JP2788640B2 - Gas concentration detection sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a gas for measuring an oxygen concentration mainly in a gas to be measured by providing an electrode on the surface of an oxygen ion conductive solid electrolyte and limiting diffusion. The sensor for concentration detection (hereinafter referred to as “oxygen sensor”) is particularly effective when used for measuring humidity based on a limit current value that changes in accordance with a change in humidity.

[Prior art] In order to measure the oxygen concentration in a gas, a pair of thick-film or thin-film electrodes 31 and 32 made of porous material such as platinum are provided on the surface of an oxygen ion-conductive solid electrolyte 30, As an oxygen sensor for applying a voltage between the electrodes and detecting the oxygen concentration based on the current value, as shown in FIG. 9, a box 34 having an oxygen diffusion hole 33 having a small diameter serves as a cathode. One electrode
32, and a void 35 formed inside a box 34, and, as shown in FIG. 10, in place of the box 34 having the oxygen diffusion holes 33, a box made of a porous body One electrode 32 at 36
There is a case in which the box 36 itself is covered with an oxygen diffusion restricting body.

Also, as a humidity measurement method using an oxygen sensor,
When a voltage is applied to the oxygen sensor, as shown in FIG. 5, a diffusion limiting current value I corresponding to the oxygen concentration in the gas to be measured is obtained.
By increasing the applied voltage further than the voltage at which _L1 is obtained, the diffusion of water vapor in the gas to be measured is again restricted by diffusion, and a diffusion-limited current value _IL2 according to the moisture concentration is obtained. There has been proposed a method of obtaining humidity based on the limited current values I _L1 and I _L2 (Japanese Patent Laid-Open No. Sho 62-1501).
No. 51, JP-A-62-150152).

[Problems to be Solved by the Invention] However, when mass-producing these oxygen sensors, in the former case, the diameter of the oxygen diffusion holes 33 provided in the case 34 is set, and in the latter case, the case as the oxygen diffusion control body is used. Since it is difficult to manufacture the body 36 itself with a certain degree of accuracy, variation occurs as an oxygen diffusion controller, and the diffusion limiting amount cannot be made constant. Therefore, the current value when oxygen is diffusion-limited cannot be made constant.

Further, in these devices, since diffusion control is performed by oxygen diffusion holes or porous boxes, the flow rate of oxygen diffused is large, and the current value is accordingly large. Further, when the voltage is increased to obtain the diffusion limiting current value _IL2 accompanying the decomposition of water vapor, the current value further increases. For this reason, the load on the electrodes due to the current increases, and the temperature of the sensor element must be increased due to the large current, so that the sensor element deteriorates quickly.

The present invention provides an oxygen sensor in which a pair of electrodes are provided on the surface of a solid electrolyte. The purpose is to provide.

[Means for Solving the Problems] In the present invention, a pair of membrane-like electrodes made of a porous body are provided in close contact with the surface of a solid electrolyte having oxygen ion conductivity, and Gas diffusion limiting means for limiting diffusion of oxygen in the gas to be measured is provided on the negative electrode, a voltage is applied between the pair of electrodes, and a diffusion limited current value accompanying the diffusion limitation by the gas diffusion limiting means is provided. In the gas concentration detection sensor for detecting the oxygen concentration in the gas to be measured, a connection portion for applying the voltage and connected to the negative electrode for energization is integrated with the negative electrode by the porous body. A gas diffusion preventing member for forming a film on the surface of the solid electrolyte and preventing gas diffusion to the cathode while the cathode and the connection are exposed only in part of the connection; By The technical means is that the connecting portion is configured as the gas diffusion restricting means for restricting diffusion of oxygen to the negative electrode by covering.

Further, the pair of electrodes may be provided on the same surface of the solid electrolyte at an interval.

Alternatively, the pair of electrodes may be provided on opposing surfaces of the solid electrolyte.

[Operation] In the gas concentration detection sensor of the present invention, when a voltage is applied between the electrodes, oxygen in the gas to be measured is ionized and pumped in the solid electrolyte.

At this time, in the negative electrode covered by the gas diffusion preventing member, the connecting portion for conducting electricity is provided integrally in a film shape by a porous body, and a part thereof is exposed from the gas diffusion preventing member to restrict gas diffusion. Since it is a means, the gas containing oxygen passes through the connection part for energization at the boundary between the part covered with the gas diffusion preventing member and the exposed part, and diffuses.

At this time, the amount of gas diffused to the negative electrode inside the gas diffusion preventing member is a cross-sectional area determined by the length of the current-carrying connection portion covered by the gas diffusion preventing member and the thickness and width of the boundary portion. Accordingly, the amount of gas diffused inside the gas diffusion preventing member is reduced because the connection part for energization is in the form of a film.

Further, the respective electrodes can be provided on the same surface of the solid electrolyte at intervals, or can be provided separately on opposing surfaces of the solid electrolyte, as necessary.

[Effect of the Invention] In the present invention, the amount of gas diffused to the inside of the gas diffusion preventing member is determined by the length of the current-carrying connection portion covered by the gas diffusion preventing member and the thickness and width of the boundary portion, Since the negative electrode and the connecting portion are in the form of a porous film provided by a platinum paste or the like,
The length of the portion covered with the gas diffusion preventing member and the width and thickness of the boundary can be easily made constant. Therefore, the reproducibility is good and there is little variation even when mass-produced.

Further, the gas is diffused only from the connection part for energization, and since the connection part for energization is in the form of a film, the amount of gas diffused can be reduced. Therefore, since the load current can be reduced, the deterioration of the interface between the solid electrolyte and each electrode is reduced. Further, the amount of diffusion gas when diffusion is limited can be reduced, so that the diffusion limiting current value is reduced. For this reason, even if the applied voltage is increased until the diffusion limiting current value _IL2 when the diffusion limiting is again performed by the decomposition of water vapor in the gas to be measured to obtain the humidity, the diffusion limiting current value _IL2 Is not as many as in the past.

Therefore, the load on each electrode is greatly reduced as compared with the conventional one, and the diffusion limiting current value is small, so that the temperature of the solid electrolyte can be lowered and the oxygen ion conductivity can be lowered, and the solid electrolyte can be used. Blacking (tissue destruction) due to thermal deterioration and increase in current hardly occurs, and a long-life oxygen concentration detection sensor can be provided.

In addition, since each electrode and each connection portion for energization can be provided integrally, and other energization members are not required, the manufacturing process can be simplified.

In the second invention, the electrodes are provided by printing on the same surface of the solid electrolyte, and the provision of the gas diffusion preventing member becomes simple. Therefore, the manufacturing process is greatly simplified.

According to the third aspect, the solid electrolyte only needs to have a size sufficient to provide each electrode, so that the size can be reduced as compared with the second aspect.

[Examples] Next, the present invention will be described based on examples.

FIG. 2 shows a first embodiment of an oxygen sensor 1 as a gas concentration detection sensor according to the present invention.

The oxygen sensor 1 includes a sensor element 10 and a ceramic heater 20.
Consists of

The sensor element 10 includes an oxygen ion conductive plate 11, a positive electrode 12, a negative electrode 13, an alumina porous layer 14, and a glaze layer 15.

The oxygen ion conductive plate 11 is a plate made of stabilized zirconia as a solid electrolyte in which zirconium oxide is added with yttrium oxide as a stabilizer to form a solid solution. In this embodiment,
The oxygen ion conductive plate 11 is 5 × 7 mm square and 0.3 mm thick.

On one surface of the oxygen ion conductive plate 11, a positive electrode 12 and a negative electrode 13 are formed at intervals. Each electrode 12, 13
Platinum paste is printed on the oxygen ion conductive plate 11 and is a porous platinum electrode fired at 1500 ° C. simultaneously with the oxygen ion conductive plate 11, and the positive electrode 12 and the negative electrode 13 have electrode portions 12a and 13a, respectively. It consists of connection parts 12b and 13b for energization.

On the oxygen ion conductive plate 11 on the cathode 13 side, an alumina porous layer formed by applying a paste of alumina powder mixed with glass is applied.
14 is provided so as to cover only a part of the electrode portion 13a and the connection portion 13b of the negative electrode 13, and further, the alumina porous layer 14 prevents the gas to be measured from touching the electrode portion 13a of the negative electrode 13. The glaze layer 15 coated with glass for the electrode part 13
The alumina porous layer 14 and the glaze layer 15 are baked on the oxygen ion conductive plate 11 at 850 ° C. to 900 ° C.

Therefore, as shown in FIG.
Is separated from the gas to be measured, and the connecting portion 13b of the negative electrode 13 is exposed from the glaze layer 15.Therefore, at the connecting portion 13b between the end 15a of the glaze layer 15 and the oxygen ion conductive plate 11, each electrode 1
When a voltage is applied to 2 and 13, the gas diffusion controller for controlling the oxygen diffusion amount and the water vapor diffusion amount is also used.

Here, each of the electrodes 12 and 13 has a thickness t of 20 μm, and each of the electrode portions 12a and 13a is a square having a side of 2.5 mm.

In the connection portion 13b, as shown in FIG.
mm, and the length L covered by the glaze layer 15 was 2 mm.

Here, assuming that the area of the electrode portions 12a and 13a is S and the cross-sectional area given by the product of the width W and the thickness t of the connection portion 13b is s, the diffusion amount of oxygen to the electrode portion 13a is It is proportional to the length L and inversely proportional to the length L.

Based on these values, the range of the ratio R between the area S of the electrode portion 13a of the negative electrode 13 and the connection portion 13b as a gas diffusion electrode, which is particularly effective for practical use to obtain the limit current value, is obtained as R = s / L / S = 1 × 10 ⁻⁵ to 8 × 10 ⁻² . In this embodiment, since s = 0.02, L = 2, and S = 6.25, the value of the ratio R is R = 1.6 × It was 10 ^-3 .

The sensor element 10 is mounted on the ceramic heater 20 by applying glass at about 800 ° C.

As shown in FIG. 4, the ceramic heater 20 is formed by printing a metal paste made of tungsten (W) on the surface of a green sheet 20A of 96% alumina (Al ₂ O ₃ ) so as to form a heater pattern 20a. Green sheet 20
B is a plate-shaped heater which is baked by coating, and both ends of the heater pattern 20a in the ceramic heater 20 are connected to the conductor pattern 20a.
b, 20c, the electrode 2 on the surface 20d of the ceramic heater 20
1 and 22 are connected respectively.

Here, since the gas diffusion is restricted by the connection portion 13b of the porous negative electrode 13, the heater pattern 20a of the ceramic heater 20 locally heats only the electrode portions 12a and 13a of the electrodes 12, 13. In addition, pumping by the connection portion 13b is prevented.

A vent 23 is formed at the center of the ceramic heater 20 in order to improve the heating efficiency of the sensor element 10, and through holes 24, 25, 26 are provided in a plurality of rows.

Further, on the surface 20d of the ceramic heater 20, sensor electrodes 27 and 28 connected to the connection portions 12b and 13b by a printed pattern of ruthenium oxide are provided for energizing the electrodes 12 and 13 of the sensor element 10. Have been. The sensor electrodes 27 and 28 are printed with a paste for pattern formation,
When the sensor element 10 is mounted by baking, the baking is performed at the same time.

The oxygen sensor 1 according to the present embodiment having the above-described configuration has the third configuration.
As shown in the figure, the sensor is used as a sensor section of a humidity measuring device A to which a voltage is applied from a variable voltage power supply E between the sensor electrodes 27 and 28, and an applied voltage and a current value are measured. At this time, the ceramic heater 20 is energized,
The sensor element 10 is maintained at 300C to 700C.

 Hereinafter, the operation of the oxygen sensor 1 will be described.

When the oxygen sensor 1 is disposed in the gas to be measured and a voltage is applied between the positive electrode 12 and the negative electrode 13, oxygen in the electrode portion 13a covered with the glaze layer 15 is ionized to become oxygen ions. Pumped to the positive electrode 12 according to the voltage.

Then, the oxygen in the gas to be measured flows into the connection portion 13b of the negative electrode 13.
From the electrode portion 13a covered with the glaze layer 15.

When the applied voltage is increased, the current value increases according to the applied voltage. At this time, the current value between the electrodes 12 and 13 with respect to the applied voltage changes according to the applied voltage as shown in FIG.

The amount of oxygen diffusion into the electrode portion 13a is limited by the connection portion 13b of the negative electrode 13 and is limited according to the oxygen concentration in the gas to be measured, so that when the amount of diffusion is limited, the current value is also limited accordingly. Thus, a diffusion limiting current value _IL1 is shown.

When the applied voltage becomes higher than the voltage value at which the diffusion limiting current value _IL1 is obtained, oxygen ions accompanying the decomposition of moisture (water vapor) in the gas to be measured are pumped to the positive electrode 12, and at this time, the moisture is also From the connecting portion 13b into the electrode portion 13a, and a current value according to the diffusion amount flows.

When the applied voltage is increased, the current value further increases in accordance with the moisture concentration, and when the amount of diffusion is limited at the connection portion 13b of the negative electrode 13, the current value is accordingly limited, and the current value is limited.
Indicates I _L2 .

As described above, in the present embodiment, oxygen and water vapor diffuse from the connection portion 13b of the porous negative electrode 13 into the electrode portion 13a,
Since the connection portion 13b has a film shape in which the diffusion amount can be easily adjusted by its width and length, the sensor element 10 is formed with good reproducibility, and the diffusion limit amount can be kept constant.

Further, since the negative electrode 13 is in the form of a film, the amount of diffusion can be reduced, and as a result, the current value can be reduced as a whole. Therefore, the load on each electrode can be reduced, and the heating temperature of the heater for assisting operation can be lowered.

Furthermore, even when the applied voltage is high, as in the case of using for humidity measurement as described above, the load on each electrode is far less than in the past.

As a result, a long-life oxygen sensor having improved durability and usable for a long time can be obtained.

In the negative electrode, since the electrode portion and the connection portion are provided integrally, there is no need to provide another conductor for energization.

Although the humidity measuring device is described in the first embodiment, the same operation is performed when measuring the oxygen concentration.

In the first embodiment, the alumina porous layer 14 is provided. However, when the width of the connecting portion 13b is about 1 mm, the inter-electrode current is set to be small and there is no significant change in characteristics.
There is no particular need to provide them.

 FIG. 6 shows a second embodiment of the present invention.

Here, without covering only the negative electrode 13, a cover plate 16 having substantially the same width as the oxygen ion conductive plate 11 is adhered to cover the two electrode portions 12 and 13, and corresponds to the electrode portion 12 a of the positive electrode 12. A circular hole 16a is formed in a portion where the electrode portion 12a comes into contact with the gas to be measured. Therefore, similarly to the first embodiment, the connection portion 13b between the end portion 16b of the cover plate 16 and the oxygen ion conductive plate 11 also serves as a diffusion controller for limiting oxygen diffusion.

Further, since there is no large difference between the width of the cover plate 16 and the width of the oxygen ion conductive plate 11, the positioning operation when covering with the cover plate 16 is simplified as compared with the case where only the negative electrode 13 is covered.

FIG. 6 shows a circular hole 16a, but the shape of the hole 16a is not limited to a circle. As a modified example, the electrode portion 13a of the negative electrode 13 does not contact the gas to be measured, and the electrode portion 12a of the positive electrode 12 does not. It may be square or any other shape as long as only the gas comes into contact with the gas to be measured.

 FIG. 7 shows a third embodiment of the present invention.

Here, instead of the glaze layer 15 in the first embodiment, an electrode portion of the negative electrode 13 is provided by a box 17 for preventing oxygen diffusion.
A void 17a is formed so as to cover 13a.

 FIG. 8 shows a fourth embodiment of the present invention.

In each of the above embodiments, both the positive electrode 12 and the negative electrode 13 are provided on one surface of the oxygen ion conductive plate 11, but in the fourth embodiment, the positive electrode 12 and the negative electrode 12 are provided on both surfaces of the oxygen ion conductive plate 11, respectively. The electrode 13 is provided, and the positive electrode 12 side is the surface of the ceramic heater 20.
It is baked with glass 18 on 20d.

In this embodiment, since s = 0.02, L = 2, and S = 25, the value of the ratio R was R = 4 × 10 ⁻⁴ .

[Brief description of the drawings]

1 to 3 show a first embodiment of the present invention, FIG. 1 is a sectional view of an oxygen sensor, FIG. 2 is a perspective view of the oxygen sensor,
FIG. 3 is a schematic diagram of a humidity measuring device, FIG. 4 is a perspective view showing a configuration of the ceramic heater of the present embodiment, FIG. 5 is a voltage-current characteristic diagram showing an example of measurement by the humidity measuring device, and FIG. Is a perspective view of an oxygen sensor showing a second embodiment of the present invention, and FIG.
FIG. 8 is a sectional view of an oxygen sensor showing a third embodiment of the present invention, FIG. 8 is a sectional view of an oxygen sensor showing a fourth embodiment of the present invention,
9 and 10 are cross-sectional views showing a conventional oxygen sensor. In the figure, 1 ... Oxygen sensor (sensor for detecting gas concentration), 11
... Oxygen ion conductive plate (solid electrolyte), 12a, 13a ... Electrode part (pair of electrodes), 13b ... Connection part (electrode for energization), 1
5 Glaze layer (gas diffusion preventing member).

Claims (3)

(57) [Claims] 1. A pair of membrane-like electrodes made of a porous body are provided in close contact with the surface of a solid electrolyte having oxygen ion conductivity, and a negative electrode of the gas to be measured is attached to a negative electrode of the pair of electrodes. A gas diffusion limiting means for limiting oxygen diffusion is provided, a voltage is applied between the pair of electrodes, and oxygen in the gas to be measured is determined based on a diffusion limiting current value accompanying the diffusion limitation by the gas diffusion limiting means. In the gas concentration detection sensor for detecting a concentration, a connecting portion connected to the negative electrode for applying the voltage for applying the voltage is provided on the surface of the solid electrolyte integrally with the negative electrode by the porous body. By forming the negative electrode and the connecting portion with a gas diffusion preventing member for preventing gas diffusion to the negative electrode in a state where only a part of the connecting portion is exposed, A gas concentration detecting sensor, wherein a connecting portion is configured as the gas diffusion restricting means for restricting diffusion of oxygen to the negative electrode. 2. The apparatus according to claim 1, wherein said pair of electrodes are provided on the same surface of said solid electrolyte at an interval.
The sensor for detecting gas concentration according to any one of the preceding claims. 3. The solid electrolyte according to claim 1, wherein said pair of electrodes are provided on opposing surfaces of said solid electrolyte.
The sensor for detecting gas concentration according to any one of the preceding claims.