, solutions, melts, from a substance in another crystalline or amorphous state.
Crystallization begins when a certain limiting condition is reached, for example, supercooling of a liquid or supersaturation of a vapor, when small crystals - "nuclei", centers of crystallization, appear almost simultaneously in a multitude of crystals. Microcrystals grow by attaching new atoms or molecules from the surrounding melt, liquid or vapor to their surface. The growth of crystal faces occurs layer by layer, the edges of incomplete atomic layers (growth steps) during crystallization move along the face in successive fronts one after another. Depending on the growth rate and conditions, the crystallization process leads to the formation of various forms of growth (multifaceted, flattened, acicular, filiform, skeletal, dendritic, and other forms) and internal crystal structures (zonal, sectorial, block, and other structures). With rapid crystallization, various internal defects of the crystal lattice inevitably arise.
If the crystal does not melt, does not dissolve, does not evaporate and does not grow, then it is in thermodynamic equilibrium with the mother medium (melt, solution or vapor). Equilibrium of a crystal with a melt of the same substance is possible only at the melting temperature, and equilibrium with a solution and vapor - if the latter are saturated. Supersaturation or supercooling of the medium is a necessary condition for the growth of a crystal in it, and the growth rate of the crystal is the greater, the greater the deviation from the PT equilibrium conditions.
Crystallization is a phase transition of a substance from the state of a supercooled (supersaturated) mother medium to a crystalline chemical compound with a lower free energy. Excess energy generated during crystallization is released in the form of latent heat. Some of this heat can be converted into mechanical work. For example, a growing crystal can lift a load placed on it, developing crystallization pressure (being different for different minerals, in some cases it can be estimated at tens of kg/cm2). In particular, antholiths of such a soft mineral as gypsum can lift pieces of rock weighing several kg. An example is also widely known with ice antholithscapable of pushing frozen ground or lifting heavy objects. And the salt crystals that form in the cracks of concrete dams in seawater sometimes cause the concrete to fail.
A supercooled medium can preserve for a long time, without crystallizing, an unstable metastable state. However, when a certain critical supercooling of the medium, which is critical for the given conditions, is reached, many small seed crystals instantly appear in it. The resulting crystals grow and, if supercooling decreases, new nuclei, as a rule, no longer appear. Much also depends on the presence of energetically active phases or particles that can play the role of "seeds" that provoke the onset of crystallization.
Plasma, as well as from amorphous substances or crystals of a different structure. In the process of crystallization, atoms, molecules or ions of a substance line up in a crystal lattice. Crystallization is a non-equilibrium phase transition of the 1st order. The conditions for the equilibrium of a crystal with a medium (melt, steam, solution, etc.) are defined as the phase equilibrium of the aggregate states of a substance during phase transitions of the 1st kind: equality of temperature, pressure, and chemical potential. A necessary condition for crystal growth is a deviation from equilibrium, determined by overcooling (difference in temperature from equilibrium) and supersaturation (difference in pressure or concentration from equilibrium values). The thermodynamic driving force of the phase transition is the higher, the greater the deviation from equilibrium. The transition of a substance to the crystalline phase is accompanied by the release of the latent heat of crystallization, and if this heat is not completely removed, the deviation from equilibrium can decrease and the process slows down. As a phase transition of the 1st kind of crystallization, it is accompanied by a jump in the specific volume relative to the initial phase, and this can lead to a change in pressure in the crystallizing system. Thus, crystallization is a complex process of heat and mass transfer, which is controlled by thermodynamic and kinetic factors. Many of them are difficult to control. The level of purity, temperature and concentration of components in the immediate vicinity of the phase boundary, mixing, heat transfer can be the main factors determining the size, number and shape of the resulting crystals.
Centers of crystallization. The crystallization process consists of two stages: the nucleation of crystallization centers and the growth of crystals. The initial stage - the nucleation of crystallization centers - is the formation of clusters with order characteristic of a crystal. But sometimes their structure may differ from the structure of a stable macroscopic crystal. The formation of such clusters in pure liquids or gases occurs below the melting point of a massive crystal as a result of random collisions during the thermal motion of atoms or molecules. At temperatures below equilibrium, the association of particles into a crystalline cluster is thermodynamically beneficial, but the appearance of its new surface requires energy, which is an obstacle to the nucleation of crystallization centers. The smaller the cluster, the greater the fraction of particles that make up its surface. Therefore, at small sizes, most clusters break up due to fluctuations in the vibrational energy of particles. As the cluster grows, the fraction of the surface energy decreases with respect to the bulk energy of particle association, which increases the stability of the cluster. At a given supersaturation, there is a critical size, above which the clusters are capable of further growth and become crystallization centers.
The numerical characteristic of the intensity of the nucleation of crystallization centers is the frequency of nucleation (nucleation) - the number of centers that arise per unit of time per unit volume of the medium. The existing theory explains the temperature dependence of the nucleation frequency and relates it to the parameters of the medium in which crystallization centers are formed. For liquids with low viscosity, for example, for most molten metals, the theory predicts large supercoolings, at which spontaneous nucleation of crystallization centers should be observed. With a further increase in supercooling, the nucleation frequency rapidly increases, reaching a maximum at a temperature approximately equal to one third of the equilibrium temperature of the crystal with the melt. The rapid decrease in the frequency of nucleation of crystallization centers at even lower temperatures is due to the slowdown in thermal motion and a strong increase in viscosity. For more viscous liquids, the frequency maximum is shifted towards lower subcoolings, and the frequency values themselves are much lower.
Since many parameters of the theory are known with insufficient accuracy for calculations, experimental data play an important role. Approximation to ideal conditions is achieved by using small drops of liquids with a diameter from several micrometers to nanometers in the experiments. Spontaneous nucleation requires large deviations from equilibrium, and crystallization centers are characterized by a critical size of the order of one nanometer. For example, for melts of pure metals, the temperature of spontaneous nucleation of crystallization centers observed in experiments is 30–50% of the melting temperature. Many silicate melts, when cooled, generally solidify without crystallization, forming glasses. It has been experimentally shown that in viscous liquids the process of nucleation of crystallization centers is nonstationary. This means that the frequency of nucleation of crystallization centers, which is characteristic of a given deviation from equilibrium, appears only after the delay time has elapsed, which can be quite large, comparable to or even exceeding the cooling time of the sample. Metal melts are characterized by a much lower viscosity, and the suppression of the spontaneous nucleation of crystallization centers for some alloys is possible only with very rapid cooling (at a rate of more than 10 6 K/s). This underlies the technology for producing amorphous metals. The stability of the amorphous state is ensured by a strong slowing down of the exchange of atoms between the crystal and the medium at low temperatures. It is possible to observe the crystallization of the amorphous state obtained in this way during heating, increasing the intensity of thermal motion, and the latent heat of the phase transition released in this case can significantly intensify the process, further raising the temperature. For some substances (germanium, silicon, amorphous ice), explosive crystallization of the amorphous state is observed.
In polluted media, crystallization centers appear on foreign crystalline particles with much smaller deviations from equilibrium. The frequency of nucleation of crystallization centers in such cases also depends on the material of the walls of the vessel and the effect of radiation. Germinal crystals on a well-wettable orienting surface are approximately dome-shaped; Therefore, such a heterogeneous nucleation of crystallization centers occurs at lower supercoolings. Controlled heterogeneous nucleation of crystallization centers is used, for example, in the epitaxial production of single-crystal films. When growing large perfect single crystals containing the minimum possible number of defects on a seed crystallization center, it is necessary to avoid the appearance of spontaneous nuclei. To do this, use a small deviation from the equilibrium conditions. In metallurgy, when obtaining crystalline materials, they strive to obtain the maximum number of crystallization centers, for which deep supercooling of the melts is created.
Mechanisms of crystal growth. Depending on whether the surface of the crystal is smooth or rough on the atomic scale, two mechanisms of crystal growth are distinguished: layered and normal. Atomically smooth surfaces usually correspond to the most developed faces with simple crystallographic indices. They contain a relatively small number of defects: vacancies and adsorbed atoms. The edges of incomplete atomic planes form steps (Fig. 1), which, in turn, have a small number of kinks. The elementary act of crystal growth consists in attaching a new particle to a fracture and does not change the surface energy. The successive attachment of particles to a fracture leads to its movement along the step, and the steps along the surface - such growth is called layer-by-layer. The density of steps during layer-by-layer growth depends on the mechanism of their generation. Steps can arise as a result of the formation and growth of two-dimensional nuclei. The process of formation of two-dimensional nuclei capable of further growth on an atomically smooth surface has some analogy with the formation of crystallization centers in a liquid. A two-dimensional nucleus also has a critical size, starting from which it is capable of further growth. When a two-dimensional nucleus is aggregated, an obstacle to its development at small sizes is the expenditure of work on the linear energy of its perimeter. But as the size increases, the fraction of the linear energy of the perimeter becomes smaller and smaller, and, starting from a certain critical size, the two-dimensional nucleus becomes the center of growth of a new step. The frequency of formation of two-dimensional nuclei is very low for small deviations from equilibrium, and the growth rate determined by two-dimensional nucleation is correspondingly low. Noticeable growth rates with this mechanism of step formation begin at a noticeable supercooling and increase very strongly (exponentially) with its increase. Another step generation mechanism is associated with screw dislocations. If the crystal contains a screw dislocation, then its growth occurs by attaching atoms to the end of the step ending in the dislocation (Fig. 2a). When growing on a screw dislocation, the step acquires a spiral shape (Fig. 2b), and a noticeable growth rate increases with supercooling according to a quadratic law and is observed already at small deviations from equilibrium.
On atomically rough surfaces (Fig. 3), the kink density is high, and the addition of new particles to the crystal occurs practically at any point on its surface. This growth is called normal. Its speed increases linearly with supercooling. The theory of crystal growth relates the packing density of the crystal surface to the binding energy between the particles of the crystal surface and the heat of crystallization. It is believed that if the binding energy is high enough, all close-packed faces are smooth. This is characteristic of crystals growing from vapor. The heat of crystallization of melts, as a rule, is much lower than the heat of crystallization from steam; therefore, the binding energy of particles in a crystal is lower compared to a melt than compared to steam. In this regard, the surface of a crystal growing from a melt is usually rough, which determines normal growth and the formation of rounded faces. The transition from roughness to faceting is possible with a change in concentration in two-component systems during crystal growth from solution. In crystals of germanium and silicon growing from a melt, one can observe the coexistence of flat and rounded faces.
The forms of crystal growth are determined by the anisotropy of their properties and the conditions of heat and mass transfer during crystallization. Crystals with rough surfaces are usually round in shape. When growing such crystals, due to the high rate of surface processes, supercooling at the boundary with the melt is small and the growing surface repeats the shape of the temperature field isotherm in the system at the equilibrium temperature. Atomically smooth surfaces appear as faces. The equilibrium form of a crystalline polyhedron is such that the distance from the center to each face is proportional to the magnitude of its surface energy. The equilibrium form is also a stationary form of growth, but in a real growth process it can be strongly distorted due to the instability of the growth surface under finite (rather than infinitesimal) supercooling and the influence of impurities.
If the melt is strongly supercooled and the temperature in the melt decreases with distance from the growth front, then the growth is unstable: a protrusion that randomly appears on the crystal surface falls into the region of greater supercooling, and its growth rate increases. Such instability for a flat crystallization front leads to the formation of a banded or cellular structure of the crystal (Fig. 4). With the growth of a small crystal, this instability manifests itself starting from a certain size of the crystal. It develops protrusions and acquires a skeletal or dendritic form, which is characterized by the appearance of secondary branches after the primary protrusion reaches a critical length (Fig. 5). The growth of large faceted crystals from immobile solution can also be unstable. Supersaturation in this case is higher at the vertices and edges of the crystal and less in the central parts of the face. Therefore, vertices become the leading sources of layer growth. With a large difference in supersaturations at the vertices and in the centers of the faces, the vertices overtake the centers of the faces, and a skeletal form of the crystal appears (Fig. 6). At a given temperature in a two-component system, equilibrium can exist for different compositions of the crystal and melt. During crystal growth, one of the components accumulates in front of the front, causing concentration supercooling, and this often leads to instability of the growth front.
Different faces of the crystal during growth capture different amounts of impurities contained in the medium. This is how its sectorial structure arises. If the crystal poorly captures the impurity, it accumulates ahead of the growth front. The periodic capture of this boundary layer by a growing crystal leads to the formation of a zonal structure (Fig. 7). The capture of impurities leads to a change in the parameters of the crystal lattice, and internal stresses arise at the boundaries of regions of different composition, which leads to the formation of dislocations and cracks. Dislocations arise as a result of relaxation of elastic stresses in an unevenly heated crystal, or they can pass from the seed to the growing crystal.
Mass crystallization - the simultaneous growth of many crystals, widely used in industry. The properties of ingots and castings during the crystallization of metallurgical melts depend to a large extent on the number of crystallization centers and the conditions for their growth. During the solidification of metal castings, crystallization centers appear first on the cooled walls of the mold, where the molten metal is poured. Of the randomly oriented crystals, those that grow perpendicular to the wall survive. They form a columnar zone near the wall. Convection currents in the melt can break off dendritic branches, supplying secondary crystallization centers to the melt. Bulk crystallization in solutions begins at heterogeneous crystallization centers or on specially introduced seed crystals. Collisions of these crystals with each other and with the walls of the vessel in a stirred solution give rise to secondary centers of crystallization. To create additional centers of crystallization, ultrasonic crushing of growing crystals or the addition of surfactants is used. Bulk crystallization is also used to purify substances from impurities.
Application of crystallization. In nature, crystallization leads to the formation of minerals, ice, plays an important role in many biological processes. Crystallization also occurs in some chemical reactions, in the process of electrolysis. It underlies many technological processes: in metallurgy, in the production of materials for electronics and optics. Massive single crystals and thin films are obtained by crystallization. Crystallization is widely used in the chemical, food, and medical industries: in the technology of purification of substances, in the production of salt, sugar, and medicines.
Lit.: Shubnikov A.V. Formation of crystals. M.; L., 1947; Lemmlein GG Morphology and genesis of crystals. M., 1973; Lodiz R. A., Parker R. L. Growth of single crystals. M., 1974; Problems of modern crystallography. M., 1975; Modern crystallography. M., 1980. T. 3; Chernov A. A. Crystallization Physics. M., 1983; Geguzin Ya. E., Kaganovsky Yu. S. Diffusion processes on the surface of a crystal. M., 1984; Skripov VP, Koverda VP Spontaneous crystallization of supercooled liquids. M., 1984; Problems of crystallography. M., 1987; Chuprunov E. V., Khokhlov A. F., Faddeev M. A. Crystallography. M., 2000.
Water is not only one of the most necessary, but also the most amazing phenomena on our planet.
It is known that virtually all substances of natural or artificial origin are able to be in different states of aggregation and change them depending on environmental conditions. And although scientists know more than a dozen phase states, some of which can only be obtained within the laboratory, only three such states are most often found in nature: liquid, solid and gaseous. Water can be in all three of these states, passing from one to another in natural conditions.
Water in its liquid state has loosely bound molecules that are in constant motion and try to cluster into a structure, but cannot do so due to the heat. In this form, water can take absolutely any form, but is not able to hold it on its own. When heated, the molecules begin to move much faster, they move away from each other, and when the water gradually turns into a gaseous state, that is, it turns into water vapor, the bonds between the molecules finally break. When water is exposed to low temperatures, the movement of molecules slows down greatly, molecular bonds become very strong, and molecules, which are no longer interfered with by heat, are ordered into a crystalline 
hexagonal structure. We have all seen similar hexagons falling to the ground in the form of snowflakes. The process of turning water into ice is called crystallization or solidification. In the solid state, water can retain any form it takes for a long time.
The process of crystallization of water begins at a temperature of 0 degrees Celsius, which has 100 units. This measuring system is used in many European countries and in the CIS. In America, the temperature is measured using the Fahrenheit scale, which has 180 divisions. Through it, water passes from a liquid state to a solid state at 32 degrees.
However, water does not always freeze at these temperatures, so very pure water can be supercooled to a temperature of -40 ° C and it will not freeze. The fact is that in very pure water there are no impurities that serve as the basis for building a crystalline structure. Impurities to which molecules are attached can be dust particles, dissolved salts, etc.
A feature of water is the fact that while other substances are compressed when freezing, it, on the contrary, expands when it transforms into ice. This happens because when water passes from a liquid to a solid state, the distance between its molecules increases slightly. And since ice has a lower density than water, it floats on its surface.
Speaking of the freezing of water, one cannot fail to mention the interesting fact that hot water freezes faster than cold water, no matter how paradoxical it may sound. This phenomenon was known back in the time of Aristotle, but neither the famous philosopher nor his followers managed to unravel this mystery and the phenomenon was forgotten for many years. They started talking about it again only in 1963, when Erasto Mpemba, a student from Tanzania, noticed that when making ice cream, a delicacy made from warmed milk hardens faster. The boy told his physics teacher about this, but he laughed at him. Only in 1969, having met with physics professor Dennis Osborne, the young man was able to find confirmation of his conjecture, after jointly conducted experiments. Since then, many hypotheses have been put forward regarding this phenomenon, for example, that hot water freezes faster due to its rapid evaporation, which leads to a decrease in the volume of water and, as a result, faster solidification. But none of them could not explain the nature of this phenomenon.
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Alexandra
24.08.2017 12:05
11.03.2015 21:11
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CRYSTALLIZATION- the formation of crystals from a gas, solution, melt, glass or crystal of other structures (polymorphic transformations). K. consists in laying atoms, molecules or ions in crystal lattice. K. determines the formation of minerals, ice, plays an important role in the atm. phenomena, in living organisms (formation of tooth enamel, bones, kidney stones). By K. receive both massive single crystals, and thin crystalline. films, dielectrics and metals. Mass K. - at the same time. the growth of many small crystals - is the basis of metallurgy and is widely used in the chemical, food and medical industries.
Thermodynamics of crystallization. The arrangement of particles in a crystal is ordered (see Fig. far and near order), and their entropy S K less entropy S c in disordered medium (steam, solution, melt). Therefore, a decrease in temperature T at post. pressure R leads to that chemical potential substances in a crystal
becomes less than its potential in the initial environment:
Here are the interaction energies of particles and sp. the volume of matter in a crystal and disordered states (phases), S K and S C - entropy. Thus, crystalline. the phase turns out to be “more profitable”, K. occurs, accompanied by the release of the so-called. latent heat K.: H=T(S C -S K)0.5-5 eV, as well as a jump in beats. volume 
(phase transition of the first kind). If p10 4 atm, then the term in relation (1) is small, and the heat value of K. is equal to 
, i.e., is a measure of the change in the binding energy between particles at K. [at K. from the melt and may have decomp. signs].
To. at polymorphic transformations (see. Polymorphism) can be a second-order phase transition. In the case of first-order transitions, the crystal-medium interface is localized within several interatomic distances, and its sp. free energy >0.
For transitions of the second kind, the boundary is not localized and
Conditions( p, T, C k) =(p, T, C c) for each of the components of the crystal and the medium, the relationship is determined p, T and concentration of components With, at which the crystal is in equilibrium with the medium, i.e. state diagram substances. Difference
, which is a measure of deviation from equilibrium, called. thermodynamics h. driving force K. Usually it is created by lowering the temperature below the equilibrium value T 0, i.e. subcooling the system T-T 0 -T. If a
T T 0 , then
If the pressure R vapor or concentration With there are more equilibrium values in solution p 0 and From 0, then they talk about abs. supersaturation ( p=p-
p 0 or C=C-C 0) or relates. supersaturation (= r/r 0 or S/S 0). In this case, in rarefied vapors and dilute solutions
In the process of growing single crystals from solutions, usually from vapors and during chemical. reactions 1, during the condensation of molecular beams 10 2 -10 4 .
To. can occur as a result or with participation of chemical. reactions. The equilibrium state of a mixture of gases with a possible chem. reactions between its constituent substances A i can be generally written as , where is stoichiometric. coefficient (<0
для прямой реакции,
>0 for reverse). In this case
Here To is the equilibrium constant of the reaction, p i- (or concentration, if the reaction proceeds in solution). In the case of electrolytes
Rice. 4. Crystallographic densest (top) and pentagonal (bottom) packings.
Lowering the temperature not only reduces the work of nucleation, but also exponentially increases the viscosity of the melt, i.e., reduces the frequency of attachment of new particles to the nucleus (Fig. 5a). As a result I(T) first reaches a maximum, and then becomes so small (Fig. 5, b) that at low temp-pax the melt solidifies, remaining amorphous. In melts with relatively low viscosity, this is only possible with very rapid (10 6 K/s) cooling. This is how amorphous metal alloys are obtained (see. amorphous metals). In liquid helium, the formation of nuclei is possible not by the transition of the system through the barrier, but by tunneling through it. When growing large perfect crystals on "seeds", the appearance of spontaneous nuclei is avoided by using slightly supersaturated solutions or overheated melts. On the contrary, in metallurgy one strives to obtain the maximum number of crystallization centers by creating deep supercoolings (see below).
Rice. Fig. 5. Temperature dependences of the rate of nucleation and crystal growth: a) solid curves - temperature dependence of the number of nuclei of citric acid in supercooled aqueous solution (saturation temperature: A-62 ° C, AT- 85 °С); dotted line - increase in viscosity (in poise) of solutions with a decrease T; b) growth rate v crystals of benzophenone from the melt as a function T.
crystal growth can be layered and normal depending on whether its surface is atomically smooth or rough. The atomic planes forming a smooth face are almost completely completed and contain a relatively small number of vacancies and atoms adsorbed in places corresponding to the sites of the crystal. gratings of the next layer. The edges of unfinished atomic planes form steps (Fig. 6c). As a result of thermal fluctuations, the step contains a certain number of three-dimensional incoming angles - kinks. The attachment of a new particle to a kink does not change the surface energy and, therefore, is an elementary act of crystal growth. With an increase in the ratio of thermal energy kT to the surface energy (per 1 atomic site on the surface), the kink density increases. Accordingly, the configuration increases. entropy and the free linear energy of the step decreases. When defining relations (close to 1, but somewhat different for different faces), the linear energy of the step turns to 0, and the step is "smeared" along the edge, the edge turns into a rough, i.e., uniformly and densely covered with fractures surface (Fig. 6, b). The connection of surface energy with the heat of K. allows us to conclude that for substances and temperatures, for which the change in entropy during K. is such that S/k>4, all close-packed faces are smooth. This situation is typical for crystal-vapor equilibrium, and also (for certain substances) for the crystal-melt interface. The transition from roughness to faceting is possible with a change in concentration in two-component systems (K. from solutions). If a S/k<2
(typical for the melting of metals), then the surfaces of any orientation are rough. At 
individual smooth faces coexist with rough surfaces (eg Ge and Si crystals in melts, garnets in melts and high-temperature solutions). The dependence of the free energy and velocity of the surface on the orientation of the surface has sharp (singular) minima for smooth (singular) faces and rounded (nonsingular) for rough surfaces.
Rice. 6. Atomically smooth ( a) and rough ( b) surfaces (computer modeling).
Rice. 7. Concentric steps on the (100) face of NaCl during growth from a molecular beam. Step height 2.82 A (decorated with small crystals of specially deposited gold).
Rice. 8. a - Spiral form of growth; b- a step ending on the surface at the point of its intersection with a screw.
The addition of a new atom in any position on the surface, except for a break, changes its energy. The filling of a few vacancies, which reduces this energy, cannot give rise to a new atomic layer, and the concentration of atoms in places corresponding to the lattice sites of the next layer increases the energy and is therefore small. As a result, the irreversible attachment of particles to a crystal, i.e., its growth, is possible only when there are breaks on its surface. On rough surfaces, the density of fractures is high, and growth along the normal to the surface is possible at almost any point. Such an increase in normal. It is limited by the speed of attaching otd. particles to fracture. His speed R increases linearly with supercooling at the front K.:
Here a- interatomic distance, l 0 - distance between kinks, - effective thermal frequency , - energy required to attach particles to a kink (activation energy). It takes into account the rearrangement of the short-range order in a liquid, desolvation builds. particles and kinks in solutions, chem. reactions, etc. In simple melts, the coefficient. are large, which ensures growth at a noticeable rate when supercooling at the K front. T 1 TO. So, for the growth of Si 10 6 cm / s R = (3-5) * 10 -3 cm / s is achieved at 10 -5 K. At sufficiently low temperatures, the particle mobility decreases and the growth rate decreases, similar to the nucleation rate (Fig. . 5, b).
If the surface is smooth, then breaks exist only on the steps, growth is followed. deposition of layers and called. layered. If the surface is formed by a ladder of identical steps and is on average deviated from the nearest singular face by an angle with tangent R, then cf. its growth rate along the normal to this singular orientation
where is the step growth rate along the face, (In solutions 10 -1 -10 -3 cm/s.)
The density of steps is determined by whether they are generated by two-dimensional nuclei or dislocations The formation of two-dimensional nuclei requires overcoming a potential barrier, the height of which is proportional to the linear energy of the steps and inversely proportional to . Correspondingly, the speed of the k. is exponentially small for small T[for the growth of face (III) Si with R=(3-5)*10 -3 cm/s, it is necessary T 0.ZK; see above]. In K. from molecular beams, if there are places of predominant formation of two-dimensional nuclei, the steps have the form of closed rings (Fig. 7). It is possible that the formation of nuclei is "facilitated" by the points of emergence of edge dislocations on the surface.
When growing on a screw dislocation, the step formed by it in the process of growth acquires a spiral shape (Fig. 8), since at the end point of the step on the dislocation, its growth rate is 0. In the process of spiral growth, the new layer "winds" on itself around exit point of the dislocation and a gently sloping (vicinal) mound of growth appears on the surface. Often mounds are formed by a group of dislocations, the total Burgers vector of which has a component in the direction of the normal to the surface b, equal to several parameters a gratings. The exit points of these dislocations can occupy a certain area on the surface (with a perimeter 2L, rice. nine, a, c). In this case, the slope of the circular vicinal mound forms an angle with the singular face with the tangent R =b/(19r c +2 h) (Fig. 9, b). The slopes of the mounds are measured by optical methods. (Fig. 10), by the method of colors of thin plates, and sometimes directly by visualization of the steps.
Rice. 9. A two-start helix forming a vicinal mound around the exit points of two dislocations to the surface: o) general view of the mound; b) its section by a plane perpendicular to the face and passing through the exit points of dislocations; c) a spiral on the (100) face of a synthetic diamond.
Rice. 10. Interference fringes from the vicinal hillock on the edge of the crystal prism ADP(growth from aqueous solution).
The radius of a 2D crit. nucleus is proportional to the linear energy of the step and inversely proportional to T. Therefore, with an increase T knoll steepness R increases linearly at small T and tends to saturation at large (at L 0). Accordingly, the normal growth rate R increases quadratically with supersaturation at low supercoolings and linearly at high supercoolings (Fig. 11). Variations of the Burgs vector and extent L dislocation sources determine the scatter in the growth rates of crystallographically identical faces (or the same face) under the same conditions. As the face grows, the exit point of a dislocation not perpendicular to it shifts and can reach one of the edges. After that, the step disappears. Further, crystallization proceeds only by two-dimensional nucleation, and the growth rate decreases at low supercoolings (at least by several orders of magnitude for crystallization from a melt and by several orders of magnitude for crystallization from solution). Due to the relatively small values of the linear energy of the steps at the crystal-melt interface and the absence of the problem of delivering the crystallizing substance , and L to several. orders of magnitude higher than for K. from solutions and the gas phase.
In view of the low density of the gas phase, layer-by-layer K. goes from it to the main. not by a direct hit of particles on the steps, but due to particles adsorbed on atomically smooth "terraces" between the steps. During the time between the moments of sticking to the surface and such a particle performs random walks on the surface and leaves the point of sticking at a distance of the order of cp. run l s . Therefore, only particles adsorbed around it in a band wide can reach the step. Most particles falling on the surface with a low density of steps evaporate - coefficient. condensation for such surfaces is small. It approaches 1 at a high density of steps, i.e. at means. supersaturations. For the same reason, the velocity of radiation from the gas phase, even at one dislocation, increases quadratically with supersaturation at low supersaturations and linearly at high supersaturations. During the condensation of molecular beams, steps are formed by two-dimensional nucleation in places where supersaturation in the adsorption layer reaches a critical level, and therefore cf. the distance between the steps is determined by the path length of the adsorbers. particles.
The supply of matter to a growing surface and the removal of heat from it K. limits the speed of K. when these processes proceed more slowly than surface ones. Such a diffusion regime is typical for crystals made from melts and unmixed solutions. The high rate of crystallization from a melt underlies all widely used methods for growing single crystals, in which the velocity of crystallization is set mechanically. the movement of the crystal relative to the independently formed thermal field. Kinetic the mode of crystallization, when the velocity of crystallization is limited by surface processes, is characteristic of crystallization from stirred solutions, from the gas phase, and the growth of crystals with high melting entropy from a stirred melt.
Rice. 11. Tilts R vicinal hillocks formed by two different dislocation sources, and the facet growth rates specified by them R depending on supersaturation.
growth forms crystals (habitus) are determined by the anisotropy of the K. velocity and the conditions of heat and mass transfer. Crystals with rough surfaces are usually round in shape. Atomically smooth surfaces appear as faces. Stationary form crystalline. polyhedron is such that the distance from the center to each face is proportional to its growth rate. As a result, the crystal is formed by faces with min. growth rates (faces with high speeds gradually decrease and disappear). They are parallel to planes with max. tight packing and max. strong bonds in the atomic structure of the crystal. Therefore, crystals with a chain and layered structure have an acicular or tabular shape. The anisotropy of growth rates and, consequently, the form of crystal growth in decomp. phases depend on the composition, T, T and strongly change under the action of surface-active impurities.
Due to the high speed of surface processes K. hypothermia T small on atomically rough surfaces, i.e. T=T 0(hence the name isotherms). In the case of non-metals, close-packed faces with simple indices often remain singular and appear on the rounded K. front in the form of a flat cut in the form of a circle, ellipse, or ring (Fig. 12, a), depending on the shape of the K isotherm. is constant and reaches a minimum at points, max. away from the isotherm T=T 0. At these points of the highest supercooling, layers are generated that determine the growth rate of the facet. Therefore, the stationary size of the face is the larger, the larger T is necessary for its growth at a rate equal to the velocity of the rounded crystal front in the direction of the crystal's elongation. Rough and faceted surfaces capture different amounts of impurities, and a crystal with coexisting faceted and rough forms grows inhomogeneous (Fig. 12, b).
Rice. 12. Formation of a flat face on the rounded front of the crystal (the crystal is pulled out of the melt): a- axial section of a crystal with a crystallization front concave towards the crystal in the center and flat along the periphery; b- longitudinal section of the Si crystal (peripheral region is enriched with impurities).
If a T decreases in the melt with distance from the front, then the front is unstable: a ledge that has arisen accidentally on it falls into a region of greater supercooling, the growth rate of the top of the ledge becomes even faster, and so on. As a result, the flat front breaks up into lamellar or needle-shaped crystals - in a section parallel to the front, a banded or cellular structure appears. The cells are characteristic of large temperature gradients and usually have a hexagonal shape, regardless of the symmetry of the crystal (Fig. 13). The instability is incompatible with the growth of perfect single crystals, since it leads to the capture of inclusions in the mother medium. Spherical a crystal growing in a supercooled melt or solution retains its shape until its radius reaches a critical value. values depending on the radius of the critical. nucleus and the rate of surface processes K. Later, protrusions develop, and the crystal acquires a skeletal structure (Fig. 14, a, b) or dendritic shape (Fig. 14, in, G). The name of the latter is associated with the appearance of secondary branches after the primary ledge reaches the critical point. length.
Rice. 13. Scheme of the cellular structure of the crystallization front.
Rice. 14. Initial round crystal of cyclohexanol in the melt ( a), the initial stage of skeletal crystal growth ( b), dendrite ( in), dendrite at high hypothermia ( G).
The impurity pushed away by the K. front from the melt accumulates in front of it and, changing the equilibrium temperature of K., causes the so-called. concentration supercooling that increases with distance from the front. If the equilibrium temperature in the melt increases with distance from the front faster than the true one, then a concentration instability arises. It disappears at sufficiently high ratios of the temperature gradient at the K. front to its velocity.
The K. front from the solution is always unstable, since supersaturation increases with distance from the growing surface. Faceted crystals are characterized by a large supersaturation near the vertices and edges, and the difference increases with the face size. With sufficiently large supersaturation and size of the facets, the vertices become the leading sources of growth steps, and in the center. dips appear in parts of the faces - skeletal growth begins (Fig. 15). Some impurities contribute to it. The instability of K. from solutions is suppressed by intensive mixing, a decrease in supersaturation, and sometimes the introduction of impurities.
Rice. 15. Skeletal crystal of spinel.
Impurity capture. The ratio of impurity concentrations in the crystal and the original substance called. coefficient capture To. At K<1 К. ведёт к очистке от примеси кристалла, при К>1 - to the purification of the original medium, K=1 corresponds to the preservation of the concentration. Coef. capture by different faces are different and do not coincide with thermodynamic. equilibrium, determined by the state diagram. Therefore, the composition of the crystal deviates from thermodynamic equilibrium. For example, during laser or electronic pulsed recrystallization of thin near-surface Si layers with K rates of up to several times. m/s concentration of impurities As, Sb, In, Bi in the Si crystal exceeds the equilibrium one by 3-600 times, and the vast majority of impurity atoms are located at the lattice sites. This is due, firstly, to the statistical selection: each site of the lattice during K. is finally filled with one or another atom after many attempts (from 10 6 -10 7 at speeds of 10 -3 cm / s and up to 10 at speeds of m / s). Second, under conditions of fast crystallization, diffusion in the melt does not have time to proceed.
The nonequilibrium impurity capture during layer-by-layer growth is associated with statistic. selection at the steps, and also with the fact that even the equilibrium impurity concentration in the surface layer of the crystal and the end of the step differs markedly from the bulk concentration. With a sufficiently rapid deposition of layers, the next layer immures the previous one together with the impurity contained in it. As a result, each face captures an impurity in the quantity corresponding to the concentration in its surface layer, and the crystal turns out to be composed of growth sectors of different faces, with decomp. impurity concentrations, etc. defects- there is a so-called. sectorial structure of the crystal (Fig. 16). The amount of impurity captured when the step moves along the face depends on the orientation of this step. Therefore, the growth sector of a given face, in turn, is divided into regions plotted vicinals different orientations with different impurity contents (vicinal sectoriality, Fig. 17).
The rate and impurity concentration at the K. front from the melt fluctuate due to the convection of the melt and the rotation of the crystal and the crucible in a usually slightly asymmetric thermal field. The corresponding positions of the K. front are imprinted in the crystal in the form of stripes (zonal structure, Fig. 16). Temperature fluctuations can be so strong that crystal growth is replaced by melting and cf. speed is an order of magnitude less than the instantaneous speed. The intensity of convection and the banding amplitude decrease when crystals are grown in weightlessness.
Rice. 16. Sectorial and zonal structure of a potassium alum crystal.
Rice. 17. A vicinal hillock formed on a face by steps of three different orientations around an edge dislocation D(a). Different slopes of the mound capture different amounts of impurities ( b).
Defect formation. Foreign gases, soluble in solutions and melts better than in crystals, are released at the K front. Gas bubbles are captured by a growing crystal if they exceed the critical value. size decreasing with increasing growth rate (similarly, solid particles are captured). In case of crystallization in weightlessness, the convective removal of bubbles from the crystallization front is difficult, and the crystal is enriched with gaseous inclusions. By specially creating bubbles, foam materials are obtained. Real crystals always have zonally and sectorially distributed impurities, to-rye change the lattice parameter, which causes ext. stresses, dislocations and cracks. The latter also arise due to the discrepancy between the lattice parameters of the seed (substrate) and the crystal growing on it. Sources of internal stresses and dislocations are also inclusions of the mother medium and foreign particles.
When K. from the melt, dislocations arise due to thermoelastic stresses caused by a non-linear temperature distribution; when cooling the already grown parts of the crystal from the outside; with a linear distribution of temperature along the normal to a sufficiently extended crystal front, if free temperature bending of the crystal is impossible; seed inheritance. Therefore, the growth of dislocation-free crystals of Si, GaAs, IP begins with seeds of small diameter and is carried out in the most uniform temperature field. Crystals may contain dislocation loops smaller than 1 µm. The loops are formed as contours of disc-shaped accumulations (clusters) of interstitial atoms (or vacancies) resulting from the decay of a supersaturated solid solution upon cooling of the grown crystal. Impurity atoms can be centers of cluster nucleation.
Bulk K. When defining conditions is possible at the same time. growth of many crystals. Spontaneous mass appearance of nuclei and their growth occur, for example, during the solidification of metal castings. Crystals are nucleated primarily on the cooled walls of the mold, where overheated metal is poured. The nuclei on the walls are randomly oriented, however, in the process of growth, those of them "survive" for which the direction of max. growth rate perpendicular to the wall (geometric selection of crystals). As a result, the surface has a so-called. a columnar zone consisting of narrow crystals elongated along the normal to the surface.
Bulk crystallization in solutions begins either on spontaneously formed nuclei or on specially introduced seeds. Colliding in a stirred solution with each other, with the walls of the vessel and the stirrer, the crystals are destroyed and give rise to new crystallization centers (secondary nucleation). The cause of secondary nucleation can also be small fragments of layers hanging over the face, "seal" flat parallel faces, mother liquor inclusions. In metallurgy, strong convective currents are used, which break off dendritic crystals and spread the centers of crystals over the entire volume; sometimes ultrasonic crushing of growing crystals is used. Bulk K. purify substances from impurities (K<1). Массовая К. из газовой фазы (в т. ч. из плазмы) используется
для получения ультрадисперсных порошков с размерами кристалликов до 10 -6
см и менее. Необходимые для этого высокие переохлаждения достигаются резким
охлаждением пара смеси химически реагирующих газов или плазмы. Известен способ
массовой К. капель, кристаллизующихся во время падения в охлаждаемом газе.
Lit.: Growing crystals from solutions, 2nd ed., L., 1983; Lemmlein G. G., Morphology and genesis of crystals, M., 1973; Lodiz R. A., Parker R. L., Growth of single crystals, transl. from English, M., 1974; Problems of modern crystallography, M., 1975; Modern crystallography, vol. 3, Moscow, 1980; Chernov A. A., Physics of crystallization, M., 1983; Geguzin Ya. E., Kaganevsky Yu. S., Diffusion processes on the surface of a crystal, M., 1984; Morokhov I. D., Trusov L. I., Lapovok V. N., Physical phenomena in ultrafine media, M., 1984; Skripov V.P., Koverda V.P., Spontaneous crystallization of supercooled liquids, M., 1984.
A phase is a homogeneous part of a thermodynamic system separated from other parts of the system (other phases) by an interface, when passing through which the chemical composition, structure and properties of the substance change abruptly.
Crystallization is the process of separating a solid phase in the form of crystals from solutions or melts; in the chemical industry, the crystallization process is used to obtain substances in a pure form.
Crystallization begins when a certain limiting condition is reached, for example, supercooling of a liquid or supersaturation of a vapor, when many small crystals appear almost instantly - crystallization centers. Crystals grow by attaching atoms or molecules from a liquid or vapor. The growth of crystal faces occurs layer by layer, the edges of incomplete atomic layers (steps) move along the face during growth. The dependence of the growth rate on crystallization conditions leads to a variety of growth forms and crystal structures (polyhedral, lamellar, acicular, skeletal, dendritic and other forms, pencil structures, etc.). In the process of crystallization, various defects inevitably arise.
The number of crystallization centers and the growth rate are significantly affected by the degree of supercooling.
The degree of supercooling is the level of cooling of a liquid metal below the temperature of its transition into a crystalline (solid) modification. S.p. necessary to compensate for the energy of the latent heat of crystallization. Primary crystallization is the formation of crystals in metals (and alloys) during the transition from a liquid to a solid state.
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Synonyms:See what "Crystallization" is in other dictionaries:
- (new lat., from Greek krystallos crystal). Such a transition of bodies from a liquid state to a solid state, in which they take on known crystalline forms. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. CRYSTALLIZATION ... Dictionary of foreign words of the Russian language
crystallization- and, well. crystallisation, lat. crystalsatio. 1. chem. The process of crystal formation. Sl. 18. Crystallization or graining. Scrap. OM 599. // Sl. 18 11 18. These mineral vapors of the whole bolle are involved in crystallization, coloring of stones and ... ... Historical Dictionary of Gallicisms of the Russian Language
The formation of crystals from vapors, p moats, melts, from v va to tv. state (amorphous or other crystalline), from electrolytes in the process of electrolysis (electrocrystallization), as well as in chemical. reactions. For K., a violation of thermodynamic ... Physical Encyclopedia
Crystallization- - the process of crystal formation during the transition of a substance from a thermodynamically less stable state to a more stable one. [Glossary of basic terms needed in the design, construction and operation of highways.] ... ... Encyclopedia of terms, definitions and explanations of building materials
Modern Encyclopedia
Exist., number of synonyms: 4 vacuum crystallization (1) hydatogenesis (2) ... Synonym dictionary
Crystallization- CRYSTALLIZATION, the process of formation of crystals from vapors, solutions, melts, from a substance in another crystalline or amorphous state. Crystallization begins when the liquid is supercooled or the vapor is supersaturated, when almost instantly ... ... Illustrated Encyclopedic Dictionary
CRYSTALLIZATION, the process of the formation of CRYSTALS from a substance that passes from a gaseous or liquid state to a solid state (sublimation or melting) or arising from a solution (evaporation or precipitation). During melting, the solid is heated ... ... Scientific and technical encyclopedic dictionary
CRYSTALLIZE, zuyu, zuesh; ovated; owls. and nesov., that. Turn (touch) into crystals. Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov
The process of transition of a body from a liquid (sometimes gaseous) state to a solid state, and it takes on a more or less regular geometric shape of a crystal ... Encyclopedia of Brockhaus and Efron
The growth process of k la since its inception. K. can occur from a liquid state (solution, melt; magma; phase transition), gaseous (see. Sublimation) and solid. See Recrystallization, Metasomatosis, Concentration flows, Regeneration ... ... Geological Encyclopedia
Books
- The Crystallization of Public Opinion, Edward Bernays. The book "The Crystallization of Public Opinion" by Edward Bernays is the first and already classic work devoted to PR as an independent discipline. Written in 1923, it is for the first time clearly...
