Title of Invention

"AN IMPROVED PROCESS FOR SINTERING OF SILICON CARBIDE".

Abstract

The invention relates to an improved process for sintering of silicon carbide. The process of the present invention can be used to produce sintered silicon carbide and silicon carbide based composite material consisting of additives of various shapes and sizes required for application as engineering as well as special refractory material.The process steps are: Silicon carbide powder is mixed with 5-25% additive by milling, milled powder is formed into shapes by different forming methods such as slip casting uniaxial pressing or isostatic pressing, the formed articles are dried in oven at 110 ± 10°C, dried articles are placed in silicon carbide crucibles., silicon carbide crucibles containing formed articles are loaded in microwave heating system, microwave power generated is 0.8 to 5 KW for 0.5 to 8 hours.

Full Text

The present invention relates to an improved process for sintering of silicon carbide. The main usage of the sintered silicon carbide are for making components in I ho 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 an 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. Takel 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 DOS 18, 1990. M. B. Trigg. also used sialon as an additive-Australian Patent 00271, 1988. Yttria-aluminium nitride combination was used by W. D. O. Docker, 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. 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 sintering of silicon carbide which obviates the drawback 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 uniformity of fired products. Accordingly, the present invention provides an improved process for sintering of silicon carbide which comprises; mixing silicon carbide powder of various fineness in the range of 1 m2/g to 16 m2/g characterised in that with 5-25 wt percent sintering additives containing 65-75% A12O3, 15-35% SiO2 and 0.5-10% CaO for a period of 1 to3 hrs, forming the said milled power into shapes by methods such as herein described, drying the formed shapes at a temperature of 110±10°C, encapsulating the dried shapes into silicon carbide crucibles, charging the encapsulated samples into a microwave heating system of a power output 0.8 to 5 KW for 0.5 to 8.0 hours to obtain sintered silicon carbide. In an embodiment silicon carbide powder used may be of a or P variety. In another embodiment of the present invention forming into shapes may be done by processes such as slip casting, uniaxial pressing, isostatic pressing. The details of the process of invention are given below: 1. Silicon carbide powder is mixed with 5-25% 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 110 ± 10°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 0.5 to 8 hours. The process of the present invention can be used to produce sintered silicon carbide and silicon carbide based composite material consisting of additives of various shapes and sizes required for application as engineering as well as special refractory material. Silicon carbide gets oxidized if heated in atmosphere containing oxygen. The oxidation start from the surface of the articles due to more availability of oxygen at that area. In the conventional heating processes, energy is transported into the system from external surfaces towards the build, thereby increasing temperature at the outer surface more than that of core area, thereby creating a temperature gradient. These two factors accelerate oxidation of the surface thus increasing the volume of the surface and slowly the entire body is oxidised before the Rinlcnng temperature is reached fbmung silica rich phases - an undesirable product by itself. In microwave sintering, core temperature increases rapidly and is always more than the surface temperature. The overall reaction initiates at the centre of the body which progresses towards the surface with respect to tune. Thus, volumetric heating not only prevents oxidation but also ensures better homogeneity in terms of mechanical factors leading to a more strainfree, mechanically stronger body. In addition to above, fuel economy is much more higher than the conventional processes that uses external heating system with inherent poor fuel economy factor. The novelty of the present invention is to obtain a strainfree mechanically stronger sintered silicon carbide. The inventive steps of the present invention are: (a) The sinteriong additive containing 65 - 75 % A12O3, 15 - 35 % SiO2 and 0.5 - 10%CaO. (b) The sintering being effected using microwave heating system. The above inventive steps result in a novel sintered silicon caibide; which is strain free and mechanically stronger. 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 90 g of silicon carbide powder and 10 g of additive containing 70 %A12O3, 25 % SiO2 and 5 % CaO are mixed in ball mill for two hours. The mixed powder is sieved through 200 mesh B.S., the sieved powder is uniaxially pressed at 250 MPa. Pressed article is dried at 110°C for 2 hours. Mod article in placed in dilicon cm bido crucible and the cruible is then placed in microwave generating system. Microwave power at 2 kW is passed lor one hour. Results: Hardness: 96 Kg/mm2 Fleiural Strength (3-point): 28 MPa Example - 2 85 g of silicon carbide powder and 15 g of additive containing 65 %A12O3, 25 %SiO2 and 10% CaO are mixed in ball mill for three hour. The mixed powder is sieved through 200 mesh B.S., sieved powder is shaped by slip casting by the known method. Slip cast shapes are slowly dried at room temperature for 24 hours. After drying at room temperature, the articles are dried at 110°C in air-oven for 24 hours. Dried specimens are placed in silicon carbide crucibles and placed in microwave generating system. Microwave power of 1.5 kW is passed through the articles for 1.5 hours. Results: Hardness: 112 Kg/mm2 Fleiural Strength (3-point): 40 MPa Example - 3 80 g of silicon carbide powder and 20 g of additives containing 75%A12O3, 20% SiO2 and 5% CaO are mixed in ball mill for one hour. The mixed powder is sieved through 200 mesh B.S., sieved powder is uniaxially pressed at 250 MPa. Pressed article is dried at 110°C for 2 hour. Dried article is placed in silicon carbide crucible and the crucible is then placed in microwave generating system. Microwave power at 2.5 kW is passed for two hrs. Results: Hardness:115 Kg/mm2 Flexural Strength (3-point): 43 MPa Example 4 75 g of silicon carbide powder and 25 g of additive containing 72%A12O3, 26% SiO2 and 2% CaO are mixed in ball mill for three hour. The mixed powder is sieved through 200 mesh B.S, the sieved powder is shaped by slip casting by the known method. Slip cast shapes are slowly dried at room temperature for 24 hours. After drying at room temperature, the articles are dried at 110°C in air-oven for 24 hours. Dried specimens are placed in silicon carbide crucibles and placed in microwave generating system. Microwave power of 3.0 kW is passed through the articles for 2.5 hours;. Results: Hardness: 118 Kg/mm2 Flexural Strength (3-point): 45 MPa Example 5 80 g of silicon carbide and 20 g of additive containing 70% A12O3, 28 %SiO2 and 2 % CaO are mixed in ball mill for two hour. The mixed powder is sieved through 200 mesh B.S., sieved powder is uniaxially pressed at 150 MPa. Pressed article is dried at 110°C for 2 hours. Dried article is placed in silicon carbide crucible and the crucible is then placed in microwave generating system. Microwave power at 0.8 kW is passed for three hours. Results: Hardness: 115 Kg/mm2 Flexural Strength (3-point): 43 MPa 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 mechanic al strain due to homogeneous microstructure is assured to a lower level. We Claim: 1. An improved process for sintering of silicon carbide which comprises; mixing silicon carbide powder of various fineness in the range of 1 m2/g to 16 m2/g characterised in that with 5-25 wt percent sintering additives containing 65-75% A12O3, 15-35% SiO2 and 0.5-10% CaO for a period of 1 to3 hrs, forming the said milled power into shapes by known methods such as herein described, drying the formed shapes at a temperature of 110±10°C, encapsulating the dried shapes into silicon carbide crucibles, charging the encapsulated samples into a microwave heating system of a power output 0.8 to 5 KW for 0.5 to 8.0 hours to obtain sintered silicon carbide. 2. A process as claimed in claim 1 wherein silicon carbide powder used is of a or p variety. 3. A process as claimed in claim 1 wherein forming methods used are slip casting, uniaxial pressing, isostatic pressing. 4. An improved process for sintering of silicon carbide substantially as herein described with reference to the examples.

Documents:

150-del-2000-abstract.pdf

150-del-2000-claims.pdf

150-del-2000-correspondence-others.pdf

150-del-2000-correspondence-po.pdf

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

150-del-2000-form-1.pdf

150-del-2000-form-19.pdf

150-del-2000-form-2.pdf