JPS58200153A - Gas detection element - Google Patents

Abstract

PURPOSE:To obtain a combustible gas detection element which is highly sensitive at a relatively low working temperature even with a high sensitivity to CH4 gas by use of a gas sensitive body mainly composed of SnO2 and containing SO4<--> within the specified range of proportion. CONSTITUTION:An SnO2 powder is mixed with an SnSO4 powder and after an organic binder is added thereto, the mixture is graded to particles of 100-200mum to make a powder. The powder is pressure-molded into a rectangular parallel piped and sintered in the air. Au or the like is evaporated on the surface of a sintered body to provide a pair of comb-shaped electrodes while a heat generating body made of Pt or the like is provided on the back thereof to obtain a combustible gas detection element. The sintered body is produced from a mixture containing SO<--> 0.05-10wt% by addition of SnSO4. The presence of SO4<--> provides an element which is sufficiently sensitive within a range several tens - a fraction of the lower limit concentration of explosion even with a high sensitivity to CH4 gas.

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 (LIEL) and a higher specific gravity than air.
Because they tend to get stuck in rooms, accidents continue to occur, resulting in numerous casualties every year.

In recent years, gas detection elements using metal oxides such as stannic oxide (5nO2) and gamma-type ferric oxide (γ-y・203) have been put into practical use, and are being applied to gas leak alarms. ing. And even if a situation such as a gas leak occurs, L
By the time the IEL is reached, the presence of propane gas can be quickly detected and an explosion can be prevented.

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, 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 since most of them use precious metal catalysts as sensitizers in the J8 reaction material, there is a problem of catalyst poisoning by three different gases. , they have problems such as low selectivity for 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, a noble metal catalyst has traditionally been added to the sensitive material.

Alternatively, efforts have been made to operate the sensitive body at a considerably high temperature. In contrast, the present invention realizes an element with a high degree of methane resistance even at a relatively low operating temperature without adding any noble metal catalyst.

The present invention was discovered in the course of research into a gas sensing element using stannic oxide (Sn02) as a gas reactant.

In other words, by containing sulfate ions (804--) in 5n02, which is the base material of the gas sensitive material, the gas sensitivity characteristics are dramatically improved, and the sensitivity to the aforementioned methane gas is sufficiently high for practical use. This was done by discovering what could be achieved.

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

[Example 1] Various amounts of stannous sulfate (SnO2) were added as an additive to a commercially available sulfuric oxide (SnO2) reagent to contain sulfate ions.
SnSO4) powders were added and mixed, and an organic binder was further added thereto to produce several powders sized into particles of 100 to 200 μm in size. The thus obtained powders containing different amounts of stannous sulfate were press-molded into a rectangular parallelepiped shape and fired in air at a temperature of aooooc for 1 hour. MuU was vapor 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 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 4oo0C and its gas sensitivity characteristics were measured.

The resistance value in air (Rg) is measured in a measuring container with a volume of 601 in which dry air is slowly stirred to the extent that no turbulence occurs.
) contains methane (CH4) with a purity of over 99% in this container.
, propane ((3Hs).

Isobutane (i-C4H10) and hydrogen (H2) gases are introduced at a volume ratio of 10 ppm/sec,
Each measurement was made when the concentration reached 0.1% by volume. The gas concentration to be measured was chosen to be 0.1% by volume because the detection concentration that is practically required for a gas detection element is from several tenths to several tenths of the lower explosive limit concentration (LiEL) of the gas. 1 and the LEL of each of the above gases is about 2% by volume to 6% by volume.

Also, sulfate ion (804-'-) contained in the gas sensitive material
The presence of the compound was confirmed by infrared absorption spectrum, and the amount contained was determined from the TG-DT curve and fluorescent X-ray analysis. Various Fi! The sensitive characteristics of a gas sensitive material containing the amount of te ions are shown. In addition, Figure 1 (IL) and (b) express this in terms of sensitivity (Ra/1g); Figure 1 (IL) shows methane and propane, and Figure 1 (b) shows isobutane and 5 characteristics for hydrogen are shown.

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

Note that the sulfate ion (804
--) The reason for limiting the amount of 0.005 to 10.0% by weight is that if the amount is less than 0.005 to 10.0% by weight, as shown in the table above, there is no effect of improving the gas sensitivity characteristics; This is because if it exceeds 10.0% by weight, it is impractical in terms of stability of properties or mechanical strength. Items marked with * in the table above correspond to this, and are listed as comparative examples in Table 5. As shown in No. 001 in the table, ordinary 5nOs+ itself does not have sufficient gas sensitivity characteristics and cannot be put to practical use as it is.

However, by containing sulfate ions, as shown in the above table and FIG. 1, a large sensitivity to various flammable gases including methane appears.

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 tin oxide used in this example, which is nearly completely crystallized, shows extremely high activity due to the presence of sulfate ions, resulting in a very large amount of gas! This results in a 1'8 response characteristic.

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 was 400°C.
Although only the case of Another important effect of the present invention, which is that selectivity can be significantly controlled, will be specifically explained in Example 2 shown below. In Example 2, ethanol was used as the test gas instead of 0 2 isobutane gas, which has almost the same characteristics as propane gas.

U Example 2] A commercially available reagent of tin oxide (SnOz) and an aqueous solution of stannous sulfate (8nSO4) adjusted to various concentrations as an additive containing sulfate ions were prepared.

Next, 1 og of the above SnO2 reagent was weighed out, and the above stannous 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 a size of 60 to 100 microns, and triethanolamine was added to form a paste.

On the other hand, the length and width of the substrate for the gas detection element are 6 mm each.
An alumina substrate having a thickness of o, smm was prepared, and a pair of comb-shaped electrodes were formed by printing gold paste on the surface in a comb shape at intervals of o, ts mm and baking it. Then, on the back side of the alumina substrate, a commercially available ruthenium oxide glaze resistor was printed between the gold electrodes and baked to form a heater.

Next, the above-mentioned paste was printed on the surface of the substrate to a thickness of about 70 μm, and after air drying at room temperature, it was gradually heated to a temperature of 4oOOC and held at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film of stannic oxide (Sn02) containing sulfate ions. The thickness of this gas sensitive member was approximately 66μ. 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, the paste itself was slowly heated at 400°C in the same manner as described above. This was subjected to TG-DT and 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 thus obtained sensing element were measured in the same manner as in Example 1 except that the operating temperatures were set at two points, 350°G and 4600G. Figure 2 (IL
) to Figure 2(d) are temporal diagrams showing the relationship between sulfate ion content and sensitivity (Ra/1g) to various flammable gases; Figure 2(IL) is for methane, Figure 2(b) is for methane; is propane,
Figure 2 (0) is hydrogen, Figure 2 ((11 is ethanol), and as is clear from Figure 2, sulfate ion (so4-) is 0.0
By containing 05% by weight or more, 36° 0C, 46
It can be seen that the gas sensitivity characteristics are dramatically improved at any operating temperature of 0°G.

(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. (The data are not listed here.) Another important point is that gas selectivity varies greatly depending on the operating temperature. As an example, FIG. 3 shows the relationship between sensitivity and operating temperature when 0.5% by weight of sulfate ions are contained. As is clear from FIG. 3, at an operating temperature of 3600C, the sensitivity to ethanol is significantly greater than that of other gases, indicating that the selectivity to ethanol is extremely high. On the other hand, at an operating temperature of 460°G, the sensitivity to ethanol is very low, and the sensitivity to other methane, propane, and hydrogen is relatively very high. In other words, this element has the feature 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 it is possible to easily distinguish between ethanol and other gases by periodically changing the operating temperature or by using two elements with different operating temperatures. 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 tin oxide (SnO2) was used as a starting material, but the present invention is not intended to limit the starting material or manufacturing method in any way. Of course, it is also possible to add further additives to improve the properties.

As described above, the gas sensing element of the present invention has dramatically improved gas sensitivity characteristics due to the inclusion of sulfuric ions in the stannic oxide (SnO2), which is the base material of the gas sensing element. 4 can achieve extremely high sensitivity even for methane gas, which has been considered difficult to detect in trace amounts. 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 various gas disaster prevention fields that are becoming increasingly important.

[Brief explanation of the drawing]

Figure 1 (&), (b) is a characteristic diagram showing the relationship between sulfate ion content and sensitivity (Ra/1g) in one embodiment of the present invention, and Figure 2 (&), (b), ( C)
, (+1) is methane in another embodiment of the present invention,
A characteristic diagram showing the relationship between sulfate ion content and sensitivity (Ra/1g) for each flammable gas such as propane, hydrogen, and -[-tar/-tar, with operating temperature as a parameter. Figure 3 shows the same results. FIG. 3 is a characteristic diagram showing the operating temperature dependence of sensitivity (Ra/Rg) in an example. Name of agent: Patent attorney Toshio Nakao and 1 other person 1st
Figure ra Low liquid acid ion content (wt% 2 Figure 1 Illusion 2nd tl! tC Figure 2 (d)

Claims (2)

[Claims] (1) A gas sensing element characterized by using as a gas sensitive material a material mainly composed of stannic oxide (Sn02) and containing 0.006 to 10% by weight of sulfate ions. (2) The gas sensitive body is a sintered body obtained by pressure molding and firing, or a sintered film obtained by printing and firing a paste, according to claim (1). Gas detection element.