Crystal bar process
The crystal bar process was developed by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925. This process was the first industrial process for the commercial production of pure ductile metallic zirconium, it is used in the production of small quantities of ultra-pure zirconium. It involves the formation of the metal iodides and their subsequent decomposition to yield pure metal; this process was superseded commercially by the Kroll process. As seen in the diagram below, impure titanium, hafnium, thorium or protactinium is heated in an evacuated vessel with a halogen at 50–250 °C; the patent involved the intermediacy of TiI4 and ZrI4, which were volatilized. At atmospheric pressure TiI4 melts at 150 °C and boils at 377 °C, while ZrI4 melts at 499 °C and boils at 600 °C; the boiling points are lower at reduced pressure. The gaseous metal tetraiodide is decomposed on a white hot tungsten filament; as more metal is deposited the filament conducts better and thus a greater electric current is required to maintain the temperature of the filament.
The process can be performed in the span of several hours or several weeks, depending on the particular setup. The crystal bar process can be performed using any number of metals using whichever halogen or combination of halogens is most appropriate for that sort of transport mechanism, based on the reactivities involved; the only metals it has been used to purify on an industrial scale are titanium and hafnium, in fact is still in use today on a much smaller scale for special purity needs. Several metals purified via this process
In chemistry, recrystallization is a technique used to purify chemicals. By dissolving both impurities and a compound in an appropriate solvent, either the desired compound or impurities can be removed from the solution, leaving the other behind, it is named for the crystals formed when the compound precipitates out. Alternatively, recrystallization can refer to the natural growth of larger ice crystals at the expense of smaller ones. In chemistry, recrystallization is a procedure for purifying compounds; the most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted, recrystallization being one of them. There are different recrystallization techniques that can be used such as: Typically, the mixture of "compound A" and "impurity B" is dissolved in the smallest amount of hot solvent to dissolve the mixture, thus making a saturated solution; the solution is allowed to cool. As the solution cools the solubility of compounds in solution drops.
This results in the desired compound dropping from solution. The slower the rate of cooling, the bigger the crystals form. In an ideal situation the solubility product of the impurity, B, is not exceeded at any temperature. In that case the solid crystals will consist of pure A and all the impurity will remain in solution; the solid crystals are collected by filtration and the filtrate is discarded. If the solubility product of the impurity is exceeded, some of the impurity will co-precipitate. However, because of the low concentration of the impurity, its concentration in the precipitated crystals will be less than its concentration in the original solid. Repeated recrystallization will result in an purer crystalline precipitate; the purity is checked after each recrystallization by measuring the melting point, since impurities lower the melting point. NMR spectroscopy can be used to check the level of impurity. Repeated recrystallization results in some loss of material because of the non-zero solubility of compound A.
The crystallization process requires an initiation step, such as the addition of a "seed" crystal. In the laboratory a minuscule fragment of glass, produced by scratching the side of the glass recrystallization vessel, may provide the nucleus on which crystals may grow. Successful recrystallization depends on finding the right solvent; this is a combination of prediction/experience and trial/error. The compounds must be more soluble at the higher temperature than at the lower temperatures. Any insoluble impurity is removed by the technique of hot filtration; this method is the same as the above but. This relies on "impurity B" being soluble in a first solvent. A second solvent is added. Either "compound A" or "impurity B" will be insoluble in this solvent and precipitate, whilst the other of "compound A"/"impurity B" will remain in solution, thus the proportion of first and second solvents is critical. The second solvent is added until one of the compounds begins to crystallize from solution and the solution is cooled.
Heating can be used. The reverse of this method can be used where a mixture of solvent dissolves both A and B. One of the solvents is removed by distillation or by an applied vacuum; this results in a change in the proportions of solvent causing either "compound A" or "impurity B" to precipitate. Hot filtration can be used to separate "compound A" from both "impurity B" and some "insoluble matter C"; this technique uses a single-solvent system as described above. When both "compound A" and "impurity B" are dissolved in the minimum amount of hot solvent, the solution is filtered to remove "insoluble matter C"; this matter may be anything from a third impurity compound to fragments of broken glass. For a successful procedure, one must ensure that the filtration apparatus is hot in order to stop the dissolved compounds crystallizing from solution during filtration, thus forming crystals on the filter paper or funnel. One way to achieve this is to heat a conical flask containing a small amount of clean solvent on a hot plate.
A filter funnel is rested on the mouth, hot solvent vapors keep the stem warm. Jacketed filter funnels may be used; the filter paper is preferably fluted, rather than folded into a quarter. It is simpler to do the filtration and recrystallization as two independent and separate steps; that is dissolve "compound A" and "impurity B" in a suitable solvent at room temperature, remove the solvent and recrystallize using any of the methods listed above. Crystallization requires an initiation step; this can be spontaneous or can be done by adding a small amount of the pure compound to the saturated solution, or can be done by scratching the glass surface to create a seeding surface for crystal growth. It is thought that dust particles can act as simple seeds. Growing crystals for X-ray crystallography can be quite difficult. For X-ray analysis, single perfect crystals are required. A small amount of pure compound is used, crystals are allowed to grow slowly. Several techniques can be used to grow these perfect crystals: Slow evaporation of a single solvent - the compound is dissolved in a suitable solvent and the solvent is allowed to evaporate.
Once the solution is saturated crystals can form. Slow evaporation of a multi-solvent system - the same as above, however as the solvent composition changes due to eva
The Verneuil process called flame fusion, was the first commercially successful method of manufacturing synthetic gemstones, developed in the late 1800s by the French chemist Auguste Verneuil. It is used to produce the ruby and sapphire varieties of corundum, as well as the diamond simulants rutile and strontium titanate; the principle of the process involves melting a finely powdered substance using an oxyhydrogen flame, crystallising the melted droplets into a boule. The process is considered to be the founding step of modern industrial crystal growth technology, remains in wide use to this day. Since the study of alchemy began, there have been attempts to synthetically produce precious stones, ruby, being one of the prized cardinal gems, has long been a prime candidate. In the 19th century, significant advances were achieved, with the first ruby formed by melting two smaller rubies together in 1817, the first microscopic crystals created from alumina in a laboratory in 1837. By 1877, chemist Edmond Frémy had devised an effective method for commercial ruby manufacture by using molten baths of alumina, yielding the first gemstone-quality synthetic stones.
The Parisian chemist Auguste Verneuil collaborated with Fremy on developing the method, but soon went on to independently develop the flame fusion process, which would come to bear his name. One of Verneuil's sources of inspiration for developing his own method was the appearance of synthetic rubies sold by an unknown Genevan merchant in 1880; these "Geneva rubies" were dismissed as artificial at the time, but are now believed to be the first rubies produced by flame fusion, predating Verneuil's work on the process by 20 years. After examining the "Geneva rubies", Verneuil came to the conclusion that it was possible to recrystallise finely ground aluminium oxide into a large gemstone; this realisation, along with the availability of the developed oxyhydrogen torch and growing demand for synthetic rubies, led him to design the Verneuil furnace, where finely ground purified alumina and chromium oxide were melted by a flame of at least 2,000 °C, recrystallised on a support below the flame, creating a large crystal.
He announced his work in 1902, publishing details outlining the process in 1904. By 1910, Verneuil's laboratory had expanded into a 30-furnace production facility, with annual gemstone production by the Verneuil process having reached 1,000 kg in 1907. By 1912, production reached 3,200 kg, would go on to reach 200,000 kg in 1980 and 250,000 kg in 2000, led by Hrand Djevahirdjian's factory in Monthey, founded in 1914; the most notable improvements in the process were made in 1932, by S. K. Popov, who helped establish the capability for producing high-quality sapphires in the Soviet Union through the next 20 years. A large production capability was established in the United States during World War II, when European sources were not available, jewels were in high demand for their military applications; the process was designed for the synthesis of rubies, which became the first gemstones to be synthetically produced, thanks to the efforts of Fremy and Verneuil. However, the Verneuil process could be used for the production of other stones, including blue sapphire, which required ferric oxide to be substituted for chromium oxide, as well as more elaborate ones, such as star sapphires, where titania was added and the boule was kept in the heat longer, allowing needles of rutile to crystallise within it.
In 1947, the Linde Air Products division of Union Carbide pioneered the use of the Verneuil process for creating such star sapphires, until production was discontinued in 1974 due to overseas competition. Despite some improvements in the method, the Verneuil process remains unchanged to this day, while maintaining a leading position in the manufacture of synthetic corundum and spinel gemstones, its most significant setback came in 1917, when Jan Czochralski introduced the Czochralski process, which has found numerous applications in the semiconductor industry, where a much higher quality of crystals is required than the Verneuil process can produce. Other alternatives to the process emerged in 1957, when Bell Labs introduced the hydrothermal process, in 1958, when Carroll Chatham introduced the flux method. In 1989 Larry P Kelley of ICT, Inc. developed a variant of the Czochralski process where natural ruby is used as the'feed' material. One of the most crucial factors in crystallising an artificial gemstone is obtaining pure starting material, with at least 99.9995% purity.
In the case of manufacturing rubies or sapphires, this material is alumina. The presence of sodium impurities is undesirable, as it makes the crystal opaque. Depending on the desired colouration of the crystal, small quantities of various oxides are added, such as chromium oxide for a red ruby, or ferric oxide and titania for a blue sapphire. Other starting materials include titania for producing rutile, or titanyl double oxalate for producing strontium titanate. Alternatively, valueless crystals of the desired product can be used; this starting material is finely powdered, placed in a container within a Verneuil furnace, with an opening at the bottom through which the powder can escape when the container is vibrated. While the powder is being released, oxygen is supplied into the furnace, travels with the powder down a narrow tube; this tube is located within a larger tube. At the point where the narrow tube opens into the larger one, combustion occurs, with a flame of at least 2,000 °C at its core.
As the powder passes through the flame, it melts into smal
Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate. The overlayer is called an epitaxial epitaxial layer; the term epitaxy comes from the Greek roots epi, meaning "above", taxis, meaning "an ordered manner". It can be translated as "arranging upon". For most technological applications, it is desired that the deposited material form a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal structure. Epitaxial films may be grown from liquid precursors; because the substrate acts as a seed crystal, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal. If the overlayer either forms a random orientation with respect to the substrate or does not form an ordered overlayer, it is termed non-epitaxial growth. If an epitaxial film is deposited on a substrate of the same composition, the process is called homoepitaxy. Homoepitaxy is a kind of epitaxy performed with only one material, in which a crystalline film is grown on a substrate or film of the same material.
This technology is used to grow a film, more pure than the substrate and to fabricate layers having different doping levels. In academic literature, homoepitaxy is abbreviated to "homoepi". Heteroepitaxy is a kind of epitaxy performed with materials. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material; this technology is used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. Examples include silicon on sapphire, gallium nitride on sapphire, aluminium gallium indium phosphide on gallium arsenide or diamond or iridium, graphene on hexagonal boron nitride. Heterotopotaxy is a process similar to heteroepitaxy except that thin-film growth is not limited to two-dimensional growth. Pendeo-epitaxy is a process in which the heteroepitaxial film is growing vertically and laterally at the same time. In 2D crystal heterostructure, graphene nanoribbons embedded in hexagonal boron nitride give an example of pendeo-epitaxy.
Epitaxy is used in silicon-based manufacturing processes for bipolar junction transistors and modern complementary metal–oxide–semiconductors, but it is important for compound semiconductors such as gallium arsenide. Manufacturing issues include control of the amount and uniformity of the deposition's resistivity and thickness, the cleanliness and purity of the surface and the chamber atmosphere, the prevention of the much more doped substrate wafer's diffusion of dopant to the new layers, imperfections of the growth process, protecting the surfaces during manufacture and handling. Epitaxy is used in semiconductor fabrication. Indeed, epitaxy is the only affordable method of high quality crystal growth for many semiconductor materials. In surface science, epitaxy is used to create and study monolayer and multilayer films of adsorbed organic molecules on single crystalline surfaces. Adsorbed molecules form ordered structures on atomically flat terraces of single crystalline surfaces and can directly be observed via scanning tunnelling microscopy.
In contrast, surface defects and their geometry have significant influence on the adsorption of organic molecules Epitaxial silicon is grown using vapor-phase epitaxy, a modification of chemical vapor deposition. Molecular-beam and liquid-phase epitaxy are used for compound semiconductors. Solid-phase epitaxy is used for crystal-damage healing. Silicon is most deposited by doping with silicon tetrachloride and hydrogen at 1200 to 1250 °C: SiCl4 + 2H2 ↔ Si + 4HClThis reaction is reversible, the growth rate depends upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, negative growth rates may occur if too much hydrogen chloride byproduct is present. An additional etching reaction competes with the deposition reaction: SiCl4 + Si ↔ 2SiCl2Silicon VPE may use silane and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way: SiH4 → Si + 2H2This reaction does not inadvertently etch the wafer, takes place at lower temperatures than deposition from silicon tetrachloride.
However, it will form a polycrystalline film unless controlled, it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide. VPE is sometimes classified by the chemistry of the source gases, such as hydride VPE and metalorganic VPE. Liquid-phase epitaxy is a method to grow semiconductor crystal layers from the melt on solid substrates; this happens at temperatures well below the melting point of the deposited semiconductor. The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition, the deposition of the semiconductor crystal on the substrate is fast and uniform; the most used substrate is indium phosphide. Other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar. Centrifugal liquid-phase epitaxy is used commercially to make thin layer
Precipitation is the creation of a solid from a solution. When the reaction occurs in a liquid solution, the solid formed is called the'precipitate'; the chemical that causes the solid to form is called the'precipitant'. Without sufficient force of gravity to bring the solid particles together, the precipitate remains in suspension. After sedimentation when using a centrifuge to press it into a compact mass, the precipitate may be referred to as a'pellet'. Precipitation can be used as a medium; the precipitate-free liquid remaining above the solid is called the'supernate' or'supernatant'. Powders derived from precipitation have historically been known as'flowers'; when the solid appears in the form of cellulose fibers which have been through chemical processing, the process is referred to as regeneration. Sometimes the formation of a precipitate indicates the occurrence of a chemical reaction. If silver nitrate solution is poured into a solution of sodium chloride, a chemical reaction occurs forming a white precipitate of silver chloride.
When potassium iodide solution reacts with lead nitrate solution, a yellow precipitate of lead iodide is formed. Precipitation may occur. Precipitation may occur from a supersaturated solution. In solids, precipitation occurs if the concentration of one solid is above the solubility limit in the host solid, due to e.g. rapid quenching or ion implantation, the temperature is high enough that diffusion can lead to segregation into precipitates. Precipitation in solids is used to synthesize nanoclusters. An important stage of the precipitation process is the onset of nucleation; the creation of a hypothetical solid particle includes the formation of an interface, which requires some energy based on the relative surface energy of the solid and the solution. If this energy is not available, no suitable nucleation surface is available, supersaturation occurs. Precipitation reactions can be used for making pigments, removing salts from water in water treatment, in classical qualitative inorganic analysis.
Precipitation is useful to isolate the products of a reaction during workup. Ideally, the product of the reaction is insoluble in the reaction solvent. Thus, it precipitates. An example of this would be the synthesis of porphyrins in refluxing propionic acid. By cooling the reaction mixture to room temperature, crystals of the porphyrin precipitate, are collected by filtration: Precipitation may occur when an antisolvent is added, drastically reducing the solubility of the desired product. Thereafter, the precipitate may be separated by filtration, decanting, or centrifugation. An example would be the synthesis of chromic tetraphenylporphyrin chloride: water is added to the DMF reaction solution, the product precipitates. Precipitation is useful in purifying products: crude bmim-Cl is taken up in acetonitrile, dropped into ethyl acetate, where it precipitates. Another important application of an antisolvent is in ethanol precipitation of DNA. In metallurgy, precipitation from a solid solution is a useful way to strengthen alloys.
An example of a precipitation reaction: Aqueous silver nitrate is added to a solution containing potassium chloride, the precipitation of a white solid, silver chloride, is observed. AgNO 3 + KCl ⟶ AgCl ↓ + KNO 3 The silver chloride has formed a solid, observed as a precipitate; this reaction can be written emphasizing the dissociated ions in a combined solution. This is known as the ionic equation. Ag + + NO 3 − + K + + Cl − ⟶ AgCl ↓ + K + + NO 3 − A final way to represent a precipitate reaction is known as a net ionic reaction. Many compounds containing metal ions produce precipitates with distinctive colors; the following are typical colors for various metals. However, many of these compounds can produce colors different from those listed. Other compounds form white precipitates. Precipitate formation is useful in the detection of the type of cation in a salt. To do this, an alkali first reacts with the unknown salt to produce a precipitate, the hydroxide of the unknown salt. To identify the cation, the color of the precipitate and its solubility in excess are noted.
Similar processes are used in sequence – for example, a barium nitrate solution will react with sulfate ions to form a solid barium sulfate precipitate, indicating that it is that sulfate ions are present. Digestion, or precipitate ageing, happens when a freshly formed precipitate is left at a higher temperature, in the solution from which it precipitates, it results in bigger particles. The physico-chemical process underlying digestion is called Ostwald ripening. Coprecipitation Salting in Salting out Effervescence Zumdahl, Steven S.. Chemical Principles. New York: Houghton Mifflin. ISBN 0-618-37206-7. Precipitation reactions of certain cations Digestion Instruments A Thesis on pattern formation in precipitation reactions
Hydrothermal synthesis includes the various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures. The term "hydrothermal" is of geologic origin. Geochemists and mineralogists have studied hydrothermal phase equilibria since the beginning of the twentieth century. George W. Morey at the Carnegie Institution and Percy W. Bridgman at Harvard University did much of the work to lay the foundations necessary to containment of reactive media in the temperature and pressure range where most of the hydrothermal work is conducted. Hydrothermal synthesis can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure; the crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave, in which a nutrient is supplied along with water. A temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end the nutrient solute dissolves, while at the cooler end it is deposited on a seed crystal, growing the desired crystal.
Advantages of the hydrothermal method over other types of crystal growth include the ability to create crystalline phases which are not stable at the melting point. Materials which have a high vapour pressure near their melting points can be grown by the hydrothermal method; the method is particularly suitable for the growth of large good-quality crystals while maintaining control over their composition. Disadvantages of the method include the need of expensive autoclaves, the impossibility of observing the crystal as it grows if a steel tube is used. There are autoclaves made out of thick walled glass, which can be used up to 10 bar; the first report of the hydrothermal growth of crystals was by German geologist Karl Emil von Schafhäutl in 1845: he grew microscopic quartz crystals in a pressure cooker. In 1848, Robert Bunsen reported growing crystals of barium and strontium carbonate at 200 °C and at pressures of 15 atmospheres, using sealed glass tubes and aqueous ammonium chloride as a solvent.
In 1849 and 1851, French crystallographer Henri Hureau de Sénarmont produced crystals of various minerals via hydrothermal synthesis. Giorgio Spezia published reports on the growth of macroscopic crystals, he used solutions of sodium silicate, natural crystals as seeds and supply, a silver-lined vessel. By heating the supply end of his vessel to 320-350 °C, the other end to 165-180 °C, he obtained about 15 mm of new growth over a 200-day period. Unlike modern practice, the hotter part of the vessel was at the top. A shortage in the electronics industry of natural quartz crystals from Brazil during World War 2 led to postwar development of a commercial-scale hydrothermal process for culturing quartz crystals, by A. C. Walker and Ernie Buehler in 1950 at Bell Laboratories. Other notable contributions have been made by Nacken, Hale and Kohman. A large number of compounds belonging to all classes have been synthesized under hydrothermal conditions: elements and complex oxides, molybdates, silicates, germanates etc.
Hydrothermal synthesis is used to grow synthetic quartz and other single crystals with commercial value. Some of the crystals that have been efficiently grown are emeralds, quartz and others; the method has proved to be efficient both in the search for new compounds with specific physical properties and in the systematic physicochemical investigation of intricate multicomponent systems at elevated temperatures and pressures. The crystallization vessels used are autoclaves; these are thick-walled steel cylinders with a hermetic seal which must withstand high temperatures and pressures for prolonged periods of time. Furthermore, the autoclave material must be inert with respect to the solvent; the closure is the most important element of the autoclave. Many designs have been developed for the most famous being the Bridgman seal. In most cases, steel-corroding solutions are used in hydrothermal experiments. To prevent corrosion of the internal cavity of the autoclave, protective inserts are used; these may have the same shape as the autoclave and fit in the internal cavity, or be a "floating" type insert which occupies only part of the autoclave interior.
Inserts may be made of carbon-free iron, silver, platinum, glass, or Teflon, depending on the temperature and solution used. This is the most extensively used method in crystal growing. Supersaturation is achieved by reducing the temperature in the crystal growth zone; the nutrient is placed in the lower part of the autoclave filled with a specific amount of solvent. The autoclave is heated; the nutrient dissolves in the hotter zone and the saturated aqueous solution in the lower part is transported to the upper part by convective motion of the solution. The cooler and denser solution in the upper part of the autoclave descends while the counterflow of solution ascends; the solution becomes supersaturated in the upper part as the result of the reduction in temperature and crystallization sets in. In this technique crystallization takes place without a temperature gradient between the growth and dissolution zones; the supersaturation is achieved by a gradual reduction in temperature of the solution in the autoclave.
The disadvantage of this technique is the difficulty in controlling the growth process and introducing seed crystals. For these reasons, this technique is seldom used; this technique is based on the difference in solub
The skull crucible process was developed at the Lebedev Physical Institute in Moscow to manufacture cubic zirconia. It was invented to solve the problem of cubic zirconia's melting-point being too high for platinum crucibles. In essence, by heating only the center of a volume of cubic zirconia, the material forms its own "crucible" from its cooler outer layers; the term "skull" refers to these outer layers forming a shell enclosing the molten volume. Zirconium oxide powder is heated gradually allowed to cool. Heating is accomplished by radio frequency induction using a coil wrapped around the apparatus; the outside of the device is water-cooled in order to keep the RF coil from melting and to cool the outside of the zirconium oxide and thus maintain the shape of the zirconium powder. Since zirconium oxide in its solid state does not conduct electricity, a piece of zirconium metal is placed inside the gob of zirconium oxide; as the zirconium melts it oxidizes and blends with the now molten zirconium oxide, a conductor, is heated by RF induction.
When the zirconium oxide is melted on the inside the amplitude of the RF induction coil is reduced and crystals form as the material cools. This would form a monoclinic crystal system of zirconium oxide. In order to maintain a cubic crystal system a stabilizer is added, magnesium oxide, calcium oxide or yttrium oxide as well as any material to color the crystal. After the mixture cools the outer shell is broken off and the interior of the gob is used to manufacture gemstones