Title of Invention	"A PROCESS FOR PREPARATION OF SINTERING ADDITIVES IN POWDER FORM USEFUL FOR SINTERING OF SILICON CARBIDE"
Abstract	The invention relates to a process for preparation of sintering additives in powder form useful for sintering of silicon carbide. The process steps comprises; mixing aluminium oxide and aluminium nitride in the molar proportion of 5 to 95, preparing a slurry by known methods using organic solvents, drying the said slurry to obtain a powder, firing the powder at a temperature in the range of 1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours, grinding the product by known methods, in presence of organic solvents and drying the said slurry to obtain a powder.

Title of Invention

"A PROCESS FOR PREPARATION OF SINTERING ADDITIVES IN POWDER FORM USEFUL FOR SINTERING OF SILICON CARBIDE"

Abstract

The invention relates to a process for preparation of sintering additives in powder form useful for sintering of silicon carbide. The process steps comprises; mixing aluminium oxide and aluminium nitride in the molar proportion of 5 to 95, preparing a slurry by known methods using organic solvents, drying the said slurry to obtain a powder, firing the powder at a temperature in the range of 1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours, grinding the product by known methods, in presence of organic solvents and drying the said slurry to obtain a powder.

Full Text	The present invention relates to a process of making sintering additives useful for sintering of silicon carbide. The main usage of the sintering aid is to produce sintered silicon carbide articles to be used for making components in the field of machines with static and moving parts where the atmosphere is oxidising or neutral or reducing and where the operation takes place at room temperature, below room temperature and at an elevated temperature up to 2000°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, corrosion, oxidative corrosion etc. are to be countered at room temperature, below room temperature and at an elevated temperature up to 2000°C. The present day method of making sintered silicon carbide essentially consists of using oxides of different metals like aluminium, yttrium, rare earth, magnesium and their combinations as additives. In some cases boron, carbon and aluminium is used in elemental form either alone or in combination with oxide additives. Aluminium and boron nitride is also used in some cases of which reference may be made to "method for preparing sintered shapes of silicon carbide-M.Omori and H.Takei, U.S. Pat. 4564490, 1986" where oxide additives in combination with alumina promote sintering of silicon carbide. Magnesia-alumina combination was used by M.B.Trigg.-Australian Patent 00518, 1990. M.B.Trigg. also used sialon as an additive-Australian Patent 00271, 1988. Yttria-aluminium nitride combination was used by W.D.G. Bocker, European Patent 419271AZ, 1990. Advantage of formation of silicon carbide-aluminium nitride was used for sintering of silicon carbide by G.Erwin(Jr.)-US Patent 3492153, 1970. The main drawbacks of all the above processes are :- 1. Silicon carbide should be essentially in the submicron range. 2. Silicon carbide should be ultra pure. 3. For most of the additives like aluminium nitride or boron nitride pressureless sintering is not possible where as hot pressing or hot isostatic pressing is cost inhibitive and non-flexible. 4. For many oxide additives like aluminium oxide decomposition of silicon carbide at sintering temperature is difficult to control. 5. Amount of additives are in a narrow zone resulting into more stringent process control parameters. 6. With aluminium oxide-aluminium nitride mixed additives sintering atmosphere must be argon or such inert gas that will not promote formation and growth of intermediate aluminium ox nitride phase. The main object of the present invention is to provide a process for preparation of sintering additives powder useful for sintering of silicon carbide which obviates the above difficulties. Accordingly the present invention provides a process for preparation of sintering additives in powder form useful for sintering of silicon carbide which comprises; mixing aluminum oxide and aluminum nitride in attritor mill using alumina balls in the molar proportion of 5 to 95, separating the milled powder from alumina balls by sieving through 100 mesh B.S., preparing a slurry by known methods using organic solvents as herein described, drying the said slurry to obtain a powder, firing the said powder containing aluminum oxide and aluminium nitride at a temperature in the range of 1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours, grinding the product by known methods in presence of organic solvents as defined above, drying the said slurry to obtain sintering additives in powder form. known methods in presence of organic solvents as defined above, drying the said slurry to obtain sintering additives in powder form. In still another embodiment of the present invention grinding of fired products may be done by using alumina balls of diameter in the range of 1-3 mm for a period in the range of 1 -5 hours. In yet another embodiment of the present invention the milled powder may be separated from alumina balls by sieving through 100 mesh B.S. The details of the processes of the present invention are given below: a) Aluminium oxide and aluminium nitride is mixed in molar proportion of 5 to 95 in organic medium such as acetone, hexane aided by alumina balls in an attrition mill or any mixer useful for mixing ceramic raw material for a period in the rnage of 15 minutes to 3 hours. b) After milling, the slurry is sieved through 100 mesh B.S., dried the powder mixture & fired at temperature in the range of 1750-1850°C under nitrogen atmosphere. c) The fired product is ground by attrition mill or any grinder useful for grinding ceramic raw material using organic solvents such as hexane, acetone and alumina balls are used of diameter in the range of 1-3 mm for a period in the range of 1-5 hours. d) After milling the product slurry was sieved through 100 mesh B.S. to separate ground powder from alumina balls. When silicon carbide is sintered by using alumina alone, silicon carbide is dissociated and impedes sintering by following reactions :-SiC + A12O3 = SiOt + Al2Ot + COt 2 SiC + SiO2 - 3 Sit + 2 COt 2 SiC + A12O3 = 2 Sit + Al2Ot +2 COt 3 SiC + A1203 = 3 Sit+ 2 Alt + COt On the other hand, when aluminium nitride is used alone, only hot pressing leads to densification, that too in argon atmosphere only as excessive grain growth of aluminium nitride in nitrogen atmosphere at sintering atmosphere impedes sintering. When a combination of both A1N and Al2O3 powder is used in the SiC system, both the above factors will operate and full densification is not achievable. The situation will become more complex if nitrogen atmosphere is used due to the formation of A1ON, an intermediate in the nitridation of Al2O3 to A1N, which competes with sintering of SiC leading to sluggish densification. Preheat-treatment of Al2O3 and A1N mixture forms series of solid solutions by which the unfavourable reactions are effectively eliminated and complete densification could be achieved under both argon and nitrogen atmosphere. The entire range of solid solutions have wide range of compatibility with silicon carbide making wide ranging compositions between SiC and additive possible. The following example are given by way of illustration and should not be construed to limit the scope of the present invention. Example -1 97.02 g of aluminium oxide is mixed with 2.98 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 3 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. The dried material is used as additive for sintering of silicon carbide. Example -2 95.7 g of aluminium oxide is mixed with 4.3 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for 2.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -3 92.5 g of aluminium oxide is mixed with 7.5 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 2.0mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1750°C in nitrogen atmosphere for 3.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -4 90.34 g of aluminium oxide is mixed with 9.66 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 2.0mm for 2 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 2.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -5 92.5 g of aluminium oxide is mixed with 7.5 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.0mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -6 81.75 g of aluminium oxide is mixed with 18.25 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for 2.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -7 38.34 g of aluminium oxide is mixed with 61.66 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1750°C in nitrogen atmosphere for 4.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.0mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -8 26.22 g of aluminium oxide is mixed with 73.78 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 3.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 3 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -9 95.7 g of aluminium oxide is mixed with 4.3 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for 2.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -10 81.75 g of aluminium oxide is mixed with 18.25 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. The main advantages of the process of the present invention are: 1. Non-requirement of stringent limitations regarding fineness of silicon carbide powder. 2. Non-requirement of purification of starting silicon carbide powder to a very high level. 3. Pressureless sintering gives full densification. 4. Decomposition of silicon carbide at sintering temperature is in the acceptable minimum. 5. Sintering atmosphere may be nitrogen, argon or any inert gas. 6. A wide composition range of SiC : additive possible making the compositional aspect flexible. We Claim: 1. A process for preparation of sintering additives in powder form useful for sintering of silicon carbide which comprises; mixing aluminum oxide and aluminum nitride in attritor mill using alumina balls in the molar proportion of 5 to 95, separating the milled powder from alumina balls by sieving through 100 mesh B.S., preparing a slurry by known methods using organic solvents as herein described, drying the said slurry to obtain a powder, firing the said powder containing aluminum oxide and aluminium nitride at a temperature in the range of 1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours, grinding the product by known methods in presence of organic solvents as defined above, drying the said slurry to obtain sintering additives in powder form. 2. A process as claimed in claims 1 wherein the organic solvents used are acetone or hexane. 3. A process as claimed in claims 1-2 wherein grinding of fired products are done by using alumina balls of diameter in the range of 1-3 mm for a period in the range of 1-5 hours. 4. A process for preparation of sintering additives in powder form useful for sintering of silicon carbide substantially as herein described with reference to the examples.

Full Text

The present invention relates to a process of making sintering additives useful for sintering of silicon carbide.
The main usage of the sintering aid is to produce sintered silicon carbide articles to be used for making components in the field of machines with static and moving parts where the atmosphere is oxidising or neutral or reducing and where the operation takes place at room temperature, below room temperature and at an elevated temperature up to 2000°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, corrosion, oxidative corrosion etc. are to be countered at room temperature, below room temperature and at an elevated temperature up to 2000°C.
The present day method of making sintered silicon carbide essentially consists of using oxides of different metals like aluminium, yttrium, rare earth, magnesium and their combinations as additives. In some cases boron, carbon and aluminium is used in elemental form either alone or in combination with oxide additives. Aluminium and boron nitride is also used
in some cases of which reference may be made to "method for preparing sintered shapes of silicon carbide-M.Omori and H.Takei, U.S. Pat. 4564490, 1986" where oxide additives in combination with alumina promote sintering of silicon carbide. Magnesia-alumina combination was used by M.B.Trigg.-Australian Patent 00518, 1990. M.B.Trigg. also used sialon as an additive-Australian Patent 00271, 1988. Yttria-aluminium nitride combination was used by W.D.G. Bocker, European Patent 419271AZ, 1990. Advantage of formation of silicon carbide-aluminium nitride was used for sintering of silicon carbide by G.Erwin(Jr.)-US Patent 3492153, 1970. The main drawbacks of all the above processes are :-
1. Silicon carbide should be essentially in the submicron range.
2. Silicon carbide should be ultra pure.
3. For most of the additives like aluminium nitride or boron nitride
pressureless sintering is not possible where as hot pressing or hot isostatic
pressing is cost inhibitive and non-flexible.
4. For many oxide additives like aluminium oxide decomposition of silicon
carbide at sintering temperature is difficult to control.
5. Amount of additives are in a narrow zone resulting into more stringent
process control parameters.
6. With aluminium oxide-aluminium nitride mixed additives sintering atmosphere must be argon or such inert gas that will not promote formation and growth of intermediate aluminium ox nitride phase. The main object of the present invention is to provide a process for preparation of sintering additives powder useful for sintering of silicon carbide which obviates the above difficulties.
Accordingly the present invention provides a process for preparation of sintering additives in powder form useful for sintering of silicon carbide which comprises; mixing aluminum oxide and aluminum nitride in attritor mill using alumina balls in the molar proportion of 5 to 95, separating the milled powder from alumina balls by sieving through 100 mesh B.S., preparing a slurry by known methods using organic solvents as herein described, drying the said slurry to obtain a powder, firing the said powder containing aluminum oxide and aluminium nitride at a temperature in the range of 1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours, grinding the product by known methods
in presence of organic solvents as defined above, drying the said slurry to obtain sintering additives in powder form.
known methods in presence of organic solvents as defined above, drying the said slurry to obtain sintering additives in powder form.
In still another embodiment of the present invention grinding of fired products may be done by using alumina balls of diameter in the range of 1-3 mm for a period in the range of 1 -5 hours.
In yet another embodiment of the present invention the milled powder may be separated from alumina balls by sieving through 100 mesh B.S.
The details of the processes of the present invention are given below:
a) Aluminium oxide and aluminium nitride is mixed in molar proportion of 5 to 95 in organic medium such as acetone, hexane aided by alumina balls in an attrition mill or any mixer useful for mixing ceramic raw material for a period in the rnage of 15 minutes to 3 hours.
b) After milling, the slurry is sieved through 100 mesh B.S., dried the powder mixture & fired at temperature in the range of 1750-1850°C under nitrogen atmosphere.
c) The fired product is ground by attrition mill or any grinder useful for grinding ceramic raw material using organic solvents such as hexane, acetone and alumina balls are used of diameter in the range of 1-3 mm for a period in the range of 1-5 hours.
d) After milling the product slurry was sieved through 100 mesh B.S. to separate ground powder from alumina balls.
When silicon carbide is sintered by using alumina alone, silicon carbide is dissociated and impedes sintering by following reactions :-SiC + A12O3 = SiOt + Al2Ot + COt 2 SiC + SiO2 - 3 Sit + 2 COt
2 SiC + A12O3 = 2 Sit + Al2Ot +2 COt
3 SiC + A1203 = 3 Sit+ 2 Alt + COt
On the other hand, when aluminium nitride is used alone, only hot pressing leads to densification, that too in argon atmosphere only as excessive grain growth of aluminium nitride in nitrogen atmosphere at sintering atmosphere impedes sintering.
When a combination of both A1N and Al2O3 powder is used in the SiC system, both the above factors will operate and full densification is not achievable. The situation will become more complex if nitrogen atmosphere is used due to the formation of A1ON, an intermediate in the nitridation of Al2O3 to A1N, which competes with sintering of SiC leading to sluggish densification. Preheat-treatment of Al2O3 and A1N mixture forms series of solid solutions by which the unfavourable reactions are effectively eliminated and complete densification could be achieved under both argon and nitrogen atmosphere. The entire range of solid solutions have wide range of compatibility with silicon carbide making wide ranging compositions between SiC and additive possible.
The following example are given by way of illustration and should not be construed to limit the scope of the present invention.
Example -1
97.02 g of aluminium oxide is mixed with 2.98 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 3 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. The dried material is used as additive for sintering of silicon carbide.
Example -2
95.7 g of aluminium oxide is mixed with 4.3 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for
2.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -3
92.5 g of aluminium oxide is mixed with 7.5 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 2.0mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1750°C in nitrogen atmosphere for 3.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -4
90.34 g of aluminium oxide is mixed with 9.66 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of
diameter 2.0mm for 2 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 2.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -5
92.5 g of aluminium oxide is mixed with 7.5 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.0mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -6
81.75 g of aluminium oxide is mixed with 18.25 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for 2.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -7
38.34 g of aluminium oxide is mixed with 61.66 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1750°C in nitrogen atmosphere for 4.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 2.0mm for 2 hours. After milling, alumina balls
were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours. Example -8
26.22 g of aluminium oxide is mixed with 73.78 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 3.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 3 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -9
95.7 g of aluminium oxide is mixed with 4.3 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of

diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1850°C in nitrogen atmosphere for 2.5 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 4 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
Example -10
81.75 g of aluminium oxide is mixed with 18.25 g of aluminium nitride in an attritor mill using acetone as liquid medium with 450g alumina ball of diameter 1.5mm for 1 hour. After milling, alumina balls were separated by sieving through 100 mesh B.S. The sieved slurry was dried slowly at room temperature. Dried mixture was fired at 1800°C in nitrogen atmosphere for 1.0 hours. The fired material was attrited in acetone medium using 450g alumina balls of diameter 1.5mm for 2 hours. After milling, alumina balls were removed by sieving through 100 mesh B.S. The mixture was dried slowly at room temperature followed by drying at 110°C for 2 hours.
The main advantages of the process of the present invention are:
1. Non-requirement of stringent limitations regarding fineness of silicon
carbide powder.
2. Non-requirement of purification of starting silicon carbide powder to a very high level.
3. Pressureless sintering gives full densification.
4. Decomposition of silicon carbide at sintering temperature is in the
acceptable minimum.
5. Sintering atmosphere may be nitrogen, argon or any inert gas.
6. A wide composition range of SiC : additive possible making the
compositional aspect flexible.

We Claim:
1. A process for preparation of sintering additives in powder form useful for
sintering of silicon carbide which comprises; mixing aluminum oxide and
aluminum nitride in attritor mill using alumina balls in the molar proportion of 5
to 95, separating the milled powder from alumina balls by sieving through 100
mesh B.S., preparing a slurry by known methods using organic solvents as herein
described, drying the said slurry to obtain a powder, firing the said powder
containing aluminum oxide and aluminium nitride at a temperature in the range of
1750-1850°C under nitrogen atmosphere for a period in the range of 1-5 hours,
grinding the product by known methods in presence of organic solvents as defined
above, drying the said slurry to obtain sintering additives in powder form.
2. A process as claimed in claims 1 wherein the organic solvents used are acetone
or hexane.
3. A process as claimed in claims 1-2 wherein grinding of fired products are done by
using alumina balls of diameter in the range of 1-3 mm for a period in the range
of 1-5 hours.
4. A process for preparation of sintering additives in powder form useful for
sintering of silicon carbide substantially as herein described with reference to the
examples.

Documents:

332-del-1999-abstract.pdf

332-del-1999-claims.pdf

332-del-1999-correspondence-others.pdf

332-del-1999-correspondence-po.pdf

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

332-del-1999-form-1.pdf

332-del-1999-form-19.pdf

332-del-1999-form-2.pdf

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Patent Number

215750

Indian Patent Application Number

332/DEL/1999

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

03-Mar-2008

Date of Filing

25-Feb-1999

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001

Inventors:

#	Inventor's Name	Inventor's Address
1	SANKAR GHATAK	CENTAL GLASS & CERAMIC RESEARCH INTITUTE, CALCUTTA 700032, INDIA.
2	AMIYASNKAR SANYAL	CENTAL GLASS & CERAMIC RESEARCH INTITUTE, CALCUTTA 700032, INDIA.
3	KAJAL KUMAR DHARGUPTA	CENTAL GLASS & CERAMIC RESEARCH INTITUTE, CALCUTTA 700032, INDIA.
4	SANTANU MANDAL	CENTAL GLASS & CERAMIC RESEARCH INTITUTE, CALCUTTA 700032, INDIA.

PCT International Classification Number

C04B 35/583

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA