Process
Crystals are formed under the condition of phase transition. There are three kinds of phases, namely gas phase, liquid phase and solid phase. Only crystals are real solids. Crystals are formed when the gas and liquid phases are transformed into solid phases, and the solid phases can also be directly transformed.
The general process of crystal formation is to generate crystal nuclei first, and then gradually grow up. It is generally believed that there are three stages of crystal growth from the liquid phase or the gas phase: ①The medium reaches the supersaturation and supercooling stage; ②The nucleation stage; ③The growth stage.
In a certain medium system, the appearance of supersaturated and supercooled states does not mean that the entire system crystallizes at the same time. Instantaneous fine crystalline particles appeared everywhere in the system. At this time, due to local changes in temperature or concentration, external impacts, or the influence of some impurity particles, areas with high local supersaturation and supercooling will appear in the system, making the size of crystal particles above the critical value. This formation of crystalline particles is called nucleation.
The particles in the medium system enter an unstable state at the same time to form a new phase, which is called uniform nucleation.
In some local areas in the system, a new phase of nuclei is first formed, which is called heterogeneous nucleation.
Uniform nucleation means that in a system, the nucleation probability is equal everywhere. This has to overcome a considerable surface energy barrier, that is, a considerable degree of supercooling is required to nucleate.
The non-uniform nucleation process is due to the existence of certain inhomogeneities in the system, such as suspended impurity particles, unevenness on the container wall, all of which effectively reduce the potential barrier for surface energy nucleation , Preferentially form crystal nuclei at these non-uniform locations. Because of this, it can nucleate locally even when the degree of subcooling is very small.
In unit time, the number of nuclei formed in unit volume is called nucleation speed. It depends on the degree of supersaturation or supercooling of the substance. The higher the degree of supersaturation and supercooling, the higher the nucleation rate. The nucleation speed is also related to the viscosity of the medium. The viscosity will hinder the diffusion of the substance and reduce the nucleation speed. After the nucleation is formed, it will grow further.
Layer growth theory
Kossel (1927) first proposed, and later developed by Stranski (Stranski), the layer growth theory of crystals is also called Kossel. Searle-Strandsky theory.
It discusses that when a layer of atomic plane is grown on the smooth surface of the crystal nucleus, the best position for the particle to enter the "seat" of the crystal lattice on the interface is the position with three concave angles. At this position, the particle combines with the crystal nucleus to form a bond and releases the greatest energy. Because when each new particle from the environmental phase is in place on the crystal lattice at the interface of the environmental phase and the new phase, the most likely combination position is the most favorable position in terms of energy. The position with the greatest energy. The possible growth positions of the particles on the surface of the growing crystal:
k is a tortuous surface with three concave angles, which is the most favorable growth position; the second is the S step surface, which has two The position where the surface is concave; the most unfavorable growth position is A. From this, it can be concluded that when the crystal grows under ideal conditions, it grows one row and column first, and then grows adjacent rows and columns. After the first layer of nets are overgrown, the second layer of nets begins to grow. The crystal planes (the outermost net) grow in parallel and move outward. This is the layer growth theory of crystals, and it can be used to explain some of the following growth phenomena.
1) Crystals often grow into flat and straight polyhedrons.
2) During the process of crystal growth, the environment may change. The crystals formed at different times may have subtle changes in physical properties (such as color) and composition, so the cross-section of the crystal is often The band structure can be seen. It shows that the crystal planes grow in parallel outwards.
3) Since the crystal planes grow outward in parallel, the angle between the corresponding crystal planes on different crystals of the same mineral remains unchanged.
4) The crystal grows from small to large, and many crystal planes move parallel to the outside to form a pyramid with the center of the crystal as the apex, which is called a growth cone or sand bell-like structure. It can often be seen in thin slices.
However, the actual situation of crystal growth is much more complicated than simple layer growth theory. The thickness of the material layer deposited on one crystal plane at a time can reach tens of thousands or hundreds of thousands of molecular layers. At the same time, it does not have to be piled up layer by layer, but one layer has not yet grown, and a new layer begins to grow. As a result of this continued growth, the crystal surface is not flat and becomes a step shape called a crystal face step. Although Kossel's theory has its correct aspects, the actual crystal growth process does not completely follow the two-dimensional layer growth mechanism. Because after the first layer of the crystal has grown, it will be difficult to grow the second layer on it. The reason is that the already-grown surface has a small gravitational force on the particles in the solution, and it is not easy to overcome the problems of the particles. Thermal vibration puts the mass point in place. Therefore, when the degree of supersaturation or the degree of supercooling is low, the growth of crystals needs to be explained by other growth mechanisms.
In the process of crystal growth, the relative growth rate of different crystal planes and which crystal planes develop on the crystal are introduced below. Several major theories about this aspect are introduced below.
Bravais’ Law
As early as 1855, the French crystallologist A. Bravis discussed the actual crystal structure from the geometric concept of the spatial lattice structure of crystals. The relationship between the surface and the surface network in the spatial lattice structure, that is, the crystal plane of the actual crystal is often parallel to the surface network with the highest node density on the network surface. This is the Bravais law.
This conclusion of Bravais is based on the inference that the relative growth rate of different crystal faces on the crystal is inversely proportional to the density of the nodes on the mesh. The so-called crystal plane growth rate refers to the thickness of the crystal plane growing in its vertical direction per unit time. The density of nodes on the mesh surface of the crystal plane AB is the largest, and the spacing between the mesh surfaces is also the largest. The attraction of the mesh to the foreign particles is small, the growth speed is slow, and the crystal plane expands laterally, and finally remains on the crystal; the CD crystal plane is the second; BC The density of nodes on the mesh surface of the crystal face is the smallest, and the spacing between the mesh surfaces is also small. The mesh faces the foreign particles with high gravitational force, and the growth rate is the fastest. The surface with greater density of nodes on the mesh surface.
In general, Bravais's law clarifies the basic law of crystal face development. However, since the specific arrangement of the particles in the crystal was still unknown at that time, Bravais based only on the spatial lattice composed of abstract nodes, rather than the real crystal structure. Therefore, there may be some deviations from the actual situation in some cases. In 1937, the American crystallologist Donnay-Harker further considered that other symmetrical elements (such as spiral axis and slip surface) other than the periodic translation (embodied in the spatial lattice) in the crystal structure compared to certain direction planes. The influence of the density of nodes on the Internet has expanded the scope of application of Bravais's law.
Another shortcoming of Bravais’s law is that it only considers the crystal itself, while ignoring the medium conditions for growing the crystal.
From the liquid phase to the solid phase From the gas phase to the solid phase From the solid phase to the solid phase recrystallization
Crystals are formed under the condition of phase transition. There are three kinds of phases, namely gas phase, liquid phase and solid phase. Only crystals are real solids. Crystals are formed when the gas and liquid phases are transformed into solid phases, and the solid phases can also be directly transformed.
Change from liquid phase to solid phase
(1) Crystallization from the melt. When the temperature is lower than the melting point, the crystals begin to precipitate, that is, only when the melt is supercooled Time crystals can happen. For example, water crystallizes into ice when the temperature is below zero degrees Celsius; the metal melt is cooled to below the melting point and crystallizes into metal crystals.
(2) Crystallization from solution When the solution reaches supersaturation, crystals can be precipitated. The methods are as follows:
1) The temperature decreases. If the hydrothermal fluid after the magmatic period is farther away from the magma source, the temperature will gradually decrease, and various mineral crystals will gradually precipitate; 2) Water evaporation, such as natural salt lake brine evaporation , 3) Through a chemical reaction, insoluble substances are generated.
Deciding the morphology of crystal growth, internal factors are fundamental, and the external environment in which it is generated has a great influence on the crystal morphology. When the same kind of crystal is grown under different conditions, the crystal morphology may be different. Several main external factors affecting crystal growth are described below.
Vortex temperature, impurity viscosity, crystallization speed
There are many external factors that affect crystal growth. For example, the order of crystal precipitation also affects the crystal morphology. The first precipitation has more free space. The crystal form is complete and becomes euhedral crystal; the later grown will form semi-automatic crystalline or other morphological crystals. When natural crystals of the same mineral are formed under different geological conditions, they may show different characteristics in terms of morphology and physical properties. These characteristics mark the growth environment of the crystal and are called typomorphic characteristics.
1. Dissolution of crystals
Put the crystals in an unsaturated solution and the crystals will begin to dissolve. Since the corners and edges have more chances to contact with the solvent, these places dissolve faster, so the crystals can dissolve into approximately spherical shapes. The octahedron, such as alum, dissolves into a nearly spherical octahedron.
When the crystal plane is dissolved, small pits will be dissolved in some weak places first, which is called etching. Observed under a microscope, these etched images are composed of various secondary small crystal planes. Etching image on crystals of calcite and dolomite (b). When the crystal faces with different mesh density are dissolved, the crystal faces with higher mesh density will dissolve first, because the crystal dough with higher mesh density has a large distance between faces and is easy to be destroyed.
2. Crystal regeneration
Destroyed and dissolved crystals can be restored to the polyhedral form in a suitable environment, which is called crystal regeneration, such as the regeneration of quartz particles in Banyan.
Dissolution and regeneration are not simply opposite phenomena. When the crystal dissolves, the dissolution rate changes gradually with the direction, so the crystal dissolves to form a nearly spherical shape; when the crystal is regenerated, the growth rate changes suddenly with the change of the direction, so the crystal can be restored to a geometric polyhedral form.
The growth of crystals in nature is often not linear. Dissolution and regeneration often occur alternately in nature, making the surface of the crystal complex. For example, some narrow crystal faces are formed on the crystal, or some special protrusions and patterns are formed on the crystal face.
Synthetic crystals
Technology development
The research on the growth of natural mineral crystals is helpful to understand the formation and development history of minerals, rocks, and geological bodies, and Provide some useful enlightening materials for the development and utilization of mineral resources. The synthetic body can not only simulate and explain the formation conditions of natural minerals, but more importantly, it can provide crystal materials urgently needed by modern science and technology.
In recent years, the experimental technology of artificial crystal synthesis has developed rapidly, and a large number of important crystal materials have been successfully synthesized, such as laser materials, semiconductor materials, magnetic materials, artificial gemstones, and many other modern technologies with special requirements. Functional crystal material. At present, synthetic crystals have become an important part of materials science, which is the main pillar of industrial development.
Method
The main way of artificially synthesizing crystals is to cultivate them in solution and prepare them through the transformation of homogeneity and multi-image under high temperature and high pressure (such as using graphite to prepare diamond). There are many specific methods, and the most commonly used methods are briefly introduced below.
(1) Hydrothermal method This is a method of cultivating crystals from a supersaturated hot water solution under high temperature and pressure. With this method, hundreds of crystals, such as crystal, corundum (ruby, sapphire), beryl (emerald, aquamarine), garnet, and many other silicates and tungstates, can be synthesized.
Crystal cultivation is carried out in an autoclave. The autoclave is made of special steel that is resistant to high temperature, pressure, acid and alkali. The upper part is the crystallization zone, with seed crystals hanging; the lower part is the dissolution zone, where the raw materials for cultivating crystals are placed, and the kettle is filled with solvent medium. Convection occurs due to the temperature difference between the crystallization zone and the dissolution zone (such as culturing crystals, the crystallization zone is 330-350°C, and the dissolution zone is 360-380°C) convection occurs, which brings the high-temperature saturated solution to the low-temperature crystallization zone to form supersaturation The solute is precipitated and the seed crystal grows. The temperature lowered and the solution that has precipitated part of the solute flows to the lower part to dissolve the culture material, and so on, so that the seed crystals can grow continuously.
(2) Czochralski method This is a method of pulling single crystals directly from the melt. The melt is placed in the tangerine collapse, and the seed crystal is fixed on a lifting rod that can be rotated and raised. Lower the pull rod, insert the seed crystal into the melt, and adjust the temperature to make the seed crystal grow. Raise the pull rod so that one side of the crystal grows and the other side is slowly pulled out. This is a common method for growing crystals from melts. This method can be used to pull out a variety of crystals, such as single crystal silicon, scheelite, yttrium aluminum garnet and uniform transparent ruby.
(3) Flame melting method This is a method of melting powder with hydrogen-oxygen flame and crystallizing it. The small hammer 1 hits the barrel 2 containing the powder material, the powder material is vibrated and falls through the screen 3, and the oxygen enters through the inlet 4 to send the powder down. 5 is the hydrogen inlet, and the hydrogen and oxygen are mixed and burned at the nozzle 6. , The powder is melted by the high temperature of the flame and falls on the crystallization rod 7, and the temperature of the rod end is controlled to gradually crystallize the molten layer falling on the rod end. In order to make the crystal grow to a certain length, the crystal rod can be gradually moved down. In this way, a variety of crystals such as ruby, sapphire, spinel, rutile, strontium titanate, and yttrium aluminum garnet have been successfully synthesized.
(4) The material used in the crucible descending method for crystal growth is placed in a cylindrical crucible, slowly descending, and passing through a heating furnace with a certain temperature gradient, the furnace temperature is controlled slightly higher than Near the melting point of the material. According to the nature of the material, the heating device can be a resistance furnace or a high-frequency furnace. When passing through the heating zone, the material in the crucible is melted. When the crucible continues to fall, the temperature at the bottom of the crucible first drops below the melting point and begins to crystallize. The crystal continues to grow as the crucible descends. This method is often used to prepare single crystals of alkali metal and alkaline earth metal halides and fluorides.
(5) Zone melting method The zone melting method uses heat energy to generate a melting zone at one end of the semiconductor bar, and then welds the single crystal seed crystal. Adjust the temperature so that the molten zone slowly moves to the other end of the rod, and grows into a single crystal through the entire rod material with the same crystal orientation as the seed crystal.
(6) The Kyropoulos method is also called the Kyropoulos method, or KY method for short. Its principle is similar to the Czochralski method, and the raw materials are heated to the melting point. After melting to form molten soup, the single crystal seed crystal (SeedCrystal, also known as seed rod) contacts the surface of the molten soup, and a single crystal with the same crystal structure as the seed crystal begins to grow on the solid-liquid interface between the seed crystal and the molten soup. , The seed crystal is pulled up at a very slow speed, but after the seed crystal is pulled up for a period of time to form a crystal neck, after the solidification rate at the interface between the molten soup and the seed crystal stabilizes, the seed crystal will no longer be pulled up. Without rotating, the single crystal is gradually solidified from above to the bottom by controlling the cooling rate, and finally solidifies into an entire single crystal ingot.
Numerical simulation
Preparation of large-scale crystal growth, especially suitable for high-tech applications, such as DRAMs, integrated circuit semiconductors, monocrystalline or polycrystalline solar cells, LEDs Illuminated gem base and so on. Generally speaking, the Czochralski method (also known as the Cz method, Czochralski method) produces monocrystalline silicon for IC and solar cells; the floating zone method (Fz method: Floating Zone) produces high-purity monocrystalline silicon; the directional solidification method (DS method, VB method, etc.) are mostly used in the production of polysilicon used in solar cells. The above several processes use effective simulation tools to complete the prediction of the thermal field, mechanical properties, and geometric structure of the single crystal furnace and the crystal during the crystal pulling process by establishing numerical models, and finally realize the evaluation of the quality of the produced crystal.
Professor François Dupret, a scholar in the field of crystal growth, from the University of Leuven, Belgium, published an article in "J. of Heat and Mass Transfer" in the 1990s: Global modelling of heat transfer in crystal growth furnaces, details Explains how to establish a global heat transfer control model in a crystal growth furnace, and verifies the accuracy of this global model.