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Boron is a chemical element with the symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in the Solar System and in the Earth's crust. It constitutes about 0.001 percent by weight of Earth’s crust. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax and kernite. The largest known boron deposits are in Turkey, the largest producer of boron minerals.
Elemental boron is a metalloid that is found in small amounts in meteoroids but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, very pure boron is produced with difficulty because of refractory contamination by carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature. The primary use of elemental boron is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all boron consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and refractory materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. Boron as sodium perborate is used as a bleach. A small amount of boron is used as a dopant in semiconductors, and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which (boron-10) has a number of uses as a neutron-capturing agent.
In biology, borates have low toxicity in mammals (similar to table salt), but are more toxic to arthropods and are used as insecticides. Boric acid is mildly antimicrobial, and several natural boron-containing organic antibiotics are known.[13] Boron is an essential plant nutrient and boron compounds such as borax and boric acid are used as fertilizers in agriculture, although it's only required in small amounts, with excess being toxic. Boron compounds play a strengthening role in the cell walls of all plants. There is no consensus on whether boron is an essential nutrient for mammals, including humans, although there is some evidence it supports bone health.
en.wikipedia.org
that is found in small amounts in meteoroids but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, very pure boron is produced with difficulty because of refractory contamination by carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature. The primary use of elemental boron is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all boron consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and refractory materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. Boron as sodium perborate is used as a bleach. A small amount of boron is used as a dopant in semiconductors, and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which (boron-10) has a number of uses as a neutron-capturing agent.
In biology, borates have low toxicity in mammals (similar to table salt), but are more toxic to arthropods and are used as insecticides. Boric acid is mildly antimicrobial, and several natural boron-containing organic antibiotics are known. Boron is an essential plant nutrient and boron compounds such as borax and boric acid are used as fertilizers in agriculture, although it's only required in small amounts, with excess being toxic. Boron compounds play a strengthening role in the cell walls of all plants. There is no consensus on whether boron is an essential nutrient for mammals, including humans, although there is some evidence it supports bone health.
Applications
Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate pentahydrate. In the United States, 70% of the boron is used for the production of glass and ceramics. The major global industrial-scale use of boron compounds (about 46% of end-use) is in production of glass fiber for boron-containing insulating and structural fiberglasses, especially in Asia. Boron is added to the glass as borax pentahydrate or boron oxide, to influence the strength or fluxing qualities of the glass fibers. Another 10% of global boron production is for borosilicate glass as used in high strength glassware. About 15% of global boron is used in boron ceramics, including super-hard materials discussed below. Agriculture consumes 11% of global boron production, and bleaches and detergents about 6%.
Elemental boron fiber
Boron fibers (boron filaments) are high-strength, lightweight materials that are used chiefly 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. The fibers can be produced by chemical vapor deposition of boron on a tungsten filament.
Boron fibers and sub-millimeter sized crystalline boron springs are produced by laser-assisted chemical vapor deposition. Translation of the focused laser beam allows production of 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.
Boronated fiberglass
Fiberglass is a fiber reinforced polymer made of plastic reinforced by glass fibers, commonly woven into a mat. The glass fibers used in the material are made of various types of glass depending upon the fiberglass use. These glasses all contain silica or silicate, with varying amounts of oxides of calcium, magnesium, and sometimes boron. The boron is present as borosilicate, borax, or boron oxide, and is added to increase the strength of the glass, or as a fluxing agent to decrease the melting temperature of silica, which is too high to be easily worked in its pure form to make glass fibers.
The highly boronated glasses used in fiberglass are E-glass (named for "Electrical" use, but now the most common fiberglass for general use). E-glass is alumino-borosilicate glass with less than 1% w/w alkali oxides, mainly used for glass-reinforced plastics. Other common high-boron glasses include C-glass, an alkali-lime glass with high boron oxide content, used for glass staple fibers and insulation, and D-glass, a borosilicate glass, named for its low Dielectric constant).
Not all fiberglasses contain boron, but on a global scale, most of the fiberglass used does contain it. Because the ubiquitous use of fiberglass in construction and insulation, boron-containing fiberglasses consume half the global production of boron, and are the single largest commercial boron market.
Borosilicate glass
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. Schott AG's "Duran" and Owens-Corning's trademarked Pyrex are two major brand names for this glass, used both in laboratory glassware and in consumer cookware and bakeware, chiefly for this resistance.
Boron carbide ceramic
Several boron compounds are known for their extreme hardness and toughness. Boron carbide is a ceramic material which is obtained by decomposing B2O3 with carbon in an electric furnace:
2 B2O3 + 7 C → B4C + 6 CO
Boron carbide's structure is only approximately B4C, and it shows a clear depletion of carbon from this suggested stoichiometric ratio. This is due to its very complex structure. The substance can be seen with empirical formula B12C3 (i.e., with B12 dodecahedra being a motif), but with less carbon, as the suggested C3 units are replaced with C-B-C chains, and some smaller (B6) octahedra are present as well (see the boron carbide article for structural analysis). The repeating polymer plus semi-crystalline structure of boron carbide gives it great structural strength per weight. It is used in tank armor, bulletproof vests, and numerous other structural applications.
Boron carbide's ability to absorb neutrons without forming long-lived radionuclides (especially when doped with extra boron-10) makes the material attractive as an absorbent for neutron radiation arising in nuclear power plants.Nuclear applications of boron carbide include shielding, control rods and shut-down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.
High-hardness and abrasive compounds
Boron carbide and cubic boron nitride powders are widely used as abrasives. Boron nitride is a material 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,is a superior abrasive. Its hardness is only slightly smaller than, but its chemical stability is superior, to that of diamond. Heterodiamond (also called BCN) is another diamond-like boron compound.
Metallurgy
Boron is added to boron steels at the level of a few parts per million to increase hardenability. Higher percentages are added to steels used in the nuclear industry due to boron's neutron absorption ability.
Boron can also increase the surface hardness of steels and alloys through boriding. Additionally 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.
For example, 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. Similarly, 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.
Detergent formulations and bleaching agents
Borax is used in various household laundry and cleaning products,[95] including the "20 Mule Team Borax" laundry booster and "Boraxo" powdered hand soap. It is also present in some tooth bleaching formulas.
Sodium perborate serves as a source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. However, despite its name, "Borateem" laundry bleach no longer contains any boron compounds, using sodium percarbonate instead as a bleaching agent.
Insecticides
Boric acid is used as an insecticide, notably against ants, fleas, and cockroaches.[97]
Semiconductors
Boron is a useful dopant for such semiconductors as silicon, germanium, and silicon carbide. Having one fewer 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 the 1970s, it was mostly replaced by ion implantation, which relies mostly on BF3 as a boron source.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.Triethylborane is also injected into vapor deposition reactors as a boron source.[citation needed] Examples are the plasma deposition of boron-containing hard carbon films, silicon nitride–boron nitride films, and for doping of diamond film with boron.
Magnets
Boron is a component of neodymium magnets (Nd2Fe14B), which are among the strongest type of permanent magnet. These magnets are found in a variety of electromechanical and electronic devices, such as magnetic resonance imaging (MRI) medical imaging systems, in compact and relatively small motors and actuators. As examples, computer HDDs (hard disk drives), CD (compact disk) and DVD (digital versatile disk) players rely on neodymium magnet motors to deliver intense rotary power in a remarkably compact package. In mobile phones 'Neo' magnets provide the magnetic field which allows tiny speakers to deliver appreciable audio power.
Shielding and neutron absorber in nuclear reactors
Boron shielding is used as a control for nuclear reactors, taking advantage of its high cross-section for neutron capture.
In pressurized water reactors a variable concentration of boronic acid in the cooling water is used as a neutron poison to compensate the variable reactivity of the fuel. When new rods are inserted the concentration of boronic acid is maximal, and is reduced during the lifetime.
Other nonmedical uses
Elemental boron is a metalloid that is found in small amounts in meteoroids but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, very pure boron is produced with difficulty because of refractory contamination by carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature. The primary use of elemental boron is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all boron consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and refractory materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. Boron as sodium perborate is used as a bleach. A small amount of boron is used as a dopant in semiconductors, and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which (boron-10) has a number of uses as a neutron-capturing agent.
In biology, borates have low toxicity in mammals (similar to table salt), but are more toxic to arthropods and are used as insecticides. Boric acid is mildly antimicrobial, and several natural boron-containing organic antibiotics are known.[13] Boron is an essential plant nutrient and boron compounds such as borax and boric acid are used as fertilizers in agriculture, although it's only required in small amounts, with excess being toxic. Boron compounds play a strengthening role in the cell walls of all plants. There is no consensus on whether boron is an essential nutrient for mammals, including humans, although there is some evidence it supports bone health.
Boron - Wikipedia
that is found in small amounts in meteoroids but chemically uncombined boron is not otherwise found naturally on Earth. Industrially, very pure boron is produced with difficulty because of refractory contamination by carbon or other elements. Several allotropes of boron exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature. The primary use of elemental boron is as boron filaments with applications similar to carbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all boron consumed globally is an additive in fiberglass for insulation and structural materials. The next leading use is in polymers and ceramics in high-strength, lightweight structural and refractory materials. Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. Boron as sodium perborate is used as a bleach. A small amount of boron is used as a dopant in semiconductors, and reagent intermediates in the synthesis of organic fine chemicals. A few boron-containing organic pharmaceuticals are used or are in study. Natural boron is composed of two stable isotopes, one of which (boron-10) has a number of uses as a neutron-capturing agent.
In biology, borates have low toxicity in mammals (similar to table salt), but are more toxic to arthropods and are used as insecticides. Boric acid is mildly antimicrobial, and several natural boron-containing organic antibiotics are known. Boron is an essential plant nutrient and boron compounds such as borax and boric acid are used as fertilizers in agriculture, although it's only required in small amounts, with excess being toxic. Boron compounds play a strengthening role in the cell walls of all plants. There is no consensus on whether boron is an essential nutrient for mammals, including humans, although there is some evidence it supports bone health.
Applications
Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate pentahydrate. In the United States, 70% of the boron is used for the production of glass and ceramics. The major global industrial-scale use of boron compounds (about 46% of end-use) is in production of glass fiber for boron-containing insulating and structural fiberglasses, especially in Asia. Boron is added to the glass as borax pentahydrate or boron oxide, to influence the strength or fluxing qualities of the glass fibers. Another 10% of global boron production is for borosilicate glass as used in high strength glassware. About 15% of global boron is used in boron ceramics, including super-hard materials discussed below. Agriculture consumes 11% of global boron production, and bleaches and detergents about 6%.
Elemental boron fiber
Boron fibers (boron filaments) are high-strength, lightweight materials that are used chiefly 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. The fibers can be produced by chemical vapor deposition of boron on a tungsten filament.
Boron fibers and sub-millimeter sized crystalline boron springs are produced by laser-assisted chemical vapor deposition. Translation of the focused laser beam allows production of 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.
Boronated fiberglass
Fiberglass is a fiber reinforced polymer made of plastic reinforced by glass fibers, commonly woven into a mat. The glass fibers used in the material are made of various types of glass depending upon the fiberglass use. These glasses all contain silica or silicate, with varying amounts of oxides of calcium, magnesium, and sometimes boron. The boron is present as borosilicate, borax, or boron oxide, and is added to increase the strength of the glass, or as a fluxing agent to decrease the melting temperature of silica, which is too high to be easily worked in its pure form to make glass fibers.
The highly boronated glasses used in fiberglass are E-glass (named for "Electrical" use, but now the most common fiberglass for general use). E-glass is alumino-borosilicate glass with less than 1% w/w alkali oxides, mainly used for glass-reinforced plastics. Other common high-boron glasses include C-glass, an alkali-lime glass with high boron oxide content, used for glass staple fibers and insulation, and D-glass, a borosilicate glass, named for its low Dielectric constant).
Not all fiberglasses contain boron, but on a global scale, most of the fiberglass used does contain it. Because the ubiquitous use of fiberglass in construction and insulation, boron-containing fiberglasses consume half the global production of boron, and are the single largest commercial boron market.
Borosilicate glass
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. Schott AG's "Duran" and Owens-Corning's trademarked Pyrex are two major brand names for this glass, used both in laboratory glassware and in consumer cookware and bakeware, chiefly for this resistance.
Boron carbide ceramic
Several boron compounds are known for their extreme hardness and toughness. Boron carbide is a ceramic material which is obtained by decomposing B2O3 with carbon in an electric furnace:
2 B2O3 + 7 C → B4C + 6 CO
Boron carbide's structure is only approximately B4C, and it shows a clear depletion of carbon from this suggested stoichiometric ratio. This is due to its very complex structure. The substance can be seen with empirical formula B12C3 (i.e., with B12 dodecahedra being a motif), but with less carbon, as the suggested C3 units are replaced with C-B-C chains, and some smaller (B6) octahedra are present as well (see the boron carbide article for structural analysis). The repeating polymer plus semi-crystalline structure of boron carbide gives it great structural strength per weight. It is used in tank armor, bulletproof vests, and numerous other structural applications.
Boron carbide's ability to absorb neutrons without forming long-lived radionuclides (especially when doped with extra boron-10) makes the material attractive as an absorbent for neutron radiation arising in nuclear power plants.Nuclear applications of boron carbide include shielding, control rods and shut-down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.
High-hardness and abrasive compounds
Boron carbide and cubic boron nitride powders are widely used as abrasives. Boron nitride is a material 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,is a superior abrasive. Its hardness is only slightly smaller than, but its chemical stability is superior, to that of diamond. Heterodiamond (also called BCN) is another diamond-like boron compound.
Metallurgy
Boron is added to boron steels at the level of a few parts per million to increase hardenability. Higher percentages are added to steels used in the nuclear industry due to boron's neutron absorption ability.
Boron can also increase the surface hardness of steels and alloys through boriding. Additionally 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.
For example, 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. Similarly, 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.
Detergent formulations and bleaching agents
Borax is used in various household laundry and cleaning products,[95] including the "20 Mule Team Borax" laundry booster and "Boraxo" powdered hand soap. It is also present in some tooth bleaching formulas.
Sodium perborate serves as a source of active oxygen in many detergents, laundry detergents, cleaning products, and laundry bleaches. However, despite its name, "Borateem" laundry bleach no longer contains any boron compounds, using sodium percarbonate instead as a bleaching agent.
Insecticides
Boric acid is used as an insecticide, notably against ants, fleas, and cockroaches.[97]
Semiconductors
Boron is a useful dopant for such semiconductors as silicon, germanium, and silicon carbide. Having one fewer 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 the 1970s, it was mostly replaced by ion implantation, which relies mostly on BF3 as a boron source.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.Triethylborane is also injected into vapor deposition reactors as a boron source.[citation needed] Examples are the plasma deposition of boron-containing hard carbon films, silicon nitride–boron nitride films, and for doping of diamond film with boron.
Magnets
Boron is a component of neodymium magnets (Nd2Fe14B), which are among the strongest type of permanent magnet. These magnets are found in a variety of electromechanical and electronic devices, such as magnetic resonance imaging (MRI) medical imaging systems, in compact and relatively small motors and actuators. As examples, computer HDDs (hard disk drives), CD (compact disk) and DVD (digital versatile disk) players rely on neodymium magnet motors to deliver intense rotary power in a remarkably compact package. In mobile phones 'Neo' magnets provide the magnetic field which allows tiny speakers to deliver appreciable audio power.
Shielding and neutron absorber in nuclear reactors
Boron shielding is used as a control for nuclear reactors, taking advantage of its high cross-section for neutron capture.
In pressurized water reactors a variable concentration of boronic acid in the cooling water is used as a neutron poison to compensate the variable reactivity of the fuel. When new rods are inserted the concentration of boronic acid is maximal, and is reduced during the lifetime.
Other nonmedical uses
- Because of its distinctive green flame, amorphous boron is used in pyrotechnic flares.
- Starch and casein-based adhesives contain sodium tetraborate decahydrate (Na2B4O7·10 H2O)
- Some anti-corrosion systems contain borax.
- Sodium borates are used as a flux for soldering silver and gold and with ammonium chloride for welding ferrous metals.They are also fire retarding additives to plastics and rubber articles.
- Boric acid (also known as orthoboric acid) H3BO3 is used in the production of textile fiberglass and flat panel displays and in many PVAc- and PVOH-based adhesives.
- Triethylborane is a substance which ignites the JP-7 fuel of the Pratt & Whitney J58 turbojet/ramjet engines powering the Lockheed SR-71 Blackbird. 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. Today SpaceX uses it to ignite the engines on their Falcon 9 rocket. Triethylborane is suitable for this because of its pyrophoric properties, especially the fact that it burns with a very high temperature. Triethylborane is an industrial initiator in radical reactions, where it is effective even at low temperatures.
- Borates are used as environmentally benign wood preservatives.
