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Model structure of glass
Fig. 1 Model structure of glass

Changing of volume of a melt when cooling
Fig. 2  Changing of volume of a melt when cooling,
(a) crystallization
(b) glass formation

Glasses  - contrary to crystalline materials - show no regular arrangement of their atoms. For example, the SiO4-tetrahedrons of silicate glasses are irregulary interlaced (cf. Fig. 1). The relatively large gaps can be filled by cations like sodium or potassium.

In order to differentiate glasses from crystalline materials, the changing of volume is described when the respective substances are cooled from melting temperature to ambient temperature (cf. Fig. 2). Here such systems are compared which (a) solidify by crystallization and (b) by formation of glass. One detects that crystalline materials go through a enormous variation in volume with the melting temperature TS, they crystallize (red line). In contrast to crystalline materials glasses condense over a wide temperature range (blue curve) also beyond TS. In the area of the transformation temperature Tg they change from the plastic state to the brittle state when cooled slowly (blue dotted line). Rapid cooling leads to an undercooled liquid (blue line). The changing of viscosity described here is fundamental for the for the processing of glasses and depends only on the chemical composition of the respective glasses.

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© 2004 Büro für angewandte Mineralogie · Dr. Stephan Rudolph · D-47918 Tönisvorst
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