JPH09269307A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor of high reliability capable of being mounted on combustion machinery, having stable output characteristics against the irregularity of an environmental factor inclusive of temp. irregularity accompanied by the load fluctuations of the combustion machinery and enhanced in durable reliability related to the deterioration of an electrode and a catalyst. SOLUTION: Two pairs of comb-shaped electrodes 2 are formed on the same substrate 1 in order to cancel fluctuation effect by providing a compensation element for cancelling a fluctuation factor other than carbon monoxide and N-type semiconductor oxide sintered material films 3 are formed on the respective electrodes 2. Further, a porous carbon monoxide and hydrogen oxidizing catalyst film 4 is formed on one N-type semiconductor oxide sintered material film while a porous hydrogen selective oxidizing catalyst film 5 having no carbon monoxide oxidizing capacity is formed on the other N-type semiconductor oxide sintered material film 3 to obtain extremely stable output characteristics.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting combustible gas, particularly carbon monoxide, in the general atmosphere or in exhaust gas of various combustion equipment that uses gas or petroleum as a fuel. A sensor that is stable against changes,
Further, the present invention relates to a gas sensor having excellent characteristics in terms of durability, which is the greatest problem in chemical sensors.

[0002]

2. Description of the Related Art Carbon monoxide (CO) is a colorless, tasteless, and odorless gas, slightly lighter than air but highly toxic, and has a pressure of 200 PPM.
Even if the concentration is as low as 2 to 3 hours, it causes headaches, etc., and when it reaches 3000 PPM or higher, it takes about 6000 minutes in 10 minutes.
Above PPM, death for a few minutes of breathing.

Since it is generated from instant water heaters, bath kettles, oil and gas heating appliances and charcoal fires even in ordinary households, it is cheap, compact and reliable to be used by being built into these devices or installed indoors. There is a strong demand for a high carbon monoxide sensor.

As a conventionally proposed gas sensor, especially a carbon monoxide detection sensor, an electrode for absorbing carbon monoxide (CO) to oxidize it is provided in an electrolytic solution, and the CO concentration is determined from a current value proportional to the CO concentration. Using a detection method (constant potential electrolysis gas sensor), a sintered body type n-type semiconductor oxide such as tin oxide (Sn02) sensitized with a trace amount of additional elements such as noble metals, these semiconductors are treated as flammable gas. A method of detecting gas by utilizing the characteristic that the electric conductivity changes when contacting (semiconductor type gas sensor), a pair of comparison of a platinum fine wire of about 20 μm with alumina and no precious metal Known as a method (catalytic combustion type gas sensor) that detects the difference in heat generation when a combustible gas comes into contact with this element to carry out a catalytic oxidation reaction by heating it to a certain temperature using an element. To have. For example, [Reference 1] Supervision by Toyoaki Omori: “Practical Encyclopedia of Sensors” by Fuji
Techno System Chapter 14 Basics of Gas Sensor (Masaki Haruta), P11
2-130 (1986) has a detailed description.

Particularly, as a semiconductor type CO sensor, a gas sensor utilizing a change in resistance value of a metal oxide semiconductor is disclosed in [Reference 2] Japanese Patent Publication No. 53-43320 [Reference 3] Japanese Patent Publication No. 61-50051. There has been proposed a method of periodically heating in a high temperature region and a low temperature region alternately and intermittently sampling the output of a gas sensor in the low temperature region to detect CO gas, and an improvement thereof. These are characterized in that the selectivity of CO detection is enhanced mainly by devising from the signal processing side.

Further, [Document 4] Japanese Patent Application Laid-Open No. 1-227951 discloses a gas sensor using a metal oxide whose resistance value changes with gas as a sensor body, in which a zeolite coating layer is provided on the surface of the sensor body. Proposed. This also aims at improving the selectivity of CO detection.

FIG. 12 shows the structure of a general semiconductor gas sensor.
Shown in FIG. 12 shows an example of a hot wire semiconductor type sensor in which a platinum wire coil 18 is used, and a semiconductor film 19 in which palladium or the like is mainly added to tin oxide is formed on the surface of the platinum wire coil 18. It energizes the platinum wire to raise the temperature of the semiconductor film and captures the change in resistance as an electrode wire.
It is characterized by high sensitivity when the carbon monoxide concentration is low.

Regarding the gas selective permeable membrane which is one of the main points of the present invention, [Reference 5] Tatsuya Okubo, Seiji Morooka, Current status and future development of inorganic separation membranes, Chemical Engineering, 12, 1 (1)
988, 1989). Here, a typical production method of a ceramic gas selective permeable membrane, that is, an inorganic separation membrane, and a membrane structure are introduced. However, application of the inorganic separation membrane to the gas sensor has never been attempted in the past.

[0009]

In the case of a gas leak alarm device which has a proven track record, it is intended to operate in an indoor environment in which humans live, so the temperature condition is gentle and the change in ambient temperature is small. On the other hand, in the case of combustion equipment, the sensor is installed in the exhaust gas flow path of the equipment. The ambient temperature of the sensor naturally depends on the installation location. However, even if the location where the temperature change is relatively gradual is selected, it varies from the strong combustion to the weak combustion with the fluctuation of the load of the combustion equipment, at least from 30 ° C to 300 ° C, and depending on the condition, 50
A large temperature change up to about 0 ° C occurs.

In the catalytic combustion type, since the combustion heat due to the oxidation of carbon monoxide is detected in principle, an output cannot be obtained when the temperature exceeds 300 ° C. In the semiconductor type, as long as the conventional technique is applied, if the temperature range is this much, the fluctuation range of the output is large, and there is a problem that carbon monoxide cannot be detected due to being buried in the temperature error.

Further, the characteristics required of the exhaust gas sensor for combustion equipment are not limited to the small temperature error. Due to the burner characteristics of the combustion equipment, hydrogen is generated like carbon monoxide. The fluctuation range of the hydrogen concentration fluctuates from a half level of carbon monoxide to a double level.

If there is no selectivity that can absorb this fluctuation range,
It becomes a system with many malfunctions and is not practical. High selectivity of carbon monoxide to hydrogen is required.
Since hydrogen is generally more reactive than carbon monoxide, hydrogen exhibits an interfering action in almost all chemical sensors. Removing hydrogen interference is a major issue.

Further, in the combustion exhaust gas, the partial pressure of water vapor,
Oxygen partial pressure and carbon dioxide partial pressure fluctuate remarkably depending on combustion conditions, which causes malfunction. Therefore, while a gas sensor with the characteristics that can withstand this change in environmental conditions is required,
There is currently no answer to this.

The most important issue of chemical sensors that have been widely used as gas sensors has been durability. this is,
This is because the electrodes and catalysts that play a central role in the function of the chemical sensor deteriorate over time as the reaction progresses. Particularly, for the purpose of detecting carbon monoxide, the sensor is generally strongly affected by not only carbon monoxide but also reducing gas such as hydrogen.

In the case of a chemical sensor which basically applies a catalyst or a chemisorption phenomenon, the durability and the selectivity of the catalyst are generally contradictory with each other, and the chemical sensor is used for the purpose of reducing the interference effect of the reducing gas. When the operation of is operated on the activation control side (low temperature side) of the catalytic reaction, the selectivity is improved,
Adsorption of various poisonous gases onto the surface of the catalyst is likely to occur, and the catalyst deteriorates rapidly and has no durability. On the contrary, if the operation is used on the diffusion control side (high temperature side), the selectivity is deteriorated.

For example, as a semiconductor type gas sensor, Ti ^{4+ is used.}
In the case of a sensor using an element in which fine gold particles are supported on αFe ₂ O ₃ doped with, the selectivity of CO for hydrogen and alcohol is excellent, but the durability of the catalyst is disadvantageous. The methods described in Documents 2-4 do not basically improve the durability of the element because the sensor itself is not protected from various poisonous gases related to durability. In this respect, zeolite can be expected to have an effect of removing poisoning gas, however, in order to actually make the effect effective, it is necessary to control the pore structure of the zeolite coating more than the pore structure of the zeolite itself. However, since this is extremely difficult to control, the practical effect of 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.

The gas sensor utilizing the resistance change of the metal oxide semiconductor is generally put into practical use as a gas leak alarm, but the durability is usually about 3 years. This is due to the deterioration of the semiconductor oxide due to the poisoning gas.
When incorporating a carbon monoxide detection gas sensor in a combustion device,
The durability of the gas sensor needs to be about 10 years, and a drastic improvement in durability over the conventional technology is required.

In any type of sensor, a noble metal is often used as an electrode, a catalyst, or a sensitizer that plays a central role in the sensor function, and it is deteriorated by a sulfur compound or a silicone compound to ensure durability. It's getting very difficult. Further, since hydrocarbons coexisting in exhaust gas of combustion equipment have a large molecular weight and a large molecule size, adsorption on the surface of platinum hinders the adsorption of carbon monoxide and adversely affects as a disturbing gas.

[0019]

In order to solve the above-mentioned problems, the sensor of the present invention comprises heating means, two pairs of comb-shaped electrodes are formed on the same substrate, and N electrodes are formed on the respective electrodes. -Type semiconductor oxide-based sintered body film is formed, and a porous carbon monoxide and hydrogen oxidation catalyst film is further formed on one N-type semiconductor oxide-based sintered body film, and the other N-type semiconductor oxide-based sintered body is formed. A porous hydrogen selective oxidation catalyst film having no hydrogen monoxide oxidizing ability but carbon monoxide oxidizing ability is formed and used on the body membrane.

The operation of the sensor in the general atmosphere or in the exhaust gas flow path of the combustion equipment will be described.

The heat source for operating the sensor is achieved by the heating means provided in the sensor. If necessary, temperature control may be performed by using a temperature detection means such as a thermistor together. As the heating means, various means such as a heating wire or a resistance heater film can be applied. As a material used for the resistance heater film, a noble metal such as platinum is preferable in terms of durability. When using a heating wire, iron-chromium, nickel-
A chrome-based material or the like can be used.

According to the first aspect of the present invention, when the combustion exhaust gas containing carbon monoxide comes in, porous carbon monoxide and hydrogen oxide are formed on one electrode side formed on the same substrate. Since the catalyst film is formed, the reducing gas such as carbon monoxide and hydrogen is all oxidized by the catalyst film, and the reducing gas does not reach the N-type semiconductor sintered body film. On the other electrode side, hydrogen is oxidized but carbon monoxide is not oxidized and reaches the N-type semiconductor sintered body film. The resistance value of the N-type semiconductor-based oxide sintered body film on the side where the porous carbon monoxide and hydrogen oxidation catalyst film is formed changes due to changes in environmental conditions such as temperature, oxygen and water vapor. On the other hand, on the side where the porous hydrogen selective oxidation catalyst film is formed, the influence of carbon monoxide is added in addition to the influence of the above environmental conditions. By using the output characteristics obtained from both of these electrodes and using the difference or ratio of the two as an output, the influence of environmental conditions other than carbon monoxide is canceled, and stable output characteristics can be obtained in response to changes in environmental conditions. In general, in an N-type semiconductor oxide-based sintered body film, the adsorbed oxygen on the surface of the film is reduced by reacting with a reducing gas, and the height and width of the potential barrier are reduced, which facilitates the transfer of electrons and increases the ratio. Resistance decreases. These have a temperature characteristic, an oxygen partial pressure characteristic, and the like, which is an obstacle, but in the present invention, the output is stabilized by the configuration in which only the carbon monoxide output is taken out.
Strictly speaking, when the device is constructed, both electrodes have minute errors because the temperature and the film thickness of the N-type semiconductor oxide-based sintered body film are different. In order to practically reduce the influence of this error, it is necessary that the sensitivity of the sensor is high.

The present invention further has a structure in which all the gas flowing into the gas sensor functional element portion is caused to flow through the ceramic gas selective permeable membrane. That is, in the gas sensor of the present invention, two pairs of comb-shaped electrodes are formed on the same substrate by using a ceramic gas selective permeation tube whose pores are controlled to 3 to 100Å, and an N-type semiconductor oxide is formed on each of the electrodes. A sintered material film of a physical type is formed, and a porous carbon monoxide and hydrogen oxidation catalyst film is further formed on one of the sintered N type semiconductor oxide film,
An element in which a porous hydrogen selective oxidation catalyst film having no hydrogen monoxide oxidizing ability but carbon monoxide oxidizing ability was formed on the other N-type semiconductor oxide-based sintered body film was provided in each ceramic gas selective permeation tube. The lead wire from the electrode is taken out, housed and sealed, and a heater is arranged outside the ceramic gas selective permeation tube.

The ceramic gas selective permeable membrane will be described below. The ceramic gas selective permeable membrane is prepared by using a commercially available porous ceramic or porous glass. Porous ceramic or porous glass is used as a ceramic filter in various applications. For example, it is well known that it is used for separating yeast from beer. The pore size is 0.05 μm
To several μm. As it is, the selective permeability of gas cannot be obtained, so that it is necessary to fill the pores to control the pore diameter. As a method of controlling the pore diameter, a method of forming a sol-gel coating in the pores, or a CVD method of forming a coating in the pores by thermal decomposition to control the pores is known. . Various conventionally known film forming methods can be applied. Among them, for example, the pore diameter can be controlled to the pore diameter of the gas molecular diffusion region by utilizing the decomposition reaction of metal alkoxide (sol-gel method) or the CVD reaction. With these methods, the pore diameter can be controlled to be uniform pores of 2Å to several Å. The size of such pores is the size of gas molecules, and the movement of the gas in the pores has the effect of interaction between the substance on the surface of the pores and the gas, but actually has a complicated diffusion property. Basically, the gas permeation rate is in the region of Knudsen diffusion to molecular sieve diffusion, and is inversely proportional to the square root of the molecular weight of the gas and regulates permeation, so that it has the property of significantly impeding the permeation of molecules of large size. Large size molecules cannot pass through the permselective membrane.

In the present invention, a gas selective permeation tube made of ceramics whose pore size is controlled to 3 to 100 Å, practically 5 to 100 Å, and preferably 3 to 10 Å is used, and it seems that the gas sensor is basically housed inside. Configure the sensor in this state.

That is, the gas contained in the general atmosphere or in the exhaust gas of a combustion device first diffuses into the pores whose pores are controlled, but a gas having a larger molecular size than the pore diameter, such as kerosene vapor or Silicone oligomers, etc.
Cannot penetrate into the sensor. Further, since reactive gases such as SO ₂ and NO ₂ have a large molecular weight, it is difficult for them to diffuse in the pores, and the amount reaching the gas detection portion decreases. Low-molecular gas molecules such as oxygen, carbon monoxide, and nitrogen can freely reach the gas detection unit in a state similar to Knudsen diffusion. The size of the pore size is extremely important, and if the pore size exceeds 100Å, selective permeability of gas cannot be obtained. On the other hand, if it is smaller than 3Å, target molecules such as CO and O ₂ cannot permeate. Moreover, 5-100Å is practically preferable. This is because the production is relatively easy when producing the gas selective permeable membrane. Furthermore, it is desirable that the pore size is 3 to 10 Å in order to exert the selective permeability of gas. In this case, the diffusion of gas is in the region of the molecular sieve, and complete selective permeability is obtained. Even in the case of 10 Å or more and 100 Å or more, it becomes a Knudsen diffusion region, and since the large-sized molecule has the effect of restricting the inflow, deterioration of the electrode or the catalyst can be prevented, and a long life can be expected to be contributed.

Further, by housing the sensor in the gas selective permeation tube, the temperature effect of the sensor is reduced. That is,
In the case of a bare sensor, temperature dependence includes complicated reaction processes such as oxidation and reduction reactions such as adsorption on the sensor surface, and these reaction processes have exponential characteristics with respect to temperature. Therefore, it has a large temperature coefficient. On the other hand, when the gas is housed in a gas selective permeation tube, the reaction rate is controlled by diffusion from the gas permeation tube when the temperature is used on the higher side in order to avoid the influence of ambient temperature. Since the sex changes with the power of 1.7 with respect to the temperature, the temperature characteristics are eventually stabilized.

Since the sensor of the present invention uses a ceramic gas selective permeation tube whose pores are controlled to 3 to 100 Å in the portion corresponding to the gas inlet / outlet to the sensor, the gas selective permeation tube of this gas selective permeation tube has. Depending on the function, the sensor detection section related to the gas detection function does not reach silicone and heavy hydrocarbons, and poisonous gases such as SO ₂ and NO _x that adversely affect the sensor life. It will have a long life.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a gas sensor provided with heating means, in which two pairs of comb-shaped electrodes are formed on the same substrate, and an N-type semiconductor oxide-based sintered body film is formed on each of the electrodes. A porous carbon monoxide and hydrogen oxidation catalyst film formed on one of the N-type semiconductor oxide-based sintered body films, and an oxidizing ability of carbon monoxide on the other N-type semiconductor oxide-based sintered body film. However, it is formed by forming a porous hydrogen selective oxidation catalyst film having a hydrogen oxidation ability.

The resistance value of the N-type semiconductor oxide-based sintered body film decreases when carbon monoxide is detected, but when used in combustion exhaust gas, the resistance value changes due to various factors other than carbon monoxide. The factors are divided into physical factors and chemical factors. Physically, it is the effect of temperature and chemically reducing gas such as hydrogen. In the present invention, one of the two resistance detection units shows a change in which other factors are added in addition to carbon monoxide, and the other shows a change in which carbon monoxide is removed and other factors are added. By taking the difference or ratio between the two, factors other than carbon monoxide are canceled and the effect of carbon monoxide can be extracted purely. Furthermore, the electrode and the N-type semiconductor oxide-based film, which are easily affected by deterioration such as specific poison, are protected by the porous catalyst film, so that a long life can be expected. If the catalyst film layer is porous, the film thickness can be increased, and the catalyst performance becomes more advantageous and compatibility with durability is achieved.

In the second configuration, the above element is housed in a gas selective permeation tube and used to regulate and control the gas diffused to the sensor function section. In other words, the total amount of gas is reduced, and the high molecular weight gas that is easily poisoned by the sensor function section is not permeated or is difficult to permeate, thereby extending the life. Further, the temperature characteristics of the sensor can be stabilized by shifting the temperature control of the sensor to the diffusion control type.

A third structure is provided with a heating means, and two pairs of comb-shaped electrodes are formed on the same substrate, and an N-type semiconductor oxide containing a carbon monoxide sensitizer is provided on each electrode. Porous carbon monoxide and hydrogen oxidation catalyst film on the other side of the N-type semiconductor oxide-based sintered body film, which is formed by further forming a sintered body film of a material and further containing a sensitizer for detecting carbon monoxide, A porous selective hydrogen oxidation catalyst film having carbon monoxide oxidizing ability but not hydrogen monoxide oxidizing ability is formed on an N-type semiconductor oxide based sintered body film containing a sensitizer for carbon monoxide detection. It is configured to be used.

A fourth structure is provided with a heating means, and two pairs of comb-shaped electrodes are formed on the same substrate, and an N-type semiconductor oxide containing a carbon monoxide sensitizer is formed on each electrode. On the N-type semiconductor oxide-based sintered body film formed by forming a material-based sintered body film and further containing a sensitizer for detecting carbon monoxide.
Forming a ceramic paper film by mixing oxides of one or more elements selected from the group of Co, Mn, Cu, Fe, Cr, Ni and composite oxides into ceramic fibers, and forming the other one of the oxidation N comprising a carbon-sensitizing sensitizer
A ceramic paper film formed by mixing oxides of one or more elements selected from the group of Zn, Sn, In and composite oxides into ceramic fibers is formed on the type-type semiconductor oxide-based sintered body film. It is configured to be used. That is, the fifth structure relates to a porous catalyst film having excellent porosity and reactivity, in which two pairs of comb electrodes are formed on the same substrate,
An N-type semiconductor formed by forming a carbon monoxide sensitizer film on each electrode and forming an N-type semiconductor oxide-based sintered body film, and further containing a carbon monoxide detection sensitizer film. Co, Mn, Cu, Fe, Cr, Ni on the oxide-based sintered body film
N containing a sensitizer for detecting carbon monoxide and forming a ceramic paper film by mixing an oxide of one or more elements selected from the group On the type semiconductor oxide-based sintered body film,
Detection of N-type semiconductor oxide-based sintered body film by using a ceramic paper film formed by mixing papers of oxides and composite oxides of one or more elements selected from the group of Sn and In The catalytic action can be highly activated without hindering the rapid diffusion of the gas to be used.
With this configuration, the thickness of the catalyst film can be increased, and actually, it is advantageous that the catalyst film is thick as long as the diffusion property is not impaired. With the catalyst film of this structure, SO
_Since the poisoning gas such as ₂ and NO ₂ is absorbed and removed, the N-type semiconductor oxide-based sintered body film and the comb-shaped electrode are protected, and the durability can be expected to be improved.

When a sintered film or the like is used as the catalyst film, it hinders the diffusion of the reaction gas even if it has a high oxidizing ability and significantly impairs the responsiveness, while the catalyst ability and the gas diffusibility, that is, the sensor. It is possible to achieve a good balance of both responsiveness.

In the fifth structure, two pairs of comb-shaped electrode films are formed on one surface of the substrate, an N-type semiconductor oxide-based sintered body film is laminated on each of the two pairs of comb-shaped electrode films, and one of the two films is laminated. A porous carbon monoxide and hydrogen oxidation catalyst film is formed on the N-type semiconductor oxide-based sintered body film, and hydrogen oxidation is performed on the other N-type semiconductor oxide-based sintered body film, although the carbon monoxide has no oxidizing ability. A porous hydrogen selective oxidation catalyst film having an ability is formed, and a heater film is further formed on the back surface of the substrate for use. The sixth configuration relates to the temperature stability of the sensor and the reduction of heating temperature energy, and the heater film is also formed on the same substrate as the sensor at the same time to stabilize the characteristics of the sensor. In particular, because of its excellent temperature following property, high-temperature heating and low-temperature heating, which are performed by ordinary semiconductor elements, are periodically repeated, and the oxide film of the N-type semiconductor oxide-based sintered body is stably formed at the time of high-temperature heating to achieve a high specific resistance. It is also possible to use the device for stabilizing the device and increasing the sensitivity by taking in the specific resistance at the time of heating at a low temperature, which has a higher sensitivity, as an output after the state.

In the sixth structure, a pair of comb-shaped electrode films are formed on the outer surface of a substantially cylindrical substrate, and an N-type semiconductor oxide-based sintered body film is laminated on the two sets of comb-shaped electrode films. On one of the N-type semiconductor oxide-based sintered body films, a porous carbon monoxide and hydrogen oxidation catalyst film, and on the other N-type semiconductor oxide-based sintered body film,
A porous hydrogen selective oxidation catalyst film having hydrogen monoxide oxidizing ability but not carbon monoxide oxidizing ability is formed, and a heater wire is arranged inside the cylinder for use.

Further, two pairs of comb-shaped electrode films are formed on one surface of the substrate, and an N-type semiconductor oxide-based sintered body film is laminated on each of the two pairs of comb-shaped electrode films. Porous carbon monoxide and hydrogen oxidation catalyst film on the sintered sintered body film,
On the other N-type semiconductor oxide-based sintered body film, a porous hydrogen selective oxidation catalyst film having no hydrogen monoxide oxidizing ability but carbon monoxide oxidizing ability was formed, and further a heater film was formed on the back surface of the substrate. The element is configured to be housed and sealed in a state in which lead wires and heater wires from each electrode are taken out inside a ceramic gas selective permeation tube having fine pores controlled to 3 to 100 Å. Further, a pair of comb-shaped electrode films is formed on the outer surface of the substantially cylindrical substrate, and an N-type semiconductor oxide-based sintered body film is laminated on the two sets of comb-shaped electrode films. Porous carbon monoxide and hydrogen oxidation catalyst film on the sintered film,
On the other N-type semiconductor oxide-based sintered body film, a porous hydrogen selective oxidation catalyst film which does not have the oxidizing ability of carbon monoxide but has a hydrogen oxidizing ability is formed, and an element having a heater wire inside the cylinder is formed. The ceramic gas selective permeation tube having pores controlled to 3 to 100 Å is constructed so that the lead wire and the heater wire from each electrode are taken out and housed and sealed. The seventh structure is expected to stabilize the temperature of the device and reduce the size of the device. Since the heater wire is arranged inside the substantially cylindrical substrate, there is an advantage that the temperature is easily stabilized in the radial direction as compared with the case of a flat plate. There is also an advantage that the gas selective permeation tube, which is housed for the purpose of further improving durability, can be housed.

In the seventh structure, a porous substrate is used as the substrate. The eighth configuration is configured to use a porous film as the N-type semiconductor film.

In both the seventh and eighth configurations, the sensitivity of the N-type semiconductor oxide-based sintered body film to carbon monoxide is high, and more stable output characteristics can be obtained.

Embodiments of the present invention will be described below with reference to the drawings. (Embodiment 1) FIG. 1 is a sectional view of a semiconductor gas sensor according to an embodiment of the present invention. In FIG. 1, reference numeral 1 is a ceramic substrate. In FIG. 1, the electrode films and the like are formed on the front and back surfaces of the ceramic substrate, but they need not be flat plates as long as they are on the same substrate. As a material of the ceramic substrate, a material generally used as a substrate such as alumina can be applied. Reference numeral 2 on the substrate 1 is a comb-shaped electrode, which is a film of a conductive metal such as gold, palladium, or platinum, and a paste is used to form a predetermined pattern by screen printing or a masking jig with a predetermined pattern is used. Vacuum deposition,
A thin film such as sputtering may be formed. It may also be formed by transfer. 3 is an N-type semiconductor oxide-based sintered body film formed on the comb shape. Aluminum, silicon, magnesium, strontium, barium, tantalum, niobium, titanium, lanthanum containing N-type semiconductor oxides such as tin oxide, zinc oxide, and indium oxide as main components,
Oxides of typical elements such as cerium, alkaline earth metals, and rare earth elements may be used alone or in combination as an additive. These films are formed mainly by a thick film method, that is, by applying or printing a paste and then baking or drying. It can also be formed by a dry method such as plasma spraying, sputtering, laser ablation, or a thin film method. For example, in the case of a thick film method, a binder such as glass for the above metal oxide powder and a polymer additive for adjusting paste characteristics such as carboxymethyl cellulose are used together with a solvent in a disperser such as an automatic mortar or a triple roll mill. Then, it is made into a paste, printed by a screen printing method, dried, and baked to prepare.

Reference numeral 4 denotes a porous carbon monoxide and hydrogen oxidation catalyst film, which is a catalyst in which a precious metal such as platinum, rhodium or palladium is supported on a porous carrier such as alumina, or iron, nickel, cobalt, nickel, chromium or the like. It is formed by the same method as the N-type semiconductor oxide-based sintered body coating, such as a method of coating and baking, using a catalyst mainly composed of an oxide mainly composed of a transition metal compound or a complex oxide such as a perovskite compound. It This porous carbon monoxide and hydrogen oxidation catalyst film has a property of completely oxidizing carbon monoxide and further hydrogen together with gas diffusibility. This is because the above-mentioned oxidation catalysts other than aluminum, manganese, copper, and gold have a stronger hydrogen oxidation ability than carbon monoxide. That is, various gases are diffused into the lower N-type semiconductor oxide-based sintered body film, but carbon monoxide, hydrogen, etc. are completely oxidized during that time. 5 is a porous hydrogen oxidation catalyst that does not have the ability to oxidize carbon monoxide but has the ability to oxidize hydrogen, and magnesium, silver, zinc, cadmium, which does not have the ability to adsorb carbon monoxide,
It is formed by a method similar to the above, such as a method of coating and baking, using a catalyst mainly composed of an oxide such as tin or indium or a complex oxide. This film has the ability to oxidize hydrogen as well as the diffusivity of gas. That is, carbon monoxide and the like reach the lower N-type semiconductor oxide-based sintered body film layer, but only hydrogen is completely oxidized and does not reach the lower layer. Reference numeral 6 denotes a heating means provided in the gas sensor, to which various heaters and the like can be applied. When this element is placed in the combustion exhaust gas containing carbon monoxide, the side where the carbon monoxide and hydrogen oxidation catalyst film is formed and the catalyst film that does not have the oxidation ability of carbon monoxide but has the hydrogen oxidation ability is formed. The resistance value change of the element due to the N-type semiconductor film is such that the resistance value changes on the carbon monoxide oxidation catalyst side due to the resistance value change factor other than carbon monoxide, while the other This shows the combined change of carbon monoxide and other factors that change the resistance value. As a result, by taking the difference or resistance ratio between the two, the factors of resistance value change other than carbon monoxide are canceled. As a method of extracting the signal, a deviation voltage can be extracted by forming a bridge circuit between the resistance values of the both. This makes it possible to detect a change in the resistance value of carbon monoxide that is nearly pure.

(Embodiment 2) FIG. 2 shows a sectional view of another embodiment of the present invention. FIG. 2 shows the element of FIG.
It is stored inside. Further, the heating means 6 has a structure in which a heater wire is wound outside the gas selective permeation tube 7 in this embodiment. The heater may be formed by patterning a resistance film on the outside of the gas selective permeation tube by plating or printing instead of winding the heating wire. The other symbols in FIG. 2 are
It is similar to FIG. The gas selective permeation tube 7 is a tubular ceramic gas selective permeation membrane in which the pore size is controlled to 3 to 100Å, practically 5 to 100Å, and preferably 3 to 10Å. Alumina, cordierite, or the like can be used as the material of the gas selective permeable membrane made of ceramics. The operation of this sensor will be described. This gas sensor has a total of 15 heaters with 6 heaters.
It is heated to 0 to 400 ° C. When brought into contact with carbon monoxide-containing air, gas molecules flow into the element through the ceramic gas selective permeation tube 7 only. The operation after that is the same as that in the case of FIG.

In the following, 3 to 100 Å, practically 5 to
A method of controlling the pore size of the porous membrane by the sol-gel method, which is a powerful method among the methods for producing a ceramic gas selective permeable membrane having pores of 100 liter, preferably 3 to 10 liter, will be described. In the basic production method, a ceramic porous body having a pore of 1 micron level, which is generally commercially available as a microfiltration membrane, is used as a base material, and the pores are closed.

After hydrolyzing a metal alkoxide such as aluminum isopropoxide or tetraethoxysilane, it is subjected to polycondensation under a catalytic condition such as hydrochloric acid to prepare a sol solution.
When this sol solution is brought into contact with a hydrophilic porous ceramic having pores penetrating the sol solution, water is sucked by the capillary force, and the sol is concentrated or gelated in the pores of the porous ceramic or glass. If necessary, a method of filtering the sol solution using a porous ceramic can also be adopted. By utilizing this phenomenon, the pore diameter can be controlled. When the pores of the porous ceramic are brought into contact with the sol aqueous solution obtained from the metal alkoxide, specifically, when the porous ceramic is dipped in the sol solution for a short time and then dried, gelation occurs in the pores. , The pores close. By adjusting the wettability of the surface of the pores of the porous ceramic or glass, the concentration of the sol, and the immersion time, uniform pore diameter control of 2 to several Å level can be achieved.

In addition to the sol-gel method, the CVD method may be used to control the pores by thermally growing the compound in the flow system while forming and growing an oxide in the pores of the porous body. This method is excellent as a method for producing a cylindrical ceramic gas selective permeation tube.

High-molecular-weight gas does not pass through the pores thus produced. In addition, the interaction with the gel film formed inside the pores increases the gas permeability. That is, the intermolecular force between the gas molecule and the gel molecule is due to the orientation force due to the interaction between the permanent dipoles, the inductive force between the permanent dipole and the induced dipole, and the dispersion force based on the Van der Waals interaction.

(Embodiment 3) FIG. 3 shows a conceptual diagram of a cross section of an embodiment in which a porous ceramic substrate 10 is used as a substrate. By using the porous ceramic substrate 10, all the films formed on the substrate become porous, and the detection sensitivity of carbon monoxide can be increased. The porous ceramic substrate is
It is produced by a sintering method in which organic fine particles are mixed in a ceramic raw material and fired. The fine pore diameter of the membrane obtained by this method is about 0.1 μm even if it is fine. The porosity is preferably about 30% in consideration of the substrate strength. When using this porous substrate, the gas can freely permeate and diffuse through the substrate.

(Embodiment 4) FIG. 4 shows a conceptual diagram of a cross section of another embodiment in which the porous N-type semiconductor oxide-based sintered body 11 is used. The method for producing the porous N-type semiconductor oxide-based sintered body 11 is, for example, in the case of a thick film method, a polymer for adjusting the paste characteristics such as the above-mentioned metal oxide powder or the like with a binder such as glass and carboxymethyl cellulose. Using an automatic mortar or three-roll mill disperser with additives, a method of adding porous fine powder such as silica gel as an additive when forming a paste, or volatile fine powder such as camphor, or fine crystals. It can be produced by printing, drying and firing by a screen printing method, which is made into a paste by a method of adding a thermally decomposable fine powder such as cellulose. The porous N-type semiconductor oxide-based sintered body produced in this manner has high reactivity with the carbon monoxide gas to be detected as in the case of the porous substrate, and has high sensitivity.

Example 5 FIG. 5 shows carbon monoxide sensitizer 1
The conceptual diagram of the principal part cross section about the N-type semiconductor oxide type sintered compact layer containing 3 is shown. The sensitizer 13 is used by being dispersed in the sintered body together with the N-type semiconductor oxide 12. The sensitizer 13 may be added by mixing it in the above paste in advance or by carrying it after forming an N-type semiconductor oxide-based sintered body. The function of the sensitizer is that carbon monoxide is once adsorbed on the surface of the sensitizer and spills over to the N-type semiconductor oxide to sensitize it, and acts on the band structure of the N-type semiconductor oxide. Although it has a function of increasing conductivity, either method may be used in the present invention. The sensitizer that can be used in the present invention, gold, platinum, palladium, noble metals such as rhodium, or iron, manganese,
Examples thereof include one or more oxides selected from the group of elements of copper, nickel, cobalt, and chromium, or composite oxides having these elements as active centers. Usually, the latter oxide has a P-type semiconductor oxide property, and therefore has the function of canceling the N-type semiconductor oxide property when the blending amount increases,
It is necessary to keep the compounding amount at most 10%.

(Embodiment 6) FIG. 6 is a sectional view showing the principal part of an embodiment relating to the catalyst membrane of the present invention. Reference numeral 14 is a catalyst film, and the catalyst film 14 has a structure in which catalyst particles 15 are contained in a ceramic fiber 16. This catalyst membrane is produced by suspending and dispersing ceramic fibers in an aqueous solution together with a binder and catalyst particles 15 and then making paper from the suspended state, compressing and drying. Since the catalyst particles 15 are present in the ceramic paper in a highly active state, when used as a catalyst film, the gas permeability is extremely good, that is, gas permeability, and the catalyst can also be used in a highly active state.

The catalyst used here is a complete oxidation catalyst for carbon monoxide on one side, and cobalt, manganese,
Oxides and complex oxides of one or more elements selected from the group of iron, copper, nickel, and chromium are highly active and inexpensive. Of course, a noble metal-based supported catalyst may be used. On the other hand,
As a selective oxidation catalyst of hydrogen, oxides and complex oxides of elements selected from the group of zinc, tin, and indium are preferable because they have excellent selectivity.

(Embodiment 7) FIG. 7 shows a sectional view of another embodiment of the present invention. In FIG. 7, two sets of sensor systems are provided on one side of the substrate 1, and the heater film 6 is formed on the back side. Figure 7
In, all symbols are the same as in FIG. Since the heater film is formed on the substrate, the heating required for the operation of the device can be efficiently performed, so that the power consumption can be small. In particular, when a method of operating the output characteristics of the sensor at two points, that is, high temperature and low temperature, is adopted, it is advantageous in terms of responsiveness.

(Embodiment 8) FIG. 8 shows a sectional view of another embodiment of the present invention. In FIG. 8, A is a sectional view thereof,
B is a cross-sectional view of an essential part of a substantially cylindrical substrate 17 which is a base material.
All other symbols are as described above. The heating means 6 is a heater wire. It is desirable to apply a transfer method or the like for producing the electrode film or the like on the cylindrical substrate. The film can be formed by plasma spraying or other dry method. The configuration of FIG. 8 has the advantage that the present element is efficiently stored in the gas selective permeation tube and a small element can be formed.

The gas selective permeation tube will be described below.
The gas permselective tube is manufactured by processing a ceramic filter as a base, which is generally used for the purpose of precision filtration and manufactured by a sintering method. Since the ceramic filter produced by the sintering method has a pore size limit of 0.1 micron, it is necessary to treat the pores in order to use it for the purpose of selective gas permeation. is there. In the present invention, a commercially available alumina microfiltration tube (2.2 / 1.8 mmφ) was used, and this was closed with an aluminum polymer to form a gas selective permeation tube. The aluminum polymer was prepared by the sol-gel method. That is, aluminum isopropoxide was hydrolyzed at 80 ° C. for 24 hours, and then hydrolyzed with hydrochloric acid catalyst in 4
It was prepared by polymerizing for 8 hours. The gas selective permeation tube was treated with about 10 alumina microfiltration membranes in the polymer solution.
Soak for 50 seconds and dry at room temperature for about 8 hours, then 50
In the method produced by firing to 773K at K / h, the pore size was as large as about 800Å. Next, the pore diameter was controlled by repeating the operation of filtering the alumina polymer with a filtration membrane for about 3 minutes, followed by drying at room temperature for about 8 hours and then firing at 50K / h to 773K several times. It was The boehmite film had changed to γ-alumina. Evaluation of the molecular weight cutoff and direct confirmation with a scanning electron microscope revealed that the pore diameter was about 50 ° after about four treatments. In this way, a ceramic gas selective permeation tube was produced. It was produced in this way. It was treated with a length of 35 cm, cut into a required size with a diamond cutter, and used as a sensor.

The sensor is 1.2 / 0.8 mmφ × 4
A comb-shaped electrode was formed on an alumina tube of mm, and the paste prepared by the following method was applied on the electrode to a film thickness of about 10 μmn, dried at 100 ° C. for 10 minutes, and further baked at 400 ° C. for 1 hour. It was made. The paste is In ₂ O ₃ powder 1
25 parts by weight of CuFe ₂ O ₄ powder and 0.03 parts by weight of particulate Au are used as a solvent in an automatic mortar with respect to 00 parts by weight,
After being dispersed together with glass and carboxymethyl cellulose for about 1 hour, it was prepared by dispersing and mixing twice with a three-roll mill.

FIG. 9 shows the relationship between the carbon monoxide and the resistance value characteristics of the sensor when carbon monoxide and hydrogen were flowed in a flow-type reaction test device using this element while changing the compounding ratio thereof. . It can be seen from FIG. 9 that this system is an element that is extremely unlikely to be affected by hydrogen interference.

FIG. 10 shows the relationship between carbon monoxide and the resistance value when the sensor is housed inside the 800 Å gas selective permeation tube, and FIG. 11 shows the relationship between the sensor prepared above inside the 50 Å. The relationship between carbon monoxide and resistance is shown.

It can be seen from FIGS. 10 and 11 that the temperature effect of the sensor is reduced by housing the sensor in the gas selective permeation tube. It is considered that this is because the temperature characteristic of the sensor is a diffusion dominant type with a small temperature coefficient. By storing the gas in the gas selective permeable membrane as in this example, the influence of temperature is reduced.

Next, in order to confirm the influence of durability, a sensor is arranged in the exhaust gas passage of the combustion equipment, and SO ₂ is added to the exhaust gas in an amount of about 100 ppm and an organic low-molecular-weight silicone is added to the exhaust gas for the purpose of an accelerated acceleration test. The change with time of the sensor was examined. The bare sensor equivalent to the prior art deteriorated in about 50 hours in the case of the semiconductor method, whereas the sensor of the present invention is stable with no characteristic change observed after 3000 hours. It has been confirmed and is under continuous evaluation.

[0060]

As described above, according to the gas sensor of the present invention, the following effects can be obtained.

(1) Regarding the detection of carbon monoxide, since two sensors are used to extract an output signal purely based on carbon monoxide with respect to environmental changes caused by changes in combustion characteristics such as temperature changes. It is highly stable and suitable for installation in combustion equipment.

(2) Regarding the durability, which has hitherto been the biggest problem in practical use of the chemical sensor, the catalyst layer capable of setting a large thickness can absorb the interference gas and reach the sensor. Since the sensor is housed in this by using a gas selective permeation tube that regulates, the life of the sensor is expected to be dramatically increased, and an extremely highly reliable sensor system can be constructed.

(3) In terms of sensor manufacturing, it is possible to apply a thick film printing method or the like, which is excellent in mass productivity, and in principle it is possible to reduce the size and power consumption. Yes,
In terms of cost, it is inexpensive and a highly practical sensor can be obtained.

In both the seventh and eighth configurations, the sensitivity of the N-type semiconductor oxide based sintered body film to carbon monoxide is increased, and more stable output characteristics can be obtained.

[Brief description of drawings]

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

FIG. 2 is a sectional view of a gas sensor according to a second embodiment of the present invention.

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

FIG. 4 is a sectional view showing a manufacturing process of a gas sensor according to a fourth embodiment of the invention.

FIG. 5 is a sectional view of a main part of a gas sensor according to a fifth embodiment of the present invention.

FIG. 6 is a sectional view of an essential part of a gas sensor according to a sixth embodiment of the present invention.

FIG. 7 is a sectional view of a gas sensor according to a seventh embodiment of the present invention.

FIG. 8A is a sectional view of a gas sensor according to an eighth embodiment of the present invention. FIG. 8B is a sectional view of essential parts of a gas sensor according to the eighth embodiment of the present invention.

FIG. 9 is a graph showing characteristics of the gas sensor according to the second embodiment of the present invention.

FIG. 10 is a graph showing the characteristics of the gas sensor according to the third embodiment of the present invention.

FIG. 11 is a graph showing the characteristics of the gas sensor according to the fourth embodiment of the present invention.

FIG. 12 is a sectional view of a conventional gas sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Pair of comb-shaped electrode films 3 N-type semiconductor oxide-based sintered body film 4 Porous carbon monoxide and hydrogen oxidation catalyst film 5 Porous hydrogen selective oxidation catalyst film 6 Heating means 7 Gas selective permeation tube 9 Lead wire 10 Porous substrate (porous ceramic substrate) 11 Porous N-type semiconductor oxide-based sintered body 12 N-type semiconductor oxide

Claims (11)

[Claims] 1. A gas sensor comprising a heating means, wherein two pairs of comb-shaped electrodes are formed on the same substrate, and an N-type semiconductor oxide-based sintered body film is formed on each of the electrodes. A porous carbon monoxide and hydrogen oxidation catalyst film is formed on the N-type semiconductor oxide-based sintered body film, and hydrogen oxidation is performed on the other N-type semiconductor oxide-based sintered body film, although the carbon monoxide has no oxidizing ability. A gas sensor formed by forming a porous hydrogen selective oxidation catalyst film having the ability. 2. A gas sensor is housed and sealed in a ceramic gas selective permeation tube having fine pores controlled to 3 to 100 Å with lead wires from each electrode taken out, and a ceramic gas as a heating means. The gas sensor according to claim 1, wherein a heater is arranged outside the selective permeation tube. 3. A gas sensor comprising heating means, wherein two pairs of comb-shaped electrodes are formed on the same substrate and a carbon monoxide sensitizer is contained on each electrode. A porous carbon monoxide and hydrogen oxidation catalyst film is formed on the N-type semiconductor oxide-based sintered film, which is formed by forming a sintered film and further containing a sensitizer for detecting carbon monoxide, and the other one. Forming a porous selective hydrogen oxidation catalyst film having hydrogen oxidization ability, but not carbon monoxide oxidization ability, on an N-type semiconductor oxide based sintered body film containing a sensitizer for carbon oxide detection. Consisting of a gas sensor. 4. A gas sensor comprising heating means, wherein two pairs of comb electrodes are formed on the same substrate and a carbon monoxide sensitizer is contained on each electrode. A group of Co, Mn, Cu, Fe, Cr, and Ni was formed on the N-type semiconductor oxide-based sintered body film formed by forming a sintered body film and further containing a sensitizer for detecting carbon monoxide. An N-type semiconductor that forms a ceramic paper film by mixing an oxide of one or more elements selected from the above and a composite oxide in a ceramic fiber, and contains a sensitizer for detecting the other carbon monoxide Zn, Sn, In on the oxide-based sintered body film
A gas sensor formed by forming a ceramic paper film by mixing an oxide of one or more elements and a composite oxide selected from the group 1) into a ceramic fiber. 5. The gas sensor according to claim 3, wherein an oxide containing In ₂ O ₃ as a main component is used as the N-type semiconductor oxide, and CuFe ₂ O ₄ and Au are used as the sensitizer. 6. A pair of comb-shaped electrode films is formed on one surface of a substrate,
An N-type semiconductor oxide-based sintered body film is laminated on each of two pairs of comb-shaped electrode films, and a porous carbon monoxide and hydrogen oxidation catalyst film is formed on one of the N-type semiconductor oxide-based sintered body films. , On the other N-type semiconductor oxide-based sintered body film, a porous hydrogen selective oxidation catalyst film having hydrogen monoxide oxidizing ability but not carbon monoxide oxidizing ability is formed, and a heater film is further formed on the back surface of the substrate. A gas sensor. 7. The gas sensor is housed in a ceramic gas selective permeation tube having a fine pore controlled to 3 to 100 Å, and the lead wire and heater wire from each electrode are taken out and sealed. 7. The gas sensor according to any one of 1 to 6. 8. A pair of comb-shaped electrode films are formed on the outer surface of a substantially cylindrical substrate, and an N-type semiconductor oxide-based sintered body film is laminated on two sets of comb-shaped electrode films, one of which is N-type. A porous carbon monoxide and hydrogen oxidation catalyst film is formed on the semiconductor oxide-based sintered body film, and a hydrogen oxidation capacity is formed on the other N-type semiconductor oxide-based sintered body film, though the carbon monoxide has no oxidizing ability. A gas sensor that has a porous hydrogen selective oxidation catalyst film with a heater wire inside the cylinder. 9. A gas sensor is housed in a ceramic gas selective permeation tube having a fine pore controlled to 3 to 100 Å, and the lead wire and heater wire from each electrode are taken out and sealed. The gas sensor described. 10. The gas sensor according to claim 1, wherein the substrate is porous. 11. The gas sensor according to claim 1, wherein the N-type semiconductor film is porous.