JPH027025B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a combustible gas detection element. In recent years, various research and developments regarding combustible gas sensing element materials 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, mainly in households, but also in various factories, which have become major social problems. In particular, propane gas has a low explosive limit (LEL) and has a higher specific gravity than air, so accidents involving propane gas tend to stagnate in rooms, resulting in numerous casualties every year. In recent years, combustible gas detection elements using metal group 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 to detect gas leaks. It is applied to utensils, etc. 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 mechan 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 that are sensitive to methane gas have already been developed, but because they use noble metal catalysts as sensitizers in the sensitive material, there are problems with catalyst poisoning by various gases, and the selectivity for methane gas is low. However, there are problems such as high dependence on the temperature and ambient humidity. Therefore,
At present, the characteristics are still insufficient for practical use. The present invention provides a gas detection element that has characteristics that can solve these problems and has a sensitivity that is sufficiently high for methane gas for practical use. 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. . However, the device according to the present invention realizes a device with high sensitivity to methane even at a relatively low operating temperature without adding any noble metal catalyst. Hereinafter, the present invention will be specifically described based on examples. Example 1 50 g of commercially available ferric sulfate (FeSO ₄ 7H ₂ O)
It was dissolved in pure water kept at 2°C and stirred well. In addition, commercially available stannic chloride (SnCl _{4.5H 2} O)
80g was dissolved in 1 pure water and stirred well. Then, this solution was poured into the ferric sulfate solution and stirred thoroughly again. At this time, the color of the solution became deep yellow. Next, 8N ammonium hydroxide (NH ₄ OH) solution was added to this solution while stirring at a rate of 60% per minute.
It was added dropwise at a rate of cc until the pH of the solution reached 7.
After the dropwise addition was completed, the solution temperature was maintained at 50°C for 10 minutes and then returned to room temperature. At this stage, a brownish black coprecipitate was obtained. This coprecipitate was taken out by suction filtration and dried at a temperature of 110° C. for 12 hours. The dried material was in the form of black-gray powder. this,
The powder was pulverized for 2 hours using a grinder to obtain a reactor raw material powder. After sizing this powder to 50 to 100 μm, polyethylene glycol was added to form a paste. On the other hand, as a substrate for the gas detection element, an alumina substrate with a length and width of 5 mm and a thickness of 0.5 mm was prepared, and gold paste was printed on the surface in a comb shape at intervals of 0.5 mm.
A pair of comb-shaped electrodes were formed by baking. and,
A commercially available ruthenium oxide glaze heater was printed on the back side 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 substrate to a thickness of about 70 μm, air-dried at room temperature, and then baked in normal air at a temperature of 650° C. for 1 hour. The paste was evaporated during this baking process, resulting in a sintered film with sufficient mechanical strength for practical use. The thickness of this gas sensitive material was approximately 40 μm. Next, a current was applied to the heater to maintain the temperature of the sensitive body (operating temperature) at 400°C, and the gas sensitivity characteristics were measured. The resistance value in air (R _a ) 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 in gas (R _g ) The sample gas with a purity of 99% or more is placed in this container as a volume ratio.
The gas sensitivity characteristics were investigated by flowing in at a rate of 10 ppm/sec and measuring when a certain gas concentration was reached. Methane (CH ₄ ), ethane (C ₂ H ₆ ), propane (C ₃ H ₈ ), isobutane (i-
C ₄ H ₁₀ ), hydrogen (H ₂ ) and ethyl alcohol (C ₂ H ₅ OH) at 0.05% each.
Measurements were made at gas concentrations of 0.2% and 1.0%. R _a is 860kΩ
The dependence of R _g on the concentration of each gas was as shown in Table 1. As can be seen from this table, while it has high sensitivity to methane, ethane, propane, isobutane, and hydrogen gases, it has very low sensitivity to ethyl alcohol. In this way, the device manufactured by the manufacturing method of the present invention can be used not only for ethane, hydrogen, and isobutane gas, which are used as components of general city gas (mixed gas), but also for propane, which is the main component of LP gas. ,
It also has great sensitivity to isobutane gas. A major feature of the present invention is that it has practically sufficient sensitivity for methane gas, the main component of natural gas, which has conventionally been difficult to detect with high sensitivity using semiconductor gas detection elements that do not use precious metal catalysts. It is to have the following. Furthermore, another noteworthy point of the present invention is that it has little dependence on ambient humidity. By the way, this sensing element was placed in an ambient temperature atmosphere of 40℃, and the surrounding relative humidity was varied in the range of 35% to 95%.
R _g (0.2) at a concentration of 0.2% for each gas was measured.
In the table, β _H indicates the ratio of R _g (0.2) at 35% and 95% relative humidity, and indicates the degree of dependence on ambient humidity. As is clear from the table, although there are some differences depending on the type of gas being tested, it can be seen that the dependence on ambient humidity is very small. This is because the effectiveness of the present invention is significant, considering that β _H of conventional semiconductor type catalysts using noble metal catalysts is about 1.25 or more.

[Table] Example 2 Commercially available ferric sulfate (FeSO ₄ 7H ₂ O),
An aqueous solution containing iron ions, tin ions, and zinc ions was prepared in the same manner as in Example 1 using stannic chloride (SnCl ₄ .5H ₂ O) and zinc sulfate (ZnSO ₄ .7H ₂ O). By changing the mixing ratio of these aqueous solutions, various mixed solutions with different concentrations were prepared. A coprecipitate was obtained using NH ₄ OH in each of these solutions. Then, these were dried and pulverized to obtain a raw material powder for the sensitive material. The obtained fine powder raw material is mixed with an organic binder to
The particles were sized to a size of 200 μm. These various powders were each shaped into a rectangular parallelepiped (2 x 1.5 x 3 mm ³ ).
It was pressure molded at a pressure of 400 kg/cm ² and fired in air at a temperature of 800° C. for 1 hour. Au on the surface of this sintered body
was vapor-deposited to form a pair of comb-shaped electrodes, and a platinum heating element was attached to the back side of the comb-shaped electrodes with an inorganic adhesive to serve as a heater, thereby producing a sensing element. A current is passed through this heating element, and the current value is adjusted to adjust the operating temperature of the element to 400%.
The gas sensitivity characteristics were measured while maintaining the temperature at ℃. The results are shown in Table 2. As in the case of Example 1, the measurement gases used were methane, ethane, propane, isobutane, hydrogen, and ethyl alcohol. R _g shows only the value when the gas concentration is 0.2%. Regarding β _H , among these measurement gases, β _H
The values are shown for isobutane gas, which produces the largest amount of gas. As is clear from Table 2, the content of Sn and Zn is 0.5 in total in terms of SnO ₂ and ZnO, respectively.
If the amount is less than mol%, no effect of the addition will be observed. On the other hand, if it exceeds 70 mol%, some materials may exhibit gas sensitivity characteristics to some extent, but the resistance value may become abnormally low, or the value may vary greatly or change over time, making it difficult to put it to practical use. It becomes something you don't get. For this reason, SnO ₂ ,
The total blend amount when converted to ZnO is 0.5 to 70 mol%
A range of is desirable.

【table】

[Comparative example]

Prepare commercially available reagent grade ferric oxide (Fe ₂ O ₃ ), tin oxide (SnO ₂ ), and zinc oxide (ZnO), and prepare Fe ₂ O ₃ :SnO ₂ :ZnO=80 mol%: 10 mol %:
They were mixed at 10 mol % in a total amount of 100 g, and wet-mixed for 5 hours using a ball mill. and,
This was dried in air at a temperature of 200°C for 20 hours.
This powder was sized, pressure-molded, and fired in the same manner as in Example 2 to prepare a gas sensing element, and its gas sensitivity characteristics were examined. At an operating temperature of 400° C., R _a was 785 kΩ, which was not significantly different from the value of Example 2, which was 696 kΩ. However, the resistance values at concentrations of methane, ethane, propane, isobutane, and hydrogen at 0.2% each are 656 kΩ, 640 kΩ, 630 kΩ, respectively.
They were 621 kΩ and 683 kΩ, and had very low sensitivity to both gases. That is, even devices manufactured using the same composition have significant differences in gas sensitivity characteristics due to differences in the raw material powder of the sensor. This difference clearly represents the effect of the present invention. As described above, the combustible gas detection element produced by the manufacturing method of the present invention can be used for other combustible gases, including methane gas, for which it has been difficult to achieve high sensitivity without using a precious metal catalyst. for,
It has great sensitivity. Furthermore, the sensitivity to alcohol, which is said to be the main cause of malfunctions in practice, is much lower than the sensitivity to flammable gases mentioned above. In other words, it can be said that it has high selectivity. On the other hand, as mentioned earlier, the dependence on water vapor (humidity), which is another cause of malfunction, is extremely small. As described above, the gas detection element manufactured by the manufacturing method of the present invention can be used at present, as the demand for natural gas is increasing.
It is expected that this product will make a significant contribution to a wide range of fields such as gas leak alarms, various kitchen units, and gas disaster prevention systems. In the examples of the present invention, FeSO ₄ .7H ₂ O and ZnSO ₄ .7H ₂ O are used as starting materials for preparing aqueous solutions.
and SnCl ₄ .5H ₂ O were used, but the present invention is not particularly limited to these, and it goes without saying that any aqueous solution containing the respective metal ions may be used. Of course, in order to further improve the properties, metal ions other than tin ions and zinc ions may also be added.

Claims (1)

[Claims] 1. A solution containing iron ions and at least one of tin ions and zinc ions is prepared, the solution is co-precipitated, and the resulting fine powder is used as a raw material to produce a sensitizer. 1. A method for manufacturing a combustible gas detection element, comprising: 2. A sensitive body consisting of a sintered body or a sintered film is obtained by press-molding and firing a fine powder raw material, or by printing and baking the paste, thereby forming a sensitive body comprising a sintered body or a sintered film. A method for manufacturing a combustible gas detection element. 3 The metal ion of at least one of tin or zinc ion was prepared so that the total amount contained in the sensitizer was 0.5 to 70 mol% in terms of stannic oxide (SnO ₂ ) or zinc oxide (ZnO). Claim 1 characterized in that a solution is used.
A method for manufacturing a combustible gas detection element as described in 2.