Several allotropes of boron exist; amorphous boron is a brown powder, though crystalline boron is black, hard (9.3 on Mohs' scale), and a weak conductor at room temperature (22-28 °C, 72-82 °F). Elemental boron is used as a dopant in the semiconductor industry, while boron compounds play important roles as light structural materials, nontoxic insecticides and preservatives, and reagents for chemical synthesis.
Boron is an essential plant nutrient, although higher soil concentrations of boron may also be toxic to plants. As an ultratrace element, boron is necessary for the optimal health of rats and presumably other mammals, though its physiological role in animals is poorly understood.
Brown amorphous boron is a product of certain chemical reactions. It contains boron atoms randomly bonded to each other without long range order.
Crystalline boron, a very hard, black material with a high melting point, exists in many polymorphs. Two rhombohedral forms, α-boron and β-boron containing 12 and 106.7 atoms in the rhombohedral unit cell respectively, and 50-atom tetragonal boron are the three most characterised crystalline forms.
Optical characteristics of crystalline/elemental boron include the transmittance of infrared light. At standard temperatures, elemental boron is a poor electrical conductor, but is a good electrical conductor at high temperatures.
Chemically boron is electron-deficient, possessing a vacant p-orbital. It is an electrophile. Compounds of boron often behave as Lewis acids, readily bonding with electron-rich substances to compensate for boron's electron deficiency. The reactions of boron are dominated by such requirement for electrons. Also, boron is the least electronegative non-metal, meaning that it is usually oxidized (loses electrons) in reactions.
Boron is also similar to carbon with its capability to form stable covalently bonded molecular networks. Boron is also used for heat resistant alloys. Boron can form compounds whose formal oxidation state is not three, such as B(II) in B2F4.
The sources for borax and boric acid were at first dry lakes in Tibet and from 1818 on the geysers in the tuscan village Larderello Italy. During the 19th century the deposits in south America, California and Turkey were discovered.
The element was not isolated until 1808 by Sir Humphry Davy , Joseph Louis Gay-Lussac, and Louis Jacques Thénard, to about 50 percent purity, by the reduction of boric acid with sodium or magnesium. These men did not recognize the substance as an element. It was Jöns Jakob Berzelius in 1824 who identified boron as an element. The first pure boron was produced by the American chemist W. Weintraub in 1909, although this is disputed by some researchers.
Nearly all boron ore extracted from the Earth is destined for refinement into boric acid and sodium tetraborate. In the United States 70% of the boron is used for the production of glass and ceramics. For scientific glassware, for example, borosilicate glass is used because of its capabilities to withstand chemical reactions and sudden changes in temperature. Duran and Pyrex are two major brand names under which this glass is available.
It is still a matter of research as to which boron compound is the hardest:
Like all superhard materials with properties similar to diamond, these boron-based materials do not possess a unique hardness value but rather a hardness range. This is because hardness tests (e.g. Knoop, Vickers, Rockwell) depend on many conditions (direction, load...) according to whether the diamond used in the test will indent more or less deeply the given material. As a result, they all scratch each other as well as diamond under certain conditions.
These borides have been primarily developed as a substitute for diamond in coated tools (CVD or PVD diamond-like coated), as well as diamond powder coated blades, since diamond becomes soluble in iron and unstable at high temperatures, thus reducing tool life.
Interestingly enough, boron nitride in its hexagonal form (h-BN), is a very soft material (only 2 in Mohs hardness scale) compared to the cubic form, h-BN being slightly more inert chemically than c-BN at very high temperatures, a feature extremely useful in advanced foundry and casting refractory applications (high end crucibles).
At a lesser degree, certain boronized (or borided) metals and alloys, through means of ion implantation or only ion beam deposition of boron ions, show a spectacular increase in surface resistance and microhardness, thus having superficial characteristics similar to the corresponding borides. Laser alloying has also been successfully used for the same purpose. Atomic penetration of materials (aforementioned laser and implantation methods) are preferred over deposition methods (CVD deposition and PVD deposition) since the borides are formed "within" the metallic substrate (the ions literally penetrate the metal), relatively deep from the surface.
Economically important sources are from the ore rasorite (kernite) and tincal (borax ore) which are both found in the Mojave Desert of California, with borax being the most important source there. The largest borax deposits are found in Central and Western Turkey including the provinces of Eskişehir, Kütahya and Balıkesir.
See also: Borate minerals.
Increasing demand for boric acid has led a number of producers to invest in additional capacity. Eti Mine Company of Turkey opened a new 100,000 tonnes per year capacity boric acid plant at Emet in 2003. Rio Tinto increased the capacity of its boron plant from 260,000 tonnes per year in 2003 to 310,000 tonnes per year by May 2005, with plans to grow this to 366,000 tonnes per year in 2006.
Chinese boron producers have been unable to meet rapidly growing demand for high quality borates. This has led to imports of disodium tetraborate growing by a hundredfold between 2000 and 2005 and boric acid imports increasing by 28% per year over the same period.
The rise in global demand has been driven by high rates of growth in fiberglass and borosilicate production. A rapid increase in the manufacture of reinforcement-grade fiberglass in Asia with a consequent increase in demand for borates has offset the development of boron-free reinforcement-grade fiberglass in Europe and the USA. The recent rises in energy prices can be expected to lead to greater use of insulation-grade fiberglass, with consequent growth in the use of boron.
Roskill Consulting Group forecasts that world demand for boron will grow by 3.4% per year to reach 21 million tonnes by 2010. The highest growth in demand is expected to be in Asia where demand could rise by an average 5.7% per year.
Boron is an essential plant nutrient, required primarily for maintaining the integrity of cell walls. Conversely, high soil concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as well as poor overall growth performance. Levels as low as 0.8 ppm can cause these same symptoms to appear in plants particularly sensitive to boron in the soil. Nearly all plants, even those somewhat tolerant of boron in the soil, will show at least some symptoms of boron toxicity when boron in the soil is greater than 1.8 ppm. When boron in the soil exceeds 2.0 ppm, few plants will perform well. Plants sensitive to boron in the soil may not survive. When boron levels in plant tissue exceed 200 ppm symptoms of boron toxicity are likely to appear.
As an ultratrace element, boron is necessary for the optimal health of rats, although it is necessary in such small amounts that ultrapurified foods and dust filtration of air is necessary to show the effects of boron deficiency, which manifest as poor coat/hair quality. Presumably, boron is necessary to other mammals. No deficiency syndrome in humans has been described. Small amounts of boron occur widely in the diet, and the amounts needed in the diet would, by analogy with rodent studies, be very small. The exact physiological role of boron in the animal kingdom is poorly understood.
Boron occurs in all foods produced from plants. Since 1989 its nutritional value has been argued. It is thought that boron plays several biochemical roles in animals, including humans. The U.S. Department of agriculture conducted an experiment in which postmenopausal women took 3 mg of boron a day. The results showed that supplemental boron reduced excretion of calcium by 44%, and activated estrogen and vitamin D. However, whether these effects were conventionally nutritional, or medicinal, could not be determined.
The US National Institute of Health quotes this source:
Boron has two naturally-occurring and stable isotopes, (80.1%) and (19.9%). The mass difference results in a wide range of δ values in natural waters, ranging from -16 to +59. There are 13 known isotopes of boron, the shortest-lived isotope is which decays through proton emission and alpha decay. It has a half-life of 3.26500x10-22 s. Isotopic fractionation of boron is controlled by the exchange reactions of the boron species B(OH)3 and B(OH)4. Boron isotopes are also fractionated during mineral crystallization, during H2O phase changes in hydrothermal systems, and during hydrothermal alteration of rock. The latter effect species preferential removal of the (OH)4 ion onto clays results in solutions enriched in (OH)3 may be responsible for the large enrichment in seawater relative to both oceanic crust and continental crust; this difference may act as an isotopic signature.
The exotic exhibits a nuclear halo.
In nuclear reactors, is used for reactivity control and in emergency shutdown systems. It can serve either function in the form of borosilicate control rods or as boric acid. In pressurized water reactors, boric acid is added to the reactor coolant when the plant is shut down for refueling. It is then slowly filtered out over many months as fissile material is used up and the fuel becomes less reactive.
In future manned interplanetary spacecraft, has a theoretical role as structural material (as boron fibers or BN nanotube material) which also would serve a special role in the radiation shield. One of the difficulties in dealing with cosmic rays, which are mostly high energy protons, is that some secondary radiation from interaction of cosmic rays and spacecraft structural materials, is in the form of high energy spallation neutrons. Such neutrons can be moderated by materials high in light elements such as structural polyethylene, but the moderated neutrons continue to be a radiation hazard unless actively absorbed in a way which dumps the absorption energy in the shielding, far away from biological systems. Among light elements that absorb thermal neutrons, and appear as potential spacecraft structural materials able to do double duty in this regard.