JPS6160385B2 - - 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 gases. 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 noble metal catalysts added to the susceptor material, or to operate the susceptor at a considerably high temperature of, 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 examines the effects of various anions contained in titanium oxide (TiO ₂ ) on the gas sensitivity characteristics and the effects of additives in a gas sensing element using titanium oxide (TiO 2 ) as a gas sensitive material. It is found within. In other words, the gas sensing element of the present invention includes TiO ₂ containing 0.005 to 10% by weight of sulfate ions, and at least one of Sn and Zr as additives, in terms of the total amount of additives in terms of SnO ₂ and ZrO ₂ , respectively.
The gas-sensitive material contains 0.1 to 50 mol% of TiO ₂ containing sulfate ions, which is the base material of the gas-sensitive material, by adding Sn or Zr to improve the gas-sensitive properties and its properties. This was achieved by discovering that the reliability was dramatically improved and that it was possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas. 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 TiO ₂ is kept constant, and the amount of Sn or Zr as an additive and the combination thereof are changed. [Example 1] 20 g of a titanium sulfate solution was added as an additive for containing sulfate ions to 200 g of a commercially available titanium oxide (TiO ₂ ) reagent, and the mixture was mixed for 2 hours using a sieve machine. These mixtures were equally divided into several parts, and commercially available ditin oxide (SnO ₂ ) and zirconium oxide (ZrO ₂ ) reagents were added thereto, either singly or in combination. Then, each powder was further dry-mixed for 3 hours using a miller. Add an organic binder to each of these to give 100%
The particles were sized to ~200μ in size. Next, these powders were pressure-molded into a rectangular parallelepiped shape and heated in air for 600 min.
It was baked for 1 hour at a temperature of °C. Next, AU was vapor-deposited on the surface of this fired body to form a pair of comb-shaped electrodes, and a platinum heating element was attached to the back side 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. 99% purity in
The above methane (CH ₄ ) and hydrogen (H ₂ ) gases were made to flow at a volume ratio of 10 ppm/sec, and each was measured when the concentration reached 0.2 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 fired sensitizers ranged from 0.12 to
It was 0.18% by weight. FIG. 1 and FIG. 2 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added alone. The sensitivity characteristics are (i) gas sensitivity (Ra/Rg), (ii) resistance change rate over time ΔR (sensor at 2000℃ at 400℃
The resistance value was evaluated based on the rate of change from the initial value when the resistance value was maintained for a certain period of time. 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 FIG. 1, FIG. 2, and Table 1, the gas sensitivity characteristics (gas sensitivity: Ra/Rg) are greatly improved by adding Sn or Zr alone 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 mol% in the present invention is that less than 0.1 mol% has the effect of improving gas sensitivity characteristics and reliability, as shown in Figures 1, 2, and Table 1. This is because, 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]

Weigh out 100 g of commercially available titanium oxide reagent and commercially available tin oxide (SnO ₂ ) and zirconium oxide (ZrO ₂ ) reagents in the proportions shown in Table 2, and weigh each in a sieve machine. Mixed for 2 hours. Next, each of the mixed powders was divided into 8 equal parts, titanium sulfate solutions prepared in advance at various concentrations were added thereto, and then each of the powders was mixed for 1 hour using a sieve machine. In this way, there are three types of oxide compositions as representative examples (Samples A to
C) A total of 24 types of samples were obtained, with 8 types for each oxide composition having different amounts of sulfate ions.

[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 material was approximately 50μ. 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 FIGS. 3 to 5 show the relationship between the gas sensitivity (Ra/Rg) and the sulfate ions contained in samples A to C having different oxide compositions, 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. 3 to 5, 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. Furthermore, as can be seen from Figures 3 to 5, if the amount of sulfate ions is less than 0.005% by weight, Sn or
There is no effect of adding Zr, 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 or Zr as an additive to titanium oxide containing sulfate ions as a sensitive body. This dramatically improves gas sensitivity, making it possible to achieve extremely high sensitivity even at a relatively low temperature of 400 degrees Celsius for methane gas, which until now has been difficult to detect in trace amounts without the use of precious metal catalysts. 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 will make an extremely large contribution to improving the reliability of any sensing element, which is the most important element.

[Brief explanation of the drawing]

Figures 1 and 2 are characteristic diagrams showing the relationship between the amount of additives, the sensitivity to methane and hydrogen (Ra/Rg), and the rate of change in resistance over time (ΔR) in one embodiment of the present invention, and Figures 3- FIG. 5 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)

[Scope of Claims] 1 Titanium oxide (TiO ₂ ) containing 0.005 to 10% by weight of sulfate ions is added with at least one of tin (Sn) and zirconium (Zr) as additives to form SnO ₂ and ZrO ₂ , respectively. 1. A gas sensing element characterized in that the gas sensing element contains a total amount of additives of 0.1 to 50 mol%. 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.