Physical properties of crystalline bodies. Solid bodies
Solid bodies.
INdifferences from liquids solid bodies possess elasticity of form . In any attempts to change the geometry of the solid body in it there are elastic forces that prevent this effect. Based on the features of the internal structure of solids, distinguish crystal and amorphous solid bodies. Crystals and amorphous bodies differ significantly among themselves in many physical properties.
Amorphous bodiesin its internal structure, liquids are very reministed, so they are often called them overcoiled fluids . Like liquids, amorphous bodies are structurally isotropic. Their properties do not depend on the direction under consideration. It is explained by the fact that in amorphous bodies, as well as in liquids persists middle order (Coordination number), and far (lengths and corners of links) is absent. These are the complete homogeneity of all the macrophysical properties of the amorphous body. Typical examples of amorphous bodies are glass, resins, bitumens, amber.
Crystal bodies, in contrast to amorphous, have a clear orderly microstructure, which is preserved on the macro level and appears externally as small grains with flat edges and sharp ribs called crystals.
Common crystal bodies (metals and alloys, sugar and table salt, ice and sand, stone and clay, cement and ceramics, semiconductors, etc.) are usually polycrystals, consisting of chaotic-oriented combined monocrystalliki (crystallites), the dimensions of which are about 1 μM (10 -6 m) However, sometimes single-crystals are found quite large sizes. For example, mountain crystal single crystals achieve human growth in modern technique Monocrystals play an important role, so the technology of their artificial cultivation has been developed.
Inside a single crystal, atoms (ions) of the substance are placed in compliance with long-range order, in the nodes of a clearly-oriented geometric structure, called the name crystal lattice Each substance is formed in a solid state, the crystal lattice is individual according to the geometry. Its form is determined by the structure of substance molecules. In the lattice can always be highlighted elementary cell, preserving all its geometrical features, but includes the minimum possible number of nodes.
Single crystals of each particular substance may have different sizes. However, they all retain the same geometry, which manifests itself in the preservation of permanent angles between the corresponding edges of the crystal. If the shape of the single crystal is forcibly disrupted, then, with subsequent cultivation of the melt, or simply, when heated, necessarily restores its previous form. The reason for such a reduction of the shape of the crystal is the known condition of thermodynamic stability - the desire to minimize the potential energy. For crystals, this condition is formulated independently of J. Gibbs, P Curie and G. Vulphoma in the form of principle: the surface energy of the crystal must be minimal.
One of the most characteristic features Monocrystals is anisotropy. there are many physical and mechanical properties. For example, hardness, strength, fragility, thermal expansion, the rate of propagation of elastic waves, electrical conductivity and thermal conductivity of many crystals may depend on the directions in the crystal. In the polycrystals, anisotropy is practically not manifested only due to the chaotic mutual orientation of the generators of their small single crystallines. It is related to the fact that in the crystal lattice of the distance between the nodes in different directions in the general case turn out to be substantially different.
Another important feature of crystals can be considered that they melt and crystallize at a constant temperature, in full accordance with the thermodynamic theory of phase transitions of the first kind. Amorphous solid bodies have no pronounced phase transition. When heated, they softened smoothly, in a wide range of temperature changes, this means that amorphous bodies do not have a certain regular structure and, when heated, it collapsing in stages, while the crystals during heating destroy the homogeneous crystal grille (with its long-term order) strictly at fixed energy conditions, And consequently, at a fixed temperature.
Some solids are capable of exist steadily both in crystalline and in amorphous states. A characteristic example is glass. With a fairly fast cooling of the melt, the glass becomes very viscous and hardens, not so much to acquire a crystal structure. However, with very slow cooling, with an excerpt at a certain temperature level, the same glass crystallizes and acquires specific properties (such glass is called satalla ). Other typical example is quartz. In nature, it usually exists in the form of a crystal, and amorphous quartz is always formed from the melt (it is also called it melted quartz ). Experience shows that the harder the substance molecules and the stronger their intermolecular bonds, the easier when cooled, get a solid amorphous modification.
The solid body is called the aggregate state of the substance characterized by the constancy of the form and volume, and the thermal movements of particles in them are chaotic oscillations of particles regarding equilibrium positions.
Solid bodies are divided into crystalline and amorphous.
Crystal bodies are solid bodies having an ordered periodically repeated arrangement of particles.
The structure for which the regular arrangement of particles with periodic repeatability in those dimensions is called a crystal lattice.
Figure 53.1.
A characteristic feature of crystals is their anisotropy - the dependence of the physical properties (elastic, mechanical, thermal, electrical, magnetic) from the direction. The anisotropy of crystals is explained by the fact that the density of the parties is not the same in different directions.
If the crystal body consists of a single crystal, it is called a single crystal. If the solid consists of a set of randomly oriented crystalline grains, it is called a polycrystal. In polycrystals, anisotropy is observed only for individual small crystals.
Solid bodies physical properties which are the same in all directions (isotropic) are called amorphous. For amorphous bodies, as well as for liquids, a nearby order is characterized in the location of the particles, but, unlike the liquids, the mobility of the particles in them satisfied.
Organic amorphous bodies whose molecules consist of big number The identical long molecular chains connected by chemical bonds are called polymers (for example, rubber, polyethylene, rubber).
Depending on the genus of particles located in the assemblies of the crystal lattice and on the nature of the interaction forces between particles, 4 physical types of crystal are distinguished:
Ionic crystals, eg, NaCl. In the nodes of the crystal lattice there are ions of different signs. The relationship between ions is due to the forces of Coulomb attraction and is called such a ton of heteropolar.
Atomic crystals, eg, FROM (diamond), GE, SI.. In the lattice nodes there are neutral atoms that are held there due to covalent bonds arising from the exchange forces having a purely quantum character.
Metal crystals. In the nodes of the crystal lattice are positive metal ions. Valence electrons in metals are poorly connected with their atoms, they are freely moving throughout the volume of the crystal, forming the so-called "electronic gas". He connects positively charged ions.
Molecular crystals, For example, naphthalene, - in solid state (dry ice). They consist of molecules interconnected by Van der Waals, i.e. The strengths of the interaction of induced molecular electric dipoles.
§ 54. Changing the aggregate state
And in liquids and in solids there is always some number of molecules, the energy of which is sufficient to overcome the attraction to other molecules, and which are capable of leaving the surface of the liquid or solid. Such a liquid process is called evaporation (or vaporization), for solid bodies - sublimation (or sublimation).
Condensation It is called the transition of a substance due to its cooling or compression from the gaseous state into liquid.
Figure 54.1.
If the number of molecules leaving the liquid per unit of time through a single surface is equal to the number of molecules moving from steam into a liquid, then a dynamic equilibrium occurs between the processes of evaporation and condensation. Couples in equilibrium with its liquid is called saturated.
Melting It is called the transition of a substance from the crystalline 9-hard) state in liquid. Melting occurs at a certain, increasing with an increase in external pressure, melting temperature T pl.
Figure 54.2.
In the process of melting the heat, the substance reported is to perform work on the destruction of the crystal lattice, and therefore (Fig. 54.2, a) before the melt of the entire crystal.
The amount of heat L, which is necessary for melting 1 kg of substance, is called specific heat melting.
If the liquid is cooled, the process will go in the opposite direction (Fig. 54.2, b), - the amount of heat released by the body during crystallization): First, the temperature of the liquid is reduced, then at a constant temperature equal to T. pL , begins crystallization.
For crystallization of the substance, the presence of crystallization centers - crystalline germs, which can be both crystals of the resulting substance and any foreign inclusions. If there are no crystallization centers in a clean fluid, it can be cooled to a temperature of a smaller crystallization temperature, forming, with a supercooled fluid (Fig. B. - dotted).
Amorphous bodies are supercooled fluids.
Details Category: Molecular Kinetic theory Posted on November 14, 2014 17:19 Views: 15569In the solid bodies of the particle (molecules, atoms and ions) are located as close to each other that the interaction forces between them do not allow them to fly away. These particles can only make oscillatory movements around the equilibrium position. therefore solid body Save the form and volume.
By its molecular structure, solid bodies are divided into crystal and Amorphous .
The structure of crystal tel
Crystal cell
Crystallic calls such solid bodies, molecules, atoms or ions in which are located in a strictly defined geometric order, forming a structure in space, which is called crystal lattice . This order is periodically repeated in all directions in three-dimensional space. It is saved by large distances And not limited in space. He's called far about order .
Types of crystal solids
The crystal lattice is a mathematical model with which you can imagine how particles in the crystal are located. Mentally connecting in the space of the straight lines of the point, in which these particles are located, we get a crystal lattice.
The distance between atoms located in the nodes of this lattice is called grid parameter .
Depending on which particles are located in nodes, crystalline lattices are molecular, atomic, ion and metal .
From the type of crystal lattice, such properties of crystalline bodies are dependent as the melting point, elasticity, strength.
When the temperature is raised to the value at which the melting of a solid substance begins, the crystal lattice is destroyed. Molecules get more freedom, and the solid crystalline substance goes into a liquid stage. The stronger the connection between molecules, the higher the melting point.
Molecular grille
IN molecular lattices Communication between molecules is not durable. Therefore, under normal conditions, such substances are in a liquid or gaseous state. The hard state is possible for them only at low temperatures. Their melting temperature (transition from solid state In liquid) also low. And under normal conditions, they are in a gaseous state. Examples - iodine (i 2), "dry ice" (carbon dioxide CO 2).
Atomic grille
In substances having an atomic crystal lattice, the connection between the atoms is durable. Therefore, the substances themselves are very hard. They melt at high temperatures. The crystal atomic grid has silicon, germanium, boron, quartz, oxides of some metals and the highest substance in nature - diamond.
Ion lattice
To substances with an ion crystal lattice include rhymes, most salts, typical metal oxides. Since the strength of the attraction of ions is very large, these substances are able to melt only at a very high temperature. They are called refractory. They have high strength and hardness.
Metal grille
In the nodes of the metal lattice, which all metals and their alloys have, and atoms, and ions are located. Due to this structure, the metals have good living and plasticity, high heat and electrical conductivity.
Most often, crystal shape - right polyhedron. The edges and ribs of such polyhedra always remain constant for a particular substance.
Single crystal called monocrystal . It has the right one geometric shape, continuous crystal lattice.
Examples of natural single crystals - Diamond, Ruby, Rhinestone, Stone Salt, Icelandic Swamp, Quartz. In artificial conditions, single crystals are obtained in the crystallization process when the solutions or melts are cooling to a certain temperature, the solid substance is isolated from them in the form of crystals. At a slow crystallization rate, the cut of such crystals has natural shape. In this way, in special industrial conditions, for example, single crystals of semiconductors or dielectrics are obtained.
Small crystals, randomly struck each other, are called polycrystals . The brightest example of polycrystal is a stone granite. All metals are also polycrystals.
Anisotropy of crystal tel
In the crystals, the particles are located with various density in different directions. If we connect the straight line atoms in one of the directions of the crystal lattice, the distance between them will be the same for all this direction. In any other direction, the distance between atoms is also constantly, but its value may already differ from the distance in the previous case. This means that in different directions between atoms there are different intensity of the interaction force. Therefore, the physical properties of the substance in these areas will also differ. This phenomenon is called anisotropy - the dependence of the properties of the substance from the direction.
Electrical conductivity, thermal conductivity, elasticity, refractive index and other properties of the crystalline substance vary depending on the direction in the crystal. Differently in different directions is carried out electricityIn different ways the substance is heated, light rays are reflected in different ways.
In polycrystals, the phenomenon of anisotropy is not observed. The properties of the substance remain the same in all directions.
Properties of liquids
1. Characteristics of the liquid state. Middle order.
2. Surface tension. Forces arising from the surface curve. Laplace formula. Wetting and capillary phenomena.
1. Characteristics of liquid state. Liquid state occupies intermediate position between gases and crystalscombines some features of both of these states. For crystal States characteristic ordered arrangement of particles (atoms or molecules) in gases In this sense full chaos. According to radiographic studies, in relation to the nature of the location of the liquid particles occupy an intermediate position.
The location of the liquid particles is the so-called middle order. It means that in relation to any particle, the location of neighbors closest to it is ordered. but as the location is removed from this particle, with respect to it, other particles becomes less or less ordered and quite quickly, the order in the location of the particles completely disappears.
In crystal occurs far order – the ordered arrangement of particles relative to any particle is observed within a significant amount..
Evaluate the structure of the substance allows radial distribution function (In some textbooks, it is called a pair function of distribution). Choose some molecule as a body of reference. The average number of molecules in the spherical layer volume at a distance r. from the selected molecule (Fig. 10.1) we denote dN (R). The probability to detect molecules in this spherical layer
case perfect Gaza No elements of volume have advantages and the likelihood of finding a particle in this volume is proportional to the volume and g (R) \u003d1.
In perfect crystal Structure Hard And all mutual distances are fixed (Fig. 10.2).
Peaks corresponds to the grid nodes, and the final line width g (R) It is a consequence of oscillations of atoms relative to the node in the real crystal.
more smoothed than the crystal). At long distances, the curve tends to 1 as for the perfect gas.
only orientation is ordered, Mutual location, as in conventional liquids, long-range order does not detect.
2. Surface tension .
Liquid molecules are located as close to each other that the forces of attraction between them have a significant amount. The interaction quickly decreases with a distance, starting at some distance r. (molecular action radius). For each molecule in the surface layer thickness r. The force will act in the liquid (Fig. 10.5).
to increase the potential energy of the molecule. I.e in the surface layer of molecules have additional potential energy - superficial .
Due to the presence of the forces acting on the molecules in the surface layer, the fluid seeks to reduce its surfaceAs if it were concluded in the elastical stretched film, seeking to gripe (no film actually no).
Representing the liquid film (for example, a soap film), stretched on a wire frame, one of the sides of which (jumper) can move (Fig. 10.6). Thanks to the desire of the surface, the force will act on the wire. It is directed by the surface tangent to the surface of the fluid, perpendicular to the contour site (the length of the jumper) to which it acts ().
equal strength of the tension of the film, i.e. . The coefficient 2 appears due to the fact that the film has two surface layers.
Liquid outside the field of external forces will take a form with a minimum surface, i.e. shara shape.
Current surface pressure.
In the case of a spontaneous surface of the surface of the surface tension, they seek to reduce this surface. (Fig. 10.7).
pressure in the case of an inexcrew surface, and\u003e 0 in the case of a convex surface, and<0, если поверхность вогнутая (в этом случае поверхностный слой, стремится сократиться, растягивает жидкость и давление уменьшается).
Calculate additional pressure for the spherical surface of the liquid. Match a mentally spherical drop of liquid with a diametral plane for two hemispheres. Due to surface tension
Laplace summarized this formula to the surface of any form.
Formula Laplaslooks like that:
Wetting and capillary phenomena.
Wetting - a phenomenon that occurs when the liquid occurs with the surface of a solid body or other liquid. Expressed in particular, in spreading fluid on a solid surface. Wetting causes the formation of a meniscus in a capillary tube, determines the shape of a drop on a solid surface, etc. (we note that usually wetting is considered as a result of intermolecurial interaction, but wetting may be the result of a chemical reaction, diffusion processes).
Measure of wetting Usually serve regional angle between the tangent to the surface of the liquid. (Fig. 10.10). If, then they say that
where the coefficients of the surface tension of the fluid at the boundaries: a solid body - gas, a solid body - liquid, liquid - gas. Cutting on, we get for edible angle ratio:
(For example, full wetting will be with).
Wetting is essential in industry. Good wetting is necessary when painted, washing, processing photographic materials, soldering. The impurities strongly affect the size of the surface tension. For example, dissolving water soap reduces its surface tension coefficient by almost 1.5 times (which, in particular, causes the use of soap as a detergent). Distribution can lead to the fact that from the solution, the threads of which are covered with paraffin (with a small level of water), water is not poured, refuting the well-known saying.
Capillary phenomena.
The existence of wetting and edge angle leads to the fact that near the vessel walls there is a curvature of the surface of the liquid. If the liquid wets the walls, the surface has a concave form, if not wetting - convex. This kind of curved liquid surfaces are called meniscus. (Fig. 10.11)
| Wetting | Distribution |
| Fig. 10.11 |
Under the visible surface in the capillary, the pressure will differ from the pressure under the flat surface by magnitude. Between the liquid in the capillary and in a wide vessel, this difference in the level is established so that the hydrostatic pressure is equilibrated capillary pressure. In the case of a spherical form of meniscus
Radius of curvature of meniscus express through the edge angle and radius of the capillary r. then
In case of wetting and the height of the lift of the fluid in the capillary is the greater, the smaller the radius of the capillary r. .
Capillary phenomenon occupies in a person's life exceptional role. The supply of plants, trees occurs with the help of capillaries that are in each plant. Capillary phenomena can play and negative role. For example, in construction. The need for waterproofing foundations of buildings is caused by capillary phenomena.
Questions for self-control
1. Keep the liquid state in comparison with crystals and gases.
2. What is such a long and morbid order?
3. What makes it possible to make a radial distribution function? Draw it for crystals, liquids and gases.
4. What is the coefficient of surface tension?
6. What is so wetting? What is a measure of wetting? Give examples of processes for which good wetting is necessary.
7. What depends the height of the fluid raising in the capillary?
Lecture number 5 (11)
The properties of solid tel
1. Amorphous and crystal bodies. Building and types of crystals. De
fekes in crystals.
2. Mechanical properties of crystals. Mechanism of plastic deform
. Deformation of elastic stretching. The law of a bitch.
Amorphous and crystal bodies.
In amorphous bodies exists middle order Atom location. Crystals possess far about order Atom location. Amorphous Body isotropic, crystalline - anisotropic.
When cooled and heated, the temperature and temperature curves are different for amorphous and crystalline bodies. For amorphous bodies, the transition from liquid in a hard state can be dozens of degrees. For crystals, the melting point is constant. Cases are possible when the same substance, depending on the cooling conditions, can be obtained both in crystalline and in amorphous solid state. For example, glass when very slow cooling melt can crystallize. At the same time, on the boundaries of small formed crystals, light scattering will occur, and the crystallized glass loses transparency.
Crystal cell. The main property of crystals is the regularity of atoms in them. On the totality of points in which atoms are located (more precisely atomic nuclei), they say crystal latticeand points themselves are called grid knots.
The main characteristic of the crystal lattice is spatial periodicityits structures: crystal as it consists of repeating parts (cells).
We can break the crystal lattice on completely identical parallelepipeds containing the same number of equally located atoms. Crystal represents a combination of parallelepiped, parallel to the shift in relation to each other. If you shift the crystal lattice in parallel to myself at the distance of the length of the rib, then the grid is combined with itself. These offsets are called broadcast, and lattice symmetries in relation to these offsets are talking to broadcast symmetry (parallel transfer, rotation relative to the axis, mirror reflection, etc.).
If at the top of any elementary cell is atom, then the same atoms should obviously be in all other vertices of this and other cells. The totality of the same and equally located atoms is called grid Brav This crystal. She represents like skeleton crystal lattice, personifying all its broadcast symmetry, i.e. All its frequency.
Classification of various types of symmetry of crystals Based primarily on classification different types of grilles Brava.
The most symmetrical lattice of Brave is a lattice having symmetry cuba (cubic system). There are three different
| Grilles Brava belonging to the cubic system: simple | , | ||
| volumetric-central (in the center of Cuba - atom), gorolstered (except atoms in the vertices - more in the atom in | |||
centers of all their faces). In addition to cubic, there is a tetragonal, rhombic, monoclinic and others (we will not consider).
Brave lattice, generally speaking, does not include all atoms in the crystal. Real crystal grillecan be represented as a combination of several lattices of Brava, persecuted alone to another.
Physical types of crystals.
By the nature of the particles, of which the crystalline lattice is constructed, by the nature of the interaction forces between them, is distinguished by ionic, atomic, metal and molecular crystals.
1. Ionic crystals. In the nodes of the crystal lattice are alternately positive and negative ions. These ions are attracted to each other by electrostatic (Coulomb) forces. Example: Stone Salt Grid (Fig. 11.1).
| Fig. 11.1. |
2. Atomic crystals. Typical representatives are graphite and diamond. Communication between atoms - covalent. In this case, each of the valence electrons enters the electron pair that connects this atom with one of the neighbors.
3. Metal crystals. Grills consist of out positively charged ionsbetween which are located "Free" electrons. These electrons are "collectivized" and can be considered as a kind of "electronic gas". Electrons play the role of "cement", holding "+" ions, otherwise the lattice would sing. Ions hold the electrons within the lattice.
4. Molecular crystals. An example is ice. In nodes - moleculesrelated to each other forces van der Waals. forces interaction Molecular electric dipoles.
There may be several types of connections (for example, in graphite - covalent, metal and van der Waals).
Defects in Crystalch.
In real crystal lattices there exists deviations from the ideal arrangement of atoms In the lattices that we have so close now. All such deviations are called defects of crystal lattice.
Spot defects - Such, in which violates the near order:
Another type of defects - dislocations - Linear defects of the crystal lattice, violating the correct alternation of atomic planes. They are violate long-range order, distorting all its structure. They play an important role in the mechanical properties of solid bodies. The simplest types of dislocations are edible and screw. In the case of the edge dislocation, the excess crystalline plane is inwarded between adjacent layers of atoms (Fig. 11.5).
In the case of a helical dislocation, part of the crystal lattice is shifted relative to the other (Fig. 11.6)
Depending on the physical properties and molecular structures, two main class solids are isolated - crystalline and amorphous.
Definition 1.
Amorphous bodies have such a feature as isotropy. This concept means that they are relatively independent of optical, mechanical and other physical properties and directions in which external forces affect them.
The main feature of the afroful bodies is the chaotic arrangement of atoms and molecules that are collected only in small local groups, not more than several particles in each.
This property brings out amorphous bodies with liquids. Such solid bodies include amber and other solid resins, various types of plastic and glass. Under the influence of high temperatures, amorphous bodies softened, however, strong effects of heat are needed to translate them into liquid.
All crystal bodies have a clear internal structure. Particle groups in the same order are periodically repeated throughout the volume of such a body. To clearly imagine such a structure, spatial crystalline grilles are usually used. They consist of a certain number of nodes that form centers of molecules or atoms of a particular substance. Typically, such a grill is constructed from ions that are part of the desired molecules. Thus, in the table salt, the internal structure consists of sodium ions and chlorine, in pairs of molecules. Such crystal bodies are called ionic.
Figure 3. 6. one . Crystal grille of a salt salt.
Definition 2.
In the structure of each substance, one minimum component can be distinguished - elementary cell.
The entire lattice, from which the crystal body consists, can be composed by broadcasting (parallel transport) of such a cell in certain directions.
The number of types of crystal lattices is not infinite. In total there are 230 species, most of which are created artificially or found in natural materials. Structural grilles can take forms of volume centered cubes (for example, in iron), grazent cubes (in gold, copper), prisms with six faces (magnesium, zinc).
In turn, crystalline bodies are divided into polycrystals and single crystals. Most substances relate to polycrystals, because They consist of the so-called crystallites. These are small crystalline, which have grown together and oriented chaotic. Monocrystalline substances are relatively rare, even among artificial materials.
Definition 3.
Polycrystals have the property of isotropy, that is, the same properties in all directions.
The polycrystalline body structure is clearly visible under the microscope, and some materials, for example, cast iron, and a naked look.
Definition 4.
Polymorphism - This is the possibility of substance to exist in several phases, i.e. Crystal modifications that differ from each other with physical properties.
The process of transition to another modification was called polyforal transition.
An example of such a phenomenon may be the conversion of graphite in a diamond, which in industrial conditions occurs at high pressure (up to 100,000 atmospheres) and high temperatures
(up to 2000 k).
To study the structure of the crystal lattice of a single crystal or a polycrystalline sample, X-ray diffraction is used.
Simple crystal lattices are shown in the figure below. It is necessary to take into account that the distance between the particles is so little, which is comparable to the sizes of these particles themselves. For clarity, only the positions of the centers are shown in the schemes.
Figure 3. 6. 2. Simple crystal lattices: 1 - simple cubic lattice; 2 - granetable cubic lattice; 3 - a centrified cubic lattice; 4 - Hexagonal grille.
The most simple is a cubic grille: such a structure consists of cubes with particles in the vertices. The grazent grille has particles not only in the vertices, but also on the edges. For example, the crystal lattice of the cooking salt is two grazent lattices attached to each other. The centrified lattice has additional particles in the center of each cube.
The lattices of metals have one important feature. Ions of the substance are held in their places due to the interaction with the gas of free electrons. The so-called electron gas is formed due to one or more electrons given by atoms. Such free electrons can move throughout the volume of the crystal.
Figure 3. 6. 3. Structure of a metallic crystal.
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