JP2846559B2 - 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]

SUMMARY OF THE INVENTION The present invention relates to a porous sintered body made of lanthanum manganite, which is made of one or more metal atoms selected from the group consisting of aluminum, cobalt, magnesium and nickel. A manganese atom at the B site is partially substituted, and a dimensional shrinkage caused by a thermal cycle between room temperature and 1000 ° C. is 0.01% or less per thermal cycle. It is related to.

[0008]

The present inventor has proposed that a porous sintered body composed of lanthanum manganite in which the A site is doped with Ca, Sr, etc. between a temperature of 900 to 1100 ° C. and a temperature of room temperature to 600 ° C. A heating-cooling cycle was applied to test its stability. As a result, it was found that the porous sintered body contracted by about 0.01 to 0.1% per thermal cycle. Moreover, the shrinkage due to this thermal cycle is 100
It was found that the thermal convergence did not converge even after several thermal cycles, and reached several percent in 100 thermal 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.

This mechanism 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 present inventor has further studied based on the above findings, and as a result, it has been found that a part of the B site of lanthanum manganite constituting the porous sintered body is replaced with aluminum, cobalt, magnesium or nickel. It has been found that the dimensional shrinkage due to the heat cycle is remarkably suppressed. This mechanism is not clear. Further, the reason why only the above-mentioned specific metal atom is effective is not clear.

[0011] 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 tenth heat cycle is indicated.

It is preferable that the substitution ratio of the B site of lanthanum manganite by one or more metal atoms selected from the group consisting of aluminum, cobalt, magnesium and nickel is 0.02% 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 coefficient of thermal expansion increases, causing a mismatch between the coefficient of thermal expansion of the zirconia solid electrolyte and the electric conductivity. Is significantly reduced.

[0013] In the lanthanum manganite constituting the porous sintered body of the present invention, the A site can be doped with an alkaline earth metal or a rare earth element. This element includes scandium, yttrium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium,
Holmium, erbium, thulium, ytterbium,
Lutetium, calcium, strontium, barium,
Choose from Radium.

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 manufacturing process of the porous sintered body are allowed. Shall be.

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 a SOFC
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 this solid electrolyte membrane is formed of yttria-stabilized zirconia, the substitution amount of calcium at the A site of lanthanum manganite is preferably 10 to 30%, and the substitution amount of strontium is 5 to 20%. It is preferable to match the thermal expansion with the solid electrolyte membrane.

JP-A-1200560 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 a 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. (Page 3, Table 1).

[0021]

EXAMPLES (Production of Experimental Samples) La ₂ O ₃ , Mn ₃ O ₄ , CaC
Powders of O ₃ , Al ₂ O ₃ , CoO, MgO, and NiO were used. For each example, a predetermined amount of starting material was weighed and mixed so that the composition ratios shown in Tables 1, 2 and 3 were obtained. This mixed powder was molded at a pressure of 1 tf / cm ² by a cold isostatic press method to produce a molded body. This compact was heat-treated in the air at 1100 ° C. for 40 hours to synthesize lanthanum manganite having the composition shown in Tables 1 to 3.

This composite was pulverized in a ball mill to produce 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 porosity of each sample was measured by a water displacement method. The results are shown in Tables 1 to 3. Further, Samples 1 to 5 shown in Table 1 were crushed again in a mortar and subjected to X-ray diffraction measurement by a powder method. FIG. 1 shows the measurement results. The numerical values attached to each curve shown in FIG. 1 indicate the sample numbers in Table 1. As can be seen from FIG. 1, the diffraction patterns of Samples 2, 3, 4, and 5 according to the example of the present invention are almost the same as the diffraction pattern of Sample 1 according to the comparative example, and show a single phase. I have. From the X-ray diffraction pattern, it can be seen that Samples 2 to 5
In the above, nickel, cobalt, magnesium, and aluminum are surely dissolved in the lanthanum manganite crystal.

Next, each sample was heated to 600 ° C. at a rate of 200 ° C./hour in the air, and then subjected to a thermal cycle between 600 ° C. and 1000 ° C. at a rate of 200 ° C./hour for 10 times. 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, 2 and 3 show the measurement results.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

[Table 3]

In Table 1, in all of the samples 1 to 5, calcium replaces 20% of the lanthanum atom at the A site. In samples 2 to 5, 5% of the manganese atoms at the B site were replaced with nickel, cobalt, magnesium, or aluminum. In Samples 2, 3, 4, and 5 according to the examples of the present invention, the average value of the dimensional shrinkage in the initial 10 times after firing is 0.01% or less per time. This value is 0.04 for sample 1.
Reaches 3%.

The present inventor measured the dimensional change of the sample 1 in Table 1 by increasing and decreasing the temperature from room temperature to 1000 ° C. using a thermal dilatometer. As a result, it was found that the dimensional shrinkage phenomenon occurred in a temperature range of 900 ° C. to 800 ° C. when the temperature was lowered. Therefore, it is presumed that absorption of oxygen atoms and migration of metal atoms occur in this temperature range. In addition, 600, which is the condition of this experiment,
The results from thermal cycling between ° C and 1000 ° C are the same as those from thermal cycling between room temperature and 1000 ° C.

The sample 1 was placed in the atmosphere at 1000 ° C. for 10 minutes.
After the temperature was maintained for a period of time and the temperature was lowered to room temperature, the dimensional change before and after the heating was measured. On the other hand, as shown in Table 1, the dimensional shrinkage rate per heat cycle was 0.0 for 10 heat cycles after firing.
43%. Therefore, shrinkage of 0.03% substantially corresponds to the amount of dimensional shrinkage for one heat cycle. From this result, the above-mentioned dimensional shrinkage of 0.03% is 1 degree at 1000 ° C.
1000 ° C, not generated during holding for 0 hours
This occurs during the cooling process in which the temperature has dropped from room temperature 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.

Table 2 shows the case where nickel, cobalt, magnesium or aluminum accounts for 0.01% of the manganese site. In Table 3, each of these metal atoms accounts for 10% of the manganese site,
The case where 5% or 20% is occupied is shown. In the graph of FIG. 2, the results are shown in Tables 1 to 3, where the horizontal axis represents the substitution amount (%) of the B site of each metal atom, and the vertical axis represents the shrinkage amount (%) per heat cycle. Was plotted.

As can be seen from Tables 1, 2 and 3, and FIG. 2, although there is a slight difference depending on the type of the substitution element,
The heat cycle shrinkage is suppressed by increasing the substitution amount. This substitution amount is particularly preferably 5 to 20%.

Further, among the samples of the respective examples shown in Tables 1 to 3, particularly the samples shown in Table 4 were selected, and the electric conductivity at 1000 ° C. was evaluated. This is a characteristic required as an SOFC air electrode. This measurement is performed in air at 10
The test was performed at 00 ° C. by a DC four-terminal method. Table 4 shows the results.
Shown in

However, in Table 4, 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 have a value at the porosity of 30%.

[0035]

[Table 4]

As can be seen from Table 4, when the B-site substitution amount increases, the electric conductivity tends to decrease. In particular, when the substitution amount exceeds 15%, in this example, the electric conductivity is lower than in the case of non-substitution. The conductivity becomes about half. This tendency is the same when the substitution atom is nickel or aluminum.

The reason for this is that the electrical conduction mechanism of lanthanum manganite is hopping conduction of holes caused by the valence of manganese being trivalent and tetravalent.
It is considered that the amount of manganese in the crystal serving as a conduction path was reduced. In any case, the B-site substitution amount of each atom is preferably 15% or less, more preferably 10% or less. Further, in consideration of the problem of the thermal cycle shrinkage, the substitution amount of each atom is more preferably 5 to 15%, and further preferably 5 to 10%.

The relationship between the substitution amount X and the coefficient of thermal expansion was investigated for the composition of La _{1 -X} AX Mn _0.95 Ni _0.05 O ₃ . For reference, the thermal expansion coefficient of 8 mol% yttria-stabilized zirconia is shown. As starting materials, La ₂ O ₃ , Mn ₃ O
₄ , powders of CaCO ₃ , SrCO ₃ and NiO were used. For each example, the composition ratio shown in Table 5 was obtained.
Predetermined amounts of starting materials were weighed and mixed. This mixed powder was subjected to 1 tf
/ Cm ^{2 at} a pressure of 1100 cm ²
Heat treatment was performed at 40 ° C. for 40 hours to synthesize lanthanum manganite having the composition shown in Table 5.

This composite was pulverized with a ball mill to prepare a lanthanum manganite powder having an average particle size of about 4 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 fired in the air at 1200 to 1500 ° C. for 5 hours, and has a porosity of 3
A 0 ± 1% sintered body was obtained, and a square bar 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 obtain an experimental sample.

[0040]

Table 5 Composition Average coefficient of thermal expansion (× 10 ⁻⁶ / K, 20 ° C. to 1000 ° C.) LaMn _0.95 Ni _0.05 O ₃ 12.3 La _0.95 Ca _0.05 Mn _0.95 Ni _0.05 O ₃ 12.1 La _0.90 Ca _0.10 Mn _0.95 Ni _0.05 O ₃ 11.3 La _0.85 Ca _0.15 Mn _0.95 Ni _0.05 O ₃ 11.0 La _0.8 Ca _0.2 Mn _0.95 Ni _0.05 O ₃ 10.8 La _0.75 Ca _0.25 Mn _0.95 Ni _0.05 O ₃ 11.1 La _0.725 Ca _0.275 Mn _0.95 Ni _0.05 O ₃ 11.2 La _0.7 Ca _0.3 Mn _0.95 Ni _0.05 O ₃ 11.2 La _0.675 Ca _0.325 Mn _0.95 Ni _0.05 O ₃ 11.4 La _0.65 Ca _0.35 Mn _0.95 Ni _0.05 O ₃ 11.5 La _0.95 Sr _0.05 Mn _0.95 Ni _0.05 O ₃ 11.2 La _0.90 Sr _0.10 Mn _0.95 Ni _0.05 O ₃ 11.0 La _0.8 Sr _0.2 Mn _0.95 Ni _0.05 O ₃ 11.1 La _0.7 Sr _0.3 Mn _0.95 Ni _0.05 O ₃ 11. 8 8 mol% yttria stabilized zirconia 10.5

The average thermal expansion coefficient is minimized when the substitution amount X of Ca is about 0.2 and the substitution amount X of Sr is about 0.1.
When it was minimal. In order to match the coefficient of thermal expansion with yttria-stabilized zirconia, which is a solid electrolyte, and to obtain a stable cell over a long period of time, the substitution amount of Ca should be 0.1 ≦ X
≦ 0.3, preferably 0.15 ≦ X ≦ 0.2
More preferably, it is set to 5. For the same reason, the substitution amount X of Sr is preferably set to 0.05 ≦ X ≦ 0.20,
More preferably, 0.10 ≦ X ≦ 0.20.

[0042]

As described above, according to the present invention, part of the manganese atoms at the B site of lanthanum manganite constituting the porous sintered body is converted from the group consisting of aluminum, cobalt, magnesium and nickel. By substituting one or more selected metal atoms, the dimensional shrinkage of the porous sintered body due to the above thermal cycle can be suppressed.

[Brief description of the drawings]

FIG. 1 is a graph showing an X-ray diffraction pattern of lanthanum manganite constituting a porous sintered body of each example.

FIG. 2 shows the substitution amount of B site of lantern manganite,
It is a graph which shows the relationship with the shrinkage amount per one thermal cycle of a porous sintered body.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) C04B 38/00-38/10 C04B 35/00-35/22 C04B 35/42-35/51

Claims (7)

(57) [Claims] 1. A porous sintered body made of lanthanum manganite, wherein one or more metal atoms selected from the group consisting of aluminum, cobalt, magnesium and nickel are used to form one of the manganese atoms at the B site of lanthanum manganite. A porous sintered body, wherein a part is replaced, and a dimensional shrinkage caused by a thermal cycle between room temperature and 1000 ° C. is 0.01% or less per thermal cycle. 2. The substitution rate of the B site by one or more metal atoms selected from the group consisting of aluminum, cobalt, magnesium and nickel is 0.02% or more and 20% or more.
The porous sintered body according to claim 1, wherein: 3. The porous sintered body according to claim 1, wherein part of the lanthanum atom at the A site of lanthanum manganite is substituted by one or more metal atoms selected from the group consisting of calcium and strontium. 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. The solid oxide fuel cell according to claim 5, wherein said air electrode also functions as a support. 7. The solid oxide fuel cell according to claim 6, wherein the air electrode is cylindrical.