JPH0412058A - Stabilized zirconia solid electrolyte and production thereof - Google Patents

Abstract

PURPOSE:To improve strength of stabilized zirconia solid electrolyte without damaging oxygen ion conductivity, by making a metal oxide exist in granules of stabilized zirconia. CONSTITUTION:A slurry of submicron stabilized airconia powder blended with a metal alkoxide, a metal salt, submicron metal powder or metal oxide powder is molded and sintered at 1,300-1,700 deg.C in such a way that zirconia has >=95% density to give a polycrystalline solid electrolyte of sintered material which has >=60% metal oxide in granules of stabilized zirconia having >=2mum average particles diameter and >=90% cubic system and has peak strength of zorconia obtained by X-ray diffraction shown by formulas I and II and >=95% theoretical density.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a solid electrolyte, and more particularly to a stabilized zirconia solid electrolyte with improved strength without impairing oxygen ion conductivity, and a method for producing the same.

[Prior Art] Stabilized zirconia is used as an electrolyte in solid electrolyte fuel cells. Stabilized zirconia has a tetragonal zirconia phase (partially 5tabilized zirconia) in the region where the amount of stabilizer is small.
con ia) is formed, and when the amount of stabilizer topes increases, the cubic zirconia phase (fully stabilized zirconia, Fu
JIy 5tabilized Zirconia (hereinafter simply referred to as stabilized zirconia) is produced. Elements such as yttrium, cerium, calcium, and magnesium are used as stabilizers.

Both stabilized zirconia and partially stabilized zirconia have oxygen ion conductivity, but stabilized zirconia has higher performance in terms of conductivity, and stabilized zirconia is also superior in terms of stability at high temperatures. There is.

On the other hand, partially stabilized zirconia has one of the highest strengths among ceramic materials, and is overwhelmingly more advantageous than stabilized zirconia as a structural material.

Therefore, industrially, stabilized zirconia is used for small-volume items such as oxygen sensors from the viewpoint of electrical conductivity, and partially stabilized zirconia is used for crushing poles, zirconia knives, etc. from the viewpoint of strength.

The electrolyte for solid electrolyte fuel cells is preferably one with high oxygen ion conductivity in order to reduce the internal resistance of the battery.
-The use of stabilized zirconia for boat fishing is being considered.

Cylindrical and flat plate structures have been proposed for fixed electrolyte fuel cells. The cylindrical type is a method of forming cells (electrolyte and electrodes) on a porous ceramic support tube.
Since the cell does not need to be self-supporting, the strength requirements for the cell material are not very strict. However, since the volume of the support does not contribute to power generation at all, high integration is difficult.

On the other hand, the lithographic type does not have a support for the cells, so it is possible to achieve high integration, but on the other hand, the strength conditions required for the cells, especially the electrolyte, are quite severe in order to support the cells themselves.

Therefore, it has been proposed to use partially stabilized zirconia in the electrolyte of flat plate solid oxide fuel cells, and to add 5 to 30% by weight of alumina powder to the stabilized zirconia powder used as the starting material to improve strength. There is.

[Problems to be Solved by the Invention] When partially stabilized zirconia is used as an electrolyte, there is a problem in that the internal resistance of the battery increases compared to stabilized zirconia. Furthermore, in the method of adding alumina powder to the stabilized zirconia powder as a starting material, the strength of the solid electrolyte is improved, but since alumina is an insulator, there is a problem in that the oxygen ion conductivity is significantly reduced.

In view of the above-mentioned circumstances, the present invention aims to provide a solid electrolyte with improved strength without impairing the high ionic conductivity of stabilized zirconia, and a method for producing the same.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a solid electrolyte characterized in that stabilized zirconia contains a metal oxide, and the metal oxide is present within the stabilized zirconia particles. Similarly, a method for producing a fixed electrolyte is characterized in that a stabilized zirconia powder slurry to which metal alkoxide, metal salt, or submicron metal powder or metal oxide powder is added is used as a starting material, and this is formed and fired. , after dispersing the stabilized zirconia powder in a metal alkoxide or metal salt solution, prehydrolyzing and/or calcining the stabilized zirconia powder to support the metal hydroxide or oxide;
The present invention provides a method for producing a solid electrolyte, which comprises using this powder as a starting material, molding and firing.

According to the present invention, since the metal alkoxide or metal salt in the form of a solution covers the zirconia particles thinly and uniformly, by hydrolyzing or thermally decomposing the metal alkoxide or metal salt, it is possible to mix and disperse the conventional zirconia powder and metal oxide (alumina) powder. A much more uniform dispersion can be achieved with a lower metal oxide fraction compared to the method. Since the metal oxide particles present within the grains are extremely small, their influence on the oxygen ion conduction of zirconia is negligible.

In the present invention, the metal oxide used to improve the strength of stabilized zirconia exists substantially within the grains of the zirconia particles (some of them also exist at the grain boundaries); Metal oxides do not exist as particles equivalent to zirconia particles.This structure is called a nanocomposite, and its structure is schematically shown in Figure 1.
In the figure, 1 is a YSZ crystal grain, and 2 is Al2O3 inside the grain.

Figure 2 shows the structure of conventional Az2o3-added YSZ.
is 737 grains, and 4 is Alt03 grains. In the nanocomposite, the 737 grains 1 are on the order of micrometers, generally several square feet or more, typically several tens to hundreds of cubes, whereas the metal oxide grains 2 are on the order of nanometers (
It is on the order of submicron. By employing such a nanocomposite structure, strength can be improved without reducing ionic conductivity. The reason for this is thought to be that the metal oxide to be added is small, and furthermore, a small amount is sufficient.

Metal oxides that can be used for this purpose include substances that do not form a solid solution with stabilized zirconia, such as alumina, chromia, and mullite, as well as substances that crystallographically form a solid solution with stabilized zirconia, such as , magnesia, calcia and other alkaline earth metal oxides, rare earth element oxides, titania, bismuth oxide, thoria, urania, etc. can also be used. This is because even substances that can form a crystallographic solid solution form a nanocomposite structure depending on the firing conditions, particularly the temperature. Moreover, in that case, it goes without saying that a solid solution may be partially formed. Furthermore, a mixture or composite of each of the above metal oxides may be used. Some composites, such as yttoria-doped ceria, have ionic conductivity in themselves, and the addition of this is preferable because the decrease in ionic conductivity is less.

Even if the amount of metal oxide is 0.1% by weight in the stabilized zirconia electrolyte, it has the effect of improving the strength, and as the amount increases, the strength increases, but if the amount of metal oxide is too large, the ionic conductivity of the stabilized zirconia will decrease. 30% by weight or less is preferable.

More preferably 0.01 to 20% by weight, even 0.1% by weight
-5% by weight.

Stabilized zirconia is made by mixing zirconia with a stabilizer such as yttria or calcia at 5 to 10 mo1%, typically 8ff.
It is effective to stabilize it by adding 1% lo.

A first step in which a metal oxide is present within the grains of stabilized zirconia.
This method is a method in which a metal alkoxide, a metal salt, or a submicron Φ metal powder or metal oxide powder is added to a stabilized zirconia slurry. The metal alkoxide has the formula M(OC,)Izn41)ffiC, where M is a metal element, n is typically an integer of 1 to 4, and m is the valence of the metal M. ] is preferable. As a solvent, Cf1F[2n. , 0H (n=1 to 4) lower alcohols, aromatic organic solvents such as Hensen, toluene, etc. can be used. Further, conventional additives such as a binder, a dispersant, an antifoaming agent, and a plasticizer can be added to the slurry.

The stabilized zirconia slurry can be shaped and fired by conventional methods.

A second process in which a metal oxide is present within the grains of stabilized zirconia.
In this method, stabilized zirconia particles are dispersed in a metal alkoxide or metal salt solution, and then hydrolyzed, followed by filtration, washing, and drying to support metal components in the form of hydroxide on the stabilized zirconia particles. This is a method in which the stabilized zirconia particles are heated, calcined if necessary, converted into a metal oxide form by thermal decomposition, supported, and then molded and fired using the stabilized zirconia particles. Also in this case, the same solvent as above can be used as the solvent for the metal alkoxide. As the metal salt, nitrate, chloride, carbonate, acetate, etc. can be used. In this case, the solvent is water, lower alcohol,
Glycol etc. are used. Hydrolysis is carried out by dropping and mixing a solution of alkali hydroxide, ammonia, basic amine, etc. in the above-mentioned solvent using a conventional method.

The stabilized zirconia particles thus obtained carry metal components in the form of fine hydroxides or oxides on their surfaces. Molding and firing using the stabilized zirconia particles can be carried out by conventional methods.

[Example] Aluminum isopropoxide (
Mw=204.25) 4.085 g was added and stirred well to completely dissolve. Completely stabilized zirconia (8Y) containing 8 mo1% of Y2O2 (Ittria) as a stabilizer.
SZ) powder (manufactured by Tosoh, TZ-8Y, average particle size 0.3
Weigh 100g of IITn) and add isopropanol 50c.
c was added and thoroughly mixed and dispersed using a ball mill.

To this, the toluene solution of aluminum isopropoxide, 10 g of polyvinyl butyral (PVB) powder,
A small amount of a dispersant, a defoaming agent, and a plasticizer were added, and the mixture was further mixed and dispersed in a ball mill. After degassing the slurry in vacuum, a green sheet was produced using a doctor blade device, punched, and fired to obtain a fired ceramic sheet. The fired body has a thickness of about 200 mm, bending strength,
The resistance and fuel cell performance are shown in the table below, and both showed extremely good performance.

When the fired body was observed using a transmission electron microscope, the presence of alumina particles within the stabilized zirconia particles was observed. Further, a part of the alumina particles also exists at the grain boundaries of the zirconia particles.

Example 100 g of indolium stabilized zirconia (YSZ) powder was dissolved in 100 cc of toluene in the same manner as in Example 1, and 100 g of indolium stabilized zirconia (YSZ) powder was added (dispersed to form a slurry, then mixed with pure water). Hydrolysis was carried out by slowly adding 5 g of isopropanol to a small amount of ammonia water to make 100 cc.The filtered and dried powder was calcined at 600°C to form alumina-supported YSZ powder. I got it.

Add 50cc of toluene and 50cc of isopropanol to this powder.
c. 10 g of polyvinyl butyral (PVB) and a small amount of a dispersant, a defoaming agent, and a plasticizer were added, mixed and dispersed in a ball mill, and sheeted using a doctor blade device. Both the strength and cell performance of the fired body are shown in Table 1 below, and almost the same performance as in Example 1 was obtained.

When the fired body was observed using a transmission electron microscope, the presence of alumina particles within the stabilized zirconia particles was confirmed. In addition, a part of the alumina particles also exists at the grain boundaries of the zirconia particles.

Comparative Coal Construction I Instead of the alumina-supported YSZ powder of Example 2, partially stabilized and fully stabilized zirconia (3ysz and 8Y) containing 3 mo1% and 8 mo1% of ittria as stabilizers were used.
A slurry was prepared in the same manner as in Example 2 using SZ (average particle size: 0.3 mm), and was molded and fired.

The characteristics of the fired body are shown in the table below.

Alumina powder (Ozeki Chemical TM-DR, average particle size 0, 2
am ) was added in an amount of 10% by weight, 20% by weight, and 30% by weight, and fired bodies were produced in the same manner as in Example 2.

The characteristics of the fired body are shown in Table 1 below.

Completely stabilized zirconia (8Y
SZ) powder (manufactured by Tosoh, TZ-8Y, average particle size 0.3
1! m) and the additive substances shown in Table 2 (particle size 0.3-0,
A total of 100 g of 5μ) was weighed out in the proportions shown in Table 2, 50 cc of isopropanol was added, and the mixture was thoroughly mixed and dispersed in a ball mill. To this, 10 g of polyvinyl butyral (PVB) powder and a small amount of a dispersant, a defoaming agent, and a plasticizer were added, and the mixture was further mixed and dispersed in a ball mill. After vacuum defoaming of the finished slurry, a green sheet was made using a doctor blade device, punched out, and
After firing at 0°C for 18 hours, a fired ceramic sheet was obtained. The fired body had a thickness of about 200 mm, and its bending strength, resistance, and fuel cell performance are shown in Table 2 below, and it showed extremely good performance in all of them.

In addition, in Table 2, the strength is 3 m x 40 mm
XO, 2l! This is the average value obtained by preparing 10 test pieces with a dimensional error within 10% in m and performing a three-point bending test with a span of 30 mm. The conductivity is determined by applying an area of 0.5 c on both sides of the specimen.
The electrode i was formed by applying Pt paste, and the current was collected using a Pt net, and the value was at 1000° C1 in the atmosphere.

The cell performance was determined by placing an anode (Ni28YSZ paste mixed at a weight ratio of 9:1) and a cathode (Lao, 9S) on both sides of the test piece.
ro, I M103 paste) was formed and current was collected using a Pt network, and pure hydrogen was used as a fuel at 1000°C, and pure oxygen was used as an oxidation side at a flow rate of 100c.
This is the output at constant IA current when flowing at c/min.

Table [Effects of the Invention] In the fully stabilized zirconia fixed electrolyte according to the present invention, a small amount of metal oxide exists in the grains, increasing the strength of the fired product.
Strength can be improved without impairing the excellent ionic conductivity of fully stabilized zirconia.

[Brief explanation of drawings]

FIG. 1 and FIG. 2 are schematic organization diagrams of stabilized zirconia solid electrolytes of the present invention and the conventional method, respectively. 1...732 grains, 2...Affiz03.
3...732 grains, 4...to! 203 grains.

Claims (3)

[Claims] 1. A solid electrolyte characterized in that stabilized zirconia contains a metal oxide, and the metal oxide is present within the grains of stabilized zirconia particles. 2. A method for producing a solid electrolyte, which comprises using as a starting material a stabilized zirconia powder slurry to which metal alkoxide, metal salt, or submicron metal powder or metal oxide powder is added, and molding and firing the slurry. 3. Stabilized zirconia powder is dispersed in a metal alkoxide or metal salt solution, then hydrolyzed and/or calcined to make the stabilized zirconia powder support a metal hydroxide or oxide, and this powder is used as a starting material to be shaped and fired. A method for producing a solid electrolyte, characterized by: