JPH0230459B2 - GASUKENCHISOSHI - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a gas detection element using a metal oxide semiconductor used to detect combustible gas. Configuration and Problems of Conventional Examples In recent years, various research and development efforts have been active regarding materials for sensing elements for flammable gases. This is strongly attributable to the fact that explosion accidents caused by flammable gases and poisoning accidents caused by toxic gases occur frequently, mainly in households and in various factories, and have become a major social problem. In particular, propane gas has a lower explosive limit (LEL).
Because it has a lower specific gravity and a higher specific gravity than air, it tends to stagnate in rooms, causing many accidents and causing many casualties every year. In recent years, gas detection elements using metal 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 in gas leak alarms, etc. It is applied. 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 methane gas, has come to be used for general household use and is gradually becoming popular. Therefore, the demand for gas detection elements that can sensitively detect methane gas, which is the main component of LNG, is increasing. Of course, gas detection elements sensitive to methane gas have already been developed, but most of them use noble metal catalysts as sensitizers in the sensitive material.
Problems include catalyst poisoning by various gases, low selectivity to methane gas, and large changes in characteristics over time. For example, since methane gas itself is a very stable gas, a sensing element with sufficient sensitivity must be extremely active. Efforts have been made to use a noble metal catalyst added to the susceptor material, or to operate the susceptor at a considerably high temperature, for example, 450° C. or higher. However, the current situation is that the properties are still insufficient for practical use. OBJECTS OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to realize a gas detection element with high sensitivity to methane even at relatively low operating temperatures without adding any noble metal catalyst. Structure of the Invention The present invention is a gas sensing element using cadmium oxide (CdO) as a gas sensitive material, and is currently investigating the effects of various anions contained therein on the gas sensitivity characteristics and the effects of additives. This is what was found in. Specifically, the present invention provides that among several anions, sulfate ions in particular significantly suppress grain growth during sintering of the metal oxide of the base material, and chemically enhance the surface of the grains. Tetravalent metals such as Sn, Zr, or Ti, which have the effect of activating and significantly increasing the gas adsorption capacity, further enhance the effect of sulfate ions, and these additives only increase the gas sensitivity. This was achieved based on the discovery of three points that could significantly improve the aging characteristics. That is, the gas sensing element of the present invention contains CdO containing 0.005 to 10% by weight of sulfate ions, and at least one of Sn, Zr, and Ti as additives, converted into SnO ₂ , ZrO ₂ , and TiO _{2 ,} respectively. Some gas sensitizers contain 0.1 to 50 mol% of additives in total, and these include Sn, Zr, and CdO containing sulfate ions, which are the base materials of gas sensitizers.
Or, it was discovered that by adding Ti, the gas sensitivity characteristics and its reliability were dramatically improved, and it was also possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas. It is. Description of Examples Examples of the present invention will be described below. First, in Example 1, a case will be described in which the amount of sulfate ions contained in CdO is kept constant, and the amounts of additives Sn, Zr, or Ti and their combinations are varied. Example 1 Commercially available cadmium oxide (CdO) reagent (which was confirmed to be entirely in the CdO phase by X-ray diffraction)
Add cadmium sulfate (CdSO ₄ −XH ₂ O) reagent to 200 g as an additive to contain sulfate ions.
40g was added and mixed for 2 hours using a rice cooker. These mixtures were equally divided into several parts, and commercially available reagents of stannic oxide (SnO ₂ ), zirconium oxide (ZrO ₂ ), and titanium oxide (TiO ₂ ) were added thereto, either singly or in combination. Then, each powder was further dry-mixed for 3 hours using a miller. Then, an organic binder was added to each of these to form particles with a size of 100 to 200μ.
Next, these powders were pressure-molded into a rectangular parallelepiped shape and fired in air at a temperature of 600°C for 1 hour. Next, Au 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 a sensing element was fabricated. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. The element temperature was maintained at 400°C and its gas sensitivity characteristics were measured. The resistance value (Ra) in air 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 (Rg) in gas was determined by 99% purity inside
The above methane (CH ₄ ) and hydrogen (H ₂ ) gases are made to flow at a volume ratio of 10 ppm/sec,
Each was measured when its concentration reached 0.2% by volume. The gas concentration to be measured was chosen to be 0.2% because
The detection concentration practically required for a gas detection element is one-tenth of the lower explosive limit concentration (LEL) of the gas.
This is because the LEL of each of the above gases is approximately 2% by volume to 5% by volume. Further, the presence of sulfate ions (SO ₄ ^-- ) contained in the gas sensitive material was confirmed by infrared absorption spectrum, and the amount contained was identified from the TG-DTA curve and fluorescent X-ray analysis. As a result, the amount of sulfate ions contained in these sintered sensitizers was 0.62 to 0.84.
It was in weight%. FIGS. 1 to 3 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added individually. Sensitivity characteristics were evaluated using (i) gas sensitivity (Ra/Rg), and (ii) resistance change rate over time ΔR (rate of change in resistance value from the initial value when the sensitive body was held at a temperature of 400°C for 2000 hours). . Table 1 also shows the gas sensitivity (Ra/
Rg) and resistance change rate over time (ΔR). Note that ΔR is written in parentheses in the table. As is clear from Figures 1 to 3 and Table 1, the gas sensitivity characteristics (gas sensitivity: Ra/Rg) are greatly improved by adding Sn, Zr, or Ti singly or in combination. . Also noteworthy is the change in resistance value over time, and the addition of these additives significantly reduces the rate of change. It can thus be seen that the addition of Sn, Zr, or Ti can dramatically improve gas sensitivity characteristics and reliability. In the present invention, the total amount of additives is limited to 0.1 to 50 mol%, because if it is less than 0.1 mol%, as shown in Figures 1 to 3 and Table 1, the effect of improving gas sensitivity characteristics and reliability is insufficient. On the other hand, if it exceeds 50 mol %, the resistance value itself becomes high and the stability of the characteristics is lacking. Those marked with * in the table correspond to these, and are listed as comparative examples in Table 1.

[Table] * Comparative example By the way, in general, metal oxides that are amorphous to some extent are more active in physicochemical phenomena such as adsorption to combustible gases than those that are crystallized. It is said to be easy. but,
Even the commercially available cadmium oxide used in this example, which was almost completely crystallized, showed extremely high activity by containing sulfide ions and adding Sn, Zr, or Ti; As a result, it can be seen that very large gas sensitivity and high reliability can be achieved. This is a combination of sulfate ion, Sn, Zr, or Ti.
The presence of valent metals plays a role in significantly suppressing grain growth during sintering of cadmium oxide, the base material of the sensitive body, thereby suppressing the decrease in specific surface area due to sintering and making the surface of the particles chemically highly active. west,
This effect is due to a marked increase in gas adsorption capacity. In Example 1, the sensitive body is a sintered body, the amount of sulfate ions contained is constant, and the amounts and combinations of additives are varied. In Example 2 shown below, the sensitive body is a sintered film, and contrary to Example 1, the type and amount of additives are kept constant and the amount of sulfate ions contained is varied. That is, in Example 2, it is confirmed that the present invention is effective even when the sensitive body is a sintered film, and the effect of the amount of sulfate ions contained on the gas sensitivity characteristics will be described. Example 2 100 g of a commercially available cadmium oxide (CdO) reagent was mixed with commercially available stannic oxide (SnO ₂ ), zirconium oxide (ZrO ₂ ), and titanium oxide (TiO ₂ ) reagents in the proportions shown in Table 2. Each sample was weighed out and mixed in a sieve for 2 hours. Next, each mixed powder was divided into 8 equal parts, and cadmium sulfate (CdSO ₄ −XH ₂ O) prepared in advance at various concentrations was added to it.
The solutions were added and then each powder was mixed for 1 hour, also in a mill. In this way, there are three types of oxide compositions as representative examples (sample A
~C) A total of 24 types of samples were obtained, with 8 types of samples having different amounts of sulfate ions for each oxide composition.

[Table] Several 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 50 to 100μ, and triethanolamine was added to form a paste. On the other hand, as a substrate for the gas detection element, the length and width are each 5 mm, and the thickness is
Prepare a 0.5mm alumina substrate, and place 0.5mm on this surface.
A pair of comb-shaped electrodes was formed by printing gold paste in a comb shape at intervals of . Then, a commercially available ruthenium oxide glaze resistor 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 board to a thickness of about 70μ, and after air drying at room temperature, it was heated to 400℃.
The mixture was gradually heated until the temperature reached , and maintained at this temperature for 1 hour. At this stage, the paste evaporated and became a sintered film of the respective oxide composition containing sulfate ions. The thickness of this gas sensitive body was approximately 55μ. 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 to 400°C in the same manner as described above. TG−DTA
It was also subjected to fluorescent X-ray spectroscopy. Further, the presence of sulfate ions was confirmed by analyzing the infrared absorption spectrum as in Example 1. Example 1 shows the gas sensitivity characteristics of each sensing element.
It was measured in the same way as in the case of . FIGS. 4 to 6 show the relationship between the gas sensitivity (Ra/Rg) and the sulfate ions contained in samples A to C having different oxide compositions, respectively. Furthermore, Figures 4, 5, and 6 show the amount of sulfate ions contained and the sensitivity to each gas of propane and methane in the case of the sensitizers having the compositions shown in Sample Nos. A, B, and C in Table 2, respectively. This shows the correlation. Table 2 also shows the rate of change in resistance over time when samples A to C containing 2 to 5% by weight of sulfate ions were evaluated using the same method as in Example 1, as representative examples of characteristics over time. shows. In Example 2, methane and propane were used as the test gases. As is clear from FIGS. 4 to 6, almost the same characteristics as those obtained in Example 1 are obtained even when the sensitive body is a sintered film. Furthermore, as is clear from Table 3, the rate of change in resistance value over time is also very small, as in Example 1. Further, as can be seen from FIGS. 4 to 6, if the amount of sulfate ions is less than 0.005% by weight, there is no effect of adding Sn, Zr or Ti, and the effects of the present invention cannot be expected. On the other hand, if it exceeds 10.0% by weight, it becomes impractical in terms of stability of properties or mechanical strength. The reason why the amount of sulfate ions contained in the gas sensing element of the present invention is limited to 0.005 to 10.0% by weight is based on the above-mentioned reason.

[Table] By the way, in Examples 1 and 2, commercially available oxide reagents were used as starting materials, but in the present invention, the final composition of the reactor may be within the above-mentioned range. There are no limitations on starting materials or manufacturing methods. In addition, although methane, hydrogen, or propane were used as the test gases in the examples, the effects of the present invention are by no means limited to these gases, and can be applied to flammable gases such as ethane, isobutane, and alcohol. Of course, it is also effective. Effects of the Invention As explained above, the gas sensing element of the present invention uses a sintered body or a sintered film obtained by adding Sn, Zr, or Ti as an additive to cadmium oxide containing sulfate ions as a sensitive body. can be,
This dramatically improves gas sensitivity, making it possible to achieve extremely high sensitivity even at a relatively low temperature of 400°C for methane gas, which until now has been considered difficult to detect in trace amounts without the use of noble metal media. It is. 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. Another effect of the present invention is a significant reduction in the life characteristics, especially the change in resistance value over time due to energization. In other words, this makes an extremely large contribution to improving the reliability of the element, which is the most important element of any sensing element.

[Brief explanation of drawings]

Figures 1 to 3 are characteristic diagrams showing the relationship between the amount of additives, sensitivity to methane and hydrogen (Ra/Rg), and rate of change in resistance over time (ΔR) in one embodiment of the present invention, and Figures 4 to 3 FIG. 6 is a characteristic diagram showing the relationship between the sulfate ion content and the sensitivity to methane and propane (Ra/Rg) for three representative oxide compositions in another example of the present invention.

Claims (1)

[Claims] 1. Cadmium oxide (CdO) containing 0.005 to 10% by weight of sulfate ions, tin (Sn), zirconium (Zr), and titanium (Ti) as additives.
At least one of them is SnO ₂ ,
Total amount of additives converted to ZrO ₂ and TiO ₂ from 0.1 to 50
1. A gas sensing element characterized in that a gas sensing element containing mol% of the gas is used as a gas sensitive material. 2. The gas detection according to claim 1, wherein 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. element.