Abstract
The crystallization kinetics of Na 2O · 3SiO 2 have been determined over the temperature range between 600°C and 800°C. The (metastable) melting point of this material has been determined as 811 ± 3°C. The growth rate vs. temperature relation exhibits the maximum generally found for glass-forming systems, with the maximum growth rate (3.3 x 10 -6 cm sec -1) observed at 735 °C. Under all conditions of growth, the crystal-liquid interfaces are faceted. This is indicative of significant growth rate anisotropy, and suggests that the entropy of fusion is large in Jackson's sense (Δ S fM &62- 4 R, where Δ S fM is the molar entropy of fusion). Assuming Δ S fM = 4 R, by analogy with the characteristics of Na 2O · 2SiO 2, the reduced growth rate versus undercooling relation has been constructed. The form of this relation — a straight line of positive slope passing through the origin — is suggestive of growth by a screw dislocation mechanism. The magnitudes of the growth rates are in close accord with the predictions of the standard model for such growth.
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