Boron

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*Boron-10 content may be as low as 19.1% and as
high as 20.3% in natural samples. Boron-11 is
the remainder in such cases.

Boron is a chemical element in the periodic table that has the symbol B and atomic number 5. A trivalent metalloid element, boron occurs abundantly in the ore borax. There are several allotropes of boron; amorphous boron is a brown powder, but metallic boron is black. The metallic form is hard (9.3 on Mohs' scale) and a poor conductor at room temperature. It is never found free in nature. Crystalline boron exists in many polymorphs. Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the three most characterised crystalline forms.

Notable characteristics

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 only non-metal that usually is oxidized (losing electrons) in reactions.

Optical characteristics of this element include the transmittance of infrared light. At standard temperatures boron is a poor electrical conductor but is a good conductor at high temperatures.

Boron nitride can be used to make materials that are almost as hard as diamond. The nitride also acts as an electrical insulator but conducts heat similarly to a metal. This compound exists in a second form that has lubricating qualities that are similar to graphite. Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks.

Applications

The most economically important compounds of boron are:

Sodium tetraborate pentahydrate (Na₂B₄O₇ · 5H₂O), which is used in large amounts in making insulating fiberglass and sodium perborate bleach,
Orthoboric acid (H₃BO₃) or boric acid, used in the production of textile fiberglass and flat panel displays or eye drops, among many uses, and
Sodium tetraborate decahydrate (Na₂B₄O₇ · 10H₂O) or borax, used in the production of adhesives, in anti-corrosion systems and many other uses.

Of the several hundred uses of boron compounds, one can cite the following ones:

Boron being an essential micronutrient, playing notably a role in plant fertilisation and in the building of cell wall structures, it is used in agriculture.
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 also traditionally used as an insecticide, notably against ants or cockroaches.
Compounds of boron are used extensively in organic synthesis and in the manufacture of borosilicate and borophophosilicate glasses.
Other compounds are used as wood preservatives, and are particularly attractive in this regard because they possess low toxicity.
¹⁰B is used to assist control of nuclear reactors, a shield against radiation and in neutron detection.
Purified ¹¹B (depleted boron) is used for borosilicate glasses in rad-hard electronics.
Research is being conducted into fusion power by interaction of hydrogen and boron. Potential benefits include relatively small and uncomplicated reactors and supposedly greater safety.
Boron filaments are high-strength, lightweight materials that are chiefly used for advanced aerospace structures as a component of composite materials.
Sodium borohydride (NaBH₄), is a popular chemical reducing agent, used (for example) for reducing aldehydes and ketones to alcohols.
Boron in trace amounts is used as dopant for P-type semiconductors.

Boron compounds are being investigated for use in a broad range of applications, including as components in sugar-permeable membranes, carbohydrate sensors and bioconjugates. Medicinal applications being investigated include boron neutron capture therapy and drug delivery. Other boron compounds show promise in treating arthritis.

Hydrides of boron are oxidized easily and liberate a considerable amount of energy. They have therefore been studied for use as possible rocket fuels, along with elemental boron. However, issues of cost, incomplete combustion, and boric oxide deposits seem to make it infeasible.

Boron possesses many interesting compounds with nitrogen. These include boron nitride (BN)(as mentioned above). This compund is composed of layers of fused hexagonal sheets (analogous to graphite). These sheets (unlike those in graphite) are in registry. This means that layers are placed directly upon one another such that a viewer looking down onto the structure would view only the top layer. The polar B-N bonds mean that boron nitride is not an electrical conductor (in contrast to graphite which is).

History

Compounds of boron (Arabic Buraq from Persian Burah) have been known of for thousands of years. In early Egypt, mummification depended upon an ore known as natron, which contained borates as well as some other common salts. Borax glazes were used in China from 300 AD, and boron compounds were used in glassmaking in ancient Rome.

The element was not isolated until 1808 by Sir Humphry Davy, Joseph Louis Gay-Lussac, and Louis Jacques Thénard, to about 50 percent purity, by the reduction of boric acid with sodium or magnesium. These men did not recognize the substance as an element. It was Jöns Jakob Berzelius in 1824 that identified boron as an element. The first pure boron was produced by the American chemist W. Weintraub in 1909, which is doubted by some researchers.

Boron was not believed to be useful to the human body until 1989 research suggested its signficance.

Occurrence

The United States and Turkey are the world's largest producers of boron. Boron does not appear in nature 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. Turkey is another place where extensive borax deposits are found.

Even a boron-containing natural antibiotic, boromycin, isolated from streptomyces, is known.

Pure elemental boron is not easy to prepare. The earliest methods used involve reduction of boric oxide with metals such as magnesium or aluminium. 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 than further purified with the Czochralski process.

In 1997 crystalline boron (99% pure) cost about States dollar|US$" target="_blank" >^*5 per gram and amorphous boron cost about US$2 per gram.

Food

Boron occurs in all foods produced by plants. Since 1989 its nutritional value has been argued. The U.S. Department of agriculture conducted an experiment in which postmenopausal women took 3 mg of boron a day. The results showed that boron can drop excretion of calcium by 44%, and activate estrogen and vitamin D.

See also Borate minerals.

Analytical quantification

For determination of boron content in food or materials the colorimetric curcumin method is used. Boron has to be transfered to boric acid or borates and on reaction with curcumin in acidic solution a red colored boron-chelate complex - rosocyanine - is formed.

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 beneficiated 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.

Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine 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 fibreglass and borosilicate production. A rapid increase in the manufacture of reinforcement-grade fibreglass in Asia with a consequent increase in demand for borates has offset the development of boron-free reinforcement-grade fibreglass in Europe and the USA. The recent rises in energy prices can be expected to lead to greater use of insulation-grade fibreglass, 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.^*

Isotopes

Boron has two naturally-occurring and stable isotopes, ¹¹B (80.1%) and ¹⁰B (19.9%). The mass difference results in a wide range of δ¹¹B values in natural waters, ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is ⁷B which decays through proton emission and alpha decay. It has a half-life of 3.26500x10^-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(O H)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 ¹⁰B(OH)₄ ion onto clays results in solutions enriched in ¹¹B(OH)₃ may be responsible for the large ¹¹B enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.

Depleted boron

The ¹⁰B isotope is good at capturing thermal neutrons from cosmic radiation or in pressurized water reactors. It then undergoes fission - producing a gamma ray, an alpha particle, and a lithium ion. When this happens inside of an integrated circuit, the fission products may then dump charge into nearby chip structures, causing data loss (bit flipping, or single event upset). In critical semiconductor designs, depleted boron -- consisting almost entirely of ¹¹B -- is used to avoid this effect, as one of radiation hardening measures. ¹¹B is a by-product of the nuclear industry.

Precautions

Elemental boron and borates are not toxic 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 handling care.

References

Los Alamos National Laboratory – Boron

External links

Chemical elements | Metalloids

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