TABLE OF CONTENTS Wolfgang Pannhorst
Glass ceramics are the result of two independent lines of research activities in the USA in the 1940s and 1950s which, when combined, opened up the view of a family of materials with a high potential for new applications. One route of research was performed at Corning Glass Works by Stookey who investigated the nucleation of glasses. While for a long time his research centered around photonucleation of opal and colored glasses with crystalline phase contents of less than 5%, he one day found accidentally that some of these photonucleated glasses can be transformed by an annealing process to highly crystalline materials with a very fine microstructure, i.e., with crystal sizes in the range of microns. In a further research effort he found that similar results may be obtained by using special additives, so-called nucleating agents, instead of the photonucleation process. His fundamental patent [1.1] discloses that TiO 2 acts as such a nucleating agent in a rather large number of glass systems.
The other route started with the discovery by Hummel in 1951 [1.2] that crystalline aggregates of ?-eucryptite (Li 2O-Al 2O 3-2SiO 2) display a negative volume expansion. People immediately realized that this observation opens up the perspective of developing materials without any expansion in some temperature intervals, thus creating thermoshock resistant or dimensionally highly stable materials. As a consequence, an intensive research activity started to find out whether this observation...
