The crystal bar process (also known as iodide process or the van Arkel–de Boer 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 titanium and zirconium. It primarily involves the formation of the metal iodides and their subsequent decomposition to yield pure metal. This process was superseded commercially by the Kroll process.
|Crystallization · Crystal growth |
Recrystallization · Seed crystal
Protocrystalline · Single crystal
|Methods and technology|
Crystal bar process
Laser-heated pedestal growth
Shaping processes in crystal growth
|Nucleation · Crystal |
Crystal structure · Solid
As seen in the diagram below, impure titanium, zirconium, hafnium, vanadium, thorium or protactinium is heated in an evacuated vessel with a halogen at 50–250 °C. The patent specifically involved the intermediacy of TiI4 and ZrI4, which were volatilized (leaving impurities as solid). 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 (1400 °C). 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.
Generally, 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, zirconium and hafnium, and in fact is still in use today on a much smaller scale for special purity needs.
Several metals purified via this process:
Anton Eduard van Arkel, ('s-Gravenzande Netherlands, 19 November 1893 – Leiden, 14 March 1976) was a Dutch chemist.
He suggested the names "pnictogen" and "pnictide".Van Arkel became member of the Royal Netherlands Academy of Arts and Sciences in 1962.Boule (crystal)
A boule is a single crystal ingot produced by synthetic means.A boule of silicon is the starting material for most of the integrated circuits used today. In the semiconductor industry synthetic boules can be made by a number of methods, such as the Bridgman technique and the Czochralski process, which result in a cylindrical rod of material.
In the Czochralski process a seed crystal is required to create a larger crystal, or ingot. This seed crystal is dipped into the pure molten silicon and slowly extracted. The molten silicon grows on the seed crystal in a crystalline fashion. As the seed is extracted the silicon solidifies and eventually a large, cylindrical boule is produced.A semiconductor crystal boule is normally cut into circular wafers using an inside hole diamond saw or diamond wire saw, and each wafer is lapped and polished to provide substrates suitable for the fabrication of semiconductor devices on its surface.The process is also used to create sapphires, which are used for substrates in the production of blue and white LEDs, optical windows in special applications and as the protective covers for watches.Bridgman–Stockbarger technique
The Bridgman–Stockbarger technique is named after Harvard physicist Percy Williams Bridgman (1882-1961) and MIT physicist Donald C. Stockbarger (1895–1952). The technique includes two similar but distinct methods primarily used for growing boules (single crystal ingots), but which can be used for solidifying polycrystalline ingots as well.
The methods involve heating polycrystalline material above its melting point and slowly cooling it from one end of its container, where a seed crystal is located. A single crystal of the same crystallographic orientation as the seed material is grown on the seed and is progressively formed along the length of the container. The process can be carried out in a horizontal or vertical orientation, and usually involves a rotating crucible/ampoule to stir the melt.The Bridgman method is a popular way of producing certain semiconductor crystals such as gallium arsenide, for which the Czochralski process is more difficult. The process can reliably produce single crystal ingots, but does not necessarily result in uniform properties through the crystal.
The difference between the Bridgman technique and Stockbarger technique is subtle: While both methods utilize a temperature gradient and a moving crucible, the Bridgman technique utilizes the relatively uncontrolled gradient produced at the exit of the furnace; the Stockbarger technique introduces a baffle, or shelf, separating two coupled furnaces with temperatures above and below the freezing point. Stockbarger's modification of the Bridgman technique allows for better control over the temperature gradient at the melt/crystal interface.
When seed crystals are not employed as described above, polycrystalline ingots can be produced from a feedstock consisting of rods, chunks, or any irregularly shaped pieces once they are melted and allowed to re-solidify. The resultant microstructure of the ingots so obtained are characteristic of directionally solidified metals and alloys with their aligned grains.
A variant of the technique known as the horizontal directional solidification method or HDSM developed by Khachik Bagdasarov starting in the 1960s in the Soviet Union uses a flat-bottomed crucible with short sidewalls rather than an enclosed ampoule, and has been used to grow various large oxide crystals including Yb:YAG (a laser host crystal), and sapphire crystals 45 cm wide and over 1 meter long.Flux method
Flux method is a method of crystal growth where the components of the desired substance are dissolved in a solvent (flux). The method is particularly suitable for crystals needing to be free from thermal strain. It takes place in a crucible made of highly stable, non-reactive material. For production of oxide crystals, metals such as platinum, tantalum, and niobium are common. Production of metallic crystals generally uses crucibles made from ceramics such as alumina, zirconia, and boron nitride. The crucibles and their contents are often isolated from the air for reaction, either by sealing them in a quartz ampoule or by using a furnace with atmosphere control. A saturated solution is prepared by keeping the constituents of the desired crystal and the flux at a temperature slightly above the saturation temperature long enough to form a complete solution. Then the crucible is cooled in order to allow the desired material to precipitate. Crystal formation can begin by spontaneous nucleation or may be encouraged by the use of a seed. As material precipitates out of the solution, the amount of solute in the flux decreases and the temperature at which the solution is saturated lowers. This process repeats itself as the furnace continues to cool until the solution reaches its melting point or the reaction is stopped artificially.
One advantage of this method is that the crystals grown often display natural facets, which often makes preparing crystals for measurement significantly easier. A disadvantage is that most flux method syntheses produce relatively small crystals. However, some materials such as the "115" heavy fermion superconductors (CeXIn5, X=Co,Ir,Rh) may grow up to a few centimeters.Fractional crystallization (chemistry)
In chemistry, fractional crystallization is a method of refining substances based on differences in solubility. It fractionates via differences in crystallization (forming of crystals). If a mixture of two or more substances in solution are allowed to crystallize, for example by allowing the temperature of the solution to decrease or increase, the precipitate will contain more of the least soluble substance. The proportion of components in the precipitate will depend on their solubility products. If the solubility products are very similar, a cascade process will be needed to effectuate a complete separation.
This technique is often used in chemical engineering to obtain very pure substances, or to recover saleable products from waste solutions.Fractional freezing
Fractional freezing is a process used in process engineering and chemistry to separate substances with different melting points. It can be done by partial melting of a solid, for example in zone refining of silicon or metals, or by partial crystallization of a liquid, as in freeze distillation, also called normal freezing or progressive freezing. The initial sample is thus fractionated (separated into fractions).
Partial crystallization can also be achieved by adding a dilute solvent to the mixture, and cooling and concentrating the mixture by evaporating the solvent, a process called solution crystallization. Fractional freezing is generally used to produce ultra-pure solids, or to concentrate heat-sensitive liquids.Hafnium tetraiodide
Hafnium tetraiodide is the inorganic compound with the formula HfI4. It is a red-orange, moisture sensitive, sublimable solid that is produced by heating a mixture of hafnium with excess iodine. It is an intermediate in the crystal bar process for producing hafnium metal.
In this compound, the hafnium centers adopt octahedral coordination geometry. Like most binary metal halides, the compound is a polymeric. It is one-dimensional polymer consisting of chains of edge-shared bioctahedral Hf2I8 subunits, similar to the motif adopted by HfCl4. The nonbridging iodide ligands have shorter bonds to Hf than the bridging iodide ligands.Industrial processes
Industrial processes are procedures involving chemical, physical, electrical or mechanical steps to aid in the manufacturng of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.Jan Hendrik de Boer
Jan Hendrik de Boer (19 March 1899 – 25 April 1971) was a Dutch physicist and chemist.
De Boer was born in Ruinen, now De Wolden, and died in The Hague. He studied at the University of Groningen and was later employed in industry.
Together with Anton Eduard van Arkel, de Boer developed a chemical transport reaction for titanium, zirconium, and hafnium known as the crystal bar process. In a closed vessel the metal reacts with iodine at elevated temperature forming the iodide. At a tungsten filament of 1700 °C the reverse reaction occurs, and the iodine and the metal are set free. The metal forms a solid coating at the tungsten filament and the iodine can react with additional metal, resulting in a steady turnover.
M + 2I2 (>400 °C) → MI4
MI4 (1700 °C) → M + 2I2De Boer became a member of the Royal Netherlands Academy of Arts and Sciences in 1940, and foreign member in 1947.Kyropoulos process
The Kyropoulos process is a method of bulk crystal growth used to obtain single crystals. The process is named for Spyro Kyropoulos, who proposed the technique in 1926 as a method to grow brittle alkali halide and alkali earth metal crystals for precision optics. The largest application of the Kyropoulos process is to grow large boules of single crystal sapphire used to produce substrates for the manufacture gallium nitride-based LEDs, and as a durable optical material.Laser-heated pedestal growth
Laser-heated pedestal growth (LHPG) or laser floating zone (LFZ) is a crystal growth technique. A narrow region of a crystal is melted with a powerful CO2 or YAG laser. The laser and hence the floating zone, is moved along the crystal. The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it. This technique for growing crystals from the melt (liquid/solid phase transition) is used in materials research.Micro-pulling-down
The micro-pulling-down (µ-PD) method is a crystal growth technique based on continuous transport of the melted substance through micro-channel(s) made in a crucible bottom. Continuous solidification of the melt is progressed on a liquid/solid interface positioned under the crucible. In a steady state, both the melt and the crystal are pulled-down with a constant (but generally different) velocity.
Many different types of crystal are grown by this technique, including Y3Al5O12, Si, Si-Ge, LiNbO3,
α-Al2O3, Y2O3, Sc2O3,
LiF, CaF2, BaF2, etc.Protocrystalline
A protocrystalline phase is a distinct phase occurring during crystal growth which evolves into a microcrystalline form. The term is typically associated with silicon films in optical applications such as solar cells.Recrystallization (geology)
In geology, solid-state recrystallization is a metamorphic process that occurs under temperature and pressure where atoms of a mineral are reorganized by diffusion and/or dislocation glide. The mineral composition may remain unchanged. This process can be illustrated by observing how snow recrystallizes to ice.
As opposed to metasomatism, which is a change in composition, recrystallization can be a purely physical process.
Limestone is a sedimentary rock that undergoes metamorphic recrystallization to form marble, and clays can recrystallize to muscovite mica.Seed crystal
A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is to be grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the otherwise slow randomness of natural crystal growth and allows manufacture on a scale suitable for industry.Skull crucible
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 even 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 then 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 also 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, and is heated by RF induction.
When the zirconium oxide is melted on the inside (but not completely, since the outside needs to remain solid) the amplitude of the RF induction coil is gradually reduced and crystals form as the material cools. Normally 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 then used to manufacture gemstones.Verneuil process
The Verneuil process, also 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 primarily 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, and crystallising the melted droplets into a boule. The process is considered to be the founding step of modern industrial crystal growth technology, and remains in wide use to this day.Zirconium(IV) iodide
Zirconium(IV) iodide is the chemical compound with the formula ZrI4. It is the most readily available iodide of zirconium. It is an orange-coloured solid that degrades in the presence of water.The compound was once prominent as an intermediate in the purification of zirconium metal.Zone melting
Zone melting (or zone refining or floating zone process or travelling melting zone) is a group of similar methods of purifying crystals, in which a narrow region of a crystal is melted, and this molten zone is moved along the crystal. The molten region melts impure solid at its forward edge and leaves a wake of purer material solidified behind it as it moves through the ingot. The impurities concentrate in the melt, and are moved to one end of the ingot. Zone refining was invented by John Desmond Bernal and further developed by William Gardner Pfann in Bell Labs as a method to prepare high purity materials, mainly semiconductors, for manufacturing transistors. Its early use was on germanium for this purpose, but it can be extended to virtually any solute-solvent system having an appreciable concentration difference between solid and liquid phases at equilibrium. This process is also known as the float zone process, particularly in semiconductor materials processing.