JPH03237348A - Gas detector - Google Patents

Abstract

PURPOSE:To provide the gas detector which is less deteriorated by a secular change by setting the spacing between an anode and a cathode at >=0.5 times the max. thickness of a gas sensitive layer existing between the electrodes. CONSTITUTION:Electrode patterns 16 are formed on a ceramics substrate 12. The spacing W between the anode side electrode pattern 16a in contact with the gas sensitive layer 24 and the cathode side electrode pattern 16b of these electrode patterns 16 is formed to >=0.5 times the max. thickness (h) of the gas sensitive layer 24 existing between the electrodes. The current density near the electrode boundary of both electrodes is decreased by specifying the distance between the electrodes in such a manner and, therefore, the migration is suppressed and the increase in internal resistance is suppressed to a lower level. The deterioration of the electrodes in an oxidation atmosphere of a high temp. is, therefore, prevented and the exact taking out of the electric signal corresponding to the partial pressure of the gas is possible. In addition, the improvement in the durability of the gas detector is made as well.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a gas detector using a gas-sensitive metal oxide whose resistance value changes depending on the surrounding gas.

[Prior Art] Conventionally, oxygen gas detectors, combustible gas detectors, etc. have been put into practical use as gas detectors for detecting the presence or concentration of gas, and these detectors include, for example, TiO
Gas-sensitive metal oxides (gas-sensitive elements) whose electrical resistance changes when they come into contact with gas, such as oxides of transition metal elements such as 2, Coo and Ni○, are used.

This transition metal oxide is
It is a non-stoichiometric compound in which the amount of internal charge carriers (holes, electrons) changes, and therefore its conductivity changes depending on the partial pressure of the surrounding oxygen gas.

In the above gas detector, as shown in FIG. 9, a resistor Rc is provided in series with the gas-sensitive element A, and a DC voltage V is applied to the series circuit of the resistor Rc and the gas-sensitive element A. Body R
The voltage vO generated across e is detected as a voltage signal, thereby detecting the gas partial pressure.

In this gas detector (when the resistance value to the intuitive gas element A is smaller than the comparison resistance Rc, the voltage Vo generated across the resistor Rc becomes large, and conversely, the resistance value of the gas sensing element A becomes smaller than the comparison resistance When it is larger than Rc, the voltage V
o becomes smaller.

[Problems to be Solved by the Invention] On the contrary, the conventional gas detector described above has a problem in that an accurate output signal may not be obtained due to migration due to the movement of ions in the overlying gas element.

For example, in a sensor using TlO2, in a high-temperature oxidizing atmosphere, an ion current flows due to the bias between the cathode and anode, so the anode side is in a state of insufficient T4 and an excess of 02-. Oxygen defects are reduced. Therefore, the resistance between the anode and the gas-sensitive element increases, and the resistance value of the gas-sensitive element differs from the initial state value and deteriorates. As a result, there was a problem in that an accurate voltage signal for gas partial pressure detection 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 present invention, which solves the above-mentioned problems, includes a pair of electrodes consisting of an anode and a cathode, a gas-sensitive metal oxide that covers the pair of electrodes, and a gas-sensitive metal oxide that covers surrounding gas components and and/or a porous gas-sensitive layer whose electrical resistance changes depending on its concentration, wherein the distance between the two electrodes is 0.000 mm of the maximum thickness of the gas-sensitive layer existing between the electrodes. The gist of the present invention is a gas detector characterized by 5 times or more.

Here, it is desirable that the distance between the anode and the cathode is 1.5 times or more the maximum thickness of the gas-sensitive element existing between the two electrodes. That is, if the distance between the electrodes is increased by more than this ratio, the increase in internal resistance between the electrodes can be suppressed to within 50% compared to a ratio of 0.5 times or less.

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

Furthermore, the gas-sensitive metal oxide used in the gas-sensitive layer may be selected depending on the gas component to be detected, but TiO2,5 is commonly used.
n02. CaO, Zn○, Nb20B, Cr2
Examples include transition metal oxides such as O3 and Ni○.In the present invention, any one or a combination of two or more of these may be used.

Further, for the purpose of protecting the gas-sensitive layer, a coating layer may be provided over the gas-sensitive layer or the upper layer. This coating layer (adsorbs and captures toxic substances such as lead on gas-sensitive metal oxides and prevents the toxic substances from reaching the gas-sensitive layer.The material of the coating layer is thermally stable. There is no particular limitation as long as the material is suitable, and for example, alumina, magnesia spinel, zirconia, etc. can be used.

[Function] In the gas detector of the present invention configured as described above (
By applying a high bias between the anode and cathode at higher temperatures, an ionic current flows through the gas-sensitive metal oxide forming the gas-sensitive layer. This results in a shortage of metal oxides (friend metal ions) near the anode and an excess of oxygen. As a result, the number of oxygen defects near the anode decreases, increasing the internal resistance on the anode side.

Therefore, if the distance between the anode and cathode in contact with the gas-sensitive layer is set to be wider than a predetermined ratio, the current density applied to the interface between the gas-sensitive layer and the electrode will decrease, which will make it difficult for the ionic current to flow. This suppresses the increase in resistance between the gas-sensitive layer and the gas-sensitive layer.

As a result, a voltage signal corresponding to the gas partial pressure is obtained, and the gas partial pressure can be accurately detected.

[Embodiment] First, an embodiment of the present invention will be described based on FIGS. 1 to 8.

This example is an oxygen gas detector (gas detector) 10 using T i 02 as the upper gas layer.

As shown in the partially broken perspective view of FIG.

13b, 13e; platinum lead wires 14a, 14b,
14e is connected. The electrode pattern 16 is composed of the detection electrode patterns 16a, 16b and the thermal resistance electrode pattern 16e.
A ceramic plate 18 integrated with a ceramic substrate 12 is laminated.

Among the electrode patterns 16, the distance W between the anode side electrode pattern 16a and the cathode side electrode pattern 16b which are in contact with the gas-sensitive layer 24 is determined by the maximum thickness h of the gas-sensitive layer 24 existing between the electrodes, as shown in FIG. 0°5 times or more (for example, 0.35
ROM).

Further, a window portion 20 is formed in the ceramic plate 18 shown in FIG. 1, and a gas-sensitive layer 24 containing E TlO2 as a main component is formed in this window portion 20. Spherical granulated particles 22 are interposed between the gas-sensitive layer 24 and the ceramic substrate 12 to prevent them from peeling off.

Further, a catalyst made of noble metals 11 Pt and Rh is supported on the gas-sensitive layer 24, and a coat layer 26 made of Al2O3 is further formed to cover this gas-sensitive layer 24.

Next, the manufacturing process of the oxygen gas detector [0] will be explained based on FIGS. 3 to 7.

(2) 92 wt% alumina, 3 wt% magnesia, and 5 wt% sintering aids (silica, calcia, etc.) are mixed in a pot mill for 20 hours. Thereafter, 12% by weight of polyvinyl butyral and 4% by weight of dibutyl phthalate are added as an organic binder to the mixture, and methyl ethyl ketone, toluene, etc. are added as a solvent. Further, the mixture is mixed in a pot mill for 15 hours 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 47.8mmX4.0mmX0°8mm', 18A green sheet for lamination is 47.8mmX4. Ornm
Xo, 26 mm'. Then, on the green sheet for lamination 18A, 3-05mm x 2. A window portion 20 of 0 mm is formed.

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

(2) Next, using the electrode paste prepared in (2) above, the electrode pattern 16 is formed on the substrate green sheet 12A by thick film printing. This electrode pattern 16 includes detection electrode patterns 16a, 16b, and gas-sensitive layer 2.
4, and a terminal pattern 13a that serves as a terminal for both patterns 16.
, 13b, 13e are formed. (Figure 3 (a), (
b)) Here, the rain detection electrode patterns 16a, 16
The interval W of b is the thickness h of the gas-sensitive layer 24 existing between the electrodes.
0. The distance W is formed to be 5 times or more, for example, 0.35 mm.

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

13e, a platinum lead wire 14a with a diameter of 0.2 mm.

14b and 14e respectively (Fig. 4 (a) and (b)).

(2) Next, the lamination green sheet 18A is laminated and thermocompressed onto the substrate green sheet 12A to form a laminate. At this time, the window 20 to the laminating green sheet 18
(The tips of the detection electrode patterns 16a and 16b are exposed.Then, in the window 20, 80 to 150 mesh spherical granulated particles (secondary particles) made of the same material as the green sheet prepared above are inserted. ) 22 is dispersed and adhered, and then the above-mentioned laminate is fired at 1500'C for 2 hours in an atmosphere that is almost the same as the air, thereby forming an integrated ceramic substrate 12 and ceramic plate 18 (see Fig. 5 ( stomach)
(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, in order to form the gas-sensitive layer 24, the inside of the window 20 of the ceramic plate 18 is filled with a gas-sensitive metal oxide whose main component is TlO2.First, the TlO2 paste is prepared. .

That is, the average particle size after calcining in the atmosphere at 1200°C for 1 hour was 1.
20 parts by weight of platinum black was added as a catalyst to 100 parts by weight of 2 μm TlO2 powder, and further 1. - adding only 2 parts by weight of 3% by weight ethyl cellulose as a binder;
These are mixed in butycarpitol (a trade name for 2-(2-butoxyethoxy)ethanol) to a viscosity of 300 poise to prepare a T i 02 paste.

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. 6(A) and 6(B)).

(2) Next, a catalyst is supported on the gas-sensitive layer 24. First, chloroplatinic acid (Pt: 200 g/l) is
．． The platinum catalyst is uniformly supported by dropping it onto the 0 μm gas-sensitive layer 24 and then rapidly thermally decomposing it at 950° C. in a propane burner.

(2) Next, on the gas-sensitive layer 24, a layer of A for the coating layer 26 is applied.
After applying the paste consisting of 12O3, the laminate which has undergone the above steps is fired in the atmosphere at 1200'C (for 21 hours) (FIGS. 7(a) and 7(b)).

Next, an example of an experiment conducted to confirm the effects of this example will be described.

(Experiment example) In this experiment, both electrode patterns 16a of the gas detector 10
, 16b (distance between the inner ends) W and the maximum thickness h of the gas-sensitive layer 24 existing between the electrodes is set as shown in Table 1. Note that both electrode patterns 16a, 1
The length and width of 6b were constant.

Table 1: The internal resistance RT of the gas detector 10 prepared in this way is measured at the air-fuel ratio λ=
Set to 0.9 and perform initial measurements.

How to measure this internal resistance RT (lead wire 1 in Figure 1)
Apply +12V voltage to 4e, connect lead wire 14a'l to ground, and connect 50Ω between leads 14a and 14b.
This is done by connecting a fixed resistor. The data obtained in this way is called initial data.

Next, a durability test of the gas detector 10 is performed in order to examine the change over time in this example.

First, a voltage of +20V is applied to the lead wire 14a in the atmosphere, a voltage of 5V is applied to the lead wire 14b, and
The lead wire 14e is connected to ground, and the gas-sensitive layer 24 is heated at about 1000° C. for 300 minutes. After heating, the internal resistance RT of the gas detector 10 is measured in the same way as the initial data measurement. The data obtained in this way is called post-endurance data.

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

Table 2 Comparing the increase in internal resistance RT between the initial data and the post-durability data from Table 2 and FIG. It is not so large, less than 5 times, and in particular, the value of Yin 6 to 10 is small, less than 2.5 times, but in the comparative example, it is 9
It has increased significantly by double. That is, the distance W between the anode-side electrode pattern 16a and the cathode-side electrode pattern 16b is set to 0.000% of the maximum thickness h of the gas-sensitive layer 24 existing between the electrodes. When the value is set to 5 times or more, the increase in internal resistance RT after durability is small. Among these, Nct6 to NclOO, in which the above ratio is set to 1.5 times or more, has an internal resistance RT after durability.
The increase in both electrode patterns 16a is particularly small.
, 16b, the larger the distance W between the two, the more remarkable the effect is. Note that if the internal resistance RT increases by a factor of 5 or more, the characteristics will be significantly degraded, but if the increase is about 2 times or more, there will be no significant effect.

[Effects of the Invention] As detailed above, in the gas detector of the present invention,
By setting the distance between the anode and the cathode (at least 0.5 times the maximum thickness of the gas-sensitive layer existing between the electrodes), the current density near the electrode interface of both electrodes is reduced, suppressing migration and increasing the internal It is possible to suppress the increase in resistance to a low level.Therefore, it is possible to prevent electrode deterioration in high-temperature oxidizing atmospheres, to accurately extract electrical signals corresponding to gas partial pressure, and to increase the durability of the gas detector. can be improved.

[Brief explanation of drawings]

Fig. 1 is a partially cutaway perspective view of a gas detector according to an embodiment of the present invention, Fig. 2 is a sectional view showing the main parts of this embodiment, and Figs. 3 to 7 are a gas detector. It is a diagram showing the assembly procedure of
In each figure, (A) is a front view @ (B) is an end view along line A-A, Figure 8 is a graph showing the change in internal resistance during one durability test, and Figure 9 is an electrical diagram showing a conventional gas detector. It is a circuit diagram. O... Gas detector 6b... Anode side electrode pattern 6a... Cathode side electrode pattern 6e... Heat resistance electrode pattern 4... Gas sensitive layer

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. A gas detector equipped with a porous gas-sensitive layer, characterized in that the distance between the two electrodes is 0.5 times or more the maximum thickness of the gas-sensitive layer existing between the electrodes. vessel.