JP7617982B2 - Solid electrolyte and solid electrolyte joint - Google Patents

Description

The present invention relates to a solid electrolyte and a solid electrolyte joint. The solid electrolyte and the solid electrolyte joint of the present invention are used in various fields that utilize their oxide ion conductivity.

Various oxide ion conductive solid electrolytes are known. Such solid electrolytes are used in various fields, for example, as oxygen permeation elements, electrolytes for fuel cells, and gas sensors. For example, Patent Document 1 describes an electrolyte-electrode assembly in which a solid electrolyte made of an apatite-type composite oxide is interposed between an anode electrode and a cathode electrode. An intermediate layer exhibiting isotropic oxide ion conduction is interposed between the cathode electrode and the solid electrolyte. The intermediate layer is made of cerium oxide doped with samarium, yttrium, gadolinium, or lanthanum. The solid electrolyte is made of La _x Si ₆ O _1.5X+12 (8≦X≦10). The document describes that this electrolyte-electrode assembly improves the power generation performance of a solid oxide fuel cell.

JP 2013-51101 A

As described in Patent Document 1, various devices using oxide-ion conductive solid electrolytes have been proposed, but when evaluated as a whole device, it cannot be said that the oxide-ion conductivity inherent to the solid electrolyte is fully utilized. In particular, even if the solid electrolyte itself has high oxide-ion conductivity, the electrical resistance at the interface between the solid electrolyte and the electrode may become high, resulting in high electrical resistance for the entire device.

The object of the present invention is therefore to provide a solid electrolyte capable of reducing the electrical resistance of a device having a solid electrolyte and a solid electrolyte junction having the solid electrolyte.

As a result of intensive research by the present inventors to solve the above problems, it was found that the above problems can be solved by using an apatite-type composite oxide having a specific crystal structure as a solid electrolyte. The present invention was made based on this finding, and solves the above problems by providing a solid electrolyte made of an apatite-type composite oxide having oxide ion conductivity, in which the value I ₀₀₂ /I ₀₀₄ , which is the ratio of the 002 peak intensity I ₀₀₂ to the 004 peak intensity I ₀₀₄ obtained by X-ray diffraction measurement, is 0.3 to 0.8.

The present invention also provides a solid electrolyte joint in which the above-mentioned solid electrolyte is joined to a metal oxide intermediate layer or a metal electrode layer.

FIG. 1 is a schematic diagram showing one embodiment of a device including a solid electrolyte of the present invention. FIG. 2 is a schematic diagram showing another embodiment of a device including a solid electrolyte of the present invention. FIG. 3 is a schematic diagram showing still another embodiment of a device including a solid electrolyte of the present invention.

The present invention will be described below based on its preferred embodiments. The solid electrolyte of the present invention is made of an apatite-type composite oxide having oxide ion conductivity. The solid electrolyte is a conductor in which oxide ions serve as carriers.

Examples of the apatite-type complex oxide include complex oxides containing lanthanum and silicon. As the apatite-type complex oxide, those containing lanthanum (La), a trivalent element, silicon (Si), a tetravalent element, and oxygen (O), and having a composition represented by La _x Si ₆ O _1.5x+12 (x represents a number of 8 or more and 10 or less), are preferred from the viewpoint of high oxide ion conductivity. The most preferred composition of this apatite-type complex oxide is La _9.33 Si ₆ O _26. This complex oxide can be produced, for example, according to the method described in JP-A-2013-51101.

Another example of the apatite-type complex oxide is a complex oxide represented by the general formula (1): _A9.33+x [ _{T6.00-yMy ] O26.0+z} . This complex oxide also has an apatite-type structure. In the formula, A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. In the formula, T is an element containing Si or Ge, or both. In the formula, M is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Ge, Zn, Sn, W, and Mo. From the viewpoint of enhancing the c-axis orientation, M is preferably one or more elements selected from the group consisting of B, Ge and Zn.

In the formula, x is preferably -1.33 or more and 1.50 or less, more preferably 0.00 or more and 0.70 or less, and even more preferably 0.45 or more and 0.65 or less, from the viewpoint of increasing the degree of orientation and oxide ion conductivity. In the formula, y is preferably 0.00 or more and 3.00 or less, more preferably 0.40 or more and 2.00 or less, and even more preferably 0.40 or more and 1.00 or less, from the viewpoint of filling the T element position in the apatite type crystal lattice. In the formula, z is preferably -5.00 or more and 5.20 or less, more preferably -2.00 or more and 1.50 or less, and even more preferably -1.00 or more and 1.00 or less, from the viewpoint of maintaining electrical neutrality in the apatite type crystal lattice.

In the above formula, the ratio of the number of moles of A to the number of moles of T, in other words (9.33+x)/(6.00-y) in the above formula, is preferably 1.33 or more and 3.61 or less, more preferably 1.40 or more and 3.00 or less, and even more preferably 1.50 or more and 2.00 or less, from the viewpoint of maintaining the spatial occupancy rate in the apatite-type crystal lattice.

Among the composite oxides represented by the general formula (1), it is preferable to use a composite oxide in which A is lanthanum, that is, a composite oxide represented by _La9.33+x [ _{T6.00-yMy ] O26.0+z} , from the viewpoint of further increasing the oxide ion conductivity. Specific examples of the composite oxide represented by La9.33 _{+ x} [ _{T6.00 - yMy} ]O26.0 _{+ z} include La9.33+x( _Si4.70B1.30 ) _O26.0+ z, _{La9.33+x (} Si4.70Ge1.30) _O26.0+z , _{La9.33+x(Si4.70Zn1.30)O26.0+z, La9.33+x(Si4.70W1.30)O26.0+z, La9.33+ x ( Si4.70Sn1.30 ) O26.0 + x , La9.33 + x ( Ge4.70B 1.30} )O _26.0+z , etc. The composite oxide represented by the general formula (1) can be produced, for example, according to the method described in US 2018/183068 A1.

Whether any of the above-mentioned apatite-type complex oxides is used as the apatite-type complex oxide, it is preferable that the complex oxide contains lanthanum, since this can effectively reduce the electrical resistance of the device containing the solid electrolyte of the present invention. In addition, regardless of which apatite-type complex oxide is used, it is preferable that the complex oxide is oriented along the c-axis. Oriented along the c-axis means that when the apatite-type complex oxide is polycrystalline, the crystal axis is aligned along the c-axis. Furthermore, when the apatite-type complex oxide exists as a single crystal, it is possible to make the c-axis direction coincide with the conduction direction of oxide ions in the device.

One of the characteristics of the apatite-type complex oxide is that, when the apatite-type complex oxide is subjected to X-ray diffraction measurement, the ratio of the peak intensity I ₀₀₂ of 002 to the peak intensity I ₀₀₄ of 004, I ₀₀₂ /I ₀₀₄ , is 0.3 or more and 0.8 or less. Until now, when the apatite-type complex oxide strongly oriented to the c-axis is subjected to X-ray diffraction measurement, the diffraction peak intensity other than 00x (x is an even number of 2 or more) is very small, making quantitative analysis difficult, which is a problem. However, the present inventor has found that the intensity ratio of the diffraction peaks of 002 and 004 strongly reflects the amount of defective sites, and further found that an apatite-type complex oxide having an intensity ratio within a specific range has a low interface resistance with the electrode layer and reduces the electrical resistance of the device as a whole. In particular, when the apatite-type complex oxide contains lanthanum, when the complex oxide maintains the apatite-type crystal structure and lanthanum is defective, oxygen in the structure is also defective in order to maintain the balance of charge. The inventors believe that when oxygen deficiency occurs, oxide ions become more mobile, improving oxide ion conductivity, and thus further lowering the interface resistance between the composite oxide and the electrode layer. In addition, A other than lanthanum is likely to enter the same crystal site as lanthanum in the apatite-type composite oxide, and therefore a similar mechanism to that described above is thought to occur. In this way, the present invention aims to increase oxide ion conductivity not due to the intermediate layer or electrode layer joined to the solid electrolyte, but due to the crystal structure of the solid electrolyte itself.

The greater the amount of vacancies in the apatite-type complex oxide, the smaller the ratio of I ₀₀₂ to I ₀₀₄ , and the lower the interfacial resistance between the apatite-type complex oxide and the electrode layer. From this viewpoint, the ratio of I ₀₀₂ to I ₀₀₄ is preferably 0.5 to 0.8, and more preferably 0.5 to 0.75.

The values of I ₀₀₂ and I ₀₀₄ are measured using an X-ray diffraction method. In particular, they can be measured using a point-converging X-ray focusing mirror (CMF optical system) and a zero-dimensional detector. The peak intensity of X-ray diffraction in the present invention refers to the integrated intensity.

In order to more accurately measure the amount of defects in the apatite crystal structure, the peak intensity ratio of 002 and 004, which provides a higher X-ray diffraction intensity, was selected as an index for this X-ray diffraction peak intensity. Note that the positions of the 002 and 004 X-ray diffraction peaks in the apatite crystal structure can be uniquely determined from a known crystal database, etc.

In order to make the ratio of I ₀₀₂ to I ₀₀₄ satisfy the above-mentioned range, it is preferable to form a deficiency site in the apatite-type complex oxide. In order to form a deficiency site, for example, it has been found as a result of the study by the present inventor that it is advantageous to form a film of a different composition adjacent to the apatite-type complex oxide and anneal the film to remove constituent elements, particularly lanthanum, from the crystal structure. In this case, it is necessary to remove constituent elements such as lanthanum while maintaining the apatite-type crystal structure. For this purpose, a film in which lanthanum can be diffused or in which lanthanum can be solid-dissolved (hereinafter also referred to as "La-absorbing film") is laminated on the surface of the apatite-type complex oxide. Then, it is preferable to obtain an apatite-type complex oxide in which lanthanum is deficient by annealing at a high temperature of 700°C or more to diffuse or dissolve lanthanum in the apatite-type complex oxide into the La-absorbing film. The La-absorbing film may then be removed by a predetermined means.

The solid electrolyte of the present invention can be used as is, or can be used in the form of a device in which it is combined with other components, depending on the specific application. An example of such a device is shown in FIG. 1. The device shown in FIG. 1 is a solid electrolyte assembly 10 that includes the solid electrolyte of the present invention. This solid electrolyte assembly 10 will be described below.

As shown in FIG. 1, the solid electrolyte junction 10 includes a layer 11 made of the above-mentioned solid electrolyte (hereinafter referred to as the "solid electrolyte layer"). An intermediate layer 12 is laminated on and in contact with the solid electrolyte layer 11 and is joined to one surface of the solid electrolyte layer 11. In the embodiment shown in FIG. 1, the solid electrolyte layer 11 and the intermediate layer 12 are in direct contact with each other, with no other layer interposed between them. The intermediate layer 12 is made of a material having oxide ion conductivity and electron conductivity.

From the viewpoint of effectively reducing the electrical resistance of the solid electrolyte assembly 10, the thickness of the solid electrolyte layer 11 is preferably 10 nm to 1000 μm, more preferably 50 nm to 700 μm, and even more preferably 100 nm to 500 μm. The thickness of the solid electrolyte layer 11 can be measured, for example, using a stylus step gauge, a vernier caliper, or an electron microscope.

As shown in FIG. 1, the solid electrolyte junction 10 may have a metal electrode layer 13 bonded to one of the two surfaces of the solid electrolyte layer 11, the surface opposite to the surface on which the intermediate layer 12 is disposed. The solid electrolyte layer 11, the intermediate layer 12, and the metal electrode layer 13 are disposed in this order to form a device 20 made of the solid electrolyte junction 10. In the embodiment shown in FIG. 1, the solid electrolyte layer 11 and the metal electrode layer 13 are in direct contact with each other, with no other layer interposed between them.

The intermediate layer 12 is composed of a metal oxide. The intermediate layer 12 is composed of only a metal oxide, or contains a metal oxide and other substances. The intermediate layer 12 is formed for the purpose of smoothly transferring oxide ions between the solid electrolyte layer 11. According to this embodiment, since the solid electrolyte having the above-mentioned specific crystal structure is used as the solid electrolyte layer 11, the interface resistance between the solid electrolyte layer 11 and the intermediate layer 12 can be reduced regardless of the type of the intermediate layer 12, and the transfer of oxide ions can be performed more smoothly.

As the intermediate layer 12, a material made of a metal oxide and having oxide ion conductivity is preferably used. As the intermediate layer 12, for example, cerium oxide containing one or more elements selected from the group consisting of samarium, yttrium, gadolinium, and lanthanum (hereinafter, these elements are also referred to as "doping elements" for convenience) can be used. In this case, the content of the doping element is preferably 0.1 mol% or more and 0.5 mol% or less, more preferably 0.15 mol% or more and 0.4 mol% or less, and even more preferably 0.2 mol% or more and 0.3 mol% or less, expressed as M/(M+Ce)×100 (M represents the number of moles of the doping element).

The intermediate layer 12 may be made of an oxide of bismuth. Examples of the oxide of bismuth include bismuth (III) oxide and a composite oxide of bismuth and another metal element. Examples of the other metal element include one or more rare earth elements. Examples of the rare earth element include lanthanum, gadolinium, yttrium, erbium, ytterbium, and dysprosium. In particular, the intermediate layer 12 is preferably made of a composite oxide of bismuth and lanthanum, gadolinium, or yttrium, from the viewpoint of effectively reducing the electrical resistance between the solid electrolyte layer 11 and the intermediate layer 12. Furthermore, this composite oxide is preferably represented by (Ln _m Bi _n ) ₂ O _3. In the formula, Ln represents a rare earth element. The sum of m and n is 1, and n>0. In addition, m is preferably 0.1 to 0.4.

A mixed conductor having oxide ion conductivity and electronic conductivity can also be used as the intermediate layer 12. A mixed conductor is a material having two conductivities, oxide ion conductivity and electronic conductivity, i.e., mixed conductivity. In particular, from the viewpoint of achieving lattice matching of a plane perpendicular to the orientation direction at the interface between the solid electrolyte layer 11 and the intermediate layer 12, it is preferable that the material constituting the intermediate layer 12 is a perovskite-type oxide. In particular, when the material constituting the solid electrolyte layer 11 is represented by the above-mentioned general formula (1), the lattice matching can be successfully achieved if the material constituting the intermediate layer 12 is a perovskite-type oxide.

In particular, it is preferable to use a material represented by general formula (2): _ABO3 as the material constituting the intermediate layer 12 from the viewpoint of further improving oxide ion conductivity. In the formula, A is preferably one or more metal elements selected from, for example, La, Sr, Ba, and Ca, and a particularly preferred metal element is at least one of La and Sr. B is a transition metal element, and is preferably one or more metal elements selected from Co, Ni, Mn, Cr, Ti, Fe, and Cu, and a particularly preferred metal element is at least one of Co and Ni. In particular, the material represented by general formula (2) is preferably a composite oxide containing La, Sr, Co, and Ni.

Among the composite oxides represented by the general formula (2), the one represented by La _0.6 Sr _0.4 Co _0.9 Ni _0.1 O _3-δ is particularly preferred.

The intermediate layer 12 made of the composite oxide represented by general formula (2) can be formed on one side of the solid electrolyte layer 11 by using, for example, various thin film formation methods. Examples of thin film formation methods include physical vapor deposition and chemical vapor deposition, and among these, the intermediate layer 12 can be formed more successfully by using the physical vapor deposition method. Among the physical vapor deposition methods, it is particularly preferable to use the PLD (Pulsed Laser Deposition) method.

Regardless of which of the above-mentioned materials constitute the intermediate layer 12, it is effective from the viewpoint of reducing the interface resistance that the material constituting the solid electrolyte layer 11 and the material constituting the intermediate layer 12 are both uniaxially oriented along the stacking direction of the solid electrolyte layer 11 and the intermediate layer 12. In order to uniaxially orient the material constituting the solid electrolyte layer 11 and the material constituting the intermediate layer 12 along the stacking direction, for example, while heating the solid electrolyte layer 11 as the substrate to a predetermined temperature, in an atmosphere with a controlled oxygen partial pressure, a physical vapor deposition method or a chemical vapor deposition method may be used to form a thin film of the intermediate layer 12 on the solid electrolyte layer 11, and locally epitaxially grow the thin film. In addition, a uniaxially oriented thin film of the intermediate layer 12 can also be formed on the solid electrolyte layer 11 using atomic layer deposition (ALD). However, the method is not limited to these methods.

Whether or not the material constituting the solid electrolyte layer 11 and the material constituting the intermediate layer 12 are both uniaxially oriented can be determined by observing the cross section of the joint interface with a transmission electron microscope (TEM). The lattice constants and interplanar spacings of the solid electrolyte layer 11 and the intermediate layer 12 can be calculated from the diffraction pattern obtained by performing X-ray diffraction measurement while rocking.

Regardless of which of the above-mentioned materials is used to compose the intermediate layer 12, the intermediate layer 12 can effectively reduce the electrical resistance between the intermediate layer 12 and the solid electrolyte layer 11 as long as it has a predetermined thickness. In particular, the thickness of the intermediate layer 12 bonded to the solid electrolyte layer 11 in the stacking direction is preferably 80 nm or more, more preferably 100 nm or more, and even more preferably 100 nm or more and 1000 nm or less. The thickness of the intermediate layer 12 can be measured using a stylus step gauge or an electron microscope.

The metal electrode layer 13 formed on the opposite side of the solid electrolyte layer 11 to the intermediate layer 12 is preferably composed of an element of the platinum group, since it has advantages such as ease of formation and high catalytic activity. Examples of elements of the platinum group include platinum, ruthenium, rhodium, palladium, osmium, and iridium. These elements can be used alone or in combination of two or more. In addition, a cermet containing an element of the platinum group can be used as the metal electrode layer 13.

The solid electrolyte junction 10 and device 20 of the embodiment shown in FIG. 1 can be suitably manufactured, for example, by the method described below. First, the solid electrolyte layer 11 is manufactured by a known method. For the manufacturing, for example, the methods described in JP 2013-51101 A and US 2018/183068 A1 can be adopted.

Next, the intermediate layer 12 is formed on one of the two faces of the solid electrolyte layer 11. Various thin film formation methods can be used to form the intermediate layer 12. As one of the thin film formation methods, for example, the PLD method described above can be used. Specifically, in order to uniaxially orient both the material constituting the solid electrolyte layer 11 and the material constituting the intermediate layer 12 along the stacking direction of the solid electrolyte layer 11 and the intermediate layer 12, the PLD method described above is used, and when the intermediate layer 12 is formed on one face of the solid electrolyte layer 11, the solid electrolyte layer 11 may be heated to a predetermined temperature. The heating temperature is preferably set to, for example, 600°C or higher and 700°C or lower in order to more successfully achieve uniaxial orientation.

After the intermediate layer 12 is formed in this manner, the metal electrode layer 13 is formed on the surface of the solid electrolyte layer 11 opposite to the surface on which the intermediate layer 12 is formed. To form the metal electrode layer 13, for example, a paste containing particles of a platinum group metal is used. The paste is applied to the surface of the solid electrolyte layer 11 to form a coating film, and the coating film is fired to form an electrode made of a porous body. The firing conditions can be a temperature of 600°C or higher and a time of 30 minutes to 120 minutes. The atmosphere can be an oxygen-containing atmosphere such as air.

The above method provides the desired solid electrolyte junction 10 and device 20. The device 20 thus obtained is suitable for use as, for example, an oxygen permeable element, various gas sensors including an oxygen sensor, steam electrolysis, or a solid electrolyte fuel cell, taking advantage of its high oxide ion conductivity. Whatever the application of the device 20, it is advantageous to use the intermediate layer 12 as a cathode, that is, as an electrode where a reduction reaction of oxygen gas occurs. For example, when the device 20 is used as an oxygen permeable element, the metal electrode layer 13 is connected to the anode of a DC power source, and the intermediate layer 12 is connected to the cathode of the DC power source, and a predetermined DC voltage is applied between the intermediate layer 12 and the metal electrode layer 13. As a result, oxygen receives electrons on the intermediate layer 12 side, and oxide ions are generated. The generated oxide ions move through the solid electrolyte layer 11 to reach the metal electrode layer 13. The oxide ions that reach the metal electrode layer 13 release electrons and become oxygen gas. This reaction allows the solid electrolyte layer 11 to transmit oxygen gas contained in the atmosphere on the intermediate layer 12 side to the metal electrode layer 13 side through the solid electrolyte layer 11. If necessary, a current collecting layer made of a conductive material such as platinum may be formed on at least one of the surfaces of the intermediate layer 12 and the metal electrode layer 13.

The applied voltage is preferably set to 0.1 V or more and 4.0 V or less from the viewpoint of increasing the amount of oxygen gas permeation. When a voltage is applied between the two electrodes, it is preferable that the oxide ion conductivity of the solid electrolyte layer 11 is sufficiently high. For example, it is preferable that the oxide ion conductivity is 1.0×10 ⁻³ S/cm or more in terms of conductivity. For this purpose, it is preferable to hold the solid electrolyte layer 11 or the entire device 20 at a predetermined temperature. This holding temperature depends on the material of the solid electrolyte layer 11, but is generally preferably set in the range of 300° C. or more and 600° C. or less. By using the device 20 under this condition, oxygen gas contained in the atmosphere on the intermediate layer 12 side can be permeated through the solid electrolyte layer 11 to the metal electrode layer 13 side.

When the device 20 is also used as a limiting current type oxygen sensor, a current is generated due to the oxide ions generated on the intermediate layer 12 side migrating to the metal electrode layer 13 side via the solid electrolyte layer 11. Since the current value depends on the oxygen gas concentration on the intermediate layer 12 side, the oxygen gas concentration on the intermediate layer 12 side can be measured by measuring the current value.

Although the present invention has been described above based on its preferred embodiment, the present invention is not limited to the above embodiment. For example, in the above embodiment, the intermediate layer 12 is disposed only on one side of the solid electrolyte layer 11. Alternatively, as shown in FIG. 2, a separate intermediate layer 12' may be disposed on the surface of the solid electrolyte layer 11 facing the intermediate layer 12. When a separate intermediate layer 12' is disposed on the surface of the solid electrolyte layer 11 facing the intermediate layer 12, the intermediate layers 12, 12' may be the same or different. In this case, it is preferable that the material constituting the solid electrolyte layer 11, the material constituting one intermediate layer 12, and the material constituting the other intermediate layer 12' are all uniaxially oriented along the lamination direction.

In addition, instead of forming intermediate layers 12, 12' on each surface of the solid electrolyte layer 11, metal electrode layers 13, 13' may be formed as shown in FIG. 3. Even in this case, the electrical resistance of the entire device 20 can be reduced. Each metal electrode layer 13, 13' may be the same or different.

The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

Example 1
_La2O3 powder and _SiO2 powder were blended in a molar ratio of 1: ₁ , and ethanol was added and mixed in a ball mill. The mixture was dried, pulverized in a mortar, and fired in an air atmosphere at 1650°C for 3 hours using a platinum crucible. Ethanol was added to the fired product and pulverized in a ball mill to obtain a fired powder. The fired powder was placed in a 20 mmφ molding machine and pressed from one direction to perform uniaxial molding. Further, cold isostatic pressing (CIP) was performed at 700 MPa for 1 minute to form a pellet. The pellet-shaped molded body was heated in air at 1600°C for 3 hours to obtain a pellet-shaped sintered body. When the sintered body was subjected to powder X-ray diffraction measurement and chemical analysis, it was confirmed that _it had a structure of _La2SiO5 having an apatite-type crystal structure.

800 mg of the pellets obtained and 140 mg of B ₂ O ₃ powder were placed in a sagger with a lid. Using an electric furnace, the pellets and B ₂ O ₃ powder were heated in the air at 1550° C. (furnace atmosphere temperature) for 50 hours. This heating generated B ₂ O ₃ vapor in the sagger and reacted the B ₂ O ₃ vapor with the pellets to obtain the desired solid electrolyte layer 11. This solid electrolyte layer 11 had a structure of La _9.33+x [Si _6.00-y B _y ]O _26.0+z , where x=0.50, y=1.17, and z=0.16, and the molar ratio of La to Si was 2.04. The thickness measured by a vernier caliper was 350 μm. Hereinafter, this compound is abbreviated as "LSBO".

[Production of La-absorbing film]
The powder _{of Bi2O3} was placed in a 50 mmφ molding machine, pressed from one direction to perform uniaxial molding, and then hot press sintering was performed. The sintering conditions were a nitrogen gas atmosphere, a pressure of 30 MPa, a temperature of 600°C, and 3 hours, to obtain a target for sputtering. Using this target, sputtering was performed on both sides of the solid electrolyte layer 11 made of LSBO by high-frequency sputtering to a thickness of 300 _nm . The sputtering conditions were an RF output of 30 W and an argon gas pressure of 0.8 Pa. After the sputtering was completed, annealing was performed in the atmosphere at 750°C for 1 hour. By this annealing, La in the solid electrolyte was diffused to the _Bi2O3 layer (La absorption layer) side, and La-deficient LSBO was formed. Next, an operation was performed to remove the layer in which La was absorbed from this solid electrolyte. In detail, the _Bi2O3 layer _in which La was absorbed was completely removed using a precision polishing device. The thickness of the La-deficient LSBO was then measured with a vernier caliper and found to be 340 μm.

Example 2
La-deficient LSBO was produced in the same manner as in Example 1, except that the La absorption layer was annealed at 800° C. for 1 hour.

Example 3
La-deficient LSBO was produced in the same manner as in Example 1, except that the La absorption layer was annealed at 850° C. for 1 hour.

Comparative Example 1
A solid electrolyte was produced in the same manner as in Example 1, except that the La absorption layer was not formed and therefore annealing was not performed.

[Evaluation 1]
The solid electrolytes obtained in the examples and comparative examples were subjected to XRD measurement by the method described below to determine the I ₀₀₂ /I ₀₀₄ value. In addition, the oxide ion conductivity was measured by the method described below. The results are shown in Table 1 below.

[XRD Measurement]
Measurement was carried out using a fully automated multipurpose X-ray diffractometer SmartLab manufactured by Rigaku Corporation under the following conditions.
Tube voltage: 40 kV
Tube current: 30mA
X-ray source: CuKα
Incident optical element: confocal mirror (CMF)
Incident side slit configuration: Collimator size 1.4mm x 1.4mm
Receiving side slit configuration: parallel slit analyzer 0.114 deg, receiving slit 20 mm
Detector: scintillation counter Measurement range: 2θ = 20 to 60 deg
Step width: 0.01 deg
Scan speed: 1 deg/min. Analysis was performed using PDXL2 from Rigaku Corporation. A straight line connecting the end points was selected for the background, and a split pseudo-Voigt function was selected for the peak shape, and profile fitting was performed to obtain the peak intensities (integral intensities) of 002 and 004. As described above, the 002 peak and the 004 peak are c-axis oriented, and are independent peaks that do not overlap with peaks from other planes. If there is an overlap with a peak from another phase, which is expected when the device is configured as a multilayer structure, the peaks can be resolved in the same way as in general XRD data analysis, so that only the peak intensities of 002 and 004 can be obtained.

[Measurement of oxide ion conductivity]
Platinum films of 150 nm thickness were formed on both sides of the solid electrolyte by sputtering to form electrodes. The solid electrolyte was placed in a heating furnace, and complex impedance analysis of the solid electrolyte was performed by changing the temperature of the heating furnace. An impedance measuring device was used for the analysis, and the frequency was set to 0.1 Hz to 32 MHz. The oxide ion conductivity (S/cm) was calculated from the total resistance component (intragranular resistance + grain boundary resistance). The oxide ion conductivity at 600°C is shown in Table 1.

As is clear from the results shown in Table 1, the solid electrolytes obtained in the examples in which the value of I ₀₀₂ /I ₀₀₄ was within a specific range had a conductivity that was about 5 times or more higher than that of the solid electrolytes of the comparative examples, and thus had a high effect of improving oxide ion conductivity.

Example 4
An intermediate layer 12 made of _Bi2O3 containing _yttrium was formed on both sides of the solid electrolyte obtained in Example 1. A target manufactured by the following method was used to form the intermediate layer 12. The target was used to perform sputtering on both sides of the solid electrolyte to form an intermediate layer. After the intermediate layer was formed, heating was performed at 700°C for 1 hour to manufacture a solid electrolyte joint 10.

[Target manufacturing]
A predetermined amount of _{Y2O3 powder} and _{Bi2O3 powder} were mixed, ethanol was added, and the mixture was mixed in a ball mill. The mixture was dried, crushed in a mortar, and fired in an alumina crucible at 700°C in air for 3 hours. Ethanol was added to the fired product and crushed in a planetary ball mill to obtain a fired powder. The fired powder was placed in a 50 mmφ molding machine and pressed from one direction to perform uniaxial molding, followed by hot press sintering. The sintering conditions were a nitrogen gas atmosphere, a pressure of 30 MPa, a temperature of 600°C, and 3 hours. In this way, a target for sputtering was obtained.

Example 5
A target made of cerium oxide containing samarium was used instead of the target used in Example 4. This target was manufactured in the same manner as the target used in Example 4. Using this target, an intermediate layer 12 made of cerium oxide containing samarium was manufactured in the same manner as in Example 4. In this manner, a solid electrolyte joint 10 was manufactured.

Comparative Example 2
An intermediate layer made of cerium oxide containing samarium was formed on both sides of the solid electrolyte obtained in Comparative Example 1. The intermediate layer was formed in the same manner as in Example 5.

[Measurement of current density]
The electrical resistance of the solid electrolyte junctions obtained in Examples 4 and 5 and Comparative Example 2 was measured. The measurement was performed at 600° C. A direct current voltage of 1 V was applied between the intermediate layers in the atmosphere, and the current density was calculated from the obtained current value. The results are shown in Table 2.

As is clear from the results shown in Table 2, the solid electrolyte junction in which an intermediate layer is provided on a solid electrolyte having a specific range of I ₀₀₂ /I ₀₀₄ value can obtain a current density value that is about 7 times or more higher than that of the solid electrolyte junction of the comparative example.

The present invention provides a solid electrolyte that can reduce the electrical resistance of a device that includes the solid electrolyte. The present invention also provides a solid electrolyte assembly that has low electrical resistance.

Claims (7)

A solid electrolyte joint is formed by joining a solid electrolyte made of an apatite-type composite oxide having oxide ion conductivity, the solid electrolyte having a ratio of a 002 peak intensity I ₀₀₂ to a 004 peak intensity I ₀₀₄ (I ₀₀₂ /I ₀₀₄₎ of 0.3 to 0.8 as determined by X-ray diffraction measurement, and an intermediate layer made of a metal oxide,
The composite oxide has a general formula: _A9.33+x [ _{T6.00-yMy ] O26.0+z} (wherein A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, and Ba. T is an element containing Si or Ge, or both. M is Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, and T is one or more elements selected from the group consisting of Ge, Zn, Sn, W and Mo. In the formula, x is a number from -1.33 to 1.50, y is a number from 0.40 to 2.00, and z is a number from -5.00 to 5.20, and a ratio of the number of moles of A to the number of moles of T is 1.33 to 3.61. The solid electrolyte junction according to claim 1, in which the solid electrolyte is oriented along the c-axis. The solid electrolyte junction according to claim 1 or 2, wherein the metal oxide is cerium oxide containing one or more elements selected from the group consisting of samarium, yttrium, gadolinium, and lanthanum. The solid electrolyte joint according to claim 1 or 2, wherein the metal oxide is an oxide of bismuth. The solid electrolyte junction according to claim 1 or 2, wherein the metal oxide is a mixed conductor having oxide ion conductivity and electronic conductivity. The solid electrolyte junction according to claim 1 or 2, wherein an intermediate layer that is the same as or different from the intermediate layer is disposed on the surface opposite to the surface on which the intermediate layer is disposed in the solid electrolyte junction. The solid electrolyte junction according to claim 1 or 2, which is used as an oxygen permeation element, a gas sensor, or a solid electrolyte fuel cell.

Images