JPH07196382A - Porous sintered product and solid electrolyte type fuel cell - Google Patents

Abstract

PURPOSE:To improve the stability of a porous sintered product against thermal cycles by grinding a lanthanum manganite obtained by calcining the mixture of raw materials, adding an organic binder and a hole-forming agent to the calcined product, molding the mixture, and subsequently sintering the molded product. CONSTITUTION:La2O3, CaCO3, Mn3O4, Nip, Cub, MgO, Al2O3 and SrCO3 are mixed with each other in a prescribed ratio, and calcined. The produced lanthanum manganite is ground, mixed with an organic binder and a hole-forming agent, molded and subsequently sintered to obtain a porous sintered product comprising a perovskite structure lanthanum manganite whose A site is substituted with Ca in an amount of 22-35%, Mn of whose B site is partially substituted with at least a metal element selected from Al, Co, Cu, Mg, Ni, Fe, Ti, and Zn, and in which a dimensional contraction caused by a thermal cycle at room temperature to 1000 deg.C is <=0% per thermal cycle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous sintered body and a solid oxide fuel cell using the same as an air electrode material.

[0002]

2. Description of the Related Art Solid oxide fuel cells (SOFC) are
Since it operates at a high temperature of 1000 ° C, the electrode reaction is extremely active, no expensive precious metal catalyst such as platinum is required, the polarization is small, and the output voltage is relatively high, so the energy conversion efficiency is higher than other fuel cells. Is extremely high. Furthermore, since the structural material is composed entirely of solid, it is stable and has a long life.

In SOFC development projects, it is important to search for stable materials at high temperatures. As an air electrode material for SOFC, a lanthanum manganite sintered body is currently considered to be promising (Energy Engineering, 13, 2, 52-6).
8 pages, 1990). As such a lanthanum manganite sintered body, one having a substantially stoichiometric composition or one having a partial loss of A site (lanthanum site) (manganese-rich composition) is also known. Further, it has been reported that the weight of the lanthanum manganite sintered body having a composition in which the A site is partially lost decreases as the temperature rises from room temperature to 1000 ° C. (J. Electrochem. S).
oc. 138, 5, 1519-1523, 1991.
Year). In this case, the weight of the sintered body begins to decrease from around 800 ° C.

[0004]

Problems to be Solved by the Invention
A porous sintered body made of lanthanum manganite doped with a and Sr is regarded as a promising material for an air electrode including a self-supporting air cathode tube. However, the present inventors have discovered for the first time that the porous sintered body has the following problems.

That is, the SOFC power generation temperature of 900-
When a heating-cooling cycle is applied between a temperature of 1100 ° C. and a temperature of room temperature to 600 ° C., a crack occurs between the air cathode tube made of the above-mentioned porous sintered body and other constituent materials of the unit cell. It was found that the battery was damaged and the unit cell was destroyed. Moreover, even if this unit cell is operated at 1000 ° C. for a long time,
Such cracks did not occur at all. Therefore, this phenomenon was considered not to be due to the firing shrinkage of the porous sintered body, but to the dimensional change due to the thermal cycle.

An object of the present invention is to provide the lanthanum manganite porous sintered body with the above-mentioned stability against thermal cycles.

[0007]

A porous sintered body according to the present invention is made of lanthanum manganite having a perovskite structure, and 22% or more of A site of lanthanum manganite,
35% or less is replaced by calcium, and aluminum, cobalt, copper, magnesium, nickel,
One or more metal atoms selected from the group consisting of iron, titanium and zinc partially replaces the manganese atoms at the B site, and the dimensional shrinkage caused by the thermal cycle between room temperature and 1000 ° C. per thermal cycle It is characterized by being 0.01% or less.

The porous sintered body according to the present invention is made of lanthanum manganite having a perovskite structure, and 20% or more and 40% or less of the A site of lanthanum manganite is replaced by strontium, and aluminum and cobalt. , A dimension generated by thermal cycling between room temperature and 1000 ° C., with some of the B-site manganese atoms being replaced by one or more metal atoms selected from the group consisting of copper, magnesium, nickel, iron, titanium and zinc. The shrinkage is characterized by being 0.01% or less per thermal cycle.

[0009]

First, the dimensional shrinkage phenomenon of the porous sintered body due to the heat cycle will be described. The inventor of the present invention has proposed a conventional lanthanum manganite porous sintered body with 900 to 11
Between a temperature of 00 ° C and a temperature of room temperature to 600 ° C,
A heating-cooling cycle was run to test its stability. In this lanthanum manganite, the B site is not particularly substituted, and is 10 mol% to 20 mol% of the A site.
Is replaced by calcium, or 10 mol% to 15 mol% of the A site is replaced by strontium.

As a result, it was found that the size of the above-mentioned porous sintered body shrank by 0.01 to 0.04% per one thermal cycle. Moreover, the shrinkage due to this thermal cycle is
It was found that the heat treatment did not converge even after 100 heat cycles, and reached several percent after 100 heat cycles. When the air electrode contracts in this way, a crack is generated between the air electrode and other constituent materials of the unit cell, which causes destruction of the unit cell.

The Applicant has conducted research to solve this problem. As a result, according to the porous sintered body made of the composite oxide having the above composition, the dimensional shrinkage caused by the thermal cycle between room temperature and 1000 ° C. is 0.01% or less per thermal cycle. I found that it can be suppressed to. Moreover, by this, 900-1100
Heating between ° C and room temperature to 600 ° C-
It was confirmed that cracks did not occur between the porous sintered body and the other constituent materials even after the cooling cycle.

"Dimensional shrinkage is 0.0 per thermal cycle
"1% or less" means that after sintering the porous sintered body,
It shall mean the average value of each dimensional shrinkage from the first heat cycle to the 10th or 100th heat cycle.

In the porous sintered body of the present invention, aluminum, cobalt, copper, magnesium, nickel, iron,
A part of the manganese atoms at the B site of lanthanum manganite is replaced with one or more metal atoms selected from the group consisting of titanium and zinc.

The substitution ratio of metal atoms other than manganese atoms at the B site is preferably 0.02% or more and 20% or less, more preferably 5% or more and 20% or less. If this substitution ratio is less than 0.02%, the effect of suppressing shrinkage is not remarkable, and if it exceeds 20%, the electrical conductivity decreases. When the substitution ratio is 5% or more, the effect of suppressing the shrinkage due to the thermal cycle becomes more remarkable, and 2
When it is 0% or less, the decrease in electric conductivity of the porous sintered body is small.

Two or more kinds of the above metal atoms can be substituted at the B site of the porous sintered body. In this case, the total amount of substitution of two or more types of substitution atoms is preferably set to the above ratio.

In the porous sintered body of the present invention, 22% or more and 35% or less of the A site is replaced with calcium. When the amount of substitution of calcium is 15% or more and less than 22%, the effect of reducing the dimensional shrinkage with respect to the heat cycle is not remarkable. If the amount of substitution is less than 15%, the amount of dimensional shrinkage due to thermal cycles tends to decrease, but the change in the thermal expansion coefficient between 900 ° C and 1000 ° C during temperature increase and decrease. Is 2% or more. If the amount of substitution of calcium exceeds 35%, the coefficient of thermal expansion of the porous sintered body will rapidly increase.

Alternatively, 20% or more and 40% or less of the A site is replaced with strontium. If the amount of strontium substitution is less than 20%, the effect of reducing dimensional shrinkage with respect to the heat cycle is not significant. When the strontium substitution amount exceeds 40%, the thermal expansion coefficient of the porous sintered body increases.

Further, since lanthanum manganite can have a non-stoichiometric composition, the composition variation of lanthanum manganite caused by some impurities inevitably mixed in the manufacturing process of the porous sintered body is allowed. It

Further, the present inventor has conducted research on the mechanism of dimensional shrinkage of the porous sintered body that accompanies the heat cycle. As a result, it was found that when the value obtained by dividing the weight at 1000 ° C. by the weight at room temperature is 0.9988 or more, the above dimensional shrinkage is significantly suppressed. That is, when the temperature of the porous sintered body is raised from room temperature to a high temperature of about 1000 ° C., a weight reduction is observed in the porous sintered body, and this weight reduction has a clear correlation with the dimensional shrinkage due to the heat cycle. Had.

The mechanism of the above phenomenon is not clear at this time. However, when the temperature is raised from room temperature to about 1000 ° C., the weight of the porous sintered body slightly decreases, and when the temperature is lowered to room temperature again, this weight is restored.

Further, as a result of further research by the present inventor, the activation energy (hereinafter referred to as activation energy) of electric conductivity of the porous sintered body is clearly dimensional shrinkage due to the above thermal cycle. It was found to have a correlation. That is, when an Arrhenius plot is created over a wide range of 200 ° C to 1000 ° C, 200 ° C to 600 °
Activation energy in the range of C and 900 to 1000
It was found that the thermal cycle dimensional shrinkage proceeded in the porous sintered body in which the difference with the activation energy in the range of ° C clearly appeared.

Specifically, if the difference between the activation energy in the range of 200 ° C. to 600 ° C. and the activation energy in the range of 900 to 1000 ° C. is 0.01 eV or less, the difference between room temperature and 1000 ° C. It was revealed that the dimensional shrinkage caused by the heat cycle during the heat cycle can be suppressed to 0.01% or less per heat cycle.

When the change in the coefficient of thermal expansion between the temperature rising and the temperature falling between 900 ° C. and 1000 ° C. is 2% or less, the dimensional shrinkage due to the thermal cycle is 1%. It was also confirmed that the amount was 0.01% or less per time, which was a remarkable decrease.

The mechanism by which these phenomena occur is currently unknown. However, it is speculated that oxygen goes in and out of the crystal in the temperature range of 800 ° C. or higher in the atmosphere due to the thermal cycle, and the crystal lattice is distorted as the oxygen goes in and out, promoting the mass transfer of metal atoms. It Further, the amount of dimensional shrinkage of the porous sintered body due to the heat cycle is slightly different depending on the grain size of the sintered body, the temperature rising / falling rate during the heat cycle, and the oxygen partial pressure in the atmosphere. That is, it was found that the smaller the crystal grain size, the lower the temperature raising / lowering rate, and the higher the oxygen partial pressure in the atmosphere, the greater the dimensional shrinkage of the porous sintered body.

The porous sintered body of the present invention can be particularly preferably used as a high temperature electrode material which is stable against heat cycles. Examples of such high-temperature electrode materials include fusion reactors, MHD
There are electrode materials for power generation.

The porous sintered body of the present invention is SOFC
It can be used particularly preferably as an air electrode material for the. Furthermore,
It is preferably used as a material for a self-supporting air electrode substrate. Such an air electrode base is used as a base material of a single cell, and each component such as a solid electrolyte membrane, a fuel electrode membrane, an interconnector and a separator is laminated on the air electrode base. At this time, the shape of the air electrode substrate may be a cylindrical shape with both ends open, a bottomed cylindrical shape with one end open and the other end closed, or a flat plate shape. Of these, any of the above-mentioned cylindrical shapes is particularly preferable because thermal stress is less likely to be applied and gas sealing is easy.

The porosity of the porous sintered body is preferably 5-40%. When this is used as an air electrode material for SOFC, the porosity is preferably 15 to 40%, and more preferably 25 to 35%. In this case, by setting the porosity of the air electrode to 15% or more, the gas diffusion resistance is reduced, and by setting the porosity to 40% or less, it is possible to secure some strength.

When the porous sintered body of the present invention is used as an air electrode material for SOFC, the thermal expansion coefficient of the porous sintered body is close to the thermal expansion coefficient of the solid electrolyte membrane or the fuel electrode membrane. Must. When the solid electrolyte membrane is yttria-stabilized zirconia, the temperature is 25 ° C to 1 ° C.
The average coefficient of thermal expansion between 000 ° C is 10.5 × 10 ^-6 K ^-1
Is known to be.

Japanese Unexamined Patent Publication (Kokai) No. 1-200560 discloses an electrode material having a composition of La _1-X A _X Mn _{1- y By} O _{3 in} a lanthanum manganite-based perovskite complex oxide. . Where A is C
a and Sr, and B is nickel and chromium. 0 ≦
X ≦ 0.4 and 0 <y ≦ 0.25. By using the electrode material having such a composition, the bulk conductivity (σ) and the interfacial conductivity (σ _E ) are improved, and the electrode resistance and the polarization potential are reduced.

However, the problem of shrinkage of the porous sintered body due to the heat cycle in the present invention has not been recognized at all, and there is no motivation to reach the present invention. Further, as is clear from the examples of JP-A-1-200560, the substitution amount of calcium at the A site is 40%, which is unsuitable as an air electrode for SOFC in terms of thermal expansion coefficient. (Table 3 page 1 of this publication).

In order to produce the porous sintered body of the present invention, preferably, the raw materials are mixed so as to have the above composition, the mixture is fired to synthesize lanthanum manganite, and the synthesized product is pulverized. To produce lanthanum manganite powder. The powder is mixed with an organic binder and a pore-forming agent to be molded, and the molded body is fired to obtain a porous sintered body.

[0032]

Example (Experiment 1) (Production of sample for experiment) As a starting material, La ₂ O ₃ ,
CaCO ₃ , Mn ₃ O ₄ , NiO, CuO, MgO, A
Powders of l ₂ O ₃ and SrCO ₃ were used. Table 1, Table 2
In each of the examples, a predetermined amount of starting materials were weighed and mixed so that the composition ratio shown in FIG. This mixed powder was subjected to 1 tf / cm ² by a cold isostatic press method.
Molding was performed under the pressure of 1 to prepare a molded body. This molded body was heat-treated in the air at 1100 ° C. for 40 hours to synthesize lanthanum manganite having the composition shown in Tables 1 and 2.

This synthetic material was crushed by a ball mill to prepare a lanthanum manganite powder having an average particle size of about 3 to 6 μm. Next, polyvinyl alcohol was dispersed as an organic binder in this lanthanum manganite powder, and a square plate was formed by a uniaxial pressing method. This molded body is placed in the atmosphere 125
A sintered body was obtained by firing at 0 ° C. for 5 hours, and a rectangular rod having a length of 3 mm, a width of 4 mm, and a length of 40 mm was cut out from this sintered body to obtain a test sample.

(Measurement) First, sample 1 of the embodiment shown in Table 1
10 to Sample 25 of Comparative Example were crushed in a mortar and subjected to X-ray diffraction measurement by a powder method. As a result, the diffraction patterns of Samples 1 to 10 according to the example of the present invention are almost the same as the diffraction pattern of Sample 25 according to the comparative example, and show a single phase.

In view of these X-ray diffraction patterns, in Samples 1 to 10, calcium, nickel, copper, magnesium, aluminum, and strontium were certainly solid-dissolved in the lanthanum manganite crystals.

Next, the porosity of each sample was measured by the water substitution method. The results are shown in Tables 1 and 2.

Next, each sample was heated to 600 ° C. at a rate of 200 ° C./hour in the atmosphere, and then heat cycled between 600 ° C. and 1000 ° C. at a temperature rising / falling rate of 200 ° C./hour for 10 times. And cooled to room temperature. At this time, in each heat cycle, a constant temperature was maintained for 30 minutes at 600 ° C. and 1000 ° C., respectively. Then, the dimension of each sample was measured using a micrometer, and the dimensional shrinkage before and after the thermal cycle was calculated. The results of these measurements are shown in Tables 1 and 2.

The electrical conductivity of each sample at 1000 ° C. was evaluated. This is a characteristic required for an SOFC air electrode and the like. This measurement is 1000
It was carried out by the DC four-terminal method at ℃. The results are also shown in Tables 1 and 2. However, in Tables 1 and 2, in order to eliminate the influence on the electric conductivity due to the difference in porosity, the measurement data was corrected by calculation so that the porosity was 30%.

For some of the samples, the temperature range from 25 ° C to
The average coefficient of thermal expansion of 1000 ° C was measured.

[0040]

[Table 1]

[0041]

[Table 2]

As can be seen from Table 2 above, sample number 25
And sample Nos. 21 to 24, when the substitution amount of calcium was 20% or the substitution amount of strontium was 10%, when part of the B site was replaced with nickel, aluminum, etc. The dimensional shrinkage of the sintered body is decreasing. However, it was found that the electric conductivity of the porous sintered body was lower than that of Sample No. 25, and the electric conductivity tended to further decrease as the amount of substitution at the B site increased. This point is disadvantageous as an electrode material.

On the other hand, comparing sample No. 1 and sample No. 25, it can be seen that there is not much difference in electrical conductivity, and on the other hand, dimensional shrinkage is small. Sample numbers 1 and 2
In comparison with Sample No. 1, in Sample No. 1 according to the example of the present invention, the electric conductivity of the porous sintered body is improved.

Further, looking at Sample Nos. 2, 3, and 4, as the amount of calcium substitution increases, the dimensional shrinkage decreases and the electrical conductivity also improves. Comparing Sample Nos. 5 and 22, again by increasing the calcium substitution to 35% as in Sample No. 5,
It can be seen that the electric conductivity is improved to the level of Sample No. 25.

Comparing Sample Nos. 7 and 23, Sample No. 7 also exhibits the effect of improving the electrical conductivity in addition to reducing the dimensional shrinkage. Comparing Sample Nos. 10 and 24, in Sample No. 10, in addition to reducing dimensional shrinkage,
It also has the effect of improving the electrical conductivity.

Further, the present inventor raised the temperature of the sample 25 of Table 1 from room temperature to 1000 ° C. and lowered it,
The dimensional change of the porous sintered body was measured by a thermal expansion meter.
As a result, the dimensional shrinkage phenomenon is
We have identified what is happening in the temperature range of 800 ° C.
Therefore, it is assumed that oxygen atoms are absorbed and metal atoms are moved in this temperature range. Further, the result of the thermal cycle between 600 ° C. and 1000 ° C., which is the condition of this experiment, is the same as the result of the thermal cycle between room temperature and 1000 ° C.

Sample 25 was also subjected to 1 atmosphere at 1000 ° C.
After maintaining for 0 hour and cooling to room temperature, the dimensional change rate before and after heating was measured, and it showed 0.03% shrinkage. On the other hand, referring to Table 2, the dimensional shrinkage ratio per thermal cycle is 0.
It was 029%.

Therefore, the shrinkage of 0.03% substantially corresponds to the dimensional shrinkage of one heat cycle. From this result, the above-mentioned 0.03% dimensional shrinkage did not occur during the holding at 1000 ° C. for 10 hours, but during the temperature lowering process in which the temperature decreased from 1000 ° C. to room temperature. . In other words, the shrinkage phenomenon of the porous sintered body due to the above-mentioned thermal cycle is caused by a mechanism completely different from the progress of sintering due to the holding of the porous sintered body at a high temperature.

(Experiment 2) In the same manner as in Experiment 1, a porous sintered body of each example was manufactured so as to have the compositions and porosities shown in Tables 3 and 4. Then, the porosity and the dimensional shrinkage of the porous sintered body of each example were measured. The results are shown in Tables 3 and 4.

[0050]

[Table 3]

[0051]

[Table 4]

As can be seen from Tables 3 and 4, sample number 41
Comparing Nos. 42 and 43 with each other, it can be seen that the dimensional shrinkage rate is remarkably reduced when the amount of calcium substitution is increased. However, when the sample numbers 31 and 32 according to the present invention are compared with the sample number 41 described above, the dimensional shrinkage rate is still more remarkably reduced despite the fact that the substitution amount of calcium is the same. Further, comparing sample Nos. 33 and 42, even if the amount of substitution of calcium was the same, the dimensional shrinkage ratio was remarkably reduced by substituting a part (5%) of B site with cobalt. You can see that

It can be seen from Sample Nos. 36 and 37 that the present invention is particularly effective in reducing the dimensional shrinkage ratio. Further, comparing sample Nos. 44 and 45, it can be seen that the dimensional shrinkage rate remarkably decreases when the substitution amount of strontium is increased. However, sample number 3 according to the invention
Comparing Samples Nos. 4 and 35 with Sample No. 44 described above, the dimensional shrinkage ratio is still more remarkably reduced although the substitution amount of strontium is the same.

(Experiment 3) Next, a porous sintered body having each composition shown in Table 5 was produced as described above, and the porosity, dimensional shrinkage ratio, electric conductivity, and The average coefficient of thermal expansion was measured. The results are shown in Table 5.

[0055]

[Table 5]

(Experiment 4) Next, as described above, a porous sintered body having each composition shown in Table 6 was manufactured, and the thermal expansion coefficient was measured at each temperature. The results are shown in Table 6.

[0057]

[Table 6]

As can be seen from the above, when the substitution amount of calcium is 15% or less, the ratio of the coefficient of thermal expansion at 900 ° C. and 1000 ° C. becomes as large as 2% or more. When the substitution amount of calcium is increased to 20% or more, the thermal expansion coefficient of the porous sintered body tends to increase. At present, zirconia is regarded as a promising solid electrolyte material for SOFC, and its thermal expansion coefficient is 10.5 × 10 ⁻⁶ /
It is about K. Therefore, from the viewpoint of matching the thermal expansion of the solid electrolyte of SOFC and the air electrode, it is preferable to reduce the substitution amount of calcium, and particularly 25 to 35%.
It is preferable that

Further, in the La—Ca—Mn—Ni system porous sintered body, the substitution amount of nickel was changed, and the electrical conductivity and the dimensional shrinkage ratio of the porous sintered body were measured. The result is shown in FIG. As a result, the substitution amount of B site is 0.0
It is particularly preferably 2% or more from the viewpoint of dimensional shrinkage, and preferably 20% or less from the viewpoint of electrical conductivity.

Further, in the La-Ca-Mn-Co type porous sintered body, the substitution amount of calcium was changed, and the average thermal expansion coefficient and the dimensional shrinkage rate of the porous sintered body were measured.
The result is shown in FIG. As a result, in order to reduce the dimensional shrinkage, the amount of calcium substitution should be 22% or more, but in order to keep the average coefficient of thermal expansion low, it is preferably 35% or less.

[0061]

As described above, according to the present invention, the dimensional shrinkage caused by the thermal cycle between room temperature and 1000 ° C. can be suppressed to a very small level, which allows the above heating-cooling. It is possible to prevent the occurrence of cracks between the porous sintered body and the other constituent materials even if the cycle is repeated.

[Brief description of drawings]

FIG. 1 is a graph showing the results of measuring electric conductivity and dimensional shrinkage of a porous sintered body of La-Ca-Mn-Ni based porous sintered body while changing the substitution amount of nickel. is there.

FIG. 2 is a graph showing the results of measuring the average thermal expansion coefficient and dimensional shrinkage of the porous sintered body by changing the amount of calcium substitution in the La-Ca-Mn-Co porous sintered body. Is.

Claims (10)

[Claims] 1. A porous sintered body composed of lanthanum manganite having a perovskite structure, wherein 22% or more and 35% or less of the A site of the lanthanum manganite is replaced by calcium, and aluminum, cobalt, copper A thermal cycle between room temperature and 1000 ° C., wherein a part of the manganese atom at the B site of the lanthanum manganite is replaced by one or more metal atoms selected from the group consisting of, magnesium, nickel, iron, titanium and zinc. The dimensional shrinkage caused by each heat cycle is 0.01
% Or less, a porous sintered body. 2. The porous sintered body according to claim 1, wherein a substitution ratio of metal atoms other than manganese atoms in the B site is 0.02% or more and 20% or less. 3. The porous calcination according to claim 1, wherein the change in the coefficient of thermal expansion between the temperature rising and the temperature falling between 900 ° C. and 1000 ° C. is 2% or less. Union. 4. The porosity is 5% or more and 40% or less,
The porous sintered body according to any one of claims 1 to 3. 5. A solid oxide fuel cell, characterized in that an air electrode is formed by the porous sintered body according to claim 1. 6. A porous sintered body made of lanthanum manganite having a perovskite structure, wherein 20% or more and 40% or less of the A site of the lanthanum manganite is replaced by strontium, and aluminum, cobalt, copper A thermal cycle between room temperature and 1000 ° C., wherein a part of the manganese atom at the B site of the lanthanum manganite is replaced by one or more metal atoms selected from the group consisting of magnesium, nickel, iron, titanium and zinc. Dimensional shrinkage caused by 0.0 per thermal cycle
A porous sintered body characterized by being 1% or less. 7. The porous sintered body according to claim 6, wherein a substitution ratio of metal atoms other than manganese atoms in the B site is 0.02% or more and 20% or less. 8. The porous calcination according to claim 6, wherein the change in the coefficient of thermal expansion between the temperature rising and the temperature falling between 900 ° C. and 1000 ° C. is 2% or less. Union. 9. The porosity is 5% or more and 40% or less,
The porous sintered body according to any one of claims 6 to 8. 10. A solid oxide fuel cell, characterized in that an air electrode is formed by the porous sintered body according to claim 6.