BORON
Boron is a chemical element with symbol B and atomic number 5. Because boron is produced
entirely by cosmic ray spallation and not by stellar nucleosynthesis,
[8]
it is a low-abundance element in
both the solar system and the 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 andkernite.
Chemically uncombined boron, which is classed as a metalloid, is found in small amounts
in meteoroids, but is not found naturally on Earth. Industrially, very pure boron is produced with
difficulty, as boron tends to form refractory materials containing small amounts of carbon or other
elements. Several allotropes of boron exist: amorphous boron is a brown powder and crystalline
boron is black, extremely hard (about 9.5 on theMohs scale), and a poor conductor at room
temperature. Elemental boron is used as a dopant in the semiconductor industry.
The major industrial-scale uses of boron compounds are in sodium perborate bleaches, and (Owens-
Corning) Borosilicate glass which it trademarked as Pyrex, with superior strength and breakage
resistace (thermal shock resistance) than ordinary soda lime glass. .
Boron polymers and ceramicsplay specialized roles as high-strength lightweight structural and
refractory materials. Boron compounds are used in silica-based glasses and ceramics to give them
resistance to thermal shock. Boron-containing reagents are used as 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 a natural boron-
containing organic antibiotic is known.
[9]
Boron is essential to life. Small amounts of boron compounds
play a strengthening role in the cell walls of all plants, making boron necessary in soils. Experiments
indicate a role for boron as an ultratrace element in animals, but its role in animal physiology is
unknown.
HISTORY AND ETYMOLOGY
The name boron originates from the Arabic word buraq or the Persian word burah;
[10]
which are
names for the mineral borax.
[11]
Sassolite
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 Persian alchemist Jbir ibn Hayyn seems to mention it in
AD700. Marco Polobrought some glazes back to Italy in the 13th century. Agricola, around 1600, reports the
use of borax as a flux inmetallurgy. In 1777, boric acid was recognized in the hot springs (soffioni)
near Florence, Italy, and became known assal sedativum, with mainly medical uses. The rare mineral is
called sassolite, which is found at Sasso, Italy. Sasso was the main source of European borax from 1827 to
1872, at which date American sources replaced it.
[12][13]
Boron compounds were relatively rarely used
chemicals until the late 1800s when Francis Marion Smith's Pacific Coast Borax Company first popularized
these compounds and made them in volume and hence cheap.
[14]
Boron was not recognized as an element until it was isolated by Sir Humphry Davy
[5]
and by Joseph Louis
Gay-Lussac and Louis Jacques Thnard.
[4]
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 to reduce boric acid instead ofelectrolysis. He produced enough boron to confirm a new element
and named the element boracium.
[5]
Gay-Lussac and Thnard used iron to reduce boric acid at high
temperatures. They showed by oxidizing boron with air that boric acid is an oxidation product of
boron.
[4][15]
Jns Jakob Berzeliusidentified boron as an element in 1824.
[16]
Pure boron was arguably first
produced by the American chemist Ezekiel Weintraub in 1909.
CHARACTERISTICS
Allotropes
Boron chunks
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.
[20][21]
Crystalline boron is a very hard, black material with a high melting
point of above 2000 C. It exists in four major polymorphs: , , and T. Whereas , and T phases are
based on B
12
icosahedra, the -phase can be described as a rocksalt-type arrangement of the icosahedra and
B
2
atomic pairs.
[22]
It can be produced by compressing other boron phases to 1220 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 18002200 C. As to the and phases, they might both coexist
at ambient conditions with the phase being more stable.
[22][23][24]
Compressing boron above 160 GPa
produces a boron phase with an as yet unknown structure, and this phase is a superconductor at temperatures
612 K.
Chemistry of the element
Elemental boron is rare and poorly studied because the material is extremely difficult to prepare. Most studies
on "boron" involve samples that contain small amounts of carbon. Chemically, boron behaves more similarly
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.
[18]
The rate of 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 formboron trioxide.
Chemical compounds
Boron (III) trifluoride structure, showing "empty" boron p orbital in pi-type coordinate covalent bonds
In its most familiar compounds, boron has the formal oxidation state III. These include oxides, sulfides, nitrides,
and halides.
[34]
The trihalides adopt a planar trigonal structure. These compounds are Lewis acids in that they readily
form adducts with electron-pair donors, which are called Lewis bases. For example, fluoride (F
) and boron
trifluoride (BF
3
) combined to give thetetrafluoroborate anion, BF
4
. Boron trifluoride is used in the
petrochemical industry as a catalyst. The halides react with water to form boric acid.
[34]
Boron is found in nature on Earth entirely as various oxides of B(III), often associated with other elements.
More than one hundredborate minerals contain boron in oxidation state +3. These minerals resemble silicates
in some respect, although boron is often found not only in a tetrahedral coordination with oxygen, but also in a
trigonal planar configuration. Unlike silicates, the boron minerals never contain boron with coordination number
greater than four. A typical motif is exemplified by the tetraborate anions of the common mineral borax, shown
at left. The formal negative charge of the tetrahedral borate centers is balanced by metal cations in the
minerals, such as the sodium (Na
+
) in borax.
[34]
Boranes are chemical compounds of boron and hydrogen, with the generic formula of B
x
H
y
. These compounds
do not occur in nature. Many of the boranes readily oxidise on contact with air, some violently. The parent
member BH
3
is called borane, but it is known only in the gaseous state, and dimerises to form diborane, B
2
H
6
.
The larger boranes all consist of boron clusters that are polyhedral, some of which exist as isomers. For
example, isomers of B
20
H
26
are based on the fusion of two 10-atom clusters.
The most important boranes are diborane B
2
H
6
and two of its pyrolysis products, pentaborane B
5
H
9
and
decaborane B
10
H
14
. A large number of anionic boron hydrides are known, e.g. [B
12
H
12
]
2
.
The formal oxidation number in boranes is positive, and is based on the assumption that hydrogen is counted
as 1 as in active metal hydrides. The mean oxidation number for the borons is then simply the ratio of
hydrogen to boron in the molecule. For example, in diborane B
2
H
6
, the boron oxidation state is +3, but in
decaborane B
10
H
14
, it is
7
/
5
or +1.4. In these compounds the oxidation state of boron is often not a whole
number.
The boron nitrides are notable for the variety of structures that they adopt. They adopt structures analogous to
various allotropes of carbon, including graphite, diamond, and nanotubes. In the diamond-like structure called
cubic boron nitride (tradename Borazon), boron atoms exist in the tetrahedral structure of carbons atoms in
diamond, but one in every four B-N bonds can be viewed as a coordinate covalent bond, wherein two electrons
are donated by the nitrogen atom which acts as the Lewis base to a bond to the Lewis acidic boron(III) centre.
Cubic boron nitride, among other applications, is used as an abrasive, as it has a hardness comparable with
diamond (the two substances are able to produce scratches on each other). In the BN compound analogue of
graphite, hexagonal boron nitride (h-BN), the positively-charged boron and negatively-charged nitrogen atoms
in each plane lie adjacent to the oppositely charged atom in the next plane. Consequently 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.
Organoboron chemistry
A large number of organoboron compounds are known and many are useful in organic synthesis. Many are
produced from hydroboration, which employs diborane, B
2
H
6
, a simple boranechemical. Organoboron(III)
compounds are usually tetrahedral or trigonal planar, for example, tetraphenylborate, [B(C
6
H
5
)
4
]
-
vs triphenylborane, B(C
6
H
5
)
3
. However, multiple boron atoms reacting with each other have a tendency to form
novel dodecahedral (12-sided) and icosahedral (20-sided) structures composed completely of boron atoms, or
with varying numbers of carbon heteroatoms.
Organoboron chemicals have been employed in uses as diverse as boron carbide (see below), a complex very
hard ceramic composed of boron-carbon cluster anions and cations, to carboranes, carbon-boron cluster
chemistry compounds that can be halogenated to form reactive structures including carborane acid,
a superacid. As one example, carboranes form useful molecular moieties that add considerable amounts of
boron to other biochemicals in order to synthesize boron-containing compounds for boron neutron capture
therapy of cancer.
Compounds of B(I) and B(II)
Although these are not found on Earth naturally, boron forms a variety of stable compounds with formal
oxidation state less than three. As for many covalent compounds, formal oxidation states are often of little
meaning in boron hydrides and metal borides. The halides also form derivatives of B(I) and B(II). BF,
isoelectronic with N
2
, is not isolable in condensed form, but B
2
F
4
and B
4
Cl
4
are well characterized.
[37]
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 boron in oxidation state less than III. Illustrative
is magnesium diboride (MgB
2
). Each boron atom has a formal 1 charge and magnesium is assigned a formal
charge of 2+. In this material, the boron centers are trigonal planar, with an extra double bond for each boron,
with the boron atoms forming sheets akin to the carbon in graphite. However, unlike the case with hexagonal
boron nitride which by comparison lacks electrons in the plane of the covalent atoms, the delocalized electrons
in the plane of magnesium diboride allow it to conduct electricity similar to isoelectronic graphite. In addition, in
2001 this material was found to be a high-temperature superconductor.
[38][39]
Certain other metal borides find specialized applications as hard materials for cutting tools.
[40]
From the structural perspective, the most distinctive chemical compounds of boron are the hydrides. Included
in this series are the cluster compoundsdodecaborate (B
12
H
12
2-
), decaborane (B
10
H
14
), and
the carboranes such as C
2
B
10
H
12
. Characteristically such compounds contain boron with coordination numbers
greater than four.
[34]
Isotopes
Boron has two naturally occurring and stable isotopes,
11
B (80.1%) and
10
B (19.9%). The mass difference
results in a wide range of
11
B values, which are defined as a fractional difference between the
11
B and
10
B 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
7
B which decays through proton emission and alpha decay. It
has a half-life of 3.510
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
H
2
O phase changes in hydrothermalsystems, and during hydrothermal alteration of rock. The latter effect
results in preferential removal of the [
10
B(OH)
4
]
ion onto clays. It results in solutions enriched in
11
B(OH)
3
and
therefore may be responsible for the large
11
B enrichment in seawater relative to both oceanic crust
and continental crust; this difference may act as an isotopic signature.
[41]
The exotic
17
B exhibits a nuclear halo,
i.e. its radius is appreciably larger than that predicted by the liquid drop model.
[42]
The
10
B isotope is good at capturing thermal neutrons (see neutron cross section#Typical cross sections).
Natural boron is about 20%
10
B and 80%
11
B. The nuclear industry enriches natural boron to nearly pure
10
B.
The less-valuable by-product, depleted boron, is nearly pure
11
B.
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.
[43]
Several industrial-scale enrichment processes have been developed, however
only the fractionated vacuum distillation of the dimethyl ether adduct of boron trifluoride (DME-BF
3
) and column
chromatography of borates are being used.
Enriched boron (boron-10)
Neutron cross section of boron (top curve is for
10
B and bottom curve for
11
B)
Enriched boron or
10
B is used in both radiation shielding and is the primary nuclide used in neutron capture
therapy of cancer. In the latter ("boron neutron capture therapy" or BNCT), a compound containing
10
B 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, however, trigger energetic and short-range secondary alpha particle and lithium-7 heavy ion
radiation that are products of the boron + neutron nuclear reaction, and this ion radiation additionally bombards
the tumor, especially from inside the tumor cells.
[46][47][48][49]
In nuclear reactors,
10
B is used for reactivity control and in emergency shutdown systems. It can serve either
function in the form ofborosilicate 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.
[50]
In future manned interplanetary spacecraft,
10
B 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 spallationneutrons. 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,
6
Li and
10
B appear as potential spacecraft structural materials which serve both for mechanical
reinforcement and radiation protection.
[51]
Depleted boron (boron-11)
Cosmic radiation will produce secondary neutrons if it hits spacecraft structures. Those neutrons will be
captured in
10
B, if it is present in the spacecraft's semiconductors, producing a gamma ray, an alpha particle,
and a lithium ion. These resultant decay products may then irradiate 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
11
B and contains almost no
10
B.
11
B is
largely immune to radiation damage. Depleted boron is a by-product of the nuclear industry.
[50]
11
B 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, thealpha particles from
11
B fusion can be
turned directly into electric power, and all radiation stops as soon as the reactor is turned off.
[52]
NMR spectroscopy
Both
10
B and
11
B possess nuclear spin. The nuclear spin of
10
B is 3 and that of
11
B 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
10
B and
11
B nuclei also cause splitting in
the resonances of attached nuclei.
Occurrence
A fragment of ulexite
Borax crystals
Boron is rare in the Universe and solar system due to trace formation in the Big Bang and in stars. It is formed
in minor amounts in cosmic ray spallation nucleosynthesis and may be found uncombined in cosmic
dust and meteoroid materials. In the high oxygen environment of Earth, boron is always found fully oxidized to
borate.
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. The worldwide
commercial borate deposits are estimated at 10 million tonnes.
[54][55]
Turkey and the United States are the
world's largest producers of boron.
[56][57]
Turkey has 63% of the worlds boron reserves.
[58]
Boron does not
appear on Earth in elemental form but is found combined in borax, boric
acid, colemanite, kernite, ulexite andborates. Boric acid is sometimes found in volcanic spring waters.
Ulexite is one of over a hundred borate minerals; it is a fibrous crystal where individual fibers can guide light
like optical fibers.
[59]
Economically important sources of boron are rasorite (kernite) and tincal (borax ore). They are both found in
the Mojave Desert of California where the Rio Tinto Borax Mine (also known as the U.S. Borax Boron
Mine) 35234.447N 1174045.412W near Boron, California is California's largest open-pit mine and the
largest borax mine in the world, producing nearly half the world's borates from this single site.
[60][61]
However,
the largest borax deposits known, many still untapped, are in Central and Western Turkey including the
provinces of Eskiehir, Ktahya and Balkesir
PRODUCTION
The production of boron compounds does not involve formation of elemental boron, but exploits the convenient
availability of borates.
The earliest routes to elemental boron 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 use in the semiconductor industry is produced by the decomposition of diborane at high temperatures and
then further purified with the zone melting or Czochralski processes.
[65]
Market trend
Estimated global consumption of boron rose to a record 1.8 million tonnes of B
2
O
3
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.
[66]
Increasing demand for boric acid has led a number of producers to invest in additional capacity. Turkey's state-
owned Eti Mine Works 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 sodium tetraborate (borax) growing by a hundredfold between 2000 and 2005 and boric acid
imports increasing by 28% per year over the same period.
[67][68]
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.
USE OF BORON
Amorphous boron is used in pyrotechnic flares to provide a distinctive green colour, and in rockets as an igniter. The most
important compounds of boron are boric (or boracic) acid, widely used as a mild antiseptic, and borax which serves as a
cleansing flux in welding and as a water softener in washing powders. Boron compounds are also extensively used in the
manufacture of borosilicate glasses. Pyrex glass is tough and heat resistant because of the boric acid used to make it.
The isotope boron 10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used
for detecting neutrons. Demand is increasing for boron filaments, a high-strength, low-density material chiefly employed
for advanced aerospace structures.
STRUCTURE
When solid, the crystal structure of boron is rhombohedral.
When solid, the STRUCFBJHJYH
B
