conductor at room temperature. At
least three crystalline forms are pos-
sible; 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. Sam-
ples 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 electri-
cally heatedÜlaments. Amorphous
boron can be obtained by reducing
the trioxide with magnesium pow-
der. Boron when heated reacts with
oxygen, halogens, oxidizing acids,
and hot alkalis. It is used in semicon-
ductors and inÜlaments for special-
ized aerospace applications.
Amorphous boron is used inÛares,
giving a green coloration. The iso-
tope boron–10 is used in nuclear re-
actor control rods and shields. The
element was discovered in 1808 by
Sir Humphry Davy and by J. L. Gay-
Lussac 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 ex-
tremely hard, over 9½on 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 be-
cause of its high proportion of
boron–10.
boron hydride See borane.
boron nitride A solid, BN, insolu-
ble in cold water and slowly decom-
posed by hot water; r.d. 2.25 (hexago-
nal); sublimes above 3000°C. Boron
nitride is manufactured by heating
boron oxide to 800°C on an acid-solu-
ble carrier, such as calcium phos-
phate, in the presence of nitrogen or
ammonia. It is isoelectronic with car-
bon 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 ab-
sorbs 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 chlo-
rine 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 labora-
tory to promote reactions that liber-
ate these donors. The compound is
important industrially as a source of
pure boron (reduction with hydro-
gen) for the electronics industry. It is
also used for the preparation of bo-
ranes by reaction with metal hy-
drides.borosilicate Any of a large number
of substances in which BO 3 and SiO 4
units are linked to form networks81 borosilicate
b