JPS6129660B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas detection element, and an object thereof is to provide a gas detection element that exhibits high sensitivity even to methane without using a catalyst. Conventionally, gas detection elements have been known that utilize phenomena such as resistance changes in platinum wires and discoloration of palladium chloride aqueous solutions, but these all have drawbacks such as low gas sensitivity and difficult detection methods. have. Furthermore, although gas chromatographs and chemical analysis methods have high accuracy, they have drawbacks such as lack of immediacy in analysis and large-scale and expensive equipment. On the other hand, stannic oxide (SnO ₂ ), zinc oxide (ZnO),
Gas sensing elements using n-type semiconductors such as cadmium oxide (CdO) are known. This type of gas detection element is capable of instantly and quantitatively detecting combustible gases, and has a high gas sensitivity. In order to increase the responsiveness of gas adsorption and desorption,
It is configured to be used while being kept at a high temperature at all times. However, in this type of sensing element, an important issue is whether the resistance value changes over time, that is, whether the life span is sufficient. Elements using the aforementioned metal oxide semiconductors cannot obtain sufficient sensitivity and responsiveness on their own, so adding various noble metal catalysts to activate the elements reduces the lifespan. This is an element that leaves a lot of questions. Recently, it has been discovered that among ferric oxides, gamma-type ferric oxide (γ-Fe ₂ O ₃ ), which has a spinel structure, exhibits excellent gas-sensitivity properties, and gas sensing elements using this as a sensitizer have been developed. Development is underway. However, at high temperatures, this γ-Fe ₂ O ₃ transforms into alpha-type ferric oxide (α-Fe ₂ O ₃ ) with a corundum-type crystal structure that does not have gas-sensitive properties, so it cannot be used at operating temperatures. Although some problems remain in terms of changes in resistance over time, this method is attracting attention as a method that provides high gas sensitivity without using a catalyst. The present invention relates to a sintered body mainly composed of cubic spinel represented by the general formula M _x Zn _4-2x O ₄ (however, M
This is based on the discovery that Sn, Ti) can provide high gas sensitivity, especially sensitivity to methane gas (CH ₄ ), without the presence of a catalyst. Currently, gas is becoming increasingly popular as an energy source in general households. However, as is well known, gas is extremely dangerous. There is an urgent need to develop gas leak alarms for home use in order to eliminate the dangers of gas and make it safe to use. Generally speaking, there are two types of gas: LP gas and city gas. Furthermore, what is called city gas includes natural gas whose main component is methane and mixed production gas (hydrogen and butane). Furthermore, the composition of these gases also differs depending on the region. Therefore,
It is extremely difficult to detect gases of all compositions with the same gas detection element. This is one of the factors delaying the development of sensing elements. In particular, this is thought to be because general sensors are sensitive to stable gases such as methane, whereas they are sensitive to hydrogen, butane, and propane. The present invention realizes such a gas detection element that does not use a noble metal catalyst that may adversely affect changes over time and has high gas sensitivity to methane. Hereinafter, the device according to the present invention will be specifically described based on Examples. Explain in detail. Example 1 Tin oxide (SnO ₂ ) powder was added to zinc oxide (ZnO) powder in various proportions, water was further added, and the mixture was thoroughly mixed in a monomaron pot and then dried. This mixed powder was calcined in air at a temperature of 900° C. for 1 hour in an electric furnace. Thereafter, the calcined powder was finely pulverized in a monomaron pot. An organic binder was added to the calcined and pulverized powder, which was then granulated and sized, and press-molded into a cylindrical shape with a length of 2 mm and a diameter of 2 mm. The pressure at this time was 700Kg/ ^cm2 , and two platinum wires with a thickness of 100μ were embedded during molding. This becomes the electrode of the element. The molded body was fired in an electric furnace in air at a temperature of 750°C for 1 hour. The tensile strength of the platinum wire of this sintered body is approximately 100 to 400 g.
It can be seen that sufficient sintering continues. Next, a coiled heater for heating this sintered body was prepared, and as shown in FIG. 1, the sintered body was placed inside the coiled heater and bonded. In the figure, 1 is the sintered body, 2, 2' are platinum electrode wires for outputting the output, 3 is a coiled heater made of nichrome wire, 4 is cement for insulating the heater 3, 5, 6, 7, 8 is the heater 3 and the sintered body 1
It is an electrical terminal for. The above structural package was covered with an explosion-proof net in the same manner as the conventional element to form a gas detection element. Table 1 shows the stannic oxide in this gas sensing element and its manufacturing conditions.

[Table] Connect an ohmmeter between terminals 7 and 9 of each terminal, turn on the heater 3, and set the temperature of the sintered body 1 to approximately 450℃.
was held at The heater power at this time was 1.3W. After confirming that the atmosphere to be measured was sufficiently clean air, the resistance value R _A of the sintered body 1 at that time was measured. Next, we will look at the resistance value R _C in air containing 3000 ppm of methane (CH ₄ ) as a typical example of natural gas, and the mixed gas of 35% isobutane (i-C ₄ H ₁₀ ) and 35% hydrogen (H ₂ ). The resistance value R _M in air containing 3000 ppm of was measured. The results are shown in FIG. FIG. 2 shows the relationship between the composition ratio of Sn _x Z _4-2x O ₄ and the resistance values _RA , _RC , and _RM in each atmosphere. As is clear from this, sensitivity to methane is high when the value of x is within the range of 0.5 to 1.5. However, for mixed gas,
As the value of x increases, the sensitivity decreases.
At this time, in order to simultaneously detect natural gas and mixed gas, it is desirable that the sensitivities for both be as similar as possible. From this point of view, when x is within the range of 0.75 to 1.25, the sensitivities of both sensors are relatively close, and it can be said that the sensors are excellent as general-purpose city gas sensors. It is estimated from X-ray analysis of the sintered body that this is obtained by containing a large amount of SnZn ₂ O ₄ . Example 2 Various amounts of titanium oxide (TiO ₂ ) powder were added to zinc oxide (ZnO) powder, water was further added and mixed well, and then it was dried. The mixed powder was calcined in an electric furnace at 900°C for 1 hour in air. The obtained calcined powder is finely pulverized, an organic binder is added thereto, granulated and sized to form a powder with a length of 2 mm and a diameter of 2 mm.
Pressure molded to mm. The pressure at this time is 700Kg/cm ²
It was hot. Hereinafter, a detection element was manufactured using the same procedure as in Example 1, and the resistance values R _A , R _C , R
_M was measured. The results are summarized in Table 2.

[Table] In the table, the resistance value R _A in air increases as the amount of TiO ₂ increases, but when methane is
The resistance value in air containing Ti _x Zn _4-2x O ₄ becomes almost constant around x = 1.00, so the value of x is
Within the range of 0.5 to 1.5, the sensitivity to methane increases.
Furthermore, the resistance value R _M to the mixed gas is greater than the increase in R _A and increases, so the sensitivity gradually decreases. From the above, it can be seen that the value of x is preferably around 0.75 to 1.25 for use in sensors for city gas and natural gas. To be more specific, x
If the value of is less than 0.75, the sensitivity for mixed gases is high but the sensitivity for methane is low, so if you set an alarm concentration for methane, the alarm concentration for mixed gases will be much lower than the required alarm concentration. This will result in an alarm being issued, and there is a possibility that many false alarms will occur. Conversely, if an alarm concentration is set for the mixed gas, the alarm will not be issued unless the concentration of methane is extremely high, which can be said to be dangerous. As is clear from the examples described above, when using SnO ₂ , TiO ₂ and ZnO alone, the cubic spinel phase was very incomplete.
MZn ₂ O ₄ (MSn, Ti), that is, effective methane sensitivity can only be obtained near x=1.00. Further, when using SnO ₂ , TiO ₂ , or ZnO alone, a catalyst is required, but in the present invention, high gas sensitivity can be obtained with various gases without using a catalyst.

[Brief explanation of the drawing]

FIG. 1 shows an example of the structure of the gas sensing element according to the present invention, and FIG. 2 shows a typical example of the resistance value change depending on the composition ratio. 1...Sintered body, 2,2'...Platinum electrode wire, 3...
...Heater, 4...Cement.

Claims (1)

[Claims] 1 General formula M _x Zn _4-2x O ₄ (M is Ti and
At least one selected from Sn0.75≦x≦
1.25). A gas sensing element characterized in that it uses a sintered body whose main component phase is a cubic spinel phase as a gas sensitive body.