conductor at room temperature. At
least three crystalline forms are possible; two are rhombohedral and the
other tetragonal. The element is
never found free in nature. It occurs
as orthoboric acid in volcanic springs
in Tuscany, as borates in kernite
(Na 2 B 4 O 7 .4H 2 O), and as colemanite
(Ca 2 B 6 O 11 .5H 2 O) in California. Samples usually contain isotopes in the
ratio of 19.78% boron–10 to 80.22%
boron–11. Extraction is achieved by
vapour-phase reduction of boron
trichloride with hydrogen on electrically heated Ülaments. Amorphous
boron can be obtained by reducing
the trioxide with magnesium powder. Boron when heated reacts with
oxygen, halogens, oxidizing acids,
and hot alkalis. It is used in semiconductors and in Ülaments for specialized aerospace applications.
Amorphous boron is used in Ûares,
giving a green coloration. The isotope boron–10 is used in nuclear reactor control rods and shields. The
element was discovered in 1808 by
Sir Humphry *Davy and by J. L. *GayLussac and L. J. Thenard.
A
• Information from the WebElements site
boron carbide A black solid, B 4 C,
soluble only in fused alkali; it is extremely hard, over 9½ on Mohs’
scale; rhombohedral; r.d. 2.52; m.p.
2350°C; b.p. >3500°C. Boron carbide
is manufactured by the reduction of
boric oxide with petroleum coke in
an electric furnace. It is used largely
as an abrasive, but objects can also
be fabricated using high-temperature
powder metallurgy. Boron nitride is
also used as a neutron absorber because of its high proportion of
boron–10.
boron hydride See borane.
boron nitride A solid, BN, insoluble in cold water and slowly decomposed by hot water; r.d. 2.25 (hexagonal); sublimes above 3000°C. Boron
nitride is manufactured by heating
boron oxide to 800°C on an acid-soluble carrier, such as calcium phosphate, in the presence of nitrogen or
ammonia. It is isoelectronic with carbon and, like carbon, it has a very
hard cubic form (borazon) and a
softer hexagonal form; unlike
graphite this is a nonconductor. It is
used in the electrical industries
where its high thermal conductivity
and high resistance are of especial
value.
boron(III) oxide (boric anhydride;
boric oxide; diboron trioxide) A
glassy solid, B 2 O 3 , that gradually absorbs water to form boric acid. It has
some amphoteric characteristics and
forms various salts.
boron trichloride A colourless
fuming liquid, BCl 3 , which reacts
with water to give hydrogen chloride
and boric acid; r.d. 1.349; m.p.
–107°C; b.p. 12.5°C. Boron trichloride
is prepared industrially by the
exothermic chlorination of boron
carbide at above 700°C, followed by
fractional distillation. An alternative,
but more expensive, laboratory
method is the reaction of dry chlorine with boron at high temperature.
Boron trichloride is a Lewis *acid,
forming stable addition compounds
with such donors as ammonia and
the amines and is used in the laboratory to promote reactions that liberate these donors. The compound is
important industrially as a source of
pure boron (reduction with hydrogen) for the electronics industry. It is
also used for the preparation of boranes by reaction with metal hydrides.
borosilicate Any of a large number
of substances in which BO 3 and SiO 4
units are linked to form networks
81
borosilicate
b
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least three crystalline forms are possible; two are rhombohedral and the
other tetragonal. The element is
never found free in nature. It occurs
as orthoboric acid in volcanic springs
in Tuscany, as borates in kernite
(Na 2 B 4 O 7 .4H 2 O), and as colemanite
(Ca 2 B 6 O 11 .5H 2 O) in California. Samples usually contain isotopes in the
ratio of 19.78% boron–10 to 80.22%
boron–11. Extraction is achieved by
vapour-phase reduction of boron
trichloride with hydrogen on electrically heated Ülaments. Amorphous
boron can be obtained by reducing
the trioxide with magnesium powder. Boron when heated reacts with
oxygen, halogens, oxidizing acids,
and hot alkalis. It is used in semiconductors and in Ülaments for specialized aerospace applications.
Amorphous boron is used in Ûares,
giving a green coloration. The isotope boron–10 is used in nuclear reactor control rods and shields. The
element was discovered in 1808 by
Sir Humphry *Davy and by J. L. *GayLussac and L. J. Thenard.
A
• Information from the WebElements site
boron carbide A black solid, B 4 C,
soluble only in fused alkali; it is extremely hard, over 9½ on Mohs’
scale; rhombohedral; r.d. 2.52; m.p.
2350°C; b.p. >3500°C. Boron carbide
is manufactured by the reduction of
boric oxide with petroleum coke in
an electric furnace. It is used largely
as an abrasive, but objects can also
be fabricated using high-temperature
powder metallurgy. Boron nitride is
also used as a neutron absorber because of its high proportion of
boron–10.
boron hydride See borane.
boron nitride A solid, BN, insoluble in cold water and slowly decomposed by hot water; r.d. 2.25 (hexagonal); sublimes above 3000°C. Boron
nitride is manufactured by heating
boron oxide to 800°C on an acid-soluble carrier, such as calcium phosphate, in the presence of nitrogen or
ammonia. It is isoelectronic with carbon and, like carbon, it has a very
hard cubic form (borazon) and a
softer hexagonal form; unlike
graphite this is a nonconductor. It is
used in the electrical industries
where its high thermal conductivity
and high resistance are of especial
value.
boron(III) oxide (boric anhydride;
boric oxide; diboron trioxide) A
glassy solid, B 2 O 3 , that gradually absorbs water to form boric acid. It has
some amphoteric characteristics and
forms various salts.
boron trichloride A colourless
fuming liquid, BCl 3 , which reacts
with water to give hydrogen chloride
and boric acid; r.d. 1.349; m.p.
–107°C; b.p. 12.5°C. Boron trichloride
is prepared industrially by the
exothermic chlorination of boron
carbide at above 700°C, followed by
fractional distillation. An alternative,
but more expensive, laboratory
method is the reaction of dry chlorine with boron at high temperature.
Boron trichloride is a Lewis *acid,
forming stable addition compounds
with such donors as ammonia and
the amines and is used in the laboratory to promote reactions that liberate these donors. The compound is
important industrially as a source of
pure boron (reduction with hydrogen) for the electronics industry. It is
also used for the preparation of boranes by reaction with metal hydrides.
borosilicate Any of a large number
of substances in which BO 3 and SiO 4
units are linked to form networks
81
borosilicate
b
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