Porous sintered lanthanum manganite bodies and method of manufacturing the same

Abstract: A porous sintered lanthanum manganite body has an alkali metal content of at least 100 ppm and the alkali metal concentration in the lanthanum manganite crystal grains is higher adjacent the grain boundaries than away from the grain boundaries. The body is made by forming a shaped green body of material and firing the shaped green body, the sinterable material has a content of alkali metal of at least 100 ppm. The sintered body has improved strength, and is suitable for example as an air electrode of a solid oxide fuel cell.

Description: The present invention relates to a method of making porous sintered lanthanum manganite bodies, such as tubes, for example bodies useful as air electrodes of solid oxide fuel cells. DESCRIPTION OF THE PRIOR ART Lanthanum manganite has recently become of increasing interest as a temperature resistant electrically conductive ceramic material. In the art, the term lanthanum manganite includes doped lanthanum manganites. For this specification and claims we define lanthanum manganite as an oxide of perovskite crystal structure having the formula La 1-x A x Mn 1-y B y O 3±δ wherein 0 ≤ x ≤ 0.75 0 ≤ y ≤ 0.5 0 ≤ δ ≤ 0.2 and A is one or more of alkaline earth metals (particularly Ca and Sr), Y and rare earth metals, and B is one or more of Cr, Ni, Co, Al and Zn. A lanthanum manganite sintered body is typically made by mixing starting components e.g. oxides, calcining the mixture to generate the lanthanum manganite, grinding the calcined product to suitable powder size, shaping the powder and firing. One use of lanthanum manganite sintered bodies is in solid oxide fuel cells ("SOFCs"). SOFCs are promising power generation systems having higher efficiency and cleanliness. SOFCs are composed of air electrodes, fuel electrodes, solid electrolytes and interconnectors. For the air electrodes, porous sintered lanthanum manganite having high electroconductivity is used. There are two types of lanthanum manganite air electrode; one is a thin film electrode on a porous support tube made of, for example, calcia-stabilized zirconia, and the other is a porous lanthanum manganite tube electrode which itself is a support. The latter type is preferable because of simplicity of cell manufacturing process, which leads to low manufacturing cost. This type of air electrode is made by processes such as extrusion, slurry casting, injection molding and so on. Extrusion is especially convenient. After extrusion of a tube, an end closure of the tube may be added, if desired. The self-support type porous air electrode is required to have two conflicting features; high air permeation and high strength. Air permeation of the electrode is usually controlled by electrode porosity; the higher the electrode porosity, the higher the permeation which is obtained. However, higher porosity results in lower electrode strength, which may lead to a breakage of the SOFC system. The porosity of the air electrode is preferably 15 to 40%, more preferably 25 to 35%, when used in a SOFC system. However, when electrode porosity is set at 25 to 35%, the electrode strength is not always enough for handling, for manufacture of cells, or for operation. SUMMARY OF THE INVENTION An object of the present invention is therefore to provide a method of making porous sintered lanthanum manganite bodies which have high strength and are usable as self-support type air electrodes for SOFCs. The present inventors have found that a porous lanthanum manganite sintered body in which alkali metal (such as Li,Na,K) as an additive is rich near the grain boundaries has enhanced strength. The reason why this sintered body is strengthened is not clear, but we assume that the enriched alkali metal at the grain boundary may suppress diffusion of atoms and thus may inhibit grain growth of lanthanum manganite. According to this invention, there is provided a method of making a porous sintered lanthanum manganite body as set out in claim 1. In the sintered body produced, desirably the alkali metal content is at least 100 ppm, preferably at least 150 ppm. The maximum limit for the alkali metal content is not critical, provided that the properties of the lanthanum manganite are not impaired so as to be unsatisfactory, but preferably the alkali metal content is not more than 300 ppm. The alkali metal is typically one or more of Li, Na and K. In the method the sinterable material preferably has a content of alkali metal of at least 100 ppm, more preferably at least 300 ppm. Generally, the alkali metal content in the sinterable material is higher than in the sintered body, because alkali metal is lost during sintering. Suitably, the alkali metal content in the sinterable material is not more than 2000 ppm. Preferably the alkali metal is present in the sinterable material in the form of at least one oxide, hydroxide or salt of at least one of Li, Na and K. The invention also provides a sintered lanthanum manganite body made by this method, and an air electrode of a solid oxide fuel cell in the form of a porous sintered body of the invention. The sinterable material may contain at least one metal compound which becomes incorporated in the lanthanum manganite crystal structure on firing. This metal compound can suitably be a partially unreacted starting component of a mixture which has been calcined to generate the lanthanum manganite, or may be a component, such as La 2 O 3 or CaO, added to the lanthanum manganite after the formation thereof. The total amount of such a metal compound or compounds (when present) is preferably in the range 1 to 50%, more preferably 10-40%, by weight of the paste or body. Suitable methods of forming the shaped green body are those conventionally available, such as extrusion, slurry casting and dry powder compaction. The sinterable material typically also contains conventional additives as required, particularly binder and pore-former. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing a relationship between tube porosity and hoop strength obtained in Example 1. Figure 2 is photograph and a schematic diagram taken from the photograph of the porous sintered body of Example 1, Sample 2, giving the measured line profile of sodium content analyzed by EDS. EXAMPLES Examples of the invention and comparative examples are described below by way of illustration. The invention is not limited to these examples. EXAMPLE 1 Powders of La 2 O 3 , CaCO 3 and Mn 3 O 4 were mixed in the dry condition so that the molar ratio of La:Ca:Mn was equal to 0.8:0.2:1.0, then the mixture was calcined at 1600°C to form calcia-doped lanthanum manganite, La 0.8 Ca 0.2 MnO 3 . Samples of the calcined powder were ground in a ball mill in a wet condition to a median diameter ranging from 0.5 to 25 µm. These process steps of grinding and calcining were repeated three times to ensure homogeneity of the lanthanum manganite powders. To form lanthanum manganite pastes, PVA (polyvinylalcohol) as a binder, cellulose as a pore-former, and ammonium naphthalenesulfonate as a dispersant or an alkali metal salt were added to the ground lanthanum manganite powder. Water fraction of all pastes was adjusted to 11.0 wt%. These paste formulations are summarized in Table 1. Paste formulation in parts by weight Examples of the invention LCM median diameter (µm) PVA cellulose disp. A disp. S salt K salt L 1 100 10.2 3.0 3.0 - 0.5 - - 2 100 10.2 3.0 3.0 - 0.1 - - 3 100 10.2 3.0 3.0 - - 1.0 - 4 100 10.2 3.0 3.0 - - - 0.5 Compara tive Example 1 100 10.2 3.0 3.0 0.5 - - - The above described pastes were kneaded and extruded into cylindrical tubes of 20 mm in outer diameter, 2000 mm in length, 2 mm in wall thickness. Extruded tubes were dried and fired at 1550°C, 1600°C or 1620°C for 4 hr. Then tube porosity, and hoop strength were measured, as summarized in Table 2. Some tubes were cut to polish and alkaline metal contents across grain boundaris were qualitatively analyzed by EDS (Energy Dispersive X-ray Spectroscopy), as shown in Fig. 2, in which 1 indicates lanthanum manganite grains, 2 indicates a grain boundary and 3 a pore. Tube porosity and hoop strength Examples of the invention firing temp. (°C) porosity (%) hoop strength (MPa) 1-1 1550 33.1 46.3±8.4 1-2 1600 29.8 61.5±5.1 1-3 1620 27.8 67.9±4.3 2-1 1550 33.8 48.2±7.3 2-2 1600 30.5 60.2±4.8 2-3 1620 27.9 68.6±3.9 3-1 1550 32.4 44.1±8.6 3-2 1600 29.5 62.3±4.9 3-3 1620 28.2 65.1±4.2 4-1 1550 34.1 42.2±9.9 4-2 1600 30.9 60.5±6.3 4-3 1620 29.3 64.6±4.8 Comparative Examples 1-1 1550 30.1 42.3±10.8 1-2 1600 27.2 49.1±9.1 1-3 1620 25.8 55.6±6.5 EXAMPLE 2 Powders of La 2 O 3 , Y 2 O 3 , CaCO 3 and Mn 3 O 4 were mixed in dry condition so that molar ratio of La:Y:Ca:Mn was equal to 0.3:0.3:0.4:1.0, then the mixture was calcined at 1600°C to form the lanthanum-site-substituted lanthanum manganite, La 0.3 Y 0.3 Ca 0.4 MnO 3 . Further processing was as described in Example 1. Paste formulation, tube porosity and hoop strength are summarized in Tables 3 and 4. Paste formulation in parts by weight Examples of the invention LYCM median diameter (µm) PVA cellulose disp.A Na 2 CO 3 5 100 12.7 2.8 3.5 - 0.5 Comparative Example 2 100 12.7 2.8 3.5 0.5 - Tube porosity and hoop strength Examples of the invention firing temp. (°C) tube porosity (%) hoop strength (MPa) 5-1 1550 34.0 36.9±9.7 5-2 1600 31.2 44.6±7.3 5-3 1620 29.9 46.1±6.9 Comparative Examples 2-1 1550 33.6 27.9±13.8 2-2 1600 30.1 36.0±10.8 2-3 1620 28.9 38.3±11.6 EXAMPLE 3 Powders of La 2 O 3 , Nd 2 O 3 , CaCO 3 , Mn 3 O 4 and NiO were mixed in the dry condition so that the molar ratio of La:Nd:Ca:Mn:Ni was equal to 0.7:0.1:0.2:0.85:0.15, then the mixture was calcined at 1600°C to form the lanthanum manganite, La 0.7 Nd 0.1 Ca 0.2 Mn 0.85 Ni 0.15 O 3 . Further processing was as described in Example 1. Paste formulation, tube porosity and hoop strength are summarized in Tables 5 and 6. Paste formulation in parts by weight Examples of the invention LM median diameter (µm) PVA cellulose disp.A disp.S 6 100 8.5 3.0 2.7 - 0.5 Comparative Example 3 100 8.5 3.0 2.7 0.5 - Tube porosity and hoop strength Examples of the invention firing temp. (°C) tube porosity (%) hoop strength (MPa) 6-1 1550 33.6 47.8±7.2 6-2 1600 30.7 59.4±3.9 6-3 1620 28.4 67.9±4.4 Comparative Example 3-1 1550 32.6 38.3±8.7 3-2 1600 28.3 45.1±7.1 3-3 1620 25.4 60.6±5.9

Claims: A method of making a porous sintered lanthanum manganite body comprising the steps of forming a shaped green body of material which is sinterable to form porous sintered lanthanum manganite and firing the shaped green body to sinter it, wherein said sinterable material in dried form has a content of alkali metal of at least 100 ppm. A method according to claim 1 wherein said alkali metal content of said sinterable material is at least 300 ppm. A method according to claim 1 or 3 wherein said alkali metal content of said sinterable material is not more than 2000 ppm. A method according to any one of claims 1 to 3 wherein the alkali metal is present in said sinterable material in the form of at least one oxide, hydroxide or salt of at least one of Li, Na and K. A solid oxide fuel cell having an air electrode in the form of a body produced by a method according to any one of claims 1 to 4.

Inventors: ARAKI KIYOSHI, NISHIOKA MASAO, ARAKI, KIYOSHI, NISHIOKA, MASAO
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: C04B35
IPC codes: C04B38, H01M4, C04B35, H01M8