Uranium (U) exhibits a high temperature body-centered cubic (b.c.c.) allotrope that is often stabilized by alloying with transition metals such as Zr, Mo, and Nb for technological applications. One such application involves U-Zr as nuclear fuel, where radiation damage and diffusion (processes heavily dependent on point defects) are of vital importance. Metallic nuclear fuels swell under fission conditions, creating fission product gases such as helium, xenon and krypton. Several systems of U are examined within a density functional theory framework utilizing projector augmented wave pseudopotentials. Two separate generalized gradient approximations of the exchange-correlation are used to calculate defect properties and are compared. The bulk modulus, the lattice constant, and the Birch-Murnaghan equation of state for the defect free b.c.c. uranium allotrope are calculated. Defect parameters calculated include energies of formation of vacancies in the α and γ allotropes, as well as self-interstitials, Zr, He, Xe and Kr interstitial and substitutional defects. The results for vacancies agree very well with experimental and previous computational studies. The most probable self-interstitial site in γ-U is the <110> dumbbell and the most probable defect location for dilute Zr in γ-U is the substitutional site. The most likely position for Xe and Kr atoms in uranium is the substitutional site. Helium atoms are likely to be found in a wide variety of defect positions due to the comparable formation energies of all defect configurations analyzed.

published_or_final_version / Statistics / Master / Master of Social Sciences

Gyvenamųjų namų apšiltinimo ir apdailos defektai – dažnas reiškinys šiuolaikinėje statyboje. Statant arba perkant namą, norima, kad būstas būtų jaukus, šiltas, gražus, ekonomiškas ir patogus. Nuo gyvenamojo namo apšiltinimo ir apdailos kokybės priklauso viso namo estetinė išvaizda, šilumos izoliaciniai, higieniniai ir kiti parametrai. Darbo tikslas – gyvenamųjų pastatų apšiltinimo ir apdailos galimų defektų diagnostikos ir eliminavimo būdų analizė. Darbą sudaro įvadas, keturi skyriai bei išvados. Įvade pateikiamas darbo tikslas ir uždaviniai tikslui įgyvendinti. Pirmame darbo skyriuje pateikiama Lietuvos ir užsienio mokslinės literatūros, susijusios su apšiltinimo ir apdailos medžiagų tyrimais ir bandymais, defektų nustatymo būdais, apžvalga. Antrame darbo skyriuje analizuojami gyvenamųjų pastatų apšiltinimo ir apdailos defektai, jų diagnostikos ir pašalinimo būdai. Trečiame darbo skyriuje analizuojamos pastatų apšiltinimo ir apdailos defektų atsiradimo priežastys. Ketvirtame darbo skyriuje pateikiamas apšiltinimo ir apdailos defektų pašalinimo racionalaus varianto parinkimo modelis. Variantams palyginti pateikiama rodiklių sistema, skaičiavimai ir gauti rezultatai. Darbo pabaigoje pateikiamos išvados. Darbo apimtis – 69 puslapiai teksto (be priedų). Darbe pateikti 22 paveikslai, 10 lentelių, 83 bibliografiniai šaltiniai ir 10 priedų. / Defects of warming and trimming of dwelling houses are very common in contemporary constructions. When buying or building a house we want it to be cosy, warm, beautiful, efficient and comfortable. The whole appearance, thermal insulation, hygiene and other parameters depend on warming and trimming quality. The purpose of this thesis is to analyse the methods for diagnosing and eliminating the possible defects of warming and trimming of dwelling houses. Thesis consists of an introduction, four chapters and a conclusion. Introduction contains the goal of the thesis and the tasks for achieving it. The first chapter gives an overview of Lithuanian and foreign literature on research and experiments with warming and trimming materials and methods to identify the defects. The second chapter focuses on analysis of the defects of trimming and warming of dwelling houses, methods for their diagnosis and elimination. The third chapter analysis the causes of the defects of warming and trimming of buildings. The fourth chapter gives a model for selection of a rational method to eliminate the warming and trimming defects. Rating system, calculations and acquired results are also given, in order to compare the methods. In the end of thesis, the conclusions are drawn. Thesis contains 69 pages of text (without annexes). There are 22 figures, 10 tables, 83 bibliographic sources and 10 annexes in this thesis.

Akademisk avhandling--Uppsala. / Extra t.p., with thesis statement, inserted. Bibliography: p. 55-60.

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[pt] É reconhecido o potencial de usar carbono como um dopante tipo p em InAlAs devido a obtenção de elevados níveis de dopagem [1,2]. Entretanto, níveis elevados de dopagem só são alcançados em baixas temperaturas de crescimento (Tg inferiores a 600°C). Nessas temperaturas, as camadas crescidas apresentam qualidade ótica inferior quando comparadas com camadas crescidas em temperaturas mais altas, o que é prejudicial para dispositivos de optoeletrônica. Neste trabalho, é apresentada uma investigação sistemática das propriedades de transporte e óticas em camadas de InAlAs dopadas com carbono para diferentes temperaturas de crescimento. É observado que quanto mais baixa for a Tg maior será a incorporação de carbono e maior a atividade elétrica. Este resultado indica que o carbono é incorporado de diversas maneiras, bem como um aceitador raso. O carbono também pode ser incorporado como um doador raso, pois é um dopante anfotérico. Entretanto, este fato, não é suficiente para explicar os resultados de transporte. A diferença entre a concentração Hall e a concentração CV indica a incorporação de doadores profundos. Provavelmente, o carbono participa na formação desses doadores profundos, uma vez que a concentração de doador profundo varia linearmente com a densidade atômica de carbono, determinada pela técnica SIMS. Por outro lado, centros não radiativos são mais facilmente incorporados em baixas Tg e a eficiência da fotoluminescência é reduzida. Essa degradação da fotoluminescência é independente da concentração de carbono, consequentemente, pode-se concluir que essa redução na eficiência da fotoluminescência não está associada à presença de doadores profundos. Com a finalidade de obter um incremento na atividade elétrica do carbono e melhoria na qualidade ótica das camadas, as amostras foram submetidas a tratamentos térmicos. Os tratamentos térmicos aumentaram a concentração de buracos mas não influenciaram na densidade de doadores profundos ou na qualidade ótica das camadas. Para a utilização de InAlAs dopado com carbono em dispositivos, deve-se obter simultaneamente uma boa qualidade ótica e elevada atividade elétrica das camadas.Então, deve-se identificar o doador profundo, que está associado ao carbono, com o objetivo de reduzí-lo ou eliminá-lo e consequentemente, obter um incremento na atividade elétrica das camadas. Desta forma as camadas podem ser crescidas a temperaturas mais altas adequadas para uma emissão de fotoluminescência eficiente. Cálculos teóricos são apresentados de modo a ajudar essa identificação. Outra possibilidade é usar diferentes fontes de arsina em que as moléculas se dissociem em temperaturas mais baixas. / [en] The potential of using carbon as a p-type dopant for InAlAs has already been recognized due to the achievable high hole concentration [1,2]. However, high doping levels are reached only for low growth teperatures (Tg below 600°C). These temperatures produce layers with poor optical quality as compared to those grown at higher temperatures, which can be detrimental for optoeletronic device. In this work we present crystal, transport and optical properties of such layers grown at different temperatures. We find that the lower Tg, the more efficient the carbon incorporation and its electrical activity are. This result indicates that carbon is incorporated in forms different from a shallow acceptor, as well. Carbon can also be incorporated as a shallow donor since it is an amphoteric dopant. However, this alone does not explain the transport results. The difference between the net free charge density determined from capacitance measurements indicates that a deep donor is also incorporated. Carbon most likely participates in the deep donor formation since the inferred deep donor concentration varies linearly with the carbon atomic density measured by SIMS. On the other hand, non- radiative deep levels are more efficiently incorporated as Tg is reduced degrading the photoluminescence characteristics. Such degration is independent of the carbon doping. Therefore, one concludes that the decrease in the photoluminescence efficiency cannot be related to the presence of the deep donor mentioned in the previous paragraph. To further probe the carbon electrical activity and its effect on the optical properties of the layers, the samples have been subjected to a heat-treatment. Annealing the samples increases the hole concentration, but neither affects the deep donor density nor improves the layers optical quality. In order to use carbon doped InAlAs in devices which simultaneously require good optical quality and high electrical activity of the layers, one should identify the deep donor involving carbon in order to try to reduce its concentration or even eliminate it, consequently improving the electrical activity of the layers. In such a way the layers can be grown at higher temperatures, adequate for an efficient photoluminescence emission. Theoretical calculations are being carried out to help with such identification. Another possibility is to use other arsine sources which crack at lower temperatures.

The casualty rate of road vehicle accidents in South Africa is one of the highest in the world. This trend has persisted with little variation over the years, despite the efforts of local road safety organisations and research institutions to decrease them. Some of these road vehicle accidents are due to a mechanical failure of the vehicle. The main goal of this study is to establish how high the incidence of mechanical failure is in these accidents. And further to assess if these percentages of mechanical failures do coincide with trends already indicated nationally as well as internationally. Detailed information on the condition of vehicles was collected in and around the Pretoria area. Surveys were conducted to obtain local road and traffic information about vehicle conditions. The one survey was defined as Potential Mechanical Defect Tests (PMDT) where vehicles were stopped and given a brief mechanical inspection, and the other was a Minibus Survey where information was obtained about the age of the vehicle and the overall condition and pressure of tyres. Additional information was obtained from the Accident Response Unit (ARU) and the Forensic Sciences Laboratories, both of the South African Police Services in Pretoria. The findings of the study are that according to the data collected by the ARU over a period of 2,5 years, on average 3,3% of the accidents reported per year in the region were caused by mechanical failures. These identified percentages correspond with values obtained for international countries. The PMDT data indicate that 40% of the vehicles surveyed in suburban areas and 29% of the vehicles surveyed on the highway had mechanical defects that contravened current road and traffic regulations. The difference between the percentages indicates that the condition of vehicles inspected in the suburban area differs significantly from the condition of those using the highway. In the Minibus Survey, large irregularities in tyre pressure were identified as cause for concern. In general, all of the above findings indicated that maintenance on older vehicles seemed to receive less attention. It is proposed that annual vehicle inspections should be introduced, especially for vehicles carrying fare-paying passengers, to improve road safety standards on South African roads. / Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

No description available.

This work presents introductory research to provide the relationship between electrical measurements and residual stresses caused by hybrid processing. X-ray diffraction is used to measure residual stresses in the ceramic substrate as well as metallization of wideband coplanar thick film probes. The probes are also tested for electrical performance using Time Domain Reflectometry (TDR) technique. This thesis presents results that indicate that significant tensile residual stresses were generated on the back of the substrate, particularly during the laser scribing phase of the finished device. Significant tensile stresses were also observed in the Silver-Palladium (Ag-Pd) metallization. TDR measurements indicated significant response variations which were attributed to these induced stresses. Attempts were also made to reduce these stresses by an annealing process in order to improve the TDR response uniformity. / Master of Science