MICROWAVE SYNTHESIS OF CORUNDUM MATERIALS
WITH VARIOUS DENSITIES
S. A. Suvorov,
I. A. Turkin,
and L. N. Printsev
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 6 – 10, December, 2000.
Materials with various densities obtained by high-temperature microwave synthesis from aluminum oxide
powders with various grain sizes and prehistories, which are modified by additives of radio-transparent and
radio-absorbing substances, are described. The process of high-temperature consolidation of the initial parti
cles by a welding mechanism provides a high level of strength in the materials.
Corundum parts (dense and heat-insulating) possess a fa-
vorable combination of thermomechanical and chemical
properties and are widely used in various fields of engineer-
ing in the form of functional elements of high-temperature
units. Stability of the operating properties of the parts is
chiefly guaranteed by the use of powders with controlled pa-
rameters of composition and structure in the production pro-
cess, which provides reproducibility of the characteristics of
the parts. High service properties are provided by the forma-
tion of structures composed of conjugate elements (grains,
crystals). The use of pure and very pure initial granular mate
rials provides the highest level of high-temperature proper
ties. The small sizes of particles of the initial powders in syn
thesis of dense structures hinder the uniform distribution of
the ingredients of the charge over the volume, which leads to
considerable scattering of the technical parameters of the
parts. To a certain extent these problems are removed by
sintering in the presence of an artificially formed liquid
phase due to the decrease in the viscosity; however, in most
cases the formation of a eutectic melt hampers the emergence
of a structure with directly bonded crystals.
High-density corundum materials are produced from alu
mina of commercial grades with particles of at most 1 – 3 mm
in size modified by additives. By controlling the micro
structure of the material we can obtain virtually poreless
corundum materials with an ultimate bending strength
> 500 MPa .
Effective heat-insulating corundum parts are produced
from void microspheres 0.3–4mminsize with a bulk den
sity of 0.5 – 0.9 g/cm
. The parts are fabricated from corun-
dum microspheres of various fractions and milled technical
alumina, which increases their apparent density to 2 g/cm
[2 – 4].
A promising way for creating effective heat insulation is
high-temperature foaming due to the viscous flow of the ma-
terial under the pressure of the formed gas; in the case of the
use of pure alumina, this means that the foaming should be
performed at a temperature close to 2000°C. Ultraporous
foam materials (natural, i.e., pumice, and artificial, i.e., foam
glass) possess a quite high abrasion strength.
EXAMPLES OF MICROWAVE SYNTHESIS
OF CORUNDUM MATERIALS
Dense corundum materials (based on alumina with a spe
cific surface of 11 m
/g and a mean particle size of 0.5 mm)
were synthesized in a microwave furnace (2.45 GHz, 6 kW)
at 1600°C. The density of the obtained materials was 98 – 99%
of the theoretical value. The microstructure of the material
was represented by closed pores and crystals 5 – 50 mmin
size. The size of the near-surface grains did not differ from
that of the grains lying in the central part of the specimen .
Materials from submicron corundum powders (the mean
crystal size was 0.3 mm) were synthesized at a frequency of
60 GHz at a temperature of 1300, 1500, and 1700°C (the
heating rate was 3.33, 2.75, and 4.60 K/sec, respectively). At
an initial density of 50%, the density of the synthesized ma
terials was 75, 88, and 96% of the theoretical value. Inde
pendently of the density, the microstructure of the specimens
was represented by crystals up to 5 mm in size .
Refractories and Industrial Ceramics Vol. 41, Nos. 11 – 12, 2000
1083-4877/00/1112-0417$25.00 © 2001 Plenum Publishing Corporation
St. Petersburg State Technological Institute (Technical Univer
sity), St. Petersburg, Russia.