Principles of Glass Formation

The earliest glasses used by man were found in nature. The ease of formation of sharp edges on obsidians, for example, allowed the production of knives, arrow heads, and other cutting tools. These naturally occurring glasses, which result from the cooling of molten rock, or lava, contain a wide variety of components, including alkali, alkaline earths, and transition metal oxides. In every case, however, silica is found to be the major constituent of these materials. 

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The tribochemical reactions based on the hard and soft acids and bases (HSAB) principle linked to the friction coefficient (p) are summarized above. The effect of tribological processes consists of the formation of mixed short-chain phosphate glasses containing sulfide precipitates. 

We give a brief description of the general principles of the sol-gel technique, with special emphasis on fiber making via this technique. Essentially the sol-gel route of making any glass or ceramic involves the formation of the appropriate glass or ceramic structure by chemical polymerization of suitable compounds in... 

Glass formation or the absence of crystallite nucleation and growth is also dependent on the confusion principle . This is an empirical observation which suggests that, in a multicomponent melt, several crystalline forms are in competition in the crystallization process the confused situation that results acts as a barrier to microcrystal growth and enhances the formation of the vitreous state. 

Evidently in such multi-component glasses of simple halides, significant lowering of the eutectic temperatures and of course, the confusion principle, together assist formation of glasses. 

Glass formation by mechanical alloying of elemental crystalline powders can be considered a special form of solid-state interdiifusion reaction. The basic principles of such a reaction [3.15] are described in Fig. 3.4. As is well known, the thermodynamic stable state of a system is determined by a minimum in the free enthalpy G. In metallic systems, the free enthalpy of the equilibrium crystalline state Gx is always lower than that of the amorphous state Ga below the melting temperature. The amorphous state is a metastable state, i.e., an energy barrier prevents the amorphous phase from spontaneous crystallization. To form an amorphous metal by a solid-state reaction, it is necessary to establish first a crystalline initial state with a high free enthalpy G0 (Fig. 3.4). Depending on the formation process, this initial state can be achieved, for example, by... 

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