JPH01157072A - Oxygen concentration cell - Google Patents

Abstract

PURPOSE:To improve the extent of strength in a specified temperature range as well as to aim at reduction in secular deterioration by using such zirconia porcelain that consists of ZrO2 and Y2O3, specifying a mole ratio of Y2O3/ZrO2, and sets mean grain size to be less than the specified one, as a solid electrolyte. CONSTITUTION:In the case of ZrO2-Y2O3 zirconia porcelain, such a tetragonal crystal grain that a mole ratio of Y2O3/ZrO2 is within the range of 2/98-4/96, and mean grain size is less than 2mu is used. This zirconia porcelain is used as a solid electrolyte, installing an electrode, and an oxygen concentration cell is produced. With this constitution, since tetragonalness stably exists there without entailing any phase transformation into a monocline at a temperature range from 500 deg.C to room temperature, higher strength is secured and, what is more, secular deterioration also becomes lessened.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides ZrO□-Y2O which has high strength and exhibits extremely little deterioration over time due to long-term use in a specific temperature range.

The present invention relates to an oxygen concentration battery using a solid electrolyte made of zirconia porcelain.

Conventionally, Zr(h-'lzO+-based zirconia porcelains include fully stabilized zirconia porcelains consisting only of cubic crystals,
Partially stabilized zirconia porcelains consisting of cubic crystals and monoclinic crystals are known, and both are used as heat-resistant materials, solid electrolytes, and the like. Fully stabilized zirconia porcelain is stable in the temperature range from room temperature to approximately 1500°C, and hardly deteriorates over time due to long-term use, but its low strength makes it useful for detecting the oxygen concentration in automobile exhaust gas, for example. When used as a solid electrolyte for oxygen sensors, it has the disadvantage of being extremely susceptible to damage due to thermal shock. On the other hand, partially stabilized zirconia porcelain made of cubic and monoclinic crystals has higher strength and better thermal shock resistance than fully stabilized zirconia porcelain, but it
The deterioration of strength over time in a specific temperature range of °C is extremely large, and when used for a long time at this temperature, many fine cracks appear on the porcelain surface and it becomes water absorbent, resulting in a significant decrease in strength and eventually It had the serious drawback of being easily damaged.

This is because in ZrO□-Y2O3-based partially stabilized zirconia porcelain, the crystal grains, which are tetragonal at a firing temperature of about 1500°C, change to 500° during cooling from about 1500°C to room temperature.
A phase transformation to monoclinic crystal occurs near C, and the resulting volume change causes excessive stress in the porcelain, resulting in many extremely small racks occurring within the crystal grains. It is thought that if it is left in a specific temperature range for a long time, it will expand and eventually lead to porcelain destruction.

The present invention eliminates these drawbacks of partially stabilized zirconia porcelain, and uses zirconia porcelain as a solid electrolyte, which has excellent strength and significantly improves deterioration of strength over time in a specific temperature range of 200°C to 300°C. ,
It is an oxygen concentration battery that has at least one pair of electrodes in contact with the solid electrolyte, and the solid electrolyte is mainly made of ZrO□.
and Y2O, the molar ratio of Y2O3/ZrO□ is in the range of 2/98 to 4/96, the crystal grains are mainly composed of tetragonal crystal grains, the average crystal grain size is 2μ or less, and the 200″C Zirconia porcelain with excellent durability at temperatures ranging from 300°C to 300°C and mainly ZrO□ and Y2O3
The molar ratio of Y2O3/ZrO□ is 2798-7
/93 range, the crystal grains are mainly composed of cubic crystal grains and tetragonal crystal grains, the average crystal grain size is 2μ or less, and has excellent durability at 200°C to 300'C. It is.

That is, the present invention sets the molar ratio of Y2O3/ZrO□ to a specific value in ZrO□-Y2O3-based zirconia porcelain,
By keeping the average crystal grain below a specific value, the conventional
Tetragonal crystals, which are unstable due to phase transformation at temperatures below 0°C, are made to exist stably without undergoing phase transformation to monoclinic crystals within the temperature range from 500°C to room temperature, and crystal grains are mainly used. Using zirconia porcelain as a solid electrolyte, which has extremely high strength due to the formation of tetragonal crystal grains or mainly cubic crystal grains and tetragonal crystal grains, and which exhibits extremely little deterioration over time in a specific temperature range. This is an oxygen concentration battery having at least one pair of electrodes in contact with each other.

To explain the present invention in more detail, in order for the tetragonal crystal to exist stably, it is extremely important that the average crystal grain size of the porcelain be 2 μm or less, preferably 1 μm or less.

In other words, the relationship between the average crystal grain size and the bending strength is as shown in Figure 1. In curve A before the durability test, no rapid decrease in strength is observed even when the average crystal grain size is 2 μ or more, but at 20° 150 to a specific temperature range of 300'C to 300'C
In curve B after a durability test held for 0 hours, when the average crystal grain size exceeds 2μ, the strength rapidly decreases and deterioration over time becomes significant due to the formation of excessive monoclinic crystals and the inherent fineness rank. . Furthermore, as described in the Examples below, if the average crystal grain size is 2μ or less, preferably 1μ or less, the crystal phase will hardly change even if left in a specific temperature range of 200°C to 300°C, and the crystal phase will be square. The crystal remains stable. In this way, in the present invention, 2 is said to have excellent durability at 200'C to 300°C.
This means that there is little deterioration over time at any temperature between 00°C and 300°C. As an example of a specific measuring method, as described in the examples, 200°C
In order to cover the entire temperature range of 00°C, we conducted an endurance test in which heating and cooling were repeated between 200'C and 300°C in the air at a rate of 10°C/min. It is better to measure the subsequent change in bending strength or crystal phase. The longer the durability time, the greater the degree of deterioration, but 1
After about 500 hours, the difference between the conventional zirconia porcelain and the zirconia porcelain of the present invention becomes clear. The reason why transformation to monoclinic crystals is more difficult to occur than tetragonal crystal grains when the crystal grain size is reduced in this way is that when crystal grains are small, tetragonal crystals are more stable than monoclinic crystals due to the surface free energy of the particles. This is considered to be the case. Note that the average crystal grains are measured by the following method. Count the number n of particles within a certain area S that contains 50 or more particles in an electron micrograph of a mirror-polished porcelain surface etched with hydrofluoric acid, and calculate the area equal to the average area S per particle. The diameter d of the circle is expressed by the formula d =
Calculated using (4s/π). Then, d is determined for three or more visual fields of the same sample, and the average value is taken as the average crystal grain size.

The number of particles n is the sum of the number of particles completely included in the constant area S and the percentage of the number of particles cut by the boundary line of the constant area.

As shown in Figure 2, the relationship between the X-ray diffraction line peak intensity ratio and the bending strength is as follows:
The intensities of the X-ray diffraction lines of the (200) plane of the cubic crystal are T(200), M(111), and C(
200), the strength of the zirconia porcelain C made of mainly tetragonal crystal grains constituting the present invention is the strength before deterioration of the conventional zirconia porcelain made of cubic crystal grains and monoclinic crystal grains. (Also, zirconia porcelain E, which is mainly composed of cubic crystal grains and tetragonal crystal grains, is a zirconia porcelain E that is mainly composed of cubic crystal grains and monoclinic crystal grains.) This strength is higher than the strength F. Zirconia porcelains C and E of the present invention have higher strength than zirconia porcelain G consisting only of cubic crystals, and the strength improves as the number of tetragonal crystals increases.

In the present invention, zirconia porcelain mainly composed of tetragonal crystals refers to zirconia porcelain mainly composed of tetragonal crystals (M(11
1) +C(200))/T(200) X-ray diffraction line peak intensity ratio of 0.4 or less, which includes monoclinic and/or cubic crystals. The above range of X-ray peak intensity ratios corresponds to approximately 20 volume percent or less of monoclinic and/or cubic crystals.

Also, zirconia porcelain, which is mainly composed of cubic crystal grains and tetragonal crystal grains, includes zirconia porcelain that is composed only of tetragonal crystal grains and cubic crystal grains, as well as zirconia porcelain that is mainly composed of cubic crystal grains and tetragonal crystal grains.
/ (T(200) + C(200)) (7) The intensity ratio is 0.05 or more, the intensity ratio of M (111) / T (200) is 1 or less, M (111) / (T (200)
+ C(200)) (7) Also includes those in which monoclinic crystals exist such that the intensity ratio is 0.4 or less. The above range of X-ray peak intensity ratio corresponds to an amount of monoclinic crystals of approximately 20% by volume or less of the total.

Also, in the present invention, zirconia porcelain mainly composed of ZrO2 and Y2O3 means zirconia porcelain mainly using Y2O3 as a stabilizer of ZrO2,
About 30 mol% or less of Y2O, other rare earth element oxides,
For example, Yb20z + S+, 03 + NbzO3,
It may be substituted with Sm2O3, CeO2, etc., or with CaO or MgO.

In addition, the zirconia porcelain according to the present invention has 5ift + Al
The porcelain may contain a sintering aid such as 2O2+clay in an amount of 30% by weight or less based on the total weight of the porcelain. The crystalline phase constituting the porcelain is identified by X-ray diffraction using a mirror-polished surface of the porcelain.

After being exposed to a temperature range of 200°C to 300°C, the porcelain is also polished again and subjected to X-ray diffraction using the mirrored surface.

In addition, the bending strength can be determined by the commonly used three-point bending method or by the four-point bending method.
Depending on the point bending method, initial measurements and 200″C to 30°C
The measurement after exposure to a temperature range of 0"C is based on the same method, and after forming the test piece into a predetermined shape,
It is designed to be exposed to a temperature range of 300'C to 300'C.

The reasons for limiting the numerical values of the present invention are as follows.

If the molar ratio of Y2O,/ZrO□ is less than 2/98, tetragonal zirconia porcelain cannot be obtained, and if it exceeds 7/93, almost no tetragonal crystals are contained, resulting in cubic zirconia porcelain. Further, if the ratio is outside the range of 2/98 to 4/96, mainly tetragonal zirconia porcelain cannot be obtained.

In the present invention, the electrode includes a perovskite-type composite oxide electrode represented by a platinum group metal such as PL, Rh, or Pd, or La, or AXBO3 (A is an alkaline earth metal, and B is one or more transition metals); Alternatively, a cermet electrode made of the platinum group metal and a ceramic such as Zr0z or He1203 may be used, and the forming method may be a screen printing method, baking method, plating, or the like.

The shape of the zirconia porcelain used as the solid electrolyte may be any shape such as a cylinder, a bag tube, or a flat plate, but the cylindrical one is made by press-compression molding zirconia powder, and the flat one is made by pressing zirconia powder. A slurry obtained by kneading an organic binder and an organic solvent may be formed by a doctor blade method or the like.

When a pair of electrodes constituting the oxygen concentration battery of the present invention is exposed to a reducing gas, the electrodes should be covered with a porous ceramic layer to prevent the reducing gas from coming into direct contact with the electrodes. is desirable. The porous protective layer is made of zirconia,
It may be made of ceramic such as alumina or spinel, and may be formed by plasma spraying, or by forming on a flat substrate by screen printing, doctor blade, etc., and then baking.

Note that the solid electrolyte of the oxygen concentration battery of the present invention is composed of mainly tetragonal crystal particles, or mainly cubic crystal particles and tetragonal crystal particles, and has an average crystal particle diameter of not more than a specific value. The production of zirconia porcelain with excellent durability in C can be easily achieved by selecting the composition, raw materials used, raw material particle size, firing conditions, cooling conditions, etc.

In the oxygen concentration battery of the present invention using zirconia porcelain mainly composed of tetragonal crystal grains and zirconia porcelain mainly composed of cubic crystal grains and tetragonal crystal grains, the ion transfer number of the solid electrolyte is approximately 1, which is the theoretical value. Because it can generate a normal electromotive force, it is used as an oxygen sensor, and because it is oxygen ion conductive, it is used as an oxygen pump or solid electrolyte fuel cell. Next, an example will be described.

Example 1 First, the properties of the solid electrolyte constituting the oxygen concentration battery as zirconia porcelain were compared.

ZrO□, YzOz or a compound thereof was prepared and mixed in a ball mill so as to have the composition shown in Table 1. The mixture was calcined at 800'C, wet-pulverized in a ball mill,
After drying, the powder was press-molded and fired at 1000°C to 1400°C for 1 to 3 hours to obtain zirconia porcelain used in the oxygen concentration battery of the present invention. The average crystal grain size, X-ray diffraction line intensity, bending strength, and volume resistivity of these porcelains were compared and measured. The X-ray diffraction line intensity ratio is the (200) plane of the cubic crystal and the (200) plane of the tetragonal crystal.
) plane and the (111) plane of the monoclinic crystal. The bending strength of porcelain is 3.5 x 3.5
It was finished into a bar shape of 50 mm and was determined by a three-point bending method. The volume resistivity was measured in the atmosphere at 400°C by a four-probe method.

In Table 1, 200°C to 300'C durability refers to a durability test in which heating and cooling were repeated between 200°C and 300°C at a rate of temperature rise and fall of 10°C/min. Table 1 shows the measurement results for various compositions. Table 1 shows 200
The X-ray diffraction line intensity ratio after the durability test at °C to 300 °C is also described. Further, in Table 1, the column rB/to The ratio of the initial value to the value after the durability test of the crystal (11r1 plane/tetragonal (200) plane), that is, the degree of phase transformation from tetragonal to monoclinic in the durability test, and in other words, the ratio of the initial value to the value after the durability test of the tetragonal (200) plane It means a reduction rate, and the closer it is to 1, the more stable the square product is.Table 1 also lists examples outside the numerical limit range of the present invention as reference examples.

FIG. 3 shows the C/D value with respect to the average crystal grain size for the examples listed in Table 1, and FIG. 4 similarly shows the B/A X100O value with respect to the average crystal grain size. Third
The numbers attached to each point in the figures and FIG. 4 indicate Nα of the example.

As is clear from Table 1 and Figures 3 and 4, the zirconia porcelain of the present invention has high strength and
Even if it is left in a specific temperature range of 0°C, there is almost no change in crystal phase or bending strength.

Furthermore, it has been found that in order to be stable in a specific temperature range, the average crystal grain size of the porcelain must be 2 microns or less, preferably 1 micron or less.

Furthermore, it was confirmed that the volume resistivity was also low.

Example 2 The zirconia powder prepared in Table 1 No. 16 was press-molded and fired at 1400°C for 3 hours to form the solid electrolyte tube 2 shown in FIG. A reference electrode 1 and a measuring electrode 3 were provided on the outer surface by a PT plating method, and a porous protective layer 4 was further provided on the outside of the measuring electrode 3 by a plasma spraying method to obtain 10 oxygen sensor elements 20.

While maintaining the temperature of the oxygen sensor element 20 at 600K, the inner atmosphere 10 is air and the outer atmosphere 11 is Hz: 10%.
, HJ: 1% residual N, and when the electromotive force generated between the reference electrode 1 and the measurement electrode 3 was measured, all 10 sensors were 1.14±0.02V, which was approximately It generated an electromotive force as expected in theory.

Further, when the atmosphere of the measurement electrode 3 was suddenly changed from the H2 atmosphere to the air atmosphere, the electromotive force became almost O (zero) in about 2 seconds.

This shows that the oxygen concentration cell of the present invention can be used as an oxygen sensor.

As described above, the oxygen concentration battery of the present invention uses zirconia porcelain as a solid electrolyte, and the zirconia porcelain has mainly tetragonal crystal particles or mainly tetragonal crystal particles and It is made of cubic crystal grains and the crystal grain size is below a certain value, so it has extremely high strength and 200%
Deterioration over time in a specific temperature range of °C to 300'C is extremely low, so it can be used in automotive oxygen sensors that require high strength and heat resistance, oxygen meters for steel, fuel cells for power generation, etc. It is extremely useful in industry.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the relationship between the average crystal particle diameter and bending strength of zirconia porcelain as a solid electrolyte constituting an oxygen concentration battery before and after a durability test, and Figure 2 is a diagram showing the relationship between the cubic (200) plane and the tetragonal plane. The relationship between the intensity ratio of X-ray diffraction lines of the (200) plane and the refraction intensity, and the relationship between the cubic crystal (200) plane and the monoclinic crystal (11〒)
FIG. 2 is an explanatory diagram showing the relationship between the intensity ratio of X-ray diffraction lines of a surface and the bending strength before and after deterioration over time. Figure 3 shows the initial value of the X-ray diffraction line intensity ratio (C
) and the value after the durability test (D) (C/I)) and the average crystal particle size. Transverse bending strength of zirconia porcelain (A) and bending strength after durability test (B)
FIG. 5 is a sectional view of the tip of an oxygen sensor which is one of the oxygen concentration batteries of the present invention.

Claims (1)

[Claims] 1. Mainly composed of ZrO_2 and Y_2O_3, Y_
A solid electrolyte made of zirconia porcelain in which the molar ratio of 2O_3/ZrO_2 is in the range of 2/98 to 4/96, the crystal grains are mainly tetragonal crystal grains, and the average crystal grain size is 2 μ or less, and the solid electrolyte and at least one pair of electrodes provided in contact with the oxygen concentration battery. 2. Mainly composed of ZrO_2 and Y_2O_3, Y_
From zirconia porcelain in which the molar ratio of 2O_3/ZrO_2 is in the range of 2/98 to 7/93, the crystal grains are mainly composed of cubic crystal grains and tetragonal crystal grains, and the average crystal grain size is 2 μ or less An oxygen concentration battery comprising a solid electrolyte and at least one pair of electrodes provided in contact with the solid electrolyte.