{"id":437,"date":"2018-12-04T07:17:52","date_gmt":"2018-12-04T07:17:52","guid":{"rendered":"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=437"},"modified":"2018-12-05T06:36:38","modified_gmt":"2018-12-05T06:36:38","slug":"experimental-crystal-growth","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/chapter\/experimental-crystal-growth\/","title":{"rendered":"Experimental Crystal growth"},"content":{"raw":"
\"epgp<\/a>\r\n<\/span><\/div>\r\n\r\n
\r\n\r\n \r\n\r\n26.1 Introduction <\/strong>\r\n\r\n \r\n

Successful growth of crystals requires strict control over phase change. There are processes involving growth of solute from phase transitions viz., liquid to solid state or vapour to solid state or solid to solid phase transitions. Main categories of crystal growth methods include the following:<\/p>\r\n \r\n\r\nA.\u00a0 \u00a0 \u00a0 Growth from Solution\r\n\r\nA.1\u00a0\u00a0\u00a0\u00a0 Growth from flux\r\n\r\nA.2\u00a0\u00a0\u00a0\u00a0 Hydrothermal growth\r\n\r\nA.3\u00a0\u00a0\u00a0\u00a0 High pressure growth\r\n\r\nA.4\u00a0\u00a0\u00a0\u00a0 Growth from water solution\r\n\r\nA.5\u00a0\u00a0\u00a0\u00a0 Growth from gel\r\n\r\nA.6\u00a0\u00a0 Other innovative techniques which fall under sub-categories of related growth techniques include:\r\n\r\n \r\n\r\n\u2022\u00a0\u00a0\u00a0 Organic\u00a0 solution growth\r\n\r\n\u2022\u00a0\u00a0\u00a0 Accelerated\u00a0 crucible growth\r\n\r\n\u2022\u00a0\u00a0\u00a0 Electro-crystallization\r\n\r\n\u2022\u00a0\u00a0\u00a0 Liquid\u00a0 phase\u00a0 epitaxy and\r\n\r\n\u2022\u00a0\u00a0\u00a0 Molten\u00a0 metal\u00a0 solution growth\r\n\r\n \r\n\r\nB. Growth from Melt\r\n\r\nB.1\u00a0\u00a0 Czochralski crystal pulling technique\r\n\r\nB.2\u00a0\u00a0 Verneuil flame fusion growth technique\r\n\r\nB.3\u00a0\u00a0 Bridgman-Stockbarger growth technique\r\n\r\nB.4\u00a0\u00a0 Crystal growth by Zone melting\r\n\r\nB.5\u00a0\u00a0 Other innovative techniques that fall under sub-category of related growth technique including:\r\n\r\n \r\n\r\n\u2022\u00a0\u00a0\u00a0 Plasma melting\r\n\r\n\u2022\u00a0\u00a0\u00a0 Liquid\u00a0 encapsulation pulling\r\n\r\n\u2022\u00a0\u00a0\u00a0 Skull melting\r\n\r\n\u2022\u00a0\u00a0\u00a0 Normal freezing\r\n\r\n\u2022\u00a0\u00a0\u00a0 Directional freezing\r\n\r\n \r\n\r\nC.\u00a0 Growth from Vapour Phase\r\n\r\n \r\n\r\nC.1\u00a0\u00a0 Gas phase reaction method\r\n\r\nC.2\u00a0\u00a0 Chemical Vapour deposition method\r\n\r\nC.3\u00a0\u00a0 Vapour phase epitaxy\r\n\r\nC.4\u00a0\u00a0\u00a0\u00a0\u00a0 Gas transport processes\r\n\r\nC.5\u00a0\u00a0\u00a0\u00a0\u00a0 Halide transport processes\r\n\r\nC.6\u00a0\u00a0\u00a0\u00a0\u00a0 Sublimation\r\n\r\nC.7\u00a0\u00a0\u00a0\u00a0\u00a0 Vacuum evaporation\r\n\r\nC.8\u00a0\u00a0\u00a0\u00a0\u00a0 Molecular beam epitaxy<\/span>\r\n\r\nC.9\u00a0\u00a0\u00a0\u00a0\u00a0 Temperature oscillation method<\/span>\r\n\r\n \r\n\r\nD.\u00a0\u00a0\u00a0\u00a0\u00a0 Solid State Growth<\/span>\r\n\r\nD.1 Sintering method<\/span>\r\n\r\nD.2 Zone heating method<\/span>\r\n\r\nD.3 Strain anneal<\/span>\r\n\r\nD.4 Polymorphic phase transition<\/span>\r\n\r\nD.5 Solid state diffusion reactions<\/span>\r\n\r\n<\/div>\r\n

\r\n\r\n \r\n\r\nSome of these techniques which are widely used may be described in a slightly more detail.\r\n\r\n \r\n\r\n26.2<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Growth from Solution<\/strong>\r\n\r\n \r\n

In this method the components of the crystal to be grown are dissolved in a solvent so as to form a saturated solution. The solution is made supersaturated by evaporation of the solvent or by changing the temperature which results into crystallization of the excess material. There are two types of solution growth, one being \u201cLow Temperature Solution Growth\u201d and the other being<\/p>\r\n \r\n

\u201cHigh Temperature Solution Growth\u201d. Growth from solution yields crystals usually with well developed habit faces as it happens with natural crystallization.<\/p>\r\n \r\n

Many commercial materials like sugar, salt, hydrated materials, several materials that decompose before melting and several good quality crystals meant for electronics and optical industries are grown from solution. Potassium dihydrogen phosphate (K D P), Ammonium dihydrogen phosphate (A D P) , optical grade sodium chloride crystals and several others are grown from aqueous solutions , quartz (optical , electronic and high Q-value grade) are grown using hydrothermal crystallization technique whereas magnetic bubble domain materials and several useful oxide crystals are grown using flux method.<\/p>\r\n \r\n\r\n26.2.1\u00a0\u00a0 Growth From Water Solution<\/strong>\r\n\r\n \r\n

In contrast to high temperature solution method to be described later, growth from water solution is directed at producing a crystal by growth from solution at low temperatures of materials which have moderate to high solubility in the temperature range ambient to somewhere around 353\u2070K and at atmospheric pressure. Growth of crystals from low - solubility systems or under high pressures falls under other specialised techniques, details of which are described in relevant sections. Advantages of growth from water solution are as follows:<\/p>\r\n \r\n

    \r\n \t
  1. Since crystal growth from solution occurs at ambient temperatures, one can exercise a better degree of control over the growth conditions<\/li>\r\n \t
  2. Temperatures can be easily stabilized in this range. As a result supersaturation can be accurately and precisely controlled.<\/li>\r\n \t
  3. The proximity to ambient temperatures reduces the possibility of major thermal shock to the crystal both during its growth and completion of growth.<\/li>\r\n \t
  4. It allows growth of crystals under closely controlled equilibrium conditions and at not very high temperatures, consequently resulting in the minimum of both equilibrium and non- equilibrium defects and in several cases to almost zero.<\/li>\r\n \t
  5. The method is particularly useful for the growth of those materials which decompose in the melt or in the solid at high temperatures and which undergo phase transformations above the working range of crystallization from solution at low temperatures. Such materials include several organic and inorganic substances.<\/li>\r\n \t
  6. Different morphologies and polymorphic forms of the same substance can be obtained by variation of growth parameters or of solvent.<\/li>\r\n<\/ol>\r\n<\/div>\r\n
    \r\n\r\n \r\n\r\nHowever, this category of growth method suffers from some major disadvantages, as for example:\r\n\r\n1.\u00a0\u00a0\u00a0 Possibility\u00a0 of\u00a0 solvent inclusion\r\n\r\n2.\u00a0\u00a0\u00a0 Slow\u00a0 rate of growth of crystals in several cases.\r\n\r\n \r\n

    The disadvantage identified at serial no.1 can be overcome to a reasonable level by better control of growth conditions.<\/p>\r\n \r\n\r\nThere are two basic techniques which are used in the growth of large crystals from water solution. In both cases, a saturated solution is first prepared and a seed crystal is inserted.\r\n\r\n \r\n

    In one technique the temperature is lowered slowly so as to reduce the solubility leading to crystallization. In the second technique, the temperature is held constant but the solvent is permitted to evaporate leading to crystallization. Vigorous stirring is required in most of the cases and many variations on these two general techniques are possible.<\/p>\r\n \r\n

    Several crystals like guanidinium aluminium sulphate hexahydrate ; Rochelle salt ; glycine sulphate ; sodium chlorate; potassium alum ; sodium bromate; sodium nitrate; triglycine sulphate, potassium dihydrogen phosphate; ammonium dihydrogen phosphate and several others have been grown by this method.<\/p>\r\n \r\n\r\nIn order to be able to achieve maximum potential of this technique of growth, it is important to make choice of a suitable solvent and use ultra pure materials. The ideal solvent should have the following properties\/characteristics:\r\n\r\n \r\n

      \r\n \t
    1. Be able to yield a prismatic habit in the crystal; the most useful crystals are the ones which grow at equivalent rates in all dimensions and lead to large bulk crystals.<\/li>\r\n \t
    2. Low viscosity<\/li>\r\n \t
    3. Density\u00a0 less\u00a0 than that of the bulk solute. It is desirable that the growing crystal does not float.<\/li>\r\n \t
    4. High solute solubility.<\/li>\r\n \t
    5. Low\u00a0 volatility\u00a0 so that uncontrolled loss of solvent during the growth period is minimized.<\/li>\r\n \t
    6. High\u00a0 and\u00a0 positive temperature coefficient of solute solubility.<\/li>\r\n<\/ol>\r\n \r\n

      The last one concerns the solubility of the material in the solvent and its dependence on temperature. These factors control supersaturation which is the driving force behind the rate of crystal growth. The supersaturation \u03b4 is defined as \u03b4 = C\/C0 , where C0 is the equilibrium concentration of solute at the temperature of growth and C represents the increase by which true concentration exceeds this. There are different ways in which solution can be supersaturated. One is \u201cTemperature Lowering Method\u201d. In this method attempt has to be made to control the temperature controlling rate so as to achieve a constant supersaturation over a wider temperature range. It is versatile and easy method to operate. The second method is known as \u201cSolvent Evaporation Technique\u201d in which slow evaporation of the solvent at constant temperature is allowed. If evaporation rate is controlled so that it is held constant, then again a constant supersaturation can be maintained. In practice, it is more difficult to achieve than is a controlled temperature lowering rate.<\/p>\r\n \r\n

      There\u00a0 may be cases\u00a0 in which the solute\u00a0 solubility\u00a0 is\u00a0\u00a0\u00a0\u00a0 very low.\u00a0 The above\u00a0described methods cannot be employed and some other methods which allow the precisely controlled slow development of supersaturation could be used instead. In these methods, the supersaturation is achieved either by the slow interdiffusion of solutions of two reacting chemicals, which on mixing react to form the solute, or by the interdiffusion of a solution with a solvent in which the solute is insoluble or less soluble The required control over the supersaturation is achieved by controlling the flux of the interdiffusing reactants, i.e., by varying temperature, concentration gradient and solvent viscosity. This is the basic principle of gel growth techniques for the growth of crystals of insoluble salts. This technique of growth will be discussed later in the relevant section.<\/span><\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n \r\n\r\n26.3<\/strong>\u00a0\u00a0\u00a0 Low Temperature Solution Growth.<\/strong>\r\n\r\n26.1\u00a0\u00a0 Introduction:<\/strong>\r\n\r\n \r\n

      Crystals\u00a0 grown from solution\u00a0 are\u00a0 by slow cooling\u00a0 or\u00a0 solvent\u00a0 evaporation\u00a0 techniques. Almost 90% of all crystals produced by low -temperature solution techniques are soluble in water. Obviously, this puts limitations on their use to applications where water and water vapour does not get involved. The requirement for such crystals, therefore, remains restricted. The crystals grow from solutions which are supersaturated. As the growth takes pace, a concentration gradient occurs near the growth face. Because of this, the growth face is inherently unstable with respect to supersaturation. In crystal growth from solution at low temperatures, the crystal is generally immersed in the solution. As a result, the latent heat evolved makes the crystal hotter than the solution. It is an additional factor which increases the supersaturation gradient. Stable growth becomes possible on account of stabilizing influence of surface free energy and the growth kinetics. It is because of this reason that stable growth faces are always singular in solution growth. However, in seeded growth from solution, non-singular faces occur around seeds obtained by cutting crystals along some suitable orientation\/planes which usually renders the growth unstable. This is the reason that cluster of inclusions is seen around seed crystals. Rates of growth from solution are much less as compared to rates of growth from melt.<\/p>\r\n \r\n

      The low-temperature solution growth has been discussed in detail by Buckley (1951). One could obtain more information by referring to H.E.Buckley (1951), Crystal Growth, published by Chapman and Hall, London.<\/p>\r\n \r\n

      In this method of growth, solvents and solution should have certain characteristics in order to be effective. An ideal solvent is the one which should:<\/p>\r\n \r\n\r\n\u25cf\u00a0 Yield a prismatic habit in the crystal,\r\n\r\n\u25cf\u00a0 Have a high, positive, temperature coefficient of solute solubility,\r\n\r\n\u25cf\u00a0 Have moderate reversible solubility,\r\n\r\n\u25cf\u00a0 Be non-corrosive,\r\n\r\n\u25cf\u00a0 Have a small vapour pressure,\r\n\r\n\u25cf\u00a0 Have low volatility,\r\n\r\n\u25cf\u00a0 Be non-toxic,\r\n\r\n\u25cf\u00a0 Be available in pure state at low price\r\n\r\n\u25cf\u00a0 Be non-inflammable,\r\n\r\n\u25cf\u00a0 Have low viscosity, and\r\n\r\n\u25cf\u00a0 Have density less than that of the bulk solute.\r\n\r\n \r\n

      The last two characteristics of the solvent are helpful in the simplification of apparatus design since it is desirable that the growing crystal should not float and that it should be well agitated. Solvents are required to have low volatility because it reduces the possibility of uncontrolled loss of\u00a0solvent\u00a0 during\u00a0 prolonged\u00a0 growth periods.\u00a0 It\u00a0 is\u00a0 particularly\u00a0 important\u00a0 where supersaturation\u00a0 is\u00a0<\/span>achieved\u00a0 by\u00a0 processes\u00a0 other\u00a0 than solvent\u00a0 evaporation.\u00a0 The first\u00a0 three requirements\u00a0\u00a0 of solvent\u00a0<\/span>characteristics are the most important characteristics which need to be described in some more detail.<\/span><\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n \r\n

      There is no such single solvent which incorporates in itself all these characteristics. Solvents that are generally used include water, ethyl alcohol, carbon tetrachloride, acetone, hexane, xylene and several others. However, water is used in most (> 90%) of the cases.<\/p>\r\n \r\n\r\n \r\n\r\n26.2<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Habit of crystals.<\/strong>\r\n\r\n \r\n

      Crystals which grow at more or\u00a0 less equivalent\u00a0 rates in all dimensions\u00a0 (popularly known as equi-dimensional\u00a0 crystals)\u00a0 are the most\u00a0 useful ones. From\u00a0 the research point\u00a0 of\u00a0 view\u00a0 als o,\u00a0 such crystals are quite informative regarding orientations, micro-topography and so on. Also, if the crystal grows with large habit faces , the defects , which generally get generated either from nucleus or\u00a0 the seed\u00a0 and propagate along\u00a0 specific\u00a0\u00a0 directions\u00a0 into\u00a0 the bulk\u00a0 of\u00a0 the growing crystal,\u00a0 usually\u00a0 become isolated\u00a0 into defective\u00a0 regions\u00a0 surrounded by large\u00a0 volumes of\u00a0 very high perfection.\u00a0 If\u00a0 the crystals grow\u00a0 in\u00a0 the form of needles or plates\u00a0 the growth dislocations\u00a0 follow\u00a0 the principal growth directions with the result that crystals get imperfect. It is more so for crystals which grow in the form of needles where the defects propagate continuously into the whole body of the crystal. Variations in the habit of a given crystal do occur with the variations in solvents.<\/p>\r\n \r\n\r\n26.3<\/strong>\u00a0Solubility<\/strong>\r\n\r\n \r\n

      The major factors involved in crystal growth from solution are the solubility of the material in the solvent and its dependence on temperature. The former provides the amount of material which is available for growth and hence defines the total size of the grown crystal. Both the solubility and the temperature define the supersaturation which is the driving force that controls the rate at which the crystal grows.<\/p>\r\n \r\n

      The supersaturation \u03b4 is defined as:\u03b4 =\u00a0\u00a0 C\/C0 , where C0 is the equilibrium concentration of the\u00a0 solute\u00a0 at\u00a0 the temperature of growth and\u00a0\u00a0\u00a0 C is\u00a0 the\u00a0 increment by which the actual\u00a0 concentration exceeds this. Supersaturation\u00a0 in\u00a0 a given\u00a0 solution\u00a0 can be achieved\u00a0 in many ways.\u00a0 One is\u00a0 to lower\u00a0 the temperature\u00a0 of the solution\u00a0 below\u00a0\u00a0 the equilibrium\u00a0 saturation\u00a0 temperature.\u00a0 It\u00a0 is\u00a0 popularly known as \u201cTemperature\u00a0 Lowering\u00a0 Method\u00a0 \u201c. In this\u00a0 method,\u00a0 the temperature is\u00a0 lowered continuously\u00a0 at\u00a0 a\u00a0controlled rate and \u0394C is determined by the rate at which temperature is lowered. In case of linear variations, \u0394C remains constant over small temperature intervals subject to the condition that the solubility-temperature curve does not change its slope very rapidly. However, it is not difficult to match the temperature lowering rate to the desired shape of the solubility curve in order to make it possible to achieve a constant supersaturation over a wide temperature range. This method is the most versatile and a convenient one to operate.<\/p>\r\n \r\n

      The second method is \u201cConstant Temperature Differential Method\u201d. In this technique, the temperature is suddenly lowered; difference between the two equilibrium values is C. The value of C and thus the growth rate will decrease as growth progresses. So, one has to replenish the solution saturated at the upper temperature constantly as growth proceeds. Though much more complicated than the first method (i.e., Temperature lowering method), it can suit under some situations and is a means of growth under rigidly constant temperature and supersaturation conditions.<\/p>\r\n \r\n

      The third method is to allow the slow evaporation of the solvent at constant temperature and\u00a0the technique is popularly known as \u201cSolvent Evaporation Technique\u201d. In this method constant supersaturation can be maintained by controlling the evaporation rate at a constant value. In terms of practical feasibility it is more difficult to achieve as compared to controlled temperature change.<\/span><\/p>\r\n \r\n

      Which of the methods of supersaturation are to be applied depends, to a great extent, on the shape of solubility curve and magnitude of the solubility. For this, one is required to have full knowledge concerning solubility-temperature data.<\/span><\/p>\r\n \r\n

      Let us consider a solubility curve of figure 26.1 in which the material changes its solubility rapidly with temperature. The curve is divided into three major regions \u2500 High, Moderate and Low. The region M corresponds to moderate to high solubility and moderate solubility-temperature gradient and so is a region which is suited for the use of \u201cTemperature Lowering Method\u201d.<\/span><\/p>\r\n \r\n

      To further discuss this, it is required to have solubility data for some materials as tabulated below in table 26.1:<\/span><\/p>\r\n \r\n

      It is found that if solubility falls in the range 200-1000g solute and that the ratio of solubilitytemperature gradient to solubility ( i.e., solubility ratio ) falls in the range 0.03-0.01 good quality<\/span>crystals can be grown at temperature lowering rates of 0.5-1 degree per day , i.e., at super saturations\u00a0<\/span>~\u00a0 2%. It requires a greater degree of temperature precision (\u00b1 0.005 K) to ensure that there are no sudden fluctuation which ensures prevention of sudden bursts of faster rate of uncontrolled growth which leads to imperfect crystals.<\/span><\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n\u00a0<\/strong>\r\n

      In case of solubility and solubility gradient exceeding this range, as for example at L, it is important that the system is controlled with great precision. Small fluctuations in temperature will lead to large fluctuations in solubility, supersaturation and growth rate. It, therefore, demands a better precision of the temperature of the order say \u00b1 0.001 K in order to bring down these variations for systems at the lower end of L range. If such a precision in the system does not become available, one may use constant temperature differential method in which the temperatures of both source and growing crystals can be maintained at precisely controlled constant values.<\/p>\r\n\u00a0<\/strong>\r\n

      Let us now come to the region N. In this region of rapidly changing solubility gradient, it is difficult to maintain constant growth conditions even for short periods of time.<\/p>\r\n \r\n

      In the region\u00a0 O the gradient\u00a0 is\u00a0 shallow\u00a0\u00a0\u00a0 and the\u00a0\u00a0\u00a0\u00a0 C\u00a0 achievable\u00a0 by\u00a0 either\u00a0 the temperature lowering or constant temperature differential techniques is greatly reduced. Obviously, the growth rates will be low and even slight fluctuations would greatly affect the growth parameters. It is desirable to use solvent evaporation technique to achieve reasonable level of supersaturation.<\/p>\r\n \r\n

      The solvent evaporation method is very effective in such cases where the overall solubility is low. In such cases, the occurrence of fluctuations brings about only small fluctuations in supersaturation and so the growth process does not get affected much.<\/p>\r\n \r\n

      Following the above said general guidelines, one can identify a suitable solvent and the appropriate technique of growth and use the same to grow a crystal of large dimensions. Let\u00a0 us\u00a0 discuss\u00a0 the low\u00a0 temperature solution\u00a0 growth further\u00a0 in\u00a0 the background of some information provided in table 26.1.There is a specific classification of materials under the categories of\u00a0 \u201cEasy to grow\u201d,\u00a0 \u201cDifficult\u00a0 to Grow\u201d and\u00a0 \u201cGrowth Improved by Varying Conditions\u201d. The first category gives examples of materials which can be relatively eas ily grown to obtain crystals of large dimensions\u00a0 by\u00a0 the \u201ctemperature lowering method\u201d (or\u00a0 \u201cconstant\u00a0 temperature\u00a0 differential method\u201d) and at temperature lowering rates in the range of 0.1-1 Kh\u25001subject to the condition that the apparatus used for growth is able to maintain temperature stability of more than \u00b1 0.005 K over long periods of time.<\/p>\r\n \r\n

      The materials like sodium chloride, Benzil, Calcium carbonate and silver iodide are relatively difficult to grow. The third category of crystals are the ones whose growth process can be improved by varying conditions of growth. The quality of crystals of anthracene , urea (with water and methanol\u00a0as solvent) and several such organic and inorganic materials is worse as compared to those belonging to first category of crystals when they are grown at similar rates close to ambient temperatures.<\/span><\/p>\r\n\"\"\r\n

      Figure 26.1: Curve of solubility versus temperature<\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n26.4 Preliminary Experiment<\/strong>\r\n\r\n \r\n

      To grow a well-formed single crystal of the desired substance one has to make a solution which is saturated at room temperature and then allow it to evaporate in order that as the solvent evaporates some of the substance in solution will have to be deposited so that the solution does not get supersaturated. If one desires to grow crystal of a particular substance, it is necessary to have knowledge about its solubility and then calculate the relative amounts of solute and solvent that is required to make a saturated solution.<\/p>\r\n \r\n

      As for example, we may take the case of Alum whose chemical composition is KAl(SO4)2.12H2O. The first step is to study the table that provide information regarding solubility of alum which is about 11g in 100 ml of water at a temperature of 20\u00b0C and at 100\u00b0C its solubility is extremely large. One can, therefore, make a solution of alum by taking ~<\/strong>15g of alum in 100ml of water. Put this solution into a beaker, heat it with constant and vigoursly stirring it. The hot solution may be filtered to obtain a pure solution. Place the solution in the beaker which is loosely covered in order to allow water to evaporate without allowing the dust to enter into the solution or get contaminated in any way. After a few days one would find some alum having deposited as small crystals. Take one of the small crystals and suspend the same with the help of a strong thread so that it remains immersed in the solution of alum as shown in figure 26.2.Allow water to evaporate. As water evaporates the crystal will start growing in size. Allow one or two weeks for the crystal to grow. One\u00a0would find the growth of alum crystal having increased in its size and developed into a regular octahedron. It would take a week or two for the crystal to grow to a large size.<\/span><\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n\"\"\"\"\r\n

      Figure 26.2: Schematic diagram showing a simple arrangement for growth from water solution<\/p>\r\n \r\n\r\n26.5<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Apparatus For Crystallization<\/strong>\r\n\r\n26.5.1<\/strong>\u00a0\u00a0\u00a0 Slow cooling technique<\/strong>\r\n\r\n \r\n

      The method for producing large single crystals from solution is the \u201cTemperature Lowering Method\u201d. Different types of apparatus designed for the production of large crystals of organic and inorganic substances from saturated solution at low temperatures are many and are described in the literature (see bibliography). Pamplin has listed a large number of references that are available on the subject. The variations in design of the apparatus depends on the ultimate size of the crystal that one may wish to grow , nature of the solvent phase and other related parameters of growth. However, there are certain basic requirements of the growth apparatus which have to be common to all. These are:<\/p>\r\n \r\n

      i) Temperature control to better than 0.01K. The thermostatic control may preferably be somewhere in the<\/p>\r\nrange of 0.005-0.001k.\r\n\r\nii)\u00a0 Vigorous stirring of solution. It would ensure prevention of layering and spurious nucleation.\r\n

      iii)\u00a0 Efficient reciprocated stirring of the crystal. It is necessary for the prevention of local super-saturations<\/p>\r\nor under-saturations which become the cause of variations (increase or decrease ) in the rate of crystal\r\n\r\ngrowth and leading to incorporation of solvent and \u201cveiling\u201d.\r\n

      iv)\u00a0 Arrangement for controlled super-saturation of the solution.<\/p>\r\n \r\n

      The use of slow cooling processes is used in a large number of crystal growth experiments from solution. They are in several ways simpler than the processes involved in solvent evaporation or temperature gradient transfer. In this process only one vessel is required and the most important requirement is to meet the major technological requirement of providing the desired rate of cooling as precisely as possible. On a commercial scale the volumes of solution used are quite large, viz., in the range of 10-100 litres. However, in the current demand of KH2PO4 of 300mm dimensions, even larger volumes of solution may be required.<\/p>\r\n\r\n<\/div>\r\n

      \r\n\r\n\"\"\r\n

      Figure 26.3: Schematic diagram of a simple crystallizer used for the growth by slow cooling technique<\/p>\r\n \r\n

      Figure 26.3 shows the design of a simple crystallizer for growth by slow cooling method. It consists of a double-vessel system, stirrer, and water inlet\/outlet, mechanical arrangement for rotation of seed crystal, seed crystal holder and insulating platform. There is an external temperature-controlled bath from which water is pumped into the water jacket so as to maintain the solution at the programmed temperature. In order to ensure circulation of the solution, multiple stirrers are used. In figure 26.3 only one stirrer is shown. There is a provision for placement of seed crystal on a holder which is in the form a disc. The seed crystal is mounted on this disc which can be rotated .It is rotated in a particular direction (say clockwise) for some time and then rotated in the reverse direction (say anti-clockwise) for some time. The number of rotations per minute (rpm) is maintained appropriately after careful calculations.<\/p>\r\n \r\n

      In using the technique of growth by slow cooling, it is necessary to maintain temperature stability. Interface instability due to any reason results into inclusion formation and spurious nucleation. The instability is mainly caused by short bursts of rapid growth as a consequence of fluctuation in temperature. Growth-rate variations can as well be caused by other parameters like fluctuations in the amount of stirring. Rotation of the seed in two opposite directions does reduce the possibility of spurious nucleation to a great extent.<\/p>\r\n \r\n\r\n26.5.2<\/strong>\u00a0\u00a0\u00a0 Solvent Evaporation technique<\/strong>\r\n\r\n \r\n

      Slow evaporation is most easily achieved by using a controlled flow gas inlet\/outlet attached to the head of the flask and a trap to collect the condensing liquid.<\/p>\r\n \r\n

      One is confronted with a practical problem and that is to ensure a constant desirable rate of\u00a0 loss\u00a0 of\u00a0 solvent\u00a0 from\u00a0 the system.\u00a0 A system\u00a0 for\u00a0 growth by solvent \u00a0evaporation\u00a0 is\u00a0 shown in\u00a0 a schematic diagram\u00a0 of\u00a0 figure\u00a0 26.4 which meets\u00a0 the above requirement\u00a0 of\u00a0 acceptable\u00a0 rate\u00a0 of\u00a0 loss\u00a0 of solvent\u00a0 from\u00a0 the system\u00a0 by providing\u00a0 a\u00a0 cooled condensation region. Alternative method\u00a0 is\u00a0\u00a0 to use porous covers separating the saturated vapour above the solution from a large and well stirred volume of air with a negligibly small partial pressure of the solvent. The porosity of the covers may be in the form\u00a0 of\u00a0 holes\u00a0 or\u00a0 tubes,\u00a0 or\u00a0 a permeable membrane.\u00a0 The porous\u00a0 covers can also\u00a0 be in\u00a0 the form\u00a0 of cellulose-fibre sheets , say Cellophane , are very useful as they allow water to go out while protecting the system from dust and external particles. The rate of loss of vapour through the membrane can be varied using a non-porous cover over part of the membrane area.<\/p>\r\n \r\n

      Solvent evaporation process produces crystals of limited size as is the case with slow cooling process. However, crystal grown by solvent evaporation are less pure as compared to crystals grown by slow cooling process. It is because with solvent evaporation, the concentration of impurities in the solution has tendency to increase as growth proceeds further.<\/p>\r\n\r\n<\/div>\r\n

      <\/div>\r\n
      \r\n\r\n\"\"\r\n\r\n \r\n

      Figure 26.4: Schematic diagram of an apparatus for the growth of crystals by solvent evaporation process<\/p>\r\n \r\n

      The apparatus shown in figure 26.4 has provision for holding seed crystals. In fact, more than one seed crystal can be used by cementing each of them to one of the holders attached to a multi -arm\u00a0 assembly\u00a0 named \u2018Spider\u201d;\u00a0 the spider\u00a0 can be rotated both in\u00a0 the clockwise as well\u00a0 as\u00a0 anti- clockwise\u00a0 directions. The solution\u00a0 is\u00a0\u00a0 thus\u00a0 continuously\u00a0 stirred which helps\u00a0 to avoid\u00a0 interface instability\u00a0 and\u00a0 spurious nucleation.\u00a0 Typically,\u00a0 the spider\u00a0 is made to make four\u00a0 revolutions\u00a0\u00a0 in\u00a0 one direction, stop for a while, and then allowed\u00a0 to make four revolutions in the opposite direction. The cycle\u00a0 is\u00a0 composed\u00a0 of making\u00a0 revolutions in\u00a0 one direction,\u00a0 pause,\u00a0 then making\u00a0 revolutions\u00a0 in\u00a0 the opposite direction, another pause before repeating the cycle. Each cycle keeps on repeating during the experimentation of crystal growth. The maximum r.p.m. depends on the width of a crystal and radius of its rotatory movement and the Reynolds number so required is calculated which usually should be at the most \u2264 2000.<\/p>\r\n \r\n\r\n26.5.3<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Temperature-difference Technique.<\/strong>\r\n\r\n \r\n\r\nA simple apparatus which works on the basis of temperature-difference process is shown in a schematic diagram of figure 26.5\r\n\r\n<\/div>\r\n

      \r\n\r\n\"\"\r\n

      Figure 26.5: Schematic diagram of simple apparatus for crystal growth by temperature-difference process.<\/p>\r\n \r\n\r\nIn this arrangement, the crystallizer is divided into two zones \u2500 one cool growth zone\r\n\r\nand the other hot nutrient zone. Seed crystal is maintained at temperature T which is lower by\u00a0 T as\r\n

      compared to the upper nutrient zone; the nutrient zone being maintained at a temperature T + T. The solution is almost saturated in the zone at T + T while the seed crystal maintained at temperature T will grow. The amount of growth is limited only by the amount of nutrient which is put into the hot zone of the crystallizer. One could, in principle, think of adding more nutrients during the growth run but then to maintain the temperature at T + T requires much more care. The above apparatus is a simpler version of temperature-difference crystallizer. In those systems which are used for production purposes, the cool growth zone is separated from the hot saturator. The solution is pumped from one vessel to the other. Supersaturated solutions have a tendency to nucleate when pumped, and if solution saturated at T + T is pumped directly to the growth vessel, it makes a way for undissolved particles to get transferred to the growth region. To avoid this from happening, three vessels are used in place of one. If this is not done, the undissolved particles will enter the growing crystals and render them imperfect.<\/p>\r\n \r\n

      It is claimed that the growth of really large crystals is most likely to be achieved in systems of this type. However, if one is to deal with large volumes of liquid exceeding 100 litres, it is extremely difficult to provide suitable degree of mixing and to avoid convective instabilities.<\/p>\r\n\r\n<\/div>\r\n \r\n\r\n\r\n\r\n\r\n
      you can view video on Experimental Crystal growt<\/strong><\/td>\r\n\"\"<\/a><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n\r\nReferences.<\/strong>\r\n
        \r\n \t
      1. Pamplin,B.R. :\u201cCrystal Growth\u201d, Pergamon Press,N.Y.,1980.<\/li>\r\n \t
      2. Brice, J.C. : \u201cCrystal Growth Processes\u201d, John Wiley & Sons,N.Y.,1986.<\/li>\r\n \t
      3. Cracknell,A.P.: \u201cCrystals and Their Structures\u201d, Pergamon Press, Oxford,1969.<\/li>\r\n \t
      4. Gilman,J.J. : \u201c The Art & Science of Growing Crystals\u201d,Wiley,N.Y.,1963.<\/li>\r\n<\/ol>\r\nSuggested Reading.<\/strong>\r\n
          \r\n \t
        1. Buckley,H.E.: \u201c Crystal Growth\u201d, Chapman & Hall, London,1951.<\/li>\r\n \t
        2. Brice,J.C.: \u201c The Growth of Crystals From Liquids\u201d,North-Holland, Amsterdam,1973.<\/li>\r\n \t
        3. Rosenberger,F.: \u201c Fundamentals Of Crystal Growth\u201d,Springer-Verlag,Berlin,1979.<\/li>\r\n \t
        4. Chernov,A.A.: \u201c Modern Crystallography III: Crystal Growth\u201d, Springer-Verlag,Berlin.<\/li>\r\n \t
        5. Van Enckevort,W.J.P.: Prog.Cryst.Growth& Charac.,9,1984.<\/li>\r\n \t
        6. Torgesen,J.L., Horton,A.T. and Saylor,C.P.:\u201dEquipment for single crystal growth from aqueous Solution,J.Res. of the NBS-C,Engineering & Instrumentation, 67C,25.<\/li>\r\n \t
        7. Hooper,R.M.;Mcardle,B.J.;Narang,R.S.,Sherwood,J.N.: in \u201cCrystal Growth\u201ded.B.R.Pamplin, Pergamon Press,1980.<\/li>\r\n<\/ol>\r\n ","rendered":"
          \"epgp<\/a>
          \n<\/span><\/div>\n
          \n

           <\/p>\n

          26.1 Introduction <\/strong><\/p>\n

           <\/p>\n

          Successful growth of crystals requires strict control over phase change. There are processes involving growth of solute from phase transitions viz., liquid to solid state or vapour to solid state or solid to solid phase transitions. Main categories of crystal growth methods include the following:<\/p>\n

           <\/p>\n

          A.\u00a0 \u00a0 \u00a0 Growth from Solution<\/p>\n

          A.1\u00a0\u00a0\u00a0\u00a0 Growth from flux<\/p>\n

          A.2\u00a0\u00a0\u00a0\u00a0 Hydrothermal growth<\/p>\n

          A.3\u00a0\u00a0\u00a0\u00a0 High pressure growth<\/p>\n

          A.4\u00a0\u00a0\u00a0\u00a0 Growth from water solution<\/p>\n

          A.5\u00a0\u00a0\u00a0\u00a0 Growth from gel<\/p>\n

          A.6\u00a0\u00a0 Other innovative techniques which fall under sub-categories of related growth techniques include:<\/p>\n

           <\/p>\n

          \u2022\u00a0\u00a0\u00a0 Organic\u00a0 solution growth<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Accelerated\u00a0 crucible growth<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Electro-crystallization<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Liquid\u00a0 phase\u00a0 epitaxy and<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Molten\u00a0 metal\u00a0 solution growth<\/p>\n

           <\/p>\n

          B. Growth from Melt<\/p>\n

          B.1\u00a0\u00a0 Czochralski crystal pulling technique<\/p>\n

          B.2\u00a0\u00a0 Verneuil flame fusion growth technique<\/p>\n

          B.3\u00a0\u00a0 Bridgman-Stockbarger growth technique<\/p>\n

          B.4\u00a0\u00a0 Crystal growth by Zone melting<\/p>\n

          B.5\u00a0\u00a0 Other innovative techniques that fall under sub-category of related growth technique including:<\/p>\n

           <\/p>\n

          \u2022\u00a0\u00a0\u00a0 Plasma melting<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Liquid\u00a0 encapsulation pulling<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Skull melting<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Normal freezing<\/p>\n

          \u2022\u00a0\u00a0\u00a0 Directional freezing<\/p>\n

           <\/p>\n

          C.\u00a0 Growth from Vapour Phase<\/p>\n

           <\/p>\n

          C.1\u00a0\u00a0 Gas phase reaction method<\/p>\n

          C.2\u00a0\u00a0 Chemical Vapour deposition method<\/p>\n

          C.3\u00a0\u00a0 Vapour phase epitaxy<\/p>\n

          C.4\u00a0\u00a0\u00a0\u00a0\u00a0 Gas transport processes<\/p>\n

          C.5\u00a0\u00a0\u00a0\u00a0\u00a0 Halide transport processes<\/p>\n

          C.6\u00a0\u00a0\u00a0\u00a0\u00a0 Sublimation<\/p>\n

          C.7\u00a0\u00a0\u00a0\u00a0\u00a0 Vacuum evaporation<\/p>\n

          C.8\u00a0\u00a0\u00a0\u00a0\u00a0 Molecular beam epitaxy<\/span><\/p>\n

          C.9\u00a0\u00a0\u00a0\u00a0\u00a0 Temperature oscillation method<\/span><\/p>\n

           <\/p>\n

          D.\u00a0\u00a0\u00a0\u00a0\u00a0 Solid State Growth<\/span><\/p>\n

          D.1 Sintering method<\/span><\/p>\n

          D.2 Zone heating method<\/span><\/p>\n

          D.3 Strain anneal<\/span><\/p>\n

          D.4 Polymorphic phase transition<\/span><\/p>\n

          D.5 Solid state diffusion reactions<\/span><\/p>\n<\/div>\n

          \n

           <\/p>\n

          Some of these techniques which are widely used may be described in a slightly more detail.<\/p>\n

           <\/p>\n

          26.2<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Growth from Solution<\/strong><\/p>\n

           <\/p>\n

          In this method the components of the crystal to be grown are dissolved in a solvent so as to form a saturated solution. The solution is made supersaturated by evaporation of the solvent or by changing the temperature which results into crystallization of the excess material. There are two types of solution growth, one being \u201cLow Temperature Solution Growth\u201d and the other being<\/p>\n

           <\/p>\n

          \u201cHigh Temperature Solution Growth\u201d. Growth from solution yields crystals usually with well developed habit faces as it happens with natural crystallization.<\/p>\n

           <\/p>\n

          Many commercial materials like sugar, salt, hydrated materials, several materials that decompose before melting and several good quality crystals meant for electronics and optical industries are grown from solution. Potassium dihydrogen phosphate (K D P), Ammonium dihydrogen phosphate (A D P) , optical grade sodium chloride crystals and several others are grown from aqueous solutions , quartz (optical , electronic and high Q-value grade) are grown using hydrothermal crystallization technique whereas magnetic bubble domain materials and several useful oxide crystals are grown using flux method.<\/p>\n

           <\/p>\n

          26.2.1\u00a0\u00a0 Growth From Water Solution<\/strong><\/p>\n

           <\/p>\n

          In contrast to high temperature solution method to be described later, growth from water solution is directed at producing a crystal by growth from solution at low temperatures of materials which have moderate to high solubility in the temperature range ambient to somewhere around 353\u2070K and at atmospheric pressure. Growth of crystals from low – solubility systems or under high pressures falls under other specialised techniques, details of which are described in relevant sections. Advantages of growth from water solution are as follows:<\/p>\n

           <\/p>\n

            \n
          1. Since crystal growth from solution occurs at ambient temperatures, one can exercise a better degree of control over the growth conditions<\/li>\n
          2. Temperatures can be easily stabilized in this range. As a result supersaturation can be accurately and precisely controlled.<\/li>\n
          3. The proximity to ambient temperatures reduces the possibility of major thermal shock to the crystal both during its growth and completion of growth.<\/li>\n
          4. It allows growth of crystals under closely controlled equilibrium conditions and at not very high temperatures, consequently resulting in the minimum of both equilibrium and non- equilibrium defects and in several cases to almost zero.<\/li>\n
          5. The method is particularly useful for the growth of those materials which decompose in the melt or in the solid at high temperatures and which undergo phase transformations above the working range of crystallization from solution at low temperatures. Such materials include several organic and inorganic substances.<\/li>\n
          6. Different morphologies and polymorphic forms of the same substance can be obtained by variation of growth parameters or of solvent.<\/li>\n<\/ol>\n<\/div>\n
            \n

             <\/p>\n

            However, this category of growth method suffers from some major disadvantages, as for example:<\/p>\n

            1.\u00a0\u00a0\u00a0 Possibility\u00a0 of\u00a0 solvent inclusion<\/p>\n

            2.\u00a0\u00a0\u00a0 Slow\u00a0 rate of growth of crystals in several cases.<\/p>\n

             <\/p>\n

            The disadvantage identified at serial no.1 can be overcome to a reasonable level by better control of growth conditions.<\/p>\n

             <\/p>\n

            There are two basic techniques which are used in the growth of large crystals from water solution. In both cases, a saturated solution is first prepared and a seed crystal is inserted.<\/p>\n

             <\/p>\n

            In one technique the temperature is lowered slowly so as to reduce the solubility leading to crystallization. In the second technique, the temperature is held constant but the solvent is permitted to evaporate leading to crystallization. Vigorous stirring is required in most of the cases and many variations on these two general techniques are possible.<\/p>\n

             <\/p>\n

            Several crystals like guanidinium aluminium sulphate hexahydrate ; Rochelle salt ; glycine sulphate ; sodium chlorate; potassium alum ; sodium bromate; sodium nitrate; triglycine sulphate, potassium dihydrogen phosphate; ammonium dihydrogen phosphate and several others have been grown by this method.<\/p>\n

             <\/p>\n

            In order to be able to achieve maximum potential of this technique of growth, it is important to make choice of a suitable solvent and use ultra pure materials. The ideal solvent should have the following properties\/characteristics:<\/p>\n

             <\/p>\n

              \n
            1. Be able to yield a prismatic habit in the crystal; the most useful crystals are the ones which grow at equivalent rates in all dimensions and lead to large bulk crystals.<\/li>\n
            2. Low viscosity<\/li>\n
            3. Density\u00a0 less\u00a0 than that of the bulk solute. It is desirable that the growing crystal does not float.<\/li>\n
            4. High solute solubility.<\/li>\n
            5. Low\u00a0 volatility\u00a0 so that uncontrolled loss of solvent during the growth period is minimized.<\/li>\n
            6. High\u00a0 and\u00a0 positive temperature coefficient of solute solubility.<\/li>\n<\/ol>\n

               <\/p>\n

              The last one concerns the solubility of the material in the solvent and its dependence on temperature. These factors control supersaturation which is the driving force behind the rate of crystal growth. The supersaturation \u03b4 is defined as \u03b4 = C\/C0 , where C0 is the equilibrium concentration of solute at the temperature of growth and C represents the increase by which true concentration exceeds this. There are different ways in which solution can be supersaturated. One is \u201cTemperature Lowering Method\u201d. In this method attempt has to be made to control the temperature controlling rate so as to achieve a constant supersaturation over a wider temperature range. It is versatile and easy method to operate. The second method is known as \u201cSolvent Evaporation Technique\u201d in which slow evaporation of the solvent at constant temperature is allowed. If evaporation rate is controlled so that it is held constant, then again a constant supersaturation can be maintained. In practice, it is more difficult to achieve than is a controlled temperature lowering rate.<\/p>\n

               <\/p>\n

              There\u00a0 may be cases\u00a0 in which the solute\u00a0 solubility\u00a0 is\u00a0\u00a0\u00a0\u00a0 very low.\u00a0 The above\u00a0described methods cannot be employed and some other methods which allow the precisely controlled slow development of supersaturation could be used instead. In these methods, the supersaturation is achieved either by the slow interdiffusion of solutions of two reacting chemicals, which on mixing react to form the solute, or by the interdiffusion of a solution with a solvent in which the solute is insoluble or less soluble The required control over the supersaturation is achieved by controlling the flux of the interdiffusing reactants, i.e., by varying temperature, concentration gradient and solvent viscosity. This is the basic principle of gel growth techniques for the growth of crystals of insoluble salts. This technique of growth will be discussed later in the relevant section.<\/span><\/p>\n<\/div>\n

              \n

               <\/p>\n

              26.3<\/strong>\u00a0\u00a0\u00a0 Low Temperature Solution Growth.<\/strong><\/p>\n

              26.1\u00a0\u00a0 Introduction:<\/strong><\/p>\n

               <\/p>\n

              Crystals\u00a0 grown from solution\u00a0 are\u00a0 by slow cooling\u00a0 or\u00a0 solvent\u00a0 evaporation\u00a0 techniques. Almost 90% of all crystals produced by low -temperature solution techniques are soluble in water. Obviously, this puts limitations on their use to applications where water and water vapour does not get involved. The requirement for such crystals, therefore, remains restricted. The crystals grow from solutions which are supersaturated. As the growth takes pace, a concentration gradient occurs near the growth face. Because of this, the growth face is inherently unstable with respect to supersaturation. In crystal growth from solution at low temperatures, the crystal is generally immersed in the solution. As a result, the latent heat evolved makes the crystal hotter than the solution. It is an additional factor which increases the supersaturation gradient. Stable growth becomes possible on account of stabilizing influence of surface free energy and the growth kinetics. It is because of this reason that stable growth faces are always singular in solution growth. However, in seeded growth from solution, non-singular faces occur around seeds obtained by cutting crystals along some suitable orientation\/planes which usually renders the growth unstable. This is the reason that cluster of inclusions is seen around seed crystals. Rates of growth from solution are much less as compared to rates of growth from melt.<\/p>\n

               <\/p>\n

              The low-temperature solution growth has been discussed in detail by Buckley (1951). One could obtain more information by referring to H.E.Buckley (1951), Crystal Growth, published by Chapman and Hall, London.<\/p>\n

               <\/p>\n

              In this method of growth, solvents and solution should have certain characteristics in order to be effective. An ideal solvent is the one which should:<\/p>\n

               <\/p>\n

              \u25cf\u00a0 Yield a prismatic habit in the crystal,<\/p>\n

              \u25cf\u00a0 Have a high, positive, temperature coefficient of solute solubility,<\/p>\n

              \u25cf\u00a0 Have moderate reversible solubility,<\/p>\n

              \u25cf\u00a0 Be non-corrosive,<\/p>\n

              \u25cf\u00a0 Have a small vapour pressure,<\/p>\n

              \u25cf\u00a0 Have low volatility,<\/p>\n

              \u25cf\u00a0 Be non-toxic,<\/p>\n

              \u25cf\u00a0 Be available in pure state at low price<\/p>\n

              \u25cf\u00a0 Be non-inflammable,<\/p>\n

              \u25cf\u00a0 Have low viscosity, and<\/p>\n

              \u25cf\u00a0 Have density less than that of the bulk solute.<\/p>\n

               <\/p>\n

              The last two characteristics of the solvent are helpful in the simplification of apparatus design since it is desirable that the growing crystal should not float and that it should be well agitated. Solvents are required to have low volatility because it reduces the possibility of uncontrolled loss of\u00a0solvent\u00a0 during\u00a0 prolonged\u00a0 growth periods.\u00a0 It\u00a0 is\u00a0 particularly\u00a0 important\u00a0 where supersaturation\u00a0 is\u00a0<\/span>achieved\u00a0 by\u00a0 processes\u00a0 other\u00a0 than solvent\u00a0 evaporation.\u00a0 The first\u00a0 three requirements\u00a0\u00a0 of solvent\u00a0<\/span>characteristics are the most important characteristics which need to be described in some more detail.<\/span><\/p>\n<\/div>\n

              \n

               <\/p>\n

              There is no such single solvent which incorporates in itself all these characteristics. Solvents that are generally used include water, ethyl alcohol, carbon tetrachloride, acetone, hexane, xylene and several others. However, water is used in most (> 90%) of the cases.<\/p>\n

               <\/p>\n

               <\/p>\n

              26.2<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Habit of crystals.<\/strong><\/p>\n

               <\/p>\n

              Crystals which grow at more or\u00a0 less equivalent\u00a0 rates in all dimensions\u00a0 (popularly known as equi-dimensional\u00a0 crystals)\u00a0 are the most\u00a0 useful ones. From\u00a0 the research point\u00a0 of\u00a0 view\u00a0 als o,\u00a0 such crystals are quite informative regarding orientations, micro-topography and so on. Also, if the crystal grows with large habit faces , the defects , which generally get generated either from nucleus or\u00a0 the seed\u00a0 and propagate along\u00a0 specific\u00a0\u00a0 directions\u00a0 into\u00a0 the bulk\u00a0 of\u00a0 the growing crystal,\u00a0 usually\u00a0 become isolated\u00a0 into defective\u00a0 regions\u00a0 surrounded by large\u00a0 volumes of\u00a0 very high perfection.\u00a0 If\u00a0 the crystals grow\u00a0 in\u00a0 the form of needles or plates\u00a0 the growth dislocations\u00a0 follow\u00a0 the principal growth directions with the result that crystals get imperfect. It is more so for crystals which grow in the form of needles where the defects propagate continuously into the whole body of the crystal. Variations in the habit of a given crystal do occur with the variations in solvents.<\/p>\n

               <\/p>\n

              26.3<\/strong>\u00a0Solubility<\/strong><\/p>\n

               <\/p>\n

              The major factors involved in crystal growth from solution are the solubility of the material in the solvent and its dependence on temperature. The former provides the amount of material which is available for growth and hence defines the total size of the grown crystal. Both the solubility and the temperature define the supersaturation which is the driving force that controls the rate at which the crystal grows.<\/p>\n

               <\/p>\n

              The supersaturation \u03b4 is defined as:\u03b4 =\u00a0\u00a0 C\/C0 , where C0 is the equilibrium concentration of the\u00a0 solute\u00a0 at\u00a0 the temperature of growth and\u00a0\u00a0\u00a0 C is\u00a0 the\u00a0 increment by which the actual\u00a0 concentration exceeds this. Supersaturation\u00a0 in\u00a0 a given\u00a0 solution\u00a0 can be achieved\u00a0 in many ways.\u00a0 One is\u00a0 to lower\u00a0 the temperature\u00a0 of the solution\u00a0 below\u00a0\u00a0 the equilibrium\u00a0 saturation\u00a0 temperature.\u00a0 It\u00a0 is\u00a0 popularly known as \u201cTemperature\u00a0 Lowering\u00a0 Method\u00a0 \u201c. In this\u00a0 method,\u00a0 the temperature is\u00a0 lowered continuously\u00a0 at\u00a0 a\u00a0controlled rate and \u0394C is determined by the rate at which temperature is lowered. In case of linear variations, \u0394C remains constant over small temperature intervals subject to the condition that the solubility-temperature curve does not change its slope very rapidly. However, it is not difficult to match the temperature lowering rate to the desired shape of the solubility curve in order to make it possible to achieve a constant supersaturation over a wide temperature range. This method is the most versatile and a convenient one to operate.<\/p>\n

               <\/p>\n

              The second method is \u201cConstant Temperature Differential Method\u201d. In this technique, the temperature is suddenly lowered; difference between the two equilibrium values is C. The value of C and thus the growth rate will decrease as growth progresses. So, one has to replenish the solution saturated at the upper temperature constantly as growth proceeds. Though much more complicated than the first method (i.e., Temperature lowering method), it can suit under some situations and is a means of growth under rigidly constant temperature and supersaturation conditions.<\/p>\n

               <\/p>\n

              The third method is to allow the slow evaporation of the solvent at constant temperature and\u00a0the technique is popularly known as \u201cSolvent Evaporation Technique\u201d. In this method constant supersaturation can be maintained by controlling the evaporation rate at a constant value. In terms of practical feasibility it is more difficult to achieve as compared to controlled temperature change.<\/span><\/p>\n

               <\/p>\n

              Which of the methods of supersaturation are to be applied depends, to a great extent, on the shape of solubility curve and magnitude of the solubility. For this, one is required to have full knowledge concerning solubility-temperature data.<\/span><\/p>\n

               <\/p>\n

              Let us consider a solubility curve of figure 26.1 in which the material changes its solubility rapidly with temperature. The curve is divided into three major regions \u2500 High, Moderate and Low. The region M corresponds to moderate to high solubility and moderate solubility-temperature gradient and so is a region which is suited for the use of \u201cTemperature Lowering Method\u201d.<\/span><\/p>\n

               <\/p>\n

              To further discuss this, it is required to have solubility data for some materials as tabulated below in table 26.1:<\/span><\/p>\n

               <\/p>\n

              It is found that if solubility falls in the range 200-1000g solute and that the ratio of solubilitytemperature gradient to solubility ( i.e., solubility ratio ) falls in the range 0.03-0.01 good quality<\/span>crystals can be grown at temperature lowering rates of 0.5-1 degree per day , i.e., at super saturations\u00a0<\/span>~\u00a0 2%. It requires a greater degree of temperature precision (\u00b1 0.005 K) to ensure that there are no sudden fluctuation which ensures prevention of sudden bursts of faster rate of uncontrolled growth which leads to imperfect crystals.<\/span><\/p>\n<\/div>\n

              \n

              \u00a0<\/strong><\/p>\n

              In case of solubility and solubility gradient exceeding this range, as for example at L, it is important that the system is controlled with great precision. Small fluctuations in temperature will lead to large fluctuations in solubility, supersaturation and growth rate. It, therefore, demands a better precision of the temperature of the order say \u00b1 0.001 K in order to bring down these variations for systems at the lower end of L range. If such a precision in the system does not become available, one may use constant temperature differential method in which the temperatures of both source and growing crystals can be maintained at precisely controlled constant values.<\/p>\n

              \u00a0<\/strong><\/p>\n

              Let us now come to the region N. In this region of rapidly changing solubility gradient, it is difficult to maintain constant growth conditions even for short periods of time.<\/p>\n

               <\/p>\n

              In the region\u00a0 O the gradient\u00a0 is\u00a0 shallow\u00a0\u00a0\u00a0 and the\u00a0\u00a0\u00a0\u00a0 C\u00a0 achievable\u00a0 by\u00a0 either\u00a0 the temperature lowering or constant temperature differential techniques is greatly reduced. Obviously, the growth rates will be low and even slight fluctuations would greatly affect the growth parameters. It is desirable to use solvent evaporation technique to achieve reasonable level of supersaturation.<\/p>\n

               <\/p>\n

              The solvent evaporation method is very effective in such cases where the overall solubility is low. In such cases, the occurrence of fluctuations brings about only small fluctuations in supersaturation and so the growth process does not get affected much.<\/p>\n

               <\/p>\n

              Following the above said general guidelines, one can identify a suitable solvent and the appropriate technique of growth and use the same to grow a crystal of large dimensions. Let\u00a0 us\u00a0 discuss\u00a0 the low\u00a0 temperature solution\u00a0 growth further\u00a0 in\u00a0 the background of some information provided in table 26.1.There is a specific classification of materials under the categories of\u00a0 \u201cEasy to grow\u201d,\u00a0 \u201cDifficult\u00a0 to Grow\u201d and\u00a0 \u201cGrowth Improved by Varying Conditions\u201d. The first category gives examples of materials which can be relatively eas ily grown to obtain crystals of large dimensions\u00a0 by\u00a0 the \u201ctemperature lowering method\u201d (or\u00a0 \u201cconstant\u00a0 temperature\u00a0 differential method\u201d) and at temperature lowering rates in the range of 0.1-1 Kh\u25001subject to the condition that the apparatus used for growth is able to maintain temperature stability of more than \u00b1 0.005 K over long periods of time.<\/p>\n

               <\/p>\n

              The materials like sodium chloride, Benzil, Calcium carbonate and silver iodide are relatively difficult to grow. The third category of crystals are the ones whose growth process can be improved by varying conditions of growth. The quality of crystals of anthracene , urea (with water and methanol\u00a0as solvent) and several such organic and inorganic materials is worse as compared to those belonging to first category of crystals when they are grown at similar rates close to ambient temperatures.<\/span><\/p>\n

              \"\"<\/p>\n

              Figure 26.1: Curve of solubility versus temperature<\/p>\n<\/div>\n

              \n

              26.4 Preliminary Experiment<\/strong><\/p>\n

               <\/p>\n

              To grow a well-formed single crystal of the desired substance one has to make a solution which is saturated at room temperature and then allow it to evaporate in order that as the solvent evaporates some of the substance in solution will have to be deposited so that the solution does not get supersaturated. If one desires to grow crystal of a particular substance, it is necessary to have knowledge about its solubility and then calculate the relative amounts of solute and solvent that is required to make a saturated solution.<\/p>\n

               <\/p>\n

              As for example, we may take the case of Alum whose chemical composition is KAl(SO4)2.12H2O. The first step is to study the table that provide information regarding solubility of alum which is about 11g in 100 ml of water at a temperature of 20\u00b0C and at 100\u00b0C its solubility is extremely large. One can, therefore, make a solution of alum by taking ~<\/strong>15g of alum in 100ml of water. Put this solution into a beaker, heat it with constant and vigoursly stirring it. The hot solution may be filtered to obtain a pure solution. Place the solution in the beaker which is loosely covered in order to allow water to evaporate without allowing the dust to enter into the solution or get contaminated in any way. After a few days one would find some alum having deposited as small crystals. Take one of the small crystals and suspend the same with the help of a strong thread so that it remains immersed in the solution of alum as shown in figure 26.2.Allow water to evaporate. As water evaporates the crystal will start growing in size. Allow one or two weeks for the crystal to grow. One\u00a0would find the growth of alum crystal having increased in its size and developed into a regular octahedron. It would take a week or two for the crystal to grow to a large size.<\/span><\/p>\n<\/div>\n

              \n

              \"\"\"\"<\/p>\n

              Figure 26.2: Schematic diagram showing a simple arrangement for growth from water solution<\/p>\n

               <\/p>\n

              26.5<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Apparatus For Crystallization<\/strong><\/p>\n

              26.5.1<\/strong>\u00a0\u00a0\u00a0 Slow cooling technique<\/strong><\/p>\n

               <\/p>\n

              The method for producing large single crystals from solution is the \u201cTemperature Lowering Method\u201d. Different types of apparatus designed for the production of large crystals of organic and inorganic substances from saturated solution at low temperatures are many and are described in the literature (see bibliography). Pamplin has listed a large number of references that are available on the subject. The variations in design of the apparatus depends on the ultimate size of the crystal that one may wish to grow , nature of the solvent phase and other related parameters of growth. However, there are certain basic requirements of the growth apparatus which have to be common to all. These are:<\/p>\n

               <\/p>\n

              i) Temperature control to better than 0.01K. The thermostatic control may preferably be somewhere in the<\/p>\n

              range of 0.005-0.001k.<\/p>\n

              ii)\u00a0 Vigorous stirring of solution. It would ensure prevention of layering and spurious nucleation.<\/p>\n

              iii)\u00a0 Efficient reciprocated stirring of the crystal. It is necessary for the prevention of local super-saturations<\/p>\n

              or under-saturations which become the cause of variations (increase or decrease ) in the rate of crystal<\/p>\n

              growth and leading to incorporation of solvent and \u201cveiling\u201d.<\/p>\n

              iv)\u00a0 Arrangement for controlled super-saturation of the solution.<\/p>\n

               <\/p>\n

              The use of slow cooling processes is used in a large number of crystal growth experiments from solution. They are in several ways simpler than the processes involved in solvent evaporation or temperature gradient transfer. In this process only one vessel is required and the most important requirement is to meet the major technological requirement of providing the desired rate of cooling as precisely as possible. On a commercial scale the volumes of solution used are quite large, viz., in the range of 10-100 litres. However, in the current demand of KH2PO4 of 300mm dimensions, even larger volumes of solution may be required.<\/p>\n<\/div>\n

              \n

              \"\"<\/p>\n

              Figure 26.3: Schematic diagram of a simple crystallizer used for the growth by slow cooling technique<\/p>\n

               <\/p>\n

              Figure 26.3 shows the design of a simple crystallizer for growth by slow cooling method. It consists of a double-vessel system, stirrer, and water inlet\/outlet, mechanical arrangement for rotation of seed crystal, seed crystal holder and insulating platform. There is an external temperature-controlled bath from which water is pumped into the water jacket so as to maintain the solution at the programmed temperature. In order to ensure circulation of the solution, multiple stirrers are used. In figure 26.3 only one stirrer is shown. There is a provision for placement of seed crystal on a holder which is in the form a disc. The seed crystal is mounted on this disc which can be rotated .It is rotated in a particular direction (say clockwise) for some time and then rotated in the reverse direction (say anti-clockwise) for some time. The number of rotations per minute (rpm) is maintained appropriately after careful calculations.<\/p>\n

               <\/p>\n

              In using the technique of growth by slow cooling, it is necessary to maintain temperature stability. Interface instability due to any reason results into inclusion formation and spurious nucleation. The instability is mainly caused by short bursts of rapid growth as a consequence of fluctuation in temperature. Growth-rate variations can as well be caused by other parameters like fluctuations in the amount of stirring. Rotation of the seed in two opposite directions does reduce the possibility of spurious nucleation to a great extent.<\/p>\n

               <\/p>\n

              26.5.2<\/strong>\u00a0\u00a0\u00a0 Solvent Evaporation technique<\/strong><\/p>\n

               <\/p>\n

              Slow evaporation is most easily achieved by using a controlled flow gas inlet\/outlet attached to the head of the flask and a trap to collect the condensing liquid.<\/p>\n

               <\/p>\n

              One is confronted with a practical problem and that is to ensure a constant desirable rate of\u00a0 loss\u00a0 of\u00a0 solvent\u00a0 from\u00a0 the system.\u00a0 A system\u00a0 for\u00a0 growth by solvent \u00a0evaporation\u00a0 is\u00a0 shown in\u00a0 a schematic diagram\u00a0 of\u00a0 figure\u00a0 26.4 which meets\u00a0 the above requirement\u00a0 of\u00a0 acceptable\u00a0 rate\u00a0 of\u00a0 loss\u00a0 of solvent\u00a0 from\u00a0 the system\u00a0 by providing\u00a0 a\u00a0 cooled condensation region. Alternative method\u00a0 is\u00a0\u00a0 to use porous covers separating the saturated vapour above the solution from a large and well stirred volume of air with a negligibly small partial pressure of the solvent. The porosity of the covers may be in the form\u00a0 of\u00a0 holes\u00a0 or\u00a0 tubes,\u00a0 or\u00a0 a permeable membrane.\u00a0 The porous\u00a0 covers can also\u00a0 be in\u00a0 the form\u00a0 of cellulose-fibre sheets , say Cellophane , are very useful as they allow water to go out while protecting the system from dust and external particles. The rate of loss of vapour through the membrane can be varied using a non-porous cover over part of the membrane area.<\/p>\n

               <\/p>\n

              Solvent evaporation process produces crystals of limited size as is the case with slow cooling process. However, crystal grown by solvent evaporation are less pure as compared to crystals grown by slow cooling process. It is because with solvent evaporation, the concentration of impurities in the solution has tendency to increase as growth proceeds further.<\/p>\n<\/div>\n

              <\/div>\n
              \n

              \"\"<\/p>\n

               <\/p>\n

              Figure 26.4: Schematic diagram of an apparatus for the growth of crystals by solvent evaporation process<\/p>\n

               <\/p>\n

              The apparatus shown in figure 26.4 has provision for holding seed crystals. In fact, more than one seed crystal can be used by cementing each of them to one of the holders attached to a multi -arm\u00a0 assembly\u00a0 named \u2018Spider\u201d;\u00a0 the spider\u00a0 can be rotated both in\u00a0 the clockwise as well\u00a0 as\u00a0 anti- clockwise\u00a0 directions. The solution\u00a0 is\u00a0\u00a0 thus\u00a0 continuously\u00a0 stirred which helps\u00a0 to avoid\u00a0 interface instability\u00a0 and\u00a0 spurious nucleation.\u00a0 Typically,\u00a0 the spider\u00a0 is made to make four\u00a0 revolutions\u00a0\u00a0 in\u00a0 one direction, stop for a while, and then allowed\u00a0 to make four revolutions in the opposite direction. The cycle\u00a0 is\u00a0 composed\u00a0 of making\u00a0 revolutions in\u00a0 one direction,\u00a0 pause,\u00a0 then making\u00a0 revolutions\u00a0 in\u00a0 the opposite direction, another pause before repeating the cycle. Each cycle keeps on repeating during the experimentation of crystal growth. The maximum r.p.m. depends on the width of a crystal and radius of its rotatory movement and the Reynolds number so required is calculated which usually should be at the most \u2264 2000.<\/p>\n

               <\/p>\n

              26.5.3<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Temperature-difference Technique.<\/strong><\/p>\n

               <\/p>\n

              A simple apparatus which works on the basis of temperature-difference process is shown in a schematic diagram of figure 26.5<\/p>\n<\/div>\n

              \n

              \"\"<\/p>\n

              Figure 26.5: Schematic diagram of simple apparatus for crystal growth by temperature-difference process.<\/p>\n

               <\/p>\n

              In this arrangement, the crystallizer is divided into two zones \u2500 one cool growth zone<\/p>\n

              and the other hot nutrient zone. Seed crystal is maintained at temperature T which is lower by\u00a0 T as<\/p>\n

              compared to the upper nutrient zone; the nutrient zone being maintained at a temperature T + T. The solution is almost saturated in the zone at T + T while the seed crystal maintained at temperature T will grow. The amount of growth is limited only by the amount of nutrient which is put into the hot zone of the crystallizer. One could, in principle, think of adding more nutrients during the growth run but then to maintain the temperature at T + T requires much more care. The above apparatus is a simpler version of temperature-difference crystallizer. In those systems which are used for production purposes, the cool growth zone is separated from the hot saturator. The solution is pumped from one vessel to the other. Supersaturated solutions have a tendency to nucleate when pumped, and if solution saturated at T + T is pumped directly to the growth vessel, it makes a way for undissolved particles to get transferred to the growth region. To avoid this from happening, three vessels are used in place of one. If this is not done, the undissolved particles will enter the growing crystals and render them imperfect.<\/p>\n

               <\/p>\n

              It is claimed that the growth of really large crystals is most likely to be achieved in systems of this type. However, if one is to deal with large volumes of liquid exceeding 100 litres, it is extremely difficult to provide suitable degree of mixing and to avoid convective instabilities.<\/p>\n<\/div>\n

               <\/p>\n\n\n\n
              you can view video on Experimental Crystal growt<\/strong><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

              References.<\/strong><\/p>\n

                \n
              1. Pamplin,B.R. :\u201cCrystal Growth\u201d, Pergamon Press,N.Y.,1980.<\/li>\n
              2. Brice, J.C. : \u201cCrystal Growth Processes\u201d, John Wiley & Sons,N.Y.,1986.<\/li>\n
              3. Cracknell,A.P.: \u201cCrystals and Their Structures\u201d, Pergamon Press, Oxford,1969.<\/li>\n
              4. Gilman,J.J. : \u201c The Art & Science of Growing Crystals\u201d,Wiley,N.Y.,1963.<\/li>\n<\/ol>\n

                Suggested Reading.<\/strong><\/p>\n

                  \n
                1. Buckley,H.E.: \u201c Crystal Growth\u201d, Chapman & Hall, London,1951.<\/li>\n
                2. Brice,J.C.: \u201c The Growth of Crystals From Liquids\u201d,North-Holland, Amsterdam,1973.<\/li>\n
                3. Rosenberger,F.: \u201c Fundamentals Of Crystal Growth\u201d,Springer-Verlag,Berlin,1979.<\/li>\n
                4. Chernov,A.A.: \u201c Modern Crystallography III: Crystal Growth\u201d, Springer-Verlag,Berlin.<\/li>\n
                5. Van Enckevort,W.J.P.: Prog.Cryst.Growth& Charac.,9,1984.<\/li>\n
                6. Torgesen,J.L., Horton,A.T. and Saylor,C.P.:\u201dEquipment for single crystal growth from aqueous Solution,J.Res. of the NBS-C,Engineering & Instrumentation, 67C,25.<\/li>\n
                7. Hooper,R.M.;Mcardle,B.J.;Narang,R.S.,Sherwood,J.N.: in \u201cCrystal Growth\u201ded.B.R.Pamplin, Pergamon Press,1980.<\/li>\n<\/ol>\n

                   <\/p>\n","protected":false},"author":3,"menu_order":26,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["prof-p-n-kotru"],"pb_section_license":""},"chapter-type":[],"contributor":[58],"license":[],"class_list":["post-437","chapter","type-chapter","status-publish","hentry","contributor-prof-p-n-kotru"],"part":3,"_links":{"self":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/437","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/users\/3"}],"version-history":[{"count":6,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/437\/revisions"}],"predecessor-version":[{"id":616,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/437\/revisions\/616"}],"part":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/437\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/media?parent=437"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapter-type?post=437"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/contributor?post=437"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/license?post=437"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}