JPH07198674A - Gas sensor - Google Patents

Abstract

PURPOSE:To provide a gas sensor with little change with the lapse of hours in zirconia solid electrolyte even if it is used for long hours and excellent in durability. CONSTITUTION:A gas sensor uses zirconia solid electrolyte containing Y2O3 as stabilizer. The molar ratio Y2O3/ZrO2 of ZrO2 and Y2O3 which constitute the zirconia solid electrolyte is within the range of 7/93 to 12/88, and the zirconia solid electrolyte has a crystal particle structure of less than 0.05 in peak strength ratio T (200)/[C (200)+T (200)] when the X-ray diffraction ray peak strength of cubic system (200) phase is C (200) and the X-ray diffraction ray peak strength of tetragonal system (200) phase is T (200). The zirconia solid electrolyte may contain at least one of Y2O3 and CaO and MgO as stabilizer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor using a zirconia solid electrolyte, and more particularly to a gas sensor having little change over time in the zirconia solid electrolyte even after long-term use and having excellent durability. .

[0002]

2. Description of the Related Art Conventionally, a gas sensor using a zirconia solid electrolyte has various gas concentrations (for example, O ₂ concentration,
It is used to measure the amount of water vapor). When the mechanical strength of the gas sensor is emphasized, it is widely practiced to use a partially stabilized zirconia solid electrolyte composed of two or more kinds of crystal particles such as cubic crystal, tetragonal crystal and monoclinic crystal. ing.

[0003]

However, when the above partially stabilized zirconia solid electrolyte is used for various gas sensors and is used in a temperature range of 300 ° C. to 700 ° C. for a long time, the solid electrolyte gradually changes with time. However, there is a problem in that the characteristics, accuracy, etc. of the sensor start to deteriorate.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a gas sensor which has little change over time in a zirconia solid electrolyte even when it is used for a long time and which is excellent in durability.

[0005]

The gas sensor according to the first aspect of the present invention is a gas sensor using a zirconia solid electrolyte containing Y ₂ O ₃ as a stabilizer (hereinafter referred to as “solid electrolyte”). Z that constitutes the zirconia solid electrolyte
The molar ratio of rO ₂ and Y ₂ O ₃ Y ₂ O ₃ / ZrO ₂ is 7/9.
3 to 12/88, and the zirconia solid electrolyte has a C (200) X-ray diffraction line peak intensity of the cubic (200) plane, and a tetragonal (200) plane X-ray diffraction line peak. Peak intensity ratio T when intensity is T (200)
(200) / [C (200) + T (200)] is 0.0
It is characterized by having a crystal grain composition of less than 5.

As shown in the second aspect of the present invention, the "solid electrolyte" may contain Y ₂ O ₃ and at least one of CaO and MgO as a stabilizer.
As described above, the molar ratio of Y ₂ O ₃ and ZrO ₂ (Y ₂ O ₃ /
The ZrO ₂ ) and the peak intensity ratio are determined in order to improve the stability of the solid electrolyte over a long period of time by making the solid electrolyte fully stabilized or substantially fully stabilized. That is, the above molar ratio Y ₂ O ₃ / ZrO ₂ is 7/9.
When it is less than 3 (when the solid electrolyte is in the partially stabilized region), the change with time of the solid electrolyte is large. On the other hand, 1
When it exceeds 2/88, the durability is improved as compared with the case where it is in the above-mentioned partially stabilized region, but it cannot be said to be sufficient. Further, when the above peak intensity ratio is 0.05 or more, the proportion of tetragonal crystals in the crystal particles constituting the solid electrolyte increases, and the change with time of the solid electrolyte increases. In addition, in the present invention, the measurement of each peak intensity is a value when the (200) plane such as a cubic crystal is used, and when the other plane is used, the peak intensity may have different values. . In the present invention, the (200) plane is used because the X-ray diffraction peaks of cubic crystal and tetragonal crystal can be clearly separated and distinguished.

[0007]

In the gas sensor of the present invention, ZrO ₂ and Y ₂ O _{3 are used.}
A solid electrolyte is used which is completely stabilized or substantially completely stabilized by controlling the molar ratio of X and the peak intensity ratio of X-ray diffraction lines. As a result, most of the crystal particles that make up the solid electrolyte are composed of cubic crystals, and there are no particles that have a different crystal structure (tetragonal crystal), or there are very few particles, so stability at the crystal particle interface is improved. , Leading to improvement in stability of the solid electrolyte over time. Further, by using the solid electrolyte of the above composition, the starting voltage of the second limiting current (steam decomposition starting voltage, etc.) is lowered, and when measuring gas such as steam, the applied voltage can be reduced, and as a result, The load on the sensor is also reduced and the durability is improved.

[0008]

EXAMPLES The present invention will be specifically described below with reference to examples. (1) Overview of gas sensor Configuration and manufacturing method of gas sensor (sensor body) Sensor body A of limiting current type gas sensor used in this example
Are shown in FIGS. The sensor body A includes a sensor chip portion 1 and a ceramic heater portion 2 as shown in FIG. The sensor chip portion 1 is a stabilized zirconia plate (5 mm × 5 mm) made of a solid electrolyte with good oxygen ion conductivity in which a predetermined amount of Y ₂ O ₃ as a stabilizer is added to ZrO ₂ to form a solid solution.
7 mm × 0.3 mm) 11, a negative electrode (thickness: 20 μm) 12 and a positive electrode (thickness: 20 μm) 13 that are closely arranged on the surface thereof, and a gas that covers most of the negative electrode 12. It is configured by the supply limiting means 14.

These electrodes 12 and 13 are both porous platinum electrodes, and have a detecting portion (2.5 mm × 2.5 m).
m) 121, 131 and connecting part (width: 1 mm, length: 2 m
m) 122 and 132. In addition, the gas supply limiting means 14 includes the detection unit 12 of the negative electrode 12.
1 and the alumina porous layer 141 that covers the portion of the connecting portion 122 near the detection portion 121 (hereinafter referred to as the “root portion”) 122a, and the alumina porous layer 141 so that the measured gas does not directly enter the detection portion 121. The glaze layer 142 covering the quality layer 141 and the connecting portion 12 not covered with the glaze layer 142.
2 other than the root portion 122a (hereinafter, "exposed portion")
Say. ) 122b and.

On the other hand, the ceramic heater portion 2 is formed by burying a heater pattern 21 mainly made of tungsten in a ceramic mainly made of alumina, and one corner portion (above-mentioned above) on the surface of the ceramic heater portion 2 is formed. A pair of exposed portions 231 and 232 for energization are provided in a corner portion where the sensor chip portion is not arranged). Then, a structure is adopted in which the exposed parts 231 and 232 for energization are energized to locally heat the detection parts 121 and 131 to 300 to 700 ° C. Further, a ventilation hole 22 for increasing the heating efficiency of the stabilized zirconia plate 11 is provided in the substantially central portion of the ceramic heater portion 2.

In the present embodiment, the above sensor body A was manufactured as follows. First, a platinum paste for forming the negative electrode 12 and the positive electrode 13 is applied on the surface of a ZrO ₂ —Y ₂ O ₃ green sheet for forming the stabilized zirconia plate 11 by a printing method, After forming the coating film for the cathode and the coating film for the anode, the coating film and the above ZrO ₂ —Y ₂ O ₃ green sheet are formed at 1450 ° C.
Simultaneous firing was performed at a temperature of 1500 ° C. Next, the alumina porous layer 14 is formed on the portion corresponding to the detection portion 121 and the root portion 122a of the negative electrode 12 of the fired product.
In order to form No. 1, a paste prepared by mixing alumina powder with glass powder was applied to form a porous layer coating film. Furthermore,
The glass powder for forming the glaze layer 142 is applied onto the porous layer coating film to form the glaze layer coating film, followed by firing at a temperature of 850 to 900 ° C. to obtain the sensor chip. Part 1 was prepared.

On the other hand, a grate sheet as shown in FIG. 3 (96 parts by weight of A when the total solid content is 100 parts by weight)
It is prepared from 1 ₂ O ₃ powder and other parts of a sintering aid powder such as SiO ₂ , CaO, MgO and the like, which is hereinafter referred to as a “lower layer side gleat sheet”. ) After forming a hollow portion h ₁ to be the ventilation hole 22 after firing in the approximate center of g ₁ , a tungsten paste is appropriately applied to the periphery thereof by a printing method to form a heater pattern coating for the heater pattern 21. A film was formed. Both ends 211 and 212 of the heater pattern 21 (coating for heater pattern) are opposed to each other on one end side (the side where the sensor chip portion is not arranged) of the surface of the green sheet g ₁ . Furthermore, these ends 2
Reference numerals 11 and 212 denote the pair of exposed portions 231, 23 for energization.
2 are respectively connected.

Further, a grate sheet (hereinafter, referred to as "upper layer side glee sheet") g ₂ which is prepared in the same manner as the lower layer side green sheet g ₁ and has a similar hollow portion h ₂ is used as the lower layer side green sheet g _1. Laminate and fire from above
A ceramic heater unit 2 as shown in FIG.
It was created. Next, the ceramic heater portion 2 is opposed to the air vent 22 provided in the substantially central portion, and
In order to form a pair of sensor electrodes 24 and 25 extending in the longitudinal direction of the ceramic heater portion 2, a ruthenium oxide paste was applied by a printing method to form an electrode forming coating film.
Incidentally, these sensor electrodes 24 and 25 also have end portions 241 and 251 facing each other similarly to the heater pattern.

Further, on the other end side (on the side where the energizing exposed portion 231 and the like are not arranged) of the surface of the ceramic heater portion 2, the sensor chip portion 1 is heated to a temperature of about 850 ° C. by using a sealing glass 3. At the same time as the bonding, the sensor electrodes 24 and 25 are fired to manufacture the sensor body A according to this embodiment.

Operating Principle Next, the operating principle of the sensor body A (limit current type gas sensor) according to this embodiment will be described. First, the limiting current type gas sensor is placed in the gas to be measured, the ceramic heater portion 2 is energized, and the negative electrode 12 and the positive electrode 13 are further provided.
Apply voltage between them. As a result, the oxygen inside the detection part 121 of the negative electrode 12 is ionized into oxygen ions, and the oxygen in the gas to be measured changes according to the applied voltage V.
2 is pumped to the positive electrode 13. At this time, only the stabilized zirconia plate 11 under the detection part 121 is locally heated, but the stabilized zirconia plate 11 under the connection part 122 is heated.
Is not sufficiently heated to exhibit oxygen ion conductivity, so oxygen is not covered by the glaze layer 142.
From the exposed portion 122b of the above to the inside of the detection portion 121 covered with the glaze layer 142.

At this time, the output current I flowing between the negative electrode 12 and the positive electrode 13 changes as shown in FIG. That is, when the applied voltage V is the voltage value V _{1 to} V ₂ , the detection unit 121
The amount of oxygen diffused into the inside is controlled by the gas supply limiting means 14 and is limited according to the oxygen concentration in the gas to be measured.
Along with this, the current value is also limited to the diffusion limited current value I _L1 and reaches the first flat portion F ₁ . When the applied voltage becomes higher than the voltage value V _{2 at} which the diffusion limiting current value I _L1 is obtained, the water vapor (moisture) in the gas to be measured is decomposed and the oxygen ions generated by the decomposition are pumped to the positive electrode 13. The water vapor diffuses from the exposed portion 122b into the detection portion 121 covered with the glaze layer 142, and the current value increases according to the amount of diffusion.

Further, the applied voltage V is increased to set the voltage value to V
₃ output current value to the ~V ₄ I is further increased depending on the water vapor concentration, but limits the amount of diffusion of water vapor in the gas supply controlling unit 14, a current value along with it also limits the diffusion restriction in accordance with the water vapor concentration The current value becomes _IL2 , and the second flat portion F2
Leading to. In this case, V ₁ ~ between the negative electrode 12 and the positive electrode 13
By applying the voltage of V ₂ and measuring the diffusion limiting current value I _L1 , the oxygen concentration can be detected from its magnitude, and V _3-
By applying the voltage of V ₄ and measuring the diffusion limiting current value I _L2 ,
From the size, the amount of water vapor (water content) can be detected.

(2) Performance test and its evaluation Performance test 1 This performance test was conducted with ZrO ₂ and Y ₂ O constituting the solid electrolyte.
_The purpose of this study is to investigate the influence of the molar ratio of _{3 and} the composition of crystalline particles on the performance of the gas sensor over time. A limiting current type gas sensor based on the above manufacturing method and operating principle was manufactured using a stabilized zirconia solid electrolyte having the composition (and crystal particle composition) shown in Table 1.

[0019]

[Table 1]

In the table, "amount of ZrO ₂ " and "Y ₂
Unit of amount "of O ₃ are both" moles ", at the stage of adjusting the green sheet, as shown in the Table, the adjustment of the respective amount has been performed. The molar ratio in the table is the molar ratio of ZrO ₂ and Y ₂ O ₃ (Y ₂ O ₃ / Zr
O ₂ ) is shown. Furthermore, the "X-ray diffraction intensity ratio" in the table is
The X-ray diffraction line peak intensity of the cubic (200) plane is C (2
00), and the X-ray diffraction line peak intensity of the tetragonal (200) plane is represented by the following formula. Peak intensity ratio = T (200) / [C (200) + T (2
00)] In the column of "Crystal grain structure" in the table, "C" indicates cubic crystal, "T" indicates tetragonal crystal, and "C + T" indicates that cubic crystal and tetragonal crystal are mixed. Furthermore, "*" in the table indicates that the value is out of the range of the present invention.

Then, the above-mentioned gas sensors (Test Examples 1 to 1)
FIG. 5 shows the voltage-current characteristics of 5) when the temperature of the elements (detection sections 121 and 131) is kept at 500 ° C. by the ceramic heater section 2. According to this, even when the amount of Y ₂ O ₃ added increases and the tetragonal crystal phase decreases, the limiting current value and the electrolysis start voltage value of steam for O ₂ and H ₂ O in the first flat portion are reduced. (Theoretical value; close to about 1.1 V) does not change, but the rise to the second flat portion becomes fastest when the amount of Y ₂ O ₃ added is about 8 to 11 mol (Test Examples 2 to 4). When the amount of Y ₂ O ₃ added is further increased to 12 mol or more (Test Example 5), on the contrary, the rise to the second flat portion begins to be delayed.

As described above, when the addition amount of Y ₂ O ₃ is changed, the electrolysis start voltage of water vapor does not change, but the electron conduction start voltage is constant at about 2.5 V, so that the second flat portion is fast. Appearing, the voltage range of the flat part of the limiting current value with respect to H ₂ O is widened. Therefore, in these test examples, since the voltage range becomes wide under the same water vapor pressure, the measurable water vapor pressure range becomes wider, and it becomes possible to further reduce the driving voltage of the gas sensor, and the load on the sensor decreases. Therefore, the durability of the sensor can be improved.

Performance Test 2 Next, the same sensor as in Test Examples 1 to 5 was used, the element temperature was kept at 500 ° C., and each atmosphere (O ₂ ; 21
%, H ₂ O; 100 mmHg), and the change with time was investigated when it was operated for a long time. The result is shown in FIG. According to this, the amount of Y ₂ O ₃ added is about 5 mol (Test Example 1).
In the above, after 100 hours, O ₂ or H
_The limiting current value for ₂ O begins to decrease. Further, in the case of adding 15 moles (Test Example 5), the durability is slightly improved as compared with Test Example 1, but even after about 2000 hours, a similar tendency of decreasing the limiting current value appears. On the other hand, with respect to the sensors (Test Examples 2 to 4) in which the amount of Y ₂ O ₃ added was 7 to 12 mol, there was almost no change in the limiting current value with respect to O ₂ or H ₂ O even after 10,000 hours. However, good characteristics were exhibited.

Performance Test 3 Next, a change over time was investigated when the device was operated for a long time under a high water vapor pressure (760 mmHg) at which the sensor load was larger than in the case of the performance test 2 described above. The result is shown in FIG. 7. According to this, when the amount of Y ₂ O ₃ added is about 5 mol (Test Example 1) and a large amount of tetragonal crystals are present, the limiting current value for H ₂ O begins to decrease after several hours. Also,
Also for the one to which 15 mol was added (Test Example 5),
A similar tendency appears after 0 hours. Against these,
A sensor in which the amount of Y ₂ O ₃ added is 7 to 12 mol (Test Examples 2 to 2)
Regarding 4), although a slight decrease in the limiting current value with respect to H ₂ O was observed after 1000 hours, it exhibited fairly good characteristics, and was stable even under a high water vapor pressure and its change with time was small.

The present invention is not limited to the specific examples described above, and various modifications may be made within the scope of the present invention depending on the purpose and application. That is,
In the present invention, was used ZrO ₂ solid electrolyte containing Y ₂ O _3, instead of Y ₂ O _3, or CaO with Y ₂ O _3, it may be used ZrO ₂ solid electrolyte containing MgO . Further, in the present embodiment, the negative electrode 12 and the positive electrode 13 are arranged in parallel on one surface of the stabilized zirconia plate 11, but as shown in FIGS. 8 and 9, the stabilized zirconia plates 11a and 11b are provided. The negative electrodes 12a and 12b and the positive electrode 1
It may be sandwiched between 3a and 13b. Furthermore, instead of the gas supply limiting means 14 shown in FIGS.
The box bodies 51a and 51b as shown in FIG. 8 may be arranged. In this case, as shown in FIG. 8, a minute hole 6a for introducing the gas to be measured may be provided in a part of the box body 51a. , Same box 5
1b itself may be composed of a porous body.

[0026]

As described above, according to the present invention, even if the sensor element is heated to a high temperature of 300 to 700 ° C. for a long time and used, the sensor characteristics hardly change. Therefore, the gas sensor of the present invention can maintain high quality and accuracy for a long time, and thus has extremely high reliability.

[Brief description of drawings]

FIG. 1 is a perspective view showing a sensor body of an embodiment.

FIG. 2 is a partial vertical cross-sectional view of the sensor body of the embodiment.

FIG. 3 is an explanatory view showing a manufacturing process of the ceramic heater part of the embodiment.

FIG. 4 is a graph illustrating the operation of the gas sensor of the example.

FIG. 5 is a graph showing voltage-current characteristics of the gas sensor of the example.

FIG. 6 is a graph showing changes with time of the gas sensor of the example.

FIG. 7 is a graph showing changes with time of a gas sensor of an example under a high water vapor pressure.

FIG. 8 is a vertical cross-sectional view of a sensor body according to a modified example of this embodiment.

FIG. 9 is a vertical sectional view of a sensor body according to a modified example of this embodiment.

[Explanation of sign]

A: Sensor body, 1; Sansa tip part, 11; Stabilized zirconia plate, 12; Cathode electrode, 13; Positive electrode, 121, 1
31; detection part, 122, 132; connection part, 122a; root part, 122b; exposed part, 14; gas supply limiting means, 1
41; Alumina porous layer, 142; Glaze layer, 2; Ceramic heater part, 21; Heater pattern, 211, 2
12; end part, 22; vent hole, 231, 232; exposed part for energization, 24, 25; sensor electrode, 3; sealing glass, g
₁ ; lower layer side gree sheet, g ₂ ; upper layer side gree sheet, h ₁ , h ₂ ; cavity, 51a, 51b; box, 6
a: Micropores.

Claims (2)

[Claims] 1. A gas sensor using a zirconia solid electrolyte containing Y ₂ O ₃ as a stabilizer, which comprises ZrO ₂ and Y ₂ O ₃ constituting the zirconia solid electrolyte.
Has a molar ratio Y ₂ O ₃ / ZrO _{2 in} the range of 7/93 to 12/88, and the zirconia solid electrolyte has an X-ray diffraction peak intensity of a cubic (200) plane of C (200),
The X-ray diffraction peak intensity of the tetragonal (200) plane is T (2
00), the peak intensity ratio T (200) / [C
A gas sensor having a crystal grain composition in which (200) + T (200)] is less than 0.05. 2. A gas sensor using a zirconia solid electrolyte containing Y ₂ O ₃ and at least one of CaO and MgO as a stabilizer, which comprises ZrO ₂ and Y ₂ constituting the zirconia solid electrolyte. O ₃
Has a molar ratio Y ₂ O ₃ / ZrO _{2 in} the range of 7/93 to 12/88, and the zirconia solid electrolyte has an X-ray diffraction peak intensity of a cubic (200) plane of C (200),
The X-ray diffraction peak intensity of the tetragonal (200) plane is T (2
00), the peak intensity ratio T (200) / [C
A gas sensor having a crystal grain composition in which (200) + T (200)] is less than 0.05.