2014-01-27
673
Lecture № 7.
1. Knitting materials in a liquid condition.
2. Hypothesis of a structure of a liquid.
3. Feature of structure силикатных расплавов.
Квазикристаллической - deformed crystal
Сиботаксическая group - strongly deformed deformed structure of crystals
As is known, key operation in manufacture силикатных and other refractory materials is the high-temperature processing, during which the initial firm substances can completely (technology flown down, enamels, глазури, плавленых of cements, огнеупоров, абразивов) or partially to pass in расплав (портландцементный клинкер, глиноземистый cement, porcelain, faience, шамотные both динасовые огнеупоры and т. д.). Knowledge of a nature расплавов силикатов and their properties therefore is rather important.
Плавление - process of transition of substance of a firm condition in liquid, made under action of temperature. Плавление of crystal substances is phase transformation caused spasmodic change of their internal structure and физико-chemical properties, connected to it(him).
From the known theorems of classical thermodynamics follows, that at balance between two condition of substance of free energy of unit of weight of substance in both condition should be identical. From here condition of balance between firm and liquid phases should be equality Gтв=Gж.
Плавление - эндотермический process, it(him) characterize by the latent heat плавления, т. е. By quantity(amount) of heat, which is necessary to inform the substance which has been heated up to temperature плавления, that it(him) расплавить. This heat is spent for overcoming of forces of interaction of atoms of a crystal lattice and characterizes a difference in energy of interaction between particles in firm and liquid condition.
Temperature плавления, being the characteristic of a crystal showing his(its) stability to thermal influences, should be in interrelation with energy of a crystal lattice.
Besides structure on a structure расплавов, in particular on structure of particles, of which they consist, the large influence is rendered by(with) temperature. Agrees
О. In. Мазурину, kinds of structural transformations in расплавах, the temperatures, occurring at change, are reduced to the following basic types:
1. Temperature флуктуации of density. This kind of heterogeneity is typical of any расплавов. In process of cooling the intensity these флуктуации decreases proportionally to decrease(reduction) of temperature so long as расплав is saved in метастабильном a condition.
2. Change of coordination. The influence of structure on coordination of ions in расплаве is investigated in detail enough, influence of temperature on a parity(ratio) in расплаве of the same atoms which are taking place in different coordination condition on oxygen, even is qualitatively investigated very little. In most cases it is necessary to expect reduction of coordination numbers at growth of temperature.
3. Education and диссоциация of structural complexes. The amplification(strengthening) of thermal fluctuations of ions with increase of temperature results in break of communications(connections) 51 and splitting of complexes. At downturn of temperature, on the contrary, there is an integration of complexes, т. е. Increase of a degree of association.
Thus, the temperature factor defines(determines) a dynamic nature of structural components расплава. It is necessary always to mean, that there, where at the certain moment the greatest correctness of mutual orientation was observed, at the following moment there can be a deformation resulting(bringing) gradually in complete infringement of an ordered arrangement of particles. The speed of mutual transition defines(determines) also parity(ratio) of volumes ordered and аморфной of parts of a liquid. And to each temperature there corresponds(meets) the certain parity(ratio) of these volumes.
At manufacturing the majority силикатных of products (glass, ceramics, цементного клинкера) during them обжига occurs private(individual) or even complete плавление of a material. The properties formed thus of a liquid phase are rendered by(with) the extremely large influence on technological process of manufacture and on final properties of ready production. Now is established, that between a structure of substances which are taking place in crystal, liquid or стеклообразном a condition, are available not only various, but also essential similarities. Basically there are three hypotheses of a structure of a liquid. The greatest recognition was received by(with) models of a structure of liquids offered in different years Берналом, И.Я. Френкелем and Г.Стюартом. Agrees Берналу, offered a hypothesis бездефектной of a liquid, the liquid has structure poorly distinguished from geometry of a crystal, from which she is received. The model of a structure of a liquid offered Френкелем, is called квазикристаллической. According to this model is especial in близи of temperature кристаллизации is considered(examined) as the deformed crystal, in which the distant order is lost, but is saved near.
According to the theory " Роев or сиботаксисов " in a liquid there are units named сиботаксическими by groups representing pseudo-crystal educations, which structure comes nearer to a structure of the appropriate crystals allocated from a liquid at кристаллизации. The structure сиботаксических of groups can be considered(examined) as the strongly deformed deformed structure of crystals. Сиботаксические of group are mobile, dynamic units, which collapse at movement and are created again. These groups are divided(shared) by areas of a chaotic arrangement of particles, however of sharp transitions between areas with partially ordered and disorder. This hypothesis in comparison with other hypotheses now has the greatest recognition.
Structure расплавов силикатов.
High temperature плавления силикатов (is usual from 500 0С up to 2000 0С) puts significant obstacles to research of their structure in расплавленном a condition. To number of the most important properties силикатных расплавов, connected with their structure and investigated with sufficient completeness concern: viscosity, superficial tension and density.
Viscosity силикатных расплавов.
Viscosity of substance in particular силикатных расплавов is understood as property of substance to render resistance mainly of one parts of a liquid concerning others. The existing methods of measurement of viscosity of glasses can be divided(shared) into 2 kinds:
1) Methods of definition of viscosity in rather low temperatures:
а) Method of a stretching of a string;
б) Method of a deflection of a string;
в) Method закручивания of a core.
2) Method of definition of viscosity at high temperatures:
а) Method of a falling ball;
б) Method of rotating cylinders.
Superficial tension расплавляющихся силикатов.
The size of a superficial tension is measured by job, which is necessary for making for education of unit of a new surface or for increase of a surface of a liquid at unit at constant temperature:
Where, - factor of a superficial tension;
And job on increase of a surface of a liquid;
S- Area.
The superficial tension plays the important role in engineering. The heating of glass products causes under influence of a superficial tension, rounding off of sharp edges(territories). At sintering fire-resistant materials and destruction them расплавленными силикатами, at кристаллизации расплавов and dissolution in them of crystals.
Density силикатных расплавов is defined(determined) by a method of measurement of loss in weight shipped in расплав of a platinum ball of known volume and weight suspended on a thin platinum string attached top end to коромыслу analytical весов. Thus density d is defined(determined) under the formula:
D = Рв - Р р
The role расплавов by manufacture силикатных of materials is rather various. Determining meaning(importance) have the properties силикатных расплавов for technology of a glass - product of cooling расплавов without them кристаллизации. The vast role расплавов and for technology of various materials received by them кристаллизации (плавленые огнеупоры, cements, абразивы, monocrystals various оксидов, халькогенидов, галогенидов and т is abundantly clear. д.). At the same time it is necessary to note and that large meaning(importance), which belongs расплавам in technology of a number(line) of the basic multitonnage products силикатной of technology, where they are that environment(Wednesday), in which there are many reactions of education of chemical connections determining at the end properties of ready products. The liquid phase in many respects defines(determines) also processes of sintering, т. е. Condensation of a material at обжиге many силикатных of products.
It is possible to result many examples from various areas of technology силикатов, which well illustrate a role расплавов in technology силикатов.
1. At обжиге портландцементного клинкера in rotating
Furnaces a liquid phase appearing in a zone of sintering, completely
Supervises process of formation main цементного of a mineral - трехкальцевого силиката - алита ЗСаО · SiO2.
Н. And. Торопов describes process of formation цементного клинкера as follows. At the first stage - stage твердофазного of sintering from a raw mix consisting from карбоната кальция and clay, three are formed клинкерных of a mineral: ЗСаО · SiO2, ЗСаО-А12О3 and 4СаО-А12O3-Fе2O3. Трехкальциевый силикат, distinguished by the highest absolute durability among цементных of minerals and speed of its(her) increase, at this stage is not formed. He is formed at the second stage - stage жидкофазного of sintering цементного клинкера. At this stage there is a dissolution superfluous оксида кальция and 2СаО · SiO2 in клинкерной of a liquid. In her from ions Са2 +, О2- also [SiO4] -4 is formed трехкальциевый силикат ЗСаО·SiO2. Considerably smaller solubility in клинкерной of a liquid in comparison with other minerals quickly results in creation расплавов, пересыщенных in relation to трехкальциевому силикату, therefore he by first and выкристаллизовывается in these conditions.
2. By manufacture of one of most important fire-resistant
Materials - динаса the liquid phase also plays determining
Role. Динас - sour огнеупор, on structure approximately on 90 % (May).
Consisting from SiO2 and 10 % (May). СаО. In the phase attitude(relation) is submitted basically тридимитом. Last is more desirable phase, than other polymorphic versions SiO2, as the transformation it(him) from the high-temperature form in низкотемпературную is accompanied considerably by smaller change of volume, than, for example, at similar transformation of quartz, and it is rather important at job тридимита as футеровочного of a material. As an initial material for manufacture динаса is the quartz, it(he) is necessary for translating at heating in тридимит. This process in a firm phase goes very slowly. The addition of small weight СаО results in occurrence at rather low temperatures (1430... 1450ЁС) in complete conformity with the diagram of a condition of system СаО- SiO2 of the significant contents расплава. Astable at such temperatures the quartz is well dissolved in a liquid phase and allocates more stable in the given conditions the form кремнезема - тридимит.
3. By manufacture of porcelain the basic phase component of this material - муллит is formed by dissolution of quartz and products of decomposition каолинита in полевошпатовом расплаве.
Thus, nature and the properties расплавов, raw materials, formed at high-temperature processing, by manufacture various силикатных of materials, have huge practical meaning(importance), enabling a choice of optimum parameters of technological process and reception of materials with the beforehand given properties.
Questions:
What factors define(determine) absolute meaning(importance) of temperature плавления of crystal substances?
Describe essence of existing modelling representations about a structure of liquids and, in particular, structure силикатных расплавов.
How influence temperature and structure силикатных расплавов their viscosity?
What defines(determines) size of a superficial tension and moistening ability расплавов?
How it is possible to classify оксиды on their influence on a superficial tension?
In what parameter and how it is possible to characterize moistening ability расплавов?
Result examples of a role of a liquid phase by manufacture силикатных of materials.
