JPH0754312B2 - Gas detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas detector for detecting a gas component or its concentration, and particularly to gas detection using a gas-sensitive metal oxide. Regarding vessels.

[Prior Art] Conventionally, as a gas detector of this type, for example, there is an oxygen sensor shown in cross section in FIG.

That is, the oxygen sensor 50 has a pair of anode and cathode 54a, 54b formed on a ceramic substrate 52, and on the substrate 52,
Laminating the ceramic laminate 60 having the window portion 58, and further,
A gas sensitive layer 62 containing TiO ₂ as a main component is laminated so as to cover the electrodes 54a and 54b and block the window 58, and the gas sensitive layer 62 is further activated at 500 ° C. to activate the gas sensitive layer 62. Heaters 64 and 65 are provided for heating to ~ 700 ° C.

[Problems to be Solved by the Invention] By the way, in the above oxygen sensor, Ti ⁴⁺ becomes insufficient on the anode side and excess O ^2- And oxygen defects are reduced. Therefore, the resistance between the anode and the gas-sensitive layer of TiO ₂ increases, and the resistance value of the gas-sensitive layer becomes different from the initial state and deteriorates. On the other hand, even on the cathode side, due to the ionic current, Ti ^{4+ is in} an excess state and O ^{2 − is in} a shortage state, but since O ^{2 −} is replenished from the atmosphere, a porous appearance made of TiO _{2 is} obtained. The gas layer sinters to reduce the surface area.
Due to such progress of sintering of the gas-sensitive layer, there is a problem that an accurate response cannot be obtained with respect to a change in gas concentration.

The present invention has been 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 which is less deteriorated due to aging.

[Means for Solving Problems] The present invention employs the following means in order to solve the above problems.

That is, the gist of the first invention is a pair of electrodes consisting of an anode and a cathode, a gas-sensitive metal oxide that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electrical resistance depending on the surrounding gas component and / or its concentration. And a heating means for heating the gas-sensitive layer, and the heating means is provided so as to heat the gas-sensitive layer on the anode side to a higher temperature than the gas-sensitive layer on the cathode side. It is a gas detector characterized in that

Here, the electrode is not particularly limited as long as it is a heat-resistant conductor, but normally, an electrode containing tungsten, molybdenum, silver, gold or a platinum group as a main component is used.

As the gas-sensitive metal oxide used in the gas-sensitive layer, the substance may be selected according to the gas component to be detected,
TiO ₂ , SnO ₂ , CoO, ZnO, Nb are usually used.
Examples thereof include transition metal oxides such as ₂ O ₅ , Cr ₂ O ₃ , 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 by a hybrid technique such as a thick film technique. Alternatively, it may be formed into a disk type, a bead type or the like used in a thermistor or the like without using the thick film technique or the like.

Then, a part of the heating element and one electrode of the gas detector may be connected to apply a voltage to the gas sensitive layer to reduce the number of terminals and simplify the measurement circuit.

The heating element as the heating means may be provided on the same ceramic substrate as the anode and the cathode, or may be provided on the opposite surface. Furthermore, the materials of the anode and cathode may be different. For example, change the material of anode and cathode to Pt, Pd
Noble metal with catalytic properties, such as W, Re
High melting point metals such as

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

[Operation] In the present invention, the temperature of the gas partial pressure is measured based on the change in the resistance value between the electrodes while the gas sensitive layer is heated to a predetermined temperature.

Further, the gas detector of the present invention is provided with heating means for setting the temperature around the anode side to be higher than that around the cathode side. By this heating means, for example, when a gas sensitive layer made of TiO ₂ is applied to the gas detector, the temperature of the gas sensitive layer on the anode side is in the range of 500 ° C. to 700 ° C. which is most suitable for detection of normal gas. Set higher than 800 ℃, higher than. When TiO ₂ is heated to this temperature, the conductivity of TiO ₂ shifts to the property as an intrinsic semiconductor.

Therefore, in the TiO ₂ layer on the anode side having the characteristics of an intrinsic semiconductor, a conductive action is performed that does not depend on the number of lattice defects, so that it is possible to suppress a change over time due to an increase in resistance between the anode and the gas-sensitive layer.

Since the gas-sensitive layer on the anode side is less deteriorated with the progress of heating and sintering, heating to a high temperature does not deteriorate the characteristics of the sensor so much, but on the cathode side, the influence of deterioration due to heating and sintering is small. Although large, since the gas sensitive layer on the cathode side is set to a normal activation temperature, deterioration due to sintering on the cathode side is not increased.

[Embodiment] An embodiment of the present invention will be described with reference to the drawings. Note that the scale of each drawing is different for the sake of explanation.

First, an embodiment of the first invention will be described with reference to FIG.

In this example, TiO ₂ supporting a silver-based alloy as a gas sensitive layer was used.
It is an oxygen gas detector 10 using.

As shown in the partially broken perspective view of FIG. 1, platinum lead wires 14a, 14 are provided on the ceramic substrate 12 with terminals 13a, 13b, 13e.
b, electrode pattern 16a such as detection electrode patterns 16a, 16b connected to 14e and thermal resistance electrode pattern 16e are formed,
Further, a ceramic laminated plate 18 integrated with the ceramic substrate is laminated on the ceramic substrate 12 and the electrode pattern 16.

Of the electrode patterns 16, the anode side electrode pattern 16a and the cathode side electrode pattern 16b which are in contact with the gas sensitive layer 24 are rectangular, and the periphery thereof is a thermal resistance electrode pattern 16 which is a heater.
e is formed in a U shape. The heat generating portion 16ea on the anode side of the thermal resistance electrode pattern 16e is the heat generating portion 16e on the cathode side.
Its width is narrower than that of b, which increases the amount of heat generated on the anode side.

A window portion 20 is formed in the ceramic laminated plate 18, and a gas-sensitive layer 24 containing TiO ₂ as a main component is formed in the window portion 20.
Are formed. This gas sensitive layer 24 and the ceramic substrate
Spherical granulated particles 22 are interposed between the particles 12 to prevent separation between the particles.

The gas-sensitive layer 24 carries a catalyst made of a noble metal such as Pt or Ph.

A coat layer 26 made of Al ₂ O ₃ is formed on the gas sensitive layer 24.

Next, the manufacturing process of the oxygen gas detector 10 will be described with reference to FIGS.
It will be described with reference to the drawing.

92 wt% alumina, 3 wt% magnesia, and 5 wt% sintering aid (silica, calcia, etc.) are mixed in a pot mill for 20 hours. Then, the mixture was mixed with polyvinyl butyral 12 wt% and dibutyl phthalate 4 wt% as an organic binder.
Was added, and methyl ethyl ketone, toluene and the like were added as a solvent. Further, the mixture is mixed in a pot mill for 15 hours to form a slurry, and the doctor blade method is used to form the green sheets 12A and 18A for substrates and for lamination.

The shape of the above green sheet is the green sheet 12A for the substrate.
47.8mm × 4.0mm × 0.8mm ^t , with laminated green sheet 18A
It has a size of 47.8 mm × 4.0 mm × 0.26 mm ^t , and a 3.05 mm × 2.0 mm window portion 18a is formed in the lamination green sheet 18A.

Next, platinum black and sponge-like platinum were blended in a ratio of 2: 1, 10 wt% of the material mixture of the green sheet used above was added, and a solvent such as butyl carbidol and Ethocel was added, Use as electrode paste.

Next, an electrode pattern 16 is formed on the green sheet 12A for the substrate by thick film printing using the electrode paste adjusted in.
To form. As the electrode pattern 16, as described above,
Detection electrode patterns 16a, 16b, and a thermal resistance electrode pattern 16e that serves as a heater for heating the gas-sensitive portion 20, and terminal patterns 13a, 13b, 13e that serve as terminals of both patterns 16 described above.
Are formed (Fig. 2 (a), (b)).

Of the thermal resistance electrode pattern 16e, the heat generating portion 16e on the anode side
The width of a is formed to have a width shown in Table 1 so as to be smaller than the width of the heating portion 16eb on the cathode side.

Then, the terminal pattern 13a, 13b, 13e, diameter 0.2
The platinum lead wires 14a, 14b, 14e of mm are connected to each other (Fig. 3 (a) (b)).

Next, the lamination green sheet 18A is laminated and thermocompression-bonded on the substrate green sheet 12A to form a laminate.
At this time, the window portion 20 of the green sheet for lamination 18A,
The tips of the detection electrode patterns 16a and 16b are exposed. Then, 80-150 mesh spherical granulated particles (secondary particles) 22 made of the same material as the green sheet prepared in step 20 are dispersed and adhered, and then the above-mentioned laminated body is exposed to the atmosphere at 1500 ° C. By firing for 2 hours in the same atmosphere, the integrated ceramic substrate 12 and ceramic laminated plate 18 are formed (FIGS. 4A and 4B).

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, the window 20 of the ceramic laminate 18 is filled with a gas-sensitive metal oxide containing TiO ₂ as a main component. First, a TiO ₂ paste is prepared.

That is, the average particle size of 1.2 after calcination in the air at 1200 ° C for 1 hour
was added 3% by weight of ethyl cellulose relative to TiO ₂ powder [mu] m, which were mixed in butyronitrile carbitol (2- (2-butoxyethoxy) tradename ethanol), TiO ₂ with respect to 300 poise viscosity Adjust the paste. Then, the window portion 18a is filled with this TiO ₂ paste by the thick film printing technique (FIGS. 5A and 5B).

Next, a catalyst is supported on the gas sensitive layer 24. First, 2.0 μl of chloroplatinic acid) PT: 200 / l) is added to the gas sensitive layer 2
Then, the platinum catalyst is uniformly supported by rapid thermal decomposition at 950 ° C. in a propane burner.

Next, after a paste made of Al ₂ O ₃ for the coat layer 26 is applied on the gas sensitive layer 20, the laminate is left standing in the air at 1200 ° C. for 1 hour to be baked. (Fig. 6 (a),
(B)).

In addition, in order to confirm the effect of the above-described embodiment, a sample was prepared in which the width of the heat generating portion 16ea on the anode side and the width of the heat generating portion 16eb on the cathode side of the thermal resistance electrode pattern 16e were changed as shown in Table 1. In addition, a comparative example corresponding to the conventional technique is also prepared and described in the same table. Here, the distance between the anode and the cathode and the thermal resistance electrode pattern 16e was set to be the same.

The internal resistance RT of the gas-sensing detector 10 thus created is first measured in the propane burner at a gas temperature of 350 ° C. with an air-fuel ratio λ =
Set to 0.9 and measure. This internal resistance RT is measured by applying a voltage of + 12V to the lead wire 14e,
Is connected to the ground, and between lead wire 14a and lead wire 14b.
Connect with a fixed resistance of 50KΩ. The data obtained by this is called initial data. Next, a durability test of the gas detector 10 is carried out in order to investigate the change with time of the embodiment of the present invention. First, apply + 14V voltage to the lead wire 14a in the atmosphere, connect the lead wires 14b and 14e to the ground, and
The gas sensitive layer 24 is heated at 00 ° C. for 1000 hours. After heating, the internal resistance RT of the gas detector 10 is measured in the same manner as the measurement of the initial data. The data obtained by this is called post-durability data.

In this experiment, the resistance value of the thermal resistance electrode pattern 16e at 20 ° C. is measured, and further, before forming the gas sensitive layer 24, the thermal resistance electrode pattern 16e heats the detection electrode patterns 16a and 16b. , Measure the surface temperature with an infrared thermometer.

The results of the above measurements are also shown in Table 1. In Table 1,
The total resistance of the thermal resistance electrode pattern 16e at 20 ° C is shown as a heater resistance, and the detection electrode pattern 16a when heated is used.
The surface temperature of 16b is displayed as the surface temperature.

Comparing the increase in internal resistance RT between the initial data and the data after endurance from Table 1, in the comparative example, the internal resistance RT is
Although this is a very large increase of 9 times or more, in the examples of the present invention, the increase of the internal resistance RT after endurance is extremely small at 1.6 or less, and the effect of the present examples is remarkable.

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.
The internal resistance R between the embedded electrode 80 and the electrode pattern 16a on the anode side and the internal resistance R between the embedded electrode 80 and the electrode pattern 16b on the cathode side were set in the same manner as in the above experiment to obtain the resistance values of the initial and endurance values. Measure the change. The data obtained by this measurement are shown in FIG.

The solid line in FIG. 8 indicates the internal resistance RT between the electrode pattern 16a on the anode side and the electrode pattern 16b on the cathode side.
Changes in the internal resistance R between the embedded electrode 80 and the electrode pattern 16a on the anode side, and those indicated by the dotted line indicate the changes in the internal resistance R between the embedded electrode 80 and the electrode pattern 16b on the cathode side. This is a change in the internal resistance R during the period.

As is clear from FIG. 8, in the embodiment of the present invention in which the width of the heat generating portion 16ea on the anode side is set to be smaller than the width of the heat generating portion 16eb on the cathode side, the increase of the internal resistance R on the anode side is small. Therefore, the increase of the total internal resistance RT is also small. As a result, a detection signal corresponding to the partial pressure of the measurement gas can be taken out, and the gas partial pressure can be accurately detected.

Further, as a gas detector 10A of another embodiment, as shown in FIG. 9, thermal resistance electrode patterns 16EA and 16EB are provided on one surface of the ceramic substrate 12 to form an anode and a cathode electrode pattern.
16A and 16B may be provided on other surfaces.

Further, as another embodiment, as shown in FIG. 10, a ceramic substrate 120 is formed from two ceramic substrates 120A and 120B, and one of the ceramic substrates 120A has an electrode pattern 12
0a, 120b are formed on the other ceramic substrate 120B, and a material different from the electrode patterns 120a, 120b (for example, W, Re, etc.)
The thermal resistance pattern 120e may be formed by and the both substrates may be thermocompression bonded. This makes it possible to form patterns having different firing conditions on an integrated substrate, and avoid costly electrodes such as platinum to reduce costs.

[Effects of the Invention] As described in detail above, according to the gas detector of the present invention, the gas sensing layer on the anode side is heated to a higher temperature than the gas sensing layer on the cathode side by the heating means. The transfer of electrons to and from the gas layer becomes active and does not depend on the number of defects. Therefore, the change in internal resistance is small, that is, the change over time can be reduced.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of an oxygen sensor according to an embodiment of the present invention, FIGS. 2 to 6 are explanatory views of the manufacture of the embodiment, and FIG. 7 is an internal resistance of each electrode. FIG. 8 is an explanatory view showing a measuring method, FIG. 8 is a graph showing a change in internal resistance in a durability test, FIG. 9 is an explanatory view showing another embodiment, and FIG. 10 is an explanatory view showing yet another embodiment. FIG. 11 is a sectional view showing a conventional gas-sensitive detector. 10, 10A …… Oxygen sensor (gas detector), 12 …… Ceramic substrate, 16a, 16b …… Detection electrode pattern, 16e …… Thermal resistance electrode pattern, 16ea …… Anode side heating part, 16eb …… Cathode Side heating part, 18 …… ceramic laminated plate, 20 …… window part,
24 …… Gas sensitive layer,

Claims (1)

[Claims] 1. A pair of electrodes consisting of an anode and a cathode, and a porous material which covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electric resistance that changes according to the surrounding gas component and / or its concentration. And a heating means for heating the gas sensitive layer, wherein the heating means is provided so as to heat the gas sensitive layer on the anode side to a higher temperature than the gas sensitive layer on the cathode side. Gas detector.