M.Sc. (Physics) / Please refer to full text to view abstract

The experimental work described in this thesis is designed to give information on the thermodynamic properties and chemical reactivity of the surface of an ionic crystal. The principal studies concern halogen exchange between hydrogen chloride gas and the surface of sodium bromide. For this reaction, like any ordinary chemical reaction, both kinetics and equilibrium may be investigated. In the present study, the most Interesting feature has been the measurement of the equilibrium constant and the calculation therefrom of the changes in thermodynamic properties attending the replacement of a bromide ion by a chloride ion in the surface. This appears to be the first reported measurement of an equilibrium constant involving concentrations in a solid surface. The work differs sharply from previous studies of isotopic exchange, which have been exclusively kinetic. The equilibrium is best represented by the equation [formula omitted] where X and (1-X) are the fractions of the surface anion sites occupied by Cl⁻ and Br⁻ respectively, ∆°S refers to the entropy changes of the exchanged ions at X = 0, and Si(X) describes the effect of any anion on the entropy of its neighbours in the surface. The equation indicates the formation of a two-dimensional solid solution of NaCl and NaBr, deviating from ideality through the term Si(X), so that the equilibrium constant K is a function of X. Since K is independent of temperature (20°-95°C.), ∆H = 0 and the equilibrium equation contains entropy terms only. From the value K = 6X10⁻³ at X = 0 it is estimated that [formula omitted] where S°Cl⁻ and S°Br⁻ refer to surface ions. The variation of K with X is used to arrive at a tentative estimate, based on some simplifying assumptions, of [formulas omitted]. Of this, about -2 cal/mole deg. K may be a simple mass effect. These large decreases in entropy are discussed in relation to the probable structure of the surface of sodium bromide; they are consistent with the "Verwey distortion" in which the anions are displaced outwards from the ideal surface, the distortion increasing, ceteris paribus, with the size and polarizability of the anions. The kinetics of the exchange reaction have been studied. It has not proved possible to analyze the rate curves in detail. The most striking feature, in comparison with previous studies, is that all reactions involving bromine (initially in either the gaseous or the solid phase) are faster than those involving chlorine isotopes only. The reason for this may be either the availability of d-orbitals of bromine for bonding in a transition complex, or the polarizability of the bromine atom. The exchange reaction has been used to study diffusion of chloride ions into sodium bromide at room temperature. It is possible to measure diffusion coefficients of about 10⁻²²cm²sec⁻¹, smaller by a factor of about 10⁵ than can be measured by most conventional methods. The results suggest that diffusion takes place principally along dislocations. The dehydration of high surface area sodium bromide has been studied, and is found to take place at a vapour pressure different from the equilibrium value for the system, NaBr.2H₂0; NaBr; H₂O. This is attributed to the formation of a "surface hydrate" overlying the lattice of anhydrous NaBr. This process may be regarded as adsorption with a discontinuous isotherm, and implies that the NaBr surface is energetically homogeneous. It has been found that color centers may readily be produced in the alkali halides by a Tesla Coil discharge. Centers thus produced in sodium chloride have been examined spetroscopically. The technique may be useful in experiments to establish the mechanism of exchange reactions such as that described in this thesis. / Science, Faculty of / Chemistry, Department of / Graduate

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Cohesive energies, bulk moduli, and equilibrium lattice constants have been calculated for the 5d transition atoms (Hf, Ta, W, Re, Os, Ir and Pt) in face–centred cubic crystal lattices. We have used the ab initio pseudopotential method for the total energy calculations within the local density approximation. Two calculations have been performed for each element, one using only the s, p and d angular momentum components and another including the s, p and d components as well as the unoccupied 5f orbital in the ionic pseudopotentials. The pseudo–wave functions and charge densities of the valence electrons have been represented by a basis of plane waves. For the 5d metals the changes in the electronic structure of the solid are small and they produce small changes in the bulk properties. / Thesis (M.Sc.) - University of KwaZulu-Natal, Pietermaritzburg, 2008.

The purpose of this investigation was to find out more about students’ spontaneous ideas about properties and transformations of matter. Examples of matter chosen are such which students know about from everyday life and focus is mainly on the questions if students know that all matter has weight and that matter is preserved in transformations. The method used is semi structured qualitative interviews with individual students. Different experiments were carried out together with the students who afterwards were interviewed. However, the students show varying answers and no generalizations can be done. However, when their conclusions from observations differ from their earlier experiences they trust their eyes. All students know that a human body has weight as well as a grain of dust. They also know that with addition of a colored substance in the water, the mass increases, but more hesitation exists when the additive is sugar. A couple of students are capable of discussing logically sometimes but not at all events. / Undersökningens syfte var att undersöka vad elever i årskurs tre har för spontana uppfattningar om materiens egenskaper och dess transformationer. Exempel på materia som valts är det eleverna känner till i vardagen och undersökningen har främst fokuserat på om eleverna dels vet att all materia väger något samt dels att materia är beständig och inte försvinner vid transformationer. Metoden som använts är semistrukturerade kvalitativa intervjuer med enskilda elever. Olika försök genomfördes tillsammans med eleverna som sedan intervjuades. Resultatet visar att eleverna varierar sina svar. Inga generella slutsatser kan dras. Dock kan man se att de grundar sina svar på det de ser och knyter sina föreställningar till sin vardag. När ögats uppfattning inte överensstämmer med erfarenheter litar flertalet elever på ögats observation. Alla elever vet att en människa väger liksom dammkorn. De vet också att vid tillsats av en färgad substans i vatten så ökar massan men fler oklarheter råder vid tillsats av socker. Ett par elever förmår resonera logiskt vid ett tillfälle men inte vid ett annat.

Anisotropy in the spatial arrangement of species and also geometric anisotropy in the components of liquid and soft matter systems gives rise to complex phase and non-trivial dynamic behaviors. Many systems are encompassed by this description including polymers, guest molecules in porous gels or nano-structured materials and also orientable fluids. In order to understand these systems, computational studies provide valuable insight. The study of such systems computationally is difficult if not prohibitive thus it is necessary to reduce these systems to simple models that capture the essential physical processes that govern the dynamics and phase behavior. Two simple models fit into this category: systems where the surrounding solvent is held stationary, a Lorentz model and also systems where a liquid crystal is formed from hard spherocylinders and is driven by an electric field. The Lorentz models studied provide a description of the dynamical regimes accessible when the probe and/or scatterers are given geometric anisotropy. The resulting dynamics are studied when order is present in the stationary solvent, i.e. structured versus isotropic solvent structure. Effective channels depending upon the competition of length scales as well as the structure of the stationary solvent leads to transitions in the dynamics of the traversing probe. Geometric anisotropy in nematogens in liquid crystals leads to the formation of mesophases indicative of a liquid crystal. When coupled to a rotating electric field, the dynamics and phase of the nematogens can be controlled.

The objectives of this work are to discuss multiscale models that are used to characterize the constitutive relations of the granular composite materials with dual functions. This is accomplished by the use of ab initio methods to obtain the constitutive relations of the structural energetic materials without conducting tests. First, it is necessary to study the quantum many body problem to quantitatively determine the internal energy of the material when subjected to different strain conditions. It is impossible to obtain an exact solution to the quantum many body problem that is modeled by the Schrödinger's equations with the current technology. It is possible to solve these equations approximately by the density functional theory which yields only energies at absolute 0ºK. Thus it becomes necessary to add both the lattice thermal contributions and electron thermal contribution. Then, resulting energy is used to bridge to the continuum level and obtain the constitutive equations. This is the procedure that is used in this work. The issues of the constitutive equations form the focus of this thesis. More specifically, the scope of the thesis is further restricted to analyze the constitutive equations of specific mixtures of nickel, aluminum with PTFE or Teflon as the binder. It is to be noted that the equations of state forms only a part of the complete constitutive relationships. This thesis presents solutions to the following problems: (1) Determination of the thermodynamically complete equation of state of the binder and the energetic material PTFE or Teflon, from ab initio methods based on the density functional theory. (2) Determination of the equations of state of the granular composite or the mixture of nickel, aluminum and PTFE from ab initio methods. (3) Determination of the complete constitutive equation of aluminum, from ab initio methods, under conditions of finite deformations, with principle of objectivity, material symmetry conditions and polyconvexity of the strain energy. All results are compared to test results whenever they are available.

Capes / Com o desenvolvimento das tecnologias de impressão tridimensionais (3D) para prototipagem rápida, novos materiais estão sendo constantemente pesquisados, porém, nem todos possuem as características necessárias para serem utilizados para esta finalidade. O custo dos materiais comumente utilizados e suas limitações de aplicação e reutilização são aspectos que devem ser levados em conta, e envolvem a busca por materiais de baixo custo, com adequadas características termomecânicas como também de manufatura, reciclagem, biodegradabilidade e que sejam provenientes de fontes renováveis. Empiricamente, a história da farmácia e da medicina é conhecida por utilizar plantas medicinais devido a suas propriedades bioativas, recentemente a comprovação científica da utilização das substâncias resultantes de seu metabolismo secundário justifica esta afirmação. Dentro deste contexto, o objetivo geral desta tese foi avaliar a bioatividade das resinas vegetais de Stirax benzoin, Commiphora myrrha e Boswellia papyrifera contra os micro-organismos Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, e Candida albicans através da metodologia de difusão em ágar, e também verificar sua possibilidade de aplicações como materiais de impressão 3D através da avaliação de suas propriedades térmicas e físico-mecânicas. Os filamentos foram confeccionados via Hot Melt Extrusion (HME) sendo posteriormente impressos via Fused Deposition Modeling (FDM). Os materiais obtidos foram caracterizados por espectroscopia no ultravioleta visível (UV-vis), infravermelho com transformada de Fourier (FTIR), difração por raios X (DRX) e calorimetria exploratória diferencial (DSC), adicionalmente testes de resistência mecânica à tração e à compressão também foram realizados. Como resultado os materiais inibiram o crescimento dos organismos patógenos em estudo como também apresentaram características adequadas de extrusão e impressão 3D utilizando a técnica de modelagem por deposição fundida. / With the development of three-dimensional (3D) printing technologies for rapid prototyping, new materials are constantly being researched, but not all of them have the necessary characteristics to be used for this purpose. The cost of the commonly used materials and their limitations of application and reuse are aspects that must be taken into account, and involve the search for low cost materials with adequate thermo-mechanical characteristics as well as manufacturing, recycling, biodegradability and that come from sources renewable sources. Empirically, the history of pharmacy and medicine is known to use medicinal plants due to its bioactive properties, recently the scientific evidence of the use of the substances resulting from its secondary metabolism justifies this statement. Withing this context, the general objective of this thesis was to evaluate the bioactivity of the plant resins of Stirax benzoin, Commiphora myrrha and Boswellia papyrifera against the microorganisms Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Candida albicans through diffusion methodology In agar, and also verify its possibility of application as 3D printing materials through the evaluation of its thermal and physical-mechanical properties. The filaments were made by Hot Melt Extrusion (HME) and later printed by Fused Deposition Modeling (FDM). The obtained materials were characterized by visible ultraviolet (UV-vis), Fourier transform infrared spectroscopy (FTIR), X rays diffraction (DRX) and differential scanning calorimetry) in addition mechanical resistance to traction and compression tests were also performed. As a result, the materials inhibited the growth of the pathogens organisms under study, as well as exhibited suitable characteristics of extrusion and 3D printing using the technique Fused Deposition Modeling.