JPH06148129A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To prevent the deterioration of an ion conductor's mechanism strength and the occurrence of distortion or microcracks inside of the ion conductor, and decrease the fluctuation in the quantity of oxygen being introduced, as well as the variation in an ionic current characteristics. CONSTITUTION:In an oxygen sensor 31, electrodes 34 and 35 wherein a predetermined voltage is applied across respective surfaces 33a and 33b an ion conductor 33 are provide. An electrode protecting layer 36 is provided at the upper position of one electrode 35 of the ion conductor 33. A sealing layer 37 with airtightness is provided all over the surfaces of the external surface 36a of the electrode protecting layer 36, and an air gap part is provided between the electrode protecting layer 36 and the ion conductor 33. The electrode protecting layer 36 is made of a material whose thermal expansion coefficient is smaller than that of the ion conductor 33, and the sealing layer 37 is made of another material whose thermal expansion coefficient is approximate to that of the ion conductor 33.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiting current type oxygen sensor suitable for use in preventing oxygen deficiency accidents in basements and in combustion management of engines and boilers.

[0002]

2. Description of the Related Art Conventionally, a limiting current type oxygen sensor using stabilized zirconia as an ionic conductor has been proposed and put into practical use. This oxygen sensor uses zirconia (ZrO ₂ ) which is a type of ceramics, yttria (Y ₂ O ₃ ),
Stabilized zirconia in which several mol% of oxides such as magnesia (MgO) and calcia (CaO) are solid-solved is used as an ion conductor, and it is possible to detect a wide range of oxygen partial pressure, and the response speed is fast Since it has various features such as stable electromotive force and can be used in a high temperature atmosphere, it prevents oxygen deficiency accidents in closed rooms such as basements, measures oxygen concentration in molten steel, manages combustion in engines and boilers, and measures pollution. It is a sensor that is suitable for various purposes.

As an example of the oxygen sensor, a thick film type oxygen sensor as shown in FIG. 4 is known. The oxygen sensor 1 is stabilized zirconia (e.g., ZrO ₂ over 8
mol% Y ₂ O ₃ ) etc. and an ionic conductor 2 made of a thin solid electrolyte having ionic conductivity, and this ionic conductor 2
Porous platinum electrodes 3 and 4 respectively formed on both surfaces of each of the electrodes and to which a constant sensor monitoring voltage is applied, and bonded to the electrode 3 side of the ionic conductor 2 by crystallized glass 5 and 5, and
It is roughly composed of a ceramic cap 7 in which a gas diffusion hole 6 that penetrates vertically is formed in a central portion, and a heater 8 for heating formed on the upper surface of the ceramic cap 7. The ceramic cap 7 may be replaced with an inorganic porous material having extremely high oxygen permeability, such as porous alumina. In the oxygen sensor 1, if a predetermined voltage is applied to the heater 8 and a predetermined voltage is applied between the electrodes 3 and 4, the oxygen sensor 1 is taken into the ionic conductor 2 through the gas diffusion hole 6. Oxygen contained in the sample gas flows as ions in the ionic conductor 2 due to the oxygen pumping action, and the oxygen concentration in the sample gas is measured from the current value using the oxygen ions as carriers.

Further, like the oxygen sensor 11 shown in FIG.
Without forming the gas diffusion hole 6 in the ceramic cap 7, the gas diffusion hole 12 penetrating vertically in the center of the ionic conductor 2.
It is also known that formed. Also in the oxygen sensor 11 having this configuration, oxygen in the sample gas taken in through the gas diffusion holes 12 of the ionic conductor 2 flows as ions in the ionic conductor 2 due to the oxygen pumping action. The oxygen concentration in the sample gas is measured from the current value using as a carrier.

Further, like the oxygen sensor 21 shown in FIG.
There is also known a structure in which the entire upper portion of the electrode 3 of the ionic conductor 2 is covered with a ceramic cap 23 having an airtight glass film 22 formed on its inner surface. Also in the oxygen sensor 21 having this configuration, oxygen in the sample gas taken in from the gas diffusion holes 12 of the ionic conductor 2 flows as ions in the ionic conductor 2 due to the oxygen pumping action. The oxygen concentration in the sample gas is measured from the current value using as a carrier.

[0006]

By the way, the above-mentioned oxygen sensors 1 and 11 are formed on the side of the electrode 3 of the ionic conductor 2 which has been machined into a predetermined shape by a machining process, and also formed into a predetermined shape by a machining process. The processed ceramic cap 7 is joined by crystallized glass 5 and 5,
Further, since the oxygen sensor 21 is formed by joining a ceramic cap 23, which is similarly machined into a predetermined shape by a machining process, to the electrode 3 side of the ionic conductor 2, the machine of the ionic conductor is machined by these machining processes. However, there is a problem in that the mechanical strength of the ionic conductor is lowered and strains, microcracks, and the like are likely to occur inside the ionic conductor. Further, in order to reduce the size, the machining accuracy must be greatly improved, and the current machining process must be greatly improved. Further, since high precision is required, the machining process becomes a process that requires a great deal of skill, and it takes a long time to complete the process, resulting in a very high cost.

In particular, in the case of the oxygen sensor 21, since the glass film 22 having a uniform thickness must be formed on the inner surface of the ceramic cap 23, the nonuniformity of the thickness of the glass film 22 becomes large when the size is reduced. There is a problem that the variation of the ion current characteristics becomes large. Also,
In the actual manufacturing process of the oxygen sensor 21, there were various problems such as low yield, complicated work, and high manufacturing cost. Since these drawbacks are obstacles when introducing an automated line, it was difficult to mass-produce these oxygen sensors 1 to 21.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an oxygen sensor having excellent characteristics such as high reliability, mass production, easy manufacture, and low cost. .

[0009]

In order to solve the above problems, the present invention employs the following oxygen sensor. That is, an electrode to which a predetermined voltage is applied is provided on each surface of the ionic conductor, and oxygen in the sample gas taken into the ionic conductor through the gas diffusion holes is ionized in the ionic conductor by the oxygen pumping action. In the oxygen sensor in which the oxygen concentration in the sample gas is measured from the current value using the oxygen ions as carriers, the electrode is placed above the electrode located on one side of the ion conductor. An electrode protective layer is provided to cover the entire surface, an airtight sealing layer is provided on the entire outer surface of the electrode protective layer, and a gap is provided between the electrode protective layer and the ionic conductor. Is made of a material having a smaller expansion coefficient than the ionic conductor,
The sealing layer is made of a material having an expansion coefficient similar to that of the ionic conductor.

[0010]

In the oxygen sensor of the present invention, the sealing layer is made of a material having an expansion coefficient similar to that of the ionic conductor, whereby the sealing layer is firmly bonded to the ionic conductor. Also,
When the electrode protection layer is made of a material having a smaller expansion coefficient than the ionic conductor, the electrode protection layer is bonded to the sealing layer without being bonded to the ionic conductor. Therefore,
A slight space is generated between the electrode protection layer and the ionic conductor, and this space has a function as a void.

In this oxygen sensor, an electrode protective layer made of a material having a smaller expansion coefficient than that of the ionic conductor covers the electrodes and voids entirely, and the ionic conductor and the expansion coefficient are provided on the entire outer surface of the electrode protective layer. By providing an airtight sealing layer composed of an approximate material, the sealing layer reinforces the mechanical strength of the ionic conductor, and the mechanical strength of the ionic conductor is reduced and the ionic conductor is reduced. Prevents internal distortion and microcracks. Further, the electrode protective layer and the sealing layer satisfactorily separate the external space from the external environment while maintaining the void portion in a predetermined shape with high accuracy. Therefore,
Variations in ion current characteristics are reduced.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the oxygen sensor of the present invention will be described below with reference to the drawings. FIG. 1 is a front sectional view showing an oxygen sensor 31 of the present invention. The oxygen sensor 31 includes a stabilized zirconia (for example, ZrO ₂ -8 mol% Y ₂ O ₃ ) in which a gas diffusion hole 32 that vertically penetrates is formed in a central portion.
Each surface 33a, 33 of the thin ion conductor 33 composed of
b is provided with porous platinum electrodes 34 and 35, respectively, and an electrode protection layer 36 that entirely covers the electrodes 35 is provided above the electrodes 35, and the entire outer surface 36a of the electrode protection layer 36 is airtight. A sealing layer 37 having a property is provided,
A heater 38 for heating the ionic conductor 33 to a predetermined temperature is integrally provided on the upper surface 37a of the sealing layer 37, and a space 39 is provided between the electrode protection layer 35 and the ionic conductor 33. Has been formed.

The electrode protection layer 36 is made of a material having a smaller expansion coefficient than the ionic conductor 33. For example, stabilized zirconia (linear expansion coefficient: 8 ×) which is an ionic conductor.
Crystallized glass having a linear expansion coefficient smaller than that of 10 ^{−6 to} 11.4 × 10 ⁻⁶ / ° C. (coefficient of linear expansion: 5 × 10 ^{−6 to} 5.5 × 1)
(0 ⁻⁷ / ° C.) is preferably used. The sealing layer 37 is
It is made of a material having a coefficient of expansion close to that of the ionic conductor 33 and a composition similar to that of the electrode protective layer 36. Expansion coefficient: 8 × 10 ^{−6 to} 11 ×
10 ⁻⁶ / ° C.) is preferably used.

Next, a method of manufacturing the oxygen sensor 31 will be described with reference to FIG. First, as shown in FIG. 2A, the electrodes 3 are formed on the surfaces 33 a and 33 b of the ionic conductor 33.
After forming 4, 35, the first crystallized glass layer 41 that covers the electrode 35 and has a smaller linear expansion coefficient than the ionic conductor 33 by screen printing, and the first crystallized glass layer 41. A second crystallized glass layer 42 having a linear expansion coefficient similar to that of the ionic conductor 33 is sequentially printed to form pellets 43 for firing.

Then, as shown in FIG. 2 (b), the pellets 43 for firing are mixed with the first crystallized glass layer 4
The temperature at which the first and second crystallized glass layers 42 melt (50
Baking is performed at a temperature of 0 to 900 ° C. or higher, for example, 1000 ° C. In this case, the ionic conductor 33 and the second crystallized glass layer 42 are firmly bonded to each other because the linear expansion coefficients are close to each other, and the first crystallized glass layer 41 and the second crystallized glass layer 42 are also firmly bonded. The layer 42 has a different coefficient of linear expansion but has a similar composition, so that it is firmly bonded. On the other hand, the ionic conductor 33 and the first crystallized glass layer 41 are not joined because they have different linear expansion coefficients and compositions, and the first crystallized glass layer 41 is melted by heating to cause the second crystal. As a result of moving to the side of the crystallized glass layer 42 and joining, a space 39 whose shape is controlled with high precision is formed between the ionic conductor 33 and the first crystallized glass layer 41. After the firing is completed, the first crystallized glass layer 41 is formed into the electrode protective layer 36 and the second crystallized glass layer 41.
The crystallized glass layer 42 is used as the sealing layer 37. A heater 38, a predetermined electric circuit, and the like are provided on the fired pellet 43 to form the oxygen sensor 31.

In this oxygen sensor 31, the electrode 3
By applying a predetermined voltage between No. 4 and No. 35, oxygen in the sample gas enters the void 39 through the gas diffusion hole 32. The oxygen absorbs electrons on the surface of the electrode 35 by the oxygen pumping action and changes into oxygen ions to flow in the ionic conductor 33. From the current value using the oxygen ions as carriers, the oxygen concentration in the sample gas is To be measured. Here, since the shape of the void 39 is controlled with high accuracy and is well isolated from the external environment, the fluctuation of the amount of oxygen taken in is small and the variation of the ion current characteristics is also small.

FIG. 3 is a diagram showing the limiting current characteristics of the oxygen sensor 31. This figure is a graph showing the magnitude of the electric current obtained when the oxygen sensor 31 is heated to 400 ° C. and the magnitude of the voltage applied between the electrodes 34 and 35 is changed. It can be seen that good limiting current characteristics are exhibited.

As described above, according to the oxygen sensor 31 of the above-described embodiment, the electrode protection layer 3 is provided at the upper position of the electrode 35.
6, a sealing layer 37 is provided on the entire outer surface 36a of the electrode protection layer 36, and a gap 39 is formed between the electrode protection layer 35 and the ion conductor 33. The electrode protection layer 36 is the ion conductor 33. Since the material having a smaller expansion coefficient is used and the sealing layer 37 is made of a material having a coefficient of expansion similar to that of the ionic conductor 33 and a composition similar to that of the electrode protection layer 36, the sealing layer 37 serves as a mechanical member of the ionic conductor 33. The ionic conductor 3 by reinforcing the physical strength
It is possible to prevent deterioration of the mechanical strength of No. 3 and generation of strain and microcracks inside the ionic conductor 33.

Further, the electrode protection layer 36 and the sealing layer 37 control the shape of the voids 39 with high accuracy, and the voids 39 are well isolated from the external environment, so that the fluctuation of the amount of oxygen taken in is reduced. Therefore, it is possible to reduce variations in ion current characteristics. Therefore, it is not necessary to control the machining process with high precision as in the conventional case, the manufacturing process can be simplified, and the manufacturing cost can be reduced.

[0020]

As described in detail above, according to the oxygen sensor of the present invention, an electrode protective layer that entirely covers the electrode is provided above the electrode located on one side of the ionic conductor. A sealing layer having airtightness is provided on the entire outer surface of the electrode protective layer, and a gap is provided between the electrode protective layer and the ionic conductor, and the electrode protective layer has a coefficient of expansion larger than that of the ionic conductor. Since the sealing layer is made of a small material and has a coefficient of expansion similar to that of the ionic conductor, the sealing layer reinforces the mechanical strength of the ionic conductor, and It is possible to prevent the reduction of the mechanical strength and the generation of strain and microcracks inside the ionic conductor.

Further, the electrode protective layer and the sealing layer control the shape of the void portion with high accuracy and well isolate the void portion from the external environment, whereby the fluctuation of the amount of oxygen taken in can be reduced. Therefore, variations in ion current characteristics can be reduced. Therefore, it is not necessary to control the machining process with high precision as in the conventional case, the manufacturing process can be simplified, and the manufacturing cost can be reduced.

[Brief description of drawings]

FIG. 1 is a front sectional view showing an oxygen sensor according to an embodiment of the present invention.

FIG. 2 is a process drawing showing a method for manufacturing an oxygen sensor according to an embodiment of the present invention.

FIG. 3 is a diagram showing a limiting current characteristic of an oxygen sensor according to an embodiment of the present invention.

FIG. 4 is a front sectional view showing a conventional oxygen sensor.

FIG. 5 is a front sectional view showing a conventional oxygen sensor.

FIG. 6 is a front sectional view showing a conventional oxygen sensor.

[Explanation of symbols]

31 ... Oxygen sensor, 32 ... Gas diffusion hole, 33 ... Ion conductor, 33a, 33b ... Surface, 34, 35 ... Electrode, 36 ...
Electrode protection layer, 36a ... Outer surface, 37 ... Sealing layer, 37a ... Top surface, 38 ... Heater, 39 ... Void portion, 41 ... First crystallized glass layer, 42 ... Second crystallized glass layer, 43 ... Firing Pellets for.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshinori Kato 1-5-1, Kiba, Koto-ku, Tokyo Fujikura Kiba Factory Ltd.

Claims (1)

[Claims] 1. An electrode to which a predetermined voltage is applied is provided on each surface of an ionic conductor, and oxygen in a sample gas taken into the ionic conductor through a gas diffusion hole is oxygen-pumping action to cause the ionic conductor to flow. Flowing as ions in the inside, from the current value using the oxygen ions as carriers, in the oxygen sensor in which the oxygen concentration in the sample gas is measured, in the upper position of the electrode located on one side of the ionic conductor, An electrode protective layer that entirely covers the electrode is provided, a sealing layer having airtightness is provided on the entire outer surface of the electrode protective layer, and a void is provided between the electrode protective layer and the ionic conductor, The oxygen sensor, wherein the electrode protective layer is made of a material having a smaller expansion coefficient than that of the ionic conductor, and the sealing layer is made of a material having a similar expansion coefficient to that of the ionic conductor.