温度・粘度
Glass Transition
Japanese: ガラス転移
The temperature range (Tg) at which the material transitions from liquid-like behavior to solid-like behavior. It is located near the slow-cooling point and is central to the annealing process. It is the transition point at which glass appears to be a “solid” while still maintaining a liquid state.
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The glass transition refers to the temperature range during the cooling process in which glass transitions from liquid-like viscous flow behavior to solid-like elastic and brittle behavior; the central temperature of this range is called the glass transition temperature (Tg) (treatise §0). Tg coincides almost exactly with the “annealing point (log η = 13)” at the viscosity fixed point and is approximately 482°C (as measured by ASTM) for the Bullseye COE90 series. At temperatures above Tg, glass can relieve stress as a liquid, while at temperatures below Tg, it retains stress as a solid. This is why holding the glass near Tg during annealing is important.
The glass transition is evidence that glass is a “supercooled liquid”; it does not appear as a sharp transition (first-order transition) like the melting point of a crystalline solid, but rather as a gradual transition region spanning several tens of degrees (second-order transition behavior). The faster the cooling rate, the more the Tg shifts toward higher temperatures; the slower the cooling rate, the more it shifts toward lower temperatures. For this reason, it is important to note that “the slow-cooling point may vary slightly depending on the measurement method”; therefore, when comparing Tg values from different brands, it is crucial to ensure that the measurement standards (such as JIS R 3103-2 or ASTM C336) are consistent. Differences in Tg cause stress to develop when dissimilar glasses are mixed (the core of the dissimilar glass problem).
Related Concepts
過冷却液体（supercooled-liquid）・粘度定点（viscosity-fixed-points）・annealing域（annealing-zone）・Adams-Williamson則（adams-williamson-rule）・Narayanaswamy構造緩和モデル（narayanaswamy-model）・COE（coe）
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.