JPS63736B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses filmy tin oxide as a NO ₂ gas sensitizer.
The purpose of this invention is to provide a NO ₂ gas detection method using an NO ₂ gas detector that has excellent sensitivity _and response speed. Conventionally, as an accurate, simple and continuous measurement method for NO ₂ gas, NO _X gas (NO ₂ and NO ) is available. This gas detector belongs to the category of so-called semiconductor-type gas detectors, and is produced by adsorbing electron-attracting oxidizing gases such as NO ₂ , NO, and O ₂ onto a tin oxide sensitizer, which is an n-type semiconductor. , which utilizes a phenomenon in which the electrical resistance of the tin oxide sensitive material increases in proportion to the concentration of those gases.
Note that this phenomenon has long been widely known in the physical and chemical fields as a property of metal oxide semiconductors. This semiconductor type 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 for operating the sensitive body at an appropriate temperature. It has an extremely simple structure consisting of a heating source. The performance (selectivity, sensitivity, response speed) of a semiconductor gas detector, which has an extremely simple structure, is determined by, on the one hand, the physical properties of the materials that make up the sensitive body, and on the other hand, This is the temperature at which the sensitive body and the gas to be detected interact with each other. In other words, even if the same material were used as a sensitive material, differences in material properties caused by differences in surface structure defined by its shape or internal structure would lead to noticeable differences in the characteristics of gas detectors. appear. Furthermore, even if the same substance and the same material properties are used, the characteristics of the gas detector will vary greatly depending on the operating temperature. This is not difficult to understand if one considers that the gas detection mechanism involves chemical interaction between the sensitive body and the gas to be detected, as is generally said. In the above-mentioned US Patent No. 4,169,369, a material specifically selected for _NO We are proposing a _NOx gas detector with excellent performance. Also,
Regarding the operating temperature, it is stated that 150 to 300°C is suitable. However, it is difficult to know the appropriate values for the material properties of tin oxide in terms of sensitivity and response speed, which are the main characteristics of a gas detector.
It only states that above 150°C, the response speed becomes faster, but "overshoot" occurs, and below 150°C, the response speed becomes slower with low concentration gas. What does "overshoot" mean?
There is no description of how "overshoot" affects the performance of the detector, and even an engineer familiar with the technical field of gas detectors does not know why the upper limit of operating temperature is 300℃. However, it is extremely difficult to know this. As mentioned above, in conventional _NO It is difficult to know the optimum operating temperature for the device, and this poses a major difficulty in putting into practical use _NOx gas detectors that use tin oxide as a sensor. In addition, if the operating temperature is kept below 300℃ as in the past, it may be necessary to cool the detector due to the actual temperature of the exhaust gas, which makes the detector It is inevitable that it will be complicated (and expensive). The present invention has been made in view of the above points, and by providing a suitable material as a tin oxide sensitive material for an NO ₂ gas detector using tin oxide as a sensitive material, it is possible to achieve excellent sensitivity and response speed. The present invention attempts to provide a NO ₂ gas detection method. That is, in the present invention, as a suitable tin oxide sensitizer material,
From the half-width of the diffraction peak corresponding to the (110) plane that appears around 2θ = 26.6° (θ is the diffraction angle) in the X-ray diffraction diagram of SnO ₂ crystal, the Debye-Schierer formula (crystal particle diameter D = Kλ / β cos θ, K is a coefficient close to 1, λ
is the X-ray wavelength 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.
Tin oxide formed in a film shape with a crystal particle diameter of 100 to 140 Å given by the half width of the diffraction peak of SiO ₂ single crystal powder is selected. And, as the operating temperature of _NO2 gas detector using tin oxide sensitizer material,
Suitably selected at 300-400°C. Examples of the present invention will be described below. Figure 1 shows one of the embodiments of the present invention, which is equipped with a tin oxide sensitizer having a crystal particle size of 100 to 140 Å.
Showing NO ₂ gas detector. In the figure, 3 is a 96% pure alumina substrate with a surface roughness of about 2.5μ, a thickness of 0.5 mm, a length of 5 mm, and a width of 5 mm. 4 is obtained by applying a conductive paste mainly composed of platinum to one side of this alumina substrate 3 and baking it.
A sheet heater consisting of a 60Ω resistor maintains the detector at a fixed operating temperature. 5 is a lead wire made of platinum wire with a wire diameter of 0.5 mm for supplying power to the sheet heater 4; 2 is a conductive paste mainly composed of platinum; The central part of the electrode is printed with a shape exposed through a 0.5 mm wide groove, and the electrical signals from the gas sensitive body are taken out to the outside. Reference numeral 8 denotes a thermocouple for measuring the temperature of the gas sensitive body, which is made of a CA (chromel-alumel) wire and is baked and fixed 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.
1 is a NO ₂ gas sensitive material made of a tin oxide film having a crystal grain size of 100 to 140 Å according to the present invention. The tin oxide film is formed using, for example, a target material with a diameter of 150 mm made of metallic tin powder with a purity of 99.99% and a 100 mesh pass, with a distance between electrodes of 40 mm, an argon gas pressure of 1.25 pa (pascal), and an oxygen gas pressure of 1.25 pa (pascal).
1.25pa, alumina substrate temperature ~120℃, power 400W,
It is produced by heating and baking a tin oxide thin film obtained by sputtering for 30 minutes at a voltage of 2.55 KV at 605°C in air for 2 hours. Figure 2 shows that the tin oxide film obtained in this way was exposed to a tube current of 200 mA.
It is a diffraction pattern of 2θ=20 to 35° obtained by X-ray diffraction using copper as an anticathode at a tube voltage of 50 KV. The diffraction peak corresponding to the (110) crystal plane peculiar to SnO ₂ is 2θ=
Obtained at 26.6°. A sharp diffraction peak near 2θ=25.6° is a peak attributed to alumina of the substrate. The crystal grain size D of the tin oxide film is determined using the Debye-Scherer equation described above as follows. D=Kλ/(B-b)cosθ……Debye-Scherer equation In this equation, K1.0, λ=1.5405Å, θ
= 26.6/2, B = 0.80 (half-width of the diffraction peak at 2θ = 26.6° shown in Figure 2), b = 0.13 (same X-ray diffraction device as shown in Figure 3 that obtained Figure 2) Substituting the half-width of the diffraction peak at 2θ = 31° of the SiO ₂ crystal powder with a particle size of ~43μ obtained in becomes. It can be seen that the crystal grain size of the tin oxide film is 135 Å. Although the crystal grain size of the tin oxide film is determined by the above method, the gas response shown in Fig. using a characteristic measuring device
The NO ₂ gas response characteristics were evaluated. In Fig. 4, 11 is a sample gas supply tank, 12 is a flow meter for sample gas, 13 is a sample gas reservoir, 14 is a carrier gas (N ₂ or dry air) supply tank, 15 is a first carrier gas reservoir, and 16 is for carrier gas. pump, 17 is a carrier gas flow meter, 18 is a second carrier gas reservoir, 19 is a gas spectrometer, 20 is a gas switching solenoid valve, 21 is a gas detection chamber, 22 is a gas detector, 23
24 is a heater temperature controller, 25 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 Figure 4,
The concentration of NO ₂ sample gas was measured using a gas spectrometer using chemiluminescence method, MODEL manufactured by Yanagimoto Seisakusho Co., Ltd.
Each measurement was calibrated according to SAE-5149. Nitrogen or the atmosphere with a temperature of about 20°C and relative humidity of 50 to 70% was used as a carrier gas (diluent gas). While the carrier gas is introduced into the gas flow path at a rate of 2 per minute by the pump 16, the operating temperature is maintained at a preset operating temperature by the heater temperature controller 24 to which the thermocouple 8 of the gas detector shown in FIG. 1 is connected. After that, open the gas switching solenoid valve 20.
By this operation, the NO ₂ sample gas, which has been diluted in advance to a predetermined concentration, is sucked from the sample gas reservoir 13 by the pump 27 and introduced into the gas flow path at a flow rate of 2/min. The change in electrical resistance of the gas detector 22 at this time is recorded as a rise characteristic via the electrical resistance measuring device 25 of the gas detector. Here, the circuit configuration of the electrical resistance measuring device of the gas detector is shown in FIG. In FIG. 5, 28 is a power source for applying a DC voltage of 1 V to the gas detector 22 through a resistor (1 MΩ) 29, 30 is an impedance converter, and 31 is a recorder. Now, after the electrical resistance value reaches a certain value R _G ,
By operating the gas switching solenoid valve 20 again, carrier gas is introduced at a flow rate of 2/min using the same pump 26 to replace the carrier gas.
The change in electrical resistance of the gas detector 22 at this time is recorded as a falling characteristic. The temperature change in the gas detector during gas replacement is regulated to the same flow rate of ga2/min for both carrier gas and sample gas. Maintained below ±1℃. Next, after the electrical resistance reaches a certain value, R _O , the same thing is done using the sample gas, which has been changed to a different concentration. Note that the gas detection chamber 22 in which the gas detector 22 is arranged
The volume of the sample gas is about 200 c.c., and the replacement of the sample gas with the carrier gas is completed in about 6 seconds at the latest. Figure 6 shows that using such a measuring device, air temperature is 20℃ and relative humidity is 62% as the carrier gas, NO ₂ gas concentration is 20ppm, and operating temperature is 190 to 500.
When the crystal grain size is 73, 81, 85, 104,
The gas sensitivity S given by R _G /R _O of a gas detector with the structure shown in Figure 1, which uses a tin oxide film of 120, 135, 142, 146, and 159 Å as a sensitive material, and the sample gas Resistance value of gas sensitive membrane R _G after 30 seconds of introduction
30 is used to show the operating temperature dependence of the product S×S _A (30) of the rise rate S _A 30 given by R _G (30)/R _O −1/(R _G /R _O −1)×100. . In addition, Fig. 7 shows that using the same carrier gas as in Fig. 6, the NO ₂ gas concentration was reduced to 0.6 at an operating temperature of 390°C.
The slope of the straight line obtained when the vertical axis is the logarithm logS of the sensitivity S given by R _G /R _O when changing between ~40 ppm, and the logarithm logC _NO2 of NO ₂ gas concentration C _NO2 is plotted on the horizontal axis. FIG. 3 is a diagram showing the relationship between ΔlogS/ΔlogC _NO2 and crystal grain size. In FIG. 7, for the case using a tin oxide film with a grain size of 100 Å or less, at an operating temperature of 390° C., S1 was obtained, and ΔlogS/ΔlogC _NO2 could not be determined. As is clear from FIGS. 6 and 7, in this gas detector, the sensitivity and response speed are determined by the crystal grain size of the tin oxide film used as the sensitive body and the sensitivity of the sensitive film to NO ₂ gas, which is the gas to be detected. It is highly dependent on the temperature at which it is applied. The present invention has been made based on the fact that the sensitivity and response speed of the gas detector of the present invention depend on the crystal grain size of the tin oxide film and on the operating temperature. but
By operating a gas detector using a 10-140 Å tin oxide film as a sensitive material at a temperature of 300-400°C, it achieved excellent sensitivity and response speed. Can provide NO ₂ gas detection method. That is, ΔlogS/ shown in FIG.
The ΔlogC _NO2 value shows a nearly constant value of ~0.4 when the grain size is 140 Å or less, and the NO ₂ gas sensitivity is extremely superior when compared to those over 140 Å, and when the grain size is 100 Å or more, As is clear from Figure 6,
At an operating temperature of 300 to 400° C., the S×S _A (30) value has a maximum value, and the gas detector can exhibit maximum performance. According to the present invention, a NO ₂ gas detector using a tin oxide film as a sensitizer with a grain size of 100-140 Å is operated at an operating temperature of
By operating at 300-400℃, NO ₂ gas detection with maximum sensitivity and maximum response speed is possible. In addition, the crystal grain size to see the effect of the present invention is
The tin oxide film with a thickness of 73, 81, 85, 104, 120, 135, 142, 146, and 159 Å was formed by the above-mentioned sputter hanging method using the metal tin powder as a target.
In the process of heating and firing this in the atmosphere, the alumina substrate temperature during sputtering and the temperature during heating and firing were selected as shown in the table below. In each case, the film thickness is 300 to 3300 Å.

[Table] Furthermore, as the tin oxide film having a crystal grain size of 100 to 140 Å according to the present invention, a film having a thickness of several thousand Å formed by the so-called sputtering method described above may be used.
In addition, the film thickness obtained by the so-called vapor deposition method in which metal tin is heated and evaporated in an oxygen gas atmosphere is several micrometers thick.
In addition, a white precipitate of α-stannic acid obtained by hydrolyzing SnO ₄ with aqueous ammonia or β-stannic acid obtained by oxidizing fine metallic tin powder in concentrated nitric acid. The NO ₂ gas detection method according to the present invention may be a tin oxide film with a thickness of several μm obtained by printing and baking a paste obtained by mixing with a low-alkali glass powder. In this case, the same effect as shown in FIGS. 6 and 7 is obtained. In addition, in the above-mentioned in-gas vapor deposition method, the oxygen gas pressure that gives the crystal grain size of 100 to 140 Å is 0.1 to 1.
0.5Torr is preferably chosen. Furthermore, in the method of printing and firing α-stannic acid, a tin oxide film having a crystal grain size of 100 to 140 Å according to the present invention can be obtained at a firing temperature of 450 to 620°C. Further, in the method of printing and firing β-stannic acid, the tin oxide film according to the present invention can be obtained at a firing temperature of 400 to 450°C. Next, FIG. 8 shows various gas characteristics in nitrogen gas at an operating temperature of 390° C. of an NO ₂ gas sensor using a tin oxide film with a crystal grain size of 120 Å obtained by sputtering as a sensitive material. As is clear from FIG. 8, according to the present invention
The NO ₂ gas detection method is performed under the atmosphere of combustion gases generated when normal fossil fuels are burned in the atmosphere (NO ₂ : 30-40 ppm, NO: ~500 ppm, CO: ~
100ppm, O ₂ ; ~10000ppm, remaining N ₂ ),
Selectively responds to changes in _NO2 gas concentration. As described above, the NO ₂ gas detection method of the present invention is
NO ₂ with high selectivity, high sensitivity, and high responsiveness
Enables gas measurement. The present invention has also been proposed in the past. Operating temperature of 150~300℃
Being able to raise the operating temperature to 300 to 400°C also has the advantage that in practice it is not necessary to cool the exhaust gas before it interacts with the gas detector.

[Brief explanation of the drawing]

FIG. 1 is used in one embodiment of the present invention.
A cross-sectional view of the NO ₂ gas detector, Figure 2 is an X-ray diffraction diagram of a tin oxide film, and Figure 3 is an X-ray diffraction diagram of a silicon oxide crystal powder.
4 is a block diagram showing the configuration of the gas response characteristic measuring device, FIG. 5 is a diagram showing the circuit configuration of the electrical resistance measuring device of the gas detector, and FIGS. 6 to 8 are the diagrams of the present invention. These figures are for explaining the effect. Of these, Figure 6 shows the relationship between the operating temperature and gas sensitivity of tin oxide films of various grain sizes, and Figure 7 shows the relationship between the crystal grain size and gas sensitivity. Figure 8 shows gas detection characteristics for various gases. DESCRIPTION OF SYMBOLS 1... Gas sensitive body, 2... Electrode, 3... Substrate, 4... Planar heater.

Claims (1)

[Claims] 1 Film-like tin oxide whose crystal grain size is calculated from the half-width of the diffraction peak attributed to the (110) plane of the SnO ₂ crystal that appears in X-ray diffraction measurements using the Debye-Scherer equation from 100 to 140 Å. 1. A method for detecting NO ₂ gas, characterized in that a sensor is used as a sensor, and the sensor is operated at a temperature of 300 to 400°C.