JPS6160384B2 - - 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 and 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 zinc oxide (ZnO) as a gas sensing 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 discovered. That is, the gas sensing element of the present invention contains ZnO 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. The gas sensitizer contains 0.1 to 50 mol% of additives in total, and this is the base material of the gas sensitizer, ZnO containing sulfate ions, plus Zn, Zr.
Alternatively, the discovery was made based on the discovery that by adding Ti, the gas sensitivity characteristics and its reliability were dramatically improved, and that it was possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas. It is. DESCRIPTION OF EMBODIMENTS The following embodiments of the present invention will be described. First, in Example 1, a case will be described in which the amount of sulfate ions contained in ZnO is kept constant, and the amounts of additives Sn, Zr, or Ti and their combinations are varied. [Example 1] 200 g of commercially available zinc oxide (ZnO) reagent (confirmed to be entirely ZnO phase by X-ray diffraction)
50g of zinc sulfate (ZnSO ₄ -7H ₂ O) reagent was added as an additive to contain sulfate ions,
The mixture was mixed for 2 hours in 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 these to form particles with a size of 100 to 200μ. Next, these powders were pressure-molded into a rectangular parallelepiped shape and heated in air at a temperature of 600°C.
Baked for an 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
Each of the above gases, methane (CH ₄ ) and hydrogen (H ₂ ), was flowed in at a volume ratio of 10 ppm/sec, and each was measured when the concentration reached 0.2% by volume. The gas concentration to be measured was chosen to be 0.2% because the practically required detection concentration for a gas detection element is several tenths 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. Furthermore, 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 ranges from 0.50 to
It was 0.64% by weight. Figures 1 to 3 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added individually.
Sensitive characteristics are evaluated by (i) gas sensitivity (Ra/Rg), (ii) rate of change in resistance over time △R (rate of change in resistance value from the initial value when the sensitive body is held at a temperature of 400°C for 2000 hours) did. Table 1 also shows the gas sensitivity (Ra/Rg) when additives are used in combination;
The rate of change in resistance over time (ΔR) is shown. 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. The reason why the total amount of additives is limited to 0.1 to 50% by mole in the present invention is that less than 0.1 mole% improves gas sensitivity characteristics and reliability, as shown in Figures 1 to 3 and Table 1. This is because no effect is observed, and on the contrary, if it exceeds 50 mol%, the resistance value itself becomes high and the stability of the characteristics is lacking. Those marked with a cross in the table correspond to these, and are listed as comparative examples in Table 1.

[Example 2]

To 100 g of a commercially available zinc oxide (ZnO) reagent, commercially available tin oxide (SnO ₂ ), zirconium oxide (ZrO ₂ ), and titanium oxide (TiO ₂ ) reagents were weighed in the proportions shown in Table 2. The mixture was mixed for 2 hours using a sieve machine. Next, each mixed powder was divided into eight equal parts, and zinc sulfate (ZnSO ₄ -7H ₂ O) solutions prepared in advance at various concentrations were added thereto.
Thereafter, each powder was mixed for 1 hour using a milling machine. In this way, a total of 24 types of samples were obtained, including 3 types of representative oxide compositions (samples A to C) and 8 types of samples with different amounts of sulfate ions for each oxide composition. Ta.

[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 . 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, we slowly heated the paste at a temperature of 400°C in the same way as described above. TG-DTA
It was also subjected to fluorescent X-ray analysis. 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 The relationship between the gas sensitivity (Ra/Rg) and the sulfate ion contained in samples A to C having different oxide compositions in FIGS. 4 to 6 is shown, respectively. Table 3 also shows that samples A to C contain 2 to 5 sulfate ions as representative examples of aging characteristics.
The rate of change in resistance value over time when evaluated using the same method as in Example 1 for those containing % by weight is shown. In Example 2, methane and propane were used as the test gases. As is clear from FIGS. 4 to 6, it can be seen that almost the same characteristics as those obtained in Example 1 are obtained even when the sensitive body is a sintered film. Also the third
As is clear from the table, 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 are applicable 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 zinc oxide containing sulfate ions as a sensitive body. This dramatically improves gas sensitivity, achieving extremely high sensitivity even at a relatively low temperature of 400 degrees Celsius for methane gas, which until now was considered difficult to detect in trace amounts without the use of precious metal catalysts. It is possible. 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 the drawing]

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 (△R) in one embodiment of the present invention, and Figure 4 ~Figure 6 is a characteristic diagram showing the relationship between sulfate ion content and sensitivity to methane and propane (Ra/Rg) for three representative oxides and compositions in another example of the present invention. It is.

Claims (1)

[Claims] 1 Zinc oxide (ZnO) containing 0.005 to 10% by weight of sulfate ions, and at least one of tin (Sn), zirconium (Zr), and titanium (Ti) as additives, each containing SnO ₂ , _ZrO2 and
A gas sensing element characterized in that the gas sensing element contains a total amount of additives of 0.1 to 50 mol% in terms of _TiO2 . 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.