JP3336851B2 - Method for producing Ni / YSZ cermet raw material powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Ni / YSZ cermet raw material 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 Ni / YSZ cermet raw material powder capable of improving the power generation characteristics and durability of an SOFC, improving the uniformity of the powder composition and structure, and appropriately controlling the sinterability. Further, the present invention relates to a method for producing an SOFC using such a Ni / YSZ cermet raw material powder.

[0002]

2. Description of the Related Art The prior art will be described using a fuel electrode material of an SOFC as an example. As a material for a fuel electrode of an SOFC, a sintered film of a composite powder in which NiO and Y ₂ O ₃ stabilized ZrO ₂ (YSZ) are mixed and compounded is mainly used (JP-A-61-153280; Showa 61-19857
0 etc.). Note that NiO in the sintered film is reduced to Ni during the operation of the SOFC, and the film becomes a Ni / YSZ cermet film.

[0003] As a method for producing such a raw material powder for Ni / YSZ cermet, generally, NiO powder and YS are used.
A method in which both powders are mixed in a solid state and then heated (calcined) and slightly sintered to form a composite (solid mixing method) is employed. Furthermore, a method is also known in which the composite powder is pulverized after calcination to obtain a raw material powder for slurry application (JP-A-6-2955731). As a mixing method,
A method using a ball mill and a method using mechanochemical mechanical mixing are known. Also, a method has been proposed in which Ni, Zr, and Y are mixed in an ionic state and thermally decomposed (JP-A-7-29575).

[0004] Ni / YSZ cermet is composed of various components (Ni
And YSZ) are crossed, but those in which Ni is connected like a network have good conductivity, and Ni or Y
Those in which the network of Ni has been cut due to aggregation of SZ grains have poor conductivity. If the conductivity of the fuel electrode of the SOFC is poor, the power generation efficiency of the SOFC decreases. Therefore, there is a demand for a NiO / YSZ composite powder that does not cause agglomeration of Ni or YSZ and can form a network structure of Ni firmly.
Furthermore, since Ni tends to cause sinter aggregation during operation of the SOFC, there is also a demand that the network of Ni must be uniform. When Ni is agglomerated, the three-phase interface between Ni, the solid electrolyte, and the gas phase (fuel gas) is reduced, and the reaction between oxygen ions and the fuel gas is reduced.

[0005]

The above-mentioned conventional manufacturing method has the following problems. Ball mill mixing method: In the mixed powder, a component having a large specific gravity (NiO) or a component having a large particle diameter is settled, and the composition of the powder tends to be uneven. In particular, when wet mixing is performed using a liquid as a medium, such a sedimentation phenomenon is likely to occur in a drying step after the mixing treatment. Even if spray drying or the like is performed from the slurry state, it is difficult to obtain a uniform powder because the deposition rate varies depending on the difference in particle diameter and the difference in specific gravity. Also, SOF
When powder having a size of several μm or less, which is considered to be suitable for the fuel electrode of C, is mixed, it is difficult to dissolve agglomeration of the powder at the primary particle level by dry mixing.

[0006] Mechanochemical mechanical mixing: This method, in one example, has a rotating blade installed in a container and promotes powder mixing by centrifugal force and agitation by rotation of the container or the rotating blade itself. At this time, the powders are mixed with each other in a state where the temperature is naturally increased by heat generated naturally or in a state where the temperature is forcibly increased. In other words, a major feature of the mechanochemical method is that the bonding between powders is promoted by heat. This method is mainly used as a surface modification method in which a fine powder is adsorbed on the surface of the coarse powder in the mixing of the coarse powder and the fine powder to create a composite powder composed of a shell and a core. It is becoming widely used today.

However, this mechanochemical surface modification technique often utilizes a difference in particle size such that fine particles are fixed to the surface of coarse particles, and therefore, the raw materials used are restricted, and materials having the same particle size are used. It is difficult to achieve the effect for the purpose of mixing between them.

A raw material powder obtained by a wet method such as a pyrolysis method has better uniformity than a powder obtained by a solid mixing method. However, if not only the particle size of the powder but also the sinterability (shrinkage ratio, BET value, etc.) is not controlled, the stress applied to the cell base due to firing shrinkage during firing of the fuel electrode and the reduction of NiO to Ni The formed film cannot function as an electrode due to cracks or the like generated by stress at the time of contraction.

The present invention provides a Ni / YSZ with improved uniformity of powder composition and structure and moderate control of sinterability, which can contribute to improvement of power generation characteristics and durability of SOFC.
An object of the present invention is to provide a method for producing cermet raw material powder.

[0010]

In order to solve the above-mentioned problems, a method for producing a Ni / YSZ cermet raw material powder according to the present invention comprises: a Ni / YSZ cermet raw material powder (NiO / YS
Z composite powder); a wet mixing step of obtaining a mixture containing Ni, Zr, Y and oxygen by a wet method; and a granular material containing an oxide of each of the above metals by decomposing the mixture. A primary calcination step of calcining the powder, a pulverizing step of pulverizing the calcined powder (NiO / YSZ composite powder), and a pulverized powder obtained in the pulverizing step. And a secondary calcining step of calcining again. That is, the dispersibility of each element is enhanced by wet mixing, and the sinterability of the raw material powder is adjusted in the calcination step and the particle size adjustment step. At this time, by performing calcination a plurality of times with the pulverization interposed therebetween, it is possible to obtain powder having a small diameter and suppressed sinterability, which is suitable for forming a fuel electrode of an SOFC. That is, the fuel electrode formed by using such a powder has excellent conductivity, air permeability, and durability.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A method for producing a Ni / YSZ cermet raw material powder according to one embodiment of the present invention comprises the steps of: producing a Ni / YSZ cermet raw material powder (NiO / YSZ composite powder) which becomes Ni / YSZ cermet after a firing / reducing step. A method for producing; a solution adjusting step of adjusting a raw material solution containing Ni ions, Zr ions, and Y ions at a desired ratio; and mixing a coprecipitated solution with the raw material solution to form at least one of the above metals; A coprecipitation step of coprecipitating a solid substance containing oxygen (coprecipitation substance) from the raw material solution; a decomposition step of decomposing the coprecipitation substance to obtain a powder containing the oxide of each of the metals; A primary calcining step of calcining the granules, and a calcined powder (N
a pulverizing step of pulverizing the iO / YSZ composite powder), and a secondary calcining step of recalcining the pulverized powder obtained in the pulverizing step. As mentioned above,
Wet mixing process → Powder or granular material containing oxides of Ni, Zr, and Y may be obtained by thermal decomposition. However, coprecipitated powder is preferred because it is excellent in the uniformity of the particle diameter and the uniformity of the structure.

In the method for producing a Ni / YSZ cermet raw material powder in this embodiment (coprecipitation method), a solution based on a nitric acid aqueous solution can be used as the raw material solution, and an oxalic acid aqueous solution can be used as the coprecipitate solution. In this case, the following coprecipitation reaction occurs. Ni ²⁺ + (COOH) ₂ → Ni (COO) ₂ ↓ + 2H ⁺ Zr ⁴⁺ +2 (COOH) ₂ → Zr (COO) ₄ ↓ + 4H ⁺ 2Y ³⁺ +3 (COOH) ₂ → Y ₂ (COO) ₆ ↓ + 6H ⁺

In the method for producing a Ni / YSZ cermet raw material powder of the above embodiment, it is preferable that the raw material solution and the coprecipitated solution are heated to 60 ° C. to the boiling point in advance and then mixed. In general, the precipitation reaction of Ni oxalate hardly occurs at normal temperature. Therefore, it is difficult to obtain a coprecipitated substance having a uniform composition. On the other hand, in the above temperature range, the above-mentioned three reactions occur almost evenly, so that a coprecipitated substance in which components are uniformly dispersed can be efficiently obtained.

When the Ni / YSZ cermet raw material powder of the present invention is used as a fuel electrode material for an SOFC, the weight ratio of NiO to YSZ in the powder is 30: 70-7.
It is preferably 0:30. If the YSZ ratio exceeds 70, the conductivity of the fired film of the powder decreases, which is not preferable. From such a viewpoint, the ratio of the nickel oxide weight (converted value) to the YSZ weight (converted value) in the mixture (raw material solution) is more preferably 50:50 to 70:30. However, for the graded layer between the solid electrolyte membrane and the fuel electrode, those with low Ni have low conductivity of the membrane itself, but the stress caused by the difference in thermal expansion between the high Ni-containing layer and the electrolyte. Is preferable because it can alleviate the problem.

When the Ni / YSZ cermet raw material powder of the present invention is used as a fuel electrode material for an SOFC, Ni / YSZ
It is preferable that the particle size of the YSZ cermet raw material powder is 0.1 to 10 μm. This is because the balance between gas permeability and conductivity is good. In this case, the upper layer of the fuel electrode may be formed using relatively coarse particles, and the lower layer may be formed using relatively fine particles. In this case, the BET value of the Ni / YSZ cermet raw material powder is preferably set to 0.8 to 12.0. If the BET value is larger than this, sintering proceeds and sintering cracks tend to occur. If the BET value is smaller than this, it is necessary to raise the sintering temperature for sintering. However, in fabricating an SOFC cell, the firing temperature must be matched with the other materials constituting the cell, and the firing temperature of the fuel electrode must be lower than the firing temperature of the substrate on which the fuel electrode is applied. Therefore, there is a limit to raising the firing temperature of the fuel electrode.

In the method for producing a Ni / YSZ cermet raw material powder according to the present invention, if the desired particle size and BET value cannot be obtained after the secondary calcination, further pulverization, tertiary calcination, pulverization, and fourth calcination , Crushing and quinting may be advanced. However, even if the number of repetitions is repeated, the particles are pulverized by pulverization, so that the powder characteristics converge to a certain particle size and BET value, so that a large change does not appear and the processing effect is small. In addition, the number of steps is increased, so that the cost is increased due to manufacturing conditions. Further, when the shape of the powder is rounded and the sinterability is lowered, there is a possibility that the fuel electrode film cannot be formed unless the temperature is raised to a temperature higher than the firing temperature of the SOFC cell substrate. . For these reasons, it is preferable to keep the number of process repetitions to 5 or less.

Regarding the calcination conditions, the condition of the primary calcination is (8)
00 to 1,250 ° C.) × (2 to 10 hours), and the condition of the secondary calcination is (700 to 1,050 ° C.) × (2 to 10 hours).
Is preferred. Furthermore, the calcination condition of the primary calcination is (900-
(1,200 ° C.) × (2 to 10 hours), and the calcination conditions for the secondary calcination are more preferably (800 to 1,000 ° C.) × (2 to 10 hours). The reason is that during primary calcination, 800
If the heat treatment (calcination) temperature is lower than ℃, the nitric acid component may remain and the purity of the raw material powder may be reduced. In such a heat treatment, the powder may not be crystallized but sintered. This is because the fine particles that do not advance remain in a state as they are, and are excessively sintered during film formation (firing) of the SOFC fuel electrode later, which causes peeling and cracking. In addition, the calcined powder at 1,250 ° C. or higher hardens the calcined powder, so that the pulverization efficiency in the next step decreases, which is not preferable in the production step. In the case of secondary calcination, the temperature is 70
Below 0 ° C., the powder is hardly sintered, so that the effect of the secondary calcination is small. On the other hand, if the temperature is 1,050 ° C. or higher, sintering proceeds and powder having a large particle diameter is likely to be formed again. Therefore,
It is important to control the particle size after the secondary calcination so as to fall within a preferable particle size range for film formation on the SOFC cell. Generally, the temperature of the secondary calcination is lower than the temperature of the primary calcination, that is, the condition of the secondary calcination is generally milder than that of the primary calcination. This is because, if the secondary calcination condition is a high temperature for a long time, particles having a larger particle size than the primary calcination finished powder are generated, which is not preferable for film formation on the SOFC cell.

When the Ni / YSZ cermet raw material powder of the present invention is used as a fuel electrode material for an SOFC,
It is preferable that the particle size is 2 μm or less at the time of pulverization after primary calcination. At this time, if the pulverized particle size is large, larger particles are generated in the secondary calcination in the next step, which is not preferable as a film forming material for the SOFC cell. Classification may be performed after calcination or pulverization, if necessary.

[0019]

Embodiments of the present invention will be described below. FIG.
4 is a flowchart illustrating steps of a method for producing a Ni / YSZ cermet raw material powder and a method for evaluating the same according to a standard example of the present invention. Description will be made with reference to this flowchart.

(1) Raw material solution adjustment: zirconium nitrate / yttrium aqueous solution (8 mol% Y ₂ O ₃
Content, oxide equivalent content 23.4 wt%), nickel nitrate hexahydrate crystal as NiO raw material, pure water for adjusting coprecipitated substance concentration, NiO / YSZ composition 65/35 wt%
And stirred well.

(2) Coprecipitation solution preparation: In this embodiment,
An oxalic acid aqueous solution was used as a coprecipitation solution. Take pure water in a container and heat to about 80 ° C. While stirring the warm water, oxalic acid dihydrate crystals were gradually added and dissolved.
It was kept at 0 ° C. Regarding the amount of the oxalic acid aqueous solution, it is preferable that the amount of the oxalic acid is slightly larger than the stoichiometric ratio so that the metal ions are completely precipitated in the coprecipitation step. The excess amount this time was about 5 mol%.

(3) Solution mixing → coprecipitation: Nickel oxalate crystals are hardly formed even when nickel nitrate and oxalic acid aqueous solution are reacted at room temperature. Therefore, the production rate of nickel oxalate is increased by adding nitric acid while heating an aqueous solution containing nickel such as nitrate. The raw material solution (NiO / YSZ composite powder aqueous solution) is heated to 80 ° C to 90 ° C, and is heated to 80 ° C to 9 ° C.
Precipitation was produced by an oxalic acid coprecipitation method by gradually adding the resulting mixture to a nitric acid aqueous solution heated and maintained at 0 ° C. with good stirring. Since powder is produced by the reaction, it is preferable to use a stirrer having a torque for stirring the solution.
Since the solution causes an exothermic reaction due to the coprecipitation reaction, the temperature of the solution usually rises by about 10 to 20 ° C. from the initial state after the reaction. After all the solutions were mixed, the solution was naturally cooled while continuing to stir to room temperature.

(4) Drying: The reactants are allowed to stand in a dryer, and
Hot air at 20 ° C. was sent to evaporate the water content of the precipitate. At this time, the powder (sediment) and the moisture may be separated using a filtration device. (5) Thermal decomposition: The dried sample was subjected to a heat treatment at 500 ° C. for 5 hours to remove a nitric acid component and residual oxalic acid. The reaction at that time is estimated as follows. Ni (COO) ₂ → NiO + CO + CO ₂ Zr (COO) ₄ → ZrO ₂ + 2CO + 2CO ₂ Y ₂ (COO) ₆ → Y ₂ O ₃ + 3CO + 3CO ₂

(6) Pulverization (disintegration): Wet pulverization using 2φ and 3φ PSZ balls was performed. This aims at crushing and homogenizing the secondary particles. However, according to the wet laser diffraction particle size distribution measurement, the powder obtained by this coprecipitation method was 1 μm.
m or less occupy about 80% of the whole, and since even secondary particles have a particle size of 10 to 20 μm, it is possible to omit this ball milling treatment.

(7) Primary calcination: The obtained powder is mixed with 1100
℃ × 5hr 、 1150 ℃ × 5hr 、 1200 ℃ × 5hr 、 14
Primary calcination (heat treatment) was performed at each condition of 00 ° C. × 5 hours.
At the time of calcination, NiO fine powder mainly causes other N
Combine with iO powder. Further, Y ₂ O ₃ gradually dissolves in ZrO ₂ and crystallizes. Table 1 shows the powder particle size and the BET value after the primary calcination.

[0026]

[Table 1]

(8) Pulverization: The primary calcined powder was pulverized to a diameter of 2 μm or less by air current pulverization of the following specifications. Table 1 also shows the particle size and BET value after pulverization. In addition, the grinding method,
If fine grinding is possible, other means such as ball mill grinding can be used. In relation to the primary calcination temperature and the pulverization, in the case of A4 powder having a calcination temperature of 1,400 ° C., the particle size is 2 μm.
It took a very long time to make the following, and it was difficult to grind.

(9) Secondary calcination: pulverized powder at 1,000 ° C.
Secondary calcination was performed under the conditions of × 5 hours. The purpose of this secondary calcination is to suppress the sinterability of the crushed powder by lightly sintering the crushed powder again. As shown in Table 1,
The BET value has decreased due to the secondary calcination. A4
The reason why the BET value of the powder after the secondary calcination is the same is that the powder is kept at 1,000 ° C. × 5
This is due to the fact that the primary calcination reduced the activity of the particle surface before sintering did not occur in the secondary calcination of hr.

(10) Film formation and firing: 27 parts of the obtained Ni / YSZ cermet raw material powder, 68 parts of ethanol as an organic solvent, 1 part of a polycarboxylic acid type polymer surfactant as a dispersant, A slurry was prepared by mixing 1 part of a dialkyl sulfosuccinate as an antifoaming agent and 3 parts of cellulose as a binder. This slurry was slurry-coated on a YSZ electrolyte substrate by dipping.
After drying, this was fired at 1,400 ° C. The thickness of the obtained cermet film was 80 μm, and the porosity was 27%.

(11) Conductivity measurement: After reducing the cermet film in a H ₂ atmosphere at 1,000 ° C. × 5 hours, the conductivity was measured by a four-terminal method. Table 1 also shows the results. The results show that the powder having the condition of A2 has the highest conductivity (1,600 s / c).
m). In the film using the raw material powder by the conventional solid mixing method, the conductivity is around 1,100 s / cm, and in the case of the example of A2, it is improved by 45%.

(12) Cell Production and Power Generation Evaluation A power generation test cell was prepared using the powder of A2 shown in Table 1 and evaluated. First, a NiO / YSZ composite powder slurry was prepared in the same manner as in (10) above. This is used as an air electrode (LaSrMn
O ₃ ) and an electrolyte (YSZ) are coated, dried, and fired at 1,400 ° C. to form a 3% H ₂ atmosphere, 1,0
Reduction treatment was performed at 00 ° C. Fuel: 11% H ₂ O, 89% H ₂
, Oxidizing agent; air 4 volume equivalents, the fuel utilization rate of 40%, a result of power generation rated at operating conditions of temperature 1,000 ° C., shows a high output with a maximum output of 0.5 W / cm ^2, the powder according to a conventional powder mix The maximum output of a cell fabricated using
² ) It was confirmed that it was higher.

[0032]

As is apparent from the above description, the method for producing a Ni / YSZ cermet raw material powder of the present invention exhibits the following effects. Fine powder with reduced sintering properties can be produced.
A porous Ni / YSZ cermet can be obtained.
Therefore, it is possible to provide a fuel electrode cermet having a fine structure, air permeability, and conductivity, and improve the power generation performance of the SOFC. Since the conductivity is higher than that of a conventional powder having the same composition,
The thickness of the OFC fuel electrode can be reduced, whereby the power generation performance of the SOFC can be improved. Since the dispersibility of the structures of the sintered body and the fired film is excellent, the aggregation of Ni and YSZ does not occur even in a high-temperature environment, so that it is possible to prevent deterioration of the conductive characteristics over time and improve reliability. Due to restrictions on the firing temperature when fabricating SOFC cells,
Powder preparation by repetition of heat treatment and pulverization is also effective in obtaining powder having desired characteristics. By changing the heat treatment and pulverization conditions for each layer component of the fuel electrode itself, a powder having optimum firing conditions can be given to each layer, and good electrode characteristics can be maintained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a process for producing and evaluating a NiO / YSZ composite powder according to one embodiment of the present invention.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/626 C01G 25 / 02,53 / 00 H01M 4/88

Claims (10)

(57) [Claims] 1. A Ni / YSZ cermet raw material powder (NiO / YSZ cermet) which becomes Ni / YSZ cermet after a firing / reducing step.
/ YSZ composite powder); a wet mixing step of obtaining a mixture containing Ni, Zr, Y and oxygen by a wet method; and a powder containing the oxide of each of the above metals by decomposing the mixture. A decomposing step of obtaining a body, a primary calcination step of calcining the powder, a pulverizing step of pulverizing the calcined powder (NiO / YSZ composite powder), and a pulverization obtained in the pulverization step. A method for producing a Ni / YSZ cermet raw material powder, comprising: a secondary calcining step of calcining the powder again. 2. A Ni / YSZ cermet raw material powder (NiO YSZ cermet) which becomes Ni / YSZ cermet after the firing / reduction step.
/ YSZ composite powder); a solution adjusting step of adjusting a raw material solution containing Ni ions, Zr ions, and Y ions at a desired ratio; and mixing a coprecipitated solution with the raw material solution. A coprecipitation step of coprecipitating a solid substance (coprecipitated substance) containing one or more metals and oxygen from the raw material solution; and decomposing the coprecipitated substance to obtain a powder and a granular material containing the oxide of each metal. A decomposition step, a primary calcination step of calcining the powder, a pulverizing step of pulverizing the calcined powder (NiO / YSZ composite powder), and re-grinding the pulverized powder obtained in the pulverizing step. A method for producing a Ni / YSZ cermet raw material powder, comprising: a secondary calcination step of calcination. 3. The method according to claim 2, wherein the raw material solution is a solution based on an aqueous nitric acid solution, the coprecipitation solution is an aqueous oxalic acid solution, and the respective aqueous solutions are heated to 60 ° C. to a boiling point before mixing. A method for producing a Ni / YSZ cermet raw material powder. 4. The calcination condition of the primary calcination is (800 to 1, 2).
The Ni / YSZ cermet according to claim 1, 2 or 3, wherein the calcination condition of the secondary calcination is (700-1,050C) x (2-10Hr). Production method of raw material powder. 5. The calcination condition of the primary calcination is (900 to 1,2,5).
4. The Ni / YSZ cermet according to claim 1, wherein the second calcination condition is (800 to 1,000 ° C.) × (2 to 10 hours). Production method of raw material powder. 6. The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the temperature of the secondary calcination is lower than the temperature of the primary calcination. 7. The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the calcining step is repeated within a total of five times with a pulverizing step interposed therebetween. 8. The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the particle size at the time of pulverization after primary calcination is 2 μm or less. 9. The NiO / YSZ composite powder of NiO /
The method for producing a Ni / YSZ cermet raw material powder according to any one of claims 1 to 8, wherein the YSZ composition is in the range of 30/70 to 70/30. 10. The Ni according to claim 1, wherein
A method for producing a solid oxide fuel cell, comprising: forming a Ni / YSZ cermet raw material powder produced by a method for producing a / YSZ cermet raw material powder on a YSZ solid electrolyte membrane by a slurry coating method.