Studies of hexacelsian and celsian barium aluminosilicates

The first part of this work (chapter 3) describes the reaction paths leading to the formation of BaAl{dollar}sb2{dollar}Si{dollar}sb2{dollar}O{dollar}sb8{dollar} (BAS) from a mixture of {dollar}gamma{dollar}-BaCO{dollar}sb3, alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3,{dollar} and amorphous SiO{dollar}sb2{dollar} powders. Heat treatments conducted from 600 to 1200{dollar}spcirc{dollar}C in air were used to transform the powder mixtures into hexacelsian BAS. The phase evolution to BAS was examined by x-ray diffraction. Several experiments were designed to microscopically reproduce the solid-solid interfaces expected during the synthesis of BAS and enabled the author to describe the different stages of the reaction.; There exist two reaction paths in formation of BAS in this study: (1) formation of a series of barium silicates leading to BaO{dollar}cdot{dollar}2SiO{dollar}sb2{dollar} (BS{dollar}sb2){dollar} which then reacts with Al{dollar}sb2{dollar}O{dollar}sb3{dollar} to form BAS and (2) formation of BaO{dollar}cdot{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} (BA) which then reacts with SiO{dollar}sb2{dollar} to form BAS. The kinetics of the latter is slower than that of the former because the reaction between BaO{dollar}cdot{dollar}Al{dollar}sb2{dollar}O{dollar}sb3{dollar} and SiO{dollar}sb2{dollar} to form BAS includes a bond breaking process.; The second part (chapter 4) of this research was undertaken to study the role of additives on the kinetics of the transformation of hexacelsian to celsian. Pre-synthesized hexacelsian powders doped with various additives were heated at temperatures ranging from 850 to 1400{dollar}spcirc{dollar}C for 4 hrs. Semi-quantitative analysis of XRD was used to determine the extent of the hexacelsian-to-celsian transformation. This work was extended further to investigate the mechanisms involved in the transformation.; Defect structures developed in the additive-containing celsian provide insights about the sites occupied by the cations added. Experimental results indicate that the doping of {dollar}sim{dollar}0.99A cations in promoting the conversion of hexacelsian to celsian is by forming an interstitial solid solution in hexacelsian and {dollar}sim{dollar}0.66A cations form a substitutional solid solution. In a kinetic study on the CaO- or MgO-enhanced transformation, values of rate constant, k, and Avlami constant, n, at various temperatures were evaluated from the Johnson-Mehl-Avrami equation.; Although the hexacelsian-celsian transformation is a framework reconstruction process, the estimated activation energies in previous researches and in this study have the values much lower than the expected value of {dollar}sim{dollar}777 kJ/mol for (Al,Si)-O bond breaking mechanism. A more reasonable mechanism is proposed in this study. The short-range diffusion of barium ions induced by a basal-plane glide in hexacelsian possibly facilitates the (Al,Si)-O bond opening. The basal-plane glide is a rate-determining step in the transformation and could be enhanced by the addition of mineralizers.; Finally, in chapter 5 the effect of additives on the in-situ crystallization of celsian and {dollar}beta{dollar}-Si{dollar}sb3{dollar}N{dollar}sb4{dollar} was investigated upon cooling from 1800{dollar}spcirc{dollar}C, in which a liquid-phase sintering of the Si{dollar}sb3{dollar}N{dollar}sb4{dollar} reinforced BAS composites is performed. It was found that the presence of additives no longer stabilizes the celsian phase and is detrimental on the mechanical properties, in particular fluoride additives. (Abstract shortened by UMI.)...