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Definitions

• the invention relates to a new method of fabricating a lanthanum-strontium-manganese (LSM) perovskite.
• LSM lanthanum-strontium-manganese
• perovskite is conventionally used to designate any substance presenting a structure of the ABO 3 type.
• a lanthanum-strontium-manganese (LSM) perovskite is a compound in which A is strontium-doped lanthanum and B is manganese. Its structure is of the following type:
• SOFC solid oxide fuel cell
• the article “Effect of spray parameters on the electrical conductivity of plasma-sprayed La 1-x Sr x MnO 3 coating for the cathode of SOFCs” describes a coating obtained by plasma spraying. As stated on page 279, such a coating presents a lamellar structure that is not presented by a fused and cast product.
• the object of the invention is to satisfy this need.
• this object is achieved by means of a method of fabricating lanthanum-strontium-manganese perovskite, the method comprising the following steps:
• the raw materials being selected in such a manner that the solid material obtained in step c), referred to as a “fused material”, presents the following chemical composition in percentages by weight for a total of 100%:
• the material may be in the form of a block or a particle.
• the method thus provides a method of fabricating particles comprising lanthanum-strontium-manganese perovskite, the method comprising the following steps:
• step a 1 the raw materials are selected in such a manner that the solid particles obtained in step d 1 ), referred to as “fused particles”, present the following chemical composition, in percentages by weight and for a total of 100%:
• the invention provides a method of fabricating a block comprising lanthanum-strontium manganese perovskite, the method comprising the following successive steps;
• step a 2 the raw materials are selected in such a manner that the unmolded block presents the following chemical composition, in percentages by weight for a total of 100%:
• the steps of the method of the invention are conventional for fabricating fused particles or fused blocks, and the person skilled in the art knows how to determine the raw materials in such a manner as to obtain said chemical composition in said fused particles or said fused blocks, this composition being identical to that of the lanthanum-strontium-manganese perovskite (La 1-x Sr x ) 1-y MnO 3 in which 0 ⁇ x ⁇ 0.5 and ⁇ 0.05 ⁇ y ⁇ 0.24. Nevertheless, the inventors were surprised to discover that such conventional steps led to a high percentage of perovskite phase.
• the fabrication method of the first and second embodiments of the invention further includes one and preferably more of the following optional characteristics:
• This composition is identical to that of the lanthanum-strontium-manganese perovskite (La 1-x Sr x ) 1-y MnO 3 with 0 ⁇ x ⁇ 0.5 and 0 ⁇ y ⁇ 0.1.
• This composition is identical to that of the lanthanum-strontium-manganese perovskite (La 1-x Sr x ) 1-y MnO 3 with 0 ⁇ x ⁇ 0.35 and 0 ⁇ y c 0.1.
• This composition is identical to that of the lanthanum-strontium-manganese perovskite (La 1-x Sr x ) 1-y MnO 3 with 0.15 ⁇ x ⁇ 0.35 and 0 ⁇ y ⁇ 0.1.
• This composition is identical to that of the lanthanum-strontium-manganese perovskite (La 1-x Sr x ) 1-y MnO 3 with 0.15 ⁇ x ⁇ 0.25 and 0 ⁇ y ⁇ 0.1.
• the fabrication method of the first embodiment of the invention further comprises one and preferably more of the following optional characteristics:
• the fabrication method of the second embodiment of the invention further comprises one and preferably more of the following optional characteristics:
• the invention also provides a fused product in the form of a fused particle or a fused block having a percentage of lanthanum-strontium-manganese perovskite ignoring impurities that is greater than 50%, preferably greater than 70%, more preferably greater than 90%, still more preferably greater than 99%, the product presenting the following chemical composition, in percentages by weight and for a total of 100%:
• this product presents, ignoring impurities, a percentage of said lanthanum-strontium-manganese perovskite of formula (La 1-x Sr x ) 1-y MnO 3 in which 0 ⁇ x ⁇ 0.5 and ⁇ 0.05 ⁇ y ⁇ 0.24, that is greater than 99.9%, with the parameters x and y in the formula being atom proportions for each element.
• the product of the invention includes one, and preferably more than one, of the following optional characteristics:
• the product of the invention may present the shape of a block having a thickness greater than 1 millimeter (mm), preferably greater than 2 mm, preferably greater than 5 centimeters (cm), more preferably greater than 15 cm, the thickness of a block being its smallest dimension.
• the block preferably presents a weight greater than 200 grams (g).
• the product of the invention may also present the shape of a particle, which may present one or more of the following optional characteristics:
• the product may also be in the form of a particle of size smaller than 4 mm, e.g. less than 3 mm.
• the sphericity of the particle may be greater than 0.5, preferably greater than 0.6, where sphericity is defined as the ratio between its smallest dimension and its greatest dimension.
• the product of the invention need not have been subjected to annealing heat treatment after solidification or cooling and/or need not be the result of grinding.
• the invention provides the use of particles or, possibly after grinding, of a block, that results from implementing a method of the invention or using a fused product, in particular fused particles, of the invention, in the fabrication of cathodes for solid oxide fuel cells (SOFC).
• SOFC solid oxide fuel cells
• T 100 ⁇ ( A LSM )/( A LSM +A 30.4 ⁇ 2 ⁇ 31.6 ) (1)
• fused product designates a solid product, possibly annealed, obtained by complete solidification of a composition in the liquid state (here referred to as a “molten liquid”).
• the “unmolded” product as obtained at the end of step e 2 ) may still include zones that have not solidified, and immediately after unmolding it is therefore not considered as being a fused product.
• fused particle designates a solid particle, possibly annealed, obtained by solidifying a composition in the liquid state.
• size of a particle designates the average of its greatest dimension dM and its smallest dimension dm: (dM+dm)/2.
• the thickness of a block is its smallest dimension.
• impurities designates inevitable ingredients necessarily introduced together with the raw materials or resulting from reactions with these ingredients.
• the starting charge is formed using the specified oxides, or precursors thereof.
• Compositions can be adjusted by adding pure oxides or mixtures of oxides and/or precursors, in particular La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, or Mn 3 O 4 .
• the person skilled in the art adjusts the composition of the starting charge so as to obtain, at the end of step d 1 ), a particle in accordance with the invention.
• the chemical analysis of the fused particle of the invention is generally substantially identical to that of the starting charge.
• the person skilled in the art knows how to adapt the composition of the starting charge, e.g. in order to take account of the presence of volatile elements or of the disappearance of certain elements during fusing.
• no element other than La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, and Mn 3 O 4 and precursors thereof is voluntarily introduced into the starting charge, with the other elements that are present being impurities.
• the sum of La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, Mn 3 O 4 , and precursors thereof represents more than 99% by weight of the starting charge.
• the raw materials can be mixed together intimately in a mixer.
• the mixture is then poured into a fusion furnace.
• step b 1 the starting charge is fused, preferably in an electric arc furnace. Electrofusion enables large quantities of particles to be fabricated with advantageous efficiencies.
• Heroult type arc furnace having two electrodes and in which the vessel has a diameter of about 0.8 meters (m), and is capable of containing about 180 kilograms (kg) of molten liquid.
• the voltage preferably lies in the range 140 volts (V) to 180 V, the power is about 240 kilowatts (kW), and the energy used lies in the range 1150 kilowatt hours per (metric) tonne (kWh/t) to 2800 kWh/t.
• any known furnace can be envisaged, such as an induction furnace, a plasma furnace, or other types of Heroult furnace, providing they are capable of causing the starting charge to melt completely, Without this necessarily being the case, it is possible to increase the quality of stirring by bubbling through an oxidizing gas (e.g. air or oxygen) as mentioned in FR 1 208 577.
• an oxidizing gas e.g. air or oxygen
• the quality with which the molten liquid is stirred can be improved in particular by bubbling through a gas containing 35% by volume of oxygen.
• step c 1 a stream of molten liquid at a temperature preferably lying in the range 1600° C. to 1800° C. is dispersed into small liquid droplets.
• Dispersion may be the result of blowing through the stream of molten liquid.
• any other method of atomizing a molten liquid known to the person skilled in the art could be envisaged.
• step d 1 the liquid droplets are transformed into solid particles by contact with an oxygenated fluid, preferably a gas, more preferably with air and/or steam.
• the oxygenated fluid preferably includes at least 20%, or even at least 25%, by volume of oxygen.
• the method is preferably adapted in such a manner that once the droplet of molten liquid has formed it comes immediately into contact with the oxygenated fluid. More preferably, dispersion (step c 1 )) and solidification (step d 1 )) are substantially simultaneous, the molten liquid being dispersed by an oxygenated fluid suitable for cooling and solidifying this liquid.
• Contact with the oxygenated fluid is preferably maintained at least until the particles have solidified completely.
• no solidification means are used other than cooling by contact with the oxygenated fluid.
• the rate of cooling is a function of particle diameter. It is about 1000 K/s for particles having a size of 0.3 mm.
• step d 1 solid particles of the invention are obtained that present a size lying in the range 0.01 mm to 3 mm, as a function of dispersion conditions.
• putting the molten liquid into contact with an oxygenated fluid enables industrial quantities of lanthanum-strontium-manganese perovskite to be obtained at reduced cost and at a percentage ignoring impurities that is remarkable, reaching more than 90%, and even more than 99.9%, without any annealing step.
• the impurities are all of the ingredients other than oxides of lanthanum, strontium, and manganese, and other than combinations of these oxides.
• impurities Al; Ca; Si; Zr; Na; Ba; and Fe.
• the total content of impurities, expressed in the form of oxides is less than 0.7% by weight, preferably less than 0.4%, more preferably;
• CaO ⁇ 0.25% preferably CaO ⁇ 0.05%
• ZrO 2 ⁇ 0.5% preferably ZrO 2 ⁇ 0.05%;
• BaO ⁇ 0.1% preferably BaO ⁇ 0.06%
• step d 1 particles of the invention are obtained.
• the particles are introduced into a furnace to be subjected to annealing heat treatment,
• annealing serves to further increase the percentage of lanthanum-strontium-manganese perovskite. This makes it possible to obtain lanthanum-strontium-manganese perovskite percentages substantially equal to 100%, ignoring impurities.
• the annealing temperature preferably lies in the range 1050° C. to 1400° C., more preferably in the range 1100° C. to 1200° C., and still preferably is about 1150° C. This temperature is preferably maintained for a duration longer than 0.5 hours, preferably longer than 2 hours, and is preferably about 5 hours.
• the annealing heat temperature is performed under an atmosphere containing at least 20% by volume of oxygen, preferably under air, preferably at ambient pressure of about 1 bar.
• the fused particles of the invention may be ground before or after annealing. Where necessary, grain size selection is then performed, as a function of the intended application.
• the particles of the invention may advantageously present a variety of sizes, the fabrication method not being limited to obtaining perovskite powders of submicron size. It is thus well suited to industrial fabrication.
• the particles obtained can advantageously be used for fabricating a cathode for solid oxide fuel cells (SOFC).
• SOFC solid oxide fuel cells
• the starting raw materials were initially mixed together intimately in a mixer:
• the starting charge as obtained in this way having a weight of 50 kg, was poured into a Heroult type arc fusion furnace. It was then melted implementing fusion with long arcs (voltage: 160 V, Power: 240 kW, the applied energy varying, depending on the example, over the range 1300 kWh/t to 2800 kWh/t) so as to melt the entire mixture in a complete and homogeneous manner. Preparation conditions were oxidizing.
• the quality of stirring was improved by bubbling through a gas containing 35% by volume of oxygen, as described in FR 1 208 577.
• the molten liquid was cast so as to form a stream.
• Dry compressed air was blown at a pressure of 3 bars to break up the stream and disperse it into droplets of molten liquid.
• the blown air cooled the droplets and froze them in the form of fused particles.
• the melted particles can be spherical or otherwise, hollow or solid. They generally present a size lying in the range 0.01 mm to 3 mm.
• the chemical analysis was performed by X-ray fluorescence.
• the percentage of lanthanum-strontium-manganese perovskite was determined using X-ray diffraction patterns acquired with a D5000 diffractometer from the supplier Bruker provided with a copper DX tube. After fusing and solidification, the products obtained can comprise the perovskite phase together with other phases, in smaller quantities, such as La 2 MnO 4 .
• lanthanum-strontium-manganese perovskite phases are identified using the conventional protocol, by X-ray diffraction, using an ICDD sheet.
• ICDD sheet No. 00-053-0058 applies to the La 0.8 Sr 0.2 MnO 3 lanthanum-strontium-manganese perovskite phase.
• the perovskite percentage is equal to 100%.
• Table 1 summarizes the results obtained prior to any annealing heat treatment.
• Table 1 shows up the effectiveness of the method of the invention.
• the product After heat treatment, the product presents a lanthanum-strontium-manganese perovskite percentage of 100%, ignoring impurities.
• step a 2 the starting charge is determined as in step a 1 ) described above, with the preferred characteristics, and in particular those relating to the selection of elements present or to their quantities, being the same as in above-described step a 1 ).
• the person skilled in the art adjusts the composition of the starting charge so as to obtain, at the end of step e 2 ), a block in accordance with the invention.
• the chemical analysis of the block of the invention is generally substantially identical to that of the starting charge.
• the person skilled in the art knows how to adapt the composition of the starting charge, e.g. in order to take account of the presence of volatile elements or of the disappearance of certain elements during fusing.
• the raw materials can be mixed together intimately in a mixer.
• the mixture is then poured into a fusion furnace.
• step b 2 the starting charge is fused, e.g. in an electric arc furnace, so as to fuse the entire starting charge in complete and homogenous manner.
• Electrofusion enables large blocks to be fabricated, possibly having thickness of 150 mm, with advantageous efficiencies.
• a Heroult type arc furnace having two electrodes and a vessel of diameter of about 0.8 m and capable of containing about 180 kg of molten liquid.
• the voltage used preferably lies in the range 140 V to 180 V, the power is about 240 kW, and the energy lies in the range 1150 kWh/t to 2800 kWh/t.
• any known furnace could be envisaged, such as an induction furnace, a plasma furnace, or other types of Heroult furnace, providing they enable the starting charge to be melted completely.
• an oxidizing gas e.g. air or oxygen
• the quality of the stirring of the molten liquid may in particular be improved by bubbling through a gas containing 35% by volume of oxygen.
• an induction furnace is most preferred, e.g. as described in FR 1 430 962.
• the block can thus be unmolded prior to solidifying completely, while the core of the block is still liquid.
• this early unmolding advantageously increases the percentage of lanthanum-strontium-manganese perovskite.
• the temperature of the molten liquid as measured during casting preferably lies in the range 1600° C. to 1800° C.
• step c 2 the molten liquid is cast into a mold suitable for withstanding the bath of molten liquid. It is preferable to use molds made of graphite, of cast iron, or as defined in U.S. Pat. No. 3,993,119. With an induction furnace, the turn is considered as constituting the mold. Casting is preferably performed in air.
• step d 2 the liquid cast into the mold is cooled to obtain a block that is solidified at least in part.
• the molten liquid is preferably put into contact with an oxygenated fluid, preferably a gas, preferably air.
• an oxygenated fluid preferably a gas, preferably air.
• This putting into contact can be performed as soon as casting begins. Nevertheless, it is preferable not to initiate this contact until after casting.
• putting into contact with the oxygenated fluid is preferably initiated only after unmolding, and preferably as soon as possible after unmolding.
• the oxygenated fluid preferably includes at least 20%, or even at least 25%, by volume of oxygen.
• contact is maintained with the oxygenated fluid until the block has solidified completely.
• the contact may be direct, e.g. for the surfaces of the molten liquid cast into the mold and forming the interface with ambient air. It may also be indirect, e.g. for the still-molten liquid in the core of a block whose outside surfaces have already solidified. Oxygen must then pass through the “walls” as constituted in this way in order to reach the molten liquid.
• Said putting of the molten liquid that is solidifying into contact with an oxygenated fluid is preferably initiated less than 1 hour, preferably less than 30 minutes, more preferably less than 20 minutes after the beginning of solidification.
• the rate of cooling during solidification is not a determining factor for improving the percentage of lanthanum-strontium-manganese perovskite.
• the rate of cooling is thus preferably always maintained below 1000 K/s, preferably below 100 K/s, more preferably below 50 K/s.
• conventional simple cooling means can thus be used.
• in order to solidify the molten liquid i.e. freeze it, use is made only of molds in contact with the surrounding air or that are cooled in particular by circulation of a heat-conveying fluid, or when the block is extracted from the mold and contains molten liquid, the block can be put into contact with the oxygenated fluid. Reliability and costs are thus improved.
• step e 2 the block is unmolded. To facilitate putting the molten liquid in contact with an oxygenated fluid, it is preferable to unmold the block as quickly as possible, if possible prior to complete solidification, Solidification then continues in step e 2 ).
• the block is unmolded as soon as it presents sufficient stiffness to be capable of substantially conserving its shape.
• the block is unmolded as quickly as possible and it is then immediately put into contact with the oxygenated fluid.
• unmolding is performed less than 20 minutes after the beginning of solidification.
• a block of the invention contains a quantity of lanthanum-strontium-manganese perovskite that is as much increased as the molten liquid has been put into contact with oxygen at an early stage during solidification, and then kept in contact therewith.
• the unmolded block is put into a furnace to be subjected to annealing heat treatment.
• annealing enables the percentage of lanthanum-strontium-manganese perovskite to be increased substantially. It is thus possible to obtain lanthanum-strontium-manganese perovskite percentages greater than 99%, preferably greater than 99.9%, and even substantially equal to 100%, ignoring impurities.
• the composition and the structure of the lanthanum-strontium-manganese perovskite can be expressed by the formula (La 1-x Sr x ) 1-y MnO 3 where 0 ⁇ x ⁇ 0.5 and ⁇ 0.05 ⁇ y ⁇ 0.24, the parameters x and y of the formula being the atom proportions of each element.
• the annealing heat treatment increases the percentage of lanthanum-strontium-manganese perovskite even if it is not possible to put any molten liquid in contact with an oxygenated fluid, e.g. because the block being fabricated is already fully solidified at the time of unmolding, and it was not possible to put it into contact with an oxygenated fluid while it was cooling in the mold, or while it was being cast.
• the parameters of the annealing heat treatment are a function of the dimensions of the blocks being treated. Preferably, these parameters as follows:
• the annealing heat treatment is preferable for the annealing heat treatment to be performed under an atmosphere containing at least 20%, or even at least 25%, by volume of oxygen, preferably under air, preferably at ambient pressure of about 1 bar.
• the annealing heat treatment must be performed after the block has solidified completely. Before being annealed, the block may nevertheless be broken into pieces or into a powder.
• the block is preferably ground into the form of particles having a size of about 5 mm or less than 5 mm.
• the method of the invention leads to a block of the invention having a majority of the lanthanum-strontium-manganese perovskite phase.
• the annealed block or particles of the invention present a lanthanum-strontium-manganese perovskite percentage, ignoring impurities, of more than 99%, preferably of more than 99.9%, preferably of 100%.
• the impurities are all the elements other than the oxides of lanthanum, strontium, and manganese and other than combinations of said oxides.
• impurities Al; Ca; Si; Zr; Na; Ba; and Fe.
• the total percentage by weight of impurities expressed in oxide form is less than 0.7%, preferably less than 0.4%. More preferably:
• CaO ⁇ 0.25% preferably CaO ⁇ 0.05%
• ZrO 2 ⁇ 0.5% preferably ZrO 2 ⁇ 0.05%;
• BaO ⁇ 0.1% preferably BaO ⁇ 0.06%
• the block of the invention may advantageously present arbitrary dimensions, the fabrication method not being limited to obtaining perovskite powders of submicron size.
• the block is thus well suited to industrial fabrication.
• the block presents a thickness greater than 1 mm, preferably greater than 2 mm, preferably greater than 5 cm, more preferably greater than 15 cm, where the thickness of a block is its smallest dimension.
• the optionally annealed block is subsequently crushed and ground to the desired grain size.
• the method of the invention enables particles to be fabricated having a variety of dimensions and at low cost.
• the unmolded block is initially crushed into pieces in the range 0 to 5 mm. Then annealing heat treatment is performed on the pieces, which are subsequently ground to the desired grain sizes.
• the starting raw materials identical to those used for the examples of the first embodiment of the invention, were initially mixed together intimately in a mixer.
• the starting charge as obtained in that way was poured into an arc fusion furnace. Fusion was performed using long arcs so as to melt the entire mixture in complete and homogeneous manner. Preparation conditions were oxidizing. The temperature of the molten liquid as measured during casting lay in the range 1600° C. to 1800° C.
• the molten liquid was then cast, in air, into a variety of molds: made of cast iron, of graphite, or as defined in U.S. Pat. No. 3,993,119.
• Vr ( Tf ⁇ Ts )/ t r
• Tf designates the temperature of the molten liquid during casting (in ° C.)
• Ts designates the temperature of the block at the instant at which it is completely solidified (in ° C.)
• t r designates the time between the beginning of casting and the moment when the block has solidified completely (in seconds).
• the chemical analyses and the determination of the lanthanum-strontium-manganese perovskite phase were performed on samples ground into powder and representative of the cast blocks.
• the powders presented a median diameter of less than 40 ⁇ m.
• the chemical analysis was performed by X-ray fluorescence.
• the lanthanum-strontium-manganese perovskite percentage was determined from X-ray diffraction patterns as described above for the examples relating to the first embodiment.
• Tables 2 and 2′ summarize the results obtained before any annealing heat treatment.
• Table 2′ shows the effectiveness on the method of the invention. It also makes it possible to observe that during fusion by induction (Examples 8 2 and 9 2 ) where the surfaces of the unmolded block come more quickly into contact with the oxygen of the air (after 20 minutes at most in these examples, while the block was not yet completely solidified), the percentage of lanthanum-strontium-manganese perovskite in the final product was very high, reaching substantially 100%, thus advantageously making it pointless to have recourse to annealing heat treatment.
• Examples 1 2 , 2 2 , 4 2 , and 5 2 were subsequently subjected to annealing heat treatment (Table 2).
• the annealing heat treatment was performed on cast blocks or on blocks crushed to the 0 to 5 mm range (Example 1).
• the heat treatment parameters used are specified in Table 3.
• the heat treatment was performed in air.
• Table 3 shows that the treatment leads to a significant increase in the percentage of lanthanum-strontium-manganese perovskite, up to substantially 100%.
• the method of the invention makes it possible in simple and inexpensive manner to fabricate industrial quantities of products, and in particular of particles and blocks, that present high percentages of lanthanum-strontium-manganese perovskite and that present the following chemical analysis, in percentages by weight for a total of 100%:
• the method even makes it possible to fabricate products that present lanthanum-strontium-manganese perovskite having the formula (La 1-x Sr x ) 1-y MnO 3 with 0 ⁇ x ⁇ 0.5 and ⁇ 0.05 ⁇ y ⁇ 0.24 at percentages, ignoring impurities, of more than 99.9%, or even of 100%.
• the dimensions of the products may subsequently be reduced, for example the products may be ground into the form of powders if that is required by their utilization.
• the invention relates to a product, in particular in the form of a particle or a block, comprising LTM perovskite, where L designates lanthanum, T is an element selected from strontium, calcium, magnesium, barium, yttrium, ytterbium, cerium, and mixtures of these elements, and X designates manganese.
• L designates lanthanum
• T is an element selected from strontium, calcium, magnesium, barium, yttrium, ytterbium, cerium, and mixtures of these elements
• X designates manganese.
• the invention also provides methods of fabricating this product.
• LTM perovskite powders are used in particular to fabricate cathodes for solid oxide fuel cells, using methods that are complex and expensive, as mentioned in the introduction. There thus exists a need for products comprising LTM perovskite that can be fabricated at reduced cost and in industrial quantities. An object of the invention is to satisfy this need.
• each electrode is generally subdivided into two layers.
• the first layer acts as a current collector layer (CCL).
• CCL current collector layer
• One of the raw materials used as a cathode material in SOFC technology is a powder of doped lanthanum-manganese (LTM) perovskite.
• the active layer in the cathode is situated between the CCL and the electrolyte, and must make it possible simultaneously to supply electrons to the system in order to reduce oxygen from the air into O 2 ⁇ ions and to transport these O 2 ⁇ ions to the electrolyte.
• the active CFL is generally made up of a mixture of ionically-conductive material and of electronically-conductive material (doped lanthanum-manganese perovskite). Contact between the two materials and air must be optimized, i.e. the number of triple points must be as large as possible and there must be grain percolation for each of the materials.
• Doped zirconias (cubic zirconia stabilized with yttrium oxide, cubic zirconia stabilized with scandium, . . . ) are commonly used as electrolyte materials or in the functional cathode layer.
• the doped lanthanum-manganese perovskite of the cathode material can react with the doped zirconium of the electrolyte to form new phases at their interface, in particular a La 2 Zr 2 O 7 pyrochlore-type phase, in particular when x is less than 0.4, or even less than 0.3 in the formula for the LTM perovskite.
• This phase reduces the performance of the cell.
• Another object of the invention is to satisfy this need.
• a fused product in particular in the form of a particle or a block, made of LTM perovskite, where L designates lanthanum, T is an element selected from strontium, calcium, magnesium, barium, yttrium, ytterbium, cerium, and mixtures of these elements, and X designates manganese.
• the product of the invention may in particular be present in the form of a powder.
• the size of the particles may in particular be greater than 1 ⁇ m, or greater than 10 ⁇ m, or greater than 0.25 mm, and/or less than 3 mm.
• This powder may in particular comprise more than 90% by weight, or more than 95% or substantially 100% of fused particles of LTM perovskite of the invention.
• the product of the invention is fused, i.e. it is obtained by being fused and then solidified.
• the merit of the inventors lies in discovering such that a technique can enable products containing LTM perovskite to be fabricated,
• the product of the invention can thus be fabricated at low cost and in industrial quantities.
• the product of the invention may be an annealed product or a product which has not been annealed, i.e. which has not been thermically treated after its solidification.
• the product of the invention when in contact with yttrium oxide doped zirconia powder always generates less polychlore-type phase La 2 Zr 2 O 7 than the prior art products that have the same chemical composition. It is therefore very well adapted to fabricating cathodes for SOFCs.
• the content and the nature of the resulting LTM perovskite depend in particular on the composition of the starting charge. Nevertheless, the product of the invention is always polycrystalline.
• the fused product has an LTM perovskite percentage greater than 50%, ignoring impurities, said perovskite presenting molar proportions l p , t p , and m p of lanthanum, of the element T, and of manganese, respectively, such that using the notation:
• variables x and y correspond to the atom proportions x and y of the (La (1-x) T x ) (1-y) MnO (3- ⁇ ) structure of the LTM perovskite of the product of the invention.
• the product of the invention includes one and preferably more of the following optional characteristics:
• these characteristics improve the electrical conductivity properties, making the products particularly suitable, possibly after grinding, for fabricating cathodes for solid oxide fuel cells (SOFC).
• SOFC solid oxide fuel cells
• the product of the invention need not be subjected to annealing heat treatment after cooling, and/or need not be the result of grinding.
• the invention also provides a method of fabricating a fused product comprising LTM perovskite, where L designates lanthanum (La), T is an element selected from the group formed by strontium, calcium, magnesium, barium, yttrium, ytterbium, cerium, and mixtures of these elements, and M designates manganese (Mn), the method comprising the following steps:
• the invention relates to a product in the form of a particle and to a method of fabricating such particles.
• the invention provides a method of fabrication comprising steps a′), b′), and c′), with step c′) comprising the following steps:
• the method is preferably adapted so as to obtain particles of an LTM perovskite product of the invention.
• the fabrication method of the invention preferably also includes one, and preferably more than one, of the general optional characteristics listed above and/or of the following particular characteristics:
• the fused particles may be ground and/or subjected to an operation of selecting grain size as a function of the intended applications, e.g. by screening, in particular in such a manner that the particles obtained present a size greater than 1 ⁇ m, or greater than 10 ⁇ m, and/or less than 3 mm.
• the invention relates to a product in the form of a block and to a method of fabricating such a block.
• the invention provides a fabrication method comprising steps a′), b′), and c′), with step c′) comprising the following steps:
• the fabrication method of the invention preferably also comprises one, and preferably more than one, of the general optional characteristics listed above and/or of the following particular characteristics:
• phases other than LTM perovskite may be present, together with impurities coming from the raw materials.
• the impurities are all the elements other than lanthanum, the element T, manganese, and combinations of those elements.
• SiO 2 ⁇ 0.1% preferably SiO 2 ⁇ 0.07%; preferably SiO 2 ⁇ 0.06; and/or
• ZrO 2 ⁇ 0.5% preferably ZrO 2 ⁇ 0.1%, preferably ZrO 2 ⁇ 0.05; and/or
• Na 2 O ⁇ 0.1% preferably Na 2 O ⁇ 0.07, preferably Na 2 O ⁇ 0.05%;
• the invention concerns the use of fused products fabricated or being suitable for being fabricated by a method of the invention or fused products of the invention in the fabrication of cathodes for solid oxide fuel cells (SOFC).
• SOFC solid oxide fuel cells
• the LTM perovskite percentage, ignoring impurities is defined using the following formula (1):
• T 100 ⁇ ( A LTM )/( A LTM +A secondary phase ) (1)
• a multiplet is the partial superposition of a plurality of peaks. For example, a multiplet made up of two peaks is a doublet, and a multiplet made up of three peaks is a triplet.
• a starting charge suitable for fabricating a particle of the invention is formed from compounds of lanthanum, of the element T, and of manganese, in particular in the form of oxides or of carbonates, or from precursors of the elements lanthanum, T, and manganese.
• the compositions can be adjusted by adding pure oxides or mixtures of oxides and/or of precursors, in particular SrO, SrCO 3 , La 2 O 3 , CaO, CaCO 3 , Y 2 O 3 , Yb 2 O 3 , MgO, MgCO 3 , CeO 2 , BaO, MnO 2 , MnO, or Mn 3 O 4 .
• the use of oxides and/or carbonates improves the availability of oxygen needed for forming perovskite, and is thus preferred.
• the quantities of lanthanum, of the element T, and of manganese in the starting charge are practically the same in the fabricated fused product. Some of these ingredients, in quantities that are variable as a function of fusion conditions, can be volatilized during the fusion step. On the basis of general knowledge, or by performing simple routine tests, the person skilled in the art knows how to adapt the quantities of these ingredients in the starting charge as a function of the quantities desired in the fused products and of the fusion conditions implemented.
• the grain sizes of the powders used may be those commonly encountered in fusion methods.
• the basic mixture may comprise, in addition to the compounds providing the elements lanthanum, T, and manganese, and the impurities, other compounds that are introduced for comparing particular properties on the fabricated particles.
• no compound other than those providing the elements lanthanum, T, and manganese and in particular no compound other than La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, Mn 3 O 4 , CaO, CaCO 3 , Y 2 O 3 , Yb 2 O 3 , MgO, MgCO 3 , CeO 2 , BaO is voluntarily introduced into the starting charge, the other elements present being impurities.
• the sum of La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, Mn 3 O 4 , CaO, Y 2 O 3 , MgO, CeO 2 , BaO and of their precursors represents more than 99% by weight of the starting charge.
• the compounds providing the elements lanthanum, T, and manganese are selected from La 2 O 31 SrO, SrCO 3 , MnO 2 , MnO, Mn 3 O 4 , CaO, CaCO 3 , and their precursors, the sum of these ingredients preferably representing more than 99% by weight of the starting charge. Still preferably, no compound other than La 2 O 3 , SrO, SrCO 3 , MnO 2 , MnO, Mn 3 O 4 , CaO, CaCO 3 is voluntarily introduced into the starting charge, the other elements present being impurities.
• the molar proportions of the elements L, T, and M in the starting charge are close to those of the perovskite it is desired to fabricate.
• the molar proportions l d , t d , and m d of the elements L, T, and N respectively, as molar percentages relative to the sum of the proportions l d , t d , and m d , to satisfy the following conditions:
• k 1 and k 2 are those to be used under established working conditions, i.e. ignoring transition phases between different compositions and ignoring starting stages. If the desired product implies a change in the composition of the starting charge compared with that used for fabricating the preceding product, it is necessary to take account of the residues of the preceding product in the furnace. Nevertheless, the person skilled in the art knows how to adapt the starting charge accordingly.
• the raw materials can be mixed together intimately in a mixer.
• the mixture is then poured into a fusion furnace.
• step b 1 ′ the starting charge is fused, preferably in an electric arc furnace. Electrofusion makes it possible to fabricate large quantities of particles with advantageous efficiencies.
• Heroult type arc furnace having two electrodes and a vessel with a diameter of about 0.8 m, and capable of containing about 180 kg of molten liquid.
• the voltage preferably lies in the range 140 V to 180 V, with power of about 240 kW, and energy lying in the range 1150 kWh/t to 2800 kWh/t.
• furnaces such as an induction furnace, a plasma furnace, or other types of Heroult furnace, providing they make it possible to melt the starting charge completely.
• an oxidizing gas e.g. air or oxygen
• the quality with which the molten liquid is stirred can be improved in particular by bubbling through a gas containing 35% by volume of oxygen.
• step b 1 ′ the entire starting charge is in liquid form.
• step c 1 ′ a stream of molten liquid at a temperature that is preferably greater than 1600° C., and preferably less than 2200° C., more preferably less than 1800° C. when the element T is strontium, is dispersed in the form of small liquid droplets.
• the dispersion may be the result of blowing through the stream of molten liquid.
• any other method of atomizing a molten liquid known to the person skilled in the art, could be envisaged.
• step d 1 ′ the liquid droplets are transformed into solid particles by contact with an oxygenated fluid, preferably a gas, more preferably with air and/or steam.
• the oxygenated fluid preferably contains at least 20%, or even at least 25%, oxygen by volume.
• the method is preferably adapted in such a manner that immediately on being formed, the droplet of molten liquid comes into contact with the oxygenated fluid. More preferably, the dispersion and solidification steps (steps c 1 ′) and d 1 ′)) are substantially simultaneous, the molten liquid being dispersed by an oxygenated fluid suitable for cooling and solidifying the liquid.
• Contact with the oxygenated fluid is preferably maintained at least until the particles have solidified completely.
• Blowing air at ambient temperature is entirely suitable.
• Cooling rate is a function of particle diameter. It is about 1000 K/s for particles having a size of 0.3 mm. The cooling rate is preferably adapted so that the particles have become hard, at least at their peripheries, before coming into contact with the recovery container.
• the solid particles of the invention are obtained presenting a size lying in the range 0.01 mm to 3 mm, or in the range 0.01 mm to 4 mm, as a function of dispersion conditions.
• putting the molten liquid into contact with an oxygenated fluid makes it possible, and at low cost, to obtain industrial quantities of LTM perovskite at percentages, ignoring impurities, that are remarkable, reaching more than 90%, and even more than 99.9%, without an annealing step.
• the fused particles are introduced into a furnace to be subjected to annealing heat treatment.
• annealing can further increase the percentage of LTM perovskite. It is thus possible to obtain LTM perovskite percentages substantially equal to 100%, ignoring impurities.
• the annealing temperature preferably lies in the range 1050° C. to 1400° C., more preferably in the range 1100° C. to 1200° C., and still preferably, is about 1150° C. This temperature is preferably maintained for a duration longer than 0.5 hours, preferably longer than 2 hours, preferably about 5 hours.
• the annealing heat treatment is preferably performed under an atmosphere containing at least 20% by volume of oxygen, preferably under air, preferably at ambient pressure of about 1 bar.
• the fused particles of the invention may be ground, before or after being annealed. If necessary, grain size selection can then be performed, depending on the intended application.
• Particles of the invention may advantageously present a variety of dimensions, the fabrication method not being limited to obtaining perovskite powders of submicron size. It is thus well adapted to industrial fabrication.
• the particles that are obtained may advantageously be used for fabricating a cathode for solid oxide fuel cells (SOFC).
• SOFC solid oxide fuel cells
• the starting charge obtained in this way was poured into a Heroult type arc fusion furnace. It was then melted using long arc fusion (voltage of 150 V, power of 225 kW, applied energy varying in the examples over the range 1380 kWh/t to 2000 kWh/t) so as to melt the entire mixture in complete and homogeneous manner.
• Working conditions were oxidizing.
• the molten liquid was cast so as to form a stream.
• the temperature of the molten liquid as measured during casting lay in the range 1730° C. to 1850° C.
• Dry compressed air at ambient temperature and blown at a pressure lying in the range 1 bar to 3 bars was used to break up the stream and disperse the molten liquid in droplets.
• the fused particles can be made to be spherical or otherwise, hollow or solid. They present a size lying in the range 0.01 mm to 3 mm, or in the range 0.01 mm to 4 mm.
• the lanthanum-strontium-manganese perovskite percentage was determined using X-ray diffraction patterns acquired with a D5000 diffractometer from the supplier Bruker, provided with a copper DX tube. After fusion, the products obtained can comprise the perovskite phase together with smaller quantities of other phases such as La 2 MnO 4 .
• the lanthanum-strontium-manganese perovskite phases are identified using the conventional protocol, by X-ray diffraction, using International Center for Diffraction Data (ICDD) sheets.
• ICDD 01-089-8084 sheet is the sheet for the La 0.7 Ca 0.3 MnO 3- ⁇ lanthanum-calcium-manganese perovskite phase.
• the perovskite percentage is 100%.
• Table 4 summarizes the results obtained before any annealing heat treatment.
• Table 4 shows the effectiveness of the method of the invention.
• the method of the invention in its first generalized embodiment makes it simple and inexpensive to fabricate industrial quantities of particles having very high levels of lanthanum-element T-manganese perovskite, where element T is an element selected from the group constituted by strontium, calcium, magnesium, barium, yttrium, ytterbium, and cerium.
• the method makes it possible to fabricate particles that are constituted, ignoring impurities, by more than 99.9% or even 100% lanthanum-element T-manganese perovskite having the formula (La 1-x T x ) 1-y MnO 3- ⁇ with 0 ⁇ x ⁇ 0.5 and ⁇ 0.1 ⁇ y ⁇ 0.24.
• the dimensions of the particles can then be reduced, e.g. by grinding to form powders, if so required for their utilization.
• step a 2 ′ a starting charge was prepared as was explained for step a 1 ′) above, step a 2 ′) presenting the same preferred characteristics as step a 1 ′).
• step b 2 ′ the starting charge was fused.
• Any fusion furnace can be used. Fusion can be performed as explained in the description relating to step b 1 ′), or preferably in the description relating to step b 2 ) above. In particular, the description of step b 2 ) explained the advantage of using an induction furnace.
• step b 2 ′ the entire starting charge is in liquid form.
• step c 2 ′ the molten liquid is cast into a mold, as explained above for step c 2 ).
• the cast molten liquid presents a temperature that is preferably greater than 1600° C. and preferably less than 2200° C., more preferably less than 1800° C. when the element T is strontium.
• step d 2 ′ the liquid cast into the mold is cooled until an at least partially solidified block is obtained, as described above for step d 2 ), step d 2 ′) possessing the same preferred characteristics as step d 2 ), in particular concerning putting into contact with an oxygenated fluid, cooling speeds, and cooling means.
• the inventors have found that the rate of cooling during solidification is not a determining factor for improving the LTM perovskite percentage.
• step e 2 ′ the block is unmolded as explained above for step e 2 ), step e 2 ′) presenting the same preferred characteristics as step e 2 ), in particular concerning the instant of unmolding and putting into contact with an oxygenated fluid.
• step f 2 ′ the unmolded block is put into a furnace to be subjected to annealing heat treatment, as explained above for step f 2 ), with step f 2 ′) presenting the same preferred characteristics as step f 2 ), in particular concerning annealing parameters.
• such annealing can significantly increase the LTM perovskite percentage. It is thus possible to obtain LTM perovskite percentages greater than 99%, preferably greater than 99.9%, and even substantially equal to 100%, ignoring impurities, and this can be done even if it is not possible to put any molten liquid into contact with an oxygenated fluid, e.g. because the fabricated block was already completely solidified at the moment of unmolding and no contact with an oxygenated fluid was possible during cooling in the mold or during casting.
• the block of the invention may advantageously present arbitrary dimensions, the fabrication method not being limited to obtaining perovskite powders of submicron size.
• the block is thus perfectly adapted to industrial fabrication.
• the block presents a thickness greater than 1 mm, preferably greater than 2 mm, preferably greater than 5 cm, more preferably than 15 cm, where the thickness of a block is its smallest dimension.
• the optionally annealed block is subsequently crushed and ground to the desired grain size.
• the method of the invention enables particles to be fabricated having a variety of dimensions and at low cost.
• the unmolded block is initially crushed into pieces in the range 0 to 5 mm. Then annealing heat treatment is performed on the pieces, which are subsequently ground to the desired grain sizes.
• the starting raw materials identical to those used for the examples of the first generalized embodiment relating to particles, were initially mixed together intimately in a mixer.
• the resulting starting charge was poured into a Heroult type arc fusion furnace (except for Example 6 2 ′). It was then melted by long arc fusion (voltage 180 V, applied energy varying between examples over the range 1150 kWh/t to 1760 kWh/t) so as to melt the entire mixture in complete and homogeneous manner. Operating conditions were oxidizing.
• the molten liquid was cast in air into cast iron molds.
• the temperature of the molten liquid measured during casting lay in the range 1560° C. to 1700° C.
• Table 5 summarizes the results obtained before any annealing heat treatment.
• the content of the element lanthanum is expressed in the form of La 2 O 3
• the content of the element calcium is expressed in the form CaO
• the content of the element manganese is expressed in the form MnO.
• Table 5′ shows the effectiveness of the method of the invention. It also shows that when fusion is performed by induction (Example 6 2 ′) where the surfaces of the unmolded block came more quickly into contact with oxygen of the air (after a maximum of 20 minutes from the beginning of solidification in this example, when the block was still not completely solidified), the lanthanum-calcium-manganese perovskite percentage in the final product is very high, reaching 99.9%, which advantageously can make it pointless to have recourse to annealing heat treatment.
• Examples 1 2 ′, 2 2 ′, 3 2 ′, and 5 2 ′ were subsequently subjected to annealing heat treatment (Table 6).
• the annealing heat treatment was performed on cast blocks or on blocks crushed into 0-5 mm pieces (Examples 1 2 ′ and 2 2 ′).
• the heat treatment parameters used are given in Table 6.
• the heat treatment was performed in air.
• the LTM perovskite percentages were determined as in the above examples relating to particles of LTM, specifically relating to particles of lanthanum-calcium-manganese.
• Table 6 shows that the annealing treatment leads to a significant increase in the lanthanum-calcium-manganese perovskite percentage, up to substantially 100%.
• the method according to the second generalized embodiment of the invention makes it simple and inexpensive to fabricate industrial quantities of blocks having very high levels of lanthanum-element T-manganese perovskite, where element T is an element selected from the group constituted by strontium, calcium, magnesium, barium, yttrium, ytterbium, and cerium.
• the method makes it possible to fabricate blocks that are constituted, ignoring impurities, by more than 99.9% or even 100%, of lanthanum-element T-manganese perovskite having the formula (La 1-x T x ) 1-y MnO 3- ⁇ with 0 ⁇ x ⁇ 0.5 and ⁇ 0.1 ⁇ y ⁇ 0.24.
• the method makes it possible in particular to fabricate blocks containing lanthanum-calcium-manganese (LCM) perovskite in which:
• the dimensions of the blocks can then be reduced, e.g. by grinding into the form of powders if that is required for their utilization.
• Fused LTM perovskite products are particularly remarkable in that the quantity of the La 2 Zr 2 O 7 pyrochlore-type phase that is generated, measured using the above-described protocol, is always less than the quantity of the La 2 Zr 2 O 7 pyrochlore-type phase that is generated under the same conditions using an LTM perovskite powder obtained by a method other than by fusion. This property would even appear to constitute a signature of the products of the invention.
• the method used for measuring this property is as follows.
• the median size of the LTM powders and the parameters of the sintering heat treatment have been determined so as to favour the formation of a La 2 Zr 2 O 7 pyrochlore-type phase, so as to highlight the behavior differences of the LTM powders when they are in contact with a zirconia powder stabilized with 8 mol % yttrium oxide.
• the quantity of La 2 Zr 2 O 7 pyrochlore-type phase contained in the sintered sample is measured by X-ray diffraction. The measurement is thus a comparative measurement, not a quantitative measurement.
• Comparisons between different LTM perovskite powders can easily be performed while taking care to use the same protocol, and also the same stabilized zirconia powder.
• all of the samples are sintered in the same furnace, in order to limit any dispersions that might arise due to the method of preparing the samples to be characterized.
• the tests consisted in intimately mixing a zirconia powder with a doped lanthanum-manganese (LTM) perovskite powder, in preparing pellets, then in raising to high temperature to encourage the creation of the La 2 Zr 2 O 7 phase.
• the quantity of said phase that has been generated relative to the quantity of the zirconia in the sample was then determined by X-ray diffraction.
• Samples comprising LTM powders for comparison were prepared as follows.
• Pellets having a diameter of 13 mm and a thickness of substantially 5 mm were then made using a pelleting-press: 2.8 g of powder were put therein and pressed under 50 kilonewtons (kN) with a manual Weber press for 1 min.
• kN kilonewtons
• pellets were then placed on an aluminum saggar provided with a cover.
• the assembly was put into a Naber 1800 furnace sold by the supplier Nabertherm, then raised to 1375° C. for 24 hours, with temperature being raised at 5° C./min and being reduced at 5° C./min.
• Each sintered pellet was then thinned in a grinder so as to remove a thickness of about 2 mm, thus revealing the core of the material, Finally the pellet was coated in a transparent resin and polished.
• X-ray diffraction measurements were achieved using a D5000 apparatus from the supplier Bruker provided with a copper DX tube.
• the X-ray diffraction pattern was made with a step size of 0.02° and an acquisition time of 4 seconds per step. In practice, those diagrams make it possible to detect:
• Pellets were made from a mixture of each of these powders with stabilized zirconia as described above.
• Table 8 shows clearly that powders of fused perovskite products of the invention present a ratio
• the La 2 Zr 2 O 7 pyrochlore-type phase can even be undetectable.
• the performance of solid oxide fuel cells using these products is improved as a result.
• Table 8 (example 1 2 ) shows that an annealing step does not change the advantageous behavior of a product of the invention. Therefore, after annealing, a product of the invention remains different than the comparative examples.
• the LTM perovskite products may also generate prejudicial T a Zr b O c type phases, a, b and c being integers, when they are in contact with zirconia, as described here above.
• the quantity of each T a Zr b O c type phase, expressed on the basis of the total quantity of this T a Zr b O c type phase and of the cubic zirconias is always less with the fused products of the invention than with not fused products according to prior art, as it is showed by the underlying examples.
• the method which is used to measure the quantity of a T a Zr b O c type phase is similar to the method used to measure the quantity of La 2 Zr 2 O 7 pyrochlore-type phase that was described previously.
• the phase to be measured is SrZrO 3 .
• the diffraction patterns show:
• Table 10 summarizes the measures of the ratio “area (SrZrO 3 )/(area(SrZrO 3 )+area (cubic zirconia))” determined for each of the fabricated samples.
• Table 10 clearly shows that the powders of the fused products of LSM perovskite according to the invention have a ratio area(SrZrO 3 )/(area(SrZrO 3 )+area(cubic zirconia)) which is comparatively much smaller than the powders of perovskite products obtained with another method than fusion.
• the SrZrO 3 phase may even be undetectable.
• the products of the invention are not limited to any particular shape or dimensions.

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