{"id":471,"date":"2018-12-04T09:26:44","date_gmt":"2018-12-04T09:26:44","guid":{"rendered":"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/?post_type=chapter&#038;p=471"},"modified":"2018-12-04T10:06:12","modified_gmt":"2018-12-04T10:06:12","slug":"high-temperature-solution-growth","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/chapter\/high-temperature-solution-growth\/","title":{"rendered":"High Temperature Solution Growth"},"content":{"raw":"<div><span style=\"float: right\"><a href=\"https:\/\/youtu.be\/_GBT6JaguWo\" target=\"_blank\" rel=\"noopener\"><img src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"epgp books\" width=\"75px\" height=\"75px;\" \/><\/a>\r\n<\/span><\/div>\r\n<div>\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n<strong>TABLE OF CONTENTS<\/strong>\r\n\r\n&nbsp;\r\n\r\n28 Growth from Flux\r\n\r\n&nbsp;\r\n\r\n28.1 Introduction\r\n\r\n&nbsp;\r\n\r\n28.2 Advantages of Flux Growth\r\n\r\n&nbsp;\r\n\r\n28.3 Disadvantages\/Limitations of Flux Method\r\n\r\n&nbsp;\r\n\r\n28.4 Equipment For Flux Growth\r\n\r\n&nbsp;\r\n\r\n28.5 Growth Processes\r\n\r\n&nbsp;\r\n\r\n28.5.1 Slow Cooling Process\r\n\r\n&nbsp;\r\n\r\n28.5.2 Process of Evaporation\r\n\r\n&nbsp;\r\n\r\n28.6 Advantages of Flux Evaporation\r\n\r\n&nbsp;\r\n\r\n28.7 Disadvantages of Flux Evaporation\r\n\r\n&nbsp;\r\n\r\n28.8 Growth Procedure\r\n\r\n&nbsp;\r\n\r\n28.9 Choice of Flux\r\n\r\n&nbsp;\r\n\r\n28.10 Example of some Suitable Solvents\r\n\r\n&nbsp;\r\n\r\n28.11 Separation of Crystals from Flux\r\n\r\n&nbsp;\r\n\r\n28.12 Some Examples of Flux Grown Crystals\r\n\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong style=\"text-align: initial;font-size: 1em\">LEARNING OBJECTIVES<\/strong>\r\n<div>\r\n<ul>\r\n \t<li>In this module we learn about a crystal growth technique known as \u201cflux method of crystal growth\u201d<\/li>\r\n \t<li>This type of growth is explained to be a high temperature solution growth.<\/li>\r\n \t<li>This method allows growth of crystals at temperature well below the melting point of the material.<\/li>\r\n \t<li>Two types of processes are involved in this method of growth; one being growth by spontaneous<\/li>\r\n \t<li>Advantages and disadvantages of flux growth are described.<\/li>\r\n \t<li>The equipment involved in flux growth are described.<\/li>\r\n \t<li>The two growth procedures viz, growth by flux evaporation and growth by slow cooling are\u00a0described<\/li>\r\n \t<li>Advantages and disadvantages of growth by flux evaporation are explained.<\/li>\r\n \t<li>Experimental arrangement for the growth of ruby crystals as perfected are described at bell telephone laboratory, USA is discussed.<\/li>\r\n \t<li>Here, one also learns as to what are the requirements of a good flux in terms of its properties.<\/li>\r\n \t<li>Some suitable solvents for the growth of some specific crystals are given as examples.<\/li>\r\n \t<li>Procedures for separation of crystals from adhered flux including (i) method of crucible inversion.(ii) hot draining technique (ii) in- built mechanism in the growth systems for inversion are described.<\/li>\r\n \t<li>Specific examples of crystals grown by flux method viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M-type hexaferrites are described.<\/li>\r\n<\/ul>\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong style=\"text-align: initial;font-size: 1em\">28\u00a0<\/strong><strong style=\"text-align: initial;font-size: 1em\">Growth from Flux<\/strong>\r\n\r\n&nbsp;\r\n\r\n<strong style=\"text-align: initial;font-size: 1em\">28.<\/strong><strong style=\"text-align: initial;font-size: 1em\">1\u00a0 Introduction<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">In general, the principles of high-temperature solution growth are more or less similar to those of low-temperature solution growth. The methods that are used in low -temperature solution growth can also be used in high temperature solution growth except the modifications that may be required because of different temperature regimes.<\/span><\/p>\r\n\r\n<div>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">Flux method is a high- temperature solution growth method. Crystals are grown from pure melt provided the material to be crystallized is stable at its melting point. However, solution methods allow growth of crystals at temperatures well below the melting point. The distinction between pure melt growth and fluxed\u2013melt growth is that in case of the former the solvent (major component) freezes whereas in the latter it is the solute which crystallizes usually below its melting point (if it be assumed that it even has one).<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">Fluxed\u2013melt growth is a method by which a wide range of crystals may be grown with the minimum equipment and information. Components of the material desired as single crystals, usually a simple or complex oxide or fluoride are dissolved in flux or solvent. Crystal<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">growth then proceeds at relatively low temperatures, often as much as 1000 \u2070 C below the melting point of the solute.<\/p>\r\n&nbsp;\r\n\r\nFluxed\u2013melt growth is achieved by using any of the following two types of processes:\r\n\r\n&nbsp;\r\n\r\n(i)\u00a0 Growth by spontaneous nucleation and\r\n\r\n(ii) Seeded growth.\r\n\r\n&nbsp;\r\n\r\n<strong>28.2<\/strong>\u00a0\u00a0 <strong>Advantages of Flux Growth<\/strong>\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">Most of the advantages of flux growth result from the fact that crystals are grown at temperatures below the melting point. The advantages are:<\/p>\r\n\r\n<ol>\r\n \t<li>Crystals are relatively free from thermal strain and it is possible to grow good quality crystals.<\/li>\r\n \t<li style=\"text-align: justify\">The container problem for refractory materials is avoided, e.g., MgO melts at 2800\u2070 C; at this temperature the only suitable container is MgO powder. Usually, flux growth takes place at 1250\u2070 C and so platinum container can be used.<\/li>\r\n \t<li style=\"text-align: justify\">It is not possible to use pure melt growth of incongruently melting materials which decompose into another solid phase and \/or liquid at temperatures above their melting point.<\/li>\r\n \t<li style=\"text-align: justify\">Such materials which are volatile at the melting point may be grown by flux method at much lower temperatures.<\/li>\r\n \t<li style=\"text-align: justify\">Such complex oxides which may have volatile components and decompose just before the melting point cannot be grown from the molten phase. As for example, in the growth of GdVO4, one of the components used is V2O3. V2O3 is volatile. ROF (where R stands for rare earth) is the other component which loses RF3 at high temperatures. As such, it is not possible to grow such oxides from the molten phase. Growth by flux method avoids this problem and make it possible to grow crystals of such complex materials. So, flux method of growth enables one to grow crystals of materials which become non\u2013stoichiometric because of the loss of a volatile constituent.<\/li>\r\n \t<li style=\"text-align: justify\">It is desirable to grow crystals of such materials which have very high vapour pressure at the melting point.<\/li>\r\n \t<li style=\"text-align: justify\">Flux method of growth permits crystal growth of materials which undergo a solid state phase transition resulting into severe strain or fracture as the growth occurs at a temperature below this transition.<\/li>\r\n \t<li style=\"text-align: justify\">Flux method is applicable to refractory materials which are technically difficult to grow as crystals from the melt because of crucible or furnace problem.<\/li>\r\n \t<li style=\"text-align: justify\">Flux method yields facetted crystals as if they were grown in nature.<\/li>\r\n \t<li style=\"text-align: justify\">The apparatus required is simple and within the scope of most of the laboratories<\/li>\r\n<\/ol>\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong style=\"text-align: initial;font-size: 1em\">28.3<\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0\u00a0\u00a0\u00a0 <\/span><strong style=\"text-align: initial;font-size: 1em\">Disadvantages\/Limitations of Flux Method.<\/strong>\r\n\r\n&nbsp;\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">The flux method, however, suffers from some disadvantages. The major disadvantages of flux method are the following:<\/span>\r\n<div>\r\n<ol>\r\n \t<li style=\"text-align: justify\"><span style=\"font-size: 1em\">The<\/span><span style=\"text-align: initial;text-indent: 1em;font-size: 1em\">\u00a0crystals grow in the presence of major impurity which comes from the solvent. To overcome this, one has to use means of slow growth rate so that capture of solvent inclusions by the growing crystal is avoided. Alternatively, one may also make choice of a suitable flux which may reduce this to a minimum.<\/span><\/li>\r\n \t<li style=\"text-align: justify\">Flux method yields relatively small size of crystals.<\/li>\r\n<\/ol>\r\n&nbsp;\r\n\r\n<strong>28.4<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Equipment For Flux Growth<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">Equipment and other materials that are required for the growth of crystals by flux method include the following:<\/p>\r\n&nbsp;\r\n\r\n\u2022Crucibles\r\n\r\n&nbsp;\r\n\r\n\u2022Adequately ventilated muffle furnaces\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">\u2022Thermocouples, Temperature controllers and Programmers \u2022Refractory muffles for flux evaporation<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">\u2022Refractory bricks and means of shaping them as may be required \u2022Materials required for mixtures of solute<\/p>\r\nand flux.\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n<strong>28.5<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Growth Procedures<\/strong>\r\n\r\n&nbsp;\r\n\r\nThere are two growth procedures that may be followed for the growth of crystals from fluxed\r\n\r\nmelt.\r\n\r\n&nbsp;\r\n\r\n1.\u00a0 Growth by flux evaporation\r\n\r\n&nbsp;\r\n\r\n2.\u00a0 Growth by slow cooling.\r\n\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong style=\"text-align: initial;font-size: 1em\">28.5.1 Slow Cooling Process.<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">The equipment for growth by fluxed-melt technique, using the process of slow cooling is typically of the type as shown in figure 28.1.<\/span><\/p>\r\n\r\n<div>\r\n\r\n<img class=\"aligncenter size-full wp-image-475\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94.png\" alt=\"\" width=\"415\" height=\"216\" \/>\r\n<p style=\"text-align: center\"><strong><em>Figure 28.1: Horizontal furnace with crucibles inside it for flux growth of materials by slow<\/em><\/strong><\/p>\r\n&nbsp;\r\n\r\n<strong><em>cooling method.<\/em><\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">Crucible made of platinum or rhodium (depending on maximum temperature to be attained) charged with the starting composition of appropriate stoichiometry is placed in the sillimanite. (Al2SiO3) furnace muffle. It is preferable to keep lid over the crucible in order to safe guard the interior of the furnace from any attack by volatile toxic material. Sometimes it becomes necessary to weld lid with the crucible, if there is any indication of the lid getting blown off during the experimental process. Tightly held lids with a pin hole are also sometimes used, if the situation so demands. The composition of the charge includes well homogenised chemicals and flux components. It is then pressed into the crucible and placed in the furnace. The contents of the crucible are raised to a temperature of 1300\u00b0C or so and held at that temperature for a soaking period of about 12-24 hours. It is followed by slow and controlled cooling at the rate of 2-4\u00b0C per hour till about 800\u00b0C with the help of electronically controlled temperature programmers. After this, the cooling rate may be increased to about 100\u00b0C per hour till room temperature. The crystals are then separated from flux either by hammering or a special technique of hot pouring (to be described later), if required. The crystals may then be cleaned in 20% HNO3 , if required.<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">The rate of growth v of a crystal of area A is determined by supersaturation. This in turn depends on the process used to produce the supersaturated state which is an essential requirement for crystal growth. If the solution is cooled at a rate dT\/dt , the growth rate v is given by:<\/p>\r\n&nbsp;\r\n\r\nv = (V\/\u03c1A)( dne\/dT ) ( dT\/dt )\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">The crystal density \u03c1 and the s lope dne\/dT of the solubility curve are fixed by the system. The solution volume V is selected depending on the total mass of crystals required and the area<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">A of the crystal faces will change as the crystals grow. According to this expression the most important parameter is the cooling rate dT\/dt which is directly responsible for the rate of mass deposition. Therefore, it is the most important factor in an experiment which will control the growth rate. In general, a constant cooling rate of about 1\u00b0C h\u25001 is most acceptable provided it can be done in the background of experimental feasibility and convenience.<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><strong style=\"text-align: initial;font-size: 1em\">28.5.2<\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\">Process of Evaporation.<\/strong><\/p>\r\n\r\n<\/div>\r\n<div>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">The process of slow evaporation is not that popular as the slow cooling process as a method of producing crystals. It is, however, used only when compound formation occurs at low temperatures. As an example, we may consider the chemical reaction:<\/p>\r\n&nbsp;\r\n\r\nPbO + TiO<sub>2<\/sub>\u00a0 \u2192\u00a0 PbTiO<sub>3<\/sub>. ( at temperature T &lt; 1200\u00b0C).\r\n\r\n&nbsp;\r\n\r\nSo, TiO<sub>2<\/sub> could be crystallized only by evaporation of PbO at temperature &gt; 1200\u00b0C. The growth rate is given by:\r\n\r\n&nbsp;\r\n\r\nv\u00a0 = (ne\/\u03c1A ).dV\/dt.\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">It means that the growth rate depends on ne (and so T) and on evaporation rate dV\/dt . Control of growth rate in this case is more difficult in practice than in case of slow cooling. However, the technique is to vary the size of the hole in an otherwise sealed crucible<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">During growth the hole gets smaller because of deposition of solute that is transported in the vapour and so the evaporation rate and hence the growth rate varies. There have been several innovations in the design of apparatus which have slightly improved the control over solvent evaporation.<\/p>\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n<strong>28.6<\/strong>\u00a0\u00a0 <strong>Advantages of Flux Evaporation.<\/strong>\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\nThe advantages of growth by flux evaporation may be summarized as follows:\r\n\r\n&nbsp;\r\n\r\n1.\u00a0 Growth of crystals occurs at a higher temperature than with the slow cooling method.\r\n\r\n2.\u00a0 Crystals having a small variation of solubility with temperature can be grown.\r\n<p style=\"text-align: justify\">3.\u00a0 If the process of crystal growth is carried out to completion, almost cent percent yield of the solute phase<\/p>\r\nis achievable. Also, crystals free from adhered flux are obtainable.\r\n\r\n4.\u00a0 In certain cases, large well formed crystals are produced at the base of the crucible.\r\n\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong>28.7\u00a0Disadvantages of Flux Evaporation.<\/strong>\r\n\r\n&nbsp;\r\n\r\nThe advantages of growth by flux evaporation may be summarized as follows:\r\n\r\n&nbsp;\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">1.\u00a0\u00a0\u00a0\u00a0 Nucleation at the melt surface may be excessive\u00a0 which is likely to result in intergrown crystals.<\/span>\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">2.\u00a0\u00a0\u00a0\u00a0 Control over the growth process is lesser as compared to slow cooling method. The latter process of <\/span>\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">growth yields better quality crystals.<\/span>\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">3.\u00a0 If the segregation coefficient between dopant present in the solvent and in the crystal differs from unity, there will be a greater concentration gradient in crystals grown by flux evaporation than by slow cooling.<\/span>\r\n<div>\r\n\r\n&nbsp;\r\n\r\nBe it growth by slow cooling or evaporation of solvent, crystallization occurs by spontaneous nucleation in both cases.\r\n\r\n&nbsp;\r\n\r\n<strong>28.8<\/strong>\u00a0<strong>Growth Procedure<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">The basic technique may best be explained by a description of the flux growth of ruby crystal as perfected by Remeika of the Bell Telephone Laboratory, U.S.A. The experimental set-up is shown in figure 28.2.<\/p>\r\n&nbsp;\r\n\r\n<img class=\"aligncenter size-full wp-image-476\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95.png\" alt=\"\" width=\"410\" height=\"405\" \/>\r\n\r\n&nbsp;\r\n<p style=\"text-align: center\"><strong><em>Figure 28.2: Experimental arrangement for the growth of ruby crystals by Flux method.<\/em><\/strong><\/p>\r\n\r\n<\/div>\r\n<strong><em>\u00a0<\/em><\/strong>\r\n<div>\r\n<p style=\"text-align: justify\">The platinum crucible of 6\u201d diameter and 10\u201d depth is loaded with the flux consisting of suitable amount of lead oxide and boron oxide (B<sub>2<\/sub>O<sub>3<\/sub>). To this is added appropriate quantity of aluminium oxide (Al<sub>2<\/sub>O<sub>3<\/sub>) and a small amount of chromium oxide (Cr<sub>2<\/sub>O<sub>3<\/sub>). The crucible charged with the flux and constituent chemicals is placed on a pedestal as shown in figure in an electrically heated furnace. The pedestal is rotated 30 turns in one direction and then 30 turns in the opposite direction to obtain uniform mixture of the molten contents of the crucible. The temperature of the furnace is raised to 1300\u00b0C. At this temperature the aluminium oxide and<\/p>\r\n<p style=\"text-align: justify\">chromium oxide get dissolved. After six hours, mixing is stopped and the temperature is lowered at a steady state of 4\u00b0c per hour for 8 days. At this stage, the furnace is shut off and the crucible is taken out. As the temperature passes 1240 \u2070 C on the way down, the solubility of ruby is exceeded and the crystals grow as the temperature is further reduced. To favour growth of crystals on the bottom rather than at the surface of the melt, top of the crucible is kept 10 \u2070 C hotter as compared to the temperature of the bottom, by adjusting power of the heater. It has to be ensured that the temperature is kept constant since a sudden drop would cause the production of many small crystals rather than the desired few large ones. Another consequence of temperature fluctuations is the occurrence of inclusions of small amount of flux into the growing crystals, rendering them useless for optical studies and adversely affecting their beauty.<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">The flux is removed from the grown crystals by dissolving the former in nitric acid which neither affects rubies nor crucible. Rhombohedral ruby crystals having size 3\/4\/\/ across and large flat\u00a0 ruby plates upto 1\/10\/\/ thick and several inches across have been obtained using this experimental set\u2013up at the Bell Telephone Laboratory, U.S.A. in the initial stages of launching this study.<\/p>\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n<strong>28.9<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Choice of Flux.<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">One of the most important requirements of growth by high temperature solution method is making choice of flux. For all solution growth techniques one is required to find a suitable solvent. Two types of solvents are used in high temperature solution technique. One is liquid metals like gallium, indium and tin which are often used for the growth of semiconducting materials whereas the other includes oxides and halides like PbO, PbF 2 which are used for ionic materials. The solutions in oxide and halide solvents are called \u201cfluxed melts\u201d which is because the solvents used are those which are used as fluxes in operations like welding, soldering and brazing. A good flux should have the following properties:<\/p>\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n(i) It should have a low melting point.\r\n\r\n(ii)\u00a0 It should be a good solvent, dissolving anywhere between 5 and 30 weight percent of solute at the\r\n\r\nmaximum temperature intended to be used. The solubility should decrease with temperature.\r\n\r\n(iii)\u00a0\u00a0\u00a0\u00a0 It should not form a compound with solute, nor a solid solution beyond a degree which depends on\r\n\r\nthe proposed use of the crystal.\r\n\r\n(iv) It should be compatible with the material, of which the crucible is made, over the proposed temperature\r\n\r\nrange.\r\n\r\n(v) For growth by slow cooling method, it should be of low volatility. For a volatile flux it is necessary to use\r\n\r\nlower temperature range, a faster rate of cooling which often results in nucleation at the melt-surface.\r\n\r\nEvaporation may be prevented if lids are welded to the crucibles, but this refinement is expensive and time\u2013\r\n\r\nconsuming.\r\n\r\n(vi)\u00a0\u00a0\u00a0 It should be of low viscosity.\r\n\r\n(vii)\u00a0\u00a0 Toxicity of flux should be low.\r\n\r\n<span style=\"font-size: 1em;text-align: initial\">(viii) The flux should be such as to be easily separable from the crystals.<\/span>\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">(ix) The supersaturation required to cause nucleation should be much larger than that required for growth of <\/span>\r\n\r\n<span style=\"text-align: initial;font-size: 1em\">crystals. It helps in creation of only a few nuclei and growth of larger crystals.<\/span>\r\n\r\n<\/div>\r\n<div>\r\n\r\n&nbsp;\r\n\r\n<strong>28.10<\/strong>\u00a0\u00a0 <strong>Example of Some Suitable Solvents.<\/strong>\r\n\r\n&nbsp;\r\n\r\nSome of the most frequently used solvents for the growth of some crystals are as given in table 28.1\r\n\r\n&nbsp;\r\n\r\n<strong>Table 28.1\u00a0<\/strong><strong>Some suitable solvents for the growth of some materials.<\/strong>\r\n\r\n&nbsp;\r\n\r\n<img class=\"aligncenter size-full wp-image-477\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96.png\" alt=\"\" width=\"580\" height=\"589\" \/><img class=\"aligncenter size-full wp-image-478\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97.png\" alt=\"\" width=\"572\" height=\"438\" \/>\r\n\r\n&nbsp;\r\n\r\n<strong>28.11 Separation of Crystals From Flux. <\/strong>\r\n\r\n<\/div>\r\n<div>\r\n<p style=\"text-align: justify\">Flux is often found to get adhered to the crystals and their separation from the crucible and\/or the grown crystals becomes essential. One method is to separate the crystals from the adhered flux by gentle hammering. If it does not work, one is required to undertake other means .Separation of crystals from flux is one of the most important steps in obtaining crystals.Wanklyn of the Crystal Growing Group at the Clarendon Laboratory; university of Oxford has grown a large number of crystals using flux method. A lot of innovation has been done by the group regarding growth of crystals by this technique. One of the several innovative techniques concerns separation of grown crystals from flux. To prolong life of a crucible and separate crystals from flux without damaging them is to remove flux from the crucible while in a molten state. One way is to invert the crucible, using long handled tongs, and then placing the same upside down on a refractory brick or a bed of alumina as shown in figure 28.3.The other technique is \u201cHot Draining of Flux\u201d as is shown in figure 28.4. The other method is to use a furnace in which there is a mechanism of inverting the crucible in the furnace itself. This arrangement is shown in figure 28.5<\/p>\r\n<img class=\"aligncenter size-full wp-image-479\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98.png\" alt=\"\" width=\"478\" height=\"314\" \/>\r\n<p style=\"text-align: center\"><strong><em>Figure 28.3: Separation of crystals from flux by the method of crucible inversion.<\/em><\/strong><\/p>\r\n\r\n<\/div>\r\n<strong><em>\r\n<img class=\"aligncenter size-full wp-image-480\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99.png\" alt=\"\" width=\"319\" height=\"403\" \/><\/em><\/strong>\r\n<div>\r\n<p style=\"text-align: center\"><strong><em>Figure 28.4: Separation of crystals from flux by hot draining technique<\/em><\/strong><\/p>\r\n\r\n<\/div>\r\n<div>\r\n\r\n<img class=\"aligncenter size-full wp-image-481\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100.png\" alt=\"\" width=\"587\" height=\"298\" \/>\r\n<p style=\"text-align: center\"><strong><em>Figure 28.5: Cross-sectional view of end of brick and crucible assembly for inversion along with device for inversion<\/em><\/strong><\/p>\r\n&nbsp;\r\n\r\n<strong>28.12<\/strong>\u00a0\u00a0\u00a0 <strong>Some Examples of Flux Grown Crystals<\/strong>.\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">Fluxed melt technique has been used for the growth of a large number of crystals, particularly those materials which are required to be grown at temperatures well below their melting points. These include a large number of materials which are very useful to electronic, magnetic and optical industries. The materials generally are complex oxides or fluorides. The list is very long and one can search for the same in the literature.<\/p>\r\n&nbsp;\r\n\r\nHowever, some typical examples may be given here:\r\n\r\n&nbsp;\r\n\r\n<strong>1. Growth of Rare Earth Orthoferrites:<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">The crystals of rare earth orthoferrites bearing composition RFeO3 (where R = Dy, Ho, Gd, Tb , Er , Yb , Sm , La and Y) are grown using appropriate \/stoichiometric starting composition of R2O<sub>3<\/sub> + Fe<sub>2<\/sub>O<sub>3<\/sub> with B2O<sub>3<\/sub> -PbO-PbF<sub>2<\/sub> -PbO<sub>2<\/sub> as flux component. The composition is homogenised by vigoursely mixing the same. The well mixed composition is pressed into a platinum crucible and placed in the furnace where it is held at a temperature of ~ 1300\u00b0C for a soaking period of 12-16 hours. It is then cooled at the rate of 2\u00b0C per hour till 800\u00b0C is reached. Thereafter it is cooled at the rate of around 100\u00b0C per hour till room temperature. Well facetted crystals are formed at the bottom and\/or walls of the crucibles. The crystals are separated from flux by gently hammering the crucible and then cleaned in 10-20 % HNO3 .<\/span><\/p>\r\n\r\n<\/div>\r\n<div>\r\n\r\n&nbsp;\r\n\r\n<strong>2. Growth of Rare Earth Orthochromites:<\/strong>\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\">The crystals of rare earth orthochromites bearing composition RCrO3\u00a0 ( where R = Y , La , Gd , Yb ) are grown using starting composition of R2O3 + Cr2 O3 with PbF2-PbO2-B2O3 as the flux component . The rest of the procedure is almost the same as in the above case.<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><strong>3.\u00a0 Growth of Rare Earth Aluminates :<\/strong><\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\">The growth of rare earth aluminates bearing composition RAlO<sub>3<\/sub>, (where=\u00a0 Gd,Eu,Sm,Nd,La,Y,Pr,Tb,Dy,Ho,Er) are grown\u00a0\u00a0 using\u00a0\u00a0 starting\u00a0 composition\u00a0\u00a0 of\u00a0\u00a0 R2O<sub>3<\/sub>- Al2O3 with flux component PbF 2-PbO-PbO<sub>2<\/sub> and B2O3+MoO3 as additives. The rest of the procedure is almost the same as in the above case.<\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><strong style=\"text-align: initial;font-size: 1em\">4. Growth of Lanthanum Borate:<\/strong><\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">Lanthanum borate crystals are grown using La2O3 with flux component PbO-B2O3. The starting composition is pressed into a platinum crucible and then maintained at a soaking temperature of 1250\u00b0C for 15 hours. The cooling rate is maintained at 3\u00b0C per hour till 700\u00b0C and thereafter faster cooling is done as in the above cases. Crystals in the form of platelets, tabular and equidimensional are produced. Some of the crystals of LaBO3 which are approximately equidimensional and grown following the above procedure are shown in figure 28.6<\/span><\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><strong><span style=\"text-align: initial;font-size: 1em\">5. Growth of M-type Hexaferrites:<\/span><\/strong><\/p>\r\n&nbsp;\r\n<p style=\"text-align: justify\"><span style=\"font-size: 1em\">Growth of M-type hexaferrite crystals of the composition SrFe12O19 is achieved by taking strontium oxide and iron oxide (ferrite composition) and sodium carbonate as flux. The ferrite and flux compositions are taken as 73.7% and 26.3% mol percent respectively and pressed into a platinum crucible which is then placed in a vertical type furnace. The material is soaked at 1320\u00b0C for 24 hours under continuous oxygen flux and then allowed to cool at 4\u00b0C hr\u25001 till it attains a temperature of 800\u00b0C. Thereafter, it is allowed to cool at a slightly faster rate. Substituted hexaferrites bearing the composition SrXaYbFe12-(a+b)O19 (where X and Y represent the substituting atoms) are grown, following the same procedure as described above, to investigate the effects of making substitutions in hexaferrites.<\/span><\/p>\r\n\r\n<\/div>\r\n<div>\r\n\r\n<img class=\"aligncenter size-full wp-image-482\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-101.png\" alt=\"\" width=\"299\" height=\"267\" \/>\r\n<p style=\"text-align: center\"><strong><em>Figure 28.6: Nearly equidimensional crystals of Lanthanum borate grown by flux method.<\/em><\/strong><\/p>\r\n\r\n<\/div>\r\n&nbsp;\r\n\r\n<strong>SUMMARY<\/strong>\r\n<ul>\r\n \t<li>Advantages and disadvantages of growth by flux evaporation are explained.<\/li>\r\n \t<li>Crystal growth setup as perfected by bell telephone laboratory, USA for the growth of ruby crystals is discussed as an example for explaining the procedure adopte d in this technique of crystal growth .<\/li>\r\n \t<li>Properties of good flux are identified.<\/li>\r\n \t<li>Suitable solvents for the growth of some specific crystals are given.<\/li>\r\n \t<li>Procedures involved in separating flux grown crystals from adhered flux including (i) method of crucible inve rsion.(ii) hot draining technique (ii) in- built mechanis m in the growth systems for inversion are described.<\/li>\r\n \t<li>Specific examples of crystals of flux melt growth viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M -type hexaferrites are described<\/li>\r\n<\/ul>\r\n<table>\r\n<tbody>\r\n<tr>\r\n<td><strong>you can view video on High Temperature Solution Growth\r\n<\/strong><\/td>\r\n<td><a href=\"https:\/\/youtu.be\/_GBT6JaguWo\" target=\"_blank\" rel=\"noopener\"><img class=\"alignnone wp-image-120\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"\" width=\"36\" height=\"36\" \/><\/a><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<strong>References.<\/strong>\r\n<ol>\r\n \t<li>Wanklyn,B.M. : \u201c Practical Aspects Of Flux Growth By Spontaneous Nucleation\u201d in \u201c Crystal Growth\u201d,ed.Pamplin,B.R.,PergamonPress,N.Y\/Oxford, Vol.1,1974.<\/li>\r\n \t<li>Elwell,D. &amp;Scheel,H.J.:\u201d Crystal Growth From High Temperature Solutions \u201c,Academic Press, N.y.,1975.<\/li>\r\n \t<li>Pamplin,B.R.: \u201c Crystal Growth\u201d II edition , Pergamon Press,N.Y.,1980.<\/li>\r\n \t<li>Gilman ,J.J. : \u201c The Art &amp; Science of Growing Crystals\u201d., Wiley, N.Y.,1963<\/li>\r\n \t<li>Brice,J.C.: \u201c Crystal Growth Processes\u201d, Blackie &amp; Son ,Ltd., Glasgow.,1986.<\/li>\r\n \t<li>Kotru,P.N.&amp;Wankyn,B.M.: \u201c J.Mat.Sci.Lett.\u201d, <strong>8<\/strong> ,1973,1055.<\/li>\r\n<\/ol>\r\n&nbsp;\r\n\r\n<strong>Suggested Reading<\/strong>.\r\n<ol>\r\n \t<li>Chase,A.B. : \u201c Exploratory Flux Growth \u201c in Preparation and Properties of Solid State Materials, Vol.1,ed.Lefever, Marcel Dekker,N.y.,1971.<\/li>\r\n \t<li>Titova,A.G.: \u201c Growth of Crystals\u201d-vol.II,eds. Shubnikov,A.V.andSheftal,N.N.,Consultant Bureau,N.Y.,1959.<\/li>\r\n \t<li>Timofeeva,V.A.: \u201c Growth of Crystals\u201d, vol.II ,eds.Shubnikov,A.V. and Sheftal,N.N.,Consultant<\/li>\r\n \t<li>Elwell,D. : in\u201d Crystal Growth &amp; Characterization \u201c,ed.Ueda,R.&amp; Mullin, J.B. ,North-Holland Publication ,1975.<\/li>\r\n \t<li>Elwell,D.: \u201c Crystal Growth &amp; Materials\u201d eds. E. Kaldis&amp;H.J.Scheel,North-Holland,Amsterdam 1976.<\/li>\r\n \t<li>Brice,J.C.: in \u201c Crystal Growth And Materials\u201d, eds. E.Kaldis&amp;H.J.Scheel , North-Holland, Amsterdam, 1976.<\/li>\r\n<\/ol>\r\n&nbsp;","rendered":"<div><span style=\"float: right\"><a href=\"https:\/\/youtu.be\/_GBT6JaguWo\" target=\"_blank\" rel=\"noopener\"><img decoding=\"async\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"epgp books\" width=\"75px\" height=\"75px;\" \/><\/a><br \/>\n<\/span><\/div>\n<div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><strong>TABLE OF CONTENTS<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>28 Growth from Flux<\/p>\n<p>&nbsp;<\/p>\n<p>28.1 Introduction<\/p>\n<p>&nbsp;<\/p>\n<p>28.2 Advantages of Flux Growth<\/p>\n<p>&nbsp;<\/p>\n<p>28.3 Disadvantages\/Limitations of Flux Method<\/p>\n<p>&nbsp;<\/p>\n<p>28.4 Equipment For Flux Growth<\/p>\n<p>&nbsp;<\/p>\n<p>28.5 Growth Processes<\/p>\n<p>&nbsp;<\/p>\n<p>28.5.1 Slow Cooling Process<\/p>\n<p>&nbsp;<\/p>\n<p>28.5.2 Process of Evaporation<\/p>\n<p>&nbsp;<\/p>\n<p>28.6 Advantages of Flux Evaporation<\/p>\n<p>&nbsp;<\/p>\n<p>28.7 Disadvantages of Flux Evaporation<\/p>\n<p>&nbsp;<\/p>\n<p>28.8 Growth Procedure<\/p>\n<p>&nbsp;<\/p>\n<p>28.9 Choice of Flux<\/p>\n<p>&nbsp;<\/p>\n<p>28.10 Example of some Suitable Solvents<\/p>\n<p>&nbsp;<\/p>\n<p>28.11 Separation of Crystals from Flux<\/p>\n<p>&nbsp;<\/p>\n<p>28.12 Some Examples of Flux Grown Crystals<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">LEARNING OBJECTIVES<\/strong><\/p>\n<div>\n<ul>\n<li>In this module we learn about a crystal growth technique known as \u201cflux method of crystal growth\u201d<\/li>\n<li>This type of growth is explained to be a high temperature solution growth.<\/li>\n<li>This method allows growth of crystals at temperature well below the melting point of the material.<\/li>\n<li>Two types of processes are involved in this method of growth; one being growth by spontaneous<\/li>\n<li>Advantages and disadvantages of flux growth are described.<\/li>\n<li>The equipment involved in flux growth are described.<\/li>\n<li>The two growth procedures viz, growth by flux evaporation and growth by slow cooling are\u00a0described<\/li>\n<li>Advantages and disadvantages of growth by flux evaporation are explained.<\/li>\n<li>Experimental arrangement for the growth of ruby crystals as perfected are described at bell telephone laboratory, USA is discussed.<\/li>\n<li>Here, one also learns as to what are the requirements of a good flux in terms of its properties.<\/li>\n<li>Some suitable solvents for the growth of some specific crystals are given as examples.<\/li>\n<li>Procedures for separation of crystals from adhered flux including (i) method of crucible inversion.(ii) hot draining technique (ii) in- built mechanism in the growth systems for inversion are described.<\/li>\n<li>Specific examples of crystals grown by flux method viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M-type hexaferrites are described.<\/li>\n<\/ul>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">28\u00a0<\/strong><strong style=\"text-align: initial;font-size: 1em\">Growth from Flux<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">28.<\/strong><strong style=\"text-align: initial;font-size: 1em\">1\u00a0 Introduction<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">In general, the principles of high-temperature solution growth are more or less similar to those of low-temperature solution growth. The methods that are used in low -temperature solution growth can also be used in high temperature solution growth except the modifications that may be required because of different temperature regimes.<\/span><\/p>\n<div>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Flux method is a high- temperature solution growth method. Crystals are grown from pure melt provided the material to be crystallized is stable at its melting point. However, solution methods allow growth of crystals at temperatures well below the melting point. The distinction between pure melt growth and fluxed\u2013melt growth is that in case of the former the solvent (major component) freezes whereas in the latter it is the solute which crystallizes usually below its melting point (if it be assumed that it even has one).<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Fluxed\u2013melt growth is a method by which a wide range of crystals may be grown with the minimum equipment and information. Components of the material desired as single crystals, usually a simple or complex oxide or fluoride are dissolved in flux or solvent. Crystal<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">growth then proceeds at relatively low temperatures, often as much as 1000 \u2070 C below the melting point of the solute.<\/p>\n<p>&nbsp;<\/p>\n<p>Fluxed\u2013melt growth is achieved by using any of the following two types of processes:<\/p>\n<p>&nbsp;<\/p>\n<p>(i)\u00a0 Growth by spontaneous nucleation and<\/p>\n<p>(ii) Seeded growth.<\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.2<\/strong>\u00a0\u00a0 <strong>Advantages of Flux Growth<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Most of the advantages of flux growth result from the fact that crystals are grown at temperatures below the melting point. The advantages are:<\/p>\n<ol>\n<li>Crystals are relatively free from thermal strain and it is possible to grow good quality crystals.<\/li>\n<li style=\"text-align: justify\">The container problem for refractory materials is avoided, e.g., MgO melts at 2800\u2070 C; at this temperature the only suitable container is MgO powder. Usually, flux growth takes place at 1250\u2070 C and so platinum container can be used.<\/li>\n<li style=\"text-align: justify\">It is not possible to use pure melt growth of incongruently melting materials which decompose into another solid phase and \/or liquid at temperatures above their melting point.<\/li>\n<li style=\"text-align: justify\">Such materials which are volatile at the melting point may be grown by flux method at much lower temperatures.<\/li>\n<li style=\"text-align: justify\">Such complex oxides which may have volatile components and decompose just before the melting point cannot be grown from the molten phase. As for example, in the growth of GdVO4, one of the components used is V2O3. V2O3 is volatile. ROF (where R stands for rare earth) is the other component which loses RF3 at high temperatures. As such, it is not possible to grow such oxides from the molten phase. Growth by flux method avoids this problem and make it possible to grow crystals of such complex materials. So, flux method of growth enables one to grow crystals of materials which become non\u2013stoichiometric because of the loss of a volatile constituent.<\/li>\n<li style=\"text-align: justify\">It is desirable to grow crystals of such materials which have very high vapour pressure at the melting point.<\/li>\n<li style=\"text-align: justify\">Flux method of growth permits crystal growth of materials which undergo a solid state phase transition resulting into severe strain or fracture as the growth occurs at a temperature below this transition.<\/li>\n<li style=\"text-align: justify\">Flux method is applicable to refractory materials which are technically difficult to grow as crystals from the melt because of crucible or furnace problem.<\/li>\n<li style=\"text-align: justify\">Flux method yields facetted crystals as if they were grown in nature.<\/li>\n<li style=\"text-align: justify\">The apparatus required is simple and within the scope of most of the laboratories<\/li>\n<\/ol>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">28.3<\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0\u00a0\u00a0\u00a0 <\/span><strong style=\"text-align: initial;font-size: 1em\">Disadvantages\/Limitations of Flux Method.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">The flux method, however, suffers from some disadvantages. The major disadvantages of flux method are the following:<\/span><\/p>\n<div>\n<ol>\n<li style=\"text-align: justify\"><span style=\"font-size: 1em\">The<\/span><span style=\"text-align: initial;text-indent: 1em;font-size: 1em\">\u00a0crystals grow in the presence of major impurity which comes from the solvent. To overcome this, one has to use means of slow growth rate so that capture of solvent inclusions by the growing crystal is avoided. Alternatively, one may also make choice of a suitable flux which may reduce this to a minimum.<\/span><\/li>\n<li style=\"text-align: justify\">Flux method yields relatively small size of crystals.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<p><strong>28.4<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Equipment For Flux Growth<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Equipment and other materials that are required for the growth of crystals by flux method include the following:<\/p>\n<p>&nbsp;<\/p>\n<p>\u2022Crucibles<\/p>\n<p>&nbsp;<\/p>\n<p>\u2022Adequately ventilated muffle furnaces<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">\u2022Thermocouples, Temperature controllers and Programmers \u2022Refractory muffles for flux evaporation<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">\u2022Refractory bricks and means of shaping them as may be required \u2022Materials required for mixtures of solute<\/p>\n<p>and flux.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.5<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Growth Procedures<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>There are two growth procedures that may be followed for the growth of crystals from fluxed<\/p>\n<p>melt.<\/p>\n<p>&nbsp;<\/p>\n<p>1.\u00a0 Growth by flux evaporation<\/p>\n<p>&nbsp;<\/p>\n<p>2.\u00a0 Growth by slow cooling.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong style=\"text-align: initial;font-size: 1em\">28.5.1 Slow Cooling Process.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">The equipment for growth by fluxed-melt technique, using the process of slow cooling is typically of the type as shown in figure 28.1.<\/span><\/p>\n<div>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-475\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94.png\" alt=\"\" width=\"415\" height=\"216\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94.png 415w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94-300x156.png 300w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94-65x34.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94-225x117.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-94-350x182.png 350w\" sizes=\"auto, (max-width: 415px) 100vw, 415px\" \/><\/p>\n<p style=\"text-align: center\"><strong><em>Figure 28.1: Horizontal furnace with crucibles inside it for flux growth of materials by slow<\/em><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><strong><em>cooling method.<\/em><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Crucible made of platinum or rhodium (depending on maximum temperature to be attained) charged with the starting composition of appropriate stoichiometry is placed in the sillimanite. (Al2SiO3) furnace muffle. It is preferable to keep lid over the crucible in order to safe guard the interior of the furnace from any attack by volatile toxic material. Sometimes it becomes necessary to weld lid with the crucible, if there is any indication of the lid getting blown off during the experimental process. Tightly held lids with a pin hole are also sometimes used, if the situation so demands. The composition of the charge includes well homogenised chemicals and flux components. It is then pressed into the crucible and placed in the furnace. The contents of the crucible are raised to a temperature of 1300\u00b0C or so and held at that temperature for a soaking period of about 12-24 hours. It is followed by slow and controlled cooling at the rate of 2-4\u00b0C per hour till about 800\u00b0C with the help of electronically controlled temperature programmers. After this, the cooling rate may be increased to about 100\u00b0C per hour till room temperature. The crystals are then separated from flux either by hammering or a special technique of hot pouring (to be described later), if required. The crystals may then be cleaned in 20% HNO3 , if required.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The rate of growth v of a crystal of area A is determined by supersaturation. This in turn depends on the process used to produce the supersaturated state which is an essential requirement for crystal growth. If the solution is cooled at a rate dT\/dt , the growth rate v is given by:<\/p>\n<p>&nbsp;<\/p>\n<p>v = (V\/\u03c1A)( dne\/dT ) ( dT\/dt )<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The crystal density \u03c1 and the s lope dne\/dT of the solubility curve are fixed by the system. The solution volume V is selected depending on the total mass of crystals required and the area<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">A of the crystal faces will change as the crystals grow. According to this expression the most important parameter is the cooling rate dT\/dt which is directly responsible for the rate of mass deposition. Therefore, it is the most important factor in an experiment which will control the growth rate. In general, a constant cooling rate of about 1\u00b0C h\u25001 is most acceptable provided it can be done in the background of experimental feasibility and convenience.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><strong style=\"text-align: initial;font-size: 1em\">28.5.2<\/strong><span style=\"text-align: initial;font-size: 1em\">\u00a0<\/span><strong style=\"text-align: initial;font-size: 1em\">Process of Evaporation.<\/strong><\/p>\n<\/div>\n<div>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The process of slow evaporation is not that popular as the slow cooling process as a method of producing crystals. It is, however, used only when compound formation occurs at low temperatures. As an example, we may consider the chemical reaction:<\/p>\n<p>&nbsp;<\/p>\n<p>PbO + TiO<sub>2<\/sub>\u00a0 \u2192\u00a0 PbTiO<sub>3<\/sub>. ( at temperature T &lt; 1200\u00b0C).<\/p>\n<p>&nbsp;<\/p>\n<p>So, TiO<sub>2<\/sub> could be crystallized only by evaporation of PbO at temperature &gt; 1200\u00b0C. The growth rate is given by:<\/p>\n<p>&nbsp;<\/p>\n<p>v\u00a0 = (ne\/\u03c1A ).dV\/dt.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">It means that the growth rate depends on ne (and so T) and on evaporation rate dV\/dt . Control of growth rate in this case is more difficult in practice than in case of slow cooling. However, the technique is to vary the size of the hole in an otherwise sealed crucible<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">During growth the hole gets smaller because of deposition of solute that is transported in the vapour and so the evaporation rate and hence the growth rate varies. There have been several innovations in the design of apparatus which have slightly improved the control over solvent evaporation.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.6<\/strong>\u00a0\u00a0 <strong>Advantages of Flux Evaporation.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>The advantages of growth by flux evaporation may be summarized as follows:<\/p>\n<p>&nbsp;<\/p>\n<p>1.\u00a0 Growth of crystals occurs at a higher temperature than with the slow cooling method.<\/p>\n<p>2.\u00a0 Crystals having a small variation of solubility with temperature can be grown.<\/p>\n<p style=\"text-align: justify\">3.\u00a0 If the process of crystal growth is carried out to completion, almost cent percent yield of the solute phase<\/p>\n<p>is achievable. Also, crystals free from adhered flux are obtainable.<\/p>\n<p>4.\u00a0 In certain cases, large well formed crystals are produced at the base of the crucible.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong>28.7\u00a0Disadvantages of Flux Evaporation.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>The advantages of growth by flux evaporation may be summarized as follows:<\/p>\n<p>&nbsp;<\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">1.\u00a0\u00a0\u00a0\u00a0 Nucleation at the melt surface may be excessive\u00a0 which is likely to result in intergrown crystals.<\/span><\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">2.\u00a0\u00a0\u00a0\u00a0 Control over the growth process is lesser as compared to slow cooling method. The latter process of <\/span><\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">growth yields better quality crystals.<\/span><\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">3.\u00a0 If the segregation coefficient between dopant present in the solvent and in the crystal differs from unity, there will be a greater concentration gradient in crystals grown by flux evaporation than by slow cooling.<\/span><\/p>\n<div>\n<p>&nbsp;<\/p>\n<p>Be it growth by slow cooling or evaporation of solvent, crystallization occurs by spontaneous nucleation in both cases.<\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.8<\/strong>\u00a0<strong>Growth Procedure<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The basic technique may best be explained by a description of the flux growth of ruby crystal as perfected by Remeika of the Bell Telephone Laboratory, U.S.A. The experimental set-up is shown in figure 28.2.<\/p>\n<p>&nbsp;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-476\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95.png\" alt=\"\" width=\"410\" height=\"405\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95.png 410w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95-300x296.png 300w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95-65x64.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95-225x222.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-95-350x346.png 350w\" sizes=\"auto, (max-width: 410px) 100vw, 410px\" \/><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: center\"><strong><em>Figure 28.2: Experimental arrangement for the growth of ruby crystals by Flux method.<\/em><\/strong><\/p>\n<\/div>\n<p><strong><em>\u00a0<\/em><\/strong><\/p>\n<div>\n<p style=\"text-align: justify\">The platinum crucible of 6\u201d diameter and 10\u201d depth is loaded with the flux consisting of suitable amount of lead oxide and boron oxide (B<sub>2<\/sub>O<sub>3<\/sub>). To this is added appropriate quantity of aluminium oxide (Al<sub>2<\/sub>O<sub>3<\/sub>) and a small amount of chromium oxide (Cr<sub>2<\/sub>O<sub>3<\/sub>). The crucible charged with the flux and constituent chemicals is placed on a pedestal as shown in figure in an electrically heated furnace. The pedestal is rotated 30 turns in one direction and then 30 turns in the opposite direction to obtain uniform mixture of the molten contents of the crucible. The temperature of the furnace is raised to 1300\u00b0C. At this temperature the aluminium oxide and<\/p>\n<p style=\"text-align: justify\">chromium oxide get dissolved. After six hours, mixing is stopped and the temperature is lowered at a steady state of 4\u00b0c per hour for 8 days. At this stage, the furnace is shut off and the crucible is taken out. As the temperature passes 1240 \u2070 C on the way down, the solubility of ruby is exceeded and the crystals grow as the temperature is further reduced. To favour growth of crystals on the bottom rather than at the surface of the melt, top of the crucible is kept 10 \u2070 C hotter as compared to the temperature of the bottom, by adjusting power of the heater. It has to be ensured that the temperature is kept constant since a sudden drop would cause the production of many small crystals rather than the desired few large ones. Another consequence of temperature fluctuations is the occurrence of inclusions of small amount of flux into the growing crystals, rendering them useless for optical studies and adversely affecting their beauty.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The flux is removed from the grown crystals by dissolving the former in nitric acid which neither affects rubies nor crucible. Rhombohedral ruby crystals having size 3\/4\/\/ across and large flat\u00a0 ruby plates upto 1\/10\/\/ thick and several inches across have been obtained using this experimental set\u2013up at the Bell Telephone Laboratory, U.S.A. in the initial stages of launching this study.<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.9<\/strong>\u00a0\u00a0\u00a0\u00a0 <strong>Choice of Flux.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">One of the most important requirements of growth by high temperature solution method is making choice of flux. For all solution growth techniques one is required to find a suitable solvent. Two types of solvents are used in high temperature solution technique. One is liquid metals like gallium, indium and tin which are often used for the growth of semiconducting materials whereas the other includes oxides and halides like PbO, PbF 2 which are used for ionic materials. The solutions in oxide and halide solvents are called \u201cfluxed melts\u201d which is because the solvents used are those which are used as fluxes in operations like welding, soldering and brazing. A good flux should have the following properties:<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>(i) It should have a low melting point.<\/p>\n<p>(ii)\u00a0 It should be a good solvent, dissolving anywhere between 5 and 30 weight percent of solute at the<\/p>\n<p>maximum temperature intended to be used. The solubility should decrease with temperature.<\/p>\n<p>(iii)\u00a0\u00a0\u00a0\u00a0 It should not form a compound with solute, nor a solid solution beyond a degree which depends on<\/p>\n<p>the proposed use of the crystal.<\/p>\n<p>(iv) It should be compatible with the material, of which the crucible is made, over the proposed temperature<\/p>\n<p>range.<\/p>\n<p>(v) For growth by slow cooling method, it should be of low volatility. For a volatile flux it is necessary to use<\/p>\n<p>lower temperature range, a faster rate of cooling which often results in nucleation at the melt-surface.<\/p>\n<p>Evaporation may be prevented if lids are welded to the crucibles, but this refinement is expensive and time\u2013<\/p>\n<p>consuming.<\/p>\n<p>(vi)\u00a0\u00a0\u00a0 It should be of low viscosity.<\/p>\n<p>(vii)\u00a0\u00a0 Toxicity of flux should be low.<\/p>\n<p><span style=\"font-size: 1em;text-align: initial\">(viii) The flux should be such as to be easily separable from the crystals.<\/span><\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">(ix) The supersaturation required to cause nucleation should be much larger than that required for growth of <\/span><\/p>\n<p><span style=\"text-align: initial;font-size: 1em\">crystals. It helps in creation of only a few nuclei and growth of larger crystals.<\/span><\/p>\n<\/div>\n<div>\n<p>&nbsp;<\/p>\n<p><strong>28.10<\/strong>\u00a0\u00a0 <strong>Example of Some Suitable Solvents.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p>Some of the most frequently used solvents for the growth of some crystals are as given in table 28.1<\/p>\n<p>&nbsp;<\/p>\n<p><strong>Table 28.1\u00a0<\/strong><strong>Some suitable solvents for the growth of some materials.<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-477\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96.png\" alt=\"\" width=\"580\" height=\"589\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96.png 580w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96-295x300.png 295w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96-65x66.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96-225x228.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-96-350x355.png 350w\" sizes=\"auto, (max-width: 580px) 100vw, 580px\" \/><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-478\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97.png\" alt=\"\" width=\"572\" height=\"438\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97.png 572w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97-300x230.png 300w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97-65x50.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97-225x172.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-97-350x268.png 350w\" sizes=\"auto, (max-width: 572px) 100vw, 572px\" \/><\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.11 Separation of Crystals From Flux. <\/strong><\/p>\n<\/div>\n<div>\n<p style=\"text-align: justify\">Flux is often found to get adhered to the crystals and their separation from the crucible and\/or the grown crystals becomes essential. One method is to separate the crystals from the adhered flux by gentle hammering. If it does not work, one is required to undertake other means .Separation of crystals from flux is one of the most important steps in obtaining crystals.Wanklyn of the Crystal Growing Group at the Clarendon Laboratory; university of Oxford has grown a large number of crystals using flux method. A lot of innovation has been done by the group regarding growth of crystals by this technique. One of the several innovative techniques concerns separation of grown crystals from flux. To prolong life of a crucible and separate crystals from flux without damaging them is to remove flux from the crucible while in a molten state. One way is to invert the crucible, using long handled tongs, and then placing the same upside down on a refractory brick or a bed of alumina as shown in figure 28.3.The other technique is \u201cHot Draining of Flux\u201d as is shown in figure 28.4. The other method is to use a furnace in which there is a mechanism of inverting the crucible in the furnace itself. This arrangement is shown in figure 28.5<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-479\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98.png\" alt=\"\" width=\"478\" height=\"314\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98.png 478w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98-300x197.png 300w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98-65x43.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98-225x148.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-98-350x230.png 350w\" sizes=\"auto, (max-width: 478px) 100vw, 478px\" \/><\/p>\n<p style=\"text-align: center\"><strong><em>Figure 28.3: Separation of crystals from flux by the method of crucible inversion.<\/em><\/strong><\/p>\n<\/div>\n<p><strong><em><br \/>\n<img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-480\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99.png\" alt=\"\" width=\"319\" height=\"403\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99.png 319w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99-237x300.png 237w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99-65x82.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-99-225x284.png 225w\" sizes=\"auto, (max-width: 319px) 100vw, 319px\" \/><\/em><\/strong><\/p>\n<div>\n<p style=\"text-align: center\"><strong><em>Figure 28.4: Separation of crystals from flux by hot draining technique<\/em><\/strong><\/p>\n<\/div>\n<div>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-481\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100.png\" alt=\"\" width=\"587\" height=\"298\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100.png 587w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100-300x152.png 300w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100-65x33.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100-225x114.png 225w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-100-350x178.png 350w\" sizes=\"auto, (max-width: 587px) 100vw, 587px\" \/><\/p>\n<p style=\"text-align: center\"><strong><em>Figure 28.5: Cross-sectional view of end of brick and crucible assembly for inversion along with device for inversion<\/em><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p><strong>28.12<\/strong>\u00a0\u00a0\u00a0 <strong>Some Examples of Flux Grown Crystals<\/strong>.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Fluxed melt technique has been used for the growth of a large number of crystals, particularly those materials which are required to be grown at temperatures well below their melting points. These include a large number of materials which are very useful to electronic, magnetic and optical industries. The materials generally are complex oxides or fluorides. The list is very long and one can search for the same in the literature.<\/p>\n<p>&nbsp;<\/p>\n<p>However, some typical examples may be given here:<\/p>\n<p>&nbsp;<\/p>\n<p><strong>1. Growth of Rare Earth Orthoferrites:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">The crystals of rare earth orthoferrites bearing composition RFeO3 (where R = Dy, Ho, Gd, Tb , Er , Yb , Sm , La and Y) are grown using appropriate \/stoichiometric starting composition of R2O<sub>3<\/sub> + Fe<sub>2<\/sub>O<sub>3<\/sub> with B2O<sub>3<\/sub> -PbO-PbF<sub>2<\/sub> -PbO<sub>2<\/sub> as flux component. The composition is homogenised by vigoursely mixing the same. The well mixed composition is pressed into a platinum crucible and placed in the furnace where it is held at a temperature of ~ 1300\u00b0C for a soaking period of 12-16 hours. It is then cooled at the rate of 2\u00b0C per hour till 800\u00b0C is reached. Thereafter it is cooled at the rate of around 100\u00b0C per hour till room temperature. Well facetted crystals are formed at the bottom and\/or walls of the crucibles. The crystals are separated from flux by gently hammering the crucible and then cleaned in 10-20 % HNO3 .<\/span><\/p>\n<\/div>\n<div>\n<p>&nbsp;<\/p>\n<p><strong>2. Growth of Rare Earth Orthochromites:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The crystals of rare earth orthochromites bearing composition RCrO3\u00a0 ( where R = Y , La , Gd , Yb ) are grown using starting composition of R2O3 + Cr2 O3 with PbF2-PbO2-B2O3 as the flux component . The rest of the procedure is almost the same as in the above case.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><strong>3.\u00a0 Growth of Rare Earth Aluminates :<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">The growth of rare earth aluminates bearing composition RAlO<sub>3<\/sub>, (where=\u00a0 Gd,Eu,Sm,Nd,La,Y,Pr,Tb,Dy,Ho,Er) are grown\u00a0\u00a0 using\u00a0\u00a0 starting\u00a0 composition\u00a0\u00a0 of\u00a0\u00a0 R2O<sub>3<\/sub>&#8211; Al2O3 with flux component PbF 2-PbO-PbO<sub>2<\/sub> and B2O3+MoO3 as additives. The rest of the procedure is almost the same as in the above case.<\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><strong style=\"text-align: initial;font-size: 1em\">4. Growth of Lanthanum Borate:<\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><span style=\"text-align: initial;font-size: 1em\">Lanthanum borate crystals are grown using La2O3 with flux component PbO-B2O3. The starting composition is pressed into a platinum crucible and then maintained at a soaking temperature of 1250\u00b0C for 15 hours. The cooling rate is maintained at 3\u00b0C per hour till 700\u00b0C and thereafter faster cooling is done as in the above cases. Crystals in the form of platelets, tabular and equidimensional are produced. Some of the crystals of LaBO3 which are approximately equidimensional and grown following the above procedure are shown in figure 28.6<\/span><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><strong><span style=\"text-align: initial;font-size: 1em\">5. Growth of M-type Hexaferrites:<\/span><\/strong><\/p>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><span style=\"font-size: 1em\">Growth of M-type hexaferrite crystals of the composition SrFe12O19 is achieved by taking strontium oxide and iron oxide (ferrite composition) and sodium carbonate as flux. The ferrite and flux compositions are taken as 73.7% and 26.3% mol percent respectively and pressed into a platinum crucible which is then placed in a vertical type furnace. The material is soaked at 1320\u00b0C for 24 hours under continuous oxygen flux and then allowed to cool at 4\u00b0C hr\u25001 till it attains a temperature of 800\u00b0C. Thereafter, it is allowed to cool at a slightly faster rate. Substituted hexaferrites bearing the composition SrXaYbFe12-(a+b)O19 (where X and Y represent the substituting atoms) are grown, following the same procedure as described above, to investigate the effects of making substitutions in hexaferrites.<\/span><\/p>\n<\/div>\n<div>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-482\" src=\"http:\/\/msp06.epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/sites\/106\/2018\/12\/2-101.png\" alt=\"\" width=\"299\" height=\"267\" srcset=\"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-101.png 299w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-101-65x58.png 65w, https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-content\/uploads\/sites\/106\/2018\/12\/2-101-225x201.png 225w\" sizes=\"auto, (max-width: 299px) 100vw, 299px\" \/><\/p>\n<p style=\"text-align: center\"><strong><em>Figure 28.6: Nearly equidimensional crystals of Lanthanum borate grown by flux method.<\/em><\/strong><\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<p><strong>SUMMARY<\/strong><\/p>\n<ul>\n<li>Advantages and disadvantages of growth by flux evaporation are explained.<\/li>\n<li>Crystal growth setup as perfected by bell telephone laboratory, USA for the growth of ruby crystals is discussed as an example for explaining the procedure adopte d in this technique of crystal growth .<\/li>\n<li>Properties of good flux are identified.<\/li>\n<li>Suitable solvents for the growth of some specific crystals are given.<\/li>\n<li>Procedures involved in separating flux grown crystals from adhered flux including (i) method of crucible inve rsion.(ii) hot draining technique (ii) in- built mechanis m in the growth systems for inversion are described.<\/li>\n<li>Specific examples of crystals of flux melt growth viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M -type hexaferrites are described<\/li>\n<\/ul>\n<table>\n<tbody>\n<tr>\n<td><strong>you can view video on High Temperature Solution Growth<br \/>\n<\/strong><\/td>\n<td><a href=\"https:\/\/youtu.be\/_GBT6JaguWo\" target=\"_blank\" rel=\"noopener\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-120\" src=\"http:\/\/epgpbooks.inflibnet.ac.in\/wp-content\/uploads\/2018\/11\/download.png\" alt=\"\" width=\"36\" height=\"36\" \/><\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><strong>References.<\/strong><\/p>\n<ol>\n<li>Wanklyn,B.M. : \u201c Practical Aspects Of Flux Growth By Spontaneous Nucleation\u201d in \u201c Crystal Growth\u201d,ed.Pamplin,B.R.,PergamonPress,N.Y\/Oxford, Vol.1,1974.<\/li>\n<li>Elwell,D. &amp;Scheel,H.J.:\u201d Crystal Growth From High Temperature Solutions \u201c,Academic Press, N.y.,1975.<\/li>\n<li>Pamplin,B.R.: \u201c Crystal Growth\u201d II edition , Pergamon Press,N.Y.,1980.<\/li>\n<li>Gilman ,J.J. : \u201c The Art &amp; Science of Growing Crystals\u201d., Wiley, N.Y.,1963<\/li>\n<li>Brice,J.C.: \u201c Crystal Growth Processes\u201d, Blackie &amp; Son ,Ltd., Glasgow.,1986.<\/li>\n<li>Kotru,P.N.&amp;Wankyn,B.M.: \u201c J.Mat.Sci.Lett.\u201d, <strong>8<\/strong> ,1973,1055.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n<p><strong>Suggested Reading<\/strong>.<\/p>\n<ol>\n<li>Chase,A.B. : \u201c Exploratory Flux Growth \u201c in Preparation and Properties of Solid State Materials, Vol.1,ed.Lefever, Marcel Dekker,N.y.,1971.<\/li>\n<li>Titova,A.G.: \u201c Growth of Crystals\u201d-vol.II,eds. Shubnikov,A.V.andSheftal,N.N.,Consultant Bureau,N.Y.,1959.<\/li>\n<li>Timofeeva,V.A.: \u201c Growth of Crystals\u201d, vol.II ,eds.Shubnikov,A.V. and Sheftal,N.N.,Consultant<\/li>\n<li>Elwell,D. : in\u201d Crystal Growth &amp; Characterization \u201c,ed.Ueda,R.&amp; Mullin, J.B. ,North-Holland Publication ,1975.<\/li>\n<li>Elwell,D.: \u201c Crystal Growth &amp; Materials\u201d eds. E. Kaldis&amp;H.J.Scheel,North-Holland,Amsterdam 1976.<\/li>\n<li>Brice,J.C.: in \u201c Crystal Growth And Materials\u201d, eds. E.Kaldis&amp;H.J.Scheel , North-Holland, Amsterdam, 1976.<\/li>\n<\/ol>\n<p>&nbsp;<\/p>\n","protected":false},"author":3,"menu_order":28,"template":"","meta":{"_acf_changed":false,"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-471","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\/471","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":7,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/471\/revisions"}],"predecessor-version":[{"id":486,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapters\/471\/revisions\/486"}],"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\/471\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/media?parent=471"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/pressbooks\/v2\/chapter-type?post=471"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/contributor?post=471"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp06\/wp-json\/wp\/v2\/license?post=471"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}