>>Physics: Amorphous bodies
Not all solids are crystals. There are many amorphous bodies. How are they different from crystals?
Amorphous bodies do not have a strict order in the arrangement of atoms. Only the nearest atoms-neighbors are arranged in some order. But there is no strict repetition in all directions of the same structural element, which is characteristic of crystals, in amorphous bodies.
According to the arrangement of atoms and their behavior, amorphous bodies are similar to liquids.
Often the same substance can be in both a crystalline and an amorphous state. For example, quartz SiO 2 can be in both crystalline and amorphous form (silica). The crystalline form of quartz can be schematically represented as a lattice of regular hexagons ( fig.12.6, a). The amorphous structure of quartz also has the form of a lattice, but irregular shape. Along with hexagons, it contains pentagons and heptagons ( fig.12.6, b).
Properties of amorphous bodies. All amorphous bodies are isotropic, that is, their physical properties are the same in all directions. Amorphous bodies include glass, resin, rosin, sugar candy, etc.
At external influences amorphous bodies exhibit both elastic properties, like solids, and fluidity, like liquids. So, with short-term impacts (impacts), they behave like solid bodies and, with a strong impact, break into pieces. But with a very long exposure, amorphous bodies flow. You can see for yourself if you are patient. Follow a piece of resin that is lying on a hard surface. Gradually, the resin spreads over it, and the higher the temperature of the resin, the faster this happens.
Atoms or molecules of amorphous bodies, like liquid molecules, have a certain time of "sedentary life" - the time of oscillations around the equilibrium position. But unlike liquids, they have a very long time.
So, for a var at t\u003d 20 ° C, the time of "sedentary life" is approximately 0.1 s. In this respect, amorphous bodies are close to crystalline ones, since jumps of atoms from one equilibrium position to another occur relatively rarely.
Amorphous bodies at low temperatures resemble solid bodies in their properties. They have almost no fluidity, but as the temperature rises, they gradually soften and their properties more and more approach those of liquids. This is because as the temperature rises, the jumps of atoms from one equilibrium position to another gradually become more frequent. certain melting point amorphous bodies, unlike crystalline ones, do not.
liquid crystals. In nature, there are substances that simultaneously have the basic properties of a crystal and a liquid, namely anisotropy and fluidity. This state of matter is called liquid crystal. Liquid crystals are mainly organic substances, the molecules of which have a long filamentous shape or the shape of flat plates.
Let us consider the simplest case, when a liquid crystal is formed by filamentous molecules. These molecules are parallel to each other, but they are randomly shifted, i.e., order, unlike ordinary crystals, exists only in one direction.
At thermal motion the centers of these molecules move randomly, but the orientation of the molecules does not change, and they remain parallel to themselves. A strict orientation of molecules does not exist in the entire volume of the crystal, but in small areas called domains. Refraction and reflection of light occur at the domain boundary, so liquid crystals are opaque. However, in a liquid crystal layer placed between two thin plates, the distance between which is 0.01-0.1 mm, with parallel recesses of 10-100 nm, all molecules will be parallel and the crystal will become transparent. If an electric voltage is applied to some parts of the liquid crystal, then the liquid crystal state is disturbed. These areas become opaque and begin to glow, while areas without tension remain dark. This phenomenon is used in the creation of liquid crystal TV screens. It should be noted that the screen itself consists of a huge number of elements and the electronic control circuit for such a screen is extremely complex.
Solid state physics. Mankind has always used and will continue to use solid bodies. But if earlier solid state physics lagged behind the development of technology based on direct experience, now the situation has changed. Theoretical research leads to the creation of solids, the properties of which are quite unusual.
It would be impossible to obtain such bodies by trial and error. The creation of transistors, which will be discussed later, is a vivid example of how understanding the structure of solids led to a revolution in all radio engineering.
Obtaining materials with specified mechanical, magnetic, electrical and other properties is one of the main directions of modern solid state physics. Approximately half of the world's physicists are now working in this area of physics.
Amorphous bodies occupy an intermediate position between crystalline solids and liquids. Their atoms or molecules are arranged in relative order. Understanding the structure of solids (crystalline and amorphous) allows you to create materials with desired properties.
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1. How do amorphous bodies differ from crystalline ones?
2. Give examples of amorphous bodies.
3. Would the profession of glass blower arise if glass were a crystalline body, and not an amorphous one?
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics Grade 10
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Unlike crystalline solids, there is no strict order in the arrangement of particles in an amorphous body.
Although amorphous solids are able to retain their shape, they do not have a crystal lattice. Some regularity is observed only for molecules and atoms located in the neighborhood. This order is called short-range order . It is not repeated across the board and is not saved on long distances like crystalline bodies.
Examples of amorphous bodies are glass, amber, artificial resins, wax, paraffin, plasticine, etc.
Atoms in amorphous bodies oscillate around points that are randomly located. Therefore, the structure of these bodies resembles the structure of liquids. But the particles in them are less mobile. The time of their oscillation around the equilibrium position is longer than in liquids. Jumps of atoms to another position also occur much less frequently.
How do solids behave when heated? crystalline bodies? They begin to melt at a certain melting point. And for some time they are simultaneously in a solid and liquid state, until all the substance is melted.
Amorphous bodies do not have a specific melting point. . When heated, they do not melt, but gradually soften.
Put a piece of plasticine near heater. After a while it will become soft. This does not happen instantly, but over a period of time.
Since the properties of amorphous bodies are similar to those of liquids, they are considered as supercooled liquids with a very high viscosity (solidified liquids). Under normal conditions, they cannot flow. But when heated, jumps of atoms in them occur more often, viscosity decreases, and amorphous bodies gradually soften. The higher the temperature, the lower the viscosity, and gradually the amorphous body becomes liquid.
Ordinary glass is a solid amorphous body. It is obtained by melting silicon oxide, soda and lime. Heating the mixture to 1400 about C, get a liquid vitreous mass. When cooling liquid glass does not solidify, like crystalline bodies, but remains a liquid, the viscosity of which increases, and the fluidity decreases. Under ordinary conditions, it appears to us as a solid body. But in fact it is a liquid that has an enormous viscosity and fluidity, so small that it can hardly be distinguished by the most ultra-sensitive instruments.
The amorphous state of matter is unstable. Over time, from an amorphous state, it gradually turns into a crystalline one. This process in different substances takes place with different speed. We see how sugar crystals cover sugar candies. This does not take much time.
For crystals to form in ordinary glass, a lot of time must pass. During crystallization, glass loses its strength, transparency, becomes cloudy, and becomes brittle.
In crystalline solids, the physical properties differ in different directions. And in amorphous bodies they are the same in all directions. This phenomenon is called isotropy .
An amorphous body equally conducts electricity and heat in all directions, and refracts light equally. Sound also propagates equally in amorphous bodies in all directions.
Properties amorphous substances used in modern technologies. Of particular interest are metal alloys that do not have a crystalline structure and are amorphous solids. They are called metal glasses . Their physical, mechanical, electrical and other properties differ from similar properties of conventional metals for the better.
So, in medicine, amorphous alloys are used, the strength of which exceeds that of titanium. They are used to make screws or plates that connect broken bones. Unlike titanium fasteners, this material gradually disintegrates and is replaced by bone material over time.
High-strength alloys are used in the manufacture of metal-cutting tools, fittings, springs, and parts of mechanisms.
An amorphous alloy with high magnetic permeability has been developed in Japan. By using it in transformer cores instead of textured transformer steel sheets, eddy current losses can be reduced by a factor of 20.
Amorphous metals have unique properties. They are called the material of the future.
It must be remembered that not all bodies that exist on planet Earth have a crystalline structure. Exceptions to the rule are called "amorphous bodies". How are they different? Based on the translation of this term - amorphous - it can be assumed that such substances differ from others in their form or appearance. We are talking about the absence of the so-called crystal lattice. The splitting process, in which faces appear, does not occur. Amorphous bodies also differ in that they do not depend on environment, and their properties are constant. Such substances are called isotropic.
From a school course in physics, one can recall that amorphous substances have a structure in which the atoms in them are arranged in a chaotic manner. Only neighboring structures can have a certain place, where such an arrangement is forced. But still, drawing an analogy with crystals, amorphous bodies do not have a strict ordering of molecules and atoms (in physics, this property is called "long-range order"). As a result of the research, it was found that these substances are similar in structure to liquids.
Some bodies (as an example, we can take silicon dioxide, whose formula is SiO 2) can simultaneously be in an amorphous state and have crystal structure. Quartz in the first version has an irregular lattice structure, in the second - a regular hexagon.
As mentioned above, amorphous bodies do not have a crystal lattice. Their atoms and molecules have a short-range order of placement, which will be the first distinctive feature these substances.
These bodies are deprived of fluidity. In order to better explain the second property of substances, we can do this using the example of wax. It's no secret that if you pour water into a funnel, it will simply pour out of it. The same will be with any other fluid substances. And the properties of amorphous bodies do not allow them to do such "tricks". If the wax is placed in a funnel, then it will first spread over the surface and only then begin to drain from it. This is due to the fact that the molecules in a substance jump from one equilibrium position to a completely different one without having a main location.
It's time to talk about the melting process. It should be remembered that amorphous substances do not have a specific temperature at which melting begins. As the degree rises, the body gradually becomes softer and then turns into a liquid. Physicists always focus not on the temperature at which this process began to occur, but on the corresponding melting temperature range.
It has already been mentioned above. Amorphous bodies are isotropic. That is, their properties in any direction are unchanged, even if the conditions of stay in places are different.
At least once every person observed that over a certain period of time the glasses began to become cloudy. This property of amorphous bodies is associated with increased internal energy (it is many times greater than that of crystals). Because of this, these substances can easily go into a crystalline state on their own.
After a certain period of time, any amorphous body passes into a crystalline state. This can be observed in the usual life of a person. For example, if you leave a lollipop or honey for several months, you will notice that both of them have lost their transparency. An ordinary person will say that they are just sugared. Indeed, if you break the body, you can see the presence of sugar crystals.
So, speaking of this, it is necessary to clarify that the spontaneous transformation into another state is due to the fact that amorphous substances are unstable. Comparing them with crystals, one can understand that the latter are many times more “powerful”. The fact can be explained thanks to the intermolecular theory. According to her, the molecules are constantly jumping from one place to another, thereby filling the voids. Over time, a stable crystal lattice is formed.
The process of melting of amorphous bodies is the moment when, with a rise in temperature, all bonds between atoms collapse. It is then that the substance turns into a liquid. If the melting conditions are such that the pressure is the same throughout the entire period, then the temperature must also be fixed.
In nature, there are bodies that have a liquid crystal structure. As a rule, they are included in the list of organic substances, and their molecules have a filamentous shape. The bodies about which in question, have the properties of liquids and crystals, namely fluidity and anisotropy.
In such substances, the molecules are parallel to each other, however, there is an unfixed distance between them. They are constantly moving, but they are not inclined to change orientation, therefore they are constantly in one position.
Amorphous metals are better known ordinary person called metallic glass.
Back in 1940, scientists started talking about the existence of these bodies. Even then it became known that metals specially obtained by vacuum deposition did not have crystal lattices. And only 20 years later the first glass of this type was produced. special attention it did not cause scientists; and only after another 10 years, American and Japanese professionals started talking about it, and then Korean and European ones.
Amorphous metals differ in viscosity, enough high level strength and resistance to corrosion.
« Physics - Grade 10 "
In addition to solids that have a crystalline structure, which is characterized by a strict order in the arrangement of atoms, there are amorphous solids.
Amorphous bodies do not have a strict order in the arrangement of atoms. Only the nearest atoms-neighbors are arranged in some order. But there is no strict repetition in all directions of the same structural element, which is characteristic of crystals, in amorphous bodies. According to the arrangement of atoms and their behavior, amorphous bodies are similar to liquids. Often the same substance can be in both a crystalline and an amorphous state.
Theoretical studies lead to the production of solids, the properties of which are quite unusual. It would be impossible to obtain such bodies by trial and error. The creation of transistors, which will be discussed later, is a vivid example of how understanding the structure of solids has led to a revolution in all radio engineering.
Obtaining materials with specified mechanical, magnetic, electrical and other properties is one of the main directions of modern solid state physics.
Amorphous solids in many of their properties and mainly in microstructure should be considered as highly supercooled liquids with a very high viscosity coefficient. The structure of such bodies is characterized only by short-range order in the arrangement of particles. Some of these substances are not able to crystallize at all: wax, sealing wax, resins. Others, under a certain cooling regime, form crystalline structures, but in the case of rapid cooling, an increase in viscosity prevents ordering in the arrangement of particles. The substance solidifies before the crystallization process is realized. Such bodies are called glassy: glass, ice. The process of crystallization in such a substance can also occur after solidification (clouding of glasses). Amorphous also include solid organic substances: rubber, wood, leather, plastics, wool, cotton and silk fibers. The process of transition of such substances from the liquid phase to the solid phase is shown in Fig. – curve I.
Amorphous bodies do not have a solidification (melting) temperature. On the graph T \u003d f (t) there is an inflection point, which is called the softening point. A decrease in temperature leads to a gradual increase in viscosity. This nature of the transition to a solid state causes the absence of a specific heat of fusion in amorphous substances. The reverse transition, when heat is supplied, there is a smooth softening to the state of a liquid.
CRYSTAL SOLID BODIES.
A characteristic feature of the microstructure of crystals is the spatial periodicity of their internal electric fields and the repeatability in the arrangement of crystal-forming particles - atoms, ions and molecules (long-range order). Particles alternate in a certain order along straight lines, which are called nodal. In any flat section of a crystal, two intersecting systems of such lines form a set of absolutely identical parallelograms, which tightly, without gaps, cover the section plane. In space, the intersection of three non-coplanar systems of such lines forms a spatial grid that divides the crystal into a set of completely identical parallelepipeds. The points of intersection of the lines forming the crystal lattice are called nodes. Distances between nodes along some direction are called translations or lattice periods. A parallelepiped built on three non-coplanar translations is called an elementary cell or a lattice repeatability parallelepiped. The most important geometric property of crystal lattices is the symmetry in the arrangement of particles with respect to certain directions and planes. For this reason, although there are several ways to choose a unit cell, for a given crystal structure, choose it so that it corresponds to the symmetry of the lattice.
Crystalline bodies can be divided into two groups: single crystals and polycrystals. For single crystals, a single crystal lattice is observed in the volume of the entire body. And although the external shape of single crystals of the same type may be different, the angles between the corresponding faces will always be the same. A characteristic feature of single crystals is the anisotropy of mechanical, thermal, electrical, optical, and other properties.
Single crystals are often found in the natural state in nature. For example, most minerals are crystal, emeralds, rubies. At present, for industrial purposes, many single crystals are grown artificially from solutions and melts - rubies, germanium, silicon, gallium arsenide.
The same chemical element can form several, differing in geometry, crystal structures. This phenomenon is called polymorphism. For example, carbon is graphite and diamond; ice five modifications, etc.
The correct external faceting and anisotropy of properties, as a rule, do not appear for crystalline bodies. This is because crystalline solids usually consist of many randomly oriented small crystals. Such solids are called polycrystalline. This is due to the mechanism of crystallization: when the conditions necessary for this process are reached, crystallization centers simultaneously appear in many places of the initial phase. The nucleated crystals are located and oriented relative to each other quite arbitrarily. For this reason, at the end of the process, we get a solid body in the form of a conglomerate of intergrown small crystals - crystallites.
From an energetic point of view, the difference between crystalline and amorphous solids is clearly seen in the process of solidification and melting. Crystalline bodies have a melting point - the temperature when the substance stably exists in two phases - solid and liquid (Fig. curve 2). The transition of a solid molecule into a liquid means that it acquires an additional three degrees of freedom of translational motion. That. unit mass of a substance at T pl. in the liquid phase has a greater internal energy than the same mass in the solid phase. In addition, the distance between particles changes. Therefore, in general, the amount of heat required to convert a unit mass of a crystalline substance into a liquid will be:
λ \u003d (U W -U cr) + P (V W -V cr),
where λ is the specific heat of melting (crystallization), (U f -U cr) is the difference between the internal energies of the liquid and crystalline phases, P is the external pressure, (V f -V cr) is the difference in specific volumes. According to the Clausius-Clapeyron equation, the melting point depends on pressure:
It can be seen that if (V W -V cr)> 0, then 
> 0, i.e. with increasing pressure, the melting point rises. If the volume of the substance decreases during melting (V W -V cr)<
0 (вода, висмут), то рост давления приводит
к понижению Т пл.
Amorphous bodies have no heat of fusion. Heating leads to a gradual increase in the rate of thermal motion and a decrease in viscosity. There is an inflection point on the process graph (Fig.), which is conventionally called the softening point.
THERMAL PROPERTIES OF SOLID BODIES
Due to the strong interaction, thermal motion in crystals is limited only by vibrations of particles around the nodes of the crystal lattice. The amplitude of these fluctuations usually does not turn 10 -11 m, i.e. is only 5-7% of the grating period along the corresponding direction. The nature of these oscillations is very complicated, since it is determined by the forces of interaction of an oscillating particle with all its neighbors.
An increase in temperature means an increase in the energy of particle motion. This, in turn, means an increase in the amplitude of particle oscillations and explains the expansion of crystalline solids upon heating.
l t = l 0 (1 + αt 0),
Where l t and l 0 - linear dimensions of the body at temperatures t 0 and 0 0 С, α - coefficient of linear expansion. For solids α has the order of 10 -5 - 10 -6 K -1 . As a result of linear expansion, the volume of the body also increases:
V t = V 0 (1 + βt 0),
here β is the volume expansion coefficient. β = 3α in the case of isotropic expansion. Single-crystal bodies, being anisotropic, have three different values of α.
Each particle that oscillates has three degrees of freedom of oscillatory motion. Considering that, in addition to kinetic energy, particles also have potential energy, the energy ε = kT should be assigned to one degree of freedom of particles of solid bodies. Now for the internal energy of the mole we will have:
U μ = 3N A kT = 3RT,
and for the molar heat capacity:
Those. the molar heat capacity of chemically simple crystalline bodies is the same and does not depend on temperature. This is the Dulong-Petit law.
As the experiment showed, this law is quite well fulfilled, starting from room temperatures. Explanations for deviations from the Dulong-Petit law at low temperatures were given by Einstein and Debye in the quantum theory of heat capacity. It was shown that the energy that falls on one degree of freedom is not a constant value, but depends on temperature and oscillation frequency.
REAL CRYSTALS. DEFECTS IN CRYSTALS
Real crystals have a number of violations of the ideal structure, which are called crystal defects:
a) point defects -
Schottky defects (nodes not occupied by particles);
Frenkel defects (displacement of particles from nodes to interstitials);
impurities (implanted foreign atoms);
b) linear - edge and screw dislocations. It's local irregular
sti in the arrangement of particles
due to the incompleteness of individual atomic planes
or due to violations in the sequence of their development;
c) planar - boundaries between crystallites, rows of linear dislocations.
