No description available.

An experimental investigation to study the influence of base metal composition on the time required to achieve complete isothermal solidification (tf) during TLP bonding of three Ni-base superalloys was performed. Alloys IN 738, DS Rene80 and DS IC 6 show similar behaviour when bonded at 1100 oC, with comparable tf. However, at higher temperatures, IN 738 requires extended period of time (as compared to DS Rene80 and DS IC 6) to achieve complete isothermal solidification. The prolonged tf in IN 738 appears to be caused by a more pronounced reduction in concentration gradient of the diffusing solute within the material during bonding. In contrast, the shorter complete isothermal solidification time experienced by alloy DS IC6 is attributable to its capability to better accommodate the diffusing solute, through the formation of densely packed second-phase precipitates in the diffusion affected zone (DAZ).

Graduate / 0485

Protonation and hydrogen bonding effects on the redox properties of verdazyl radicals were investigated. Verdazyls with various substituents at the C3 position were synthesized, including species with no basic site, a basic site distant from the verdazyl heterocycle, a basic site proximal to the verdazyl heterocycle but with strained hydrogen bonding geometry, and a basic site proximal to the verdazyl heterocycle with favourable hydrogen bonding geometry. Treatment of these verdazyls with trifluoroacetic acid resulted in several general changes to their cyclic voltammograms: the oxidation and reduction potentials shifted to higher potentials, with concurrent loss of reversibility of the reduction process due to protonation of the verdazylate anion. These changes were likely a result of intermolecular interactions between the substrate and acidic medium rather than specific protonation or intramolecular hydrogen bonding effects. Acid treatment of the verdazyl with favourable hydrogen bonding geometry resulted in emergence of a new reversible process in the CV that was found to match the oxidation process of the corresponding leuco verdazyl. Attempts to synthesize this leuco verdazyl were successful, however purification was not completed. Spectroscopy of the crude product suggests that it may be stable to air, an uncommon feature in leuco species due to their propensity to be easily oxidized by O2. Preliminary investigations of the potential of verdazyls as alcohol oxidation catalysts were made. Several previously reported and novel species were investigated as stoichiometric and catalytic oxidants akin to nitroxides and phenoxyls. It was found that the verdazyls, verdazylium cations and verdazyl-metal complexes tested did not stoichiometrically or catalytically oxidize alcohols. / Graduate

Graduate / 0485

No description available.

Structural effects in hydrogen and non-hydrogen bonded liquids and their mixtures have been studied using several experimental techniques and theoretical approaches. Apparent heat capacities and volumes of linear alcohols in hydrocarbons were determined at very low alcohol concentrations and their self-association in solution discussed in terms of the Treszczanowicz-Kehiaian theory. An extension of this theory was used to describe cholesterol self-association and its interactions with tripalmitin and lecithin. Heat capacities of water-organic mixtures are reported. It is found that water behaves as a lower alcohol at the organic-rich concentration range. At the water-rich end, Shinoda's views on water structuring around hydrophobes are supported. Thermal pressure coefficients of cyclohexane + normal and branched alkanes are consistent with the presence of orientational order in the long pure n-alkanes. Excess volumes for mixtures of alkanes with liquids of different internal pressures are predicted using Flory theory. The anomalous thermodynamic behaviour of cyclopentane mixed with cyclic and branched alkanes has been studied through the measurement of cyclopentane spin-lattice relaxation times in these mixtures. An extension of Sanchez-Lacombe theory for pure liquids is described and the molecular parameters obtained for sixty common substances. An equation of state for pure n-alkanes with correlations of molecular orientations is presented.

The demand for posterior aesthetic restorations has dramatically increased in recent years. Several new materials have been developed with improved aesthetic, adhesive and mechanical properties. However, microleakage elimination, especially if the margins are on dentine or cementum, and attainment of optimum mechanical properties for posterior use, are still questionable. The aim of this study was to investigate microleakage and surface microhardness of a nanofilled, a nanohybrid and an ormocer based nano-ceramic composite restorative material.

<p>The use of provisional acrylic crowns is common practice with prosthodontic treatment. Provisional crowns are prone to marginal leakage and poor retention due to weak interim cements. The aim of this study was to assess the effect of the application of two different dentine-bonding agents on the microleakage of provisional crowns.</p>

<p>Amalgam bonding agents have been shown to enhance retention of amalgam restorations by mechanical means. However, recent studies showed that the use o glass ionomer cements and resin cements as lining and bonding materials to amalgam restorations will increase the retentive strength of the amalgam restorations, hence reducing the micro leakage and secondary caries. The purpose of this study was to compare the relative retentive strength of conventional amalgam restorations and bonded amalgam restorations using resin adhesive, glass ionomer cements and resin cements.</p>