boron

beryllium ← boron → carbon - ↑ B ↓ Al 5B Periodic table
Appearance
black/brown
General properties
Name, symbol, number	boron, B, 5
Element category	metalloid
Group, period, block	13, 2, p
Standard atomic weight	10.811(7) g·mol−1
Electron configuration	[He] 2s2 2p1
Electrons per shell	2, 3 (Image)
Physical properties
Phase	solid
Liquid density at m.p.	2.08 g·cm−3
Melting point	2349 K, 2076 °C, 3769 °F
Boiling point	4200 K, 3927 °C, 7101 °F
Heat of fusion	50.2 kJ·mol−1
Heat of vaporization	480 kJ·mol−1
Specific heat capacity	(25 °C) 11.087 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K 2348 2562 2822 3141 3545 4072
Atomic properties
Oxidation states	4,[1] 3, 2, 1[2] (mildly acidic oxide)
Electronegativity	2.04 (Pauling scale)
Ionization energies (more)	1st: 800.6 kJ·mol−1
	2nd: 2427.1 kJ·mol−1
	3rd: 3659.7 kJ·mol−1
Atomic radius	90 pm
Covalent radius	84±3 pm
Van der Waals radius	192 pm
Miscellanea
Magnetic ordering	diamagnetic[3]
Electrical resistivity	(20 °C) ~106Ω·m
Thermal conductivity	(300 K) 27.4 W·m−1·K−1
Thermal expansion	(25 °C) (ß form) 5–7 [4] µm·m−1·K−1
Speed of sound (thin rod)	(20 °C) 16,200 m/s
Mohs hardness	~9.5
CAS registry number	7440-42-8
Most stable isotopes
Main article: Isotopes of boron
iso NA half-life DM DE (MeV) DP 10B 19.9(7)%* 10B is stable with 5 neutrons[5] 11B 80.1(7)%* 11B is stable with 6 neutrons[5] *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.[6]
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Boron (pronounced /ˈbɔrɒn/) is the 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.

Several allotropes of boron exist: amorphous boron is a brown powder; whereas crystalline boron is black, extremely hard (about 9.5 on Mohs' scale), and a poor conductor at room temperature. Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, insecticides and preservatives, and reagents for chemical synthesis.

Boron is an essential plant nutrient. Whereas lack of boron results in boron deficiency disorder, 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 not yet fully understood.

Characteristics

Allotropes

Amorphous boron

Boron is similar to carbon in its capability to form stable covalently bonded molecular networks. Even nominally disordered (amorphous) boron contains regular boron icosahedra which are, however, bonded randomly to each other without long-range order.^[7][8] Crystalline boron is a very hard, black material with a high melting point of above 2000 °C. It exists in four major polymorphs: α, β, γ and T. Whereas α, β and T phases are based on B₁₂ icosahedra, the γ-phase can be described as a rocksalt-type arrangement of the icosahedra and B₂ atomic pairs.^[9] It can be produced by compressing other boron phases to 12-20 GPa and heating to 1500-1800 °C; it remains stable after releasing the temperature and pressure. The T phase is produced at similar pressures, but higher temperatures of 1800-2200 °C. As to the α and β phases, they might both coexist at ambient conditions with the β phase being more stable.^[10][9] Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures 6-12 K.^[11]

Chemical

Chemically, boron is closer to silicon than to aluminium. Crystalline boron is chemically inert and resistant to attack by boiling hydrofluoric or hydrochloric acid. When finely divided, it is attacked slowly by hot concentrated hydrogen peroxide, hot concentrated nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids.^[20][21]

Oxidation of boron depends upon the crystallinity, particle size, purity and temperature. Boron does not react with air at room temperature, but at higher temperatures it burns to form boron trioxide:

4 B + 3 O₂ (g) → 2 B₂O₃ (s)

Boron reacts with sulfur to produce boron sulfide:

2 B + 3 S (g) → B₂S₃ (s)

The first synthesis was performed by Jöns Jakob Berzelius in 1824. Another reaction, starting from boron and hydrogen sulfide, was conducted by Friedrich Wöhler and Henri Etienne Sainte-Claire Deville and published in 1858.^[22][23]

2 B + 3 H₂S → B₂S₃ (s) + 3 H₂

Wöhler and Deville also documented vigorous reactions between boron and the halogens resulting in boron trichloride, boron trifluoride and boron tribromide.^[23] For example:

2 B + 3 Br₂ → 2 BBr₃

Boron can form compounds whose formal oxidation state is not three, such as B(IV) in boron carbide BC,^[1] B(II) in B₂F₄,^[24] and B(I) in boron fluoride BF.^[2] Boron compounds such as BCl₃ behave as electrophiles or Lewis acids in their reactions.^[25] Boron is the least electronegative non-metal.^[26]

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, which are defined as a fractional difference between the ¹¹B and ¹⁰B and traditionally expressed in parts per thousand, 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 3.5×10⁻²² 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 results in preferential removal of the ¹⁰B(OH)₄ ion onto clays. It results in solutions enriched in ¹¹B(OH)₃ and therefore 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. ^[27] The exotic ¹⁷B exhibits a nuclear halo, i.e. its radius is appreciably larger than that predicted by the liquid drop model.^[28]

Enriched boron (boron-10)

Neutron cross section of boron (top curve is for ¹⁰B and bottom curve for ¹¹B)

The ¹⁰B isotope is good at capturing thermal neutrons. Natural boron is about 20% ¹⁰B and 80%¹¹B. The nuclear industry enriches natural boron to nearly pure ¹⁰B. The waste product, or depleted boron, is nearly pure ¹¹B. ¹¹B is a candidate as a fuel for aneutronic fusion and is used in the semiconductor industry. Enriched boron or ¹⁰B is used in both radiation shielding and in boron neutron capture therapy. In the latter, a compound containing ¹⁰B 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.^[29][30][31]

In nuclear reactors, ¹⁰B 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.^[32]

In future manned interplanetary spacecraft, ¹⁰B has a theoretical role as structural material (as boron fibers or BN nanotube material) which would also 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 materials is high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in the shielding. Among light elements that absorb thermal neutrons, ⁶Li and ¹⁰B appear as potential spacecraft structural materials which serve both for mechanical reinforcement and radiation protection.^[33]

Depleted boron (boron-11)

Cosmic radiation will produce secondary neutrons if it hits spacecraft structures; and neutrons cause fission in ¹⁰B if it is present in the spacecraft's semiconductors, producing a gamma ray, an alpha particle, and a lithium ion. The resultant fission products may then dump charge into nearby semiconductor 'chip' structures, causing data loss (bit flipping, or single event upset). In radiation hardened semiconductor designs, one countermeasure is to use depleted boron which is greatly enriched in ¹¹B and contains almost no ¹⁰B. ¹¹B is largely immune to radiation damage. Depleted boron is a by-product of the nuclear industry.^[32]

¹¹B is also a candidate as a fuel for aneutronic fusion. When struck by a proton with energy 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, which weakens reactor structures and induces long term radioactivity thereby endangering operating personnel. Whereas, the alpha particles from ¹¹B fusion can be turned directly into electric power, and all radiation stops as soon as the reactor is turned off.^[34]

NMR spectroscopy

Both ¹⁰B and ¹¹B possess nuclear spin. The nuclear spin of ¹⁰B is 3 and that of ¹¹B is 3/2. These isotopes are, therefore, of use in nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to detecting the boron-11 nuclei are available commercially. The ¹⁰B and ¹¹B nuclei also cause splitting in the resonances of attached nuclei.^[35]

Occurrence

A fragment of ulexite

Borax crystals

Boron is a relatively rare element in the Earth's crust, representing only 0.001%. The worldwide commercial borate deposits are estimated as 10 million tonnes.^[36][37] Turkey and the United States are the world's largest producers of boron.^[38][39] Turkey has almost 72% of the world’s boron reserves.^[40] 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; it is a fibrous crystal where individual fibers can guide light like optical fibers.^[41]

Economically important sources of boron are rasorite (kernite) and tincal (borax ore). They are both found in the Mojave Desert of California, but the largest borax deposits are in Central and Western Turkey including the provinces of Eskişehir, Kütahya and Balıkesir.^[42][43][44]

History and etymology

Sassolite

The name boron originates from the Arabic word buraq or the Persian word burah;^[45] which are names for the mineral borax.^[46]

Boron compounds were known thousands of years ago. Borax was known from the deserts of western Tibet, where it received the name of tincal, derived from the Sanskrit. Borax glazes were used in China from AD300, and some tincal even reached the West, where the Arabic alchemist Geber seems to mention it in 700. Marco Polo brought some glazes back to Italy in the 13th century. Agricola, around 1600, reports its use as a flux in metallurgy. In 1777, boric acid was recognized in the hot springs (soffioni) near Florence, Italy, and became known as sal sedativum, with mainly medical uses. The rare mineral is called sassolite, which is found at Sasso, Italy. This was the main source of European borax from 1827 to 1872, at which date American sources replaced it.^[47][48]

Boron was not recognized as an element until it was isolated by Sir Humphry Davy, Joseph Louis Gay-Lussac and Louis Jacques Thénard in 1808 through the reaction of boric acid and potassium. Davy called the element boracium.^[49] Jöns Jakob Berzelius identified boron as an element in 1824. The first pure boron was arguably produced by the American chemist W. Weintraub in 1909.^[50][21]

Production

Pure elemental boron is difficult to extract. The earliest methods involved reduction of boric oxide with metals such as magnesium or aluminium. However the product is almost always contaminated with metal borides. Pure boron can be prepared by reducing volatile boron halides with hydrogen at high temperatures. Ultrapure boron, for the use in semiconductor industry, is produced by the decomposition of diborane at high temperatures and then further purified with the zone melting or Czochralski processes.^[51]

Isotope enrichment

Because of its high neutron cross-section, boron-10 is often used to control fission in nuclear reactors as a neutron-capturing substance. ^[52] Several industrial-scale enrichment processes have been developed, however only the fractionated vacuum distillation of the dimethyl ether adduct of boron trifluoride (DME-BF₃) and column chromatography of borates are being used. ^[53]

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 and boric acid that have a lower pollutant content. The average cost of crystalline boron is $5/g.^[54]

Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine Company of Turkey opened a new boric acid plant with the production capacity of 100,000 tonnes per year at Emet in 2003. Rio Tinto Group 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.^[55][56]

The rise in global demand has been driven by high growth rates 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 may lead to greater use of insulation-grade fiberglass, with consequent growth in the boron consumption. 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.^[55][57]

Applications

Glass and ceramics

Borosilicate glassware. Displayed are two beakers and a test tube.

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.^[58] Borosilicate glass, which is typically 12%-15% B₂O₃, 80% SiO₂, and 2% Al₂O₃, has a low coefficient of thermal expansion giving it a good resistance to thermal shock. Duran and Pyrex are two major brand names for this glass.^[59]

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.^[60][61] The fibers can be produced by chemical vapor deposition of boron on a tungsten filament.^[38][62]

Boron fibers and sub-millimeter sized crystalline boron springs are produced by laser-assisted chemical vapor deposition. Translation of the focused laser beam allows to produce even complex helical structures. Such structures show good mechanical properties (elastic modulus 450 GPa, fracture strain 3.7 %, fracture stress 17 GPa) and can be applied as reinforcement of ceramics or in micromechanical systems.^[63]

Semiconductor industry

Boron is an important technological dopant for such important semiconductors as silicon, germanium and silicon carbide. Having one less valence electron than the host atom, it donates a hole resulting in p-type conductivity. Traditional method of introducing boron into semiconductors is via its atomic diffusion at high temperatures. This process uses either solid (B₂O₃), liquid (BBr₃) or gaseous boron sources (B₂H₆ or BF₃). However, after 1970s, it was mostly replaced by ion implantation, which relies mostly on BF₃ as a boron source.^[64] Boron trichloride gas is also an important chemical in semiconductor industry, however not for doping but rather for plasma etching of metals and their oxides.^[65]

Engineering materials

Boron carbide is used for inner plates of ballistic vests

Boron carbide, a ceramic material which is obtained by decomposing B₂O₃ with carbon in the electric furnace:

2 B₂O₃ + 7 C → B₄C + 6 CO

It is used in tank armor, bulletproof vests, and numerous other structural applications. Its ability to absorb neutrons without forming long lived radionuclides makes the material attractive as an absorbent for neutron radiation arising in nuclear power plants. Nuclear applications of boron carbide include shielding, control rod and shut down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.^[66]

Magnesium diboride is an important superconducting material with the transition temperature of 39 K. MgB₂ wires are produced with the powder-in-tube process and applied in superconducting magnets.^[67][68]

Boron is a part of neodymium magnet (Nd₂Fe₁₄B), which is the strongest type of permanent magnet. It is found in all kinds of domestic and professional electromechanical and electronic devices, such as magnetic resonance imaging (MRI), various motors and actuators, computer HDDs, CD and DVD players, mobile phones, timer switches, speakers, etc.^[3]

High-hardness compounds

Several boron compounds are known for their extreme hardness and toughness, including

• Heterodiamond (also called BCN);
• Boron nitride. This material is isoelectronic to carbon. Similar to carbon, it has both hexagonal (soft graphite-like h-BN) and cubic (hard, diamond-like c-BN) forms. h-BN is used as a high temperature component and lubricant. c-BN, also known under commercial name borazon,^[71] is a superior abrasive. Its hardness is only slightly smaller, but chemical stability is superior to that of diamond.
• Rhenium diboride can be produced at ambient pressures, but is rather expensive because of rhenium. The hardness of ReB₂ exhibits considerable anisotropy because of its hexagonal layered structure. Its value is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.^[72]
• AlMgB₁₄ + TiB₂ composites possess high hardness and wear resistance and are used in either bulk form or as coatings for components exposed to high temperatures and wear loads. ^[73]

Boron carbide and cubic boron nitride powders are widely used as abrasives. Metal borides are used for coating tools through chemical vapor deposition or physical vapor deposition. Implantation of boron ions into metals and alloys, through ion implantation or ion beam deposition, results in a spectacular increase in surface resistance and microhardness. Laser alloying has also been successfully used for the same purpose. These borides are an alternative to diamond coated tools, and their (treated) surfaces have similar properties to those of the bulk boride. ^[74]

In chemistry

Navy emergency flare

Launch of Apollo 15

Sodium tetraborate pentahydrate (Na₂B₄O₇ • 5H₂O) is used in large amounts in making insulating fiberglass and sodium perborate bleach.^[75] Sodium tetraborate decahydrate (Na₂B₄O₇ • 10 H₂O) can be found in adhesives and in anti-corrosion systems.^[76] Sodium borates are used as a flux for soldering silver and gold and with ammonium chloride for welding ferrous metals.^[77] They are also fire retarding additives to plastics and rubber articles.^[78] 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.^[75]

Boric acid (also known as orthoboric acid) H₃BO₃ is used in the production of textile fiberglass and flat panel displays.^[79] It also has antiseptic, antifungal, and antiviral properties and for this reasons is applied as a water clarifier in swimming pool water treatment.^[80] Boric acid is also traditionally used as an insecticide, notably against ants, fleas, and cockroaches.^[81]

Triethylborane is a substance which ignites the JP-7 fuel of the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71 Blackbird.^[82] It was also used to ignite the F-1 Engines on the Saturn V Rocket utilized by NASA's Apollo and Skylab programs from 1967 until 1973. Triethylborane is suitable for this because of its pyrophoric properties, especially the fact that it burns with very high temperature.^[83] Triethylborane is an industrial initiator in radical reactions, where it is effective even at low temperatures. It is also injected into vapor deposition reactors as a boron source. Examples are the plasma deposition of boron-containing hard carbon films, silicon nitride-boron nitride films, and for doping of diamond film with boron.^[84]

Boron compounds show promise in treating arthritis.^[85] Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.^[86] It is also used as a melting point depressant in nickel-chromium braze alloys.^[87]

Biological role

There is a boron-containing natural antibiotic, boromycin, isolated from streptomyces.^[88][89] 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 content in the soil is greater than 1.8 ppm. When this content exceeds 2.0 ppm, few plants will perform well and some may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear.^[90][91][92]

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.^[93]

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.^[94] 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 Institutes 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. ^[95][96]

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.^[97]

Health issues

Elemental boron and borates are non-toxic to humans and animals (approximately similar to table salt). The LD₅₀ (dose at which there is 50% mortality) for animals is about 6 g per kg of body weight. Substances with LD₅₀ above 2 g are considered non-toxic. The minimum lethal dose for humans has not been established, but an intake of 4 g/day was reported without incidents, and medical dosages of 20 g of boric acid for neutron capture therapy caused no problems. Fish have survived for 30 min in a saturated boric acid solution and can survive longer in strong borax solutions.^[98] Borates are more toxic to insects than to mammals. The boranes and similar gaseous compounds are quite poisonous. As usual, it is not an element that is intrinsically poisonous, but toxicity depends on structure.^[47][48]

The boranes (boron hydrogen compounds) are toxic as well as highly flammable and require special care when handling. Sodium borohydride presents a fire hazard due to its reducing nature, and the liberation of hydrogen on contact with acid. Boron halides are corrosive.^[99]

Congenital endothelial dystrophy type 2, a rare form of corneal dystrophy, is linked to mutations in SLC4A11 gene that encodes a transporter reportedly regulating the intracellular concentration of boron.^[100]

See also

References

External links

Wikimedia Commons has media related to: Boron

Look up boron in Wiktionary, the free dictionary.

• Boron
• WebElements.com – Boron
• National Pollutant Inventory - Boron and compounds