Pores: the plague of glass-ceramics

Reference Presenter Authors
11-035 Oscar Peitl Peitl, O.(Federal University of Sao Carlos); Zanotto, E.D.(Federal University of São Carlos (UFSCar)); Heide, K.(Friedrich Schiller Universität); We dwell on the two main mechanisms that lead to pore nucleation and growth in glass-ceramics: (i) widely different densities between the parent glass and the crystals (e.g., as found in Diopside glass), and (ii) nucleation and growth of bubbles from dissolved gases in the glass matrix, which become supersatured at the crystal / liquid interface with the increase in the crystallized fraction. To establish the conditions that lead to pore formation due to mechanism ii, we analyzed two model glass compositions that exhibit internal crystallization, 1.07Na2O.2CaO.3SiO2 (1.07N2C3S) and Li2O.2SiO2 (L2S). Using optical microscopy analysis, we construct for the first time a “pore map” - an experimental diagram showing regions of pore formation as a function of crystallinity and average crystal size. Both systems show pores having the same geometry that emerge in the residual glass phase and snake across the crystal interfaces. The pore diagram shows that bubbles (of the order of the crystal size) tend to nucleate and grow for glass-ceramics containing high volume fractions crystallized (>50%) and relatively large crystal sizes. These results provide a way to avoid pore generation in glass-ceramics by fine tuning the heat treatment schedule to obtain small grain sizes. We observed that pores only arise in L2S glass-ceramics containing crystals ~45 ?m, which are larger than in the 1.07N2C3S (~10 ?m). Hereby it is easier to produce fully crystallized, pore-free L2S glass-ceramic than 1.07N2C3S glass-ceramics. Mass spectroscopy experiments revealed that pore formation in the 1.07N2C3S system is caused by bubble nucleation owing to super saturation of O2 gas (which was dissolved in the glass phase) at the crystal/liquid interfaces as crystallization proceeds.
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