No description available.

No description available.

A study was made of the thermoreflectance spectra of a series of five thin film samples spanning the range of composition of the amorphous Ge(x)Te(1-x) binary semiconductors. The experiment was performed over photon energies ranging from 0.5 ev in the infrared to 6 ev in the ultraviolet. The results are plotted for energies above the absorption edge. In the Ge-rich materials there was little structure in the thermoreflectance spectrum. However, there was the development of two peaks in the spectrum as the Te content was increased beyond a 50:50 mixture. The results are analyzed in terms of optical constants and electronic structure. It was found that the energy separation of the peaks in the thermoreflectance corresponded closely to the separation of peaks in the valence band density of states seen in photoemission experiments. The existence of the two peaks indicates a "lone pair" band of energy levels positioned between the valence and conduction bands. This is aditional evidence of two-fold coordination of the Te atoms in these materials. At energies below the band gap there was strong interference due to increased transimission of the film. This made the analysis more complicated. A procedure is discussed for extracting from the modulated interference the specific changes occurring in the optical constants with temperature modulation.

Amorphous metallic alloys are produced by a variety of techniques some of which involve rapid solidification of the alloying constituents. In these methods the solidification occurs so rapidly that the atoms are frozen-in and partially retain their liquid configuration. There are clear structural and other indications from their various properties that amorphous metallic alloys possess short range order but lack long range order. In general, amorphous alloys are not in a thermodynamic equilibrium state and, therefore, relax structurally whenever atoms attain an appreciable mobility. Associated with structural relaxation, many physical properties change; some significantly and others only slightly. Relaxation experiments in amorphous metallic alloys often display approximate In(t) kinetics which can be understood in terms of various models. In the present work the model by Primak (1955), for which the kinetic behaviour of a system depends on processes that are distributed over a range of activation energies, is used as a basis for further development. The Primak model allows, in principle, for the identification of the order of the relaxation reaction and for the determination of an initial activation energy spectrum Po(Єo), where Єo is a characteristic activation energy. Although the model provides for a qualitative explanation of the In(t) law, it has no predictive power as to the quantitative changes accompanying the various relaxing properties. Furthermore, an estimation of Po( Єo), inferred from various isothermal annealing procedures, reveals the approximate shape but does not fix its location on the activation energy axis. These shortfalls are attributed to complications in the frequency factor v, inherent to the Primak model. Also, the Primak model does not include consideration of the entropy involved in a 'configurational jump' of any particular atom during the relaxation process. Inclusion of the configurational entropy through the frequency factor v, in the present treatment, leads to a 'relaxation equation'. Structural relaxation measurements of density (in practice length - from which density can be approximately inferred) and electrical resistivity, in an Fe4oNi40B20 alloy, have been obtained and fitted to this relaxation equation. The fitting parameters are found, within experimental error, to be the same for both length and resistivity relaxation. The initial activation energy spectrum Po(Єo), as inferred from the fits, over the energy range 1.4 to 2.0 eV, reveals roughly three regimes, namely below 1.5 eV, from 1.5 to 1.8 eV, and above 1.8 eV, respectively, over which the initial activation energy spectrum Po( Єo) assumes different approximately constant values. Previous treatments have, however, implicitly assumed that Po( Єo) is constant throughout a temperature range over which In(t) kinetics is observed. The behaviour observed in this work is associated with the intrinsic relaxation mechanism involving consecutive diffusion of the metallic and metalloid atoms, respectively. A configurational entropy change inferred from this work is found to be negative as a consequence of contraction of the spread-out free volume resulting from thermal fluctuations. Within the framework of the 'present model', other related behaviour of amorphous metallic alloys, including the glass transition, crystallization and diffusion, are discussed. Where direct comparison between theory and experiment is possible for the various observed phenomena, the agreement is good and shows an overall consistency in our approach. Finally, the analysis considered here gives an expression which can be easily used to make quantitative predictions about the experimental relaxation behaviour. An immediate understanding of some of the main features of experimental data on relaxation can, therefore, be obtained through application of the present model. / Thesis (Ph.D.)-University of Natal, Durban, 1998.

No description available.

This thesis consists of two completely separate parts: In part I some problems related to phonons in amorphous solids are considered, whilst Part II is devoted to the study of excitons in Cuprous Oxide (Cu₂O). Part I.- A theoretical model, suitable to treat vibrations in tetrahedrally coordinated amorphous systems is developed and permits the study of a number of situations of current interest. Three of these situations are studied in detail: 1) The local response of hydrogen in amorphous silicon when a single silicon is attached to one, two or three hydrogens. The differences between these three configurations are discussed and a direct connection with experimental results is suggested. 2) The interesting case of an amorphous alloy, where both, topological and substitutional disorder are present. This is treated within the spirit of the Coherent Potential Approximation. The particular alloy chosen (Si-Ge) is readily tractable because oithe similar bond characteristics of both components, which allows the neglect of force constant changes. 3) The Raman spectrum of AX₂, glasses. The model adopted permits the investigation of the local response at the defect sites for a number of defects. In order to explain the defect lines observed in the experiments, four plausible defect configurations are considered: a missing A-X bond, a X-A double bond, an A-A bond, and a square ring (two tetrahedra sharing an edge). A simple model to calculate the Raman response in amorphous solids is also outlined. Part II.- The valence band of Cu₂0 is studied in detail to account for the deviations from the hydrogenic law of the exciton spectrum. The appearance of the two series of excitons is explained in terms of a spin-orbit splitting of the valence band in the centre of the Brillouin Zone, using a Tight-Binding Approximation. The deviations of the lowest exciton levels from their expected values are seen to arise from an admixture of the two components of the split -off valence band due to direct Coulomb and exchange interactions. The Hamiltonian used corresponds to the so called "Spherical Approximation" and the results obtained are in remarkable agreement with the experiments.

Azithromycin, an azalide and member of the macrolide group, is a broad spectrum antimicrobial, representing one of the bestselling antimicrobials worldwide. It is derived from erythromycin and exhibits improved acidic stability as a result of its structural modifications. The stable solid form of azithromycin is its dihydrate, although it also naturally occurs in its metastable forms, i.e. the monohydrate and anhydrate. Because azithromycin is poorly soluble in water, its absorption from the gastro-intestinal tract is negatively influenced, which ultimately affects its bioavailability following oral administration (37 %). Polymorphic (monohydrates and dihydrates) and anhydrous forms of azithromycin were screened and investigated. One anhydrous form also proved to be amorphous, which shifted the focus of this study from polymorphism to amorphism. An amorphous glassy azithromycin was subsequently prepared and fully characterised to present its solid state profile. The stability of this amorphous glassy form was established at a high temperature and relative humidity over a period of four weeks. Exposure to increased relative humidity (up to 95 %) and increased water content (up to 50 %) also served as stability indicating tests. Its solubility in various aqueous media was determined. A solid dosage form (tablet), containing the azithromycin glass, was prepared, whereafter these tablets were subjected to dissolution studies in different aqueous media. The stability of azithromycin glass in tablet form was determined over a period of three months. The permeability of azithromycin glass across excised pig intestinal tissue was further established at various pH values. This amorphous glassy form of azithromycin (AZM-G) proved to be very stable at high temperature and relative humidity, whilst also remaining stable after prolonged exposure to 95 % of relative humidity, as it only adsorbed moisture onto its surface. Water content (up to 50 %) had no plasticising effect on azithromycin glass. It demonstrated a significantly higher water solubility (339 % improvement) in comparison with the commercially available azithromycin dihydrate and was it also 39 % more soluble in phosphate buffer (pH 6.8) than its dihydrate counterpart. The prepared azithromycin glass tablets showed a promising dissolution profile in water, due to the improved water solubility of this glass form. The transport of azithromycin glass at higher pH values (6.8 and 7.2) across the membrane proved to be significantly higher than that of azithromycin dihydrate, thus also illustrating its pH dependence for its transport across pig intestinal tissue. The improved water solubility of the azithromycin glass, together with its faster dissolution rate, its superior stability and its increased permeability, may ultimately result in a higher azithromycin bioavailability following oral administration. These research outcomes hence give rise to the need for investigating the effect of administering lower dosages of azithromycin and to determine whether the same antimicrobial efficacy would possibly be achieved, due to maintaining the same tissue concentration levels at these lower dosages. / Thesis (PhD (Pharmaceutics))--North-West University, Potchefstroom Campus, 2013

Azithromycin, an azalide and member of the macrolide group, is a broad spectrum antimicrobial, representing one of the bestselling antimicrobials worldwide. It is derived from erythromycin and exhibits improved acidic stability as a result of its structural modifications. The stable solid form of azithromycin is its dihydrate, although it also naturally occurs in its metastable forms, i.e. the monohydrate and anhydrate. Because azithromycin is poorly soluble in water, its absorption from the gastro-intestinal tract is negatively influenced, which ultimately affects its bioavailability following oral administration (37 %). Polymorphic (monohydrates and dihydrates) and anhydrous forms of azithromycin were screened and investigated. One anhydrous form also proved to be amorphous, which shifted the focus of this study from polymorphism to amorphism. An amorphous glassy azithromycin was subsequently prepared and fully characterised to present its solid state profile. The stability of this amorphous glassy form was established at a high temperature and relative humidity over a period of four weeks. Exposure to increased relative humidity (up to 95 %) and increased water content (up to 50 %) also served as stability indicating tests. Its solubility in various aqueous media was determined. A solid dosage form (tablet), containing the azithromycin glass, was prepared, whereafter these tablets were subjected to dissolution studies in different aqueous media. The stability of azithromycin glass in tablet form was determined over a period of three months. The permeability of azithromycin glass across excised pig intestinal tissue was further established at various pH values. This amorphous glassy form of azithromycin (AZM-G) proved to be very stable at high temperature and relative humidity, whilst also remaining stable after prolonged exposure to 95 % of relative humidity, as it only adsorbed moisture onto its surface. Water content (up to 50 %) had no plasticising effect on azithromycin glass. It demonstrated a significantly higher water solubility (339 % improvement) in comparison with the commercially available azithromycin dihydrate and was it also 39 % more soluble in phosphate buffer (pH 6.8) than its dihydrate counterpart. The prepared azithromycin glass tablets showed a promising dissolution profile in water, due to the improved water solubility of this glass form. The transport of azithromycin glass at higher pH values (6.8 and 7.2) across the membrane proved to be significantly higher than that of azithromycin dihydrate, thus also illustrating its pH dependence for its transport across pig intestinal tissue. The improved water solubility of the azithromycin glass, together with its faster dissolution rate, its superior stability and its increased permeability, may ultimately result in a higher azithromycin bioavailability following oral administration. These research outcomes hence give rise to the need for investigating the effect of administering lower dosages of azithromycin and to determine whether the same antimicrobial efficacy would possibly be achieved, due to maintaining the same tissue concentration levels at these lower dosages. / Thesis (PhD (Pharmaceutics))--North-West University, Potchefstroom Campus, 2013

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This thesis contains two main parts: the first part focusses on an electron diffraction study on unsupported antimony (Sb) clusters, while in the second part the design and development of a time-of-flight mass spectrometer (TOFMS) is discussed. Electron diffraction is an ideal tool to study the structure of clusters entrained in a beam. The main advantage of this technique is the ability to study the clusters in situ and in an interaction-free environment. It is therefore not necessary to remove the particles from the vacuum system which would lead to oxidation. Since the particles do not have to be deposited on a sample for further investigation, there is also no substrate which could influence the cluster structure. An additional advantage is the short exposure to the electron beam, thereby minimising the likelihood of damaging the particles. Sb clusters were produced using an inert-gas aggregation source. To control the cluster properties the source temperature, pressure and type of cooling gas can be adjusted. In the range of source parameters tested, Sb clusters with three different structures were observed: a crystalline structure corresponding to the rhombohedral structure of bulk Sb, an amorphous structure equivalent to the structure of amorphous Sb thin films, and a structure with the same diffraction signature as Sb4 (Sb evaporates mainly as Sb4). This last structure was found to belong to large particles consisting of randomly oriented Sb4 units. In order to study the size distributions and morphologies of the Sb clusters, the clusters were deposited onto substrates and studied under an electron microscope. The crystalline particles showed a wide variety of strongly faceted shapes. Depending on source conditions, the average cluster diameters ranged from 15 to 130 nm. There was a considerable disagreement between these values and the size estimates from the diffraction results with the latter being smaller by an order of magnitude. This might be due to the existence of domains inside the clusters. The amorphous particles were all found to be spherical with mean sizes between 27 and 45 nm. The Sb4 particles showed a liquid-like morphology and tended to coalesce easily. Their sizes ranged from 18 to 35 nm. To obtain an independent method for determining the cluster size, a TOFMS was designed and developed in collaboration with Dr Bernhard Kaiser. However, the TOFMS failed to detect a cluster signal in the original set-up which is most likely due to a defective ioniser and underestimated cluster energies. Further tests were performed in a new vacuum system and mass spectra for palladium clusters were successfully recorded.