JP3191544B2 - Thick film type gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

This invention relates to LP gas, city gas,
The present invention relates to a thick-film gas sensor for a gas leak alarm that detects hydrogen gas and the like, and more particularly to a configuration of a first oxidized combustion layer and a second oxidized combustion layer of a thick-film gas sensor.

[0002]

2. Description of the Related Art As one of gas sensors, a sensor using a metal oxide semiconductor such as tin oxide or zinc oxide is known. When these metal oxide semiconductors are heated to about 300 to 500 ° C. in the air, the oxygen in the air is activated and adsorbed on the particle surface to increase the resistance, but the reducing gas is a reducing gas in the test gas which is a reducing gas. Is adsorbed by the metal oxide semiconductor instead of adsorbed oxygen, and the electric resistance value decreases. A gas sensor using a metal oxide semiconductor utilizing such properties is used for a gas leak alarm device for LP gas, city gas, and the like.

FIG. 5 is a plan view showing a conventional thick film type gas sensor. FIG. 6 is a cross-sectional view of the conventional thick film type gas sensor taken along the line BB of FIG. The conventional thick film type gas sensor is formed by laminating a second oxidized combustion layer 3B, a gas-sensitive layer 4, and a first oxidized combustion layer 3A on one main surface of an insulating substrate 1 such as alumina. A heater 8 is formed on the other main surface of the substrate 1 to heat the thick-film gas sensor to a predetermined temperature. The gas-sensitive layer 4 is a layer made of, for example, an n-type metal oxide semiconductor such as tin oxide. The first and second oxidation combustion layers 3A and 3B are formed by supporting a noble metal catalyst such as platinum on tin oxide. These oxidation combustion layers 3A and 3B enhance the selectivity for combustible gas and selectively burn alcohol and the like. Gas sensing layer 4
Alternatively, commercial power is directly applied to the heater 8 via the electrodes 2 and 2A.

[0004]

However, in the above-described conventional three-layer type thick film type gas sensor, the temperature of the thick film type gas sensor rises due to the fluctuation of the power supply voltage, and therefore the resistance value of the sensor decreases. There is a problem that an alarm is issued when there is no flammable gas in the atmosphere. Further, the conventional three-layer thick-film gas sensor has a problem that the aging period becomes long because of carrying a noble metal catalyst such as platinum, and the resistance value gradually decreases with time in the flammable gas. Was.

The present invention has been made in view of the above points, and an object of the present invention is to improve the catalyst of the oxidizing combustion layer so that the resistance value does not fluctuate when the voltage fluctuates, the aging period is short, and the resistance value increases with time. An object of the present invention is to provide a thick film type gas sensor which is stable in terms of quality.

[0006]

According to the present invention, there is provided a thick-film gas sensor for detecting the presence or absence of gas by utilizing a change in the resistance value of a metal oxide semiconductor.
A substrate, (2) a pair of electrodes, (3) a first oxidizing combustion layer, (4) a gas-sensitive layer, and (5) a second oxidizing combustion layer.
(6) A heater is included, the substrate is a support for the gas sensor, and a pair of electrodes are directly attached on the substrate at a distance from each other;
The second oxidation combustion layer is selectively laminated on the substrate and the pair of electrodes, the gas-sensitive layer is selectively laminated on the second oxidation combustion layer and the pair of electrodes, and the first oxidation combustion layer is sensitive. The first oxidized combustion layer is formed by coating at least one selected from the group consisting of nickel oxide, iron oxide, aluminum oxide, and chromium oxide and a noble metal on a carrier made of an n-type metal oxide semiconductor. Supported as a catalyst, and the second oxidized combustion layer is formed on a carrier made of an n-type metal oxide semiconductor by nickel oxide,
At least one selected from the group consisting of iron oxide, aluminum oxide, and chromium oxide and a noble metal are uniformly supported as a catalyst, the gas-sensitive layer is made of an n-type metal oxide semiconductor, and the heater is laminated on the substrate. This is achieved by heating the thick-film gas sensor to a predetermined temperature.

The oxidation combustion layer oxidizes and combusts an interference gas such as alcohol to eliminate the influence of the interference gas on the thick film type gas sensor. The gas-sensitive layer adsorbs a test gas such as LP gas, reduces the electric resistance value, and detects the test gas.

[0008]

The oxides such as nickel oxide, iron oxide, aluminum oxide and chromium oxide contained in the first oxidizing combustion layer or the second oxidizing combustion layer are composed of noble metal and n-type metal oxide semiconductor in the gas-sensitive layer. Functions as a catalyst for adsorption. Oxygen is supplied from the oxidized combustion layer carrying oxides such as nickel oxide, iron oxide, aluminum oxide and chromium oxide and the noble metal to the n-type metal oxide semiconductor of the gas-sensitive layer by the oxidized combustion layer carrying only the noble metal. Good up to higher temperatures compared to properties.

In addition, since oxides such as nickel oxide, iron oxide, aluminum oxide, and chromium oxide and noble metals are uniformly supported on the n-type metal oxide semiconductor, the catalytic activity is increased and stabilized. After mixing oxides such as nickel oxide, iron oxide, aluminum oxide and chromium oxide with the n-type metal oxide semiconductor, the noble metal is supported on these mixtures, so that nickel oxide, iron oxide, aluminum oxide, chromium oxide, etc. Is uniformly dispersed in the n-type metal oxide semiconductor.

[0010]

Next, an embodiment of the present invention will be described with reference to the drawings. Embodiment 1 FIG. 1 is a plan view showing a thick-film gas sensor according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of the thick-film gas sensor according to the embodiment of the present invention, taken along line AA of FIG.

Parts performing the same functions as those of the conventional thick film type gas sensor are denoted by the same reference numerals. The structure of the thick film type gas sensor according to the embodiment of the present invention is as follows. Substrate 1
Two electrodes 2 are provided on one main surface of the first electrode 2 so as to be separated from each other. A second oxidation combustion layer 3Y is selectively provided on the substrate 1 and the electrode 2. The gas-sensitive layer 4 is selectively formed on the second oxidation combustion layer 3Y and the electrode 2. The first oxidized combustion layer 3X is provided by completely covering the gas-sensitive layer 4. The lead wire 7 is connected to the electrode 2. An electrode 2A is formed on the other main surface of the substrate 1, a heater 8 is selectively formed on the electrode 2A and the substrate 1, and a lead wire 9 is connected to the electrode 2A.

The substrate 1 is 0.5 mm thick, 3 mm long and 3 mm wide.
mm polished alumina substrate. The heater comprises a ruthenium oxide resistor. Such a thick film type gas sensor is prepared as follows. Screen printing of a platinum electrode paste on the two main surfaces of the substrate 1 in a predetermined pattern,
After drying, it was fired at a temperature of about 1100 ° C. A paste for a heater made of ruthenium oxide was screen-printed in a predetermined pattern and fired.

The stannic acid powder is dried, and nickel oxide NiO is added to the obtained stannic acid powder at a ratio of 500 ppm, mixed with ethyl alcohol as a dispersion medium in a ball mill for 20 hours, dried, and dried in the air at 730. Heat treatment was performed at a temperature of 2 ° C. for 2 hours. In this way, a tin oxide SnO ₂ powder containing nickel oxide NiO as an impurity was obtained. After pulverizing the obtained powder with a ball mill, an aqueous solution of chloroplatinic acid H ₂ PtCl ₆ was added,
The mixture was kneaded and dried to obtain a powder containing 2% of platinum Pt. This powder was heat-treated at 600 ° C. for 3 hours to obtain chloroplatinic acid H ₂ PtCl ₆
Was disassembled. Thus, nickel oxide NiO and platinum Pt
To give a tin oxide SnO ₂ powder containing as a catalyst. The obtained powder was pulverized with a ball mill, ethyl silicate, ethyl cellulose, carbitol and the like were added in appropriate amounts and kneaded to obtain a paste for an oxidized combustion layer. The obtained paste for an oxidized combustion layer was screen-printed in a predetermined pattern to a thickness of 20 μm and dried at 120 ° C. for 2 hours to obtain a second molded layer of an oxidized combustion layer.

The stannic acid powder is dried, heat-treated at 730 ° C. for 2 hours in dry air, and an appropriate amount of ethyl silicate, ethyl cellulose, carbitol, etc. is added to the obtained tin oxide powder and kneaded to form a gas-sensitive layer. Was obtained. The obtained paste was screen-printed in a predetermined pattern to a thickness of 50 μm and dried at 120 ° C. for 2 hours to obtain a molded layer of a gas-sensitive layer.

The paste for oxidizing combustion layer is screen-printed to a thickness of 20 μm according to a predetermined pattern.
It dried at 2 degreeC for 2 hours, and obtained the molded layer of the 1st oxidation combustion layer. A molded layer composed of three layers of a second oxidized combustion layer, a gas-sensitive layer, and a first oxidized combustion layer is simultaneously fired at a temperature of 630 ° C. for 3 hours for 3 hours. Oxidation combustion layer 3
Y was formed.

Subsequently, platinum lead wires 7, 9 were connected to the electrodes 2, 2A, respectively, to obtain a thick film type gas sensor. Each lead was connected to the alarm circuit. In the first and second oxidized combustion layers formed by the above-described method, the platinum Pt crystal is finely dispersed by the nickel oxide NiO crystal and the crystal growth is suppressed, and the combustion of the interference gas and the oxygen to the gas-sensitive layer are suppressed. It becomes a catalyst layer that functions as a supply and has excellent activity and stability.

FIG. 3 is a diagram showing the dependency of the resistance value on the number of days elapsed in the thick-film gas sensor according to the embodiment of the present invention (A) in comparison with the characteristic (A) of the conventional thick-film gas sensor. The measurement was performed in 0.2% isobutane gas diluted with air. It can be seen that the thick film gas sensor of the present invention in which the nickel oxide NiO and platinum catalysts are supported in the first and second oxidized combustion layers has better resistance stability than the conventional thick film gas sensor. This is because the catalytic activity of the oxidation combustion layer is stable. The aging period has also been shortened. This is because the catalytic activity of the oxidation combustion layer is large.

FIG. 4 is a diagram showing the voltage dependence (c) of the resistance value of the thick-film gas sensor according to the embodiment of the present invention in comparison with the characteristic (d) of the conventional thick-film gas sensor. The resistance value of the thick-film gas sensor is that in air. It can be seen that the thick-film gas sensor of the present invention has less voltage dependence of the resistance value than the conventional gas sensor. The addition amount of nickel oxide NiO is suitably in the range of 100 ppm to 5000 ppm, and the addition amount of platinum Pt is suitably 1 to 5% by weight.

Further, tin oxide Sn which is a metal oxide semiconductor
The oxide added to O ₂ is replaced by nickel oxide NiO, ferric oxide Fe ₂ O ₃ , aluminum oxide Al ₂ O ₃ , chromium oxide Cr ₂ O ₃
Etc. can be used. As the n-type metal oxide semiconductor, zinc oxide or the like can be used in addition to tin oxide SnO ₂ . Example 2 Tin oxide powder was dried, and nickel oxide NiO was added to the obtained tin oxide powder at a ratio of 500 ppm, and the mixture was mixed with ethyl alcohol as a dispersion medium for 20 hours using a ball mill and dried. An aqueous solution of chloroplatinic acid, H ₂ PtCl _6, was added to the obtained powder, kneaded and dried to obtain a powder of tin oxide SnO ₂ containing 2% of platinum Pt. This powder was heat-treated at 730 ° C. for ₂ hours to decompose chloroplatinic acid H ₂ PtCl ₆ . Thus, a tin oxide SnO ₂ powder containing nickel oxide NiO and platinum Pt was obtained. The obtained powder was pulverized with a ball mill, an appropriate amount of ethyl silicate, ethyl cellulose, carbitol, etc. were added and kneaded to obtain a thick film type gas sensor in the same manner as in Example 1 except that a paste for an oxidized combustion layer was obtained. did.

When preparing a thick film type gas sensor by this method, a catalyst can be separately supported on tin oxide SnO ₂ and nickel oxide NiO. Example 3 A stannic acid powder was dried, and an aqueous solution of nickel chloride was added to the obtained stannic acid powder, kneaded, dried, and dried in air.
Heat treatment was performed at a temperature of 30 ° C. for 2 hours. Thus, tin oxide SnO ₂ powder containing nickel oxide NiO at a ratio of 500 ppm was obtained.

After the obtained powder was pulverized with a ball mill, an aqueous solution of chloroplatinic acid H ₂ PtCl ₆ was added, kneaded and dried, and heat-treated at 600 ° C. for 3 hours to decompose chloroplatinic acid H ₂ PtCl ₆ . Thus, nickel oxide NiO 500ppm
And SnO ₂ powder containing tin Pt 2% as a catalyst. The obtained powder is pulverized with a ball mill, and an appropriate amount of ethyl silicate, ethyl cellulose, carbitol, etc. is added,
A thick-film gas sensor was manufactured in the same manner as in Example 1 except that kneading was performed to obtain a paste for an oxidation combustion layer.

When a thick film type gas sensor is prepared by the present method,
The dispersion state of nickel oxide NiO is good, and the activity of the catalyst is increased.

[0023]

According to the present invention, the thick-film gas sensor is of a three-layer type including a first oxidizing combustion layer, a gas-sensitive layer, and a second oxidizing combustion layer, and the oxidizing combustion layer is an n-type metal oxide. Combustion, in which at least one selected from the group consisting of nickel oxide, iron oxide, aluminum oxide, and chromium oxide and a noble metal are uniformly dispersed and supported as a catalyst on the oxide semiconductor, so that oxygen is supplied from the oxidation combustion layer to the gas-sensitive layer. Even if the catalytic activity of the layer is higher than that of the noble metal alone and the temperature of the thick-film gas sensor rises due to the fluctuation of the heater voltage, the fluctuation of the resistance value of the thick-film gas sensor is small and an alarm is issued by the fluctuation of the heater voltage A thick film type gas sensor which is excellent in reliability and free from defects can be obtained.

Further, the increase in the catalytic activity and stability of the oxidized combustion layer makes it possible to obtain a thick-film gas sensor in which the resistance value in the combustible gas is more stable than the conventional thick-film gas sensor and the aging period is shortened. .

[Brief description of the drawings]

FIG. 1 is a plan view showing a thick film gas sensor according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of the thick-film gas sensor according to the embodiment of the present invention, taken along line AA of FIG. 1;

FIG. 3 is a graph showing the dependence of the resistance value on the number of days elapsed for the thick-film gas sensor according to the embodiment of the present invention (A) in comparison with the characteristic (A) of the conventional thick-film gas sensor.

FIG. 4 is a diagram showing the voltage dependence (c) of the resistance value of the thick-film gas sensor according to the embodiment of the present invention in comparison with the characteristic (d) of the conventional thick-film gas sensor.

FIG. 5 is a plan view showing a conventional thick film type gas sensor.

FIG. 6 is a cross-sectional view of the conventional thick-film gas sensor taken along the line BB in FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Electrode 2A Electrode 3A First oxidized combustion layer 3B Second oxidized combustion layer 3X First oxidized combustion layer 3Y Second oxidized combustion layer 4 Gas-sensitive layer 7 Lead wire 8 Heater 9 Lead wire

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-7-43331 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/12

Claims (5)

(57) [Claims] 1. A thick film type gas sensor for detecting the presence or absence of a gas by utilizing a change in the resistance value of a metal oxide semiconductor, comprising: (1) a substrate; (2) a pair of electrodes; (4) a gas-sensitive layer, (5) a second oxidizing combustion layer, and (6) a heater, wherein the substrate is a support for the gas sensor, and a pair of electrodes are provided on the substrate. The second oxidized combustion layer is selectively laminated on the substrate and the pair of electrodes, and the gas-sensitive layer is selectively laminated on the second oxidized combustion layer and the pair of electrodes. The first oxidizing and burning layer covers the entire gas-sensitive layer and is laminated. The first oxidizing and burning layer is formed of nickel oxide, iron oxide, aluminum oxide, and chromium oxide on a carrier made of an n-type metal oxide semiconductor. At least one selected from the group and the noble metal are uniformly supported as a catalyst, and the second oxidation combustion layer Comprises a carrier made of an n-type metal oxide semiconductor and at least one selected from the group consisting of nickel oxide, iron oxide, aluminum oxide and chromium oxide and a noble metal as a catalyst, and the gas-sensitive layer is an n-type metal oxide semiconductor. Wherein the heater is stacked on the substrate to heat the thick-film gas sensor to a predetermined temperature. 2. A thick-film gas sensor according to claim 1, wherein at least one of the nickel oxide, iron oxide, aluminum oxide and chromium oxide is carried in an amount of 100.
A thick film type gas sensor characterized by being in the range of 5,000 ppm to 5,000 ppm. 3. The thick-film gas sensor according to claim 1, wherein the amount of the noble metal carried is in the range of 1 to 5% by weight. 4. The thick-film gas sensor according to claim 1, wherein the first oxidized combustion layer or the second oxidized combustion layer is formed of nickel oxide, iron oxide, aluminum oxide, chromium oxide on an n-type metal oxide semiconductor. A thick-film gas sensor characterized in that at least one selected from the group consisting of: 5. The thick-film gas sensor according to claim 1, wherein the n-type metal oxide semiconductor is tin oxide.