Title of Invention

A PROCESS FOR THE PREPARATION OF SILICON NITRIDE POWDER.

Abstract

This invention relates to a process for the preparation of silicon nitride powder. The process comprises homogenizing and powdering by conventional methods a composition essentially consisting of: 44-60 weight% SiO2, 19-27 weight% C, 3.5-13 weight% p-Si3N4, and 11-26 weight% Fe(NO3)3. 9H2O, 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 to 1550°C in nitrogen atmosphere, grinding by conventional methods to obtain silicon nitride powder.

Full Text

This invention relates to a process for the preparation of silicon nitride powder. This invention particularly relates to a process for the preparation of silicon nitride powder from the synergistic composition as described and claimed in our co-pending patent application no. 0175del2002. The silicon nitride powder 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 the 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: 3Si02 + 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 Si02- 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 silica: carbon 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 jam) silica: carbon 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 folds. 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 js 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 process 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 process for the preparation of silicon nitride powder wherein pure silica, a source of carbon such as activated charcoal, ß-silicon nitride, a source of iron such as ferric nitrate and pure nitrogen gas are used as starting materials. Still another object of the present invention is to provide a process for the preparation of silicon nitride powder from a synergistic composition wherein iron is used as a catalyst and ß -silicon nitride as seeding material in the starting materials. A further object is to provide a process for the preparation of silicon nitride powder from the synergistic composition as described and claimed in our co-pending application no. 0175del2002. Yet another object of the present invention is to provide a process wherein a lower temperature of firing is required thus making the process cost effective. The present invention relates to a process for the preparation of silicon nitride powder which involves carbothermal reduction and nitridation of silica by inventive steps of introducing p-Si3N4 preferably rich in the ß- phase with the advantages such as cost effectiveness. Accordingly, the process provides a process for the preparation of silicon nitride powder which comprises homogenizing and powdering by conventional methods a composition essentially consisting of: Milling by known methods 44-60 weight % Si02, 19-27 weight % C, 3.5-13 weight % p-Si3N4, and 11-26 weight % Fe(NO3)3. 9H2O to obtain powder, 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 to 1550°C in nitrogen atmosphere, grinding by conventional methods to obtain silicon nitride powder. In an embodiment of the present invention, Si3N4 may contain In another embodiment of the present invention the powder may be pressed uniaxially at a pressure in the range of 1 to 50 kg/cm2. In another embodiment of the present invention, the nitrogen gas may contain The process of the present invention for the preparation of silicon nitride powder is described below in detail: 1. Pure and powdered SiO2, C and p-Si3N4 and Fe (N03)3. 9H2O were taken as starting materials. 2. The accurately weighed Fe (NO3)3 was made into a solution of acetone and was mixed with the above mixture. 3. Accurately weighed appropriate proportions of starting materials 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. 4. After milling the powder was separated from the balls, sieved and was dried. 5. The milled powder was taken in a steel mould and was uniaxially pressed with pressure ranging from 1 to 50 Kg/cm2. 6. 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 to1550°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 nucleation's are favoured by the decrease in particle size of the starting solid reactants. When a small amount of finely divided a-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 p- SisN4 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 >1540°C. Thus, the novelty of the present invention is that the product obtained contains atleast 90% of Si3N4 from a reaction which commences at lower temperature than the existing processes by using a lower amount of the catalyst. The inventive step lies in introducing p-Si3N4 powder simultaneously with iron. 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 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 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 (N03)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 (NO3)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 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 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(N03)3- 23.91 weight % was ball milled for 5 hour, dried, coldpressed 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 Si02- 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 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 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 SisN4 as the major crystalline phase along with some SiC present in the product. The main advantages of the present process are: 1} Use of lower amount of iron which is beneficial so far as the quality of the produced powder be concerned. 2) The complete reduction and nitridation is possible under lower reaction temperature thereby making the process cost effective. 3) The process allows the 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 thereby making the process still economic. 4) The process allows the 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 thereby making the process further economic. 5) The obtained precursor powder is sinterable with appropriate additives to produce dense material suitable for use in refractory and other applications. We Claim: 1. A process for the preparation of silicon nitride powder which comprises homogenizing and powdering by conventional methods a composition essentially consisting of: -60 weight % SiO2, 19-27 weight % C, 3.5-13 weight % p-Si3N4, arid 11-26 weight % Fe(NO3)3. 9H2O to obtain powder, 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 to 1550°C in nitrogen atmosphere, grinding by conventional methods to obtain silicon nitride powder. 2. A process as claimed in claim 1 wherein Si3N4 contains 3. A process as claimed in claims 1 -2 wherein the powder is pressed uniaxially at a pressure in the range of 1 to 50 kg/cm2. 4. A process as claimed in claims 1-3 wherein the nitrogen gas contains oxygen and water vapour each. 5. A process for the preparation of silicon nitride powder substantially as herein described with reference to the examples.

Documents:

176-del-2002-abstract.pdf

176-del-2002-claims.pdf

176-del-2002-correspondence-others.pdf

176-del-2002-correspondence-po.pdf

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

176-del-2002-form-1.pdf

176-del-2002-form-18.pdf

176-del-2002-form-2.pdf

176-del-2002-form-3.pdf