Crystallization Mechanism of MAS‐osumilite with Composition Mg₂Al₄Si₁₁O₃₀ from Glass

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 (3-quartz, of composition MgAl,O,*nSiO,. BaO concentrates in a residual glass phase between the @quartz, crystals. In a second stage small amounts of Ba-osumilite (BaMg,Al,-Si,O,) 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 P-quartz,, grains. 'hvo 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 P-quartz,, crystals having a composition close to that of MAS-osumilite.

Microstructures of alkoxide-derived barium osumilite (BaMg2Al6Si9O30) glass ceramics

“…The microstructures observed in PAR and AAR Baosumilite glass ceramics are similar to those observed by Winter [8,9] and Varshal [10] in their crystallisation studies of melt-derived osumilite. The only obvious difference is in the formation of the first precursor phases which give rise to regions of Ba-rich residual glass from which barium osumilite nucleates and grows.…”

“…(ot-cordierite: a = 0.9770 nm, c = 0.9352 nm; Ba-osumilite: a= 1.0137 nm, c= 1.4341 nm). The close values of a according to Winter and Berger [8] explain the formation of cordierite in the crystallisation of osumilite. The presence of cordierite as a secondary phase has also been reported in other studies on melted aluminosilicate systems with osumilite compositions [9,10].…”

“…The marked decrease in the amounts of mullite and hexacelsian and the steady increase in the concentration of barium osumilite phase in both gels suggest that some interactions or reactions are occurring though not to completion between mullite, hexacelsian and the residual glass to form the more stable barium osumilite. This glass would be expected to be Ba and Mg rich in PAR and only Mg rich in AAR because the formation of mullite and hexacelsian as the main precursor phases in PAR and AAR respectively would draw some A1 in PAR and Ba plus A1 in AAR from the surrounding glass, from which according to Winter and Berger [8] Ba-osumilite crystals nucleate and grow. Varshal and Levitina [10] reported that metastable crystal phases formed in the initial stages of the crystallisation of aluminosilicate glasses containing Ba and Mg, interact with Ba and Mg phases to form osumilite.…”

“…Barium osumilite (BaMg2Al6Si9030) has a crystal structure related to that of cordierite with similar properties and is also potentially useful as a refractory material and CMC matrix [5][6]. Crystallisation studies of osumilite-based glasses [7][8][9][10][11][12] prepared by conventional melting report complex crystallisation sequences with the formation of several precursor phases before the nucleation and growth of the stable osumilite phases. The present study describes and compares the crystallisation of stoichiometric barium osumilite derived from two sol-gel routes using alkoxides and organic salts as the starting materials and the microstructures of the.…”

Crystallization of 60SiO2–20MgO–10Al2O3–10BaO Glass Ceramics

“…These materials are based on the beneficial properties of two or three oxide crystalline phases: aluminum silicate (mullite), magnesium silicates (enstatite and forsterite), aluminum–magnesium silicate (cordierite) barium silicates, and barium–aluminum silicate (celsian) 1–8 . Several four‐component glass ceramics have been prepared 9–11 . Chaim and Heuer 10 crystallized a 52SiO 2 –13MgO–32Al 2 O 3 –2BaO glass and observed the primary growth of denderitic μ‐cordierite that were consumed by secondary nucleation of denderitic β‐cordierite.…”