JP2593823Y2 - Gas detector - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector of a limiting current type, and more particularly to a gas detector which does not give an erroneous detection output.

[0002]

2. Description of the Related Art Conventionally, various types of sensors have been proposed and used for measuring the concentration of various gases in air. One of the representative sensors is a limiting current type sensor, which has electrodes on both sides of a solid electrolyte substrate which is a conductor such as oxygen ions and hydrogen ions, and a detection target gas is one electrode. Is reached by applying a voltage between the electrodes to generate an ionic current of the gas to be detected, and measuring a current value (limit current value) flowing between the electrodes to detect the object. It measures the concentration of gas.

[0003] There are four types of gas detection elements of this type when roughly classified according to their structure. The first type, for example, a device as shown in Japanese Patent Publication No. 49-19838 has the structure shown in FIG. That is, a porous electrode 3a, 3b is provided on both surfaces of the solid electrolyte substrate 2, and a voltage can be applied between the electrodes 3a, 3b.

In the case of the gas sensing element shown in FIG. 3 , for example, if the solid electrolyte substrate 2 is a zirconia-based substrate and an element for measuring the oxygen gas concentration, the oxygen gas passes through the porous electrode 3a and passes through the solid electrolyte. It reaches the substrate 2, where it is ionized to oxygen ions. Since a voltage is applied between the electrodes 3a and 3b (the electrode 3a is a negative electrode), the oxygen ions move toward the electrode 3b. And electrode 3
At b, the oxygen ions become oxygen molecules again and pass through the holes of the electrode 3b and are released to the outside. As described above, charge transfer occurs due to oxygen ionization and ion gasification, so that current flows through a circuit connecting the power supply and the element. The current flowing in the circuit increases with an increase in the applied voltage, but the amount of gas flowing into the solid electrolyte substrate 2 is limited by the porous electrode, and the voltage applied to the electrode is increased. However, the current value flowing through the circuit does not substantially change above a certain voltage value. This state is called a limit current state, and the current value (limit current value) in this limit current state reflects the partial pressure (concentration) of the gas to be detected. Make a detection.

In this type of gas detection element, as described above, the porosity of the porous electrodes 3a and 3b is reduced to limit the amount of gas flowing through the electrodes, thereby obtaining a limit current state. As the porosity of the electrode becomes smaller, the area of the three-phase interface between the electrode, the solid electrolyte, and the gas phase decreases, and the ionization rate decreases. Has the disadvantage that the accuracy of the method is greatly reduced.

A second type of gas detection element is disclosed, for example, in Japanese Patent Publication No. 59-26895.
It has a structure shown in FIG. In this gas sensing element, porous electrodes 3a are provided on both sides of the solid electrolyte substrate 2,
3b is provided and its cathode side (electrode 3a
The sealing member 4 is installed so as to form the internal chamber 6 on the side). Fine gas diffusion holes 5 communicating with the internal chamber 6 are formed in the sealing member 4, and the gas diffusion holes 5 restrict the flow of gas into the surface of the electrode 3 a. This type of gas detection element can measure up to a higher gas partial pressure than the first type of gas detection element described above.

A third type is disclosed, for example, in Japanese Patent Application No. 62-23603.
Are those shown in No. 4, having the structure shown in FIG. Here, minute gas diffusion holes 5 are formed in the solid electrolyte substrate 2.
Are provided, and both surfaces of the solid electrolyte substrate 2 are porous electrodes 3
a, 3b. Further, the cathode (electrode 3 a) is in a state of being sealed by the sealing member 4. After passing through the porous electrode 3b, the detection target gas reaches the internal chamber 6 through the gas diffusion holes 5 and the porous electrode 3a. In this third type of gas detection element, the porous electrode serves as a dust filter, and thus is resistant to dirt. In addition, since the outer surface 41 of the sealing member 4 is free, there is an advantage that a gas sensing element having a multi-function can be formed by forming a sensitive film of another kind of gas on this surface.

The fourth type is, for example, Japanese Patent Publication No. 2-48.
It is those shown in No. 58, and has a structure as shown in FIG. That is, the solid electrolyte porous electrode 3a on both sides of the substrate 2, provided with a 3b, covering the one electrode in the ceramic material 17 of the porous, this porous ceramic member 17, the electrodes (in the example of FIG. 6 The configuration is such that the gas flow into the electrode 3a) is restricted. In the gas detection element having this configuration, the porosity of the porous ceramic material 17 must be manufactured with good reproducibility, and the manufacturing conditions must be strictly controlled. This is because if the porosity of the ceramic material 17 changes for each product, the accuracy of the gas detection element varies.

In any of the above-described types, the limit current state is created by restricting the flow of the gas to be detected to the electrode surface. Compared with the second and third ones (that is, those with minute gas diffusion holes in the solid electrolyte substrate) from the viewpoint of performance variations coming from above and the reproducibility of the detection signal (sensor output) Many are used. However, in the second and third types of gas detection elements, foreign substances such as dust may be clogged in the minute gas diffusion holes. In this case, the gas diffusion resistance increases and the sensor output (limit current value) decreases. Therefore, there is a disadvantage that the accuracy and accuracy of the detection device are reduced. In fact, in a single-signal detection device using one sensor element, whether the concentration of the test gas has decreased and the sensor output has decreased, or whether the sensor output has decreased due to clogging of minute gas diffusion holes by foreign matter, Cannot determine. Usually, in this type of sensor element, the diameter of the gas diffusion hole is about several microns to several tens of microns, and there is no effective means for preventing clogging of the gas diffusion hole by foreign matter.

In order to reliably detect gas concentration, a method of simultaneously operating two or more sensors of the same type to extract respective detection signals and comparing these signals to obtain an accurate gas concentration can be considered. Is not economical.
In particular, in this type of sensor element, since the element is usually maintained at an operating temperature of about 300 to 450 ° C. by using a heater, only an increase in cost due to installing a plurality of sensor elements at one detection site is required. In addition, the power consumption of the heater is considerably increased.

Accordingly, it is an object of the present invention to provide a gas detection device that does not give an erroneous detection output and does not increase power consumption.

[0012]

As a result of intensive studies in view of the above-mentioned objects, the present inventor has found that a gas diffusion hole is provided on both sides of a single sensor substrate made of ceramics having a high electrical insulation in which a heater is embedded. In addition, two or more gas sensing element portions each having an internal chamber formed by the solid electrolyte substrate having positive and negative porous electrodes on both surfaces thereof, the sensor substrate, and the spacer are formed. At the same time test from
If the knowledge signal is taken out, reliable gas detection can be performed. Also, since only one heater is used, the power consumption of the gas detection device does not increase, and the heater is embedded in ceramics. We discovered that there was no deterioration, and came to the present invention.

That is, the gas detection device according to the present invention comprises: (a) a solid electrolyte substrate having gas diffusion holes on both surfaces of an electrically insulating ceramic sensor substrate having a heater embedded therein; Positive and negative porous electrodes formed on both sides, (c) a spacer disposed between the sensor substrate and the solid electrolyte substrate to form an internal chamber in combination with the sensor substrate and the solid electrolyte substrate. Are formed one by one.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a schematic sectional view showing a gas detection device according to an embodiment of the present invention. This gas detection device 1
In the first embodiment, a gas detecting element portion having the following configuration is formed on each of both surfaces of one sensor substrate 7 made of an electrically insulating ceramic having a heater 8 embedded therein. Gas detection element portions 1 formed on both sides of sensor substrate 7
Since 0 and 10 'have substantially the same configuration, only the structure of one of the gas detection element sections 10 will be described.

On one surface of the sensor substrate 7 in which the heater 8 is embedded, a solid electrolyte substrate 2 having minute gas diffusion holes 5 and a porous electrolyte formed on each surface of the solid electrolyte substrate 2 are provided. A gas detection element unit 10 having electrodes 3a and 3b and a spacer 9 provided between the solid electrolyte substrate 2 and the sensor substrate 7 and arranged to form an internal chamber 6 on the surface on which the electrode 3a is formed is provided. Is formed. Also, the electrodes 3a,
Leads 11 and 12 are connected to 3b, and are connected to a power supply and an ammeter arranged in series. Further heater 8
Lead wires 13 and 14 are also connected to an external power supply.

As described above, the internal chamber 6 is provided with the solid electrolyte substrate 2.
, The spacer 9 and the sensor substrate 7,
The internal chamber 6 is provided with the minute gas diffusion holes 5 (and the electrodes 3a,
Only the hole 3b) communicates with the outside.

As described above, the other surface of the sensor substrate 7 is formed with another gas detecting element portion 10 'substantially identical to the above-described structure. Therefore, as can be seen from FIG. 1, the sensor substrate 7 (and the heater 8) is a common part in the two gas detection element sections 10, 10 '.

When the gas to be detected is oxygen, a zirconia-based substrate that is an oxygen ion conductor is used as the solid electrolyte substrate 2. At this time, it is preferable to use zirconia to which at least one of yttria, calcia, ceria, and the like is added as a stabilizer. The solid electrolyte substrate is not limited to a zirconia-based substrate, and a material that conducts ions of the gas species may be appropriately selected and used according to a target gas to be used as a sensing element for a gas other than oxygen.

The electrodes 3a and 3b formed on both sides of the solid electrolyte substrate 2 are made of a porous material. Since the electrodes 3a and 3b function as catalyst activation electrodes, Pt, Pd, Ag, Rh, In
In order to prevent sintering, it is preferable to use a mixture of at least one of these metal materials, an alloy material thereof, and a sintering material such as zirconia and boron nitride. In particular, it is preferable to use Pt or a mixture of Pt and zirconia. The electrode 3a formed in the inner chamber 6 becomes a negative electrode.

As the spacer 9, it is preferable to use an inorganic substance (a sealing material such as a vitreous material) having the same thermal expansion coefficient as that of the solid electrolyte substrate 2.

The sensor substrate 7 is preferably formed from a ceramic member containing oxide ceramic, carbide ceramic or nitride ceramic as a main component. Specifically, oxide ceramics include Al ₂ O ₃ , MgO
, SiO ₂ , BeO, CaO, ThO ₂ , HfO ₂ , TiO ₂ , Cr ₂
O _{3 and the} like. The carbide ceramics include Cr ₂ C ₃ , Be ₂ C, TiC, ZrC, VC, NbC, HfC,
TaC, SiC, B ₄ C, Mo ₂ C, WC and the like. Further, as nitride ceramics, TiN, Si ₃ N ₄ , ZrN
, HfN, AlN and the like. Preferred ceramics include Al ₂ O ₃ , MgO, and SiO _{2 which} are industrially easily available and have relatively high strength.

As in the gas detector 1 shown in FIG. 1, the sensor substrate 7 has a heater embedded therein. As a material of the heater 8, platinum, a platinum-based alloy, nichrome, or the like can be used.

A sensor substrate 7 having a heater 8 embedded therein as shown in FIG. 1 is provided on a green sheet of ceramics (for example, alumina) which is a constituent material of the above-mentioned sensor substrate 7 on a green sheet of platinum, a platinum-based alloy, nichrome or the like. The paste can be manufactured by screen-printing the paste, further laminating a ceramic green sheet on the paste, and firing the green sheet. A through hole may be used to take out a lead wire connected to the embedded heater 8 from the sensor substrate 7.

A lead wire connected to the electrodes 3a and 3b and the heater 8 may be a platinum wire or the like.

When the gas concentration is detected by the gas detector 1 having the above structure, the heater 8 is heated to keep the gas detector 1 at a predetermined operating temperature. Electrode 3a, when a voltage is applied to 3b, the detection target gas is ionized at the surface of the electrodes 3a in the interior chamber 6 (when the detection target gas is oxygen, oxygen molecular ion O ₂ ^- a). Electrodes 3a, 3
Since a voltage is applied between the electrodes b, the ionized molecules move inside the solid electrolyte substrate 2 toward the other electrode 3b. The ionized molecules reaching the electrode 3b emit electrons there, return to the original molecules, and are emitted to the atmosphere. As a result, a current flows between the electrodes 3a and 3b. As described above, since the flow rate of the gas molecules to be detected flowing into the internal chamber is limited by the gas diffusion holes 5, the above-described current does not change even when the voltage applied to the electrodes 3a and 3b is changed. A limiting current condition occurs.
Since the magnitude of the limiting current changes according to the concentration of the gas to be detected in the atmosphere, the gas concentration can be known.

The gas detecting device 1 has one gas detecting element on each side of the sensor substrate 7. The characteristics of the solid electrolyte substrate in each of the gas detecting elements 10 and 10 ′, If the size, the configuration of the electrodes, and the like are the same, the limit current values obtained from the two gas detection element units 10, 10 'have substantially the same value. The two limit current values thus obtained are constantly compared, and if only one of the two signal values (limit current value) significantly decreases, it is determined that the decrease in the signal value is due to clogging of the gas diffusion holes. And adopt only high signal values. Since the probability of two small gas diffusion holes clogging at the same time is extremely low, adoption of an erroneous signal value due to the small gas diffusion hole clogging is substantially avoided.

To obtain a plurality of signals relating to the concentration of the gas to be detected at the same time, a plurality of independent sensors of the same quality may be used, but this is extremely disadvantageous in power consumption and cost. That is, in order to improve the ionic conductivity of the solid electrolyte substrate, the gas (detection) temperature of this type of gas detection element is usually set to 300 to 450 ° C., but the electric power consumed by the heater is low. This will account for the majority of the total power consumption of the sensor. If two independent sensors are used, the power consumption of the heater is substantially doubled. Further, the number of peripheral parts such as the storage cap of the sensor is also doubled.

However, in the gas detection device 1 according to the present invention described above, only one heater is required, which is almost equal to the power consumption of the conventional sensor element. In addition, since it is a gas detection device integrating two elements,
Except for a circuit section for receiving a plurality of signals, peripheral components do not increase.

Although the present invention has been described with reference to the accompanying drawings, the present invention is not limited to this, and various modifications can be made.

The present invention will be described in more detail with reference to the following specific examples. Example 1 A gas detector having the structure shown in FIG. 1 was prepared in the following manner. As a raw material of the solid electrolyte substrate 2, zirconia powder containing 8 mol% of yttria as a stabilizer was used. A rectangular parallelepiped block was formed by press molding using this raw material powder, and the block was sintered at about 1450 ° C. A minute gas diffusion hole of a predetermined size is formed in the obtained block, and the block is sliced and polished perpendicular to the axis of the minute gas diffusion hole to obtain a plate-like solid electrolyte substrate of 7 mm × 7 mm × 0.3 mm. I got

A conductive paste obtained by adding an organic binder and an organic solvent to platinum powder was applied to both sides of the substrate to a size of 4 mm × 4 mm by a screen printing method and fired to form electrodes. A platinum lead wire was attached to this electrode.

In the manufacture of the sensor substrate 7 in which the heater 8 is embedded, first, a green sheet is prepared from alumina powder, a platinum paste is screen-printed on one surface of the green sheet, and then a green sheet is formed on the surface to which the platinum paste is applied. One alumina green sheet was stacked and fired.

Also, using a silicate glass material as a spacer,
The sensor substrate 7, the spacer 9, and the solid electrolyte substrate 2 having electrodes were combined so as to have the structure shown in FIG. Glass paste was used for joining these members.

Using the gas detection device thus prepared, oxygen gas was detected in the following manner.

First, a DC voltage of 7 volts was applied to the heater 8, and the element temperature was maintained at 420.degree. The current flowing through the heater at this time was about 150 mA. In this state, a predetermined DC voltage was applied between the electrodes 3a and 3b of each gas detection element portion, and a current value flowing between the electrodes was measured with respect to the applied voltage. In this case, the current flowing between the electrodes is a current generated when oxygen dissolved from the negative electrode into the solid electrolyte substrate is ionized, moves to the positive electrode, and is gasified there. The measurement results are shown in Figure 2, the output of the two gas sensing element portion formed on the both surfaces of the sensor substrate 7 (current value) becomes substantially the same value, they were both expressed in the line of FIG. 2 (a). In the line (a), the current value I in the so-called plateau region where the current value hardly changes even when the applied voltage is changed.
₁ is an index indicating the oxygen partial pressure of the atmosphere in which this sensor is placed. In the case where fine gas diffusion holes of the solid electrolyte substrate is clogged by dust or the like, the output of the gas sensing element (current value), for example as shown in (b) of FIG.

[0036]

As described above, since the gas detecting device according to the present invention has two or more gas detecting elements, even if a minute gas diffusion hole in one of the gas detecting elements is clogged, accurate gas detection is possible. A detection output can be obtained. Further, since the heater in the gas detection device may be almost the same as that provided in a conventional element of this type, power consumption in the heater does not increase. Further, since the heater is embedded in the sensor substrate made of an electrically insulating ceramic, it does not deteriorate even when used for a long period of time, and can maintain a stable heating temperature at all times.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a gas detection device according to an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between an applied voltage and a current flowing between electrodes in Example 1, and also shows a graph showing a current value when a minute gas diffusion hole of a solid electrolyte substrate is clogged. .

FIG. 3 is a schematic sectional view showing an example of a conventional limiting current type gas detection device.

FIG. 4 is a schematic sectional view showing another example of a conventional limiting current type gas detection device.

FIG. 5 is a schematic cross-sectional view showing another example of a conventional limiting current type gas detection device.

FIG. 6 is a schematic sectional view showing still another example of the conventional limiting current type gas detector.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas detection device 2 Solid electrolyte substrate 3a, 3b electrode 5 Micro gas diffusion hole 6 Inner chamber 7 Sensor substrate 8 Heater 9 Spacer 10, 10 'Gas detection element 11, 12, Lead wire 17 Porous ceramic material

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-264858 (JP, A) JP-A-61-155751 (JP, A) JP-A-60-260842 (JP, A) JP-A 64-64 78148 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G01N 27/41

Claims (1)

(57) [Scope of request for utility model registration] 1. A solid electrolyte substrate having gas diffusion holes on both surfaces of a sensor substrate made of an electrically insulating ceramic having a heater embedded therein, and (b) positive and negative pores formed on both surfaces of the solid electrolyte substrate. A cathode electrode, and a gas detection element portion (c) disposed between the sensor substrate and the solid electrolyte substrate and having a spacer that forms an internal chamber in combination with the sensor substrate and the solid electrolyte substrate, A gas detection device formed one by one.