No description available.

No description available.

Lightweight (Ca,Mg)Zr₄(PO₄)₆(CMZP) ceramics have been fabricated with relative densities (P_lightweight/p_dense) of less than 0.35 by the polymer foam technique, and higher than 0.35 by the polymer powder technique. The polymer powder method forms an inhomogeneous pore structure having average pore sizes of 30-80 μm compared with the polymer foam method, which yields a large and uniform pore structures with pores 250-300 μm in diameter. The thermal expansion of the lightweight CMZP ceramics varies from positive (+2.7x1 0⁻⁶/â C) to negative (-2.6x10⁻⁶/ â C) with increasing grain size. The lightweight CMZP ceramics exhibit an excellent thermal shock resistance and this resistance is improved by increasing porosity. Mechanical properties including tensile strength, compressive strength, Modulus of rupture (MOR), and Young's moduli have been determined and vary exponentially with porosity. The strengths of the lightweight CMZP ceramics are superior to those of lightweight Zr0₂ with the same bulk density. Young's moduli, which were determined using a sonic velocity technique, range from about 5 GPa to 30 GPa, depending on the composition and the relative density. Furthermore, corrosion studies show that lightweight CMZP ceramics possess high resistance to acid corrosion at ambient temperature. / Master of Science

Abstract not available

There is no abstract available for this thesis. / Department of Art

A model for the dynamic response of a laminated composite arch is developed from classical shell theory. The problem is reduced from a shell to an arch by making an assumption that the variation of the field of variables in the direction of the width of the arch is small compared to those in other directions. Standard separation of variables is used to change from a system of partial differential equations to that of ordinary differential equations. Several methods of solution are explored, namely the Laplace transformation, the method of particular solutions, and the eigensolution. The eigensolution is chosen as the the most efficient in terms of computer time and is the easiest to modify. The free vibration of the arch is explored and the natural frequencies of the system are determined. The response of the arch to general forcing functions is also considered, by the use of the Fourier transformation technique. Damping through material viscoelasticity and use of the model in evaluation of experimental data are also discussed. / Master of Science / incomplete_metadata

Glass, sapphire and polymer fibers transparent to visible and infrared electromagnetic frequencies are extensively used in communication and sensing. The lifetimes of these waveguides are extended considerably by suitably coating them. Plastic coated silica waveguides are gradually replacing metal coaxial cables used in communications and they have been used successfully in various types of sensing. Unfortunately plastic coatings cannot withstand very high temperatures. In order to perform contact or invasive sensing in the medium to high temperature range and in harsh environments, other appropriate coating materials have to be used. This thesis examines various refractory materials as candidate coating materials. Coating materials should not react chemically with the waveguide material but should have matching thermal expansion coefficients. Refractory materials are examined in detail for thermodynamic suitability for both sapphire and silica waveguide cores and claddings. The candidate coating materials selected are alumina, silicon carbide, zirconia and metal niobium. Experimental verification of the chemical inertness of these materials with silica and sapphire in very low pressure and at 857°C temperature is studied. The materials found suitable for coating can be coated using the various methods discussed. Fibers suitably coated with these materials would be suitable for high temperature sensing in harsh environments and in situ within advanced high temperature composites. Metal niobium does not react with sodium and is thermodynamically compatible with alumina which is also a very stable refractory material. Multilayer coatings of niobium and alumina on sapphire exposed to harsh environmental conditions can prolong the life of the sapphire waveguide. X-ray diffraction and electron microprobe analyses of the single oxides and carbides, namely, alumina, silicon carbide and zirconia and the metal niobium, were conducted. It was found that sapphire did not react with any of the selected ceramics; the silica fiber underwent structural change in the silicon carbide matrix and the change was macroscopic. Within the restricted environment, the silica fiber appeared not to react with the alumina, zirconia and niobium matrices. This thesis specifically considers the possibility of using the various ceramics as coating materials without analyzing the nature of the phases present. Hence detailed analyses of phases were not made when macroscopic change in fiber structure was observed or as observed during the x-ray analyses and microprobe analyses. / Master of Science

The purpose of the research was to develop a translucent low fired porcelain casting slip using South African raw materials, due to the ever increasing electricity tariffs in South Africa as well as the physical deterioration put on the elements and brickwork in electric kilns when fired to traditional porcelain temperatures. Traditional porcelain bodies that can be purchased from South African suppliers are required to be fired to between 12000C and 13000C. The commercially prepared porcelains when tested produced white vitrified bodies but were lacking in translucency. Local ceramic artists are therefore compelled to import their porcelains from overseas suppliers if they require a white translucent porcelain but this is still requires a firing temperature well above 12000C. It has been shown that by using South African ceramic raw materials and adjusting a Parian ceramic formula using a selected frit; a low fired translucent porcelain can be made that matures below 12000C. The addition of paper fibres to the non-plastic porcelain was necessary to reduce the high shrinkage rate and prevented the clay from cracking and tearing in the firing process. With the further adjustments to the formula by the addition of calcium triphosphate true white translucent porcelain was produced. Without this last adjustment the porcelain would be an off-white colour due to the impurities found in the South African ceramic raw materials which are mainly contaminated with iron oxide. It was found that the following formula produced a white translucent porcelain which vitrified at 11900C and satisfies the original concept in the title stated above.

A novel method of impregnation was used to make thermoplastic prepregs with aqueous suspension of polymer powder and AS-4 carbon fibers. The prepregging, consolidation, and laminate evaluation procedures are described in this report. The need and importance of a dispersing and binding agent and its interactions with the composite matrix are assessed. A time-temperature-pressure consolidation cycle was designed for PEEK prepreg. Two thermoplastic polymers were studied : LaRC-TPI, provided by Mitsui Toatsu Chemicals, and Poly(etheretherketone), synthesized in Virginia Tech Chemistry Department. Comparison of LaRC-TPI composites to those made at NASA-Langley Research Center demonstrated good reproducibility of the techniques used in this study. The last PEEK composites fabricated compared well to the ICI standards. / Master of Science

<p dir="ltr">To decrease the use of fossil fuels that generate greenhouse gases, there has been a push to find alternative processes for electricity generation. An attractive renewable alternative is to use solar-thermal energy for grid level electricity production. One method used to generate electricity from the conversion of solar-thermal energy is concentrated solar power (CSP) via the power tower paradigm, which involves an array of mirrors that concentrate sunlight to a spot on a tower. The light heats up a heat transfer fluid which later transfers the thermal energy to a working fluid that expands so as to spin a turbine to generate electricity. Current CSP plants have a peak operation temperature of 550℃, but improvements to the heat exchanger are integral to increasing the peak operation temperature of such plants to a 750℃ target. Ceramic/metal composites (cermets) have been proposed for use as heat exchangers in these CSP plants due to the creep resistance of the ceramic component and toughness of the metal component. One potential material that has an attractive combination of properties for this application is the alumina/chromium (Al2O3/Cr) cermet, given the rigidity and creep resistance of the Al2O3 component and the high-temperature toughness of the Cr phase. Compared to other oxidation-resistant oxide/metal cermets, the Al2O3 and Cr components of this cermet have a relatively close average linear thermal expansion match from 25℃ to 750℃, which is advantageous due to the thermal gradients and thermal cycling of the heat exchanger during operation.</p><p dir="ltr">In this dissertation, the Al2O3/Cr cermet was produced via reaction forming (RF) or reactive melt infiltration (RMI). The RF method involves the reaction of Cr2O3 and Al constituent powder mixtures at high temperature and modest pressures to obtain dense Al2O3/Cr plates. The RMI method involves immersing a shaped porous Cr2O3 preform into an Al or Al-Cr alloy bath to infiltrate and react to form Al2O3/Al-Cr plates. For both methods, the plate microstructure was analyzed for the various reaction conditions. The adiabatic temperature increase for the reaction between Cr2O3 and Al liquid or Al-Cr liquid alloys was calculated. Thermal properties (linear coefficient of thermal expansion, heat capacity, thermal diffusivity, thermal conductivity) and mechanical properties for the RF Al2O3/Cr plates were also measured. Lastly, the reaction kinetics between dense, polycrystalline Cr2O3 and a liquid Al-35at% Cr alloy were experimentally determined at various temperatures and compared to models based on different rate-limiting steps.</p>