JPS6365350A - Gas detector - Google Patents

Abstract

PURPOSE:To improve the response speed of a detector and to perform fine control by carrying a conductor which accelerates the ionization of gas on a gas sensitive layer. CONSTITUTION:The electrode pattern 16 of electrodes 16a and 16b for detection, a heat resistance electrode 16e, etc., is formed on a ceramic substrate 12 and connected to platinum lead wires 14a, 14b, and 14e by terminals 13a, 13b, and 13e. A ceramic laminate plate 18 is laminated on the substrate 12 and electrode pattern 16 and united with the substrate 12, and has a window part 18a. The gas sensing layer 20 which carries silver-based alloy, on the other hand, is so charged in the widow part 18a as to cover the electrode patterns 16a and 16b. Further, spherical granulated particles 22 are dispersed between the substrate 12 and gas sensitive layer 20 to prevent the both from separating from each other, and a coating layer 24 made of Al2O3 is laminated on the gas sensitive layer 20. The conductor carried on the gas sensitive layer 20 accelerates the ionization of the gas, but has no strong catalyzing operation, so the reaction to partial pressure variation of the objective gas never becomes slow. Therefore, the response to environmental gas components and/or their concentration is good.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a gas detector for detecting gas components or their concentrations, and in particular to gas detection using gas-sensitive metal oxides. Concerning vessels.

[Prior Art] Conventionally, oxygen gas detectors, combustible gas detectors, and the like have been put into practical use as gas detectors for detecting the presence or concentration of gases in the atmosphere.

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 i 02, C00, Ni
Oxides of transition metal elements such as ○ can be used as oxygen senna.

The transition metal oxides exemplified here are non-stoichiometric 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.

By the way, in order to accurately detect the partial pressure of oxygen gas in a non-equilibrium gas such as the exhaust gas of an internal combustion engine, the above-mentioned oxygen sensor needs to equilibrate the non-equilibrium gas. Conventionally, a noble metal catalyst such as PT is powdered and uniformly dispersed and supported in the angry gas layer.

The present inventors further investigated the action of the noble metal catalyst and found that the noble metal catalyst not only equilibrates non-equilibrium gas but also assists in the ionization of gas.

That is, in order for the gas-sensitive metal oxide to detect the partial pressure of oxygen gas, it is necessary to donate electrons to oxygen gas molecules, as in 02 + 46-2202-.

However, since gas-sensitive metal oxides have relatively low excitation energy for donating electrons, it is difficult to donate sufficient electrons. Especially at low temperatures, it is impossible.

However, when a metal with high electrical conductivity, such as the noble metal catalyst mentioned above, exists on the surface of a gas-sensitive metal oxide, electrons are actively donated to oxygen gas molecules, and the gas-sensitive metal oxide easily absorbs oxygen. Gas partial pressure can be detected.

To summarize, in conventional gas detectors, as mentioned above, noble metals have two roles: as a catalyst to promote equilibrium of non-equilibrium gases and as a conductor to promote ionization of gases. I was letting it go.

[Problems to be Solved by the Invention] However, catalysts do not bring non-equilibrium gases into equilibrium by adsorbing non-equilibrium gases, including oxygen gas, on the catalyst surface, causing them to react on the catalyst surface, and desorbing them as equilibrium gases. Therefore, even if a change in oxygen gas partial pressure occurs in the gas, the output is delayed by the time from when the gas is adsorbed by the catalyst until it is desorbed. In particular, the stronger the catalytic action, that is, the stronger the ability to equilibrate a non-equilibrium gas, the longer the time from adsorption to desorption becomes. Therefore, when attempting to improve the equilibrium effect by using a catalyst with strong catalytic activity, the reaction to changes in oxygen gas partial pressure was delayed. In addition, the catalytic action is weak,
That is, if a large amount of a catalyst with a short time from adsorption to desorption is used, the equilibrium effect can be improved without deteriorating the response, but in this case, the resistance of the gas-sensitive layer itself decreases, which is undesirable.

[Means for solving the problems] The present invention employs the following means to solve the above problems.

That is, the gist of the first invention is as follows: a pair of electrodes, a porous material covering the pair of electrodes, containing a gas-sensitive metal oxide, and having an electrical resistance that changes depending on the surrounding gas component and/or its concentration. A second aspect of the invention resides in a gas detector comprising: a gas-sensitive layer; the gas-sensitive layer carries a conductor in a dispersed manner; The gas-sensitive layer comprises a porous gas-sensitive layer containing a gas-sensitive metal oxide and whose electrical resistance changes depending on the surrounding gas components and/or its concentration, and an upper layer covering the gas-sensitive layer, wherein the gas-sensitive layer is The present invention provides a gas detector characterized in that a conductor is supported in a dispersed manner, and the upper layer also carries a catalyst in a dispersed manner.

The electrode is particularly limited as long as it is a heat-resistant conductor.

The gas-sensitive metal oxide used in the gas-sensitive layer may be selected depending on the gas component to be detected.
Commonly used materials include Ti02, SnO2,
Coo, ZnO, Nb2O5, Cr2O3, Ni
Examples include transition metal oxides such as O, and in the present invention, any one or a combination of two or more of these may be used.

The conductor supported on this gas-sensitive layer is not particularly limited as long as it is a heat-resistant conductor;
Gold whose wire resistance at °C is 6.8 μΩ・4.7
Preferably, silver is μΩ, and more preferably gold or an alloy of silver and platinum group metal, which has high conductivity and excellent heat resistance. The wire resistance of platinum at 500°C is 27.
The wire resistance of Pd is 9μΩ·country and 27.95μΩ·country. In addition, even if the conductivity is excellent, if the catalytic property is high, the time from adsorption to dellf will become longer as mentioned above, which will actually reduce the response, so it is better to have a low catalytic property. Also, gold, silver, gold, or an alloy of silver and a platinum group metal are preferable as conductors.

This conductor is supported on the gas-sensitive layer, for example, by the following method.

・Precipitation method A precipitant is added to a solution of a salt containing a conductor element to cause precipitation. There are cases where it is precipitated on a gas-sensitive layer serving as a carrier, and cases where both the gas-sensitive layer component and the conductor component are precipitated simultaneously (co-precipitated). The conductor component precipitated in this way is fixed to the cass-sensitive layer by firing.

・Impregnation method After impregnating a porous gas layer that serves as a carrier with a solution containing conductor component elements, the conductor is dispersed and supported on the sensitive gas layer by thermal decomposition, reduction, etc.

In addition to the above, an ion exchange method that utilizes the ion exchange properties of a gas-sensitive layer as a carrier, an adsorption method that utilizes adsorption from a solution containing a conductor component element, etc. can also be used.

The catalyst used in the second invention may be selected depending on the state in which the gas detector is used and the gas to be detected. When used for gas partial pressure measurement, one or more selected from platinum group elements is preferred. This catalyst is
It is supported on the upper layer in the same manner as the conductor described above.

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. Alternatively, it may be formed into a disk shape, bead shape, etc. used in a thermistor, etc., without using thick film technology or the like.

Furthermore, a heating element may be provided near 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, 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 over the gas-sensitive layer or the upper layer. This coating layer adsorbs and captures toxic substances such as lead on the gas-sensitive metal 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.

[Effect] The operation of the first invention will be explained.

In the present invention, the conductor supported on the gas-sensitive layer promotes the ionization of the gas, but since it does not have a strong catalytic effect, the reaction to changes in the partial pressure of the target gas is not delayed. Therefore, responsiveness to surrounding gas components and/or their concentrations is good.

The second invention will be explained in detail.

In addition to the structure of the first invention, the gas detector of the present invention has an upper layer supporting a catalyst. Therefore, for example, the partial pressure of oxygen gas in non-equilibrium gas such as exhaust gas from an internal combustion engine can also be accurately measured. Furthermore, as described above, in conventional gas detectors, a small amount of a catalyst with a strong catalytic action is used because the use of a large amount of catalyst lowers the resistance, which is undesirable.

However, in the present invention, since the catalyst is supported only on the upper layer, it has no relation to the resistance of the gas-sensitive layer itself, and can be supported as much as necessary. Therefore, if a large amount of a catalyst with a relatively weak catalytic action is used, the gas detector will have good response and excellent equilibration of non-equilibrium gases, as described above.

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

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

In this example, a Ti film supporting a silver thread alloy was used as a gas-sensitive layer.
This is an oxygen gas detector 10 that uses O2.

As shown in the partially broken perspective view of FIG. 1, the gas detector 10 includes a ceramic substrate 12, and terminals 13a, 13 formed on the ceramic substrate 12.
b, 13e and platinum lead wire 14a.

Detection electrode 16a connected to 14b and 14e.

16b and a heat resistance electrode 16e, a ceramic laminate 18 which is laminated on the ceramic substrate 12 and the electrode pattern 16 and is integrated with the ceramic substrate 12 and has a window 18a; The detection electrode pattern 1 is placed on the window 18a of the laminated plate 18.
6a and 16b and carrying a silver-based alloy, and spherical granules that are dispersed between the ceramic substrate 12 and the gas-sensitive layer 20 to prevent them from peeling off. Particles 22 and Al2 layered on the gas-sensitive layer 20
A coat Jm24 made of O3.

Next, an embodiment of the second invention will be described with reference to FIG.

In this example, a Ti film supporting a silver thread alloy was used as a gas-sensitive layer.
Al2 using O2 and supporting noble metal catalyst as upper layer
This is an oxygen gas detector 30 using O3. The same figure numbers are used for common parts between this embodiment and the embodiment of the first invention,
and its explanation will be omitted.

The gas detector 30 of this embodiment has the following features, except that an upper layer 34 carrying a noble metal catalyst is further provided on the gas-sensitive layer 32 supporting a silver thread alloy provided in the window 18a of the ceramic laminate 18. It has the same structure as the embodiment of the first invention.

Next, an oxygen gas detector 1 which is an embodiment of the first invention described above will be described.
The manufacturing process of 0 will be explained with reference to FIGS. 3 to 6.

(2) 92 wt% alumina, 3 vt% magnesia, and 5 wt% sintering aids (silica, calcia, etc.) are mixed in a bot mill for 20 hours. Thereafter, 12% by weight of polyvinyl butyral and 4% by weight of dibutyl phthalate were added as organic binders to the mixture, and methyl ethyl ketone, toluene, etc. were added as solvents. Further, the mixture is mixed in a bot 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 above green sheet is board green sheet 12.
A is 47°8mmX4.0mmX0.8mm', green sheet for lamination 18A is 47.8mmX4. QmmXO
, 26 mm', and the green sheet for lamination 18A has a size of 3°05 mm x 2. Omm window 18a
form.

■Next, platinum black and spongy platinum were mixed at a ratio of 2:1, and 1Qvt% 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, 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 and 16b, the thermal resistance electrode pattern 16e that serves as a heater for heating the gas sensing section 20, and the terminal pattern 13a that serves as the terminals of both of the patterns 16. , 13b, 13e are formed. (Fig. 3 (a), (b)) ② After that, the terminal patterns 13a and 13b are formed.

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

14b and 14e are connected respectively.
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 portion 1 of the green sheet for lamination 18A
8a, the tips of the detection electrode patterns 16a and 16b are exposed. Then, 80 to 150 meshes of spherical granulated particles (secondary particles) 22 made of the same material as the green sheet prepared in step (3) are dispersed and adhered to the window portion 18a, and then the above-mentioned laminate is exposed to the atmosphere at 1500°C. The ceramic substrate 12 and the ceramic laminate 18 are integrally formed by firing in almost the same atmosphere for 2 hours (see FIG. 5).
b), (b)).

When the spherical granulated particles 22 are dispersed and adhered as described above and fired, the particles 22 are dispersed on the ceramic substrate 12 to form an uneven surface, as shown in an enlarged view in FIG. 5(C).

■ Next, ceramic? ! The window portion 18a of the 71 plate 18 is filled with a gas-sensitive metal oxide whose main component is TiO2. 3 for TiO2 powder of
% by weight of ethylcellulose was added and these were mixed in butycarpitol (a trade name for 2-(2-butoxyethoxy)ethanol) to a viscosity of 300 voids to form TiO
2 Adjust the paste. Then, this TiO2 paste is filled into the window portion 18a using a thick film printing technique, and a coat layer 24 of Al2O3 is laminated.

Thereafter, the laminate that has undergone the above steps is left to stand in the atmosphere at 1200° C. for 1 hour and fired. (Figure 6 (a), (b)).

■ A metal salt aqueous solution, which will be described later, is dropped onto the window 18a of the fired laminate and thermally decomposed in a hydrogen furnace at 700°C for 2 hours to support the metal on the sensitive gas M20, thereby detecting the gas in this example. A container 10 is manufactured.

Next, an oxygen scum detector 30 which is an embodiment of the second invention will be explained.
The manufacturing process will be explained.

The manufacture of this oxygen gas detector 30 is based on the above-mentioned gas detector 10.
The manufacturing process is almost the same as that of step (2) above, but a step of providing an upper layer 34 on the gas-sensitive layer 32 formed in step (2) above is added.

That is, after the above-mentioned steps (1) to (2), the next steps illustrated in FIGS. 7(a) and 7(b) are performed.

However, the coating layer 24 in (2) is not provided.

■ Gas-sensitive layer 3 inside the window 18a of the ceramic laminate 18
2, the upper layer 134 supporting the catalyst is laminated,
First, the average particle size is 0. After adding 16 parts by weight of platinum powder and 4 parts by weight of rhodium powder to 80 parts by weight of a mixed powder consisting of 96% by weight of 5 μm Al2O3 powder, 23% by weight of 5i0, and 11% by weight of Ca, 3 parts by weight of ethyl cellulose was added. These were added to butycarpitol (2-(2
- butoxyethoxy) (trade name of ethanol) to prepare an alumina paste to a viscosity of 300 voids. Then, this alumina paste is applied to a thickness of approximately 100 μm on the gas-sensitive layer 32 of the window portion 18a using thick film printing technology.
Once again, the laminate that has undergone the above steps is left in the atmosphere at 1200° C. for 1 hour and fired.

In step (1) above, gas detector samples are prepared by changing the composition of the metal salt aqueous solution used as shown in Table 1. After this sample was assembled into an oxygen sensor, the response speed and durability of each sample were measured in the following experiment. The experimental results are also shown in Table 1. In Table 1, the unit of each component in the metal salt aqueous solution is g/1. The unit of response speed is m5ec. Further, the sample numbers of the examples of the first invention are 1-1 to 10, and the sample numbers of the examples of the second invention are 2-1 to 2-5.

・Response speed■ Expose the sample assembled as an oxygen sensor to the exhaust gas of a propane gas burner. In this propane gas burner, the exhaust temperature is 350°C, and the air-fuel ratio increases every second with excess fuel (hereinafter referred to as rich, air-fuel ratio λ = 0.9).
and fuel shortage (hereinafter referred to as lean, λ = 1.1).

■Adjust the voltage applied to the sensor so that the output of the gas detector 10.30 is 1■ when the exhaust gas is rich, and the output is 0■ when the exhaust gas is lean.

■ As for the response speed, the output of the gas detector 10.30 when the atmosphere changes from lean to rich changes from 300mV to 6.
Measure the time for the output to change from 600 mV to 300 mV when the atmosphere changes from rich to lean.

・Durability ■ A sample assembled as an oxygen sensor is attached to an actual vehicle, driven in a predetermined durability pattern, and durability is examined from changes in response speed before and after driving. That is, oxygen sensor S
is a commercially available 2000cc EFI as shown in Figure 8.
It is installed between the engine Eng and the three-way catalyst THC in the exhaust pipe Man of a vehicle with a three-way catalyst. The control unit Uni then controls the operating state of the engine according to the output of the oxygen sensor S. The output of the sensor S is detected by a circuit as shown in FIG. Here, B is a power supply and Re is a comparison resistance.

(2) The above engine Eng is operated for 300 hours according to the durability pattern shown in FIG. In addition, the solid line in the figure shows the temperature of the sample, and the broken line shows the temperature of the exhaust gas.

The above-mentioned response speed was measured before and after this operation, and the change was taken as the durability result. That is, a sample with little change in response speed before and after operation is judged to have excellent durability. In Table 1, before operation is referred to as initial stage, and after operation is referred to as after durability test.

Table 1 To Ji ← one eleven two The following was found from the above experiment.

(2) When silver is contained in the metal supported on the gas-sensitive layer, as in samples Nos. 1-4 to 10.2-3 to 5, the responsiveness is excellent both before and after the durability test. In particular, the response speed when changing from rich to lean was that of sample No.
, 1-1~3.2 It is much faster than -1~2. This means that the response speed from rich to lean is 0.
Considering that it shows a reaction rate of 2+4e-→202-, it is thought that this is because the metal supported on the gas-sensitive layer of the example has high conductivity, making it easy to supply electrons.

(2) When an upper layer containing a catalyst is further provided on the gas-sensitive layer as in Sample Nos. 2-1 to 2-5, the response time changes less before and after the durability test. This is thought to be because by providing this layer, poisonous substances such as lead and phosphorus do not reach the gas-sensitive layer, and the reactivity of the gas-sensitive layer is not impaired.

[Effects of the Invention] The present invention was achieved by studying the effect of the metal supported on the gas-sensitive layer as described above. That is, in the first invention, the ionization of the gas is applied to the gas-sensitive layer. A promoting conductor was supported. Therefore, the response speed of the gas detector is greatly improved.

Therefore, for example, by using the present invention to control the air-fuel ratio of a gas burner, etc., the reaction becomes very fast and more fine-grained control becomes possible.

In the second invention, the non-equilibrium gas is equilibrated using a catalyst supported on the upper layer, and the gas is ionized using a conductor supported on the gas-sensitive layer.

Therefore, there is no need to consider the resistance of the gas-sensitive layer when selecting the type and amount of catalyst. Therefore, by using a large amount of a catalyst with a weak catalytic effect, that is, a short time from adsorption to desorption, it is possible to achieve excellent equilibrium of non-equilibrium gases.
As with the first invention, it is possible to provide a gas detector with excellent response speed.

Furthermore, even if the gas to be measured contains poisonous substances such as lead or phosphorus, the presence of the upper layer prevents the poisonous substances from reaching the gas-sensitive layer, resulting in excellent durability with little deterioration over time. It becomes a gas detector.

[Brief explanation of drawings]

FIG. 1 is a partially cutaway perspective view of an embodiment of the first invention, FIG. 2 is a partially cutaway perspective view of an embodiment of the second invention, and FIGS. 3 to 7 are explanatory views of manufacturing the embodiment. FIGS. 8 and 9 are explanatory diagrams of a durability test for using an oxygen sensor in an internal combustion engine, and FIG. 10 is a diagram of its durability pattern. 10.30... Gas detector, 12... Ceramic substrate, 16.16a, 16b... Detection electrode, 16e...
Heat resistance electrode, 18a... Window portion, 18... Ceramic laminate, 20.32... Gas sensitive layer, 22... Spherical granulated particles, 24... Coating layer, 34...
...upper layer,

Claims (1)

[Claims] 1. A pair of electrodes, and a porous gas-sensitive material that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electrical resistance that changes depending on the surrounding gas components and/or their concentration. A gas detector comprising: a gas-sensitive layer, wherein the gas-sensitive layer supports a conductor in a dispersed manner. 2. The gas detector according to claim 1, wherein the conductor is an alloy of silver and a platinum group metal. 3 a pair of electrodes, a porous gas-sensitive layer that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electrical resistance that changes depending on the surrounding gas component and/or its concentration; and the gas-sensitive layer. A gas detector comprising: an upper layer covering the gas-sensitive layer, wherein the gas-sensitive layer carries a conductor in a dispersed manner, and the upper layer carries a catalyst in a dispersed manner. 4. The gas detector according to claim 3, wherein the conductor is an alloy of silver and platinum group metal. 5. The gas detector according to claim 3 or 4, wherein the catalyst comprises one or more selected from platinum group elements.