Browsing by Author "Mbise, Godfrey Wilson"

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    Angular- selective window coatings: some theoretical and experimental studies
    (University of Dar es Salaam, 1989) Mbise, Godfrey Wilson
    This dissertation proposes a potential solution to the problem of seasonal and diurnal control of radioactive throughput across windows. The variation of solar transmission with the sun's altitude can be controlled by window coatings with angular selective transmittance. A theoretical study, based on several model investigations, on angular selectivity of homogeneous films of Ag, Al and Cr was made. A computer simulation optimized the transmittance of single films and of 3- & 5-layer stacks including these metals and ZnS or SiO2 in the visible and near IR regions. The latter constructions have improved transmittance performance. It is demonstrated that more pronounced angular selectivity is attainable with five (than with three or single) layer coatings. In order to devise means to obtain enhanced angular selectivity, an initial experimental study investigated silver films with inclined columnar microstructures. These films, characterized by an off-normal optical axis, were fabricated by resistive evaporation at oblique angle of vapour incidence onto glass. Scanning electron microscopy revealed 3nm-resolved microstructures comprising distinct, more or less well defined, columns. Transmission electron microscopy and surface profilometry showed that the degree of inhomogeneity and porosity in the films increased with evaporation angle, a. Electron diffraction showed a decreasing crystallinity with increasing a. Effects of self-shadowing and ad-atom mobility are thought to be responsible for these features. Spectrophotametry yielded strongly angular selective transmittance. For the first time, a "Transition region", which defines the span of attainable angular selective transmittance, is defined and identified in silver films. Possible explanations for some of the salient optical features are discussed. The universality of the "tangent rule" for defining the columnar inclination angle, especially for a >_ 60°, is discredited.
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    Optical and structural properties of obliquely evaporated thin films.
    (University of Dar es Salaam, 1996) Mbise, Godfrey Wilson
    for seasonal and diurnal control of radiative throughput across windows. Emphasis is presently given to window coatings whose optical properties are strongly angular dependent. Besides other conceivable applications, these kind of window coatings can potentially control the variation of solar transmission with, for example, the sun's altitude. Basically, two types of window coating systems, namely the multilayer stacks and single metal based coatings were investigated in this study, as summarized in the following paragraphs. Multilayer stacks with up to 7 layers for provision of low solar energy throughput together with high levels of angular-dependent visible transmittance have been designed. Computer simulation based on various experimental and theoretical investigations was employed to optimize the desired optical characteristics. (i.e., high luminous transmittance coupled with low near-infrared transmittance for unpolarized light incident at oblique angles). Two novel design-types of multilayer stacks were studied. These are the D/M/D and the 'Random D-M' designs (where D = Dielectric, M = Metal). Six different dielectric materials (ZnS, Si02, MgF2, Ti02, CaF2 and WO) and three metals (Ag, Al and Cr) were used in this study. The information from the simulation (in particular the physical thickness, the type of material for each layer as well as the arrangement of the layers) was utilized to experimentally fabricate the respective design(s). These optical coatings were made by thermal evaporation. Qualitatively, the experimental data were in good agreement with the theoretical ones. Quite substantial angular performance in luminous transmittance was obtained from both theory and experiment. Optical anisotropy of Cr thin films was investigated as a function of deposition conditions. The emphasis was on attempts to optimize the angular selectivity of Cr thin films prepared by oblique angle evaporation. Substantial angular selectivity together with a good degree of illumination were empirically obtained. The deposition angle and the film thickness were concluded to be the most important deposition parameters for angular selectivity. The background pressure, the rate of deposition and the substrate temperature were cited as other determining parameters for angular selectivity. A systematic study of the surface roughness of obliquely deposited Cr and CaF2 deposits on glass was carried out by the use of Atomic Force Microscopy (AFM), capable of resolving surface features down to atomic scale. The traditional roughness parameters - namely the root-mean-square roughness, the average roughness and the peak-to-valley roughness have been accurately determined. The dependence of these roughness parameters on deposition conditions was carefully investigated. The roughness statistics generally agree with most of the earlier work. However, both the static and dynamic roughness exponents were larger than values predicted for ballistic deposition and are likely to be governed by surface diffusion. The results are interpreted in terms of unstable growth of columnar structures due to Schowoebel barriers for diffusing atoms in the interface. Further structural characterization of obliquely deposited thin films were performed by various techniques, namely scanning electron microscopy, transmission electron microscopy, X-ray diffractometry and auger electron spectroscopy. Electrical resistivity of the films was investigated as a function of deposition angle and film thickness. The ensuing results were found to complement other previous studies in this research area. Finally, we made a systematic evaluation on the suitability of the Effective Medium (EM) formalism in correctly explaining the optical properties of obliquely deposited films. A biaxial model, based on the Maxwell-Garnett and Bruggeman E M approaches, was exploited for that purpose. The model, valid at any metal concentration, assumes that the medium under consideration is optically anisotropic and absorbing. The current model takes into account: (i) the real morphology of the microstructures in the film, (ii) the inclination of the columns from the surface normal, (iii) the volume fraction of the columns (iv) the thickness of the film, as well as (v) the polarization state of the incoming radiation (light). The predicted effective electrical permittivity were used to calculate the optical properties of the respective medium via modified Fresnel relations. The validity of the model was tested by considering a case study of obliquely evaporated chromium and nickel films. Thus the empirically observed microstructure parameters corresponding to these films were used to predict the effective dielectric functions. The latter were then used to calculate the optical properties of the respective films on glass, in the visible and near-infrared regions, and for various angles of beam incidence. Indeed, the model could well describe the experimentally observed optical properties in these films as well as define the limits of angular selectivity in obliquely deposited chromium films.