JPS62206441A - Threshold current type gas sensor - Google Patents

Abstract

PURPOSE:To reduce heat loss while raising mechanical strength, by sandwitching a sheet-like ion conductor with a pair of holding members from both sides. CONSTITUTION:A thick film heater 11 connected to a lead 13 is provided between sheet-like ion conductors 1 and 6 and a cathode 2 and an anode 5 are provided separately on the opposite surface of the heater 11. Frames 14 and 32 and shielding plates 15 and 24 are provided on the cathode 2 side and on the anode 5 side of the conductors 1 and 6 to form a closed spaces 17 and 25 as measuring space. Then, communicating holes 18 and 26 are made in the shielding plates 15 and 24 and moreover, the communicating hole 18 communicates with a communicating hole 21 formed with a top plate 16 to serve as supply hole for a gas to be measured. The frames 14 and 24, the shielding plates 15 and 24 and the top plate 16 composed holding members 31 and 32 to sandwitch the ion conductors 1 and 6 from both sides. This reduces heat loss while improving mechanical strength.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] The present invention relates to the structure of a limiting current type oxygen sensor using an oxygen ion conductor for combustion control of an automobile engine or boiler or for oxygen deficiency monitoring.

[Prior art and its problems]

Stabilized zirconia, a solid electrolyte ceramic that becomes an oxygen ion conductor at high temperatures, is widely used as an oxygen sensor, and we will first provide an overview of it. There are two types of oxygen sensors using stabilized zirconia: concentration cell type and limiting current type. The concentration cell type is an oxygen sensor that has been widely used in the past. Stabilized zirconia heated to a high temperature of around 850°C is used as a partition wall between the measurement atmosphere and the reference atmosphere. Measure the electromotive force generated proportionally. On the other hand, the limiting current sensor is an oxygen sensor that has recently been put into practical use, and as shown in FIG. A small hole 53 provided in a cap 55 communicates with the external measurement atmosphere to restrict the diffusion and entry of oxygen from the outside.

The stabilized zirconia plate 50 is heated by a heater (not shown), and the oxygen that reaches the cathode 51 combines with electrons by electrolysis to become oxygen ions, and when a voltage is applied between the cathode 51 and the anode 54, the stabilized zirconia plate 50 is heated. The electrons move with the ions at t7, give electrons to the anode 54 provided on the opposite side of the cathode 51 of the stabilized zirconia plate 50, and are emitted to the outside as oxygen molecules, so that a current output is obtained.

When the applied voltage exceeds a certain value, diffusion from the small holes 53 comes to dominate the transport rate of oxygen, so the value of the output current is determined by the size of the small holes 530 and the oxygen concentration. Once the dimensions are determined, a flat region is created in the voltage-current characteristics, and the value of the current (this is called the limiting current) in that region is proportional to the oxygen concentration. In this way, a current output proportional to the oxygen concentration can be obtained, so by applying an appropriate voltage,
It is used as a sensor for air-fuel ratio control and oxygen deficiency monitoring in the combustion of automobile engines and boilers by measuring the output current. Since the limiting current type sensor forcibly moves oxygen ions using an external voltage, it can be used at a relatively low temperature (nearly 350°C) at which it can provide ionic conductivity. This is an overview of oxygen sensors.

There are two problems with the conventional method in constructing this limiting current type oxygen sensor.

One of these is to suppress heat loss from the sensor element. As mentioned above, the limiting current type oxygen sensor has a temperature of 350°C.
It is necessary to heat the sensor element nearby, and in the case of a sensor element with a heating heater built into the sensor element, it is desirable to minimize heat loss to the outside of the element and heat it effectively with less power consumption. There are two types of heat loss: heat radiation, and heat removal due to gas convection around the element. Heat loss due to convection is particularly important. Conventionally, this has been dealt with by downsizing the device to reduce its absolute heat dissipation area, or by wrapping the device itself in a heat insulating material such as asbestos or glass coat [7]. There are the following problems in reducing the size of the sensor element from t2. That is, the heater and cathode 51 are made into one type together with the sensor element.
Since electrodes such as the anode 54 are made by screen printing using a paste of a conductive material, there is a limit to their miniaturization, including connection of leads for signal extraction. Moreover, since it is a rhinoceros membrane, if it is formed by vapor deposition or sputtering instead of the screen printing described above, it will take a long time to obtain the required thickness, making it unsuitable for mass production. Furthermore, oxygen is allowed to diffuse into the atmosphere. Due to the problem of clogging of the J hole 53, the possibility of forming the J hole 53 into a J% shape is also limited. In order to obtain the limiting current characteristics, it is necessary that this small hole 53 is sufficiently small compared to the closed space 52 surrounding the cathode.
If there is a limit to the diameter of the small hole 53, the reduction in size of the closed space 52 is also limited, which also limits the reduction in size of the sensor element as a whole.

On the other hand, wrapping the sensor element with a heat insulating material requires a wrapping process in addition to the sensor element fabrication process.
Care must be taken not to damage the lead wires already attached to the sensor element during wrapping, which poses a problem in mass production.

Another problem is the strength of the oxygen ion conductor stabilized zirconia plate 50. The stabilized zirconia plate 50 is used with a thin thickness and a low electrical resistance value between the cathode 51 and the anode 54 so as to provide good conductivity even at a relatively low temperature of around 350°C. t
7. Therefore, the mechanical strength of the plate was insufficient and it was easily broken.

[Purpose of the invention]

It is an object of the present invention to solve the above-mentioned problems and to provide a limiting current type gas sensor having a structure that causes less heat loss and less risk of damage to the oxygen ion conductor.

[Key points of the invention]

In this invention, a thin stabilized zirconia plate, which is a heated ionic conductor such as an oxygen ionic conductor, is attached from both sides.
By holding the ion conductor between the pair of holding members, the ion conductor is thermally insulated from the outside, and the ion conductor is reinforced and protected.

The thin stabilized zirconia plate, which is an ionic conductor, has l.
A pair of electrode layers is provided as a cathode and an anode on each surface, and these electrode layers are heated using a thick film heater provided on a stabilized zirconia plate as a heating means, and oxygen ions as a measurement gas are heated. Provides electrical conductivity.

The holding member encloses a space composed of, for example, a frame-shaped body part and a shielding plate part, and this space becomes a heat-retaining space that keeps the heated electrode layer part of the ion conductor warm, and also serves as a heat-retaining space for keeping the heated electrode layer part of the ion conductor warm. It serves as a measurement space for holding the measurement gas that is brought into contact with the anode to ionize it, or that returns the measurement gas from ions to gas molecules by giving electrons to the anode. The heat-retaining space can also serve as a measurement space. Since the holding member having such a space surrounds and holds from both sides the region including the heated electrode layer portion of the ionic conductor and the thick film heater as the heating means, the region including the electrode layer portion and the heating means is held from both sides. is thermally insulated from the outside. Furthermore, the holding member is a member such as the above-mentioned frame-shaped body that constitutes the heat-retaining space and the measurement space, and firmly holds the peripheral part of the area including the electrode layer part of the ion conductor and the heating means, thereby conducting ion conduction. Reinforce your body,
Also, a shielding plate protects the ionic conductor from direct external force.

One of the holding members, for example, the holding member on the cathode side, is equipped with a measurement gas restriction flat step, such as a communication path with an extremely small cross-sectional area that communicates with the outside, provided on the shield plate, to prevent the measurement gas from entering the measurement space. The introduction is limited t2 to give a limiting current characteristic.

Further, the other holding member, for example, the holding member on the anode side, is also provided with a lead-out means such as a communication path in the shielding plate portion, so that the measurement gas, which has changed from ions to gas molecules on the anode side, is led out to the outside.

Further, the holding member may be made of a porous breathable material such as a breathable thermal insulator, and the breathable holes thereof can be used as a heat retention space, a measurement space, a restriction means, and a derivation means. In this case, a structure can be adopted in which the breathable thermal insulator firmly sandwiches the ionic conductor from both sides, and the ionic conductor is sandwiched by a non-breathable reinforcing frame provided with an opening, for example, It is also possible to use a structure in which a breathable heat insulating material such as a synthetic fiber is inserted.

[Embodiments of the invention]

FIG. 1 shows an example of the structure of an oxygen sensor as a limiting current type gas sensor according to the present invention.
An exploded perspective view is shown to make the structure easier to understand. 1 for the thin plate-like stabilized zirconia plate 1 as an ionic conductor
A thick film cathode 2 as one of the pair of electrode layers and a conductive thick film 3 for taking out a signal current are provided. This conductive thick film 3 is applied to the stabilized zirconia plate 1 which is also formed on the inner surface of the through hole 4 provided in the stabilized zirconia plate 1, according to the inventor's invention described in Japanese Patent Application No. 132,658/1980. The stabilized zirconia plate reaches the back surface [2] and when the stabilized zirconia plate 1, which is a thin plate-like stabilized zirconia plate 6 as an ion conductor and provided with the anode 5 as the other electrode layer, is stacked, the stabilized zirconia plate The cathode 2 and the cathode thick film terminal 7 are electrically connected by contacting the cathode thick film terminal 7 provided on the cathode 6 . Also provided on the same side of the stabilized zirconia plate 6 as the anode 5 is a conductive thick film 8, only shown in cross section.
Similarly to the above case, the anode 5 is also formed on the inner surface of the through hole 9 of the stabilized zirconia plate 6, and the anode 5 is electrically connected to the anode side thick film terminal 10 provided on the surface opposite to the anode 5. Further, on the surface of the stabilized zirconia plate 6 opposite to the anode 5, a thick film heater 11 which is an electric heating element as a heating means is provided together with a thick film terminal 12 for heater. Thick film terminal for cathode 7. A platinum foil lead 13 is connected to the anode thick film terminal 10 and the heater thick film terminal 12 for connection to the outside of the sensor. Both the cathode 2 and the anode 5 are formed by screen printing a platinum-based paste, and include conductive thick films 3 and 8, thick film terminals for the cathode 7. Thick film terminal 10 for anode, thick film heater 1
1. Both thick film terminals 12 for heaters are screen printed with platinum-rhodium alloy paste,
It is baked into the stabilized zirconia surface during the sintering process of stabilized zirconia.

On the stabilized zirconia plate 1 provided with the cathode 2, two frames 14 as frame-like body parts surrounding the cathode 2 are superimposed. In this embodiment, there are two frames 14, but the number can vary depending on the design conditions of the sensor. Furthermore, a shielding plate 15 and a top plate 16 are stacked as a shielding plate part on the opposite side of the frame-shaped body to the stabilized zirconia plate 1 side, forming a closed space 17 as a measurement space surrounding the cathode. Shielding plate 1
5 has a communication hole 18 open therein, and the top plate 16 is provided with an extremely shallow guide groove 19 and an opening 20 connected thereto. Guide groove 19. The shielding plate 15 and the opening 20 create a communication path 2 with an extremely small cross-sectional area.
1 is formed and constitutes a restricting means for restricting the introduction of oxygen gas as a measurement gas into a closed space 17 as a measurement space.

The anode side has almost the same configuration, and the stabilized zirconia plate 6 is stacked with a frame-like body part, 1 and 2 frames 23, and a shielding plate part as a shielding plate part, and is stacked on the anode side. A closed space 25 is formed between the measurement space and L2, and a communication path 26 as a derivation means provided on the shielding plate 24 is also formed.
Oxygen gas, which has become oxygen molecules at the anode 5, is led out of the sensor from the closed space 25 through the sensor.

Frame 14. The shielding plate 15 and the top plate 16 form a cathode-side holding member 31 later shown in FIG. 2, and the frame 23° and the shielding plate 24 form an anode-side holding member 32 shown later in FIG. The oxidized zirconia plates 1 and 6 are sandwiched from both sides.

Stabilized zirconia is also an excellent thermal insulator material that is an oxygen ion conductor. Frames 14 and 23. Shielding plates 15 and 24. and the top plate 16 is stabilized zirconia plates 1 and 6
Both are made of stabilized zirconia, which matches the coefficient of thermal expansion with the zirconia, and has excellent thermal insulation properties. Furthermore, the amount of oxygen as the measurement gas contained in the closed spaces 17 and 25 is extremely small compared to the amount of air held in the closed spaces 17 and 25 due to the construction principle of the current-limited gas sensor. In this respect, it is safe to assume that the closed spaces 17 and 25 are filled with air having extremely high thermal insulation properties. Therefore, the closed space 17 as a measurement space
and 25 function as a momempo mixed space.

The stabilized zirconia plates 1 and 6 heated by the thick film heater 11 are held in holding members 31 and 3 made of a thermally insulating material, which enclose a space serving as a measurement space and a heat retention space.
2 is sandwiched between the stabilized zirconia plates 1 and 6, which prevents heat from being taken away from the stabilized zirconia plates 1 and 6 due to heat conduction to the air around the sensor and air convection. Heat loss due to thermal radiation can also be suppressed. Furthermore, the stabilized zirconia plates 1 and 6 are reinforced by firmly sandwiching the peripheral edges of the area including the cathode 2) anode 5 and thick film heater 11 between frames 14 and 23, and are further protected by shielding plates 15 and 24. Therefore, even if the thickness of the plate is made thin to improve the conductivity between the cathode 2 and the anode 5, there is no risk of damage due to external force.

Furthermore, frames 14 and 23. Since the shielding plates 15 and 24 have exactly the same shape, using the same material, stabilized zirconia, improves the JCf productivity for both.

For reference, the manufacturing process of the sensor according to the present invention will be briefly described. Top plate 16. The shielding plate 15, the frames 4 and 23, and the stabilized zirconia plates 1 and 6 are stacked in the state of green sheets before sintering. At this time, each green sheet is formed into a required shape, and the stabilized zirconia plates 1 and 6 have a cathode 2. Anode 5° conductive thick film 3 and 8. Thick film terminal for cathode 7. Thick film terminal for anode 10. Thick film heater 11
．． The thick film terminal 12 for the heater has already been marked with a screen mark.
j. Also, the shielding plate 15
Although it is shown here separately to help understand the shape, a thick styrene resin flammable film 28 formed into the shape of a guide groove is screen printed at the dotted line in the figure, and then hot pressed. A guide groove 19 is formed in the top plate 16 in the process. Furthermore, in order to maintain the shape of the closed spaces 17 and 25 during the hot pressing process described above, a styrene resin combustible plate having the same shape as the closed spaces is inserted, although not shown here. The stacked pieces as described above are pressure-molded using a hot press and then fired at a temperature of 1500° C. for 1 hour to sinter the zirconia. In this process, the combustible thick film 28 is burned out, and the top plate 16
A guide groove 19 is formed in. The combustible plate is also burnt out to provide the required closed space. The above is an overview of the production process.

FIG. 2 shows the outer shape of a limiting current type oxygen sensor according to an embodiment of the present invention, in which (a) is the top surface and (b) is the side surface. FIG. 3 schematically shows a cross section taken along line A-A in the top view of FIG. 2(a), and shows stabilized zirconia plates 1 and 6.
The frames 14 and 23 . Shielding plates 15 and 24, top plate 1
6, including the closed spaces 17 and 25, are referred to as holding members 31 and 32. In this configuration, when a gas with a relatively high oxygen concentration such as oxygen in the atmosphere is targeted, the thickness of the screen-printed flammable thick film 28 shown in FIG. By making the guide groove 19 longer, the cross-sectional area of the guide groove 19 can be narrowed and the length can be increased. The passage 21 can be easily formed.

FIG. 4 shows schematic cross-sectional views and t7 of other different embodiments of the present invention. FIG. 4(a) shows a vertical hole 30 with a minute diameter in which the communication path 21 shown in FIG. 3 is provided in the shielding plate 29.
That is. This vertical hole 3o can be provided when measuring gas with a relatively low oxygen concentration, and has the advantage that the top plate 16 can be omitted. In the figure, the number of vertical holes 30 is one, but there may be a plurality of vertical holes that provide equivalent limiting current characteristics. (b) is a case in which the holding member 32 in FIG. 3 is replaced with a porous air-permeable thermal insulator 33 such as porous ceramics, and the measurement including the means for extracting the air-permeable holes of the air-permeable thermal insulator 330 is performed. It constitutes a space and a heat insulation space. The breathable thermal insulator 33 holds the stabilized zirconia plates 1 and 6 in the holding member 3.
1 along with cathode 2. Anode 5 and thick film heater 1
The stabilized zirconia plate 6 is firmly sandwiched between the periphery or the entire surface of the region including the stabilized zirconia plate 6, and can be configured simply by simply stacking the breathable thermal insulator 33 on the stabilized zirconia plate 6. This breathable thermal insulator 33 further includes a glass wool-like heat insulator as a breathable thermal insulator in the opening of a reinforcing frame 35 having an opening to the outside similar to frame 14 or 23, as shown in (C). If it has a non-uniform structure with insulating fibers 36 inserted,
It has the advantage of being able to be replaced with a new one when it becomes clogged. In this configuration, the ventilation holes of the thermally insulating fibers 36 constitute a measurement space including a lead-out means and a heat retention space, and the reinforcing frame 35 serves to reinforce and protect the stabilized zirconia plates 1 and 6. , the thermally insulating fibers 36 also play a role in mitigating external forces. (d) is different from (b) in that the holding member 31 on the cathode side is also made of a breathable thermal insulator 34.

Regarding this, as in the case of (C), although the figure and illustration are not 1 ton, it is possible to insert heat insulating fibers into the reinforcing frame to create a structure that can be replaced in case of clogging. In either case, the cathode side is made to have lower air permeability than the anode side to provide limiting current characteristics. In (e), the ionic conductor is a single thin stabilized zirconia plate 37. , a cathode 38 is provided on one side, an anode 39 is provided on the other side, and a thick film heater 40 is provided on the side where the cathode 38 is provided.The thick film heater 40 may be provided on the anode side.

In this example, since the distance between the cathode 38 and the anode 39 is shortened, even better conductivity can be obtained.

In this embodiment, there are four types of holding members 31 on the cathode side (FIGS. 3, 4(a), 4(d), and thermally insulating fibers), and three types of holding members 32 on the anode side ( Figure 3, Figure 4 (b), Figure 4 (C)), oxygen ion conductor and two types of t7 (Figure 3, Figure 4 (e))
You can select and configure the sensor. Furthermore, many more embodiments are possible by considering the shape of the vertical hole 30 shown in FIG. 4(a) and the variety of positions in which the thick film heater 40 is provided as shown in FIG. 4(e).

〔Effect of the invention〕

This invention has a structure in which a thin stabilized zirconia plate as a heated ion conductor is held between a pair of holding members from both sides. The holding member consists of a frame and a shielding plate made of stabilized zirconia, which is an ionic conductor and also a good thermal insulator, and the closed space created by these forms a heat-retaining space that also serves as a measurement space. Since the stabilized zirconia plate is sandwiched so as to surround the electrode layer portion of the stabilized zirconia plate and the thick film heater, heat is removed from the ionic conductor by heat transfer to the surrounding air, convection effects, and thermal radiation. The ionic conductor can be effectively heated with less power consumption. In addition, the frame constituting the holding member firmly holds the thin plate of the ion conductor from both sides at the periphery of the area including the electrode layer part and the thick film heater, so the ion conductor is reinforced by the frame and at the same time by the shielding plate. Since they are protected from the outside, the ionic conductor and electrode layer can be prevented from being damaged by impact, vibration, or gas erosion caused by gas containing metal powder. Therefore, the thickness of the stabilized zirconia plate as an ion conductor can be reduced, and the conductivity to oxygen ions, which is the measurement gas, can be improved. In addition, if the cathode side shielding plate section is composed of two sheets, a shielding plate and a top plate, and a groove provided in the top plate forms a communication path that restricts the introduction of the measurement gas, the measurement gas with a high oxygen concentration is effective. It is possible to easily construct a communication path with a small cross-sectional area and a long length that can be narrowed down to a small area.

[Brief explanation of drawings]

Fig. 1 is an exploded perspective view of an embodiment of the present invention, Fig. 2 is an external view of the same embodiment, Fig. 3 is a sectional view taken along line A-A in Fig. 2(a), and Fig. 4 is an exploded perspective view of an embodiment of the present invention. Cross-sectional views of different embodiments,
FIG. 5 is a sectional view of a limiting current type gas sensor according to the prior art. 1.6, 37, 50: Stabilized zirconia plate, 2, 38° 51: Cathode, 5, 39, 54 near node, 11,
40: Thick film heater, 14, 23: Frame, 15, 24: Shielding plate, 16: Top plate, 17.25.52: Closed space, 18
: Communication hole, 19: Guide groove, 20: Opening, 21, 26
: Communication path, 31, 32: Holding'' (22) member, 33, 34: Breathable heat insulator, 35: Reinforcement frame, Fig. 2, Fig. 3, 37 Stabilizing sill] Contradiction between 397/- Mr A
PuR

Claims (1)

[Claims] 1) A thin plate-shaped ionic conductor made of a solid electrolyte having a pair of electrode layers, a heating means for heating the electrode layer portion of the ionic conductor to a measurement temperature, and heating by the means. A heat-retaining space that keeps the heated electrode layer portion warm from the outside and a measurement space that holds the measurement gas that is to be brought into contact with the electrode layer are each enclosed, and the ionic conductor is sandwiched from both sides so as to surround the electrode layer portion and the heating means. a pair of holding members, the ionic conductor is firmly held by the pair of holding members at least at the peripheral edge of the region including the electrode layer portion and the heating means, and one of the measurement spaces is provided with a space for introducing the measurement gas. 1. A limiting current type gas sensor, characterized in that one of the gases is communicated with the outside through a limiting means for restricting the amount of gas to be measured, and the other is communicated with the outside through a means for deriving the measured gas. 2) In the gas sensor according to claim 1,
A limiting current type gas sensor characterized in that the gas sensor is an oxygen sensor and the ionic conductor is made of stabilized zirconia. 3) In the gas sensor according to claim 2,
A limiting current type gas sensor characterized in that a holding member is made of stabilized zirconia. 4) In the gas sensor according to claim 1,
The ionic conductor is composed of two thin plates, and the heating means is composed of two thin plates.
A limiting current chamber gas sensor characterized by being an electric heating element sandwiched between two thin plates. 5) In the gas sensor according to claim 1,
1. A limiting current type gas sensor characterized in that electrode layers are provided on both sides of a thin plate-like ion conductor, and measurement gas restriction means and derivation means are provided in separate holding members. 6) In the gas sensor according to claim 1,
A limiting current type gas sensor characterized in that a measurement space within each holding member is provided in common with at least a portion of a heat retention space. 7) In the gas sensor according to claim 1,
a frame-shaped body part in which at least one of the holding members is disposed on the ion conductor side and defines a measurement space; and a shielding plate part arranged in the anti-ion conductor side of the frame-shaped body part to close and shield the measurement space from the outside. A limiting current type gas sensor comprising: a limiting current type gas sensor, characterized in that the shielding plate portion is provided with a communication path for measuring gas as a limiting means or a deriving means. 8) In the gas sensor according to claim 7,
The shielding plate portion of the holding member on the side where the limiting means is provided further includes a shielding plate on the side of the frame-shaped body and a top plate on the side opposite to the frame-shaped body, and the shielding plate side has a communication hole communicating with the measurement space. A limiting current type gas sensor, characterized in that the top plate side is provided with a narrow guide groove on the shielding plate side as a restricting means communicating with the communication hole, and an opening for communicating the guide groove with the outside. 9) In the gas sensor according to claim 7,
A limiting current type gas sensor characterized in that the shielding member on the side where the deriving means is provided consists of a single shielding plate, and the shielding plate is provided with a deriving hole that communicates the measurement space with the outside as the deriving means. 10) A limiting current type gas sensor according to any one of claims 7 to 9, characterized in that the heat retention space is shared with the measurement space. 11) In the gas sensor according to claim 1, at least one of the holding members is composed of a porous, air-permeable thermal insulator, and the heat retention space, the measurement space, and the restricting means or deriving means are formed of the thermal insulator. A limiting current type gas sensor characterized by being composed of ventilation holes. 12) In the gas sensor according to claim 1, at least one of the holding members is inserted into the non-breathable reinforcing frame body having an opening to the outside at a portion corresponding to the electrode layer portion. A limiting current type gas sensor comprising a breathable thermal insulator.