JP3564535B2 - Method for producing high-density sintered lanthanum-based composite oxide having perovskite-type crystal structure - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure that can be used as an electrolyte member of a solid oxide fuel cell (hereinafter referred to as an SOFC), a steam electrolyzer, an oxygen separator, and the like. It relates to a manufacturing method.
[0002]
[Prior art]
Conventionally, a sintered body of a lanthanum-based composite oxide having a perovskite-type crystal structure has been prepared by a solid-state reaction method using starting materials such as oxides, carbonates, nitrates, and hydroxides of metal elements constituting the sintered body. It is produced by a method of synthesizing a composite oxide powder by a hydrolysis method, a sol-gel method, a hydrothermal method, a spray pyrolysis method or the like, and finally sintering it in an electric furnace. Among these composite oxides, lanthanum gallate (LaGaO ₃ ) -based oxides having high oxide ion conductivity can be used as electrolyte members for SOFCs, steam electrolyzers, oxygen separators, and the like. Requires a dense sintered body without gas permeation.
[0003]
However, in general, a lanthanum-based composite oxide having a perovskite-type crystal structure has poor sinterability, and a conventional method of manufacturing a sintered body using an electric furnace has a relative density of about 95% or more required as an electrolyte member. Is limited to lanthanum gallate (LaGaO ₃ ) -based oxides, lanthanum indate (LaInO ₃ ) -based oxides, and the like. Even with these oxides, firing at a high temperature of about 1500 ° C. or more for 10 hours or more is required to obtain a high-density sintered body, and a large amount of energy is required to produce a sintered body.
[0004]
[Problems to be solved by the invention]
A main object of the present invention is to provide a lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure having high oxide ion conductivity, which can be used as an electrolyte member of an SOFC, a steam electrolyzer, an oxygen separator, and the like. Is to provide a production method that can be produced in a short time at a low temperature with good reproducibility.
[0005]
[Means for Solving the Problems]
The present inventor has conducted intensive studies in view of the above-described problems of the prior art, and as a result, a method of sintering a lanthanum-based composite oxide having a specific composition by applying a DC pulse current under pressure. For example, they have found that a high-density sintered body having high oxide ion conductivity and no gas permeation can be manufactured at a relatively low temperature in a short time, and the present invention has been completed.
[0006]
That is, the present invention provides a lanthanum-based composite oxide high-density sintered body having the following perovskite-type crystal structure and a method for producing the same.
1. General formula:
_{(A x B 1-x)} (C y D 1-y) O 2.8 ~ 3 (1)
(Where A is lanthanum, B is at least one selected from calcium and strontium, C is at least one selected from aluminum, gallium, scandium and indium, D is at least one selected from magnesium and cobalt, x is 0.8 to 1 and y is 0.8 to 1.) A method for producing a lanthanum-based composite oxide high-density sintered body having a perovskite crystal structure represented by the following general formula ( A perovskite-type crystal structure characterized in that a composite oxide powder containing the elements A to D in 1) in the same ratio as in the general formula (1) is used as a raw material and is sintered by applying a DC pulse current under pressure. A method for producing a lanthanum-based composite oxide high-density sintered body having:
2. A method for producing a high-density sintered lanthanum-based composite oxide having a perovskite-type crystal structure, wherein the sintered body is heat-treated after being electrically sintered by the method of the above item 1.
3. The method for producing a lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure according to the above item 1 or 2, which is electrically sintered at 1000 to 1600 ° C. under a pressure of 20 to 60 MPa.
4. A lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure obtained by any one of the above items 1 to 3.
5. Item 5. The high-density lanthanum-based composite oxide sintered body having a perovskite-type crystal structure according to Item 4, having a density of 95% or more with respect to the theoretical density.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
According to the production method of the present invention, a high-density lanthanum-based composite oxide having a perovskite-type crystal structure is sintered by using a lanthanum-based composite oxide powder as a raw material and conducting sintering with a DC pulse current under pressure. You can get union.
[0008]
Hereinafter, the method for producing a lanthanum-based composite oxide sintered body of the present invention will be described in detail.
[0009]
Lanthanum-based composite oxide powder In the method of the present invention, as a raw material, a composite oxide powder containing a metal element in the same ratio as the target lanthanum-based composite oxide sintered body is used. That is, in the present invention, the target compound represented by the general formula:
_{(A x B 1-x)} (C y D 1-y) O 2.8 ~ 3 (1)
(Where A is lanthanum, B is at least one selected from calcium and strontium, C is at least one selected from aluminum, gallium, scandium and indium, D is at least one selected from magnesium and cobalt, x is 0.8-1 and y is 0.8-1.)
A composite oxide powder containing the elements A to D in the lanthanum-based composite oxide sintered body represented by the following formula in the same ratio as in the general formula (1) is used as a raw material.
[0010]
The method for producing the composite oxide powder is not particularly limited. For example, using various compounds containing each of the elements A to D alone or two or more as the raw materials, the elements A to D are mixed at a predetermined ratio. Various known synthesis methods such as a solid phase reaction method, a hydrolysis method, a sol-gel method, a hydrothermal method, and a spray pyrolysis method can be adopted by mixing the raw material compounds as described above. Since the composite oxide powder has a perovskite crystal structure due to current sintering described later, it does not need to have a complete perovskite crystal structure when used as a raw material. It may have an amorphous structure.
[0011]
The particle size of the composite oxide powder is not particularly limited, but is preferably about 10 μm or less, and about 5 μm from the viewpoint of sinterability during electric current sintering described below. The average particle diameter is more preferably about 1 μm or less.
[0012]
Electric current sintering method In the method of the present invention, after forming the above-mentioned composite oxide powder into a predetermined shape, by sintering by applying a DC pulse current under pressure, the composite oxide powder has a perovskite-type crystal structure. A high-density sintered body of a lanthanum-based composite oxide can be obtained.
[0013]
As the electric sintering method, for example, a pressure sintering method for applying a DC pulse current, such as a discharge plasma sintering method, a discharge sintering method, or a plasma activated sintering method, can be employed. Specifically, for example, a raw material powder is filled in a jig having a predetermined shape and compressed to form a green compact. The green compact is preferably applied at about 20 to 60 MPa, more preferably about 30 to 50 MPa. For example, a pulsed ON-OFF DC current having a pulse width of about 2 to 3 milliseconds and a cycle of about 3 Hz to 300 kHz, preferably about 10 Hz to 100 Hz may be applied while applying pressure.
[0014]
By applying a DC pulse current in such a manner, the discharge phenomena generated in the gaps between the particles of the filled raw material powder are utilized, and the cleaning action of the particle surface by discharge plasma, discharge impact pressure, etc. and the electric field are generated. The electric field diffusion effect, the thermal diffusion effect by Joule heat, the plastic deformation pressure by pressurization, and the like serve as a driving force for sintering, thereby promoting sintering.
[0015]
The sintering temperature is preferably about 1000 to 1600C, more preferably about 1200 to 1500C. An appropriate temperature range may be selected from the range of the sintering temperature according to the kind of the target composite oxide sintered body.
[0016]
The sintering temperature can be adjusted by the peak current value of the DC pulse current.If the same sintering jig is used, increasing the peak current value of the pulse current increases the sintering temperature. The current value may be increased or decreased while monitoring the temperature of the jig, and the peak current value may be controlled to a predetermined temperature. Usually, in order to set the sintering temperature in the above range, a pulse current having a peak current value of about 800 to 1200 A may be applied.
[0017]
The sintering time may be generally in the range of about 1 minute to 30 minutes, and more preferably about 3 minutes to 10 minutes.
[0018]
The atmosphere during sintering is not particularly limited, and may be an oxygen-containing atmosphere such as in the air. However, when sintering at a high temperature of about 1300 ° C. or higher using a graphite sintering jig, In order to avoid this, the sintering is preferably carried out under a reduced pressure atmosphere of, for example, about 10 Pa or less, preferably about 7 Pa or less.
[0019]
By the above-described current sintering method, a composite oxide sintered body having a target high-density perovskite crystal structure can be obtained.For example, when a graphite sintering jig is used, In the vicinity of the surface of the sintered body to be obtained, conductive graphite which is a component of the jig is included. Such impurities such as graphite contained in the vicinity of the surface of the sintered body can be obtained by polishing the surface of the sintered body or by sintering the sintered body in air at about 500 to 1200 ° C., preferably about 500 to 1000 ° C. It can be easily removed by holding and heating for about minutes to 6 hours. At the time of heat treatment, in order to prevent a change in characteristics, the sintered body is accommodated in a container such as alumina which does not react with the sintered body and is subjected to a predetermined speed of about 1 to 50 ° C./min, preferably about 2 to 15 ° C./min. After the temperature is raised to and maintained at the heat treatment temperature, it is preferable to lower the temperature at the same rate as when the temperature was raised.
[0020]
Further, when using a graphite sintering jig, when the sintered body is partially reduced by the carbon content in the jig during sintering, and the oxygen content in the obtained sintered body is insufficient. There is. In such a case, the obtained sintered body is further heat-treated in an oxygen-containing atmosphere such as the air at about 1000 to 1300 ° C. and re-oxidized to obtain the target compound represented by the general formula (1). A sintered body to be obtained can be obtained. The heat treatment time is not particularly limited, but may be generally about 1 to 5 hours.
[0021]
In the heat treatment for removing the graphite component, if the heat treatment temperature is set to about 1000 ° C. or higher, the sintered body can be reoxidized simultaneously with the removal of the graphite.
[0022]
According to the method described above, the general formula:
_{(A x B 1-x)} (C y D 1-y) O 2.8 ~ 3 (1)
(Where A is lanthanum, B is at least one selected from calcium and strontium, C is at least one selected from aluminum, gallium, scandium and indium, D is at least one selected from magnesium and cobalt, x is 0.8 to 1 and y is 0.8 to 1). Thus, a lanthanum-based composite oxide sintered body having a perovskite crystal structure is obtained. The resulting composite oxide sintered body is a sintered body having a high oxide ion conductivity, and generally has a high density of 95% or more of the theoretical density and has no gas permeation. It can be effectively used as an electrolyte member for a steam electrolyzer, an oxygen separator or the like.
[0023]
【The invention's effect】
According to the method of the present invention, a high-density sintered body of a lanthanum-based composite oxide can be stably produced at a lower temperature and in a shorter time than in a conventional external heat sintering method using an electric furnace or the like. The sintered body obtained by the method of the present invention is a high-density sintered body having a perovskite-type crystal structure having high oxide ion conductivity, and is effectively used as an electrolyte member for a SOFC, a steam electrolyzer, an oxygen separator, or the like. It can be used.
[0024]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0025]
Example 1
Using lanthanum oxide, strontium carbonate, and scandium oxide as raw materials, the respective compounds were mixed so that La: Sr: Sc (atomic ratio) = 0.9: 0.1: 1. After mixing these compounds, the mixture was calcined in air at 1350 ° C. for 10 hours, pulverized for 1 hour using a mortar, and further calcined and pulverized under the same conditions to obtain a composite oxide powder. .
[0026]
Electric current sintering was performed by the following method using the composite oxide powder obtained by the above method as a starting material.
[0027]
As the electric current sintering machine, a discharge plasma sintering machine SPS-515S manufactured by Izumi Tech Co., Ltd. was used. As the sintering jig, a cylindrical one made of graphite and having a diameter of 1.5 cm was used. About 1 g of the raw material powder was uniformly charged into this jig, a pressure of about 40 MPa was applied, and the inside of the sintering chamber was evacuated to about 7 Pa. Next, a DC pulse current of about 1100 A was applied to the jig to heat the periphery of the sample to 1400 to 1550 ° C. at a rate of about 170 ° C./min. After maintaining this state for 5 minutes, the application of current and pressure was stopped, the sample was cooled to room temperature, and the inside of the sintering chamber was returned to atmospheric pressure.
[0028]
The sintered body taken out in this state was black and had electrical conductivity, and it was found from X-ray diffraction that it contained graphite of a jig.
[0029]
This sintered body is heat-treated in air at 1300 ° C. for 2 hours to obtain a lanthanum strontium scandium oxide (hereinafter referred to as LSS) represented by the general formula: La _0.9 Sr _0.1 ScO _2.95. A sintered body having a perovskite-type crystal structure (hereinafter, referred to as an SPS sintered body) was obtained.
[0030]
Starting material (a), SPS sintered body (sintered body by spark plasma sintering method) (b), sintered body using ordinary electric furnace (fired at 1625 ° C. for 10 hours) (hereinafter, CS sintered body) FIG. 1 shows the X-ray diffraction pattern for (c). Although the starting material has a slight impurity phase, the SPS sintered body (sintered body by the discharge plasma sintering method) is an LSS single phase having an orthorhombic perovskite-type crystal structure and its lattice constant. Was a = 0.5787 (2) nm (representing 0.5787 ± 0.0002 nm; the same applies hereinafter), b = 0.8103 (2) nm, and c = 0.5691 (2) nm. These values are as follows: CS sintered body (sintered body using electric furnace) (a = 0.5789 (1) nm, b = 0.8106 (2) nm, c = 0.5689 (1) nm) And showed good agreement.
[0031]
FIG. 2 shows the relationship between the sintering temperature and the relative density of the SPS sintered body (sintered body by the discharge plasma sintering method) and the CS sintered body (sintered body using an electric furnace). As can be seen from FIG. 2, the density of the SPS sintered body is, for example, 5.5 g / cm ³ for a sintered body obtained by sintering at 1500 ° C. for 5 minutes, which is the theoretical density (5.6 g / cm 3). ³ ) was 98%.
[0032]
On the other hand, the density of the CS sintered body (sintered body using an electric furnace) when sintered at 1625 ° C. for 10 hours was 4.3 g / cm ³ (77% of the theoretical density).
[0033]
From these results, it can be seen that according to the spark plasma sintering method, a high-density sintered body can be obtained in a very short time at a temperature lower by about 100 ° C. than in the case where sintering is performed using a normal electric furnace.
[0034]
Further, an SPS sintered body (relative density 98%, diameter 15 mm, thickness 0.89 mm) obtained by a discharge plasma method and a CS sintered body (relative density 77%, diameter 15 mm, Relationship between temperature and ion transport number (ratio of ionic conductivity to total conductivity) by air-hydrogen (4% H ₂ -N ₂ balance, 30 ° C. H ₂ O saturation) battery for thickness 0.89 mm) Is shown in FIG. As can be seen from FIG. 3, the ion transport number of the SPS sintered body is 0.94 at 500 ° C. and 0.75 at 900 ° C., which is lower than that of the CS sintered body (relative density of 77%). It can be seen that the ion transport number is improved by about 0.5. This means that gas permeation through the electrolyte was eliminated by increasing the relative density from 77% to 98%. In addition, a stable current could be obtained from the battery manufactured using the SPS sintered body.
[0035]
As described above, the method of the present invention is suitable as a method for sintering a high-density electrolyte material that can be used in an SOFC, a steam electrolyzer, an oxygen separator, or the like at a low temperature and at a high speed.
[Brief description of the drawings]
FIG. 1 shows a starting material (a) used in Example 1, a sintered body (SPS sintered body) obtained by a spark plasma sintering method (b), and a sintered body obtained by a normal sintering method The figure which shows the X-ray diffraction pattern of (CS sintered compact) (c).
FIG. 2 Sintering temperature dependence of relative density of sintered body (SPS sintered body) obtained by spark plasma sintering method and sintered body (CS sintered body) obtained by normal sintering method The graph which shows sex.
FIG. 3 shows air-hydrogen (4% H ₂ ) of a sintered body (SPS sintered body) obtained by a spark plasma sintering method and a sintered body (CS sintered body) obtained by a normal sintering method. -N ₂ balance, graph showing the temperature dependence of the ion transference number due to 30 ° C. _{H 2} O humidified) batteries.

Claims (5)

General formula:
_{(A x B 1-x)} (C y D 1-y) O 2.8 ~ 3 (1)
(Where A is lanthanum, B is at least one selected from calcium and strontium, C is at least one selected from aluminum, gallium, scandium and indium, D is at least one selected from magnesium and cobalt, x is 0.8 to 1 and y is 0.8 to 1). A method for producing a lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure represented by the following general formula ( A perovskite-type crystal structure characterized in that a composite oxide powder containing the elements A to D in 1) in the same ratio as in the general formula (1) is used as a raw material and is sintered by applying a DC pulse current under pressure. A method for producing a lanthanum-based composite oxide high-density sintered body having: A method for producing a high-density lanthanum-based composite oxide sintered body having a perovskite-type crystal structure, wherein the sintered body is heat-treated after being electrically sintered by the method of claim 1. The method for producing a lanthanum-based composite oxide high-density sintered body having a perovskite-type crystal structure according to claim 1, wherein the sintering is carried out at 1000 to 1600 ° C. under a pressure of 20 to 60 MPa. A high-density lanthanum-based composite oxide sintered body having a perovskite crystal structure obtained by the method according to claim 1. The high-density lanthanum-based composite oxide sintered body having a perovskite-type crystal structure according to claim 4, having a density of 95% or more of the theoretical density.

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