Glass is one of the oldest and most versatile materials known to man.
Man has known glass for a very long time. Faience decorations found by archaeologists dating back to the period of the first dynasty of the pharaohs indicate that glass was known in Egypt as early as 5 thousand years ago. A glass cylinder seal discovered during excavations in Mesopotamia dates back to the period of the Akkadian dynasty, that is, it is over 4,000 years old. Glassware found in Japan and India was made about 2,000 years ago. But scientists do not have a common opinion about the time and place of the appearance of glass.
How did glass come about?
One of the legends says that the Phoenician merchants cooked food on the sandy shore while they were staying. They built the hearth not from stones, but from their pieces of African soda. Straw served as fuel. Waking up in the morning, they found a glass ingot on the ashes.
Russian craftsmen knew the secrets of glass production more than a thousand years ago. In those days, alkali, sand and lime were the raw materials for glass production. Plant ash or soda was used as an alkali.
Chemical composition of glass
Glasses are natural and artificial. Natural glass can be formed, for example, during a volcanic eruption or when lightning strikes deposits of quartz sand. But in nature there are so few opportunities for the formation of natural glass that mankind has long since learned to obtain artificial glass for its needs.
Glass- an amorphous body obtained by supercooling the melt, which consists of various oxides.
Depending on which oxide is the main component, silicate glasses (SiO2), borate (B203), phosphate (P205) and combined (borosilicate, etc.) glasses are distinguished.
silicate glass
The most common is silicate glass. Its main component– silicon dioxide (SiO2). 70-75% glass consists of it. Silicon dioxide is obtained from quartz sand. Calcium oxide (CaO) is the second component of glass, giving it chemical resistance and brilliance. In ancient times, sea shells or tree ash served as a source of calcium oxide, since people were not familiar with limestone. In addition to these two components, glass contains sodium oxide (Na2O) and potassium oxide (K2O), which are necessary for glass melting. The sources of oxides are soda (Na2CO3) and potash (K2CO3). If the glass consists only of high purity silica, it is called quartz.
Physical properties of glass
According to the physical properties of the glass are divided into ordinary, heat-resistant and colored.
Ordinary glasses
Three groups of ordinary glasses are known: lime-sodium, lime-potassium and lime-sodium-potassium.
Lime-sodium, or soda, glass is used for the production of window glass, dishes.
High temperature resistance lime-potassium, or potash, glass allows its use in the production of equipment and high-quality dishes.
Lime-sodium-potassium glass has a high chemical resistance. Most often used in the manufacture of dishes.
Fragility is the main disadvantage of conventional glass. To expand the scope of ordinary glass, it is tempered and tempered glass is obtained, which is called stalinite. Ordinary glass is also used to create triplex - laminated glass.
Heat-resistant glasses
Heat-resistant glasses are called refractory, heat-resistant. They are used in products that are operated in special conditions. Heat-resistant glasses include borosilicate glass, laboratory glass and ceramic glass.
The high corrosion resistance of borosilicate glass and its heat resistance make it possible to use this glass for creating special installations in chemical engineering. This glass also makes excellent heat-resistant cookware. The same high-quality tableware can be made from laboratory glass. And sitalls are successfully used in mechanical engineering.
Colored glasses
After hardening, the glass mass has a bluish-green or yellowish-green tint. But if various metal oxides are introduced into the mixture, which change its structure during glass melting, then after cooling the glass will be able to highlight certain colors from the light spectrum passing through it.
Such glasses are used for the manufacture of art products, stained-glass windows, dishes.
Glass combines two elements: fire and ice. Fire helps the glass to come into being. Glass becomes like ice when it solidifies in the form of a product.
It is impossible for modern people to imagine their life without glass. It surrounds us everywhere: at home, in transport, at work and on vacation. It is impossible to name at least one industry in which glass would not be used.
All solids are divided into crystalline and amorphous. Amorphous have a disordered structure and can melt at a sufficiently high temperature. In science, everything is called glass. amorphous bodies, which are formed as a result of supercooling of the melt.
Glass in everyday life is called a transparent fragile material. Depending on one or another component that is part of the initial glass mass, the following types of glass are distinguished in the industry: silicate, borate, borosilicate, aluminosilicate, boroaluminosilicate, phosphate and others.
Basic Method glass is obtained by melting a mixture of quartz sand (SiO2), soda (Na2CO3) and lime (CaO). The result is a chemical complex with the composition Na2O*CaO*6SiO2.
Physical, mechanical and chemical properties of glass:
Density glasses depends on the components that make up their composition. Thus, glass mass containing large amounts of lead oxide is denser than glass consisting, among other materials, of oxides of lithium, beryllium or boron.
Compressive strength - the ability of the material to resist internal stresses when exposed to any loads from the outside. In this case, the degree of strength of a particular type of glass depends on chemical included in its composition. Glasses containing oxides of calcium or boron are more durable. Glasses with lead and aluminum oxides are characterized by low strength. Various damages (cracks, deep scratches) significantly reduce the strength of the material. To artificially increase the strength index, the surface of some glass products is coated with an organosilicon film.
fragility - the mechanical property of bodies to collapse under the action of external forces. The magnitude of the fragility of glass mainly depends not on the chemical composition of its constituent components, but to a greater extent on the homogeneity of the glass mass (the components included in its composition must be pure, pure) and the wall thickness of the glass product.
Hardness - the mechanical property of one material to resist the penetration of another, harder one into it. It is possible to determine the degree of hardness of a particular material using a special scale table that reflects the properties of some minerals, which are arranged in ascending order, starting with the less hard talc, the hardness of which is taken as one, and ending with the hardest - diamond with a hardness of 10 conventionally accepted units. The degree of hardness of a particular type of glass mainly depends on the chemical composition of its constituent components. Thus, the use of lead oxide in the creation of glass mass significantly reduces the hardness of glass. And, on the contrary, silicate glasses are quite difficult to mechanically process.
Heat capacity - the property of bodies to receive and store a certain amount of heat in any process without changing the state. The heat capacity of glass directly depends on the chemical composition of the components that make up the initial glass mass. The higher the content of lead and barium oxides in the glass mass, the lower the thermal conductivity. And light oxides, such as, for example, lithium oxide, can increase the thermal conductivity of glass. Glass with low heat capacity cools much more slowly.
Thermal conductivity
- the property of bodies to pass heat through themselves from one surface to another, provided that they have different temperature. Glass does not conduct heat well. Moreover, the highest thermal conductivity was noted for quartz glass. With a decrease in the proportion of silicon oxide in the total mass of glass or when it is replaced by any other substance, the level of thermal conductivity decreases.
Softening start temperature is the temperature at which an amorphous body begins to soften and melt. The hardest - quartz - glass begins to deform only at a temperature of 1200-1500 ° C. Other types of glass soften already at a temperature of 550-650 0C. The value of the temperature of the beginning of melting of a particular grade and type of glass is determined by the chemical composition of the components. So, refractory oxides of silicon or aluminum increase the temperature level of the onset of softening, and low-melting ones (sodium and potassium oxides), on the contrary, lower it.
thermal expansion - the phenomenon of expansion of the size of a body under the influence of high temperatures. Finishing materials should be selected so that the value of their thermal expansion corresponds to the same indicator of the glass mass of the main product. The coefficient of thermal expansion of glasses directly depends on the chemical composition of the initial mass. The more alkali oxides in the glass mass, the higher the thermal expansion index, and, conversely, the presence of silicon, aluminum and boron oxides in the glass reduces this value.
Heat resistance - the ability of glass to resist corrosion and destruction as a result of a sharp change in external temperature. This coefficient depends not only on the chemical composition of the mass, but also on the size of the product, as well as on the amount of heat transfer on its surface.
Chemical resistance - the ability of a body not to succumb to the effects of water, salt solutions, gases and atmospheric moisture. Indicators of chemical resistance depend on the quality of the glass mass and the influencing agent. Thus, glass that does not corrode when exposed to water can be deformed when exposed to alkaline and saline solutions.
Light refraction - changing the direction of the light beam when it passes through the boundary of two transparent media. The value indicating the refraction of glass light is always greater than one.
reflection of light - this is the return of a light beam when it falls on the surface of two media having different refractive indices.
Light dispersion - the decomposition of a light beam into a spectrum when it is refracted. The value of glass light dispersion directly depends on the chemical composition of the material. The presence of heavy oxides in the glass mass increases the dispersion index.
light absorption - the ability of a medium to reduce the intensity of the passage of a light beam. The light absorption rate of glasses is low. It increases only in the manufacture of glass using various dyes, as well as special methods of processing finished products.
light scattering is the deflection of light rays in different directions. The light scattering index depends on the quality of the glass surface. So, passing through a rough surface, the beam is partially scattered, and therefore such glass looks translucent.
The Terminology Commission of the Academy of Sciences of the USSR gave the following definition to glass:
"Glass refers to all amorphous bodies obtained by supercooling the melt, regardless of the chemical composition and temperature range of solidification, and possessing, as a result of a gradual increase in viscosity, the mechanical properties of solids, and the process of transition from a liquid state to a glassy state must be reversible."
Glass is considered a technical term as opposed to the scientific term "glassy state". There may be bubbles, small crystals in the glass. In a glassy material, a very large number of tiny crystals can even be specially formed, making the material opaque or giving it a different color. Such material is called "milky" glass, colored glass, etc.
Modern concepts distinguish between the terms "glass" and "glassy state". "Vitreous state": "A solid, non-crystalline substance formed by cooling a liquid at a rate sufficient to prevent crystallization during cooling." N.V. Solomin, "glass is a material that mainly consists of a glassy substance."
All substances in the glassy state have several common physical and chemical characteristics. Typical glassy bodies:
1. isotopes, i.e. their properties are the same in all directions;
2. when heated, they do not melt like crystals, but gradually soften, passing from a brittle to a viscous, highly viscous and drop-liquid state;
3. melt and harden reversibly, regaining their original properties.
The reversibility of the presses and properties indicates that glass-forming melts and solidified glass are true solutions. The transition of a substance from a liquid state to a solid state with a decrease in temperature can occur in two ways: the substance crystallizes or solidifies in the form of glass.
Almost all substances can follow the first path. However, the crystallization path is common only for those substances that, being in a liquid state, have a low viscosity and whose viscosity increases relatively slowly, up to the moment of crystallization.
To the second group, they depend to a decisive extent on the concentration of alkalis or on the concentration of any other selected components. Their dependence on the composition affects: viscosity, electrical conductivity, ion diffusion rate, dielectric loss, chemical resistance, light transmission, hardness, surface tension.
Physical properties glass
The density of ordinary sodium-potassium-silicate glass, including window glass, fluctuates between 2500-2600 kg/m3. With an increase in temperature from 20 to 1300 ° C, the density of most glasses decreases by 6-12%, that is, by 100 ° C, the density decreases by 15 kg / m3. The compressive strength of conventional annealed glass is 500-2000MPa, window glass is 900-1000MPa.
The hardness of glass depends on the chemical composition. Glasses have different hardness within 4,000-10,000 MPa. The hardest is quartz glass, with an increase in the content of alkali oxides, the hardness of glasses decreases.
Fragility. Glass, along with diamond and quartz, is a perfectly brittle material. Since brittleness is most pronounced on impact, it is characterized by impact strength. The impact strength of glass depends on the specific viscosity.
Thermal conductivity. Quartz glasses have the highest thermal conductivity. Ordinary window glass has 0.97 W/(m.K). As the temperature rises, the thermal conductivity increases, the thermal conductivity depends on the chemical composition of the glass.
The high transparency of oxide glasses has made them indispensable for glazing buildings, mirrors and optical devices, including laser, television, film and photographic equipment, and so on. For building sheet glass, window glass, display glass, it must be taken into account that the light transmission coefficient directly depends on the reflectivity of the glass surface and on its absorbing ability. Theoretically, even perfect glass that does not absorb light cannot transmit more than 92% of light.
Optical properties of glass: refractive index is the ability of glass to refract light falling on it. For the production of ceramic dyes, the refractive index is very important. It depends on how much light it reflects. ceramic product and how it will look.
Mechanical properties: elasticity is the property of a solid body to restore its original shape after the termination of the load. Elasticity is characterized by such quantities as the modulus of normal elasticity, which determines the magnitude of stresses arising under the influence of a load in tension (compression).
Internal friction: Glassy systems have the ability to absorb mechanical, in particular sonic and ultrasonic vibrations. The damping of oscillations depends on the composition of the inhomogeneities in the glass.
The thermal properties of silicate systems are the most important properties both in the study and in the manufacture of ceramic and glass products.
Specific heat capacity: - determined by the amount of heat Q required to heat a unit mass of glass by 1°C.
Chemical resistance - resistance to various aggressive media - one of the very important properties of glasses is important for medicine. Tempered glasses break down 1.5-2 times faster than well-annealed glasses. In modern construction, special glasses with sun and heat-shielding properties are used for window, door and other openings. For these glasses, it is important the spectral nature of the light flux that has passed through clarification, the assessment of the color tone. Based on these characteristics, a certain type of glass is selected, as well as the determination of thermal and lighting properties, their influence on working conditions, the design of buildings and structures.
For a long time, windows have been made to lighten and give comfort to a living space. Since glass was a rarity, other materials were used instead. Fortunately, nowadays glass is not uncommon: it is used everywhere and for different purposes. Moreover, you can buy not only ordinary window glass, but also colored glass for making stained-glass windows.
All solids are divided into crystalline and amorphous. The latter have the property of melting at a sufficiently high temperature. Unlike crystalline bodies, they have a structure with only small areas of ordered ions, and these areas are interconnected so that they form an asymmetry.
In science (chemistry, physics), it is customary to call glass all amorphous bodies that are formed as a result of supercooling of the melt. These bodies, due to the gradual increase in the degree of viscosity, are endowed with all the features of solid bodies. They also have the property of a reverse transition from a solid to a liquid state.
Glass in everyday life is called a transparent fragile material. Depending on one or another component that is part of the initial glass mass, the following types of glass are distinguished in the industry: silicate, borate, borosilicate, aluminosilicate, boroaluminosilicate, phosphate and others.
Like any other physical body, glass has a number of properties.
Physical and mechanical properties of glass
Glass density depends on the components included in their composition. Thus, glass mass containing large amounts of lead oxide is denser than glass consisting, among other materials, of oxides of lithium, beryllium or boron. As a rule, the average density of glass (window, container, high-quality, heat-resistant) ranges from 2.24×10 cubic meters - 2.9×10 cubic meters kg/m3. The density of crystal is somewhat higher: from 3.5 x 10 in a cube - 3.7 x 10 in a cube of kg / m3.
Strength. Under the compressive strength in physics and chemistry, it is customary to understand the ability of a material to resist internal stresses when exposed to any loads from the outside. The tensile strength of glass is from 500 to 2000 MPa (crystal - 700-800 MPa). Let's compare this value with the strength of cast iron and steel: 600-1200 and 2000 MPa, respectively.
At the same time, the degree of strength of a particular type of glass depends on the chemical substance that is part of it.
Glasses containing oxides of calcium or boron are more durable. Glasses with lead and aluminum oxides are characterized by low strength.
Tensile strength glass tensile strength is only 35-100 MPa. The degree of tensile strength of glass largely depends on the presence of various defects formed on its surface. Various damages (cracks, deep scratches) significantly reduce the strength of the material. To artificially increase the strength index, the surface of some glass products is coated with an organosilicon film.
fragility- the mechanical property of bodies to collapse under the action of external forces. The magnitude of the fragility of glass mainly depends not on the chemical composition of its constituent components, but to a greater extent on the homogeneity of the glass mass (the components included in its composition must be pure, pure) and the wall thickness of the glass product.
hardness denote the mechanical property of one material to resist the penetration of another, harder one into it. It is possible to determine the degree of hardness of a particular material using a special scale table that reflects the properties of some minerals, which are arranged in ascending order, starting with the less hard talc, the hardness of which is taken as one, and ending with the hardest - diamond with a hardness of 10 conventionally accepted units.
Often the hardness of glass is "measured" by grinding, using the so-called abrasive hardness method. In this case, its value is set depending on the peeling rate of a unit surface of the glass product under certain grinding conditions.
Degree of hardness one or another type of glass mainly depends on the chemical composition of its constituent components. Thus, the use of lead oxide in the creation of glass mass significantly reduces the hardness of glass. And, on the contrary, silicate glasses are quite difficult to mechanically process.
Heat capacity is the property of bodies to receive and store a certain amount of heat in any process without changing state.
Heat capacity of glass directly depends on the chemical composition of the components that make up the initial glass mass. His specific heat at an average temperature is 0.33-1.05 J / (kgxK). Moreover, the higher the content of lead and barium oxides in the glass mass, the lower the thermal conductivity index. But light oxides, such as, for example, lithium oxide, can increase the thermal conductivity of glass.
In the manufacture of glass products, it should be remembered that amorphous bodies with a low heat capacity cool down much more slowly than bodies with a high heat capacity. In such bodies, there is also an increase in the amount of heat capacity with an increase in external temperature. Moreover, in the liquid state, this figure grows somewhat faster. This is also true for glasses of various types.
Thermal conductivity. This term in science denotes the property of bodies to pass heat through themselves from one surface to another, provided that the latter have different temperatures.
It is known that glass conducts heat poorly (by the way, this property is widely used in the construction of buildings). The level of its thermal conductivity averages 0.95-0.98 W / (m x K). Moreover, the highest thermal conductivity was noted for quartz glass. With a decrease in the proportion of silicon oxide in the total mass of glass or when it is replaced by any other substance, the level of thermal conductivity decreases.
Softening start temperature- this is the temperature at which the body (amorphous) begins to soften and melt. The hardest - quartz - glass begins to deform only at a temperature of 1200-1500 ° C. Other types of glass soften already at a temperature of 550-650 0C. These indicators are important to take into account in various works with glass: in the process of blowing products, when processing the edges of these products, as well as during thermal polishing of their surfaces.
Value melting start temperature one or another grade and type of glass is determined by the chemical composition of the components. So, refractory oxides of silicon or aluminum increase the temperature level of the onset of softening, and low-melting ones (sodium and potassium oxides), on the contrary, lower it.
thermal expansion. This term is used to denote the phenomenon of expansion of the size of a body under the influence of high temperatures. This value is very important to take into account in the manufacture of glass products with various overlays on the surface. Finishing materials should be selected so that the value of their thermal expansion corresponds to the same indicator of the glass mass of the main product.
Thermal expansion coefficient glass directly depends on the chemical composition of the initial mass. The more alkali oxides in the glass mass, the higher the thermal expansion index, and, conversely, the presence of silicon, aluminum and boron oxides in the glass reduces this value.
heat resistance the ability of glass to resist corrosion and destruction as a result of a sharp change in external temperature is determined. This coefficient depends not only on the chemical composition of the mass, but also on the size of the product, as well as on the amount of heat transfer on its surface.
Optical properties of glass
Light refraction- so in science they call the change in the direction of a light beam when it passes through the border of two transparent media. The value indicating the refraction of glass light is always greater than one.
reflection of light- this is the return of a light beam when it falls on the surface of two media having different refractive indices.
Light dispersion- the decomposition of a light beam into a spectrum when it is refracted. The value of glass light dispersion directly depends on the chemical composition of the material. The presence of heavy oxides in the glass mass increases the dispersion index. It is this property that explains the phenomenon of the so-called play of light in crystal products.
By absorbing light determine the ability of a medium to reduce the intensity of the passage of a light beam. The light absorption rate of glasses is low. It increases only in the manufacture of glass using various dyes, as well as special methods of processing finished products.
light scattering is the deflection of light rays in different directions. The light scattering index depends on the quality of the glass surface. So, passing through a rough surface, the beam is partially scattered, and therefore such glass looks translucent. This property, as a rule, is used in the manufacture of glass lampshades for lamps and plafonds for lamps.
Chemical properties of glass
Among chemical properties It is necessary to highlight the chemical resistance of glass and products made from it.
Chemical resistance in science is called the ability of a body not to succumb to the effects of water, salt solutions, gases and atmospheric moisture. Indicators of chemical resistance depend on the quality of the glass mass and the influencing agent. Thus, glass that does not corrode when exposed to water can be deformed when exposed to alkaline and saline solutions.
Silicate glasses are distinguished by an unusual combination of properties, transparency, absolute water tightness and universal chemical resistance. All this is explained by the specific composition and structure of glass.
Density glass depends on the chemical composition and for conventional building glass is 2400...2600 kg/m 3 . The density of window glass is 2550 kg / m. Glasses containing lead oxide (“Bohemian crystal”) are distinguished by high density - more than 3000 kg / m 3. The porosity and water absorption of glass are almost equal to 0%.
Mechanical properties. Glass in building structures is more often subjected to bending, stretching and impact and less often to compression, therefore, the main indicators that determine it mechanical properties, should be considered tensile strength and brittleness.
theoretical glass tensile strength - (10...12) 10 3 MPa. In practice, this value is 200...300 times lower and ranges from 30 to 60 MPa. This is explained by the fact that there are weakened areas in the glass (microinhomogeneities, surface defects, internal stresses). The larger the size of the glass products, the more likely the presence of such areas. An example of the dependence of the strength of glass on the size of the test product is glass fiber. Fiberglass with a diameter of 1 ... 10 microns has a tensile strength of 300 ... 500 MPa, i.e., almost 10 times higher than that of sheet glass. Strongly reduce the tensile strength of the glass scratches; cutting glass with a diamond is based on this.
Glass compressive strength high - 900 ... 1000 MPa, i.e. almost like steel and cast iron. In the temperature range from - 50 to + 70 ° C, the strength of glass practically does not change.
Glass at normal temperatures is characterized by the fact that it has no plastic deformations. When loaded, it obeys Hooke's law up to brittle fracture. Elastic modulusglass E=(7...7.5) 10 4 MPa.
fragility - the main disadvantage of glass. The main indicator of brittleness is the ratio of the modulus of elasticity to tensile strength E/R p . For glass, it is 1300 ... 1500 (for steel 400 ... 460, rubber 0.4 ... 0.6). In addition, the uniformity of the structure (homogeneity) of glass contributes to the unhindered development of cracks, which is a necessary condition for the manifestation of brittleness.
glass hardness, which, in terms of chemical composition, is a substance close to feldspars, is the same as that of these minerals, and, depending on the chemical composition, is in the range of 5 ... 7 on the Mohs scale.
Optical properties glasses are characterized by light transmission (transparency), light refraction, reflection, scattering, etc. Ordinary silicate glasses, except for special ones (see below), transmit the entire visible part of the spectrum (up to 88 ... 92%) and practically do not transmit ultraviolet and infrared rays. Refractive index of building glass (P= 1.50...1.52) determines the strength of the reflected light and the light transmission of the glass at different angles of light incidence. When the angle of incidence of light is changed from 0 to 75°, the light transmission of glass decreases from 90 to 50%.
Thermal conductivity of various types of glass depends little on their composition and is 0.6 ... 0.8 W / (m K), which is almost 10 times lower than that of similar crystalline minerals. For example, the thermal conductivity of a quartz crystal is 7.2 W / (m K).
Coefficient of linear thermal expansion (CLTE) of glass is relatively small (for ordinary glass 9 10 -6 K -1). But due to the low thermal conductivity and high modulus of elasticity, the stresses developing in the glass during sharp one-sided heating (or cooling) can reach values that lead to glass failure. This explains the relatively small heat resistance(ability to withstand sudden temperature changes) of ordinary glass. It is 70 ... 90 ° C.
Soundproofing ability glass is quite high. Glass with a thickness of 1 cm in terms of sound insulation approximately corresponds to a brick wall in half a brick - 12 cm.
Chemical resistance silicate glass is one of its most unique properties. Glass well resists the action of water, alkalis and acids (with the exception of hydrofluoric and phosphoric). This is explained by the fact that under the action of water and aqueous solutions, Na + and Ca ++ ions are washed out from the outer layer of glass and a chemically resistant film enriched with SiO 2 is formed. This film protects the glass from further damage.
