JP3046387B2 - Gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[Object of the invention]

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated gas sensor using a metal oxide semiconductor, and more particularly to a gas sensor having excellent gas sensitivity characteristics and long-term stability for ventilation.

[0003]

2. Description of the Related Art Conventionally, a gas sensor using a metal oxide semiconductor as a gas sensing element has been used for the purpose of leakage inspection of combustible gas and detection of contamination of indoor air. Among these gas sensors, those using an n-type semiconductor such as zinc oxide, tin oxide, or indium oxide as a gas sensitizer show that the electric resistance of the n-type semiconductor decreases by contact with a reducing gas. Gas is detected using
In the case where the type semiconductor is used as a gas sensing element, the gas is detected by utilizing the fact that the electrical resistance of the p-type semiconductor increases due to the contact with the reducing gas. Such a gas sensor is called a semiconductor gas sensor, and has features of being excellent in sensitivity, response speed, and economy.

However, semiconductor gas sensors that are currently in widespread use have the problems that the electrical resistance is not sufficiently reduced or increased with a semiconductor alone, and that it is difficult to selectively identify and detect a specific component. ing. There is also a problem that the electric resistance changes with time due to poisoning deterioration and humidity and temperature. For the former, generally, a catalyst layer composed of a porous metal oxide supporting a catalytic substance such as a noble metal or a metal oxide is formed on a gas-sensitive body by applying (stacking type), or as described above. For example, such a catalyst substance is added to a semiconductor. Such a catalyst provides a field of adsorption, reaction, diffusion and permeation for the gas to be detected, and not only improves the sensitivity of the gas sensitizer, but also increases the sensitivity to a specific component, that is, a moderate gas. It has the function of expressing selectivity. In addition, the latter problem relating to reliability is not only a semiconductor type but also a problem of the fate of a chemical sensor, and is caused by a principle that a sensitive body of a gas sensor reacts by directly contacting a chemical component.

[0005] Therefore, in order to obtain a gas sensor with little change over time and excellent stability, the degradation mechanism has been studied. For example, in a tin oxide sintered type semiconductor gas sensor, it has been found that growth of metal oxide semiconductor particles, a decrease in the surface area of a metal catalyst, a decrease in surface hydroxyl groups, and the like are the causes of deterioration (500 Hyakuzo, Electrochemistry 50, (No.
1), 99, (1982), Matsuura, Takahata, Gohyakuzo, Proceedings of the 6th [Sensor Basics and Applications] Symposium, B-1, P63, (1986)
, Nakamura et al., Proceedings of the 5th Chemical Sensor Research Presentation, Z-
26, P55, (1986), etc.), and good results have been obtained in some cases by changing the sintering conditions and the binder.

On the other hand, it has been reported that the life of the stacked tin oxide semiconductor gas sensor was extended several times by changing the firing conditions of the semiconductor thin film or changing the electrode material in order to improve the reliability. ing. But,
Even when these methods are used, sufficient control of deterioration in high humidity has not been achieved. Further, from a ripening test of a semiconductor using a pressure cooker tester, no correlation was recognized between the length of processing time or the presence or absence of processing and the reliability in high humidity. In other words, in the stacked semiconductor gas sensor, the gas reacts with the semiconductor and the catalyst layer formed thereon, and the degradation in high humidity is not caused by the semiconductor film, but mainly due to the catalytic activity. It is thought to be due to the decline. Therefore, in general, it is considered that as a measure for extending the life of the catalyst, it is effective to improve the dispersibility of the metal catalyst on the carrier, adjust the amount of the catalyst carried, and control the particle size of the catalyst.

Here, in particular, the required characteristics of a gas sensor used as a ventilation sensor for generating a control signal for automatically performing ventilation according to the indoor atmosphere include:
(a) It is important to have uniform sensitivity to various gases that cause discomfort or are harmful to the human body, especially sensitivity to the harmful gas carbon monoxide (CO) gas. (CO gas has a detection standard of about 200 ppm due to its danger), and (b) it is excellent in moisture resistance because it is exposed to a general atmosphere and is greatly affected by humidity.

For example, a paste obtained by adding an aluminum resin salt, turpentine oil, ethyl hydroxyethyl cellulose, or the like to a tungsten-copper / alumina powder obtained by supporting copper and tungsten on alumina by an impregnation method is formed on a flat substrate. A gas sensor element manufactured by screen printing on a gas sensitive body made of a tin oxide semiconductor, drying and firing, and applying a catalyst layer thereon, satisfies the sensitivity characteristic (a) described above. The screen printing method used for the method of manufacturing a thick film integrated circuit is an excellent method with high mass productivity, and the stacked gas sensor manufactured by this method is a combination of a semiconductor that is a gas sensitive body and the above catalyst, Furthermore, since it is also possible to apply a surface treatment or the like to each of the semiconductor and the catalyst, there is an advantage that the sensor characteristics can be easily changed and adjusted.

However, a stacked tin oxide semiconductor gas sensor having a catalyst layer formed by this screen printing,
In the moisture resistance test, in addition to the increase in the resistance value of the semiconductor in the air, the sensitivity to CO as the detection target may change depending on the change of the catalyst, etc. On the other hand, good results have not been obtained.

[0010]

As described above, the conventional stacked semiconductor gas sensor used for the automatic control of ventilation and the like has a problem that the resistance of the semiconductor increases in air due to continuous use in a high humidity atmosphere. There was a problem in stability over time, for example, and the sensitivity to various gases changed.

The present invention has been made in order to solve such problems, and has excellent sensitivity to miscellaneous gases such as ethanol, propane, and carbon monoxide, and has been used for a long time or in a high humidity atmosphere. It is an object of the present invention to provide a stacked semiconductor gas sensor that suppresses a change in gas sensitivity during use in a semiconductor device and has excellent stability over time.

[0012]

Configuration of the Invention

[0013]

The gas sensor of the present invention comprises an insulating substrate, a pair of electrodes formed on the insulating substrate, and a metal oxide semiconductor formed over the pair of electrodes. In a gas sensor comprising a gas-sensitive film consisting of: and a catalyst layer formed so as to cover the gas-sensitive film, the catalyst layer comprises alumina as a carrier, and copper, tungsten and phosphorus or an oxide thereof are deposited on the carrier. It is characterized by being constituted by a supported catalyst.

[0014]

In the gas sensor of the present invention, a catalyst in which three kinds of metals of copper, tungsten and phosphorus or their oxides are simultaneously supported on alumina as a carrier is used as a catalyst layer of a gas-sensitive film. As described above, by using a mixture of the above three kinds of metals, it is possible to effectively suppress catalyst deterioration particularly in a high-humidity atmosphere. Copper and tungsten, as catalysts, mainly exhibit gas sensitivity characteristics and selectivity desirable as ventilation sensors, whereas phosphorus addition has little effect on gas sensitivity characteristics and contributes to improved stability of characteristics. . It is considered that the addition of phosphorus changes the properties of the catalyst and the semiconductor surface, and acts, for example, to promote the adsorption / desorption reaction of water vapor or a hydroxyl group. In particular,
It exhibits the function of reducing the resistance of an element once increased in a high-humidity atmosphere for a long time when returned to a low-humidity atmosphere.
Thus, the reduction of phosphorus is an effective method for improving the humidity characteristics.

[0015]

Next, embodiments of the present invention will be described with reference to the drawings.

FIGS. 1A and 1B are diagrams showing a configuration of a gas sensor according to an embodiment of the present invention, wherein FIG. 1A is a plan view thereof, and FIG.
It is sectional drawing which followed the -A line. In FIG. 1, reference numeral 1 denotes an insulating substrate made of alumina or the like, and a heater 2 is built in the insulating substrate 1. On the surface of the insulating substrate 1, a pair of electrodes 3, 4 provided to face each other is formed. These opposing electrodes 3, 4 are composed of comb-shaped electrode bodies 3a, 4a and electrode pads 3b, 4b, and are placed on the electrode bodies 3a, 4a located near the center of the insulating substrate 1. A gas-sensitive film 5 is formed so as to be in contact with each of them. The gas-sensitive film 5 is made of a metal oxide semiconductor, and mainly includes, for example, tin oxide, zinc oxide, indium oxide, and the like.

A catalyst layer 6 is formed on the gas-sensitive film 5 so as to cover it. This catalyst layer 6
It consists of a porous layer of catalyst particles in which three kinds of metal elements of copper, tungsten and phosphorus are supported on alumina particles. Here, as the content ratio of copper, tungsten and phosphorus in the catalyst layer, the range of 0.1 to 10.0 parts by weight of copper and the range of 0.1 to 20.0 parts by weight of tungsten is Is preferably in the range of 0.01 to 10.0 parts by weight. Phosphorus added to alumina
If the amount is less than 0.01 part by weight, the effect of improving moisture resistance and the like cannot be sufficiently obtained, and if the amount exceeds 10.0 parts by weight, sensitivity may be reduced. Further, when the weight ratio of tungsten to copper, for example, the weight ratio represented by W / Cu exceeds 10, the gas sensitivity decreases, so it is practically desirable that W / Cu be in the range of 0.1 to 10. .

The electrode pads 3 for the opposing electrodes 3 and 4
A lead wire 7 is attached to b and 4b, a heater pad 8 is connected to the heater 2, and a lead wire 9 is further attached to the heater pad to constitute a gas sensor.

The heater 2 burns steam, dust, oil, and the like adhering to the gas sensor, and also increases the sensitivity and responsiveness of the element by increasing the gas adsorption / desorption speed. As described above, generally, the heater is often built in the insulating substrate of the gas sensor.

The gas sensor of this embodiment having the above-described configuration is manufactured, for example, as follows.

FIG. 2 is a flowchart showing an example of the manufacturing process of the gas sensor according to the present invention. For example, a multi-piece substrate is used as the insulating substrate. Hereinafter, the manufacturing process will be described with reference to FIG.

First, a multi-cavity insulating substrate is prepared (step 101). A counter electrode is formed on the surface of the insulating substrate (Step 102). Next, a pattern of a gas-sensitive film is formed on the counter electrode by screen printing (step 103). After the printed pattern is dried (step 104), it is baked (step 105). These steps 103 to 105 are usually repeated two or more times to form a desired gas-sensitive film.

Subsequently, on the laminated gas-sensitive film,
Form a porous catalyst layer by screen printing (Step
106), after drying (step 107), baking (step 108) to form a desired catalyst layer.

Thereafter, the multi-cavity insulating substrate is divided into chips (step 109), and a lead wire is connected to each chip (step 110) to complete the gas sensor.

Here, the catalyst layer forming step (step 10)
6) will be described in more detail with reference to FIG. FIG. 3 is a flowchart showing a method for forming a catalyst layer in the gas sensor of the present invention.

First, a predetermined amount of alumina fine powder serving as a carrier is weighed (step 201), and this is converted into an aqueous solution containing tungsten and copper, for example, an aqueous solution containing ammonium tungstate or copper sulfate, among the three types of catalyst metals. Immerse (step 202). After sufficiently stirring and mixing (Step 203), the mixture is dried under reduced pressure (Step 204) and further dried by heating (Step 205). Thereafter, the dried product is made into a powder using a crusher or the like (Step 206), and is stored in a quartz crucible or the like and fired (Step 207). Further, the particles are sieved to adjust the particle size, and secondary particles of the catalyst are taken out (Step 208). Next, a binder is added to the secondary particles of the catalyst to form a paste, and phosphorus is added, for example, as a phosphorus resinate, and mixed well to obtain a paste for printing a catalyst layer (step 209). This catalyst layer printing paste is pattern-formed on a predetermined gas-sensitive film by screen printing (Step 210) and dried (Step 211).
Calcination (step 212) to form the desired catalyst layer. Of these steps, step 211 corresponds to step 107 in FIG. 2, and step 212 corresponds to step 108 in FIG.

As described above, phosphorus may be added to the binder as a phosphorus screwate, or may be added as an aqueous phosphoric acid solution during the preparation of the catalyst powder (step 202).

Next, a specific production example of the above-described gas sensor and its characteristic evaluation results will be described.

Example 1 First, on an alumina substrate with a built-in heater for multi-cavity production,
The counter electrode was formed by printing and firing a gold conductor paste. Next, a paste for a gas-sensitive film consisting of 50.5% by weight of tin 2-ethylhexanoate, 0.5% by weight of 2-niobium resinate, 5.0% by weight of ethylhydroxyethylcellulose (EHEC), and 44.5% by weight of terpineol was prepared. The pattern of the gas-sensitive film was printed on the counter electrode by screen printing. Thereafter, the film was dried at 120 ° C. for 15 minutes and heat-treated by a firing program having a peak at 500 ° C. for 10 minutes to form a first gas-sensitive film. A paste having the same composition was printed again on the first-layer sensitive film thus formed, followed by drying and heat treatment to form a second-layer gas-sensitive film.

Next, an alumina-supported catalyst layer was formed on the upper side of the gas-sensitive film as follows.

First, a predetermined amount of fine alumina powder is weighed,
This was immersed in an aqueous solution containing ammonium tungstate and copper sulfate. After sufficiently stirring and mixing, the mixture was dried under reduced pressure for 1 hour and further dried by heating at 120 ° C. Thereafter, the dried product was made into a powder using a crusher (step 206), housed in a quartz crucible, and fired at a temperature of 400 ° C to 600 ° C. Furthermore, secondary particles of the catalyst having a particle size remaining between meshes of # 400 to # 100 were taken out through a sieve.

To the secondary particles of the catalyst, aluminum resinate, ethylhydroxyethylcellulose (EHEC) and terpineol were added as a binder to form a paste, and phosphorus resinate was further added and mixed well to prepare a paste for printing a catalyst layer. This catalyst layer printing paste is
Copper was adjusted to 1 part by weight, tungsten to 5 parts by weight, and phosphorus to 1 part by weight based on 100 parts by weight of alumina. Next, the above-mentioned catalyst layer printing paste is pattern-formed by screen printing on the above-described gas-sensitive film having a two-layer structure, dried at 120 ° C. for 15 minutes, and then subjected to a belt furnace having a peak at 600 ° C. × 10 minutes. To form a catalyst layer.

Thereafter, the insulating substrate on which the catalyst layer is formed is divided into a predetermined number of chips, lead wires are respectively connected to the electrode pads for the counter electrode and the heater pads, and the chips are mounted. The area around the chip was covered with an explosion-proof net to obtain the desired gas sensor.

Example 2 A catalyst layer was prepared in the same manner as in Example 1 except that 1 part by weight of Cu, 5 parts by weight of W, and 0.1 part by weight of P were contained (supported) with respect to 100 parts by weight of alumina. To produce a gas sensor.

Comparative Example 1 In the catalyst layer, 1 part of Cu was added to 100 parts by weight of alumina.
A gas sensor was manufactured in the same manner as in Example 1 except that 5 parts by weight and W were contained (supported).

The sensitivity characteristics of the gas sensors obtained in the examples and the comparative examples thus obtained were measured for carbon monoxide, tobacco smoke, hydrogen and ethanol. Further, a change in the resistance value of the gas sensor in a high humidity atmosphere (40 ° C., 90% RH) (after 2000 hours) was measured. The results are shown in Table 1 and FIG. Further, FIG. 5 shows how the gas sensor of Example 1 changes when the power is supplied for 1000 hours in a high humidity atmosphere (40 ° C., 90% RH) for 1000 hours and then supplied at normal temperature and normal humidity.

Table 1 Supported amount of catalyst Supported amount Sensitivity (Rair / Rgas) Resistance metal (parts by weight) change rate * 2 * 1 CO tobacco hydrogen ethanol Rair Sco Example 1 Cu-WP 1: 5: 1 3.79 1.61 5.76 7.71 36 12 Example 2 Cu-WP 1: 5: 0.1 3.27 1.38 7.39 8.51 45 11 Comparative Example 1 Cu-W 1: 5 3.54 1.39 5.10 5.08 97 122 * 1: Ratio of supported metal to 100 parts by weight of alumina It is.

* 2: The rate of change of the resistance value is the rate of change after lapse of 2000 hours, and was obtained from the following formula. (R [2000] −R [0] / R [0]) × 100 (%) where R [2000] is the resistance value in air after 2000 hours (R
air), R [0] is the resistance value (Rai
r).

As is clear from the results shown in Table 1, the gas sensors according to the embodiments have good sensitivity to various gases.
Moreover, the resistance value in air in a high humidity atmosphere (Rair)
And the resistance value (Rco) in the presence of 200 ppm of carbon monoxide was stable.

As is apparent from FIG. 4, the resistance value of the conventional gas sensor (comparative example) fluctuates greatly with the passage of time, and the resistance value gradually increases. It can be seen that the change in the resistance value with the passage of time is very small and the stability is high. further,
FIG. 5 shows that the resistance value once increased in the high-humidity atmosphere tends to decrease when returned to the low-humidity atmosphere.

As described above, in the gas sensor according to the embodiment, the performance deterioration over time can be suppressed as compared with the conventional gas sensor, the CO sensitivity is high, and the deterioration in a high humidity atmosphere can be reduced. .

[0042]

As described above, according to the present invention,
By providing a catalyst layer in which three metal elements of copper, tungsten and phosphorus are supported on alumina, high sensitivity to carbon monoxide, little deterioration over time even in a high-humidity atmosphere, and high reliability over a long period of time can be obtained. It is possible to provide a gas sensor that can be used.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a gas sensor according to one embodiment of the present invention.

FIG. 2 is a flowchart showing a manufacturing process of the gas sensor of the present invention.

FIG. 3 is a flowchart showing a catalyst layer forming step in FIG. 2 in detail.

FIG. 4 is a diagram illustrating a change in resistance value of a gas sensor according to an example of the present invention and a comparative example in a high humidity atmosphere.

FIG. 5 is a diagram showing a change in resistance value when a gas sensor according to an embodiment of the present invention is energized in a high-humidity atmosphere, then returned to normal temperature and normal temperature and humidity, and energized.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Insulating substrate 2 ... Heater 3, 4 ... Counter electrode 5 ... Gas sensitive film 6 ... Catalyst layer

Continuation of the front page (72) Inventor Masayuki Shiratori 1 Toshiba-cho, Komukai, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture (56) References JP-A-56-43550 (JP, A) JP-A-56- 43548 (JP, A) JP-A-62-204148 (JP, A) JP-A-60-149957 (JP, A) JP-A-63-83652 (JP, A) JP-A-57-57251 (JP, A) JP-A-60-7352 (JP, A) JP-A-54-145192 (JP, A) JP-A-54-145193 (JP, A) JP-A-54-145194 (JP, A) JP-A-62-126341 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G01N 27/12 JICST file (JOIS) WPI (DIALOG)

Claims (1)

(57) [Claims] An insulating substrate; a pair of electrodes formed on the insulating substrate so as to face each other; a gas-sensitive film formed of a metal oxide semiconductor formed over the pair of electrodes;
In a gas sensor comprising a catalyst layer formed so as to cover the gas-sensitive film, the catalyst layer is constituted by a catalyst having alumina as a carrier and copper, tungsten and phosphorus or an oxide thereof supported on the carrier. A gas sensor, comprising: