TOPAZ-ACTIVATED SOLID-PHASE SINTERING OF MULLITE
T. V. Vakalova,
A. V. Ivanchenkov,
and V. M. Pogrebenkov
Translated from Novye Ogneupory, No. 1, pp. 40 – 45, January, 2005.
Original article submitted November 24, 2004.
The solid-phase topaz-activated sintering of mullite from stoichiometric mixtures of pure aluminum and sili
con oxides is studied. The role of topaz in the mechanism of synthesis and sintering of mullite is discussed
It has been reported in the previous paper  that topaz
added at a concentration of 2% to a mixture of alumina and
silica produces a mineralizing effect on the solid-phase
mullitization involving products of thermal decomposition of
topaz, viz. mullite and gaseous fluorides.
Our goal in this study was to see in what manner a topaz
concentrate might affect the sintering of mullite synthesize
from pure oxides. Fabrication of a dense ceramic from
high-purity mullite by combining synthesis with sintering is
an arduous task; therefore the activated sintering of mullite
was explored using the following scheme: (i) synthesis of
mullite from a stoichiometric mixture of the precursor oxi-
des; (ii) morphologic analysis (for size and shape) of the
mullite formed; (iii) activation of the sintering process by us
ing highly pulverized briquette material; (iv) preparation of
compositions (synthetic mullite + topaz concentrate); (v) sin
tering of the compositions prepared.
SYNTHESIS OF MULLITE AND PROPERTIES
OF THE MULLITE CAKE
Topaz concentrate was added to precursor mixtures of
stoichiometric composition at a concentration of 1% (in ex
cess of 100%) for the synthesis of mullite at 1500 and
1650°C and 1 – 2% for the synthesis at 1400°C. The silica
component for all mixtures was pulverized veined quartz
with average particle diameter d
=18mm. The alumina
components were Al
(analytical grade) d
= 19.4 mm
(composition 1 ), amorphous Al
= 16.8 mm (composi
tion 2 ), and alumina hydrate (analytical grade) d
= 31.1 mm
(composition 3 ).
The precursor mixtures with no topaz concentrate added
(subcompositions 1-0, 2-0, and 3-0 ) and with 1% (sub
compositions 1-1, 2-1, and 3-1 ) and 2% topaz concentrate
added (subcompositions 1-2, 2-2, and 3-2 ) were used to pre-
pare briquettes by semi-dry pressing under a pressure of
100 MPa; the briquettes were sintered in a furnace fitted with
lanthanum-chromite heaters, at 1400, 1500, and 1650°C,
held for2hatafinal temperature, and then furnace-cooled.
The temperature for preparing the mullite cake was se-
lected with allowance for the conditions of mullite synthesis
in each particular case. It was established previously in 
that the mullitization sets in at 1400°C, proceeds extensively
at 1500 – 1550°C (especially in the presence of topaz), and
virtually terminates at 1650°C. It was assumed that with the
mullitization completed at 1400 and 1500°C, the mullite
crystal lattice developed a high degree of imperfection and
no stable structure (framework) was formed to prevent the
subsequent sintering. As regards the synthesis temperature of
1650°C, it was assumed that the synthesis of mullite would
produce no adverse effect on sintering since the structure ex
pansion had occurred during the pre-sintering.
A study has shown that, irrespective of the synthesis tem
perature, adding topaz produced a marked effect on the pro
perties of the briquette that was molded using the highly-re
active alumina component — amorphous Al
tion 2 ); to a lesser extent, the effect was observed with alu
mina hydrate (composition 3 ), and it was vanishingly small
with the low-activity coarsely crystalline Al
tion 1 ).
The completeness of mullitization was controlled by
x-ray diffractometry using a DRON-3M instrument. The ma
jor crystalline phase in sintered products was mullite; under
1650°C synthesis conditions, mullite only was formed in all
Refractories and Industrial Ceramics Vol. 46, No. 2, 2005
1083-4877/05/4602-0132 © 2005 Springer Science+Business Media, Inc.
Tomsk Polytechnical University, Tomsk, Russia.