Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part II: Characterization of devitrification and glass‐ceramic phase assemblages

The devitrification process and formation of crystalline phases from commercial alkaline earth boroaluminosilicate glasses containing 48‐61 mol% SiO2, 18‐28 mol% CaO, 1‐7 mol% MgO, 7‐10 mol% Al2O3, 1‐11 mol% B2O3 plus minor amounts of Na2O, K2O, FeO, and TiO2 were quantified through analysis of phase assemblages as function of heat treatments above the glass transition temperatures, using the electron microprobe and powder X‐ray diffraction. Treatments at 800 and 850°C lasted up to 6 weeks. Results indicate that devitrification was strongly activated through presence of heterogeneous nucleation, and that the growth mechanism gradually changed from three‐dimensional growth at the onset of devitrification toward one‐dimensional growth in later stages, when heterogeneous nucleation was absent or less dominating. Most glasses developed entangled and fibrous microstructures with little or no residual glass phase, which are adequate for rigid sealants, and only one of the laboratory analogue glasses, MCAS, developed microstructures with both more equiaxed grains and a considerable amount of residual glass phase, which may be adequate for more compliant and self‐healing sealants as often required in SOC‐applications. Even though the glasses lie within a relatively narrow compositional range, resulting phase assemblages differed significantly. Anorthite (plagioclase) developed as the main crystalline phase in all samples together with pyroxene (or pyroxenoide) and cristobalite. Calcium‐magnesium‐silicate pyroxene (diopside) was in a large part replaced by the calcium‐silicate pyroxenoid (wollastonite) in the samples where the mol‐proportion MgO:CaO was 1:5 or lower. In samples with a very low MgO proportion and consequently a high CaO proportion, calcium metaborate and calcium aluminum borosilicate (okayamalite) crystallized among the main phases and these glasses crystallized completely within the period of heat treatment. Although cristobalite is metastable at the annealing temperatures, both α and β forms were rapidly formed in most of the samples, likely due to kinetic reasons. The presence of the latter is explained by the stabilization effect of Al and B substitution for Si compensated by Ca stuffing in the structure. The stuffed cristobalite transformed with time to quartz (at 800°C) or quartz plus tridymite (at 850°C). Boron was incorporated in the first crystallizing phases, especially diopside, substituting for Al and Si, but the so established substitut