阅读说明：本技术 取向性磷灰石型氧化物离子导体及其制造方法 (Oriented apatite oxide ion conductor and method for producing same ) 是由 井手慎吾 城勇介 于 2019-02-14 设计创作，主要内容包括：本发明的取向性磷灰石型氧化物离子导体由复合氧化物形成,所述复合氧化物以A<Sub>9.33+x</Sub>[T<Sub>6.00-y</Sub>M<Sub>y</Sub>]O<Sub>26.0+z</Sub>(式中的A为选自La、Ce、Pr、Nd、Sm、Eu、Gd、Tb、Dy、Be、Mg、Ca、Sr及Ba中的一种或两种以上的元素。式中的T为包含Si或Ge或其两者的元素。式中的M为选自B、Ge、Zn、Sn、W及Mo中的一种或两种以上的元素。)表示,式中的x为-1.00～1.00,式中的y为0.40以上且低于1.00,式中的z为-3.00～2.00。(The oriented apatite oxide ion conductor of the present invention is formed of a composite oxide in which A is the component 9.33+x [T 6.00‑y M y ]O 26.0+z (wherein A is one or more elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr and Ba, T is an element containing Si or Ge or both of them, M is one or more elements selected from B, Ge, Zn, Sn, W and Mo), x is-1.00 to 1.00, y is 0.40 to less than 1.00, and z is-3.00 to 2.00).)

1. An oriented apatite oxide ion conductor formed of a composite oxide represented by formula A_9.33+x[T_6.00-yM_y]O_26.0+zWherein A is one or more elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr and Ba, T is an element containing Si or Ge or both of them, M is one or more elements selected from B, Ge, Zn, Sn, W and Mo, x is-1.00 to 1.00, y is 0.40 or more and less than 1.00, and z is-3.00 to 2.00.

2. The oriented apatite oxide ion conductor according to claim 1, wherein the degree of orientation as measured by the Lotgering method is 0.60 or more.

3. An electrode assembly comprising the oriented apatite oxide ion conductor according to claim 1 or 2 and electrodes laminated on both surfaces thereof.

4. A method for producing an oriented apatite oxide ion conductor, comprising the steps of: by mixing with A_{2.00+ x}TO_5.00+zHeating a precursor in a gas phase containing an M element, wherein A is 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, T is an element containing Si or Ge or both, and reacting the M element with the precursor to give an oriented apatite crystal structureWherein x is-1.00, z is-2.00, and M is one or more elements selected from B, Ge, Zn, Sn, W and Mo; and an annealing step.

Technical Field

The present invention relates to an oriented apatite oxide ion conductor which can be used as a solid electrolyte, a sensor, a catalyst, a separation membrane, and the like of various cells such as a solid electrolyte fuel cell (SOFC), an ion cell, and an air cell, and a method for producing the same.

Background

The applicant previously proposed to use A_9.33+x[T_6-yM_y]O_26.00+zThe oriented apatite oxide ion conductor is shown (see patent document 1). In the formula, A represents an element such as La. T represents Si or the like. M represents B, etc. The oxide ion conductor is prepared by mixing A_2+xTO_5+zThe precursor is obtained by heating a gas phase containing an M element to react the M element with the precursor. In order to obtain the above oxide ion conductor having an apatite crystal structure by this method, the amount of M element required needs to be 1.00 or more in y depending on the composition of the above precursor.

Disclosure of Invention

The oxide ion conductor represented by the above formula is a substance having excellent oxide ion conductivity. Oxide ion conductors are widely used in solid electrolytes, oxygen sensors, catalysts, separation membranes, and the like of various batteries such as solid electrolyte fuel cells, ion batteries, and air batteries, and further improvement in oxide ion conductivity is required. Accordingly, an object of the present invention is to further improve the conductivity of a conventionally known oriented apatite oxide ion conductor.

The present inventors have conducted intensive studies to solve the above problems, and as a result, have found that: by controlling the composition of the elements constituting the oriented apatite oxide ion conductor, the conductivity of oxide ions can be further improved.

The present invention has been made in view of the above-mentioned findings, and the above-mentioned problems can be solved by providing an oriented apatite oxide ion conductor formed of a composite oxide in which A is the component A_9.33+x[T_6.00-yM_y]O_26.0+z(wherein A is one or more elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr and Ba, T is an element containing Si or Ge or both of them, M is one or more elements selected from B, Ge, Zn, Sn, W and Mo), x is-1.00 to 1.00, y is 0.40 to less than 1.00, and z is-3.00 to 2.00).

The present invention also provides a method for producing an oriented apatite oxide ion conductor, comprising the steps of: by mixing with A_2.00+xTO_5.00+z(wherein A is 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; T is an element containing Si or Ge or both of them; x is-1.00 to 1.00; z is-2.00 to 2.00.) A precursor represented by the following formula is heated in a gas phase containing an M element (M is one or more elements selected from the group consisting of B, Ge, Zn, Sn, W and Mo) and the M element reacts with the precursor to produce an oriented apatite crystal structure; and an annealing step.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described. The inventors of the present invention have studied the oxide ion conductor described in patent document 1, and as a result, have found that: if the element M is introduced into the oxide ion conductor in an excessive amount, a decrease in conductivity is observed. Therefore, it is advantageous to prevent the excessive introduction of the M element from the viewpoint of suppressing the decrease in conductivity. The present inventors have made extensive studies and as a result, found a method for producing an oriented apatite oxide ion conductor having a small amount of M element introduced. The oxide ion conductor will be described in detail below.

< Oriented apatite oxide ion conductor >

An oriented apatite oxide ion conductor (hereinafter also referred to as "present oxide ion conductor") according to an example of the present embodiment is formed of a composite oxide having an apatite crystal structure (hereinafter also referred to as "present apatite composite oxide") represented by formula (1): a. the_9.33+x[T_6.00-yM_y]O_26.0+z(wherein A is one or more elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr and Ba, T is an element containing Si or Ge or both of them, M is one or more elements selected from B, Ge, Zn, Sn, W and Mo), x is-1.00 to 1.00, y is 0.40 to less than 1.00, and z is-3.00 to 2.00).

In the present specification, the term "orientation" of an oriented apatite oxide ion conductor means that an apatite oxide ion conductor as a polycrystalline body has an orientation axis having a uniform crystal axis, and includes uniaxial orientation and biaxial orientation. The apatite-type composite oxide preferably has c-axis orientation.

In the formula (1), La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr and Ba which are listed as A are elements having the following common points: the metal oxide is a positive ion, and can be a lanthanide or an alkaline earth metal that can form an apatite hexagonal structure. Among them, from the viewpoint of further improving the oxide ion conductivity, a combination with one or two or more elements of La, Nd, Ba, Sr, Ca, and Ce is preferable, and among them, a combination of one of La or Nd, or La and one or two or more elements of Nd, Ba, Sr, Ca, and Ce is preferable. T in formula (1) may be an element containing Si, Ge, or both of them.

The M element in the formula (1) passes through a metastable precursor (A described later) in the gas phase_2.00+xTO_5.00+z) The precursor is introduced, and as a result, the crystals of the precursor can be changed to an apatite crystal structure and oriented in one direction. From such a viewpoint, as the M element, any of the following elements may be used: the precursor is in a vapor phase at a temperature of 1000 ℃ or higher at which the precursor has an apatite crystal structure, and a desired vapor pressure can be obtained. The "required vapor pressure" means a vapor pressure that can move in a gas phase state in an atmosphere and can promote a reaction by diffusing grain boundaries or intragranular from the surface of the precursor toward the inside.

From such a viewpoint, the M element may be one or two or more elements selected from B, Ge, Zn, W, Sn, and Mo, for example. Among them, B, Ge and Zn are particularly preferable from the viewpoint of high degree of orientation and high productivity (orientation speed).

In formula (1): a. the_9.33+x[T_6.00-yM_y]O_26.0+zIn view of improving the degree of orientation and the oxide ion conductivity, x is preferably-1.00 to 1.00, more preferably 0.00 or more or 0.70 or less, and still more preferably 0.45 or more or 0.65 or less. From the viewpoint of burying the T element site in the crystal lattice of the apatite-type crystal and from the viewpoint of improving the conductivity of the target oxide ion conductor, y in formula (1) is preferably 0.40 or more and less than 1.00, of which 0.40 to 0.90 is preferable, further preferably 0.40 or more or 0.80 or less, particularly preferably 0.40 or more or 0.70 or less, and particularly preferably 0.50 to 0.70. From the viewpoint of maintaining the electroneutrality in the crystal lattice of the apatite form crystal, z in the formula (1) is preferably-3.00 to 2.00, and particularly preferably-2.00 or more or 1.50 or moreAmong them, more preferred is-1.00 or more or 1.00 or less.

In the formula (1), from the viewpoint of ensuring the space occupancy in the crystal lattice of the apatite-form crystal, the ratio (a/M) of the number of moles of a to the number of moles of M, that is, (9.33+ x)/y in the formula (1), is preferably 10.0 to 26.0, more preferably 10.0 or 26.0 or less, further preferably 11.0 or more or 26.0 or less, and particularly preferably 12.0 or more or 26.0 or less.

As formula (1): a. the_9.33+x[T_6.00-yM_y]O_26.0+zSpecific examples of (3) include La_9.33+xSi_5.2Zn_0.80O_26.0+z、La_9.33+xGe_5.09B_0.91O_26.0+z、Nd_9.33+xSi_5.15B_0.85O_26.0+zAnd the like. However, it is not limited to these.

The apatite composite oxide can be set to have a Lotgering orientation degree, which is an orientation degree measured by the Lotgering method, of 0.60 or more, of these, 0.80 or more, further 0.90 or more, and particularly 0.97 or more.

In order to set the degree of Lotgering orientation of the present apatite composite oxide to 0.60 or more, it is preferable to set the degree of Lotgering orientation to A_2.00+xTO_5.00+zThe precursors represented were prepared as a single phase and were high in density (relative density of 80% or more). However, it is not limited to such a method.

The apatite-type composite oxide can set the oxide ion conductivity to 10 at 500 DEG C^-4S/cm or more, wherein it can be set to 10^-3S/cm or more, particularly 10^-2And more than S/cm. In order to set the oxide ion conductivity at 500 ℃ of the present apatite-type composite oxide to 10^-4S/cm or more, and the Lotgering orientation degree is preferably set to 0.60 or more. However, it is not limited to such a method.

The apatite-type composite oxide can have a mobility of 0.8 or more, preferably 0.9 or more, and particularly preferably 0.95 or more. In order to transfer the apatite composite oxideThe ratio is set to 0.8 or more, and A is preferably set to_9.33+x[T_6.00-yM_y]O_26.0+zThe purity of (2) is set to 90 mass% or more. However, it is not limited to such a method.

< method for producing oxide ion conductor >

A method for producing an oxide ion conductor according to an example of the present embodiment (hereinafter also referred to as "the present production method") is a production method including the steps of: by mixing the compound represented by the formula (2): a. the_2.00+xTO_5.00+z(wherein A is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Sr and Ba; T is an element containing Si or Ge or both of them; x is-1.00 to 1.00, and z is-2.00 to 2.00.) A precursor represented by the following formula is heated in a gas phase containing an M element (M is one or more elements selected from the group consisting of B, Ge, Zn, Sn, W and Mo) and the precursor is reacted with the M element to form an oriented apatite crystal structure (hereinafter, also referred to as "gas-solid phase diffusion step"); and a step of annealing (hereinafter, also referred to as "annealing step"). The present production method may include a gas-solid phase diffusion step and an annealing step, and may optionally include additional steps.

According to the present production method, it is possible to suppress the occurrence of cracks and the like in the crystal, and therefore, not only can an oriented apatite oxide ion conductor having a larger area be produced, but also an oxide ion conductor having an apatite crystal structure in which the crystal is strongly oriented in one direction can be obtained. As a result, an oriented apatite composite oxide having a higher oxide ion conductivity can be obtained.

The precursor is reacted with the precursor from the surface thereof by the M element (cation) in the gas phase to form an oriented apatite composite oxide, and the reaction proceeds at the interface between the precursor and the produced apatite phase, whereby the entire precursor can be made into the oriented apatite composite oxide. In particular, if the oriented apatite composite oxide obtained by the gas-solid phase diffusion step is heated in a gas phase containing no M element in the annealing step performed after the step, the M element is re-vaporized, so that the content of the M element in the targeted oriented apatite composite oxide can be reduced, and a high degree of orientation can be achieved by crystal rearrangement. Therefore, the present oxide ion conductor described above can be produced by the present production method. However, the oxide ion conductor that can be produced by the present production method is not limited to the present oxide ion conductor described above.

(precursor)

The precursor in the present production method may be a non-oriented substance as long as it is a compound represented by the above formula (2).

The precursor may be, for example, a sintered body, a molded body, or a film body.

The precursor may be a compound obtained by a wet synthesis method such as a sol-gel method or a hydrothermal synthesis method using a target compound containing the elements a and T as a raw material, a compound obtained by sintering a compound containing the elements a and T, or a film formed by sputtering or the like, for example.

The sintered body of the precursor may be, for example, a composite oxide sintered body obtained by mixing two or more kinds of oxides by a solid phase method and heating, a green compact obtained by pressure-molding a powder obtained by pulverizing the sintered body, or a sintered body obtained by further heating and sintering the green compact (referred to as "composite oxide green compact sintered body"). Among these, the composite oxide compact sintered body is preferable in terms of the density of the final oriented apatite oxide ion conductor, and among these, a compact sintered body obtained by heat sintering a compact formed by pressure molding by Cold Isostatic Pressing (CIP) is particularly preferable, and a compact sintered body obtained by polishing the surface of the compact sintered body is more preferable. As a method for producing the precursor, it is preferable to heat and sinter the precursor at 1100 to 1700 ℃ in the air, and among them, it is more preferable to heat a mixture containing compounds a and T as raw materials at 1200 to 1700 ℃ in the air, and then heat and sinter the mixture again at 1300 to 1700 ℃ in the air in the form of a green compact. As the respective firing actions in the case of performing the two-time firing in this manner, the first firing mainly has an action of synthesizing the composite oxide, and the second firing mainly has an action of sintering the composite oxide.

The amount of the M element doped from the gas phase is determined according to the composition ratio of the precursors. Therefore, the amount of the M element in the apatite silicate, germanate or germanosilicate produced by the vapor phase method, i.e., the present apatite composite oxide, depends on the composition ratio of the precursor.

From such a viewpoint, x in the formula (2) is preferably-1.00 to 1.00, more preferably-0.40 or more or 0.70 or less, and still more preferably 0.00 or more or 0.60 or less.

From the viewpoint of maintaining electroneutrality in the crystal lattice of the precursor crystal and chemically maintaining the crystal structure, z in formula (2) is preferably-2.00 to 2.00, more preferably-0.60 or more or 1.00 or less, and still more preferably 0.00 or more or 0.70 or less.

As a specific composition example of the precursor, for example, La can be cited₂SiO₅、Nd₂SiO₅、LaNdSiO₅、La₂GeO₅And the like. However, it is not limited to these.

(gas phase-solid phase diffusion step)

The vapor-solid phase diffusion step in the present production method is characterized in that the oriented crystal grows from the vapor-solid phase interface. The M element is introduced from the gas phase, and an oriented sintered body having a desired composition can be obtained. At this time, the crystal is oriented in the process in which the M element in the gas phase enters the crystal through the surface of the precursor. Therefore, the orientation direction can be controlled by masking a part of the surface of the sintered precursor compact.

The M element may be an element that can be brought into a gas phase at 1000 ℃ or higher at which the precursor changes to the crystal structure of the apatite form to obtain a desired vapor pressure. Here, the term "required forThe vapor pressure "means a vapor pressure that can move in a gas phase in an atmosphere and can react by diffusing from the surface of the precursor toward the inside of the grain boundary or the intragranular portion. From such a viewpoint, the M element may be one or two or more elements selected from B, Ge, Zn, W, Sn, and Mo. These sintered bodies have an oriented apatite crystal structure in which an M element is introduced into a T site by a reaction between the M element in a gas phase and the surface of a precursor. For example, when the M element is B, B is an example of a compound containing the M element₂O₃、H₃BO₃、LaBO₃、LaB₆And the like. Amorphous materials such as borosilicate glass may also be used. On the other hand, when the M element is Zn, ZnO, Zn metal, Zn are exemplified₂SiO₄In the case of Ge, GeO may be mentioned₂Ge Metal, and in the case of W, WO may be mentioned₃、WO₂Examples of the metal include Sn, SnO₂Metals such as SnO and Sn, and in the case of Mo, MoO₂、MoO₃、MoSi₂Mo metal, etc.

The gas phase containing the M element may contain any one of ions containing the M element, vapor containing the M element, gas containing the M element, and the like. For example, the vapor phase may contain oxygen and vapor containing the M element. Therefore, the heating atmosphere in this case, that is, the atmosphere in the container containing the M element may be any of an atmospheric atmosphere, an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere, and may be in a pressurized state or a vacuum state.

In the vapor-solid phase diffusion step, as a specific method for heating the precursor in a vapor phase containing an M element, for example, the method may be carried out by using the above-mentioned A_2.00+xTO_5.00+zThe precursor and the compound containing the M element are charged into a container, for example, a closed container or a container with a lid, and heated to vaporize the compound containing the M element, and the atmosphere in the container is set to a gas phase atmosphere containing the M element to vaporize the M elementAnd reacting with the surface of the precursor. However, it is not limited to such a method. The "container" in the gas-solid phase diffusion step is a member that defines a space required to obtain the above-described "required vapor pressure", and examples thereof include a reaction tube, a chamber, and a covered sagger. However, it is not limited to these.

More specifically, by mixing La₂SiO₅Sintered body of composition and B₂O₃Heating the powder in the same alumina container with a cover at 1200-1600 deg.C to obtain B₂O₃The powder is gasified, and the atmosphere in the container is set to a gas phase atmosphere containing B element, thereby synthesizing c-axis oriented apatite La with B substituted at Si sites_9.33+x(Si_4.7B_1.3)O_26.0+z。

The heating temperature (set temperature of the furnace) in the gas-solid phase diffusion step is preferably set to 1000 ℃ or higher, more preferably 1100 ℃ or higher, and particularly preferably 1200 ℃ or higher. The upper limit of the heating temperature is not particularly limited, but can be understood as follows: the temperature of about 1700 ℃ at which the crystal structure of the apatite composite oxide can be maintained under atmospheric pressure becomes the upper limit temperature.

(annealing step)

This step is a step of further heating the oriented apatite composite oxide obtained in the gas-solid phase diffusion step. The oriented apatite composite oxide is preferably heated in a gas phase atmosphere containing no M element. By doing so, the M element contained in the crystal structure of the oriented apatite composite oxide obtained by the gas-solid phase diffusion step can be removed, and the content thereof can be reduced. In the annealing step, the oriented apatite composite oxide obtained in the gas phase-solid phase diffusion step may be taken out, and the oriented apatite composite oxide may be heated in a container that does not contain a gas containing an M element or a compound containing an M element. In the gas-solid phase diffusion step, when the M element is supplied by the gas flow, the annealing step may be performed continuously with the gas-solid phase diffusion step by stopping the gas flow. The atmosphere for annealing may be any of an atmospheric atmosphere, an oxidizing atmosphere, a reducing atmosphere, and an inert atmosphere, and may be a pressurized state or a vacuum state thereof.

Specifically, the c-axis oriented apatite A in which the amount y of substitution by B is preferably less than 1.00, more preferably 0.90 or less can be produced by heating the oriented apatite composite oxide at 1000 to 1600 ℃ in the atmosphere_9.33+x[T_6.00-yMy]O_26.0+z。

The heating temperature of the oriented apatite composite oxide in the annealing step is preferably set to 1000 ℃ or higher, more preferably 1100 ℃ or higher, and particularly preferably 1200 ℃ or higher. The upper limit of the heating temperature is not particularly limited, but the temperature in the vicinity of 1700 ℃ at which the crystal structure of the apatite-type composite oxide can be maintained is the upper limit temperature under atmospheric pressure. The annealing time is set to a temperature within this range, and is preferably set to 0.5 hours or more, or 3.0 hours or less, and more preferably set to 1.0 hours or more, or 2.0 hours or less.

In the annealing step, the oriented apatite composite oxide is heated at a temperature of preferably 1000 ℃ or higher as described above, thereby vaporizing the M element contained in the crystal structure and removing the M element from the oriented apatite composite oxide. Therefore, the container used in the annealing step is preferably an open-type non-closed container. When a closed container is used, it is preferable to remove the M element in the gas phase by heating while flowing a gas containing oxygen in the container.

< use >

As an example of the usage of the oxide ion conductor, a usage of a solid electrolyte having an electrode assembly having a structure in which electrodes are laminated on both surfaces of the oxide ion conductor is given. The shape of the oxide ion conductor is not limited. For example, the film may be in a cylindrical shape, in addition to a flat film shape. For example, when the oxide ion conductor has a cylindrical shape, electrodes are generally stacked on the inner and outer circumferential surfaces thereof.

When the electrode assembly as described above using the oxide ion conductor is used as a unit cell of a fuel cell (SOFC), for example, if a fuel gas is supplied to an anode electrode of the electrode assembly and an oxidizing agent (air, oxygen, or the like) is supplied to a cathode electrode and the electrode assembly is operated at 350 to 1000 ℃^2-The ions reach the anode electrode through the solid electrolyte, and are combined with hydrogen to release electrons, thereby generating power.

On the other hand, when the electrode assembly as described above using the present oxide ion conductor is used as an oxygen sensor, if one side of the electrode assembly is exposed to a reference gas and the opposite side is exposed to a measurement atmosphere, for example, an electromotive force is generated depending on the oxygen concentration of the measurement atmosphere. Therefore, for example, the reference gas is set to the atmosphere and the measurement atmosphere is set to the exhaust gas from the internal combustion engine, so that the reference gas and the measurement atmosphere can be used for the air-fuel ratio control of the exhaust gas.

In addition, when the electrode assembly as described above using the oxide ion conductor is used as an oxygen separation membrane, if air is supplied to the cathode electrode and the fuel cell (SOFC) is operated at 350 to 1000 ℃, oxygen molecules that have received electrons in the cathode become O molecules^2-The ions reach the anode electrode through the solid electrolyte, where they release electrons to O^2-The ions are bonded to each other, thereby allowing only oxygen molecules to permeate.

When the electrode assembly as described above using the present oxide ion conductor is used as a limiting current type oxygen sensor, the current value between the electrodes obtained from the oxygen concentration on the cathode electrode side of the electrode assembly is measured, and the electrode assembly can be used as an oxygen sensor.

In these applications, the thickness of the oxide ion conductor is preferably 0.01 μm to 1000 μm, more preferably 0.1 μm or more or 500 μm or less, from the viewpoint of suppressing the electric resistance and the production stability. The electrode used for the above application is preferably porous. The electrode material can be suitably used as a known material for the purpose, and the thickness thereof is preferably about 0.01 to 70 μm.

< description of terms >

In the present specification, the expression "X to Y" (X and Y are arbitrary numbers) includes the meaning of "X or more and Y or less" and also includes the meaning of "preferably more than X" or "preferably less than Y" unless otherwise specified. In addition, when the expression "X or more" (X is an arbitrary number) or "Y or less" (Y is an arbitrary number), the meaning of "preferably more than X" or "preferably less than Y" is also included.