Glass and glass-ceramic materials
8. Glass and glass-ceramic materials
Glass is an amorphous material which acquires mechanical properties of solid after cooling of mineral fusion.
The basic products of industry are sheet glass as: unpolished, polished, multi-layered, tempered, etc. Last years an assortment of glass products is considerably extended and mastered a number of new products from glass, particularly colored, reinforced, structural profile and plate. The increase in production of window pane 3...4 mm in thick was caused by the increase of area of the light openings. The various products of the architectural-construction assignment are widely used: glass blocks, structural glass, glass packs. They are applied as effective transluent materials for external and internal enclosing structures in housings, civil and industrial buildings.
The glass carpet tessellated tiles, marbled glass - coloured tiles from opaque glass mass, sheet glass coated by ceramic paints on one side, patterned stained glass are assigned for revetment of buildings.
Glass pipes, glass-fiber waterproofed and heat-insulating materials are widespread.
The production of sheet and pressed glass crystalline material - slag glass-ceramic is developed. The small cost of raw materials and highly-mechanized technology of production of slag glass-ceramic along with high operating qualities promotes this material to become one of the most effective construction materials.
Glass with overcoated on its surface thin transparent metallic or plastic film; sheet glasses, agglutinate transparent synthetic films belong to progressive composition materials. The production of structural profile glass box-like section, stained reinforced glass, glass with selective light penetration for all ranges of waves’ lengths, stained rolled glass and other new perspective construction glass products is advanced.
Enamels and glazes are similar to glass by composition and materials structure, which are sheeted by thin layer on the surface of metallic and ceramic products. They are intended for a giving to the products decorativeness, protection of them against corrosion, improvement of mechanical and dielectric strength.
8.1. Vitreous state. Compositions and properties of glass
Vitreous state. A number of the theories, which explain the vitreous state of substance, is known at present. Crystallite theory and theory of continuous disordered lattice are obtained the greatest acknowledgement.
Before the crystalline theory there were ideas about the glass as about the completely amorphous supercooled liquid. According to the crystalline theory glass has the ordered zones of submicron sizes - crystallites. Crystallites consist of the tetrahedrons [SiO4] and polyhedrons [MeOn], their sizes are within the limits (15...25).10-10 m.
The author of the theory of disordered lattice - American researcher W.H.Zakhariasen advanced it in 1932. He considered the glass- as continuous atomic three-dimensional lattice, deprived of symmetry and of periodicity. This lattice, according to Zachariasen, is the infinitely large cell, in the units of which the atoms or ions are located, not one pair of which is structurally equivalent.
The hypothesis of Zakhariasen was proved to be unable to explain many experimental facts, established late. Least convincing proved to be the assertion of Zachariasen about the chemical uniformity of multi-component glass.
Along with basic theories examined above others were proposed: polymeric, microheterogeneous and others.
Types and compositions of glass. Glass classifies depending on composition and assignment.
Oxide, chalcogenide and halide glasses on composition are distinguished. Silicate (quartz), aluminosilicate, borosilicate, alumino-phosphate and other glasses, which names are determined by the glass-forming oxides are included into the most numerous group of oxide glass.
In the construction silicate and aluminosilicate glass is used frequently united into one group - silicate glass, taking into account that in their composition SiO2 predominates. The most of industrial glass include the silicates. Phosphate glass fusions are used for the production of optical, electric vacuum glass; borate - for the special forms of glass. The mixed borosilicate glass is used for manufacturing the optical and thermally resistant glass products.
The oxides of lithium, potassium, sodium, beryllium, calcium, magnesium, strontium and other elements can be added in the composition of many glasses for regulating the properties.
The chemical composition of construction glass is within the limits (%): SiO2 - 71,5...72,5, Al2O3 - 1,5...2, Na2O - 13...15, СаО - 6,5...9, MgO - 3,8...4,3.
The basic component of glass is silica which introduced into the glass charge with the quartz sand, ground quartzite or sandstone. Alumina is introduced in the form the feldspar, kaolin, the sodium oxide - soda and sodium sulfate, and the potassium oxide - in the form potash and potassium nitrate. The oxide of sodium accelerates the process of glass formation, reducing melting point and facilitating the clarification of glass mass, but increases the coefficient of thermal expansion and decreases the chemical stability. The oxide of potassium decreases the tendency of glass toward the crystallization, gives luster to it and improves light transmission. The oxides of calcium and magnesium are ensured in the glass charge by different varieties of the natural carbonates of calcium and magnesium. These oxides increase the chemical stability of glass, and MgO - decreases the tendency of glass toward the crystallization.
Boric anhydride is introduced into the charge with the borax and boric acid. It increases the melting speed, contributes to the purification of glass, it increases thermal and chemical stability, decreases the tendency to crystallization and reduces thermal expansion coefficient.
Basic requirement for all types of raw material is the absence of impurities and uniformity in the content of basic oxide.
Different auxiliary raw materials are introduced together with the basic into the glass charge: lighters, opaquers, dyes. Lighters contribute to removal of the gas bubbles from the glass mass (chlorides and sodium sulfates, fluorspar). As the opaquers, fluorine and phosphorus compounds and sometimes antimony of tin are used for obtaining the opaque glass. Dyes for the glass are divided into the molecular, which are dissolved in the glass mass, and colloidal, dispersed in the form of colloidal particles. The first include the compounds of cobalt (dark-blue color), chromium (green), manganese (violet), uranium (yellow), iron (brown and blue-green tones), and the second - gold (ruby), silver (golden yellow), selenium (pink) and other
The classification of construction glass according to the designation and the basic types of the glass products, produced by industry, are given in Table 8.1.
Table 8.1
Classification of construction glass and glass products
Form and the designation of the glass products |
Types of the glass products |
Sheet transparent |
Window, plate glass, reinforced figured, heat absorbing, colored |
Structural |
Glass blocks, glass packet, profile the glass-, door leafs |
Facing |
Carpet- mosaic tiles, enameled tiles, glass-crystallite, smalt, glass crumb |
Heat- and soundproof |
Products from the glass staple fiber, mats construction, the foam glass |
Non-woven fiberglass materials |
Fiberglass roofing, moisture-proof, thermal insulating linens, glass paper, filtering materials |
Glass with the special properties is obtained by the regulation of their chemical composition and, using the appropriate methods of treatment in the production process. Thus, figure glass is obtained from the fusion of metal or salts. In this case one side of glass is thermally polished, and deep relief with the intermittent pattern and the alternation of sections with the polished and lusterless surface is formed on another.
For obtaining the uviol glass, which has the ability to transmit not less than 25% ultraviolet rays, the raw materials with the especially high value of the degree of purity (less than 0.03% oxides of iron) are used. A bsorbing heat of the long-wave part of the spectrum of solar rays' glass with the lowered light transmission are made with the introduction the contribution of the oxides of cobalt, nickel and iron.
A number of special glasses are obtained during the putting at the surface of glass of the metal films and their oxides. Film coatings are brought by electrochemical treatment, by chemical precipitation from the solutions, by cathode sputtering, by evaporation in the vacuum. The transparent plastic films belong also to the surface of glass, which change the natural vibration frequency of glass. Such glass is used for the soundproof glazing.
The large group of glass products are based on the fiberglass. There are distinguished continuous and staple fibers. Fiberglass is made from the alkali-free aluminum-borosilicate glass, the alkaline and neutral glass. High-temperature-resisting fibers are made from the vitreous, siliceous and kaolin. Fiberglass differs of high tensile strength, relatively low density, heat- and chemical stability. They are used for preparing of fiberglass fabrics, electro and thermal insulation materials, sand paper, glass-fiber-reinforced plastics. Glass wool, which consists of short glassy fibers and has high thermal insulation properties are obtained by processing of mineral fusions and glass mass.
Properties of glass. The most important properties of glass are optical properties. For ordinary structural glass the refraction coefficient, that is ratio of speed of light transmission in vacuum to speed of light transmission in glass, is 1.52...1.53. Depending on chemical composition, structure of glass, character of its surface, refraction coefficient hesitates from 1.47 up to 2.05. Optical transmission of ordinary window-pane is 83...90%.
Ability of glass of light passing is characterized by the ratio between amount of light energy which passes through the glass and total light energy.
Strength of glass is not equal at the different types of loading - at bending and tensile strength in 7...10 times less than at compression.
Ordinary glass has poor impact resistance, its strength at impact bend is only 0.15...0.20 MPa. The additive of oxides of magnesium, silica, iron increases the impact resistance at 5...20%, adding of boric anhydride - at 50%.
Fragility - main failure of glass, it is predefined by high relation of the modulus of elasticity to the value of tensile strength, and also by absence of plastic deformation of glass before bursting and high-rate of cracks spreading. Fragility of glass diminishes if content of В2О3, SіО2, Аl2О3 is increased and also at heat treatment.
Glass has comparatively low thermal conductivity
[λ= 0,4...0,8 W/(m•K)],
the temperature coefficient of linear expansion hesitates from
to
,
it diminishes at addition into the composition of glass SіО2,
AI2O3,
MgO and rises due to alkaline oxides.
Glass is more proof to the action of the sharp heating, than sharp cooling, because in its superficial layers compression strains appear at heating, and at cooling – tensile strains. The most heat-resistant - quartz glass stands at cooling the overfall of temperatures to 1000°C, low alkaline borosilicate glass – 150...300°C, ordinary structural glass - 80...100 °C.
Glass is characterized by high firmness to the action of acids (except for hydrofluoric and phosphoric), neutral and acid salts. Chemical firmness of glass in 10...20 times goes down under the action of solutions of alkalis, phosphates, phosphoric and especially hydrofluoric acid. Chemical corrosion of glass sharply increases in the case of increase of temperature and pressure.
Ordinary construction glass well skips neutrons and gamma-radiation. Protective properties of glass in relation to gamma- and x-ray radiation, as well as other materials, rise with the increase of density. Heavy-weight glass is obtained by increasing the content of lead, boron and caesium in it. Flow of neutrons loss is provided by the oxides of boron, lithium, cadmium.
A number of the physical-mechanical properties of glass is calculated from the additive formulas, which assume the calculation of shared participation in the synthesis of the properties of separate oxides. Following formula can be used:
(8.1)
where Р - the property of glass; Рі - coefficient of shared participation; mi - content of the i-th oxide in mass %.
The computed values of the indices of properties are averaged; they should be corrected taking into account the special features of technology, method of treatment.
The density of construction glass depending on composition varies in interval of 2.47…2.56, foam glass density of 0.15…0.80 g/cm3.
The theoretical compressive strength of silicate glass is equal to 7000… 12000 MPa, quartz of 1200… 2500 MPa, actual 500… 800 MPa. This significant difference is explained by the presence on the surface of glass and in it volume the scratches and heterogeneities. Micro-scratch and microscopic cracks create in the glass the wedging efforts, which facilitate its destruction. The values of the strength of the glass specimens with a thickness of 5 mm in the dependence on the surface condition are given below:
natural (fiery polishing) - 218 MPa;
scratched by fine-grit emery paper - 131 MPa;
scratched by coarse-grit emery paper - 41 MPa;
slicked and polished - 71 MPa;
polished and non-slicked - 215 MPa;
slicked and polished with the subsequent hardening - 180 MPa.
With the decreasing of size of specimens and diameter of glass fibers the influence of defects decreases and the strength of glass grows. This effect of scale factor follows from the statistical theory of strength, in accordance which with increasing in the sizes of specimens grows the probability of the appearance in them of dangerous defects and microstresses.
Residual stresses in the glass are mainly removed by annealing - the heat treatment of material, i.e. its heating to the specific temperature and slow cooling.
Reduction in the strength of glass is conditioned the characteristic defects, caused by the insufficient uniformity of glass mass, by the disturbances of the technological parameters. Filament like cords in state of tension and located on the surface of products are especially dangerous.
For the glass the influence of "fatigue", caused by the lasting effect of loads is characteristic. Safe constant load for the glass taking into account fatigue is considerably less calculated.
To the development of microcracks surface-active media and, first of all, water contribute. Effect of the environment grows with an increasing in the duration of contact, temperature, value of the accompanying stress. Established, for example, if the strength of glass is in air 5.3 MPa, then in the water with 20°С it is reduced to 4.2 MPa, and with an increase of the temperature of water to 80°С to 3.9 MPa.
There are a number of methods of improvement of the mechanical and other properties of the glass: hardening in the air flow and in the special liquid media, etching in the hydrofluoric acid, ion exchange, crystallization of surface layer, reinforcement, coating glass with films and others. At the hardening the resistance of glass to bending rises in 4…5, etching and the coating with films - 5…10 times.
Hardening glass consists in its heating to 700… 900С and subsequent sharp, but uniform cooling. They for the first time revealed the phenomenon of hardening glass in 17th century, but for strengthening the glass products in industrial scales began to use in the 30th years of 20th century. In the hardening operation on the surface of glass the evenly distributed compressive stresses, which increase its ultimate strength under the action of the external bending or impact loads, appear. The surface of glass is cooled by air or by some organic, for example, silicon liquids. Liquids with the high boiling point, which especially considerably increase strength as a result of the formation, on the surface of glass of durable films are used. The comparative characteristic of the basic properties of initial (burned) and hardened sheet glass is given in Table 8.2.
In the magnitude of the strengthened effect to hardening the method of the etching of the surface of glass by the solution of hydrofluoric acid in is dissolved the weakened surface layer not yield. The method of the three-stage treatment of glass - hardening in the liquid at the conditions of the ultrasonic field, etching and then application of protective coatings is also effective.
Table 8.2
Comparative characteristic of the basic properties of the initial (annealed)
and hardened sheet glass
Property |
Glass |
|
Annealed |
Hardened |
|
Impact strength |
The impact of sphere with a mass of 800 g withstands from the height 150 mm |
The impact of sphere with a mass of 800 g withstands from the height more that 1200 mm |
Bending strength, MPa |
To 50 |
To 250 |
Elasticity |
- |
Bending deflection is 4…5 times more than in annealed |
Heat resistance, С |
60...70 |
To 175 |
Electrical conductivity |
- |
It is 2…3 times more than in annealed |
Density, g/cm3 |
2.5 |
It is reduces insignificantly |
Coefficient of the linear expansion, 10-6/K |
8.8…9.5 |
It is increases insignificantly |
At the method of ion exchange in the surface layer of glass are created the stresses of compression as a result of the diffusion substitution of the ions Na+ by the alkaline ions, which pass from the salt fusion.
Triplex method consists in the production of the three-layered sheet glass, which consists of two external sheets of glass, firmly glued between itself by the intermediate inner layer, which consists of the transparent filler plate of elastic organic material. The basic merit of triplex consists in its nonshatterability. The group of shatterproof glass includes also the hardened and reinforced by wire mesh glass.
The thermal resistance of glass characterizes its ability to maintain sharp temperatures without the destruction. It is measured by the temperature, to which it is possible to cool suddenly glass specimen without its destruction.
With the chilling of glass as a result of the unequal rate of cooling in the surface layers the stresses of tension appear, in the internal - compression, while with the heating - vice versa. Considering that the destruction of glass starts from the surface and compressive strength is much more than tensile strength, the chilling of glass products is more dangerous than sharp heating.
The thermal resistance of different glass is within the limits 90...1000°С. For example, it is not above for the sheet window glass 90°, chemical-laboratory - 120...140°С, and quartz - 800...1000°С.