Structural changes during the melting of crystals and glasses
JSGT 1959 V43 T417-T437
The kinetics of melting of a substance can follow several patterns. The melting of quartz and of albite is an example of one extreme. These crystals melt as nearly perfect crystals by gradually changing into viscous liquids. Their melting points can be determined only by static methods because the crystals can be overheated easily and the heating above the melting temperature does not produce drastic changes in their rheology. Sodium chloride is an example of the other extreme. Above the Tammann temperature defects migrate into the crystals which the authors assume will produce fissures and cause the binding forces to disproportionate into stronger and weaker forces. The stronger forces (smaller internuclear distances) lead to ionic clusters which may be free to oscillate and to rotate. These clusters are separated by fluctuating fissures which permit the salt to become very fluid right above the melting point. The tendency of a substance to form a glass is related to the concentration of defects, in particular of vacant anion sites, in the crystal close to the melting point. SiO2 in contrast to TiO2 or SnO2 is not likely to develop anion vacancies even at high temperature. The structures of molten substances determine their nucleation behaviour, their viscosities and their abilities to form fibres and films.
W. A. Weyl & E. Chostner Marboe
