JPH11258192A - Semiconductor gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a semiconductor gas sensor whose mechanical strength is high, whose sensitivity is high, whose moistureproofness and time-dependent stability are excellent and whose durability against a poisoning gas is high. SOLUTION: A sensing part 6 is formed to be oval spherically shaped, a heater 25 which is composed of an electrode coil of a noble metal wire such as a platinum wire or the like is buried, and a core wire 20 which is composed of a noble metal wire such as a platinum wire or the like is installed at the inside of the heater 25. The heater 25 is installed between a lead wire 201 and a lead wire 203 . The core wire 20 is formed of a lead wire 202 . The sensing part 6 is formed in such a way that a metal oxide semiconductor 6b such as tin oxide or the like is stuck to the surface of a carrier 6a composed of metal oxide particles such as aluminum particles or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor gas sensor in which a metal oxide semiconductor is used as a sensitive part and a gas is detected by a change in the electric resistance of the sensitive part.

[0002]

2. Description of the Related Art Heretofore, semiconductor gas sensors using a metal oxide semiconductor as a sensitive portion (gas detection material) have been studied and developed in various places. As this kind of semiconductor gas sensor,
As shown in FIG. 12, a sensitive portion 6 'made of a metal oxide semiconductor thin film is formed on an insulating substrate 1 to increase the sensitivity, and a pair of electrodes 4 for measuring electrical resistance are formed on the sensitive portion 6'. Numerous studies have been made on a so-called flat-type thin film gas sensor (hereinafter, abbreviated as a thin film gas sensor) provided with a sensor. In which a sensitive part 6 'is formed by a sol-gel method (for example,
No. 145933), and those in which the sensitive portion 6 ′ is formed into a film by using an organometallic compound thermal decomposition method.

[0003]

In general, it is difficult for the thin film gas sensor to sufficiently increase the adhesive strength of the sensitive portion 6 'to the substrate 1 or the electrode 4, and the sensitive portion 6' is separated from the substrate 1 or the electrode 4. There is a problem (hereinafter, referred to as a first problem) that the sensing portion 6 'cracks. As a solution to the first problem, the gas pressure at the time of sputtering is set higher than that of the conventional film forming condition, and the sensitive portion 6 'is formed.
(See Japanese Patent Application Laid-Open No. 58-143255) or a film obtained by alternately laminating metal oxide layers and insulating oxide layers and performing an annealing treatment (Japanese Patent Application Laid-Open No. 7-190977).
And the like, have been proposed.

In addition to the first problem, the thin-film gas sensor is inferior in humidity resistance, poisoning gas resistance and the like as compared with a bulk gas sensor and a thick film gas sensor (hereinafter referred to as a second problem). There is). As a solution to the second problem, a device in which an SiO ₂ film is laminated on the sensitive portion 6 ′ has been proposed (see JP-A-6-50919).

Further, the thin-film gas sensor has a problem that thermal degradation is large and stability with time is low (hereinafter, referred to as a third problem). As a solution to the third problem, a method in which heat treatment is performed at a temperature of 350 ° C. or more in an air stream containing water vapor (see Japanese Patent Application Laid-Open No. 6-213853). A thin gold film formed so as to prevent atmospheric gas from entering the interface (Japanese Unexamined Patent Application Publication No.
-86767), Sn constituting the sensitive part 6 '
O ₂ thin film containing tungsten oxide, molybdenum oxide, vanadium oxide, etc.
Japanese Patent Application Laid-Open No. 4-127047), and those in which an insulating thin film is applied on the sensitive portion 6 '(see Japanese Patent Application Laid-Open No. 4-127047).

On the other hand, a bulk-type gas sensor or a thick-film gas sensor having high mechanical strength has a problem that the sensitivity is lower than that of a thin-film gas sensor (hereinafter, referred to as a fourth problem). The present invention has been made in view of the above circumstances,
An object of the present invention is to provide a semiconductor gas sensor having high mechanical strength, high sensitivity, excellent moisture resistance and stability over time, and high durability against poisoning gases.

[0007]

According to a first aspect of the present invention, there is provided a semiconductor gas sensor in which a pair of electrodes for measuring electrical resistance is provided on a sensitive portion made of a metal oxide semiconductor to achieve the above object. The sensitive part is characterized by being formed by adhering the metal oxide semiconductor on the surface of a carrier composed of oxide particles, and the metal oxide semiconductor is formed on the surface of the carrier composed of oxide particles. By forming the sensitive part by attaching, even if the sensitive part is made thicker and the mechanical strength is increased, the change in resistance value related to gas detection is caused by the metal oxide attached to the surface of the carrier. Since it is dominated by semiconductors, it is possible to increase the mechanical strength by making the sensitive part a thick film or sintered body and to increase the sensitivity in the same manner as a thin film gas sensor. Do By similarly moisture resistance and conventional thick film type gas sensor or a bulk-type gas sensor, stability over time, thereby enhancing the durability against the poison gas.

According to a second aspect of the present invention, in the first aspect, the metal oxide semiconductor is supported on the carrier. According to a third aspect of the present invention, in the first aspect, the metal oxide semiconductor is formed so as to cover a surface of the carrier. According to a fourth aspect of the present invention, in the first to third aspects of the present invention, the oxide particles are particles having an electric resistance sufficiently larger than an electric resistance of the metal oxide semiconductor.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A semiconductor gas sensor S according to the present embodiment.
Is a base 3 having a configuration as shown in FIG. 2 and also serving as a bottom of a sensor housing (not shown) formed of a bottomed cylindrical body.
0, three terminals 10 ₁ , 10 ₂ , 10 ₃ that penetrate the base 30 and protrude into and out of the sensor housing, and terminals 10 ₁ , 1
0 _2, 10 ₃ and a lead wire 20 _1, 20 _2, 20 ₃ is connected fixed to support sensitive part 6. Sensitive part 6
Is formed in an elliptical spherical shape, and a heater 25 made of an electrode coil of a noble metal wire such as platinum is embedded, and a core wire 20 made of a noble metal wire such as platinum is provided inside the heater 25. Here, the heater 25 is connected to the lead wire 2 described above.
0 _1, provided between the 20 _3, the core wire 20 is formed by a lead wire 20 ₂ described above. In the configuration of FIG. 2,
A lead wire 20 _2, constitute electrodes for electric resistance measurement between one of the leads 20 _1, 20 _3, lead 20
₁ and the lead wire 20 ₃ constitutes the electrode for the heater.

In the semiconductor gas sensor S of this embodiment, as shown in FIG. 1, the sensitive portion 6 is formed by attaching a metal oxide semiconductor 6b to a surface of a carrier 6a made of metal oxide particles. There is a feature in the point. Hereinafter, three examples (1) to (3) of the method of forming the sensitive portion 6 will be described. (1) was added Echiruekisan stannous a 20 wt% inclusive ethanol solution 5cc respect 1g of Al ₂ O ₃ (alumina), after evaporation of the solvent, and calcined at eg 400 ° C. in air, Al ₂ Carrier 6a composed of O ₃ particles
A powder having a metal oxide semiconductor 6b made of SnO ₂ adhered to the surface of is obtained. Here, the form of attachment of the metal oxide semiconductor 6b to the surface of the carrier 6a varies depending on the firing temperature, and may be a form in which the metal oxide semiconductor 6b is carried on the surface of the carrier 6a, or may cover the surface of the carrier 6a. The metal oxide semiconductor 6b thin film may be formed as described above. Next, the obtained powder is pasted using ethylene glycol to obtain a paste-like gas-sensitive material. The paste-like gas-sensitive material is applied to the heater 25 and the core wire 20, and a voltage is applied to the heater 25. About 400-500 ° C
To obtain the sensitive part 6.

It should be noted that P together with tin ethylexanate
By adding a noble metal catalyst such as t, Rh, or Au or a salt such as zinc chloride, it is possible to impart appropriate sensitivity to each gas and improve the response speed. Further, after firing, silica sol and alumina sol are applied, and then fired again, whereby the mechanical strength of the sensitive portion 6 is increased. (2) Al ₂ O ₃ is pasted using ethylene glycol, and a paste-like gas-sensitive material is applied to the heater 25 and the core wire 20, and then tin ethyl oxalate is added to the heater 20.
Impregnation with an ethanol solution containing t% and firing. Thereafter, the operation of impregnating with an ethanol solution containing 20 wt% of tin ethylexanoate and firing is repeated several times to form the sensitive portion 6. Here, the resistance value of the sensitive part 6 decreases as the number of repetitions increases, but the sensitivity does not change.

[0012] By impregnating with an aqueous solution of a noble metal salt such as platinum or an aqueous solution of a metal salt such as zinc chloride, and then sintering, it is possible to impart appropriate sensitivity to each gas or improve the response speed. Can be. Further, after firing, a silica sol or an alumina sol is applied and then refired, whereby the mechanical strength of the sensitive portion 6 is increased. (3) 5 cc of an ethanol solution containing 20% by weight of tin ethylexanoate is added to 1 g of Al ₂ O ₃ (alumina), and the solvent is evaporated, followed by firing. The obtained powder is pasted using ethylene glycol, the paste-like powder is applied to the heater 25 and the core wire 20, and a voltage is applied to the heater 25 to sinter the gas-sensitive material. After that, it is impregnated with a 20 wt% ethanol solution of tin ethylexanoate and fired. Thereafter, the operation of impregnating with an ethanol solution containing 20 wt% of tin ethylexanoate and firing is repeated several times to form the sensitive portion 6.

In the above (1) to (3), alumina is used as the carrier 6a.
a is not limited to alumina, but may be any material having a sufficiently large resistance value as compared with the metal oxide semiconductor 6b, and may be silica, zirconium oxide, tin oxide fired at a high temperature, or the like. Further, the metal oxide semiconductor 6b is not limited to tin oxide, but may be indium oxide, tungsten oxide, zinc oxide, or the like.

The above-mentioned semiconductor gas sensor S is a sintered body type gas sensor in which the heater 25 is embedded in the sensitive portion 6, but is not limited to the sintered body type gas sensor. It may be a membrane gas sensor.
In the configuration shown in FIG. 3, a pair of gold electrodes 4A 'and 4B' for measuring electric resistance and a pair of gold electrodes 2A and 2B for heater are provided on the back surface of the square alumina substrate 1 as shown in FIG.
And a heater 25 'made of ruthenium oxide is formed between the gold electrodes 2A and 2B. Also, gold electrodes 4A, 4B connected to the gold electrodes 4A ', 4B' on the back surface are provided on the front surface using through holes as shown in FIG.
A thick film sensitive portion 6 is provided so as to extend over the gold electrodes 4A and 4B. Here, the sensitive part 6 is formed by applying and baking the above-mentioned paste-like powder. 5 in FIG.
Indicates a lead wire.

In the semiconductor gas sensor according to the present embodiment, since the sensitive portion 6 is formed by attaching the metal oxide semiconductor 6b to the surface of the carrier 6a, the sensitive portion 6 is formed as a thick film or a sintered body to provide a mechanical effect. Even when the strength is increased, the change in resistance value related to gas detection is governed by the metal oxide semiconductor 6b attached to the surface of the carrier 6a. The sensitivity can be increased as in the case of the thin film gas sensor. In addition, by forming the sensitive portion 6 into a thick film or a sintered body, the moisture resistance,
Stability with time and durability against poisoning gas can be improved.

(Example 1) In this example, a gas sensor was formed in which the sensitive portion 6 was formed by setting the temperature of the gas-sensitive material at the time of firing to 500 ° C. in the method (1) described in the above embodiment. Here, in the present embodiment, the Al ₂ O ₃ particles constitute the carrier 6a, and the metal oxide semiconductor 6b made of tin oxide adheres to the surface of the carrier 6a. In the present embodiment, 3000 to 300 mesh Al ₂ O ₃ particles are used.

(Embodiment 2) In this embodiment, the above-described embodiment
Gas-sensitive material at the time of firing in the method of (1) described in the above aspect
Temperature of 500 ° C and Al_TwoO_ThreeCarrier 6 consisting of particles
SnO on the surface of a _TwoMetal oxide semiconductor 6b
After wearing, 0.6 mol / l of zinc chloride aqueous solution
The sensitive part 6 is formed by adding zinc and firing.
The formed sintered body type gas sensor was manufactured. In this embodiment,
Then, Al_TwoO_ThreeNo 3000 mesh as particles
And a mesh of 300 mesh is used.

Comparative Example 1 50 wt% of SnO ₂ powder
The powder obtained by mixing the alumina is converted into a paste by using ethylene glycol, and the paste-like gas-sensitive material is
Then, a general sintered body type gas sensor in which a sensitive portion was formed by applying a gas-sensitive material to the core wire 20 and sintering the gas-sensitive material at 700 ° C. was manufactured. Note that the basic configuration is substantially the same as that of the first embodiment, and only the configuration of the sensitive unit 6 is different.

(Comparative Example 2) A flat thin film having a sensitive portion formed by repeating an operation of applying an ethanol solution containing 20 wt% of tin ethylexanoate on a flat insulating substrate and firing at 500 ° C. four times. A gas sensor was manufactured.
Here, a description will be given of the results of measurement of the characteristics of Example 1 and Example 2, Comparative Example 1, and Comparative Example 2 described above.

FIGS. 4 to 7 show the ratio (Rs / Rsair) of the resistance Rs of the sensitive part 6 in the gas to the resistance Rsair of the sensitive part 6 in the air of the semiconductor gas sensor.
5 shows Example 1, FIG. 5 shows Example 2, FIG. 6 shows Comparative Example 1,
FIG. 7 shows Comparative Example 2. 4 to 7, the horizontal axis represents gas concentration, and the vertical axis represents Rs / Rsair.
In each figure, a (○) indicates the measurement result for H ₂ S gas, b (△) indicates the measurement result for CH ₃ SH gas, and c (▼).
Shows the measurement results for C ₂ H ₅ OH gas, respectively. Here, the smaller the value of Rs / Rsair at the same gas concentration, the higher the sensitivity. By comparing FIG. 4, FIG. 5, FIG. 6, and FIG. 7, the semiconductor gas sensors of Example 1 and Example 2 are H ₂ S gas and CH ₃ SH compared to the semiconductor gas sensors of Comparative Example 1 and Comparative Example 2. Gas, C
It was found that the sensitivity was high for any of the ₂ H ₅ OH gases.

8 and 9 show response characteristics of the semiconductor gas sensor.
8 shows Example 1, FIG. 9 shows Comparative Example 1, and FIG.
Shown respectively. 8 and 9, the horizontal axis is time, and the vertical axis is
FIG. 8 and FIG. 9 show the resistance value Rs of the sensitive part 6 in the gas.
9 (a) is H_TwoThe response characteristics to S gas, (b)
CH_ThreeThe response characteristic to SH gas is shown in FIG._TwoH _Five
The response characteristics to OH gas are shown respectively. In addition,
The measurement temperature of the response characteristics was kept constant at 320 ° C.

Here, the first rapid change (decrease) of the resistance value Rs in FIG. 8A and FIG.
This is because the atmosphere was changed from air to a H ₂ S gas concentration of 0.3 ppm, and the second rapid change (decrease)
Is caused by changing the H ₂ S gas concentration from 0.3 ppm to 1 ppm. The first rapid change in the resistance value Rs in (b) of each of FIGS. 8 and 9 is due to the change of the atmosphere from air to a CH ₃ SH gas concentration of 0.3 ppm. The rapid change is C
This is because the H ₃ SH gas concentration was changed from 0.3 ppm to 1 ppm.

The first rapid change of the resistance value Rs in FIG. 8C and FIG. 9C is caused by changing the atmosphere from air to a C ₂ H ₅ OH gas concentration of 3 ppm. The second rapid change is caused by changing the C ₂ H ₅ OH gas concentration from 3 ppm to 10 ppm. By comparing FIG. 8 and FIG.
It has been found that the semiconductor gas sensor of Example 1 has a steep change in the resistance value Rs and quicker response than the semiconductor gas sensor of Comparative Example 1.

FIG. 10 shows the results of examining the moisture resistance of each of Examples 1 and 2, Comparative Example 1 and Comparative Example 2. The resistance values Rsair and Rs / Rsair before and after the test were compared. Things. The resistance value Rs was measured with the concentration of the H ₂ S gas being 1 ppm. Here, as the test conditions, the temperature of the atmosphere was kept constant at 40 ° C., the relative humidity of the atmosphere was kept constant at 90%, and the test was left for 3 days. From the measurement results of FIG. 10, Example 1 and Example 2
Compared with Comparative Example 2, the resistance values Rsair and R
It was confirmed that the rate of change of each of s / Rsair was small and excellent in moisture resistance and poisoning gas resistance.

FIG. 11 shows Example 1, Example 2, Comparative Example 1,
Comparative Example 2 are graphs showing changes with time of the resistance value Rs in a H ₂ S gas atmosphere resistance Rsair and 1ppm in air, the horizontal axis represents elapsed days, and the vertical axis represents the resistance value. In FIG. 11, a (●) represents the resistance Rsair of the first embodiment, B (▲) represents the resistance Rsair of the second embodiment,
C indicates the resistance Rsair of Comparative Example 1, and D indicates the resistance Rsair of Comparative Example 2. Further, FIG.
E (●) shows the resistance value Rs of the first embodiment, F (▲) shows the resistance value Rs of the second embodiment, and G (○) shows the resistance value Rs of the first comparative example.
s and チ indicate the resistance Rs of Comparative Example 2, respectively.

In FIG. 11, the resistance value Rsair in air is shown.
When comparing the changes over time, Examples 1 (a) and 2 (b) show good stability over time, similar to Comparative Example 1 (c), which is a conventional sintered body type gas sensor, and are flat plate type thin film gas sensors. It was confirmed that the stability with time was higher than that of Comparative Example 2 (d). In FIG. 11, 1 ppm of H
Comparing the change with time of the resistance value Rs in the ₂ S gas atmosphere, the first embodiment (e) and the second embodiment (f) showed the same aging as the comparative example 1 (g) which is a conventional sintered body type gas sensor. Comparative Example 2 which has good stability and is a flat plate type thin film gas sensor
It was confirmed that the stability with time was higher than that of (h).

[0027]

According to the first aspect of the present invention, there is provided a semiconductor gas sensor in which a pair of electrodes for measuring electric resistance is provided on a sensitive portion made of a metal oxide semiconductor, wherein the sensitive portion has a surface of a carrier made of oxide particles. Since the metal oxide semiconductor is attached to the carrier, the sensitive portion is formed by attaching the metal oxide semiconductor to the surface of the carrier made of oxide particles. Even if the mechanical strength is increased, the change in resistance value related to gas detection is governed by the metal oxide semiconductor attached to the surface of the carrier,
By making the sensitive part a thick film or sintered body, the mechanical strength can be increased and the sensitivity can be increased like a thin film gas sensor. As in the case of the type gas sensor and the bulk type gas sensor, there is an effect that moisture resistance, stability over time, and durability against poisoning gas can be improved.

[Brief description of the drawings]

FIGS. 1A and 1B show an embodiment, and FIG.
(B) is an enlarged view of a main part B of (a).

FIG. 2 is a schematic configuration diagram of the above.

3A and 3B show another configuration of the above, wherein FIG. 3A is a perspective view seen from the front side, and FIG. 3B is a perspective view seen from the back side.

FIG. 4 is a graph showing a gas concentration dependency of a resistance value change in Example 1.

FIG. 5 is a graph showing the gas concentration dependency of a resistance value change in Example 2.

FIG. 6 is a graph showing a gas concentration dependency of a resistance value change in Comparative Example 1.

FIG. 7 is a graph showing a gas concentration dependency of a resistance value change in Comparative Example 2.

FIG. 8 is a response characteristic diagram of the first embodiment.

FIG. 9 is a response characteristic diagram of Comparative Example 1.

FIG. 10 is a comparison diagram of evaluation results of moisture resistance of each of the above examples.

FIG. 11 is a graph showing a measurement result of a temporal change in a resistance value of each of the above examples.

FIG. 12 is a schematic sectional view of a conventional thin film gas sensor.

[Explanation of symbols]

6 sensitive part 6a carrier 6b metal oxide semiconductor 20 core 20 _1, 20 _2, 20 ₃ leads 25 heater

Claims (4)

[Claims] 1. A semiconductor gas sensor comprising a sensitive portion made of a metal oxide semiconductor and a pair of electrodes for measuring electric resistance provided on a sensitive portion made of a metal oxide semiconductor. A semiconductor gas sensor formed by being attached. 2. The semiconductor gas sensor according to claim 1, wherein the metal oxide semiconductor is supported on the carrier. 3. The method according to claim 1, wherein the metal oxide semiconductor is formed so as to cover a surface of the carrier.
A semiconductor gas sensor as described in the above. 4. The semiconductor gas sensor according to claim 1, wherein the oxide particles are particles having an electric resistance sufficiently larger than an electric resistance of the metal oxide semiconductor.