JPH05170444A - Stabilized zirconia-alumina powder for solid electrolyte fuel cell - Google Patents

Abstract

PURPOSE:To form a sintered material by sintering at relatively low temp. such as <=1400 deg.C in flat planer shape end concretely to form a cell structural material by a wet process such as the tape casting method so as to enable to make an electrolyte for the solid electrolyte fuel cell by the co-sintering method. CONSTITUTION:A high purity alumina of <=5wt.% is dispersed into a zirconia stabilized in crystalline structure with an yttria of 8-10mol%. Particle diameter ratio (y) of the stabilized zirconia powder to the alumina powder {(y)=(zirconia particle diameter)/(alumina particle diameter)} is (y)>0.63 and particle diameter of the zirconia powder is $ 0.3mum and particle diameter of the alumina powder is <=0.7mum. Particularly when adding amount of the high purity alumina is 1wt.%, a unique phenomenon is exhibited.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a stabilized zirconia-alumina powder for a solid electrolyte fuel cell. More specifically, the present invention provides a stabilized zirconia suitable for use in preparing an electrolyte for a solid oxide fuel cell, particularly by a co-sintering method.
Regarding alumina powder.

[0002]

2. Description of the Related Art Yttria-stabilized zirconia has oxygen ion conductivity and is stable in a high temperature and redox atmosphere. Therefore, a dense sintered body is used as an electrolyte for a solid electrolyte fuel cell or an oxygen sensor. However, zirconia (ZrO ₂ ) does not yield yttria (Y
Not only when ₂ O ₃ ) is used as a stabilizer, but generally the dense sintering temperature is high. For example, when the average particle of yttria-stabilized zirconia is about 0.2 to 0.3 μm and does not include impurities that inhibit sintering, such as chromium oxide, 1600
C., 5-10 hours are required.

On the other hand, in the case of a flat plate type solid electrolyte fuel cell,
It is conceivable to form an electrode by, for example, slurry coating or the like on a solid electrolyte obtained by densely sintering zirconia. By using such a method, a high-performance single cell of a solid oxide fuel cell can be produced without deteriorating the electrode characteristics. However, in the case of this slurry coating method, since the electrode material is made into a slurry and applied to the electrolyte, the electrode cannot be made thick. For this reason, when the unit cells are stacked via the separator, electricity flows through the thin electrodes during current collection, causing a large voltage drop due to the electrical resistance of the electrodes, and thus abnormally reducing the efficiency of the battery. It will be.

Therefore, a co-sintering method for producing a thick electrode single cell by producing a single cell formed of an electrolyte material and each electrode material by a single sintering process using a wet method such as a tape casting method. , In other words, fuel electrode / electrolyte / air electrode 3
A simultaneous sintering method was devised in which a unit cell composed of layers was sintered at one time. If this co-sintering method is used, it is considered possible to manufacture a high-performance solid electrolyte fuel cell having thick electrodes at low cost.

[0005]

However, when a unit cell is manufactured by the co-sintering method, the sintering temperature is high (1
(High temperature over 400 ° C) and loss of catalytic activity of the air electrode, it is necessary to make the contraction rate of the electrolyte and other electrode materials the same at each temperature, and the thermal expansion coefficients must match. There is a problem.

Therefore, alumina is used as a sintering aid to lower the sintering temperature of zirconia or to synthesize finer zirconia particles (for example, particles having a primary particle diameter of 200 to 500 angstroms) The dense sintering temperature is also lowered) to compensate for these drawbacks. Among these conditions, the one having the lowest lowest dense sintering temperature is, for example, stabilized zirconia (trade name: TZ-8Y: primary average particle diameter 25, manufactured by Tosoh Corporation).
0 angstrom) and then the dense sintering condition is 13
It is to hold at 50 to 1400 ° C. for 5 hours. However, the particles of this powder are too fine, and when an electrolyte is produced by a wet method, an unsintered green sheet is warped or it is difficult to uniformly form the green sheet. This impairs the yield rate of the solid oxide fuel cell product, and eventually lacks reliability, which is the most important item for putting the solid oxide fuel cell into practical use, and impedes commercialization.

Therefore, the present invention has a relatively low temperature of, for example, 14
An object of the present invention is to provide a stabilized zirconia-alumina powder which can be sintered at 00 ° C or lower and can be molded into a flat plate without warping. More specifically, the present invention is a stabilized zirconia-alumina suitable for an electrolyte of a solid electrolyte fuel cell which can be formed by a co-sintering method by molding a cell constituent material using a wet method such as a tape casting method. The purpose is to provide a powder.

[0008]

In order to achieve the above object, the zirconia-alumina powder of the present invention has a high zirconia content of 8 to 10 mol% of yttria whose crystal structure is stabilized at 5% by weight or less. Purity alumina is dispersed, and the particle size ratio y [y = (zirconia particle size / alumina particle size)] of the stabilized zirconia powder to the alumina powder is y>.
The powder is 0.63 and the size of the zirconia is 0.3 μm or less and the size of the alumina particles is 0.7 μm or less.

The powder of the present invention can be obtained by a method of mixing powders of yttria-stabilized zirconia and alumina, a coprecipitation method, a combustion synthesis method, or the like.

When yttria is less than 8 mol%, the conductivity (reciprocal of electric resistance) is low, and two different crystals, tetragonal and monoclinic, are contained in zirconia having a cubic crystal structure. The one with the structure is included,
This causes a volume change, which causes a problem that the electrolyte plate made of zirconia is destroyed. For example, in cubic zirconia, the amount of 1 cm ³ per mole is changed to tetragonal,
When the monoclinic zirconia changes to 1.5 cm ³ per mol, it collapses itself (the numerical values shown in the text are appropriate values that are easy to explain). Also,
When yttria is more than 10 mol%, the conductivity is low, the crystal structure of yttria becomes dominant, and the structure of zirconia (fluorite type structure) is lost, and finally the excellent properties of zirconia are lost. Resulting in.

Further, as shown in FIG. 4, the conductivity of yttria-stabilized zirconia in which the amount x of alumina added is within the range of 0 <x ≦ 5% by weight is almost the same as the conductivity when alumina is not added. Or, it is specifically improved depending on the amount added. This conductivity is a value at 1000 ° C. which is the operating temperature of the solid oxide fuel cell. However,
Since alumina is an insulator, addition of a large amount causes a decrease in conductivity. In this case, the conductivity of the zirconia-alumina composite electrolyte added in an amount of more than 5% by weight is lowered. This means that adding more than 5% by weight of alumina lowers the conductivity of the zirconia-alumina composite electrolyte, making it unsuitable for practical use.

On the other hand, the conductivity should originally be the highest when alumina is not added, but according to the measurement results, when alumina is not added (0% by weight), 1 to 1% of alumina is added.
It was found that the conductivity was lower than when 5% by weight was added. Although this is not clear in detail, by adding alumina, alumina may form a solid solution in zirconia or form a melt, and the relative density of the sintered body whose conductivity is measured is improved. Probably the cause. However, the sinterability of this zirconia-alumina composite electrolyte is
As shown in FIG. 1, the added amount of alumina (x: weight percent) helps to reduce the dense sintering temperature in the region of 0 <x ≦ 5, and the pure sintering amount of pure zirconia to which nothing is added is In comparison, the temperature can be lowered by 200 ° C. In particular, when 1 wt% was added, it was possible to sinter densely (94% or more) without holding time at about 1400 ° C., and the conductivity was also improved specifically. Alumina basically does not react with zirconia that is an electrolyte, lanthanum manganite that is an air electrode, and nickel zirconia cermet that is a fuel electrode, and does not deteriorate the performance of the solid electrolyte fuel cell.

Next, the particle size ratio y [y = (zirconia particle size / alumina particle size)] of the stabilized zirconia powder to the alumina powder is y> 0.63 and the size of the zirconia is 0. 0.3 μm or less, the size of alumina particles is 0.7 μm
The condition is that the powder is the following. This is because if the particles of zirconia have a particle size larger than this, the temperature at which a dense zirconia sintered body is formed does not fall below 1400 ° C., no matter how much alumina is added. Also,
When the particle size of alumina becomes larger than 0.7 μm, the amount of alumina added (x: weight percent) requires an amount of alumina added in the range of 0 <x ≦ 5 or more. Problems such as a decrease in the product quality and an increase in the defective product rate appear. This is considered to be because when the alumina particles are large, the surface area of the alumina per volume becomes small, the contact area with the zirconia particles also decreases, and the effect of adding alumina becomes small.

The method for producing the zirconia-alumina composite electrolyte used in this experiment is as follows: Distilled water or pure water is mixed without adding a dispersant and the like, followed by rapid drying to obtain zirconia particles and alumina particles. And were mixed uniformly. It has also been found that a simple method such as mixing with a regular bee increases the defective rate.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the present invention will be described in detail below with reference to the embodiments shown in the drawings.

In the zirconia-alumina powder of the present invention, zirconia whose crystal structure is stabilized with 8 to 10 mol% yttria and high-purity alumina are used as raw materials. Moreover, these zirconia powder and high-purity alumina powder are
The particle size ratio y (zirconia particle size / alumina particle size) is y>
A powder having a size of 0.63 and a size of the zirconia and alumina powder of 0.7 μm or less is prepared. The amount of alumina added to the zirconia powder (x:% by weight) is 0 <
The region was set to x ≦ 5. For example, the present powder is a uniform zirconia-alumina composite electrolyte prepared by mixing zirconia-alumina with a ball mill for 24 hours using pure water without using a dispersant or the like, and then rapidly drying it with a spray dryer. A powder was obtained.

Next, the influence of the added amount and particle size of alumina on the sinterability and conductivity will be described with reference to examples.

The yttria-stabilized zirconia used in the experiment before addition of alumina had a ZrO ₂ content of 85.62.
Wt%, a Y ₂ O ₃ is 14.2 wt%, which corresponds to nearly 8 mol% yttria -84% stabilized zirconia. However, at present, it is not possible to reduce the impurities to 0 by any method. Therefore, both the samples and impurities used in this experiment were 0.03% by weight for Al ₂ O _{3 and} 0.04% for NaO _2. %, MgO is 0.01% by weight, CaO is 0.08% by weight, Fe ₂ O ₃ is 0.01% by weight, and SiO ₂ is 0.01% by weight, which are very small. Among them, the conductivity is affected by F
There are two values, e ₂ O ₃ and SiO ₂ , but it can be seen that it is the smallest value and has a value that hardly affects the conductivity. The average particle size of this zirconia is 0.25 μm.

Example 1: 84.80% by weight of ZrO ₂ , Y
₂ O ₃ 14.0 wt%, Al ₂ O ₃ 1.01 wt%, N
aO ₂ 0.03% by weight, MgO 0.01% by weight, CaO
0.07 wt%, Fe ₂ O ₃ 0.01 wt%, SiO ₂
<0.01 wt% composition. This composition was obtained by adding 1% by weight alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. It should be noted that 8 mol% yttria-stabilized zirconia has SiO ₂ and Fe in order to make the electric resistance extremely high.
₂ O ₃ is ruthless and there are almost no impurities. The zirconia has an average particle size of 0.25 μm, and the alumina has an average particle size of 0.23 μm. The sinterability of a powder of this composition is represented in FIGS. 1 and 2 as a 1% Al ₂ O ₃ additive.

Example 2: 83.08% by weight of ZrO ₂ , Y
₂ O ₃ 13.8% by weight, Al ₂ O ₃ 3.01% by weight, N
aO ₂ 0.04% by weight, MgO 0.01% by weight, CaO
0.05% by weight, Fe ₂ O ₃ 0.01% by weight, SiO ₂
<0.01 wt% composition. This composition was obtained by adding 3 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. In this case, zirconia has an average particle size of 0.25 μm and alumina has an average particle size of 0.23 μm. The sinterability of the powder of this composition is shown in Fig. 2 as 3% Al ₂ O ₃
Represented as an additive.

Example 3: 81.29% by weight of ZrO ₂ , Y
₂ O ₃ 13.4% by weight, Al ₂ O ₃ 5.03% by weight, N
aO ₂ 0.03% by weight, MgO 0.01% by weight, CaO
0.05% by weight, Fe ₂ O ₃ 0.01% by weight, SiO ₂
<0.01 wt% composition. This composition was obtained by adding 5 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. In this case, zirconia has an average particle size of 0.25 μm and alumina has an average particle size of 0.23 μm. The sinterability of the powder of this composition is shown in Fig. 2 as 5% Al ₂ O ₃
Represented as an additive.

Comparative Example 1: ZrO ₂ 76.71% by weight, Y
₂ O ₃ 13.1% by weight, Al ₂ O ₃ 10.06% by weight,
0.03 wt% NaO ₂ , 0.01 wt% MgO, Ca
O0.07% by weight, Fe ₂ O ₃ 0.01% by weight, SiO
₂ <0.01 wt.% Composition. This composition was obtained by adding 20 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. The average particle size of alumina is 0.2.
It is 3 μm. The sinterability of the powder of this composition is 10 in FIG.
% Al ₂ O ₃ additive.

Comparative Example 2: 68.07% by weight of ZrO ₂ , Y
₂ O ₃ 11.8% by weight, Al ₂ O ₃ 20.23% by weight,
0.03 wt% NaO ₂ , 0.01 wt% MgO, Ca
0.05 wt% O, 0.01 wt% Fe ₂ O ₃ , SiO
₂ <0.01 wt.% Composition. This composition was obtained by adding 20% by weight alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. The average particle size of alumina is 0.2.
It is 3 μm. The sinterability of the powder having this composition is shown in FIG.
% Al ₂ O ₃ additive.

Comparative Example 3: 59.41% by weight of ZrO ₂ , Y
₂ O ₃ 9.9% by weight, Al ₂ O ₃ 30.57% by weight, N
aO ₂ 0.04% by weight, MgO 0.01% by weight, CaO
0.05% by weight, Fe ₂ O ₃ 0.01% by weight, SiO ₂
<0.01 wt% composition. This composition was obtained by adding 30% by weight alumina to yttria-stabilized zirconia, and was achieved using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. The average particle size of alumina is 0.23.
μm. The sinterability of powder of this composition is 30% in Fig. 1.
It is represented as an Al ₂ O ₃ additive.

Comparative Example 4: 68.07% by weight of ZrO ₂ , Y
₂ O ₃ 11.8% by weight, Al ₂ O ₃ 20.02% by weight,
0.03 wt% NaO ₂ , 0.01 wt% MgO, Ca
0.05 wt% O, 0.01 wt% Fe ₂ O ₃ , SiO
₂ <0.01 wt.% Composition. This composition was obtained by adding 20% by weight alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Sumitomo Chemical Co., Ltd. The average particle size of alumina is 0.3.
It is 9 μm. The sinterability of the powder having this composition is particularly good in Comparative Example 2
I didn't show it because it was the same as that of No. Even if the average particle size of alumina was increased from 0.23 μm to 0.39 μm, the sinterability did not change.

Comparative Example 5: 67.98% by weight of ZrO ₂ , Y
₂ O ₃ 11.6% by weight, Al ₂ O ₃ 20.28% by weight,
0.04 wt% NaO ₂ , 0.01 wt% MgO, Ca
O0.07% by weight, Fe ₂ O ₃ 0.01% by weight, SiO
₂ <0.01 wt.% Composition. This composition was obtained by adding 20% by weight alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Sumitomo Chemical Co., Ltd. The average particle size of alumina is 0.6.
8 μm. The sinterability of the powder having this composition is also particularly Comparative Example 2.
I didn't show it because it was the same as that of No. That is, even if the average particle size of alumina was increased from 0.23 μm to 0.68 μm, the sinterability did not change particularly. Therefore, the sinterability is not affected even if the average particle size of alumina is increased to form it thick by a wet method such as a tape casting method and then fired. Moreover, it can be formed into a flat plate without warpage.

The powders of these examples and comparative examples were evaluated by the method shown in FIG. First, in order to produce zirconia-alumina powder as uniformly as possible, pure water such as distilled water was added to these powders, wet-mixed for 24 hours using a ball mill, and rapidly dried with a spray dryer to obtain evaluation powders. did. Then, the powder of each example was press-molded at a pressure of 92 MPa. Next, an evaluation sample was obtained by firing in the temperature range of 1000 to 1600 ° C. for 0.5, 10 hours. Then, the size of the sample was measured to determine the relative density.

The change in conductivity of each evaluation sample with respect to the amount of alumina added was measured by a measuring device using an AC impedance method as shown in FIG. In this device, an alternating current is made to flow from the potentiostat 1 and the frequency response analyzer 2 to the sample 5 via the electrode 4, and the change of the alternating current is measured by the digital oscilloscope 3 to obtain the conductivity. The electrode 4 was coated with platinum paste and baked at 1000 ° C. for 1 hour. The value measured by this device was analyzed by Cole-Cole plot to analyze the impedance of the sample and obtain the electric resistance. The result is shown in FIG.

In FIG. 2, 1100 ° C. to 1600 ° C. in the respective regions where the amount of alumina added is 0, 1, 3, 5% by weight.
Shows relative density without holding time at the temperature of. It can be seen that the dense sintering temperature is lowered by 150 ° C. or more in the region of 1 to 5 wt% as compared with the case where the added amount of alumina is 0 wt%.
This result also shows that addition of a fixed amount of alumina lowers the dense sintering temperature of zirconia.

In FIG. 1, 0, 1, 5, 10, 2 alumina was added.
1100 ° C to 1600 ° C when 0,30% by weight is added
The respective relative densities of each of the above-mentioned samples without the holding time are shown. From this, it can be seen that the dense sintering temperature is higher in the region of 10% by weight or more than in the case of not adding alumina (8YZS), as compared with the case of adding 1 to 5% by weight of alumina. That is, addition of a large amount of alumina does not necessarily lead to a decrease in the dense sintering temperature, but only in a specific range (0 <x ≦ 5% by weight: x is the amount of alumina added), a decrease in the dense sintering temperature occurs. Understand that you can contribute.

FIG. 3 shows the relationship between each firing temperature and the holding time. 1400 ° C for zirconia without alumina
In the case of firing at 10, it was finally densely sintered by holding for 10 hours. On the other hand, when adding 20% by weight of alumina,
When fired at 0 ° C., a dense sintered body of 94% or more was not formed even if held for 10 hours, and finally a dense sintering was performed at 1500 ° C. for a holding time of 10 hours.

FIG. 4 shows the relationship between the amount of alumina added and the conductivity of zirconia to which the alumina was added. The conductivity of the zirconia-alumina composite electrolyte containing less than 5% by weight of alumina is almost the same as that of the one without addition, or on the contrary improved, but if more than 5% by weight of alumina is added, it will be compared to the case without addition. It can be seen that the electric resistance is also high. From this, it can be understood that the addition of alumina did not cause a problem with respect to the electric conductivity if it was 5% by weight or less, and on the contrary had the effect of improving it.

The above embodiment is one example of the preferred embodiment of the present invention, but the present invention is not limited to this, and various modifications can be made without departing from the gist of the present invention. For example, in the present embodiment, the powder was produced by the wet mixing method using pure water, but it can be produced by the coprecipitation method or the combustion synthesis method. Further, the mixing time, the mixing method, the mixing apparatus, and the drying method at the time of producing the powder are not particularly limited.

[0034]

As is apparent from the above description, the stabilized zirconia-alumina powder of the present invention has a zirconia particle or alumina particle size of 1400 even if the diameter thereof is relatively larger than that of the conventional one.
It can be sintered at a low temperature around ℃. Moreover, even if the particle size of each particle is made larger than that of the conventional product, the sinterability is not affected. Therefore, even if a wet method such as a tape casting method is used, the particle does not warp or crack during the firing process. Therefore, if the powder of the present invention is used as a raw material for a zirconia-alumina composite electrolyte, a solid electrolyte fuel cell having an electrode having a thick film thickness can be produced by a co-sintering method, and a high-performance one can be produced. You can

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the firing temperature and the relative density of a powder of the present invention and a comparative product.

FIG. 2 is a graph showing the relationship between the firing temperature and the relative density of the powder of the present invention and yttria-stabilized zirconia without addition of alumina.

FIG. 3 is a graph showing a relationship between holding time and relative density for a comparative example.

FIG. 4 is a graph showing the effect of the amount of alumina added to yttria-stabilized zirconia on the conductivity.

FIG. 5 is a process diagram showing a method for producing a zirconia-alumina composite material and a method for producing an evaluation sample thereof.

FIG. 6 is a schematic view of a conductivity measuring device.

Claims (2)

[Claims] 1. Grains of the stabilized zirconia powder and the alumina powder, in which 5 wt% or less of high-purity alumina is dispersed in zirconia whose crystal structure is stabilized by 8 to 10 mol% of yttria. Diameter ratio y [y = (zirconia particle size /
Stabilized zirconia for solid electrolyte fuel cells, characterized in that the alumina particle size)] is y> 0.63, the size of the stabilized zirconia particles is 0.3 μm or less, and the alumina particles are 0.7 μm or less. -Alumina powder. 2. The stabilized zirconia-alumina powder for a solid electrolyte fuel cell according to claim 1, wherein the amount of high-purity alumina added is 1% by weight.