Title of Invention	A SYNERGISTIC COMPOSITON FOR THE PREPARATION OF SILICON NITRIDE POWDER.
Abstract	The present invention relates to a synergistic composition for the preparation of silicon nitride powder. The inventive step resides in introducing ß- Si3N4 powder simultaneously with some iron in the composition. The precursor powder after complete reduction and nitridation contains atleast 90% Si3N4 preferably rich in the ß- phase with the advantages such as cost effectiveness.

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This invention relates to a synergistic composition for the preparation of silicon nitride powder. The silicon nitride powder prepared from the synergistic composition of the present invention is useful for the preparation of dense products suitable for some refractory and engineering applications. The present day method consists of firing an intimately mixed green mixture of fine silica (SiO2) with carbon (C) under flowing nitrogen gas atmosphere for which reference may be made to M. Mehner in German Patent no. 88 999, 30 September, 1896. Reference may also be made to Zhang et al. in J. Am. Ceram. Soc., Vol. 67, No. 10, 1984, pp. 421-429 entitled "Preparation of silicon nitride from silica" wherein the importance of using the fineness of both silica and carbon initial particle size have been studied. It was recommended that monodisperse, spherical silica powder with surface area of >150 m2/g was effective for silicon nitride preparation. In this study, a large excess of carbon to silica molar ratio upto 16:1 was used while the stoichiometric ratio is 2:1 as per the following overall nitridation reaction: 3SiO2 + 6C + 2N2 = Si3N4 + 6CO. (1) A maximum nitridation yield of 40% of theoretical was obtained from a reaction effected at 1400°C for 16h by using a carbon with surface area in range of 35 to 55 m2/g. In case where an extremely fine sized carbon with surface area >650 m2/g was used, the yield of the nitrogen uptake in the product increased to 80% of theoretical under similar conditions of reaction. Further reference may be made to Komeya et al. in J. Mater. Sci., vol. 10, No. 7, 1975, pp. 1243-1246, entitled "Synthesis of the a- form of silicon nitride from silica" wherein a still larger silica:carbon molar ratio in the range of 1:20 to 1:60 were used. A maximum nitrogen uptake of 30 wt.% under the reaction conditions of 1400°C for 5h was obtained against the theoretical value of 40 wt.%. To improve the yield of nitrogen uptake in the product in an another attempt a higher reaction temperature >1450°C has been used for which reference may be made to Lee et al. in Nitrogen Ceramics, edited by F.L. Riley, Noordhoff, Leyden, 1977, pp.175-181 entitled "Reactions in the SiO2- C- N2 systems". It was found that there exists a critical boundary temperature of 1450°C above which silicon carbide appears in the product in place of silicon nitride when the starting silicaicarbon ratio exceeds the value of 1:3. In still another attempt for which reference may be seen in Inoue et al. in J. Am. Ceram. Soc, Vol. 65, No. 12, 1982, C205, entitled "Synthesis of silicon nitride powder from silica reduction", extremely fine (particle size -0.02 µm) silicaicarbon with a molar ratio ranging in between 1:2 and 1:4 was mixed with preformed a-silicon nitride in a weight ratio of 0.1 to 1.0 wt.%. The nitrogen uptake of 34 to 35 wt.% in a reaction at 1400°C for 5h could be obtained. In further attempt for which reference may be made to Hanna et al. in Brit. Ceram. Trans. J., Vol. 84, No. 1,1985, pp. 18-21, entitled "Silicon carbide and nitride from rice hulls-Ill: Formation of silicon nitride" a source of iron was used in the starting composition under a flow of ammonia gas in place of nitrogen gas. The pure silicon nitride as a reaction phase only appeared at a temperature of 1300°C while above """ ~~^: 1350°C silicon carbide also started producing. A maximum of 90% of the starting silica could be reduced upto 1500°C resulting in a mixture of silicon nitride and silicon carbide where an amount >6 wt.% was used in the starting mixture. Another process using iron upto a maximum of 5 wt.% could produce pure silicon nitride but at a very high temperature of >1540°C under a nitrogen gas flow for which reference may be made to Bandyopadhyay et al. in Ceram. Intern., Vol. 17, 1991, pp. 171-170, entitled "Reaction sequences in the synthesis of silicon nitride from quartz". The drawbacks of the above processes were many fold. Firstly, an extremely fine grain sized silica was required which had to be prepared in a process by following the sol-gel technique and was evidently a very costly process. Secondly, the use of large excess of carbon in the starting mixture involves an additional firing at above 500°C in air of the post reacted product where the unreacted carbon has to be burnt off. Additionally, the failure of achieving nitrogen uptake upto the theoretical value signifies some residual silica in the product where no other phase appeared in the reaction product. The residual silica may be harmful in the ultimate use of the material. Moreover, the yield of reaction is difficult to be increased by using higher reaction temperature above 1450°C because there appears an additional phase, silicon carbide, which becomes competitive to the amount of silicon nitride formation. The use of iron produces silicon nitride along with silicon carbide where an addition of -7 wt.% iron is required in case ammonia is the reacting gas. In case of nitrogen as the reacting gas, a little lesser amount of iron ~5 wt.% can be used to obtain pure silicon nitride but a high reaction temperature >1540°C is required. Thus the general drawbacks of the above process are: 1. The starting silica particle size should be extremely small with surface area of the powder at least greater than 150 m2/g which is produced following a very expensive sol-gel process. 2. The starting carbon particle size should also be extremely fine with surface area of the powder preferably greater than 150 m2/g. 3. The production of silicon nitride requires the use of large amount of iron when used as a catalyst or very high temperature of above 1540°C. The main object of the present invention is to provide a synergistic composition for the preparation of silicon nitride powder containing atleast 90 wt.% of the phase which obviates the above disadvantages. Another object of the present invention is to provide a synergistic composition wherein pure silica, a source of carbon such as activated charcoal, ß-silicon nitride, a source of iron such as ferric nitrate are used. Yet another object of the present invention is to provide a synergistic composition which allows the use of lower amount of iron thus enhancing the quality of silicon nitride powder produced from this composition. The present invention relates to a synergistic composition for the preparation of silicon nitride powder. The inventive step resides introducing ß- Si3N4 powder simultaneously with some iron in the starting composition. The precursor powder after complete reduction and nitridation containing atleast 90% Si3N4 preferably rich in the ß- phase with the advantages such as cost effectiveness. Accordingly, the process provides a synergistic composition for the preparation of silicon nitride powder which comprises : 44-60 weight% SiO2, 19-27weight% C, 3.5-13 weight % ß- Si304, and 11 - 26 weight% Fe(NO3)3.9H2O In an embodiment of the present invention pure and powdered natural silica activated charcoal and ß- Si3N4 may be used. In another embodiment of the present invention, the Si3N4 used may contain In yet another embodiment of the present invention, a source of carbon such as activated charcoal powder with surface area of around 35 m2/g may be used. The composition of the present invention is not a mere admixture but is a synergistic mixture and the property of the final product is not the mere aggregation of properties of the individual ingredients. In our co-pending patent application no. 176/DEL/2002 , we have described and claimed a process for the preparation of silicon nitride powder from the synergistic composition which comprises preparing a homogeneous mixture by conventional methods of the composition of the present invention, passing the powder through 100 mesh, pressing the powder so obtained by conventional methods to form green compacts, sintering the green compacts at a temperature in the range of 1475°C to 1550°C in nitrogen atmosphere, grinding by conventional methods to obtain silicon nitride powder. The process as described and claimed in our co-pending pending patent application no. 176/DEL/2002 for the preparation of silicon nitride powder from the synergistic composition of the present invention is described below in detail: 1. Pure and powdered SiO2l C and - Si3N4and Fe (N03)3. 9H20 were taken as starting materials. 2. The accurately weighed Fe(N03)3 was made into a solution of acetone and was mixed with the above mixture. 3. Accurately weighed appropriate proportions of starting materials of compositions of the present invention were taken in an alumina pot of a ball mill along with alumina balls (size around 5 to 15 mm) for ball milling wherein the ball : powder ratio were kept in the range of 6:1 to 15:1, preferably around 9:1 and wherein the milling was done in a liquid medium of acetone for which the water content was 0.2%. The milling time was for a period between 2 to 8 hours. 3. After milling the powder was separated from the balls, sieved and was dried. 4. The milled powder was taken in a steel mould and was uniaxially pressed with pressure ranging from 1 to 50 Kg/cm2. 5. The pressed green billets were taken in a graphite resistance heating furnace and were fired in nitrogen gas atmosphere at a temperature in the range of 1475° to 1550°C. In general, the carbothermic reduction and nitridation of silica is sensitively guided by the initial particle size of the reactants. Under extreme reducing condition, a solid- solid reaction is taking place where SiO2 is reduced by solid Carbon to form a mixture of vapour phase of SiO and CO. In a second set of reaction, SiO vapour reacts with gaseous nitrogen to form solid Si3N4. The formation of Si3N4 is started from a heterogeneous nucleation on C and SiO2 surface followed by growth from the gas phase reaction. Both the first phase of reactions as well as the nucleations are favoured by the decrease in particle size of the starting solid reactants. When a small amount of finely divided α-Si3N4 is used, these act as seeding material. These like phase itself act as the heterogenously nucleated sites and favours strongly the Si3N4 formation. On the other hand, the nitride formation is presumed to be related to the existence of a Fe-Si liquid phase when iron is used in the starting mixture. The appearance of Fe and Si in the reaction site are due to the reduction of their respective oxides during firing. When the reaction proceeds, the liquid becomes saturated with nitrogen causing the precipitation of Si3N4. A continuous growth of the nitride occurs with simultaneous dissolution of silicon and nitrogen into the liquid to make it saturated. Except for solubility, the growth is assumed to be controlled by the diffusivity of the constituent elements in the liquid after their dissolution. A larger amount of iron favours therefore the formation of a larger amount of liquid which can make the SiO2 and C particles wet enough to serve as centres where the nucleation can take place. In the present case, it may be believed that the added ß- Si3N4 particles in the starting mixture itself serve as the "like"- nucleation sites where from growth can occur. Therefore, the reaction does not require large amount of iron and produces similar yield at lower temperature of around 1500°C which otherwise results from a reaction temperature of >1540°C. Thus the novelty of the present invention is that the synergistic composition composition consists of a lower amount of the catalyst. The inventive step lies in the synergistic composition having ß- Si3N4 powder simultaneously with lower amount of iron, as catalyst. The following examples are given by way of illustration of the present invention and should not be construed to limit the scope of the invention: Example 1 A composition containing SiO2- 48.26 weight%, C- 20.27 weight%, Si3N4 6.76 weight% and Fe(NO3)3- 24.72 weight% was ball milled for 5 hours, dried, cold pressed under uniaxial pressing and was fired at 1525°C for 5 hours in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh'1 at a pressure of 0.12 MPa. The firing weight loss was 122% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the only crystalline phase present in the product. Example 2 A composition containing SiO2- 48.26 weight%, C- 20.27 weight%, Si3N4 6.76 weight% and Fe(NO3)3- 24.72 weight% was ball milled for 5 hour, dried, cold pressed under uniaxial pressing and was fired at 1500°C for 5 hour in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh'1 at a pressure of 0.12 MPa. The firing weight loss was 116% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the only crystalline phase present in the product. Example 3 A composition containing SiO2- 48.26 weight%, C- 20.27 weight%- Si3N4- 6.76 weight% and Fe(N03)3- 24.72 weight% was ball milled for 5 hour, dried, cold pressed under uniaxial pressing and was fired at 1480°C for 5 hour in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh"1 at a pressure of 0.12 MPa. The firing weight loss was 102% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the major crystalline phase along with some SiC present in the product. Example 4 A composition containing SiO2- 46.68 weight%, C- 19.61 weight%, Si3N4- 9.80 weight% and Fe(NO3)3- 23.91 weight% was ball milled for 5 hour, dried, cold pressed under uniaxial pressing and was fired at 1525°C for 5 hour in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh-1 at a pressure of 0.12 MPa. The firing weight loss was 124% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the only crystalline phase present in the product. Example 5 A composition containing SiO2- 46.68 weight%, C- 19.61 weight%, Si3N4- 9.80 weight% and Fe(NO3)3- 23.91 weight% was ball milled for 5 hour, dried, cold pressed under uniaxial pressing and was fired at 1500°C for 5 hour in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh-1 at a pressure of 0.12 MPa. The firing weight loss was 114% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the only crystalline phase present in the product. Example 6 A composition containing SiO2- 46.68 weight%, C- 19.61 weight%, Si3N4- 9.80 weight% and Fe(NO3)3- 23.91 weight% was ball milled for 5 hour, dried, cold pressed under uniaxial pressing and was fired at1480°C for 5 hour in a nitrogen gas atmosphere under a linear gas flow rate of 40 mh'1 at a pressure of 0.12 MPa. The firing weight loss was 107% of theoretical, calculated by following the equation no.1 as stated above. The sample was completely grey in colour containing soft agglomerates, grindable to produce fine sized powder. The x-ray diffraction shows Si3N4 as the major crystalline phase along with some SiC present in the product. The main advantages of the synergistic composition of the present invention are : 1) Use of lower amount of iron which is beneficial so far as the quality of the produced powder be concerned. 2) Use of starting silica which may be prepared only by grinding of naturally occurring and abundantly available silica in a mill rather than fine silica produced from a sol-gel technique, which is costly. 3) Use of starting carbon with surface area only in the range of around 35 m2/g in comparison to that in the range of 150-650 m2/g used in majority of the prior arts. We claim: 1. A synergistic composition for the preparation of silicon nitride powder which comprises: 44-60 weight% SiO2, 19-27weight%C, 3.5-13 weight% ß-Si3N4, and 11-26 weight% Fe(N03)3.9H2O. 2. A synergistic composition as claimed in claim 1, wherein pure and powdered natural silica, activated charcoal and ß-Si3N4 is used. 3. A synergistic composition as claimed in claims 1-2 wherein Si3N4 contains 4. A synergistic composition as claimed in claims 1-3 wherein the source of carbon such as activated charcoal powder with surface area of around 35 m2/g is used. 5. A synergistic composition for the preparation of silicon nitride powder substantially as herein described with reference to the examples.

Documents:

175-del-2002-abstract.pdf

175-del-2002-claims.pdf

175-del-2002-correspondence-others.pdf

175-del-2002-correspondence-po.pdf

175-del-2002-description (complete).pdf

175-DEL-2002-Form-1.pdf

175-del-2002-form-18.pdf

175-del-2002-form-2.pdf

175-del-2002-form-3.pdf