Title of Invention

A PROCESS OF MANUFACTURING β-SILICON NITRIDE

Abstract

The present infection relates to a process of the manufacturing β-silicon nitride. In the process, α-silicon nitride is converted to β-silicon nitride by an unstable phase like yttrium aluminum pervoskite (YAP) in the Y2O3 - Al2O3 system. The novelty of the present process lies in increasing the level of homogeneity of mixture precursor and reactivity of Y2O3by the inventive step of performing the mixing in the form of slurry which resulted uniform mixing of the raw materials. Due to this higher reactivities of Y2O3 and A12O3, the overall temperature of the conversion of α-Si3N4 to β-Si3N4 decreases and degree of conversion increases.

Full Text

The present invention relates to a process of manufacturing p-silicon nitride. Sintered β-silicon nitride has many attractive properties. Its high hardness and wear resistance make it very useful as cutting tools, bearing and grinding media. It is very useful in burners, welding nozzles, heat exchangers and sheilding tubes due to its stability at high temperature and resistance toward chemical attack together with its high thermal conductivity. Again, its good thermal shock resistance facilitates its use as a refractory crucibles, thermocouple sheilding tube, riser pipe, jet and nozzle, taphole plug, stirrer and cladding material. These all properties along with high specific strength and suitable electrical properties make it of use in the construction of engines as material for valves, tarbocharger rotors, gas turbines, catalyst carriers and spark plug. Reference may be made to Coe, R. F., Lumby, R. J. and Pawson, M. P. (1972) Some Properties and Applications of Hot-pressed Silicon Nitride, Special Ceramics, 5 (ed. P. Popper), Brit. Ceramic Research Association, pp. 377-84, where in a method of making P-silicon nitride essentially consists of hot pressing a-silicon nitride powder with small percent of additive such as MgO by hot-pressing at 1740°C. They reported a linear relationship between %p-silicon nitride and log time for silicon nitride powder. Reference may also be made to William E. Lee and W. Mark Rainforth, Published by Chapman and Hall, London, UK, (1994) pp. 406, wherein reaction transformation of a to β-silicon nitride involving a chemical reaction between α-silicon nitride and enstatite was reported. Another method of making β-silicon nitride powder is by using yttria and alumina to form yttrium aluminium garnet in the form of solid as an additive for conversion of a-Si3N4 to β-Si3N4 for which references may be made to M. K. M. Hruschka, W. Si. S. Tosatti, T. J. Graule and L. J. Gauckler, in J. Am. Ceram. Soc. 82[8] 2039-43 (1999), wherein a-silicon nitride powder was intimately mixed with A12O3 and Y2O3 in the proportion that leads to YAG. In all the above processes the main drawbacks may be listed as below. 1. Homogeneous mixing of solid ingredients like a-silicon nitride powder and additives require special processing techniques and expensive equipments. 2. Hot pressing technique is not economically viable and lacks flexibility as far as the fabrication of intricated shapes are concerned. 3. Solid state powder mixing would lead to non-uniform mixing and hence probability of coating of each silicon carbide grain with the liquid-phase forming composition is very poor and it require high amount of liquid phase. 4. The presence of high amount of liquid phase would deteriorate the inherent property of silicon nitride. In the prior art process only yttrium aluminum garnet (YAG) was used in Y2O3-A12O3 system as an additive of remanufacturing of β-silicon nitride. To the best of our knowledge and belief yttrium aluminum perovskite (YAP) and yttrium aluminum monoclinic (YAM) were not used as an additive for the conversion of a-silicon nitride to β-silicon nitride. The main object of the present invention is to provide a process of manufacturing P-silicon nitride which obviates the drawbacks as detailed above. Another objective of the present invention is to effect the conversion of a-silicon nitride to P-silicon nitride by an unstable phase like yttrium aluminum perovskite (YAP) in the Y2O3-A12O3 system. Still another object of the present invention is to eliminate the process of hot pressing. Yet another object of the present invention is to ensure the homogeneous mixing of the additive composition with α-silieon nitride powder. Still another object of the present invention is to ensure the maximum conversion of a-silicon nitride to P-silicon nitride with minimum of yttrium-alumina additive. Still another object of the present invention is to reduce the α toβ conversion temperature. Accordingly, the present invention provides a process of manufacturing P-silicon nitride which comprises characterized in that preparing a slurry of a-silicon nitride powder with aluminum nitrate solution and yttrium nitrate solution in such a proportion that corresponds to Y203 : Al2O3 in the range of 9:1 to 1:9 in molar proportion in the final composition, converting the slurry into a gel like mass by increasing the pH of the slurry in the range of 7.0 to 12.0 by adding soluble hydroxide solution, filtering the gel like mass, drying the filtered mass at a temperature in the range of 90-110°C, calcining the dried mass at temperature in the range of 400 to 800°C, grinding the calcined mass by known processes, forming the ground powder into shapes by known methods, firing the shapes in nitrogen atmosphere at a temperature in the range of 1600 to 1850°C for a period in the range of 30 to 300 minutes. In an embodiment of the present invention yttria-alumina additive used for a to p conversion of silicon nitride may be in the range of 1 -25 weight percent. In another embodiment of the present invention the concentratuion of aluminium nitrate solution may be in the range of 0.5 (M) to 5.0 (M). In yet another embodiment of the present invention the concentration of yttrium nitrate solution may be in the range of 0.5 (M) to 2.5 (M) concentration. In still another embodiment of the present invention the molar ratio of A12O3 to Y2O3 may be 1:1 in the fired product. In yet another embodiment of the present invention soluble hydroxides used may be such as ammonium hydroxide, In another embodiment of the present invention the firing may be effected by loading the sample in a graphite crucible provided with a threaded lid. In still another embodiment of the present invention the shapes may be packed into crucible with the powder of same composition of the shapes to be fired. The details of the process of the present invention are given below : 1. a -silicon nitride powder is mixed with aluminium nitrate solution and yttrium nitrate solution to for a slurry in such a ratio that corresponds to ¥263: A^Oj in the range of 1:9 to 9:1 molar proportion in the fired product. 2. The whole suspension is treated with liquour ammonia to reach the pH of the mixture in the range of 7.0 to 12.0. 3. The whole mass is filtered to separate the gel-like material. 4. The gel-like material is dried at a temperature in the range of 90° - 110°C. 5. The dried mass is calcined at a temperature in the range of 400° - 800°C. 6. The calcined mass is ground by known methods. 7. The sieved powder is formed into shapes by known methods such as slip casting, uniaxial pressing, isostatic pressing. 8. The formed shape is packed into a graphite crucible provided with a threaded lid and covered with the powder of the identical composition to that of the materials to be sintered. 9. The packed shape is fired in nitrogen at a temperature in the range of 1600° - 1850°C for a period in the range of 30 - 300 minutes. In the prior art processses α-Si3N4 is converted to p-Si3N4 either by alkali-earth oxide such as MgO, rare-earth oxidesuch as Y2O3 or a solid solution yttrium aluminium garnet (YAG) consisting of Y2O3 and A12O3 in the molar proportion 3:5. So far YAG is considered to be the best additive for the conversion of α-Si3N4 to β-Si3N4 as far as the properties of the end product are concerned. Solid state mixing of Y2O3 and A12O3 with α-Si3N4 is a method practiced for achieving the same. But the solid state mixing always produces less homogeneous products and conversion temperature of α-Si3N4->β-Si3N4 is also high as Y2O3 and A12O3 reacts to form YAG at high temperature. The novelty of the present process lies in increasing the level of homogeneity of mixture precursor and reactivity of Y203 and A12O3 by the inventive step of performing the mixing in the form of slurry which resulted uniform mixing of the raw materials. Due to this higher reacivities of ¥203 and A^Os the overall temperature of the conversion of a-Si3N4 to p-Si3N4 decreases and degree of conversion increases. The inventive step lies in (a) use of solid solution of Y2O3 : A12O3 in the 1:1 molar ratio which instead of stable YAG, produces structurally less stable perovskite which helps in a to P conversion, (b) the use of Y+3 and A1+3 in the reactive hydroxy-hydrogel form instead of Y2O3 and A12O3, which decreases both solid-solution formation temperature as well as a to P conversion temperature. The following examples are given by 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 30.28 ml. of yttrium nitrate 2(M)solution =3.445gm. of Y2O3, 61.0 ml aluminium nitrate 1(M) solution = 1.554gm of Al2O3 and 95 gm. of α-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 8.5 and stirred vigorously. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 800°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1850°C for a soaking time of 30 minutes in nitrogen atmosphere. EXAMPLE 2 45 ml. of yttrium nitrate 1 (M)solution =3.445gm. of Y2O3, 22 ml aluminium nitrate 2(M) solution s 1.554gm of A12O3 and 95 gm. of α-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 11.5 and stirred vigorously. It is then dried at 90°C. The dried mass was ground in acetone medium & calcined at 500°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1600°C for a soaking time of 300 minutes in nitrogen atmosphere. EXAMPLES 120 ml. of yttrium nitrate 0.5(M)solution =G.445gm. of Y2O3, 45 ml aluminium nitrate 1(M) solution = 1.554gm of A12O3 and 95 gm. of a-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 9.5 and stirred vigorously. It is then dried at 95°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1900°C for a soaking time of 60 minutes in nitrogen atmosphere. EXAMPLE 4 30.48 ml. of yttrium nitrate 2(M)solution = 6.89gm. of Y203, 60.96 ml aluminium nitrate 1(M) solution = 3.108 gm of A12O3 and 90 gm. of a-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 10.5 and stirred vigorously. It is then dried at 100°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1850°C for a soaking time of 150 minutes in nitrogen atmosphere. EXAMPLE 5 60.96 ml. of yttrium nitrate l(M)solution =6.89 gm. of Y2O3, 30.48 ml aluminium nitrate 2(M) solution = 3.108gm of A12O3 and 90 gin. of α-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 9.5 and stirred vigorously. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 700°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1750°C for a soaking time of 200 minutes in nitrogen atmosphere. EXAMPLE 6 91.0 ml. of yttrium nitrate l(M)solution =6.89 gm. of Y2O3, 45 ml aluminium nitrate 2(M) solution = 3.108gm of A12O3 and 90 gm. of a-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 9.0 and stirred vigorously. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 900°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1800°C for a soaking time o£240 minutes in nitrogen atmosphere. EXAMPLE7 121.9 ml. of yttrium nitrate l(M)solution =13.78 gm. of Y2O3, 60.96 ml aluminium nitrate 2(M) solution = 6.216gm of Al2O3 and 80 gm. of a-silicon nitride powder was taken and to which 48.5 ml of distilled water, sufficient amount of ammonium hydroixde(l:l) was added to maintain pH at 7.5 and stirred vigorously. It is then dried at 110°C. The dried mass was ground in acetone medium & calcined at 750°C to drive-off the chemically bonded water. The calcined mass is ground again in acetone medium & pelletised under a pressure of 200 MPa. The pellets are sintered at 1750°C for a soaking time of 60 minutes in nitrogen atmosphere. The main advantages of the present invention are: 1. The homogeneity in mixing lead to the complete conversion of a- silicon nitride to p - silicon nitride. We Claim: 1. A process of manufacturing p-silicon nitride which comprises characterized in that preparing a slurry of α-silicon nitride powder with aluminum nitrate solution and yttrium nitrate solution in such a proportion that corresponds to Y2O3 : A12O3 in the range of 9:1 to 1:9 in molar proportion in the final composition, converting the slurry into a gel like mass by increasing the pH of the slurry in the range of 7.0 to 12.0 by adding soluble hydroxide solution, filtering the gel like mass, drying the filtered mass at a temperature in the range of 90-110°C, calcining the dried mass at temperature in the range of 400 to 800°C, grinding the calcined mass by known processes, forming the ground powder into shapes by known methods, firing the shapes in nitrogen atmosphere at a temperature in the range of 1600 to 1850°C for a period in the range of 30 to 300 minutes. 2. A process as claimed in claim 1 wherein yttria-alumina additive used for a to P conversion of silicon nitride is in the range of 1-25 weight percent. 3. A process as claimed in claim 1 and 2 wherein the concentration of aluminum nitrate solution is in the range of 0.5 (M) to 5.0 (M). 4. A process as claimed in claims 1-3 wherein the concentration of yttrium nitrate solution is in the range of 0.5 (M) to 2.5 (M) concentration. 5. A process as claimed in claims 1-4 wherein the molar ratio of Al2O3 to Y2O3 is 1:1 in the fired product. 6. A process as claimed in claims 1-5 wherein soluble hydroxides used is such as ammonium hydroxide. 7. A new process of manufacturing β-silicon nitride substantially as herein described with reference to the examples.

Documents:

312-del-2001-abstract.pdf

312-del-2001-claims.pdf

312-del-2001-correspondence-others.pdf

312-del-2001-correspondence-po.pdf

312-del-2001-description (complete).pdf

312-del-2001-form-1.pdf

312-del-2001-form-18.pdf

312-del-2001-form-2.pdf

312-del-2001-form-3.pdf