Title of Invention

A PROCESS OF MAKING ALUMINATES OF LANTHANIDES

Abstract

The present invention relates to a process of making aluminates of lanthanides. Lanthanides series include the elements such as La, Ce, Pr, Nd and Sm. The present process was initiated based on the ability of formation of hetro-metallic hydroxy complex by Ln"x cation in aqueous medium. The giant three-dimensional structure formed by such cations may provide sufficient space for accommodating fine particles within the structural cages they form. This ensures homogeneous distribution of reactants and after partial dehydration reactive M-O bonds helps in rapid reaction at lower temperature. The novelty of the present process lies in the formation of polynuclear hydroxy-hydrogel complex of Ln"x ion in which solid A12O3 particles were uniformly embedded inside the hydroxy-hydrogel polynuclear network.

Full Text

The present invention relates to a process of making aluminates of lanthanides. Lanthanides series include the elements such as La, Ce, Pr, Nd and Sm. The present invention particularly relates to aluminates of La, Ce, Pr, Nd and Sm. More particularly lanthanum aluminate. Lanthanum aluminate is used in such high performance application as in thin film buffer layers; on silicon. It is a promising candidate for ferroelectric substrate material as well as effective compositing material in silicon carbide ceramics used as structural and engineering components. Bulk lanthanum aluminate has been prepared by direct combination of oxide references for which may be made to Gun Yong Sung, Kwang Yong Kang and Sin-Chong Park "Synthesis and properties of lanthanum aluminate target for radio-frequency magnetron sputtering" in J. Am. Ceram. Society, 74(2)437(1991) wherein La2O3 and A12O3 powders (1:1 by mol %) were mixed in acetone for 24 h using an alumina ball mil. After acetone was removed the mixture was dried for 24 h and calcined for 3 h in air at 1600°C. The calcined powders were ground and ball milled in acetone for 24 h and dried. The main draw backs of the above process is the non-homogeneity of mixing of solid reactants and poor reactivity that requires a high temperature for complete reaction to produce phase pure material. Reference may also be made to A.M. Golub, T.N. Maidukova and T.F Limar "Preparation of lanthanum aluminate by coprecipitation" in Izv. Akad, Nauk SSSR, Neorg. Mater. 2(9)1608 (1996) wherein partial lanthanum aluminate formation was reported at 900°C with complete conversion occurring at 1300°C when lanthanum and aluminium salts were coprecipitated from ammonium carbonate solution. The draw backs of the process mentioned herein suffers from deviation from stoichiometry due to the incomplete precipitation of one the components at a fixed PH of the precipitation system. References may also be made to K. Vidsayagar, J. Gopalakrishnan and C.N.R. Rao "Synthesis of complex metal oxides using hydroxides cyanide and nitrate solid solution precursors" in J. Solid State Chem. 58, 29 (1985) and V.S. Kvylov, I.L. Belova, R.L. Magunov, V.D. Kozlov, A.V. Kalinchenko and N.P. Krotko, "Preparation of rare earth aluminates from aqueous solution" in Izv. Akad. SSSR. Neorg. Material, 9 1388(1973) wherein partial aluminate products formation was reported when lanthanum and aluminium were precipitated with ammonium hydroxide followed by heat treatment in the temperature range of 1100° to 1350°C. Reference may further be made to B.C. Lux, R.D. Clark, A. Salazar, L.K. Sveum and M.A. Krebs " Aerosol generation of lanthanum aluminate" in J. Am. Ceram. Soc. 76 (10) 2669 (1993) wherein partial formation of lanthanum aluminate was reported from water soluble lanthanum and aluminium salts at an temperature 1200° - 1500°C using aerosol furnace technique. Reference may further be made to Ercan Taspinar and A. Cuneyt Tas "Low temperature chemical synthesis of lanthanum monoaluminate" in J. Am. Ceram. Soc. 80(11) 133(1997) wherein two separate powder preparation techniques (i) homogeneous precipitation from aqueous solution containing urea in the presence of nitrate salts and (ii) self propagating combustion synthesis from aqueous solutions of urea and the nitrate salts was reported to produce lanthanum aluminate at 850°C and 750°C respectively. In the homogenous precipitation process a total of 50 ml of cation stock solution (i.e. lanthanum nitrates and aluminium nitrates) was thoroughly mixed in a glass beaker. Distilled water (750 ml) containing 13 gm of urea was then mixed with above solution. The final clear solution was slowly heated at 95°C on a hot plate. Precipitation started in ~2 h with a slight turbidity in the clear solution, as the temperature did reach 90°C. Precipitates were aged for 4 h at 95°C on the hot plate under slow stirring. After ageing, the precipitates in the opaque solution were separated from the mother liquor either by conventional filtration or centrifugation (at 2500 rpm) in polycarbonate bottles. The precipitates were washed once with water and then twice with isopropyl alcohol. The washed precipitates were oven dried at 85°C over night. In the self propagating combustion synthesis a total of 50 ml of the cation stroke solution in appropriate amount to give the stoichiometric lanthanum aluminate was mixed with a magnetic stirrer in an uncovered 100 ml capacity glass beaker for 1 h at room temperature. 2 g urea was then added to this solution. Following dissolution of urea in the cation solution, the beaker contents were immediately transferred into a 250 ml capacity pyrex beaker. The pyrex beaker containing the liquid mixure was placed in a muffle furnace maintained at 510° ± 10°C. Initially, the mixture boils and and undergoes dehydration followed by decomposition, with swelling and froathing resulting in a foam that ruptures with a flame and glows to incandescence. The product of combustion was a volumetric foamy and amorphous powder. This foamy powder was ground and calcined in air at 1100°C for 6 to 24h to yield crystalline and pure lanthanum aluminate phase. The main drawbacks of the above process are low yield per run and complex multi-step unit operations of the process. The general drawbacks of the above processes are: 1. Non-homogeneity of mixing of solid reactants. 2. Deviation from stoichiometry in case of co-precipitation. 3. General partial conversion of incomplete reactions 4. More number of unit operations. The main object of the present invention is to provide a process of making aluminates of lanthanides which obviate the drawbacks as detailed above. Another object of the present invention is to make phase-pure lanthanide aluminate at lower temperature. Accordingly the present invention provides a process of making aluminates of lanthanide which comprises; characterized in that mixing water soluble lanthanum nitrate salt solution with AbCb powder to obtain suspension having final composition of Ln2Oa : Al2O3=l:l (molar), pouring the said suspension into liquor ammonia with constant flow of gaseous ammonia to maintain the pH of the suspension in the range of 7.1 to 10.0 to obtain a gel like mass, ageing the gel-like mass for a period in the range of 1 to 24 hrs, separating the gel-like mass from the liquid medium by known methods such as filtration, calcining the filtered mass at a temperature in the range of 300 to 450°C for a time period of 1 to 5 hours, grinding the calcined mass, shaping the ground mass by known methods, heat-treating the shapes in atmosphere at a temperature in the range of 700-1450°C for a period in the range of 1 to 12 hours, to obtain the product. In an embodiment of the present invention lanthanides such as La, Ce, Pr, Nd and Sm may be used. In an embodiment of the present invention any water soluble salts of lanthanum such as lanthanum nitrate, lanthanum chloride may be used. In another embodiment of the present invention any water soluble salts of cerium such as cerium-nitrate, cerium-chloride may be used. In still another embodiment of the present invention any water soluble salts of praseodymium such as praseodymium nitrate, praseodymium chloride may be used. In still another embodiment of the present invention any water soluble salts of neodymium such as neodymium nitrate, neodymium chloride may be used. In yet another embodiment of the present invention any water soluble salts of samarium such as samarium chloride, samarium nitrate may be used. In still yet another embodiment of the present invention the atmosphere provided during heat-treating may be argon, nitrogen, ambient. The details of the process of the present invention are given below with reference to preparation of La-aluminate: 1. Aluminium oxide powder is mixed with lanthanum nitrate solution by constant stirring to prepare a suspension. 2. The entire suspension is poured into aqueous ammonia solution along with the constant flow of gaseous ammonia to reach the pH and maintained in the range of 7.1 to 10 to form the gel like mass. 3.The gel-like mass is aged for 1-24 hours. 4. The gel-like mass is filtered and calcined at 300°- 450°C for a period in the range of 1- 5hrs. 5. The ground mass is pelletised uniaxially to form the pellet under a pressure in the range of 10-100 MPa. 6. The pellets are heat-treated at a temperature in the range of 700-1450°C for 0.25-6 hour in argon /nitrogen/ambient atmosphere. In the conventional processes mainly three basic techniques were adopted: (a) mixing of solid oxides followed by reaction in solid state, (b) coprecipitation of La and Al-hydroxides followed by dehydration and reaction in solid state and (c) aerosol furnace technique. In all the above methods, only partial conversion into aluminates were reported, which was due to low reactivity of the powder precursors and non-homogeneity in the mixing process. In addition to this two new methods were also used, where nitrates of La and Al were reacted with urea by either coprecipitation or by self-propagation combustion process. In the last process, though phase pure materials were obtained at lower temperature but the process is complicated and suffers economically as the yield per batch is very low. The present process was initiated based on the ability of formation of hetro-metallic hydroxy complex by Ln+x cation in aqueous medium. The giant three-dimensional structure formed by such cations may provide sufficient space for accommodating fine particles within the structural cages they form. This ensures homogeneous distribution of reactants and after partial dehydration reactive M-O bonds helps in rapid reaction at lower temperature. The novelty of the present process lies in the formation of polynuclear hydroxy-hydrogel complex of Ln+x ion in which solid A^Oa particles were uniformly embedded inside the hydroxy-hydrogel polynuclear network. Due to the thermodynamically non-equilibrium placement of A^Os in the extremely reactive hydroxy-hydrogel network the reactivity of the system increases after ensuring homogeneity in mixing. The inventive step lies in the formation of Ln+x -aquo complex with uniform distribution of solid A^Oa particles into it and by flash polymerisation of the system. Though the basic theory is rather complex but the process is simple with much less number of unit operation which add on the novelty of the process. 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 23.84 g of A12O3, 88 ml. of 2 (M) La(NO3)3.6H2O solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 325°C for 3 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in Ar atmosphere at 1450°C for eight hours. Example 2 11.54 g of A12O3, 43.70 ml. of 2.0 (M) Sm(NO3)3.6H2O solution and were mixed thoroughly to obtain a slurry. 26.22 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 8 hours, filtered and calcined at 350°C for 2 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 125 MPa to form the shapes. The shapes were fired in ambient atmosphere at 1400°C for two hours. Example 3 23.60 g of A12O3, 176 ml. of 1.0 (M) Pr(N03)3.6H2O solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 325°C for 4 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in N2 atmosphere at 1350°C for eight hours. Example 4 23.84 g of A1203, 88 ml. of 2 (M) Nd(NO3)3.6H2O solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 325°C for 2 hours. The calcined powder is sieved through 100 mesh'B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in ambient atmosphere at 1450°C for four hours. Example 5 23.84 g of A1203, 88 ml. of 2 (M) Ce(NO3)3.6H2O solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 300°C for 1 hour. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in Ar atmosphere at 1450°C for eight hours. Example 6 23.84 g of A1203, 176 ml. of 1 (M) La(NO3)3.6H20 solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 325°C for 3 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in Ar atmosphere at 1450°C for eight hours. Example 7 11.54'g of AbOs, 87 ml. of 1 (M) Sm(N03)3.6H2O solution and were mixed thoroughly to obtain a slurry. 30 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 8 hours, filtered and calcined at 350°C for 2 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 125 MPa to form the shapes. The shapes were fired in ambient atmosphere at 1400°C for two hours. Example 8 23.60 g of A12O3, 176 ml. of 0.5 (M) Pr(NO3)3.6H2O solution and were mixed thoroughly to obtain a slurry. 53 ml. of liquor ammonia is added into it along with constant flow of gaseous ammonia to obtain a gel-like mass. The gel-like mass is aged for 12 hours, filtered and calcined at 325°C for 4 hours. The calcined powder is sieved through 100 mesh B.S. and pressed uniaxially under a pressure of 100 MPa to form the shapes. The shapes were fired in N2 atmosphere at 1350°C for eight hours. In all the above examples, lanthanum aluminate of respective lanthanides were obtained as phase-pure material. The advantages of the present invention are: 1. Homogeneity of mixing of chemicals is possible. 2. Complete conversion of aluminate is possible. 3. Less number of unit operation is required. 4. The synthesis at lower temperature is possible. We Claim: 1. A process of making aluminates of lanthanide which comprises; characterized in that mixing water soluble lanthanum nitrate salt solution with A12O3 powder to obtain suspension having final composition of Ln2O3 : Al2O3=l:l (molar), pouring the said suspension into liquor ammonia with constant flow of gaseous ammonia to maintain the pH of the suspension in the range of 7.1 to 10.0 to obtain a gel like mass, ageing the gel- like mass for a period in the range of 1 to 24 hrs, separating the gel-like mass from the liquid medium by known methods such as filtration, calcining the filtered mass at a temperature in the range of 300 to 450°C for a time period of 1 to 5 hours, grinding the calcined mass, shaping the ground mass by known methods, heat-treating the shapes in atmosphere at a temperature in the range of 700-1450°C for a period in the range of 1 to 12 hours, to obtain the product. 2. A process as claimed in claim 1 wherein lanthanides used are selected from La, Ce, Pr, Nd and Sm. 3. A process as claimed in claiml and 2 wherein water-soluble salts of lanthanum, used are lanthanum nitrate, lanthanum chloride. 4. A process as claimed in claim 1-3, wherein water-soluble salts of cerium, used are cerium nitrate, cerium chloride. 5. A process as claimed in claim 1-4 wherein water-soluble salts of praseodymium used are praseodymium nitrate, praseodymium chloride. 6. A process as claimed in claim 1-5 wherein water-soluble salts of neodymium used are neodymium nitrate, neodymium chloride. 7. A process as claimed in claim 1-6 wherein water-soluble salts of samarium used are samarium chloride, samarium nitrate. 8. A process as claimed in claim 1-7 wherein the atmosphere provided during heat-treating are argon, nitrogen, ambient. 9. A process of making aluminates of lanthanides substantially as herein described with reference to examples.

Documents:

308-del-2001-abstract.pdf

308-del-2001-claims.pdf

308-del-2001-correspondence-others.pdf

308-del-2001-correspondence-po.pdf

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

308-del-2001-form-1.pdf

308-del-2001-form-18.pdf

308-del-2001-form-2.pdf

308-del-2001-form-3.pdf