The structure of glass
The physicochemical properties of glass are not only determined by its chemical composition, but also closely related to its structure. Only by understanding the internal relationship among the structure, composition, structure and performance of glass, can it be possible to make glass materials or products with predetermined physicochemical properties by changing the chemical composition, thermal history or using some physical and chemical treatment methods.
Characteristics of glass
Glass is a branch of amorphous solid, which is an amorphous material with solid mechanical properties. It is often called “supercooled liquid”. In nature, there are two states of solid matter: good state and non good state. The so-called nonproductive state is the state of solid matter obtained by different methods and characterized by structural disorder. Glassy state is a kind of non-standard solid. The atoms in glass do not have long-range ordered arrangement in space like crystal, but they are similar to liquid and have short-range ordered arrangement. Glass can maintain a certain shape like a solid, but not like a liquid flowing under its own weight. Glassy substances have the following main characteristics.
(1) The arrangement of particles of isotropic glassy material is irregular and statistically uniform. Therefore, when there is no internal stress in the glass, its physical and chemical properties (such as hardness, elastic modulus, thermal expansion coefficient, thermal conductivity, refractive index, conductivity, etc.) are the same in all directions. However, when there is stress in the glass, the structural uniformity will be destroyed, and the glass will show anisotropy, such as obvious optical path difference.
(2) Metastability
The reason why the glass is in metastable state is that the glass is obtained by rapid cooling of the melt. Due to the sharp increase of viscosity during the cooling process, the particles have no time to form regular arrangement of crystals, and the internal energy of the system is not at the lowest value, but in metastable state; However, although the glass is in a higher energy state, it can not spontaneously transform into the product due to its high viscosity at room temperature; Only under certain external conditions, that is to say, we must overcome the potential barrier of material from glassy state to crystalline state, can the glass be separated. Therefore, from the point of view of thermodynamics, the glass state is unstable, but from the point of view of kinetics, it is stable. Although it has the tendency of self releasing heat transforming into crystal with low internal energy, the probability of transforming into crystal state is very small at room temperature, so the glass is in metastable state.
(3) No fixed melting point
The transformation of glassy substance from solid to liquid is carried out in a certain temperature range (transformation temperature range), which is different from crystalline substance and has no fixed melting point. When a substance is transformed from melt to solid, if it is a crystallization process, new phases will be formed in the system, and the crystallization temperature, properties and many other aspects will change abruptly
As the temperature decreases, the viscosity of the melt increases, and finally the solid glass is formed. The solidification process is completed in a wide temperature range, and no new crystals are formed. The temperature range of transition from melt to solid glass depends on the chemical composition of glass, which generally fluctuates in tens to hundreds of degrees, so glass has no fixed melting point, but only a softening temperature range. In this range, glass gradually transforms from viscoplastic to viscoelastic. The gradual change process of this property is the basis of glass with good processability.
(4) Continuity and reversibility of property change
The property change process of glassy material from melting state to solid state is continuous and reversible, in which there is a section of temperature region which is plastic, called “transformation” or “abnormal” region, in which the properties have special changes.
In the case of crystallization, the properties change as shown in the curve ABCD, t. It is the melting point of the material. When the glass is formed by supercooling, the process changes as shown in the abkfe curve. T is the glass transition temperature, t is the softening temperature of the glass. For oxide glass, the viscosity corresponding to these two values is about 101pa · s and 1005p · s.
Structure theory of broken glass
“Glass structure” refers to the geometric configuration of ions or atoms in space and the structure formers they form in glass. The research on glass structure has materialized the painstaking efforts and wisdom of many glass scientists. The first attempt to explain the essence of glass is g. tamman’s supercooled liquid hypothesis, which holds that glass is supercooled liquid, The process of glass solidifying from melt to solid is only a physical process, that is, with the decrease of temperature, the molecules of glass gradually approach due to the decrease of kinetic energy, and the interaction force gradually increases, which makes the degree of glass increase, and finally form a dense and irregular solid substance. Many people have done a lot of work. The most influential hypotheses of modern glass structure are: product theory, random network theory, gel theory, five angle symmetry theory, polymer theory and so on. Among them, the best interpretation of glass is the theory of product and random network.
Crystal theory
Randell l put forward the crystal theory of glass structure in 1930, because the radiation pattern of some glasses is similar to that of the crystals of the same composition. He thought that glass is composed of microcrystalline and amorphous material. The microproduct has regular atomic arrangement and obvious boundary with amorphous material. The microproduct size is 1.0 ~ 1.5nm, and its content accounts for more than 80%. The orientation of microcrystalline is disordered. In studying the annealing of silicate optical glass, Lebedev found that there was a sudden change in the curve of glass refractive index with temperature at 520 ℃. He explained this phenomenon as the homogeneous change of quartz “microcrystalline” in glass at 520 ℃. Lebedev believed that glass is composed of numerous “crystals”, which are different from microcrystalline, The transition from “crystal” to amorphous region is completed step by step, and there is no obvious boundary between them.
Post time: May-31-2021