JPH07149521A - Zirconia electrolyte powder - Google Patents

Abstract

PURPOSE:To control the sintering behavior in the preparation of a ceramic substance by sintering a zirconia electrolyte powder doped with a dopant for improving the electrical conductivity. CONSTITUTION:The amounts of the compounds or mixtures of SiO2 (xwt.%), Al2O3 (ywt.%) and Na2O (zwt.%) in a zirconia electrolyte powder doped with a conductivity-improving dopant are adjusted to 0<x<=1, 0<y<=5 and 0<z<=2, respectively. A single cell 4 having high reliability can be produced taking advantage of the low-temperature sinterability of the powder by placing the electrolyte 1 produced from the zirconia electrolyte powder between an air electrode 3 and a fuel electrode 2 and co-sintering the laminate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composition for improving the sintering behavior of zirconia electrolyte powder to which a dopant for improving conductivity such as yttria is added. More specifically,
The present invention particularly relates to a zirconia electrolyte powder suitable for producing a zirconia electrolyte membrane for a solid oxide fuel cell by a co-sintering method.

[0002]

2. Description of the Prior Art Zirconia electrolyte powder, for example 8-10.
Zirconia (ZrO ₂ ) whose crystal structure is stabilized or partially stabilized by adding mol% of yttria (Y ₂ O ₃ ).
Has almost no electronic conductivity even at high temperature, has high oxygen ion (O ²⁻ ) conductivity, and is extremely chemically stable even in an oxidizing / reducing atmosphere at high temperature. Therefore, the dense sintered body is used as an electrolyte of a solid electrolyte fuel cell or an oxygen sensor.

Not only when such zirconia is used as an electrolyte in a solid electrolyte fuel cell, but generally, the lower the temperature at which ceramics are densely sintered, the less energy is required at the time of production, which is suitable for mass production. There is. Therefore, zirconia powder that is sintered at a low temperature (about 1300 ° C.) has already been developed and put on the market.

Generally, ceramic powder such as zirconia shows various relative densities depending on temperature and other firing conditions. If this sintering behavior can be controlled (for example, the conditions such as the temperature required for dense sintering can be reduced), zirconia can be effectively applied to a wider range of fields.

For example, when zirconia is used as the electrolyte of a flat plate type solid electrolyte fuel cell as shown in FIG. 1, a single cell is generally produced by the process as shown in FIG. First, an organic solvent, an additive and the like are added to zirconia powder to form a slurry. Then, using a doctor blade (a kind of tape casting method) method, 30
It is necessary to process into a thin flat plate (green sheet) of 0 μm or less. The plate processing is performed in this manner because the conductivity of zirconia is not high enough to be used as an electrolyte of a solid oxide fuel cell. After that, the green sheet is slowly heat-treated to be degreased and fired at a high temperature to obtain a flat and dense electrolyte 1. And
First, this electrolyte 1
The fuel electrode 2 is formed on the plate of No. 1 and baked under the baking conditions of 1400 ° C. and about 2 hours. After that, the air electrode 3 is again formed into a film on the side opposite to the fuel electrode 2 by slurry coating or the like, and the single cell 4 is obtained by firing at 1200 ° C. for about 4 hours. The performance of the single cell 4 thus obtained is extremely excellent because the deterioration of the electrode characteristics can be minimized. However, in this manufacturing method, the unbaked electrodes 2 and 3 are thermally contracted during the baking process of the electrodes 2 and 3, and the baked electrolyte 1 is thermally expanded. Thermal stress is generated in the plate No. 1 and the electrolyte 1 is warped or cracked. In order to reduce the warpage and cracks generated in the electrolyte 1 as much as possible, it is necessary to form the electrodes 2 and 3 extremely thinly. When the thickness of the electrodes 2 and 3 is about 4 μm or less, It is possible to manufacture the single cell 4 with less warpage. The separator 5 is made of the same material as the fuel electrode 3. The separator 6 is made of the same material as the air electrode 2.

[0006]

However, the unit cell 4 having such thin electrodes 2 and 3 has an extremely large electric resistance until the generated electricity reaches the collector electrode (that is, the generated electrons are Since the current is collected through the thin electrodes 2 and 3, the specific resistance of the current collecting path is high), so it is calculated that power is being generated only near the collector electrode, and stacks were manufactured by stacking to increase the voltage. In this case, as shown in FIG. 3, the stack does not show the same power generation performance as the single cell. Therefore, the method of manufacturing a high-performance single cell having a thick electrode is extremely important for obtaining a high-performance stack.

Therefore, as shown in FIG. 4, tapes of an electrolyte material, an air electrode material, and a fuel electrode material formed by a wet method such as a doctor blade method are adhered in a green (unfired) state. A co-sintering method in which a single cell is produced from this tape by a single firing step, in other words, a multi-tape consisting of a fuel electrode material / electrolyte material / air electrode material is fired at once and manufactured simultaneously. A sintering method has been devised. If this co-sintering method is used, the three layers shrink at the same time during the firing process, so that thermal stress does not occur and a high-performance solid electrolyte fuel cell single cell having a thick electrode can be produced. It is thought that it can be done.

However, when a single cell of a solid oxide fuel cell is manufactured by this co-sintering method, the catalytic activity (O) of lanthanum manganite, which is an air electrode, is increased by firing at a high manufacturing temperature (1400 ° C. or higher for a long time). _The ability to turn ₂ into 2O ^2- ) is lost, or a low conductivity of 10 ^-3 S / cm or less is provided at the interface between the air electrode of lanthanum manganite ((La, Sr) MnO ₃ ) and the zirconia electrolyte. A step of firing three layers of an electrolyte, an air electrode, and a fuel electrode, in which a layer of lanthanum zirconate (La ₂ Zr ₂ O ₇ ) and a layer of strontium zirconate (SrZrO ₃ ) are formed, which greatly deteriorates the power generation characteristics of the single cell. It is necessary to solve many problems such as the need to make the thermal contraction rate the same at each temperature and the thermal expansion coefficient to be the same.

Therefore, (1) by using a secondary component such as alumina (Al ₂ O ₃ ) or silica (SiO ₂ ) as a sintering aid, the temperature at which zirconia is densely sintered is lowered.
(2) Finer zirconia particles (for example, a primary particle diameter of 200 to 500 angstroms (0.02 to 0.05
μm) particles) to lower the dense sintering temperature (fine particles have a lower dense sintering temperature),
(3) Attempts have been made to solve the problems such as removing impurities that affect the sinterability and achieving high purity.

Under these circumstances, for example, as the one corresponding to (2), the zirconia powder having the lowest temperature for dense sintering is, for example, yttria-stabilized zirconia (trade name, manufactured by Tosoh Corporation). : TZ-3Y, TZ-8
Y, average primary particle size 250 Å (0.0
25 μm)), and the temperature is maintained at 1300 ° C. to 1400 ° C. for about 5 hours for dense sintering. However, since this powder has too small particles, the filling rate when forming a green sheet formed by the doctor blade method is low, and the difference between the upper and lower heat shrinkages in the green sheet in the drying step and the firing step Due to a slight temperature difference, the green sheet that has not been fired may warp or the ceramic sheet that has been fired may warp. by this,
For example, it causes a decrease in product yield of a solid electrolyte fuel cell and a decrease in power generation performance, and eventually lacks reliability, which is the largest factor in putting a solid electrolyte fuel cell into practical use, which is a drawback that significantly impedes practical use.

It is an object of the present invention to provide a zirconia electrolyte powder having a composition capable of improving the sintering behavior when sintering a zirconia electrolyte powder to which a dopant for improving conductivity is added to obtain a ceramic. .

[0012]

In order to achieve the above object, the zirconia electrolyte powder of the present invention comprises SiO in the zirconia electrolyte powder to which a dopant for improving conductivity is added.
_When the contents of the compound or mixture of ₂ , ₂ , Al ₂ O ₃ and Na ₂ O are x, y and z respectively, SiO ₂ 0 <x ≦ 1 wt% Al ₂ O ₃ 0 <y ≦ 5 wt% Na ₂ O It is adjusted within the range of 0 <z ≦ 2% by weight.

Here, as the dopant for improving the conductivity of zirconia (ZrO ₂ ), stabilizers that stabilize or partially stabilize the zirconia crystal structure, such as yttria (Y ₂ O ₃ ) and calcia (CaO), are used. Can be mentioned.

Further, the content of SiO ₂ —Al ₂ O ₃ —Na ₂ O is adjusted because these components are present at the grain boundaries (around the grains) of zirconia and have a great influence on the sinterability of zirconia. Is given. That is, the SiO ₂ —Al ₂ O ₃ —Na ₂ O layer existing at the zirconia grain boundary hinders the solid phase diffusion between the zirconia particles and raises the temperature required for dense sintering. However, Al ₂ O _{3 is} contained in the zirconia powder.
If added, it reacts with and absorbs SiO ₂ —Al ₂ O ₃ —Na ₂ O, promotes solid phase diffusion between zirconia particles, and lowers the temperature required for dense sintering.

SiO ₂ --Al ₂ O ₃ --Na ₂ O may be either originally contained in the zirconia electrolyte powder or added thereto, and may be contained in any form of compound or mixture. Good. Usually SiO ₂ −
Al ₂ O ₃ —Na ₂ O is contained as an impurity of zirconia and is removed by repeatedly refining it, but SiO ₂ (0 <x ≦ 1 wt%), Na ₂
The desired sintering behavior is obtained when O is excluded to the range of (0 <z ≦ 2% by weight). When added, high-purity alumina powder is dispersed in the zirconia electrolyte powder by a mechanical method or a chemical method.

If the addition amount of Al ₂ O ₃ also exceeds 5% by weight, the thermal expansion coefficient of alumina (Al ₂ O ₃ ) particles and zirconia particles are slightly different from each other, so that thermal stress acts during firing.
Microcracks are generated in the fired body to prevent sintering.
Further, Al ₂ O ₃ does not deteriorate the conductivity as long as 0 <y ≦ 5% by weight. Rather, if Al ₂ O ₃ is 1% by weight, the conductivity will be improved.

In addition, MgO, CaO, Fe ₂ O _3, etc. may be present as impurities in the zirconia.
These can be ignored because they do not form a solid solution in the zirconia particles and are not present at the grain boundaries, or they are present at the grain boundaries in an amount that does not affect the sinterability.

The present invention can be applied to various powder production methods such as a coprecipitation method for synthesizing stabilized zirconia containing various dopants and a hydrothermal synthesis method.

[0019]

[Function] SiO ₂ -Al formed between zirconia particles
_{The 2} O ₃ —Na ₂ O layer prevents solid phase diffusion between the zirconia particles and increases the temperature required for dense sintering. Since the added alumina (Al ₂ O ₃ ) particles have very good reactivity with SiO ₂ and Na ₂ O, this SiO ₂ —Al ₂ O ₃ —
It reacts with the Na ₂ O layer, absorbs it, promotes solid phase diffusion between zirconia particles, and lowers the temperature required for dense sintering. Al ₂ O ₃ is in the range of 0 <y ≦ 5% by weight,
It does not deteriorate the conductivity.

[0020] SiO ₂ in this way -Al ₂ O ₃ -Na ₂
O increases the temperature required for dense sintering of zirconia, while addition of Al ₂ O ₃ lowers the temperature required for dense sintering of zirconia. Therefore, SiO ₂ , Al ₂ O ₃ , Na ₂
If the contents of O are x, y, and z, respectively, (0 <x ≦
The sintering behavior can be controlled by adjusting the content within the range of 1% by weight), (0 <y ≦ 5% by weight), and (0 <z ≦ 2% by weight).

Next, the present invention will be explained by taking an 8 mol% yttria-stabilized zirconia powder as an example.

Alumina (Al ₂ O ₃ ) is added to zirconia to which a dopant for improving conductivity is added, and the addition amount x is 0 <x ≦ 5.
The conductivity of those added in the range of wt% is almost equal to the conductivity in the case where alumina is not added as shown in FIG. 5, or specifically improves depending on the addition amount. still,
The conductivity in FIG. 5 is the value when alumina is added to 8 mol% yttria-stabilized zirconia, and is the value at 1000 ° C. which is the operating temperature of the solid electrolyte fuel cell.

However, since alumina is an insulating material that does not conduct electricity, if it is added in a too large amount, it will impede the oxygen ion conductivity in zirconia and reduce the intrinsic conductivity of zirconia. For example, 8 mol%
In the case of yttria-stabilized zirconia, the conductivity of zirconia added with 10% by weight of alumina sharply decreases. This means that when more than 5% by weight of alumina is added, the conductivity of zirconia becomes low and it becomes unsuitable as an electrolyte of a solid oxide fuel cell.

On the other hand, the conductivity of zirconia should be the highest when alumina is not added at all, but according to the measurement results, the conductivity when alumina is not added (0% by weight alumina) is 1% by weight of alumina. It was found that the conductivity was lower than when added.

Further, as shown in FIG. 6, in the composition region of 0 <x ≦ 5, the sinterability of this zirconia containing a small amount of alumina shows that the amount of alumina added (x =% by weight) is finely sintered. It helps to lower the sintering temperature and can be up to 200 ° C. lower than the dense sintering temperature of pure zirconia without added alumina. Especially 1% by weight
In the case of the added zirconia, it can be densely sintered (relative density 94%) under a firing condition without holding time at a low temperature of about 1400 ° C. Under such a low-temperature firing condition that enables dense sintering, the lanthanum manganite ((La, Sr) MnO ₃ ) of the air electrode material does not react with the zirconia electrolyte, and the catalytic activity of the air electrode itself is low. Does not fall. This means that it is possible to manufacture a high performance flat plate solid oxide fuel cell by using the co-sintering method.

Next, it will be clarified why such a peculiar phenomenon appears when alumina is added to zirconia.

Table 1 shows the results of the composition analysis of the 8 mol% yttria-stabilized zirconia (8YSZ) powder manufactured by Toray and the alumina powder manufactured by Daimei Kagaku, which were used in this experiment.

[Table 1]

Further, FIG. 7 shows changes in relative densities of the Toray 8YSZ simple substance and the Daimei Chemical alumina simple substance at each firing temperature. Sintering of alumina started around 1000 ° C. and its relative density reached 1400 ° C. and 95% without holding time. On the other hand, 8YSZ progresses above 1200 ° C,
It did not sinter densely even under conditions of 00 ° C. and no holding time. FIG. 6 shows the relative densities of 8YSZ and 8YSZ added with alumina at each firing temperature. Zirconia containing 1% by weight of alumina had the highest sinterability, and it was densely sintered even at 1400 ° C. without holding time. It was found that the sinterability of zirconia gradually decreased as the amount of alumina added increased in the alumina addition composition region of 1% by weight or more. In particular, it was found that the zirconia added with 10 wt% had a lower sinterability than the zirconia without added alumina. This is because the alumina particles and the zirconia particles have slightly different coefficients of thermal expansion, so that thermal stress acts during firing to generate microcracks in the sintered body, which hinders sintering.

In order to confirm these things, impurities in the yttria-stabilized zirconia used in the experiment were examined. Then, although it is a very small amount in the powder, SiO ₂ , MgO, CaO, Fe ₂ O ₃ , Al ₂ O ₃ ,
It was found that Na ₂ O was mixed as an impurity (SiO ₂ was below the detection limit of the analyzer, but it was always present). It is considered that among these impurities, those which have a great influence on the sinterability are those existing around the 8YSZ particles (grain boundaries). 8Y for MgO and CaO
Since the solid solution region with SZ is wide, it is already present in the 8YSZ particles at the time of synthesizing the zirconia powder (calcination conditions: 1000 ° C., 2 hours). Therefore, these elements do not exist as impurities in the grain boundaries of zirconia. On the other hand, Fe ₂ O ₃ is about 2 mol% at 1200 ° C.,
Al ₂ O ₃ forms a very small amount of solid solution at less than 0.5 mol% at 1700 ° C., and SiO ₂ and Na ₂ O
Is known to have no solid solution region with 8YSZ. However, the content of Fe ₂ O ₃ is smaller than that of other impurities by one digit or more, and the amount of solid solution in 8YSZ is more than 4 times as large as that of Al ₂ O ₃ , so that it is suitable for sintering zirconia. It is considered that it can be ignored as an influencing impurity. In fact, it has been found that adding a small amount of Fe ₂ O ₃ to zirconia has little effect on sinterability. As a result, it can be inferred that the main impurities are SiO ₂ , Al ₂ O ₃ , and Na ₂ O, and they are present at the grain boundaries and have a great influence on the sintering.

Next, using a thermal analyzer, this SiO ₂
The behavior of Al ₂ O ₃ —Na ₂ O-based impurities was investigated. still,
Here, the amounts of Al ₂ O ₃ and Na ₂ O are clear, but S
Since the amount of iO ₂ was below the detection limit, the amount of SiO ₂ was the detection limit value (sample), and the amount of SiO ₂ was the detection limit value 1
The experiment was performed using two samples of / 2 (sample). The analysis method was performed using two methods, thermogravimetric measurement and differential thermal analysis. The results are shown in FIGS. In the differential thermal analysis curve of the sample shown in FIG. 8, an endothermic peak at around 50 ° C. appears, but this peak is considered to be due to the dehydration reaction of water contained in the sample. The second endothermic peak (922 ° C.) is due to the peritectic temperature of the sample (the temperature at which the solid separates into solid and liquid). Then 1123
A weak endothermic peak appears at around 0 ° C and changes to an exothermic peak before reaching 1200 ° C. This complex behavior is 1
This is because evaporation of Na ₂ O starts from around 120 ° C. and the equilibrium state of the phases in the sample changes accordingly. In other words, as the amount of sodium in the sample decreases, the peritectic temperature of the sample also rises, and the solid phase and the liquid phase move to the one in which the solid phase increases (solidify). It is a peak of heat generation due to. 1/2 of SiO ₂
In the sample prepared as above, the peritectic temperature was 746 as shown in FIG.
It was found that the temperature dropped to around ℃. Also, about 1135 ℃
As described above, it is clear from the differential thermal analysis curve and the thermogravimetric measurement curve that the evaporation of Na ₂ O has started, as in the case of the sample.
The peak of the change from exothermic to endothermic near 1200 ° C is larger than that of the sample because SiO _{2 in} the sample is large.
This is because Na ₂ O is evaporated until the composition in the sample changes to the composition of the solidification condition because the amount of Na ₂ O is small.

Further, 1300 ° C. and 1
The relationship between the sodium content in the sample after firing at 400 ° C. (no holding time) and the amount of alumina added was examined.
The horizontal axis of FIG. 10 represents the amount of alumina added to zirconia,
The vertical axis represents the amount of sodium contained in the sample after firing. The sodium content in each sample calcined at 1300 ° C. was 0.0093% by weight for 8YSZ and was 1% by weight.
In the case of zirconia to which alumina is added, the content of sodium in the sample increases as the amount of alumina added increases once the amount decreases. The reason for this increase is considered to be that sodium oxide reacts with the added alumina to form a β-alumina-based compound, and evaporation of sodium oxide is suppressed. The amount of sodium oxide in 8YSZ with the addition of 1 wt% alumina, which had the highest sinterability, was the lowest.

From the various experiments described above, the sintering model as shown in FIGS. 11 and 12 can be considered. SiO ₂
In the -Al ₂ O ₃ -Na ₂ O system, a melt appears in the sample at a considerably low temperature due to its composition. It is known that the peritectic temperature of the impurities in the sample used in this experiment is 746 to 922 ° C. This temperature is the temperature at which the initial sintering of 8YSZ has occurred, and it is highly possible that the liquid phase in the SiO ₂ —Al ₂ O ₃ —Na ₂ O system is involved. However, in liquid phase sintering, if the liquid phase does not affect the diffusion of the base material (8YSZ in this case), it does not work very effectively for densification of the base material. This is because the densification of ceramic particles is rapidly promoted in the process of changing the appearing liquid phase into a solid phase (dissolution-precipitation). In particular, as is clear from the fact that the compound (or glass) layer containing SiO ₂ inhibits the diffusion of oxygen ions in the zirconia sintered body,
Does not have a positive effect on sintering.

From these facts, it can be inferred that the present 8YSZ powder is being sintered as shown in FIG. The zirconia powder to which alumina is not added has a partially thin SiO ₂ —Al ₂ O part of the zirconia particles as shown in (A).
_{The 3-} Na ₂ O-based compound layer is attached. Next, when the temperature rises to the peritectic temperature, as shown in (B), the glass or the melt (or the liquid phase containing the melt) having high plasticity is used.
The mixed phase of the solid phase) changes so as to pull the zirconia particles. This layer leaks the zirconia particles of the base material, but since it is an impurity, the precipitation reaction does not occur (does not solidify) as it is. However, when the temperature becomes higher, vapor of sodium is generated from the glass as shown in (C), the sodium in the glass decreases, and SiO ₂ —Al ₂ O ₃ —N
When the melting point of the a ₂ O-based material also rises and solidifies, sintering proceeds as in (D). However, at this time, it is highly possible that the vapor of sodium is condensed again on the neck of the zirconia particles (evaporation-condensation mechanism) to delay the condensation.

On the other hand, the zirconia added with Al ₂ O _3, SiO ₂ and Na ₂ O and very reactive Al ₂ O
_As shown in FIG. 12, ₃ reacts and absorbs the SiO ₂ —Al ₂ O ₃ —Na ₂ O layer that inhibits the sintering of zirconia. Then, rearrangement of the zirconia particles easily occurs and promotes solid phase diffusion between the zirconia particles.

To confirm this, SiO ₂ , Al ₂ O ₃ and Na ₂ O, which are considered to affect the sintering, were added to TZ-8Y manufactured by Tosoh, which has the highest sinterability. , Their sinterability was measured. The result is shown in FIG.
14 and 15. As shown in FIG. 13, SiO ₂
In the case of 1% by weight, the sinterability of TZ-8Y hardly changed. As shown in FIG. 14, when 1 wt% of Al ₂ O ₃ is added, it becomes 1% as in the case of Toray 8YSZ.
A significant improvement in sinterability was observed at around 300 to 1400 ° C. However, as shown in FIG. 15, the addition of only 0.1% by weight of sodium oxide Na ₂ O increased the sintering behavior by about 100 ° C. as compared with pure TZ-8Y (sinterability was high). Became low).

Further, by washing the 8YSZ powder with ultrapure water, 8YSZ when Na ₂ O was removed
FIG. 16 shows changes in the respective firing temperatures and relative densities. As is clear from FIG. 16, sodium oxide Na ₂ O
Is a major cause of deterioration of sinterability. From these facts, the sintering of high-purity zirconia powder should be performed as shown in FIG.
It means that the amount of impurities such as Na ₂ O in the powder is greatly involved, as in the sintering model of No. 1.

Among the constituent materials of the solid electrolyte fuel cell, the zirconia electrolyte has the lowest electric conductivity, and its characteristics greatly affect the cell performance. Figure 5 shows 8Y at 1000 ° C
The relationship between the added amount of alumina in SZ and the electrical conductivity is shown. The conductivity was highest when 1 wt% alumina was added, and the value was 0.16 S / cm. It was found that up to 5% by weight, the conductivity was almost the same as that of 8YSZ to which alumina was not added, but when it was added up to 10% by weight, it was found that the conductivity was sharply reduced. In addition, FIG. 17 shows the temperature dependence of the conductivity of alumina added weight%. When the activation energy, which is an index of the mobility of oxygen ions, is calculated from the conductivity of 250 to 700 ° C. in this graph, it is 8YSZ.
43.4 kJ / mol was obtained, which was in agreement with the previous report. The activation energy obtained from the graph of the temperature dependence of the conductivity of alumina-added 8YSZ is 4
It was found that the value changed in the region of 3.7 to 44.3 kJ / mol and increased slightly as the amount of alumina added increased.

To clarify these causes, 25
The electrical conductivity in zirconia was measured and analyzed by the impedance method at a low temperature of 0 ° C. The measurement results of 8YSZ and 8YSZ containing 1% by weight of alumina are shown in FIG. The semicircle on the left side in this figure means the impedance inside the grain in the zirconia sintered body, and the semicircle on the right side means the grain boundary impedance. 1% by weight compared to an 8YSZ arc
It can be seen that the 8YSZ arc to which alumina is added has no difference in the impedance inside the grain, but unlike 8YSZ, there is almost no impedance at the grain boundary. Considering that a sample having almost no difference in relative density is used, 8YSZ shown in FIG. 5 and 8YS containing 1% by weight of alumina are added.
It can be said that the difference in the electrical conductivity of Z at 1000 ° C. is the difference in the grain boundary resistance.

From these experimental results, it can be considered that the zirconia sintered body to which alumina is added has a model as shown in FIG. 8Y without added alumina
The particles 7 of the SZ sintered body are surrounded or partially covered by an extremely thin glass layer 8 (FIG. 19).
(A)). In other words, the glass layer 8 exists at the grain boundaries because it cannot dissolve in 8YSZ. This layer 8 is probably a SiO ₂ —Al ₂ O ₃ based glass (or mullite) containing almost no Na ₂ O and prevents diffusion of oxygen ions in zirconia. However, when the alumina particles 9 are added,
Since the glass layer 8 has high reactivity, the alumina layer 9 reacts with and absorbs the glass layer 8 and solidifies it. The alumina particles 9 added in this way remove the glass layer 8 which inhibits oxygen ions (see FIG. 19).
(B)) It is considered that the electrical resistance at the grain boundaries in 8YSZ with a slight addition of alumina is reduced. However, when the amount of alumina added increases, the alumina particles 9
Is an insulator, so that it again interferes with the movement of oxygen ions at the grain boundaries (FIG. 19C). Therefore, if alumina is added more than necessary, the conductivity will be lowered and the activation energy will be increased.

It was also found that the same effect can be obtained by adding alumina up to 2% by weight of Na ₂ O.

The samples used in these thermal analysis measurements and experiments on the influence of impurities on sinterability were prepared as follows. [Example of Sample Preparation for Experiment of Thermal Analysis Measurement] (a) Al ₂ O ₃ 0.6989 g, Na ₂ O 0.5
931g, SiO ₂ 0.1776g mixing weighed-after pressure molding into pellets, and fired at 1000 ° C. 10 hours. After repeating this mixing and firing work twice,
The sample was used for thermal analysis measurement. (B) Al ₂ O ₃ 0.6989 g, Na ₂ O 0.5
931 g and 0.0888 g of SiO ₂ were weighed and mixed, pressure-molded into pellets, and then baked at 1000 ° C. for 10 hours. After repeating this mixing and firing work twice,
The sample was used for thermal analysis measurement. [Sample preparation example of experiment for investigating influence of extremely small amount of impurities on sinterability of zirconia] (c) Tosoh TZ-8Y (Y ₂ O ₃ 13.5% by weight, Al ₂ O ₃ <0.005% by weight) %, Na ₂ O <0.0
26% by weight, Fe ₂ O ₃ <0.004% by weight, SiO ₂
<0.002 wt%, other ZrO ₂ composition. ) 19.000 g Al ₂ O ₃ 0.2000
g (corresponding to 1 wt%) was added and wet-mixed with a ball mill to prepare a sample. The result is shown in FIG. In this case, sufficient sinterability was obtained at 1400 ° C. (D) Tosoh _{TZ-8Y (Y 2 O 3} 13.5 wt%, Al ₂ O ₃ <0.005 wt%, Na ₂ O <0.0
26% by weight, Fe ₂ O ₃ <0.004% by weight, SiO ₂
<0.002 wt%, other ZrO ₂ composition. ) 9.9900 g to Na ₂ O 0.0100 g
(Corresponding to 0.1 wt%) was added and wet-mixed with a ball mill to prepare a sample. The result is shown in FIG.
In this case, it is understood that the addition of Na ₂ O alone causes a problem in sinterability. (E) Tosoh TZ-8Y (Y ₂ O ₃ 13.5% by weight, Al ₂ O ₃ <0.005% by weight, Na ₂ O <0.0
26% by weight, Fe ₂ O ₃ <0.004% by weight, SiO ₂
<0.002 wt%, other ZrO ₂ composition. ) 9.000 g to Na ₂ O 0.1000 g
(Corresponding to 1 wt%) was added and wet-mixed with a ball mill to prepare a sample. With Na ₂ O alone, even 1 wt% addition was inhomogeneous and measurement became impossible. Therefore, in order to remove Na ₂ O, conventional purification was repeated,
Manufacturing costs are high. (F) Tosoh _{TZ-8Y (Y 2 O 3} 13.5 wt%, Al ₂ O ₃ <0.005 wt%, Na ₂ O <0.0
26% by weight, Fe ₂ O ₃ <0.004% by weight, SiO ₂
<0.002 wt%, other ZrO ₂ composition. ) 9.000 g SiO ₂ 0.1000 g
(Corresponding to 1 wt%) was added and wet-mixed with a ball mill to prepare a sample. The result is shown in FIG. As a result, with addition of SiO ₂ alone, even 1 wt% is 140
At 0 ° C, sufficient sinterability was not obtained. (G) ZrO ₂ 81.29% by weight, Y ₂ O ₃ 13.
4% by weight, Al ₂ O ₃ 5.03% by weight, Na ₂ O 0.
03% by weight, MgO 0.01% by weight, CaO 0.05
% By weight, 0.01% by weight of Fe ₂ O ₃ , SiO ₂ <0.
It consists of 01% by weight. This composition was obtained by adding 5 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (8YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. This Toray zirconia 10.0000g
0.1355 g of NaHCO ₃ (equivalent to 0.5 wt%)
After adding and wet-mixing with a ball mill, a sample was prepared. Although this result is not shown, a sufficient sintering result was obtained at 1400 ° C. (H) 81.29% by weight of ZrO ₂ , Y ₂ O ₃ 13.
4% by weight, Al ₂ O ₃ 5.03% by weight, Na ₂ O 0.
03% by weight, MgO 0.01% by weight, CaO 0.05
% By weight, 0.01% by weight of Fe ₂ O ₃ , SiO ₂ <0.
It consists of 01% by weight. This composition was obtained by adding 5 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (8YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. This Toray zirconia 10.0000g
0.2710 g (corresponding to 1 wt%) of NaHCO ₃ was added to the mixture, and the mixture was wet-mixed with a ball mill to prepare a sample. Although this result is not shown, a sufficient sintering result was obtained at 1400 ° C. (I) ZrO ₂ 81.29% by weight, Y ₂ O ₃ 13.
4% by weight, Al ₂ O ₃ 5.03% by weight, Na ₂ O 0.
03% by weight, MgO 0.01% by weight, CaO 0.05
% By weight, 0.01% by weight of Fe ₂ O ₃ , SiO ₂ <0.
It consists of 01% by weight. This composition was obtained by adding 5 wt% alumina to yttria-stabilized zirconia, and was achieved by using 8 mol% yttria-stabilized zirconia (8YSZ) manufactured by Toray Industries, Inc. and alumina manufactured by Daimei Chemical Co., Ltd. This Toray zirconia 10.0000g
0.5420 g (corresponding to 2 wt%) of NaHCO ₃ was added to the mixture, and the mixture was wet-mixed with a ball mill to prepare a sample. Although this result is not shown, a sufficient sintering result was obtained at 1400 ° C.

[0042]

DETAILED 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, the zirconia powder having a crystal structure stabilized with 8 mol% yttria and the step of removing the sodium component is reduced, and the high-purity alumina powder are used as raw materials. To do. The amount of alumina added to the zirconia powder (x:% by weight) was 0 <x
The region was ≦ 5. The zirconia powder may be zirconia stabilized or partially stabilized with a stabilizer other than yttria in order to improve the electric conductivity. For example, this powder is prepared by mixing zirconia-alumina using a ball mill for 24 hours with pure water without using a dispersant, and then rapidly drying it with a spray dryer to obtain a uniform zirconia-alumina composite. An electrolyte powder was obtained.

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

The yttria-stabilized zirconia used in the experiment before adding alumina had a ZrO ₂ content of 85,6.
2 wt%, Y ₂ O ₃ is 14.2 wt%, which corresponds to nearly 8 mol% yttria -84 mol% zirconia. However, at present, it is not possible to reduce the impurities to 0 by any method. Therefore, both the sample and the impurities used in this experiment contain 0.03% by weight of Al ₂ O ₃ and Na.
₂ O is 0.04% by weight, MgO is 0.01% by weight, Ca
O is 0.08% by weight, Fe ₂ O ₃ is 0.01% by weight, S
The iO ₂ is present in a very small amount of 0.01% by weight but is present. Among these, the conductivity is affected by Fe ₂ O
There are two, ₃ and SiO ₂ , but it can be seen that it is the smallest. The average particle size of this zirconia is 0.25 μm.
Is.

Example 1 ZrO ₂ 84.80% by weight, Y ₂ O ₃ 14.0% by weight, Al ₂ O ₃ 1.01
Wt%, Na ₂ O 0.03 wt%, MgO 0.01
% By weight, CaO 0.07% by weight, Fe ₂ O ₃ 0.0
1% by weight, SiO ₂ <0.01% by weight.
This composition was obtained by adding 1 wt% alumina to yttria-stabilized zirconia, and was achieved by using Toray Industries, Inc. 8 mol% yttria-stabilized zirconia (8YSZ) and Daimei Chemical Co., Ltd. alumina. 8 mol%
Yttria-stabilized zirconia is considered to be affected by SiO ₂ and Fe _{2 which} are considered to have an effect because the electric resistance becomes extremely high.
Very little O _{3 and} almost no impurities. The average particle size of zirconia is 0.25 μm, and the average particle size of alumina is 0.23 μm. The sinterability of the powder of this composition is represented as 1A8YSZ in FIG. Further, the conductivity is represented as 1A8YSZ in FIG.

[Example 2] ZrO ₂ 83.08% by weight, Y ₂ O ₃ 13.8% by weight, Al ₂ O ₃ 3.01
Wt%, Na ₂ O 0.04 wt%, MgO 0.01
Wt%, CaO 0.05 wt%, Fe ₂ O ₃ 0.0
1% by weight, SiO ₂ <0.01% by weight.
This composition was obtained by adding 3% by weight alumina to yttria-stabilized zirconia, and was achieved by using Toray Industries, Inc. 8 mol% yttria-stabilized zirconia (8YSZ) and Daimei Chemical Co., Ltd. alumina. In this case, the average particle size of zirconia is 0.25 μm and the average particle size of alumina is 0.23 μm. The conductivity is 3A8Y in FIG.
It is represented as SZ.

[Example 3] ZrO ₂ 81.29% by weight, Y ₂ O ₃ 13.4% by weight, Al ₂ O ₃ 5.03
Wt%, Na ₂ O 0.03 wt%, MgO 0.01
Wt%, CaO 0.05 wt%, Fe ₂ O ₃ 0.0
1% by weight, SiO ₂ <0.01% by weight.
This composition was obtained by adding 5 wt% alumina to yttria-stabilized zirconia, and was achieved by using Toray Industries, Inc. 8 mol% yttria-stabilized zirconia (8YSZ) and Daimei Chemical Co., Ltd. alumina. In this case, the average particle size of zirconia is 0.25 μm and the average particle size of alumina is 0.23 μm. The sinterability of the powder of this composition is represented as 5A8YSZ in FIG. In addition, the conductivity is represented as 5A8YSZ in FIG.

Comparative Example 1 76.71% by weight of ZrO _2, 13.1% by weight of Y ₂ O ₃ , Al ₂ O ₃ 10.0
6% by weight, Na ₂ O 0.03% by weight, MgO 0.0
1 wt%, CaO 0.07 wt%, Fe ₂ O ₃ 0.0
1% by weight, SiO ₂ <0.01% by weight.
This composition is 10% by weight of yttria-stabilized zirconia.
Alumina added, Toray Industries, Inc. 8 mol%
It was achieved using yttria-stabilized zirconia (8YSZ) and Daimei Chemical Co. alumina. The average particle size of alumina is 0.23 μm. The sinterability of the powder of this composition is represented as 10A8YSZ in FIG.
In addition, the conductivity is represented as 10A8YSZ in FIG.

FIG. 6 shows the relative densities without holding time at the temperature of 1100 to 1600 ° C. in the respective regions where the amount of alumina added is 0, 1, 5% by weight. Amount of alumina added is 0
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 of wt%. Under such low temperature firing conditions, it becomes possible to manufacture a high performance flat plate solid electrolyte fuel cell by using, for example, a co-sintering method. Particularly, the zirconia containing 1% by weight of alumina of Example 1 is optimal because it can be densely sintered at a low temperature of about 1400 ° C. under a firing condition without holding time. However, in the alumina addition composition region of 1% by weight or more, the sinterability of zirconia gradually decreases as the alumina addition amount increases. In particular, as in Comparative Example 1, 1
The sinterability of zirconia added with 0 wt% is lower than that of zirconia without added alumina.

The zirconia particles are TZ- manufactured by Tosoh Corporation.
It is about 10 times larger than 3Y and TZ-8Y, but the sinterability and conductivity do not deteriorate.

The evaluation of these examples and comparative examples is shown in FIG.
It carried out by the method shown in 0. 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 and the relative density was calculated. The result is shown in FIG.

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. This device consists of a potentiostud 11 and a frequency response analyzer 12.
An alternating current is applied to the sample 15 through the electrode 14 and the change in the alternating current is measured by the digital oscilloscope 13 to obtain the conductivity. The electrode 14 was coated with platinum paste and baked at 1000 ° C. for 1 hour. The impedance of the sample was analyzed by the Cole-Cole plot of the value measured by this device, and the electric resistance was obtained. The results are shown in FIGS.

[0054]

As is apparent from the above description, the zirconia electrolyte powder of the present invention contains SiO ₂ , Al ₂ in the zirconia electrolyte powder to which a dopant for improving conductivity is added.
If the contents of the compound or mixture of O ₃ and Na ₂ O are x, y, and z, respectively, SiO ₂ 0 <x ≦ 1 wt% Al ₂ O ₃ 0 <y ≦ 5 wt% Na ₂ O 0 <z ≦ Since the range of 2% by weight is set, the SiO ₂ —Al ₂ O ₃ —Na ₂ O layer raises the temperature required for dense sintering of zirconia, while
Al ₂ O ₃ was added _{SiO 2 -Al 2 O 3 -Na 2} O
SiO ₂ -Al ₂ O and so absorb it reacts with the layer or the like to lower the temperature required for the dense sintered zirconia
_Since the interaction of _3- Na ₂ O is used, the sintering characteristics can be controlled without affecting the conductivity characteristics of the zirconia electrolyte. Therefore, according to the zirconia electrolyte powder of the present invention, it is possible to obtain a highly reliable single cell having a low temperature of dense sintering when producing a single cell of a solid electrolyte fuel cell from the zirconia electrolyte powder by a co-sintering method. The range of applications of zirconia can be expanded.

[Brief description of drawings]

FIG. 1 is an exploded perspective view of an example of a flat plate solid oxide fuel cell.

FIG. 2 is a conventional single cell manufacturing process diagram.

FIG. 3 is a model diagram showing a power generation state of a single cell.

FIG. 4 is a process drawing of a single cell by a co-sintering method.

FIG. 5 is a graph showing changes in conductivity of stabilized zirconia to which alumina is added.

FIG. 6 is a graph showing the relationship between the firing temperature and the change in relative density of stabilized zirconia containing alumina.

FIG. 7 is a graph showing changes in firing temperature and relative density of Toray 8YSZ (8% yttria-stabilized zirconia) powder and Daimei Chemical alumina powder.

FIG. 8 is a graph showing the results of thermal analysis of the SiO ₂ —Al ₂ O ₃ —Na ₂ O system (when the amount of SiO ₂ is the analytical detection limit value).

FIG. 9 is a graph showing the results of thermal analysis of a SiO ₂ —Al ₂ O ₃ —Na ₂ O system (when the amount of SiO ₂ is 1/2 of the analytical detection limit value).

FIG. 10 is a graph showing the relationship between the amount of alumina added and the content of sodium in calcined zirconia.

FIG. 11 is a sintering mechanism model of 8YSZ when alumina is not added, in which (A), (B) and (C) are before sintering.
(D) is after sintering.

FIG. 12 is a sintering mechanism model of 8YSZ when alumina is added, in which (A), (B) and (C) are before sintering, and (D) is after sintering.

FIG. 13 is a graph showing the relationship between the firing temperature and the relative density when 1 wt% SiO ₂ was added to Tosoh TZ-8Y (yttria-stabilized zirconia).

FIG. 14 is a graph showing the relationship between firing temperature and relative density when 1 wt% Al ₂ O ₃ was added to Tosoh TZ-8Y.

FIG. 15: Tosoh TZ-8Y with 0.1 wt% Na ₂ O
3 is a graph showing the relationship between the firing temperature and the relative density when is added.

FIG. 16 is a graph showing changes in relative density and firing temperature of Toray 8YSZ from which Na ₂ O was removed by ultrapure water treatment.

FIG. 17 is a graph showing firing temperature and relative density change of Toray 8YSZ to which alumina is added.

FIG. 18 is a graph showing a Cole plot of impedance of 8YSZ manufactured by Toray Co., Ltd. to which alumina is not added and which is added.

FIG. 19 is a model diagram of a microstructure of a sintered body of 8YSZ with and without addition of alumina, (A) without alumina and (B) with alumina. , (C) are those in which alumina was excessively added.

FIG. 20 is a process drawing showing a method for producing alumina-added zirconia powder and a method for evaluating relative density.

FIG. 21 is a schematic view of an apparatus for evaluating the conductivity of an alumina-added zirconia body.

[Explanation of symbols]

 1 Electrolyte 2 Fuel electrode 3 Air electrode 4 Single cell of flat plate type solid electrolyte fuel cell

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Abe 2-6-1 Nagasaka, Yokosuka City, Kanagawa Foundation Central Research Institute of Electric Power Industry Yokosuka Research Center

Claims (1)

[Claims] 1. SiO ₂ , Al ₂ O ₃ , and Na _{2 in} zirconia electrolyte powder to which a dopant for improving conductivity is added.
The contents of the compound or mixture of O are x, y,
The zirconia electrolyte powder is characterized in that SiO ₂ 0 <x ≦ 1% by weight Al ₂ O ₃ 0 <y ≦ 5% by weight Na ₂ O 0 <z ≦ 2% by weight, where z.