Acceleration of the Crystallization of Highly Refractory Glass Ceramic Fiber by Salt Treatments

Abstract

To obtain highly refractory mineral wool (glass fiber), authors tried to produce those which crystallize without softening during heating. The wool, the composition of which is within the system CaO⋅Al2O3⋅2SiO2-2MgO⋅2Al2O3⋅5SiO2-CaO⋅TiO2⋅SiO2, was easily produced by arc smelting of raw materials and by blowing compressed air to the melt. These samples, however, softened between 700°C and 800°C during heating before the crystallization took place. Surface treatment with various salts, especially such as Cu(NO3)2 and AgNO3, was found to be effective to accelerate the crystallization and decrease the shrinkage during heating (compare Fig. 1 and 2).By X-ray analysis it was observed that these nitrates decompose to the oxides at relatively lower temperatures and react with the glass fiber at the surface and accelerate the crystallization (Fig. 4, 5, 6 and 7). Microscopic observation showed also the decomposition of the nitrate and the reaction with the glass fiber at the surface. It was observed that the reacted oxides diffused into glass phase and formed dense metallic nuclei (Fig. 3 and 8). The reaction at the surface seemed also to remove Griffith flaw and to decrease the effect of the orientation of crystals by the crystallization from these flaws. Dense core of crystals thus formed around each fiber seemed to support the inner glass which should soften at high temperature by itself.The shrinkage of the properly salt-treated wool was only within a few per cent at 1000°C.