Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 48

RANDOM NETWORK

RANDOM COIL

RANDOM CLOSE‐PACKING

The atomic structure of glasses can be very complex. Three overarching structural models have arisen to describe most ideal amorphous materials.

1 The continuous random‐network model: This model describes a three‐dimensional network where an irregular network is formed through bridging atoms, but the bonding pattern at network sites is repeated. It is most applicable to covalently bonded glasses, such as oxide‐based glasses (Zachariasen 1932). In oxide‐based glasses, the oxygen atoms form bridges and network formers, such as silicon, phosphorus, or germanium, form strong bonds with them in a randomly arranged network structure. In addition, network modifiers, such as sodium and calcium, generally sit in ionic form within interstitial holes and can drastically alter the properties of the bulk material. The continuous random‐network model describes an ideal – fully amorphous – glass. In Section 2.4, we explore the structure of real glasses in more detail.

2 The random‐coil model: This model describes the disorder created by entangled chains through the use of three‐dimensional random walks (Flory 1949). In fact, the model presents a statistical distribution of conformations taken by all the chains in a population of macromolecules and is further complicated by chain length distributions, branching, and constrained rotation in real material. It is most applicable to glassy polymers, such as polystyrene and many plastics.

3 The random close‐packing model: This model describes an irregular structure of molecules that does not contain any short‐ or long‐range order or holes large enough to admit another molecule. It is based upon crystallographic models for liquid structure (Bernal 1959) and is most applicable to metallic glasses.

An exhaustive review of the knowledge gained on the structure of inorganic oxide and nonoxide glasses and the liquids they are derived from was carried out by Greaves and Sen (2007). They looked at the atomic structure, ranging from the local environments of individual atoms to the long‐range order, which can cover many interatomic distances. In this book, we will concentrate only on inorganic amorphous materials and, in particular, on silicate glass that is used extensively in domestic and industrial buildings, automobiles, and for household purposes. Even though silica‐based glasses have satisfied demands for a remarkable range of consumer goods for a long time throughout human history, the mechanism of glass cracking has not been well understood and is keeping glass scientists active (Michalske and Bunker 1987).