Title of Invention | AN IMPROVED PROCESS FOR THE PREPARATION OF MULLITE-ZIRCONIA COMPOSITES" |
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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. |
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791-del-1997- correspondence-others.pdf
791-del-1997-correspondence-po.pdf
791-del-1997-description (complete).pdf
Patent Number | 194359 | ||||||||||||
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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:
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PCT International Classification Number | C01G 25/00 | ||||||||||||
PCT International Application Number | N/A | ||||||||||||
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