JPS58200150A - Gas detection element - Google Patents

Abstract

PURPOSE:To obtain a combustible gas detection element which is operable at a relatively low temperature even with a high sensitivity to CH4 gas by use of a gas sensitive body main composed of CdO and containing SO4<--> within the specified range of proportion. CONSTITUTION:A CdO powder is mixed with a CdSO4-xH2O powder. After a organic binder is added thereto, the mixture is graded to particles of 100-200mum and pressure-molded and sintered in the air at about 600 deg.C to form a gas sensitive body containing SO4<--> 0.05-10wt%. A pair of comb-shaped electrodes are formed on the surface of the sensitive body while a heat generating body on the back thereof to make a detection element. This enables the detection of the gas concentration at a level below a fraction to several tenths of the lower limit concentration of explosion of combustible gas. Thus, a detection element even with a high sensitivity to CH4 gas can be obtained employing a sensitive body containing no precious metal catalyst.

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 explosions caused by flammable gases and poisoning accidents caused by toxic gases occur frequently in ordinary households and in various factories, which have become major social problems. In particular, propane gas has a low lower explosive limit (LEL) and a specific gravity greater than air, so it is easy to store it in a room. After 8 years, I won't cut it.
It causes many casualties every year.

In recent years, flammable gas detection elements using metal oxides such as stannic oxide (81102) and gamma-type ferric oxide (γ-Fe2O3) have been put into practical use, and are being applied to gas leak alarms. ing. Even if a situation such as a gas leak occurs, 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 mechan gas has come to be used for general household use and is gradually becoming popular. Therefore, this LN
The demand for gas detection elements that can detect methane gas, which is the main component of G, with high selectivity is also increasing.

Of course, gas detection elements sensitive to methane gas have already been developed, but most of them use a sensitizer and (2)
Since this method uses a Kankin Kaede catalyst, it has problems such as 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 this situation, and it is an object of the present invention to provide a gas detection element that has a sensitivity 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 techniques 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 a gas sensing element using cadmium oxide (CaO) as a gas sensing material on gas sensitivity characteristics.

That is, by containing sulfate ions (804-) in CdO, which is the base material of the gas sensitive material, the gas sensitivity characteristics are dramatically improved, and in addition, it is possible to realize a sensitivity that is sufficiently high for practical use even to the aforementioned methane gas. This was done by discovering that.

The present invention will be described below based on specific examples.

[Example 1] Various amounts of cadmium sulfate (CdSO4-LH20) powder were added and mixed as an additive to contain sulfate ions to a commercially available cadmium oxide (CaO) reagent, and an organic binder was further added to this. Te100-200
Several powders were prepared that were sized into μ-sized particles. The thus obtained powders containing different amounts of cadmium sulfate were press-molded into a rectangular parallelepiped shape and fired in air at a temperature of 600°C for 1 hour. A pair of comb-shaped electrodes were formed by vaporizing K on the surface of this sintered body, and a platinum heating element was attached to the back surface with an inorganic adhesive to serve as a heater and to produce a sensing element. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element.

The temperature of the element body was maintained at 400°C, and its gas sensitivity characteristics were measured.

The resistance value (Ra) in air is measured in a measurement container with a volume of 601 in which dry air is slowly stirred to the extent that no turbulence occurs.
) is in this container methane (CHS,
Propane (05H8), isobutane (i-(4H+) and hydrogen (H2) gases in volume ratio 1 OFF
/second, and each measurement was made when the concentration reached 0.1 volume. The gas concentration to be measured was chosen to be 0.1 volume because the detection concentration that is practically required for a gas detection element is several tenths to several tenths of the lower explosive limit concentration (LICL) of the gas. This is because the LEL of each of the above gases ranges from approximately 2 volumes to 6 volumes.

Also, sulfate ions (Soa-) contained in the gas sensitive material
The presence of - was confirmed by infrared absorption spectrum, and the amount contained was identified by TG-DT five-convex curve and fluorescence X-ray analysis. The following table shows the sensitivity characteristics of gas sensitizers containing various amounts of sulfate ions. Also, FIG. 1(a).

(b) shows this in terms of sensitivity (Ra 7 Rg); FIG. 1 (IL) shows the characteristics for methane and propane, and FIG. 1 (b) shows the characteristics for isobutane and hydrogen.

As is clear from the above table and Figure 1, sulfuric acid 7
It can be seen that by containing 0.005-10.0% by weight of (804-), the gas sensitivity characteristics, especially the sensitivity to methane, are dramatically improved.

Note that the sulfate ion (804
--) c7) The reason for limiting the amount to 0.005-10.0% by weight is that, as shown in the above table, there is no effect of improving the gas sensitivity characteristics below the o, o o s weight ratio. On the other hand, if it exceeds 10.0% by weight, it will lack practicality in terms of stability of properties or mechanical strength. Items marked with 4 in the table above correspond to this category, and are described as comparative examples in the table. As shown in A1 in the table, ordinary C(10) itself has extremely low gas sensitivity characteristics and cannot be put to practical use as it is.

However, due to the inclusion of sulfate ions,
As seen in the above table and FIG. 1, a large sensitivity appears to various flammable gases including methane.

Additionally, it is generally said that physicochemical phenomena such as adsorption/desorption phenomena for combustible gases are more likely to become active in metal oxides that are amorphous to some extent than those that are crystallized. However, even the commercially available reagent cadmium oxide used in this example, which is almost completely crystallized, exhibits extremely high activity due to the inclusion of sulfate ions, and as a result (( will be shown.

In Example 1 above, the case where the sensitive body is a sintered body was described, but the following example shows that the present invention is effective not only when a sintered body is used as the sensitive body but also when a sintered film is used. This will be explained using 2. In addition, in Example 1, the operating temperature is 400'
Although only case C has been described, the gas selectivity (a factor indicating the ability to selectively detect a certain gas) of the device according to the present invention changes significantly by changing the operating temperature. Another important effect of the present invention, which is that gas selectivity can be significantly controlled, will be specifically explained in Example 2 below. In Example 2, ethanol was used as the test gas instead of isofrine gas, which has almost the same characteristics as propane gas.

[Example 2] A commercially available cadmium oxide (CdO) reagent and cadmium sulfate (CdSO4-XH20) aqueous solutions prepared at various concentrations as an additive containing sulfate ions were prepared. Next, 10 g of the above CaO reagent was weighed out, and the above cadmium sulfate aqueous solution was added dropwise to each of these and mixed. Some of the mixed powders thus obtained were heat treated in air at a temperature of 400°C for 2 hours. Further, this powder was sized to 60 to 100p, and triethanolamine was added to form a paste. On the other hand, as the substrate of the gas detection element, an alumina substrate of 5 mm in length and f'i7i and thickness Q of 6 mm was used.
A pair of comb-shaped electrodes were formed by printing gold paste in a comb shape at intervals of . Then, on the back surface of the alumina substrate, a commercially available ruthenium oxide glaze resistor was printed 121 between the gold electrodes and baked to serve as a heater.

Next, the above-mentioned paste was printed on the surface of the substrate to a thickness of about 70 μm, air-dried at room temperature, and then gradually heated to a temperature of 400° C. and held at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film of cadmium oxide (CaO) containing sulfate ions. The thickness of this gas sensitive member was approximately 66μ. A gas sensing element was thus obtained.

In addition, it is difficult to identify the amount of sulfate ions contained in the gas-sensitive membrane by printing a portion of each of the above pastes on an alumina substrate, but by slowly heating the paste as is at a temperature of 400'C in the same manner as described above. This was then subjected to TG-DT and fluorescent X-ray analysis. In addition, the presence of sulfate ions was confirmed in Example 1.
This was done by analyzing the infrared absorption spectrum in the same way as in .

The gas sensitivity characteristics of the thus obtained sensing element were measured in the same manner as in Example 1 except that the operating temperatures were set at two points, 360°C and 460°C. Figure 2 e)
(d) is a characteristic diagram showing the relationship between sulfate ion content and sensitivity to various combustible gases (R-71g);
a) is methane, Figure 2 (b) is propane, Figure 2 (C)
shows the characteristics for hydrogen, and FIG. 2(d) shows the characteristics for ethanol.

As is clear from Figure 2, by containing 0.006% by weight or more of sulfate ions (SO4-), the temperature at 36°C
It can be seen that the gas sensitivity characteristics are dramatically improved at any operating temperature of , 450'C.

(However, if this sulfate ion is contained in an amount exceeding 10% by weight, the characteristics will not be stable as in the case of Example 1, and the mechanical strength will also be weakened, making it unsuitable for use as a practical device. [No data are listed here.] Another important point is that gas selectivity varies greatly depending on the operating temperature.For example, the sensitivity when 0.5% by weight of sulfate ions is included. The relationship between operating temperatures is shown in Figure 3.As is clear from Figure 3, at an operating temperature of 350'C, the sensitivity to ethanol is significantly greater than that of other gases, and the selectivity to ethanol is extremely high. On the other hand, at an operating temperature of 460'C, the sensitivity to ethanol is very small, and the sensitivity to other methane, propane, and hydrogen is relatively large. This means that the device has the characteristic that the relative sensitivity between ethanol and other gases can be easily controlled by changing the operating temperature.

From a practical standpoint, this means that by periodically changing the operating temperature or by using two elements with different operating temperatures, it is possible to easily distinguish between ethanol and other gases. This also means that it is possible to form a gas sensing element with functions that can be achieved. This point is also one of the great effects of the present invention, and greatly expands the scope of the present invention.

In each of the above Examples, a commercially available reagent of cadmium oxide (CdO) was used as a starting material, but the present invention does not limit the starting material or manufacturing method. Of course, it is also possible to further add JJII additives to improve the properties.

As described above, the gas sensing element of the present invention has dramatically improved gas sensitivity characteristics because cadmium oxide (CaO), which is the base material of the gas sensing element, contains sulfate ions.
It is possible to achieve extremely high sensitivity even for methane gas, which until now has been considered difficult to detect in trace amounts without the use of noble metal catalysts. 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. Furthermore, as already mentioned, the fact that gas selectivity can be greatly controlled by the operating temperature is also a great advantage from a practical standpoint of the present invention. As described above, the gas detection element of the present invention is expected to make an extremely large contribution to the field of gas disaster prevention, which is becoming increasingly important.

[Brief explanation of drawings]

Figure 1 (Δ), (b) is a characteristic diagram showing the relationship between sulfate ion content and sensitivity (Ra 71g) in one example of the present invention, and Figure 2 (&), (b), (C
), ((1) represents the relationship between the sulfate ion content and sensitivity U (RIL 71g) for each flammable gas of methane, propane, hydrogen, and ethanol in another embodiment of the present invention, using the operating temperature as a parameter. The characteristic diagram, FIG. 3, is a characteristic diagram showing an example of the operating temperature dependence of the sensitivity (R&7 Rg) in the same example. Name of agent: Patent attorney Toshio Nakao and one other person No. 1
Figure (Q + λ circle -, ion force content (tf'/-) in Figure 1 (b) Each nine points (Ioshi + t*f/, > Figure 2 ((1)) ; Yes -t<11%) No. 21!1 +A) Ishikureiio〉4 content (! DoγON
-Figure 2 (t'1 Fuyuan Kuionme content i <1 amount %> Figure 2 1 (1)

Claims (2)

[Claims] (1) A gas sensing element characterized in that a gas sensing element containing cadmium oxide (CaO) as a main component and containing 0.005 to 10% by weight of sulfate ions is used as a gas sensing element. (2) Claim (1) characterized in that the sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by printing and firing a paste. Gas detection element described in section.