Title of Invention

"A PROCESS FOR MAKING SILICON CARBIDE -MULLITE-ALUMINA NANOCOMPOSITE"

Abstract

The present invention relates to a process for making silicon carbide - mullite - alumina nanocomposite, Process steps are : making suspension of silicon carbide powder of surface area in the range of Im2/gm to 16m2/gm in water, adding solution of soluble aluminium salt selected from nitrate inslphate, chloride in the range of 0.5 - 1.5 (M), wherein silicon carbide - alumina molar ratio used is in the range of 0.1 to 1.4, stirring for a period in the range of 3 hrs. to 5 hrs., adding ammonium hydroxide solution with constant stirring up to the attainment of pH in the range of 10.5 - 11.5 that forms a gel like mass, ageing the gel like mass for a period in the range of 24 hrs. to 48 hrs., and washing, drying at a temperature in the range of 110 + 5°C for a period in the range of 6 to 48 hrs. heat treating the dried gel-like mass at a temperature in the range of 800 - 850°C for 2 to 4 hrs, grinding, sieving, forming shapes, heat treating the shapes at temperature in the range of 1200 - 1400°C for a period in the range of 2 to 10 hrs. in ambient atmosphere, sintering the products formed at temperature in the range of 1600 to 1750°C for a period in the range of 0.5 to 4 hrs.

Full Text

The present invention relates to a process for making silicon carbide - mullite - alumina nanocomposite. The main usage of the silicon carbide - mullite - alumina nanocomposites is in the field of machines with static and moving parts where the atmosphere is oxidising or neutral and where the operation take places at room temperature, below room temperature and elevated temperature upto 1800°C. The component may act as component of a machine, as nozzle or in any shape as may be deemed fit. The said article may also be used as special refractory material where thermal shock, abrasion, erosion, oxidative corrosion etc. are to be countered at room temperature, below room temperature and at elevated temperature upto 1800°C. The present day methods of making silicon carbide base nanocomposites consists of mixing silicon carbide powder along with different metal and/or metal oxide powders in acetone medium followed by sieving, pressing and firing in inert atmosphere by using HIPing at the temperature in the range of 1700 - 1850°C references for which are made to "Effect of milling conditions on the strength of Alumina - Silicon Carbide Nanocomposites", Jeong, Y. K., Nakahira, A., Morgan, P. E. D., and Nihara K., J. Am Ceram., Soc., 80 (5) 1307-309 (1997); "Tensile creep of Alumina - Silicon Carbide 'Nanocomposites', Thompson, A. M., Chan, H. M., & Harmer, M. P., J. Am. Ceram., Soc., 80 (9) 2221-28 (1997); "In situ processing and properties of SiC/MoSia Nanocomposites", Lee, J. I., & Hecht, L. N., J. Am., Ceram., Soc., 81 (2) 421-24 (1998); "Silicon Nitride/ Silicon Carbide Nanocomposites Materials : II, Hot Strength, Creep, and Oxidation Resistance", Rendtel, A., Hubner, H., Hermann, M., & Schubert, C., J. Am. Ceram., Soc., 81(5) 1109 - 20, 1998, or it may be made by using metal or by metal oxide powder with silicon carbide powder, forming into shape followed by pre-firing in ambient atmosphere and by sintering in inert atmosphere references of which may be made to "New design concepts of structural ceramics - ceramics nanocomposites, Nihara, K., J. Ceram., Soc., Jpm 99 (10) 974-82 (1991); "Improvement of mechanical properties of natural mullite/SiC nanocomposites", Takada, H., Nakahira, A., Ohnishi, H., & Nihara, K., Jpn., J. Powder powder met, 38, 348-51 (1991), "Processing of Silicon Carbide - Mullite - Alumina Nanocomposites", Sakka, Y., Bidinger, D. D., & Aksay, I. A., J. Am. Ceram., Soc., 78(2) 479-86, 1995. The main drawbacks of the present day processes are listed below : 1. Silicon carbide powder should be very fine, more than 90 percent in the submicron range or in nano-size. 2. Silicon carbide powder should be ultra pure. 3. Aluminium powder is required in some processes which is costly and require special protective handling of the material. 4. Mixing process require costly equipment like attrition milling with costly solvent like acetone as milling medium. 5. Many of the processes require hot isostatic pressing which is not only a cost inhibitive process but also have several limitations. The main object of the present invention is to provide a process for making silicon carbide - mullite - alumina nanocomposite which obviates the drawbacks as detailed above. Still another object of the present invention is to use micron size SiC particles instead of nano size material. Yet another object of the present invention is to reduce the firing temperature of the green product. Still another object of the present invention is to increase the oxidation resistance of the component produced. Yet another object of the present invention is to eliminate the process of purification of the silicon carbide particles to a ultra pure material to impart sinterability to the material. Still another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite - alumina nanocomposites as ceramic engineering material. Yet another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite - alumina nanocomposites as refractory material with improved quality. Still another object of the present invention is to provide a process to make consolidated mass of / silicon carbide - mullite - alumina nanocomposites as engineering ceramic material to be used upto 1800°C. Yet another object of the present invention is to provide a process to make consolidated mass of silicon carbide - mullite - alumina nanocomposites as refractory material to be used upto 1800°C. Accordingly the present invention provides a process for making sintered silicon carbide -mullite - alumina nanocomposite which comprises; making suspension of silicon carbide powder of surface area in the range of Im2/gm to 16m2/gm in water, adding solution of soluble aluminium salt selected from nitrate, sulphate, chloride in the range of 0.5 - 1.5 (M), wherein silicon carbide - alumina molar ratio used is in the range of 0.1 to 1.4, stirring for a period in the range of 3 hrs. to 5 hrs., adding ammonium hydroxide solution with constant stirring up to the attainment of pH in the range of 10.5 - 11.5 that forms a gel like mass, ageing the gel like mass for a period in the range of 24 hrs. to 48 hrs., filtering and washing, drying at a temperature in the range of 110 + 5°C for a period in the range of 6 to 48 hrs., heat treating the dried gel-like mass at a temperature in the range of 800 - 850°C for 2 to 4 hrs, grinding, sieving, forming shapes, heat treating the shapes at temperature in the range of 1200 - 1400°C for a period in the range of 2 to 10 hrs. in ambient atmosphere, sintering the products formed at temperature in the range of 1600 to 1750°C for a period in the range of 0.5 to 4 hrs. In an embodiment of the present invention soluble aluminium salt may be such as nitrate, chloride, sulphate. Yet in another embodiment of the present invention silicon carbide - alumina molar ratio may be in the range of 0.1 to 1.4 Still in another embodiment of the present invention ammonia may be such as aqueous or in gaseous state. Yet in another embodiment of the present invention the forming processes used may be such as slip casting, uniaxial pressing, isostatic pressing. In still another embodiment of the present invention the inert atmosphere used in the sintering stage may be such as Ar, He. The details of the process of the present invention are given below : a) Silicon carbide powder is mixed with dispersing agents and water by constant stirring. b) Soluble aluminium salt solution is added in the silicon carbide slip prepared in step 'a' . c) The entire mixture is stirred for 3 - 5 hrs. d) Ammonium hydroxide solution is slowly added or ammonia is passed in the mixture produced by step 'a' - c' till pH is 10.5 -11.5. e) The entire mass converts to a semisolid gel like mass which is allowed to age for 24 - 48 hrs. f) Product formed in step 'e' is filtered and washed by warm water. g) The washed material is dried at 110 + 5 C for 24 hrs. h) The dried material obtained in step 'g' is heat treated at 800 -,850°C for 2-4 hrs. in air environment. i) The heat treated product is ground and sieved through 100 mesh. j) The sieved powder is formed into green shape by slip casting or uniaxial pressing or cold isostatic pressing. k) The green pressed product is fired in air environment at 1200 - 1400°C for 2 -10 hrs. 1) The product obtained in step 'k' is sintered at temperature in the range of 1600 - 1750°C in AT or He atmosphere for a period in the range of 0.5 to 4 hrs. The process of the present invention can be used to produce sintered silicon carbide - mullite -alumina nanocomposite of various green shapes and sizes required for application as engineering and refractory material. Silicon carbide particles are always associated with a surface layer of silica which forms highly reactive silicic acid in the aqueous suspension state. Soluble aluminium salt mixed thoroughly with the silicon carbide suspension, will be homogeneously distributed throughout the entire suspending mass. Ammonia solution will convert aluminium nitrate into aluminium hydroxide hydrogel that will remain intimately in contact with silicic acid perhaps forming alumino silicate hydrogel rich in aluminium. On drying, the gel like mass will form an uniform layer of alumino silicate gel over the surface of the silicon carbide grains. Due to extremely high reactivity, this will form mullite and reactive alumina. This reactive alumina will combine with freshly formed silica during firing again forming mullite as the silica alumina system is in the alumina rich region. Through this process particle size of SiC will be progressively reduced to lower sizes which will be embedded in oxide matrix of mullite and alumina. The following examples are given by way of illustration of the process of the present invention and should not be construed to limit the scope of the present invention. EXAMPLE -1 5 gms Silicon carbide of surface area 1.2 m2/gm is mixed with 4071 ml of 0.5(M)solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1628 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110 ± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1200°C for 6 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1650°C with Ihr. soaking. EXAMPLE - 2 10 gms Silicon carbide of surface area 1.6 m2/gm is mixed with 3856 ml of 0.5(M)solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1542 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5 C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1300°C for 8 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1700°C with 2hrs. soaking. EXAMPLE -3 15 gms Silicon carbide of surface area 2.2 m2/gm is mixed with 3642 ml of 0.5(M) solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1457 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hts. followed by filtration and washing with warm water. The entire mass is dried at 110± 5*C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by uniaxial pressing at 30 MPa pressure. The compacted articles are fired at 1400 C for 10 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1700°C with 3 hrs. soaking. EXAMPLE - 4 20 gms Silicon carbide of surface area 4.5 m2/gm is mixed with 1046 of 0.75(M) solution of aluminium sulphate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1371 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5 C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by slip casting. The compacted articles are fired at 1400°C for 4 hrs in ambient atmosphere followed by sintering in Ar atmosphere at 1600°C with 4 hrs. soaking. EXAMPLE - 5 25 gms Silicon carbide of surface area 6.4 m2/g is mixed with 2142 ml of 0.75(M) solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1286 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1350°C for 6 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1700°C with 2hrs. soaking. EXAMPLE - 6 30 gms Silicon carbide of surface area 8.2 m2/gm is mixed with 2000 ml of 0.75(M) solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1200 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by slip casting. The compacted articles are fired at 1300°C for 5 hrs. in ambient atmosphere followed by sintering in He atmosphere at 1650°C with 4 hrs. soaking. EXAMPLE - 7 35 gms Silicon carbide of surface area 10.8 m /gm is mixed with 889 ml of 1(M) solution of aluminium nitrate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v ) solution 1114 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1200 C for 8 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1700° with 3 hrs soaking. EXAMPLE - 8 20 gms Silicon carbide of surface area 12.5 m2/gm is mixed with 784.3 ml of 1(M) solution of aluminium sulphate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1371 ml is added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature 200 MPa pressure. The compacted articles are fired at 1350°C for 10 hrs. in ambient atmosphere followed by sintering in He atmosphere at 1700°C with 4hrs. soaking. EXAMPLE - 9 25 gms Silicon carbide of surface area 14.6 m2/gm is mixed with 735 ml of 1(M) solution of aluminium sulphate and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1286 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5 C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1350°C for 10 hrs. in ambient atmosphere followed by sintering in He atmosphere at 1750°C with 4hrs. soaking. EXAMPLE -10 15 gms Silicon carbide of surface area 16 m2/gm is mixed with 1667 ml of 1(M) solution of aluminium chloride and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1457 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by isostatic pressing at room temperature at 200 MPa pressure. The compacted articles are fired at 1300°C for 10 hrs. in ambient atmosphere followed by sintering in Ar atmosphere at 1750 C with 4hrs. soaking. EXAMPLE -11 25 gms Silicon carbide of surface area 16 m2/gm is mixed with 1961 ml of 0.75(M) solution of aluminium chloride and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1457 ml is slowly added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass through 100 mesh sieve. Articles of desired shapes are formed from the powder by uniaxial pressing at 30 MPa pressure. The compacted articles are fired at 1400°C for 10 hrs. in ambient atmosphere followed by sintering in He atmosphere at 1750°C with 2hrs. soaking. EXAMPLE -12 20 gms Silicon carbide of surface area 12.5 m2/gm is mixed with 3137 ml of 0.5 (M) solution of aluminium chloride and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1371 ml is added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110± 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 through mesh sieve. Articles of desired shapes are formed from the powder by uniaxial pressing at 30 MPa pressure. The compacted articles are fired at 1400°C for 8 hrs in ambient atmosphere followed by sintering in Ar atmosphere at 1750°C with 3 hrs. soaking. EXAMPLE -13 20 gms Silicon carbide of surface area 12.5 m2/gm is mixed with 3137 ml of 0.5 (M) solution of aluminium chloride and stirred for 3 hrs. Ammonium hydroxide of (1:1 v/v) solution 1371 ml is added into the mixture. The gel like mass that forms is aged for 24 hrs. followed by filtration and washing with warm water. The entire mass is dried at 110+ 5°C for 48 hrs. followed by heat treatment in air environment at 850°C for 2 hrs. The heat treated material is ground to pass 100 through mesh sieve. Articles of desired shapes are formed from the powder by uniaxial pressing at 30 MPa pressure. The compacted articles are fired at 1300°C for 10 hrs in ambient atmosphere followed by sintering in Ar atmosphere at 1750°C with 3 hrs. soaking. The main advantages of the process of the present invention are : 1. non-requirement of stringent limitations regarding fineness of silicon carbide powder, 2. purification of starting SiC powder is not required, 3. no aluminium powder is required for this process, 4. mixing process is simpler. The results obtained by the present invention are given below by way of illustration (Table Removed) WE CLAIM : 1. A process for making sintered silicon carbide - mullite - alumina nanocomposite which comprises; making suspension of silicon carbide powder of surface area in the range of Im2/gm to 16m2/gm in water, adding solution of soluble aluminium salt selected from nitrate, sulphate, chloride in the range of 0.5 - 1.5 (M), wherein silicon carbide - alumina molar ratio used is in the range of 0.1 to 1.4, stirring for a period in the range of 3 hrs. to 5 hrs., adding ammonium hydroxide solution with constant stirring up to the attainment of pH in the range of 10.5 - 11.5 that forms a gel like mass, ageing the gel like mass for a period in the range of 24 hrs. to 48 hrs., filtering and washing, drying at a temperature in the range of 110 + 5°C for a period in the range of 6 to 48 hrs., heat treating the dried gel-like mass at a temperature in the range of 800 - 850°C for 2 to 4 hrs, grinding, sieving, forming shapes, heat treating the shapes at temperature in the range of 1200 - 1400°C for a period in the range of 2 to 10 hrs. in ambient atmosphere, sintering the products formed at temperature in the range of 1600 to 1750°C for a period in the range of 0.5 to 4 hrs. 2. A process as claimed in claim 1 wherein ammonia used is in aqueous solution or in the gaseous state. 3. A process as claimed in claim 1 - 2 wherein forming processes used are slip-casting, uniaxial pressing, isostatic pressing. 4. A process as claimed in claims 1-3 wherein inert atmosphere used in the sintering stage are Ar or He. 5. A process for making silicon carbide - mullite - alumina nanocomposite substantially as herein described with reference to the examples.

Documents:

1458-del-1999-abstract.pdf

1458-del-1999-claims.pdf

1458-del-1999-correspondence-others.pdf

1458-del-1999-correspondence-po.pdf

1458-del-1999-description (complete).pdf

1458-del-1999-form-1.pdf

1458-del-1999-form-19.pdf

1458-del-1999-form-2.pdf