When a liquid phase is cooled to below its freezing temperature, it usually transforms
into a crystalline solid, i.e. it crystallizes. Some liquids, because of complex molecular
configuration or slow molecular transport, do not “crystallize” (assume an ordered
configuration) upon being cooled to low temperatures, but instead form a rigid
disordered network, known as glass, which is very similar in structure to that of the
liquid. Most metals and alkali halides crystallize easily upon cooling through the
freezing temperature because the structural rearrangement from the liquid to the
crystalline state is simple and bonding is nondirectional. At temperatures just above the
freezing temperature, most metals and molten salts have fluidities approximately
equivalent to that of water at room temperature. Thus, the required structural
rearrangements can take place relatively easily.
In contrast, many inorganic silicates form glasses upon cooling because the fluidity at
and even above the freezing temperature is very low. This glass formation is related to
the high silicon-oxygen single-bond energies and the directional bonding requirements
imposed by sp3 hybridization of silicon. The disordered liquid cannot flow easily and
thus cannot undergo the rearrangements required for crystallization. Moreover, the
LN–7
2
fluidity decreases very rapidly as the temperature is lowered. [For large organic and
polymeric liquids, crystallization is difficult because of their chain lengths. In polymer
systems the intermolecular bonding (Van der Waals) is weak and expected to permit
individual molecules to readily slide past one another. Thus, it is the “difficult” structural
rearrangement required for crystallization that induces glass formation in these
systems.]
The
When a liquid phase is cooled to below its freezing temperature, it usually transforms
into a crystalline solid, i.e. it crystallizes. Some liquids, because of complex molecular
configuration or slow molecular transport, do not “crystallize” (assume an ordered
configuration) upon being cooled to low temperatures, but instead form a rigid
disordered network, known as glass, which is very similar in structure to that of the
liquid. Most metals and alkali halides crystallize easily upon cooling through the
freezing temperature because the structural rearrangement from the liquid to the
crystalline state is simple and bonding is nondirectional. At temperatures just above the
freezing temperature, most metals and molten salts have fluidities approximately
equivalent to that of water at room temperature. Thus, the required structural
rearrangements can take place relatively easily.
In contrast, many inorganic silicates form glasses upon cooling because the fluidity at
and even above the freezing temperature is very low. This glass formation is related to
the high silicon-oxygen single-bond energies and the directional bonding requirements
imposed by sp3 hybridization of silicon. The disordered liquid cannot flow easily and
thus cannot undergo the rearrangements required for crystallization. Moreover, the
LN–7
2
fluidity decreases very rapidly as the temperature is lowered. [For large organic and
polymeric liquids, crystallization is difficult because of their chain lengths. In polymer
systems the intermolecular bonding (Van der Waals) is weak and expected to permit
individual molecules to readily slide past one another. Thus, it is the “difficult” structural
rearrangement required for crystallization that induces glass formation in these
systems.]
The
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