Title of Invention

A PROCESS FOR PRODUCING DISILICIDES OF MOLYBDENUM, TUNGSTEN AND THEIR SOLID SOLUTION

Abstract

A process for producing compounds of disilicide having the formula M - Si2 where M represents Mo, W, MoxWi-x (0 < x < 1), the process comprising the steps of i) providing a dry powder mixture consisting of oxide(s) of formula M - O3 or precursors of M - O3 , where M is as defined above, carbon or its precursor and silicon carbide; ii) compacting the dry powder mixture in a press to obtain green compacts; iii) heating the green compact mass to a temperature in the range of 1600 - 1800°C in a controlled manner in a furnace under dynamic vacuum or under flowing inert gas chosen from the group 0 of the periodic table; and iv) cooling the furnace to ambient temperature to get the product.

Full Text

FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10) TITLE A Process for Producing Disilicides of Molybdenum, Tungsten and their Solid Solution APPLICANT Department of Atomic Energy, Government of India, Anushakti Bhavan, Chhatrapati Shrvaji Maharaj Marg Mumbai 400 039 The following specification particularly describes the nature of the invention and the manner in which it is to be performed:- GRANTED 23-4-2004 Field of Invention: This invention relates a process for the production of molybdenum disilicide or tungsten disilicide or molybdenum disilicide/tungsten disilicide solid solution by a modified carbothermic process. Bacl^round and Prior Art: J.J. Petrovic has described various properties of molybdenum disilicide in the article "High Temperature Structural Silicides", published in Ceramic Engineering Science Proceedings, vol. 18, No. 3, 1997, on pages 3-17. Molybdenum disilicide being an intermetallic compoimd offers the benefits of both ceramics and metals. Like most ceramic materials, molybdenum disilicide has high melting point (2030°C), relatively low density (6.24 g/cm^) and brittleness (at low temperature). On the other hand, molybdenum disilicide behaves as a metal at high temperature. In polycrystalline form, molybdenum disilicide undergoes brittle to ductile transition at around 1000°C. Apart from the above properties, molybdenum disilicide has excellent high temperature oxidation resistance by way of forming a thin coherent and adherent protective silica layer. The material is thermodynamically stable with a wide range of structural ceramics, such as, silicon nitride (Si3N4), silicon carbide (SiC), titanium diboride (TiB2), alumina (AI2O3), zirconia (Zr02), mullite (3 AI2O3. 2 Si02). Thus, there is a significant potential for composite development. It can be alloyed with other high melting point sificides, such as, tungsten disilicide (WSi2), niobium disilicide (NbSi2), and other form of molybdenum silicide (Mo5sSi3). Molybdenum disilicide based alloys and composites exhibits enhancement in some of the desirable properties. Due to metallic nature of its bonding, molybdenum disificide or its alloys and composites can be electro discharge machined, thus making it easier to machine than most structural ceramics. Because of the above mentioned properties, molybdenum disilicide and its alloys and composites are promising materials for application as high temperature structural components particularly in high performance gas turbines and engines. Other potential applications include electrical resistance flimace heating element [WIPO patent document 8902647], molten metal lances, industrial gas burners etc. The commercial application of tungsten disilicide is limited. However,' these are produced on a small scale for sputtering experiments to form thin surface layers on semiconductor materials with the aim of improving the electrical conductivity of these materials. Tungsten disilicide when alloyed with molybdenum disilicide improves low temperature oxidation resistant of molybdenum disilicide and it can be used as heating element [European Patent document EP 0886458 A2]. Molybdenum disilicide is usually produced by the reaction of the elemental molybdenum and silicon powders. One most popular process is known as the Self-propagating High-temperature Synthesis (SHS) process or Combustion synthesis process. S. C. Deevi has described the process in the article "Self-propagating High-temperature Synthesis of Molybdenum Disilicide", published in Journal of Materials Science, vol. 26,1991, on pages 3343 - 3353. In this process, a preform of molybdenum and silicon powder mixture is ignited from one end, combustion occurs and combustion front propagates throughout the preform. Density close to 90 percent of theoretical density has been achieved by proper combination of processing parameters, such as, heating rate, addition of alloying elements, processing atmosphere and molybdenum and silicon powder size used. In another process as described by R. B. Schwarz et al. in the article "Synthesis of Molybdenum Disilicide by Mechanical Alloying", published in Materials Science and Engineering, vol. A155, 1992, on pages 75 - 83, molybdenum and silicon powders are milled in a high-energy ball mill to produce molybdenum disilicide powders. Above methods of production of molybdenum disilicide use expensive raw materials, elemental molybdenum and silicoa Special care is required for handling these elemental powders to avoid oxygen contamination. Although the product of SHS process can have density close to 90% of theoretical density, it may contain oxygen as impurity as the reaction time is very less for removing 05cygen present in the metal powders. Mechanically alloyed product powders may also end up with large oxygen content as the product is highly reactive. Further, mechanical alloying process does not produce dense product. Densification involves separate step. In another process, as described in US patent document 5011800, molybdenum disilicide or molybdenum disilicide / alumina composite has been prepared by combustion route. In the process, a mixture of molybdenum trioxide, silicon dioxide, a metallic component (aluminum or magnesium powder) and an inorganic diluent react in a self-sustaining, highly exothermic mode. However, the product is not pure molybdenum disilicide and consists of molybdenum disilicide, alumina and diluent. The product is ground to powder form and diluent and alumina are leached out with suitable leachant to get molybdenum disilicide powders. J. Hojo et al. have described a process for production of timgsten disilicide in the article 'Tormation of WSi2 Powder by Carbothermal Reduction Route", published in Journal of Ceramic Society of Japan, vol. 105, No. 12, 1997, on pages 925 - 927. In the process the starting materials, namely, ammonium paratungstate, silica and carbon powder mixture are heated under (argon + hydrogen) atmosphere to a temperature of 1400°C to produce tungsten disilicide powders. However, the process requires silica and carbon in excess of the stoichiometric requirement. Use of 100% excess silica has been reported in the above article to produce single-phase tungsten disilicide. Excess silicon is lost through formation of silicon monoxide (SiO). Since, SiO loss is not controllable, the process may not have reproducibility in producing single-phase tungsten disilicide. Molybdenum disilicide/tungsten disilicide solid solution can also be produced starting from the elemental powders. However, as mentioned earlier the process becomes expensive. The solid solution can ako be obtained by mixing molybdenum disilicide and tungsten disilicide in desirable ratio and heating to affect alloy formation. J. Hojo et al. have described a process for the production of molybdenum disilicide/tungsten disilicide alloy in the article "Formation of MoSi2-WSi2 Alloy Powder by Carbothermal Reduction Route", published in Journal of Ceramic Society of Japan, vol. 105, No. 12,1997, on pages 1053 - 1056. The starting materials, namely, ammonium paramolybdate, ammonium paratungstate, siUca and carbon powder mixture are heated under argon + hydrogen atmosphere to a temperature of 1400°C. However, the process requires silica and carbon in excess of the stoichiometric requirement similar to the process of production of tungsten disilicide. Use of 75% excess silica has been reported in the above article to produce single-phase tetragonal molybdenum disilicide/tungsten disilicide solid solution. Object: The main object of the invention is to provide a process for the production of single phase molybdenum disilicide [MoSi2] or tungsten disilicide [WSi2] or solid solution thereof [(MoxWi-x) Si2, where, x Another object of the invention is to provide a process for the production of molybdenum disilicide or tungsten disilicide or solid solution thereof where synthesis and densification (greater than 90% of theoretical density) can be achieved in a single step by proper combination of processing parameters. Summary of the Invention: Accordingly the present invention relates to a process for producing compounds of disilicide having the formula M - Si2 where M represents Mo, W, MoxWi-x (where x is less than 1); the process comprising the steps of i) providing a dry powder mixture consisting of oxide(s) of formula M - O3 or precursors of oxide(s), where M is as defined above, carbon or its precursor and silicon carbide ii) compacting the dry powder mfacture in a press to obtain green compacts; iii) heating the green compact mass in a controlled manner in a ftimace under dynamic vacuum or under flowing inert gas chosen from the group 0 of the periodic table; and iv) cooling the furnace to ambient temperature to get the product. Detailed Description of the Invention: For the production of molybdenum disilicide or tungsten disilicide or solid solution thereof, carbothermic process starting with the oxides of molybdenum and / or tungsten and silicon is expected to be economical. These materials are easily available and easy to handle imlike elemental powders of molybdenum, tungsten and silicon. However, as described earlier in some of the prior art processes, carbothermic reduction of silica has the inherent drawback in loss of silicon as its suboxide (SiO) as per the following reactioa Si02 + C = SiO + CO As a result, the silicon to molybdenum atomic ratio in the product remains well below 2.0. In addition, as part of oxygen comes out with silicon, unreacted carbon remains in the product. Carbothermic reduction of 1:2 molar mixture of molybdenum trioxide and silica has been studied in our previous work (unpublished). Silicon to molybdenum atomic ratio was 1.22; carbon content was as high as 7.3 wt.% and the product composed of phases like MoSi2, Mo5Si3C (Nowotny phase) and graphite. The loss of silicon is generalfy compensated by using excess silica and carbon. However, silicon loss in this form is not controllable and obtaining sUicides with silicon to metal (Si/M) atom ratio close to 2.0 becomes difficult. To prevent the loss of silicon through the above-mentioned reaction, silicon carbide (SiC) has been used as source of silicon in the process of our present investigation. Since, it is bound with carbon, it is expected that there will be no loss of silicon through the above reaction in this process. Not bound by theory, two distinct reaction steps are observed in the process of our inventioa In the first step, dioxides of molybdenum and / or tungsten are formed according to the following reactions. M0O3 + 0.5 C = MoO2 + 0.5 CO2 WO3 + 0.5 C = WO2 + 0.5 CO2 These reactions are allowed to complete at temperature preferably below 750°C, to prevent vaporization of the trioxides. The second steps of reactions start above 1200°C. In this step the overall reactions can be expressed as follows. M0O2 + 2 SiC = MoSi2 + 2 CO WO2 + 2 SiC = WSi2 + 2 CO MoSi2 and WSi2 form solid solution in the course of the reaction. In most of the processes involving gas evolution, the product contains numerous pores. We call this as spongy material. The spongy product is of low density and needs reprocessing by making powders followed by densification (sintering). In the process of our present investigation, we can obtain dense product by proper choice of raw materials and processing conditions. The product with density greater than 85% of theoretical density has been termed as dense products in this document. In the process of present invention, precursor of molybdenum trioxide when used is chosen from molybdic acid or ammonium molybdate. Precursor of tungsten trioxide when used is chosen fi-om tungstic acid or ammonium tungstate. Powders of molybdenum trioxide or its precursor and tungsten trioxide or its precursor need to have purity about 97% and above i.e. 97 - 100% (preferably about above 98.5% i.e. 98.5 - 100%). The process uses molybdenum trioxide in its powder form. This raw material may also be obtained by thermal decomposition of molybdic acid or ammonium molybdate. Molybdic acid or ammonium molybdate of similar purity may also be directly used as precursor for molybdenum trioxide. In these cases, the quantity of precursor material taken has to generate required amount of molybdenum trioxide, and that can be found out by heating the precursor material sample to 800°C in the air. Tungsten trioxide is also used in its powder from. This raw material may also be obtained by thermal decomposition of tungstic acid or ammonium tungstate. Tungstic acid or ammonium tungstate of similar purity may also be directly used as precursor for tungsten trioxide. In these cases, the quantity of precursor material taken has to generate required amount of tungsten trioxide, and that can be found out by heating the precursor material sample to 800°C in the air. When the product required has to be in dense form, trioxides of molybdenum and/or tungsten or molybdic acid and/or tungstic acid are preferably used as raw materials. Use of ammonium molybdate and/or ammonium tungstate lead to gas evolution during heating which results in spongy product. Silicon carbide is used in its powder form with particle size in the range 0.1 - 45 micron (-325 mesh). Narrow particle size distribution with median in the range of 5 to 12 microns is preferred. The total amount of transition metal impurities should be less than 1.5 wt%. Oxygen content in silicon carbide powder is an important parameter in the process. It is preferable to use silicon carbide with lowest oxygen content. In general, oxygen content in silicon carbide powder increases with decrease in particle size. For use in the process of invention, oxygen content in the silicon carbide powder should be below 5 wt% i.e. 0 to 5 wt% preferably below 3 wt% i.e. 0 to3 wt%. Other compositional parameter of interest is the silicon to carbon weight ratio. Silicon to carbon weight ratio should be in the range of 2.3 to 2.5, preferably close to 2.35. If the available silicon carbide powders are coarse, the same may be grounded and milled in high energy ball mill using mild steel or tungsten carbide balls as grinding media. The subject of milling in high energy ball mill has been described in R.H.Perry's "Chemical Engineers' Handbook", fifty edition, McGrSaw Hill Book Company, NY, 1973, on pages 8.25 - 8.30. However, the milled silicon carbide powders may get contaminated with iron particularly when using mild steel balls or vial. In such a situation, the milled powders must be leached with 30 - 50% hydrochloric acid. In case, the available silicon carbide powders are of wide particle size distribution, the powders may be air classified or sieved to get a fraction of narrow size distribution with median size in the range of 5 to 12 micron. Details on the air classifier system has been described in R.H.Perry's "Chemical Engineers' Handbook" fifth edition, McGraw Hill Book Company, NY, 1973, on pages 8.30-8.33 In the process of invention, carbon or its precursor used is in powder form with particle size in the range 0.1 - 8 }j.m, preferably in the range 0.1 - 2 i^m. A narrow particle size distribution with median close to 0.5 jam is preferred. Purity of carbon or its precursor should be in the range 95 - 100% wt. preferably better than 98.5% . Carbon powder used may be chosen from group of materials such as graphite, charcoal and carbon black. Carbon precursor when used may be chosen from group of materials such as starch, sucrose. Reagent mixture compositions for the production of molybdenum disilicide and tungsten disilicide and molybdenum disilicide/tungsten disicilicide solution are given below. Mixture composition for production of 100 gm molybdenum disilicide : Molybdenum trioxide 95 gm Silicon carbide 53 to 61 gm Carbon 4 to 6 gm When the used silicon carbide contains more oxygen, the quantity of silicon carbide to be taken will be on the higher side of the range. Not bound by theory, we feel that the quantity may be calculated from the following relation. Silicon carbide (in gm) = 5300 / (100 - 2.5 x Wo) where Wo = weight percent of oxygen present in silicon carbide. Thus, for silicon carbide containing 2 wt.% oxygen this value in the proportional mixture composition will be 55.8 gm. In terms of molar ratio, the constituents of the mixture are to be taken as : Molybdenum trioxide : Silicon carbide : Carbon = 1 : 2.15 ± 0.15 : 0.6 ± 0.1. When using precursor(s) equivalent amount of respective precursor(s) is takea Mixture composition for production of 100 gm tungsten disilicide : Tungsten trioxide 97 gm Silicon carbide 33.5 to 38.5 gm Carbon 2.5 to 3.5 gm As in the case of preparation of molybdenum disilicide, when the used silicon carbide contains more oxygen, the quantity of silicon carbide to be taken will be on the higher side of the range. Not bound by theory, we feel that the quantity may be calculated from the following relation SiUcon carbide (in gm) = 3350 / (100 - 2.5 x Wo) Thus, for silicon carbide containing 2 wt.% oxygen this value in the proportional mixture composition will be 3 5.3 gm. In terms of molar ratio the constituents of the mixture are to be taken as : Tungsten trioxide : Silicon carbide : Carbon = 1 : 2.15 ± 0.15 : 0.6 ± 0.1. When using precursor(s) equivalent amount of respective precursor(s) is taken. For the preparation of molybdenum disilicide/tungsten disilicide solid solution, proportional quantity from each mixture composition has to be taken. In general, for the preparation of 100 gm (molybdenum disilicide-X wt% tungsten disilicide) alloy, the mixture composition can be formulated as Molybdenum trioxide 95 x (1- X/100) gm Tungsten trioxide 97 x (X/100)gm Silicon carbide 57 ± 4 x (1-X/100) + 36 ±2.5 x(X/100)gm (considering 2wt % oxygen in silicon carbide) Carbon 5 ± 1 x (1 -X/100) + 3 ±0.5 x (X/100)gm In terms of molar ratio, the constituents of the mixture are to be taken as : Molybdenum trioxide : Tungsten trioxide : Silicon carbide : Carbon = 1-X: X: 2.15 ± 0.15 : 0.6 ± 0.1. When using precursor(s) equivalent amount of respective precursor(s) is taken. Organic binder may be added with these mixtures particularly when the product is required in dense form. Addition of organic binder helps in producing defect free green compact and gives the compact masses the necessary strength for handling. Use of organic binders has been described in J.S.Reed's "Introduction to the Principles of Ceramic Processing", John Wiley and Sons, NY, 1988, on pages 152-173. Organic binders such as polyvinyl alcohol, polyvinyl acetate, camphor may be used. The amount of binder used is below 5.0 wt%, preferably 2 - 3 wt % of the total mixture. Mixing of the constituents is one of the important steps in the process. The purpose is to get homogeneous mixture. Mixture is preferably done in wet condition in a ball mill when binder is used. Binder is added to the mixture after dissolving it in a solvent having low boiling point, preferably less than 100°C. Solvent used for wet mixing is chosen from a group of solvents (preferably polar solvents) of low boiling point (preferably less than 100°C i.e. 60 - 100°C), such as ethyl alcohol, acetone. In such case, the solvent is evaporated readily after wet mixing is over. It is preferable not to use aqueous medium. If mixing is done in dry condition, there is a possibility of oxygen pick up by silicon carbide powders from the environment. Dry milling can be carried out under inert gas atmosphere without addition of any organic binder. Tungsten carbide or zirconia is useful as grinding media in this mixing process. Tungsten carbide balls are preferred. The wt. ratio of the grinding balls to the mixture taken for mixing in the grinder is about 7 - 15 in case tungsten carbide and about 4 - 10 in case of zirconia balls. Diameters of the balls should be preferably less than (1/20) times of the diameter of the diameter of the milling pot. Speed of rotation of the pot may be chosen anywhere in the range of 30-150 revolutions per minute. Time of mixing should be more than 6 hours for rpm more than 100 and more than 10 hours for rpm less than 50. Drying of the mixture is the next step. Natural drying or oven drying (preferably at temperature less than 80°C) or vacuum oven drying (preferably at temperature less than 150°C) may be employed for the purpose removal of the solvent and moisture. Drying of the mixture is done to an extent so that residual solvent and moisture content in the mixture is less than 1.0 wt.% i.e. 0-1 wt.% and preferably less than 0.5 wt% i.e. 0-0.5 wt% in case of producing dense form and less than 3.0 wt% i.e. 0-3 wt.% in case of not so dense or spongy form. Compaction of the mixture can be carried out in uniaxial press or cold isostatic press or extrusion press, depending on the shape and size of the job. The pressing of dry mixture is carried out under pressure in the range of 30 to 150 Mpa to form green compacts. Simple shapes like sold cylinder with height to diameter ratio less than 1.5, preferably less than 1.0, are made convenient in uniaxial pressing. For smaller diameter pellets, friction wdth the wall is considerable and the applied pressure should be in the upper side of the range. Lubricants such as, stearic acid, may be used during uniaxial pressing. Complex green shapes with more uniform density can be made by cold isostatic pressing. The art of cold isostatic pressing has been described in P. J. James's "Isostatic Pressing Technology", Applied Science Publishers, NY, 1983. The choice of mold material and mold design is most important in this pressing technique. Mostly it is based on the experience of the user. In case of production of dense product of complex shapes pressing can be done in cold isostatic pressing. Production of green wares by extrusion is well documented in the literature. The process is used to produce green bodies in the form of tubes and rods. In this case the mixture is plastcised by kneading with binder/plasticiser and green compacts are formed by extrusion in the form of tubes and rods through extrusion press. For production of material in dense form, i.e. density greater than 85 % of the theoretical density, by the process of present invention, the green compact is required to be free of defects, such as, cracks, laminations etc. Heating of the compact mass is the most important step in the process. Heating has to be carried out under dynamic vacuum or flowing inert gas chosen from group O of the periodic table. The furnace may be resistance heated or inductively heated furnace where heating can be controlled automatically or manually. When heating is done under vacuum, the vacuum system connected to the chamber has to have capacity to evacuate the chamber to a vacuum 10-4 mbar. The operation schedule is as follows. i) placing the compact mass in constant temperature zone of the furnace. ii) evacuating the cold chamber to a vacuum level better than 10-3 mbar. ill) heating to a temperature in the range of 650 to 825°C, at a rate in the range of 200 to 500°C/hr. iv) holding at a temperature in the range of 650 to 825°C for 1 to 2 hours or till the vacuum improves better than 5 x 10-4 mbar. v) heating to a maximum temperature in the range of 1600 - 1800°C and holding at this temperature till chamber vacuum improves to 5 x 10-4 mbar. vi) cooling the furnace to ambient temperature. When heating is done under flowing inert gas, the furnace chamber may be purged for 2 to 3 times before heating. In practice, argon gas is used as it is cheaper in comparison with other inert gases. The purity of inert gas chosen is above 99.5%. The heating schedule is given below. i) heating at a rate less than 500°C/hr. to reach a temperature in the range of 650 - 825°C under flowing inert gas. ii) holding at a temperature in the range of 650 to 825°C for 1 to 2 hours. ill) heating to a maximum temperature in the range of 1600 - 18000C and holding at this temperature until carbon monoxide partial pressure in the outgoing gas becomes less than 0.01 mbar under flowing inert gas condition. iv) cooling to ambient temperature. In the process for making molybdenum disilicide or tungsten disilicide or solid solution thereof in dense from, molybdenum trioxide or molybdic acid or tungsten trioxide or tungstic acid or mixtures thereof are mixed with silicon carbide and carbon, preferably with binder. Subsequently drying is carried out to an extent so that residual solvent and moisture content in the mixture is less than 1.0wt% i.e. 0 -1wt% and preferably less than 0.5 wt% i.e. 0 - 0.5 wt.%. After compacting the dry mixture, heating is controlled in such a manner that the furnace chamber dynamic vacuum is maintained better than 10-2 mbar during the heating process; a temperature in the range of 650 to 825°C is held for 1 to 2 hours or till the vacuum improves to 5 x 10-4 mbar; heating to a maximum temperature of 1700 - 1900°C and holding at this temperature till gas evolution stops, as indicated by the improvement of vacuum to better than 10-4 mbar; cooling the furnace to ambient temperature. Examples: The invention will now be illustrated with the help of examples. The examples are by way of illustration only and not for restricting the scope of the invention. The equipment required is given below, and the raw materials used in these examples are given in Table I. Equipment: i. Conventional wet grinding mill with tungsten carbide grinding balls was used as a nuxer. ii. Conventional vacuum drying oven was used for drying. iii. Conventional manually operated hydraulic press was used for palletizing the powder into green compacts. iv. Vacuum induction fiimace (40 kW, 3 kHz) using graphite (200 mm ht. x 150 mm outer dia. x 15 mm thickness) as susceptor was used for heating the green compacts. Max. temperature - 18 2400°C, max. vacuum 10-4 mbar. Table I: Raw materials used in these examples Source Remarks Molybdenum Trioxide 1 99% purity Tungsten Trioxide 1 99% purity Molybdic acid 1 85 % M0O3 Tungstic acid 1 99 % purity Ammonium paramolybdate 1 99 % purity Silicon Carbide 2 Oxygen = 3 wt.% ; Si/C weight ratio = 2.35 Metalic impurity = 0.7wt.% ; Particle size = 9.7 ()jm) Natural Graphite 3 Purity 98.5 % Particle size 0.6(|jm) (ground and leached inhouse) 1. M/s Thomas Baker Chemicals Ltd., Mumbai, India. 2. M/s Grindwell Norton Ltd., Bangaolre, India. 3. M/s Graphite India Ltd., Example 1: Preparation of Molybdenum Disilicide : 72 grams of molybdenum trioxide powder, 43.6 grams of silicon carbide powders, 3 grams of natural graphite powder were taken in a 250 ml capacity porcelain grinding pot. About 2.5 grams of polyvinyl alcohol (dissolved in water in concentrated form) was added to the above reactants. Zirconia balls of 3 mm diameter were used as grinding media. Acetone was used for mixing under wet condition. Mixing was done by rotating the pot at 50 rpm in a ball mill for 12 hours using 3 mm diameter zirconia balls as grinding media. Acetone was used as the solvent. The mixture was vacuum oven dried at a temperature of 110°C. The dried mixture was pelletised under uniaxial pressing at a pressure of 70 MPa to form pellets of size 15 mm diameter x 12 mm height. The pellets were charged inside an induction heated graphite vacuum fiimace. The fiimace chamber was evacuated to 10-4 mbar and heated at a rate of 350°C/hour upto 800°C, held at that temperature for 60 minutes. By this time, the vacuum improved upto 2-x 10-4 mbar. The temperature was fiirther raised with a heating rate of 500°C/hour until it reached to 1800°C. This temperature was held for 90 minutes. By this time, vacuum improved upto 3-x 10-4 mbar. The ftimace was cooled to the ambient temperature and the pellets were taken out. Percent weight loss in the heating step of the process was 36.47% which is close to the expected value (37.24%). The product was analysed and showed the following chemical composition and x-ray diffraction data is given in Table II. The product contains 63.0% molybdenum, 36.1% silicon, 0.4% carbon, 0.1% oxygen and 0.18% iron. This composition corresponds to Si/Mo atom ratio 1.96 (close to 2.0). Table II: XRD data for product of Example 1 Peak No d-values Relative intensity All the peaks in the XRD pattern of the product of example 1 closely matches to standard data of tetragonal molybdenum disilicide. The density (determined by water displacement method) of the product is 5.95 g/cm3, which corresponds to 95.% of theoretical density. Microstructural observations also confirm absence of porosity. Example 2 : Preparation of Tungsten Disilicide : The mixture composition consists of 46.4 gm timgsten trioxide, 17.3 gm silicon carbide and 1.2 gm carbon and 1.5 gm polyvinyl alcohol (binder). Mixing and compaction procedures were identical with Example 1. Heating schedule was as described in Example 1, except that maximum heating temperature was 1750°C. The weight loss in the heating step of the process was 26.02% which is close to the expected value (27.7%). The x-ray diffraction data of the product is given in Table III. Table IIIrXRD data for product of Example 2 Peak No d-values Relative intensity All the peaks in the XRD pattern of the product of example 2 closely matches with standard data of tetragonal tungsten disilicide. The density (determined by water displacement method) of the product is 9.03 g/cm3 which corresponds to 91.4% of theoretical density. Example 3: Preparation of Molybdenum Disilicide - 50 wt.% Tungsten Disilicide Solid Solution The mixture composition consists of 18.95 gm molybdenum trioxide, 19.33 gm timgsten trioxide, 18.74 gm silicon carbide and 1.29 gm natural graphite and 1.5 gm polyvinyl alcohol. Mixing, compacting and heating schedule are as described in Example 1. The weight loss in the heating step of the process was 32.50%, which is close to the expected value (33.12%). The x-ray diffraction data of the product is given in Table IV. Table IV : XRD data for product of Example 3 Peak No d-values Relative intensity The density (determined by water displacement method) of the product is 7.08 g/cm^, which corresponds to 92.6% of theoretical density. Examples 4, 5 and 6 : These examples describe preparations of molybdenum disilicide and tungsten disilicide starting from precursors of molybdenum trioxide and tungsten trioxide. In these examples, the process followed is as decribed in example 1; the details of the reagent mixture composition and results are given in Table V. Table V: Details of Examples 4,5 and 6. * Expected weight loss based on theoretical calculations. ** Percent theoretical density. CLAIM: 1. A process for producing compounds of disilicide having the formula M - Si2 where M represents Mo, W, MoxWi-x (0 i) providing a dry powder mixture consisting of oxide(s) of formula M - O3 or precursors of M - O3 , where M is as defined above, carbon or its precursor and silicon carbide; ii) compacting the dry powder mixture in a press to obtain green compacts; iii) heating the green compact mass to a temperature in the range of 1600 - 1800°C in a controlled manner in a furnace under dynamic vacuum or under flowing inert gas chosen from the group 0 of the periodic table; and iv) cooling the furnace to ambient temperature to get the product. 2. A process for producing compounds of disilicide as claimed in claim 1, wherein the dry powder mixture is provided preferably by mixing oxide(s) of formula M - O3 or precursors of M - O3, where M is as defined above, carbon or its precursor and silicon carbide with a binder, in wet condition using a solvent and evaporating the solvent thereafter. 3. A process for producing compounds of disilicide as claimed in any claim 1 or 2, wherein the residual solvent and moisture content in the mixture is brought to between 0.0 and 3.0 wt.%. 4. A process for producing compounds of disilicide as claimed in any claim 1-3, wherein the residual solvent and moisture content in the mixture is brought to between 0.0 and 0.5 wt.%. 5. A process for producing compounds of disilicide as claimed in any claim 1-4, wherein the solvent used for wet mixing is a polar solvent of low boiling point in the range of 60 tol00°C. 6. A process for producing compounds of disilicide as claimed in any claim 1-5, wherein the solvent is selected from ethyl alcohol and acetone. 7. A process for producing compounds of disilicide as claimed in any claim 1 - 6, wherein M - O3, silicon carbide and carbon powders are mixed in molar ratio 1 : 2.15 ± 0.15 : 0.6 ± 0.1 respectively for making the dry mixture, when precursors of M - O3 are used, amount equivalent to M - O3 is taken for making the mixture. 8. A process for producing compounds of disilicide as claimed in any claim 1-7, wherein M - O3, silicon carbide and carbon powders are mixed in molar ratio 1 : 2.15 + 0.15 : 0.6 + 0.1 respectively for making the dry mixture, when precursor of carbon is used, amount equivalent to carbon is taken for making the mixture. 9. A process for producing compounds of disilicide as claimed in any claim 1 - 8, wherein the precursor of molybdenum trioxide is chosen from molybdic acid or ammonium molybdate. 10. A process for producing compounds of disilicide as claimed in any claim 1-9, wherein the precursor of tungsten trioxide is chosen from tungstic acid or ammonium tungstate. 11. A process for producing compounds of disilicide as claimed in any claim 1-10, wherein the powders of molybdenum trioxide or its precursor and tungsten trioxide or its precursor have purity in the range of 97 to 100%. 12. A process for producing compounds of disilicide as claimed in any claim 1-11, wherein the powders of molybdenum trioxide or its precursor and tungsten trioxide or its precursor have purity in the range of 98.5 to 100%. , 13. A process for producing compounds of disilicide as claimed in any claim 1-12, wherein silicon carbide powders have particle size in the range 0.1 to 45 micron and oxygen content in the range of 0 to 5 wt.%. 14. A process for producing compounds of disilicide as claimed in any claim 1 -13, wherein silicon carbide powders used have particle size in the range of 0.1 to 12 micron and oxygen content in the range of 0 to 3 wt.%. 15. A process for producing compounds of disilicide as claimed in any claim 1 - 14, wherein the carbon powder used is chosen from group of materials such as graphite, charcoal and carbon black with particle size in the range 0.1 to 8 micron and purity in the range 95 to 100 wt%. 16. A process for producing compounds of disilicide as claimed in any claim 1-15, wherein the carbon powder used is chosen from group of materials such as graphite, charcoal and carbon black with particle size in the range 0.1 to 2 micron. 17. A process for producing compounds of disilicide as claimed in any claim 1-16, wherein the carbon precursor is chosen from group of materials such as starch, sucrose. 18. A process for producing compounds of disilicide as claimed in any claim 1-17, wherein pressing of the dry mixture is carried out under pressure in the range of 30 to 150 MPa to form the said green compact. 19. A process for producing compounds of disilicide as claimed in any claim 1-18, wherein the heating / cooling sequence involve (i) heating the green compact at a rate in the range of 200 to 500°C/hr to reach a temperature in the range of 650 - 825°C under dynamic vacuum, (ii) maintaining the temperature in the range of 650 to 825°C for 1 to 2 hours, (iii) heating the mass to a maximum temperature in the range of 1600 - 1800°C and maintaining this temperature till such time that chamber vacuum improves to 10-4mbar or less, (iv) cooling the furnace to ambient temperature. 20. A process for producing compounds of disilicide as claimed in any claim 1-18, wherein the heating / cooling sequence involve (i) heating the green compact at a rate in the range of 200 to 500°C/hr to reach a temperature in the range of 650 - 825 °C under flowing inert gas, (ii) maintaining the temperature in the range of 650 to 825 °C for 1 to 2 hours. (iii) heating the mass to a maximum temperature in the range of 1600 - 1800°C and maintaining this temperature till carbon monoxide partial pressure in the outgoing gas becomes 0.01 mbar or less, (iv) cooling the fiamace to ambient temperature. 21. A process for producing compounds of disilicide as claimed in any claim 1 - 18, in dense form, wherein oxides or their precursors are selected from molybdenum trioxide, molybdic acid, tungsten trioxide and tungstic acid, and are mixed with silicon carbide and carbon, in wet condition preferably adding a binder; subsequent drying is carried out to the extent so that residual solvent and moisture content in the mixture is 0.5 wt% or less; and after compacting the dry mixture the green compact is heated in a controlled manner that i) the fiimace chamber dynamic vacuum is maintained better than 10'^ mbar during the heating process, ii) a temperature in the range of 650 to 825°C is held for 1 to 2 hours or till the vacuum improves to 5 x 10-4 mbar or less. iii) heating to a maximum temperature in the range of 1700 - 1900°C and holding at this temperature till chamber vacuum improves to better than 10-4 mbar, iv) cooling the fiirnace to ambient temperature. 22. A process for producing compounds of disilicide having the formula M - Si2, where M represents Mo, W, MoxWi-x (0 Dated this 7TH day of August, 2001 SiddharthaNag S Majumdar & Co. Applicant's Agent

Documents:

779-mum-2001-cancelled pages (23-04-2004).pdf

779-mum-2001-claim(granted)-(23-4-2004).doc

779-mum-2001-claims (granted) (23-04-2004).pdf

779-mum-2001-correspondence (21-12-2004).pdf

779-mum-2001-correspondence (ipo) (21-12-2004).pdf

779-mum-2001-form 1(09-08-2001).pdf

779-mum-2001-form 19(30-10-2003).pdf

779-mum-2001-form 2(granted) (23-04-2004).pdf

779-mum-2001-form 2(granted)-(23-4-2004).doc

779-mum-2001-form 3(09-08-2001).pdf

779-mum-2001-power of attorney (23-04-2004).pdf