Abstract
MAS‐osumilite is a metastable double‐ring silicate which is attractive for use in glass‐ceramics because of its low thermal expansion coefficient. The phase can be crystallized from glass powders of MAS‐osumilite composition containing small amounts of BaO. The nucleation and crystallization process has been investigated by means of TEM and SEM. Crystallization starts with the precipitation of β‐quartzss of composition MgAl2O4·nSiO2. BaO concentrates in a residual glass phase between the β‐quartzss crystals. In a second stage small amounts of Ba‐osumilite (BaMg2Al6‐Si9O30) nucleate and crystallize from this glass phase, In a third stage BaO‐free MAS‐osumilite crystals are formed by epitactic growth on the Ba‐osumilite surfaces and grow into adjacent β‐quartzss grains. Two factors are essential for the suppression of undesired further phases to develop a monophase (besides some Ba‐osumilite) MAS‐osumilite glassceramic: a residual glass phase with a composition close to Ba‐osumilite, and β‐quartzss crystals having a composition close to that of MAS‐osumite.
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