JPS618654A - Production of gas detecting element - Google Patents

Abstract

PURPOSE:To decrease the erroneous announcement of gas leakage and the missing in announcement by exposing preliminarily a gas detecting element to an SO2 atmosphere (SO2 aging) to change the sensitivity of the gas detecting element. CONSTITUTION:The semiconductor type gas detecting element is a metallic oxide semiconductor element consisting of SnO2, FeO3, ZnO, etc. Such detecting element is exposed to the SO2 atm. of a suitable concn. in an ordinary using state for suitable time to change the sensitivity characteristic of the element to a specified state in short time so that the succeeding change with lapse of time is decreased. The concn. of the SO2 in the stage of aging is preferably larger than the concn. of the SO2 in the atmosphere to which the gas detecting element is exposed. On the other hand, the change in the sensitivity characteristic of the detecting element by SO2 aging is larger with gaseous hydrogen than with gaseous methane. The detecting element subjected to the SO2 aging is preferably coated with a filter consisting of active alumina, etc. The setting region of the alarming concn. is widely taken in the same way as prior to the SO2 aging if the element is manufactured in the above-mentioned manner.

Description

[Detailed description of the invention] 〔Technical field〕 The present invention relates to a method for manufacturing a gas sensing element.

[Background technology]

Many of the current simple gas alarms have a 5n0
A semiconductor type using a metal oxide semiconductor element such as 2, F132'03, or ZnO, and a catalytic combustion type using an element in which a combustion catalyst such as Pd or Pt is supported on an Al2O2°SiO2 carrier are used. The semiconductor type uses the property that the conductivity of the element changes due to the adsorption of flammable gas, while the catalytic combustion type uses the element temperature to change due to heat generated when flammable gas comes into contact with the element and burns. However, it takes advantage of the characteristic that the conductivity of the element changes.

The sensing targets of the gas detection element are mainly L, P, and G.

(liquefied petroleum gas; main components propane, butane, etc.) and city gas (hydrogen gas, methane gas, -carbon oxide gas, others, P, G, etc.). However, due to its operating principle, gas detection elements often sense flammable gases, organic solvents, and the like other than the above-mentioned components, causing malfunctions of gas leak alarms.

In fact, 1 edible oil is a seasoning used in home kitchens.

When cooking fish, meat, vegetables, etc., alcohol, smoke (gas-sensitive components in smoke), etc. that are temporarily generated can cause a gas leak alarm to give a false alarm.

Furthermore, if these gas leak alarms are used in places with poor ventilation, such as underground cafeterias, where the amount of cooking and cooking time is much greater than in a typical home, there is a large amount of various gases generated during cooking. , as well as the above misinformation,
Its frequency tends to increase over time. As an example of the cause, Fig. 1 shows the alarm concentration (M) for each of methane gas, hydrogen gas, and ethanol gas of a semiconductor gas leak alarm in an underground dining area.

Denoted by H and A. ) shows the change over time. As can be seen from the graph in FIG. 1, the alarm concentration of the gas leak alarm is decreasing for each gas component. That is, the sensitivity of the gas detection element of the gas leak alarm to each gas component has increased.

The cause of the increase or decrease in sensitivity of a gas detection element over time as described above has not been known in the past.

However, semiconductor-type gas detection elements used in harsh atmospheres such as underground cafeterias tend to lose their sensitivity to target gases (methane, butane, hydrogen, etc.) and non-target gases (ethanol, etc.) over time. On the other hand, with catalytic combustion type products, the target gas (
Considering that the sensitivity to gases (mainly methane) will decrease and the alarm will not be issued in the event of a gas leak, a false alarm will occur.In order to selectively detect only the target gas, the gas that reaches the gas detection element is first filtered through a gas discrimination filter. A method was developed as seen in Japanese Patent Application Laid-open No. 129596/1983, in which non-target gases are removed by passing through the gas, and only target gases are sent to the gas detection element. In this adsorption removal method, zeolite, diatomaceous earth, silica gel, activated alumina, activated carbon, etc. are used as the adsorbent, and in the adsorption reaction and combustion removal method, platinum, palladium, rhodium, etc. are used.

The above filter method is effective in reducing alarms (false alarms) regarding ethanol and gas-sensitive components in smoke that are temporarily generated during cooking. However, this method is not a sufficient countermeasure because it does not specifically understand what causes the increase or decrease in sensitivity of the gas detection element over time. The purpose of the present invention is to provide a method for manufacturing a gas detection element that significantly reduces false alarms and false alarms that frequently occur in underground cafeterias, etc., as well as false alarms caused by ethanol, etc., which rarely occur in ordinary homes.

[Disclosure of the invention]

In order to achieve the above object, the inventors first conducted an atmosphere analysis of underground malls in order to understand the substances that cause the sensitivity of gas detection elements to increase or decrease over time in underground malls and the like. As a result, it was determined that sulfur dioxide gas (hereinafter referred to as S02) is the main component that exists in larger amounts and for longer periods of time than in the atmosphere of a typical home, and that irreversibly affects the gas detection element. Furthermore, it was found that high temperature and high humidity are involved in the effects on the gas sensing element of 302. Of course, the underground atmosphere contains fuel gas components such as methane gas, hydrogen gas, -carbon oxide gas, carbon dioxide gas and nitrogen oxides (NOx,
), ethanol and aldehydes generated during cooking, but these did not cause irreversible changes to the gas detection element.

Based on these facts, the inventors proposed that if the sensitivity of the gas detection element was changed by exposing it to an S02 atmosphere in advance (hereinafter referred to as S02 aging), and then used in an atmosphere such as an underground mall, , then 80
I thought that the change in sensitivity caused by 2 could be so small that it could be ignored.

To confirm this, the following experiment was conducted.

A gas detection element placed in a test chamber was exposed to a normal air atmosphere for 7 to 10 days with a predetermined circuit voltage and heater voltage applied. Resistance value Ra and methane gas concentration are both 0.
．． 05. 0.15. The element resistance value Rm when 0.45vo1% and the hydrogen gas concentration are each 0.0
5. 0.15. The element resistance value Rh at 0.45vo1% was measured. Next, in the test tank, S0
2 Gas with a concentration of 5 ppm was continuously flowed for 66 hours, and 1
The 802nd aging was performed. Thereafter, the element resistance value of the gas detection element for each of the above gases was measured. After the first 802 aging, the gas sensing element was exposed to a normal air atmosphere for 1 to 3 days while being heated with electricity, and then exposed to an S02 atmosphere again. At this time, S0
2 concentration was 15 ppm and duration was 70 hours. Even after this second SO2 aging, the element resistance values of the gas sensing element for each of the above gases were measured. Furthermore, the gas sensing element was exposed to a normal air atmosphere for 4 days while being heated with electricity, and during that time, the element resistance value of the gas sensing element was measured in the same manner as before. These results are shown in Figure 2a. In addition, the SO2 concentration during the first SO2 aging was changed to 15 ppm + 20 ppm, and 5
The same measurements as for ppI1) were performed. - The results are shown in Figures 2b and 2C, respectively. In addition, in these figures, the three Rm and Rh indicate that the methane gas concentration is 0.05, 0.15, and 0.45y, respectively from the top.
o1%, hydrogen gas concentration 0.05, 0.15, -0.4
The value is shown at 5vo1%, and the reason why Rh is shifted from Rm in the horizontal axis direction is not due to a shift in the measurement time point, but to make it easier to express the graph. The respective regions and days shown indicate the period during which the gas sensing element was exposed to a normal air atmosphere (A) and an SO2 atmosphere (B), respectively.

As shown in Figure 2a, the first SO2 aging (S
O25ppm), Rm and Rh are reduced.

That is, the sensitivity has become more sensitive to each gas. The second S02 aging (50215 ppm) has further sharpened the sensitivity, but its effect has decreased slightly.

As shown in Figure 2b, the first SO2 aging (3
0215 ppm), Rm and Rh decreased, and the sensitivity became more sensitive. Second SO2 aging (SO2
15 ppm), the effect is hardly noticeable.

As shown in Figure 2C, the first S02 aging (S
2. Rm and Rh have decreased, and sensitivity has become more sensitive. Second SO2 aging (SO2
220 ppm), there is no effect at all.

That is, even if the second 802 aging is performed at a concentration of SO2 that is approximately the same or lower than the SO2 concentration during the first SO2 aging, the sensitivity characteristics of the gas sensing element do not change and remain stable. The 502 concentration in the underground shopping mall atmosphere is 0. Since the O concentration is about 7 ppm, if the gas sensing element is exposed to a 302 atmosphere with a concentration of 1 ppn+ or more, the sensitivity characteristics of the element will converge to a stable region.
Sensitivity increase due to S02 in an underground shopping mall atmosphere can be prevented.

Based on the above findings, the present invention provides a gas detection device characterized in that a semiconductor type gas detection element is exposed to a sulfur dioxide gas (S02) atmosphere while being heated with electricity to change the sensitivity of the gas detection element to a target gas. The gist is the manufacturing method of the device.

The manufacturing method of this invention will be described in detail below.

The semiconductor type gas detection element used in this invention is 5n0
2, Fe203, ZnO, etc., are metal oxide semiconductor elements obtained according to normal manufacturing methods. By exposing the aforementioned semiconductor type gas detection element to an SO2 atmosphere with an appropriate SO2 concentration for an appropriate period of time under normal operating conditions, that is, under electrically heated conditions, the sensitivity characteristics of the element are changed to a constant state in a short period of time, and then Significantly reduces changes over time. The SO2 concentration during this SO2 aging is preferably higher than the SO2 concentration in the atmosphere to which the gas sensing element is exposed afterwards.

By the way, 1. As seen in FIGS. 2a to 20, changes in the sensitivity characteristics of the gas detection element due to SO2 aging are larger for hydrogen gas than for methane gas. Which phenomena are disadvantageous for city gas alarms. Set the alarm concentration to 1 of each LEL for methane gas and hydrogen gas.
This is because, although the concentration must be set within the range of /100 to 1/4, this setting range becomes smaller due to increased sensitivity to hydrogen gas. If the setting range of the alarm concentration becomes small, during actual use, it will not be possible to fully absorb fluctuations in sensitivity characteristics over time, and false alarms or missed alarms will likely occur.

In such a case, it is preferable to cover the SO2 aged gas sensing element with a filter such as activated alumina. That is, the filter reduces the sensitivity of the gas detection element to hydrogen gas. This is because the passing speed of hydrogen gas is slower than that of methane gas due to the filter, and the hydrogen gas that has passed through the filter is not combusted on the surface of the gas detection element.
02→2H20) and becomes water vapor, which passes through the filter at a low speed, which suppresses the reaction, which is thought to lead to a decrease in sensitivity to hydrogen gas. No combustion of methane gas occurs by the gas detection element.

In this way, sensitivity to hydrogen gas is reduced and S
The increase in sensitivity due to SO2 aging is offset, and the setting range for the alarm concentration becomes wider than before SO2O aging. This filter is used to adjust the hydrogen gas sensitivity of the gas detection element, so any material such as activated alumina or activated carbon can be used.

Examples are shown below.

(Example 1) A semiconductor type gas detection element was placed in a test chamber, and was placed in a normal air atmosphere for 7 to 70 minutes with predetermined circuit voltage and heater voltage applied.
After exposure for 10 days, SO2 aging was performed by continuously flowing gas with an SO2 concentration of 5 ppm into the test tank for 66 hours. After that, cover this gas detection element with an activated alumina filter? The device was shaped into a device as shown in FIG. 3, and exposed to a normal air atmosphere while being heated with electricity. In the figure, 1 is a semiconductor gas detection element, 2 is a coil heater, 3 is an explosion-proof net, and 4 is an active alumina filter.

(Example 2) After subjecting a semiconductor type gas detection element to SO'2 aging in the same manner as in Example 1, the element was shaped as shown in Fig. 4 without being covered with a filter, and placed in a normal air atmosphere under electrical heating. Exposed. In the figure, 1 is a semiconductor type gas detection element, 2 is a coil heater, and 3 is an explosion-proof net.

(Example 3) After a semiconductor type gas sensing element was subjected to SO2 aging in the same manner as in Example 1, it was covered with an activated carbon filter and exposed to a normal air atmosphere while being heated with electricity.

(Example 4) A semiconductor type gas detection element was subjected to SO2 aging in the same manner as in Example 2, and then exposed to a normal air atmosphere while being heated with electricity without being covered with a filter.

Regarding the gas detection elements of Examples 1 to 4, methane gas (concentrations were Olo 5 and Olo 0, respectively) before and after SO2 aging.
．． 15. Element resistance value (Rm
), hydrogen gas (concentrations are 0.05° and 0.15, respectively)
The element resistance value (Rh) with respect to 0.45vo1%) and the element resistance value (Ra) in air were measured. The results are shown in Figures 5 to 8, respectively. In the figure, the three Rm and Rh indicate that the methane gas concentration is 0 from the top.
．． 05. 0.15. The values are shown when the hydrogen gas concentration is 0°45vo1% and the hydrogen gas concentration is 0.05.0.15.0°45vo1%.<'Rh has been shifted from Rm to make it easier to express. Regions 1 and 1, which are distinguished by broken lines, are periods in which the gas sensing element was exposed to a normal air atmosphere (1) and an S02 atmosphere (1), respectively. Indicates the period of exposure to normal air atmosphere.

As shown in FIGS. 5 to 8, when the gas detection element is subjected to S02 aging, Rm and Rh decrease in a short time. Also, by comparing Figures 5 and 7 with Figures 6 and 8, if the gas detection element is covered with a filter after So2 aging, the difference between Rm and Rh remains almost the same after aging. . That is, if the gas detection element is subjected to S02 aging and then covered with a filter, the alarm concentration setting range can be widened.

The instruments used in Examples 1 to 4 above are as follows.

Gas detection element: Gas element for city gas, manufactured by Matsushita Electronics Parts, circuit voltage Vc = 5 VDC, heater voltage Vh = 3.69 DC Test tank: Tabletop gas corrosion tester set GLP
-91 type, ■Activated alumina manufactured by Yamazaki Seiki Laboratory...5AP-1) (MD-050),
Activated carbon manufactured by Sumitomo Aluminum Smelting @ Microlite A prototype, manufactured by Kanebo [Effects of the invention] The manufacturing method of this invention changes the sensitivity in advance by exposing a semiconductor type gas detection element to an 802 atmosphere while heating it with electricity. This is to bring it into a stable state. 'For this reason, if a gas detection element manufactured by this method is used, the sensitivity will not change due to the surrounding 802, and false alarms due to increased sensitivity can be prevented. In addition, if the element is covered with a filter after S02 aging, there will be almost no difference in sensitivity change to methane gas and hydrogen gas after aging, so the LEL of methane gas and hydrogen gas will be 1/100.
When setting the alarm level in the concentration range of ~1/4, the alarm concentration can be set over a wide range. Using this, it is possible to create a gas leak alarm without false alarms or missed alarms.

[Brief explanation of drawings]

Figure 1 is a graph showing changes in alarm concentration over time;
Figure 20 is a graph showing the change in element resistance before and after SO2 aging, Figure 3 is a diagram showing the shape of a semiconductor type gas sensing element with a filter, and Figure 4 is a graph showing the shape of a semiconductor type gas sensing element without a filter. The figures shown in Figures 5 and 7 are So2o-
Graphs showing changes in element resistance of a gas sensing element with a filter attached after aging; FIGS. 6 and 8 are graphs showing changes in element resistance of a gas sensing element without a filter attached after S02 aging. 1...Semiconductor type gas detection element, 4...Activated alumina filter Representative Patent Attorney Takehiko Matsumoto Figure 1! ! I! :Past days/[d] Number of days elapsed/[d] Figure 2a Elapsed Japan/Europe/[d] Number of elapsed days/[d] Figure 2b Elapsed Japan/Europe/[d] Number of elapsed days/[d] Figure 2c Number of elapsed days , / [d] Fig. 3 Fig. 4 Fig. 5 Number of elapsed days/[d] Fig. 6 Number of elapsed days, /C (j) Fig. 7 Number of elapsed days/[d] Fig. 8 Number of elapsed days/[d]

Claims (5)

[Claims] (1) A method for manufacturing a gas sensing element, which comprises exposing a semiconductor type gas sensing element to a sulfur dioxide gas (SO_2) atmosphere while heating it with electricity to change the sensitivity of the gas sensing element to a target gas. (2) The target gas is methane gas and hydrogen gas,
2. The method of manufacturing a gas detection element according to claim 1, wherein the sensitivity of the gas detection element is varied to such an extent that an alarm level can be set within a range of 1/100 to 1/4 of the lower explosive limit of each gas. (3) The method for manufacturing a gas sensing element according to claim 1 or 2, wherein the gas sensing element is covered with a porous filter after being exposed to a sulfur dioxide gas atmosphere. (4) The method for manufacturing a gas sensing element according to claim 3, wherein the filter material is activated alumina. (5) The method for manufacturing a gas sensing element according to claim 3, wherein the filter material is activated carbon.