Random network theory, glass

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... 

Glass-forming systems other than silica have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the structural unit of vitreous B203 (22,29). The intermediate-size boroxyl ring represents a compromise between the crystallite and the random-network theory (29) (see Analytical methods). 

In the random network theory of glass d, the atoms form a three-dimensional connected structure without periodic order and with energy content comparable to that of the corresponding crystalline material. The coordination number of an atom determines its role in a glass structure, and the fulfillment of four rules determines whether an oxide is to be a glass former ... 

In this sense, glass can be viewed as an assembly of subunits [10] which is not in opposition with the random network theory. 

A number of other statements by Zachariasen have become the basis for the models for glass structures termed the Random Network Theory. These ideas will be discussed later under the topic of glass structure. It is interesting to note, however, that the term random network does not occur in the original work of Zachariasen, who referred to the glass structure as a vitreous network . Furthermore, Zachariasen specifically states that the vitreous network is not entirely random due to the restriction of a minimum value for the internuclear distances. As a result, all internuclear distances are not equally probable, and X-ray patterns of the type observed for glasses are a natural consequence of the vitreous network. 

Zachariasen could successfully explain why certain CNs are favoured for glass formation. Zachariasen random network theory formed the basis of glass formation. During his studies, he observed that the silicate crystals have a tetrahedral network and readily form glass. These tetrahedral networks are symmetrical, non-periodic, and connected with each other at the comers. This leads to the 3-D network extended over aU directions with the isotropic property attributed to the amorphous nature of glasses. He elucidated the following points during his studies. 

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