JP3748084B2 - Method for producing NiO / YSZ composite powder - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a method for producing a NiO / YSZ composite powder used for a fuel electrode material of a solid oxide fuel cell (hereinafter also referred to as SOFC). In particular, the present invention relates to a method for producing a NiO / YSZ composite powder capable of forming a SOFC fuel electrode having a uniform composition and excellent durability.
[0002]
[Prior art]
The prior art will be described taking SOFC fuel electrode material as an example. The fuel electrode material of the SOFC, sintered film of NiO and Y2O3 stabilized ZrO2 (YSZ) and the mixture complexed composite powder that has been mainly used. Note that NiO in the sintered membrane is reduced to Ni during the operation of the SOFC.
[0003]
In general, such a NiO / YSZ composite powder is produced by mixing NiO powder and YSZ powder in a solid state, then raising the temperature and sintering slightly to form a composite. ing. As a mixing method, a method using a ball mill, a mechanochemical mechanical mixing method, or the like is generally known.
[0004]
[Problems to be solved by the invention]
The conventional manufacturing method described above has the following problems.
(1) Ball mill mixing method:
Among the mixed powder, a component having a large specific gravity (NiO) or a component having a large particle size settles, and the composition of the powder tends to be uneven. In particular, when wet mixing is performed using a liquid as a medium, the above-described sedimentation phenomenon tends to occur in the drying step after the mixing process.
[0005]
(2) Mechanochemical mechanical mixing:
This method is one of mechanical mixing methods. For example, a rotary blade is installed in a container that rotates at a high speed (or the blade itself may rotate), and the surface of the rotary blade. Has a curved surface such that the powder rolls on the blade, and promotes mixing of the powder by centrifugal force and stirring by the rotation of the container or the rotary blade itself. A major feature of the mechanochemical method is that the powders are mixed with each other in a state where the heat is naturally generated or the temperature is forcibly increased, that is, the bonding between the powders is promoted by heat (conventional mechanical mixing). The main method was to remove the generated heat by cooling as seen in attritors). And this method is widely used today as a surface modification method to create a composite powder composed of shells and nuclei by adsorbing fine powder on the surface of coarse powder mainly in mixing powder body and fine powder. It is getting to be.
[0006]
As described above, in this mechanochemical mechanical mixing, the surface modification method is based on a mechanism using a powder particle size difference in which fine particles are adsorbed on the surface of coarse powder particles. For this reason, the mechanochemical mechanical mixing cannot exert a great effect when the raw materials that can be used are limited to a combination of a coarse powder and a fine powder, and it is desired to mix and combine fine powders.
[0007]
Moreover, there existed the following problems in terms of surface modification of the powder. That is, the temperature rise temperature at the time of mixing with stirring is limited in the material due to the use conditions of heating and high speed rotation, and the temperature is about 400 ° C. at most. At higher temperatures, the material of the container is limited, and contamination of the container material due to mechanical reaction between the powder and the container becomes a problem. Therefore, if etc. you mix one material powder Chiraka is a low melting point substance, but mechanochemical approach is effective, if the total material is a high melting point material as the fuel electrode material Ni O / YSZ of SOFC However, the effect of heating is small, and finally it is necessary to strengthen the bonding between the powders by a method of firing, but at this time, grain growth generally occurs, When it is desired to obtain a fine powder, the pulverization process must be performed again, and the meaning of mechanochemical mixing is reduced.
[0008]
In addition, there are the following problems in terms of composite powder. That is, in general, the powder is more agglomerated as it becomes finer. Therefore, it is difficult to dissolve the agglomeration and to uniformly adsorb the fine powder onto the coarse powder. When viewed at the powder level , as a result, uneven composition occurs and the product becomes non-uniform.
[0009]
An object of the present invention is to provide a method for producing a NiO / YSZ composite powder that has a uniform composition, is capable of obtaining fine particles, and does not require any special requirements for the particle size of the raw material powder used. Further, the present invention are shorted with to provide such a NiO / YSZ composite powder, and also aims to improve the durability of the SOFC cell.
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a method for producing a NiO / YSZ composite powder according to the present invention comprises a mixing step of preparing a mixture of nickel oxide powder and zirconium nitrate / yttrium aqueous solution, and heat-treating the resulting mixture to treat moisture and NOx. And a heat treatment step of obtaining a nickel oxide / yttria stabilized zirconia (NiO / YSZ) composite powder.
[0011]
[Action]
In the method for producing the NiO / YSZ composite powder of the present invention, the starting materials are powder and liquid. And since a solid phase is deposited from a liquid phase, a NiO / YSZ composite powder having a uniform composition can be produced.
[0012]
In the method for producing the NiO / YSZ composite powder of the present invention, the ratio of the nickel oxide weight (converted value) to the YSZ weight (converted value) in the mixture is preferably 30 to 80 to 70 to 20. . Here, the converted value means, for example, converted to the weight of ZrO ₂ contained in zirconium nitrate Zr (NO ₃ ) ₄ . The reason why the above numerical range is preferable will be described. When the ratio of YSZ exceeds 70, the conductivity of the powder fired film is lowered, which is not preferable. When the ratio of YSZ is less than 20, the frequency (number) of bonds between YSZ particles in the fired film of the powder is reduced, so that the merit of liquid phase synthesis is small.
[0013]
From such a viewpoint, the ratio of the nickel oxide weight (converted value) and the YSZ weight (converted value) in the mixture is more preferably 40 to 70 to 60 to 30.
[0014]
The production method of the present invention further includes a pulverization step of pulverizing the obtained NiO / YSZ composite powder, and in this pulverization step, the final particle size of the composite powder is preferably 10 μm or less.
[0015]
The primary particles of the powder obtained after pyrolysis depend on the NiO powder used as a raw material and the deposited YSZ particle diameter, but are often several μm or less at most. However, the final product powder is an aggregate of powder called secondary particles. When the secondary particles are coarse powder, even if the primary particles are fine powder, a film as an electrode is used. The bonding between the powders when formed into a shape causes a decrease in the overall bonding area depending on the secondary particle size, and this ultimately reduces the conductivity of the fuel electrode in the case of an SOFC cell. Cause.
[0016]
From such a viewpoint, it is more preferable that the final particle size of the powder is 5 μm or less in the pulverization step.
[0017]
【Example】
Examples of the present invention will be described below.
FIG. 1 is a flowchart showing steps of a method for producing a NiO / YSZ composite powder according to a standard embodiment of the present invention. First, description will be given with reference to this flowchart.
[0018]
(1) Nickel oxide raw material:
It can be used powder of nickel oxide powder.
[0019]
(2) YSZ raw material:
A zirconium nitrate / yttrium aqueous solution can be used. The contents of both are representative examples of ZrO 2 92 mol% and Y 2 O 3 8 mol% at the stage of becoming an oxide. Unlike the case of the present invention, have use of nickel nitrate aqueous solution as nickel oxide raw material, the use of solid powder as a YSZ material, deposited in the form of NiO takes in YSZ particles, resulting, NiO when molded as an electrode The frequency of bonding between them increases, and large cracks occur due to volume shrinkage when it becomes metallic Ni by reduction, resulting in a decrease in conductivity even though the bonding between Ni is firmly formed. The result was obtained.
[0020]
(3) Weighing:
The raw materials are weighed according to the NiO / YSZ composition ratio of the composite powder to be obtained.
[0021]
(4) Mixing:
When Ni 2 O powder is used as a raw material, it is preferable to mix and stir using a ball mill. The reason for this is to solve the aggregation of the NiO powder and promote the precipitation of YSZ particles on the primary particle surface of NiO.
[0022]
(5) Evaporation to dryness:
Moisture (mainly) of the mixture is evaporated to form a solid (mainly a mixture of NiO, Ni (NO ₃ ) ₂ , ZrO ₂ , Zr (NO ₃ ) ₄ , Y ₂ O ₃ , Y (N 0 ₃ ) _{3 in} the chemical formula) Take out. The temperature at this time is preferably 180 to 250 ° C. It is also preferable to evaporate and dry the mixture (solution or slurry) while stirring with a stirrer or the like. This is because a uniform composite powder is produced by preventing the sedimentation of the powder and the accompanying aggregation. Evaporation to dryness is carried out on a hot stirrer and using an evaporating dish. Therefore, even if the maximum temperature reached by the stirrer itself is 400 ° C, the sample temperature actually rises only up to 200 ° C due to thermal diffusion. At this time, if the sample temperature is forcibly increased, the container often breaks due to thermal expansion strain due to the temperature distribution of the evaporating dish itself.
[0023]
(6) Thermal decomposition:
In order to remove (fly away) the nitric acid component from the solid material, heat treatment is performed at about 400 to 600 ° C. At the stage of thermal decomposition, the solid material is a mixture of NiO, ZrO ₂ and Y ₂ O ₃ . In other words, the main reaction formula is as follows.
Ni (NO ₃ ) ₂ → NiO + NO ₂ + NO ₃
Zr (NO ₃ ) ₄ → ZrO ₂ + 2NO ₂ + 2NO ₃
2Y (NO ₃ ) ₃ → Y ₂ O ₃ + 3NO ₂ + 3NO ₃
[0024]
(7) Coarse grinding:
The obtained solid is roughly pulverized to a size of about 1 mm square. In the fine pulverization performed in the next step, there is a limit to the diameter of the sample that can be charged into the fine pulverizer, and many of them can reduce the size of the sample mass to a certain size (for example, 500 μm or less) in advance. is necessary. In this example, the coarsely pulverized particle diameter was set to 1 mm because of the ability of the model used (CJ-10 of Nisshin Flour Milling).
[0025]
(8) Spray pyrolysis treatment:
The steps between the above (5) evaporation to dryness to (7) coarse pulverization can also be performed in one step by spray pyrolysis treatment. Examples of processing conditions for this spray pyrolysis treatment are as follows. In the case of spray pyrolysis, both solution and slurry materials can be used. While stirring the raw material solution, a part of the solution is taken out little by little through the pipe, and is sprayed from the nozzle into the reaction vessel (mainly the upper portion of the reaction vessel) with compressed air. At this time, the temperature of the reaction vessel was set to 800 ° C. to 1000 ° C. The raw material mist is pyrolyzed and collected in the middle of dropping in the reaction vessel, whereby fine powder can be obtained. The size of the particles obtained depends on the metal oxide concentration in the raw material container, the spray rate and the pyrolysis temperature.
[0026]
(9) Calcination:
Next, calcination is performed at a temperature of 900 to 1200 ° C. for 2 hours to 10 hours. By controlling the calcining temperature and time, the particle size of the primary particles of the component can be controlled. When calcined at 900 ° C. for 10 hours, the primary particle diameter of NiO becomes about 1 μm. When calcined at 1100 ° C. for 2 hours, the primary particle diameter of NiO becomes about 2 μm. In the case of YSZ particles, there was almost no difference in particle size under the calcining conditions in this temperature range, and a powder of about 0.5 μm was obtained.
[0027]
(10) Fine grinding:
Finely pulverize to the final required particle size. As the method, there are a method by ball milling using ceramic balls, a method of airflow crushing (crushing by colliding raw material particles in an airflow), and the like. In the case of a ball mill, in order to obtain a sample having a required particle size distribution, it is necessary to obtain optimum conditions from the type of ball used, the ball diameter and amount, the processing time, and the like. In the case of airflow pulverization, fine powder can be cut during the pulverization, so it is necessary to find the optimum conditions from the feed rate of the raw material powder, the airflow speed, the coarsely pulverized particle size, and the like.
[0028]
Hereinafter, specific examples will be described.
In this example, NiO was used for the solid phase and an aqueous 8 mol yttria-containing zirconium nitrate solution was used for the liquid phase. There is no problem even if this aqueous solution is prepared from nitric acid-based raw materials of yttria and zirconia components. The NiO powder has an average particle size of 1.5 μm and the aqueous solution contains about 20% by weight in terms of 8-YSZ oxide. Each component is weighed so as to achieve a weight-target composition, and mixed and stirred in a ball mill for 24 hours. At this time, it is preferable to use nylon spheres as the stirring balls so as not to pulverize NiO. In some cases, a peptizer may be added to prevent the clay clay from rising.
[0029]
The mud is transferred to a container, and it is divided into evaporating scrapes with stirring, and then evaporated to dryness. At this time, stirring with a stirrer is performed for the purpose of preventing sedimentation of the powder. Remove the stir bar when the liquid starts to evaporate and solidify and the stirrer stops rotating. The treated evaporating dish is heat-treated in an oven at about 400 to 600 ° C. to remove the nitric acid component (NOx).
[0030]
The obtained solid was roughly pulverized to a size of about 1 mm square and calcined at 900 to 1200 ° C. The particle size of the primary particles of the component can be controlled by setting the calcining temperature and the processing time. Next, the target powder is obtained by pulverizing so that the maximum particle size of the composite powder is 5 μm or less.
[0031]
In the above, an example is shown in which liquid phase moisture is evaporated and dried, but solid precipitation can also be performed by spray pyrolysis even in the case of both solid-liquid and liquid raw materials. In this case, unlike the evaporation to dryness method, the obtained powder was a fine powder having a secondary particle size of about 100 μm or less, and thus the target powder can be obtained by performing direct calcination and final pulverization.
[0032]
A slurry was prepared from the powder thus obtained, and a film was formed as a fuel electrode of a SOFC cell by a dip coating method. In order to examine the material properties and durability, film forming was performed on a pipe having a YSZ dense film formed on the outer surface of the CSZ tube.
[0033]
The formed fuel electrode was subjected to reduction treatment at 1000 ° C. in an H ₂ atmosphere. The conductivity of the fuel electrode was measured by the four probe method. Further, a heat treatment at 1050 ° C./300 hr was performed, and the change in conductivity characteristics was examined.
[0034]
For comparison, a film-forming sample and a press-molded body using a powder prepared by ball mill mixing of powder raw materials were also prepared. The component composition was NiO / YSZ = 70/30 (weight ratio). Powder 1 synthesized from a solid / liquid raw material and powder prepared by powder mixing were designated as Powder 2.
[0035]
In the film-forming sample after hydrogen reduction, powder 2 was used, and a network-like crack was generated on the surface. As a result, the conductivity was reduced to about 100 S / cm. In the case of using the powder 1, almost no cracks are generated, and the electrical conductivity is as large as 450 S / cm. Even in the case of powder 1, when comparing the conductivity of particles having a particle size of 0.5 to 5 μm with the conductivity of particles having a particle size of 1 to 10 μm, the former was higher.
[0036]
After the treatment at 1050 ° C., the powder 1 had a conductivity of 700 to 750 S / cm, which was larger than the initial value, while the powder 2 had a characteristic deterioration of about 30%.
[0037]
When observed with an electron microscope, the powder 1 has a high YSZ bonding frequency surrounding Ni, and this is considered to function to prevent sintering shrinkage due to high temperature treatment of Ni. On the other hand, in the case of the powder 2, in addition to the partial aggregation of the components, it is presumed that such a bond of YSZ is smaller than that of the powder 1 and that the sintering preventing power of Ni is small. It is thought that characteristic deterioration appeared.
[0038]
Unlike the case of the present invention, different results can be obtained by exchanging the raw material species in the solid / liquid synthesis. A film sample prepared from a YSZ powder with an average particle size of 0.7 μm and a powder synthesized from nickel nitrate aqueous solution has large cracks after hydrogen reduction, and the conductivity is reduced to about 200 S / cm. Deterioration was seen. Such a result is considered to be due to the difference in compositing depending on the solid precipitation form from the liquid and the solid raw material particle size.
[0039]
This suggests that composite powders with different characteristics can be obtained depending on the powder particle size when NiO powder / zirconium nitrate aqueous solution is used. Actually, the powder synthesized from fine powder with a particle size of about 0.6 μm as NiO powder is used for the lower layer (immediately above the electrolyte membrane) of the film-forming body. The powder synthesized from NiO powder with a particle size of 2 μm is By using it for the upper layer, it was possible to improve the overall electric conduction characteristics and power generation characteristics. In the lower layer, Ni fine particles are preferable in order to increase the three-phase interface of YSZ-Ni-fuel gas, and in the upper layer, NiO coarse particles are preferable in order to increase air permeability. The above-mentioned value of 750 S / cm is a combination of these, and even if a similar structure is made from powders synthesized from all component solids, there is almost no effect, and the occurrence of cracks cannot be suppressed. It was.
[0040]
【The invention's effect】
As is clear from the above description, the present invention exhibits the following effects.
(1) A NiO / YSZ composite powder having a uniform composition and capable of obtaining fine particles and requiring no special requirement for the particle size of the raw material powder to be used can be provided.
(2) Another object of the present invention is to improve the durability of the SOFC cell when an SOFC fuel electrode is prepared using the NiO / YSZ composite powder of the present invention.
[Brief description of the drawings]
FIG. 1 is a flowchart showing steps of a method for producing a NiO / YSZ composite powder according to a standard embodiment of the present invention.

Claims (5)

A mixing step of creating a mixture of nickel oxide powder and zirconium nitrate / yttrium aqueous solution;
A heat treatment step of removing the moisture and NOx by heat-treating the obtained mixture to obtain a nickel oxide / yttria stabilized zirconia (NiO / YSZ) composite powder;
Of NiO / YSZ composite powder containing   The method for producing a NiO / YSZ composite powder according to claim 1, wherein the ratio of nickel oxide weight (converted value) to YSZ weight (converted value) in the mixture is 30 to 80 to 70 to 20. The method for producing a NiO / YSZ composite powder according to claim 1, wherein the ratio of nickel oxide weight ( converted value ) to YSZ weight (converted value) in the mixture is 40 to 70 to 60 to 30. Furthermore, a pulverization step of pulverizing the obtained NiO / YSZ composite powder is included,
The method for producing a NiO / YSZ composite powder according to any one of claims 1 to 3, wherein in the pulverization step, the final particle size of the composite powder is 10 µm or less. Furthermore, a pulverization step of pulverizing the obtained NiO / YSZ composite powder is included,
The method for producing a NiO / YSZ composite powder according to any one of claims 1 to 3, wherein the final particle size of the composite powder is 5 µm or less in the pulverization step.

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