Devitrification behaviour of alkaline-earth silicate fibre.

The alkaline-earth silicate fibres are a new generation of man-made insulation materials. The materials are amorphous on manufacture and have been shown to be soluble in physiological solutions and to be cleared from the lung in animal exposure trials.This reported study provides a thorough investigations of the devitrification behaviour of Superwool X-607 (Morgan Materials Technology), and two further compositions, code names A2 and B3.Thermal exposures were made within the region extending from 700 to 1250°C and 10 minutes to 3240 hours in clean furnace environments. The devitrified microstructures and products were identified using X-ray powder diffraction and analytical electron microscopy. Details are provided of the development of specimen preparation techniques to enable fibre cross-sections to be analyzed in the TEM.The devitrification products are presented as a function of exposure temperature and time for all 3 compositions. The amorphous glass separated into a silica-rich phase and an alkaline-earth silicate rich phase and the development of these amorphous phases is presented and discussed. The subsequent devitrification of these separated phases into associated silica crystalline phases and alkaline-earth silicate crystalline phases, and, in each case, the subsequent phases and transformations with increasing thermal exposure are also presented and discussed.For the crystalline silica phases, the following unusual transformation situation was identified in all 3 compositions: amorphous silica → alpha-quartz → alpha-cristobalite → tridymite. In comparison with established understanding of silica phase transformations, the following anomalies were identified and explained: a) the formation of quartz as the primary crystalline silica phase at temperatures ≥1000°C, and b) the subsequent formation of alpha-cristobalite, the low-temperature form, other than B-cristobalite, the high-temperature form, which was the only silica phase identified in the devitrified aluminosilicate fibres.For the alkaline-earth silicate phases, two forms of wollastonite solid solution were characterised. The low-temperature form, containing more Mg[2+], transforms to the high-temperature form and diopside at temperatures above 900°C. The low-temperature anomaly, ie the formation of the immature high-temperature form below 900°C is possibly due to a secondary phase separation. Pseudowollastonite was found to be metastable in the investigated temperature region. It is suggested that the detailed transformation process among these alkaline-earth silicate phases has been identified for the first time.This thesis also includes a detailed review of published studies concerning this materials systems as well as recommendations for further work.