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Boron
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Boron is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite. Boron is never found as a free element on Earth.
Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, extremely hard (9.3 on Mohs' scale), and a weak conductor at room temperature (22-28 °C, 72-82 °F).
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Encyclopedia
Boron is a chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite. Boron is never found as a free element on Earth.
Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, extremely hard (9.3 on Mohs' scale), and a weak conductor at room temperature (22-28 °C, 72-82 °F). Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, nontoxic insecticides and preservatives, and reagents for chemical synthesis.
Boron is an essential plant nutrient, although high soil concentrations of boron may also be toxic to plants. As an ultratrace element, boron is necessary for the optimal health of rats and presumably other mammals, though its physiological role in animals is poorly understood.
Characteristics
Brown amorphous boron is a product of certain chemical reactions. It contains boron atoms that are randomly bonded to each other without long range order.
Crystalline boron, a very hard, black material with a high melting point, exists in four major polymorphs:
- Rhombohedral a-boron containing 12 atoms in the unit cell.
- Rhombohedral ß-boron -- 106.7 atoms per cell.
- Tetragonal boron -- 192 atoms per cell.
- Orthorhombic ?-boron -- 28 atoms per cell.
The ? phase is the densest (2.52 g/cm3) and the hardest (Vickers hardness 50 GPa). It can only be produced by compressing a-boron or ß-boron to 12-20 GPa and heating to 1500-1800 0C and then bringing the product to the ambient conditions. This phase was possibly produced back in 1965, but its structure has been solved only recently. It can be described as a NaCl-type arrangement of two types of clusters, B12 icosahedra and B2 pairs. There is significant charge transfer (~0.5 electrons) between these clusters, making ?-boron the only elemental solid (by 2009) with significantly ionic type of bonding.
Optical characteristics of crystalline/elemental boron include the transmittance of infrared light. At standard temperatures, elemental boron is a poor electrical conductor, but is a good electrical conductor at high temperatures.
Chemically boron is electron-deficient, possessing a vacant p-orbital. It is an electrophile. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances to compensate for boron's electron deficiency. The reactions of boron are dominated by such requirement for electrons. Also, boron is the least electronegative non-metal, meaning that it is usually oxidized (loses electrons) in reactions.
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks. Boron is also used for heat resistant alloys. Boron can form compounds whose formal oxidation state is not three, such as B(II) in B2F4.
History
Jöns Jakob Berzelius identified boron as an element in 1824. It was isolated by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808. The first pure boron was produced by the American chemist W. Weintraub in 1909, although this is disputed by some researchers.
Applications
The only major use of metallic boron is as boron fiber. The fibers are used to reinforce the fuselage of fighter aircraft, for example the B-1 bomber. The fibers are produced by vapor deposition of boron on a tungsten filament.
Glass and ceramics
Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate. In the United States 70% of the boron is used for the production of glass and ceramics. For scientific glassware, for example, borosilicate glass is used because of its capabilities to withstand chemical reactions and sudden changes in temperature. Duran and Pyrex are two major brand names under which this glass is available.
Soaps and detergents
Sodium perborate serves as a source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. It is also present in some tooth bleaching formulas.
Fire retardants
Zinc borate is used as fire retardant for plastics and rubber articles and also for cellulose insulation and in cotton mattresses.
Other uses
- After the discovery that magnesium diboride becomes superconductive at temperatures below 39 K in 2001 it took some time before the first MgB2 wires could be produced with the powder-in-tube (PIT) process. By 2008 the metallic superconductors based on niobium alloys dominated the market.
- Sodium tetraborate pentahydrate (Na2B4O7 • 5H2O), which is used in large amounts in making insulating fiberglass and sodium perborate bleach.
- Boron carbide, a ceramic material, is used to make armor materials, especially in bulletproof vests for soldiers and police officers.
- Orthoboric acid (H3BO3) or boric acid is used in the production of textile fiberglass, flat panel displays and eye drops.
- Sodium tetraborate decahydrate (Na2B4O7 • 10H2O) or borax is used in the production of adhesives and in anti-corrosion systems.
- Boron nitride is a material in which the extra electron of nitrogen (with respect to carbon) enables it to form structures that are isoelectronic with carbon allotropes.
- Boron reacts with ammonia at high temperatures to give a compound called borazole (B3N3H6), also known as inorganic benzene.
- Triethylborane was used to ignite the JP-7 fuel of the Pratt / Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird.
- Boron is an essential plant micronutrient.
- Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.
- Boric acid is an important compound used in textile products.
- Boric acid is traditionally used as an insecticide, notably against ants, fleas, and cockroaches.
- A mixture of borax (sodium tetraborate decahydrate) and ammonium chloride is used as a flux for welding ferrous metals.
- Borax is also used alone as a flux for soldering silver and gold.
- Borax is sometimes found in laundry detergent.
- Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures as a component of composite materials, as well as limited production consumer and sporting goods such as golf clubs and fishing rods.
- Boron is used as a melting point depressant in nickel-chromium braze alloys.
- Boron slurry is used as an energetic material with very high energy density like rocket fuels and fuel for jet engines.
- Boron compounds show promise in treating arthritis.
- Due to its high neutron cross-section, boron is often used to control fission in nuclear reactors.
- Sodium tetraborate pentahydrate is used as a water clarifier in swimming pool water treatment.
- Boron pellets could be used to power non-polluting cars equipped with "tame combustion" engines that generate heat through the chemical reaction of boron and oxygen. The spent fuel pellets, boria, would be recycled at solar-powered generating stations.
Hardest boron compounds
The hardest boron compounds are created synthetically. Rhenium diboride (ReB2) and cubic (or beta)-boron nitride can actually scratch diamond, but are still not as hard as diamond although rhenium diboride surpasses diamond in certain directions. Rhenium diboride has been reported to be nearly as hard as cubic boron nitride and boron suboxide, and much harder than osmium diboride (which was the first step towards rhenium diboride synthesis ).
It is still a matter of research as to which boron compound is the hardest:
- Heterodiamond (called also BCN, it is supposed to be an improvement of boron nitride, although it contains carbon).
- Cubic boron nitride (CBN or borazon, the latter being the commercial name. Discovered in 1957).
- Rhenium diboride (as ReB2 synthesis requires considerably less pressure compared to cubic boron nitride. It is seen as an interesting economical alternative to CBN for the industry). However, the validity of the hardness measurements published for rhenium diboride have been questioned by some since an unusually light indenter load was used in the measurements published.
- AlMgB14 + TiB2 composites possess high hardness and wear resistance and are used in either bulk form or as coatings for components exposed to severe environments.
Like all superhard materials with properties similar to diamond, these boron-based materials do not possess a unique hardness value but rather a hardness range. This is because hardness tests (e.g. Knoop, Vickers, Rockwell) depend on many conditions (direction, load...) according to whether the diamond used in the test will indent more or less deeply the given material. As a result, they all scratch each other as well as diamond under certain conditions.
These borides have been primarily developed as a substitute for diamond in coated tools (CVD or PVD diamond-like coated), as well as diamond powder coated blades, since diamond becomes soluble in iron and unstable at high temperatures, thus reducing tool life.
Interestingly enough, boron nitride in its hexagonal form (h-BN), is a very soft material (only 2 in Mohs hardness scale) compared to the cubic form, h-BN being slightly more inert chemically than c-BN at very high temperatures, a feature extremely useful in advanced foundry and casting refractory applications (high end crucibles).
At a lesser degree, certain boronized (or borided) metals and alloys, through means of ion implantation or only ion beam deposition of boron ions, show a spectacular increase in surface resistance and microhardness, thus having superficial characteristics similar to the corresponding borides. Laser alloying has also been successfully used for the same purpose. Atomic penetration of materials (aforementioned laser and implantation methods) is preferred over deposition methods (CVD deposition and PVD deposition) since the borides are formed "within" the metallic substrate (the ions literally penetrate the metal), relatively deep from the surface.
Ultrahard fullerite, aggregated diamond nanorods and synthetic flawless diamond are still being considered as the undisputed hardest materials. None of these contain boron.
Boron compounds
The boron oxygen compounds sodium tetraborate, boric acid and sodium perborate are the compounds with the largest production capacities. For some special applications the carbide and nitride boron carbide and boron nitride are produced. Sodium borohydride is one of the few hydrides produced on industrial scale as reduction reagent in chemical synthesis.
Occurrence
The world wide commercial borate deposits are estimated to be 1010 kg of boron.
Turkey and the United States are the world's largest producers of boron.Turkey has almost 72% of the world’s boron potential and boron reserves. Boron does not appear on Earth in elemental form but is found combined in borax, boric acid, colemanite, kernite, ulexite and borates. Boric acid is sometimes found in volcanic spring waters. Ulexite is a borate mineral that naturally has properties of fiber optics.
Economically important sources are from the ore rasorite (kernite) and tincal (borax ore) which are both found in the Mojave Desert of California, with borax being the most important source there. The largest borax deposits are found in Central and Western Turkey including the provinces of Eskisehir, Kütahya and Balikesir.
See also: Borate minerals.
Production
Pure elemental boron is not easy to prepare. The earliest methods used involve reduction of boric oxide with metals such as magnesium or aluminum. However the product is almost always contaminated with metal borides. (The reaction is quite spectacular though.) Pure boron can be prepared by reducing volatile boron halogenides with hydrogen at high temperatures. The highly pure boron, for the use in semiconductor industry, is produced by the decomposition of diborane at high temperatures and then further purified with the Czochralski process.
Isotope enrichment
Due to the use of boron-10 in nuclear reactors as neutron-capturing substance, several industrial-scale enrichment process have been developed. Although nearly all possible enrichment methods are adaptable to boron enrichment, only the fractionated vacuum distillation of the dimethyl ether adduct of boron trifluoride (DME-BF3) and column chromatography of borates are used.
Market trend
Estimated global consumption of boron rose to a record 1.8 million tonnes of B2O3 in 2005, following a period of strong growth in demand from Asia, Europe and North America. Boron mining and refining capacities are considered to be adequate to meet expected levels of growth through the next decade.
The form in which boron is consumed has changed in recent years. The use of ores like colemanite has declined following concerns over arsenic content. Consumers have moved towards the use of refined borates or boric acid that have a lower pollutant content. The average cost of crystalline boron is $5/g.
Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine Company of Turkey opened a new 100,000 tonnes per year capacity boric acid plant at Emet in 2003. Rio Tinto increased the capacity of its boron plant from 260,000 tonnes per year in 2003 to 310,000 tonnes per year by May 2005, with plans to grow this to 366,000 tonnes per year in 2006.
Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. This has led to imports of disodium tetraborate growing by a hundredfold between 2000 and 2005 and boric acid imports increasing by 28% per year over the same period.
The rise in global demand has been driven by high rates of growth in fiberglass and borosilicate production. A rapid increase in the manufacture of reinforcement-grade fiberglass in Asia with a consequent increase in demand for borates has offset the development of boron-free reinforcement-grade fiberglass in Europe and the USA. The recent rises in energy prices can be expected to lead to greater use of insulation-grade fiberglass, with consequent growth in the use of boron.
Roskill Consulting Group forecasts that world demand for boron will grow by 3.4% per year to reach 21 million tonnes by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.
Boron in biology
A boron-containing natural antibiotic, boromycin, isolated from streptomyces, is known.
Boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. Conversely, high soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron in the soil is greater than 1.8 ppm. When boron in the soil exceeds 2.0 ppm, few plants will perform well. Plants sensitive to boron in the soil may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear.
As an ultratrace element, boron is necessary for the optimal health of rats, although it is necessary in such small amounts that ultrapurified foods and dust filtration of air is necessary to show the effects of boron deficiency, which manifest as poor coat/hair quality. Presumably, boron is necessary to other mammals. No deficiency syndrome in humans has been described. Small amounts of boron occur widely in the diet, and the amounts needed in the diet would, by analogy with rodent studies, be very small. The exact physiological role of boron in the animal kingdom is poorly understood.
Boron occurs in all foods produced from plants. Since 1989 its nutritional value has been argued. It is thought that boron plays several biochemical roles in animals, including humans.
The U.S. Department of agriculture conducted an experiment in which postmenopausal women took 3 mg of boron a day. The results showed that supplemental boron reduced excretion of calcium by 44%, and activated estrogen and vitamin D. However, whether these effects were conventionally nutritional, or medicinal, could not be determined.
The US National Institute of Health quotes this source:
- Total daily boron intake in normal human diets ranges from 2.1–4.3 mg boron/kg body weight (bw)/day.
Analytical quantification
For determination of boron content in food or materials the colorimetric curcumin method is used. Boron has to be transferred to boric acid or borates and on reaction with curcumin in acidic solution, a red colored boron-chelate complex, rosocyanine, is formed.
Isotopes
Boron has two naturally-occurring and stable isotopes, (80.1%) and (19.9%). The mass difference results in a wide range of d values in natural waters, ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is which decays through proton emission and alpha decay. It has a half-life of 3.5×10-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)3 and B(OH)4. Boron isotopes are also fractionated during mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect species preferential removal of the (OH)4 ion onto clays results in solutions enriched in (OH)3 may be responsible for the large enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.
The exotic exhibits a nuclear halo.
Enriched boron (boron-10)
The isotope is good at capturing thermal neutrons. Natural boron is about 20% and 80%. The nuclear industry enriches natural boron to nearly pure . The waste product, or depleted boron, is nearly pure . is a candidate as a fuel for aneutronic fusion and is used in the semiconductor industry. Enriched boron or is used in both radiation shielding and in boron neutron capture therapy. In the latter, a compound containing is attached to a muscle near a tumor. The patient is then treated with a relatively low dose of thermal neutrons. This causes energetic and short range alpha radiation from the boron to bombard the tumor.
In nuclear reactors, is used for reactivity control and in emergency shutdown systems. It can serve either function in the form of borosilicate control rods or as boric acid. In pressurized water reactors, boric acid is added to the reactor coolant when the plant is shut down for refueling. It is then slowly filtered out over many months as fissile material is used up and the fuel becomes less reactive.
In future manned interplanetary spacecraft, has a theoretical role as structural material (as boron fibers or BN nanotube material) which also would serve a special role in the radiation shield. One of the difficulties in dealing with cosmic rays, which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft structural materials, is in the form of high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as structural polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in a way which dumps the absorption energy in the shielding, far away from biological systems. Among light elements that absorb thermal neutrons, and appear as potential spacecraft structural materials able to do double duty in this regard.
Depleted boron (boron-11)
Cosmic radiation produces secondary neutrons when it hits spacecraft structures. Neutrons cause fission in if it is present in the spacecraft's semiconductors. This produces a gamma ray, an alpha particle, and a lithium ion. The resultant fission products may then dump charge into nearby chip structures, causing data loss (bit flipping, or single event upset). In radiation hardened semiconductor designs, one measure is to use depleted boron which is greatly enriched in and contains almost no . is largely immune to radiation damage. Depleted boron is a by-product of the nuclear industry.
is also a candidate as a fuel for aneutronic fusion. When struck by a proton of about 500 keV, it produces three alpha particles and 8.7 MeV of energy. Most other fusion reactions involving hydrogen and helium produce penetrating neutron radiation. This induces long term radioactivity in reactor structures and weakens them, as well as endangering operating personnel. The alpha particles from fusion can be turned directly into electric power and all radiation stops as soon as the reactor is turned off.
Boron in NMR spectroscopy
Both and possess nuclear spin. The nuclear spin of boron-10 is 3 and that of boron-11 is 3/2. These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nucleus are available commercially. The boron-10 and boron-11 nuclei also cause splitting in the resonances of attached nuclei.
Precautions
Elemental boron is nontoxic and common boron compounds such as borates and boric acid have low toxicity (approximately similar to table salt with the lethal dose being 2 to 3 grams per kilogram) and therefore do not require special precautions while handling. Some of the more exotic boron hydrogen compounds, however, are toxic as well as highly flammable and do require special care when handling.
See also
External links
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