article posted 14 April 2016
Christian Rüssel studied chemistry in Erlangen University and made a PhD in Physical Chemistry in 1983. Then we joint the Fraunhofer Society Freiburg and in 1984 the Institute of Werkstoffwissenschaften (Glass and Ceramics) in Erlangen. He completed his Habilitation in 1991 (Dr.-Ing. Habil.) and is full professor of Glass Chemistry at the Otto-Schott-Institut of Jena University since 1992. Since 2014 he is Dr. h.c. of the University of Chemical Technology and Metallurgy in Sofia, Bulgaria. He is author or co-author of 446 scientific publications (web of science). His main research areas are glass ceramics and glasses for Optics and Photonics.
Percolation, Phase Separation and Crystallisation
In a binary glass melt with statistically arranged structural units, heterogeneities occur which can be described by percolation theory. By contrast,
heterogeneities denoted as liquid-liquid phase separation are thermodynamically more advantageous than heterogeneities caused by statistical arrangement, i.e.
percolation. Depending on the chemical composition, the size of heterogeneities with high mobility, often denoted as “floppy” structures may vary notably up to
completely floppy networks. Nevertheless, the latter heterogeneities might act as precursors for liquid-liquid phase separation. Heterogeneities caused by percolation
and liquid-liquid phase separation might trigger nucleation and subsequent crystal growth.
The growth of once formed amorphous or crystalline heterogeneities, in a non-isochemical system, leads to a change in the chemical composition of the matrix phase.
In a first step, the chemical composition changes near the heterogeneities and hence a shell around a growing core is formed. If this core is less mobile than the
matrix, the growth of the heterogeneities is decelerated and might also be completely stopped. In principle, this is the same for crystalline and amorphous inclusions.
Thin cores may also be formed, if they are thermodynamically more advantageous than statistical arrangements.
Experimental evidences of core-shell formation are given by TEM and ASAXS. The effect of additives which decelerate nucleation and crystal growth and the mechanism of
nucleation inhibition are described.