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Theme: Crystallization silicate fusion. Formation of the centres crystallization and growth of crystals.
Lecture № 8.
1. Crystallization silicate fusion.
2. Homogeneous and heterogeneous formation of the centres crystallization
3. Growth of crystals.
Crystalline glass - glassceramic material.
Nucleation center - centre crystallization
Crystallization fusion or glass the process of transition of substance from thermodynamically of a unstable condition with disorder or few regulating by structure in a steady condition with the ordered crystal lattice is called.
Crystallization silicate fusion and glasses is the important stage of technological processes of reception many silicate of materials, influences the basic properties crystalline glass, crystal enamels, deafen glaze, dairy and colloidal -painted glasses; essentially influences properties of ceramics, refractory, portland cement clinker etc.
Structure, number and order of allocation of crystal phases depend on chemical structure fusion or glass and rule(situation) of initial structure on the diagram of a condition of the appropriate system.
Agrees V. N. Philipovich, fusion and glass on character crystallization и it is possible to divide(share) into two groups: 1) fusion and glass, in which began crystallization и is accompanied by disintegration on phases distinguished on structure from initial structure; 2) fusion and glass, which at crystallization и give crystals or firm solutions of the same structure, as initial fusion. From fusion of the first group it is possible to receive glass or finely crystalline structure at the appropriate thermal processing. Crystallization fusion of the second group is connected only to structural reorganizations, the origin and growth of crystals is not limited diffusional by processes, therefore at crystallization и such fusion the large crystals are formed, from which it is difficult to receive glass.
According to G. Tamman, in the field of temperatures equilibrium melting there is a temperature interval - metastable a zone of overcooling, in which the speed of formation of the centres of a new phase is insignificant. Below than temperature metastable of a zone of overcooling the spontaneous process crystallization и is possible(probable) and depends on speed of formation or number of the centres crystallization (germs) and from growth rate of crystals.
The process crystallization и of a new phase consists of two basic stages: nucleation - formation of the centres (germs) crystallization and further crystal growth at the expense accumulation of structural elements on germs down to volumetric crystallization и of all weight fusion or glass.
The formation of germs (centres) crystallization и can be homogeneous (spontaneous), when the germs of a new phase have the same structure, as the future crystals, and heterogeneous, when as germs are used substances (impurity) distinguished on structure from кристаллизующейся of a phase.
Homogeneous formation of the centres crystallization. The formation of the centres crystallization can be explained on the basis molecular-kinetic of the theory. At certain temperature of a molecule are in continuous thermal movement and have the appropriate energy. At downturn of temperature the energy of system decreases, however kinetic the energy of molecules is still high enough also any new formation breaks up owing to thermal movement of particles. The further downturn of temperature results in decrease kinetic of energy and formation of steadier congestions of molecules. At certain temperature there are steady enough groups of molecules, which become germs of a new phase. For process crystallization и it is rather essential, that the arrangement of molecules or atoms in germs corresponded(met) to their rule(situation) in a crystal lattice.
Heterogeneous formation of the centres crystallization и. Is experimentally established, that the formation of the centres crystallization и of a new phase is accelerated, if into system to enter the initiators crystallization - impurity promoting more effective and fast overcoming of a power barrier nucleation and acceleration of processes of phase transition. Such impurity are called as catalysts crystallization.
One of the main and decisive influences of the catalyst on crystallization at heterogeneous nucleation is his(its) influence on reduction of size of a superficial tension between the catalyst and primary crystal phase, that provides good wetting of the catalyst with a phase forming a germ, and serves a necessary condition of heterogeneous process.
To catalysts crystallization и the following requirements are showed: the catalyst crystallization и should have high solubility in fusionе at high temperatures and limited solubility near to temperature of a softening, to have low energy of activation at formation of the centres crystallization from fusion in the field of the lowered temperatures. Ions or the atoms of the catalyst at the lowered temperatures should have higher speed diffusion in comparison with the basic components fusion or glass. The distinction of parameters of a crystal lattice of a new crystal phase and parameters of a crystal lattice of the catalyst should not exceed 10... 15 %.
As catalysts crystallization и apply metals, oxides, fluoride and sulphide of metals or their combination.
Growth of crystals. The growth of crystals is the second step of process crystallization after formation of germs - centres crystallization. The arisen crystal continues to grow at small overcooling. At increase of overcooling or introduction of impurity the growth rate of different sides of a crystal can change The growth rate of separate sides of a crystal is various. The growth of sides occurs fiberwise, consecutive growing of layers. In result fluctuation formation on a surface two-dimensional of a germ arise steps. Arisen at edges or units of a crystal steps moves along a side with speed, in hundreds time exceeding speed of moving in a direction, perpendicular side. The connection of atoms to steps occurs to smaller power expenses, than connection to smooth sites of a side. For realization of such mechanism overcoming insignificant threshold overcooling is necessary. The growth of a crystal can occur and at formation on a surface of a crystal screw dislocation.
The total process of formation of the centres crystallization and growth of crystals in silicate fusionе characterizes crystallization ability fusion, determined by experimental methods.
1. What such crystallization fusion?
2. How much consists of the process crystallization?
3. Characterize homogeneous and heterogeneous formation of the centres crystallization.
4. How there is a growth of crystals?