuseria, and Ernzerhof in the framework of density functional theory was used. Defects formation energies, charge state transition levels and minimum energy configurations of the AlGe, IAl and IAlAlGe were obtained for 2, 1, 0, þ1 and þ2 charge states. The calculated formation energy shows that for the neutral charge state, the IAl is energetically more favourable in the T than the H configuration. The IAlAlGe forms with formation energies of 2.37 eV and 2.32 eV, when the interstitial atom is at the T and H sites, respectively. The IAlAlGe is energetically more favourable when the interstitial atom is at the T site with a binding energy of 0.8 eV. The IAl in the T configuration, induced a deep donor (þ2/þ1) level at EV þ 0:23 eV and the AlGe induced a single acceptor level (0/1) at EV þ 0:14 eV in the band gap of Ge. The IAlAlGe induced double-donor levels are at EV þ 0:06 and EV þ 0:12 eV, when the interstitial atom is at the T and H sites, respectively. The IAl and IAlAlGe exhibit properties of charge state-controlled metastability.This work is based on the research supported partly by the National Research foundation (NRF) of South Africa [Grant-specific Unique Reference Number (UID) 98961].http://link.springer.com/journal/116642018-07-30hb2016Physic