Unit 12 Structure of Glass
The Structure of Glass
In spite of the world wide development of glass technology we still have found no definite answer to the question: what is glass? The following three different definitions are given to demonstrate how to describe the state of glass:
Glass is an inorganic product of a melt which solidifies without crystallizing;
Glass is an undercooled liquid with a very high viscosity;
Glass is an intermediate substance between the liquid and crystalline states. It consists of a crystal like network combined with each other randomly.
An easy idea of the structure of glass was based on the concept of the term supercooled liquid. Molten glass was regarded as a solution of one oxide within another oxide, or rather a mutual solution of constituents within each other. The molten liquid is cooled without crystallization to form form a rigid body at room temperature.
The Network Theory
In recent years, the theory of the atomic structure of glass advanced by Zachariasen has become widely accepted. His random network theory assumes unit building blocks placed together in a random network manner. Our understanding of the constitution and the behavior of glasses is based on the descriptions of the structures of oxide glasses and on the conditions of glass formation as interpreted by a few pioneers in glass research. Tammann began to explore systematically the phenomena of the glassy state stating that the rate of cooling is most important for the understanding of the glassy state. Glasses are rigid solids and not liquids. In 1926, Goldschmidt presented his ideas on modern crystal chemistry.
A glass may resemble the crystal of the same composition with respect to the short range order, but its typical glass properties are the result of the lack of long range order. The ability of a molten substance to solidify on cooling without crystallization was explained on the basis of the geometry of the polyhedra, in particular of the ratio of the sizes of anions and cations. Thus, Goldschmidt introduced crystal chemical concepts into the field of glass technology. A decisive step in the development of glass structures was made by Zachariasen in 1932 who presented a picture of the atomic structure of witreous silica in which the silicon atoms are surrounded by four oxygens in the same way as in the different structures of crystalline silica and silicates. The only difference between glass and crystal was the absence of periodicity. The continuous, random, three dimensional structure shows the same short range order as the crystalline modifications of silica but without their long range order.
The introduction of alkali oxides into vitreous silica loosens its structure by decreasing the number of corners which each tetrahedron has to share with others. This feature could account for the lower viscosity of an alkali silicate glass as compared to pure silica. The classification of the constituents of a glass into network forming oxides and those which modify the network was simple and useful. The "intermediates" were introduced years later. There was a nearly universal acceptance of Zachariasen's rules concerning glass formation. Warren and his school using X-ray techniques for determining the atomic structures of glasses agreed with Zachariasen's picture. The application of X-rays made by for the most important contribution to the present concept of the constitution of glass.
The network formers belong to a group of elements having an energy of the element oxygen bond of 340 to 500 kJ/mol. The network modifiers, for example Mg, Ca, Li, K, Na, Cd, and Cs have an ionic oxygen bond with a bonding energy of 40 to 250 KJ/mol. Their rates of diffusion are higher especially for the Me+ ions. They break the continuous Si-O-Si network decreasing the viscosity of the melt at all temperatures. Me[2+] ions act as a bridge between two oxygen atoms. They also break the network structure, but are less movable, influencing the flow of the melt less than Me+ ions.
The main difference between a crystal and a glass is the kind of linkage of the tetrahedra. The crystalline structure repeats itself continuously in three dimensions, while in silica glass the tetrehedra are linked in a random network without an orderly repetition. In this network the SiO2 units are called glassformers. Such a glassformer determines the overall structure of the network. Commercial soda silica and soda lime silica glasses contain other materials added to the silica glass. These materials are called network modifiers, because they open the network up and introduce into it a number of oxygens larger than can be contained in the structure where all oxygens are bridges between two silicons. This produces some non bridging oxygens which are connected to only one silicon and cause a breaking of the structure.
Selected from "Process Mineralogy of Ceramic Materials", W. Baumgart, A. C. Dumham, G. C. Amstutz, Heidelberg and Hull, May 1984
Words and Expressions
solidify 固化
undercooled 过冷的
supercooled 超冷的
rigid body 刚体
short range order 短程序
long range order 长程序
be surrounded by 被…包围
periodicity 周期律
account for 由于…原因
net work former 网络形成体
net work modifier 网络调整体
