RAPID SYNTHESIS OF FINELY DISPERSED SIALON POWDER
BY CARBOTHERMAL NITRIDATION OF KAOLIN
S. A. Suvorov,
I. V. Dolgushev,
and A. V. Zabolotskii
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 4, pp.2–5,April, 2002.
A powder based on b-sialon is prepared by carbothermal nitridation. A scheme for rapid synthesis of b-sialon
with controlled dispersity at high temperature is developed. The critical size range of b-sialon particles is de
termined. Experimental data on phase composition and particle size of the synthetic sialon powders are in
agreement with theory.
Sialons are complex phases that conventionally are con-
sidered as aluminosilicates in whose molecules oxygen is
partially replaced by nitrogen . Depending on the struc-
ture, the following species are distinguished: a-SiAlON (a
close structural analog of a-Si
), b-SiAlON (b-Si
structural type), O¢-SiAlON (Si
O structural type), H and
R polytypes (AlN structural type), and X-sialon (mullite
structural type). Regions of existence of some or other types
in the constitution diagram have been discussed in .
The general formula of b-sialons is Si
where x = 0 – 4.2. Presumably, the extended region of exis
tence of b-sialons bears relevance to the closeness of their
bond lengths Si – N (0.173 nm) and Al – O (0.175 nm) .
The interest in sialons and sialon-based materials comes
from the advantageous combination of their physicochemical
properties. First, this is the high level of thermomechanical
characteristics which makes them strong competitors of sili
con and aluminum nitrides. Second, sialons exhibit a high
chemical resistance close to the chemical resistance of ox
ides. The use of naturally occurring minerals of the kaolinite
subgroup for fabrication of sialons allows one to extend sub
stantially the raw material base and to make their production
A method of carbothermal nitridation of kaolin for prepa
ration of highly dispersed sialon powders has been discussed
in . The preparation of highly dispersed sialon powders
with a given particle size has required the development of a
model for interaction of reagents that would allow one to elu-
cidate factors that determine the dispersivity of end products.
The carbothermal reduction of kaolin involves a series of
chemical reactions in which the kaolinic oxygen becomes re-
placed by nitrogen. The following stages of the carbothermal
process are distinguished [3 – 5]:
– formation of mullite and amorphous silica;
– formation of O¢-sialon;
– formation of X-sialon;
– formation of 15R-sialon.
Along with sialons, other compounds — aluminum oxi
de, aluminum nitride, and silicon carbide — are present in
the products of sialon synthesis. An exception to the rule was
the synthesis of b-sialon carried out at a nitrogen pressure of
50 atm: in this case, a practically pure sialon was obtained
(see Table 1, rows 14, 17, and 18) .
Compositionally, the phases in the sialon synthesis are
determined by the synthesis temperature: at temperatures be
low 1500°C, silicon carbide and aluminum oxide are formed;
at higher temperatures, silicon carbide and aluminum carbide
are formed. The formation of silicon carbide is observed at
the stage of mullitization at 1300°C (Table 1, row 1) accord
ing to the reactions below :
+C® SiC + 2CO. (2)
Refractories and Industrial Ceramics Vol. 43, Nos.3–4, 2002
1083-4877/02/0304-0113$27.00 © 2002 Plenum Publishing Corporation
St. Petersburg State Technological Institute (Technical Univer
sity), St. Petersburg, Russia.