Page 153 - Materials Chemistry, Second Edition

140 2 Solid-State Chemistry

(iv) Firing/sintering – quite often, precursor compounds at earlier stages of ceramic
processing are at least partially amorphous. The final firing/sintering stage is
used to fuse the particles together and convert the material into a (poly)
crystalline product, which has the bulk form and physical properties desired
for a particular application. Firing is usually performed at a temperature below
the melting point of the ceramic. Most importantly, as we will see in the next
chapter (powder metallurgy), the microstructure of the final product is strongly
related to the morphology of the green body.
Porcelain, used for applications that range from toilets to decorative plates, is

formed by firing the green ceramic comprised of the clay mineral kaolinite
(Al 2 Si 2 O 5 (OH) 4 ), and a variety of other crystalline and amorphous materials
such as feldspars (KAlSi 3 O 8 /NaAlSi 3 O 8 /CaAl 2 Si 2 O 8 ), glass, ash, and quartz. At a
temperature of ca. 1,200–1,400 C, glass and an aluminosilicate mineral known as

mullite (or porcelainite) are formed, resulting in the familiar high strength and
translucence of porcelain.
Non-oxide ceramics are typically synthesized via high-temperature routes, which
convert molecular precursors into the desired structures. For instance, SiC (carbo-
rundum) may be produced from the direct reaction of silica sand with carbon in an
electric furnace (Eq. 53). Industrially, a mixture of 50 wt% SiO2, 40 wt% coke, 7 wt
% sawdust, and 3 wt% NaCl is heated together at ca. 2,700 C – known as the

Acheson process. The purpose of the salt is to remove metallic impurities via
formation of volatile metal chlorides (e.g., FeCl 3 , MgCl 2 , etc.). To yield highly
crystalline SiC, the Lely process uses the sublimation of SiC powder or lumps at
2,500 C under argon at atmospheric pressure.

2000 2500 C
(electric furnace)
ð53Þ SiO 2 þ 2C ! SiC + CO 2
A lower-temperature route involves the reduction of dichlorodimethylsilane with Na
or Na/K alloys in an organic solvent (Eqs. 54 and 55):

350 C; toluene ðautoclaveÞ
ð54Þ x (MeÞ SiCl 2 þ 2x Na ! ðMe 2 Si) + 2x NaCl
2 x
800 C 1500 C

ð55Þ (Me 2 Si) ! SiC amorphous ! b SiC
x
Chlorinated silanes may also be used to synthesize silicon nitride (Si 3 N 4 ) ceramics,
via reaction with an amine (Eq. 56):
500 C

ð56Þ x MeSiCl 3 þ 3x NH 3 ! ðMeSiN 3 Þ þ 3x HCl
x
Of course, the “brute force” method of reacting silica with ammonia or N 2 /H 2

gases at temperatures in excess of 1,200 C will also yield crystalline silicon
nitride ceramics. Another route that does not involve chlorinated precursors
consists of sintering a polymeric precursor such as poly ðmethylvinylÞsilazane½ Š

ðCH 3 SiHNHÞ ðCH 3 SiCH = CH 2 NHÞ .
0:8 0:2 n