The sintering of aluminum nitride in a microwave induced plasma was investigated. The plasma furnace consisted of a quartz tube inserted into a waveguide connected to a 2450 MHz microwave generator. After evacuating the tube to about 1.33 mbar, nitrogen gas was introduced, generating a steady plasma. Processing parameters such as gas pressure, power level, and time were optimized to yield maximum densification of aluminum nitride. Sintering of pure and doped AlN compacts was performed in the nitrogen plasma at temperatures up to 2000 S C. Undoped specimens reached densities of only 81% theoretical, while densities in excess of 95% theoretical were achieved for yttria doped specimens in less than 15 minutes. Microstructural investigations revealed a smaller grain size in the plasma sintered specimens (about 2μ) than are present in conventionally sintered AlN (about 8μ).

Thesis (PhD)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: This study involves the analysis of crystallization and melting behavior of polymers in solution, using Solution crystallization by laser light scattering (Scalls) as analytical tool. Various other techniques such as differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS) were used for further characterization of the polymer materials. For the first time the solution behavior (crystallization and dissolution kinetics) of two biopolymers namely, poly(lactic acid) (PLA) and poly(butylene succinate) (PBS), was monitored by Scalls. Non-isothermal solution crystallization and dissolution behavior of both enantiomers (D and L) PLA and their blends were studied. It was found that addition of the D- enantiomer (PDLA) to the L- enantiomer (PLLA) in solution resulted in the formation of stereocomplex crystals (SC) and the nucleation-effect of the crystals were intensified with an increase in PDLA content leading to an earlier onset of crystallization and increased crystallization peak area. DSC analysis confirmed the formation of SC during solution crystallization. Overall, results obtained by Scalls provided promising information regarding PLA crystallization kinetics which significantly influences practical applications of this biopolymer. Binary blends consisting of PLLA and PBS were prepared in various blend ratios. Scalls was used to investigate the non-isothermal crystallization and melting behavior of these PLLA/PBS blends from dilute solutions. Addition of PBS up to 30 wt% increased crystallization temperatures of PLLA through a nucleation effect in the molten PBS state. Results indicated that the crystallization rate of PLLA can be improved by additions of small amounts of PBS, and this is of significant interest for practical applications due to the low crystallization rate that PLLA is known to exhibit. DSC thermograms showed an increase in PLLA crystallization exotherm at PBS loadings below 50 wt%, providing further evidence of the enhancement in PLLA crystallization kinetics. A series of linear polyethylene samples with varying molecular weights were studied by Scalls. The effect of molecular weight (Mw) on the solution crystallization temperature (Tc) were monitored and compared to literature where the conventional Crystaf technique was used. Experimental Scalls data correlated well with reported theoretical models. Dissolution studies were also done to study the effect of Mw on the solution melting temperature (Tm) and for the first time, an equation was generated to model solution melting temperatures of linear polyethylenes in Scalls. The solution behavior of a PE-1-octene linear low-density polyethylene (LLDPE) copolymer in a range of solvents including trichlorobenzene (TCB), ortho-dichlorobenzene (o-DCB), decalin and xylene were successfully studied. Scalls allowed for rapid analysis of crystallization and dissolution during a single experiment and offered clearer insight into crystallization kinetics when using different solvents for fractionation. Peak temperatures for analyses in TCB and o-DCB were almost identical but remarkably different to those observed when decalin and xylene was used. The study revealed that solvent type plays an important role in the solution behavior of polymers as well as ultimately the interpretation of these results. / AFRIKAANSE OPSOMMING: Hierdie studie behels die ontleding van kristallisasie en smelt gedrag van polimere in oplossing, met behulp van Oplossing kristallisasie deur laser lig verstrooiing (Scalls) as analitiese instrument. Verskeie ander tegnieke soos differensiële skandeer kalorimetrie (DSC), fourier-tranform infrarooi spektroskopie (FTIR) en dinamiese lig verstrooiing (DLS) was gebruik word vir verdere Karakterisering van die polimeer materiaal. Vir die eerste keer is die oplossingsgedrag (kristallisasie en smelting kinetika) van twee biopolimere naamlik, poli(melksuur) (PLA) en poli(butileen suksinaat) (PBS), deur Scalls gemonitor. Nie-isotermiese oplossing kristallisasie en smelt gedrag van beide enantiomere (D en L) van PLA en hul mengsels was ge-analiseer. Daar is gevind dat byvoeging van die D-enantiomeer (PDLA) tot die L-enantiomeer (PLLA) in oplossing gelei het tot die vorming van stereokompleks kristalle (SC) en die nukleasie-effek van die kristalle is versterk met 'n toename in PDLA inhoud wat gelei het tot 'n vroeër aanvang van kristallisasie en toename in kristallisasie piek-area. DSC analise het die vorming van SC gedurende oplossing kristallisasie bevestig. Algeheel, die resultate wat verkry was deur Scalls het belowende inligting oor PLA kristallisasie kinetika verskaf wat aansienlik praktiese toepassing van hierdie biopolimeer beïnvloed. Binêre mengsels bestaande uit PLLA en PBS was voorberei in verskillende mengsel verhoudings. Scalls was gebruik om die nie-isotermiese kristallisasie en smelt gedrag van hierdie PLLA/PBS mengsels, vir die eerste keer, in verdunde oplossings te ondersoek. Byvoeging van PBS tot 30 wt% het ‘n toename in kristallisasie temperature van PLLA veroorsaak deur 'n nukleasie-effek in die gesmelte PBS toestand. Resultate dui aan dat die tempo van PLLA kristallisasie versnel kan word deur byvoegings van PBS in klein hoeveelhede, en dit is van beduidende belang vir praktiese toepassings weens die lae kristallisasie-tempo waarvoor PLLA bekend is. DSC termogramme het 'n toename in PLLA kristallisasie eksoterm aangedui by PBS inhoud onder 50 wt%, wat 'n verdere bewys is van die verbetering in PLLA kristallisasie kinetika. 'n Reeks lineêre poliëtileen monsters met wisselende molekulêre massas was met Scalls geanaliseer. Die effek van molekulêre massa (Mw) op die oplossing kristallisasie temperatuur (Tc) was gemonitor en vergelyk met literatuur waar die konvensionele Crystaf tegniek gebruik is. Eksperimentele Scalls data het goed gekorreleer met gerapporteerde teoretiese modelle. Smelting studies is ook gedoen om die effek van Mw op die oplossing smelt temperatuur (Tm) te bestudeer en vir die eerste keer, is 'n vergelyking ge-genereer vir die modellering van oplossing smelt temperature van lineêre poliëtileen met behulp van Scalls. Die oplossing-gedrag van ‘n PE-1-okteen lineêre lae-digtheid poliëtileen (LLDPE) kopolimeer was suksesvol bestudeer in 'n verskeidenheid oplosmiddels, insluitend trichlorobenseen (TCB), ortodichlorobenseen (o-DCB) , decalin en xileen. Scalls het toegelaat vir vinnige analise van die kristallisasie en smelting in 'n enkele eksperiment en het dieper insig gegee oor die kristallisasie kinetika tydens fraksionering met die gebruik van verksillend oplosmiddels. Piek temperature vir analises in TCB en o-DCB was byna identies, maar merkwaardig verskillend van dié wat waargeneem was in die geval waar decalin en xileen gebruik was. Hierdie studie het getoon dat die tipe oplosmiddel 'n belangrike rol speel in die oplossing-gedrag van polimere en uiteindelik asook die interpretasie van hierdie resultate. Verder blyk Scalls na ‘n nuttige metode vir die ontleding van polimere in oplossing te wees.

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published_or_final_version / Civil Engineering / Master / Master of Philosophy

The H₂O critical point defines the parabolic vertex of the p(T) vaporization boundary and, as a geometric consequence, a positive vertical asymptote for first partial derivatives of the equation of state. Convergence of these derivatives, isothermal compressibility and isobaric expansivity, to the critical asymptote effectively controls thermodynamic, electrostatic, and transport properties of fluid H₂O and dependent transport and chemical processes in hydrothermal systems. The equation of state for fluid H₂O developed by Levelt Sengers et a1. (1983a) from modern theories of revised and extended scaling affords accurate prediction of state and thermodynamic properties in the critical region. This formulation has been used together with the virial equation of state proposed by Haar et a1. (1984) and predictive equations for the static dielectric constant (Uematsu and Franck, 1980), thermal conductivity (Sengers et a1., 1984), and dynamic viscosity (Sengers and Kamgar-Parsi, 1984) to present a comprehensive summary of fluid H₂O properties within and near the critical region. Specifically, predictive formulations and computed values for twenty-one properties are presented as a series of equations, three-dimensional P-T surfaces, isothermal and isobaric crosssections, and skeleton tables from 350°-475°C and 200-450 bar. The properties considered are density, isothermal compressibility, isobaric expansivity, Helmholtz and Gibbs free energies, internal energy, enthalpy, entropy, isochoric and isobaric heat capacities, the static dielectric constant, Z, Y, and Q Born functions (Helgeson and Kirkham, 1974a), dynamic and kinematic viscosity, thermal conductivity, thermal diffusivity, the Prandtl number, the isochoric expansivity-compressibility coefficient, and sound velocity. The equations and surfaces are analyzed with particular emphasis on functional form in the near-critical region and resultant extrema that persist well beyond the critical region. Such extrema in isobaric expansivity, isobaric heat capacity, and kinematic viscosity delineate state conditions that define local maxima in fluid and convective heat fluxes in hydrothermal systems; at the critical point, these fluxes are infinite in permeable media. Extrema in the Q and Y Born functions delineate state conditions that define local minima in the standard partial molal volumes and enthalpies of aqueous ions and complexes; at the critical point, these properties are negative infinite. Because these fluxes and thermodynamic properties converge to vertical asymptotes at the critical point, seemingly trivial variations in near-critical state conditions cause large variations in fluid mass and thermal energy transfer rates and in the state of chemical equilibrium.

Numerical investigations of heat transfer in single and multiple thick-walled pipes and countercurrent pairs are used to deduce relationships between fluid and solid temperatures needed to develop more accurate thermal models of living tissue in the extremities. A structural model of heat transfer in living tissue is developed using currently available anatomical and physiological data for the extremities. In order to improve the heat transfer basis of thermal modeling under in vivo conditions, four heat transfer problems based on structures found in the extremities are solved using in vivo parameters-the thick-walled pipe and countercurrent pair, and the multiple thick-walled pipes and countercurrent pairs-are studied. Low resolution numerical models are devised to approximate the thick-walled pipe and the non-concentric thick-walled countercurrent pair in square geometries. A constant heat transfer coefficient at the fluid-solid interface adequately approximates the fluid and solid temperatures for moderate flow conditions (Peclet number of 10 < °Pe < 1000). In the thick-walled countercurrent pair, countercurrent exchange and fluid-solid thermal interaction are found to act simultaneously, giving rise to imperfect countercurrent exchange. Fluid and solid temperatures in the multiple thick-walled pipes and pairs near the outer boundary resemble those of the single thick-walled pipes and pairs. The countercurrent pairs near the center also exhibit imperfect countercurrent exchange. In cylinders with L* > 1 containing multiple countercurrent pairs, the shapes of the temperature profiles cannot be distinguished from the temperature profile shapes of cylinders containing multiple thick-walled pipes. Fluid and solid temperatures in multiple parallel pipes may be approximated with a field equation which has the same form as the Pennes' bioheat equation. Unlike Pennes' equation, the coefficients for the blood thermal energy term quantify the dependence of the amount of thermal energy transferred between blood and tissue with the geometry of the blood the flow rate through the dimensionless axial length L*, and the dimensionless axial coordinate x* . Comparisons of structural model temperatures with available in vivo temperature studies show that blood and tissue temperatures are consistent with fluid and solid temperatures of either multiple unpaired pipes or multiple countercurrent pairs embedded in a solid cylinder. Further improvements of the basis for in vivo heat transfer modeling are crucially dependent upon more extensive comparison with three-dimensional in vivo studies.

This dissertation combines aspects of lasers, nonlinear optics and interferometry to measure the linear optical properties involved in phase matched second harmonic generation, (SHG). A new measuring technique has been developed to rapidly analyze the phase matching performance of potential SHGs. The data taken is in the form of interferograms produced by the self referencing nonlinear Fizeau interferometer (NLF), and correctly predicts when phase matched SHG will occur in the sample wedge. Data extracted from the interferograms produced by the NLF, allows us to predict both phase matching temperatures for noncritically phase matchable crystals and crystal orientation for angle tuned crystals. Phase matching measurements can be made for both Type I and Type II configurations. Phase mismatch measurements were made at the fundamental wavelength of 1.32 (mu)m, for: calcite, lithium niobate, and gadolinium molybdate (GMO). Similar measurements were made at 1.06 (mu)m. for calcite. Phase matched SHG was demonstrated in calcite, lithium niobate and KTP, while phase matching by temperature tuning is ruled out for GMO.

A mathematical model study was made to compare simulated soil temperature variations with a field situation. The study involved the use of two plots, one of which was cleared of all vegetation and the other left in its natural vegetative state. Each plot was watered at the beginning of three different trial periods and as drying occurred, the following atmospheric parameters were measured on an hourly basis: incoming solar radiation, air temperature, relative humidity, cloud cover, and wind speed. The above parameters provided input for solution of the energy budget equation to determine soil surface temperature variation. Using the soil surface temperature variation as an upper boundary condition, and assuming heat movement by conduction, the temperature at 5 cm increments down to 70 cm in the soil was calculated. The two plots provided model testing for a bare soil surface and also for a soil surface beneath a 30 percent and a 40 percent plant cover. Actual and calculated temperature difference was in most cases less than 2°F. The mathematical model is applicable to any area where the above parameters may be monitored.

The solubility of the noble gases is temperature dependent. Other factors influencing solubility are the system pressure, the solute content of the water, and gravitational potentials. Most of the noble gases dissolved in ground water are from chemical equilibrium with the atmosphere. This equilibrium takes place in the recharge zone of the aquifer, typically in the soil. The final noble-gas concentrations are determined by the temperature, the elevation, the alteration of soil-gas composition by organisms, and soil-temperature gradients (which are in part a result of water-table depth and recharge rates). If the effects of temperature can be separated from the other influences, and if the noble-gas concentrations are not altered after recharge water enters the saturated zone, variations in recharge temperature with time may be determined by measuring the noble gases in dated ground-water samples. However, analysis of available data indicates that noble-gas concentrations frequently change after recharge. This change is usually the result of reequilibration with a biogenicgas phase produced within the aquifer, or from contact with air. In order to extend the calculation of recharge history to samples with complex histories of gas equilibrium a general equation for two stage equilibrium was derived. The variables in this equation are the initial temperature and pressure of equilibrium, the final temperature and pressure of equilibrium, and the molar water-to-gas ratio at the second equilibration. An equation of this type is constructed for each of the gases: neon, argon, krypton and xenon. These equations are solved simultaneously for four of the variables listed above while the value of one is assumed. Graphical techniques for determining which assumptions to use are presented. Ground-water samples were collected in glass tubes and analyzed by gas chromatograph-mass spectrometer, using double isotope dilution standardization. The analytical method is still developmental. Field sampling was undertaken in two locations, the Milk River aquifer of southern Alberta, Canada, and the Carrizo sand aquifer in southern Texas, in order to test the method. The preliminary data obtained may show correlation with known Holocene-Pleistocene climatic fluctuations, encouraging further development of the analytical technique and field research. Finally, a method for interpreting the climatic implications of oxygen and hydrogen isotope information from ground water in conjunction with noble-gas data is given.