JPH06294767A - Limiting current type oxygen sensor - Google Patents

Abstract

PURPOSE:To obtain a limiting current type oxygen sensor which is easy to manufacture, enables reduction of the cost of a product and has an excellent limiting current characteristic and also high reliability. CONSTITUTION:A cathode electrode 2 and an anode electrode 3 are formed on the opposite surfaces of a solid electrolyte base 1 of stabilized zirconia or the like. Electrode extension parts 2a and 3a are provided from these electrodes 2 and 3 to the edge part of the base and the cathode electrode 2 is covered with a buffer layer 4 formed of a porous member. This buffer layer 4 is covered with a dome layer 5 constituted of crystallized glass or the like. The electrode extension part 2a operates as a diffusion rate determining part and thereby a limiting current characteristic can be obtained. The buffer layer 4 may be a space provided between the cathode electrode 2 and the dome layer 5.

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 having a pair of electrodes arranged with a solid electrolyte substrate sandwiched therebetween and detecting the oxygen concentration in a gas by utilizing the limiting current characteristic.

[0002]

2. Description of the Related Art FIG. 4 is a sectional view showing an example of a conventional limiting current type oxygen sensor.

In the conventional limiting current type oxygen sensor,
A cathode electrode 42 and an anode electrode 43 are formed sandwiching a solid electrolyte substrate 41 made of stabilized zirconia. Each of these electrodes 42 and 43 is made of porous platinum, and gas can flow through the inside thereof. A cap 44 having a shape in which the lower end is opened is formed on the surface of the solid electrolyte substrate 41 on the cathode electrode 42 side.
Is provided with its lower end fixed to the substrate 41, whereby an internal space surrounded by the cap 44 and the substrate 41 is formed. The cathode electrode 42 is in contact with this internal space, and the anode electrode 43 is in contact with the external atmosphere.

The cap 44 is provided with one or a plurality of fine gas diffusion holes 44b so that the external atmosphere and the internal space communicate with each other through the gas diffusion holes 44b. Further, a heater 46 is provided on the outer surface of the cap 44, and when the heater 46 is energized, the heater 46 is resistively heated to heat the oxygen sensor. Further, the cathode electrode 42 and the anode electrode 43
A power source 49 is connected between the power source and the power source, and an ammeter 48 and a voltmeter 47 are connected to the power source 49 in series and in parallel, respectively.

In the conventional oxygen sensor having the structure described above, the oxygen sensor is heated to a high temperature by supplying electric power to the heater 46 to generate resistance heat, and at the same time, a predetermined voltage is provided between the cathode electrode 42 and the anode electrode 43 by the power source 49. Voltage (V) is applied. Then, due to the oxygen pumping action, oxygen molecules (O ₂ ) contained in the gas existing in the internal space surrounded by the solid electrolyte substrate 41 and the cap 44 obtain electrons through the cathode electrode 42, and oxygen is obtained. It becomes ions and enters the solid electrolyte substrate 41. The oxygen ions move in the substrate 41 in the thickness direction through the oxygen ion holes in the substrate 41. Then, the oxygen ions reach the anode electrode 43 to release electrons, and become oxygen molecules again to be released to the outside. Due to this movement of oxygen ions, a current (A) flows between the anode electrode 43 and the cathode electrode 42.

By the way, due to the movement of oxygen, the internal space of the oxygen sensor becomes a negative pressure, and the gas flows from the outside through the gas diffusion hole 44b. At this time, since the inflow amount of the gas is limited by the gas diffusion hole 44b, the current of the oxygen sensor-
In the voltage characteristics, a so-called flat region in which the current does not change even when the voltage applied between the cathode electrode 42 and the anode electrode 43 is increased is observed. This characteristic is called the limiting current characteristic, and the current at this time is called the limiting current.

The value of the limiting current of the limiting current type oxygen sensor is
If the shape of the gas diffusion hole, the temperature of the sensor during use, and the pressure of the atmosphere are constant, it depends on the oxygen concentration in the atmosphere. Therefore, if the relationship between the limiting current value and the oxygen concentration is obtained in advance, the oxygen concentration in the atmosphere of unknown oxygen concentration can be known by measuring the limiting current value.

[0008]

However, in the above-mentioned conventional limiting current type oxygen sensor, the cap 44 having the gas diffusion hole 44b is formed, and the cap 4 is formed.
4 needs to be bonded to the solid electrolyte substrate 41 provided with the electrodes 42 and 43, and there is a problem that the manufacturing is complicated and the manufacturing cost is high. Further, since the gas diffusion holes are formed by machining, there is a drawback that the variation of the limiting current characteristic is large. Furthermore, if foreign matter adheres to the gas diffusion holes, the flow of gas may be obstructed and the limiting current characteristics may change significantly. Therefore, the conventional limiting current type oxygen sensor is not sufficiently reliable when used for a long period of time.

The present invention has been made in view of the above problems, and it is easy to manufacture, the product cost can be reduced, the variation in the characteristics is small, and the long-term reliability can be secured. It is intended to provide a sensor.

[0010]

A limiting current type oxygen sensor according to the present invention includes a solid electrolyte substrate, first and second porous electrodes arranged with the solid electrolyte substrate interposed therebetween, and the first and second porous electrodes. A porous electrode extension extending from a porous electrode toward the edge of the substrate, a buffer layer provided on the first porous electrode, a base of the buffer layer and the electrode extension. And a dome layer that hermetically covers the end side portion.

[0011]

In the limiting current type oxygen sensor according to the present invention, the first and second porous electrodes are arranged with the solid electrolyte substrate sandwiched therebetween, and the edge of the substrate is separated from the first porous electrode. A porous electrode extension portion extending toward the portion is provided. A buffer layer is provided on the first porous electrode, and the buffer layer and the base end side portion of the electrode extension are hermetically covered with a dome layer. The buffer layer is made of, for example, a member having a higher gas permeability than the first porous electrode. Alternatively, the buffer layer is a space formed by, for example, a method of forming a carbon layer between the first porous electrode and the dome layer and subjecting the carbon layer to heat treatment or the like to gasify the carbon layer. is there.

In the limiting current type oxygen sensor according to the present invention configured as described above, for example, it is assumed that the first porous electrode is a cathode electrode and the second porous electrode is an anode electrode. When a voltage is applied between the anode electrodes, oxygen molecules in the gas that have penetrated into the cathode electrodes acquire electrons and become oxygen ions, and move in the thickness direction of the solid electrolyte substrate. Then, when the oxygen ions reach the anode electrode, they emit electrons and become oxygen molecules, which are released into the atmosphere. This movement of oxygen reduces the pressure of the gas inside the dome layer, and as a result, new gas flows into the dome layer through the porous electrode extension. In this case, the inflow amount of gas is regulated by the base end side portion of the electrode extension portion. That is, in the limiting current type oxygen sensor according to the present invention, the electrode extension portion base end side portion of the portion covered with the dome layer corresponds to the gas diffusion hole in the conventional limiting current type oxygen sensor, and this electrode extension portion The part on the base end side acts as a diffusion-controlling part and the inflow of gas is limited,
The limiting current characteristic can be obtained.

By the way, it may be considered that the buffer layer is not provided on the first porous electrode. In this case as well, since the first porous electrode itself has gas permeability, the gas flowing in through the base end side portion of the electrode extension portion diffuses in the first porous electrode and has a limiting current characteristic. Can be obtained. However, usually, the porous electrode is a platinum electrode formed by firing a platinum paste and has a relatively low gas permeability, so unless a buffer layer is provided, the electrode efficiency will be high in the part separated from the diffusion-controlling part. In other words, the effective area of the electrode becomes small, the limiting current characteristics of the limiting current type oxygen sensor deteriorate, and the reliability also decreases. Also,
When the dome layer is directly formed on the first porous electrode, the dome layer and the first porous electrode are firmly bonded to each other, so that when the thermal shock or the like is applied, the first porous electrode and the substrate are separated from each other. The stress is concentrated on the bonding portion and the first porous electrode is separated from the substrate, or a component forming the dome layer (for example,
There is a possibility that glass) may enter the porous electrode and deteriorate the limiting current characteristics. Therefore, it is necessary to provide a buffer layer on the first porous substrate.

In the limiting current type oxygen sensor according to the present invention, as described above, the base end side portion of the electrode extension portion is covered with the dome layer, and the base end side portion of the electrode extension portion extends to the inside of the dome layer. Because the limiting current characteristics are obtained by limiting the gas flow rate, the process of drilling fine holes by machining that was required when manufacturing the conventional limiting current type oxygen sensor is not required, and the manufacturing is easy. Is. Further, since the gas to be measured flows into the porous electrode through the surface of the electrode extension portion not covered by the dome layer, even if foreign matter adheres to a part of the surface of the electrode extension portion. Therefore, the fluctuation of the gas inflow amount is small, and a large fluctuation of the limiting current characteristic can be avoided. Therefore, the limiting current type oxygen sensor according to the present invention,
High reliability even when used for a long period of time. Furthermore,
The limiting current type oxygen sensor according to the present invention does not need to be provided with a gas diffusion hole, can be manufactured using a technique such as screen printing, and is easy to manufacture,
The manufacturing process can be easily automated, and variations in the limiting current characteristics can be suppressed.

[0015]

Embodiments of the present invention will now be described with reference to the accompanying drawings.

FIG. 1 is a plan view showing a limiting current type oxygen sensor according to an embodiment of the present invention, and FIG. 2 is a sectional view taken along line AA of FIG. However, in FIG. 2, the heater 6 and the terminals 6a, 6b, 7, 8 on the dome layer 5 are not shown.

The solid electrolyte substrate 1 is formed by molding stabilized zirconia into a substantially disc shape, and the cathode 2 and the anode 3 are formed in a substantially circular shape on the front surface and the back surface of the solid electrolyte substrate 1, respectively. It is formed so as to face each other with sandwiching. Each of these electrodes 2 and 3 is made of porous platinum. The shapes of the solid electrolyte substrate 1 and the electrodes 2 and 3 are not particularly limited to circles. A porous electrode extension 2 a extends from the cathode electrode 2 toward the edge of the substrate 1. Similarly, a porous electrode extension 3a extends from the anode electrode 3 toward the edge of the substrate 1. In addition, the cathode electrode 2 has a buffer layer 4 that is superior in gas permeability to the cathode electrode 2.
Is covered with.

The dome layer 5 is formed on the substrate 1 by hermetically covering the base layer side portion of the buffer layer 4 and the electrode extension portion 2a, and the tip portion of the electrode extension portion 2a is formed on the dome layer 5. Exposed from. The dome layer 5 is formed of, for example, glass mixed with ceramic powder.

A heater 6 is provided on the dome layer 5 in a bellows shape, and the heater 6 is electrically connected to heater terminals 6a and 6b also formed on the dome layer 5. An anode terminal 7 and a cathode terminal 8 are provided on the dome layer 5, and these terminals 7 and 8 are provided.
Are electrically connected to the electrode extension portions 2a and 3a through wirings (not shown) provided on the sides of the substrate 1 and the dome layer 5.

In the limiting current type oxygen sensor according to the present embodiment having the above-mentioned structure, when a voltage is applied between the heater terminals 6a and 6b, the heater 6 generates resistance heat and the sensor is heated to a high temperature. When a predetermined voltage is applied between the cathode electrode 2 and the anode electrode 3 in this state, the cathode electrode 2
Oxygen molecules contained in the gas inside the cathode electrode 2
The electrons are obtained through the above to become oxygen ions, and move in the thickness direction of the solid electrolyte substrate 1. Then, when the oxygen ions reach the anode electrode 3, they release electrons and become oxygen molecules again, and are released into the atmosphere. Due to this movement of oxygen, the inside of the buffer layer 4 has a negative pressure, and new gas flows into the buffer layer 4 through the electrode extending portion 2a. At this time, the inflow amount of gas is limited by the electrode extension portion 2a, and the limiting current characteristic is obtained as in the conventional limiting current type oxygen sensor.

In this embodiment, the base end side portion of the electrode extension portion 2a covered with the dome layer 5 corresponds to the gas diffusion hole in the conventional limiting current type oxygen sensor, and the dome layer is formed when the dome layer is formed. The limiting current characteristic of the sensor can be changed by changing the thickness, width or length of the electrode extension portion covered with. In addition, since gas flows into the inside of the dome layer through the surface of the electrode extension portion that is not covered by the dome layer, even if foreign matter adheres to a part of the surface of the electrode extension portion 2a, the gas The change in the inflow is small.
Further, since the buffer layer 4 having good gas permeability is provided between the electrode 2 and the dome layer 5, the gas flowing into the dome layer 5 is easily diffused over the entire surface of the electrode 2, so that the electrode efficiency is improved. High and good limiting current characteristics can be obtained. Furthermore, since the electrode 2 and the dome layer 5 are not directly joined, there is no risk of the electrode 2 peeling from the substrate 1 even if a thermal shock is received. Therefore, the limiting-current type oxygen sensor according to the present embodiment is highly reliable because fluctuations in limiting-current characteristics are suppressed even when measuring over a long period of time, and it is not necessary to form gas diffusion holes by machining. Therefore, there is an effect that manufacturing is easy.

Next, the result of actually manufacturing the limiting current type oxygen sensor according to this embodiment and examining its characteristics will be described.

First, as the solid electrolyte substrate 1, a zirconia substrate having a square shape of 6.5 mm on a side and a thickness of 0.14 mm was prepared. Next, a platinum paste is printed on the upper surface of the substrate 1 and then fired to have a diameter of 4.0 m.
A cathode electrode 2 having a porous structure of m and a thickness of about 5 μm and a porous electrode extension 2a extending from the electrode 2 toward the edge of the substrate were formed. In addition, in the same manner as this, the substrate 1
Platinum paste is printed on the lower surface of the substrate and then fired to form a porous anode electrode 3 having a diameter of 4.0 mm and a thickness of about 5 μm and a porous electrode extending from the electrode 3 toward the edge of the substrate. The extension 3a was formed.

Next, a paste prepared by mixing 1 g of crystallized glass and 5 g of steatite powder is printed on the surface of the cathode electrode 2 side and then fired to form a porous buffer layer 4 covering the cathode electrode 2 of about 100 μm. Formed to a thickness of. Further, a paste obtained by mixing crystallized glass and ceramic powder is printed and then fired to form about 3 glass dome layers 5 covering the buffer layer 5 and the base end side portion of the electrode extension 2a.
It was formed to a thickness of 00 μm.

Next, platinum paste is printed on the dome layer 5 and then fired to form the heater 6 and the heater terminals 6a, 6a.
b, the anode terminal 7 and the cathode terminal 8 were formed.

The limiting current characteristics of the thus manufactured limiting current type oxygen sensor according to the example of the present invention at a temperature of 450 ° C. were examined. Further, as a comparative example, the limiting current characteristics of the oxygen sensor manufactured in the same manner as in the example except that the buffer layer was not provided were also examined. FIG. 3 is a graph showing the limiting current characteristics of the oxygen sensors of the above-described examples and comparative examples, in which the horizontal axis represents the sensor voltage and the vertical axis represents the current value. As is clear from this graph, the oxygen sensor according to the present example exhibited better limiting current characteristics than the comparative example. In particular, it can be seen that the rising portion is good and the electrode efficiency is high as compared with the comparative example.

In the above-mentioned embodiments, the case where the buffer layer 4 is made of a member having a higher gas permeability than the porous platinum electrode has been described. However, as the buffer layer, the space between the cathode electrode and the dome layer is used. It may be a space provided in. The space as such a buffer layer can be formed, for example, as shown below. That is, first, after forming a cathode electrode on a substrate, a carbon paste is printed on the cathode electrode and dried to form a carbon layer. Next, a glass paste is printed on the carbon layer to form a glass dome layer that covers the carbon layer. Then, the glass dome layer is fired. The carbon layer is gasified by the heat during this firing,
A space can be formed between the dome layer and the cathode electrode.

[0028]

As described above, according to the present invention, the first and second porous electrodes are arranged with the solid electrolyte substrate sandwiched therebetween, and the edge of the substrate is separated from the first porous electrode. A porous electrode extension portion extending toward the first portion, a buffer layer is provided on the first porous electrode, and the buffer layer and the base end side portion of the electrode extension portion are dome-shaped. Since the layer is airtightly covered, the amount of gas flowing into the inside of the dome layer is limited by the base end side portion of the electrode extension portion, and a limiting current characteristic can be obtained. Therefore, the limiting current type oxygen sensor according to the present invention does not require the mechanical processing for forming the gas diffusion holes, which is conventionally required, and is easy to manufacture. In addition, since the gas flowing into the dome layer is diffused through the buffer layer, the electrode efficiency is high, and the gas between the dome layer and the first layer is high.
Since it is not directly bonded to the porous electrode, it is possible to prevent peeling of the electrode due to thermal shock or the like and invasion of components constituting the dome layer into the electrode. Therefore, the limiting current type oxygen sensor according to the present invention has an effect that it has good limiting current characteristics and is highly reliable even when used for a long period of time.

[Brief description of drawings]

FIG. 1 is a plan view showing a limiting current type oxygen sensor according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is a graph showing limiting current characteristics of examples and comparative examples.

FIG. 4 is a sectional view showing an example of a conventional limiting current type oxygen sensor.

[Explanation of symbols]

 1, 41; solid electrolyte substrate 2, 42; cathode electrode 2a, 3a; electrode extension part 3, 43; anode electrode 4; buffer layer 5; dome layer 6, 46; heater 44; cap 44b; gas diffusion hole

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Issei Ishibashi 1-5-1 Kiba, Koto-ku, Tokyo Fujikura Ltd. (72) Inventor Yoshinori Kato 1-1-5 Kiba, Koto-ku, Tokyo Shares Inside Fujikura

Claims (3)

[Claims] 1. A solid electrolyte substrate, first and second porous electrodes arranged with the solid electrolyte substrate sandwiched therebetween, and extending from the first porous electrode toward an edge portion of the substrate. A porous electrode extension portion, a buffer layer provided on the first porous electrode, and a dome layer that hermetically covers the buffer layer and the base end side portion of the electrode extension portion. A limiting current type oxygen sensor characterized in that 2. The limiting current type oxygen sensor according to claim 1, wherein the buffer layer is made of a member having higher gas permeability than that of the first porous electrode. 3. The limiting current type oxygen sensor according to claim 1, wherein the buffer layer is a space provided between the first porous electrode and the dome layer.