تحضير ودراسة خزفيات ذات أساس من السيليس

Abstract

In this work, the sintering of silica in its quartz state was studied at different temperatures using tricalcium phosphate additions by following easy preparation methods. The selected starting materials for in this application were natural quartz sand and phosphate ore. First, a literature revue study was carried out on silica, its phases polymorphic and its transformations of phases. Additionally, the favorable conditions for the transformation, sintering, and the use of bricks of silica were presented. The equilibrium diagram phases in the binary and ternary systems concerning silica were also taken into account. Secondly, highly pure quartz sand and additions of tricalcium phosphate or calcium hydroxide have been used. Three mixtures consisting of quartz and tricalcium phosphate were prepared. Furthermore, the other three consisting of quartz and calcium hydroxide were prepared in order to make a comparison of the effect of each addition. Thirdly, the sintering of samples was carried out at different temperatures. Their obtained bulk densities were ranged between 1.58 and 1.72 g/cm3 with a considerable porosity situated in the interval 32 - 36% at 1400 ° C for 2 hours for silica and phosphate tricalcium samples. This density is higher than that of silica and calcium oxide samples and is acceptable when compared to that of silica bricks. The addition of alumina improves the sintering and an intake of 5% by weight gives good sintering with acceptable porosity without altering the thermal properties. The diffractograms have shown that the silica in the quartz form changes to cristobalite with the addition of tricalcium phosphate and the formation of the β - phase tricalcium phosphate. The transformation was completed by increasing the temperature or the holding time. On the other hand, quartz is transformed into cristobalite and then trydimite with calcium oxide addition. Adding alumina with tricalcium phosphate; a liquid phase appears with cristobalite. This liquid phase results from the melting of anorthite, aluminum phosphate and calcium oxide at high temperatures which form during heating. The microstructural study shows considerable porosity with a random distribution and volume; and interconnecting pores between them. It also illuminates the number of phases formed and is consistent with the study of phase transformations. The DTA and TGA confirmed the transformation of quartz to cristobalite and the stabilization of the latter in the β form at height temperatures and in the α form, at low temperatures (lower than 210 °C). There is no evidence or indication of a chemical reaction between the silica and tricalcium phosphate phases. The results of thermal expansion are in concordance with previous ones. A sudden expansion due to transformation of α cristobalite to β form about 210 ° C; and a low expansion up to 1000 ° C in samples containing silica + tricalcium phosphate have been noticed. The thermal expansion of the samples containing silica + 20 wt% tricalcium phosphate + 20 or 15 wt% 108 alumina confirmed the stabilization of the β-cristobalite at low temperatures. Finally, a very attractive observation that requires more monitoring and research is the sudden expansion at a lower temperature than 200 ° C in samples containing silica + 10 wt% tricalcium phosphate + 10 wt% alumina.