Freestanding thin film, method of manufacturing same, and electrochemical cell

Abstract

A freestanding thin film (1) contains a solid electrolyte membrane (10) constituted by apatitic compound oxides. The solid electrolyte membrane (10): has one or more oriented polycrystalline layers (11) in the membrane-thickness direction Z, in which oriented crystalline particles (C) of apatitic compound oxides are gathered with their grain boundaries (C1) contacting in the film-plane X and Y directions; has a membrane density d of 4.2 g / cm2 or greater; and has a c-axis orientation ratio with respect to the membrane-thickness Z direction Z of 0.9 or greater as a value calculated by the Lotgering method.

Description

Freestanding thin film, its manufacturing method and electrochemical cell CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on Patent Application No. 2024-112901, filed on July 12, 2024, the contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a free-standing thin film having an apatite structure, a method for producing the same, and an electrochemical cell.

[0003] Electrochemical cells used in fuel cells and the like have a single cell structure in which a cathode and an anode are arranged on either side of an electrolyte membrane. Known oxide ion conductors that serve as electrolyte membranes include zirconia-based oxides such as stabilized zirconia and perovskite-type oxides. Furthermore, apatite-type compounds, which have excellent oxide ion conductivity, have recently attracted attention. Apatite-type compounds have a hexagonal crystal structure, and it is expected that, for example, by imparting orientation to them, they can facilitate the movement of oxide ions and enhance ion conductivity.

[0004] Oriented apatite-type complex oxides include, for example, lanthanum silicate-based complex oxides, which are known to have high ionic conductivity in the c-axis direction and high electrical insulation. To improve ionic conductivity by using such materials in the electrolyte membrane of an electrochemical cell, it is desirable to thin the film and increase the c-axis orientation. For example, a method for thinning the film has been proposed in which an amorphous film that will become the oriented apatite-type complex oxide is formed by sputtering or the like, followed by heat treatment to produce a composite with a substrate.

[0005] Furthermore, Patent Document 1 discloses a method for producing an apatite-type lanthanum silicate thin film by forming a precursor thin film of the apatite-type lanthanum silicate on a substrate and then firing the film at a temperature of 800 to 1200° C. The precursor thin film is obtained by applying a raw material solution of the apatite-type lanthanum silicate by spin coating or the like, and then heating the applied solution at a temperature of 350 to 600° C., and the proportion of pores with a pore diameter of 16 nm or more is set to 1% (volume fraction) or less of the entire thin film so that the c-axes are sufficiently aligned during firing.

[0006] JP 2017-024931 A

[0007] In the method of Patent Document 1, the precursor thin film of apatite-type lanthanum silicate is formed on a substrate as a thin film of amorphous composite oxide particles, and it is preferable that the average particle size is 50 nm or less so as to reduce the proportion of large pores that inhibit crystal growth. However, if the average particle size is small, many grain boundaries will be formed in the thin film, which will easily become paths for electrical breakdown in the film thickness direction. In addition, as the substrate, a Si (100) substrate or a quartz glass substrate is used, which is easy to form a thin film with a flat surface, and is not a configuration suitable for an electrochemical cell.

[0008] Therefore, efforts are being made to develop freestanding electrolyte membranes for use in electrochemical cells that are not supported by a substrate. However, methods using plate-shaped single crystals or polycrystals have limitations on how thin the membrane can be. In contrast, there is a method in which an electrolyte membrane is formed on a substrate via a release layer, and then peeled off after heat treatment. While this method is effective in thinning the membrane, it does not provide sufficient c-axis orientation. Therefore, it is desirable to improve the membrane properties of freestanding thin films and realize electrochemical cells that fully utilize the properties of apatite-type complex oxides, which have high ionic conductivity in the c-axis direction.

[0009] An object of the present disclosure is to provide a free-standing thin film made of a composite oxide having an apatite structure, which is capable of achieving both thinning and improved ionic conductivity, a method for producing the same, and an electrochemical cell.

[0010] One aspect of the present disclosure is a self-supporting thin film including a solid electrolyte membrane made of an apatite-type complex oxide, wherein the solid electrolyte membrane has one or more oriented polycrystalline layers in a film thickness direction, in which oriented crystal grains of the apatite-type complex oxide are aggregated with their grain boundaries in contact with each other in a film plane direction, and the film density d is 4.2 g / cm 2 The free-standing thin film has a c-axis orientation rate in the film thickness direction of 0.9 or more as calculated by the Lotgering method.

[0011] Another aspect of the present disclosure is a method for producing a free-standing thin film having the above configuration, comprising: a release layer forming step of forming a release layer on a surface of a substrate, the release layer being made of a composition that can be removed after crystallization of the apatite-type complex oxide; an oxide layer forming step of forming an oxide layer made of an oxide or complex oxide having oxygen absorbing and releasing properties on the surface of the release layer; a solid electrolyte film forming step of forming an amorphous complex oxide film having the same composition as the apatite-type complex oxide on the surface of the oxide layer to a thickness of 200 nm or less, and heat-treating the film to form a first polycrystalline layer, which is an oriented polycrystalline layer; a second solid electrolyte film forming step of forming an amorphous complex oxide film having the same composition as the apatite-type complex oxide on the surface of the first polycrystalline layer, and heat-treating the film to form a second polycrystalline layer, which is an oriented polycrystalline layer; and a peeling step of removing the release layer to peel the solid electrolyte film from the substrate.

[0012] Yet another aspect of the present disclosure is a method for producing a free-standing thin film having the above configuration, comprising: a release layer forming step of forming, on the surface of a substrate, a release layer made of a composition that can be removed after crystallization of the apatite-type complex oxide; an oxide layer forming step of forming, on the surface of the release layer, an oxide layer made of an oxide or complex oxide that has oxygen absorbing and releasing properties and whose integral multiple of the lattice constant is within ±5% of the lattice constant of the apatite-type complex oxide; a solid electrolyte film forming step of forming, on the surface of the oxide layer, an amorphous complex oxide film having the same composition as the apatite-type complex oxide and heat-treating the film to form the oriented polycrystalline layer; and a peeling step of removing the release layer to peel off the solid electrolyte film from the substrate.

[0013] Yet another aspect of the present disclosure is an electrochemical cell comprising a solid electrolyte membrane made of an apatite-type composite oxide, a cathode, and an anode, wherein the solid electrolyte membrane has one or more oriented polycrystalline layers in the thickness direction, in which oriented crystal grains of the apatite-type composite oxide are aggregated with their grain boundaries in contact in the film plane direction, and the film density d is 4.2 g / cm 2or more, a c-axis orientation rate in the film thickness direction is 0.9 or more as calculated by the Lotgering method, the cathode and the anode are a pair of porous electrodes arranged between two opposing film surfaces of the solid electrolyte membrane, with an oxide layer made of an oxide or composite oxide having oxygen absorbing / releasing properties interposed therebetween, the thickness of the oxide layer is 20 nm or more and 100 nm or less, and the film thickness of the solid electrolyte membrane is 0.2 μm or more and 10 μm or less.

[0014] The self-supporting thin film having the above-mentioned structure has one or more oriented polycrystalline layers of an apatite structure that become a solid electrolyte film, and has a film density d of 4.2 g / cm 2 The film is dense as described above, and has a c-axis orientation rate of 0.9 or more as measured by the Lotgering method. The reason why such high orientation is achieved is not entirely clear, but it is presumed that the high film density d of the solid electrolyte film including the oriented polycrystalline layer suppresses the introduction of defects during crystallization of the apatite-type composite oxide and makes it easier to align the crystal orientation, contributing to the alignment of the crystal grains. This results in an oriented polycrystalline layer in which crystal grains are aggregated with a higher c-axis orientation than conventional films, making it possible to improve the ionic conductivity of the solid electrolyte film.

[0015] Such a freestanding thin film can be obtained by forming a release layer on a substrate in advance, the release layer being made of a composition that can be removed after the formation of the solid electrolyte film. After forming an oriented polycrystalline layer made of an apatite-type complex oxide that will become the solid electrolyte film, the freestanding thin film can be peeled off from the substrate. In this case, forming an oxide layer having oxygen absorbing / releasing properties integrally with the oriented polycrystalline layer facilitates the fabrication of an electrochemical cell. Furthermore, a solid electrolyte film with high c-axis orientation can be formed by forming two or more oriented polycrystalline layers, with the first layer being an amorphous complex oxide film formed to a thickness of 200 nm or less and crystallizing it by heat treatment, followed by the formation of the second layer.

[0016] Alternatively, by forming an oxide layer having oxygen absorbing / releasing properties from an oxide or composite oxide that matches the lattice constant of the apatite-type composite oxide, it becomes possible to enhance the c-axis orientation of the oriented polycrystalline layer formed thereon. In this case, there is no limitation on the thickness of the oriented polycrystalline layer, and it becomes possible to form a solid electrolyte membrane including a single oriented polycrystalline layer having a thickness of, for example, 200 nm or more.

[0017] Such a freestanding thin film can be combined with optional porous electrodes serving as cathodes and anodes to form an electrochemical cell of a desired configuration. In this case, by disposing an oxide layer having oxygen absorbing and releasing properties between the solid electrolyte membrane and the porous electrodes, it is possible to reduce the ionic conduction resistance at the interface between the solid electrolyte membrane and the porous electrodes. Therefore, by making the solid electrolyte membrane have a predetermined thickness that achieves the desired membrane strength and ionic conductivity, and by making the oxide layer have a thinner predetermined thickness, it is possible to further improve the performance of the electrochemical cell.

[0018] As described above, according to the above aspect, it is possible to provide a free-standing thin film made of a composite oxide having an apatite structure, which can achieve both a thin layer and improved ionic conductivity, a method for manufacturing the same, and an electrochemical cell.

[0019] The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which Fig. 1 is a general perspective view schematically illustrating an example of the configuration of a freestanding thin film in embodiment 1, and a partially enlarged view thereof, Fig. 2 is a general perspective view schematically illustrating another example of the configuration of the freestanding thin film in embodiment 1, Fig. 3 is a general perspective view schematically illustrating another example of the configuration of the freestanding thin film in embodiment 1, Fig. 4 is a general perspective view schematically illustrating the configuration of an electrochemical cell using the freestanding thin film in embodiment 1, and Fig. 5 is a diagram illustrating a method for manufacturing the freestanding thin film in embodiment 1. 6 is a schematic process diagram for explaining a method for manufacturing a freestanding thin film in embodiment 1; FIG. 7 is a schematic cross-sectional view for explaining a freestanding thin film constituting an electrochemical cell and a method for manufacturing the same in embodiment 2; FIG. 8 is an overall perspective view showing a schematic configuration of a freestanding thin film in Example 1; FIG. 9 is a diagram showing a comparison of the film configurations and observation images of the film surfaces in Example 1 and Comparative Example 2; and FIG. 10 is a diagram showing a comparison of observation images of the film cross sections in Example 1 and Comparative Example 2.

[0020] (Embodiment 1) An embodiment relating to a free-standing thin film, a method for manufacturing the same, and an electrochemical cell using the free-standing thin film will be described with reference to the drawings. As shown schematically in Figures 1 and 2, in this embodiment, the free-standing thin film 1 is a free-standing film including a solid electrolyte film 10 made of an apatite-type complex oxide. The solid electrolyte film 10 has one or more oriented polycrystalline layers 11 containing oriented crystalline particles C of an apatite-type complex oxide.

[0021] 1 and 2, the oriented polycrystalline layer 11 constituting the solid electrolyte membrane 10 is a dense, thin-plate-like polycrystalline body in which numerous oriented crystalline grains C are continuously aggregated with their grain boundaries C1 in contact with each other in a film plane direction (e.g., the X-axis or Y-axis direction shown in FIG. 1) perpendicular to the film thickness direction Z. The oriented crystalline grains C are particles of a composite oxide having a known apatite-type crystal structure, and the c-axis of the hexagonal apatite is oriented along the film thickness direction Z. The c-axis direction of the oriented crystalline grains C is indicated by a dotted arrow in the enlarged view of FIG. 1.

[0022] In the oriented polycrystalline layer 11, each of the numerous oriented crystal grains C extends in a columnar shape with a relatively large particle diameter between one surface (e.g., the lower surface shown in FIG. 1 ) 101 of the solid electrolyte membrane 10 and the other surface (e.g., the upper surface shown in FIG. 1 ) 102. In the film plane direction, a grain boundary C1 extending in the film thickness direction Z is formed between adjacent particles, and the numerous oriented crystal grains C are oriented in the film thickness direction Z and closely aligned with one another. This results in the solid electrolyte membrane 10 being a dense membrane with few defects and having a high film density d.

[0023] The solid electrolyte membrane 10 has a membrane density d of 4.2 g / cm 2 The film density d is 4.2 g / cm or more. 2 A dense film with a film density of 4.2 g / cm or more can provide a high c-axis orientation rate. 2 If the ratio is less than 1 / 2, the average particle size becomes small and the orientation in the film thickness direction Z tends to decrease, making it difficult to obtain the desired c-axis orientation rate. Such a configuration can be achieved by using a predetermined material or undergoing a predetermined process when forming the solid electrolyte membrane 10 on the substrate via a release layer. The method for manufacturing the solid electrolyte membrane 10 will be described in detail below.

[0024] Furthermore, the solid electrolyte membrane 10 has a c-axis orientation ratio in the film thickness direction Z of 0.9 or more as calculated by the Lotgering method. Here, the c-axis orientation ratio is defined as the ratio of crystal grains having c-axis orientation to all of the crystal grains constituting the solid electrolyte membrane 10. The c-axis orientation ratio can be calculated, for example, using the known Lotgering equation from the peak intensity in an X-ray diffraction pattern obtained by an X-ray diffractometer.

[0025] That is, when the c-axis orientation rate is 0.9 or more, almost the entire area of ​​the solid electrolyte membrane 10 is composed of oriented crystal grains C whose c-axes are aligned along the film thickness direction Z, forming an oriented polycrystalline layer 11 with little deviation from the film thickness direction Z. In this case, the C-plane of the apatite is easily exposed as the film surfaces 101, 102 of the solid electrolyte membrane 10, which is preferable from the viewpoint of improving oxide ion conductivity and film strength.

[0026] The term "freestanding membrane" refers to a freestanding thin film 1 in which two membrane surfaces 101 and 102 opposing each other in the membrane thickness direction Z can exist without being supported by a substrate or the like. The two membrane surfaces 101 and 102 in Figures 1 and 2 are exposed surfaces of the solid electrolyte membrane 10.

[0027] The detailed structure of the self-supporting thin film 1 in this embodiment will be described below. The self-supporting thin film 1 is mainly composed of a solid electrolyte film 10 made of an apatite-type complex oxide. The solid electrolyte film 10 constituting the self-supporting thin film 1 may have a single-layer structure with one oriented polycrystalline layer 11, or may have a multi-layer structure with two or three or more oriented polycrystalline layers 11 stacked together.

[0028] 1, the oriented polycrystalline layer 11 has a single-layer structure. In this case, two particle surfaces of the oriented crystalline grains C in the film thickness direction Z are exposed, and adjacent particle surfaces are connected to form two film surfaces 101 and 102.

[0029] In the configuration example shown in FIG. 2 , the solid electrolyte membrane 10 has a multilayer structure with two oriented polycrystalline layers 11. In this case, two oriented polycrystalline layers 11 (11A, 11B) are stacked in the thickness direction Z of the solid electrolyte membrane 10 (i.e., the Z-axis direction shown in FIG. 2 ). The first polycrystalline layer 11A and the second polycrystalline layer 11B have substantially the same crystal shape and crystal orientation at the layer interface 103. That is, the oriented crystalline grains C constituting the two oriented polycrystalline layers 11 are continuous in the thickness direction Z and are integrally formed with the same structure across the layer interface 103. The exposed surfaces of the first and second polycrystalline layers 11A and 11B located on the opposite side of the layer interface 103 form the two film surfaces 101 and 102 of the solid electrolyte membrane 10.

[0030] The apatite-type complex oxide constituting the solid electrolyte membrane 10 has oxide ion conductivity. The apatite-type complex oxide has oxide ions (O 2-) has a tendency to conduct ions easily, which is advantageous when using the freestanding thin film 1 as an oxide ion conductor. The a-axis and b-axis of apatite lie in a plane perpendicular to the c-axis. The oriented polycrystalline layer 11, which is an aggregation of such oriented crystal grains C, is formed so that the grain boundaries C1 are aligned along the film thickness direction Z, and are evenly distributed across the entire film plane.

[0031] 3, the free-standing thin film 1 may have a structure in which an oxide layer 2 is laminated on at least one side of an oxide-ion conductive solid electrolyte membrane 10 in the thickness direction Z. The oxide layer 2 may be a layer made of an oxide or composite oxide having oxygen absorbing / releasing properties, and is preferably disposed on both sides of the solid electrolyte membrane 10. In this case, the solid electrolyte membrane 10 is sandwiched between a pair of a first oxide layer 2A and a second oxide layer 2B, resulting in a free-standing structure in which neither side is supported by a substrate or the like.

[0032] 3 show an example in which the oriented polycrystalline layer 11 constituting the solid electrolyte membrane 10 is a single layer (see FIG. 1) and a double layer (see FIG. 2), respectively, and have the same configuration in which a pair of oxide layers 2 are disposed on either side of the solid electrolyte membrane 10. In these configuration examples, one or the other of the first oxide layer 2A and the second oxide layer 2B is disposed adjacent to one or the other of the two end faces 104 of the solid electrolyte membrane 10, and their exposed surfaces form the two membrane surfaces 101, 102 of the free-standing thin film 1.

[0033] In this way, the freestanding thin film 1 can be pre-fabricated with a desired laminated film structure depending on the intended use. This configuration is advantageous, for example, when the freestanding thin film 1 is used as an oxide ion conductor in the electrochemical cell 1C shown in Fig. 4, and the presence of the oxide layer 2 between the solid electrolyte membrane 10 and the electrode materials that will become the cathode 3 and anode 4 can improve the cell output.

[0034] Although the thickness of the freestanding thin film 1 is not necessarily limited, the thickness t of the solid electrolyte membrane 10 made of the oriented polycrystalline layer 11 is, for example, 0.2 μm or more, so that the freestanding thin film can be maintained. Furthermore, the thickness t is, for example, 10.0 μm or less, so that the migration distance of oxide ions can be shortened and ion conductivity can be increased while maintaining the desired film strength. Preferably, the thickness t can be appropriately set within the range of 1.0 μm or more and 5.0 μm or less so that the desired film strength and ion conductivity can be obtained.

[0035] The film thickness t is the thickness of the entire film constituting the solid electrolyte membrane 10, i.e., in the case of a single-layer film, it is the thickness of one of the oriented polycrystalline layers 11, and in the case of a multi-layer film, it is the total thickness of the multiple oriented polycrystalline layers 11. The film thickness t can be calculated as the average film thickness based on cross-sectional observation using a transmission electron microscope (TEM), for example.

[0036] The average grain size of the solid electrolyte membrane 10 is not necessarily limited, but may be, for example, 300 nm or more, and preferably about 400 nm or more. The average grain size can be calculated as the average grain size (D50) of oriented crystal grains C observed on the membrane surfaces 101 and 102 of the solid electrolyte membrane 10. As the average grain size increases, the grain boundaries C1, which contribute little to ionic conductivity in the c-axis direction, decrease, making it easier to obtain desired cell characteristics. More preferably, the average grain size can be appropriately set within a range of 400 nm to 700 nm.

[0037] In the free-standing thin film 1, the membrane area S of the solid electrolyte membrane 10 is, for example, 1.0 cm 2 Preferably, the membrane area S is 20.0 cm or more. 2 More preferably, 25.0 cm 2 The upper limit of the membrane area S is not particularly limited, and the membrane area S can be obtained according to the substrate area. Therefore, the membrane area S can be appropriately set depending on the application, etc., but from the viewpoint of manufacturing, for example, 500 cm 2The membrane area S is preferably in the range of about 200 cm or less. 2 The film area S can be calculated based on the results of measuring the planar shape with a laser microscope using laser light, for example.

[0038] As the composite oxide having an apatite structure constituting the solid electrolyte membrane 10, an apatite composite oxide represented by the following formula 1 can be preferably used. Formula 1: A 10-x B 6-y M y O 27-z In formula 1, A represents one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr, and Ba; B represents an element containing Si or Ge, or both; M represents one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo; and x, y, and z represent numbers satisfying -0.5≦x≦2.0, 0.0≦y≦3.0, and -6.0≦z≦4.2, respectively.

[0039] The element A in formula 1 is a lanthanoid or alkaline earth metal that can form positively charged ions and form an apatite-type hexagonal crystal structure. For example, from the viewpoint of further increasing oxide ion conductivity, it is preferably a combination with one or more elements selected from the group consisting of La, Ce, Nd, Ca, Sr, and Ba, and particularly preferably a combination of one element of La or Nd, or La with one or more elements selected from the group consisting of Ce, Nd, Ca, Sr, and Ba. Furthermore, the element B in formula 1 may be an element containing Si, Ge, or both.

[0040] The M element in formula 1 is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo. For example, from the viewpoint of increasing oxide ion conductivity, it is preferable to use Al, Ti, Y, Zr, B, and the like.

[0041] In formula 1, x is preferably -0.5 or more and 2.0 or less from the viewpoint of increasing the degree of orientation and oxygen ion conductivity. In formula 1, y is preferably 0.0 or more and 3.0 or less from the viewpoint of filling the B element position in the apatite-type crystal lattice. In formula 1, z is preferably -6.0 or more and 4.2 or less from the viewpoint of maintaining electrical neutrality in the apatite-type crystal lattice.

[0042] As the apatite-type composite oxide represented by the above formula 1, for example, a rare earth silicate-based composite oxide in which the element A in formula 1 is a rare earth element and the element B in formula 1 is Si can be used. Examples of rare earth silicate-based composite oxides include La 9.33 Si 6.0 O 26 , La 9.33 Si 5.3 B 0.7 O 25.65 , La 9.33 Si 5.3 Ti 0.7 O 26 , La 9.33 Si 5.3 Al 0.7 O 25.65 , La 9.33 Si 5.3 Y 0.7 O 25.65 , La 9.33 Si 5.3 Zr 0.7 O 26 , La 9.00 Ce 0.33 Si 6.0 O 26 However, the present invention is not limited to these.

[0043] For example, an oxide or composite oxide containing one or more elements including a transition metal element is used as the oxide or composite oxide having oxygen absorbing / releasing properties that constitutes the oxide layer 2. Examples of the transition metal element include elements such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, and Mo, and lanthanoid elements, such as Ce, Pr, and Gd.

[0044] In Fig. 3 (upper or lower diagram), the first oxide layer 2A on one side of the solid electrolyte membrane 10 and the second oxide layer 2B on the other side can be arbitrarily selected from oxides or composite oxides having oxygen absorbing / releasing properties. The first oxide layer 2A and the second oxide layer 2B may be the same oxide or composite oxide or different oxides. Specifically, CeO, PrO 11 , TiO2, VO5, Cr2O3, Mn2O3, Fe2O3, Co2O3, NiO, ZnO, ZrO2, Nb2O5, MoO2, and compounds thereof can be used. Furthermore, these oxides may be partially substituted with a dopant element, and for example, ceria-based or zirconia-based oxides such as Gd-doped CeO2 and Y-doped ZrO2 can be used.

[0045] As shown in Fig. 5 (upper diagram), when manufacturing a free-standing thin film 1 having such a structure, a method can be employed in which a release layer 300 for releasing the free-standing thin film 1 is formed on the substrate 200 in advance, and then, as shown in Fig. 5 (lower diagram), the free-standing thin film 1 is formed and then separated from the substrate 200. When the free-standing thin film 1 includes an oxide layer 2, an oxide layer 2 (e.g., a first oxide layer 2A) can be formed on the surface of the release layer 300, and a solid electrolyte membrane 10 made of an apatite-type complex oxide can be further formed on the surface of the oxide layer 2 (e.g., a first oxide layer 2A).

[0046] As shown in the figure, the solid electrolyte membrane 10 is a multilayer membrane, and the oriented polycrystalline layer 11 (e.g., the first polycrystalline layer 11A) adjacent to the first oxide layer 2A formed on the release layer 300 preferably has a thickness t1 of 200 nm (0.2 μm) or less. By setting the thickness t1 to 200 nm or less, the oriented polycrystalline layer 11 in contact with the first oxide layer 2A can be a high-quality layer with a high c-axis orientation.

[0047] In this case, the oriented polycrystalline layer 11 (e.g., the second polycrystalline layer 11B) provided adjacent to and integral with the first polycrystalline layer 11A in the thickness direction Z can also be formed as a layer of similar high quality. The thickness t2 of the second polycrystalline layer 11B can be appropriately set so that the entire solid electrolyte membrane 10 has the desired thickness t.

[0048] Preferably, the thickness t1 of the first polycrystalline layer 11A and the thickness t2 of the second polycrystalline layer 11B can be selected so that the ratio t2 / t1 therebetween is within a range of 1 to 50. When the ratio t2 / t1 is 1 or greater, it is possible to obtain a membrane with desired characteristics by increasing the membrane strength while maintaining the membrane density d and c-axis orientation of the solid electrolyte membrane 10. Furthermore, when the ratio t2 / t1 is 50 or less, it is possible to obtain a membrane with stable quality with good manufacturability while suppressing an increase in the ionic conduction resistance of the solid electrolyte membrane 10 itself.

[0049] Specifically, in FIG. 5 (upper diagram), the process includes the following steps (1) to (4): (1) forming a release layer 300 made of a composition that can be removed after crystallization of the apatite-type complex oxide on the surface of the substrate 200 (release layer forming step); (2) forming an oxide layer 2 (first oxide layer 2A) made of an oxide or complex oxide having oxygen absorbing / releasing properties on the surface of the release layer 300 (oxide layer forming step); (31) forming an amorphous complex oxide film 100 (first amorphous complex oxide film 100A) having the same composition as the apatite-type complex oxide on the surface of the oxide layer 2 (first oxide layer 2A) to a thickness of 200 nm or less, and heat-treating the film to form a first polycrystalline layer 11A, which is an oriented polycrystalline layer 11 (first solid electrolyte film forming step); (32) A step of forming an amorphous complex oxide film 100 (second amorphous complex oxide film 100B) having the same composition as the apatite-type complex oxide on the surface of the first polycrystalline layer 11A, followed by heat treatment to form a second polycrystalline layer 11B, which is an oriented polycrystalline layer 11 (second solid electrolyte film forming step), and (4) A step of removing the release layer 300 to peel the solid electrolyte film 10 from the substrate 200 (peeling step), thereby obtaining a laminated film of the first oxide layer 2A and the solid electrolyte film 10, as shown in Figure 5 (lower diagram). Furthermore, a second oxide layer 2B is formed on the opposite side of the solid electrolyte film 10 from the first oxide layer 2A, thereby producing a free-standing thin film 1 having the configuration shown in Figure 3 (lower diagram).

[0050] In this configuration in which the free-standing thin film 1 includes the oxide layers 2 on both sides in the film thickness direction Z, it is desirable to form the solid electrolyte membrane 10 as a multilayer membrane in which two oriented polycrystalline layers 11 (11A, 11B) are stacked, and then perform stepwise film formation and heat treatment. That is, after forming the release layer 300 and the oxide layer 2 on the substrate 200 in steps (1) and (2), it is preferable to form the first amorphous complex oxide membrane 100A, which will become the first polycrystalline layer 11A, to a predetermined thickness in step (31), followed by heat treatment. Thereafter, in step (32), the second amorphous complex oxide membrane 100B, which will become the second polycrystalline layer 11B, is formed and heat treated, thereby forming a high-quality oriented polycrystalline layer 11 and obtaining a solid electrolyte membrane 10 having a desired film thickness t.

[0051] Alternatively, when the free-standing thin film 1 does not have the oxide layer 2 (see FIG. 1), as shown in FIG. 6 (upper diagram), a release layer 300 can be formed in advance on the substrate 200, and an amorphous complex oxide film 100 to become the solid electrolyte film 10 can be formed as a single layer on the surface of the release layer 300. In this case, too, as shown in FIG. 6 (lower diagram), after the solid electrolyte film 10 is formed, the release layer 300 can be removed, thereby separating the solid electrolyte film 10 from the substrate 200 to obtain the free-standing thin film 1.

[0052] In this configuration, by appropriately selecting the release layer 300, the oriented polycrystalline layer 11 that becomes the solid electrolyte membrane 10 can be formed with good orientation. Therefore, the free-standing thin film 1 made of the solid electrolyte membrane 10 can be efficiently formed as a single layer membrane with a predetermined thickness t.

[0053] Furthermore, as shown in Fig. 3 (top diagram), in a configuration in which the free-standing thin film 1 includes the oxide layer 2, the solid electrolyte membrane 10 can be a single-layer membrane. In that case, the thickness t of the solid electrolyte membrane 10 disposed between the first oxide layer 2A and the second oxide layer 2B is desirably 0.2 µm (200 nm) or less. That is, the configuration is the same as that in the free-standing thin film 1 of Fig. 3 (bottom diagram), in which the solid electrolyte membrane 10 consists only of the first polycrystalline layer 11A.

[0054] A freestanding thin film 1 having such a configuration can be formed by performing only step (31) for forming the first polycrystalline layer 11A among the steps (1) to (4) described above, without performing step (32). The other steps can be performed in the same manner. That is, steps (1) and (2) form a peeling layer 300 and an oxide layer 2 on the substrate 200, and step (31) forms an amorphous complex oxide film 100 to a predetermined thickness that will become the oriented polycrystalline layer 11. Thereafter, step (4) is performed to separate the film from the substrate 200.

[0055] Next, each of the steps (1) to (4) above will be described in detail. In the peeling layer forming step (1), the material of the substrate 200 is not particularly limited, and examples thereof include metal materials such as Si and Pt, and oxide materials such as sapphire (Al2O3) and ZrO. It is preferable to use a substrate 200 made of an oxide material from the viewpoint of suppressing cracks and the like that occur due to differences in thermal expansion coefficients during heat treatment.

[0056] The composition that becomes the peeling layer 300 may be any composition that can be removed after the crystallization of the apatite-type composite oxide to separate the solid electrolyte film 10 from the substrate 200, and may be, for example, a sublimable composition or a water-soluble composition. In the former case, by using a composition with a high vapor pressure that can be removed by sublimation during the heat treatment in the solid electrolyte film formation step (3), the apatite-structured solid electrolyte film 10 can be peeled off from the substrate 200 after the heat treatment. In the latter case, the composition may be any composition that can be dissolved in water and removed after the heat treatment.

[0057] Examples of compositions with high vapor pressure include bismuth oxide, zinc oxide, boron oxide, sulfur, and phosphorus. From the viewpoint of reducing element diffusion and internal stress during heat treatment of apatite, it is preferable to use an oxide composition. Also, from the same viewpoint, it is preferable to use an oxide composition for water-soluble compositions. Examples of oxide compositions with high solubility in water include strontium aluminate and strontium chromate, and specifically, SrAlO.

[0058] Preferably, the composition that becomes the release layer 300 is an oxide or composite oxide that has a hexagonal crystal structure that extends in the plane direction of the film to be formed and that is oriented in the film thickness direction Z. In this case, if the substrate 200 is a hexagonal c-axis oriented substrate, such as a sapphire (AlO) substrate with a C-plane as its principal surface, and the release layer 300 is made of zinc oxide (ZnO), a c-axis oriented hexagonal material, the orientation of the release layer 300 is more likely to be aligned. This also works advantageously when forming a solid electrolyte membrane 10 with c-axis orientation on top of the release layer 300.

[0059] As the substrate 200, in addition to a hexagonal c-axis oriented substrate, a substrate having a tetragonal (111) plane, for example, a substrate having a SrTiO3 (111) plane as the main surface, can also be used.

[0060] The apatite-type composite oxide constituting the solid electrolyte membrane 10 is, for example, the above-mentioned rare earth silicate-based composite oxide (La 9.33 Si 6.0 O 26 (hereinafter referred to as LSO where appropriate), it is desirable to dispose a composition with a matching lattice constant underneath. Preferably, the integral multiple of the lattice constant is within a range of ±5% of the lattice constant of LSO (9.82 angstroms), for example, and ZnO (lattice constant: 3.20 angstroms), which is a sublimable oxide, is used as the release layer 300.

[0061] In this way, by forming the c-axis oriented release layer 300 of a composition whose lattice constant is close to that of LSO, a periodic structure that easily induces c-axis oriented LSO is formed even in a configuration in which an oxide layer 2 with low lattice constant matching (hereinafter referred to as lattice matching as appropriate) is interposed between the solid electrolyte membrane 10. Therefore, for example, a plane close to a hexagonal crystal is easily formed or the lattice constant is likely to be a value close to that of LSO, which is thought to contribute to an improvement in the c-axis orientation rate of the solid electrolyte membrane 10.

[0062] Examples of methods for forming the release layer 300 include atomic layer deposition, ion plating, pulsed laser deposition, plating, sputtering, and vapor deposition. Any film-forming method can also be used, such as screen printing or spin coating of an amorphous oxide prepared by a wet method such as coprecipitation or sol-gel. From the viewpoints of film quality and productivity, it is preferable to select the sputtering method.

[0063] The film formation by sputtering can be performed using an existing sputtering device, and a target made of a composition that forms the release layer 300 and the substrate 200 are placed opposite each other to perform sputtering, thereby forming a film made of a desired composition on the substrate 200. Alternatively, the substrate 200 may be preheated and sputtered while being maintained at a predetermined temperature in the range of 300° C. or higher and 800° C. or lower.

[0064] The thickness of the release layer 300 is not necessarily limited, but can be appropriately selected, for example, in the range of 200 nm to 500 nm. If the release layer 300 of the substrate 200 is 200 nm or more, the entire film plane can be easily peeled off in step (4), facilitating separation of the freestanding thin film 1 from the substrate 200. If the release layer 300 is 500 nm or less, unevenness in the film plane is suppressed, and when film formation is performed in the subsequent steps, the LSO thin film that becomes the solid electrolyte film 10 can be easily obtained as a high-density c-axis oriented film.

[0065] In the oxide layer forming step (2), the oxide layer 2 (first oxide layer 2A) can be formed by the same method as the release layer 300. Preferably, the oxide layer 2 (first oxide layer 2A) can be formed on the substrate 200 by a sputtering method using a target made of a composition for forming the oxide layer 2. For example, by using a sapphire substrate as the substrate 200 and sequentially forming the lattice-matched release layer 300 and the oxide layer 2 on the C-plane, which is the main surface of the substrate 200, it becomes possible to enhance the c-axis orientation of the solid electrolyte film 10 formed thereon.

[0066] The thickness of the oxide layer 2 is not necessarily limited, but can be selected, for example, in the range of 20 nm to 100 nm. If the thickness of the oxide layer 2 is less than 20 nm, when applied to the electrochemical cell 1C, there is a risk that the effect of reducing the ionic conduction resistance at the interface between the solid electrolyte membrane 10 and the electrode will vary. Furthermore, if the thickness exceeds 100 nm, the resistance of the oxide layer 2 itself will likely increase, which may affect the cell characteristics.

[0067] In the first solid electrolyte film forming step (31), the first amorphous complex oxide film 100A can be formed by the same method as that for the release layer 300 and the oxide layer 2, and preferably by sputtering. Specifically, the first amorphous complex oxide film 100A can be formed by sputtering using a target made of an apatite-type complex oxide on the surface layer of the first oxide layer 2A formed on the substrate 200. The composition of the complex oxide (sintered body) used as the sputtering target can be the same as that of the target apatite-type complex oxide.

[0068] The thickness of the first amorphous complex oxide film 100A formed in step (31) corresponds to the film thickness t1 of the first polycrystalline layer 12A and can be a predetermined thickness of 200 nm or less. The lower limit of the thickness of the first amorphous complex oxide film 100A is not necessarily limited, but is, for example, 20 nm or more. A high-density c-axis oriented thin film can be obtained by forming a high-density thin film that covers the entire peeling layer 300 and then crystallizing it by heat treatment.

[0069] Next, in the second solid electrolyte membrane formation step (32), a second amorphous complex oxide membrane 100B is formed in a similar manner. The thickness of the second amorphous complex oxide membrane 100B formed in this step corresponds to the membrane thickness t2 of the second polycrystalline layer 12B and can be a predetermined thickness of, for example, 800 nm or more. As a result, the second amorphous complex oxide membrane 100B that will become the second polycrystalline layer 12B is formed on the first polycrystalline layer 12A, and by further heat treatment, a high-quality solid electrolyte membrane 10 having a desired thickness of 1 μm or more can be obtained without reducing the c-axis orientation.

[0070] When the freestanding thin film 1 has oxide layers 2 on both sides of the solid electrolyte membrane 10, it is desirable to further form the second oxide layer 2B after forming the second amorphous complex oxide film 100B that will become the second polycrystalline layer 12B. The second oxide layer 2B can be formed in the same manner as the first oxide layer 2A. Furthermore, when the freestanding thin film 1 is applied to an electrochemical cell 1C, an electrode material that will become the cathode 3 or anode 4 can be further formed, followed by heat treatment to crystallize the second amorphous complex oxide film 100B.

[0071] The second oxide layer 2B formed on the second amorphous complex oxide film 100B may be made of the same material as or a different material from the first oxide layer 2A, and can be appropriately selected from the oxides or complex oxides having oxygen absorbing and releasing properties described above. The second oxide layer 2B can be formed in the same manner as the first oxide layer 2A.

[0072] When sputtering is employed in steps (1) to (31) and (32), radio frequency (RF) sputtering is preferably used. This allows for the use of a high-resistance oxide target. Specifically, a replacement gas is introduced into a vacuum chamber in which a sputtering target and a substrate 200 (opposing electrode) are placed, and the chamber is maintained at a predetermined pressure. In this state, a radio frequency voltage is applied between the electrodes to perform plasma processing. The degree of vacuum in the vacuum chamber is, for example, 1×10 -5 1x10 or more -2 The pressure for the plasma treatment can be in the range of 0.5 Pa or more and 30 Pa or less. Inert gases such as Ar, He, Ne, Kr, Xe, and Rn are used as replacement gases, and reactive sputtering can also be performed using gases such as O and N.

[0073] Preferably, in order to improve the film density, it is preferable to perform film formation while applying reverse sputtering power to the substrate side. Furthermore, it is preferable to introduce O2 gas into the chamber during film formation. By introducing O2 gas, the occurrence of large structural defects such as voids in the film can be suppressed, and a more suitable film can be formed when applied to highly active cells used in, for example, solid oxide fuel cells (SOFCs). The proportion of O2 gas in the atmosphere is preferably selected within the range of 5% to 20%. If the O2 gas content is less than 5%, the effect of suppressing the occurrence of voids may be insufficient. If the O2 gas content is higher than 20%, the rate of formation of the amorphous complex oxide film 100 on the substrate 200 may be slow, potentially reducing production efficiency.

[0074] Here, the output of the reverse sputtering can be, for example, 10 W or more and 50 W or less. It is believed that reverse sputtering not only forms a film of the composition by sputtering, but also removes molecules with low adhesive strength, which are thought to be the cause of reduced film density, from the film formation site. If the output of the reverse sputtering is too weak, it is difficult to achieve the effect of removing molecules with low adhesive strength. If the output of the reverse sputtering is too strong, most of the molecules formed in the film are removed, slowing the film formation rate and simultaneously releasing molecules from the inside of the film, making it more likely to leave defects such as voids. More preferably, the output of the reverse sputtering is appropriately selected from the range of 20 W or more and 40 W or less. However, the appropriate output of the reverse sputtering also depends on the equipment used for sample preparation, so this range is not exhaustive.

[0075] The power density during plasma treatment is, for example, 1.0 to 4.0 W / cm 2 The power is selected within the following range, and film formation can be performed efficiently while preventing abnormal deposition and sparks. Preferably, the power is 1.5 W / cm 2 3.0W / cm or more 2 The method for producing the sputtering target is not particularly limited, and for example, a method such as that described in Patent Document 1 in which raw materials for the composition to be the target are mixed and fired to form a sintered body can be employed.

[0076] In step (31), the first amorphous complex oxide film 100A formed on the upper layer of the substrate 200 is further heat-treated to crystallize it into an apatite structure and become c-axis oriented, thereby becoming an oriented polycrystalline layer 11 (first polycrystalline layer 11A) made of an apatite-type complex oxide. Similarly, in step (32), the second amorphous complex oxide film 100B formed on the first polycrystalline layer 11A is further heat-treated to crystallize it into apatite structure and become c-axis oriented, thereby becoming an oriented polycrystalline layer 11 (second polycrystalline layer 11B) made of an apatite-type complex oxide.

[0077] Following the heat treatment in step (32), or after step (32), the peeling step (4) is performed to remove the release layer 300, thereby obtaining a free-standing thin film 1 peeled from the substrate 200. If the release layer 300 is a sublimable composition, the heat treatment in step (32) can be performed by switching the atmospheric gas in the same temperature range.

[0078] From the viewpoint of the efficiency of crystallizing the amorphous complex oxide film 100 into an apatite structure, the heat treatment temperature is preferably 800°C or higher, more preferably 850°C or higher and 1200°C or lower, and more preferably 900°C or higher and 1100°C or lower. A complex oxide film formed by a sputtering method is obtained as an amorphous film and crystallizes by heat treatment, but a long time is required to allow the crystallization to proceed at temperatures below 800°C. Furthermore, when the heat treatment furnace is a muffle furnace, it is not easy to set the temperature above 1200°C, and generally, heating furnaces that can be set to temperatures above 1200°C are expensive and therefore not practically advantageous.

[0079] The heat treatment atmosphere can be an air atmosphere for steps (31) and (32). Furthermore, if step (4) is subsequently performed, the heat treatment for peeling the amorphous complex oxide film 100 from the substrate 200 can be performed in the same temperature range as the heat treatment for crystallizing the amorphous complex oxide film 100 into an apatite structure, and can be performed more efficiently by flowing the above-mentioned inert gas or a reducing gas such as hydrogen. Preferably, the heat treatment is performed in an air atmosphere for a time sufficient to crystallize the amorphous complex oxide film 100 into an apatite structure, and then the atmosphere is replaced with a reducing gas atmosphere for heat treatment, thereby enabling efficient production of the free-standing thin film 1.

[0080] During this heat treatment, crystallization of the amorphous complex oxide film 100 progresses, and an oriented polycrystalline layer 11 is formed in which oriented crystal grains C are aligned in the c-axis direction. At this time, atomic rearrangement is facilitated by the contribution of the lattice-matched peeling layer 300. Furthermore, steps (31) and (32) are performed in stages, and the amorphous complex oxide film 100 is formed with high density, thereby suppressing the introduction of defects during atomic rearrangement. In this way, dense films having an apatite-type crystal structure are sequentially formed, which is presumably responsible for improving the c-axis orientation rate of the solid electrolyte membrane 10 including the first and second polycrystalline layers 11A and 11B.

[0081] Furthermore, following the heat treatment for crystallization, the atmosphere is replaced with a reducing gas atmosphere, whereby steps (3) and (4) are carried out consecutively, and the peeling layer 300 formed on the substrate 200 is removed by sublimation. In this manner, the laminated film of the solid electrolyte film 10, which is a single layer film, and the oxide layer 2 can be peeled off from the substrate 200 to form the free-standing thin film 1.

[0082] 6 (upper diagram), even in the case of a free-standing thin film 1 consisting only of a solid electrolyte membrane 10, the free-standing thin film 1 can be manufactured through similar steps, except that the oxide layer 2 is not formed. That is, after the above-described step (1), an amorphous complex oxide film 100 can be formed to a desired thickness on the surface layer of the release layer 300 formed on the substrate 200 through step (3) without performing step (2).

[0083] Next, by similarly performing heat treatment, the amorphous complex oxide film 100 can be crystallized and c-axis oriented. In this case, the amorphous complex oxide film 100 is formed directly on the surface layer of the peeling layer 300, and atomic rearrangement is likely to occur during the heat treatment process due to the effect of lattice matching. Therefore, it is not necessary to form the amorphous complex oxide film 100 in stages and then perform the heat treatment.

[0084] Thereafter, in the same manner as above, in step (4), the release layer 300 is removed to peel the solid electrolyte membrane 10 from the substrate 200, thereby obtaining a free-standing thin film 1 in which the solid electrolyte membrane 10 is a single-layer membrane, as shown in FIG. 6 (lower diagram).

[0085] The method for converting the oriented apatite-type complex oxide film into a freestanding thin film 1 in step (4) is not limited to the method of forming the release layer 300 using a sublimable composition and then separating it from the substrate 200, as in steps (1) to (4) above, but any method can be used. For example, even when a water-soluble composition is used, the release layer 300 can be formed on the substrate 200 using the water-soluble composition in step (1), and then steps up to step (3) can be carried out in the same manner. Thereafter, in step (4), the release layer 300 is dissolved in water, thereby obtaining a freestanding thin film 1 peeled from the substrate 200.

[0086] Even in this case, the electrode material to be the cathode 3 or the anode 4 can be formed as a film on one side of the freestanding thin film 1. By using a water-soluble composition, it becomes possible to provide an electrode structure that cannot be exposed to a reducing atmosphere to remove a sublimable composition, for example, integrally with the freestanding thin film 1. In this way, the manufacturing process can be simplified by using a more appropriate composition for the release layer 300 depending on the material, structure, etc. of the electrodes of the applied electrochemical cell 1C.

[0087] Next, a configuration example of an electrochemical cell 1C using the freestanding thin film 1 of this embodiment will be described. As shown in Fig. 4, the electrochemical cell 1C includes the freestanding thin film 1 including the solid electrolyte membrane 10, a cathode 3, and an anode 4. The cathode 3 and the anode 4 are configured as a pair of porous electrodes and are arranged to sandwich the two membrane surfaces 101, 102 of the freestanding thin film 1.

[0088] The freestanding thin film 1 here has a solid electrolyte membrane 10 with a multilayer structure and a pair of oxide layers 2 disposed on either side of the membrane. The solid electrolyte membrane 10 is composed of two oriented polycrystalline layers 11. An anode 4 is laminated on a first polycrystalline layer 11A on one side in the thickness direction Z, via a first oxide layer 2A forming a membrane surface 101. A cathode 3 is laminated on a second polycrystalline layer 11B on the other side, via a second oxide layer 2B forming a membrane surface 102.

[0089] The electrochemical cell 1C is composed of a dense solid electrolyte membrane 10 that selectively conducts oxide ions, and a porous body made of a catalytically active electrode material that is provided on each of the opposing membrane surfaces 101, 102 and serves as the cathode 3 or anode 4. When the electrochemical cell 1C is operated for the purpose of fuel synthesis, for example, water vapor or carbon dioxide is introduced as a raw material gas into one of the porous bodies, and electrons are supplied using electrical energy to synthesize a fuel gas such as hydrogen, carbon monoxide, or hydrocarbon. In this case, the porous electrode on the fuel synthesis side serves as the cathode 3, and the opposite porous electrode serves as the anode 4. Oxide ions generated on the cathode 3 side migrate inside the solid electrolyte membrane 10 and are released as oxygen on the anode 4 side.

[0090] In this case, the solid electrolyte membrane 10, which serves as an oxide ion conductor, is desirably thinned so as to shorten the distance between the cathode 3 and the anode 4, from the viewpoint of reducing the conduction resistance of oxide ions. Furthermore, it is desirable that both the cathode 3 and the anode 4 be formed of a porous body, from the viewpoint of improving the diffusibility of the source gas or the synthesis gas. Since increasing the porosity of the porous body facilitates gas diffusion and improves the energy density of the electrochemical cell 1C, it is desirable that the cathode 3 and the anode 4 be configured to have a porosity of preferably 20% or more, more preferably 40% or more. On the other hand, since a high porosity tends to decrease the strength, it is desirable to appropriately select a porosity within the range of 80% or less so as to obtain the desired strength.

[0091] The cathode 3 and anode 4 each preferably have a thickness of 15 μm or more, and their combined thickness is preferably 1000 μm or less. The closer the cathode 3 and anode 4 are to the solid electrolyte membrane 10, the more likely they are to serve as a reaction field. Therefore, by reducing the thickness of the porous electrodes, the source gas can be concentrated in a location close to the solid electrolyte membrane 10 that contributes most to the reaction field, thereby achieving a highly active cell. On the other hand, if the thickness of the porous electrodes is reduced, there is a concern that the source gas may be concentrated and the reaction field may become insufficient. Therefore, it is preferable that each thickness be 15 μm or more. Furthermore, a highly active cell can be achieved by setting the combined total thickness of the cathode 3 and anode 4 to 1000 μm or less. Preferably, the thicknesses of the two porous electrodes do not need to be the same, and it is desirable that the thickness of at least one of the porous electrodes be appropriately selected so that it is in the range of 30 μm or more and 500 μm or less.

[0092] The porous electrodes that become the cathode 3 and anode 4 are formed by agglomerating particles of the electrode material with voids between the particles. The particles of the electrode material preferably have an average particle size (D50) of 100 nm or more. If the particle size is less than 100 nm, the void distance becomes smaller than the mean free path of the gas, making it difficult to ensure gas diffusibility. Furthermore, as the particle size increases, gas diffusibility improves, but the opportunity for the gas to collide with and react with the electrode material decreases. Therefore, it is preferable to appropriately select a particle size in the range of 100 nm to 10 μm so as to obtain the desired gas diffusibility and reactivity.

[0093] The porous electrodes constituting the cathode 3 and anode 4 are made of an electrode material that has catalytic activity and electronic conductivity for transmitting the electrical energy required for the electrode reaction, and it is desirable that they have good bonding strength with the solid electrolyte membrane 10 made of an apatite-type complex oxide. Examples of such electrode materials include noble metal materials such as Pt and complex oxide materials, and a porous body containing these electrode materials can be bonded to the membrane surfaces 101, 102 of the solid electrolyte membrane 10 to form the cathode 3 or the anode 4.

[0094] Preferably, the electrode material made of a composite oxide is a composite oxide material in which a part of a metal oxide containing one or more elements selected from alkali metal elements, alkaline earth metals, and lanthanoid elements is substituted with one or more transition metal elements. By adjusting the types of constituent elements and the amount of substitution of the elements in such a composite oxide material, a porous electrode having desired properties can be obtained.

[0095] Specifically, such a composite oxide material is represented by the following general formula 2: Formula 2: (M1) 1-x1 (M2) ｘ1 O 2-y1In formula 2, M1 represents one or more elements selected from alkali metal elements, alkaline earth metals, and lanthanoid elements; M2 represents one or more elements selected from the group consisting of Sc, Y, Zr, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pa, Ag, Au, and Pt; and x1 and y1 are numbers that satisfy 0.1≦x1≦0.6 and 0.0≦y1≦1.0, respectively.

[0096] In the above formula 2, examples of alkali metal elements serving as the M1 element include Li, Na, K, etc., examples of alkaline earth metal elements include Mg, Ca, Sr, Ba, etc., and examples of lanthanoid elements include La, Ce, etc. The composite oxide of the above formula 2 has a structure in which at least one of these alkali metal elements, alkaline earth metals, and lanthanoid elements is included as the M1 element, and the M2 element is added in the form of substituting a portion of the M1 element. Preferably, La, Ce, Sr, etc. is used as the M1 element, and Ti, Mn, Fe, Co, Mo, Rh, etc. is used as the M2 element.

[0097] In formula 2, x1 is preferably 0.1 or more and 0.6 or less, more preferably 0.2 or more and 0.4 or less, from the viewpoint of being substituted at the M1 element position to enhance electrode properties. In formula 2, y1 is preferably 0.0 or more and 1.0 or less, from the viewpoint of maintaining electrical neutrality. Preferably, one or both of the M1 element and the M2 element are a combination of multiple element species, which allows the above properties to be adjusted and makes it possible to obtain a more suitable electrode material. Examples of such composite oxides include La 0.8 Sr 0.3 MnO3, La 0.8 Sr 0.2 CoO3, Sr 2.0 Fe 1.5 Mo 0.5 O6, La 0.8 Sr 0.2 MnO3 / alumina, La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O3, Ce 0.6 Mn 0.3 Fe0.1 O 1.8 , La 0.4 Sr 0.1 Ti 0.45 Rh 0.05 O 1.63 , La 0.33 Sr 0.33 Co 0.33 O 2.0 However, the present invention is not limited to these.

[0098] (Embodiment 2) A second embodiment of a freestanding thin film and a method for manufacturing the same will be described with reference to the drawings. The freestanding thin film 1 of this embodiment differs from the first embodiment in the configuration of the oxide layer 2 disposed on top of the release layer 300. The basic configurations of the freestanding thin film 1 and the electrochemical cell 1C are the same as those of the first embodiment, and the following description will focus on the differences. Note that, of the symbols used in the second and subsequent embodiments, those that are the same as those used in the previous embodiments represent the same components, etc. as those in the previous embodiments, unless otherwise specified.

[0099] 7 , the freestanding thin film 1 may have an oxide layer 2 on at least one side of the solid electrolyte membrane 10, and the solid electrolyte membrane 10 may be a single-layer membrane having a predetermined thickness t. In this case, it is desirable that the oxide layer 2 (e.g., the first oxide layer 2A) on one side of the solid electrolyte membrane 10 be made of an oxide or composite oxide whose lattice constant matches that of the solid electrolyte membrane 10.

[0100] Here, matching of the lattice constant means that the integral multiple of the lattice constant is approximately the same as or close to (for example, within a range of ±5%) the lattice constant of the apatite-type composite oxide that constitutes the solid electrolyte membrane 10. Therefore, in Fig. 7 (upper diagram), the oxide layer 2 formed on the upper layer of the substrate 200 and in contact with the solid electrolyte membrane 10 is preferably selected from oxides or composite oxides that have oxygen absorbing and releasing properties and whose integral multiple of the lattice constant is within a range of ±5% of the lattice constant of the apatite-type composite oxide.

[0101] In this case, solid electrolyte membrane 10 can be formed as a single layer membrane of a desired thickness t (e.g., 0.2 μm to 10 μm) without being formed stepwise as a multilayer membrane as in Embodiment 1, or without being limited to a thickness of 200 nm or less. In this case, as shown in FIG. 7 (lower diagram), when oriented crystalline grains C constituting oriented polycrystalline layer 11 grow from film surface 101 side to film surface 102 side, first oxide layer 2A forming film surface 101 is a layer with a matched lattice constant (hereinafter referred to as a lattice matching layer), and therefore, the desired orientation can be easily obtained in oriented polycrystalline layer 11 formed on the surface layer.

[0102] In this case, as in embodiment 1, the peeling layer 300 can be configured as a lattice-matching layer, and by forming a lattice-matched oxide layer 2 on top of it, it becomes possible to efficiently form an oriented polycrystalline layer 11.

[0103] Preferably, the oxide or composite oxide constituting the oxide layer 2 is an oxide or composite oxide containing one or more elements selected from, for example, transition metal elements and Group 13 elements. Examples of transition metal elements include Sc, Ti, V, Fe, Co, Cu, and Zr, and lanthanoid elements, such as Ce, Pr, and Gd. Examples of Group 13 elements include Al, Ga, and In.

[0104] Specifically, when the apatite-type composite oxide is the rare earth silicate composite oxide (e.g., LSO) described above, the following oxides or composite oxides, whose lattice constants are integer multiples within ±5% (9.33 angstroms to 10.31 angstroms) of the lattice constant of LSO (9.82 angstroms), can be used for the first oxide layer 2A. The numbers in parentheses are lattice constants in angstroms: CoO (3.20), GaO (3.34), TiO (4.99), PtO2 (3.13), WO2 (3.11), ScCuO2 (3.22), InFeO3 (3.40), ScGaO3 (3.30), AlVO3 (3.18), and AlFeO3 (3.19).

[0105] In this way, if the first oxide layer 2A on one side of the solid electrolyte membrane 10 is an oxide or composite oxide that matches the lattice constant of the solid electrolyte membrane 10, it becomes easy to manufacture a free-standing thin film 1 having a laminated film structure including the oxide layer 2. In this case, as shown in FIG. 7 (top diagram), a release layer 300 is formed in advance on the substrate 200, and the solid electrolyte membrane 10 can be formed with a desired thickness thereon with the first oxide layer 2A interposed therebetween. Thereafter, by removing the release layer 300, the solid electrolyte membrane 10 can be peeled off from the substrate 200 together with the first oxide layer 2A, as shown in FIG. 7 (bottom diagram).

[0106] Specifically, in FIG. 7 (upper diagram), the process includes the following steps (10) to (40): (10) forming a release layer 300 on the surface of the substrate 200, the release layer 300 being made of a composition that can be removed after crystallization of the apatite-type complex oxide (release layer forming step); (20) forming an oxide layer 2 (first oxide layer 2A) on the surface of the release layer 300, the oxide or complex oxide having oxygen absorbing / releasing properties and an integral multiple of the lattice constant of which is within ±5% of the lattice constant of the apatite-type complex oxide (oxide layer forming step); (30) forming an amorphous complex oxide film (100) having the same composition as the apatite-type complex oxide on the surface of the oxide layer 2 (first oxide layer 2A), and heat-treating the film to form an oriented polycrystalline layer 11 (solid electrolyte film forming step); (40) removing the release layer 300 to release the solid electrolyte film 10 from the substrate 200 (release step); 7 (lower diagram), a laminated film of the first oxide layer 2A and the solid electrolyte membrane 10 is obtained. Furthermore, a second oxide layer 2B is formed on the opposite side of the solid electrolyte membrane 10 from the first oxide layer 2A, thereby producing a free-standing thin film 1.

[0107] Here, steps (10) and (40) are the same as steps (1) and (4) in the above-mentioned embodiment 1, but step (20) differs from step (2) in the oxide or complex oxide that becomes the oxygen compound layer 2. As a result, a lattice-matched oxide layer 2 is formed, and therefore in step (30), the amorphous complex oxide film (100) is formed in a single film formation step, and the subsequent heat treatment is also performed only once, simplifying the manufacturing process.

[0108] Example 1 An electrochemical cell 1C including a freestanding thin film 1 having the configuration shown in the above-described first embodiment was fabricated based on the method shown in Fig. 5 and evaluated. As shown in Fig. 8 (lower diagram), a sapphire substrate was used as the substrate 200, and a freestanding thin film 1 including a multilayered solid electrolyte film 10 made of oriented apatite and first and second oxide layers 2A and 2B was formed on the upper layer thereof. In addition, one of the cathode 3 and anode 4 of the electrochemical cell 1C (here, cathode 3) was placed on the surface of the second oxide layer 2B of the freestanding thin film 1 to form a half cell.

[0109] As shown in Figure 8 (top diagram), first, a release layer 300 was formed on the surface of a substrate 200 by radio frequency sputtering, with a protective layer 201 interposed therebetween to prevent a reaction (release layer formation step). The release layer 300 was made of ZnO, a sublimable composition, and a first oxide layer 2A was formed on the surface (oxide layer formation step). A first amorphous complex oxide film 100A having an apatite composition was then formed and heat-treated (first solid electrolyte film formation step). A second amorphous complex oxide film 100B having an apatite composition was then formed (second solid electrolyte film formation step). A second oxide layer 2B and an electrode layer 31 to serve as a cathode 3 were then formed and heat-treated, resulting in a half cell including a free-standing thin film 1 not supported by a substrate 200.

[0110] Thereafter, the other of the cathode 3 and anode 4 (here, anode 4) was formed in a step not shown, and then heat treatment was performed to obtain an electrochemical cell 1C. Note that the first and second oxide layers 2A and 2B were made of 10 at % Gd-doped CeO (hereinafter, abbreviated as 10% GDC, as appropriate), and the solid electrolyte membrane 10 was constructed as a multi-layered membrane including first and second polycrystalline layers 11A and 11B. The specific method is described below. The compositions used to form each layer are shown in Tables 1 and 2.

[0111] <Fabrication of freestanding thin film 1> First, a protective film for preventing a reaction and a release layer 300 were sequentially formed on the surface of a sapphire substrate 200. The substrate 200 was a square substrate (50 mm x 50 mm x 1.0 mm) using the C-face of hexagonal sapphire, and a known high-frequency sputtering device was used for film formation. The substrate 200 and a sputtering target consisting of a 10% GDC composition were placed facing each other in a chamber, and a vacuum of 1 x 10 -4 After evacuating the chamber to a pressure of 1.0 Pa or less, Ar gas mixed with 10% O was introduced, and the pressure inside the chamber was maintained at 1.0 Pa. In this state, the high-frequency power supply was operated, and a high-frequency power of 600 W (3.28 W / cm 2 ) and the film formation time was 10 minutes.

[0112] In this way, a substrate 200 was obtained on which a protective layer 201 (thickness: 25 nm) having a 10% GDC composition was formed. Next, the substrate 200 on which the protective layer 201 was formed and a sputtering target made of ZnO were placed facing each other in a chamber, and a vacuum of 1×10 -4 The chamber was evacuated to a pressure of 1.0 Pa or less. Ar gas was introduced into the chamber, and while maintaining the pressure at 1.0 Pa, the high-frequency power supply was operated to apply high-frequency power. The high-frequency power was 400 W, and the film formation time was 30 minutes, to form a release layer 300 made of ZnO with its c-axis oriented in the direction perpendicular to the plane of the 50 mm square substrate. The thickness of this release layer 300 was 300 nm.

[0113] Next, the substrate 200 on which the release layer 300 was formed and a sputtering target having a 10% GDC composition were placed opposite each other in a chamber, and a vacuum of 1×10 -4After the chamber was evacuated to a pressure of 1.0 Pa or less, Ar gas mixed with 10% O was introduced. With the chamber pressure maintained at 1.0 Pa, a high frequency power of 600 W (3.28 W / cm 2 ) and for a film formation time of 10 minutes, an amorphous thin film having a 10% GDC composition was formed on the release layer 300 to form a first oxide layer 2A (film thickness: 25 nm).

[0114] Thereafter, the substrate 200 on which the first oxide layer 2A and the peeling layer 300 have been formed and a sputtering target made of a rare earth silicate-based apatite composition (here, LSO) are placed opposite each other in a chamber, and a vacuum of 1×10 -4 After evacuating the chamber to a pressure of 1.0 Pa or less, Ar gas mixed with 10% O was introduced. With the pressure inside the chamber maintained at 1.0 Pa, a high frequency power of 500 W (2.73 W / cm 2 ), a reverse sputtering power of 30 W was applied to the substrate side, and a first amorphous complex oxide film 100A having an apatite composition was formed on the first oxide layer 2A for a film formation time of 40 minutes.

[0115] In this way, the substrate 200 on which the first amorphous complex oxide film 100A was formed via the first oxide layer 2A on the release layer 300 was heat-treated to crystallize the first amorphous complex oxide film 100A. The heat treatment was performed under conditions that would prevent the release layer 300 made of oriented ZnO from sublimating. First, the temperature was raised to 1000°C in an air atmosphere over 5 hours, held at that temperature for 4 hours, and then lowered to room temperature over 2 hours. This resulted in apatite crystallization of the first amorphous complex oxide film 100A, imparting orientation to it, and forming a first polycrystalline layer 11A made of an LSO thin film with a high c-axis orientation ratio. The thickness t1 of this first polycrystalline layer 11A was 200 nm.

[0116] Similarly, the substrate 200 on which the first polycrystalline layer 11A was formed and a sputtering target made of a rare earth silicate-based apatite composition were placed opposite each other in a chamber, and a vacuum of 1×10 -4 After evacuating the chamber to a pressure of 1.0 Pa or less, Ar gas mixed with 10% O was introduced. With the pressure inside the chamber maintained at 1.0 Pa, a high frequency power of 500 W (2.73 W / cm 2), sputtering was carried out for a film formation time of 160 minutes while applying a reverse sputtering power of 30 W to the substrate side, to form a second amorphous complex oxide film 100B of an apatite composition on the first polycrystalline layer 11A.

[0117] Next, in the same manner as in the formation of the first oxide layer 2A, the substrate 200 on which the second amorphous complex oxide film 100B was formed and a sputtering target having a 10% GDC composition were placed opposite each other in a chamber, and the vacuum was set to 1×10 -4 After the chamber was evacuated to a pressure of 1.0 Pa or less, Ar gas mixed with 10% O was introduced. With the chamber pressure maintained at 1.0 Pa, a high frequency power of 600 W (3.28 W / cm 2 ) and for a film formation time of 10 minutes, an amorphous thin film having a 10% GDC composition was formed on the second amorphous complex oxide film 100B to form a second oxide layer 2B (film thickness: 25 nm).

[0118] In this way, a membrane-substrate composite including the substrate and the thin film laminate that would become the freestanding thin film 1 was produced, and subsequently, an electrochemical cell 1C was produced as follows.

[0119] <Preparation of Electrochemical Cell 1C> First, one of the electrode layers 31 and 41 that will become the cathode 3 and anode 4 of the electrochemical cell 1C was formed to prepare an electrode-substrate composite that will become a half cell. Here, La was used as the electrode material for the electrode layer 31 that will become the cathode 3. 0.8 Sr 0.2 MnO powder was mixed in a ball mill with 10 mm diameter zirconia beads in terpineol containing 1% by mass of acrylic resin as a dispersant for 16 hours. The mixture was then kneaded to adjust the viscosity, resulting in a printing paste. The resulting printing paste was applied to the surface of the film-substrate composite prepared as described above by printing to a predetermined film thickness, and then dried to form an electrode layer 31 (film thickness: 20 nm). The film thickness was adjusted by the number of printings.

[0120] The resulting electrode-substrate composite was then heat-treated to crystallize the second amorphous complex oxide film 100B and separate it from the substrate 200. The heat treatment consisted of first raising the temperature to 1000°C in an air atmosphere over 5 hours, maintaining the temperature for 4 hours, and then introducing a 4% H mixed N gas mixture over 100 minutes to completely replace the air atmosphere with a reducing atmosphere. After maintaining this condition for 10 hours, the temperature was lowered to room temperature over 2 hours. This resulted in apatite crystallization of the second amorphous complex oxide film 100B, imparting orientation to the film, forming a second polycrystalline layer 11B (thickness t2: 800 nm) made of an LSO thin film with a high c-axis orientation. Furthermore, the peeling layer 300 made of oriented ZnO was removed by sublimation, thereby separating the film from the substrate 200.

[0121] In this way, the solid electrolyte membrane 10 (thickness t: 1.0 μm) having the first and second polycrystalline layers 11A, 11B with an oriented apatite structure laminated thereon was integrated with the first and second oxide layers 2A, 2B disposed on both sides thereof, to obtain a free-standing thin film laminate not supported by the substrate 200. Furthermore, using this thin film laminate as the free-standing thin film 1, a half cell was simultaneously formed in which a porous electrode membrane to serve as the cathode 3 was disposed on one surface in the thickness direction Z (here, the side of the second oxide layer 2B).

[0122] Furthermore, a porous electrode film that would become the anode 4 was formed on the other surface of this half cell. The electrode material for the electrode layer 41 that would become the anode 4 was La. 0.6 Sr 0.4 Co 0.8 Fe 0.2 In this way, the powder used was LaO3. 0.6 Sr 0.4 Co 0.8 Fe 0.2 A printing paste was prepared in the same manner as for the electrode layer 31, except that O3 was used instead. The obtained printing paste was applied to the other surface of the half cell (here, the side of the first oxide layer 2A) by printing to a predetermined film thickness, and then dried to form the electrode layer 41 (film thickness: 20 nm). The film thickness was adjusted by the number of times of printing.

[0123] The powders of the electrode compositions to be the cathode 3 and the anode 4 were obtained by preparing powders of nitrate hydrates of the elements excluding oxygen for each composition, dissolving them in pure water to a desired concentration, thoroughly stirring the resulting solution, and then drying and heat-treating the resulting solution.

[0124] The half-cell with the electrode layer 41 formed thereon was then subjected to a heat treatment in the same manner. The heat treatment was carried out by raising the temperature to 1000°C in an air atmosphere over 5 hours, maintaining the temperature for 4 hours, and then lowering the temperature to room temperature over 2 hours. This resulted in an electrochemical cell 1C in which a porous electrode film serving as the anode 4 was disposed on the opposite side of the freestanding thin film 1 from the cathode 3.

[0125]

[0126]

[0127] <Evaluation of Freestanding Thin Film 1 and Electrochemical Cell 1C> The obtained freestanding thin film 1 was subjected to X-ray diffraction pattern analysis by X-ray diffraction method and elemental analysis by ICP optical emission spectrometry. Furthermore, the film density d was measured by X-ray reflectivity method, and the film shape and sample cross section were observed using various microscopes. Specifically, the planar shape was measured using a laser microscope, and structural analysis in the cross-sectional direction was performed using a transmission electron microscope (TEM). These results are shown in Table 1 as Example 1.

[0128] As shown in Table 1 as the membrane structure (parent phase), the obtained solid electrolyte membrane 10 was 9.33 Si 6.0 O 26 It was confirmed that the solid electrolyte membrane 10 was a membrane formed of an aggregate of oriented crystalline particles C of an apatite structure having a composition of 1.0 μm (1000 nm) and a film area S of 25.0 cm. The solid electrolyte membrane 10 had a thickness t of 1.0 μm (1000 nm) and a film area S of 25.0 cm. 2 , the film density d is 4.23 g / cm 2The c-axis orientation rate was 0.98, and the average particle size was 410 nm. The instruments used for these measurements are as follows: X-ray diffraction apparatus: SmartLab manufactured by Rigaku Corporation ICP emission spectrometer: ICPS-7510 manufactured by Shimadzu Corporation Laser microscope: LEXT OLS4100 manufactured by Olympus Corporation Transmission electron microscope: GRAND-ARM300 manufactured by JEOL Ltd.

[0129] The film thickness t in Table 1 is a calculated value based on a TEM image of a cross-sectional sample, and is the arithmetic mean value of film thicknesses measured at 40 arbitrary points within a 10 μm long range along the film plane. The c-axis orientation rate is a ratio calculated by the Lotgering method from the X-ray diffraction patterns of 40 arbitrary particles observed by the cross-sectional TEM, and the average particle size is the arithmetic mean value calculated from the diameters of 40 arbitrary particles observed by the cross-sectional TEM.

[0130] Example 2 A free-standing thin film 1 and an electrochemical cell 1C were produced using a manufacturing method similar to that of Example 1, but with a different film structure for the solid electrolyte film 10 serving as the parent phase. That is, instead of sequentially forming two layers of amorphous complex oxide film 100 (100A, 100B) and then heat-treating them as in Example 1, a single layer of amorphous complex oxide film 100 of the same thickness was continuously formed. Furthermore, the free-standing thin film 1 and the electrochemical cell 1C were produced without forming the first oxide layer 2A and the second oxide layer 2B.

[0131] Specifically, in the same manner as in Example 1, a release layer 300 made of an oriented ZnO layer was formed on a substrate 200 on which a protective layer 201 was formed, and then an amorphous complex oxide film 100 of an apatite composition was formed without forming a first oxide layer 2 A. Here, the substrate 200 on which the release layer 300 was formed and a sputtering target made of a rare earth silicate-based apatite composition were disposed opposite each other in a chamber, and sputtering was performed under the same conditions as in Example 1, except that the film formation time was 200 minutes.

[0132] Next, without forming the second oxide layer 2B, an electrode layer 31 to become the cathode 3 was formed on the amorphous complex oxide film 100 in the same manner as in Example 1. Thereafter, by carrying out a heat treatment under the same conditions, the amorphous complex oxide film 100 was crystallized, and the peeling layer 300 made of oriented ZnO was sublimated and removed, and the oriented polycrystalline layer 11 made of the LSO thin film was peeled off from the substrate 200. As a result, a solid electrolyte film 10 (film thickness t: 1.0 μm) made of one layer of oriented polycrystalline layer 11 was obtained.

[0133] In this way, a half cell was obtained in which a cathode 3 was formed on one side of a free-standing thin film 1 made of a solid electrolyte membrane 10. Thereafter, an anode 4 was formed on the side of the half cell opposite to the cathode 3 in the same manner, to obtain an electrochemical cell 1C.

[0134] Comparative Example 1 For comparison, a free-standing thin film 1 including a solid electrolyte membrane 10 having the same membrane configuration as in Example 2 and first and second oxide layers 2A and 2B was produced by the same manufacturing method as in Example 1. That is, an amorphous complex oxide film 100 that would become the solid electrolyte membrane 10 was continuously formed to form a solid electrolyte membrane 10 consisting of a single oriented polycrystalline layer 11, with a first oxide layer 2A and a second oxide layer 2B formed on both sides of the solid electrolyte membrane 10.

[0135] Specifically, in the same manner as in Example 1, a release layer 300 made of an oriented ZnO layer was formed on a substrate 200 on which a protective layer 201 was formed, and then a first oxide layer 2A was formed, and further, an amorphous complex oxide film 100 having an apatite composition was formed. At this time, in the same manner as in Example 2, the substrate 200 on which the first oxide layer 2A was formed and a sputtering target made of a rare earth silicate-based apatite composition were placed opposite each other in a chamber, and sputtering was performed for a film formation time of 200 minutes.

[0136] Next, a second oxide layer 2B was formed on the amorphous complex oxide film 100 in the same manner as in Example 1, and further an electrode layer 31 serving as the cathode 3 was formed. Thereafter, by carrying out a heat treatment under the same conditions, the amorphous complex oxide film 100 was crystallized and the peeling layer 300 made of oriented ZnO was sublimated and removed, thereby leaving a solid electrolyte film 10 (film thickness t: 1.0 μm) made of a single oriented polycrystalline layer 11.

[0137] In this way, a half cell was obtained in which a cathode 3 was formed on one side of a free-standing thin film 1 including a solid electrolyte membrane 10 and first and second oxide layers 2A and 2B. Thereafter, an anode 4 was formed on the opposite side of the half cell from the cathode 3 in the same manner, to obtain an electrochemical cell 1C.

[0138] The self-supporting thin films 1 obtained in Example 2 and Comparative Example 1 were evaluated for film structure (matrix) in the same manner as in Example 1. The results are also shown in Table 1.

[0139] As shown in Table 1 as the membrane structure (parent phase), it was confirmed that the solid electrolyte membrane 10 obtained in Example 2 consisted of a single LSO thin film with the same composition as in Example 1. In addition, the average particle size was 470 nm, which was larger than in Example 1, and the other characteristics were the film thickness t of 1.0 μm (1000 nm) and the membrane area S of 25.0 cm 2 , the film density d is 4.23 g / cm 2 The c-axis orientation rate was 0.98, which was the same as in Example 1.

[0140] In contrast, the solid electrolyte membrane 10 obtained in Comparative Example 1 consisted of a single LSO thin film with the same composition as in Example 1, but had a slightly smaller average grain size of 380 nm and a c-axis orientation rate of 0.22, which was significantly lower than in Examples 1 and 2. The film thickness t was 1.0 μm (1000 nm), and the membrane area S was 25.0 cm 2 , the film density d is 4.23 g / cm 2 This was the same as in Example 1.

[0141] Examples 3 to 5 A free-standing thin film 1 and an electrochemical cell 1C were fabricated using the same manufacturing method as in Example 1, and including a solid electrolyte membrane 10 having the same membrane configuration as in Example 2, and first and second oxide layers 2A and 2B. That is, an amorphous complex oxide film 100 that becomes the solid electrolyte membrane 10 was continuously formed to form a solid electrolyte membrane 10 consisting of a single oriented polycrystalline layer 11, and the first oxide layer 2A was changed to CoO (lattice constant: 3.20 angstroms). The second oxide layer 2B was 10% GDC, the same as in Example 2.

[0142] Specifically, in the same manner as in Example 1, a release layer 300 made of an oriented ZnO layer was formed on a substrate 200 on which a protective layer 201 was formed, and then sputtering was performed using a sputtering target made of CoO instead of 10% GDC as the first oxide layer 2A to form an amorphous thin film of CoO composition. Furthermore, in the same manner as in Example 2, the substrate 200 on which the first oxide layer 2A was formed and a sputtering target made of a rare earth silicate-based apatite composition were placed opposite each other in a chamber, and sputtering was performed for 200 minutes.

[0143] Next, an amorphous thin film having a 10% GDC composition was formed as the second oxide layer 2B on the amorphous complex oxide film 100 in the same manner as in Example 1, and further an electrode layer 31 to become the cathode 3 was formed. Thereafter, by carrying out a heat treatment under the same conditions, the amorphous complex oxide film 100 was crystallized and the peeling layer 300 made of oriented ZnO was sublimated and removed, thereby forming a solid electrolyte film 10 (film thickness t: 1.0 μm) made of a single oriented polycrystalline layer 11.

[0144] In this way, a half cell was obtained in which a cathode 3 was formed on one side of a free-standing thin film 1 including a solid electrolyte membrane 10 and first and second oxide layers 2A and 2B. Thereafter, an anode 4 was formed on the opposite side of the half cell from the cathode 3 in the same manner, to obtain an electrochemical cell 1C (Example 3).

[0145] A half-cell including a free-standing thin film 1 was obtained in the same manner as in Example 3, except that InFeO (lattice constant: 3.40 angstroms) was used as the first oxide layer 2A, and then an electrochemical cell 1C was fabricated in the same manner (Example 4). A half-cell including a free-standing thin film 1 was obtained in the same manner as in Example 3, except that AlFeO (lattice constant: 3.19 angstroms) was used as the first oxide layer 2A, and then an electrochemical cell 1C was fabricated in the same manner (Example 5).

[0146] The self-supporting thin films 1 obtained in Examples 3 to 5 were evaluated for their film structures (matrix) in the same manner as in Example 1. The results are also shown in Table 1.

[0147] As shown in Table 1 as the membrane structure (parent phase), it was confirmed that the solid electrolyte membranes 10 obtained in Examples 3 to 5 consisted of a single LSO thin film with the same composition as in Example 1. The average particle size was 450 to 460 nm, which was larger than in Example 1 and equivalent to that in Example 2. Other than that, the film thickness t was 1.0 μm (1000 nm), the membrane area S was 25.0 cm 2 , the film density d is 4.23 g / cm 2 The c-axis orientation rate was 0.98, which was the same as in Example 1.

[0148] Thus, even in Examples 3 to 5 in which the first oxide layer 2A made of a lattice-matched oxide was formed on the surface of the release layer 300, an LSO thin film with a thickness t of 1.0 μm was formed in one step, and a high c-axis orientation rate was obtained.

[0149] Examples 6 to 8 A free-standing thin film 1 and an electrochemical cell 1C including a solid electrolyte membrane 10 with a different membrane configuration were fabricated using the same manufacturing method as in Example 1. That is, in Example 6, the solid electrolyte membrane 10 was formed by sequentially depositing two layers of first and second amorphous complex oxide films 100A and 100B, which became first and second polycrystalline layers 11A and 11B, and was configured in the same manner as in Example 1 except that the thickness ratio between the first and second polycrystalline layers 11A and 11B was different.

[0150] Specifically, in the same manner as in Example 1, a first oxide layer 2A was formed on a substrate 200 on which a protective layer 201 and a release layer 300 were formed, and the substrate 200 on which the first oxide layer 2A was formed and a sputtering target made of a rare earth silicate-based apatite composition were disposed opposite each other in a chamber, and a vacuum of 2×10 -５ After evacuating the chamber to a pressure of 1.0 Pa or less, Ar gas mixed with 20% O was introduced. With the pressure inside the chamber maintained at 1.0 Pa, a high frequency power of 400 W (2.73 W / cm 2 ), a first amorphous complex oxide film 100A having an apatite composition was formed for 60 minutes while applying a reverse sputtering power of 30 W to the substrate side. Thereafter, a heat treatment was carried out in the same manner as in Example 1 to form a first polycrystalline layer 11A (thickness t1: 100 nm).

[0151] Furthermore, a second amorphous complex oxide film 100B having an apatite composition was formed on the surface of the first polycrystalline layer 11A under the same conditions except that the film formation time was set to 2,940 minutes. Thereafter, a second oxide layer 2B and an electrode layer 31 serving as the cathode 3 were formed in the same manner as in Example 1, followed by heat treatment. As a result, the second amorphous complex oxide film 100B was crystallized into apatite and given orientation, forming the second polycrystalline layer 11B (thickness t2: 4.9 μm). The peeling layer 300 was then removed and separated from the substrate 200. After obtaining a half cell including the freestanding thin film 1 in this manner, an electrochemical cell 1C was further fabricated.

[0152] Example 7 A free-standing thin film 1 and an electrochemical cell 1C including a solid electrolyte membrane 10 with a different membrane configuration were also fabricated using the same manufacturing method as in Example 1. That is, the solid electrolyte membrane 10 was composed of a single oriented polycrystalline layer 11 corresponding to the first polycrystalline layer 11A in Example 1, with first and second polycrystalline layers 11A and 11B formed on both sides of the oriented polycrystalline layer 11 to form the free-standing thin film 1.

[0153] Specifically, a first oxide layer 2A was formed on a substrate 200 on which a protective layer 201 and a release layer 300 had been formed in the same manner as in Example 1, and then an amorphous composite oxide film 100 having an apatite composition was formed on the first oxide layer 2A under the same conditions as those for forming the first polycrystalline layer 11A in Example 1. That is, a sputtering target made of a rare earth silicate-based apatite composition was used, and the chamber was maintained at 1.0 Pa with a high-frequency power of 500 W (2.73 W / cm 2 ), and a reverse sputtering power of 30 W was applied to the substrate side, while sputtering was carried out for a film formation time of 40 minutes.

[0154] Thereafter, a second oxide layer 2B and an electrode layer 31 serving as a cathode 3 were formed, followed by heat treatment. As a result, the amorphous complex oxide film 100 was crystallized into apatite and given orientation, forming an oriented polycrystalline layer 11 (film thickness t: 200 nm), and the peeling layer 300 was removed to separate it from the substrate 200. After obtaining a half cell including the freestanding thin film 1 in this manner, an electrochemical cell 1C was further fabricated.

[0155] Example 8 Furthermore, a free-standing thin film 1 and an electrochemical cell 1C including a solid electrolyte membrane 10 with a different membrane configuration were fabricated using the same manufacturing method as in Example 1. That is, the solid electrolyte membrane 10 was formed by sequentially depositing two layers of amorphous complex oxide membranes 100 (100A, 100B) that would become first and second polycrystalline layers 11A and 11B, and the substrate 200 had a size (10 cm × 10 cm × 1.0 mm). The electrode composition used in the electrode layer 31 was a mixture of powder of an electrode material and alumina powder. Otherwise, a half cell including a free-standing thin film 1 with a membrane area S larger than that of Example 1 was obtained in the same manner as in Example 1. Then, an electrochemical cell 1C was fabricated in the same manner as in Example 1.

[0156] The self-supporting thin films 1 obtained in Examples 6 to 8 were evaluated for their film structures (matrix) in the same manner as in Example 1. The results are also shown in Table 1.

[0157] As shown in Table 1 as the membrane structure (parent phase), it was confirmed that the solid electrolyte membranes 10 obtained in Examples 6 to 8 were composed of a single-layer or double-layer LSO thin film having the same composition as Example 1. Furthermore, the solid electrolyte membrane 10 of Example 6 had a thickness t of 5.0 μm (t1 / t2=49), an average particle size of 620 nm, and a membrane density d of 4.92 g / cm 2 The solid electrolyte membranes 10 of Examples 7 and 8 had an average particle size of 400 to 410 nm and a membrane density d of 4.23 g / cm. 2 The c-axis orientation ratio was 0.98 in all cases.

[0158] In this way, in the configuration in which the oxide layer 2 is formed on both sides of the solid electrolyte membrane 10, Example 6 is formed by forming the thin first polycrystalline layer 11A (thickness t1: 200 nm) and then forming the second polycrystalline layer 11B, Example 7 is formed by forming only the thin oriented polycrystalline layer 11 (film thickness t: 200 nm), and Example 8 is formed by forming the thin oriented polycrystalline layer 11 (film thickness t: 200 nm). 2 In both of Example 8, in which the diameter was increased to 100 mm and 110 mm, good results were obtained.

[0159] For comparison, a free-standing thin film 1 and an electrochemical cell 1C including a solid electrolyte membrane 10 of a similar configuration were fabricated by changing the film formation conditions using the same manufacturing method as in Example 7. That is, the solid electrolyte membrane 10 was composed of a single oriented polycrystalline layer 11 corresponding to the first polycrystalline layer 11A in Example 1, with first and second polycrystalline layers 11A and 11B formed on both sides thereof to form the free-standing thin film 1.

[0160] Specifically, a first oxide layer 2A was formed on a substrate 200 having a protective layer 201 and a peeling layer 300 formed thereon in the same manner as in Example 1. Then, an amorphous complex oxide film 100 having an apatite composition was formed on the first oxide layer 2A under the same conditions as in Example 1, except that the reverse sputtering power of 30 W on the substrate side was not applied and the film formation time was 24 minutes when forming the first polycrystalline layer 11A. Thereafter, a second oxide layer 2B and an electrode layer 31 serving as a cathode 3 were formed and heat-treated. As a result, the amorphous complex oxide film was crystallized into apatite and given orientation, forming an oriented polycrystalline layer 11 (film thickness t: 200 nm). The peeling layer 300 was then removed and separated from the substrate 200. After obtaining a half cell including a freestanding thin film 1 in this manner, an electrochemical cell 1C was further fabricated.

[0161] The self-supporting thin film 1 obtained in Comparative Example 2 was evaluated for film structure (parent phase) in the same manner as in Example 1. The results are also shown in Table 1.

[0162] As shown in Table 1 as the membrane structure (parent phase), it was confirmed that the solid electrolyte membrane 10 obtained in Comparative Example 2 was composed of a single LSO thin film having the same composition as in Example 1. In addition, it had the same thickness t (0.2 μm) and membrane area S (25.0 cm) as in Example 7. 2 ), whereas the average particle size is 150 nm and the film density d is 4.15 g / cm 2 The c-axis orientation rate was significantly reduced to 0.23.

[0163] 9 compares the membrane configurations (schematic diagrams) and TEM observation images of the membrane surfaces of Example 1 and Comparative Example 2, and shows that the oriented crystal grains C constituting solid electrolyte membrane 10 of Example 1 have a larger average grain size than crystal grains C2 constituting solid electrolyte membrane 10 of Comparative Example 2, and also have a high c-axis orientation in the film thickness direction Z, with the crystal orientation in the film plane (X, Y) being nearly uniform. In contrast, crystal grains C2 of Comparative Example 2 have a large deviation from the film thickness direction Z, with the crystal orientation not being uniform, and the proportion of particles having c-axis orientation has decreased to 23% (i.e., c-axis orientation rate: 0.23).

[0164] 10 compares TEM observation images of the membrane cross sections of Example 1 and Comparative Example 1, and shows that the solid electrolyte membrane 10 of Example 1 is composed of two oriented polycrystalline layers 11 (11A, 11B) and has a laminated structure with c-axis orientation. Also, in the figure, a high-resolution STEM (scanning transmission electron microscope) image of a portion of the surface side of the second polycrystalline layer 11B was confirmed to have a karamar structure in which c-axis texture is concentrated. In contrast, the solid electrolyte membrane 10 of Comparative Example 1 is composed of a single polycrystalline layer, and as shown in the diffraction pattern in the figure, a portion of the surface side of the polycrystalline layer is confirmed to have a c-axis texture, but has a structure in which many crystals with misaligned crystal orientation are aggregated.

[0165] Example 9 A free-standing thin film 1 and an electrochemical cell 1C were produced in the same manner as in Example 1, except that in the same manufacturing method as in Example 1, water-soluble SrAlO was used as the composition for release layer 300 instead of sublimable oriented ZnO, and no oxide layer 2 was formed. That is, a solid electrolyte membrane 10 was produced by sequentially depositing two amorphous complex oxide films 100A and 100B, which would become first and second polycrystalline layers 11A and 11B, on release layer 300, and no oxide layer 2 was formed on either side of the films, and a free-standing thin film 1 and an electrochemical cell 1C were produced.

[0166] Specifically, similarly to Example 1, substrate 200 having protective layer 201 formed thereon and a target having a SrAlO composition for separation layer 300 were disposed facing each other in a chamber, and separation layer 300 made of SrAlO was formed on substrate 200 with protective layer 201 interposed therebetween under the same conditions as in Example 1, except that the film formation time was set to 60 minutes. Thereafter, amorphous complex oxide film 100 was formed on separation layer 300 under the same conditions as in Example 1, without forming first oxide layer 2A, and heat-treated to form first polycrystalline layer 11A (thickness t1: 200 nm).

[0167] Furthermore, under the same conditions as in Example 1, an amorphous complex oxide film 100 was formed, and an electrode layer 31 to become the cathode 3 was formed without forming the second oxide layer 2B, followed by heat treatment to form a second polycrystalline layer 11B (thickness: 800 nm). Thereafter, the film was left to stand in pure water at room temperature for 72 hours to dissolve the peeling layer 300, thereby separating it from the substrate 200. Thereafter, an electrode layer 41 to become the anode 4 was formed, thereby completing the electrochemical cell 1C.

[0168] The self-supporting thin film 1 obtained in Example 9 was evaluated for film structure (parent phase) in the same manner as in Example 1. The results are also shown in Table 1.

[0169] As shown in Table 1 as the membrane structure (parent phase), it was confirmed that the solid electrolyte membrane 10 obtained in Example 9 was composed of two LSO thin films with the same composition as in Example 1. In addition, it had the same thickness t (1.0 μm) and membrane area S (25.0 cm) as in Example 1. 2 ), film density d (4.23 g / cm 2 ), and the c-axis orientation rate (0.98), while the average particle size was slightly larger at 420 nm. Thus, even when a water-soluble composition that is not lattice-matched is used as the release layer 300, by forming the solid electrolyte membrane 10 into a two-layered membrane and making the first polycrystalline layer 11A a thin film of 200 nm, a freestanding thin film 1 with high density and a high c-axis orientation rate can be obtained.

[0170] Next, for the electrochemical cells 1C of Examples 1 to 9 and Comparative Examples 1 and 2 obtained as described above, the oxide ion conductivity [S / cm] of the solid electrolyte membrane 10 of the freestanding thin film 1 was measured. The measurement was performed by joining Pt lead wires to the surfaces of the cathode 3 and anode 4 of the obtained electrochemical cell 1C, respectively, and based on the complex impedance method. The measurement conditions are shown below. The measurement results are also shown in Table 3. - Apparatus used: ModuLab XM ECS, manufactured by Hokuto Denko Corporation - Applied voltage: 0 V - Measurement amplitude: 10 mV - Measurement frequency: 1 MHz to 0.1 Hz - Measurement environment: 500°C air atmosphere (oxygen concentration 20.9%)

[0171] Furthermore, electrochemical cells 1C of Examples 1 to 9 and Comparative Examples 1 and 2 were subjected to hydrogen production tests by electrolysis of water vapor to evaluate their electrolysis capacity (hydrogen production capacity). The electrolysis device was configured so that a supply flow path for gas containing water vapor and a recovery flow path for gas containing the generated hydrogen were provided on the cathode 3 side of the electrochemical cell 1C. When water vapor was supplied to the electrolysis device at a constant gas flow rate, the components of the recovered gas were analyzed, and the amount of hydrogen produced per unit time and unit area [ml / min / cm] was calculated. 2 The Faraday efficiency, which is the proportion of current that contributed to hydrogen production, was calculated from the theoretical production amount for the input current and the calculated hydrogen production amount. These results are also shown in Table 2.

[0172]

[0173] As shown in Table 3, in the electrochemical cells 1C of Examples 1 to 9, the oxide ion conductivity in the solid electrolyte membrane 10 portion of the freestanding thin film 1 was in the range of −2.1 to −2.5 (S / cm). Compared with the oxide ion conductivity of Comparative Examples 1 and 2, which was −4.0 to −4.1 (S / cm), all of these were favorable values.

[0174] The electrolysis capacity (hydrogen generation capacity) of Examples 1 to 9 was 1.3 to 3.8 ml / min / cm 2 In Examples 3 to 5 in which the first oxide layer 2A was used as a lattice matching layer and in Example 6 in which the film thickness t was 5.0 μm, the 2 The Faraday efficiencies of Examples 1 to 9 were in the range of 90 to 98% and the Faraday efficiencies of Examples 3 to 6 were in the range of 95 to 98%, which indicates that hydrogen generation was carried out efficiently.

[0175] In contrast, the electrolysis capacity (hydrogen generation capacity) of Comparative Examples 1 and 2 was 0.2 to 0.5 ml / min / cm 2 The Faraday efficiency was 84 to 85% which were all lower than those of Examples 1 to 9.

[0176] The free-standing thin film 1 thus obtained is configured as a single-layer or multi-layer solid electrolyte membrane 10 with oxide layers 2 disposed on both sides thereof, and is suitable for use in electrochemical cells 1C and the like, contributing to improved cell characteristics. Furthermore, there is a high degree of freedom in the configuration of each layer, the selection of materials, and the formation method, etc., making it possible to realize a high-quality free-standing thin film 1 having a desired configuration.

[0177] In the above embodiment, one oxide layer 2 is provided on each side of the solid electrolyte membrane 10 in the thickness direction Z, but two or more layers may be provided on each side. For example, when applied to an electrochemical cell 1C, if there is a difference in thermal expansion coefficient between the oxide layer 2 and the electrode material, multiple oxide layers 2 made of different materials may be combined to reduce the difference in thermal expansion coefficient. Furthermore, multiple oxide layers 2 may be formed not only in the thickness direction Z but also in the in-plane direction. Furthermore, the thickness of the solid electrolyte membrane 10 may vary in the in-plane direction. In addition, the configuration, materials, etc. of the freestanding thin film 1 or the electrochemical cell 1C can be appropriately changed depending on the desired cell characteristics, etc.

[0178] The present disclosure is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present disclosure. Furthermore, the obtained freestanding thin film 1 can be used for any purpose, such as an exhaust gas sensor or an electrochromic display, in addition to the electrochemical cell 1C for a fuel cell.

[0179] Although the present disclosure has been described based on the embodiments, it is understood that the present disclosure is not limited to those embodiments or structures. The present disclosure also encompasses various modifications and modifications within equivalent ranges. In addition, various combinations and forms, and even other combinations and forms including only one element, more than one element, or less than one element, are also within the scope and concept of the present disclosure. The features of the present disclosure are as follows: [1] A free-standing thin film (1) including a solid electrolyte membrane (10) made of an apatite-type complex oxide, wherein the solid electrolyte membrane has one or more oriented polycrystalline layers (11) in the thickness direction (Z) in which oriented crystalline grains (C) of the apatite-type complex oxide are aggregated with their grain boundaries (C1) in contact with each other in the film plane direction (X, Y), and the film density d is 4.2 g / cm 2a first polycrystalline layer (11A) adjacent to the oxide layer in the thickness direction has a thickness t1 of 200 nm or less, and a second polycrystalline layer (11B) positioned on the opposite side of the oxide layer with respect to the first polycrystalline layer sandwiched therebetween has a thickness t2 / t1 of 1 or more and a thickness t2 / t1 ratio of 1 or more to 50 or less. [4] The free-standing thin film according to [2], wherein the solid electrolyte film is a monolayer film including one of the oriented polycrystalline layers, and the oxide layer is made of an oxide or a composite oxide whose lattice constant is an integer multiple of the lattice constant of the apatite-type composite oxide within 5% of that of the apatite-type composite oxide. [5] The free-standing thin film according to [1] or [2], wherein the solid electrolyte film is a monolayer film including one of the oriented polycrystalline layers, and the thickness of the solid electrolyte film is 200 nm or less. [6] The free-standing thin film according to [1] or [2], wherein the solid electrolyte film is a multilayer film including two or more of the oriented polycrystalline layers, and the thickness t1 of a first polycrystalline layer (11A) forming one surface (101) in the thickness direction is 200 nm or less, and the ratio t2 / t1 of the thickness t2 to the thickness t1 of a second polycrystalline layer (11B) located on the opposite side of the first polycrystalline layer from the one surface is 1 or more and 50 or less. [7] The oriented crystal grains have an average grain size of 400 nm or more, and the apatite-type composite oxide is a free-standing thin film according to any one of [1] to [6], which is represented by the following formula 1: Formula 1: A 10-x B 6-y M y O 27-zIn the formula, A represents one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr, and Ba; B represents an element containing Si or Ge, or both; M represents one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo; and x, y, and z represent numbers satisfying -0.5≦x≦2.0, 0.0≦y≦3.0, and -6.0≦z≦4.2, respectively. [8] A method for producing a freestanding thin film according to any one of [1] to [7], comprising: a release layer forming step of forming a release layer (300) made of a composition removable after crystallization of the apatite-type complex oxide on the surface of a substrate (200); an oxide layer forming step of forming an oxide layer (2) made of an oxide or complex oxide having oxygen absorbing / releasing properties on the surface of the release layer; a first solid electrolyte film forming step of forming an amorphous complex oxide film (100A) having the same composition as the apatite-type complex oxide on the surface of the oxide layer to a thickness of 200 nm or less, and heat-treating the film to form a first polycrystalline layer (11A) which is the oriented polycrystalline layer; and a second solid electrolyte film forming step of forming an amorphous complex oxide film (100B) having the same composition as the apatite-type complex oxide on the surface of the first polycrystalline layer, and heat-treating the film to form a second polycrystalline layer (11B) which is the oriented polycrystalline layer. and a peeling step of removing the release layer to peel off the solid electrolyte membrane from the substrate.[9] A method for producing a free-standing thin film according to any one of [1] to [7], comprising: a release layer forming step of forming a release layer (300) made of a composition that can be removed after crystallization of the apatite-type complex oxide on the surface of a substrate (200); an oxide layer forming step of forming an oxide layer (2) made of an oxide or complex oxide that has oxygen absorbing and releasing properties and whose integral multiple of the lattice constant is within ±5% of the lattice constant of the apatite-type complex oxide on the surface of the release layer; a solid electrolyte film forming step of forming an amorphous complex oxide film (100) having the same composition as the apatite-type complex oxide on the surface of the oxide layer, and heat-treating the film to form the oriented polycrystalline layer; and a peeling step of peeling off the solid electrolyte film from the substrate by removing the peel layer.

[10] An electrochemical cell (1C) comprising a solid electrolyte membrane (10) made of an apatite-type complex oxide, a cathode (3), and an anode (4), wherein the solid electrolyte membrane has one or more oriented polycrystalline layers (11) in the thickness direction (Z) in which oriented crystalline particles (C) of the apatite-type complex oxide are aggregated with their grain boundaries (C1) in contact with each other in the film plane direction (X, Y), and has a film density d of 4.2 g / cm. 2 or more, a c-axis orientation rate in the film thickness direction is 0.9 or more as calculated by the Lotgering method, the cathode and the anode are a pair of porous electrodes arranged between two opposing film surfaces (101, 102) of the solid electrolyte membrane, with an oxide layer (2) made of an oxide or composite oxide having oxygen absorbing / releasing properties interposed therebetween, the oxide layer having a thickness of 20 nm or more and 100 nm or less, and the film thickness of the solid electrolyte membrane is 0.2 μm or more and 10 μm or less.

Claims

1. A self-supporting thin film (1) comprising a solid electrolyte membrane (10) made of an apatite-type complex oxide, wherein the solid electrolyte membrane has one or more oriented polycrystalline layers (11) in the thickness direction (Z) in which oriented crystalline particles (C) of the apatite-type complex oxide are aggregated in the film plane direction (X, Y) with their grain boundaries (C1) in contact with each other, and the membrane density d is 4.2 g / cm 2 or more, and the c-axis orientation rate in the film thickness direction is 0.9 or more as calculated by the Lotgering method.

2. The freestanding thin film according to claim 1, wherein the apatite-type complex oxide has oxide ion conductivity, and an oxide layer (2) made of an oxide or complex oxide having oxygen absorbing / releasing properties is laminated in contact with at least one side of the solid electrolyte membrane in the thickness direction of the membrane.

3. The free-standing thin film according to claim 2, wherein the solid electrolyte film is a multilayer film including two or more of the oriented polycrystalline layers, and the thickness t1 of a first polycrystalline layer (11A) adjacent to the oxide layer in the film thickness direction is 200 nm or less, and the ratio t2 / t1 of the thickness t2 to the thickness t1 of a second polycrystalline layer (11B) located on the opposite side of the first polycrystalline layer from the oxide layer is 1 or more and 50 or less.

4. The free-standing thin film according to claim 2, wherein the solid electrolyte film is a single layer film including one of the oriented polycrystalline layers, and the oxide layer is made of an oxide or composite oxide whose lattice constant is an integer multiple of the lattice constant of the apatite-type composite oxide within 5% of the lattice constant of the apatite-type composite oxide.

5. The free-standing thin film according to claim 2, wherein the solid electrolyte film is a single layer film including one of the oriented polycrystalline layers, and the film thickness of the solid electrolyte film is 200 nm or less.

6. The free-standing thin film according to claim 1, wherein the solid electrolyte film is a multi-layer film including two or more of the oriented polycrystalline layers, and the thickness t1 of a first polycrystalline layer (11A) forming one surface (101) in the film thickness direction is 200 nm or less, and the ratio t2 / t1 of the thickness t2 to the thickness t1 of a second polycrystalline layer (11B) located on the opposite side of the first polycrystalline layer from the one surface is 1 or more and 50 or less.

7. The self-supporting thin film according to any one of claims 1 to 6, wherein the oriented crystal grains have an average grain size of 400 nm or more, and the apatite-type composite oxide is represented by the following formula 1. Formula 1: A 10-x B 6-y M y O 27-z In the formula, A represents one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr, and Ba; B represents an element containing Si or Ge, or both; M represents one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Y, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo; and x, y, and z represent numbers satisfying -0.5≦x≦2.0, 0.0≦y≦3.0, and -6.0≦z≦4.2, respectively.

8. A method for producing a freestanding thin film according to claim 1, comprising: a release layer forming step of forming a release layer (300) made of a composition removable after crystallization of the apatite-type complex oxide on the surface of a substrate (200); an oxide layer forming step of forming an oxide layer (2) made of an oxide or complex oxide having oxygen absorbing / releasing properties on the surface of the release layer; a first solid electrolyte film forming step of forming an amorphous complex oxide film (100A) having the same composition as the apatite-type complex oxide on the surface of the oxide layer to a thickness of 200 nm or less, and heat-treating the film to form a first polycrystalline layer (11A) which is the oriented polycrystalline layer; and a second solid electrolyte film forming step of forming an amorphous complex oxide film (100B) having the same composition as the apatite-type complex oxide on the surface of the first polycrystalline layer, and heat-treating the film to form a second polycrystalline layer (11B) which is the oriented polycrystalline layer. and a peeling step of removing the release layer to peel off the solid electrolyte membrane from the substrate.

9. A method for producing a free-standing thin film according to claim 1, comprising: a release layer forming step of forming a release layer (300) on the surface of a substrate (200) made of a composition that can be removed after crystallization of the apatite-type complex oxide; an oxide layer forming step of forming an oxide layer (2) on the surface of the release layer made of an oxide or complex oxide that has oxygen absorbing and releasing properties and whose integral multiple of the lattice constant is within ±5% of the lattice constant of the apatite-type complex oxide; a solid electrolyte film forming step of forming an amorphous complex oxide film (100) of the same composition as the apatite-type complex oxide on the surface of the oxide layer and heat-treating it to form the oriented polycrystalline layer; and a peeling step of peeling the solid electrolyte film from the substrate by removing the peel layer.

10. An electrochemical cell (1) comprising a solid electrolyte membrane (10) made of an apatite-type complex oxide, a cathode (3), and an anode (4), wherein the solid electrolyte membrane has one or more oriented polycrystalline layers (11) in the thickness direction (Z) in which oriented crystal grains (C) of the apatite-type complex oxide are aggregated in the film plane direction (X, Y) with their grain boundaries (C1) in contact with each other, and the film density d is 4.2 g / cm 2 or more, a c-axis orientation rate in the film thickness direction is 0.9 or more as calculated by the Lotgering method, the cathode and the anode are a pair of porous electrodes arranged between two opposing film surfaces (101, 102) of the solid electrolyte membrane, with an oxide layer (2) made of an oxide or composite oxide having oxygen absorbing / releasing properties interposed therebetween, the oxide layer having a thickness of 20 nm or more and 100 nm or less, and the film thickness of the solid electrolyte membrane is 0.2 μm or more and 10 μm or less.

Images