Occupational Structure of Yerwa in the 1920s

Yerwa is the last of the Borno capitals. Although established in the first decade of colonial administration, it cannot be compared with the many other towns like Fort Lamy, Jos, Kaduna, Niamey et al. which all developed about the same time. Colonial interference with the development of Yerwa appears restricted, mainly, to insistence upon wider roads than a Borno town otherwise would have featured and resettlement schemes, e.g. Mafoni, Ari Askeri. The following is based on the premise that as the town - despite time and political circumstances of its emergence - is a distinctive Borno town, also occupational diversification and structure are distinctively related to urban Borno culture

Complete Corrections of O(\alpha\alpha_s) to the Decay of the Z Boson into Bottom Quarks

For the vertex corrections to the partial decay rate $\Gamma(Z \to b\bar{b})$ involving the top quark only the leading terms of order $\alpha\alpha_s$ in the $1/M_t$ expansion are known. In this work we compute the missing next-to-leading corrections. Thus at $O(\alpha\alpha_s)$ the complete corrections to the decay of the Z boson into bottom quarks are at hand.Comment: Latex, 11 pages, 1 figure included as ps-file. Two references changed. The complete paper is also available via anonymous ftp at ftp://ttpux2.physik.uni-karlsruhe.de/ttp97/ttp97-52/ or via www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints

Design, fabrication and test of prototype furnace for continuous growth of wide silicon ribbon

A program having the overall objective of growing wide, thin silicon dendritic web crystals quasi-continuously from a semi-automated facility is discussed. The design considerations and fabrication of the facility as well as the test and operation phase are covered; detailed engineering drawings are included as an appendix. During the test and operation phase of the program, more than eighty growth runs and numerous thermal test runs were performed. At the conclusion of the program, 2.4 cm wide web was being grown at thicknesses of 100 to 300 micrometers. As expected, the thickness and growth rate are closely related. Solar cells made from this material were tested at NASA-Lewis and found to have conversion efficiencies comparable to devices fabricated from Czochralski material

Silicon ribbon study program

The feasibility is studied of growing wide, thin silicon dendritic web for solar cell fabrication and conceptual designs are developed for the apparatus required. An analysis of the mechanisms of dendritic web growth indicated that there were no apparent fundamental limitations to the process. The analysis yielded quantitative guidelines for the thermal conditions required for this mode of crystal growth. Crucible designs were then investigated: the usual quartz crucible configurations and configurations in which silicon itself is used for the crucible. The quartz crucible design is feasible and is incorporated into a conceptual design for a laboratory scale crystal growth facility capable of semi-automated quasi-continuous operation

Microplastic-Contaminant Interactions: From Experimental Data to Environmental Implications

Microplastic particles are ubiquitously detected in all environmental compartments. Despite intensive public and scientific discussions, their potential to transport contaminants in rivers and oceans is still under assessment. To consider such particle facilitated transport, this thesis aims to quantify the underlying sorption mechanisms and to develop a comprehensive mechanistic model with parameter values derived from experimental data. The developed models consider material characteristics, physico-chemical properties of chemical compounds as well as different types of sorption isotherms. The sorption kinetics was modeled as a combination of external mass transfer governed by diffusion through an aqueous boundary layer and intraparticle diffusion within the plastic particles. Which of these processes controls the kinetics depends on the sorption strength, particle size, diffusion coefficients, and time. Based on the determined sorption isotherm, a semi-analytical model was developed for linear sorption and a numerical approach was applied to simulate coupled mass transfer for the case of non-linear sorption. Both model types were successfully validated for several plastic types, allowing to accurately describe the measured kinetics. To broaden the scope and environmental relevance of this thesis, further experiments were performed. It was revealed that changing pH conditions and the presence of additional natural sorbents significantly influenced both equilibrium partitioning and desorption kinetics. Due to the combination of experimental and mechanistic modelling tools, it was possible to elucidate coupled mass transfer processes for different experimental and field settings. Eventually, it was ascertained that time scales observed under experimental conditions may not be transferred to field conditions without an appropriate mechanistic model accounting for coupled mass transfer and the specific boundary conditions. Appropriate hydrodynamic relationships coupled to a thorough mass transfer analysis can serve to assess the vector function of pollutant-loaded particles and to evaluate whether microplastics rather act as a passive sampler or show potential to facilitate long-range contaminant transport. Moreover, as the theoretical mass transfer considerations also apply to other suspended particles, well-defined microplastic particles are ideally suited to perform in-depth mass transfer studies and to act as surrogates for particles occurring in the environment, including microplastics in urban runoff and contaminated sediment

Plura non habui. Senecas Medea und der Comparativus Senecanus

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