Title of Invention	"A PROCESS FOR MAKING SINTERED SILICON CARBIDE COMPOSITES USEFUL AS ENGINEERING CERAMICS "
Abstract	A process for making sintered silicon carbide composites useful as engineering ceramics by preparing a suspension of a mixture of SiC & Al2O3 in cerium nitrate solution, maintaining a pH in the range of 10 to 12, pouring this suspension in ammonium hydroxide solution to obtain a gel-like mass, ageing the gel-like mass for a period in the range of 8-24 hours, filtering and washing the aged gel-like mass, drying the material at a temperature in the range of 110±5°C for 8-12 hours, grinding the dried mass to pass through 100 mesh B.S., pallatising the above said dried mass isostatically under 180-250 MPa, sintering the pressed pellets so obtained at a temperature in the range of 1750-1950°C for a period in the range of 0.25 hour to 1 hour in an inert atmosphere to obtain silicon carbide composites.

Title of Invention

"A PROCESS FOR MAKING SINTERED SILICON CARBIDE COMPOSITES USEFUL AS ENGINEERING CERAMICS "

Abstract

A process for making sintered silicon carbide composites useful as engineering ceramics by preparing a suspension of a mixture of SiC & Al2O3 in cerium nitrate solution, maintaining a pH in the range of 10 to 12, pouring this suspension in ammonium hydroxide solution to obtain a gel-like mass, ageing the gel-like mass for a period in the range of 8-24 hours, filtering and washing the aged gel-like mass, drying the material at a temperature in the range of 110±5°C for 8-12 hours, grinding the dried mass to pass through 100 mesh B.S., pallatising the above said dried mass isostatically under 180-250 MPa, sintering the pressed pellets so obtained at a temperature in the range of 1750-1950°C for a period in the range of 0.25 hour to 1 hour in an inert atmosphere to obtain silicon carbide composites.

Full Text	The present invention relates to a process for making sintered Silicon Carbide composites useful as engineering ceramics. The main application of the Silicon Carbide-Cerium oxide- Aluminium Oxide composits are in the field of engineering ceramics for making component parts in the field of automobile industries, fluid transportation systems, space shuttle and in various other applications where superior thermo-mechanical properties are specified. The said material may also be used as special refractory material where thermal shock, abrasion, erosion, oxidative corrosion etc. are to be encountered. No known method of making Silicon Carbide-Cerium Oxide-Aluminium Oxide composites are available at present however several methods are available for making Silicon Carbide based composites containing Aluminium Oxide in combination with other rare earth oxides like La2O3, Sm2O3, HoO2, Gd2O3, & Y2O3 which essentially consists of using powder rare earth oxides and aluminium oxides for which references may be made to Zhao Chen, "Pressureless Sintering of SiC with Additives of Samarium Oxide & Alumina, Materials Letters, 17 (1993), pp-27-30, Zhao Chen & Lingfang Zeng , "Pressureless sintering of Silicon Carbide with additives of HoO2 and alumina, Mat. Res. Bull., vol. 30, No. 3, pp-265-70, (1995), Zhao Chen, "Effects of Gadolinia & Alumina Addition on The Densification & Toughening Behaviour of Silicon Carbide", J. Am. Ceram. Soc., & 79 (2), pp-530-32, (1996), M. Omorie & H. Takei, "Pressureless Sintering of SiC", J. Am. Ceram. Soc., 65, C-92 (1982), leading to a product formed at much higher temperature and with considerable inhomogeneity. In all the above processes the main drawbacks may be listed as below. 1. Homogeneous mixing of solid ingredients like Silicon Carbide, rare earth oxide, aluminium oxide etc. require special processing techniques and expensive equipments. 2. Many of the processes require hot pressing, the technique has several techno-economic problems. 3. All of the above processes require sintering temperature in the viscinity of 2000°C, too high a temperature for normal industrial operation. 4. High rigidity in fixing compositions, which are normally in a narrow range. 5. High level of stringency in process conditions making the process less flexible. 6. Reproducibility is lower. The main objective of the present invention is to provide a process for making sintered silicon carbide composites useful as engineering ceramics which obviates the drawbacks as detailed above., Another object of the present invention is to provide a silicon carbide composite containing cerium oxide and aluminium oxide in the starting raw materials. Yet another object of the present invention is to use gel-derived cerium oxide as additive for making the material. Still another object of the present invention is to use only pressureless sintering technique for making the material. Yet another object of the present invention is to lower down the sintering temperature from 2000 to below 1800°C. Still another object of the present invention is to provide a process wherein a wide rang of composition of ingredients allows increase in process flexibility. Yet another object of the present invention is to provide a process for making sintered shapes using the composite prepared by the process of the present invention. Accordingly, the present invention provides a process for making sintered silicon carbide composites useful as engineering ceramics which comprises preparing a suspension of a mixture of CeO2 , SiC& Al2O3 in cerium nitrate solution, pouring this suspension in ammonium hydroxide solution to obtain a gel-like mass, maintaining a pH in the range of 10 to 12, ageing the gel-like mass for a period in the range of 8-24 hours, filtering and washing the aged gel-like mass, drying the material at a temperature in the range of 110±5°C for 8-12 hours, grinding the dried mass to pass through 100 mesh B.S., pelletising the above said dried mass isostatically under 180-250 MPa, sintering the pressed pellets so obtained at a temperature in the range of 1750-1950°C for a period in the range of 0.25 hour to 1 hour in an inert atmosphere to obtain silicon carbide composites. In an embodiment of the present invention the weight ratio of cerium nitrate to alumina may be in the range of 0.10 to 2.20. In another embodiment of the present invention the ceria and alumina used may be 5 to 25 percent of SiC. In still another embodiment of the present invention the inert atmosphere provided may be such as N2, Ar, He. In yet another embodiment of the present invention the inert atmosphere may preferably be N2. The details of the process of the invention are given below : 1. Silicon carbide and aluminium oxide is mixed with cerium nitrate solution by constant stirring to prepare a suspension. 2. The entire suspension is poured in concentrated ammonia solution the pH of which is maintained at in the range of 10 to 12 by addition of ammonia. 3.The gel-like mass is aged for 8-24 hours. 4. The gel-like mass is filetred, washed and dried at a temperature in the range of a period of 110±5°C for a period in the range of 8-12 hours. 5. The dried mass is ground and passed through 100 mesh B. S. 6. The ground mass is pelletised isostatically under a pressure in the range of 180-250 Mpa. 7. The pellets are fired at 1750-1950°C for 0.25-1 hour in argon /nitrogen atmosphere, preferably in nitrogen atmosphere. The process of the present invention can be used to produce sintered silicon carbide and silicon carbide based composit materials consisting of cerium oxide and aluminium oxide of various shapes and sizes required for application as engineering as well as refractory material. Silicon carbide is difficult to sinter due to its high covalency factor. For effective densification, volume or grain boundary diffusivity needs to be activated. Some rare earth oxides including that of cerium are anticipated to be effective sintering aid for silicon carbide, as suggested from thermodynamic point of view. Aluminium oxide is well established as effective sintering aids for silicon carbide. However the use of aluminium oxide require very stringent process control otherwise reaction between silicon carbide and aluminium oxide will lead to excessive loss of material due to the formation of several volatile material at the sintering temperature resulting into a porous product. As regard to cerium oxide, though a theoretically recommended material, no record of its ability to act as sintering aid for silicon carbide is found. When cerium oxide and aluminium oxide mixture is used in the viscinity of the the eutectic zone (1835°C), it is expected that both oxide will react to form aluminate of various Ce4+ : A13+ ratio. In this way reactivity of A12O3 is reduced to a large extent in relation to its reaction towards silicon carbide at sintering temperature. In the present system, sintering or densification of compacts are much faster which competes with dissociation reaction as a result of which compact mass with no measurable open porosity forms. Microstructural evaluation reveals the presence of cerium aluminate at the grain boundary. An well distributed cerium aluminate phase, which is formed during heat treatment, increases fracture toughness of the material. Hydrogel derived cerium oxide helps in the formation of cerium aluminate at lower temperature, far below the sintering temperature of the material. The following examples are given by the 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 1.23 ml. of cerium nitrate solution s 0.41 gm. of CeO2, 4.59 gm. of Al2O3 & 95 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere. EXAMPLE 2 6.31 ml. of cerium nitrate solution s 2.10 gm. of CeO2, 2.90 gm. of A12O3 & 95 gm. of p-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.5 hour in nitrogen atmosphere. EXAMPLE 3 8.92 ml. of cerium nitrate solution 2.96 gm. of CeO2, 7.04 gm. of A12O3 & 90 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1750°C for a soaking time of 1.0 hour in argon atmosphere. EXAMPLE 4 15.92 ml. of cerium nitrate solution  5.30 gm. of CeO2, 4.70 gm. of A12O3 & 90 gm. of p-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere. EXAMPLE 5 18.93 ml. of cerium nitrate solution =6.30 gm. of CeO2, 8.70 gm. of A12O3 & 85 gm. of P-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.5 hour in argon atmosphere. EXAMPLE 6 17.84 ml. of cerium nitrate solution  5.92 gm. of CeO2, 14.08 gm. of A12O3 & 80 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.25 hour in nitrogen atmosphere. EXAMPLE 7 25.24 ml. of cerium nitrate solution 8.40 gm. of CeO2, 11.60 gm. of A12O3 & 80 gm. of p-SiC was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere. EXAMPLE 8 22.30 ml. of cerium nitrate solution s-7.40 gm. of CeO2, 17.60 gm. of A12O3 & 75 gm. of (3-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.75 hour in nitrogen atmosphere. EXAMPLE 9 31.55 ml. of cerium nitrate solution 10.50 gm. of CeO2, 14.50 gm. of A12O3 & 75 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in argon atmosphere. EXAMPLE 10 39.80 ml. of cerium nitrate solution =13.25 gm. of CeO2, 11.75 gm. of A12O3 & 75 gm. of (3-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere. The results obtained by the process of present invention are given below by way of illustration. TABLE: Sintering Data of SiC Samples of Different Additive Composition Containing 5 to 25 wt. % of Additive Content (Table Removed) The main advantages of the present invention are: 1. Production of fully dense material by a simple processing technique, 2. Reduction of firing temperature to 1750°C from above 2000°C, 3. Wide ranging compositional tolerances making the process more flexible, 4. Sintering without going for hot pressing, 5. Imparting homogeneity to the product formed. We claim : 1. A process for making sintered silicon carbide composites useful as engineering ceramics which comprises preparing a suspension of a mixture of CeO2, SiC& AI203 in cerium nitrate solution, pouring this suspension in ammonium hydroxide solution to obtain a gel-like mass, maintaining a pH in the range of 10 to 12,, ageing the gel-like mass for a period in the range of 8- 24 hours, filtering and washing the aged gel-like mass, drying the material at a temperature in the range of 110±5°C for 8-12 hours, grinding the dried mass to pass through 100 mesh B.S., pelletising the above said dried mass isostatically under 180-250 MPa, sintering the pressed pellets so obtained at a temperature in the range of 1750-1950°C for a period in the range of 0.25 hour to 1 hour in an inert atmosphere to obtain silicon carbide composites. 2. A process as claimed in claim 1 wherein the weight of cerium nitrate to alumina are in the range of 0.10 to 2.20. 3. A process as claimed in claims 1-2 wherein the ceria and alumina used are 5 to 25 percent of SiC. 4. A process as claimed in claims 1-3 wherein the inert atmosphere provided by the gases are N2, Ar, He. 5. A process for making sintered silicon carbide composites useful as engineering ceramics substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process for making sintered Silicon Carbide composites useful as engineering ceramics.
The main application of the Silicon Carbide-Cerium oxide- Aluminium Oxide composits are in the field of engineering ceramics for making component parts in the field of automobile industries, fluid transportation systems, space shuttle and in various other applications where superior thermo-mechanical properties are specified. The said material may also be used as special refractory material where thermal shock, abrasion, erosion, oxidative corrosion etc. are to be encountered.
No known method of making Silicon Carbide-Cerium Oxide-Aluminium Oxide composites are available at present however several methods are available for making Silicon Carbide based composites containing Aluminium Oxide in combination with other rare earth oxides like La2O3, Sm2O3, HoO2, Gd2O3, & Y2O3 which essentially consists of using powder rare earth oxides and aluminium oxides for which references may be made to Zhao Chen, "Pressureless Sintering of SiC with Additives of Samarium Oxide & Alumina, Materials Letters, 17 (1993), pp-27-30, Zhao Chen & Lingfang Zeng , "Pressureless sintering of Silicon Carbide with additives of HoO2 and alumina, Mat. Res. Bull., vol. 30, No. 3, pp-265-70, (1995), Zhao
Chen, "Effects of Gadolinia & Alumina Addition on The Densification & Toughening Behaviour of Silicon Carbide", J. Am. Ceram. Soc., & 79 (2), pp-530-32, (1996), M. Omorie & H. Takei, "Pressureless Sintering of SiC", J. Am. Ceram. Soc., 65, C-92 (1982), leading to a product formed at much higher temperature and with considerable inhomogeneity. In all the above processes the main drawbacks may be listed as below.
1. Homogeneous mixing of solid ingredients like Silicon Carbide, rare earth
oxide, aluminium oxide etc. require special processing techniques and
expensive equipments.
2. Many of the processes require hot pressing, the technique has several
techno-economic problems.
3. All of the above processes require sintering temperature in the viscinity of
2000°C, too high a temperature for normal industrial operation.
4. High rigidity in fixing compositions, which are normally in a narrow
range.
5. High level of stringency in process conditions making the process less
flexible.
6. Reproducibility is lower.
The main objective of the present invention is to provide a process for
making sintered silicon carbide composites useful as engineering ceramics which obviates the drawbacks as detailed above.,
Another object of the present invention is to provide a silicon carbide
composite containing cerium oxide and aluminium oxide in the starting raw
materials.
Yet another object of the present invention is to use gel-derived cerium oxide
as additive for making the material.
Still another object of the present invention is to use only pressureless
sintering technique for making the material.
Yet another object of the present invention is to lower down the sintering
temperature from 2000 to below 1800°C.
Still another object of the present invention is to provide a process wherein a
wide rang of composition of ingredients allows increase in process
flexibility.
Yet another object of the present invention is to provide a process for
making sintered shapes using the composite prepared by the process of the
present invention.
Accordingly, the present invention provides a process for making sintered silicon carbide composites useful as engineering ceramics which comprises preparing a suspension of a mixture of CeO2 , SiC& Al2O3 in cerium nitrate solution, pouring this suspension in ammonium hydroxide solution to obtain a gel-like mass, maintaining a pH in the range of 10 to 12, ageing the gel-like mass for a period in the range of 8-24 hours, filtering and washing the aged gel-like mass, drying the material at a temperature in the range of 110±5°C for 8-12 hours, grinding the dried mass to pass through 100 mesh B.S., pelletising the above said dried mass isostatically under 180-250 MPa, sintering the pressed pellets so obtained at a temperature in the range of 1750-1950°C for a period in the range of 0.25 hour to 1 hour in an inert atmosphere to obtain silicon carbide composites.
In an embodiment of the present invention the weight ratio of cerium nitrate to alumina may be in the range of 0.10 to 2.20.
In another embodiment of the present invention the ceria and alumina used may be 5 to 25 percent of SiC.
In still another embodiment of the present invention the inert atmosphere provided may be such as N2, Ar, He.
In yet another embodiment of the present invention the inert atmosphere may preferably be N2.
The details of the process of the invention are given below : 1. Silicon carbide and aluminium oxide is mixed with cerium nitrate solution
by constant stirring to prepare a suspension.
2. The entire suspension is poured in concentrated ammonia solution the pH of which is maintained at in the range of 10 to 12 by addition of ammonia. 3.The gel-like mass is aged for 8-24 hours.
4. The gel-like mass is filetred, washed and dried at a temperature in the
range of a period of 110±5°C for a period in the range of 8-12 hours.
5. The dried mass is ground and passed through 100 mesh B. S.
6. The ground mass is pelletised isostatically under a pressure in the range of
180-250 Mpa.
7. The pellets are fired at 1750-1950°C for 0.25-1 hour in argon /nitrogen
atmosphere, preferably in nitrogen atmosphere.
The process of the present invention can be used to produce sintered silicon carbide and silicon carbide based composit materials consisting of cerium oxide and aluminium oxide of various shapes and sizes required for application as engineering as well as refractory material. Silicon carbide is difficult to sinter due to its high covalency factor. For effective densification, volume or grain boundary diffusivity needs to be activated. Some rare earth oxides including that of cerium are anticipated to be effective sintering aid for silicon carbide, as suggested from thermodynamic point of view.
Aluminium oxide is well established as effective sintering aids for silicon carbide. However the use of aluminium oxide require very stringent process
control otherwise reaction between silicon carbide and aluminium oxide will lead to excessive loss of material due to the formation of several volatile material at the sintering temperature resulting into a porous product. As regard to cerium oxide, though a theoretically recommended material, no record of its ability to act as sintering aid for silicon carbide is found. When cerium oxide and aluminium oxide mixture is used in the viscinity of the the eutectic zone (1835°C), it is expected that both oxide will react to form
aluminate of various Ce4+ : A13+ ratio. In this way reactivity of A12O3 is
reduced to a large extent in relation to its reaction towards silicon carbide at sintering temperature. In the present system, sintering or densification of compacts are much faster which competes with dissociation reaction as a result of which compact mass with no measurable open porosity forms. Microstructural evaluation reveals the presence of cerium aluminate at the grain boundary. An well distributed cerium aluminate phase, which is formed during heat treatment, increases fracture toughness of the material. Hydrogel derived cerium oxide helps in the formation of cerium aluminate at lower temperature, far below the sintering temperature of the material.
The following examples are given by the 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
1.23 ml. of cerium nitrate solution s 0.41 gm. of CeO2, 4.59 gm. of Al2O3 & 95 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere.
EXAMPLE 2
6.31 ml. of cerium nitrate solution s 2.10 gm. of CeO2, 2.90 gm. of A12O3 & 95 gm. of p-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in
acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.5 hour in nitrogen atmosphere.
EXAMPLE 3
8.92 ml. of cerium nitrate solution 2.96 gm. of CeO2, 7.04 gm. of A12O3 & 90 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1750°C for a soaking time of 1.0 hour in argon atmosphere.
EXAMPLE 4
15.92 ml. of cerium nitrate solution  5.30 gm. of CeO2, 4.70 gm. of A12O3 & 90 gm. of p-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground
in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere.
EXAMPLE 5
18.93 ml. of cerium nitrate solution =6.30 gm. of CeO2, 8.70 gm. of A12O3 & 85 gm. of P-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.5 hour in argon atmosphere.
EXAMPLE 6
17.84 ml. of cerium nitrate solution  5.92 gm. of CeO2, 14.08 gm. of A12O3 & 80 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground
in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.25 hour in nitrogen atmosphere.
EXAMPLE 7
25.24 ml. of cerium nitrate solution 8.40 gm. of CeO2, 11.60 gm. of A12O3 & 80 gm. of p-SiC was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in nitrogen atmosphere.
EXAMPLE 8
22.30 ml. of cerium nitrate solution s-7.40 gm. of CeO2, 17.60 gm. of A12O3 & 75 gm. of (3-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground
in acetone medium & calcined at 900°C to drive-off the chemically bonded

water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1900°C for a soaking time of 0.75 hour in nitrogen atmosphere.
EXAMPLE 9
31.55 ml. of cerium nitrate solution 10.50 gm. of CeO2, 14.50 gm. of A12O3 & 75 gm. of ß-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a soaking time of 0.5 hour in argon atmosphere.
EXAMPLE 10
39.80 ml. of cerium nitrate solution =13.25 gm. of CeO2, 11.75 gm. of A12O3 & 75 gm. of (3-SiC powder was taken & to which 1.5 ml. of NH3 & 48.5 ml. of distilled water was added & stirred vigorously. The whole mixture was kept for 24 hours. It is then dried at 110°C. The dried mass was ground
in acetone medium & calcined at 900°C to drive-off the chemically bonded
water. The calcined mass is ground again in acetone medium & pelletised
under a pressure of 200 Mpa. The pellets are sintered at 1950°C for a
soaking time of 0.5 hour in nitrogen atmosphere.
The results obtained by the process of present invention are given below by
way of illustration.
TABLE: Sintering Data of SiC Samples of Different Additive Composition
Containing 5 to 25 wt. % of Additive Content

(Table Removed)
The main advantages of the present invention are:
1. Production of fully dense material by a simple processing technique,
2. Reduction of firing temperature to 1750°C from above 2000°C,
3. Wide ranging compositional tolerances making the process more flexible,
4. Sintering without going for hot pressing,
5. Imparting homogeneity to the product formed.

We claim :
1. A process for making sintered silicon carbide composites useful as
engineering ceramics which comprises preparing a suspension of a mixture
of CeO2, SiC& AI203 in cerium nitrate solution, pouring this suspension in
ammonium hydroxide solution to obtain a gel-like mass, maintaining a pH in
the range of 10 to 12,, ageing the gel-like mass for a period in the range of 8-
24 hours, filtering and washing the aged gel-like mass, drying the material at
a temperature in the range of 110±5°C for 8-12 hours, grinding the dried
mass to pass through 100 mesh B.S., pelletising the above said dried mass
isostatically under 180-250 MPa, sintering the pressed pellets so obtained at
a temperature in the range of 1750-1950°C for a period in the range of 0.25
hour to 1 hour in an inert atmosphere to obtain silicon carbide composites.
2. A process as claimed in claim 1 wherein the weight of cerium nitrate to
alumina are in the range of 0.10 to 2.20.
3. A process as claimed in claims 1-2 wherein the ceria and alumina used are 5
to 25 percent of SiC.
4. A process as claimed in claims 1-3 wherein the inert atmosphere provided by
the gases are N2, Ar, He.
5. A process for making sintered silicon carbide composites useful as
engineering ceramics substantially as herein described with reference to the
examples.

Documents:

290-del-1999-abstract.pdf

290-del-1999-claims.pdf

290-del-1999-correspondence-others.pdf

290-del-1999-correspondence-po.pdf

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

290-del-1999-form-1.pdf

290-del-1999-form-19.pdf

290-del-1999-form-2.pdf

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Patent Number

216550

Indian Patent Application Number

290/DEL/1999

PG Journal Number

13/2008

Publication Date

28-Mar-2008

Grant Date

14-Mar-2008

Date of Filing

19-Feb-1999

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	SANKAR GHATAK	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032.
2	KAJAL KUMARDHARGUTA	CENTRAL LEATHER RESEARCH INSTITUTE, ADYAR, CHENNAI-600 020, INDIA,
3	SANTANUMANDAL	CENTRAL LEATHER RESEARCH INSTITUTE, ADYAR, CHENNAI-600 020, INDIA,

PCT International Classification Number

C04B 35/56

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA