Title of Invention

"AN IMPROVED PROCESS FOR MAKING SINTERED SILICON CARBIDE COMPOSITES"

Abstract

An improved process for sintering of silicon carbide composites with ceramisable glass powder additive An improved process for sintering of silicon carbide composites with ceramisable glass powder additive by mixing of silicon carbide powder of various fineness such as 1 m2/g to 16 m2/g with 5 to 50 weight percent of ceramisable glass powder additive of the kind essentially containing a nucleating agent selected from TiO2, and ZrO2, forming green shapes, drying the green shapes at a temperature in the range of 40°C to 150°C for a time of 10 to 200 minutes, encapsulating the dried sample into silicon carbide crucible, loading the encapsulated samples into a microwave generating system with a power output of 0.8 kW to 5 kW, fired for 0.5 to 8.0 hours and heat treating the fired shapes at a temperature in the range of 600°C to 900°C for 60 to 1200 minutes in an inert atmosphere.

Full Text

The present invention relates to an improved process for making sintered silicon carbide composites. The present invention particularly relates to an improved process for making sintered silicon carbide composites with ceramisable glass p owder additive. The sintered SiC with ceramisable additive has application as nozzle, pump-seals, kiln-furniture, and 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 said article may also be used as special refractory material where thermal shock, abrasion, corrosion, oxidativc corrosion etc. are to be countered at room temperature, below room temperature and at an elevated temperature up to 200()°C. The present day method of making sinlcicd silicon carbide essciilially coiutinl of using oxides of different metals like aluminium, yttrium, rare earth, magnesium and their combinations as additives, hi 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. D. Trigg.-Australia Patent 00518, 1990. M. D. Trigg. also used sialon as an additive-Australian Patent 00271, 1988. Yima-.iluminium nitride combination was used by W. D. G. Bocker, European Patent 419271AZ, 1990. Advantage of formation of silicon carbide-aluminium nitride was used for sintciing of silicon carbide by G. Erwin(Jr.)-US Patent 3492153, 1970. In all the above processes the main drawbacks may be listed as below: 1. An inert atmosphere for firing is to be maintained. 2. The fuel consumption per unit mass of products are high. 3. The environmental pollution due to fuel cannot be avoided except in electrical heating. 4. Non uniform heating of components in the furnace are very common . The main object of the present invention is to provide an improved process for making of silicon carbide composites with ceramisable glass powder additive which obviates the drawbacks as detailed above. Another object of the present invention is to eliminate the use of inert atmosphere for firing the products. Still another object of the present invention is to increase the fuel efficiency of the process by sintering in microwave generating furnace. Yet another object of the present invention is to reduce the level of environmental pollution. Still another object of the present invention is to ensure higher level of uniformity of fired products. Accordingly the present invention provide an improved process for making sintered silicon carbide composites with ceramisable glass powder additive, which comprises 1) mixing of silicon carbide powder of various fineness such as 1 m2/g to 16 rn2/g with 5 to 50 weight percent of ceramisable glass powder additive of the kind as herein described essentially containing a nucleating agent selected from TiO2, and ZrO2 2) forming green shapes, 3) drying the green shapes at a temperature in the range of 40°C to 150°C for a time of 10 to 200 minutes, 4) encapsulating the dried sample into silicon carbide crucible, 5) loading the encapsulated samples into a microwave generating system with a power output of 0.8 kW to 5 kW, fired for 0.5 to 8.0 hours and 6)heat treating the fired shapes at a temperature in the range of 600°C to 900°C for 60 to 1200 minutes in an inert atmosphere. encapsulating the dried sample in silicon carbide crucible, loading the encapsulated samples into a microwave generating system with a power output of 0.8 kW to 5 kW, fired for 0.5 to 8.0 hours and heat treating (lie fired shapes at a temperature in the range of 600°C to 900°C for 60 to 1200 minutes in ambient atmosphere. In an embodiment of the present invention silicon carbide powder used may be such as a or p-variety. In another embodiment of the present invention the ceramisable glass powder additive used, may be such as Li2O-Al2O3 -SiO2, MgO- ;U2O3 -SiO2, Na2O-BaO A12O3 -SiO2, essentially containing a nucleating agent such as TiO2, ZrO2. In another embodiment of the present invention forming may be done by the processes such as uniaxial pressing, isostatic pressing, slip casting. hi yet another embodiment of the present invention the heat treatment may be done in an inert atmosphere, such as nitrogen, argon, helium. The details of the process of invention are given below: 1. Silicon carbide powder is mixed with 5-50% of ceramisable glass powder additive by milling. 2. Milled powder is formed into shapes by different forming methods such as slip casting, uniaxial pressing or isostatic pressing. 3. The formed articles are dried in oven at 40 - 150°C. 4. Dried articles are placed in silicon carbide crucibles. 5. Silicon carbide crucibles containing formed articles are loaded in microwave heating system. 6. Microwave power generated is 0.8 to 5 kW for O.'i to 8 hours. 7. The fired shapes were heat treated at a temperature in the range of 6GO°C to 900°C for 60 to 1200 minutes. The process of the present invention can be used to produce sintered silicon carbide and silicon carbide based composite materials consisting of additives of various shapes and sizes required for application as engineering as well as special refractory material. Silicon carbide can only be sintered by the help of additives due to its high covalency. Moreover, for efficient sintering or consolidation particle size of SiC powder should be very small. SiC is easily oxidised, and extent of oxidation increases rapidly with increasing surface area of silicon carbide powder. This require some inert atmosphere like N2, Ar, or He during firing of SiC green compacts. Due to high covalency factor, sintering should be conducted at a very high temperature. SiC may be compacted or consolidated at lower temperature by using a low melting; glass. But due to low melting point of the bonding materials, the properties of such products will render it unusable for most of the specialised applications. If such glass may be crystallised in a controlled way, after consolidation of the products, the properties will be improved to a large extent and will make the product usable for specialised applicatioas. , In the present system SiC was consolidated by a ceramisable glass powder in a microwave generating system. As the volumetric heating of the samples occur during microwave firing, the silicon carbide particles remained away from atmospheric oxygen and consequently undesired oxidation of silicon carbide could be prevented. The novelty of the process of the present invention resides in avoiding oxidation of SiC during consolidation as well as presence of undesirable low melting glassy phase. Tims the present process enables to consolidate SiC at lower temperature without the requirement of any inert atmosphere and converting the undesirable glassy sintering aid into desirable crystalline phases in the final consolidated products. The inventive steps of the present invention are: (a) The sintering additive containing ceramisable glass powder. (b) The sintering being effected using microwave heating system. The above inventive steps result in avoidance of the presence of low melting glassy phase in the sintered silicon carbide composites and the sintering being carried out at lower temperature without the requirement of any inert atmosphere. The following examples arc given by way of illustration and therefore should not be construed the limit of the scope of the present invention. Exam pie-1 95 gm SiC powder of surface area Im2/g and 5 gm of Li2p-Al2O3-SiO2 additive powder along with Tio2 nucleants in the form of powder having wt% composition of Li2O-2.6, Al2O3-lS, SiOr70 and TiO2-4.5 powder was mixed thoroughly. The tnixed powder was isostatically pressed to form the green shapes. 1'he green shapes were dried in an air oven at 80°C for 200 minutes. The dried green shapes wen; encapsulated in a silicon carbide crucibles. The encapsulated samples were loaded in a microwave generating furnace. Microwave power of 0.8 kW is passed for one hour. After cooling, the samples were heat treated at 600°C for 2 hours in ambient atmosphere. Results: % Oxidation of SiC = 1.2 ExampIe-2 75 gm SiC powder of surface area 4m2/g and 25 gm of Li2O-Al2O3-SiO2 additive powder along with TiO2 nucleants in the form of powder having wr% composition of Li2O-2.6, Al2O3-18, SiO2-70 and TiO2-4.5 was mixed thoroughly. The specimens were fabricated by slip casting method. The green shapes were dried in air. The air dried samples were dried in an oven at 110°C for 60 minutes. The dried green shapes were encapsulated in a silicon carbide crucibles. The encapsulating samples were loaded in a microwave generating furnace. Microwave power of 1.0 kW is passed for 60 minutes. After cooling, the samples were heat treated at 900°C for 4 hours in Ar atmosphere. Results: % Oxidation of SiC = 2.85 Example-3 50 gm SiC powder of surface area 12m2/g was mixed with 50 gin of Li2O-Al2O3-SiO2 additive powder along with TiO2 nucleants in the form of powder having wt% composition of Li2O-2.6, A12O3-18, SiO2-70 and TiO2-4.5 was mixed thoroughly.. The specimens were prepared by isostatic pressing. The green shapes were dried in an air oven at 150°C for 200 minutes. The dried green shapes were encapsulated in a silicon carbide crucibles. The encapsulated samples were loaded in a microwave generating furnace. Microwave power at 2.0 kW is passed for three hours.After cooling, the samples were heat treated at 900°C for 10 hours in N2 atmosphere. Results: % Oxidation of SiC = 3.65 Example-4 95 gm SiC of surface area 16m2/g (p-varicly) wa« mixed willi 5 gin of MgO-AltiH. additive powder along with TiO2 nucleants in the form of powder having wt% composition of MgO-13, A12O3-30, SiO2-47 and TiO2-10 was mixed thoroughly. The mixed powder was pressed in a mould to form the green shapes. The green shapes were dried in an air oven at 120°C for 200 minutes. The dried green shapes were encapsulated in a silicon carbide crucibles. The encapsulated samples were loaded in a microwave generating furnace. Microwave power at 3.0 kW is passed for one hour. After cooling, the samples were heat treated at 800°C for 6 hours in He atmosphere. Results: % Oxidation of SiC - 0.9 Example-5 75 gm SiC of surface area 16m2/g (p-variety) was mixed with 25 gm of MgO-Al2O3-SiO2 additive powder along with TiO2 nucleants in the form of powder having wt% composition of MgO-13, A12O3-30, SiO2-47 and TiO2-10 was mixed thoroughly. The specimens were prepared by isostatic pressing. The green shapes were dried in an air oven at 110°C for 200 minutes. The dried green shapes were encapsulated in a silicon carbide crucibles. The encapsulated samples were loaded in a microwave generating furnace. Microwave power atl.O kW is passed for six hours. After cooling, the samples were heat treated at 800°C for 12 hours in ambient atmosphere. Results: % Oxidation of SiC = 2.36 Example-6 90 gm SiC oi surface area 16m2/g (ß-varicly) was mixed with 10 gin of Na2O-DaO-Al2O V SiO2 additive powder along with TiO2 nucleants in the form of powder having wr% composition of Na2O-13, BaO-9, Al2O3-29, SiO2-41 and TiO2-7 was mixed thoroughly. The specimens were fabricated by slip casting method. Tie green shapes were dried in an air oven at 80°C for 400 minutes. The dried green shapes were encapsulated in a silicon carbide crucibles. The encapsulated samples were loaded in a microwave generating furnace. Microwave power at 3.0 kW is passed for three hours. After cooling, the samples were heat treated at 800°C for 10 hours in Ar atmosphere. Results: % Oxidation of SiC = 1.4 The main advantages of the present invention are : 1. The process require no inert atmosphere to be maintained in the reaction system. 2. The fuel efficiency of the process is much higher than the conventional processes. 3. The environmental pollution created by the conventional fuel is totally eliminated. 4. The product quality in terms of reduced mechanical strain due to homogeneous microstructure is assured to a lower level. 5. Very low oxidation of SiC and hence low silica formation during sintering. We claim : 1. An improved process for making sintered silicon carbide composites with ceramisable glass powder additive, which comprises 1) mixing of silicon carbide powder of various fineness such as 1 m2/g to 16 m2/g with 5 to 50 weight percent of ceramisable glass powder additive of the kind as herein described essentially containing a nucleating agent selected from TiO2, and ZrO2 2) forming green shapes, 3) drying the green shapes at a temperature in the range of 40°C to 150°C for a time of 10 to 200 minutes, 4) encapsulating the dried sample into silicon carbide crucible, 5) loading the encapsulated samples into a microwave generating system with a power output of 0.8 kW to 5 kW, fired for 0.5 to 8.0 hours and 6)heat treating the fired shapes at a temperature in the range of 600°C to 900°C for 60 to 1200 minutes in an inert atmosphere. 2. An improved process as claimed in claim 1 wherein silicon carbide powder used is of a or p-variety. 3. An improved process as claimed in claim 1 and 2 ceramisable glass powder additive used is Li2O-AL2O3-SiO2, AI2O3-SiO2, Na2O-BaO-AI2O3-SiO2. 4. An improved process as claimed in claims 1-5 the heat treatment is done in an inert atmosphere using nitrogen, argon, helium. 5. An improved process for making sintered silicon carbide composites with ceramisable glass powder additive substantially as herein described with reference to the examples.

Documents:

162-del-2000-abstract.pdf

162-del-2000-claims.pdf

162-del-2000-correspondence-others.pdf

162-del-2000-correspondence-po.pdf

162-del-2000-description (complete).pdf

162-del-2000-form-1.pdf

162-del-2000-form-19.pdf

162-del-2000-form-2.pdf

162-del-2000-form-5.pdf