Title of Invention

"A PROCESS FOR MAKING SINTERED SILICON CARBIDE-SAMARIUM OXIDE-ALUMINIUM OXIDE COMPOSITES"

Abstract A process for making sintered silicon carbide-samarium oxide-aluminum oxide composite useful as engineering ceramics which comprises preparing a suspension of a mixture SiC &. Al2O2 in samarium nitrate solution containing samarium oxide, maintaining a pH the range of 6.9 to 7.5, 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 said aged gel-like mass, followed by drying the material at temperature in the range of 110± 5°C for 8-12 hours, grinding the said dried mass pass through 100 mesh B.S., pelletising the said ground 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
Full Text The present invention relates to a process for making sintered silicon carbide-samarium oxide-aluminium oxide composites useful as engineering ceramics.
The main application of the silicon carbide-samarium oxide-aluminium oxide composites 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-samarium 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 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

teramics which obviates the drawbacks as detailed above.
Another object of the present invention is to provide a silicon carbide
composite containing samarium oxide and aluminium oxide in the
starting raw materials.
Yet another object of the present invention is to use gel-derived
samarium 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 1750°C.
Still another object of the present invention is to provide a process
wherein a wide range 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 SiC & A12O3 in
samarium nitrate solution, maintaining a pH in the range of 6.9 to 7.5,
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 a period in the range of 8-
12 hours, grinding the dried mass to pass through 100 mesh B.S.,
pelletising the ground mass isostatically under 180-250 MPa, sintering
the pressed pellets so obtained at a temperature in the range of 1750-
1950°C for 0.25 hour to 1 hour in an inert atmosphere.
In an embodiment of the present invention the weight ratio of samarium
nitrate to alumina may be in the range of 0.45 to 4.35.
In another embodiment of the present invention the samarium oxide 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 samarium 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 6.9 to 7.5 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 composite materials consisting of samarium 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 samarium 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 samarium oxide, though a theoretically recommended material, no record of its ability to act as sintering aid for silicon carbide is found. When samarium oxide and aluminium oxide mixture is used in the viscinity of the lower eutectic zone (1755°C), it is expected that both oxide will react to form aluminate of various Sm3+ : 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 samarium aluminate at the grain boundary. An well distributed samarium aluminate phase, which is formed during heat treatment, increases fracture toughness of the material. Hydrogel derived samarium oxide helps in the formation of samarium 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
17.59 ml. of samarium nitrate solution a 5.504 gm. of Sm2O3, 14.496 gm. of A12O3 & 80 gm. of p-SiC powder was taken & to which 20 ml. of NH3 & 30 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
47.45 ml. of samarium nitrate solution = 14.85 gm. of Sm2O3, 10.15 gm. of A12O3 & 75 gm. of P-SiC powder was taken & to which 40 ml. of NH3 & 10.0 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 1.0 hour in argon atmosphere.
EXAMPLE 3
55.5 ml. of samarium nitrate solution si 7.37 gm. of Sm2O3, 7.63 gm. of A12O3 & 75 gm. of p-SiC powder was taken & to which 45.0 ml. of NH3 & 10.0 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 nitrogen atmosphere.
EXAMPLE 4
47.48 ml. of samarium nitrate solution = 14.85 gm. of Sm2O3, 10.15 gm. of A12O3 & 75 gm. of ß-SiC powder was taken & to which 40.0 ml. of NH3 & 10.0 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 1.0 hour in nitrogen atmosphere.
EXAMPLE 5
21.98 ml. of samarium nitrate solution =6.88 gm. of Sm2O3, 18.12 gm. of A12O3 & 75 gm. of p-SiC powder was taken & to which 20.0 ml. of NH3 & 30.0 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 1.0 hour in nitrogen atmosphere.
TABLE: Sintering Data of SiC Samples (as Referred in the Examples):

(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
lexible,
Sintering without going for hot pressing,
4. Imparting homogeneity to the product formed.



We claim
1. A process for making sintered silicon carbide-samarium oxide-aluminium oxide composites useful as engineering ceramics which comprises preparing a suspension of a mixture of SiC & A12O3 in samarium nitrate solution, maintaining a pH in the range of 6.9 to 7.5, 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., pelletising the ground 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.
2. A process as claimed in claim-1 wherein the weight of samarium
nitrate to alumina are in the range of 0.45 to 4.35.
3. A process as claimed in claims 1&2 wherein the samarium oxide and
alumina used are 5 to 25 percent of SiC.
4. A process as claimed in claims 1-3 wherein the inert atmosphere
provided are such as N2, Ar, He.
5. A process as claimed in claims 1-4 wherein the inert atmosphere
provided is preferably N2.
6. A process for making sintered silicon carbide-samarium oxide-aluminium oxide composites useful as engineering ceramics substantially as herein described with reference to the examples.

Documents:

1494-del-1999-abstract.pdf

1494-del-1999-claims.pdf

1494-del-1999-correspondence-others.pdf

1494-del-1999-correspondence-po.pdf

1494-del-1999-description (complete).pdf

1494-del-1999-form-1.pdf

1494-del-1999-form-19.pdf

1494-del-1999-form-2.pdf


Patent Number 221459
Indian Patent Application Number 1494/DEL/1999
PG Journal Number 31/2008
Publication Date 01-Aug-2008
Grant Date 23-Jun-2008
Date of Filing 18-Nov-1999
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110 001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SANKAR GHATAK CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
2 SANTANU MANDAL CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
3 KAJAL KUMAR DHARGUPTA CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, CALCUTTA 700 032, INDIA.
PCT International Classification Number B05D 3/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA