JPS628136B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing a gas sensitive material used in a gas sensing element for detecting combustible gases and the like. Conventional configurations and their problems In recent years, with the spread of gas appliances, accidents due to gas leaks have been occurring frequently, and various methods to prevent these accidents have been studied. 2. Description of the Related Art As one of the typical gas detection elements that have been used in the past, one using an n-type metal oxide semiconductor is known.
Since semiconductor type gas sensing elements are usually required to have a fast response speed, the gas sensitive body is used while being maintained at a high temperature in the atmosphere. Therefore, an oxide that is stable against an oxidizing atmosphere is selected as the gas sensitive material. Various oxides have been used as gas sensitive materials, but recently alpha-type ferric oxide, which has a corundum-type crystal structure that was thought to be hardly sensitive to gases, has been used. It has been discovered that (α-Fe ₂ O ₃ ) can exhibit excellent gas-sensitive characteristics, and studies are underway on gas sensing elements using this as a sensitive material. Gas sensing elements using this α-Fe ₂ O ₃ have remarkable gas sensitivity characteristics when the temperature of the element is in the range of 350 to 450°C, and the sensitivity (resistance value in normal air) is remarkable.
Ra and the resistance value Rg in the gas concentration to be detected), and the rate of change in the resistance value per unit gas concentration in the concentration range to be detected is large, so the gas concentration to be detected can be determined with a quantitative degree. It has the excellent feature that it can be easily detected as a change in resistance value. In general, in such a gas sensing element, it is necessary to heat the sensitive body efficiently with as little electric power as possible, so the sensitive body naturally becomes small. The same applies to the ceramic semiconductor type.
Therefore, in the case where the sensitive material is composed of two or more types of components, if the components are not uniformly contained in the sensitive material, it will cause variations between devices. In addition, since the element itself is directly exposed to the outside air and used under harsh conditions, it is likely to become unstable, especially over a long life when charged, so the gas sensitive material should have a microstructure that is as homogeneous as possible. There is a need.
For this reason, an aqueous solution method (coprecipitation method, homogeneous precipitation method, etc.) is an effective manufacturing method. This method has the advantage that not only is dispersion mixing excellent, but fine particle powder can be obtained, which leads to an increase in specific surface area, that is, high activation, and it is possible to obtain a material that can detect stable gases such as methane. be. However, even the aqueous solution method has various problems depending on the method. For example, in the case of coprecipitation method,
When a precipitant is added to a solution, even if the concentration is dilute and the solution is sufficiently stirred, the precipitant may temporarily become highly concentrated and accelerate the formation of crystal nuclei. The size of the precipitated particles may become non-uniform or coagulation may occur. This has resulted in increased variations between elements and accelerated deterioration of characteristics in energized life tests and the like. Purpose of the Invention The present invention eliminates the drawbacks of these conventional methods and aims to homogenize the gas sensor material by obtaining uniform precipitated particles without agglomeration, resulting in less variation between elements and a lifespan of electrification. The present invention aims to provide a gas sensing element with significantly improved characteristics. Structure of the Invention _The present invention was made as a result of a series of studies regarding various precipitants and _other manufacturing processes. By heating this, it was possible to prepare uniform and homogeneous particles by taking advantage of the gradual increase in pH due to the slow hydrolysis of urea. DESCRIPTION OF EXAMPLES The effects of the present invention will be described below using several examples while comparing with comparative examples. Comparative Example 1 80 g of commercially available ferric chloride (FeCl ₃ 6H ₂ O) and 40 g of ammonium sulfate [(NH ₄ ) ₂ SO ₄ ] were each
of water and stirred while maintaining the temperature at 80°C. Further, while maintaining the temperature at 80° C., an 8N ammonium hydroxide (NH ₄ OH) solution was added dropwise to this solution at a rate of 60 c.c./min until the hydrogen ion concentration of the solution reached 7. After the dropwise addition was completed, the temperature of the solution was maintained at 60° C. for 10 minutes, and the coprecipitate was filtered off by suction. The powder thus obtained was placed in a vacuum container and vacuum dried. After pulverizing the obtained dried material for 2 hours using a grinder, it is pulverized to 100 to 200 μm using an organic binder.
The particles were sized into particles with a size of m. Two platinum wires were embedded in this powder, which was then pressure-molded into a cylindrical shape with a diameter of 2 mm and a height of 3 mm, and fired at 550° C. for 2 hours in air. The obtained porous sintered body was attached to a sensing element base, a coil-shaped heater was placed around the sintered body, and an explosion-proof stainless steel net was covered to obtain a sensing element. FIG. 1 shows the structure of a gas detection element. In the figure, 1 is a sintered body in which electrodes 3 and 4 made of two platinum wires are embedded. 2 is a heater for heating the sintered compact 1, and the heater pin 1
Electric power is supplied to the heater from heater frames 7 and 8 from heater frames 7 and 8. The resistance of the sintered body 1 is measured from the electrodes 3 and 4 through the frames 5 and 6 to the pins 9 and 10.
It is configured to be measured between Heater pins 11 and 12 and pins 9 and 10 are attached to the base 13
The stainless steel wire mesh 14 is attached to the base. Regarding the sensing element obtained as above,
We investigated the gas sensitivity characteristics, the energized life at normal operating temperature (400°C), and the overload energized life at a temperature much higher than the normal operating temperature (600°C). To measure the gas sensitivity characteristics, a current is applied to the heater part of the sensing element in advance, and the temperature of the sensing element is set to 400°C.
The sintered sensitive material was adjusted to have a temperature of 0.degree. C., and then inserted into a measuring box of known volume. A test gas was injected into the measuring box using a syringe, and the resistance value of the sintered sensitive material was measured. The normal energized life is determined by constantly applying current to the heater part of the sensing element to maintain the temperature of the sensing element at 400°C, and measuring the gas sensitivity characteristics using the method described above over time.Methane (CH ₄ ) and hydrogen ( H ₂ ) resistance change rate over time, i.e. {initial resistance Rg
(5000ppm) - Resistance Rg after t hours of energization
(5000 ppm)}/The value ΔR/R (%) of the initial resistance Rg (5000 ppm) was determined. For overload life, set the temperature of the sensitive body to normal operating temperature (400℃).
The temperature was maintained at 600°C, which is much higher than that of the current temperature, and the temperature was measured using the method described above over time, and the rate of change in resistance over time was determined using the same method as the normal energized lifespan. The initial gas sensitivity characteristics and the standard deviation of 50 sensing elements are shown in the sample No. A column of the table below _.
Figure 2 shows the change in resistance change rate for H ₂
Fig. 3 shows the change in resistance change rate for CH ₄ during the overload life, Fig. 4 shows the change in resistance for CH 4, and Fig. 5 shows the change for H ₂ . The experimental results for Comparative Example 1 are indicated by sample No. A in each figure. What can be seen from Table 1 and Figures 2, 3, 4, and 5 is that during normal energization life and overload energization life, the resistance for CH ₄ is on the positive side, that is, it tends to deteriorate. , for H ₂ gas, there is a significant change in the negative side, that is, a tendency towards sensitization, so it can be seen that it is undesirable to attach it to an actual alarm device and use it as is, as this will directly lead to a change in the detected concentration. It can also be seen that the variation among the 50 sensing elements is quite large. In addition, in the table, the average value, S indicates the standard deviation,
Ra is the resistance value in normal air. Comparative Example 2 Commercially available ferric ammonium sulfate [Fe ₂ (SO ₄ ) ₃
(NH ₄ ) ₂ SO ₄・24H ₂ O] Dissolve 96 g in 1 part of water,
Hereinafter, in the same manner as in Comparative Example 1, a sensing element was prepared from the formation of a precipitate and its gas sensitivity characteristics were evaluated.
The initial gas sensitivity characteristics are shown in the sample No. B column of the table below.
Further, the changes in the resistance change rate during the energized life and the overload energized life are shown in FIGS. 2 to 5 for sample No. B, respectively. What can be seen from the table and Figures 2 to 5 is that during normal energization life and overload energization life, CH ₄
The resistance is positive for H ₂
Since there is a significant change in the negative side, that is, a tendency towards sensitization, it can be seen that it is not preferable to attach this to an actual alarm device and use it as it is, as this will directly lead to a change in the detected concentration. In addition, the detection element 50
It can be seen that the variation among the pieces is also quite large. Example 1 Commercially available ferric chloride 80g and ammonium sulfate 40g
and 60 g of urea [(NH ₂ ) ₂ CO] were dissolved in 1 part of water and stirred while heating on a hot plate.
Heating was carried out at 95°C or higher for 3 hours. The hydrogen ion concentration (PH) at this time was 7.0. Comparative example 1 below
A detection element was prepared from suction filtration and its gas sensitivity characteristics were evaluated using the same method as described above. The initial gas sensitivity characteristics are shown in the sample No. C column of the table below, and the changes in resistance change rate during the energized life and overload energized life are shown in the second column.
They are shown as sample No. C in Figs. From the above results, it can be seen that the characteristics are extremely stable compared to the comparative example in the normal energization life and the overload energization life. It can also be seen that the variation among the 50 sensing elements has been significantly reduced. Example 2 Commercially available ferric ammonium sulfate 96g and urea 60g
was dissolved in 1 water and stirred while heating on a hot plate. Thereafter, treatment was carried out in the same manner as in Example 1, including suction filtration, fabrication of a detection element, and evaluation of gas sensitivity characteristics. The initial gas sensitivity characteristics are shown in the sample No. D column in the table below, and the changes in resistance change rate during the life test are shown in the sample No. D column in Figures 2 to 5.
Each is indicated by D. From the above results, it can be seen that the characteristics are extremely stable compared to the comparative example in the normal energization life and the overload energization life. In addition, the detection element 50
It can be seen that the variation among the pieces has also been significantly reduced.

[Table] In each of the above embodiments, the case of an element using a sensitive body obtained by sintering a pressure-molded body was explained, but in the present invention, the sensitive body is formed by pressure-molding as described above. The present invention is not only effective in the case of a sintered body, but also when a sintered film obtained by making a paste of this sintered body raw material and applying it onto a substrate and baking it is used as a sensitive body. No effect is lost. Regarding the raw material salt, although ferric chloride was used in Comparative Example 1 and Example 1, it is not limited to this, and water-soluble iron salts (ferrous chloride, ferrous and ferric sulfate, etc.) All are valid. In addition, although Comparative Example 2 and Example 2 were explained using ferric ammonium sulfate, the present invention is not limited to this, and water-soluble ferrous sulfate salts (ferric sulfate, ferric sulfate, ferrous ammonium sulfate, etc.) All are valid even if used. Further, in the examples, the case was described in which firing was performed at 550°C, but this firing temperature is not particularly limited, and may be any firing temperature that provides a strength of the sintered body that can withstand practical use. Effects of the Invention As described in detail above, the production method of the present invention skillfully applies a precipitation-forming reaction using urea hydrolysis to the preparation of raw material powder for a gas sensitive material, and the resulting powder Because the particle shape and particle size are extremely uniform, the variation in gas sensitivity characteristics between each sensing element is extremely small, and we provide an element that can maintain extremely stable characteristics even during long-term high-temperature operation. It is something to do. By this,
It is possible to realize a compatible and highly reliable combustible gas detection element, and it is expected that it will make an extremely large contribution to the field of various gas disaster prevention technologies.

[Brief explanation of the drawing]

FIG. 1 is a front view showing an example of the structure of a combustible gas detection element according to the present invention, and FIGS. 2 and 3 show changes in the rate of change of the resistance value of the gas detection element with respect to its initial value over the life of energization. The figures shown in FIGS. 4 and 5 are diagrams showing the rate of change in the resistance value of the gas detection element during the life of overload application. 1... Sintered body, 2... Heater, 3, 4... Electrode, 5, 6, 7, 8... Frame, 9, 10...
Electrode pins, 11, 12... Heater pins, 13
...Base, 14...Stainless steel wire mesh.

Claims (1)

[Claims] 1. Obtaining a gas sensitive material by drying a precipitate obtained by adding urea [(NH ₂ ) ₂ CO] to an aqueous solution containing iron ions and at least sulfate ions as anions. A method for producing a gas sensitive material characterized by: 2. The method for producing a gas susceptor material according to claim 1, characterized in that the gas susceptor material is obtained by adding urea to an aqueous solution of iron sulfate salt and drying the precipitate. 3. The method for producing a gas sensitive material according to claim 1, wherein the aqueous solution is heated when obtaining the precipitate.