JPS628137B2 - - 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.
Up to now, various oxides have been used as gas sensitive materials. Among these, it has been discovered that zinc oxide (ZnO) can exhibit excellent gas-sensing characteristics, and studies are underway on gas sensing elements using this as a sensing material. This gas detection element using ZnO has remarkable gas sensitivity characteristics when the temperature of the element is in the range of 350 to 450℃, and the sensitivity (resistance value Ra in normal air and resistance value in the gas concentration to be detected) is Rg) and the rate of change in resistance per unit gas concentration in the concentration range to be detected is large.
It has the excellent feature of being able to detect the gas concentration to be detected as a change in resistance value with good quantification. 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 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 its 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 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 is possible to prepare uniform and homogeneous particles by taking advantage of the gradual increase in pH due to the slow hydrolysis of this 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 14 g of commercially available zinc chloride (ZnCl ₂ ) and 40 g of ammonium sulfate [(NH ₄ ) ₂ SO ₄ ] were each dissolved in 1 part of water and stirred while maintaining the temperature at 80°C. Further temperature
While maintaining the temperature at 80℃, add 8N ammonium hydroxide (NH ₄ OH) solution to this solution at a rate of 60c.c./min.
The solution was added dropwise until the hydrogen ion concentration (PH) of the solution reached 7.0. After the dropwise addition was completed, the temperature of the solution was maintained at 80° 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 body was adjusted to have a temperature of 0.degree. Normal energization 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 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
For gases, there is a significant change in the negative side, that is, a tendency towards sensitization, so it can be seen that it is not preferable 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 29g of commercially available zinc sulfate (ZnSO ₄ 7H ₂ O)
A sensing element was prepared from the formation of a precipitate and its gas sensitivity characteristics were evaluated in the same manner as in Comparative Example 1. The initial gas sensitivity characteristics are shown in the column for sample No. B in the table below, and the changes in resistance change rate during energization life and overload energization life are shown in Figures 2 to 5 for sample No. B, respectively. Indicated. 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 14 g of commercially available zinc chloride, 40 g of ammonium sulfate, 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 performed at 95°C or higher. The hydrogen ion concentration (PH) at the end of the test was 7.0. Thereafter, in the same manner as in Comparative Example 1, a detection element was prepared by suction filtration and its gas sensitivity characteristics were evaluated. The initial gas sensitivity characteristics are shown in the column for sample No. C in the table below, and the change in resistance change rate over the life of energization is shown for sample No. C in FIGS. 2 to 5. 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 29 g of commercially available zinc sulfate and 60 g of urea were dissolved in 1 part of water and stirred while heating on a hot plate. The following treatment was carried out in the same manner as in Example 1,
We created a sensing element using suction filtration and evaluated its gas sensitivity characteristics. The initial gas sensitivity characteristics are shown in the column for sample No. D in the table below, and the changes in resistance change rate in the life test are shown for sample No. D in FIGS. 2 to 5, respectively. 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 zinc chloride and zinc sulfate were used in the above embodiments, the present invention is not limited to these, and any water-soluble zinc salt (such as zinc nitrate or zinc bromide) is effective. 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. Because the particle shape and diameter of the particles are extremely uniform, the variation in gas sensitivity characteristics between each sensing element is extremely small, and the element can maintain extremely stable characteristics even during long-term high-temperature operation. This is what we provide. This makes it possible to realize a compatible and extremely reliable combustible gas detection element, which is expected to make an extremely large contribution to the field of various gas disaster prevention technologies.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway front view showing an example of the structure of a combustible gas detection element according to the present invention, FIG.
The figure shows the transition of the rate of change of the resistance value of the gas detection element with respect to the initial value during the energized life.
FIG. 5 is a diagram 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 zinc 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 sensitive material according to claim 1, wherein the aqueous solution is heated when obtaining the precipitate.