JP3260947B2 - Porous sintered body and solid oxide fuel cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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 (SOFCs)
Since it operates at a high temperature of 1000 ° C, the electrode reaction is extremely active, does not require any expensive noble metal catalyst such as platinum, has a small polarization, and has a relatively high output voltage, so the energy conversion efficiency is higher than other fuel cells. Remarkably high. Further, since all the structural materials are composed of solids, they are stable and have a long life.

In the development of SOFCs, it is important to search for materials that are stable at high temperatures. Lanthanum manganite sintered bodies are currently considered promising as cathode materials for SOFCs (Energy Engineering, 13, 2, 52-6).
8, p. 1990). Among such lanthanum manganite sintered bodies, those having an almost stoichiometric composition and those having a composition in which A site (lanthanum site) is partially deleted (manganese-rich composition) are known. In addition, it has been reported that a lanthanum manganite sintered body having a composition in which the A site is partially deleted decreases in weight when the temperature is increased 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 starts to decrease around 800 ° C.

[0004]

In particular, site A has C
a, A porous sintered body made of lanthanum manganite doped with Sr is considered to be promising as a material for an air electrode including a self-supporting air electrode tube. However, the present inventors have discovered for the first time that such a porous sintered body has the following problems.

[0005] That is, the power generation temperature of the SOFC is 900 to
When a heating-cooling cycle is performed between the temperature of 1100 ° C. and the temperature of room temperature to 600 ° C., cracks occur between the air electrode tube made of the porous sintered body and other constituent materials of the unit cell. , And it was found that the cell was destroyed. Moreover, even if this cell is operated at 1000 ° C. for a long time,
Such cracks did not occur at all. Therefore, it is considered that this phenomenon is not due to the shrinkage of the porous sintered body during firing, but to the dimensional change due to the thermal cycle.

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

[0007]

The porous sintered body according to the present invention is made of lanthanum manganite having a perovskite structure, and at least 22% of the A site of lanthanum manganite.
35% or less is replaced by calcium, aluminum, cobalt, copper, magnesium, nickel,
One or more metal atoms selected from the group consisting of iron, titanium and zinc replace a part of the manganese atom 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.

Further, the porous sintered body according to the present invention is composed of lanthanum manganite having a perovskite structure, wherein 20% or more and 40% or less of the A site of lanthanum manganite are replaced by strontium, and aluminum, cobalt One or more metal atoms selected from the group consisting of copper, magnesium, nickel, iron, titanium and zinc, wherein a portion of the manganese atoms at the B site is replaced, and the dimensions resulting from thermal cycling between room temperature and 1000 ° C. The shrinkage is not more than 0.01% per heat cycle.

[0009]

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

As a result, it has been found that the dimensions of the porous sintered body shrink by about 0.01 to 0.04% per heat cycle. Moreover, shrinkage due to this heat cycle
It was found that it did not converge even after 100 heat cycles, and it reached several percent in 100 heat cycles. When the air electrode shrinks in this way, cracks occur between the unit cell and other constituent materials, causing the unit cell to break.

[0011] The present applicant has conducted research to solve this problem. As a result, according to the porous sintered body composed of the composite oxide having the above-described composition, the dimensional shrinkage caused by the thermal cycle between room temperature and 1000 ° C. is reduced by 0.01% or less per one thermal cycle. Found that it can be suppressed. Moreover, this allows 900 to 1100
° C and between room temperature and 600 ° C
It was confirmed that cracks did not occur between the porous sintered body and other constituent materials even after a cooling cycle.

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

In the porous sintered body of the present invention, aluminum, cobalt, copper, magnesium, nickel, iron,
One or more metal atoms selected from the group consisting of titanium and zinc replace a part of the manganese atom at the B site of lanthanum manganite.

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 the substitution ratio is less than 0.02%, the effect of suppressing shrinkage is not remarkable, and if it exceeds 20%, the electrical conductivity is reduced. When this substitution ratio is 5% or more, the effect of suppressing shrinkage due to thermal cycling becomes more remarkable.
When the content is 0% or less, the decrease in electric conductivity of the porous sintered body is small.

At the B site of the porous sintered body, two or more kinds of the above metal atoms can be substituted. In this case, it is preferable that the total of the substitution amounts of two or more types of substitution atoms be the above ratio.

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

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

Further, since lanthanum manganite can have a non-stoichiometric composition, fluctuations in the composition of lanthanum manganite due to some impurities that are inevitably mixed in the production process of the porous sintered body are allowed. You.

Further, the present inventor has studied on the mechanism of the dimensional shrinkage of the porous sintered body caused by the above-mentioned thermal 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 was 0.9988 or more, the dimensional shrinkage was 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., the weight of the porous sintered body is reduced, and this weight reduction has a clear correlation with the dimensional shrinkage due to the above-mentioned thermal cycle. Had.

The mechanism of the above phenomenon is not clear at present. 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 again to room temperature, the weight returns to the original value.

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

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 ° C. to 1000 ° C. is 0.01 eV or less, the difference between room temperature and 1000 ° C. It was found 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 900 ° C. and 1000 ° C. between the time of temperature rise and the time of temperature fall is 2% or less, the dimensional shrinkage due to the above thermal cycle is 1%. It was 0.01% or less per operation, and it was also confirmed that the amount significantly decreased.

The mechanism by which these phenomena occur is currently unknown. However, it is presumed that oxygen enters and exits 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 enters and exits, thereby promoting mass transfer of metal atoms. You. In addition, the amount of dimensional shrinkage of the porous sintered body accompanying the thermal cycle slightly varies depending on the crystal grain size constituting the sintered body, the temperature rise / fall rate during the thermal 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 rise / fall 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 preferably used particularly as a high-temperature electrode material that is stable against heat cycles. Such high-temperature electrode materials include fusion reactors, MHD
There are electrode materials for power generation and the like.

Further, the porous sintered body of the present invention is
As an air electrode material for use. Furthermore,
Preferably, it is used as a material for a self-supporting air electrode substrate. Such an air electrode substrate is used as a base material of a unit cell, and various components such as a solid electrolyte membrane, a fuel electrode film, an interconnector, and a separator are laminated on the air electrode substrate. At this time, the shape of the air electrode base may be a cylindrical shape with both ends opened, a bottomed cylindrical shape with one end open and the other end closed, a flat plate shape, or the like. Among them, any one of the above-mentioned cylindrical shapes is particularly preferable because thermal stress is hardly applied and gas sealing is easy.

The porosity of the porous sintered body is preferably 5 to 40%. When this is used as an air electrode material for an SOFC, the porosity is further preferably set to 15 to 40%, more preferably 25 to 35%. In this case, the gas diffusion resistance is reduced by setting the porosity of the air electrode to 15% or more, and a certain degree of strength can be secured by setting the porosity to 40% or less.

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

Japanese Unexamined Patent Publication No. 2005-560 discloses an electrode material having a composition of La ₁ -XA _X Mn _{1 -y By} O _{3 in} a lanthanum manganite-based perovskite-type composite oxide. . Where A is C
a and Sr, and B is nickel and chromium. 0 ≦
X ≦ 0.4 and 0 <y ≦ 0.25. By employing an electrode material having such a composition, the bulk conductivity (σ) and the interface 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 apparent 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 the coefficient of thermal expansion. (Table 1, page 3 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-mentioned composition, and the mixture is fired to synthesize lanthanum manganite. To produce lanthanum manganite powder. This powder is mixed with an organic binder and a pore-forming agent and molded, and the molded body is fired to obtain a porous sintered body.

[0032]

EXAMPLES (Experiment 1) (Production of experimental sample) La ₂ O ₃ ,
CaCO ₃ , Mn ₃ O ₄ , NiO, CuO, MgO, A
Each powder of l ₂ O ₃ and SrCO ₃ was used. Table 1, Table 2
For each example, a predetermined amount of the starting material was weighed and mixed so that the composition ratios shown in Table 1 were obtained. This mixed powder was subjected to a cold isostatic press method to obtain 1 tf / cm ²
To obtain a molded body. This compact was heat-treated at 1100 ° C. for 40 hours in the air to synthesize lanthanum manganite having the composition shown in Tables 1 and 2.

This composite was pulverized with 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 the lanthanum manganite powder, and a square plate was formed by a uniaxial pressing method. This molded body is placed in the atmosphere 125
It was fired at 0 ° C. for 5 hours to obtain a sintered body, 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 the sintered body to prepare an experimental sample.

(Measurement) First, the sample 1 of the embodiment shown in Table 1 was used.
Sample Nos. 10 to 25 and Comparative Example 25 were pulverized in a mortar and subjected to X-ray diffraction measurement by a powder method. As a result, each diffraction pattern of the samples 1 to 10 according to the example of the present invention is almost the same as the diffraction pattern of the sample 25 according to the comparative example, and shows a single phase.

According to these X-ray diffraction patterns, in samples 1 to 10, calcium, nickel, copper, magnesium, aluminum and strontium were surely dissolved in the lanthanum manganite crystals.

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

Next, the temperature of each sample was raised to 600 ° C. at a rate of 200 ° C./hour in the atmosphere, and thereafter, a heat cycle was performed 10 times at a rate of 200 ° C./hour between 600 ° C. and 1000 ° C. And cooled to room temperature. At this time, in each thermal cycle, a constant temperature was maintained for 30 minutes at 600 ° C. and 1000 ° C., respectively. Thereafter, the dimensions of each sample were measured using a micrometer, and the dimensional shrinkage before and after the heat cycle was calculated. Tables 1 and 2 show the measurement results.

The electrical conductivity of each sample at 1000 ° C. was evaluated. This is a characteristic required as an air electrode or the like of the SOFC. This measurement was carried out in air at 1000
C., at a direct current four-terminal method. 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 the porosity, the measurement data was corrected by calculation so as to be a value at the porosity of 30%.

Further, for some of the samples, a temperature of 25 ° C.
The average coefficient of thermal expansion at 1000 ° C was measured.

[0040]

[Table 1]

[0041]

[Table 2]

As can be seen from Table 2 above, Sample No. 25
In comparison with Sample Nos. 21 to 24, when the substitution amount of calcium is set to 20% or the substitution amount of strontium is set to 10%, when a part of the B site is replaced with nickel, aluminum, or the like, the porous material becomes porous. The dimensional shrinkage of the sintered body has decreased. However, compared to Sample No. 25, it was found that the electric conductivity of the porous sintered body was reduced, and that the electric conductivity tended to be further reduced as the substitution amount at the B site increased. This point is disadvantageous as an electrode material.

On the other hand, a comparison between Sample No. 1 and Sample No. 25 shows that there is not much difference in the electrical conductivity, while the dimensional shrinkage is small. Sample numbers 1 and 2
In comparison with No. 1, the electrical conductivity of the porous sintered body is improved in Sample No. 1 according to the example of the present invention.

Further, looking at Sample Nos. 2, 3, and 4, the dimensional shrinkage was reduced and the electrical conductivity was improved as the calcium replacement amount was increased. Comparing Sample Nos. 5 and 22, again by increasing the calcium replacement to 35% as in Sample No. 5,
It can be seen that the electric conductivity is improved to the same degree as that of the sample No. 25.

Comparing Sample Nos. 7 and 23, Sample No. 7 exhibited an effect of improving electrical conductivity in addition to reducing dimensional shrinkage. Comparing Sample Nos. 10 and 24, in Sample No. 10, in addition to the reduction in dimensional shrinkage,
The effect of improving electric conductivity is also exhibited.

Further, the present inventor raised and lowered the temperature of the sample 25 in Table 1 from room temperature to 1000 ° C.
The dimensional change of the porous sintered body was measured by a thermal dilatometer.
As a result, the dimensional shrinkage phenomenon starts at 900 ° C. when the temperature falls.
What happened in the temperature range of 800 ° C. was determined.
Therefore, it is presumed that absorption of oxygen atoms and migration of metal atoms occur 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.

Further, the sample 25 was heated at 1000.degree.
After holding for 0 hour and lowering the temperature to room temperature, the dimensional change before and after heating was measured. As a result, the shrinkage was 0.03%. On the other hand, as shown in Table 2, the dimensional shrinkage rate per heat cycle was 0.1 for 10 heat cycles after firing.
029%.

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

(Experiment 2) In the same manner as in Experiment 1, porous sintered bodies of the respective examples were manufactured so as to have the compositions and porosity 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 No. 41
Comparing the results with 42 and 43, it can be seen that the dimensional shrinkage is significantly reduced when the calcium replacement amount is increased. However, when the sample numbers 31 and 32 according to the present invention are compared with the above-mentioned sample number 41, the dimensional shrinkage is still more remarkably reduced even though the calcium substitution amount is the same. Also, comparing Sample Nos. 33 and 42, even if the amount of calcium replaced was the same, the dimensional shrinkage was significantly reduced by replacing part (5%) of the B site with cobalt. You can see that

Sample Nos. 36 and 37 also show that the present invention is particularly effective in reducing the dimensional shrinkage. Comparing Sample Nos. 44 and 45, it can be seen that when the amount of strontium substitution is increased, the dimensional shrinkage is significantly reduced. However, sample no.
Comparing Sample Nos. 4 and 35 with Sample No. 44 described above, the dimensional shrinkage is still more remarkably reduced even though the strontium substitution amount is the same.

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

[0055]

[Table 5]

(Experiment 4) Next, as described above, porous sintered bodies having the compositions shown in Table 6 were manufactured, and the thermal expansion coefficients were measured at each temperature. Table 6 shows the results.

[0057]

[Table 6]

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

In the La—Ca—Mn—Ni porous sintered body, the electric conductivity and the dimensional shrinkage of the porous sintered body were measured by changing the amount of nickel substitution. The result is shown in FIG. As a result, the replacement amount of the B site is 0.0
It is preferably at least 2% from the viewpoint of dimensional shrinkage, and more preferably at most 20% from the viewpoint of electric conductivity.

In the La-Ca-Mn-Co porous sintered body, the average amount of thermal expansion and the dimensional shrinkage of the porous sintered body were measured while changing the amount of substitution of calcium.
The result is shown in FIG. As a result, in order to reduce the dimensional shrinkage, the replacement amount of calcium is set to 22% or more, but in order to keep the average thermal expansion coefficient low, it is preferably set to 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 value. Even if the cycle is applied, it is possible to prevent cracks from being generated between the porous sintered body and other constituent materials.

[Brief description of the drawings]

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

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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/00-35/51 C04B 38/00-38/10

Claims (10)

(57) [Claims] 1. A porous sintered body composed of lanthanum manganite having a perovskite structure, wherein 22% or more and 35% or less of A sites of the lanthanum manganite are replaced by calcium, and aluminum, cobalt, copper One or more metal atoms selected from the group consisting of magnesium, nickel, iron, titanium and zinc, wherein a part of the manganese atoms at the B site of the lanthanum manganite is replaced, and a thermal cycle between room temperature and 1000 ° C. Dimensional shrinkage caused by heat cycle of 0.01
% Or less. 2. The porous sintered body according to claim 1, wherein a substitution ratio of a metal atom other than a manganese atom in the B site is 0.02% or more and 20% or less. 3. The porous sintering apparatus according to claim 1, wherein a change in a coefficient of thermal expansion between 900 ° C. and 1000 ° C. between a temperature rise and a temperature drop is 2% or less. Union. 4. A porosity of not less than 5% and not more than 40%,
The porous sintered body according to claim 1. 5. A solid oxide fuel cell, wherein an air electrode is formed by the porous sintered body according to claim 1. 6. A porous sintered body composed of lanthanum manganite having a perovskite structure, wherein 20% or more and 40% or less of A sites of the lanthanum manganite are replaced by strontium, and aluminum, cobalt, copper One or more metal atoms selected from the group consisting of magnesium, nickel, iron, titanium and zinc, wherein a part of the manganese atoms at the B site of the lanthanum manganite is replaced, and a thermal cycle between room temperature and 1000 ° C. Dimensional shrinkage caused by heat cycle of 0.0
A porous sintered body having a content of 1% or less. 7. The porous sintered body according to claim 6, wherein a substitution ratio of a metal atom other than a manganese atom in the B site is 0.02% or more and 20% or less. 8. The porous sintering method according to claim 6, wherein a change in a coefficient of thermal expansion between 900 ° C. and 1000 ° C. between a temperature rise and a temperature drop is 2% or less. Union. 9. The porosity is not less than 5% and not more than 40%,
The porous sintered body according to any one of claims 6 to 8. 10. A solid oxide fuel cell, wherein an air electrode is formed by the porous sintered body according to claim 6.