THERMAL CHANGES OF AMORPHOUS SiO<I>₂</I> AS A CONSEQUENCE OF A PRECIPITATION TEMPERATURE

Abstract

Amorphous precipitate SiO₂ as a subject of this research is obtained by reaction between Na-silicate, water solution (water glass) and sulphuric acid. The influence of precipitation temperature, upon the characteristics of obtained samples is analysed. The temperature is choosen as a parameter, which has a strong influence upon the structure and characteristics of a product, but it has a very narrow interval at the same time, where the precipitation process under an atmospheric pressure is possible. Some more important characteristics of the samples, obtained at 80-95 °C (values close to the limits of temperature interval where the precipitation reaction is possible), were analysed to show the essential differences, caused by the temperature. The analysis referred to determination of the dimensions of primary and secondary particles, density, specific surface area, and level of amorphous state (all this in green state). Then, the morphological and structural changes during heating at the characteristic temperature are followed. The changes are determined by the above characteristics and also by measuring the energetic values of some of them. Significant differences are noticed between samples where the higher precipitation temperature points to obtaining of amorphous precipitate with bigger primary particles, less specific surface area, higher density and higher molecular order which is manifested through faster and easier crystallization (when heating at 1100 °C).