Unit 17 Glass Ceramics
Introduction
The emergence of synthetic ceramics as a prominent class of materials with a unique combination of properties has been an important part of the materials science scene over the past 20 years.
These high technology ceramics have varied applications in areas utilizing their exceptional mechanical, thermal, optical, magnetic or electronic properties. A notable development of the 1970s was that of Si based ceramics as high temperature engineering solids. More recently the zirconia based ceramis have evolved as a class of material with significant improvements in fracture toughness. In the 1980s we are on the threshold of development of ceramic matrix composites with the promise of overcoming major limitations in engineering design with brittle ceramics and the development of novel properties unattainable with monolithic microstructures. Throughout this period there have been significant but less well publicized developments in the field of glass ceramics and glasses. It is the purpose of this publication to review selected topics within this important area of materials science.
Devitrification
Glass ceramics are derived from the controlled crystallization of glasses to give a material consisting of one or two crystal phases plus some residual glass depending on the starting composition and the heat treatment given.
During the crystallization process, molecular rearrangements occur to produce the appropriate crystalline structures. These structures are often themselves metastable polymorphs which under further heat treatment can transform to the thermodynamically stable crystal phases. These molecular rearrangements present a changing environment for the various nuclei in the glass and MAS NMR provides a technique for observing these various environments and following the crystallization process.
Volume Nucleation in Silicate Glass
The last twentyfive years or so have seen steady advances in the science and technology of glass ceramics, materials prepared by the controlled crystallization of glass. Peter McMillan made many outstanding contributions to these advances and the publication of the first edition of his new classic textbook in 1964 remains a landmark in the development of the field.
The preparation of a glass ceramic involves several stages. First, a glass is melted and formed into the appropriate shape. The glass article is then given a heat treatment schedule to nucleate and grow crystals in its volume until a material with the desired degree of crystallinity is produced. The kinetics of crystal nucleation and growth are thus critical in determining those compositions which can be cored into glasses reasonable stable towards devitrification, and which subsequently can be economically converted into fine grained glass ceramics by suitable heat treatment.
This chapter is concerned with crystal nucleation in glass, although no attempt is made to present a comprehensive discussion of the whole subject. Rather, certain topics, which are believed to be of particular interest, are highlighted. Throughout, the emphasis is on studies of simple silicate systems involving quantitative measurements of volume nucleation kinetics. However, such studies are considered helpful in identifying the various factors influencing nucleation behaviour in glasses in general. Moreover, these factors also apply to the more complex compositons used in glass ceramic manufacture.
After a general outline of the relevant theories, experimental studies of volume nucleation in various systems, in which the crystallizing phase has the same composition as the parent glass, are discussed. Both steady state and nonsteady state nucleation are considered. The results form a remarkably consistent pattern and indicate that the nucleation in these systems is predominantly homogeneous. The more complex case when the crystallizing phase has a different composition from that of the parent glass is then discussed. Studies of heterogeneous nucleation on metallic particles and the roles of nonmetallic nucleation agents are described. Finally, recent investigations of the effects of amorphous phase separation on crystal nucleation kinetics are reviewed.
Selected from "Glasses and Glass Ceramics", M. H. Lewis, Chapman and Hall, 1988
Words and Expressions
unattainable 达不到的
devitrification 析晶
metastable 亚稳的
polymorph 同质多形体
landmark 里程碑
