Crystals are solid bodies that have a regular geometric shape. The structure within which ordered particles are located is called a crystal lattice. The points of arrangement of particles at which they oscillate are called nodes of the crystal lattice. All these bodies are divided into single crystals and polycrystals.
What are single crystals
Single crystals are single crystals in which the crystal lattice has a clear order. Often a single crystal has the correct shape, but this feature is not required when determining the type of crystal. Most minerals are single crystals.
The external form depends on the growth rate of the substance. With a slow increase and uniformity of the material, the crystals have the correct cut. At medium speed, the cut is not pronounced. At a high crystallization rate, polycrystals consisting of many single crystals grow.
Classical examples of single crystals are diamond, quartz, topaz. In electronics, single crystals having the properties of semiconductors and dielectrics are of particular importance. Single crystal alloys are characterized by increased hardness. Ultrapure single crystals have the same properties, regardless of origin. The chemical composition of minerals depends on the speed of cultivation. The slower the crystal grows, the more perfect its composition.
Polycrystals
Single crystals and polycrystals are characterized by a high molecular interaction. A polycrystal consists of many single crystals and has an irregular shape. They are sometimes called crystallites. They appear as a result of natural growth or are grown artificially. Polycrystals can be alloys, metals, ceramics. The main characteristics consist of the properties of single crystals, but grain sizes, the distance between them, and grain boundaries are of great importance. In the presence of boundaries, the physical parameters of polycrystals change significantly, strength decreases.
Polycrystals are generated as a result of crystallization, changes in crystalline powders. These minerals are less stable than single crystals, which leads to uneven growth of individual grains.
Polymorphism
Single crystals are substances that can exist in two states at once, which will differ in their physical properties. This feature is called polymorphism.
In this case, a substance in one state can be more stable than another. When environmental conditions change, the situation may change.
Polymorphism is of the following types:
- Reconstruction - decay occurs to atoms and molecules.
- Deformational - the structure is modified. Compression or tension occurs.
- Shear - some elements of the structure change their location.
The properties of the crystal can change with a sharp change in composition. A classic example of polymorphism is carbon modification. In one state it is diamond, in the other - graphite, substances with different properties.
Some forms of carbohydrate turn into graphite when heated. Changes in properties can occur without deformation of the crystal lattice. In the case of iron, the substitution of some components leads to the disappearance of magnetic properties.
Crystal Strength
Any material used in modern technology has a final strength. The greatest strength is the alloy of nickel, chromium and iron. Increasing the strength of metals will improve military and civilian equipment. Increased wear resistance will lead to a longer service life. For this reason, scientists have long studied the strength of single crystals.
Pure single crystals are crystals with an ideal crystal lattice, contain a small number of defects. With a decrease in the number of defects, the strength of metals increases several times. In this case, the density of the metal remains almost the same.
Single crystals with an ideal lattice are resistant to mechanical stress up to the melting temperature. Do not change over time. Most often, such single crystals have a zero dislocation. But this is an optional condition. Strength is explained by the fact that microcracks form in places where there is the greatest number of dislocations. And in their absence, there is nowhere for cracks to appear. This means that the single crystal will last until its strength threshold is exceeded.
Artificial single crystals
Single crystal growth is possible at the current level of science. When processing metal, without changing its composition, you can create a single crystal, which has a high margin of safety.
There are 2 known methods for the production of single crystals:
- ultrahigh pressure and metal casting;
- cryogenic pressure.
The first method is popular in the processing of light metals. Given the purity of the metal and the increase in pressure, a new metal will gradually appear that has the same properties, but with increased strength. Under certain conditions, it is possible to obtain a single crystal with an ideal lattice. In the presence of impurities, there is a possibility that the crystal lattice will not be ideal.
In heavy metals, an increase in pressure results in a process of structural change. The single crystal has not yet turned out, and the substance has changed properties.
Cryogenic casting is based on the production of cryogenic fluids. Under the influence of a magnetic field, crystallization does not occur. The semi-crystalline form becomes a crystal after an electric charge.
Diamond and quartz
The properties of diamond are based on the fact that it is a substance with an atomic crystal lattice. The bond between atoms determines the strength of diamond. Under constant conditions, the diamond does not change. When it enters a vacuum, it gradually turns into graphite.
Crystal sizes vary significantly. Synthetically grown diamonds have facets of a cube and externally differ from their counterparts. The properties of diamond are used for cutting glass.
Quartz crystals are ubiquitous. Mineral is one of the most common. Quartz is usually colorless. If there are many cracks inside the stone, then it is white. When other impurities are added, it changes color.
Quartz crystals are used in the manufacture of glass, to create ultrasound, in electrical, radio, and television equipment. Some varieties are used in jewelry.
Single crystal structure
Solid metals have a crystalline structure. The structure of single crystals is endless rows of alternating atoms. In reality, the ordering of atoms can be disturbed due to thermal effects, mechanical or for a number of other reasons.
There are 3 types of crystal lattices:
- type of tungsten;
- type of copper;
- type of magnesium.
Application
Artificial single crystals are an opportunity to receive material with new properties. The area of application of single crystals is very large. Quartz and spar were created by nature, and sodium fluoride is grown artificially.
Single crystals are materials that are used in optics and electronics. Quartz and mica are used in optics, but are expensive. In artificial conditions, it is possible to grow a single crystal, which will be distinguished by purity and strength.
Diamond is used where high strength is required. But it is successfully synthesized in artificial conditions. Three-dimensional single crystals are grown from melts.