The synthesis of glasses by hot-pressing of gels was studied for pure SiO2, as well as for La2 03 -SiO2 and B2 03 -SiO2 systems. It was recognized that gels prepared by hydrolysis and polycondensation of organometallic compounds crystallize less quickly than those obtained from hydrosols, and are, therefore, particularly suited for hot-pressing. Different methods of pressing were investigated; they influence the residual OH-content of resultant glasses. Murray et a/.'s densification model based on viscous flow was found to apply in the case of the gels under study and a dynamic viscosity evaluation is proposed to determine rapidly the pressing parameters for a given gel-glass conversion.The method was used, in particular, for the refractory system La2 03 -Si02 and the devitrification behaviour of hot-pressed glasses was studied quantitatively. It was found that the residual OH-groups catalyse the conversion to a glass-ceramic material. The influence of pressure was also studied in some cases, and it was found to favour the precipitation of dense phases.
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