SCIENTIFIC RESEARCH AND DEVELOPMENTS
KINETICS OF THE SOLID-PHASE SYNTHESIS OF MULLITE
IN THE PRESENCE OF TOPAZ
T. V. Vakalova,
A. V. Ivanchenkov,
and V. M. Pogrebenkov
Translated from Novye Ogneupory, No. 10, pp. 83 – 90, October, 2004.
Original article submitted September 17, 2004.
Results of a kinetic study of solid-phase interactions in stoichiometric oxide mixtures of mullite composition
are reported. The mechanism of mullitization, irrespective of the oxide form (crystalline or amorphous), is dif
fusive in character. Topaz added in small quantities accelerates the process through generating surface and
structure imperfections and activating the diffusion of reactants owing to the assistance of gaseous fluorides
released by topaz. The secondary mullite produced by thermal degradation of topaz facilitates the formation of
nucleation centers for topochemical reactions and activates the mullite crystallization.
Earlier, a topaz-activated synthesis of mullite from natu-
ral raw minerals (refractory clays), mixtures of high-purity
kaolin and alumina, and synthetic oxide-based products has
been reported [1 – 3]. It was established that, irrespective of
the composition of the raw mixture, the mineralizing action
of topaz is confined to the activation of mullite synthesis by
products of thermal decomposition of topaz — mullite and
gaseous fluorides. The chemically active fluorides increase
imperfection of the crystal lattice of reactants and thereby
provide favorable conditions for their high-temperature in
teraction. The topaz-generated mullite thus plays the role of
an activator for mullitization.
Of all variants considered, the best one, both from theo
retical and experimental standpoints, seems to be the synthe
sis of mullite from oxides. As is known, the chemical inter
action in a heated mixture of solid-state reagents takes place
at the interface of co-existing phases and is thus heteroge
neous in character, and, as any topochemical process, is con
trolled by the spatial arrangement of reacting components.
The overall process may involve a range of physical and
chemical elementary processes (step), such as the occurrence
of imperfections and softening of the crystal lattice; rear
rangement of the crystal lattice owing to polymorphic trans
formation; diffusion (external, internal, or interfacial);
sintering and crystallization; fusion and dissolution of the
system’s components in a melt; chemical interaction proper,
etc. In this aspect, the rate and intensity of a reaction in a
solid-state mixture is controlled by a variety of factors such
as the state (degree of perfection) of the crystal structure of
mixture components; imperfection of the shape and surface
of granular reagents (their surface energy); involvement of
liquid and gaseous phases in the reaction; chemical composi
tion (both quantitative and qualitative) of the mixture; dis
persion of components and degree of homogenization of the
mixture; degree of compaction of the mixture; temperature
and duration of the process; occurrence of promoters (mine
ralizers) in the reaction mixture, etc. .
Mineralizers by the effect they produce on solid-phase
reactions are divided into three groups: (a) those controlling
the nucleation (formation of crystallization centers);
(b ) those affecting the rate of crystallization (by changing
the system’s viscosity, through heat withdrawal from the sys
tem, etc.) and (c) those affecting the crystal lattice and prop
erties of crystalline bodies .
The effect of a mineralizer is targeted to the “elemen
tary” steps of a reaction. Still, the final result is primarily de
termined by the effect of an additive on the rate-limiting step
of a reaction. In particular, a mechanism was considered in
 by which flux additives could exert an influence on the
diffusion step of a solid-state reaction involving factors such
as the optimum (typically low) concentration of an additive,
Refractories and Industrial Ceramics Vol. 46, No. 1, 2005
1083-4877/05/4601-0049 © 2005 Springer Science+Business Media, Inc.
For previous papers, see Novye Ogneupory,Nos.7–9(2004).
Tomsk Polytechnical University, Tomsk, Russia.