SYNTHESIS OF MULLITE FROM OXIDES DOPED WITH TOPAZ
T. V. Vakalova,
A. V. Ivanchenkov,
V. M. Pogrebenkov,
and E. V. Alekseev
Translated from Novye Ogneupory, No. 9, pp. 41 – 47, September, 2004.
Original article submitted July 8, 2004.
Results of a study of the mullitization in stoichiometric pure oxides Al
doped with topaz are re
ported. A mineralizing topaz-mediated effect is manifested in the activated synthesis of mullite of short pris
matic habit involving products of thermal degradation of topaz (mullite and volatile fluorides). The volatile
active fluorides enhance structural imperfections in the reactants and promote their high-temperature interac
tion. Mullite produced by thermal degradation of topaz plays the role of a seeding agent in the solid-state syn
thesis of mullite from oxides.
Nowadays, progress in science and technology is to a
significant extent based on the use of ceramic materials with
a high level of functional properties. In this respect, a prom-
ising material is high-alumina ceramics with a mullite crys-
talline phase, which imparts to products high mechanical
strength, refractoriness, thermal and chemical corrosion sta-
bility, high incipient softening temperature, etc. However,
the superior qualities of mullite-containing aluminosilicate
ceramics are determined not only by the high concentration
of mullite, but also by its structure and morphology (pris
matic or acicular habit of mullite particles).
The habit of mullite is determined by the composition of
the raw mixture and by the temperature of synthesis. The
acicular mullite is commonly synthesized from commercial-
grade chemicals via gas-phase mass transfer or crystalliza
tion from the molten state. In amorphous oxides, mulliti
zation starts at 900°C, proceeds actively at 1000 – 1200°C,
and is completed at 1400°C within 2 h. If crystalline SiO
are used, the temperatures for the onset and termi
nation of mullitization increase to 1400 and 1600°C, respec
tively. Lengthy heating at above 1650°C causes complete
disappearance of corundum from mixtures of mullitic com
position (silica + alumina) .
Ineffective synthesis and sintering of mullite in a sin
gle-step calcination is explained by unidirectional (one-
sided) diffusion of SiO
into the Al
grain (the so-called
Kirkendall – Frenkel effect); the diffused SiO
leaves a pore, which finally results in the buildup of a rigid
crystalline mullite framework which in the absence of a suf-
ficient liquid phase (that is, for a total of impurities
0.2 – 0.5% or less), prevents the sintering and closure of
pores. Under these conditions, the solid-phase mullitization
reaction proceeds via diffusion, with the partial diffusion co-
efficient for silicon much higher than that for aluminum. For
this reason, obtaining a dense ceramic from high-purity
mullite by a combined process of synthesis and sintering
presents a very serious problem .
To intensify the synthesis of minerals, mineralizers are
employed in ceramic technologies; in each particular case,
the mechanism of mineralization is determined by the condi
tions of synthesis . Mineralizers are promoters for mulli
tization at low temperatures (about 1350°C), while at higher
temperatures (about 1700°C) they are inactive ; they exert
a specific effect on the structure of newly formed materials
and molten phases produced by sintering and, ultimately, are
responsible for the quality of finished products. With due re
gard for the role which mineralizers may play in the structure
formation, their judicious choice provides a route towards
improving properties of ceramic materials and economic
consumption of energy during sintering.
It was thought of interest to test topaz, a fluorine-con
taining mineral, as a mineralizer for the synthesis of mullite.
Topaz subjected to thermal degradation releases gaseous flu
orides which perform, according to the silicate technology
expertise, as efficient mineralizers.
In was reported in  that Al
, rather than SiO
duced a predominant effect on mullitization. Therefore in
Refractories and Industrial Ceramics Vol. 45, No. 6, 2004
1083-4877/04/4506-0441 © 2004 Springer Science+Business Media, Inc.
Continuation of a series of communications started in Novye
Ogneupory, No. 7, 2004.
Tomsk Polytechnical University, Tomsk, Russia.