Title of Invention	"AN IMPROVED PROCESS FOR THE PREPARATION OF BETA-SILICON CARBIDE WHISKERS USEFUL FOR MAKING METAL/CEREMICS/GLASS MATRIX COMPOSITIONS"
Abstract	This invention relates to an improved process for the preparation beta silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods, such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers.

Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATION OF BETA-SILICON CARBIDE WHISKERS USEFUL FOR MAKING METAL/CEREMICS/GLASS MATRIX COMPOSITIONS"

Abstract

This invention relates to an improved process for the preparation beta silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods, such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers.

Full Text	This invention relates to an improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites. The beta-sic whiskers prepared by the process of the invention contain high concentration of whiskers above 80% and less particulates, in addition, the whiskers posses highly undulated surface and are free from woolly fibers. These properties are very much required for making metal/ceramic/glass matrix composites useful for automobile, armour; cutting tool and structural engineering applications. There is a growing interest for the synthesis of SiC with high whisker content (> 80%), least amounts of particles and hollow/woolly type of short fibers because of the stringent requirements of whisker reinforced composites. In addition to the above, high aspect-ratio of whiskers improve the wear resistance and fracture toughness of the composites. The alumina-Sic whisker composite is reported to have fracture toughness twice greater than monolithic alumina and finds applications in aerospace industry and as a cutting tool material for machining of super alloys [K.M. Prewo, "Ceramic Bull."68(2) 1989, p.395]. Chemically pure and ordered structure to the whiskers make them more strong and stable compared to polycrystalline fibers. As a result ceramic whiskers, particularly SiC whiskers find great interest to generate a new class of high strength metal/ceramic and glass matrix composite materials. The biggest limiting factor in diversifying the use of SiC whisker reinforcements is their high costs ranging from US $ 200 to 1000 per kilogram. There are references by many for the preparation of SiC whiskers by several methods. In this regard reference may be made to the publication by W.F. Knippenberg and G. Verspui in the book "Silicon Carbide-1972" ed. by R,C,Marshall, J.W.Faust Jr. and C.E.Ryan (Univ. of South Carolina Press, 1972). In this method, alpha-SiC whiskers were produced by vapourising SiC along with catalyst-lanthanum under reduced pressure. In another publication of CE.Ryan, I.Herman, R.C. Marshall, D.P.Considine and J.J.Hawley in the J. Crystal Growth 1 (1967) p.255 reference had been made to the production of beta-SiC whiskers by hydrogen reduction of methyl tri chloro-silane at 1500 C. As on today synthesis of SiC whiskers draw major attention as they are made by costly processes involving chemical vapour deposition (CVD), sol-gel, polyacryl-nitryl (PAN) precursor. Different kinds of SiO2 viz.- crushed quartz(99.98% Si02), precipitated SiO2 (99.98% pure) and crushed silica gel glass (99.9% pure) along with graphite/lamp black were used in the ratio of Si:C == 1:2 and subjected to carbothermic reduction process in presence of CO for a specific period of time to grow SiC whiskers. The product is found to be predominated by beta form but still a small percentage of alpha form is observed. In this regard reference may be made to the publications by AiChirysanthou, P.Grieveson and A.Jha, J.Mater.Sci. 26 (1991), 3463-3476; Steven R.Nutt, J.Amer.Ceram.Soc. 71 (1988), p.149; J.V.Milewsky, F.D.Gac, J.J.Petrovic and S.R.Skaggs, J.Mater.Sci. 20 (1985) 1060-1066. Similarly N.J.Paratt reported the production of beta-SiC by controlled heating of chloro-silanes, methane and CO. Reference may be made to N.J.Paratt "Fiber reinforced materials technology" Van Nostrand Reinhold, London, (1972). J.J.Shyne and J.V.Milewsky had grown beta-SiC whiskers by VLS (V-Vapour feed gases, L- Liquid catalyst and S- Solid crystalline whisker growth) process. Reference may be made to US patent 3 622 272 (November 1971). I.G.Cutler . reported the conversion of rice hulls by pyrolysis into SiC submicron particles and whiskers in the temperature region 1300-1500 C. The product contained a mixture of alpha and beta modifications of SiC (US patent 3 754 076, August 1973). Some investigators used palladium, iron, cobalt and nickel as catalysts in converting rice hulls to SiC whiskers. Reference may be made to J.G.Lee and I.B.Cutler, Ceram.Bull. 54 (1975) 195; Steven R.Nutt, J.Amer.Ceram.Soc. 71 (1988) 149. It has been shown that iron as Fe2O3 upto 5 wt. percent in the rice hulls based mixture is helpful for the formation of SiC whiskers owing to the following equation which is due to a decrease in activation energy for the gasification of silica with iron: (Formula Removed) (Reference: Publication by J.G.Lee and I.B.Cutler in Amer.Ceram.Soc.Bull. 54 (1975) p.195 and Patrick R.Taylor and S.A.Pirzada in "Metallurgical Transactions" 23-B (1992), p.443. Subsequently many workers both in India and abroad filed patents on the production of SiC whiskers from rice hulls. Rice hulls are mixed with iron/iron salts, charred and heated in inert atmosphere of N2 in the temperature range 1200-1500 C. Other catalysts such as Co, Ni, Pt, Pd in the form of salts were used in place of Fe. Palladium catalyst had shown best results in terms of SiC whiskers growth and yield. Reference may be made to M.Patel, C.B.Raju, A.K.Ray and A.Karera Indian patent No. 590/DEL 86. Alternatively charred rice husk after acid treatment and addition of catalysts Fe, Ni, Pd or Pt was heated in an inert atmosphere at 1400- 1750 C. Reference may be made to A.K.Ray, Indian patent 486/DEL 88; A.K.Ray, G.Mohanty and A.Ghose Indian patent 485/DEL 88; A.K.Ray, G.Mohanty, A.Ghose, J.Mater.Sci.Lett. 10 (1991) 227-229. In the existing processes catalysts such as Fe, Co, Ni, Pd, Pt were reported to be used and the atmosphere was that of inert gases. The process resulted in the formation of both alpha and beta type of whiskers aiong with the particulates in considerable amounts above 30%. In addition, these whiskers include wooly type and also impurities such as C, Fe which are detrimental to the properties of both metal matrix or ceramic matrix composites. Therefore, it is necessary to develop a new process which will give maximum amount of needle whiskers and minimum particles, free from woolly type whiskers, C and other metallic impurities like Fe. It is an object of the present invention to develop an improved process for preparing beta-SiC whiskers above 80% of volume with minimum SiC particles and without any woolly/side branched whiskers from an abundantly available raw material like rice hulls which will be easily amenable for carbothermic reduction and useful for reinforcement in metal/ceramic/glass matrix composites. Another objective of the present invention is to provide an improved process for the preparation of pure beta-SiC whiskers without the use of catalyst from abundantly available raw material containing both Si and C in amorphous state and in its structural net work. Accordingly, the present invention provides an improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers. In India, rice is produced annually over 65 million tones (as per "Handbook of Agriculture" published by Indian Council of Agricultural Research, New Delhi, Reprinted 1992) and the quantity of rice hulls/husk is available in abundance. The present invention uses rice hulls to obtain SiC whiskers predominantly in beta form with least amount of particulates. Rice hulls mainly comprises of silica in the amorphous form contributing Si and cellulose carbon in the proportion of Si:C = 1:2 necessary for complete carburisation of Si leading to SiC. In addition, both Si and C in rice hulls are intimately combined by nature and need no additional mixing. Further it has larger surface area due to its typical morphology. The above physical features make the rice hulls easily amenable for carbothermic reduction resulting in the formation of SiC whiskers. In addition rice hulls contain small amount of iron which favours the formation of whiskers. The present invention provides an improved process for making pure and more quantity of beta- SiC whiskers above 80% from rice hulls. The charred rice hulls are pelletized and the pellets are heated in the temperature range 1000 to 1500 C for a period ranging from 1 hour to 10 hours under vacuum and reduced atmosphere. The resultant product is purified by conventional methods. The product contains a mixture of particles and whiskers of beta-Sic. The particle size of the reaction product ranges from l to 2 microns. The percentage of organic binder varies from 2 to 5 wt percent. The reducing atmosphere is effected by passing of flow of gases like N2. The process of making beta-SiC whiskers according to the present invention is described in the following examples which are provided for illustration only and should not be construed to Limit the scope of the invention. The improvements of the process are described in the following lines. It consumes lesser N2 gas. The method involves thermal reduction of pyrolysed rice hulls in vacuum followed by minimum flow rate of inert gas (Nitrogen) at lower temperatures and avoiding costlier inert gases and complex gas mixtures. The method also excludes the use of extraneaous carbon matter and catalysts. The method employs the available carbon in the celulosic carbon form to the maximum extent. The whiskers thus obtained are guite lengthy upto 50 microns and diameter Example-1 Composition : Rice hulls - 250 grams Organic binder (10% V/V in acetone) - 50 grams Temperature (max.) - 1600 C Duration at max. temp. - 8 hours Atmosphere - Nitrogen Pressure - 0.5 kg/cm2 Flow rate - 30 lit./hr. 250 grams of clean rice hulls are mixed with an organic binder (10% V/V in acetone) and pelletised by hand in the form of cylinders. After preliminary drying the pellets are subjected to carbothermic reduction at 1600 C for 8 hours under a flow of N2 at the rate of 30 lit. per hour. Example- 2 Composition : Charred rice hulls - 100 grams Aluminium nitrite - 1.5 grams Charcoal - 15 grams Temperature (maximum) - 1400 C Duration at max. temp. - 4 hours Atmosphere - Argon flow rate - 6 lit./hr. Pressure - 0.1 kg/cm2 100 grams of charred rice hulls are mixed with 1.5 grams of aluminium nitride and 15 grams of finely powdered charcoal. Several pellets have been made by pressing the mixture at 2 tons load to form small pellets. The pellets are heated to 1400 C, with a holding of 4 hours at maximum temperature. The inert atmosphere is maintained by passing Argon gas at a flow rate of 6 liters per hour. Example-3 Composition : Charred rice hulls - 50 grams Temperature (maximum) - 1100 C Duration at max. temp. - 12 hours Atmosphere - Vacuum 10~2m bar 50 grams of charred rice hulls are weighed into a graphite container and placed in the middle of the vacuum furnace. When the vacuum inside the furnace chamber reached 10~2 m bar, the furnace was set to heating. The sample was heated to 1100 C for 12 hours and the products checked for the formation of Sic whiskers. Example- 4 Composition : Rice hulls - 200 grams Cobalt Chloride - 0.2 gram Temperature (maximum) - 1300 C Duration at max. temp. - 2 hours Atmosphere - Ammonia Flow rate - 5 lit. per hour Rice hulls are heated with a solution of Cobalt chloride containing 0.2 gram salt and dried to room temperature. The moisture is completely removed by drying the hulls at 110 C in an air oven. The material is transferred to alumina container for heat treatment at 1300 C for 2 hours in ammonia atmosphere at a flow rate of 5 1its/hours. Advantages of present invention 1 Rice hulls contain silica and carbon in poorly crystalline state. This enables to provide C and Si simultaneously and also render the carbothermic reduction process much easier. 2) The temperature of the reaction is as low as 1100-1400 C leading to formation of beta-SiC in whisker and particulate forms. 3) The particle size of the SiC particles is maximum 2-4 microns which need relatively less grinding and can be readily used for making composites of SiC + SiCw. By extending the duration of carburisation or increasing temperatures, whiskers can be grown to maximum length of 80-100 microns with particles less than 20 wt. percent. 4) The atmosphere of the reaction is carbonaceous gases like CO or N2 and causes less handling problems compared to ammonia/ N2 + H2 used in hitherto known process. 5) All the raw materials used for on the process are less expensive and are easily available. We claim : 1. An improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers. 2. An improved process as claimed in claim 1 wherein the rice hulls has a composition of SiO2 (99.0 wt%), and Fe2O3 less than 0.5 wt.% (calculated-as per rice hull ash basis). 3. An improved process as claimed in claims 1-2 wherein the inert atmosphere for the reaction is maintained, by allowing vacuum or by passing gasses like nitrogen. 4. An improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites substantially as herein described with reference to the examples.

Full Text

This invention relates to an improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites.
The beta-sic whiskers prepared by the process of the invention contain high concentration of whiskers above 80% and less particulates, in addition, the whiskers posses highly undulated surface and are free from woolly fibers. These properties are very much required for making metal/ceramic/glass matrix composites useful for automobile, armour; cutting tool and structural engineering applications. There is a growing interest for the synthesis of SiC with high whisker content (> 80%), least amounts of particles and hollow/woolly type of short fibers because of the stringent requirements of whisker reinforced composites. In addition to the above, high aspect-ratio of whiskers improve the wear resistance and fracture toughness of the composites. The alumina-Sic whisker composite is reported to have fracture toughness twice greater than monolithic alumina and finds applications in aerospace industry and as a cutting tool material for machining of super alloys [K.M. Prewo, "Ceramic Bull."68(2) 1989, p.395].
Chemically pure and ordered structure to the whiskers make them more strong and stable compared to polycrystalline fibers. As a result ceramic whiskers, particularly SiC whiskers find great interest to generate a new class of high strength metal/ceramic and glass matrix composite materials. The biggest limiting factor in diversifying the use of SiC whisker reinforcements is their high costs ranging from US $ 200 to 1000 per kilogram.
There are references by many for the preparation of SiC whiskers by several methods. In this regard reference may be made to the publication by W.F. Knippenberg and G. Verspui in the book "Silicon
Carbide-1972" ed. by R,C,Marshall, J.W.Faust Jr. and C.E.Ryan (Univ. of South Carolina Press, 1972). In this method, alpha-SiC whiskers were produced by vapourising SiC along with catalyst-lanthanum under reduced pressure. In another publication of CE.Ryan, I.Herman, R.C. Marshall, D.P.Considine and J.J.Hawley in the J. Crystal Growth 1 (1967) p.255 reference had been made to the production of beta-SiC whiskers by hydrogen reduction of methyl tri chloro-silane at 1500 C.
As on today synthesis of SiC whiskers draw major attention as they are made by costly processes involving chemical vapour deposition (CVD), sol-gel, polyacryl-nitryl (PAN) precursor. Different kinds of SiO2 viz.- crushed quartz(99.98% Si02), precipitated SiO2 (99.98% pure) and crushed silica gel glass (99.9% pure) along with graphite/lamp black were used in the ratio of Si:C == 1:2 and subjected to carbothermic reduction process in presence of CO for a specific period of time to grow SiC whiskers. The product is found to be predominated by beta form but still a small percentage of alpha form is observed. In this regard reference may be made to the publications by AiChirysanthou, P.Grieveson and A.Jha, J.Mater.Sci. 26 (1991), 3463-3476; Steven R.Nutt, J.Amer.Ceram.Soc. 71 (1988), p.149; J.V.Milewsky, F.D.Gac, J.J.Petrovic and S.R.Skaggs, J.Mater.Sci. 20 (1985) 1060-1066.
Similarly N.J.Paratt reported the production of beta-SiC by controlled heating of chloro-silanes, methane and CO. Reference may be made to N.J.Paratt "Fiber reinforced materials technology" Van Nostrand Reinhold, London, (1972).
J.J.Shyne and J.V.Milewsky had grown beta-SiC whiskers by VLS (V-Vapour feed gases, L- Liquid catalyst and S- Solid crystalline whisker growth) process. Reference may be made to US patent 3 622
272 (November 1971).
I.G.Cutler . reported the conversion of rice hulls by pyrolysis into SiC submicron particles and whiskers in the temperature region 1300-1500 C. The product contained a mixture of alpha and beta modifications of SiC (US patent 3 754 076, August 1973). Some investigators used palladium, iron, cobalt and nickel as catalysts in converting rice hulls to SiC whiskers. Reference may be made to J.G.Lee and I.B.Cutler, Ceram.Bull. 54 (1975) 195; Steven R.Nutt, J.Amer.Ceram.Soc. 71 (1988) 149. It has been shown that iron as Fe2O3 upto 5 wt. percent in the rice hulls based mixture is helpful for the formation of SiC whiskers owing to the following equation which is due to a decrease in activation energy for the gasification of silica with iron:
(Formula Removed)
(Reference: Publication by J.G.Lee and I.B.Cutler in Amer.Ceram.Soc.Bull. 54 (1975) p.195 and Patrick R.Taylor and S.A.Pirzada in "Metallurgical Transactions" 23-B (1992), p.443. Subsequently many workers both in India and abroad filed patents on the production of SiC whiskers from rice hulls. Rice hulls are mixed with iron/iron salts, charred and heated in inert atmosphere of N2 in the temperature range 1200-1500 C. Other catalysts such as Co, Ni, Pt, Pd in the form of salts were used in place of Fe. Palladium catalyst had shown best results in terms of SiC whiskers growth and yield. Reference may be made to M.Patel, C.B.Raju, A.K.Ray and A.Karera Indian patent No. 590/DEL 86. Alternatively charred rice husk after acid treatment and addition of catalysts Fe, Ni, Pd or Pt was heated in an inert atmosphere at 1400- 1750 C. Reference may be made to A.K.Ray, Indian patent 486/DEL 88; A.K.Ray, G.Mohanty and A.Ghose Indian patent 485/DEL 88; A.K.Ray, G.Mohanty, A.Ghose,
J.Mater.Sci.Lett. 10 (1991) 227-229.
In the existing processes catalysts such as Fe, Co, Ni, Pd, Pt were reported to be used and the atmosphere was that of inert gases. The process resulted in the formation of both alpha and beta type of whiskers aiong with the particulates in considerable amounts above 30%. In addition, these whiskers include wooly type and also impurities such as C, Fe which are detrimental to the properties of both metal matrix or ceramic matrix composites. Therefore, it is necessary to develop a new process which will give maximum amount of needle whiskers and minimum particles, free from woolly type whiskers, C and other metallic impurities like Fe.
It is an object of the present invention to develop an improved process for preparing beta-SiC whiskers above 80% of volume with minimum SiC particles and without any woolly/side branched whiskers from an abundantly available raw material like rice hulls which will be easily amenable for carbothermic reduction and useful for reinforcement in metal/ceramic/glass matrix composites. Another objective of the present invention is to provide an improved process for the preparation of pure beta-SiC whiskers without the use of catalyst from abundantly available raw material containing both Si and C in amorphous state and in its structural net work.
Accordingly, the present invention provides an improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers.
In India, rice is produced annually over 65 million tones (as per
"Handbook of Agriculture" published by Indian Council of Agricultural Research, New Delhi, Reprinted 1992) and the quantity of rice hulls/husk is available in abundance. The present invention uses rice hulls to obtain SiC whiskers predominantly in beta form with least amount of particulates. Rice hulls mainly comprises of silica in the amorphous form contributing Si and cellulose carbon in the proportion of Si:C = 1:2 necessary for complete carburisation of Si leading to SiC. In addition, both Si and C in rice hulls are intimately combined by nature and need no additional mixing. Further it has larger surface area due to its typical morphology. The above physical features make the rice hulls easily amenable for carbothermic reduction resulting in the formation of SiC whiskers. In addition rice hulls contain small amount of iron which favours the formation of whiskers.
The present invention provides an improved process for making pure and more quantity of beta- SiC whiskers above 80% from rice hulls.
The charred rice hulls are pelletized and the pellets are heated in the temperature range 1000 to 1500 C for a period ranging from 1 hour to 10 hours under vacuum and reduced atmosphere. The resultant product is purified by conventional methods. The product contains a mixture of particles and whiskers of beta-Sic. The particle size of the reaction product ranges from l to 2 microns. The percentage of organic binder varies from 2 to 5 wt percent. The reducing atmosphere is effected by passing of flow of gases like N2.
The process of making beta-SiC whiskers according to the present invention is described in the following examples which are provided for illustration only and should not be construed to
Limit the scope of the invention.
The improvements of the process are described in the following lines. It consumes lesser N2 gas. The method involves thermal reduction of pyrolysed rice hulls in vacuum followed by minimum flow rate of inert gas (Nitrogen) at lower temperatures and avoiding costlier inert gases and complex gas mixtures. The method also excludes the use of extraneaous carbon matter and catalysts. The method employs the available carbon in the celulosic carbon form to the maximum extent. The whiskers thus obtained are guite lengthy upto 50 microns and diameter Example-1
Composition :
Rice hulls - 250 grams
Organic binder (10% V/V in acetone) - 50 grams
Temperature (max.) - 1600 C
Duration at max. temp. - 8 hours
Atmosphere - Nitrogen
Pressure - 0.5 kg/cm2
Flow rate - 30 lit./hr.
250 grams of clean rice hulls are mixed with an organic binder (10% V/V in acetone) and pelletised by hand in the form of cylinders. After preliminary drying the pellets are subjected to carbothermic reduction at 1600 C for 8 hours under a flow of N2 at the rate of 30 lit. per hour.
Example- 2
Composition :
Charred rice hulls - 100 grams
Aluminium nitrite - 1.5 grams
Charcoal - 15 grams
Temperature (maximum) - 1400 C
Duration at max. temp. - 4 hours
Atmosphere - Argon
flow rate - 6 lit./hr.
Pressure - 0.1 kg/cm2
100 grams of charred rice hulls are mixed with 1.5 grams of aluminium nitride and 15 grams of finely powdered charcoal. Several pellets have been made by pressing the mixture at 2 tons load to form small pellets. The pellets are heated to 1400 C, with a holding of 4 hours at maximum temperature. The inert

atmosphere is maintained by passing Argon gas at a flow rate of 6 liters per hour.
Example-3
Composition :
Charred rice hulls - 50 grams
Temperature (maximum) - 1100 C
Duration at max. temp. - 12 hours
Atmosphere - Vacuum
10~2m bar
50 grams of charred rice hulls are weighed into a graphite
container and placed in the middle of the vacuum furnace. When
the vacuum inside the furnace chamber reached 10~2 m bar, the
furnace was set to heating. The sample was heated to 1100 C for
12 hours and the products checked for the formation of Sic
whiskers.
Example- 4
Composition :
Rice hulls - 200 grams
Cobalt Chloride - 0.2 gram
Temperature (maximum) - 1300 C
Duration at max. temp. - 2 hours
Atmosphere - Ammonia
Flow rate - 5 lit. per hour
Rice hulls are heated with a solution of Cobalt chloride containing 0.2 gram salt and dried to room temperature. The moisture is completely removed by drying the hulls at 110 C in an air oven. The material is transferred to alumina container for heat treatment at 1300 C for 2 hours in ammonia atmosphere at a flow rate of 5 1its/hours.
Advantages of present invention
1 Rice hulls contain silica and carbon in poorly crystalline state. This enables to provide C and Si simultaneously and also render the carbothermic reduction process much easier.
2) The temperature of the reaction is as low as 1100-1400 C leading to formation of beta-SiC in whisker and particulate forms.
3) The particle size of the SiC particles is maximum 2-4 microns which need relatively less grinding and can be readily used for making composites of SiC + SiCw. By extending the duration of carburisation or increasing temperatures, whiskers can be grown to maximum length of 80-100 microns with particles less than 20 wt. percent.
4) The atmosphere of the reaction is carbonaceous gases like CO or N2 and causes less handling problems compared to ammonia/ N2 + H2 used in hitherto known process.
5) All the raw materials used for on the process are less expensive and are easily available.

We claim :
1. An improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites which comprises palletizing the charred rice hulls by conventional methods such as herein described, heating the pellets to a temperature in the range of 1000-1500°C in vacuum followed by an inert atmosphere for a period ranging from 1 hour to 10 hours to obtain beta-SiC whiskers.
2. An improved process as claimed in claim 1 wherein the rice hulls has a composition of SiO2 (99.0 wt%), and Fe2O3 less than 0.5 wt.% (calculated-as per rice hull ash basis).
3. An improved process as claimed in claims 1-2 wherein the inert atmosphere for the reaction is maintained, by allowing vacuum or by passing gasses like nitrogen.
4. An improved process for the preparation of beta-silicon carbide whiskers useful for making metal/ceramic/glass matrix composites substantially as herein described with reference to the examples.

Documents:

1364-del-1995-abstract.pdf

1364-del-1995-claims.pdf

1364-del-1995-complete specification (granted).pdf

1364-del-1995-correspondence-others.pdf

1364-del-1995-correspondence-po.pdf

1364-del-1995-description (complete).pdf

1364-del-1995-form-1.pdf

1364-del-1995-form-2.pdf

1364-del-1995-form-4.pdf

1364-del-1995-form-9.pdf

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Patent Number

191807

Indian Patent Application Number

1364/DEL/1995

PG Journal Number

1/2004

Publication Date

03-Jan-2004

Grant Date

30-Nov-2004

Date of Filing

20-Jul-1995

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	SHISHIR VERMA	REGIONAL RESEARCH LABORATORY, CSIR , BHOPAL-462 026, INDIA
2	CHAMARTHY BUTCHI RAJU	REGIONAL RESEARCH LABORATORY, CSIR , BHOPAL-462 026, INDIA

PCT International Classification Number

C01B 31/16

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA