JPS58105049A - Detector of no2 gas and detecting method therefor - Google Patents

Abstract

PURPOSE:To detect NO2 gas with the highest sensitivity and at the highest response speed by a method wherein an NO2 gas detector provided with a sensitive body having a specified particle size is made to operate at a specific temperature, in an NO2 gas detector wherein a tin oxide film is used as a sensitive body. CONSTITUTION:Electroconductive paste formed of platinum in the main is applied on one surface of an alumina substrate 3 and baked so as to form a plane- shaped heater 4, while electroconductive paste formed of platinum in the main is printed and baked on the other surface of the alumina substrate 3 to form an electrode 2. A thermocouple 8 for measuring the temperature of a gas-sensitive body, which is formed of CA wires, is baked and fixed together with the electrode 2. A tin oxide film having a crystal particle size of 100Angstrom or less and constituting an NO2 gas-sensitive body 1 is obtained by baking, in air, a thin film of tin oxide which is obtained by sputtering metal tin having the purity of 99.99% and used as a target material. Suitable operation temperature is 200- 300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a NO2
The present invention relates to a gas detector and a detection method, and an object thereof is to easily detect NO2 gas with excellent sensitivity and response speed.

BACKGROUND ART Conventionally, a so-called semiconductor gas detector using a tin oxide thin film as a gas sensor has been proposed as an accurate, simple, and continuous method for measuring NO2 gas. The properties of metal oxide semiconductors include electron-accepting oxidizing gases, such as No. 2. No,
This method utilizes the phenomenon that when 02 and the like are adsorbed onto the tin oxide sensitive material, which is an n-type semiconductor, the electrical resistance of the tin oxide sensitive material increases in proportion to the concentration of these gases.

This semiconductor gas detector consists of a sensitive body whose electrical resistance increases or decreases in proportion to the gas concentration, a pair of electrodes for extracting this electrical resistance change as an external signal, and a sensor that operates at an appropriate temperature. It has an extremely simple configuration consisting of a heating source.

The performance (sensitivity, response speed 1 selectivity) of this extremely simple semiconductor device detector is determined by
One is the material properties of the substances that make up the sensitive body.
Another factor is the temperature at which the sensitive body and the gas to be detected interact (the operating temperature of the gas detector). In other words, if now,
Even if the same material is used as a sensitive material, differences in the physical properties of the material resulting from differences in its shape or surface structure defined by its internal crystal structure will manifest as noticeable differences in the characteristics of the gas detector. Furthermore, even if the same substance and the same material properties are used, the characteristics of the gas detector may vary greatly depending on the operating temperature of the gas detector. Understanding this is not difficult considering that the gas detection mechanism involves chemical interaction between the sensitive body and the gas to be detected on the sensitive body surface, as is generally said. do not have.

Conventionally, as a NO2 gas detector using tin oxide as a sensitive material,
NOx (
NO2, and No) gas detectors have been proposed, and
It is said that a suitable operating temperature is 150 to 300°C, but these are not clearly defined based on sensitivity and response speed, which are the main characteristics of a gas detector. Furthermore, it is difficult to know the suitable values of the material properties of the tin oxide film that will give the highest sensitivity and the highest response speed, and it is also difficult to know the suitable values for the operating temperature. That is, it has not been known at all as to what kind of material properties a tin oxide sensitive material has and at what operating temperature it is possible to detect N02 gas with good sensitivity and response speed.

In view of the above points, the present invention provides a NO2 gas detector that has excellent sensitivity and response speed by providing a material suitable for the tin oxide sensitive material for a NO2 gas detector using tin oxide as a sensitive material. and a detection method. As a suitable tin oxide sensitizer material, S n
In X-ray diffraction measurement of O2 crystal, 2θ-26,6° (θ
The Debye-Sierra set (grain size D
-KA/Bcosθ.

is a coefficient close to IK, λ is the wavelength of the X-ray used for diffraction measurement,
β-B b, B is the half-width of the specimen, and b is a value specific to the diffraction device.
A tin oxide film formed in the form of a film having a crystal grain size of 100 Å or less given by the half width of the diffraction peak of a 50 μm SiO 2 single crystal powder is selected as the sensitive material.

The present invention will be explained below using the drawings.

Figure 1 shows one of the embodiments of the present invention, with a crystal grain size of 10
Figure 2 shows a NO2 gas detector with a tin oxide sensor that is less than 0 Å. In the figure, 3 has a surface roughness of 2.6μ and a thickness of 0.
6mm 1 length 6mm % width 5mm size purity 96%
This is an alumina substrate. 4 is a planar heater made of about 609 resistors obtained by applying and baking a conductive paste mainly composed of platinum on one side of this alumina substrate 3, which makes the detector Maintained at constant operating temperature. 6 is a wire diameter ○ for supplying power to the planar heater 4. A 6 mm platinum wire, 2 is a platinum-based conductive paste printed and baked on the other side of the alumina substrate 3 in a shape that exposes the center of the alumina substrate 3 in a 0.6 mm wide groove. This is an electrode for extracting the electrical signal from the gas sensitive body to the outside. Reference numeral 8 designates a thermocouple for measuring the temperature of the gas sensitive body, which is made of a CA (chromel-alumel) wire with fixed seizure together with the electrode 2. 7 is a lead wire for the electrode 2 made of platinum wire with a wire diameter of 0.5 mm. Reference numeral 1 denotes an NO2 gas sensitive body made of a tin oxide film having a crystal grain size of 1.08 or less according to the present invention. The tin oxide film is, for example, made of metal tin with a purity of 99.99% as a target material with a diameter of 150 mm, and the distance between the electrodes is 4.
0mm, argon gas pressure 1.25pa (Pascal).

Oxygen gas pressure 1.25pa, alumina substrate temperature ~210-
C1 A tin oxide thin film obtained by sputtering for 3θ minutes at a power of 400W and a voltage of 2.55KV was heated in air for 40 minutes.
Obtained by firing at 0°C for 2 hours. Figure 2 shows the tin oxide film obtained in this way at a tube current of 200 mA.
, is a diffraction pattern of 2θ=20 to 35° obtained by X-ray diffraction measurement using copper as an anticathode at a tube voltage of 60 KV. Sn○
The diffraction peak attributed to the (110) crystal plane unique to 2 is
Obtained at θ=26.6°. In addition, 2θ-26,6°
The sharp diffraction peak in the vicinity is a peak attributed to alumina of the substrate. The crystal grain size of the tin oxide film is determined using the Debye-Scherer set described above as follows.

D- to λ/(B-b)cosθ...Debye
A set of schera where K"1.0.λ-1.54
05 people, θ=26.6/2 B=1.20 (half width of the diffraction peak at 2θ=26,60 shown in Figure 2), b=0.13 (shown in Figure 3, Figure 2) By substituting the half-width of the diffraction peak of 20=31° of 5IO2 crystal powder with a particle size of ~43μ obtained using the same X-ray diffraction apparatus, the following is obtained. It can be seen that the crystal grain size of the tin oxide film is 89 mm.

The crystal grain size of the tin oxide film is determined by the above method, but
In order to see the effects of the present invention, a gas detector having the structure shown in FIG. 1, which uses tin oxide films with different crystal grain sizes as a sensitive material, was equipped with the gas response characteristic device shown in FIG. 4. ,
The N○2 gas response characteristics were evaluated.

In Fig. 4, 11 is a sample gas supply tank;
12 is a sample gas flow meter, 13 is a sample gas reservoir, 14 is a carrier gas (N2 or dry air) supply tank, 16 is a first carrier gas reservoir, 16 is a carrier gas pump, 17 is a carrier gas flow meter, 18 is the second
carrier gas reservoir, 19 a gas analyzer, 20 a gas switching solenoid valve, 21 a gas detection chamber, 22 a gas detector, 2
3 is a heater power supply, 24 is a heater temperature controller, 2
6 is a gas detector resistance measuring device, 26 is a flow meter for measuring gas, and 27 is a pump for suctioning gas to be measured.

Now, when using the device shown in Fig. 4, the pump 1
6 to 22/min flow rate, nitrogen gas or air temperature approx.
While the atmosphere at 0°C and relative humidity of 60 to 70% is introduced into the gas flow path, the gas detector is maintained at a predetermined operating temperature by the sheet heater 4 (Fig. 1), and then diluted to a predetermined concentration. The NO2 sample gas that had been
By operating the gas switching solenoid valve 2o, sample gas is introduced into the gas flow path from the sample gas reservoir 13 at a flow rate of 2 Q/min.

This change in the electrical resistance of the gas detector is recorded as a rise characteristic via the electrical resistance measuring device 26 of the gas detector.

Here, the circuit configuration of the electrical resistance measuring device of the gas detector is explained in the fifth section.
As shown in the figure. In addition, in FIG. 6, 28 is the gas detector 22.
30 is an impedance converter, and 31 is a recorder.

Next, after the electrical resistance value reaches a certain value RG, by operating the gas switching solenoid valve 20 again, carrier gas is introduced at a flow rate of 22/min using the same pump 26, and the gas other than this is detected. The change in electrical resistance of the device 22 is recorded as a falling characteristic.

Next, after the electrical resistance value reaches a constant value R8, the same process is performed using a sample gas that has been changed to a different degree from the previous one.

Next, the present invention will be specifically explained in detail.

〈Example 1〉 Fig. 6 shows a measurement using the above-mentioned measuring device, using air at a temperature of 20°C and a relative humidity of 62% as a carrier gas, and a NO2 gas concentration of 20 pP”y on the operating side of 190-6o○°C. When this happens, the grain size is 73,81°89.104,
120, 135, 142, 146° 159 The gas sensitivity S given by RG/Ro of a gas detector having the structure shown in Fig. 1, which uses a tin oxide film as a sensitizer, and the sample gas introduction 30 Resistance value of the gas sensitive membrane after seconds RG (30
) to show the operating temperature dependence of the product 5XSA(30) of the rise rate SA(30).

In addition, the crystal grain size is 73, 81.89, 104.120
, 135, 142, 146, and 159 were all fabricated by the sputtering method using the aforementioned gold maple tin as a target. All the settings are as described above except for the substrate temperature during sputtering and the heating and baking temperature for 2 hours in the atmosphere, which are shown in the table below. All film thicknesses are 300
0 to 3,300 people.

As is clear from Figure 6, N
In an O2 gas detector, the sensitivity and response speed largely depend on the crystal grain size of the tin oxide film used as the sensitive material. In particular, N0 according to the present invention and equipped with a susceptor having a particle size of 100 Å or less formed by a sputtering method
In a detection method that uses tin oxide as a sensitizer to detect NO2 gas by operating the 2-gas detector at an operating temperature of 200 to 300''C, NO2 has the highest sensitivity and highest response speed.
Note that the tin oxide film having a crystal grain size of 100 Å or less according to the present invention may be formed by the above-mentioned so-called sputtering method to a thickness of several thousand people, but in this case, a binary operating temperature is required. It is. ),Also,
A d-tin oxide film with a thickness of several μm obtained by heating and evaporating metallic tin in an atmosphere surrounded by oxygen gas, the so-called in-gas evaporation method, has a thickness of several μm, however, the oxygen gas pressure at this time is 0.
It is important that the temperature be 1 to 0.5 Torr, and that the temperature should not be heated above 450'C in the atmosphere after film formation.

), and furthermore, the white precipitate of α-stannic acid obtained by hydrolyzing S n Cfl 4 with aqueous ammonia or β-stannic acid obtained by oxidizing metallic tin powder in concentrated nitric acid is treated with a low-melting alkali. The paste obtained by mixing with glass powder is applied by printing, and in the case of a-stannic acid, it is heated and baked at 45°C or less, and in the case of β-stannic acid, it is heated and baked at 400°C or less. Even if it is a tin oxide film with a thickness of several micrometers, it will not work, and it will work no matter what film it is.

<Example 2> Figure 7 CI! L) and (b) have a purity of 99.99 w
A tin oxide film obtained by an evaporation method in oxygen gas (oxygen gas pressure 0.5 Torr, no need to heat the substrate) using metallic tin as an evaporation source was used as the gas sensitive body 1 shown in FIG. Shows the gas characteristics of the gas detector. The operating temperature is 390°C,
As a carrier gas, nitrogen is used in the case of Fig. 7 (a), and air at 20°C and 62% temperature is used in the case of Fig. 7 (in Fig. 7).
R, /R when changing the concentration between 6 and 40 ppm
The vertical axis is the logarithm jloqF3 of the sensitivity S given by o, and N
The slope of the straight line obtained when the logarithm ROCICNO2 of the O2 gas concentration CNo2 is plotted on the horizontal axis ΔQoqS/ΔAddq
The relationship between CNo2 acidification and CNo2 crystal grain size is shown.

Incidentally, tin oxide films having different crystal grain sizes can be obtained by forming the film by the above-mentioned in-gas vapor deposition method and then firing it in the atmosphere at different temperatures for 2 hours. The table below shows an example of firing temperature and crystal grain size.

As is clear from Fig. 7(a) and (b), when the grain size exceeds 1008, ΔIlage/Δ”0qCN
O2 values drop extremely. That is, it can be seen that the gas sensitivity is extremely reduced.

<Example 3> Fig. 8 shows a NO2 gas detector A in which a tin oxide film with a crystal grain size of 81 obtained by the above-mentioned sputtering method is used as a sensitizer, and a NO2 gas detector A using a tin oxide film obtained by the above-mentioned oxygen gas vapor deposition method. Crystal grain size 81
N02 gas detector B, a- using human tin oxide film as a sensitive body
When using NO2 gas detector C, which uses a tin oxide film as a sensitive material with a crystal grain size of 86 obtained by printing and baking a base plate containing stannic acid as a product, at an operating temperature of 390°C and using nitrogen as the carrier gas. The relationship between the sensitivity S and the N02 gas concentration CNo2 is shown.

For detectors A, B, and C, the slope of the straight line (Δ
120qs/Δ”oqCNO2) shows an almost equal value of about 0.98.In other words, the absolute value of the sensitivity S of the NO2 gas detector using a tin oxide film as a sensor with almost the same crystal grain size is Although different, the N02 gas concentration characteristics are
It turns out that they are roughly equal.

In addition, the magnitude relationship of the absolute value of sensitivity S is A'''>>B and C.
This is due to the external shape of the tin oxide film, and the film obtained by vapor deposition in gas has a large number of adsorption sites due to its porosity and large specific surface area. It is thought that a greater sensitivity S was obtained compared to detectors B and C, which had dense tin oxide films.

As described above, the present invention enables NO2 gas detection to be performed with high sensitivity and high response speed.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of the NO2 gas detector according to the present invention, Fig. 2 is a sectional view of the NO2 gas detector according to the present invention;
The figure is an X-ray diffraction diagram of a tin oxide film, Figure 3 is an X-ray diffraction diagram of a silicon oxide crystal powder, Figure 4 is a block diagram showing the configuration of the gas response characteristic measuring device, and Figure 5 is an electrical diagram of a gas detector. Diagrams showing the circuit configuration of the resistance measuring device, FIG. 6, FIG. 7(a),
(b) and FIG. 8 are diagrams for explaining the present invention in detail. Of these, FIG. 6 shows the relationship between the operating temperature and gas response sensitivity of tin oxide films of various particle sizes, and FIG. Figure (a),
(b) shows the relationship between crystal grain thickness and gas sensitivity,
FIG. 8 shows the gas concentration dependence of the gas sensitivity in various tin oxide films. 1...Gas sensitive body, 2...Electrode, 3.
... Substrate, 4 ... Planar heater, 8 ...
···thermocouple. Name of agent: Patent attorney Toshio Nakao (1st person)
Figure 2 2θ (Fake 2 Figure 3 201)

Claims (3)

[Claims] (1) A film in which the crystal grain size is calculated using the Debye-Scherer set from the half-width of the diffraction peak attributed to the (110) plane of the S n O2 crystal that appears in X-ray diffraction measurement, and is 100 Å or less A NO2 gas detector characterized by having a sensitive body made of tin oxide. (2) The crystal grain size, calculated using the Debye-Scherer set from the half-width of the diffraction peak attributed to the (110) plane of the S n O2 crystal that appears in X-ray diffraction measurement, is 1008 or less. A method for detecting N02 gas, characterized in that film-like tin oxide is used as a sensitive material and the sensitive material is operated at 200 to 300°C. (3) The NO2 gas detection method according to claim 2, characterized in that a tin oxide film formed by evaporation in oxygen gas or sputtering is used.