1. Introduction: Some Essential Attributes of Glassiness Regarding the Nature of Non-crystalline Solids

Glass products have long been used from ancient times not only in our daily life but also in some laboratory experiments such as the U tube for the measurement of volume of a gas as a function of pressure. Liquefaction of the last “permanent gas helium” was done successfully with an apparatus entirely made of glass. Formerly the glasses have been produced by cooling the melts of silicate minerals without crystallization until they becomes hard and brittle solids. Later the glasses were found to exhibit hallo diffraction patterns similar to those of the liquids. Some important concepts are involved in this description. The first is the method of preparation. The melt-cooling was used in some of the modern definitions of glasses. The second is the starting materials of inorganic origin. Organic substances such as glycerol and synthetic polymers were found to behave similarly. Thus the term glasses can be extended to a wide range of substances that easily undercool to form amorphous solids. The third is the metastability of the undercooled liquids and glasses compared to the corresponding crystalline solids. If the cooling rate is adequately slow to induce nucleation, the melt becomes crystalline solid possessing regular lattice with lower Gibbs energy. Thus the formation of glass is a problem of bypassing or avoiding the crystallization. Although the main subjects of this book are the structures and properties of ordinary network glasses of inorganic origin, it will be instructive to start with the description of the general features of glassiness exhibited by various kinds of condensed matters in which the constituents are held together by interaction forces, such as the van der Waals, hydrogen bonding, ionic or covalent bonds.