Chemical transport reaction

In chemistry, a chemical transport reaction describes a process for purification and crystallization of non-volatile solids.[1] The process is also responsible for certain aspects of mineral growth from the effluent of volcanoes. The technique is distinct from chemical vapor deposition, which usually entails decomposition of molecular precursors (e.g. SiH4 → Si + 2H2) and which gives conformal coatings.

The technique, which was popularized by Harald Schäfer,[2] entails the reversible conversion of nonvolatile elements and chemical compounds into volatile derivatives.[3] The volatile derivative migrates throughout a sealed reactor, typically a sealed and evacuated glass tube heated in a tube furnace. Because the tube is under a temperature gradient, the volatile derivative reverts to the parent solid and the transport agent is released at the end opposite to which it originated (see next section). The transport agent is thus catalytic. The technique requires that the two ends of the tube (which contains the sample to be crystallized) be maintained at different temperatures. So-called two-zone tube furnaces are employed for this purpose. The method derives from the Van Arkel de Boer process[4] which was used for the purification of titanium and vanadium and uses iodine as the transport agent.

Titan-crystal bar
Crystals of titanium grown using the Van Arkel-de Boer process with I2 as the transport agent.

Cases of the exothermic and endothermic reactions of the transporting agent

Transport reactions are classified according to the thermodynamics of the reaction between the solid and the transporting agent. When the reaction is exothermic, then the solid of interest is transported from the cooler end (which can be quite hot) of the reactor to a hot end, where the equilibrium constant is less favorable and the crystals grow. The reaction of molybdenum dioxide with the transporting agent iodine is an exothermic process, thus the MoO2 migrates from the cooler end (700 °C) to the hotter end (900 °C):

MoO2 + I2 ⇌ MoO2I2 ΔHrxn < 0 (exothermic)

Using 10 milligrams of iodine for 4 grams of the solid, the process requires several days.

Alternatively, when the reaction of the solid and the transport agent is endothermic, the solid is transported from a hot zone to a cooler one. For example:

Fe2O3 + 6 HClFe2Cl6+ 3 H2O ΔHrxn > 0 (endothermic)

The sample of iron(III) oxide is maintained at 1000 °C, and the product is grown at 750 °C. HCl is the transport agent. Crystals of hematite are reportedly observed at the mouths of volcanoes because of chemical transport reactions whereby volcanic hydrogen chloride volatilizes iron(III) oxides.[5]

Halogen lamp

A similar reaction like that of MoO2 is used in halogen lamps. The tungsten is evaporated from the tungsten filament and converted with traces of oxygen and iodine into the WO2I2, at the high temperatures near the filament the compound decomposes back to tungsten, oxygen and iodine. [6]

WO2 + I2 ⇌ WO2I2, ΔHrxn < 0 (exothermic)

References

  1. ^ Michael Binnewies, Robert Glaum, Marcus Schmidt, Peer Schmidt "Chemical Vapor Transport Reactions – A Historical Review" Zeitschrift für anorganische und allgemeine Chemie 2013, Volume 639, pages 219–229. doi:10.1002/zaac.201300048
  2. ^ Günther Rienäcker, Josef Goubeau (1973). "Professor Harald Schäfer". Zeitschrift für anorganische und allgemeine Chemie. 395 (2–3): 129–133. doi:10.1002/zaac.19733950202.
  3. ^ Schäfer, H. "Chemical Transport Reactions" Academic Press, New York, 1963.
  4. ^ van Arkel, A. E.; de Boer, J. H. (1925). "Darstellung von reinem Titanium-, Zirkonium-, Hafnium- und Thoriummetall". Zeitschrift für anorganische und allgemeine Chemie (in German). 148 (1): 345–350. doi:10.1002/zaac.19251480133.
  5. ^ P. Kleinert, D. Schmidt (1966). "Beiträge zum chemischen Transport oxidischer Metallverbindungen. I. Der Transport von α-Fe2O3 über dimeres Eisen(III)-chlorid". Zeitschrift für anorganische und allgemeine Chemie. 348 (3–4): 142–150. doi:10.1002/zaac.19663480305.
  6. ^ J. H. Dettingmeijer, B. Meinders (1968). "Zum system W/O/J. I: das Gleichgewicht WO2, f + J2, g = WO2J2,g". Zeitschrift für anorganische und allgemeine Chemie. 357 (1–2): 1–10. doi:10.1002/zaac.19683570101.
Group 4 element

Group 4 is a group of elements in the periodic table.

It contains the elements titanium (Ti), zirconium (Zr), hafnium (Hf) and rutherfordium (Rf). This group lies in the d-block of the periodic table. The group itself has not acquired a trivial name; it belongs to the broader grouping of the transition metals.

The three Group 4 elements that occur naturally are titanium, zirconium and hafnium. The first three members of the group share similar properties; all three are hard refractory metals under standard conditions. However, the fourth element rutherfordium (Rf), has been synthesized in the laboratory; none of its isotopes have been found occurring in nature. All isotopes of rutherfordium are radioactive. So far, no experiments in a supercollider have been conducted to synthesize the next member of the group, unpentoctium (Upo, element 158), and it is unlikely that they will be synthesized in the near future.

Hafnium

Hafnium is a chemical element with symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869, though it was not identified until 1923, by Coster and Hevesy, making it the last stable element to be discovered. Hafnium is named after Hafnia, the Latin name for Copenhagen, where it was discovered.Hafnium is used in filaments and electrodes. Some semiconductor fabrication processes use its oxide for integrated circuits at 45 nm and smaller feature lengths. Some superalloys used for special applications contain hafnium in combination with niobium, titanium, or tungsten.

Hafnium's large neutron capture cross-section makes it a good material for neutron absorption in control rods in nuclear power plants, but at the same time requires that it be removed from the neutron-transparent corrosion-resistant zirconium alloys used in nuclear reactors.

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.

List of superconductors

The table below shows some of the parameters of common superconductors. X:Y means material X doped with element Y, TC is the highest reported transition temperature in kelvins and HC is a critical magnetic field in tesla. "BCS" means whether or not the superconductivity is explained within the BCS theory.

Mond process

The Mond process, sometimes known as the carbonyl process, is a technique created by Ludwig Mond in 1890, to extract and purify nickel. The process was used commercially before the end of the 19th century. This process converts nickel oxides into pure nickel.

This process involves the fact that carbon monoxide combines with nickel readily and reversibly to give nickel carbonyl. No other element forms a carbonyl compound under the mild conditions used in the process.This process has three steps:

1. Nickel oxide reacts with Syngas at 200 °C to give nickel, together with impurities including iron and cobalt.

NiO(s) + H2(g) → Ni(s) + H2O(g)2. The impure nickel reacts with carbon monoxide at 50–60 °C to form the gas nickel carbonyl, leaving the impurities as solids.

Ni(s) + 4 CO(g) → Ni(CO)4(g)3. The mixture of nickel carbonyl and Syngas is heated to 220–250 °C, resulting in decomposition back to nickel and carbon monoxide:

Ni(CO)4(g) → Ni(s) + 4 CO(g)Steps 2 and 3 illustrate a chemical transport reaction, exploiting the properties that (1) carbon monoxide and nickel readily combine to give a volatile complex and (2) this complex degrades back to nickel and carbon monoxide at higher temperatures. The decomposition may be engineered to produce powder, but more commonly an existing substrate is coated with nickel. For example, nickel pellets are made by dropping small, hot pellets through the carbonyl gas; this deposits a layer of nickel onto the pellets.

This process has also been used for plating nickel onto other metals, where a complex shape or sharp corners have made precise results difficult to achieve by electroplating. Although the results are good, the toxicity makes it impractical as an industrial process. Such parts are now plated by electroless nickel plating instead.

Niobium-germanium

Niobium-germanium (Nb3Ge) is a intermetallic chemical compound of niobium (Nb) and germanium (Ge). It has A15 phase structure.

It is a superconductor with a critical temperature of 23.2 K.

Sputtered films have been reported to have an upper critical field of 37 teslas at 4.2 K.

Solid-state chemistry

Solid-state chemistry, also sometimes referred as materials chemistry, is the study of the synthesis, structure, and properties of solid phase materials, particularly, but not necessarily exclusively of, non-molecular solids. It therefore has a strong overlap with solid-state physics, mineralogy, crystallography, ceramics, metallurgy, thermodynamics, materials science and electronics with a focus on the synthesis of novel materials and their characterisation. Solids can be classified as crystalline or amorphous on basis of the nature of order present in the arrangement of their constituent particles.

Vanadium(III) iodide

Vanadium(III) iodide is the inorganic compound with the formula VI3. This paramagnetic solid is generated by the reaction of vanadium powder with iodine at around 500 °C. The black hygroscopic crystals dissolve in water to give green solutions, characteristic of V(III) ions.

The purification of vanadium metal by the chemical transport reaction involving the reversible formation of vanadium(III) iodides in the presence of iodine and its subsequent decomposition to yield pure metal:

2 V + 3 I2 ⇌ 2 VI3VI3 crystallizes in the motif adopted by bismuth(III) iodide: the iodides are hexagonal-closest packed and the vanadium centers occupy one third of the octahedral holes.

When solid samples are heated, the gas contains VI4, which is probably the volatile vanadium component in the vapor transport method. Thermal decomposition of the triiodide leaves a residue of vanadium(II) iodide:

2 VI3 → VI2 + VI4 ΔH = 36.6 kcal/mol; ΔS = 38.7 eu

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