JPH09227212A - Production of ni/ysz cermet raw material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for producing Ni/YSZ cermet raw material powder which is capable of contributing to an improvement in the power generation characteristic and durability of a solid-state electrolyte type fuel battery, is improved in the uniformity of the compsn. and structure of the powder and is adequately controlled in sinterability. SOLUTION: In the processing for producing the Ni/YSZ cermet raw material powder (NiO/YSZ composite powder) after firing and reducing stages; a mixture contg. Ni, Zr, Y and oxygen is obtd. by a wet process. Powder and granular materials contg. the oxides of the various metals are obtd. by decomposing the mixture. The process includes a primary calcining stage for calcining the powder and granular materials, a pulverizing stage for pulverizing the powder and granular materials (NiO/YSZ composite powder) after calcination and a secondary calcining stage for again calcining the pulverized powder obtd. in this pulverizing stage. The dispersibility of the respective elements is enhanced by the wet process mixing and the sinterability of the raw material powder is adjusted by the calcining stage and the grain size adjusting stage. At this time, the raw material powder is calcined plural times including the pulverization. The powder which is small in sizes, is suppressed in the sinterability and is suitable for film formation of fuel electrodes of SOFC is thereby obtd.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a Ni / YSZ cermet raw material powder used as a fuel electrode material for 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 which can contribute to the improvement of the power generation characteristics and durability of SOFC and which has an improved uniformity of the composition and structure of the powder and an appropriately controlled sinterability. Furthermore, the present invention relates to a method for manufacturing SOFC using such Ni / YSZ cermet raw material powder.

[0002]

2. Description of the Related Art A conventional technique will be described by taking an SOFC fuel electrode material as an example. As a fuel electrode material for SOFC, a sintered film of a composite powder obtained by mixing and compositing NiO and Y ₂ O ₃ stabilized ZrO ₂ (YSZ) is mainly used (Japanese Patent Laid-Open No. 61-153280, Japanese Patent Laid-Open No. 63-153280). Sho 61-19857
0 etc.). The NiO in the sintered film is reduced to Ni during the operation of SOFC, and the film becomes a Ni / YSZ cermet film.

As a method for producing such a raw material powder for Ni / YSZ cermet, NiO powder and YS are generally used.
A method (solid mixing method) in which both Z powders are mixed in a solid state and then heated (calcined) and slightly sintered to form a composite (solid mixing method) is adopted. Further, a method of pulverizing the composite powder after calcination to obtain a raw material powder for slurry application is also known (JP-A-6-295731). As a mixing method,
The one using a ball mill and the one using mechanochemical mechanical mixing are known. Further, a method of mixing Ni, Zr, and Y in an ionic state and thermally decomposing the mixture has also been proposed (JP-A-7-29575).

Ni / YSZ cermet is composed of various components (Ni
And YSZ) have a microstructure in which Ni and YSZ are connected to each other like a mesh has good conductivity, and Ni or Y
If the SZ grains are aggregated and the Ni network is cut, the conductivity is poor. If the conductivity of the SOFC fuel electrode is poor, the power generation efficiency of the SOFC decreases. Therefore, there is a demand for a NiO / YSZ composite powder in which Ni and YSZ are not aggregated and a Ni network structure can be firmly formed.
Further, since Ni tends to cause sinter aggregation even during the operation of SOFC, there is also a demand that the mesh of Ni should be uniform. Note that when Ni is aggregated, the three-phase interface of Ni, the solid electrolyte, and the gas phase (fuel gas) decreases, and the reaction between oxygen ions and the fuel gas decreases, which is also disadvantageous.

[0005]

The above-mentioned conventional manufacturing method has the following problems. Ball mill mixing method: Among the mixed powders, a component having a large specific gravity (NiO) or a component having a large particle size is precipitated, 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 the drying step after the mixing process. 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 due to the difference in particle size and the difference in specific gravity. Also, SOF
When powders of several μm or less, which are considered to be suitable for the fuel electrode of C, are mixed, it is difficult to deagglomerate the powders at the primary particle level by dry mixing.

Mechanochemical mechanical mixing: In this method, for example, a rotary blade is installed in a container, and the powder mixing is promoted by centrifugal force and agitation by the rotation of the container or the rotary blade itself. At this time, the powders are mixed with each other in a state where the temperature is naturally raised by the heat generated naturally or the state where the temperature is forcibly raised. In other words, it is a major feature of the mechanochemical method that the bonding between the powders is promoted by heat. Then, 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 nucleus. It is becoming widely used today.

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

Raw material powders obtained by a wet method such as a thermal decomposition method are more uniform than powders obtained by a solid mixing method. However, if not only the particle size of the powder but also the sinterability (shrinkage rate, BET value, etc.) is controlled, the stress applied to the cell substrate 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 a crack or the like generated by the stress at the time of contraction.

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

[0010]

In order to solve the above 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) which becomes a Ni / YSZ cermet after a firing / reduction step. / 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 the oxide of each metal by decomposing the mixture. A primary calcination step of calcination of the powder and granules, a pulverization step of pulverizing the powder and granules (NiO / YSZ composite powder) after calcination, and a pulverized powder obtained in this pulverization step And a secondary calcination step of calcination again. That is, the dispersibility of each element is increased by wet mixing, and the sinterability of the raw material powder is adjusted in the calcining step and the grain size adjusting step. At this time, by performing calcination a plurality of times through pulverization, it is possible to obtain a powder having a small diameter and suppressed sinterability, which is suitable for film formation of the SOFC fuel electrode. That is, the fuel electrode formed by using such 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 aspect of the present invention is a method for producing a Ni / YSZ cermet raw material powder (NiO / YSZ composite powder) which becomes Ni / YSZ cermet after a firing / reduction step. A method for producing; a solution adjusting step of adjusting a raw material solution containing Ni ions, Zr ions, and Y ions in a desired ratio; and a coprecipitation solution is mixed with the raw material solution, and one or more of each metal and A coprecipitation step in which a solid substance containing oxygen (coprecipitation substance) is coprecipitated from the raw material solution; a decomposition process in which the coprecipitation substance is decomposed to obtain a granular material containing the oxide of each metal; Primary calcination step of calcining the granules, and powder and granules (N
The iO / YSZ composite powder) is pulverized, and the pulverized powder obtained in this pulverization step is secondarily calcined again. As mentioned above,
Wet mixing step → Pyrolysis may be performed to obtain a powder or granular material containing oxides of Ni, Zr, and Y. However, the coprecipitated powder is preferable because it is excellent in the uniformity of the particle size and the uniformity of the structure.

In the method for producing the Ni / YSZ cermet raw material powder of this aspect (coprecipitation method), a nitric acid aqueous solution-based solution can be used as the raw material solution, and an oxalic acid aqueous solution can be used as the coprecipitation 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 the Ni / YSZ cermet raw material powder of the above aspect, it is preferable that the raw material solution and the coprecipitation solution are heated to 60 ° C. to the boiling point in advance and then mixed. Of the above reactions, the precipitation reaction of Ni oxalate is generally unlikely to occur at room 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 uniformly, so that the coprecipitated substance in which the 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 SOFC, the weight ratio of NiO and YSZ in the powder is 30: 70-7.
It is preferably 0:30. If the YSZ ratio exceeds 70, the conductivity of the powder fired film becomes low, which is not preferable. From this point of view, 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, a low Ni one has a low electric conductivity of the membrane itself, but a stress caused by a difference in thermal expansion between the high Ni content layer and the electrolyte. Can be relaxed, which is preferable.

When the Ni / YSZ cermet raw material powder of the present invention is used as a SOFC fuel electrode material, Ni / YSZ
The particle size of the YSZ cermet raw material powder is preferably 0.1 to 10 μm. This is because the balance between gas permeability and conductivity is good. At this time, 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 that case, it is preferable that the BET value of the Ni / YSZ cermet raw material powder is 0.8 to 12.0. If the BET value is larger than this value, the sintering proceeds and firing cracks easily occur. If the BET value is smaller than this, it is necessary to raise the firing temperature for sintering. However, in the fabrication of SOFC cells, the firing temperature must be matched with the other materials that make up the cell, and the firing temperature of the fuel electrode must be below the firing temperature of the substrate on which the fuel electrode is applied. Therefore, raising the firing temperature of the fuel electrode is limited.

In the method for producing the Ni / YSZ cermet raw material powder of the present invention, when the desired particle size and BET value cannot be obtained after the secondary calcination, further pulverization, tertiary calcination, pulverization and quaternary calcination are carried out. You may proceed to crushing and fifth calcination. However, even if the number of times is repeated, the particles are pulverized into fine particles, and the powder characteristics converge to a certain particle size and BET value, so that a large change does not appear and the treatment effect is small. Further, since the number of steps is increased, the cost is increased due to manufacturing conditions. Further, if the powder has a rounded shape and the sinterability is low, there is a possibility that the fuel electrode membrane cannot be formed unless the temperature is raised to the firing temperature of the SOFC cell substrate or higher. . For this reason, 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
0 to 1,250 ° C.) × (2 to 10 hr), and the secondary calcination condition is (700 to 1,050 ° C.) × (2 to 10 hr)
Is preferred. Furthermore, the calcination conditions for the primary calcination (900 ~
1,200 ° C.) × (2 to 10 hr), and the calcination condition of the secondary calcination is more preferably (800 to 1,000 ° C.) × (2 to 10 hr). The reason is 800 at the time of primary calcination
If the heat treatment (calcination) temperature is ℃ or less, the nitric acid component may remain and the purity of the raw material powder decreases, and in the case of such heat treatment, the powder does not crystallize and is not sintered. This is because the fine particles that do not progress remain as they are, and excessively sinter during the subsequent SOFC fuel electrode film formation (calcination), which causes peeling and cracks. Further, calcination at 1,250 ° C. or higher causes the calcinated powder to become hard, which reduces the pulverization efficiency in the next step, which is not preferable in the manufacturing process. During the secondary calcination, the temperature is 70
Below 0 ° C., the powder hardly sinters, so the effect of secondary calcination is small. On the other hand, when the temperature is 1,050 ° C. or higher, the sintering proceeds and the powder having a large particle size is likely to be formed again. Therefore,
It is important to control the particle size after the secondary calcination so that it falls within a particle size range suitable for film formation on an SOFC cell. It is general that the temperature of the secondary calcination is lower than that of the primary calcination, that is, the condition of the secondary calcination is milder than that of the primary calcination. This is because if the secondary calcination conditions are high temperature and 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 SOFC,
At the time of pulverization after the primary calcination, the particle size is preferably 2 μm or less. 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. After calcination or crushing, classification may be carried out if necessary.

[0019]

Embodiments of the present invention will be described below. FIG.
It is a flowchart which shows the process of the manufacturing method of Ni / YSZ cermet raw material powder and its evaluation method which concern on the standard Example of this invention. Description will be made with reference to this flowchart.

(1) Preparation of raw material solution: zirconium nitrate / yttrium aqueous solution (8 mol% Y ₂ O ₃ as a YSZ raw material)
Content, oxide conversion content 23.4 wt%), nickel nitrate hexahydrate crystals as NiO raw material, pure water for adjusting the coprecipitating substance concentration is NiO / YSZ composition 65/35 wt%
And mixed well so that

(2) Preparation of coprecipitation solution: In this example,
An oxalic acid aqueous solution was used as a coprecipitation solution. Pure water is taken into the container and heated 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 oxalic acid is slightly in excess of 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 difficult to form even when nickel nitrate and an aqueous solution of oxalic acid are reacted at room temperature. Therefore, the production rate of nickel oxalate is increased by adding nitric acid in a state where a nickel-containing aqueous solution such as nitrate is heated. The raw material solution (NiO / YSZ composite powder aqueous solution) is heated to 80 ° C to 90 ° C, and this is heated to 80 ° C to 9 ° C.
A precipitate was formed by the oxalic acid coprecipitation method by gradually adding it to an aqueous nitric acid solution which was 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 reaction product was allowed to stand in a dryer for 1
Hot air at 20 ° C. was sent to evaporate the water content of the precipitate. At this time, the powder (precipitation) and the water may be separated by using a filtering device. (5) Pyrolysis: The dried sample was heat-treated at 500 ° C. for 5 hours to remove nitric acid components 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 (crushing): Wet pulverization treatment was performed using PSZ balls of 2φ and 3φ. This aims at crushing and homogenizing the secondary particles. However, the powder obtained by this coprecipitation method has a particle size of 1 μm according to the wet laser diffraction particle size distribution measurement.
It is also possible to omit this ball milling process, since particles of m or less account for about 80% of the total, and even large secondary particles have a particle size of 10 to 20 μm.

(7) Primary calcination: The obtained powder was treated with 1100
℃ × 5hr, 1150 ℃ × 5hr, 1200 ℃ × 5hr, 14
Primary calcination (heat treatment) was performed under each condition of 00 ° C. × 5 hr.
At the time of calcination, NiO fine powder is mainly mixed with other N due to the sintering phenomenon.
Combine with iO powder. Further, Y ₂ O ₃ is gradually solid-dissolved in ZrO ₂ and crystallized. Table 1 shows the powder particle size and the BET value after the primary calcination.

[0026]

[Table 1]

(8) Pulverization: The powder preliminarily calcined by air stream pulverization having the following specifications was pulverized to a diameter of 2 μm or less. Table 1 also shows the particle size and BET value after pulverization. The crushing method is
If fine pulverization is possible, other means such as ball mill pulverization can be used. Regarding the relationship between the primary calcination temperature and crushing, the particle size is 2 μm for A4 powder with a calcination temperature of 1400 ° C.
It took an extremely long time to make the amount below, and it was difficult to pulverize.

(9) Secondary calcination: milled powder at 1,000 ° C
Secondary calcination was performed under the condition of × 5 hr. This secondary calcination aims to suppress the sinterability of the pulverized powder by lightly sintering the pulverized powder again. As shown in Table 1,
The BET value has decreased due to the secondary calcination. In addition, A4
The BET value of the powder remains the same after the secondary calcination, even though the powder is agglomerated by the secondary pulverization.
This is because the activity of the particle surface decreased due to the primary calcination before the sintering occurred 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 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.
This was dried and then baked at 1,400 ° C. The obtained cermet film had a thickness of 80 μm and a porosity of 27%.

[0030] (11) conductivity measurement: cermet film of H ₂ atmosphere, and the conductivity was measured by the four probe method after reduction at 1,000 ° C. × 5 Hr. The results are also shown in Table 1. This result shows that the powder of A2 has the highest conductivity (1,600s / c
m). In the case of the film using the raw material powder by the conventional solid mixing method, the conductivity is around 1100 s / cm, so 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 (10) above. This is the air electrode (LaSrMn
O ₃₎ and coating the substrate tube consisting of the electrolyte (YSZ), dried, and calcined at 1,400 ℃, 3% H ₂ atmosphere, 1,0
Reduction treatment was carried out at 00 ° C. Fuel: 11% H ₂ O, 89% H ₂
, Oxidizer; 4 times equivalent amount of air, fuel utilization rate 40%, power generation evaluation under the operating conditions of temperature 1,000 ℃, the maximum output was 0.5 W / cm ² and high output was shown. The maximum output of the cell made by using (about 0.42W / cm
² ) It was confirmed that it was higher.

[0032]

As is clear from the above description, the method for producing the Ni / YSZ cermet raw material powder of the present invention exhibits the following effects. Since fine powder with suppressed sinterability can be produced,
A porous Ni / YSZ cermet can be obtained.
Therefore, it is possible to provide a cermet for a fuel electrode having a fine structure, air permeability, and conductivity, and it is possible to improve the power generation performance of SOFC. Since the conductivity is higher than that of the conventional powder of the same composition, S
The OFC fuel electrode can be thinned, and thereby the power generation performance of the SOFC can be improved. Since the sintered body and the fired film are excellent in dispersibility, Ni and YSZ do not aggregate even in a high temperature environment, so that deterioration of the conductive characteristics with time can be prevented and reliability can be improved. Due to the limitation of the firing temperature when manufacturing an SOFC cell,
In order to obtain a powder having desired characteristics, it is effective to adjust the powder by repeating heat treatment and pulverization. By changing the heat treatment and pulverization conditions for each layer constituent element of the fuel electrode itself, it is possible to give powder having optimum firing conditions to each layer, and it is possible to maintain good electrode characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a manufacturing and evaluation process of NiO / YSZ composite powder according to an example of the present invention.

Claims (10)

[Claims] 1. A raw material powder of Ni / YSZ cermet (NiO that becomes Ni / YSZ cermet after the firing / reduction process).
/ YSZ composite powder); a wet mixing step of obtaining a mixture containing Ni, Zr, Y and oxygen by a wet method, and powder particles containing an oxide of each of the above metals by decomposing the mixture. Decomposing step for obtaining a body, primary calcination step for calcination of the powder and granules, crushing step for crushing the powder and granules (NiO / YSZ composite powder) after calcination, and crushing obtained by this crushing step A secondary calcination step of calcining the powder again, and a method for producing a Ni / YSZ cermet raw material powder, comprising: 2. A raw material powder of Ni / YSZ cermet (NiO which becomes a 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 in a desired ratio; A coprecipitation step of coprecipitating a solid substance (coprecipitated substance) containing one or more kinds of metals and oxygen from the raw material solution, and decomposing the coprecipitated substance to obtain a granular material containing an oxide of each metal. The decomposition step, the primary calcination step of calcination of the powder and granules, the pulverization step of pulverizing the powder and granules (NiO / YSZ composite powder) after the calcination, and the pulverized powder obtained in this pulverization step again 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 a nitric acid aqueous solution, the coprecipitation solution is an oxalic acid aqueous solution, and each aqueous solution is heated to 60 ° C. to a boiling point in advance and then mixed. A manufacturing method of Ni / YSZ cermet raw material powder. 4. The calcination condition of the primary calcination is (800 to 1,2)
Ni / YSZ cermet according to claim 1, 2 or 3, wherein the calcination condition of the secondary calcination is (700 to 1,050 ° C) x (2 to 10Hr). Method of manufacturing raw material powder. 5. The calcination condition of the primary calcination is (900 to 1,2)
Ni / YSZ cermet according to claim 1, 2 or 3, wherein (00 ° C) x (2 to 10 hr) and the secondary calcination calcination condition is (800 to 1,000 ° C) x (2 to 10 hr). Method of manufacturing raw material powder. 6. The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the secondary calcination temperature is lower than the primary calcination temperature. 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 crushing step interposed therebetween. 8. The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the particle size is 2 μm or less when pulverized after the primary calcination. 9. The NiO / YSZ composite powder of NiO /
The method for producing a Ni / YSZ cermet raw material powder according to claim 1, wherein the YSZ composition is in the range of 30/70 to 70/30. 10. Ni according to any one of claims 1 to 9.
/ YSZ cermet raw material powder manufacturing method, Ni / YSZ cermet raw material powder is formed on the YSZ solid electrolyte membrane by a slurry coating method.