Page 206 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GSS Final Pages
Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

Main Group Elements 15

Victims of asbestosis suffer acute shortness of breath, I. Silicon Nitride
which in extreme cases may occasion heart failure. There
Silicon nitride (Si 3 N 4 ) has long been considered a promis-
is little agreement among researchers on the nature of the
ing material for use in ceramic gas turbines. A hard
toxicity and carcinogenicity of asbestos. Particle size is
network solid (∼9 on the Mohs scale), Si 3 N 4 features a
probably a factor, and there is some evidence that as-
structure in which each nitrogen atom is surrounded by
bestos may facilitate entry of carcinogenic hydrocarbons
three silicon atoms, and each silicon atom is tetrahedrally
into cells. For instance, exposure to asbestos enhances the
surrounded by four nitrogen atoms.
risk of lung cancer among cigarette smokers.
Si 3 N 4 can be synthesized by the direct combination of
A safe replacement for asbestos might be Saffil, spe-
the elements at 1250–1450 C:
◦
cially prepared fibrous forms of Al 2 O 3 and ZrO 2 , for
which, fortunately, there is no documented evidence of (a) 3Si (s) + 2N 2(g) −−−−→ Si 3 N 4(s)
∼1,300 C
◦
toxicity. The soft, silky fibers of Saffil can be made into
yarn, cloth, fiber mats, blankets, solid heat shields, and (b) 3SiCl 4(g) + 4NH 3(g) → Si 3 N 4(s) + 12HCl (g)
high-temperature insulation. Because axial turbine rotors made of Si 3 N 4 are still not
completely reliable, research efforts continue to focus
both on improving injection molding techniques and on
H. Silicon Carbide
strengthening the silicon nitride material systems. A ce-
Although silicon carbide, carborundum, was first synthe- ramic composite of Si 3 N 4 reinforced with silicon carbide
sized before the turn of the century, new syntheses and whiskers may reduce structural failures and provide better
applications for this important compound are still being high-temperature performance than monolithic ceramic
developed. The name carborundum derives from its hard- parts.
ness (9.3 on the Mohs hardness scale) and was coined to
indicate that SiC falls between carbon (diamond = 10.0) V. GERMANIUM, TIN, AND LEAD
and corundum (sapphire, Al 2 O 3 = 9.0).
SiC exists in several crystalline modifications, one of Germanium, tin, and lead complete the members of
which, α-SiC, is prepared commercially by reacting sand
Group 14 (IV A) elements, and the change from nonmetal-
and coke in an electric furnace at 2000 to 5000 C:
◦
lic properties in carbon to weakly metallic properties in
lead is readily observable.
SiO 2 + 3C → SiC + 2CO
Slightly iridescent and dark blue–black, the hard impure A. Occurrence, Preparation, and Uses
crystalline product is an excellent abrasive used in grind-
Germanium minerals are not abundant, and the element
ing wheels and powders.
is usually recovered from coal ash or the flue dust of
Another modification of silicon carbide, β-SiC, results
zinc ores, with which germanium often occurs. Recovery
◦
from heating very pure graphite and silicon at 1500 C.
of germanium is effected by preparing the tetrachloride
This purer modification is almost colorless and is similar
(bp, 83 C), distilling, hydrolyzing to GeO 2 , and reducing
◦
to diamond in crystal habit.
with charcoal:
In recent years, ceramic fibers and whiskers of SiC have
gainedinimportance.Whenaddedtoaluminum,thesema- GeCl 4 (l) + 2H 2 O (l) → GeO 2 (s) + 4HCl
terials greatly enhance tensile strength and nearly double
GeO 2 (s) + 2C (s) → Ge (s) + 2CO (g)
stiffness.
By means of a complex series of steps, SiC fibers can The cooled germanium crystallizes with a diamond
be fabricated from dimethyldichlorosilane, (CH 3 ) 2 SiCl 2 , structure and, when highly purified, finds its greatest use in
the starting material for silicones. SiC whiskers, which transistors. It is a brittle, gray–white metalloid.
are much shorter and finer than fibers, can be prepared by Cassiterite, SnO 2 , is readily reduced by hot carbon, a
heating rice hulls to temperatures of ∼2000 C. process known to the ancients and still employed in the
◦
The direct synthesis of high-purity, ultrafine SiC pow- pyrometallurgy of the element:
ders by means of a sophisticated plasma technology is cur-
SnO 2 (s) + 2C (s) → 2CO (g) + Sn (s)
rentlyunderdevelopment.Thesehigh-technologyceramic
powders, as well as whiskers, will find ever-increasing use Tin exists in two allotropic forms: a gray form and a
in automobile engines and turbochargers. Engine parts re- white metallic form. The white or metallic form (density,
3
◦
inforced with ceramic materials can withstand high oper- 7.31 g/cm ) is stable from the melting point, 232 C, down
◦
ating temperatures and thus enhance engine efficiency. to 13 C, at which temperature it changes to crumbly, gray