JPS6160379B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas detection element for detecting combustible gas. In recent years, various research and development activities regarding materials for sensing elements for flammable gases have become active. This is strongly attributable to the fact that explosion accidents caused by flammable gases and poisoning accidents caused by toxic gases occur frequently, mainly in households and in various factories, and have become a major social problem. In particular, propane gas has a lower explosive limit (LEL).
Because it has a lower specific gravity and a higher specific gravity than air, it tends to stagnate in rooms, causing many accidents and causing many casualties every year. In recent years, gas detection elements using metal oxides such as stannic oxide (SnO ₂ ) and gamma-type ferric oxide (γ-Fe ₂ O ₃ ) have been put into practical use, and are used in gas leak alarms, etc. It is applied. Even in the event of a gas leak, the presence of propane gas can be quickly detected before reaching the LEL, making it possible to prevent an explosion. Incidentally, in Japan, liquefied natural gas (LNG), whose main component is methane gas, has come to be used for general household use and is gradually becoming popular. Therefore, the demand for gas detection elements that can detect methane gas, which is the main component of LNG, with high selectivity is increasing. Of course, gas detection elements sensitive to methane gas have already been developed, but most of them use noble metal catalysts as sensitizers in the sensitive material.
Problems include catalyst poisoning by various gases, low selectivity to methane gas, and high dependence on ambient humidity.
Therefore, the current situation is that the properties are still insufficient for practical use. The present invention has been made in view of these circumstances, and it is an object of the present invention to provide a gas detection element that has a sensitivity that is sufficiently high for practical use even to methane gas. Since methane gas itself is a very stable gas, a detection element that has sufficient sensitivity for methane gas needs to be extremely active. Therefore, in order to achieve high sensitivity to methane gas, conventional methods have been used such as adding a noble metal catalyst to the sensitive material or operating the sensitive material at a considerably high temperature. . In contrast, the present invention realizes an element with high sensitivity to methane even at a relatively low operating temperature without adding any noble metal catalyst. The present invention was discovered while studying the effects of various anions contained in ferric oxide (Fe ₂ O ₃ ) on gas sensitivity characteristics in gas detection elements using ferric oxide (Fe 2 O 3 ) as a gas sensitive material. It is something that
That is, alpha-type ferric oxide (α-
Fe ₂ O ₃ ), gamma-type ferric oxide (γ-Fe ₂ O ₃ ), or a mixture thereof containing sulfate ions (SO ₄ ^-- ) dramatically improves gas sensitivity characteristics, and also has the aforementioned properties. This discovery was made based on the discovery that a sufficiently high sensitivity for practical use can be achieved even for methane gas. The present invention will be described below based on specific examples. (Example 1) Various amounts of ferrous sulfate (FeSO ₄ -7H ₂ O) were added as an additive to a commercially available alpha-type ferric oxide (α-Fe ₂ O ₃ ) reagent to contain sulfate ions. Powders were added and mixed, and an organic binder was further added thereto to produce several powders that were sized into particles with a size of 100 to 200μ. Obtained in this way,
Powders containing different amounts of ferrous sulfate were press-molded into a rectangular parallelepiped shape and fired in air at a temperature of 600°C for 1 hour. Au was deposited on the surface of this sintered body to form a pair of comb-shaped electrodes, and a platinum heating element was attached to the back side with an inorganic adhesive to serve as a heater and a sensing element was fabricated. A current was passed through this heating element, and the operating temperature of the element was controlled by adjusting the current value. The element temperature was maintained at 350°C and its gas sensitivity characteristics were measured. The resistance value (Ra) in air was measured in a measuring container with a volume of 50 mm in which dry air was stirred slowly to the extent that no turbulence occurred, and the resistance value (Rg) in gas was determined by 99% purity inside
The above gases of methane (CH ₄ ), propane (C ₃ H ₈ ), isobutane (i-C ₄ H ₁₀ ), and hydrogen (H ₂ ) are introduced at a volume ratio of 10 ppm/sec, and the concentration Measurements were taken when the amount reached 0.1% by volume. The gas concentration to be measured was chosen to be 0.1% by volume because the detection concentration practically required for a gas detection element is a number 10 below the lower explosive limit concentration (LEL) of the gas.
This is because the LEL of each of the above gases is approximately 2% to 5% by volume. Further, the presence of sulfate ions (SO ₄ ^-- ) contained in the gas sensitive material was confirmed by infrared absorption spectrum, and the amount contained was identified from the TG-DTA curve and fluorescent X-ray analysis. Table 1 shows the sensitivity characteristics of a gas sensitive material containing the amount of sulfate ions using Ra and Rg. Moreover, FIG. 1 shows this in terms of sensitivity (Ra/Rg). Note that isobutane in FIG. 1 is omitted because it exhibits almost the same characteristics as hydrogen.

[Example 2]

A commercially available triiron tetroxide (Fe ₃ O ₄ ) reagent and ferrous sulfate (FeSO ₄ -7H ₂ O) aqueous solutions prepared at various concentrations as an additive containing sulfate ions were prepared. Next, 10 g of the above Fe ₃ O ₄ reagent was weighed out, and the above ferrous sulfate aqueous solution was added dropwise to each of them and mixed. Some of the mixed powders thus obtained were heat treated in air at a temperature of 400°C. Further, this powder was sized to a size of 50 to 100μ, and triethanolamine was added to form a paste. On the other hand, as a substrate for the gas detection element, 5
Prepare an alumina substrate with a thickness of 0.5 mm and print gold paste in a comb shape at intervals of 0.5 mm on this surface.
A pair of comb-shaped electrodes were formed by baking. and,
A commercially available ruthenium oxide glaze resistor was printed on the back of the alumina substrate between the gold electrodes and baked to form a heater. Next, the above paste was printed on the surface of the board to a thickness of about 70μ, and after air drying at room temperature, it was heated to 400℃.
The mixture was gradually heated until the temperature reached , and maintained at this temperature for 1 hour. At this stage the paste evaporates and also
Fe ₃ O ₄ was oxidized and became a sintered film of γ-Fe ₂ O ₃ . The thickness of this gas sensitive body was approximately 55μ. A gas sensing element was thus obtained. In addition, to identify the amount of sulfate ions contained in the gas-sensitive membrane, rather than printing a portion of each of the above pastes on an alumina substrate, we slowly heated the paste at a temperature of 400°C in the same way as described above. TG-DTA
It was also subjected to fluorescent X-ray analysis. Further, the presence of sulfate ions was determined by analyzing the infrared absorption spectrum as in Example 1. The gas sensitivity characteristics of the gas detection element thus obtained were measured in the same manner as in Example 1. The results are shown in Table 2. The items marked with 〓 in the table are comparative examples for reference as in Table 1. Also, like FIG. 1, FIG. 2 shows this in terms of sensitivity (Ra/Rg), and like FIG. 1, isobutane is omitted because it shows almost the same characteristics as hydrogen.

【table】

[Table] As is already known, γ-Fe ₂ O ₃ itself has high sensitivity to combustible gases, and gas detection elements using it are already in practical use.
However, the problem with γ-Fe ₂ O ₃ itself is that its sensitivity to methane gas is lower than to other gases, as seen in No. 1 of Table 2. However, it can be seen that by containing sulfate ions as in the present invention, the sensitivity to methane gas is also significantly improved. Note that in Example 1, α-Fe ₂ O ₃ was used as the base material of the sensitive material, and in Example 2, γ-Fe ₂ O ₃ was used as the base material of the sensitive material, but α-Fe ₂ O ₃ and γ-Fe ₂ O Similar experiments confirmed that the present invention is effective even when a mixture of ₃ is used as the base material. Furthermore, in Examples 1 and 2, commercially available reagents of α-Fe ₂ O ₃ and Fe ₃ O ₄ were used as starting materials;
The present invention is not limited to starting materials or manufacturing methods. It is of course also possible to add further additives to improve the properties. As described above, the gas sensing element of the present invention has α
-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , or their mixtures contain sulfate ions, which dramatically improves their gas sensitivity, making it difficult to detect trace amounts without using precious metal catalysts. It is possible to achieve extremely high sensitivity even to methane gas. This precisely responds to social needs, which are becoming increasingly demanding as city gas is replaced with natural gas (main component: methane gas), and its effects are extremely significant.

[Brief explanation of the drawing]

FIGS. 1 and 2 are characteristic diagrams showing the relationship between the sensitivity (Ra/Rg) and the amount of sulfate ion (SO ₄ ^-- ) contained in the receptor in each example of the present invention.

Claims (1)

[Claims] 1 The main component is at least one of alpha-type ferric oxide (α-Fe ₂ O ₃ ) and gamma-type ferric oxide (γ-Fe ₂ O ₃ ), and sulfate ion (SO ₄ ^-- ) is 0.005~
A gas sensing element characterized in that a gas containing 10.0% by weight is used as a gas sensitive material.