Title of Invention	A PROCESS FOR MAKING LANTHANUM CHROMITE DENSE PRODUCTS IN AIR AT LOW TEMPERATURE PARTICULARLY SUITABLE FOR APPLICATION IN SOLID OXIDE FUEL CELLS
Abstract	A process for making calcium substituted lanthanum chromite useful for application in solid oxide fuel cells by mixing lanthanum nitrate, chromium nitrate calcium nitrate, glycine & citric acid having metal ion ratio of La:Ca: Cr =0.70:0.30:1.0 and a fixed glycine/nitrate ratio ranging between 0.50 to 0.7 and a fixed citric acid/nitrate ratio ranging between 0.05 to 0.15 evaporating the above mixture solution on a hot plate under continuous stirring, continuing the evaporation till the volume of the solution is reduced to 80-90% of the original volume, atomizing the above said solution with spray nozzles with sizes in the range of 0.5 mm to 1mm at a flow rate in the range of 20-25 ml/minute at air pressure in the range of 20-25 psi to obtain the spray pyrolysed powder, calcining the above said powder at 650-1150°C for 6 hours milling the above said calcined powder with solvent for 3-8 hours compacting the said powder at a pressure of 135-490 MPa to obtain calcium substituted lanthanum chromite.

Title of Invention

A PROCESS FOR MAKING LANTHANUM CHROMITE DENSE PRODUCTS IN AIR AT LOW TEMPERATURE PARTICULARLY SUITABLE FOR APPLICATION IN SOLID OXIDE FUEL CELLS

Abstract

A process for making calcium substituted lanthanum chromite useful for application in solid oxide fuel cells by mixing lanthanum nitrate, chromium nitrate calcium nitrate, glycine & citric acid having metal ion ratio of La:Ca: Cr =0.70:0.30:1.0 and a fixed glycine/nitrate ratio ranging between 0.50 to 0.7 and a fixed citric acid/nitrate ratio ranging between 0.05 to 0.15 evaporating the above mixture solution on a hot plate under continuous stirring, continuing the evaporation till the volume of the solution is reduced to 80-90% of the original volume, atomizing the above said solution with spray nozzles with sizes in the range of 0.5 mm to 1mm at a flow rate in the range of 20-25 ml/minute at air pressure in the range of 20-25 psi to obtain the spray pyrolysed powder, calcining the above said powder at 650-1150°C for 6 hours milling the above said calcined powder with solvent for 3-8 hours compacting the said powder at a pressure of 135-490 MPa to obtain calcium substituted lanthanum chromite.

Full Text	The present invention relates to a process for making dense products of calcium substituted lanthanum chromite in air at low temperature particularly suitable for application in solid oxide fuel cells This invention relates to a novel process for the production of dense ceramics particularly calcium doped lanthanum chromite by utilizing the sinteractive precursor powder developed through spray pyrolysis of a precursor solution containing metal salts such as nitrates and a mixture of citric acid and glycine together acting as fuel that can undergo an exothermic anionic oxidation- reduction reaction leading to auto-ignition (self-ignition) wherein a part of the heat evolved during the auto-ignition is utilized for oxide formation. 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 and converts the chemical energy of the fuel into electrical energy. It's highly efficient 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 cell components 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 LaCrO.v These doped LaCrO3 presents significant difficulties due to its low availability, high cost and a high sintering temperature. Development of inexpensive processing technologies for the production of ultrafine ceramic powders with high density, purity, good homogeneity, and fine grain size are important for their technological exploitation. The present invention could be useful in fabricating dense products of lanthanum chromite based perovskite oxides that find immense application in the fabrication of state of the art interconnect material in the zirconia based high temperature solid oxide fuel cell. This electrical])' conducting refractory may also have a value as heating element in high temperature furnace Lanthanum chromite is a refractory material with a melting point of 2510 °C. which requires very high temperatures and controlled atmospheres. 1 e extremel) 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 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, 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 investigators, LaCrO3 could be sintered to 95.3% TD only at 1740 °C and in an atmosphere of nitrogen having an oxygen partial pressure of 10"" atm The oxidation and volatilization 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) wherein they described the oxidation of Cr(ITI) to Cr(VI) and formation of fugitive CrC»3 which is a gas at the high temperatures of sintering. Therefore, the preparation of lanthanum chromite powders, which sinter to at high density at temperatures below about 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. The most promising approach in achieving all these requirements is to use solution chemistry, and improved powder separation and processing technology. One example of a method of manufacturing lanthanum chromite (LaCrO3) electrodes is disclosed in U S. Pat. No. 3,974.108. This patent discloses that strontium doped LaCrO3 can be produced by preparing slurry of lanthanum oxide, strontium carbonate and chromic acid, drying said slurry in air and then preferably calcining at a temperature of 1200-1500 °C to give a strontium doped LaCrO3 powder Sinterihg of this material occurs at temperatures above 1700 °C. Attempts to lower the sintering temperature of this material by adding fluxes have had limited success A key drawback of this approach is the negative effect the added flux(es) can have on the materials present in the other layers, especially on their electrochemical properties and thermal expansion coefficients 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. A method for preparing LaCrO3 (lanthanum chromite) precursors is disclosed in U.S. Pat. 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 the 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. An improved sol-gel method has been disclosed by U.S. Pat. No. 4,830,780 by Olson et al., for the preparation of lanthanum chromite doped with the divalent ions of magnesium, strontium, calcium or barium by co-precipitation from salt solutions of lanthanum, chromium and dopant ions with ammonium hydroxide. In this patent disclosure, extensive washing of the precipitated gel is not needed because residual ammonium ion is removed via the gas phase during powder calcination. Upon calcination at temperatures of about 600 °C the gel converts to a single compound with the huttonite structure, LaCrO4, which upon further calcination at 900 °C converts to pure lanthanum chromite, LaCrO3, with average particle size of about 0.5 µm. This powder could be sintered to 95.7% of theoretical density when fired at 1650 °C for 4 hours in a graphite furnace and to 78 % theoretical density at 1600 °C for 2 hours in a furnace with oxygen partial pressure of 10 "I0 atmospheres. Densification of this lanthanum chromite to the indicated densities was much better than what was achievable by the prior art as, for example, stated by Groupp and Anderson. However, the sintering temperature of this reactive powder is still higher than what is needed for monolithic solid oxide fuel cell applications. Densification of lanthanum chromite at much lower temperatures has been disclosed in U.S. Pat. No. 4,749,632 by Flandermeyer et al. This was achieved by the incorporation into the lanthanum chromite of a sintering aid, that is, a compound or mixture of compounds which have melting points much lower than 1400 °C. For example, lanthanum chromite mixed with 10 wt. percentage boric acid powder was formed into a tape and fired at 1377 °C to a density of about 94% of theoretical density. In another example, the sintering aid was made up of 8 wt.% (Ca,Cr) oxide, which has a eutectic point at about 1022 °C and 6 w% B2O3 and, because of the low melting point of B2O3 the melting point of this sintering aid mixture would be expected to be very much lower than 1000 °C. A mixture of lanthanum chromite with the B203 (Ca, Cr) oxide sintering aid was fired to about 90% of theoretical density at 1277 °C. Thus, U.S. Pat. No. 4,749,632 discloses sintering of lanthanum chromite at low temperatures by the incorporation of relatively large quantities of sintering aids, which melt at low temperatures. However, the use of relatively large quantities of low-temperature melting compounds would be expected to result in migration of some of the sintering aid ions into the adjacent layers during sintering and, therefore, affect the sintering behavior and electrical performance of these layers. These sintering aids may be deleterious to the fabrication and operational performance of the monolithic solid oxide fuel cell. United States Patent No 5,169,811 by Cipollini, et al. provides a lanthanum chromite powder, which is so sinter reactive that it sinters at temperatures lower than 1650 °C. The sintering temperature of this powder is decreased further, in accordance with the invention, by incorporation therein of small amounts of compounds that have melting temperatures of about 1300 °C. In this manner there is provided a lanthanum chromite powder mixture which sinters at a temperature as low as 1400 °C and which consists essentially of 1 mol of LaCri_x Mx O3 where M is a divalent metal selected from the group consisting of magnesium, calcium and mixtures and y mols of B2O3 where y ranges from 0.005 to 0.04; and z mols of La203 where the ratio z/y ranges from 1.1 to 3. The mixture is formed into powder compacts and sintered to near full density at temperatures as low as HOOT. Reference is made to the formation and sinterability of Lai_xCaxCrO3 powder by a complex polymerization method using metal nitrates, citric acid and ethylene glycol by H. S. Shin, M. Uehara, N. Enomoto and J Hojo, Journal of Ceramic Processing Research 4 (2003) 45-48, wherein a ceramic body of >96% density was obtained at 1700 °C in Ar atmosphere Reference may again be made to the Mechanical properties of calcium and strontium substituted lanthanum chromite prepared by a glycine-nitrate combustion process (Praxair Specialty Ceramics, Seattle, WA, USA) by S. W. Paulik, S. Baskaran and T R Armstrong, Journal of Materials Science 33 (1998) 2397-2404, where a ceramic body of >96% density was obtained only after sintering in air between 1600 and 1690 °C for 2-6 hours using the powders made through glycine-nitrate combustion process. N. Saka, T. Kawada, H. Yokokawa and M. Dokiya in sinterability and electrical conductivity of calcium-doped lanthanum chromites, Journal of Materials Science 25 (1990) 4531-4534, sintered the pellets at 1600 °C for 5-10 hours to obtain dense ceramics. The sintering temperature of most of the reactive powders reported in the prior art are still higher than what is needed for efficient fabrication of monolithic solid oxide fuel cell applications. Reference may be made to 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 ultra fine powders of a single phase multielement oxide such as Ca-doped lanthanum chromite is described by using metal nitrates, citric acid and ethylenediamine wherein it is mentioned that it may be possible to prepare the desired compositions of multielement oxides by (a) semicontinuous process of spraying the liquid suspension into a heated chamber followed by (b) calcinations of the product obtained there from". Thus, in 263/DEL/97 a two stage operation of (a) merely spray drying and (b) further calcination is suggested to produce multielement oxides. Furthermore, the patent 263/DEL/97 dated 30/01/97 by A Chakraborty. R N Basu and H S. Maiti describes sintering of the said powders at temperatures in the range of 1200-1250 °C resulting in dense ceramics with 99% density- The same authors in their article on low-temperature sintering of LaCaCrO, prepared by an auto-ignition process (Materials Letters 45(2000) 162-166) further reports the electrical conductivity of 99% dense Lao 7Cao 3CrO3 ceramic to be around 43S/cm at 1000 °C Attention may be made to the point that the electrical conductivity of a 95% dense phase pure La« yCao 1C1O3 ceramic was reported to be around 60S/cm by Sakai et al, on " Sinterability and electrical conductivity of calcium-doped lanthanum chromites", Journal of Materials Science 25 (1990) 4531-4534 The low conductivity reported by A Chakraborty, R. N. Basu and H. S. Maiti (Materials Letters 45(2000) 162-166) probably indicates phase inhomogeneity in the sintered product Therefore, it is critical to have a single-phase ceramic product since the single-phase dense powder compact does possess the desired electrical properties in addition to its long-term stability In the present invention the above disadvantages is taken care of by adopting a process of producing single phase homogeneous dense product. Several processes have been proposed to produce ultra-fine ceramic particles in large scale. Aerosol techniques such as spray drying, spray pyrolysis, and flame oxidation/hydrolysis of halides, plasma spraying, combustion spraying etc are the most common large scale production techniques for ceramic oxides. Of the aerosol processing techniques available for production of ceramic powders, spray pyrolysis and flame oxidation of halides are the primary methods used to produce ultrafme powders. In both methods, submicron to micron sized droplets of solutions of metal salts or alkoxides are produced by standard aerosol techniques. Spray pyrolysis is most commonly used for the preparation of monometallic ceramic powders. The resultant powders typically have sizes in the 0.1 to 100 um range and are frequently polycrystalline. Attempts to produce multicomponent oxide powders often lead to crystallites of different stoichiometry within each particle because of differences in the rates of precipitation of the metals salts or hydroxides from solution. In addition, inhomogeneity within the individual particles is often encountered while producing multicomponent ceramic powders, as disclosed by G L. Messing, S-C. Zhang, G. V. Jayanthi, "Ceramic Powder Synthesis by Spray Pyrolysis", J. Am Ceram Soc, 76, 2707- 26 (1993) and by K. Okuyama and I. W Lenggoro "Preparation of nanoparticles via spray route". Chemical Engineering Science 58 (2003) 537-547 Flame oxidation of halide aerosols is also a common technique for the large-scale production of ultrafine ceramic powders An aerosol droplet of halide is oxidized/hydrolyzed in a flame to form a ceramic vapor of metal-oxide molecules. This vapor then condenses to form particles via a nucleation and growth mechanism coupled with significant particle coalescence U.S. Pat. No. 5,075,090 by Duane J. Lewis and Galen K. Madderra discloses the formation of particles in the micron range (0.2-0.3 urn) by flame spray pyrolysis of metal alkyls dissolved in a combustible solvent such as hexane or kerosene. The metal alkyls, such as triethylaluminum is very expensive as well as highly and spontaneously flammable The choice of solvent is also limited, as metal alkyls react rapidly and often explosively with solvents such as methanol and ethanol, and even with water. Joseph J. Helble, Gary A. Moniz and Joseph R. Morency in U.S. Pat. No. 5,358,695 disclosed a process for the manufacture of nanosize metal oxide and metal carbide ceramic particles from the corresponding metal nitrate or acetate associated with a solid carbonaceous support, and introducing these particles into a high temperature, oxygen rich combustion zone, combusting the solid support and forming metal oxide powders by condensation of low vapor pressure ceramic species. Alternately, the solid fuel, typically a carbohydrate, is dissolved in water with the metal salt and sprayed into a heated zone that evaporates the water and then the resulting dry particles are combusted. In a continuous process, an aqueous solution of sucrose and zirconium or magnesium nitrate is dispersed into a low temperature (300-350° C) drying and carbonization zone, producing an aerosol of dry particles, following which the aerosol continues its passage into a high temperature oxidation zone maintained at around 977 °C to 1227 °C. The drawbacks of U S. Pat. No. 5,358,695 lies in forming an aerosol of dry, carbonaceous particles containing metal nitrates which are potentially explosive, and if the residence time in the oxidizing atmosphere is insufficient or the temperature too low, the desired metal oxide particles may be contaminated with residual carbon The yield of the process disclosed is also low, being in the range of only 0.1 to 0 2 grams per minute. Finally, since the ceramics are said to condense from the gas phase, heterogeneous multi-metal oxide particles are expected to form, in addition to variation in the stoichiometry between particles Moreover, a high furnace temperature is also maintained during processing. Prior art methods of preparing nanosized powders are particularly problematic when ceramic materials containing two or more metals in specific stoichiometric ratios are desired. In US patent no. 4,065,544, Hamling et al. describes a process wherein finely-divided metal oxides are prepared by the steps of (a) contacting a compound of a metal with a carbohydrate material to obtain an intimate mixture thereof, (b) igniting this mixture to oxidize the same and to insure conversion of substantially all of said metal compound to a fragile agglomerate of its metal oxide, and (c) pulverizing the product of step (b) to form a finely-divided metal oxide powder having a mean particle size below about 1 0 micron. The finely-divided metal oxide powders produced by this process have the useful property of sinterability at temperatures significantly lower than metal oxide powders heretofore readily available. The powders are useful in the preparation of high strength compacted shapes for use in high temperature and/or corrosive environment, in the preparation of refractory cements, catalysts, catalysts supports and the like Aksay et al. in US patent no. 5,061,682 describes a process to prepare ceramic precursor mixtures containing a metal cation, a carbohydrate, and an anion capable of participating in an anionic oxidation-reduction reaction with the carbohydrate for continuous or batchwise drying and pyrolyzing to provide superconducting ceramic powders. It has been reported by H. B. Wang, G. Y. Meng and D. K. Peng " aerosol and plasma assisted chemical vapour deposition process for multicomponent oxide Lao8Sr02Mn03 thin film" Thin Solid Films 368, 275-278 (2000), that nitrogen containing compounds such as urea or glycine can be used as a fuel to provide combustion on a batchwise scale or by aerosol and plasma assisted process for the conversion of pastes or solutions of metal nitrates and glycine or urea to ceramics The glycine is reported to complex with the metal cations, allowing the solution to be thickened to a honey-like consistency before the solvent is evaporated Upon evaporation of the solvent, viscous foam is formed and eventually ignites. The urea/metal nitrate solution is reported to form a polymeric gel upon combustion. The polymeric gel is converted into foam by the gases produced by the combustion. The production of intermediates with a honey-like consistency or a polymeric gel prior to or during combustion is dangerous and could lead to partially reacted intermediates in addition to creating an uncontrollable explosive reaction Reference may be made to "ultrasonic spray pyrolysis of a chelated precursor into spherical YBa2Cu3O)7.x high temperature superconductor powders by C. H. Chao and P. D. Own, J. Mater Sci.,30 (1995) 6136-6144 and " Morphology of particles prepared by spray pyrolysis from organic precursor solution " by H. S. Kang, Y. C. Kang, H. D. Park, Y. G. Shul, Materials Letters 57 (2003) 1288-1294, wherein YBa2Cu3O7-x powders and ZrC>2 powders respectively were prepared by ultrasonic spray-pyrolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. In this process, citric acid and ethylene glycol were added for increasing the viscosity, in addition to the addition of NH4OH for adjusting the pH to 8 and the dried gel or highly viscous solution was chosen for spray pyrolysis. The presence of organics such as ethylene glycol in the precursor could leave undecomposed organic residues in the final product that may require a higher calcination temperature for phase formation in addition to affecting the properties considerably. The smaller particle size and single phase of the powder gives three very desirable results. First the smaller particle size of the powder allows the powder to be sintered into a ceramic article (having a density of at least 95% of theoretical density) at a lower temperature than a powder with larger particles. Secondly, a multiphase powder, will affect the thermal expansion coefficient of the final ceramic articles. If the thermal expansion coefficient does not match that of the other components of the fuel cell the entire fuel cell may fail upon repeated thermal cycling. Therefore, it is critical to have a single phase sintered ceramic product. Thirdly, the multiphase dense powder compact does not have the proper electrical properties for use as an interconnect material, while the single phase dense homogeneous powder compact possess the desired electrical properties. Hence, it would be desirable to provide a process for developing highly dense ceramic product with excellent homogeneity and phase purity with desirable properties at lower temperature in air for their technological exploitation. In accordance with the patents (Application Nos NF-458/03 and NF-459/03), a ceramic precursor mixture containing a mixture of citric acid and nitrate ions can be converted into a single phase ceramic oxide by forming droplets of the said ceramic precursor mixture followed by inducing self-ignition (auto-ignition) to convert the precursor droplets to their metal oxide counterparts. Such a process was established to prepare nano structured single-phase lanthanum manganite based oxides in a large scale by auto-ignition induced spray pyrolysis of citrate-nitrate precursors. However, such a precursor was not effective in inducing a self-ignition (auto-ignition) within the droplets to convert them to their corresponding metal oxides in the case of lanthanum chromite based oxides. Reference may again be made to 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 multielement oxide such as Ca-doped Lanthanum chromite is described by using metal nitrates, citric acid and ethylenediamine wherein it is mentioned that it may be possible to prepare the desired compositions of multielement oxides by (a) semicontinuous process of spraying the liquid suspension into a heated chamber followed by (b) calcinations of the product obtained therefrom". Infact, it was found that spray pyrolysing such a precursor solution containing metal nitrates, citric acid and ethylenediamine did not induce any auto-ignition within the droplets as expected. Hence, we have modified the above process by using a mixture of citric acid and glycine that together acts as a fuel for combustion giving rise to a controlled exothermic reaction. The glycine is reported to complex with the metal cations, like citric acid but the use of glycine alone as a complexing agent produces uncontrollable explosive reaction and hence was found to be unsafe. Thus, in this invention a partially polymerized precursor solution consisting of lanthanum nitrate, calcium nitrate and chromium nitrate in water with citric acid and glycine was used as a precursor solution for spray pyrolysis The mixed solution containing appropriate quantities of lanthanum nitrate, calcium nitrate and chromium nitrate in water with citric acid and glycine was heated on a magnetic hot plate around 80-90°C to induce complexation and consequent polymerization reaction. Heating and stirring was continued and polymerization was arrested at an intermediate stage to control the viscosity of the solution suitable for easy spraying. Here, both citric acid and glycine contains carboxyl (COOH) and hydroxyl (OH) groups, and hence can participate in the complexation of metal ions, which in turn can prevent partial precipitation of the metal ions during water evaporation and thus could enhance the local homogeneity of the precursor solution. Secondly, both citric acid and glycine could also serves as fuels for the combustion of the precursor gel, where nitrate ion acts as the oxidant and citrate ion and glycine together acts as the fuel resulting in an anionic oxidation-reduction reaction between the fuel and the oxidant leading to a controlled auto-ignition process. Thus, spray-pyrolysis of a chelated multicomponent precursor solution containing a mixture of citrate, nitrate and glycine in the present invention, where the polymerization or chelation of the cation could result in atomic level intermixing between the cations and thus maintain local homogeneity at an atomic scale within individual droplets, in addition to inducing a controlled exothermic auto-ignition reaction within individual droplets and thus leading to the continuous production of nano crystalline multicomponet ceramic oxides. Such powders could be calcined to have precursor powders of fine particle size, high surface area and reactivity. The novelty of the present invention resides in developing dense calcium substituted lanthanum chromite ceramic products of density more than 95% of theoretical density with electrical conductivity in the range of 55-60 S/cm at a temperature of 1350 °C. The inventive step of the present invention lies in providing a novel sinter-reactive precursor powder that can be sintered to a dense ceramic product at a lower temperature by spray pyrolysis of a solution consisting of metal nitrates, citric acid and glycine that can induce auto-ignition within individual droplets to produce nano crystalline lanthanum chromite based oxides. However, the present invention is not limited to the production of dense products of substituted lanthanum chromite perovskite oxide, but could be extended to any other multicomponent ceramic oxide system in general This process has the potential of 1 Making dense Lai.xCaxCr03 products where x varies from 0 1 to 0 3 at a temperature as low as 1350 °C in air 2. Producing green powder compacts with density in the range of 55-60% 3. Producing dense ceramic of the said composition with density more than 95 % of theoretical density. 4. Producing dense ceramic of the said composition with conductivity in the range of 55-60 S/cm. The present invention relates to the sintering of a precursor powder with the nominal composition Lao 7Ca0 3CrO3 to be used as an interconnect plate for SOFC, developed by atomization of a precursor solution consisting of the corresponding metal salts and a mixture of complexing agents like citric acid and glycine together acting as fuels and forming fine droplets of the same in a spray chamber and inducing auto-ignition within individual droplets to produce nano particles of the corresponding metal oxides. The interconnect plate serves as an internal electrical connection between the individual cells and is formed in a laminated structure sandwiched between anode and cathode layers. While the anode and cathode layers need to retain high porosity to facilitate gas-solid reactions, the electrolyte and interconnect layers must be sintered to closed porosity to prevent the intermixing of fuel and oxidant gases The monolithic solid oxide fuel cell offers lower material costs, the potential for reduced manufacturing costs, and a higher efficiency over other geometries and designs. However, fabrication of these cells is expected to be more complicated because the individual components must be sintered separately or co-sintered at relatively low temperature. Of particular importance is the sintering behavior of the interconnect material, that is, lanthanum chromite which must be sintered to close porosity or about 94% of its theoretical density at temperatures of about 1400 °C in air or oxygen-containing atmospheres. The main inhibition to densification appears to be the volatilization of chromium oxides in oxidizing atmospheres at temperatures higher than 1400 °C. At higher temperatures it is necessary to employ an inert atmosphere during the sintering in order to prevent the volatilization of chromium. Therefore, the preparation of lanthanum chromite powders which sinter to close porosity at temperatures below about 1450 °C so that Cr volatilization is insignificant is critical for the development of fuel cell fabrication technology Thus, the main object of the present invention is to have a process for making dense products of calcium substituted lanthanum chromite at low temperature in air particularly suitable for application in solid oxide fuel cells. Yet another object of the present invention is to compact the calcined powders to green density in the range of 55-60% Still another object of the present invention is to sinter the green compacts in air at a temperature lower than the prior art. Yet another object of the present invention is to sinter the green compacts at temperatures in the range of 1300-1350 °C to produce dense ceramic of the said composition with density in the range of 95-97 % of theoretical density. Still another object of the present invention is to sinter the green compacts at temperatures in the range of 1300-1400 °C to produce dense ceramic of the said composition with density in the range of 95 to 97 % of theoretical density with conductivity in the range of 55-60 S/cm. Accordingly the present invention provides a process for making dense products of calcium substituted lanthanum chromite in air at low temperature particularly suitable for application in solid oxide fuel cells by using a sinteractive precursor powder obtained by the process as described in the previous patent application No.218 wherein the process comprises preparing standard lanthanum nitrate solution in acidified distilled water of varying molar concentrations in the range of 0.7 to 1M and preparing standard aqueous chromium nitrate solutions of varying molar concentrations in the range of 0 7 to 1M and mixing the two solutions so that the molar ratio of La:Cr is maintained as 0.7:1.0, though not limited to this and to obtain a homogeneously mixed solution of lanthanum nitrate and chromium nitrate by stirring for a period in the range of 10-15 minutes on a magnetic stirrer (200±5 °C) and by slowly adding a calculated quantity of calcium nitrate tetrahydrate to this mixed solution of lanthanum nitrate and chromium nitrate so that the ratio of (La+Ca) to Cr is equal to unity followed by adding and dissolving a known amount of citric acid monohydrate and glycine so that the ratio of citrate to nitrate (C/N) ratio is kept at 0.05 and glycine to nitrate (G/N) ratio is kept at 0.6 and allowing the mixed clear greenish blue colored solution consisting of lanthanum nitrate, calcium nitrate, chromium nitrate, glycine and citric acid having a metal ion ratio of La:Ca:Cr= 0.70:0.30:1 and an overall cation concentration i.e., sum of the concentration of the individual metal cations equals to 1 mole to evaporate by heating on a magnetic stirrer (200±5°C) with continuous stirring, allowing the evaporated solution to cool down to room temperature and atomizing the said solution with an atomizing nozzle having a 1 mm nozzle size, at a set air pressure of 20 psi, a flow rate of 25ml/min, and a set inlet temperature of 450 °C inducing auto-ignition within individual precursor droplets, burning of the droplets in the hot chamber and finally collecting the decomposed ash colored powder at the collection chamber and calcining the collected powder at a temperature of 650 °C and ball milling of the said powder for 6 hours in an acetone medium and mixing of the said powder with 2.5 wt.% polyvinyl butyral dissolved in methyl ethyl ketone and compaction of the said powder at a pressure of 258 MPa and sintering of the said green compact at a temperature of 1350 °C for 6h. In an embodiment of the present invention calcium content in the lanthanum chromite may be varied between 10-30 mol %. In another embodiment of the present invention, Ca may be replaced with Sr, Mg, or a mixed composition. In another embodiment of the present invention, the calcined powders may be compacted into rectangular bars, cylindrical shapes or plates. In another embodiment of the present invention, the calcined powders may be compacted into rectangular bars or cylindrical shapes by applying uniaxial pressures in the range of 135-485 MPa. In yet another embodiment of the present invention the green compacts may be sintered at temperatures in the range of 1300-1400 °C for 3-6 hours. Complete description of all the processing steps are given below. Preparing standard lanthanum nitrate solutions of varying molar concentration (0.7- 1.0M) in a volumetric flask with acidified distilled water. Preparing standard chromium nitrate solutions of varying molar concentration (0.7- 1.0M) in a volumetric flask with distilled water. Mixing a calculated quantity of lanthanum nitrate and chromium nitrate solutions in a one litre beaker so that the ratio of La.Cr is maintained as 0.70:1.0 but not limited to this. The above mixed solution is heated on a magnetic stirrer (200±5°C) with continuous stirring. To the above mixed solutions of lanthanum nitrate and chromium nitrate calculated amount of calcium nitrate tetrahydrate is added slowly and dissolved so that the molar ratio of (Ca+La):Cr is equal to unity. To the above mixed solution calculated quantity of glycine is added and dissolved so that the ratio of glycine to nitrate is maintained between 0.5 to 0.70, though not limited to this. To the above mixed solution, calculated quantity of citric acid monohydrate is added and dissolved so that the ratio of citrate to nitrate is maintained between 0.05 to0.15 though not limited to this. The mixed clear greenish blue colored solution consisting of lanthanum nitrate, chromium nitrate, calcium nitrate, glycine and citric acid having metal ion ratio of La:Ca:Cr=0.70:0.30:1.0 and a fixed G/N ratio but ranging between 0.50-0.7and a fixed C/N ratio but ranging between 0.05-0.15 was allowed to evaporate on a hot plate with continuous stirring. Stirring and evaporation was continued until the volume of the solution was reduced to about 80-90% of the original volume. Atomization of the said precursor solution with spray nozzles with sizes in the range of 0.5 mm to 1 mm. Atomization of the said precursor solution at a flow rate in the range of 20-25ml/min. Atomization of the said precursor solution wherein the air pressure is fixed in the range of 20-30 psi Atomization of the said precursor solution in a spray chamber wherein the inlet air temperature is fixed in the range of 450-600 °C Ball milling of the said calcined powders with acetone for 3-8 hours for deagglomeration of the particles. Mixing of the ball milled powder with 2.5 wt% polyvinyl butyral dissolved in methyl ethyl ketone. Compaction of the ball milled powders with pressures in the range of 135- 485 MP Compaction of the ball milled powders into rectangular bars or cylindrical shapes by uniaxial pressing. Sintering of the green compacts with density in the range of 55-60% at temperatures in the range of 1250-1400 °C for 3-6 hours. Complete description of all the processing steps are given below. The following examples illustrate the invention in the manner in which it may be carried out in practice. However, this will not limit the scope of the present invention The Examples also illustrate the feasibility of operating it on a continuous basis. Example 1 A standard 1 M lanthanum nitrate solution is prepared in a volumetric flask with acidified distilled water. A standard 1 M chromium nitrate solution is prepared in a volumetric flask with distilled water. 210 ml of 1M lanthanum nitrate and 300 ml of 1M chromium nitrate solutions are mixed in a 1L beaker so that the ratio of La:Cr is maintained as 0 7:1.0 This mixed solution is heated on a magnetic stirrer (200±5 °C) with continuous stirring. To the above mixed solutions of lanthanum nitrate and chromium nitrate. 21.4968 gm of calcium nitrate tetrahydrate is added slowly and dissolved so that the molar ratio of (La+Ca):(Cr) is equal to unity To this mixed metal ion solution 77gms of glycine monohydrate and 17 667 gms of citric acid monohydrate are added and dissolved so that the ratio of glycine to nitrate equals 0.60 and that of citrate to nitrate is equals to 0.05. This mixed clear greenish blue colored solution consisting of lanthanum nitrate, chromium nitrate, calcium nitrate, glycine and citric acid having a metal ion ratio of La-Ca:Cr=0.70-0 30:1.0, and a G/N ratio of 0.60 and a C/N ratio of 0.05 is allowed to evaporate on a hot plate with continuous stirring Stirring and evaporation was continued until the volume of the solution was reduced to about 90% of the original volume. This solution was atomized using a nozzle size of 1mm, a flow rate of 26ml/min, a set air pressure of 20psi and a set inlet temperature of 450 °C and the moment the droplets enter the hotzone of the chamber, drying and auto-ignition reaction was initiated so fast as evident by the flashing or sparking of the particles during ignition in the chamber. The ash colored powder collected at the collection chamber is calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrC>3 and CaCr04 The said precursor powder was ball milled with acetone for 6h followed by drying at 90+10 °C. The ball milled powder was mixed with 2.5 wt. % of Polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1250 °C. The percentage densification achieved was -86.69% (±1%). Example 2 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of Polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1300 °C for 6h The percentage densification achieved was -94.36% (±1%) Example 3 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C The ball milled powder was mixed with 2 5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97.047% (±1%). This dense calcium substituted lanthanum chromite plate exhibited a conductivity of 59.66 S/cm. Example 4 The as-sprayed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -94.18% (±1%). Example 5 The as-sprayed precursor powder was prepared as given in Example 1. The green powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97.04% (±1%). Example 6 The as-spra>ed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482 MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97 67% (±1%) Example 7 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -95.60% (±1%). Example 8 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482 MPa, and sintered at a temperature of 1300 °C for 6h. The percentage densification achieved was -95.31%) (±1%) Example 9 The as-sprayed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C The ball milled powder was mixed with 2 5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1300 °C for 6h The percentage densification achieved was -93 32% (±1%) Example 10 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -94.92% (±1%). The advantages of the present invention are: 1. Sintering of calcium substituted lanthanum chromite in air at a temperature as low as 1350 °C. 2. This low sintering temperature help in co-sintering of the monolithic solid oxide fuelcell components and hence would be highly advantageous in the efficient fabrication of SOFC. 3. To have highly sintered and homogeneous dense plates of calcium substituted lanthanum chromite with conductivities of the order of 55-60 S/cm. 4 To produce dense plates of calcium substituted lanthanum chromite with density more than 95 % of theoretical density 5 To have green powder compacts with high packing efficiency and green density of 55-60% of theoretical density. 6 Since the powder compacts in the present invention, sinter to near full density in air at temperatures as low as 1350 °C these powders are expected to facilitate the process of fabricating monolithic solid oxide fuel cells. We claim :- A process for making calcium substituted lanthanum chromite useful for application in solid oxide fuel cells by mixing lanthanum nitrate, chromium nitrate calcium nitrate, glycine & citric acid having metal ion ratio of La:Ca: Cr =0.70:0.30:1.0 and a fixed glycine/nitrate ratio ranging between 0.50 to 0.7 and a fixed citric acid/nitrate ratio ranging between 0.05 to 0.15 evaporating the above mixture solution on a hot plate under continuous stirring, continuing the evaporation till the volume of the solution is reduced to 80-90% of the original volume, atomizing the above said solution with spray nozzles with sizes in the range of 0.5 mm to 1mm at a flow rate in the range of 20-25 ml/minute at air pressure in the range of 20-25 psi to obtain the spray pyrolysed powder, calcining the above said powder at 650-1150°C for 6 hours milling the above said calcined powder with solvent for 3-8 hours compacting the said powder at a pressure of 135-490 MPa to obtain calcium substituted lanthanum chromite. 2. A process as claimed in claim 1 wherein calcination of the powder collected in the collection chamber at 600-650 °C for 3-6 h. 3. A process as claimed in claims 1-2 wherein ball milling of the said calcined powders with acetone for 4 hours for deagglomeration of the particles. 4. A process as claimed in claims 1-3 wherein mixing of the ball milled powder with 2.5 wt.% poly vinyl butyral dissolved in methyl ethyl ketone. 5. A process as claimed in claims 1-4 wherein uniaxial compaction of the ball milled powder with pressures of 258 MPa.

Full Text

The present invention relates to a process for making dense products of calcium substituted lanthanum chromite in air at low temperature particularly suitable for application in solid oxide fuel cells
This invention relates to a novel process for the production of dense ceramics particularly calcium doped lanthanum chromite by utilizing the sinteractive precursor powder developed through spray pyrolysis of a precursor solution containing metal salts such as nitrates and a mixture of citric acid and glycine together acting as fuel that can undergo an exothermic anionic oxidation- reduction reaction leading to auto-ignition (self-ignition) wherein a part of the heat evolved during the auto-ignition is utilized for oxide formation.
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 and converts the chemical energy of the fuel into electrical energy. It's highly efficient 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 cell components 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 LaCrO.v These doped LaCrO3 presents significant difficulties due to its low availability, high cost and a high sintering temperature. Development of inexpensive processing technologies for the production of ultrafine ceramic powders with high density, purity, good homogeneity, and fine grain size are important for their technological exploitation. The present invention could be useful in fabricating dense products of lanthanum chromite based perovskite oxides that find immense application in the fabrication of state of the art interconnect material in the zirconia based high temperature solid oxide fuel cell. This electrical])' conducting refractory may also have a value as heating element in high temperature furnace
Lanthanum chromite is a refractory material with a melting point of 2510 °C. which requires very high temperatures and controlled atmospheres. 1 e extremel) 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 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, 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 investigators, LaCrO3 could be sintered to 95.3% TD only at 1740 °C and in an atmosphere of nitrogen having an oxygen partial pressure of 10"" atm The oxidation and volatilization 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) wherein they described the oxidation of Cr(ITI) to Cr(VI) and formation of fugitive CrC»3 which is a gas at the high temperatures of sintering. Therefore, the preparation of lanthanum chromite powders, which sinter to at high density at temperatures below about 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. The most promising approach in achieving all these requirements is to use solution chemistry, and improved powder separation and processing technology.
One example of a method of manufacturing lanthanum chromite (LaCrO3) electrodes is disclosed in U S. Pat. No. 3,974.108. This patent discloses that strontium doped LaCrO3 can be produced by preparing slurry of lanthanum oxide, strontium carbonate and chromic acid, drying said slurry in air and then preferably calcining at a temperature of 1200-1500 °C to give a strontium doped LaCrO3 powder Sinterihg of this material occurs at temperatures above 1700 °C. Attempts to lower the sintering temperature of this material by adding fluxes have had limited success A key drawback of this approach is
the negative effect the added flux(es) can have on the materials present in the other layers, especially on their electrochemical properties and thermal expansion coefficients 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.
A method for preparing LaCrO3 (lanthanum chromite) precursors is disclosed in U.S. Pat. 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 the 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.
An improved sol-gel method has been disclosed by U.S. Pat. No. 4,830,780 by Olson et al., for the preparation of lanthanum chromite doped with the divalent ions of magnesium, strontium, calcium or barium by co-precipitation from salt solutions of lanthanum, chromium and dopant ions with ammonium hydroxide. In this patent disclosure, extensive washing of the precipitated gel is not needed because residual ammonium ion is removed via the gas phase during powder calcination. Upon calcination at temperatures of about 600 °C the gel converts to a single compound with the huttonite structure, LaCrO4, which upon further calcination at 900 °C converts to pure lanthanum chromite, LaCrO3, with average particle size of about 0.5 µm. This powder could be sintered to 95.7% of theoretical density when fired at 1650 °C for 4 hours in a graphite furnace and to 78 % theoretical density at 1600 °C for 2 hours in a furnace with oxygen partial pressure of 10 "I0 atmospheres. Densification of this lanthanum chromite to the indicated densities was much better than what was achievable by the prior art as, for example, stated by Groupp and Anderson. However, the sintering temperature of this reactive powder is still higher than what is needed for monolithic solid oxide fuel cell applications.
Densification of lanthanum chromite at much lower temperatures has been disclosed in U.S. Pat. No. 4,749,632 by Flandermeyer et al. This was achieved by the incorporation
into the lanthanum chromite of a sintering aid, that is, a compound or mixture of compounds which have melting points much lower than 1400 °C. For example, lanthanum chromite mixed with 10 wt. percentage boric acid powder was formed into a tape and fired at 1377 °C to a density of about 94% of theoretical density. In another example, the sintering aid was made up of 8 wt.% (Ca,Cr) oxide, which has a eutectic point at about 1022 °C and 6 w% B2O3 and, because of the low melting point of B2O3 the melting point of this sintering aid mixture would be expected to be very much lower than 1000 °C. A mixture of lanthanum chromite with the B203 (Ca, Cr) oxide sintering aid was fired to about 90% of theoretical density at 1277 °C.
Thus, U.S. Pat. No. 4,749,632 discloses sintering of lanthanum chromite at low temperatures by the incorporation of relatively large quantities of sintering aids, which melt at low temperatures. However, the use of relatively large quantities of low-temperature melting compounds would be expected to result in migration of some of the sintering aid ions into the adjacent layers during sintering and, therefore, affect the sintering behavior and electrical performance of these layers. These sintering aids may be deleterious to the fabrication and operational performance of the monolithic solid oxide fuel cell.
United States Patent No 5,169,811 by Cipollini, et al. provides a lanthanum chromite powder, which is so sinter reactive that it sinters at temperatures lower than 1650 °C. The sintering temperature of this powder is decreased further, in accordance with the invention, by incorporation therein of small amounts of compounds that have melting temperatures of about 1300 °C. In this manner there is provided a lanthanum chromite powder mixture which sinters at a temperature as low as 1400 °C and which consists essentially of 1 mol of LaCri_x Mx O3 where M is a divalent metal selected from the group consisting of magnesium, calcium and mixtures and y mols of B2O3 where y ranges from 0.005 to 0.04; and z mols of La203 where the ratio z/y ranges from 1.1 to 3. The mixture is formed into powder compacts and sintered to near full density at temperatures as low as HOOT.
Reference is made to the formation and sinterability of Lai_xCaxCrO3 powder by a complex polymerization method using metal nitrates, citric acid and ethylene glycol by H. S. Shin, M. Uehara, N. Enomoto and J Hojo, Journal of Ceramic Processing Research 4 (2003) 45-48, wherein a ceramic body of >96% density was obtained at 1700 °C in Ar atmosphere
Reference may again be made to the Mechanical properties of calcium and strontium substituted lanthanum chromite prepared by a glycine-nitrate combustion process (Praxair Specialty Ceramics, Seattle, WA, USA) by S. W. Paulik, S. Baskaran and T R Armstrong, Journal of Materials Science 33 (1998) 2397-2404, where a ceramic body of >96% density was obtained only after sintering in air between 1600 and 1690 °C for 2-6 hours using the powders made through glycine-nitrate combustion process.
N. Saka, T. Kawada, H. Yokokawa and M. Dokiya in sinterability and electrical
conductivity of calcium-doped lanthanum chromites, Journal of Materials Science 25
(1990) 4531-4534, sintered the pellets at 1600 °C for 5-10 hours to obtain dense
ceramics.
The sintering temperature of most of the reactive powders reported in the prior art are still
higher than what is needed for efficient fabrication of monolithic solid oxide fuel cell
applications.
Reference may be made to 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 ultra fine powders of a single phase multielement oxide such as Ca-doped lanthanum chromite is described by using metal nitrates, citric acid and ethylenediamine wherein it is mentioned that it may be possible to prepare the desired compositions of multielement oxides by (a) semicontinuous process of spraying the liquid suspension into a heated chamber followed by (b) calcinations of the product obtained there from". Thus, in 263/DEL/97 a two stage operation of (a) merely spray drying and (b) further calcination is suggested to produce multielement oxides. Furthermore, the patent 263/DEL/97 dated 30/01/97 by A Chakraborty. R N Basu and H S. Maiti describes sintering of the said
powders at temperatures in the range of 1200-1250 °C resulting in dense ceramics with 99% density- The same authors in their article on low-temperature sintering of LaCaCrO, prepared by an auto-ignition process (Materials Letters 45(2000) 162-166) further reports the electrical conductivity of 99% dense Lao 7Cao 3CrO3 ceramic to be around 43S/cm at 1000 °C Attention may be made to the point that the electrical conductivity of a 95% dense phase pure La« yCao 1C1O3 ceramic was reported to be around 60S/cm by Sakai et al, on " Sinterability and electrical conductivity of calcium-doped lanthanum chromites", Journal of Materials Science 25 (1990) 4531-4534 The low conductivity reported by A Chakraborty, R. N. Basu and H. S. Maiti (Materials Letters 45(2000) 162-166) probably indicates phase inhomogeneity in the sintered product Therefore, it is critical to have a single-phase ceramic product since the single-phase dense powder compact does possess the desired electrical properties in addition to its long-term stability In the present invention the above disadvantages is taken care of by adopting a process of producing single phase homogeneous dense product.
Several processes have been proposed to produce ultra-fine ceramic particles in large scale. Aerosol techniques such as spray drying, spray pyrolysis, and flame oxidation/hydrolysis of halides, plasma spraying, combustion spraying etc are the most common large scale production techniques for ceramic oxides.
Of the aerosol processing techniques available for production of ceramic powders, spray pyrolysis and flame oxidation of halides are the primary methods used to produce ultrafme powders. In both methods, submicron to micron sized droplets of solutions of metal salts or alkoxides are produced by standard aerosol techniques. Spray pyrolysis is most commonly used for the preparation of monometallic ceramic powders. The resultant powders typically have sizes in the 0.1 to 100 um range and are frequently polycrystalline. Attempts to produce multicomponent oxide powders often lead to crystallites of different stoichiometry within each particle because of differences in the rates of precipitation of the metals salts or hydroxides from solution. In addition, inhomogeneity within the individual particles is often encountered while producing multicomponent ceramic powders, as disclosed by G L. Messing, S-C. Zhang, G. V. Jayanthi, "Ceramic Powder Synthesis by Spray Pyrolysis", J. Am Ceram Soc, 76, 2707-
26 (1993) and by K. Okuyama and I. W Lenggoro "Preparation of nanoparticles via
spray route". Chemical Engineering Science 58 (2003) 537-547
Flame oxidation of halide aerosols is also a common technique for the large-scale production of ultrafine ceramic powders An aerosol droplet of halide is oxidized/hydrolyzed in a flame to form a ceramic vapor of metal-oxide molecules. This vapor then condenses to form particles via a nucleation and growth mechanism coupled with significant particle coalescence
U.S. Pat. No. 5,075,090 by Duane J. Lewis and Galen K. Madderra discloses the formation of particles in the micron range (0.2-0.3 urn) by flame spray pyrolysis of metal alkyls dissolved in a combustible solvent such as hexane or kerosene. The metal alkyls, such as triethylaluminum is very expensive as well as highly and spontaneously flammable The choice of solvent is also limited, as metal alkyls react rapidly and often explosively with solvents such as methanol and ethanol, and even with water.
Joseph J. Helble, Gary A. Moniz and Joseph R. Morency in U.S. Pat. No. 5,358,695 disclosed a process for the manufacture of nanosize metal oxide and metal carbide ceramic particles from the corresponding metal nitrate or acetate associated with a solid carbonaceous support, and introducing these particles into a high temperature, oxygen rich combustion zone, combusting the solid support and forming metal oxide powders by condensation of low vapor pressure ceramic species. Alternately, the solid fuel, typically a carbohydrate, is dissolved in water with the metal salt and sprayed into a heated zone that evaporates the water and then the resulting dry particles are combusted. In a continuous process, an aqueous solution of sucrose and zirconium or magnesium nitrate is dispersed into a low temperature (300-350° C) drying and carbonization zone, producing an aerosol of dry particles, following which the aerosol continues its passage into a high temperature oxidation zone maintained at around 977 °C to 1227 °C.
The drawbacks of U S. Pat. No. 5,358,695 lies in forming an aerosol of dry, carbonaceous particles containing metal nitrates which are potentially explosive, and if the residence
time in the oxidizing atmosphere is insufficient or the temperature too low, the desired metal oxide particles may be contaminated with residual carbon The yield of the process disclosed is also low, being in the range of only 0.1 to 0 2 grams per minute. Finally, since the ceramics are said to condense from the gas phase, heterogeneous multi-metal oxide particles are expected to form, in addition to variation in the stoichiometry between particles Moreover, a high furnace temperature is also maintained during processing. Prior art methods of preparing nanosized powders are particularly problematic when ceramic materials containing two or more metals in specific stoichiometric ratios are desired.
In US patent no. 4,065,544, Hamling et al. describes a process wherein finely-divided metal oxides are prepared by the steps of (a) contacting a compound of a metal with a carbohydrate material to obtain an intimate mixture thereof, (b) igniting this mixture to oxidize the same and to insure conversion of substantially all of said metal compound to a fragile agglomerate of its metal oxide, and (c) pulverizing the product of step (b) to form a finely-divided metal oxide powder having a mean particle size below about 1 0 micron. The finely-divided metal oxide powders produced by this process have the useful property of sinterability at temperatures significantly lower than metal oxide powders heretofore readily available. The powders are useful in the preparation of high strength compacted shapes for use in high temperature and/or corrosive environment, in the preparation of refractory cements, catalysts, catalysts supports and the like
Aksay et al. in US patent no. 5,061,682 describes a process to prepare ceramic precursor mixtures containing a metal cation, a carbohydrate, and an anion capable of participating in an anionic oxidation-reduction reaction with the carbohydrate for continuous or batchwise drying and pyrolyzing to provide superconducting ceramic powders.
It has been reported by H. B. Wang, G. Y. Meng and D. K. Peng " aerosol and plasma assisted chemical vapour deposition process for multicomponent oxide Lao8Sr02Mn03 thin film" Thin Solid Films 368, 275-278 (2000), that nitrogen containing compounds such as urea or glycine can be used as a fuel to provide combustion on a batchwise scale or by aerosol and plasma assisted process for the conversion of pastes or solutions of
metal nitrates and glycine or urea to ceramics The glycine is reported to complex with the metal cations, allowing the solution to be thickened to a honey-like consistency before the solvent is evaporated Upon evaporation of the solvent, viscous foam is formed and eventually ignites. The urea/metal nitrate solution is reported to form a polymeric gel upon combustion. The polymeric gel is converted into foam by the gases produced by the combustion. The production of intermediates with a honey-like consistency or a polymeric gel prior to or during combustion is dangerous and could lead to partially reacted intermediates in addition to creating an uncontrollable explosive reaction
Reference may be made to "ultrasonic spray pyrolysis of a chelated precursor into spherical YBa2Cu3O)7.x high temperature superconductor powders by C. H. Chao and P. D. Own, J. Mater Sci.,30 (1995) 6136-6144 and " Morphology of particles prepared by spray pyrolysis from organic precursor solution " by H. S. Kang, Y. C. Kang, H. D. Park, Y. G. Shul, Materials Letters 57 (2003) 1288-1294, wherein YBa2Cu3O7-x powders and ZrC>2 powders respectively were prepared by ultrasonic spray-pyrolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. In this process, citric acid and ethylene glycol were added for increasing the viscosity, in addition to the addition of NH4OH for adjusting the pH to 8 and the dried gel or highly viscous solution was chosen for spray pyrolysis. The presence of organics such as ethylene glycol in the precursor could leave undecomposed organic residues in the final product that may require a higher calcination temperature for phase formation in addition to affecting the properties considerably.
The smaller particle size and single phase of the powder gives three very desirable results. First the smaller particle size of the powder allows the powder to be sintered into a ceramic article (having a density of at least 95% of theoretical density) at a lower temperature than a powder with larger particles. Secondly, a multiphase powder, will affect the thermal expansion coefficient of the final ceramic articles. If the thermal expansion coefficient does not match that of the other components of the fuel cell the entire fuel cell may fail upon repeated thermal cycling. Therefore, it is critical to have a single phase sintered ceramic product. Thirdly, the multiphase dense powder compact does not have the proper electrical properties for use as an interconnect material, while
the single phase dense homogeneous powder compact possess the desired electrical properties.
Hence, it would be desirable to provide a process for developing highly dense ceramic product with excellent homogeneity and phase purity with desirable properties at lower temperature in air for their technological exploitation.
In accordance with the patents (Application Nos NF-458/03 and NF-459/03), a ceramic precursor mixture containing a mixture of citric acid and nitrate ions can be converted into a single phase ceramic oxide by forming droplets of the said ceramic precursor mixture followed by inducing self-ignition (auto-ignition) to convert the precursor droplets to their metal oxide counterparts. Such a process was established to prepare nano structured single-phase lanthanum manganite based oxides in a large scale by auto-ignition induced spray pyrolysis of citrate-nitrate precursors. However, such a precursor was not effective in inducing a self-ignition (auto-ignition) within the droplets to convert them to their corresponding metal oxides in the case of lanthanum chromite based oxides. Reference may again be made to 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 multielement oxide such as Ca-doped Lanthanum chromite is described by using metal nitrates, citric acid and ethylenediamine wherein it is mentioned that it may be possible to prepare the desired compositions of multielement oxides by (a) semicontinuous process of spraying the liquid suspension into a heated chamber followed by (b) calcinations of the product obtained therefrom". Infact, it was found that spray pyrolysing such a precursor solution containing metal nitrates, citric acid and ethylenediamine did not induce any auto-ignition within the droplets as expected. Hence, we have modified the above process by using a mixture of citric acid and glycine that together acts as a fuel for combustion giving rise to a controlled exothermic reaction. The glycine is reported to complex with the metal cations, like citric acid but the use of glycine alone as a complexing agent produces uncontrollable explosive reaction and hence was found to be unsafe. Thus, in this invention a partially polymerized precursor solution consisting of lanthanum nitrate, calcium nitrate and chromium nitrate in water with citric acid and glycine was used as a precursor solution for spray pyrolysis The mixed solution containing appropriate quantities of lanthanum nitrate, calcium nitrate and
chromium nitrate in water with citric acid and glycine was heated on a magnetic hot plate around 80-90°C to induce complexation and consequent polymerization reaction. Heating and stirring was continued and polymerization was arrested at an intermediate stage to control the viscosity of the solution suitable for easy spraying. Here, both citric acid and glycine contains carboxyl (COOH) and hydroxyl (OH) groups, and hence can participate in the complexation of metal ions, which in turn can prevent partial precipitation of the metal ions during water evaporation and thus could enhance the local homogeneity of the precursor solution. Secondly, both citric acid and glycine could also serves as fuels for the combustion of the precursor gel, where nitrate ion acts as the oxidant and citrate ion and glycine together acts as the fuel resulting in an anionic oxidation-reduction reaction between the fuel and the oxidant leading to a controlled auto-ignition process.
Thus, spray-pyrolysis of a chelated multicomponent precursor solution containing a mixture of citrate, nitrate and glycine in the present invention, where the polymerization or chelation of the cation could result in atomic level intermixing between the cations and thus maintain local homogeneity at an atomic scale within individual droplets, in addition to inducing a controlled exothermic auto-ignition reaction within individual droplets and thus leading to the continuous production of nano crystalline multicomponet ceramic oxides. Such powders could be calcined to have precursor powders of fine particle size, high surface area and reactivity. The novelty of the present invention resides in developing dense calcium substituted lanthanum chromite ceramic products of density more than 95% of theoretical density with electrical conductivity in the range of 55-60 S/cm at a temperature of 1350 °C. The inventive step of the present invention lies in providing a novel sinter-reactive precursor powder that can be sintered to a dense ceramic product at a lower temperature by spray pyrolysis of a solution consisting of metal nitrates, citric acid and glycine that can induce auto-ignition within individual droplets to produce nano crystalline lanthanum chromite based oxides. However, the present invention is not limited to the production of dense products of substituted lanthanum chromite perovskite oxide, but could be extended to any other multicomponent ceramic oxide system in general
This process has the potential of
1 Making dense Lai.xCaxCr03 products where x varies from 0 1 to 0 3 at a temperature as low as 1350 °C in air
2. Producing green powder compacts with density in the range of 55-60%
3. Producing dense ceramic of the said composition with density more than 95 % of theoretical density.
4. Producing dense ceramic of the said composition with conductivity in the range of 55-60 S/cm.
The present invention relates to the sintering of a precursor powder with the nominal composition Lao 7Ca0 3CrO3 to be used as an interconnect plate for SOFC, developed by atomization of a precursor solution consisting of the corresponding metal salts and a mixture of complexing agents like citric acid and glycine together acting as fuels and forming fine droplets of the same in a spray chamber and inducing auto-ignition within individual droplets to produce nano particles of the corresponding metal oxides. The interconnect plate serves as an internal electrical connection between the individual cells and is formed in a laminated structure sandwiched between anode and cathode layers. While the anode and cathode layers need to retain high porosity to facilitate gas-solid reactions, the electrolyte and interconnect layers must be sintered to closed porosity to prevent the intermixing of fuel and oxidant gases
The monolithic solid oxide fuel cell offers lower material costs, the potential for reduced manufacturing costs, and a higher efficiency over other geometries and designs. However, fabrication of these cells is expected to be more complicated because the individual components must be sintered separately or co-sintered at relatively low temperature. Of particular importance is the sintering behavior of the interconnect material, that is, lanthanum chromite which must be sintered to close porosity or about 94% of its theoretical density at temperatures of about 1400 °C in air or oxygen-containing atmospheres. The main inhibition to densification appears to be the volatilization of chromium oxides in oxidizing atmospheres at temperatures higher than 1400 °C. At higher temperatures it is necessary to employ an inert atmosphere during the
sintering in order to prevent the volatilization of chromium. Therefore, the preparation of lanthanum chromite powders which sinter to close porosity at temperatures below about 1450 °C so that Cr volatilization is insignificant is critical for the development of fuel cell fabrication technology
Thus, the main object of the present invention is to have a process for making dense products of calcium substituted lanthanum chromite at low temperature in air particularly suitable for application in solid oxide fuel cells.
Yet another object of the present invention is to compact the calcined powders to green density in the range of 55-60%
Still another object of the present invention is to sinter the green compacts in air at a temperature lower than the prior art.
Yet another object of the present invention is to sinter the green compacts at temperatures in the range of 1300-1350 °C to produce dense ceramic of the said composition with density in the range of 95-97 % of theoretical density.
Still another object of the present invention is to sinter the green compacts at temperatures in the range of 1300-1400 °C to produce dense ceramic of the said composition with density in the range of 95 to 97 % of theoretical density with conductivity in the range of 55-60 S/cm.
Accordingly the present invention provides a process for making dense products of calcium substituted lanthanum chromite in air at low temperature particularly suitable for application in solid oxide fuel cells by using a sinteractive precursor powder obtained by the process as described in the previous patent application No.218 wherein the process comprises preparing standard lanthanum nitrate solution in acidified distilled water of varying molar concentrations in the range of 0.7 to 1M and preparing standard aqueous chromium nitrate solutions of varying molar concentrations in the range of 0 7 to 1M and mixing the two solutions so that the molar ratio of La:Cr is maintained as 0.7:1.0, though not limited to this and to obtain a homogeneously mixed solution of lanthanum nitrate and chromium nitrate by stirring for a period in the range of 10-15 minutes on a magnetic
stirrer (200±5 °C) and by slowly adding a calculated quantity of calcium nitrate tetrahydrate to this mixed solution of lanthanum nitrate and chromium nitrate so that the ratio of (La+Ca) to Cr is equal to unity followed by adding and dissolving a known amount of citric acid monohydrate and glycine so that the ratio of citrate to nitrate (C/N) ratio is kept at 0.05 and glycine to nitrate (G/N) ratio is kept at 0.6 and allowing the mixed clear greenish blue colored solution consisting of lanthanum nitrate, calcium nitrate, chromium nitrate, glycine and citric acid having a metal ion ratio of La:Ca:Cr= 0.70:0.30:1 and an overall cation concentration i.e., sum of the concentration of the individual metal cations equals to 1 mole to evaporate by heating on a magnetic stirrer (200±5°C) with continuous stirring, allowing the evaporated solution to cool down to room temperature and atomizing the said solution with an atomizing nozzle having a 1 mm nozzle size, at a set air pressure of 20 psi, a flow rate of 25ml/min, and a set inlet temperature of 450 °C inducing auto-ignition within individual precursor droplets, burning of the droplets in the hot chamber and finally collecting the decomposed ash colored powder at the collection chamber and calcining the collected powder at a temperature of 650 °C and ball milling of the said powder for 6 hours in an acetone medium and mixing of the said powder with 2.5 wt.% polyvinyl butyral dissolved in methyl ethyl ketone and compaction of the said powder at a pressure of 258 MPa and sintering of the said green compact at a temperature of 1350 °C for 6h.
In an embodiment of the present invention calcium content in the lanthanum chromite may be varied between 10-30 mol %.
In another embodiment of the present invention, Ca may be replaced with Sr, Mg, or a mixed composition.
In another embodiment of the present invention, the calcined powders may be compacted into rectangular bars, cylindrical shapes or plates.
In another embodiment of the present invention, the calcined powders may be compacted into rectangular bars or cylindrical shapes by applying uniaxial pressures in the range of 135-485 MPa.
In yet another embodiment of the present invention the green compacts may be sintered at temperatures in the range of 1300-1400 °C for 3-6 hours.
Complete description of all the processing steps are given below.
Preparing standard lanthanum nitrate solutions of varying molar concentration (0.7-
1.0M) in a volumetric flask with acidified distilled water.
Preparing standard chromium nitrate solutions of varying molar concentration (0.7-
1.0M) in a volumetric flask with distilled water.
Mixing a calculated quantity of lanthanum nitrate and chromium nitrate solutions in a
one litre beaker so that the ratio of La.Cr is maintained as 0.70:1.0 but not limited to this.
The above mixed solution is heated on a magnetic stirrer (200±5°C) with continuous
stirring.
To the above mixed solutions of lanthanum nitrate and chromium nitrate calculated
amount of calcium nitrate tetrahydrate is added slowly and dissolved so that the molar
ratio of (Ca+La):Cr is equal to unity.
To the above mixed solution calculated quantity of glycine is added and dissolved so that
the ratio of glycine to nitrate is maintained between 0.5 to 0.70, though not limited to
this.
To the above mixed solution, calculated quantity of citric acid monohydrate is added and
dissolved so that the ratio of citrate to nitrate is maintained between 0.05 to0.15 though
not limited to this.
The mixed clear greenish blue colored solution consisting of lanthanum nitrate,
chromium nitrate, calcium nitrate, glycine and citric acid having metal ion ratio of
La:Ca:Cr=0.70:0.30:1.0 and a fixed G/N ratio but ranging between 0.50-0.7and a fixed
C/N ratio but ranging between 0.05-0.15 was allowed to evaporate on a hot plate with
continuous stirring.
Stirring and evaporation was continued until the volume of the solution was reduced to
about 80-90% of the original volume.
Atomization of the said precursor solution with spray nozzles with sizes in the range of
0.5 mm to 1 mm.
Atomization of the said precursor solution at a flow rate in the range of 20-25ml/min.
Atomization of the said precursor solution wherein the air pressure is fixed in the range of 20-30 psi
Atomization of the said precursor solution in a spray chamber wherein the inlet air temperature is fixed in the range of 450-600 °C
Ball milling of the said calcined powders with acetone for 3-8 hours for deagglomeration of the particles.
Mixing of the ball milled powder with 2.5 wt% polyvinyl butyral dissolved in methyl ethyl ketone.
Compaction of the ball milled powders with pressures in the range of 135- 485 MP
Compaction of the ball milled powders into rectangular bars or cylindrical shapes by uniaxial pressing.
Sintering of the green compacts with density in the range of 55-60% at temperatures in the range of 1250-1400 °C for 3-6 hours.
Complete description of all the processing steps are given below. The following examples illustrate the invention in the manner in which it may be carried out in practice. However, this will not limit the scope of the present invention The Examples also illustrate the feasibility of operating it on a continuous basis.
Example 1
A standard 1 M lanthanum nitrate solution is prepared in a volumetric flask with acidified distilled water. A standard 1 M chromium nitrate solution is prepared in a volumetric flask with distilled water. 210 ml of 1M lanthanum nitrate and 300 ml of 1M chromium nitrate solutions are mixed in a 1L beaker so that the ratio of La:Cr is maintained as 0 7:1.0 This mixed solution is heated on a magnetic stirrer (200±5 °C) with continuous stirring. To the above mixed solutions of lanthanum nitrate and chromium nitrate. 21.4968 gm of calcium nitrate tetrahydrate is added slowly and dissolved so that the molar ratio of (La+Ca):(Cr) is equal to unity To this mixed metal ion solution 77gms of
glycine monohydrate and 17 667 gms of citric acid monohydrate are added and dissolved so that the ratio of glycine to nitrate equals 0.60 and that of citrate to nitrate is equals to 0.05. This mixed clear greenish blue colored solution consisting of lanthanum nitrate, chromium nitrate, calcium nitrate, glycine and citric acid having a metal ion ratio of La-Ca:Cr=0.70-0 30:1.0, and a G/N ratio of 0.60 and a C/N ratio of 0.05 is allowed to evaporate on a hot plate with continuous stirring Stirring and evaporation was continued until the volume of the solution was reduced to about 90% of the original volume. This solution was atomized using a nozzle size of 1mm, a flow rate of 26ml/min, a set air pressure of 20psi and a set inlet temperature of 450 °C and the moment the droplets enter the hotzone of the chamber, drying and auto-ignition reaction was initiated so fast as evident by the flashing or sparking of the particles during ignition in the chamber. The ash colored powder collected at the collection chamber is calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrC>3 and CaCr04 The said precursor powder was ball milled with acetone for 6h followed by drying at 90+10 °C. The ball milled powder was mixed with 2.5 wt. % of Polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1250 °C. The percentage densification achieved was -86.69% (±1%).
Example 2 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of Polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1300 °C for 6h The percentage densification achieved was -94.36% (±1%)
Example 3
The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C The ball milled powder was mixed with 2 5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97.047% (±1%). This dense calcium substituted lanthanum chromite plate exhibited a conductivity of 59.66 S/cm.
Example 4 The as-sprayed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 258MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -94.18% (±1%).
Example 5
The as-sprayed precursor powder was prepared as given in Example 1. The green powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97.04% (±1%).
Example 6
The as-spra>ed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482 MPa, and sintered at a temperature of 1350 °C for 6h The percentage densification achieved was -97 67%
(±1%)
Example 7 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -95.60% (±1%).
Example 8 The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 482 MPa, and sintered at a temperature of 1300 °C for 6h. The percentage densification achieved was -95.31%) (±1%)
Example 9
The as-sprayed precursor powder was prepared as given in Example 1 The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C The ball milled powder was mixed with 2 5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1300 °C for 6h The percentage densification achieved was -93 32% (±1%)
Example 10
The as-sprayed precursor powder was prepared as given in Example 1. The ash colored powder collected at the collection chamber was calcined at 650 °C for 6h the XRD of which confirms the formation of a mixed oxide containing partially reacted LaCaCrO3 and CaCrO4 The said precursor powder was ball milled with acetone for 6h followed by drying at 90±10 °C. The ball milled powder was mixed with 2.5 wt. % of polyvinyl butyral dissolved in methyl ethyl ketone, then compacted into 5x10x15 mm rectangular bars by applying a uniaxial pressure of 137 MPa, and sintered at a temperature of 1400 °C for 6h. The percentage densification achieved was -94.92% (±1%).
The advantages of the present invention are:
1. Sintering of calcium substituted lanthanum chromite in air at a temperature as low
as 1350 °C.
2. This low sintering temperature help in co-sintering of the monolithic solid oxide fuelcell components and hence would be highly advantageous in the efficient fabrication of SOFC.
3. To have highly sintered and homogeneous dense plates of calcium substituted lanthanum chromite with conductivities of the order of 55-60 S/cm.
4 To produce dense plates of calcium substituted lanthanum chromite with density
more than 95 % of theoretical density
5 To have green powder compacts with high packing efficiency and green density of 55-60% of theoretical density.
6 Since the powder compacts in the present invention, sinter to near full density in air at temperatures as low as 1350 °C these powders are expected to facilitate the process of fabricating monolithic solid oxide fuel cells.

We claim :-
A process for making calcium substituted lanthanum chromite useful for application in solid oxide fuel cells by mixing lanthanum nitrate, chromium nitrate calcium nitrate, glycine & citric acid having metal ion ratio of La:Ca: Cr =0.70:0.30:1.0 and a fixed glycine/nitrate ratio ranging between 0.50 to 0.7 and a fixed citric acid/nitrate ratio ranging between 0.05 to 0.15 evaporating the above mixture solution on a hot plate under continuous stirring, continuing the evaporation till the volume of the solution is reduced to 80-90% of the original volume, atomizing the above said solution with spray nozzles with sizes in the range of 0.5 mm to 1mm at a flow rate in the range of 20-25 ml/minute at air pressure in the range of 20-25 psi to obtain the spray pyrolysed powder, calcining the above said powder at 650-1150°C for 6 hours milling the above said calcined powder with solvent for 3-8 hours compacting the said powder at a pressure of 135-490 MPa to obtain calcium substituted lanthanum chromite.
2. A process as claimed in claim 1 wherein calcination of the powder collected in
the collection chamber at 600-650 °C for 3-6 h.
3. A process as claimed in claims 1-2 wherein ball milling of the said calcined
powders with acetone for 4 hours for deagglomeration of the particles.
4. A process as claimed in claims 1-3 wherein mixing of the ball milled powder
with 2.5 wt.% poly vinyl butyral dissolved in methyl ethyl ketone.
5. A process as claimed in claims 1-4 wherein uniaxial compaction of the ball milled powder with pressures of 258 MPa.

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Patent Number

270189

Indian Patent Application Number

1222/DEL/2004

PG Journal Number

49/2015

Publication Date

04-Dec-2015

Grant Date

30-Nov-2015

Date of Filing

30-Jun-2004

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	HIMADRI SEKHAR MAITI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032
2	ABHOY KUMAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032
3	PARUKUTTYAMMA SUJATHA DEVI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032

PCT International Classification Number

C04B 35/2

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA