Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF MULLITE-ZIRCONIA COMPOSITES"

Abstract An improved process for the preparation of mullite-zirconia composite. The present invention relates to an improved process for the preparation of dense mullite-zirconia composite employing reaction sintering method, using zircon flour and calcined alumina as raw materials and a new dopant "dysprosia" . Dysprosium (Dy) with atomic no. 66 is one of the members of the Lanthanide group
Full Text This invention relates to an improved process for the
preparation of mullite-zirconia composites
The present invention particularly relates to an
improved process for the preparation of dense mullite-zirconia
composite employing reaction sintering method, using zircon
flour and calcined alumina as raw materials and a
new dopant "dysprosia".Dysprosium (Dy) with atomic no. 66 is
one of the members of the Lanthanide group (Cerium
Lutetiume). The oxide of Dysprosium is known as dysprosium
oxide or dysprosia, Dy2O3 The density of Dy2O3 is 7.81 gm/cc.

The study on mullite-zirconia composites has
received major attention because of its superior properties
with respect to refractoriness, chemical inertness, mechanical
strength at elevated temperature and also low thermal expansion.
Most of the zirconia bearing composites are expensive because
in most of the cases pure zirconia is used as one of
the ingredients. On the other hand sintering of pure mullite is
difficult due to its low lattice diffusion coefficient and
requires temperature as high as 1650° to 1700°C. The
preparation
of mullite-zirconia composites through reaction sintering
method has attracted attention of many workers since good
quality zircon sand is readily available from beach sand which
makes the process cost effective. India is fortunately having a
vast reserve of zircon sand that can be commercially exploited
for such development.
These composites are now finding application as refractory material in steel industry for making well blocks, continuous casting nozzles and their inserts. Due to the high melting point and excellent chemical properties, one of the major fields of application is in glass industries for lining of port necks, pavings, monolithic subpavings, checkers, burner blocks etc.
The use of the sintering additives such as magesia,
calcium oxide, titania and yttria for effective sintering at
o lower temerature (1400 C) and for enhancing the sintering
during preparation of mullite-zirconia composites have
been extensively studied.
1. P. Pena, J.S. Moya, S. de Aza, E. Cambier F. Camlier, C. Lablud and M.R. Anseau, J. Mater, Lett. 2 (12)772-4 (1983).
2. P. Miranzo, P. Pena, J.S. Moya and S. de Aza, J. Mater. Sc. 20(8) , 2702 - 10(1983) .
3. B. Joliet , F. Camlier, L. Dapra, C. Leblvd and A. Leriche,
S.J. de Physique, 47, C-l-723 - 28 (1986) ed. P. Odier, F.
Cabanes and B. Cales, Les Edition.
4. P. Pena, P. Maranjo, J.S. Moya and S. de Aza, J. Mater Sc, 20(6) 2011-22 (1985)
5. J. Rinkon, G. Thomas, P. Pena, S. de Aza and J.S. Moya, J. de. Physique, Vol 47, C1- 423 - 27 (1986).
6. P. Pena, M.A. Rodiguez, J.S. Maya and S. de Aza, High Tech Ceramics, Part-A, 861-70 (1987) ed. P. Vincenzini Pub. Elsevier Sc, Amsterdam.
7. M.F. Melo, J.S. Moya, P. Pena and S. de Aza, J. Mater Sc. 20 (8) 2711-18 (1985)
8. J. Ma Rincon, J.S. Moya and M.F. Melo, Trans Br. Ceram. Soc. 85(6), 201 - 06 (1986)
9. M. Fatima Melo adnd J.S. Moya, J. Mater. Sc, 25(4), 2082 - 86 (1990).
10.J. Wu and C. Lin, J. Mater. Sc. 26(12) 4631-36 (1991).
By the use of a new additive/dopant, dysprosia it has been successful in bringing about the overall improvement in the sintering, reaction and thermo-mechanical properties of sintered mullite-zirconia composites.
^Reaction sintering' is defined as a particular firing process in which chemical reaction of the starting - materials (reactants) and the elimination of porosity in the powder compact i.e. densification are both achieved in a single heat treatment step. This process has been found to be one of the simplest and least expensive methods. Depending upon the processing condition, temperature and particle size both the processes of reaction and densification (sintering) can occur either in sequentially or simultaneously. Zircon is rather a cheap material in comparison with zirconia suggesting its use for the preparation of zirconia based ceramics by reaction sintering routes. Many zirconia bearing ceramics are prepared from naturally occuring zircon. It is an established fact that the mineral zircon as a chemically pure substance decomposes in a solid state at a temperature of 1676 C giving
a mixture of tetragonal zirconia and -cristobalite. According
to the system ZrO2-Sio2

(Equation Removed)
The reaction sintering involves an in situ reaction between zircon and alumina to form mullite with dispersed zirconia as a second phase.
(Equation Removed)
Alumina and zircon react at temperature greater than o 1400 C to form mullite with zirconia particles distributed
primarily along the grain boundary. The alumina to zircon
mol ratio varies between 1 and 2. But maximum - amount of
crystalline mullite and zirconia were obtained when the ratio
is 1.5. The number of contacts between zircon and alumina
grains were at a maximum.
The main object of the present invention is to provide an improved process for the preparation of mullite-zirconia composites with high density, high tetragonal zirconia content, low thermal expansion co-efficient, high flexural strength and high fracture toughness.
Another objective of this invention is to provide a simple, inexpensive and easily adaptable process for the
manufacture of highly dense mullite-zirconia composites employing zircon flour instead of pure expensive zirconia
Another objective of the present invention is to use an additive / dopant, dysprosia (Dy2O3) to accelerate the reaction and sintering processes.
Accordingly the present invention provides an improved process for the preparation of mullite-zirconia composite which comprises:
a) mixing calcined alumina with the zircon flour in a ratio which ranges from 1 to 2,
b) adding dysprosia in the range of 2.5 to 4.5 wt% to the mixture obtained in step (a),
c) attritor milling the mixture obtained in step (b), for a period over 6 hr , in the presence of an organic solvent to obtain submicrometer powder,
d) drying the powder and adding a binder such as herein described, in the range from 5-15 wt% to the dried product,
e) pressing the obtained product uniaxially at a pressure in the range of 70 to 100 Mpa followed by isostatic pressing at a pressure in the range of 200 to 300 MPa, followed by
f) drying and firing at a temperature in the range of 1400 to 1650°C for a period ranging from 1 to 4 hrs to get the mullite-zirconia composite.
The mixture of calcined alumina and zircon flour was taken
in the ratio which ranges from 1 to 2. Dysprosium oxide was
added as an additive to the mixture in the range of 2.5 to 4.5
wt%. The mixture obtained in step (b) was attritor milled for 6
hrs in the presence of an organic solvent. The organic solvent
used in step (c) may be isopropyl alcohol, ethyl alcohol, hexane
or ethanol. This dried powder was mixed with the binder. The
binder used in step (d) may be polyvinyl alcohol, dextrine,
glycol and the amount used may range from 5 to 15 wt%
o The pressed material obtained in step (e) was dried at 110 +
o ' o o
5 C and fired at temperatures ranging from 1400 to 1650 C for
2 hours soaking period.
The characteristics of the composite prepared by the
hitherto known process and the process of the present
invention are summarised below in TABLE - 1.
TABLE 1:
(Table Removed)
Table 1 shows the relative density of the invention, which was measured by Archimedes Principle, to be comparatively higher than the existing ones. Room temperature strength was measured in an Instron machine with three point loading. Fracture Toughness was measured by indenting the samples with Vick-ers' microhardness indenter. Fracture Toughness is much higher and signifies that it can survive a lot more wear and tear in
use. Thermal expansion coefficient which was measured in a
dilotometer upto 1400 C shows the significantly lower coefficient of expansion. Lower thermal expansion coefficient improves the thermal spalling resistance of the material and it can withstand much larger number of cycles of temperature fluctuations. High temperature flexural strength measured in a 3 point bending strength tester is much higher for the -invention, compared to the products available. One of the factors for the improvement of the properties is the retention of higher percentage tetrogonal zirconia phase, at room temperature measured by X-ray diffraction.
The invention will now be described in detail with the help of following example for carrying out the process in actual practice. However, these examples should not be considered as to limit the scope of the invention as these are produced to illustrate the invention only.
Example-1
Zircon flour (59wt%) and calcined alumina (41wt%) was
attritor milled for 6 hours with isopropyl alcohol as the
o dispersing medium. After drying the slurry at 110 C the
powder was mixed with 5 wt% poly vinyl alcohol solution to
form discs and bars. Uniaxial pressing at 100 MPa was
followed by isostatic pressing around 275 MPa. The pressed
o discs and bars were sintered at temperature 1550 C for 2 hrs.
soaking time. The sintered mullite-zirconia composites showed
relative density 98% and porosity 2.6% . Tetragonal
zirconia content was found to be 22.2%. The thermal
expansion coefficient was 4.87x10-6/°C with martensitic
transformation temperature (Ms) at 725 The average
intergranular zirconia particle size was 2.0um. Room
temperature flexural strength varied between 22 MPa to 155
MPa and depended on the sintering temperature. Hot flexural
strength at 1200 C was 215MPa. Hardness and fracture toughness
was about 6 GPa and 3.5 MPa respectively.
Example-2
Zircon flour (58.0wt%), calcined alumina (39.5wt%)
and dysprosia around 2.5wt% were attritor milled for 6
hours with isopropyl alcohol as the dispersing medium. After
o drying the slurry at 110 C the powder was mixed with 5 wt%
poly vinyl alcohol solution to form discs and bars. Uniaxial
pressing at 100 MPa was followed by isostatic pressing around
275 MPa. The pressed discs and bars were sintered at temper-
o ature 1550 C for 2 hr soaking time. The relative density
of the product was 98.9% with porosity 1.5%. The tetragonal
zirconia content was found to be 3 5% Thermal expansion coef-
-6 o ficient was 3.83x10 / C with transformation temperature (Ms)
of 725 C. Intergranular zirconia grain size was 3.52um. Room
temperature flexural strength varied between 26 MPa to 328
MPa depending on the sintering temperature. Hot flexural
strength at 1200 C was 290 MPa. Hardness and fracture
1/2 toughness were 8 GPa and 4.9MPa.m respectively.
Example - 3
Zircon flour (57.0 wt%), calcined alumina (39.0 wt%) and
dysprosia around 3.5 wt% were attritor milled for 6 hours with
isopropyl alcohol as the dispersing medium. After drying the
0 slurry at 110 C the powder was mixed with 5 wt% polyvinyl
alcohol solution to form discs and bars. Uniaxial pressing
at 100 MPa was followed by isostatic pressing around 275
MPa. The pressed discs and bars were sintered at temperature
2 hr soaking time. The relative density of the product was
98.5% with the porosity of 2% The tetragonal zirconia content
was found to be 25%. The thermal expansion coefficient obtained
-6 o was 3.8 x 10 / C and Ms temperature was 690 C. Flexural
strength at ambient temperature was between 21MPa to 189 MPa depending on the temperature of sintering. The average inter-granular zirconia particle was found to be of 4.3um size. Hot
flexural strength at 120°C was 218 MPa. The hardness and
1/2 fracture toughness values were 8 GPa and 7.2 MPa.m
respectively.
Example - 4
About (56.5 wt%) zircon flour and calcined alumina (39 wt%) and
around 4.5 wt% dysprosia was attritor milled for 6 hours
in isopropyl alcohol as a dispersing medium. After drying the
slurry at 110° C the powder was mixed with 5 wt% poly vinyl
alcohol solution to form discs and bars. Uniaxial pressing at
100 MPa was followed by isostatic pressing around 275 MPa. The
pressed discs and bars were sintered at temperature 1550°C
with 2 hr soaking time. The relative density of the product
was found to be 99.0% with porosity of 1%. The relative
tetragonal zirconia content in the sintered product was 34%. The
average intergranular zirconia particle size was about 4.75
urn size. The thermal expansion coefficient was found to be
3.52x10-6/°C with a Ms temperature of 725 C.
Flexural strength at ambimbient temperature varied between 19
MPa to 197 MPa depending on the sintering temperature.
The hot flexural strength at 1200 C was found to be 220 MPa.
The Vicker's hardness and fracture toughness was about 6 GPa and
1/2 4.3 MPa.m respectively.
Advantages:
(1) Composites obtained by the process of the present invention
have high density, high tetragonal zirconia content, high
flexural strength, high fracture toughness and low thermal
expansion coefficient.
(2) The process makes use of naturally occuring zircon as a
starting material making the process economic.




We claim:
1. An improved process for the preparation of mullite-zirconia
composite which comprises:
a) mixing calcined alumina with the zircon flour in a ratio which ranges from 1 to 2,
b) adding dysprosia in the range of 2.5 to 4.5 wt% to the mixture obtained in step (a),
c) attritor milling the mixture obtained in step (b), for a period over 6 hr , in the presence of an organic solvent to obtain submicrometer powder,
d) drying the powder and adding a binder such as herein described, in the range from 5-15 wt% to the dried product,
e) pressing the obtained product uniaxially at a pressure in the range of 70 to 100 Mpa followed by isostatic pressing at a pressure in the range of 200 to 300 MPa, followed by
f) drying and firing at a temperature in the range of 1400 to 1650°C for a period ranging from 1 to 4 hrs to get the mullite-zirconia composite.
2. An improved process as claimed in claim 1 wherein the organic
solvent used in step © is such as isopropyl alcohol, ethyl alcohol,
hexane, ethane.
3. An improved process as claimed in claims 1 and 2 wherein the binder used in step (d) is such as polyvinyl alcohol, dextrin, glycerol, CMC and the amount used ranges from 5 wt% to 15 wt%.
4. An improved process for the preparation of mullite-zirconia composite substantially as herein described with reference to the examples.

Documents:

791-del-1997- correspondence-others.pdf

791-del-1997-abstract.pdf

791-del-1997-claims.pdf

791-del-1997-correspondence-po.pdf

791-del-1997-description (complete).pdf

791-del-1997-form-1.pdf

791-del-1997-form-19.pdf

791-del-1997-form-2.pdf

791-del-1997-form-3.pdf


Patent Number 194359
Indian Patent Application Number 791/DEL/1997
PG Journal Number 43/2004
Publication Date 23-Oct-2004
Grant Date 10-Feb-2006
Date of Filing 27-Mar-1997
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA
Inventors:
# Inventor's Name Inventor's Address
1 GOUTAM BANERJEE CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE CALCUTTA, INDIA
2 KABERI DAS (ROY) CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE CALCUTTA, INDIA
3 BARUNDEB MUKHERJEE CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE CALCUTTA, INDIA
PCT International Classification Number C01G 25/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA