JPH052005A - Gas sensor and its manufacture - Google Patents

Abstract

PURPOSE:To increase the sensor output and gas sensitivity of a semiconductor gas sensor at the measuring time at high temperatures, while enhancing the recovery and response properties at the measuring time at low temperatures. CONSTITUTION:A pair of arch-shaped Au electrodes 3, 3 are burnt on are alumina substrate 1 of a gas sensor 1. An SnO2 film 4 connected by the Au electrodes 3, 3 is similarly burnt on the surface of the alumina substrate 2. Moreover, a heater 5 is built in the alumina substrate 2. The thickness of the SnO2 film 4 is not. larger than 1mum, and the average diameter of minute openings of the film is not smaller than 100Angstrom .

Description

Detailed Description of the Invention

[0001]

The present invention relates to H ₂ S (hydrogen sulfide gas)
The present invention relates to a gas sensor excellent in detecting gas and a manufacturing method thereof.

[0002]

2. Description of the Related Art A semiconductor gas sensor has been conventionally used in which an electrode is connected to a metal oxide semiconductor (SnO ₂ ) whose resistance value changes due to adsorption and desorption of gas, and the presence or absence of gas is detected by measuring the resistance value. It is used as a gas leak alarm.

On the other hand, recently, there has been a demand for development of a gas sensor for performing automatic ventilation in a house such as a toilet or a kitchen. That is,
The main offensive odor components in toilets and kitchens are hydrogen sulfide, ammonia, amines and mercaptans,
To maintain a comfortable living environment, these gas concentrations are several pp
A sensor capable of detecting in the range of b to several ppm is required. However, the detectable concentration by the conventional metal oxide semiconductor gas sensor is several hundred ppm or more.

Therefore, in JP-A-63-313048 and JP-A-63-313049, an Sn alkoxide solution is applied to an insulating substrate and then the alkoxide solution is thermally decomposed to form a SnO ₂ film. It has been proposed to add another metal (usually in the form of an oxide) to the SnO ₂ film to enhance the gas detection sensitivity. However, in this method, the response speed at the time of removing H ₂ S is slow, and a method of heat cleaning is used.

[0005]

FIG. 9 is a graph showing the pore distribution of the SnO ₂ film of the conventional gas sensor described above, and FIG.
Is a graph showing the pore area distribution of the SnO ₂ film of the same gas sensor. From FIG. 9, the pore distribution of the conventional SnO ₂ film lacks uniformity, and from FIG. 10, most of the pore surface has 30 Å It can be seen that they are present in the pores below.

FIG. 11 is a graph showing the response curve of the above-mentioned SnO ₂ gas sensor, and the vertical axis represents the reciprocal of the gas sensitivity on a logarithmic scale. From this, it can be seen from the response curves at 200 ° C. and 300 ° C. that the gas sensitivity is relatively good, but the recovery response is poor when the atmosphere is switched from H ₂ S to air. On the other hand, if the sensor temperature is increased to 400 ° C., the recovery response is improved, but as shown in FIG. 12, SnO ₂ and In ₂ O ₃ have a sharp decrease in gas sensitivity.
In FIG. 12, the element temperature on the low temperature side without gas sensitivity indicates that the element resistance is at the measurement limit.

Heat cleaning is also known in which a sensor is heated to improve recovery response, but in this case continuous measurement cannot be performed and a separate circuit for heat cleaning is required.

[0008]

In order to solve the above problems, the present inventors have paid attention to gas diffusion in a porous material. That is, gas diffusion in a porous material such as a semiconductor gas sensor is very slow and is called Knudsen diffusion.
The diffusion constant in the pores at this time is represented by the following (Equation 1).

[0009]

[Equation 1]

Further, it is considered that the diffusion rate equation at this time is satisfied by Gumkeller's diffusion equation shown in the following (Equation 2).

[0011]

[Equation 2]

In order for adsorption and desorption to reach equilibrium in a shorter time than the above (Equation 2), it is advantageous that the radius (r) of the pore is large, the film thickness L is small, and the temperature T is large.

Based on the above findings, the present invention sets the thickness of the SnO ₂ film formed on the insulating substrate to 1 μm or less and the average pore diameter to 100 Å or more.

[0014]

By applying SnO ₂ in the form of a sol on an insulating substrate and firing it at a temperature of 600 ° C. or higher, the distribution of pores can be made uniform and the average pore diameter can be 100 Å or higher.

[0015]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Here, FIG. 1 is a perspective view showing an example of a gas sensor according to the present invention, and FIG. 2 is an enlarged sectional view of the gas sensor.

In the gas sensor 1, a pair of comb-shaped Au electrodes 3, 3 are formed on an alumina substrate 2 by firing, and the Au electrodes 3, 3 are formed.
A SnO ₂ film 4 to which 3 is connected is similarly formed on the surface of the alumina substrate 2 by firing, and a heater 5 is provided in the alumina substrate 2.
Is buried.

The SnO ₂ film 4 has a thickness of 1 μm or less and an average pore diameter of 100 Å or more. In order to form such SnO ₂ film 4, SnO ₂ sol is applied to the alumina substrate 2 and then baked. Specifically, add an aqueous solution of ammonium bicarbonate to an aqueous solution of stannic chloride, wash the resulting gel with water, add ammonia water to adjust the pH to 10, and then perform hydrothermal treatment at 200 ° C in an autoclave to about 20Å. The SnO ₂ sol having the crystal form is produced, and the SnO ₂ sol is applied to the alumina substrate 2, dried and then fired.

The method of producing the SnO ₂ sol is not limited to the above, and a method of adding and mixing sodium hydroxide to a hydrochloric acid acidic aqueous solution of stannic chloride, a method of treating the stannic chloride aqueous solution with an anion exchange resin, and a tin alkoxide are used. Examples thereof include a method of hydrolyzing by various means.

Incidentally, FIG. 3 is a graph showing the relationship between the firing temperature of SnO ₂ sol and the pore distribution, and FIG. 4 is a graph showing the relationship between the firing temperature of SnO ₂ sol and the pore area distribution. Considering that it is advantageous that the distribution of pores is uniform and the radius (r) is large in order to reach the equilibrium of adsorption / desorption in a short time, the firing temperature should be 600 ° C. or higher.
By setting the firing temperature to 600 ° C. or higher, an SnO ₂ film having an average pore size of 100 Å or higher can be formed.

FIG. 5 is a graph showing the relationship between the baking temperature of SnO ₂ sol, the measurement temperature and the gas sensitivity, FIG. 6 is a graph showing the relationship between the baking temperature of SnO ₂ sol and the response curve, and FIG. 7 is the measurement of the gas sensor. Graph showing the relationship between temperature and response curve,
FIG. 8 is a graph showing the relationship between the gas concentration and the gas sensitivity when fired at 700 ° C. From FIG. 5, it can be seen that the gas sensitivity is dramatically improved when fired at 600 ° C. or higher. Further, it can be seen from FIG. 6 that the sensor output and the recovery response are improved when firing at 600 ° C. or higher. And
It can be seen from FIGS. 7 and 8 that sufficient gas sensitivity, sensor output, and recovery response can be exhibited even if the temperature is measured at 160 ° C. to 300 ° C. by firing at 600 ° C. or higher (700 ° C.).

[0021]

As described above, according to the present invention, since the SnO ₂ sol is applied onto the insulating substrate, the thickness of the SnO ₂ film after firing can be controlled to 1 μm or less. Since the firing temperature was 600 ° C or higher, Sn
The average pore diameter of the O ₂ film can be 100 Å or more. By setting the thickness of the SnO ₂ film to 1 μm or less and setting the average pore diameter to 100 Å, a gas sensor excellent in gas sensitivity, sensor output and recovery response can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of a gas sensor according to the present invention.

FIG. 2 is an enlarged sectional view of the gas sensor.

FIG. 3 is a graph showing the relationship between the firing temperature of SnO ₂ sol and the pore distribution.

FIG. 4 is a graph showing the relationship between the firing temperature of SnO ₂ sol and the pore area distribution.

FIG. 5 is a graph showing the relationship between the firing temperature of SnO ₂ sol, the measurement temperature, and the gas sensitivity.

FIG. 6 is a graph showing the relationship between the firing temperature of SnO ₂ sol and the response curve.

FIG. 7 is a graph showing the relationship between the measured temperature of the gas sensor and the response curve.

FIG. 8 is a graph showing the relationship between gas concentration and gas sensitivity.

FIG. 9 is a graph showing the pore distribution of a conventional SnO ₂ film.

FIG. 10 is a graph showing the pore area distribution of a conventional SnO ₂ film.

FIG. 11 is a graph showing a response curve of a conventional gas sensor.

FIG. 12 is a graph showing the relationship between H ₂ S sensitivity and temperature of various metal oxide semiconductors.

[Explanation of symbols]

1 ... Gas sensor, 2 ... Insulating substrate, 3 ... Electrode, 4 ... SnO ₂
film.

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[Submission date] August 30, 1991

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   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takahiro Suzuki             2-1-1 Nakajima, Kokurakita-ku, Kitakyushu City, Fukuoka Prefecture             No. Totoki Equipment Co., Ltd. (72) Inventor Noriyuki Tsuchida             2-1-1 Nakajima, Kokurakita-ku, Kitakyushu City, Fukuoka Prefecture             No. Totoki Equipment Co., Ltd.

Claims (2)

[Claims] 1. A gas sensor using a change in resistance value due to adsorption and desorption of gas with respect to SnO ₂ , wherein the SnO ₂ is formed as a thin film on an insulating substrate, the thickness thereof is 1 μm or less, and the average pore diameter is 100 Å or more. A gas sensor characterized by being present. _2. A method of manufacturing a gas sensor using a change in resistance value due to adsorption and desorption of gas with respect to SnO ₂ , wherein the SnO ₂ is applied on an insulating substrate in the form of SnO ₂ sol, and then at a temperature of 600 ° C. or higher. A method of manufacturing a gas sensor, characterized by forming by firing.