Structures of Glasses

Technically, a glass is a material that can be cooled from the melt without crystallizing. Since it does not crystallize, there is no distinct melting point. Instead, the viscosity 

Icosahedral duster of boron atoms (T-50). These dusters occupy five symmetry sites in the primitive cell of the Fi i/nnm space group. 

The 8-atom molecular rings of sulfur (A-16). The space group is Fddd. 

In 1932, Zachariasen proposed a contmuous random network model that predicted that oxide glasses of the form A 0 could exist with cordigurational energies that were only slightly higher than the crystalline state if the following conditions were met  

Number of O atoms surroimding an A-atom must be no more than three in a triangle or four m a tetrahedron. 

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F. L. Galeener, in R. A. Weeks and D. L. Kinser, eds.. Effects of Modes of Formation on the Structure of Glasses, Trans Tech PubHcations Ltd., Brookfield,... 

Fig. 5.28 Structure of glass-like carbon. (According to G. M. Jenkins and K. 

The first theory of the structure of glass to become widely accepted was that of Zachariasen (1932), called the random network theory [now commonly referred to as the continuous random network (CRN) theory]. This arose... 

The phase structure of glasses has a significant effect on their physical properties, which is discussed below with reference to chemical durability. The magnitude of the phase separation can be altered by heat treatment, and enhanced or reduced by the addition of various oxides to the melt. In particular, the addition of alumina to commercial soda-lime-silica glasses reduces the tendency to phase separation, improving chemical resistance (Doremus, 1973). A detailed study of the microstructure of soda-lime-silica glasses has been published by Burnett and Douglas (1970). The control of phase separation in the melt is now commercially important for processes such as the... 

Porai-Koshits, E.A. (1977). The structure of glass. Journal of Non-Crystalline Solids 25 87-128. 

Stevels, J.M. (1960-1961). New light on the structure of glass. Philips Technical Review 22 300-311. 

Fig. 8.4 Device structures of Glass/TCO/p-i-n/contacts and glass/contacts/ n-i-p/TCO a-Si H solar cells.

Newtonian Viscosity in Glasses. As we saw in Chapter 1, the structure of glasses is fundamentally different from metals. Unlike metals and alloys, which can be modeled as hard spheres, the structural unit in most oxide glasses is a polyhedron, often a tetrahedron. As a result, the response of a structural unit to a shear force is necessarily different in molten glasses than in molten metals. The response is also generally more complicated, such that theoretical descriptions of viscosity must give way completely to empirical expressions. Let us briefly explore how this is so. 

Figure 15-11 Schematic structure of glass, which consists of an irregular network of Si04 tetrahedra connected through oxygen atoms. = O, = Si, = cation. Cations such as Li+, Na+, K+. and Ca2+ are coordinated to the oxygen atoms. The silicate network is not planar. This diagram is a projection of each tetrahedron onto the plane of the page. [Adapted tram G. A Perley, Glasses for Measurement of pH," Anal. Chem. 1949,21,394.]...

Ainsworth L., 1954, The diamond pyramid hardness of glass in relation to the strength and structure of glass, Jour. Soc. Glass Techrt., 38, 479-500. 

Bach. H. and D. Krause Amdvsis of the Camptrsiiion and Structure of Glass and Gloss Cerattues. Springer-Verlug Inc.. New York. NY. 1999. 

Figure 4.5. Atomic Structure of Glass vs. Crystalline-Solid Atomic Structure.

Do both glass globules have the same shape If both glass globules do not have the same shape, what does this mean about the atomic structure of glass ... 

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