Theme: Кристаллизация силикатных расплавов and knitting substances. Education of the centres кристаллизации and growth of crystals.

Lecture № 8.

 

1. Кристаллизация силикатных расплавов.

2. Homogeneous and heterogeneous education of the centres кристаллизации.

3. Growth of crystals.

 

 

Ситалл - стеклокристаллический a material.

Germ - centre кристаллизации

 

 

Кристаллизацией расплава or glass the process of transition of substance from термодинамически of a unstable condition with disorder or малоупорядоченной by structure in a steady condition with the ordered crystal lattice is called.

Кристаллизация силикатных расплавов and glasses is the important stage of technological processes of reception many силикатных of materials, influences the basic properties ситаллов, crystal enamels, глушеных глазурей, dairy and коллоидно-painted glasses; essentially influences properties of ceramics, огнеупоров, портландцементного клинкера etc.

Structure, number and order of allocation of crystal phases depend on chemical structure расплава or glass and rule(situation) of initial structure on the diagram of a condition of the appropriate system.

Agrees In. Н. Филиповичу, расплавы and glass on character кристаллизации it is possible to divide(share) into two groups: 1) расплавы and glass, in which began кристаллизации is accompanied by disintegration on phases distinguished on structure from initial structure; 2) расплавы and glass, which at кристаллизации give crystals or firm solutions of the same structure, as initial расплав. From расплавов of the first group it is possible to receive glass or мелкокристаллическую structure at the appropriate thermal processing. Кристаллизация расплавов of the second group is connected only to structural reorganizations, the origin and growth of crystals is not limited диффузионными by processes, therefore at кристаллизации such расплавов the large crystals are formed, from which it is difficult to receive glass.

According to one @. Тамману, in the field of temperatures equilibrium плавления there is a temperature interval - метастабильная a zone of overcooling, in which the speed of education of the centres of a new phase is insignificant. Below than temperature метастабильной of a zone of overcooling the spontaneous process кристаллизации is possible(probable) and depends on speed of education or number of the centres кристаллизации (germs) 1 and from growth rate of crystals.

The process кристаллизации of a new phase consists of two basic stages:

нуклеации - education of the centres (germs) кристаллизации and further р about with т and crystals at the expense of a congestion of structural elements on germs down to volumetric кристаллизации of all weight расплава or glass.

The education of germs (centres) кристаллизации 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 education of the centres кристаллизации. The education of the centres кристаллизации can be explained on the basis молекулярно-кинетической 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 кинетическая the energy of molecules is still high enough also any новообразование breaks up owing to thermal movement of particles. The further downturn of temperature results in decrease кинетической of energy and education of steadier congestions of molecules. At certain temperature there are steady enough groups of molecules, which become germs of a new phase. For process кристаллизации it is rather essential, that the arrangement of molecules or atoms in germs corresponded(met) to their rule(situation) in a crystal lattice.

Heterogeneous education of the centres кристаллизации. Is experimentally established, that the education of the centres кристаллизации of a new phase is accelerated, if into system to enter the initiators кристаллизации- impurity promoting more effective and fast overcoming of a power barrier зародышеобразования and acceleration of processes of phase transition. Such impurity are called as catalysts кристаллизации.

One of the main and decisive influences of the catalyst on кристаллизацию at heterogeneous зародышеобразовании 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 кристаллизации the following requirements are showed: the catalyst кристаллизации should have high solubility in расплаве at high temperatures and limited solubility near to temperature of a softening, to have low energy of activation at education of the centres кристаллизации from расплава in the field of the lowered temperatures. Ions or the atoms of the catalyst at the lowered temperatures should have higher speed диффузии in comparison with the basic components расплава 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 кристаллизации apply metals, оксиды, фториды and сульфиды of metals or their combination.

Growth of crystals. The growth of crystals is the second step of process кристаллизации after education of germs - centres кристаллизации. 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, т. е. The growth rate of separate sides of a crystal is various. The growth of sides occurs послойно, consecutive escalating of layers. In result флуктуационного of education on a surface двумерного of a germ arise ступеньки. Arisen at edges or units of a crystal ступенька moves along a side with speed, in hundreds time exceeding speed of moving in a direction, perpendicular side. The connection of atoms to ступеньке 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 education on a surface of a crystal screw дислокации.

The total process of education of the centres кристаллизации and growth of crystals in силикатном расплаве characterizes кристаллизационную ability расплава, determined by experimental methods.

 

Questions:

 

 

1. What such кристаллизация расплавов?

2. Of what stages the process кристаллизации consists?

3. Characterize homogeneous and heterogeneous education of the centres кристаллизации.

4. How there is a growth of crystals?