Boron is synthesized entirely bycosmic ray spallation andsupernovas and not bystellar nucleosynthesis, so it is a low-abundance element in theSolar System and in theEarth's crust.[14] It constitutes about 0.001 percent by weight of Earth's crust.[15] It is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, theborate minerals. These are mined industrially asevaporites, such asborax andkernite. The largest known deposits are inTurkey, the largest producer of boron minerals.
Elemental boron is found in small amounts inmeteoroids, but chemically uncombined boron is not otherwise found naturally on Earth.
Severalallotropes exist:amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard (9.3 on theMohs scale), and a poorelectrical conductor at room temperature (1.5 × 10−6 Ω−1 cm−1 room temperature electrical conductivity).[16] The primary use of the element itself is asboron filaments with applications similar tocarbon fibers in some high-strength materials.
Boron is primarily used in chemical compounds. About half of all production consumed globally is an additive infiberglass for insulation and structural materials. The next leading use is inpolymers andceramics in high-strength, lightweight structural andheat-resistant materials.Borosilicate glass is desired for its greater strength and thermal shock resistance than ordinary soda lime glass. Assodium perborate, it is used as ableach. A small amount is used as adopant insemiconductors andreagent intermediates in thesynthesis 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.
Borates have low toxicity in mammals (similar totable salt) but are more toxic toarthropods and are occasionally used asinsecticides. Boron-containing organic antibiotics are known. Although only traces are required, boron is an essentialplant nutrient.
History
Amorphous boron powder
The wordboron was coined fromborax, the mineral from which it was isolated, by analogy withcarbon, which boron resembles chemically.[17]
Borax in its mineral form (then known as tincal) first saw use as a glaze, beginning inChina circa 300 AD. Some crude borax traveled westward, and was apparently mentioned by the alchemistJabir ibn Hayyan around 700 AD.Marco Polo brought some glazes back to Italy in the 13th century.Georgius Agricola, in around 1600, reported the use of borax as a flux inmetallurgy. In 1777,boric acid was recognized in the hot springs (soffioni) nearFlorence, Italy, at which point it became known assal sedativum, with ostensible medical benefits. The mineral was namedsassolite afterSasso Pisano in Italy. Sasso was the main source ofEuropean borax from 1827 to 1872, whenAmerican sources replaced it.[18][19] Boron compounds were rarely used until the late 1800s whenFrancis Marion Smith'sPacific Coast Borax Company first popularized and produced them in volume at low cost.[20]
Boron was not recognized as an element until it was isolated by SirHumphry Davy[12] and byJoseph Louis Gay-Lussac andLouis Jacques Thénard.[11] In 1808, Davy observed that electric current sent through a solution of borates produced a brown precipitate on one of the electrodes. In his subsequent experiments, he used potassium toreduce boric acid instead ofelectrolysis. He produced enough boron to confirm a new element and named itboracium.[12] Gay-Lussac and Thénard used iron to reduce boric acid at high temperatures. By oxidizing boron with air, they showed that boric acid is its oxidation product.[11][21]Jöns Jacob Berzelius identified it as an element in 1824.[22] Pure boron was arguably first produced by the American chemist Ezekiel Weintraub in 1909.[23][24][25]
Boron has two naturally occurring and stableisotopes,11B (80.1%) and10B (19.9%). The mass difference results in a wide range of δ11B values, which are defined as a fractional difference between the11B and10B 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 is7B which decays throughproton emission andalpha decay with ahalf-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 inhydrothermal systems, and duringhydrothermal alteration ofrock. The latter effect results in preferential removal of the [10B(OH)4]−ion onto clays. It results in solutions enriched in11B(OH)3 and therefore may be responsible for the large11B enrichment in seawater relative to bothoceanic crust andcontinental crust; this difference may act as anisotopic signature.[26]
Both10B and11B possessnuclear spin. The nuclear spin of10B is 3 and that of11B is3/2. These isotopes are therefore of use innuclear magnetic resonance spectroscopy, and spectrometers specially adapted to detecting the boron-11 nuclei are available commercially. The10B and11B nuclei also cause splitting in theresonances of attached nuclei.[28]
Boron forms four majorallotropes: α-rhombohedral[29] and β-rhombohedral[30] (α-R and β-R), γ-orthorhombic[31] (γ), and β-tetragonal[32] (β-T). All four phases are stable atambient conditions. β-rhombohedral is the most stable and the most common. An α-tetragonal phase also exists (α-T) but is very difficult to produce without significant contamination. Most of the phases are based on B12 icosahedra, but the γ phase can be described as arocksalt-type arrangement of the icosahedra and B2 atomic pairs.[33] 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 β-T phase is produced at similar pressures, but higher temperatures of 1800–2200 °C. The α-T and β-T phases might coexist at ambient conditions, with the β-T phase being the more stable.[33][34][35] Compressing boron above 160 GPa produces a boron phase with an as yet unknown structure, and this phase is asuperconductor at temperatures below 6–12 K.[36][37]
Atomic boron is the lightest element having anelectron in ap-orbital in its ground state. Its first threeionization energies are higher than those for heavier group III elements, reflecting its electropositive character.[46]
Elemental boron is rare and poorly studied because the pure material is extremely difficult to prepare. Most studies of "boron" involve samples that contain small amounts of carbon. Very pure boron is produced with difficulty because of contamination by carbon or other elements that resist removal.[47]
Some early routes to elemental boron involved the reduction ofboric oxide with metals such asmagnesium oraluminium. However, the product was often contaminated withborides of those metals.[48] Pure boron can be prepared by reducing volatile boron halides withhydrogen at high temperatures. Ultrapure boron for use in the semiconductor industry is produced by the decomposition ofdiborane at high temperatures and then further purified by thezone melting orCzochralski processes.[49]
Since elemental boron is very rare, its chemical reactions are of little significance. The elemental form is not typically used as a precursor to compounds. Instead, boron compounds are produced from borates.[50]
When exposed to air, under normal conditions, aprotective oxide or hydroxide layer forms on the surface of boron, which prevents further corrosion.[51] The rate of oxidation of boron depends on the crystallinity, particle size, purity and temperature. At higher temperatures boron burns to formboron trioxide:[52]
4 B + 3 O2 → 2 B2O3
Ball-and-stick model of tetraborate anion, [B4O5(OH)4]2−, as it occurs in crystalline borax, Na2[B4O5(OH)4]·8H2O. Boron atoms are pink, with bridging oxygens in red, and four hydroxyl hydrogens in white. Note two borons are trigonally bonded sp2 with no formal charge, while the other two borons are tetrahedrally bonded sp3, each carrying a formal charge of −1. The oxidation state of all borons is III. This mixture of boron coordination numbers and formal charges is characteristic of natural boron minerals.
In some ways, boron is comparable tocarbon in its capability to form stablecovalently bonded molecular networks (even nominally disordered (amorphous) boron contains boronicosahedra, which are bonded randomly to each other withoutlong-range order.[53][54]). In terms of chemical behavior, boron compounds resemblesilicon due to theirdiagonal relationship.Aluminium, the heavier congener of boron, does not behave analogously to boron: it is far more electropositive, it is larger, and it tends not to form homoatomic Al-Al bonds. In the most familiar compounds, boron has the formal oxidation state III. These include the common oxides, sulfides, nitrides, and halides, as well as organic derivatives[52]
Boron forms the complete series of trihalides, i.e. BX3 (X = F, Cl, Br, I). The trifluoride is produced by treating borate salts withhydrogen fluoride, while the trichloride is produced bycarbothermic reduction of boron oxides in the presence of chlorine gas:[50][52]
The trihalides adopt a planar trigonal structures, in contrast to the behavior of aluminium trihalides. All charge-neutral boron halides violate the octet rule; hence, they are typicallyLewis acidic. For example,boron trifluoride (BF3) combines eagerly with fluoride sources to give thetetrafluoroborate anion, BF4−. Boron trifluoride is used in the petrochemical industry as a catalyst. The halides react with water to formboric acid.[52] Other boron halides include those with B-B bonding, such asB2F4 and B4Cl4.[56]
Oxide derivatives
Boron-containing minerals exclusively exist as oxides of B(III), often associated with other elements. More than one hundredborate minerals are known. These minerals resemble silicates in some respect, although it is often found not only in a tetrahedral coordination with oxygen, but also in a trigonal planar configuration. The borates can be subdivided into two classes, anhydrous and the far more common hydrates. The hydrates contain B-OH groups and sometimes water of crystallization. A typical motif is exemplified by the tetraborate anions of the common mineralborax. The formal negative charge of the tetrahedral borate center is balanced by sodium (Na+).[52] Some of the complexity of the borates is exemplified by the inventory of zinc borates, which are commonwood preservatives andfire retardants:[57] 4ZnO·B2O3·H2O, ZnO·B2O3·1.12H2O, ZnO·B2O3·2H2O, 6ZnO·5B2O3·3H2O, 2ZnO·3B2O3·7H2O, 2ZnO·3B2O3·3H2O, 3ZnO·5B2O3·14H2O, and ZnO·5B2O3·4.5H2O.[58]
As illustrated by the preceding examples, borate anions tend to condense by formation of B-O-B bonds. Borosilicates, with B-O-Si, and borophosphates, with B-O-P linkages, are also well represented in both minerals and synthetic compounds.[59]
Related to the oxides are thealkoxides andboronic acids with the formula B(OR)3 and R2BOH, respectively. Boron forms a wide variety of such metal-organic compounds, some of which are used in the synthesis of pharmaceuticals. These developments, especially theSuzuki reaction, was recognized with the 2010Nobel Prize in Chemistry toAkira Suzuki.[60]
Sodium borohydride is a white, fairly air-stable salt, and it converts to diborane by treatment withboron trifluoride:[50]
3 NaBH4 + 4 BF3 → 2 (BH3)2 + 3 NaBF4
Diborane is the dimer of the parent calledborane, BH3. Having a formula akin to ethane's (C2H6), diborane adopts a very different structure, featuring a pair of bridging H atoms. This unusual structure, which was deduced only in the 1940s, was an early indication of the complexity of boron chemistry.[50]
Structure of diborane
Pyrolysis of diborane givesboron hydride clusters such aspentaborane(9)B5H9 anddecaboraneB10H14.[56]: 164, 170, 173 A large number of anionic boron hydrides are also known, e.g.[B12H12]2−. In thesecluster compounds, boron has acoordination number greater than four.[52] The analysis of the bonding in these polyhedra clusters earnedWilliam N. Lipscomb the 1976 Nobel Prize in Chemistry for "studies on the structure of boranes illuminating problems of chemical bonding". Not only are their structures unusual, many of the boranes are extremely reactive. For example, a widely used procedure forpentaborane states that it will "spontaneously inflame or explode in air".[62]
A large number of organoboron compounds, species with B-C bonds, are known. Many organoboron compounds are produced fromhydroboration, the addition ofB-H bonds toC=C and C≡C bonds.[63]Diborane is traditionally used for such reactions, as illustrated by the preparation of trioctylborane:[64]
B2H6 + 6 H2C=CH(CH2)5CH3 → 2 B((CH2)7CH3)3
Thisregiochemistry, i.e. the tendency of B to attach to the terminal carbon - is explained by the polarization of the bonds in boranes, which is indicated as Bδ+-Hδ-.[56]: 144, 166
Hydroboration opened the doors for many subsequent reactions, several of which are useful inthe synthesis of complex organic compounds.[65] The significance of these methods was recognized by the award ofNobel Prize in Chemistry toH. C. Brown in 1979. Even complicated boron hydrides, such asdecaborane, undergo hydroboration.[66] Like the volatile boranes, the alkyl boranes ignite spontaneously in air.
In the 1950s, several studies examined the use ofboranes as energy-increasing "Zip fuel" additives for jet fuel.[67]
The boron-nitrides follow the pattern of avoiding B-B and N-N bonds; only B-N bonding is observed generally. Theboron nitrides exhibit structures analogous to variousallotropes of carbon, including graphite, diamond, and nanotubes. This similarity reflects the fact that B and N have eight valence electrons as does a pair of carbon atoms. In cubic boron nitride (tradenameBorazon), boron and nitrogen atoms are tetrahedral, just like carbon indiamond. Cubic boron nitride, among other applications, is used as an abrasive, as itshardness is comparable with that of diamond. Hexagonal boron nitride (h-BN) is the BN analogue of graphite, consisting of sheets of alternating B and N atoms. These sheets stack with boron and nitrogen in registry between the sheets. Graphite and h-BN have very different properties, although both are lubricants, as these planes slip past each other easily. However, h-BN is a relatively poor electrical and thermal conductor in the planar directions.[70][71] Molecular analogues of boron nitrides are represented byborazine, (BH)3(NH)3.[72]
Carbides
Unit cell of B4C. The green sphere andicosahedra consist of boron atoms, and black spheres are carbon atoms.[73]
Boron carbide's structure is only approximately reflected in its formula of 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 withempirical 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.[citation needed]
Borides
Ball-and-stick model of superconductor magnesium diboride. Boron atoms lie in hexagonal aromatic graphite-like layers, with a charge of −1 on each boron atom. Magnesium(II) ions lie between layers
Binary metal-boron compounds, the metal borides, contain only boron and a metal. They are metallic, very hard, with highmelting points.TiB2,ZrB2, andHfB2 have melting points above 3000 °C.[71] Some metal borides find specialized applications as hard materials for cutting tools.[75]
Boron is rare in the universe and solar system. The amount of boron formed in theBig Bang was negligible. Boron is not generated in the normal course of stellar nucleosynthesis, and is destroyed in stellar interiors.[76]
In the high oxygen environment of the Earth's surface, boron is always found fully oxidized to borate. Boron does not appear on Earth in elemental form. Extremely small traces of elemental boron were detected in Lunar regolith.[77][78]
Although boron is a relatively rare element in the Earth's crust, representing only 0.001% of the crust mass, it can be highly concentrated by the action of water, in which many borates are soluble. It is found naturally combined in compounds such asborax andboric acid (sometimes found involcanic spring waters). Over a hundredborate minerals are known.[79][80]
Production
A fragment of ulexite
Economically important sources of boron are the mineralscolemanite, rasorite (kernite),ulexite, andtincal. Together, these constitute 90% of mined boron-containing ore. The largest global borax deposits known, many still untapped, are in Central and WesternTurkey, including the provinces ofEskişehir,Kütahya andBalıkesir.[81][82][83] Global proven boron mineral mining reserves exceed one billion metric tonnes, against a yearly production of about four million tonnes.[84]
Turkey and the United States are the largest producers of boron products. Turkey produces about half of the global yearly demand throughEti Mine Works (Turkish:Eti Maden İşletmeleri), aTurkishstate-ownedmining andchemicals company focusing on boron products. It holds agovernment monopoly on the mining ofborate minerals in Turkey, which possesses 72% of the world's known deposits.[85] In 2012, it held a 47%share of production of global borate minerals, ahead of its main competitor,Rio Tinto Group.[86]
The average cost of crystalline elemental boron was US$5/g in 2008.[89] Elemental boron is chiefly used in making boron fibers, where it is deposited bychemical vapor deposition on atungsten core (see below). Boron fibers are used in lightweight composite applications, such as high strength tapes.[90] This use is a very small fraction of total boron use. Boron is introduced into semiconductors as boron compounds by ion implantation.[91]
Estimated global consumption of boron (almost entirely as boron compounds) was about 4 million tonnes of B2O3 in 2012. The cost of boron compounds such as borax and kernite was cumulatively US$377/tonne in 2019.[92]
Increasing demand for boric acid has led a number of producers to invest in additional capacity. Turkey's state-ownedEti Mine Works opened a new boric acid plant with the production capacity of 100,000 tonnes per year atEmet 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.
The rise in global demand has been driven by high growth rates inglass fiber,fiberglass, andborosilicate glassware production. A rapid increase in the manufacture of reinforcement-grade boron-containing fiberglass in Asia has offset the development of boron-free reinforcement-grade fiberglass in Europe and the US. 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.[93][94]
Applications
Nearly all boron ore extracted from the Earth is refined asboric acid andsodium tetraborate pentahydrate. In the United States, 70% of the boron is used for the production of glass and ceramics.[95][96] The major global industrial-scale use of boron compounds (about 46% of end-use) is in production ofglass fiber for boron-containing insulating and structuralfiberglasses, 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.[97] Another 10% of global boron production is forborosilicate 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. Bleaches and detergents consume about 6%.[98]
Fiberglasses, afiber reinforced polymer, sometimes contains borosilicate, borax, or boron oxide that is added to increase the strength of the glass. The highly boronated glasses, E-glass (named for "Electrical" use), are alumino-borosilicate glass. Another common high-boron glass, C-glass, also has a high boron oxide content and is used for glass staple fibers and insulation. D-glass is aborosilicate glass named for its low dielectric constant.[99]
Because of the ubiquitous use of fiberglass in construction and insulation, boron-containing fiberglasses consume over half the global production of boron and are the single largest commercial boron market.[100]
Boron fibers and sub-millimeter sized crystalline boron springs are produced bylaser-assistedchemical vapor deposition. Translation of the focused laser beam allows production of 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 inmicromechanical systems.[106]
Boron carbide's ability to absorb neutrons without forming long-livedradionuclides (especially when doped with extra boron-10) makes the material attractive as anabsorbent for neutron radiation arising in nuclear power plants.[107] 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.[108]
Boron carbide and cubic boron nitride powders are widely used as abrasives.Boron nitride is a material isoelectronic tocarbon. 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 nameborazon, is a superior abrasive. Its hardness is only slightly smaller than that of diamond, while its chemical stability is greater.[111]Heterodiamond (also called BCN) is another diamond-like boron compound.[112]
Boron is added toboron steels at the level of a few parts per million to increasehardenability. Higher percentages are added to steels used in thenuclear industry due to boron's neutron absorption ability.[113]
Boron can also increase the surface hardness of steels and alloys throughboriding. Additionally metalborides are used for coating tools throughchemical vapor deposition orphysical vapor deposition. Implantation of boron ions into metals and alloys, throughion implantation orion beam deposition, results in a spectacular increase in surface resistance and microhardness. Laser alloying has also been used successfully for the same purpose. These borides are an alternative to diamond coated tools, and their surfaces have similar properties to those of the bulk boride.[114]
For example,rhenium diboride can be produced at ambient pressures but is rather expensive because of the cost of rhenium. The hardness of ReB2 exhibits considerableanisotropy because of its hexagonal layered structure. Its value is comparable to that oftungsten carbide,silicon carbide,titanium diboride, orzirconium diboride.[110] 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.[115]
Detergent formulations and bleaching agents
Borax is used in various household laundry and cleaning products.[116] It is also present in sometooth bleaching formulas.[96]
Zinc borates and boric acid, popularized asfire retardants, are widely used as wood preservatives and insecticides. Boric acid is also used as a domestic insecticide.[118][119][120]
Semiconductors
Boron is a usefuldopant for such semiconductors assilicon,germanium, andsilicon carbide. Having one fewer valence electron than the host atom, it donates ahole, resulting inp-type conductivity. The traditional method of doping semiconductors with boron is viaatomic 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 byion implantation, which relies mostly on BF3 as a boron source.[121] Boron trichloride gas is also an important chemical in the semiconductor industry for theplasma etching of metals and their oxides.[122]Triethylborane is also injected intovapor deposition reactors as a boron source.[123] Examples are the plasma deposition of boron-containing hard carbon films, silicon nitride–boron nitride films, and thedoping ofdiamond film with boron.[124]
Magnets
Boron is a component ofneodymium 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 inmagnetic resonance imaging (MRI) medical imaging systems and in compact and relatively small motors andactuators. As examples, computerhard disk drive,CD, andDVD players rely on neodymium magnet motors to deliver intense rotary power in a compact package. In mobile phones, 'Neo' magnets provide the magnetic field which allows tiny speakers to deliver appreciable audio power.[125]
Shielding and neutron absorber in nuclear reactors
Inpressurized water reactors, a variable concentration of boronic acid in the cooling water is used as aneutron poison to compensate the variable reactivity of the fuel. When new rods are inserted, the concentration of boronic acid is maximal. It then is reduced during its lifetime.[127]
Other nonmedical uses
Launch ofApollo 15 Saturn V rocket, using triethylborane ignitor
Sodium borates are used as aflux for soldering silver and gold and withammonium chloride for welding ferrous metals.[130] They are also fire retarding additives to plastics and rubber articles.[131]
Boron plays a role in pharmaceutical and biological applications as it is found in various antibiotics produced by bacteria, such asboromycins,aplasmomycins,borophycins, andtartrolons. These antibiotics have shown inhibitory effects on the growth of certain bacteria, fungi, and protozoa. Boron is also being studied for its potential medicinal applications, including its incorporation into biologically active molecules for therapies like boron neutron capture therapy for brain tumors. Some boron-containing biomolecules may act as signaling molecules interacting with cell surfaces, suggesting a role in cellular communication.[140]
Boric acid has antiseptic, antifungal, and antiviral properties and, for these reasons, is applied as a water clarifier in swimming pool water treatment.[141] Mild solutions of boric acid have been used as eye antiseptics.[142]
Boron appears as an active element in the organic pharmaceuticalbortezomib, a new class of drug called the proteasome inhibitor, for treating myeloma and one form of lymphoma (it is currently in experimental trials against other types of lymphoma). The boron atom in bortezomib binds the catalytic site of the26S proteasome[143] with high affinity and specificity.
Dioxaborolane chemistry enables radioactivefluoride (18F) labeling ofantibodies orred blood cells, which allows forpositron emission tomography (PET) imaging ofcancer[147] andhemorrhages,[148] respectively. A Human-Derived, Genetic, Positron-emitting and Fluorescent (HD-GPF) reporter system uses a human protein,PSMA and non-immunogenic, and a small molecule that is positron-emitting (boron bound18F) and fluorescence for dual modality PET and fluorescent imaging of genome modified cells, e.g.cancer,CRISPR/Cas9, orCAR T-cells, in an entire mouse.[149] The dual-modality small molecule targetingPSMA was tested in humans and found the location of primary andmetastaticprostate cancer, aided the fluorescence-guided removal of cancer, and detected single cancer cells in tissue margins.[150]
Research
MgB2
Magnesium diboride (MgB2) is asuperconductor with a transition temperature of 39 K.[151][152] MgB2 wires are produced with thepowder-in-tube process and applied in superconducting magnets.[153][154] A project atCERN to make MgB2 cables has resulted in superconducting test cables able to carry 20,000 amperes for extremely high current distribution applications, such as the contemplated high luminosity version of theLarge Hadron Collider.[155]
Commercial 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.[156] Several industrial-scale enrichment processes have been developed; however, only the fractionated vacuum distillation of thedimethyl ether adduct ofboron trifluoride (DME-BF3) and column chromatography of borates are being used.[157][158]
Radiation-hardened semiconductors
Cosmic radiation produces secondary neutrons when it hits spacecraft structures. Those neutrons are captured in10B if it is present in the spacecraft'ssemiconductors, producing agamma ray, analpha particle, and alithium ion. Those resultant decay products may then irradiate nearby semiconductor "chip" structures, causing data loss (bit flipping, orsingle event upset). Inradiation-hardened semiconductor designs, one countermeasure is to usedepleted boron, which is greatly enriched in11B and contains almost no10B. This is useful because11B is largely immune to radiation damage. Depleted boron is a byproduct of thenuclear industry (see above).[159]
11B is also a candidate as a fuel foraneutronic 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. Thealpha particles from11B fusion can be turned directly into electric power, and all radiation stops as soon as the reactor is turned off.[160]
Enriched boron (boron-10)
Neutron cross section of boron (top curve is for10B and bottom curve for11B)
Enriched boron or10B is both used in radiation shielding and is the primary nuclide used inneutron capture therapy of cancer. In the latter ("boron neutron capture therapy" or BNCT), a compound containing10B is incorporated into a pharmaceutical which is selectively taken up by a malignant tumor and tissues near it. The patient is then treated with a beam of low energy neutrons at a relatively low neutron radiation dose. The neutrons trigger energetic and short-range secondaryalpha particle and lithium-7 heavy ion radiation that are products of the boron-neutronnuclear reaction, and this ion radiation additionally bombards the tumor, especially from inside the tumor cells.[162][163][164][165]
In nuclear reactors,10B is used for reactivity control and inemergency shutdown systems. It can serve either function in the form ofborosilicatecontrol rods or asboric acid. Inpressurized water reactors,10Bboric acid is added to the reactor coolant after the plant is shut down for refueling. When the plant is started up again, the boric acid is slowly filtered out over many months asfissile material is used up and the fuel becomes less reactive.[159]
Nuclear fusion
Boron has been investigated for possible applications innuclear fusion research. It is commonly used for conditioning the walls infusion reactors by depositing boron coatings on plasma-facing components and walls to reduce the release of hydrogen and impurities from the surfaces.[166] It is also being used for the dissipation of energy in the fusion plasma boundary to suppress excessive energy bursts and heat fluxes to the walls.[167][168]
Neutron capture therapy
Inneutron capture therapy (NCT) for malignant brain tumors, boron is researched to be used for selectively targeting and destroying tumor cells. The goal is to deliver higher concentrations of the non-radioactive boron isotope (10B) to the tumor cells than to the surrounding normal tissues. When these10B-containing cells are irradiated with low-energy thermal neutrons, they undergo nuclear capture reactions, releasing high linear energy transfer (LET) particles such as alpha particles and lithium-7 nuclei within a limited path length. These high-LET particles can destroy the adjacent tumor cells without causing significant harm to nearby normal cells. Boron acts as a selective agent due to its ability to absorb thermal neutrons and produce short-range physical effects primarily affecting the targeted tissue region. This binary approach allows for precise tumor cell killing while sparing healthy tissues. The effective delivery of boron involves administering boron compounds or carriers capable of accumulating selectively in tumor cells compared to surrounding tissue. BSH and BPA have been used clinically, but research continues to identify more optimal carriers. Accelerator-based neutron sources have also been developed recently as an alternative to reactor-based sources, leading to improved efficiency and enhanced clinical outcomes in NCT. By employing the properties of boron isotopes and targeted irradiation techniques, NCT offers a potential approach to treating malignant brain tumors by selectively killing cancer cells while minimizing the damage caused by traditional radiation therapies.[169]
BNCT has shown promising results in clinical trials for various other malignancies, includingglioblastoma,head and neck cancer,cutaneous melanoma,hepatocellular carcinoma,lung cancer, andextramammary Paget's disease. The treatment involves a nuclear reaction between nonradioactive boron-10 isotope and low-energy thermal or high-energy epithermal neutrons to generate α particles and lithium nuclei that selectively destroy DNA in tumor cells. The primary challenge lies in developing efficient boron agents with higher content and specific targeting properties tailored for NCT. Integration of tumor-targeting strategies with NCT could potentially establish it as a practical personalized treatment option for different types of cancers. Ongoing research explores new boron compounds, optimization strategies,theranostic agents, and radiobiological advances to overcome limitations and cost-effectively improve patient outcomes.[170][171][172]
Boron is an essential plantnutrient, required primarily for maintaining the integrity of cell walls. However, high soil concentrations of greater than 1.0 ppm lead to marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm produce these same symptoms in plants that are particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of soil boron, will show at least some symptoms of boron toxicity when soil boron content is greater than 1.8 ppm. When this content exceeds 2.0 ppm, few plants will perform well and some may not survive.[173][174][175]
In 2013, chemist and synthetic biologistSteve Benner suggested that the conditions onMars three billion years ago were much more favorable to the stability ofRNA and formation of oxygen-containing[note 1] boron andmolybdenum catalysts found in life. According to Benner's theory, primitive life, which is widely believed to haveoriginated from RNA,[182][183] first formed on Mars beforemigrating to Earth.[184]
In human health
It is thought that boron plays several essential roles in animals, including humans, but the exact physiological role is poorly understood.[185][186]Boron deficiency has only been clearly established inlivestock.[187][188] In humans, boron deficiency may affectbone mineral density, though it has been noted that additional research on the effects of bone health is necessary.[189]
Boron is not classified as an essential human nutrient because research has not established a clear biological function for it.[190][191] The U.S.Food and Nutrition Board (FNB) found the existing data insufficient to derive aRecommended Dietary Allowance (RDA),Adequate Intake (AI), orEstimated Average Requirement (EAR) for boron and the U.S.Food and Drug Administration (FDA) has not established a daily value for boron for food and dietary supplement labeling purposes.[190][191] While low boron levels can be detrimental to health, with studies suggesting that they increase the risks ofosteoporosis, poor immune function, and cognitive decline, high boron levels are associated with cell damage and toxicity.[192]
Still, studies suggest that boron may exert beneficial effects on reproduction and development,calcium metabolism,bone formation, brain function,insulin and energy substrate metabolism, immunity, andsteroidhormone (includingestrogen), andvitamin D function, among other functions.[193][191] In a small human trial published in 1987 reported on postmenopausal women first made boron deficient and then repleted with 3 mg/day, boron supplementation markedly reduced urinary calcium excretion and elevated the serum concentrations of17 β-estradiol andtestosterone.[194] Environmental boron appears to beinversely correlated witharthritis.[195]
The exact mechanism by which boron exerts its physiological effects is not fully understood but may involve interactions withadenosine monophosphate (ADP) andS-adenosyl methionine (SAM-e), two compounds involved in important cellular functions. Furthermore, boron appears to inhibit cyclicADP-ribose, thereby affecting the release of calcium ions from theendoplasmic reticulum and affecting various biological processes.[192] Some studies suggest that boron may reduce levels ofinflammatory biomarkers.[193]Congenital endothelial dystrophy type 2, a rare form ofcorneal dystrophy, is linked to mutations inSLC4A11 gene that encodes a transporter reportedly regulating the intracellular concentration of boron.[196]
Elemental boron,boron oxide,boric acid,borate, and manyorganoboron compounds are relatively nontoxic to humans and animals (with toxicity similar to that of table salt). TheLD50 (dose at which there is 50% mortality) for animals is about 6 g per kg of body weight. Substances with an LD50 above 2 g/kg are considered nontoxic. An intake of 4 g/day of boric acid was reported without incident, but more than this is considered toxic in more than a few doses. Intakes of more than 0.5 grams per day for 50 days cause minor digestive and other problems suggestive of toxicity.[200]
Theboranes (boron hydrogen compounds) and similar gaseous compounds are quite poisonous. As usual, boron is not an element that is intrinsically poisonous, but the toxicity of these compounds depends on structure (for another example of this phenomenon, seephosphine).[18][19] The boranes are also highly flammable and require special care when handling. Some combinations of boranes and other compounds are highly explosive. Sodium borohydride presents a fire hazard owing to its reducing nature and the liberation of hydrogen on contact with acid. Boron halides are corrosive.[203]
Boron toxicity in rose leaves.
Boron is necessary for plant growth, but an excess of boron is toxic to plants, and occurs particularly in acidic soil.[204][205] It presents as a yellowing from the tip inwards of the oldest leaves and black spots in barley leaves, but it can be confused with other stresses such as magnesium deficiency in other plants.[206]
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