文档介绍：53
5
Glass Structure
Introduction
Glass structure is out of equilibrium. It is obtained when a liquid is cooled down
in such a way that on passing the melting temperature or liquidus temperature
(TL), ‘freezing’ happens instead of crystallization (Tammann, 1925). The final
temperature must be so low that molecules or atoms move too slowly to rearrange
to the more stable crystalline form. In Chapter 6 we shall discuss extensively the
atomic mobility in terms of viscosity. In particular, we shall see that glass structure
may depend upon thermal history. For instance, a liquid will yield denser glasses
when the cooling rate decreases. This chapter shows the prominent characteristics of
glass disordered structure. Alternatively, glass can be built up onto a substrate, either
by deposition or chemical reaction from the vapour, the substrate being cold enough
that rearrangement cannot take place. In fact, although they are named differently,
amorphous silica (a-SiO2) films and silica glass bulk are the same materials having
the same position and a work. Ryu and Tomozawa (2006)
suggest that the concept of fictive temperature can be used to describe a-SiO2 film
structure as well (see also Chapter 6). However, amorphous layers have been achieved
with a limited number of substances pared to the usual route (Doremus,
1994). Many definitions of glass have been proposed: we shall use the most
commonly used one that defines it as an amorphous solid showing a glass transition
(Zarzycki, 1982).
The glass is frozen in a structure showing only short-range order that makes its
characterization quite difficult pared to crystalline structure that presents
long-range order (periodical structure with the repetition of a unit cell; see also
Table ). As discussed below, diffraction experiments on glasses yield results that
most closely resemble those of liquids (see also Chapter 2).
Glass; Mechanics and Technology. Eric Le Bourhis
Copyright ß 2007 WILEY-VCH Ve