JPH07107522B2 - 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 a gas using a gas-sensitive transition metal oxide. Regarding the detector.

[Prior Art] Conventionally, an oxygen gas detector, a flammable gas detector, and the like have been put into practical use as gas detectors for detecting the presence or concentration of gas.

Among these, there is one using a gas-sensitive metal oxide having a characteristic that its electric resistance changes when it comes into contact with gas. For example, oxides of transition metal elements such as TiO ₂ CoO and NiO can be used as oxygen gas detectors.

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

[Problems to be Solved by the Invention] Among the above oxygen gas detectors, for example, in a TiO ₂ sensor, an excess current of Ti ⁺⁴ is present around the cathode due to an ion current due to a bias between the cathode and the anode. together it becomes, becomes the O ^2- starved of, the O ^2- is replenished from the atmosphere to sinter the porous gas-sensitive layer composed of TiO ₂ proceeds.
That is, as the sintering proceeds, the particles are gradually fused and bonded to each other, so that the total surface area of the particles around the cathode among the particles forming the porous gas-sensitive layer is reduced. Such progress of sintering of the gas-sensitive layer causes a delay in response to changes 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 present invention provides a pair of electrodes consisting of an anode and a cathode, and a porous material which covers the electrodes and contains a gas-sensitive transition metal oxide, and whose electric resistance changes depending on the gas component and / or the gas concentration. And the average particle size of the particles forming the porous gas-sensitive layer around the cathode, and the average particle size of the particles forming the porous gas-sensitive layer around the anode. The gist is a gas detector characterized by being made larger than the diameter.

Here, the above-mentioned electrode is not particularly limited as long as it is a heat-resistant conductor, but in general, one containing tungsten, molybdenum, gold or 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 ₂ O ₅ ,
Examples thereof include transition metal oxides such as 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.

Furthermore, a heating element may be provided in the vicinity of the gas-sensitive layer 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 to reduce the number of terminals and simplify the measurement circuit.

[Operation] In the present invention, the average particle size of the particles forming the porous gas-sensitive layer around the cathode is made larger than the average particle size of the particles forming the porous gas-sensitive layer around the anode in advance. That is, since the adsorption area of the gas sensitive layer around the cathode is made small in advance, the rate of reduction of the adsorption area due to sintering can be made small, and therefore deterioration due to aging such as responsiveness can be suppressed. .

[Advantages of the Invention] As described above, according to the gas detector of the present invention, deterioration due to use can be reduced and stable characteristics can be obtained. That is, according to the present invention, the porous structure around the cathode can be obtained. Since the average particle size of the particles forming the high quality gas-sensitive layer is made larger than the average particle size of the particles forming the porous gas-sensitive layer around the anode in advance, that is, Since the specific surface area of the gas layer (with respect to the gas-sensitive layer around the anode) has been made small in advance, the change in specific surface area due to sintering from the initial state is small, and deterioration of responsiveness can be reduced.

[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 present invention will be described with reference to FIG.

The present embodiment is an oxygen gas detector 10 using TiO ₂ as a gas sensitive layer.

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.
Detection electrodes 16a and 16b and thermal resistance electrodes connected to b and 14e
An electrode pattern 16 such as 16e is formed, and a ceramic substrate is further provided on the ceramic substrate 12 and the electrode pattern 16.
First ceramic laminate 18 integrated with 12 and first
The second ceramic laminated plates 19 are laminated on the ceramic laminated plates 18 of FIG. This first ceramic laminate
Anode side and cathode side lower window portions 20a, 20b divided by the partition portion 18a are formed in the partition 18, and the second ceramic laminated plate 19 further includes the window portions in addition to the lower window portions 20a, 20b. 20
The upper window portion 20c that forms the is formed.

The anode side and cathode side lower window portions 20a, 20b of the window portion 20 are filled with the anode side and cathode side paste having the composition shown in Table 1 to form the anode side and cathode side gas-sensitive lower layers 24a, 24.
b is formed on the upper window portion 20c, and normal TiO2
The gas sensitive upper layer 24c is formed by being filled with the TiO2 paste forming the gas sensitive layer. The gas sensitive layer 24 is formed by these layers. In addition, spherical granulated particles 22 are interposed between the ceramic substrate 12 and the gas-sensitive layer 24 to prevent them from separating.

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

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 12 wt% of polyvinyl butyral as an organic binder and 4 wt% of dibutyl phthalate.
%, 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 make a slurry, and the green sheets 12A, 18A, 19A for substrates and for lamination are formed by the doctor plate method.

The shape of the above green sheet is the green sheet for substrate 12
The A is 47.8 mm × 4.0 mm × 0.8 mm ^t , and the first and second lamination green sheets 18A and 19A are 47.8 mm × 4.0 mm × 0.28 mm ^t . Then, the first green sheet for lamination 18A has the partition portion 1
Two anode side and cathode side lower windows separated by 8a are 3.05
It is formed to have a size of mm × 0.8 mm, and further, the upper window portion 20c of the second stacking green sheet 19A is formed to have a size of 3.05 mm × 2.0 mm.

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 layer 24, and terminal patterns 13a, 13b, 13e that serve as terminals of both patterns 16 described above.
To form. (Fig. 2 (a) (b)) After that, a diameter of 0.2 is applied to the terminal patterns 13a, 13b, 13e.
mm platinum lead wires 14a, 14b, 14e are connected to each other (Fig. 3 (a) and (b)). Next, the first green sheet 18A for lamination and the second green sheet 12A for lamination on the green sheet 12A for substrates. The green sheet 19A is laminated and thermocompression-bonded to form a laminate. At this time, the tips of the detection electrode patterns 16a and 16b are exposed in the window portion 20 of the stacking green sheets 18A and 19A. 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. Ceramic substrate 12 and ceramic laminated plates 18 and 19 integrated by firing for 2 hours in the same atmosphere
Are formed (Fig. 4 (a), (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 the window portion 20 of the ceramic laminated plates 18 and 19, TiO ₂
It is filled with a gas-sensitive metal oxide containing as a main component. Here, first, an anode-side paste as shown in sample No. 5 in Table 1 is prepared.

That is, the average particle size of 1.2 after calcination in the air at 1200 ° C for 1 hour
To 100 parts by weight of TiO ₂ powder of μm, 5 parts by weight of Ta ₂ O ₅ having an average particle size of 0.5 μm was added as a donor additive for enhancing electron conductivity, and 20 parts by weight of platinum black was added as a catalyst. Furthermore, only 2 parts by weight of 3% by weight of ethyl cellulose was added as a binder, and these were added to butycarbitol (2-
(2-Butoxyethoxy) ethanol (trade name) is mixed to a viscosity of 300 poise to prepare a TiO ₂ paste. Then, apply this anode side TiO ₂ paste to the anode side lower window
A 20-50 μm thick film is applied on the anode pattern 16a in 20a (FIG. 5 (a) (b)).

On the other hand, the cathode paste is ZrO ₂ containing 7 mol% Y ₂ O ₃ as a sintering inhibitor based on 100 parts by weight of TiO 2 powder having an average particle size of 3.5 μm, which is larger than the particle size of TiO 2 powder on the anode side. 5 parts by weight and 20 parts by weight of platinum black as a catalyst are added, and the same procedure as in the anode paste is performed. Then, the cathode-side paste is applied to the cathode-side lower window portion 20b in a thickness of 20 to 50 μm.

Next, the paste applied to both the anode and cathode electrodes is adjusted, but first, the average particle size is 1.2μ.
m TiO2 powder 100 parts by weight, platinum black 10 as a catalyst
Add 1 part by weight of rhodium black and 1 part by weight of rhodium black to prepare a paste in the same manner as the above paste. Then, this paste is printed in a thickness of 50 to 500 μm in the upper window portion 20c on both layers.

After that, the laminated body which has undergone the above steps is left to stand in the air at 1200 ° C. for 1 hour to be fired (FIG. 6 (a) (b)).

The composition of the paste on the anode side and the paste on the cathode side used in the above steps are changed as shown in Table 1 to prepare a sample of an oxygen gas detector. After assembling this sample into the oxygen gas detector,
The response speed and durability of each sample are measured by the following experiments. The results of the experiment are shown in Table 2.

In this experimental example, the average particle size of the raw material TiO ₂ powder was changed, and thus the average particle size of the gas sensitive layer around the anode was changed. Specifically, since the firing conditions are the same, the larger the average particle size of the raw material TiO ₂ powder, the larger the average particle size of the gas-sensitive layer particles.

○ Response speed test A sample assembled as an oxygen gas detector is exposed to the exhaust gas of a propane gas burner. This propane gas burner has an exhaust temperature of 350 ° C and an air-fuel ratio between a fuel excess (hereinafter rich, air fuel ratio λ = 0.9) and fuel shortage (hereinafter lean, λ = 1.1) every second. It is controlled to change with.

When the exhaust gas is rich, the output of the oxygen gas detector 10
Adjust the voltage applied to the oxygen gas detector so that the output at 1V and lean becomes 0V.

As the response speed, measure the time when the output of the oxygen gas detector 10 changes from 300 mV to 600 mV when the atmosphere changes from lean to rich, and the time when the output changes from 600 mV to 300 mV when the atmosphere changes from rich to lean. To do.

○ Durability test A sample assembled as an oxygen gas detector is attached to an actual vehicle, operated according to a specified durability pattern, and durability is examined from the change in response speed before and after operation. That is, as shown in FIG. 7, the oxygen gas detector S is an exhaust pipe between the engine Eng and the three-way catalyst THC of a commercially available 2000cc three-way catalyst vehicle with EFI.
Attached to Man. Then, the control unit Uni controls the operating state of the engine according to the output of the oxygen gas detector S. The output of the oxygen gas detector S is detected by the circuit shown in FIG. Here, B is a power supply and Rc is a comparison resistance.

The engine Eng described above is used in the endurance pattern shown in FIG.
Drive for 00 hours. The solid line in the figure shows the temperature of the sample, and the broken line shows the temperature of the exhaust gas.

Before and after this operation, measure the response speed Tlr, Trl described above,
The change is taken as the result of durability. That is, it is determined that the sample having less change in response speed before and after the operation has more excellent durability. In Table 1, before operation is described as initial and after operation as after durability test.

From the above experiment, as shown in Table 2, the following was found.

As is clear from comparison between sample Nos. 1 to 3 of the comparative example and sample Nos. 4 to 6 of the example (especially sample No. 3 and sample No. 4),
The particle size (average particle size) of TiO ₂ is made larger around the cathode than around the anode, and the average particle size of the gas-sensitive layer particles around the cathode is
By making the gas sensitive layer around the anode larger than the average particle size in advance, the progress of sintering is suppressed and the response time Tlr, Trl
Can be reduced.

As described above, the changes in the response times Tlr and Trl of the resistance value RT can also be confirmed by the following experiment.

That is, as shown in FIG. 10, a third electrode 16f is provided outside the anode pattern 16a and the cathode pattern 16b in the gas sensitive layer, and (a) the third electrode 16f is on the negative side, and both electrode patterns are provided. 16a, 16b are connected to the plus side, (b) The third electrode 16f is connected to the plus side, both electrode patterns 16a, 16b are connected to the minus side, and the resistance value RT and the response time Tlr, Trl are measured. To do.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view showing an oxygen gas detector according to an embodiment of the present invention, FIGS. 2 to 6 are explanatory views of the manufacture of the embodiment, and FIGS. 7 and 8 are oxygen gas detectors. FIG. 9 is a diagram for explaining the procedure of a durability test in which is used in an internal combustion engine, FIG. 9 is a diagram of its durability pattern, and FIG. 10 is a diagram for explaining the operation of the same embodiment. 10 ... Oxygen gas detector, 12 ... Ceramic substrate, 16,16a, 16b ... Electrode pattern (electrode) 18 ... First ceramic laminated plate 19 ... Second ceramic laminated plate, 20 ... Window 24 …… Gas sensitive layer,

Claims (1)

[Claims] 1. A pair of electrodes consisting of an anode and a cathode, and a porous electrode which covers the electrodes and contains a gas-sensitive transition metal oxide, and whose electric resistance changes depending on the gas component and / or the gas concentration. And a gas-sensitive layer, wherein the average particle size of the particles forming the porous gas-sensitive layer around the cathode is the average particle size of the particles forming the porous gas-sensitive layer around the anode. Gas detector characterized by being made larger.