Multi-scale Modeling of Chemical Vapor Deposition: From Feature to Reactor Scale

Multi-scale modeling of chemical vapor deposition (CVD) is a very broad topic because a large number of physical processes affect the quality and speed of film deposition. These processes have different length scales associated with them creating the need for a multi-scale model. The three main scales of importance to the modeling of CVD are the reactor scale, the feature scale, and the atomic scale. The reactor scale ranges from meters to millimeters and is called the reactor scale because it corresponds with the scale of the reactor geometry. The micrometer scale is labeled as the feature scale in this study because this is the scale related to the feature geometries. However, this is also the scale at which grain boundaries and surface quality can be discussed. The final scale of importance to the CVD process is the atomic scale. The focus of this study is on the reactor and feature scales with special focus on the coupling between these two scales. Currently there are two main methods of coupling between the reactor and feature scales. The first method is mainly applied when a modified line of sight feature scale model is used, with coupling occurring through a mass balance performed at the wafer surface. The second method is only applicable to Monte Carlo based feature scale models. Coupling in this second method is accomplished through a mass balance performed at a plane offset from the surface. During this study a means of using an offset plane to couple a continuum based reactor/meso scale model to a modified line of sight feature scale model was developed. This new model is then applied to several test cases and compared with the surface coupling method. In order to facilitate coupling at an offset plane a new feature scale model called the Ballistic Transport with Local Sticking Factors (BTLSF) was developed. The BTLSF model uses a source plane instead of a hemispherical source to calculate the initial deposition flux arriving from the source volume. The advantage of using a source plane is that it can be made to be the same plane as the coupling plane. The presence of only one interface between the feature and reactor/meso scales simplifies coupling. Modifications were also made to the surface coupling method to allow it to model non-uniform patterned features. Comparison of the two coupling methods showed that they produced similar results with a maximum of 4.6% percent difference in their effective growth rate maps. However, the shapes of individual effective reactivity functions produced by the offset coupling method are more realistic, without the step functions present in the effective reactivity functions of the surface coupling method. Also the cell size of the continuum based component of the multi-scale model was shown to be limited when the surface coupling method was used. Thanks to the work done in this study researchers using a modified line of sight feature scale model now have a choice of using either a surface or an offset coupling method to link their reactor/meso and feature scales. Furthermore, the comparative study of these two methods in this thesis highlights the differences between the two methods allowing their selection to be an informed decision

Simulation of rear surface contamination for a simple bluff body

Predicting the accumulation of material on the rear surfaces of square-backed cars is important to vehicle manufacturers, as this progressively compromises rear vision, vehicle visibility and aesthetics. It also reduces the effectiveness of rear mounted cameras. Here, this problem is represented by a simple bluff body with a single sprayer mounted centrally under its rear trailing edge. A Very Large Eddy Simulation (VLES) solver is used to simulate both the aerodynamics of the body and deposition of contaminant. Aerodynamic drag and lift coefficients were predicted to within +1.3% and −4.2% of their experimental values, respectively. Wake topology was also correctly captured, resulting in a credible prediction of the rear surface deposition pattern. Contaminant deposition is mainly driven by the lower part of the wake ring vortex, which advects material back onto the rear surface. This leads to a maximum below the rear stagnation point and an association with regions of higher base pressure. The accumulation of mass is linear with time; the relative distribution changing little as the simulation progresses, implying that shorter simulations can be compared to longer experiments. Further, the rate of accumulation quickly reaches a settled mean value, suggesting utility as a metric for assessing different vehicles

Pancreatic fistulae after pancreatic resections for neuroendocrine tumours compared with resections for other lesions

AbstractBackgroundResection for pancreatic neuroendocrine tumours (PNET) is suggested to be associated with an increased risk of a post-operative pancreatic fistula (POPF). The aim of this study was to describe morbidity after resections for PNET, focusing on POPF. Outcomes were compared with resections for other lesions.MethodsPatients undergoing an elective pancreatic resection during a 12-year period were retrospectively analysed. Morbidity was defined according to the International Study Group of Pancreatic Surgery (ISGPS) definitions.ResultsEighty-eight out of 832 patients (10.6%) underwent a resection for PNET. Atypical pancreatic resections (enucleation and central pancreatectomy) and distal pancreatectomies were more frequently performed for PNET. The POPF rate was 22.7% in patients operated for PNET compared with 17.2% in other patients (P = 0.200). In univariate analysis, body mass index (BMI), pancreatic duct diameter, somatostatin analogue administration, type of resection and type of pathology were associated with a POPF. In multivariate analysis, BMI, a pancreatic duct diameter <3 mm and central pancreatectomy remained independent risk factors [odds ratio (OR) 1.93, 95% confidence interval (CI) 1.22–3.07 and OR 3.04, 95% CI 1.05–8.82, respectively].ConclusionsHigh rates of POPF were found in patients operated for PNET. However, this was mainly owing to the fact that atypical resections, known to be associated with a higher fistula rate, were performed more frequently in these patients

Surgical management of pancreatic neuroendocrine tumors

This thesis gives an overview of the surgical management and prognosis of patients with pancreatic neuroendocrine tumors (pNET). A systematic review including 2600 studies, was performed on complications and survival after different surgical procedures for pNETs. The overall pancreatic fistula rate after enucleation was high (45%) compared to standard resections. The 5-year survival of resected pNETs without resected liver metastases was 85%. In our own patient data, we analyzed the diagnostic value of chromogranin A and a Octreoscan. In patients with small tumors without suspicious metastases on CT scan, an additional Octreoscan is not mandatory. Chromogranin A was elevated in only 27% of patients preoperatively. Further analysis revealed that the overall complication rate and the need for re-interventions/readmissions was not lower for enucleation compared to other resections. In addition, investigating 832 patients with different types of pancreatic pathology, a pNET was not a risk factor for the development of pancreatic fistula. Interestingly, in our operated patients, the incidence of lymph node metastasis after pancreatoduodenectomy was high, even for small tumors. Risk factors for recurrent disease after curative resection were identified: tumor size >2cm, positive lymph nodes and perineural invasion. Based on these risk factors we developed a nomogram to predict tumor recurrence in order to identify patients, which could benefit from adjuvant treatment and to adjust follow-up programs. In a nationwide study on the outcome after laparoscopic or open distal pancreatectomy, we found laparoscopic distal pancreatectomy to be safe, especially after structured surgical training

Climate Change and Migration to South Africa : Exploring the Role of Climate- and Environment-Related Adversities in Mobility Decision-Making

This dissertation examines the impacts of climate- and environment-related stress on migration from other sub-Saharan African countries to South Africa, which is a prominent destination for migrants. It describes the factors and processes that influenced migration decisions and provides insights into the experiences of these individuals before, during, and after migration. Data was gathered through semi-structured interviews with migrants from several sub-Saharan African countries now residing in South Africa’s Gauteng province, as well as key informants with expertise on migration, climate change, and environmental problems in Africa. Additional information was gathered from secondary material, such as reports, grey literature, and academic publications. The principle finding is that, although climatic and environmental stresses are not the primary drivers for migration to South Africa, they play a clear contributing role, both directly and indirectly. The direct contributions included drought, land degradation, floods, and erratic rainfall. Such environmental drivers for migration did not occur in isolation, instead, they were found to frequently intersect with various economic, political, social, and demographic drivers. Indirect contributions were largely through negative impacts on economic and political factors that became direct drivers for migration. Whether people respond to these adverse conditions by migrating depends on a number of factors that can be divided into three areas: intervening obstacles and facilitators of migration, personal and household characteristics, and expectations of the destination. Although some migrants in the re-search sample had experienced improvements in their quality of life since they had migrated to South Africa, the majority of migrants indicated that their lives were still characterised by insecurity, precariousness, and hopelessness.Dissertation (MSocSci (Development Studies))--University of Pretoria, 2020.Anthropology and ArchaeologyMSocSci (Development Studies)Unrestricte