JP2000028562A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide stable performance and a stable service life as to a semiconductor gas sensor applied to a passage for combustion exhaust gas for detecting incomplete combustion. SOLUTION: A pair of comb-shaped electrodes 3, elements provided with semiconductor oxide coating films 4 on respective surfaces of the electrodes 3, a ceramic gas-permable selective body 5 provided to be layered on a semiconductor oxide coating film 4 side of the elements, sealed by a sealing member 7 and having a controlled micro-pore size, and a porous catalyst provided on a surface corresponding to one of the comb-shaped electrodes 3 on the ceramic gas permable selective body 5 are provided on a surface of a heat-resistant insulating base material 2 provided with a heating means 1 in its back face. Poisoning gas is prevented from flowing into the semiconductor oxide coating films 4 by the gas permable selective body 5, and a stable sensitivity characteristics is provided to monitor zero gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting carbon monoxide, and more particularly, to a gas sensor which is excellent in stability of operation in a severe use environment and durability.

[0002]

2. Description of the Related Art Carbon monoxide is a colorless, tasteless and odorless gas, slightly lighter than air but highly toxic, and causes headaches when breathing for 2-3 hours even at a low concentration of about 200 PPM.
When the concentration becomes 000PPM or more, it takes about 10 minutes,
At concentrations above PPM, they die within minutes.

[0003] Even in ordinary households, carbon monoxide is generated from instantaneous water heaters, bath kettles, oil heating appliances, gas heating appliances, charcoal fires, etc., and should be used in these appliances or installed indoors. There is a strong demand for an inexpensive, compact, and highly reliable carbon monoxide gas detection sensor that can be used.

As a conventionally proposed gas sensor, particularly a carbon monoxide detection sensor, an electrode for absorbing and oxidizing carbon monoxide (CO) in an electrolytic solution is provided, and the CO concentration is calculated from a current value proportional to the CO concentration. Using a sensing method (a potentiostatic electrolytic gas sensor), a sintered body type n-type semiconductor oxide such as tin oxide (Sn02) sensitized with a small amount of an additional element such as a noble metal, these semiconductors are used as a combustible gas. A method of detecting gas by using the property that the electric conductivity changes when contacted (semiconductor type gas sensor), a pair of comparative elements in which alumina is attached to a thin platinum wire of about 20 μm and noble metal is supported A method of detecting the difference in heat generated when a combustible gas comes into contact with this element to perform a catalytic oxidation reaction (a contact combustion type gas sensor) is known. There. These are described, for example, in [Reference 1].
Supervised by Toyoaki Omori: "Practice Encyclopedia of Sensors": Fuji Techno System Chapter 14 Basics of Gas Sensors (Masatoshi Haruta), P
112-130 (1986).

[0005] Particularly, as a semiconductor type CO sensor, [Reference 2] Japanese Patent Publication No. 53-43320 and [Reference 3]
Japanese Patent Application Laid-Open No. Sho 61-50051 discloses that a gas sensor utilizing a change in the resistance value of a metal oxide semiconductor is periodically and alternately heated to a high temperature region and a low temperature region, and the output of the gas sensor in a low temperature region is intermittently sampled. There has been proposed a method of detecting CO gas by using the method and an improvement thereof. These are characterized in that the selectivity of CO detection is enhanced mainly by devising the signal processing aspect. [Reference 4] Japanese Patent Application Laid-Open No. 1-227951 proposes a gas sensor in which a metal oxide whose resistance value changes with gas is used as a sensor main body, in which a zeolite coating layer is provided on the surface of the sensor main body. I have. This also aims at improving the selectivity of CO detection.

As a gas sensor used by forming an electrochemical cell, an oxygen sensor using stabilized zirconia as an electrolyte has been put to practical use for controlling exhaust gas purification of automobiles and is well known. This is a system utilizing the electromotive force of an oxygen concentration cell. A gas sensor that measures carbon monoxide and carbon dioxide simultaneously with oxygen has been proposed. For example,
[Reference 5] JP-A-4-320955 and [Reference 6] JP-A-4-320956. However,
In the case of this method, it is necessary to introduce a sample gas of a known concentration to one of the electrodes to form a concentration cell. Even if the analytical instrument has high applicability, it is not necessary to introduce the sample gas every measurement. Not suitable for household appliances. However, this method can correct electrode deterioration and the like each time.

[0007]

All of these chemical sensors have the following disadvantages. In other words, even if a constant potential electrolytic gas sensor, a semiconductor type gas sensor, and a catalytic combustion type gas sensor in principle react indiscriminately to reducing gas (combustible gas) indiscriminately, even if various measures are taken, basically,
It has the property of detecting hydrogen, alcohol, etc. other than carbon monoxide (CO). That is, there is a disadvantage that the selectivity of CO is poor. In addition, the sensor and the sensor system are disadvantageous in that they are generally expensive and the signal processing circuit of the sensor is complicated. Except for the catalytic combustion type, there is also a disadvantage that the controllability is poor because the output of the sensor is non-linear with respect to the CO concentration.

[0008] Basically, in the case of a chemical sensor utilizing a catalyst or a chemical adsorption phenomenon, the durability and selectivity of the catalyst are generally in conflict with each other, and the operation of the chemical sensor is increased by increasing the selectivity. When the catalyst is used on the side where the formation is controlled (low temperature side), various poisonous gases are easily adsorbed on the surface of the catalyst, and the catalyst deteriorates rapidly and does not have durability. Conversely, if the operation is used on the diffusion dominant side (high temperature side), selectivity may deteriorate.
For example, in the case of a sensor using a device in which gold particles are supported on Ti ₄ + -doped αFe ₂ O ₃ as a semiconductor type gas sensor, the selectivity to hydrogen and alcohol of CO is extremely excellent, but the durability of the catalyst is disadvantageous. There is. The methods described in Documents 2 to 7 do not basically improve the durability of the sensor element because the sensor element itself is not protected from various poisoning gases related to durability. At this point, zeolite can be expected to have an effect of removing poisonous gas. However, in order to actually make the effect effective, it is necessary to control the pore structure of the zeolite film more than the pore structure of the zeolite itself. However, since this control is extremely difficult, the effect of the actual zeolite is reduced. That is,
Although this method has the effect of improving the selectivity of the gas sensor, it does not contribute much to the improvement of durability.

In the case of a gas sensor utilizing a change in resistance of a metal oxide semiconductor, it is generally put into practical use as a gas leak alarm, but its durability is usually about three years. This is due to the deterioration of the semiconductor oxide due to the poisoning gas.
When the carbon monoxide detection gas sensor is built into the combustion equipment,
The durability of the gas sensor is required to be about 10 years, and it is required to greatly improve the durability compared with the conventional technology.

When a gas sensor is mounted on a combustion device and used for detecting incomplete combustion, the risk of incomplete combustion increases more often in a state after the combustion device has been considerably used. At that time, there is a risk that the gas sensor is deteriorating, and there is a problem that incomplete combustion cannot be detected if the output signal is reduced due to the deterioration of the gas sensor.

The reason why the output of the chemical sensor is reduced, that is, the deterioration is caused by the deterioration of the electrode or catalyst which plays a central function of the chemical sensor with time as the reaction proceeds, This deterioration is caused by the reduction of the catalyst by reducing gases such as hydrogen and hydrocarbons present in the exhaust gas of combustion, and the strong adsorption of sulfur-based compounds on the electrode surface, resulting in the detection reaction of carbon monoxide. By being inhibited. In these chemical sensors, noble metals are often used for electrodes or catalysts that play a central role in the sensor function,
These noble metals are susceptible to sulfur-based compounds and silicone-based compounds and are susceptible to deterioration, making it extremely difficult to ensure durability. In addition, hydrocarbons that coexist in the exhaust gas of combustion equipment have a large molecular weight and a large molecular size, so if they are adsorbed on the surface of a noble metal such as platinum, the adsorption of carbon monoxide is hindered, which has a negative effect as an interfering gas. There were also problems.

Furthermore, since the sensor system is essentially not fail-safe, a highly reliable sensor with extremely high durability is required in order to be able to put it to practical use with high reliability. Even at the level, there was a problem that a sensor system that could properly establish durability was not realized.

[0013]

In order to solve the above-mentioned problems, a gas sensor according to the present invention comprises a pair of electrodes on a surface of a heat-resistant insulating substrate, and a semiconductor oxide provided on the pair of electrodes. A ceramic gas selective permeable body provided on the side of the coating and the semiconductor oxide film and having a controlled pore diameter; and a porous catalyst provided on a surface corresponding to one electrode on the ceramic gas selective permeable body. The basic configuration is the configuration described above.

Further, in addition to this basic structure, a heating means for heating the semiconductor oxide film and a gas selective permeable member for preventing the gas to be detected from directly reaching the semiconductor oxide film without passing through the gas selective permeable member. A seal member for closing a space between the semiconductor oxide film and the semiconductor oxide film is provided in this space.

[0015] The basic idea of the present invention is that most of the deterioration of the sensor is caused by the coexisting gas, so that coexisting gases other than the gas necessary for detecting carbon monoxide should be allowed to touch the gas detecting element. The idea is that if semi-permanent durability can be realized, and if semi-permanent durability can be realized, fail-out does not actually become a problem. That is,
The configuration in which the gas containing the gas to be detected comes into contact with the gas detection element via the gas selective permeable member restricts the arrival of the coexisting gas that adversely affects the sensor life to the gas detection element. In other words, the biggest problem of gas sensors using semiconductor metal oxides is that the deterioration of the electrode due to a gas other than carbon monoxide, such as sulfurous acid gas, is suppressed or blocked by a gas selective permeator whose pore size is controlled by a gas selective permeator. Is to prevent it.

The gas selective permeator uses a porous body whose pores are controlled, and utilizes the difference in gas permeation rate in the porous body to separate a gas to be detected from a coexisting gas unnecessary for detection. Do.

In general, the permeation mechanism of gas molecules in the pores of a porous body changes as follows. The flow in the gas phase is reduced from the viscous flow region where the collision between molecules is dominant, and the pore size is reduced and the Knu where the collision between the molecule and the pore wall is dominant.
Move to the dsen (Knussen) diffusion region. At this time,
The individual properties of the molecule become apparent, and the ratio of the permeation rates is theoretically given by the square root of the ratio of the molecular weights. Furthermore, as the pores become smaller and the size of the molecules becomes smaller, the gas molecules lose their freedom of movement in the direction perpendicular to the flow and cannot exist as a gas. This state is called a molecular sieve. In addition, surface diffusion in which molecules are transported while being adsorbed on the pore wall surface coexists with the gas phase flow. In particular, when the pressure exceeds the capillary condensation pressure, the surface diffusion shifts to the capillary condensation flow because the adsorbent layer covers all pores.

In the case of carbon monoxide, the coexisting gas such as a sulfur dioxide gas and a hydrocarbon gas such as kerosene vapor, which have a bad influence on carbon monoxide and oxygen necessary for the operation of the gas detection element, has a molecular weight. Since they are different, the flow into the gas detection element can be regulated by the pore size of the Knudsen diffusion region. Further, the use of surface diffusion, capillary condensed flow, and molecular sieve can enhance the separation ability. In the present invention, in particular, by controlling the pore diameter on the order of angstrom and forming a film in the pores for the purpose of chemically modifying the surface of the pores, an effective gas selective permeability can be obtained in the porous body. Used by giving.

In this case, the gas is brought into contact with the gas detecting element through a gas selective permeable body having a controlled pore size by a film containing at least one of silica and zirconia. When the pore diameter is 10 mm or less, the gas molecule exhibits molecular sieve type or surface diffusion type permeability, and the inflow is regulated by the size of the gas molecule or the affinity between the gas molecule and the inner wall of the pore. It has the property that the diffusivity into the porous body is determined. The behavior of gas in and out required for operation of the gas detection element differs depending on the principle of the gas detection element.In the case of a semiconductor type, it means that carbon monoxide and oxygen flow in and come into contact with the gas detection element. When the element is bare, other than the above, nitrogen and water vapor, which are not directly related to the operation of the detection element, sulfur dioxide or kerosene vapor which becomes an interfering gas of the detection element, and a silicone-based compound enter. Come.

Since kerosene vapor and silicone compounds have a large molecular size, the inflow can be effectively controlled, and the inflow of sulfur dioxide can be largely controlled, but the water vapor has the same level as the carbon monoxide molecule. In general, it is not possible to control the inflow, and depending on the conditions, there is a concern that capillary condensation occurs in the pores of the porous body and the pores are blocked. In this regard, by coating the surface of the gas selective permeable material with a film containing at least one of silica and zirconia, strong hydrophobicity can be imparted, and the surface diffusivity of water vapor can be inhibited to prevent condensation of water vapor. Similarly, it also inhibits hydrophilic surface diffusion of sulfur dioxide and can block the inflow of sulfur dioxide.

As described above, it is possible to reduce the influence of poisoning on elements constituting the gas detection element such as a semiconductor metal oxide. The material of the porous base material must be made of ceramic from the viewpoint of heat resistance.

As the heating means, various means such as a heating wire and a resistance heater film can be applied. As a material used for the resistance heater film, a noble metal-based material such as platinum is preferable in terms of durability. The iron-chromium-based and nickel-chromium-based ones can be used. The temperature control is carried out in combination with a heating means and, if necessary, with a temperature detecting means such as a thermistor or a thermocouple.

The ceramic porous substrate serving as the base of the gas selective permeable body will be described below. The ceramic porous substrate is prepared using a commercially available porous ceramic or porous glass. It is well known that porous ceramic or porous glass is used for various applications as a ceramic filter, for example, for separation of yeast from beer.
Although the pore size is about 0.05 μm to several μm, gas permeability cannot be obtained as it is, and it is necessary to control the pore size by filling the pores.

The method for controlling the pore diameter is to form a sol-gel film on the surface of the pores. Alternatively, a CVD method or the like in which a film is formed in the pores by thermal decomposition to control the pores is a promising method, but various known film forming methods can be applied.

Among them, for example, a method utilizing a decomposition reaction of metal alkoxide (sol-gel method) or a CVD reaction is effective, and the pore diameter can be controlled to the diameter of the gas diffusion region of gas. According to these methods, the pore diameter can be controlled to uniform pores having an average pore diameter of 10 ° or less. The CVD method can control the pore diameter finer and more uniformly than the sol-gel method. In this case, the size of the pores must be reduced to the size of the gas molecules, and the movement of gas in the pores of the porous body is affected by the interaction between the gas and the substance on the surface of the pores. Has complex diffusion properties, but basically the gas permeation process is in the region of molecular sieve diffusion or surface diffusion, which significantly impedes the permeation of large molecules or has an affinity with the inner wall of the pore. Therefore, it is necessary to prevent molecules having a large size or molecules having low affinity with the inner wall of the pore from passing through the gas selective permeator so as to have a property such that the diffusion property is regulated by the gas.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a heat-insulating base material having a pair of electrodes, a semiconductor oxide film provided on the pair of electrode surfaces, and a pore diameter provided on the semiconductor oxide film side. And a porous catalyst provided on a surface corresponding to one of the electrodes on the ceramic gas selective permeable body. By adding heating means to this basic configuration, the semiconductor gas sensor
Operation under a predetermined temperature condition and catalytic activity for oxidation of carbon monoxide in the porous catalyst body are ensured. Further, a seal member is provided in a space between the gas selective permeable body and the semiconductor oxide to prevent the gas to be detected from reaching the semiconductor oxide film directly without passing through the gas selective permeable body. Stability can be ensured.

The operation of the gas sensor will be described. Regarding the semiconductor film on the electrode on which the porous catalyst is not disposed, in the case of air containing no flammable gas such as carbon monoxide, the air arriving at the semiconductor oxide film via the gas selective permeable body is a semiconductor. In order to keep the oxide film in a stable oxidized state, the resistance of the semiconductor oxide film on the electrode does not change on the high resistance side. Next, when the carbon monoxide-containing air arrives, the carbon monoxide reduces the semiconductor oxide, the trapping of electrons or holes in the semiconductor oxide is released, the conductivity appears, and the resistance value decreases. Down, carbon monoxide is detected. Since this resistance value is related to the carbon monoxide concentration, the carbon monoxide concentration can be detected from the functional relationship between the two.
On the other hand, carbon monoxide was oxidized by the porous catalyst to the semiconductor oxide film on the other electrode, and air not containing carbon monoxide always arrived, so that the resistance value maintained a high resistance. It remains as it is. That is, the latter resistance value indicates a resistance value corresponding to the zero point. By calculating and using the ratio of the two resistance values, it is possible to cancel a characteristic change due to a subtle temperature change. In this case, the detection of carbon monoxide is intermittent data. However, in order to remove the instability of the characteristics due to the influence of adsorption of carbon monoxide to the semiconductor oxide film, the semiconductor oxide film is placed in an oxidation stable state. Periodic temperature change to return to
High-temperature operation and low-temperature operation). The setting of the high temperature and the low temperature of the heating means can be set by setting the power of the heating means in two stages by a microcomputer. Thereby, the oxidative regeneration of the semiconductor oxide film proceeds at a high temperature, and the resistance value of the semiconductor oxide film becomes high. On the other hand, if the temperature setting at low temperature is set to a temperature at which the sensitivity of the semiconductor oxide film is large, carbon monoxide can be detected with good sensitivity by measuring the resistance value at the low temperature side. Further, since the resistance value of the semiconductor oxide film on the side where the porous catalyst is disposed corresponds to the zero point, if the ratio between the two is determined, the characteristics of the sensor in which the influence of the temperature characteristics has been canceled can be obtained. According to such an embodiment, a stable sensor output can be obtained.

Examples of the semiconductor oxide film used for the gas sensing portion include n-type semiconductor oxides such as tin, indium and zinc, and noble metal sensitizers such as palladium and platinum for improving the sensitivity, and porous materials for the films. Used in combination.

With the above configuration, deterioration of the metal oxide semiconductor film portion, which is the biggest problem of the semiconductor type carbon monoxide sensor, is prevented as follows. The gas that deteriorates the metal oxide semiconductor film is prevented from being deteriorated by restricting or blocking the inflow of the gas into the surface by the ceramic gas selective permeable member having a controlled pore diameter.

As described above, the zero point largely shifts with time or the sensor detection sensitivity decreases due to instability, which is the greatest weak point of the conventional chemical sensor, that is, the deterioration of the element performing the central function of the gas sensor. It is possible to eliminate problems related to durability such as rubbing.

Further, the outside of the above-mentioned structure is wrapped with a sheet of ceramic fiber, and a heating wire is arranged outside the outside. According to the present embodiment, the temperature required for the operation of the gas sensor is provided by an external heating wire. By using these heating wires, the operating temperature necessary for driving the sensor can be realized with high reliability. The sensor thus configured is excellent in durability and operation stability.

As a heating means, a resistance film selected from the group consisting of a sputtering film, an electron beam evaporation film, and a printed film is used on the back side of the heat-resistant insulating substrate.

By using a noble metal such as platinum as a resistor, these resistive films can realize an operating temperature necessary for driving the sensor with high reliability. The sensor thus configured is excellent in durability and operation stability.

Further, the porous catalyst body is constituted by using a ceramic sheet in which an oxidation catalyst is mixed in ceramic fibers.

As the ceramic fiber, a fiber combining one or more of silica, alumina, silica / alumina, zirconia and the like is used. These inorganic fibers are 1000
It has high heat resistance and stability at the level of ° C. As the oxidation catalyst, any of transition metal oxides and complex oxides such as iron, manganese, copper, nickel, and cobalt, and catalysts in which noble metals such as platinum and palladium are supported on a heat-resistant porous carrier such as γ-alumina are used. be able to. From the viewpoint of water vapor stability, a noble metal is preferable. With this configuration,
The temperature required for the operation of the porous catalyst and the semiconductor gas sensor element is obtained. The sensor thus configured is excellent in durability and operation stability.

Further, the semiconductor oxide film is formed using a film containing at least one oxide selected from the group consisting of tin, indium and zinc, an iron compound and gold. As the semiconductor oxide film, tin, indium, n-type semiconductor oxide such as zinc and palladium for improving the sensitivity thereof, a noble metal sensitizer such as platinum is used in combination, in the case of these compositions, There is a problem that the temperature at which the maximum sensitivity is obtained is as low as 100 to 150 ° C. This is because the temperature of the exhaust gas of the combustor may reach about 250 ° C. In contrast, a coating containing at least one oxide selected from the group consisting of tin, indium and zinc, an iron compound and gold has a stable sensitivity even at a constant temperature of 200 to 300 ° C. It has desirable characteristics to be applied to the exhaust gas environment of the combustor. A gas sensor formed using this semiconductor oxide film is excellent in durability and operation stability.

[0037]

FIG. 1 to FIG. 5 show an embodiment of the present invention.
This will be described with reference to FIG.

(Embodiment 1) FIG. 1 shows a conceptual sectional view of a gas sensor according to Embodiment 1 of the present invention. In FIG. 1, reference numeral 1 denotes a heating unit which is formed, for example, of a resistance film. Reference numeral 2 denotes a heat-resistant insulating substrate using a ceramic substrate such as alumina. 3 is a comb-shaped electrode as an electrode,
A pattern is formed using gold or the like. This comb-shaped electrode may use a thin film type such as sputtering or a thick film printing type such as a paste. On the surface of the comb-shaped electrode 3, a semiconductor metal oxide film as a semiconductor oxide film 4 is formed. Various types of N
Among oxides having type semiconductor characteristics, a film containing tin oxide, indium oxide, zinc oxide, or the like has good gas sensitivity. A P-type semiconductor oxide such as iron, manganese, copper, nickel, chromium, or cobalt may be mixed in a film containing these oxides and used after sensitization.

The semiconductor oxide film is formed by adding a semiconductor oxide powder to a binder such as glass, a polymer additive for adjusting paste properties such as carboxymethylcellulose, and a porogen such as camphor together with a solvent in an automatic mortar or a three-roll mill. It is made into a paste by using a disperser, printed by a screen printing method, dried, and fired. A ceramic gas selective permeable body 5 having a controlled pore diameter is formed by laminating the pair of semiconductor oxide films in close contact with each other. A porous catalyst body 6 is arranged in a region corresponding to one of a pair of comb-shaped electrodes of a ceramic gas selective permeable body 5 whose pore diameter is controlled.

The sealing member 7 not only joins the heat-resistant insulating substrate 2 to the ceramic gas selective permeable body having a controlled pore diameter but also prevents the gas from reaching the semiconductor oxide film 4 by bypass. In addition, the gas is provided so as to flow through only the ceramic gas selective permeable body 5 whose pore diameter is controlled. Bonding is performed using various inorganic adhesives and glass compositions. In any case, it is necessary to use a composition whose thermal expansion coefficient is adjusted. Although the seal 7 is provided on the outside in FIG. 1, the seal 7 may be provided on the opposed contact surface of the heat-resistant insulating substrate 2 which does not include the semiconductor oxide film 4 and the ceramic gas selective permeable body 5 whose pore diameter is controlled.

The porous catalyst may be any one having a gas permeation property and a property of oxidizing carbon monoxide when a carbon monoxide-containing gas permeates therethrough. What carried the oxidation catalyst in the porous body can be used. In FIG. 1, the heating means is disposed on the back side of the heat-resistant insulating substrate 2, but is not limited thereto, and may be disposed on the side of the porous catalyst.

A temperature of 250 to 350 ° C. for operation as a semiconductor gas sensor is provided by the heating means 1. One of the semiconductor oxide films without a porous catalyst is
The air containing carbon monoxide and the air on which the carbon monoxide has been removed by the porous catalyst are passed through the semiconductor oxide film having the other porous catalyst, and the ceramic gas is selectively permeated through a controlled pore diameter. It reaches after diffusing through the pores of the body. When the gas containing carbon monoxide arrives,
It detects carbon monoxide and shows a low resistance value, while the other does not change at a high resistance value. The ratio between the two indicates the sensitivity of the zero gas and the detection gas. Even if the temperature setting slightly changes with the lapse of time, the influence can be canceled by comparison with the characteristics of the zero gas. In addition, the gas component adsorbed on the semiconductor oxide film of the semiconductor gas sensor and adversely affecting its life is prevented or prevented from reaching the electrode surface by the ceramic gas selective permeable member 5 having a controlled pore diameter, so that the gas component has a long life. Can be achieved.

The ceramic gas selective permeable member 5 is formed by a sol-gel method or a CVD method using a ceramic porous substrate having a pore diameter of 0.1 to 1 μm produced by a sintering method such as alumina or zirconia. A pore control film is formed on the pore surface and used. That is, the ceramic raw material powder is formed as it is or mixed with an organic substance such as a resin to form a predetermined shape, and then sintered at a temperature lower than the temperature for complete sintering. The average pore diameter of the porous body produced by the sintering method is limited to 0.1 μm. Therefore,
In order to use for the purpose of the present invention, it is necessary to use a porous substrate produced by a sintering method and treat its pores with a coating film. Since the porous body produced by the sintering method is generally commercially available as a microfiltration membrane, the commercially available ceramic porous substrate can be used in the present invention.

Next, a method of controlling pores by the sol-gel method will be described below. After hydrolyzing a metal alkoxide such as zirconium isopropoxide or tetraethoxysilane, it is subjected to polycondensation under catalytic conditions such as hydrochloric acid to prepare a desired sol solution. When the porous ceramic is contacted with a porous ceramic having pores penetrating the sol solution, for example, when the porous ceramic is immersed in the sol, the sol solution is sucked by capillary force, and when the sol is dried, the sol solution is dried in the pores of the porous ceramic. Concentration of the sol and further gelation occur. Further, when the heat is further advanced, sintering proceeds from gelation to form a coating film. If necessary, a method of filtering the sol solution using a porous ceramic can also be adopted. Using this method, the pore size can be controlled. By adjusting the wettability of the pore surface of the porous ceramic, the solvent of the sol, the concentration of the sol, the immersion time, the pulling speed of the ceramic, and the like, a gas selective permeable material having a relatively uniform pore diameter can be obtained.

In addition to the sol-gel method, the pores may be controlled by forming and growing an oxide film in the pores of the porous body while thermally decomposing the compound in a flow system by a CVD method.

Regarding the material of the base material of the gas selective permeable body 5,
Describe. From the viewpoint of the coefficient of thermal expansion, it is desirable that the base material of the gas selective permeable body is matched with the material of the heat-resistant insulating substrate 1.

It is desirable to use zirconia, silica or a mixture thereof for the material of the pore control coating used for controlling the pores of the gas selective permeable material. In particular, the gas selective permeator
This is effective when the average pore diameter is 0 ° or less. 10Å
The following pores show an effective molecular sieving effect that does not allow passage of a high molecular weight gas in a gas flow flowing from outside to inside of the gas selective permeable body. In addition, the interaction with the gel film formed inside the pores, that is, the pore control treatment film, increases selectivity in gas permeability. That is, the intermolecular force between the gas molecule and the gel molecule is determined by the orientation force due to the interaction between the permanent hyperboloids, the induced force between the permanent hyperboloid and the induced hyperboloid, and the gas permeation due to the dispersion force based on the van der Waals interaction. Has a selectivity, that is, a surface diffusivity, but a hydrophobic pore control film containing one or more of silica and zirconia is a problem in applying a porous body having a pore diameter of a region of 10 ° or less. There is no problem of blockage of pores due to capillary condensation, and it is possible to completely block the entry of a gas such as SO2 which degrades a platinum electrode.

(Embodiment 2) FIG. 2 is a schematic perspective view of a main part of a heating means of a gas sensor according to Embodiment 2 of the present invention. In FIG. 2, a resistance film 8 selected from the group of a sputtering film, an electron beam evaporation film, and a printing film is formed on the surface of a heat-resistant insulating substrate 2 and used. The resistance film 8
A predetermined circuit pattern can be formed by using a printing screen pattern or a masking jig. For convenience in manufacturing, the resistive film can be formed by forming a resistive film in an appropriate size that can take a large number of pieces, and then cutting it into a required size later. Further, the volume specific resistance can be adjusted to a resistance value that enables proper operation as a heater by adjusting the resistor composition, the film thickness, the pattern width, the pattern length, and the like. A porous body may be used as the heat-resistant insulating substrate for high activation of the semiconductor oxide film. Although the gas permeability of the portion where the resistance film is formed is reduced or impaired, the resistance film can be formed by adjusting the aperture ratio without significantly impairing the gas permeability. When a wet method such as a plating film is used as a heater film forming means, the pores are closed,
Gas permeability is lost. Therefore, the wet method is not suitable in the present invention.

The operation of the gas sensor of the present embodiment and the effect of extending the life thereof are the same as those of the first embodiment. Among the film forming methods, sputtering and electron beam evaporation using vacuum technology have problems in terms of productivity in batch processing, but basically this gas sensor adopts a method that always detects zero gas. Therefore, there is an advantage that the sensor characteristics do not basically change even if the size of the gas sensor is reduced. Reducing the size can solve the cost and productivity problems.

(Embodiment 3) FIG. 3 is a conceptual sectional view of a gas sensor according to Embodiment 3 of the present invention. 3, the configuration of the internal gas sensor is the same as that of FIG. The gas sensor shown in FIG. 1 has a configuration in which the outer surface is covered with a ceramic sheet 9 and a heating wire 10 is arranged outside the outer surface. As the ceramic sheet, a sheet made using ceramic heat-resistant fibers such as silica, alumina, and zirconia is used. As the heating wire, either nickel chromium or iron chromium can be used. It is manufactured by using a coil-shaped member that has been formed in advance, and then covering the outer member with a ceramic sheet. It is desirable that a part of the end of the ceramic sheet be joined using an inorganic adhesive or the like.

(Embodiment 4) FIG. 4 is a schematic sectional view of a porous catalyst body which is a main part of a gas sensor of a gas sensor according to Embodiment 4 of the present invention. In FIG. 4, the porous catalyst body has a configuration in which a ceramic sheet 11 in which an oxidation catalyst is mixed in ceramic fibers, a heating wire 10 and a ceramic fiber sheet 12 are laminated. The heating sheet and the ceramic sheet 11 in which the oxidation catalyst was mixed in the ceramic fiber were obtained in Example 3.
Is the same as The method for producing the porous catalyst body provided with the heating means of this embodiment is the same as that of the third embodiment. The purpose of arranging the ceramic sheet 12 is to ensure insulation of the heating wire. The gas sensor characterized by the porous catalyst provided with the heating means manufactured in this manner has stable operation and excellent durability as in the previous embodiment.

Fifth Embodiment FIG. 5 shows a characteristic graph of a gas sensor according to a fifth embodiment of the present invention. FIG.
70 parts by weight of indium oxide, 30 parts by weight of CuFe ₂ O ₄ ,
4 is a graph showing an evaluation of resistance value characteristics with respect to a carbon monoxide concentration of a semiconductor oxide film containing 0.01 part by weight of gold as a main component. FIG. 5 shows data at 250 ° C. Usually, in a semiconductor gas sensor, the ratio of the resistance value of the gas of 1000 ppm of carbon monoxide gas to the zero gas is evaluated as the sensitivity, but the sensitivity is high for the carbon monoxide gas, in the case of palladium-tin oxide. A sensitivity of 100 to 200 times is known, and the sensitivity is maximum at a temperature of 100 to 150 ° C., whereas the sensitivity of the composition of FIG. 5 is in the range of 150 to 350 ° C. Is stable. In the case of FIG. 5, the resistance value at zero gas is about 60 MΩ, which is about 170 times as high as the sensitivity of palladium tin monoxide. When applied to a combustion exhaust gas system, the exhaust temperature may be about 250 ° C., so that a conventional palladium-tin oxide system or the like is difficult to use.

The reason why the present configuration has the same high sensitivity as the conventional one will be considered. Among oxides having various N-type semiconductor characteristics, the coatings of tin oxide, indium oxide, and zinc oxide are:
Has good gas sensitivity. Furthermore, when a small amount of P-type semiconductor oxides such as iron, manganese, copper, nickel, chromium, and cobalt are dispersed and mixed in a film containing these oxides, these oxides have high gas adsorption properties and can be used for gas detection. In addition to the chemical sensitizing effect of causing a gas to spill over to the main N-type semiconductor oxide, a physical sensitizing effect of changing the Fermi level of the N-type semiconductor oxide is obtained, and an effect of improving sensitivity is obtained. If the amount of P-type material is too large, N
The characteristic of the mold is impaired, and the characteristic that the resistance value is increased by detecting gas strongly appears and becomes unstable. Among P-type semiconductor oxides typified by various transition metal oxides, if the sensitizing action has a high reactivity, it becomes difficult to use it. Therefore, it is desirable to select a material having an appropriate reactivity. From this viewpoint, the iron compound has an appropriate reactivity and is optimal. Iron compounds that can be used for this purpose include compounds in various forms. In addition to a single compound of iron, a composite oxide or a mixture having a structure such as spinel or perovskite can be used. Furthermore, they have found that gold has a specific sensitizing effect when used in combination with an iron compound.

The results of experiments relating to the effects of the present invention will be described below. A commercially available sintered product of alumina (dimensions: 10 mm × 10 mm × 0.35 mm) was used as the heat-resistant insulating substrate. A 10 Ω platinum heater film is formed on the back surface, and 3 × 8
A pair of comb-shaped electrodes were formed using a platinum paste with a size of mm. The electrodes were formed at a thickness of 10 μm by baking at 1050 ° C. by thick film printing.

The semiconductor oxide film was prepared using a paste having the following composition. That is, indium oxide (In)
_{20 3} ) as a main component (indium oxide: 20 parts by weight, a copper-iron spinel compound (CuFe ₂
O ₄ ); 5 parts by weight, glass; 1 part by weight, methyl cellulose: 1 part by weight, mixed solvent: 25 parts by weight), 10 μm
m at a film thickness of 100 m
After drying for 0 minutes, a film was prepared.

The ceramic gas selective permeable member was manufactured in the following procedure. A zirconia porous substrate (average pore diameter of 0.2 μm and dimensions of 10 mm × 10 mm × 0.5) prepared by adjusting the particle size distribution and firing temperature by a doctor blade method.
Using the zirconia-isolated alkoxide solution containing zirconia isopropoxide as a main component, the pores were controlled. The pore control is 20 Wt% of alkoxide.
Using the solution, fine pores were created by over-coating. The average pore diameter was evaluated by a mercury porosimetry using a porosimeter. The average pore diameter was 0.1 μm for one application and 0.08 μm for three applications, and the sample applied three times was used as a sample.

Next, a porous catalyst body was prepared by the following procedure. First, as for the catalyst, 0.1 wt% of platinum and palladium are supported on gamma alumina having a particle size of 40/60 mesh (by adsorbing aqueous solutions of chloroplatinic acid, nitric acid of palladium chloride and hydrochloric acid, and then borohydride). (Reduction with sodium aqueous solution) An oxidation catalyst powder was prepared. This oxidation catalyst powder was mixed with silica-alumina fiber at a basis weight of 500 g / m ² using colloidal silica as a binder to obtain a 30 cm square sheet having a thickness of 1 mm. Next, this sheet was cut into 10 mm squares for use.

A gas sensor having the structure shown in FIG. 1 was prototyped by laminating the respective components produced as described above. The bonding agent for laminating gas sensor elements is a commercially available inorganic adhesive "Sumiceram"
(Product name). The sensitivity of this gas sensor was about 155 times at 250 ° C.

The durability of the prototype sensor was evaluated by an acceleration test using a flow-type test apparatus through a sulfur dioxide gas of 100 ppm. The exam is
Air containing sulfur dioxide gas at a concentration of 100 ppm was continuously ventilated into the general atmosphere, intermittently stopped with sulfur dioxide, and the stability of the zero point was confirmed by ventilating only general air.
The sensor sensitivity is confirmed by feeding air containing ppm carbon monoxide. Ordinary ceramic filter that does not control the pore diameter, which was simultaneously manufactured as a comparative sample,
In the case of the device having an average pore diameter of 0.2 μm, the sensitivity was lost in about 3 hours.

On the other hand, in the case of the above-mentioned prototype sensor, the sensitivity is maintained at about 145 times even after about 450 hours. It is still under ongoing testing. Meanwhile, the zero point of the paired gas sensor is stable.

In this embodiment, the case where a pair of comb-shaped electrodes is provided on the surface of the heat-resistant insulating substrate has been described. However, the electrode structure is not limited to the comb shape, but may be any shape that can implement the present invention. It is preferable that the configuration is such that the resistance of the electrode can be easily changed.

[0062]

The gas sensor of the present invention is embodied in the form described above, and has the following effects.

(1) Regarding the detection of carbon monoxide, the weak point of fail-out can be covered, and practical fail-safe can be expected. As described above, the element configuration has high reliability and is suitable for installation in a combustion apparatus or the like.

(2) In terms of the practical use of chemical sensors, a configuration using a ceramic gas selective permeable body with a controlled pore diameter for restricting the reaching of an interfering gas to the electrode surface with respect to durability, which has been the greatest problem in the past. As a result, it is possible to completely block the acidic gas that has a poisoning effect on the gas sensor, and it is expected that the service life will be drastically prolonged, so that a highly reliable gas sensor system can be constructed.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a gas sensor according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a main part of a heating unit of a gas sensor according to a second embodiment of the present invention.

FIG. 3 is a sectional view showing a gas sensor according to a third embodiment of the present invention.

FIG. 4 is a sectional view showing a porous catalyst body of a gas sensor according to Embodiment 4 of the present invention.

FIG. 5 is a characteristic evaluation graph of a gas sensor according to a fifth embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating means 2 Heat resistant insulating base material 3 Comb electrode 4 Semiconductor oxide film 5 Gas selective permeable body 6 Porous catalyst 7 Sealing member 8 Resistive film 9 Ceramic fiber sheet 10 Heating wire

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2G046 AA11 BA01 BA06 BA09 BB02 BB04 BC03 BE03 BF01 BG01 EA02 EA04 EA07 EA12 FB00 FB02 FB03 FB04 FE00 FE03 FE09 FE10 FE11 FE12 FE15 FE21 FE25 FE48

Claims (7)

[Claims] 1. A heat-resistant insulating substrate, a pair of electrodes provided on a surface of the heat-resistant insulating substrate, a semiconductor oxide film provided on the pair of electrodes, and a semiconductor oxide film provided on the semiconductor oxide film. A ceramic gas selective permeable body having a controlled pore diameter,
A gas sensor comprising: a porous catalyst body provided on a surface corresponding to one of a pair of electrodes on the ceramic gas selective permeable body. 2. The gas sensor according to claim 1, wherein a heating means is provided on a surface of the heat-resistant insulating substrate corresponding to a surface on which the pair of electrodes are provided. 3. The gas sensor according to claim 1, wherein the gas sensor is enveloped by a ceramic fiber sheet, and a heating wire is disposed outside the envelope. 4. A seal member is provided between the semiconductor oxide film and the gas selective permeable member so that the gas to be detected bypasses and does not directly reach the semiconductor oxide film.
The gas sensor according to any one of claims 1 to 3. 5. The gas sensor according to claim 2, wherein a resistance film selected from the group consisting of a sputtering film, an electron beam evaporation film, and a printed film is used as the heating means. 6. A porous catalyst body comprising a ceramic sheet obtained by mixing an oxidation catalyst into ceramic fibers.
6. The gas sensor according to any one of items 5 to 5. 7. A film containing at least one oxide selected from the group consisting of tin, indium and zinc, an iron compound and gold, as the semiconductor oxide film.
The gas sensor according to any one of the preceding claims.