Title of Invention

A PROCESS FOR THE PRODUCTION OF LANTHANUM CHROMITE BASED OXIDES USING A MLTIPURPOSE CHROMIUM SOURCE

Abstract

The present invention provides a process for the production of ultrafine, highly sinteractive lanthanum chromite based perovskite oxides by using a cost-effective chromium source ammonium dichromate in place of chromium nitrate. The process uses metal nitrate-dichromate-glycine-citric acid solution mixture in which the dichromate by virtue of its self-decomposition (volcanic reaction), can initiate a controlled exothermic anionic oxidation-reduction reaction leading to a self-propagating auto-ignition reaction within individual components and utilizing the thermal energy of the reaction to convert the precursors to their corresponding oxides and thus lower the external energy need of the overall process as compared to the process where conventional chromium source is used as one of the precursors. The highly sinteractive lanthanum chromites based oxides find application in the fabrication of high density sintered ceramic interconnect plates for solid oxide fuel cea application.

Full Text

The present invention relates to a process for the production of lanthanum chromite based oxides using a multipurpose chromium source. The present invention particularly relates to the use of a novel multipurpose metal salt for the production of single and/or multi component chromium oxides, more specifically calcium substituted lanthanum chromite (La1-xCaxCrO3, where x varies from 0.1 to 0.5) through auto-ignition (self-ignition) of a gel that is formed by heating a precursor solution containing metal salts such as nitrates and dichromate and a single or mixed fuel consisting of a carboxylic acid such as citric acid and an amino acid such as glycine that can undergo a controlled anionic oxidation-reduction reaction leading to the formation of an ash precursor which upon calcination yield the phase pure oxide. Solid oxide fuel cell (SOFC) technology has considerable promise for meeting many long-term worldwide energy requirements. This technology uses cheap, readily available fuels, such as hydrogen, methane or simple alcohols. It's highly effiecient power production capabilities give rise to outstanding opportunities for this technology as a future energy source for many commercial, aerospace and defense-related applications. In order to be successful, however, methods must be developed for fabricating solid oxide fuel cells reliably and cost effectively. Of particular importance is the ceramic material that is used as interconnect material in SOFC. Commonly used interconnect materials are Mg or Ca doped LaCrOs. These doped LaCrOa presents significant difficulties due to its low availability, high cost and a high sintering temperature. Lanthanum chromite is a refractory material with a melting point of 2510°C which requires very high temperatures and controlled atmospheres, i.e., extremely low partial pressures of oxygen for sintering to near theoretical density. Typical sintering conditions required to sinter Ca doped LaCrO3 or Mg doped LaCrO3 to full density (or closed porosity) are extremely low oxygen partial pressures and a temperature of 1750°C. The low oxygen partial pressure is needed to reduce volatilization of chromium due to oxidation, which has been found to inhibit the sintering of these materials. Groupp and Anderson (L.Groupp and H.U.Anderson), J.Am.Ceram.Soc., 59, 449 (1976) have shown that LaCrO3 does not sinter in air even at temperatures as high as 1720°C. According to the data reported by these investigatiors, LaCrO3 could be sintered to 95.3% theoretical density only at 1740°C and in an atmosphere of nitrogen having an oxygen partial pressure of 10"" atm. The oxidation and volatization of lanthanum chromite in oxidizing atmospheres at temperatures higher than 1400 °C has indeed been reported by Meadowcroft and Wimmer, Am. Ceram. Soc. Bull. 58, 610 (1979) that involves the oxidation of Cr (III) to Cr (VI) and formation of fugitive CrO3 which is a gas at high temperatures of sintering. Therefore the preparation of lanthanum chromite powders which sinter to close porosity at temperatures below 1450°C, so that Cr volatilization is insignificant, is critical for the development of fuel cell fabrication technology.Reduction in the sintering temperature of a ceramic powder is achieved by controlling the composition, homogeneity, grain size, and morphology of the powder. Wet-chemical synthesis routes, which begin with the preparation of homogeneous liquid solution of the cation ingredients, produce single or multi component oxide powders with high sinterability, high surface area, well-defined chemical compositions and homogeneous distribution of the elements.Reference may be made to the work as disclosed in U. S. Patent No. 3,974,108. This patent discloses that strontium doped LaCrO3 can be produced by preparaing slurry of lanthanum oxide, strontium carbonate and chromic acid, drying the said slurry in air and then preferably calcining at a temperature of 1200-1500°C to give a strontium doped LaCrOa powder. Sintering of this material occurs at temperatures above 1700°C. An alternative approach is to use sol-gel technology to prepare high surface area, reactive LaCrO3 powders that sinter to full density below 1700°C. The reduction in sintering temperature is accomplished by controlling the size, composition, morphology, homogeneity, and reactivity of the material. Such control is achieved by tailoring the solution chemistry and powder processing parameters. One such method for preparing LaCrO3 (lanthanum chromite) precursors is disclosed in U.S. Patent No. 3,330, 697. This process involves dissolving two or more metal salts (i.e., carbonates, hydroxides) in citric acid and ethylene glycol. The resulting sol is then filtered, dried to a gel, and calcined to remove organics. However, this process results in some residual carbon being present in the material, which can have a detrimental effect on the sintering properties of the material.Reference may also be made to the work of Azegami, Yoshinaka, Hirota and Yamaguchi, Mat. Res. Bull, 33(2), 341, 1998 wherein the individual components taken were chloride salts of the constituent metal ions (e.g., LaCl3 and CrCl3 respectively) along with hydrazine monohydrate as the starting materials. The initial mixture was stirred at room temperature followed by drop wise addition of hydrazine hydrate at 70°C until the resulting suspension reached pH 11. Then the suspension was heated for 2h at the same temperature and the product was separated from the suspension by centrifugation and then dried at 120°C under reduced pressure. Finally the calcined powder was sintered at 1600°C/2h to get a density of 94% of the theoretical density. However, as the starting raw materials are taken in the form of chloride, the removal of the same from the final product is very difficult and, is a major drawback of the process is. Another drawback of the process is the high temperature required to sinter the material for densification.Reference may be made to another U.S. Patent no.: 5,286,686 which describes the preparation of doped LaCrO3 and the process "consists of the preparation of a fine hydroxide gel by coprecipitation techniques from salt solutions of lanthanum and chromium, which are taken in the required stoichiometry, with ammonia. The resultant hydroxide gel is washed with deionized water/alcohol to remove the anions while the pH is maintained highly alkaline (i.e. pH greater than 8). To the washed slurry of the hydroxide gel is then added a solution of a divalent metal salt, such as zinc nitrate, which does not precipitate quantitavely at the high pH required for quantitative lanthanum and chromium precipitation.The slurry is then subjected to a supercritical solid/liquid separation in a batch or, preferably, continuous process. The supercritical powder/liquid separation ensures that the dissolved zinc dopant is uniformly and quantitatively distributed on the lanthanum chromite precursor and, therefore, eliminates loss of zinc during washing. In addition, the separation of the powder from the liquid under supercritical conditions eliminates the effects of surface tension completely and yields an extremely fine powder. This fine powder upon calcination yields high surface area, submicron grain crystalline powders which sinter to near full density at temperatures as low as 1400°C. However, the patented process is clumsy and requires long processing time.The combustion synthesis is of particular interest because it offers several attractive advantages over other wet-chemical routes as referred above. The major advantages of combustion synthesis are: simplicity of experimental set-up, relatively short processing time and cheapness due to energy saving. However, all the prior art reports on synthesis of fine sinteractive doped LaCrO3 powders through combustion synthesis are primarily based on the use of chromium nitrate [Cr(NO3)3, 9H2O], a costly material, as the source of chromium. Reference may be made for the synthesis of lanthanum chromite (LaCrO3) interconnect using auto combustion process which is disclosed in U. S. Pat. No. 5,114,702. The work describes the preparation method of Lao.84Sro.16CrO3 by dissolving all the constituent metal nitrates in water along with glycine as the fuel. The mixture temperature was raised to approximately at 200°C and autoignition of the evaporated precursor mixture occurred. The residual ash left in the beaker after autoignition consisted of low density ash containing very fine particles of LaSrCrO3. The ash generated in this process was calcined at 650°C to obtain the final oxide powder. However, the major drawbacks of the patented process was a sintering temperature as high as1550°C was required to get a density higher than 95% of the theoretical density. Reference may be made to Indian patent application no. 263/DEL/97 dated 30/01/97 by A.Chakraborty, R.N.Basu and H.S.Maiti wherein a process for the preparation of ultrafine powders of a single phase multi-element oxides such as calcium doped lanthanum chromite is described by using metal nitrates, citric acid and ethylenediamine. Here the process adopted was more or less same as in the previous report, the only exception is the use of ethylenediamine as the extra complexing agent. The process was quite useful for generating fine powders of La1-xCaxCrO3 which could be sintered at relatively lower 1250°C temperature but the as usual the expensive and hygroscopic chromium nitrate was used as the chromium source. Reference may also be made to Indian patent application no. 1214/DEL/04 by A.Kumar, P. Sujatha Devi, A.Das Shama, J. Mukherjee and H.S. Maiti wherein a process for the continuous production of sinteractive lanthanum chromite based oxides has been described by spray pyrolising metal nitrates and a mixed fuel of glycene and citric acid followed by calcination in the range of 650°C to 1150 C. However, this process also utilizes the expensive and hygroscopic chromium nitrate as the chromium source.The cost of interconnect can be reduced if a less expensive chromium source is utilized without seriously compromising the material properties. In this regard, the use of ammonium dichromate, (NH4)2Cr2O7 as the alternate chromium source, for synthesizing doped LaCrO3 is worth trying. Apart from easy availability in India, the use of ammonium dichromate is more advantageous than the use of chromium nitrate because of several other reasons. Firstly the cost of highly pure (99.9%) ammonium dichromate is much lower than chromium nitrate of same purity. For e.g., the cost of imported Cr(NO3)3, 9H2O (E. Merck,Germany, 99.9%) is Rs. 24000/- per kg; whereas the cost of an equal amount of (NH4)Cr2O7 (Merck, India, 99.9%) is only Rs 800/-. Secondly, the availability of moles of chromium per mole of the compound is twice for ammonium dichromate as compared to chromium nitrate. Thirdly, ammonium dichromate is a primary standard, easy to handle and can be weighed more accurately as compared to highly hygroscopic chromium nitrate. Above all (NH4)2Cr2O7 itself acts as a fuel undergoing exothermic and autocatalytic decomposition with a heat of formation of -1795.4 ± 1.3 kJ/mol and a heat of formation of 476.4 ± 0.4 kJ/mol, thus reducing the total fuel utilization in the auto combustion reaction.Reference may be made to the work of Kingsley and Pederson (Materials Letters, 18 (1993) 89 - 96) who reported "the use of ammonium dichromate, (NH4)2Cr2O7, in the place of chromium nitrate in the combustion synthesis of perovskite LnCrOa, where Ln = rare earth ions." They have also reported that "stoichiometric fuel/oxidant mixtures containing ammonium dichromate required less fuel, ignited at a lower temperature, and yielded a product of higher surface area than precursors containing only metal nitrates and glycine". However, in their work, Kingsley et al (Materials Letters, 18 (1993) 89 - 96) failed to show the densification behavior of the synthesized powders and also the materials synthesized are pure LnCrO3 and so, can not be used for SOFC interconnect applications as the later requires some doping in the Ln and/or Cr site for proper electrical conductivity.Reference may again be made to Indian patent application no. 1214/DEL/04, wherein as an embodiment of the invention, it has been mentioned that "...metal ion such as chromium may be obtained from chromium nitrate or ammonium dichromate". Thus in 1214/DEL/04, the ammonium dichromate has been suggested only as a chromium source and no mention has been made about its role as the fuel. As a result, under the experimental conditions of the invention of 1214/DEL/04 that uses a high Glycine to Nitrate (G/N) ratio (in the range 0.5 to 0.7), use of ammonium dichromate leads to an uncontrollable exothermic reaction.Thus, development of inexpensive processing technologies for the production of ultrafine ceramic powders with high densities, high purities, good homogeneity, and fine grain size by using comparatively less expensive material are important in their te~ aological exploitation.The main object of the present invention is to provide a process for the production of lanthanum chromite based oxides using a multipurpose chromium source.Another object of the present invention is to provide a process to prepare ultra fine, sinteractive ceramic oxide particles with high crystallinity using polymerized precursor solution consisting the corresponding metal salts e.g; La(NO3)3, Ca(NO3)2, and a multipurpose chromium source such as ammonium dichromate (NH4)2Cr2O7 together with complexing agents like citric acid and glycine taken individually or in a mixture acting as fuel that has potential to undergo a controlled exothermic and self igniting anionic oxidation reduction reaction within the individual components. Yet another object of the present invention is to utilize the chromium source which is ammonium dichromate as a fuel by using the nature of exothermic reaction as it undergoes thus converting the metal salts to their corresponding oxides and thus lowering the external energy need for the process.Still another object of the present invention is to utilize the thermal energy of the anionic oxidation-reduction reaction between the anion of the metal salt and the citric acid glycine mixture, to convert metal salts to their corresponding oxides and thus lowers the external energy need of the process.Still yet another object of the present invention is to use relatively less amount of the compexing agent as well as the reducing fuel e.g; glycine and citric acid to ignite the reaction mixture at relatively lower temperature, thus yielding ultratfme powder with n higher surface area than the precursor mixture containing all metal nitrates, glycine c a citric acid as the reaction mixture.A further object of the present invention is to generate the fine powders of the corresponding perovskite by using inexpensive starting materials such as metal nitrates, ammonium dichromate, citric acid, glycine and water.A still further object of the present invention is to have a clean process with the feasibility of operating it on a continuous basis to produce ceramic oxides.Another object of the present invention is to initiate an auto ignition reaction within the reaction system leading to the evolution of considerable heat, and utilizing the heat evolved during auto ignition to produce a fully decomposed ceramic oxide powder at a relatively lower temperature of only 225°C in a single step. Yet another object of the present invention is to calcine the precursor powder at temperatures in the range of 650-700°C to convert them to reactive La i-xCa 1-xCrO3, where x varies from 0.1 to 0.5. Still another object of the present invention is to density the desired doped lanthanum chromite interconnect material to more than 96% of its theoretical density at a sintering temperature as low as 1400°C. The present invention could be used to produce ultrafine powders of ceramic oxides, especially the lanthanum chromite based perovskite oxides that could find immense application in the fabrication of state of art interconnect material in the zirconia based high temperature solid oxide fuel cell. This electrically conducting refractory may also have a value as heating element in high temperature furnace. In .ummary, the present invention relates to a process to synthesize lanthanum chromite based oxides using a multipurpose chromium source through auto-ignition ( self-ignition) of a gel that is formed by heating a precursor solution containing lanthanum nitrate, calcium nitrate and ammonium dichromate together with citric acid and glycine, taken individually or as a mixture that can undergo a controlled anionic oxidation-reduction reaction leading to the formation of an ash precursor which upon calcination yield the phase pure oxide. The novelty of the present invention is the use of a cheaper and chemically more stable chromium source, viz: ammonium dichromate than the conventionally used chromium nitrate that is very costly and hygroscopic in nature. The ammonium dichromate not only acts as a source of chromium but also acts as a reducing fuel due to the high valency ofchromium (+6) in this compound compared to chromium (+3) in the final product (Ca doped LaCrO3). This supports the auto combustion reaction to take place with a much higher energy efficiency thus requiring less consumption of external fuels like citric acid and glycine.Accordingly the present invention provides a process for the production of lanthanum chromite based oxides using a multipurpose chromium source, which comprises preparing standard lanthanum nitrate hexahydrate, calcium nitrate tetrahydrate and ammonium dichromate solutions in distilled water of varying concentrations in the range of 0.1 to 0.5M, mixing the said solutions in the desired proportion to obtain a homogeneously mixed solution by stirring with a magnetic stirrer for a period in the range of 10-15 minutes on a hot plate maintained at a temperature of 190 ±10 °C, followed by slowly adding a calculated amount of citric acid monohydrate and glycine dissolved in water so that the ratio of citrate to nitrate (C/N) ratio is in the range of 0.0-0.28 and the glycine to nitrate (G/N) ratio is in the range of 0.0-0.56 and subjecting to evaporation by heating under continuous stirring on a hot plate maintained at a temperature of 190 ±10 °C the mixed clear black co.oured solution consisting of lanthanum nitrate, calcium nitrate, ammonium dichromate, glycine and citric acid having a metal ion ratio of La:Ca:Cr=0.7:0.3:l and an overall cation concentration being a sum of the concentration of the individual metal cations equal to 1 mole, allowing the evaporated solution to form a clear viscous gel without any precipitation, followed by further heating till ignition of the said gel to produce light brownish ash, grinding lightly the ash so obtained and calcining at a temperature in the range of 650-700°C to obtain a calcined powder, finally sintering the said calcined powder at a temperature of 1400 °C to get a density of more than 96% of its theoretical density.In an embodiment of the present invention, the lanthanum metal ion is such as obtained from lanthanum oxides, lanthanum carbonate, lanthanum hydroxide, lanthanum chloride and lanthanum nitrate. In another embodiment of the present invention, the calcium metal ion is such as obtained from calcium hydroxyl carbonate, calcium carbonate, calcium hydroxide, calcium chloride and calcium nitrate. In still another embodiment of the present invention, the chromium metal ion is such as obtained from chromium oxides, chromium nitrate and ammonium dichromate. In yet another embodiment of the present invention, the citric acid monohydrate is replaced with anhydrous citric acid or other polycarboxylic acids like acetic acid or lactic acid for complexation. In another embodiment of the present invention, the glycine is replaced with other amino acids such as alanine, valine, leucine, iso-leucine. In another embodiment of the present invention, the citrate to nitrate ratio is in the range of 0-0.28, preferably kept at 0.14:1. In another embodiment of the present invention, the glycine to nitrate ratio is in the range of 0-0.56, preferably kept at 0.28:1. The complete description of all the processing steps are given herein below: Preparing standard lanthanum nitrate solutions of varying molar concentration (0.1-0.5M) in volumetric flask with distilled water.Preparing standard calcium nitrate solutions of varying molar concentration (0.1-0.5M) in a volumetric flask with distilled water. Preparing standard ammonium dichromate solutions of varying molar concentration (0.1-0.5M) in a volumetric flask with distilled water. Mixing a calculated amount of lanthanum nitrate and calcium nitrate in a one litre beaker so that the ratio of La:Ca is maintained as 0.7:0.3 but not limited to this. To the above mixed solutions of lanthanum nitrate and calcium nitrate, ammonium dichromate solution is added slowly and dissolved so that the molar ratio of (La+Ca):Cr is equal to unity. To the above mixed solution calculated quantity of citric acid is added and dissolved so that the ratio of citrate to nitrate is maintained between 0.0 to 0.28. To the above mixed solution calculated quantity of glycine is added and dissolved so tluit the ratio of glycine to nitrate is maintained between 0.0 to 0.56. The mixed clear black colored solution consisting of lanthanum nitrate, calcium nitrate, ammonium dichromate, glycine and citric acid having metal ion ratio of La:Ca:Cr=0.7:0.3:l and a fixed G/N ratio ranging between 0-0.56 and a fixed C/N ratio between 0-0.28 was allowed to evaporate on a hot plate (190 ±10 °C) with continuous stirring till the formation of a viscous gel.Heating further, the gel starts foaming, swelling and finally burning to produce light brownish ash.Grinding lightly the ash obtained above and calcining within a temperature range of 650-70Q°C to obtain the powder. Sintering of the above said calcined powder at a temperature of 1400 OC to get a density of more than 96% of its theoretical density. The following examples are given by the way of illustration of the working of the invention in actual practice and should not be construed to limit the scope of the present invention. Example - 1 Stock solutions of 0.1 M La(NO3)3, 0.1M Ca(NO3)2 and 0.1 M (NH4)2Cr2O7 were prepared by dissolving the respective salts in water. A mixture of the salt solutions were prepared by adding 70 ml of 0.1 M La(N03)3, 30 ml of 0.1 M Ca(NO3)3 and 50 ml of 0.1M (NH4)2Cr2O7 solution. The mixture was stirred for 5 minutes with a magnetic stirrer. To this mixture calculated amount of citric acid by dissolving in \.ater was added keeping the citrate to nitrate ratio at 0.14:1 and also calculated amount of glycine by dissolving in water was added keeping the glycine to nitrate ratio at 0,28:1. Heating (190-200°C) was carried out simultaneously with stirring till evaporation to dryness. The liquid slowly turned viscous and began to set into a gel. The gel was sticky in nature. On further heating the gel started foaming and swelling and finally burnt on its own with the appearance of a flame and evolution of large amounts of gases. The ash obtained was lightly ground followed by heating at 650°C for 6 hrs. Heating and cooling rates were 100°C/h and 200°C/h respectively. The powder thus obtained was found to be ca-substituted LaCrO3 with a small amount of secondary phase of CaCrC>4 as determined by X-ray diffraction analysis. The surface area of the powder obtained from the process said was ~17m2/g and the bulk sample was 97.2% dense of its theoretical density at 1400°C temperature. Example - 2 The experiment was repeated as in Example - 2 except that the citrate to nitrate ratio was maintained at 0.07:1 and the glycine to nitrate ratio at 0.41:1. The surface area of the resulting powder was ~13m2/g and the bulk sample was 95.3% dense of its theoretical density at 1400°C temperature. .Example - 3 The experiment was repeated as in Example - 1 except that the citrate to nitrate ratio was maintained at 0.28:1 and no glycine was used during the process. The surface area of the resulting powder was ~6.5m /g and the bulk sample was 90.5% dense of its theoretical density at 1400°C temperature. Example - 4 The experiment was repeated as in Example - 1 except that the glycine to nitrate ratio was maintained at 0.56:1 and no citric acid was used during the process. The surface area of the resulting powder was ~9.8m2/g and the bulk sample was 91.8% dense of its theoretical density at 1400°C temperature. Example - 5 The experiment was repeated as in Example-1 except no CaC(NO3)2 was used during the entire process and thus only undoped LaCrO3 was synthesized. In this case 100ml of 0.1M La(NO3)3 was mixed with the 50ml of 0.1 M (NH4)2Cr2O7 solution keeping the citrate to nitrate ratio at 0.068:1 and the glycine to nitrate ratio at 0.43:1. Heating was carried out at 150-160°C and stirring continued till evaporation to dryness. The gel generated on further heating foamed, swelled and finally burnt on its own to generate the ash. Calcination temperature was fixed at 650°C/6h and thus the powder obtained was a single phase LaCrO3 as confirmed from XRD. The surface area was determined ~16m2/g and the bulk sample was 87.4% dense of its theoretical density at 1400°C temperature. The main advantages of the present invention are: 1) The chromium source used during the process is ammonium dichromate which is nearly 20 times cheaper than the conventional source of chromium i.e., chromium nitrate. Ammonium dichromate is also easy to handle as it is a primary standard and non-hygroscopic in nature. 2) Since (NH4)2Cr2O7 itself acts as a fuel, the auto-ignition reaction required less amount of externally supplied fuel such as citric acid and glycine. 3) The reaction product is formed at relatively lower combustion temperature i.e., for Lai.xCaxCrO3 it is ~225°C and for LaCrO3 it is ~180°C. In both the cases the powder generated is relatively fine as compared to other synthesis route as reported in the literature. 4) The ultra-fine powder obtained in Example-1 can be sintered to more than 97% of its theoretical density with a sintering temperature of 1400°C. 5) It is possible to prepare any desired compositions of multielement oxides of ABO3 where A is the rare earth ion or alkaline-earth metal ion and B is any transition metal ion. We claim: 1. A process for the production of lanthanum chromite based oxides using a multipurpose chromium source, which comprises preparing standard lanthanum nitrate hexahydrate, calcium nitrate tetrahydrate and ammonium dichromate solutions in distilled water of varying concentrations in the range of 0.1 to 0.5M, mixing the said solutions in the desired proportion to obtain a homogeneously mixed solution by stirring with a magnetic stirrer for a period in the range of 10-15 minutes on a hot plate maintained at a temperature of 190 ±10 °C, followed by slowly adding a calculated amount of citric acid monohydrate and glycine dissolved in water so that the ratio of citrate to nitrate (C/N) ratio is in the range of 0.0-0.28 and the glycine to nitrate (G/N) ratio is in the range of 0.0-0.56 and subjecting to evaporation by heating under continuous stirring on a hot plate maintained at a temperature of 190 ±10 °C the mixed clear black coloured solution consisting of lanthanum nitrate, calcium nitrate, ammonium dichromate, glycine and citric acid having a metal ion ratio of La:Ca:Cr=0.7:0.3:l and an overall cation concentration being a sum of the concentration of the individual metal cations equal to 1 mole, allowing the evaporated solution to form a clear viscous gel without any precipitation, followed by further heating till ignition of the said gel to produce light brownish ash, grinding lightly the ash so obtained and calcining at a temperature in the range of 650-700°C to obtain a calcined powder, finally sintering the said calcined powder at a temperature of 1400 °C to get a density of more than 96% of its theoretical density. 2. A process as claimed in claim 1, wherein the lanthanum metal ion is such as obtained from lanthanum oxides, lanthanum carbonate, lanthanum hydroxide, lanthanum chloride and lanthanum nitrate.A process as claimed in claim 1-2, wherein the calcium metal ion is such as obtained from calcium hydroxyl carbonate, calcium carbonate, calcium hydroxide, calcium chloride and calcium nitrate. 3. A process as claimed in claim 1-3, wherein chromium metal ion is such as obtained from chromium oxides, chromium nitrate and ammonium dichromate. 4. A process as claimed in claim 1-4, wherein the citric acid monohydrate is replaced with anhydrous citric acid or other polycarboxylic acids like acetic acid or lactic acid for complexation. 5. A process as claimed in claim 1-5, wherein the glycine is replaced with other amino acids such as alanine, valine, leucine, iso-leucine. 6. A process as claimed in claim 1-6, wherein the citrate to nitrate ratio is in the range of 0-0.28, preferably kept at 0.14:1. 7. A process as claimed in claim 1-7, wherein the glycine to nitrate ratio is in the range of 0-0.56, preferably kept at 0.28:1. 9. A process for the production of lanthanum chromite based oxides using a multipurpose chromium source, substantially as herein described with reference to the examples.

Documents:

773-del-2006-abstract.pdf

773-del-2006-Claims-(07-04-2014).pdf

773-DEL-2006-Claims-(27-05-2013).pdf

773-del-2006-claims.pdf

773-del-2006-Correspondence Others-(07-04-2014).pdf

773-DEL-2006-Correspondence-Others-(27-05-2013).pdf

773-del-2006-correspondence-others-1.pdf

773-del-2006-correspondence-others.pdf

773-del-2006-description (complete).pdf

773-del-2006-form-1.pdf

773-del-2006-form-18.pdf

773-del-2006-form-2.pdf

773-DEL-2006-Form-3-(27-05-2013).pdf

773-del-2006-form-3.pdf

773-del-2006-form-5.pdf