JPS6366449A - Gas detector - Google Patents

Abstract

PURPOSE:To achieve an accurate measurement of a gas partial pressure, by covering a pair of electrodes with a gas sensing layer varying in the electric resistance with gas components to make the contact area between the gas sensing layer and the electrode on the anode side larger than that between the layer and the electrode on the cathode side. CONSTITUTION:Electrode patterns 16 such as detection electrode patterns 16a and 16b and a thermal resistance electrode pattern 16e or the like are formed on a ceramic substrate 12 and a ceramic laminate plate 18 is laminated thereon integrally with the substrate 12. A porous gas sensing layer 24 which contains a gas sensitive metal oxide to vary in the electric resistance with components and concentration of a gas in the perimeter is formed within a window 20 of the laminate plate 18 and the gas sensing layer 24 supports a precious metal catalyst such as Pt and Rh. Among the electrode patterns 16, the anode side electrode pattern 16a contacting the gas sensing layer 24 and the cathode side electrode pattern 16b are rectangular and the contact area of the electrode pattern 16a is larger than that of the electrode pattern 16b. Here, the gas sensitive metal oxide used for the gas sensing layer 24 is selected according to a gas detected by a transition metal oxide such as TiO2.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to gas detection that detects surrounding gas using a gas-sensitive metal oxide whose resistance value changes depending on the concentration of a specific gas component. It is related to vessels.

[Conventional technology] The existence of gas or not! As gas detectors for detecting U, oxygen gas detectors, combustible gas detectors, etc. have been put into practical use.

Some of these use gas-sensitive metal oxides that have the property of changing their electrical resistance when they come into contact with gas. For example, T! Oxides of transition metal elements such as 02Coo and NiO can be used as oxygen sensors.

The transition metal oxides exemplified here are non-schemiochemical compounds. The amount of charge carriers (holes, electrons) in this non-stoichiometric compound changes depending on the partial pressure of the surrounding oxygen gas. Therefore, the conductivity changes depending on the partial pressure of the surrounding oxygen gas.

The gas detector described above is configured as shown in FIG. 19 in order to output the change in the electrical resistance value of the gas detection element due to the partial pressure of the gas to be detected as a voltage signal. A resistor Rc@Ku is placed in series with the gas detection element A, and a DC voltage V is applied to the series circuit of the resistor RC and the gas detection element A, and the voltage O generated across the resistor RC is used as a voltage signal. To detect. In other words, when the resistance value of the gas detection element A is smaller than the comparison resistor RC due to a change in the partial pressure of the gas to be measured, the voltage 0 generated across the resistor RC increases, and conversely, the resistance value of the gas detection element A increases The resistance value of A is the comparison resistance R
When ■ is larger than e, the voltage VO becomes smaller.

[Problems to be Solved by the Invention] Among the oxygen sensors mentioned above, for example, in the case of the Ti(h sensor), in a high-temperature oxidizing atmosphere, Ti" As O2 becomes deficient, oxygen vacancies decrease as 02- becomes excessive.For this reason, the resistance between the anode and the Ti0z gas-sensitive layer increases, and the resistance value of the gas-sensitive layer becomes different from the initial state. As a result, there was a problem that an accurate voltage signal for detecting gas partial pressure could not be obtained.

The present invention was made as a result of investigating the problems of the above-mentioned conventional techniques, and an object of the present invention is to provide a gas detector that is less likely to deteriorate due to changes over time.

[Means for Solving the Problems] The structure of the present invention as a means for solving the above problems includes: a pair of electrodes consisting of an anode and a cathode; and a gas-sensitive metal oxide covering the pair of electrodes. and a porous gas-sensitive layer whose electrical resistance changes depending on the surrounding gas components and/or their concentration, the contact area between the anode-side electrode and the gas-sensitive layer being The gist of the present invention is a gas detector characterized in that the contact area is larger than the contact area between the cathode side electrode and the gas-sensitive layer.

Here, the electrode is not particularly limited as long as it is a heat-resistant conductor, but it is usually made of tungsten, molybdenum, gold, or a platinum group metal as a main component.

The gas-sensitive metal oxide used in the gas-sensitive layer may be selected depending on the gas component to be detected.
TiO2,5n02, c
oo, zno, Nb:z 05 .

Examples include transition metal oxides such as Cr2O3 and NiO, and in the present invention, any one or a combination of two or more of these may be used.

The gas detector of the present invention can be produced, for example, by providing a gas-sensitive layer or the like on a ceramic substrate using a hybrid technique such as a thick film technique. Ori is used in thermistors and the like without using thick film technology or the like. It may be formed into a disk shape, a beat shape, or the like.

Furthermore, a heating element may be provided near the sensitive mass m for the purpose of reducing fluctuations in the temperature characteristics of the gas detector during measurement. Then, a part of this heating element and one electrode of the gas detector may be connected to apply a voltage to the gas-sensitive layer, thereby reducing the number of terminals and simplifying the measurement circuit.

Further, for the purpose of protecting the gas-sensitive layer, a coating layer may be provided on the gas-sensitive layer or an upper layer. This coating layer adsorbs and captures toxic substances such as lead on the gas-sensitive gold oxide, and prevents the toxic substances from reaching the gas-sensitive layer. The material of the coating layer is not particularly limited as long as it is thermally stable, and for example, alumina, magnesia spinel, zirconia, etc. can be used.

Further, the contact area between the anode side electrode and the gas sensitive layer is 1 of the contact area between the cathode side electrode and the gas sensitive layer.
．． It is desirable to be 5 times the amount - 1 d.

This is because if the area ratio is above, the increase in internal resistance between the two electrodes will be suppressed to within 40% compared to the case where both electrodes are the same, which will lessen the effect on gas partial pressure detection. It is.

[Function] In the gas detector of the present invention configured as described above, by applying a high bias between the anode and the cathode at high temperature, the gas-sensitive gold oxide which forms the gas-sensitive layer is heated. As an ionic current flows, the metal oxide near the anode becomes deficient in metal ions and in excess of oxygen, and the number of oxygen defects near the anode decreases.

As a result, the internal resistance on the positive side increases, but since the area of the anode in contact with the gas-sensitive layer is larger than the area of the cathode in contact with the gas-sensitive layer, even if oxygen defects are reduced due to the applied bias, the internal resistance on the anode side increases. Sufficient oxygen defects remain in the contacting gas-sensitive layer.

This suppresses an increase in the resistance value between the anode and the gas-sensitive layer, and provides a voltage signal corresponding to the gas partial pressure. As a result, gas partial pressure can be detected accurately.

[Example] An example of the present invention will be described using the drawings. Note that the scales of each figure are different for explanation purposes.

First, one embodiment of the present invention will be described with reference to FIG.

In this example, T! This is an oxygen gas detector 10 using Oz.

As shown in the partially broken perspective view of FIG. 1, terminals 13a and 13b are provided on the ceramic substrate 12.

Electrode patterns 16 such as detection electrode patterns 16a, 16b and a thermal resistance electrode pattern 168 connected to the platinum lead wires 14a, 14b, 14e at 13e are formed, and the electrode patterns 16 are further formed on the ceramic substrate 12 and on the electrode patterns 16.
A ceramic laminate plate 18 integrated with the ceramic substrate 12 is laminated thereon.

Among the electrode patterns 16, an anode-side electrode pattern '16a and a cathode-side electrode pattern 16 in contact with the gas-sensitive layer 24
b is a rectangle, and the area of the anode side electrode 14b is larger than the area of the cathode side electrode 14a.

A window 20 is formed in the ceramic laminate 18, and within this window 20, a sensitive gas @24 whose main component is T i O2 is formed. Spherical granulated particles 22 are interposed between the gas-sensitive layer 24 and the ceramic substrate 12 to prevent them from peeling off.

A catalyst consisting of Pt and Rh membered metal catalyst is supported on the gas-sensitive tube 24 .

Further, a coating layer 26 made of AΩ203 is formed on the gas-sensitive layer 24.

Next, the manufacturing process of the oxygen gas detector 10 will be explained with reference to FIGS. 2 to 6.

(2) Mix 2 wt% of alumina, 3 wt% of magnesia, and 5 wt% of sintering aids (silica, calcia, etc.) in a pot mill for 20 hours. Thereafter, 12 wt% of polyvinyl butyral and 4 wt% of dibutyl phthalate are added as organic binders to the mixture, and methyl ethyl ketone, toluene, etc. are added as solvents. 15 more with pot mill
The mixture is mixed for a period of time to form a slurry, and green sheets 12A and 18A for substrates and lamination are formed by a doctor blade method.

The shape of the green sheet above is board green sheet 1.
2A is 47.8mm x 4.0mm x 0°8mmt, green sheet for lamination 18A is 47°8mm x 4. Ommx
o, 26mmt. Then, a window portion 20 of 3.05 mm x 2.0 mm is formed on the green sheet for lamination 18A.

■ Next, mix platinum black and spongy platinum at a ratio of 2:1, add 10 wt% of the green sheet material mixture used in step (■) above, and add solvents such as butyl calpitol and ethocel. and use it as an electrode paste.

(2) Next, the electrode pattern 16 is formed on the substrate green sheet 12A by thick film printing using the electrode paste prepared in (2). As described above, the electrode patterns 16 include the detection electrode patterns 16a, 16b and the thermal resistance electrode pattern 16e which serves as the end for heating the gas-sensitive layer 24, and the terminal pattern which serves as the terminals of both of the patterns 16. '13a, 13b, 13e are formed. (Second
Figures (A) and (B)) The detection electrode patterns 16a and 16b are formed in proportions shown in Table 1 so that the area on the back anode side is larger than the area on the cathode side.

(2) After that, the terminal patterns 13a and 13b.

13e, a platinum lead wire 14a with a diameter Q and 2 mm.

14b and 14e respectively (Fig. 3 (a) and (b)).

(2) Next, the green sheets for lamination 18A are laminated and bonded by thermocompression onto the green sheet for substrate 12A to form a laminate. At this time, the window portion 2 of the green sheet for lamination 18A
0, the tips of the detection electrode patterns 16a and 16b are exposed. Then, after dispersing and adhering 80 to 150 meshes of spherical granulated particles (secondary particles) 22 made of the same material as the green sheet prepared in step (3) into the window portion 20,
Ceramic substrate 1 made into one piece by firing the above laminate at 1500″C in an atmosphere almost the same as the air for 2 hours.
2 and a ceramic laminate 18 are formed (FIGS. 4(A) and 4(B)).

When the spherical granulated particles 22 are dispersed and adhered and fired as described above, each particle 22 is dispersed on the ceramic substrate 12 to form an uneven surface.

■ Next, Tl is placed inside the window 20 of the ceramic laminate 18.
First, T! is filled with a gas-sensitive metal oxide whose main component is O2. Adjust Oz paste.

That is, T! with an average particle size of 1.2 μm calcined in the air at 1200°C for 1 hour! For 100 @ 1 part of Oz powder,
20 parts by weight of platinum black was added as a catalyst, 2 parts by weight of ethyl cellulose with 3% ethyl cellulose was added as a binder, and these were mixed in butycarpitol (trade name of 2-(2-butoxyethoxy)ethanol). Mix at 300
Adjust the TlO2 paste to a poise viscosity.

Then, this TiO2 paste is applied to a thickness of 20 to 50 μm on the electrode patterns 16a and 16b within the window portion 20 (FIGS. 5(A) and 5(B)).

(2) Next, a catalyst is supported on the gas-sensitive layer 24. First, chloroplatinic acid (Pt: 200q/Q) is supported on the gas-sensitive layer 24.
was dropped onto the 2.0 μΩ gas-sensitive layer 24, and then rapidly thermally decomposed at 950° C. in a propane burner to uniformly support the platinum catalyst.

(2) Next, on the gas-sensitive layer 20, a layer of A for the coating layer 26 is applied.
After applying the paste made of Q203, the laminate that has undergone the above steps is left to stand in the atmosphere at 1200° C. for 1 hour and fired (FIGS. 6(a) and 6(b)).

Incidentally, in order to confirm the effect of the embodiment of the present invention, the ratio of the areas of both electrode patterns 16a and 16b was set as shown in Table 1. At the same time, the comparative examples shown in Table 1 are also set in the same manner. This change in the area ratio of both electrode patterns 16a, 16b is performed by assuming that both electrode patterns '1 (3a, 16b have a constant length and interval), and that both electrode patterns 16a, 16b are
This is done by increasing the width wc of b toward the outside.

Internal resistance RT of the gas detector 10 created in this way
is first measured in a propane burner with a gas temperature of 350°C, with the air-fuel ratio set to λ = 0.9.

The method for measuring this internal resistance R1 is to add +1 to the lead wire 14e.
A voltage of 2V is applied, the lead wire 14a is connected to the ground, and a fixed resistor of 50Ω is connected between the lead wires 14a and 14b. The data obtained in this way is called initial data.

Next, a durability test of the gas detector will be conducted to examine changes over time in the examples of the present invention. First, a voltage of +20V is applied to the lead wire 14a in the atmosphere, and the lead wires 14b and 14e are
is connected to ground, and the 10100 On volume sensitive gas layer 24 is heated at about 1000°C. After heating, eight internal resistances of the gas detector 10 are measured in the same manner as the initial data measurement. The data thus obtained is referred to as post-endurance data.

The results of both of the above measurements are shown in Table 2.

Comparing the increase in internal resistance RT between the initial data and the post-durability data from Table 2, in the example of the present invention, No.
3 is 3.9 times, N004 is 1.8 times, and 1.1 times, which is not a significant increase, but in the comparative example, No. 1 is 41 times, N004 is 1.8 times, and 1.1 times
o,2 has increased significantly by 10 times. That is, the area of the anode side electrode pattern 16a is the cathode side electrode pattern 1.
The increase in internal resistance RT after durability is smaller when the area is set larger than that of 6b, and this effect is particularly noticeable as the area ratio on the anode side is larger.

Further, as shown in FIG. 7, a buried electrode 80 made of a platinum wire of 0.1φ is provided in the gas-sensitive layer 24 at a position equidistant from the electrode pattern 16a on the anode side and the electrode pattern 16b on the cathode side. This buried electrode 80 and the electrode pattern 1 on the anode side
The internal resistance RΦ between the electrode pattern 6a and the internal resistance Re between the buried electrode 80 and the cathode side electrode pattern 16b are measured in the same manner as in the above experiment, and changes in resistance values are measured at the initial stage and after durability. The data obtained by this measurement is shown in FIG.

What is shown by a solid line in FIG. 8 is the change in internal resistance RT between the electrode pattern 16a on the anode side and the electrode pattern 16b on the cathode side, and what is shown by a dashed line is the change in the internal resistance RT between the buried electrode 80 and the electrode pattern on the anode side. The change in internal resistance R between the patterns 16a, shown by dotted lines, is the buried electrode 80.
This is the change in internal resistance Re between the electrode pattern 16b on the cathode side and the electrode pattern 16b on the cathode side.

As is clear from FIG. 8, the increase in internal resistance RT due to the durability test is due to the increase in internal resistance R on the anode side. That is, in the embodiment of the present invention in which the area of the electrode pattern 16a on the anode side is set to be larger than the area of the electrode pattern 16b on the cathode side, the increase in the internal resistance RΦ on the anode side is small, and therefore the overall internal resistance RT is reduced. The increase is also small. Thereby, a detection signal corresponding to the partial pressure of the gas to be measured can be extracted, and the gas partial pressure can be accurately detected.

[Table 1] [Table 2] [Effects of the invention] As detailed above, in the gas detector of the present invention,
Since the area of the anode is larger than the cathode, there is little change in internal resistance between the two electrodes. Therefore, a voltage signal corresponding to the gas partial pressure can be extracted, and the gas partial pressure can be detected accurately.

[Brief explanation of the drawing]

FIG. 1 is a partially cut-away oblique view of a gas detector according to an embodiment of the present invention. Figure J2 and Figures 2 to 6 are diagrams showing the assembly procedure of the gas detector. In each figure, (A) is a front view, (B) is an end view taken along line A-A, and Figure 7 is a diagram showing the assembly procedure of the gas detector. FIG. 8 is an explanatory diagram showing a method for measuring the internal resistance of each electrode, FIG. 8 is a graph showing changes in internal resistance during a durability test, and FIG. 9 is an electric circuit diagram showing a conventional gas detector. 10...Gas detector 16b...Anode side electrode pattern 16a...Cathode side electrode pattern 16e...Heater

Claims (1)

[Claims] 1. A pair of electrodes consisting of an anode and a cathode, which covers the pair of electrodes, contains a gas-sensitive metal oxide, and whose electrical resistance changes depending on the surrounding gas component and/or its concentration. and a porous gas-sensitive layer, wherein the contact area between the anode-side electrode and the gas-sensitive layer is larger than the contact area between the cathode-side electrode and the gas-sensitive layer. gas detector. 2 The contact area between the anode-side electrode and the gas-sensitive layer is
1.5 of the contact area between the cathode side electrode and the gas-sensitive layer
The gas detector according to claim 1, which is more than twice as large.