JPS5957151A - gas detection element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a gas detection element using a metal oxide semiconductor used to detect combustible gases.

Structure and Problems of Conventional Examples In recent years, various research and development activities have been active regarding materials for sensing elements for flammable gases. This is strongly attributable to the fact that explosions caused by flammable gases and poisoning accidents caused by toxic gases occur frequently, mainly in households and in various factories, and these have become major social problems.

Propane gas, in particular, has a low explosive limit (LEL) and a higher specific gravity than air, so it tends to stagnate in rooms, resulting in numerous accidents and injuries every year.

In recent years, gas detection elements using metal oxides such as stannic oxide (5n02) and gamma-type ferric oxide (γ-Fe2O3) have been put into practical use and are being applied to gas leak alarms, etc. . Even if a situation such as a gas leak occurs, the presence of propane gas can be quickly detected during the first stage leading to the LEL, and an explosion can be prevented.

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, this LNG
The demand for gas detection elements that can sensitively detect methane gas, the main component of methane gas, is also increasing.

Of course, gas detection elements that are sensitive to methane gas have already been developed, but most of them use noble metal catalysts as sensitizers in the sensitive material, so there are problems with catalyst poisoning by various gases and methane gas. However, there are problems such as low selectivity for caustic substances and large changes in causticity over time.

For example, since methane gas itself is a very stable gas, a detection element with sufficient sensitivity must be extremely active. In order to overcome this problem, efforts have been made to use a noble metal catalyst added to the sensitive material, or to operate the sensitive material at a considerably high temperature of, for example, 460.degree. C. or higher. However, at present, the characteristics 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 that has 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 sensitive material, and the effects of various anions contained therein on the gas sensitivity characteristics, as well as the effects of additives, have been investigated. This was discovered during the process.

That is, in the gas sensing element of the present invention, sulfate ions are 0.
006 to 10% by weight of ZnO containing at least one of Sn, Zr and Ti as an additive in a total amount of 0.1 to 50 mol in terms of SnO2, ZrO2 and TiO2, respectively. This material was used as a gas sensitive material, and was made by adding Zn, Zr, or Ti to ZnO containing sulfate ions, which is the base material of the gas sensitive material.
This was based on the discovery that by adding , 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. .

DESCRIPTION OF EMBODIMENTS The present invention will be described in detail below. 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] Commercially available zinc oxide (ZnO) (from X-ray diffraction, all ZnO
Zinc sulfate (Zn
5 of SO4-7H20) reagent was added and mixed for 2 hours using a sieve machine. These mixtures were equally divided into several parts, and commercially available stannic oxide (SnO2), zirconium oxide (ZrO2), and titanium oxide (TiO2) were added to each part.
Reagents were added singly or in combination. Then, each powder was further dry-mixed for 3 hours using a miller.

Then add an organic binder to each of these to give 100%
The particles were sized to a size of ~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, hu 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 produce a heater and a detection element. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. Increase the body temperature to 400'
The gas sensitivity characteristics were determined at 611° C.

The resistance value (Ha') in air is defined in a measurement container with a volume of 601 in which dry air is slowly stirred to the extent that no turbulence occurs, and the resistance value (R
q) contains methane (CH4) with a purity of 99% or higher in this container.
) and hydrogen (H2) in a volume ratio of 10pp.
It was made to flow in at a rate of m/sec, and each measurement was made when the concentration reached 0.2% by volume. The gas concentration to be measured is 0.2
% was chosen because the detection concentration practically required for a gas detection element is the number 1 of the lower explosive limit concentration (LEL) of the gas.
This is because the LEL of each of the above gases is approximately 2 volumes, by 5% to 5 volumes.

Further, the presence of one sulfate ion (S04) contained in the gas sensitive material was confirmed by infrared absorption spectrum, and the amount contained was identified from the TG-DTA curve and fluorescence x&! analysis.

As a result, the amount of sulfate ions contained in these sintered sensitive bodies was 0.50 to 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.

The sensitivity characteristics are (:) gas sensitivity (Ra/Rcr), (1
1) Resistance change rate over time ΔR (Evaluated by the change rate of resistance value with respect to the initial value when the sensitive body is held at 400°C iVL degree for 20oO hours. Table 1 also shows that When used, the graph shows the gas sensitivity (Ra/Rq) and the resistance change rate over time (△R). Note that △R is (
).

As is clear from Figures 1 to 3 and Table 1, S
By adding n, Zr, or Ti singly or in combination, gas sensitivity characteristics (gas sensitivity: Ra/Rc
r) has been greatly improved. Also noteworthy is the change in resistance value over time, and the rate of change is greatly reduced by adding these additives. In this way, 8n, Z
It can be seen that the addition of r or Tl can dramatically improve gas sensitivity characteristics and reliability.

In the present invention, the total amount of additives is limited to 0.1 to 5 O-E-Rue, because at 0.1 mol/mole, as shown in Figures 1 to 3 and Table 1, No effect on improving characteristics and reliability was observed, and on the contrary, 50 mol%
This is because, if it exceeds the value, the resistance L itself becomes worse, and the characteristics lack stability. Those marked with * in the table correspond to these, and are listed as comparative examples in Table 1.

(Details below) Table 1 *: Comparative Examples By the way, metal oxides in a somewhat amorphous state are generally more sensitive to physical properties such as adsorption phenomena for combustible gases than crystallized ones. It is said that chemical phenomena are likely to become active. However, even the commercially available zinc oxide reagent used in this example, which is almost completely crystallized, shows extremely high activity by containing sulfate ions and further adding Sn, Zr, or Ti. It can be seen that this is stable over time, resulting in extremely high gas sensitivity and high reliability.

In Example 1, the sensitive body is a sintered body, the amount of sulfate ions contained is constant, and nine combinations of amounts of additives are varied.

In Example 2 shown below, the sensitive body is a sintered film.
Contrary to I41, a case will be described in which 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] Commercially available zinc oxide (ZnO) reagent 100/- Commercially available tin oxide (5n02'), Zirconium oxide (ZnO)
rO2) and titanium oxide (Ti02) reagent in the second
The ingredients were weighed out in the proportions shown below, and each was mixed on a rocky shore for 2 hours. Next, add 8 pieces of each mixed powder.
The powder was divided into equal parts, and zinc sulfate (ZnSO4-7H20) solutions prepared in advance at various concentrations were added thereto, and then the respective powders were mixed for 1 hour on a clay rock. In this way, there are three types of oxide compositions as representative examples (
A total of 24 types of samples were obtained, including samples A and c), and 8 types for each oxide composition with different amounts of sulfate ions.

Table 2 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 60 to 100 microns, and triethanolamine was added to form a paste. On the other hand, there are 5 vertical and horizontal boards for the gas detection element.

Prepare an alumina substrate with a thickness of 0.6H, and coat the surface with 0.6H.
A pair of comb-shaped electrodes was formed by printing gold paste in a comb shape at 6 m intervals and baking it. 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-described paste was printed on the surface of the substrate to a thickness of about 70 μm, air-dried at room temperature, and then gradually heated to a temperature of 400° C. and held at this temperature for 1 hour. At this stage, the paste evaporated to form a sintered film of the respective oxide composition containing sulfate ions. The thickness of this gas sensitive member 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. In the case of T (r-DTA), the sample was subjected to fluorescent X-ray analysis. The presence of sulfate ions was confirmed by analyzing the infrared absorption spectrum as in Example 1.

The gas sensitivity characteristics of each sensing element were measured in the same manner as in Example 1. FIG. 4 to FIG. 6 show the relationship between the sample A-CO gas sensitivity (Ra/Rq) and the sulfate ion contained therein, each having a different oxide composition. Also, in Table 3,
As a representative example of the aging characteristics, Example 1 is shown for samples A-C containing 2 to 6 weight percent of sulfate ions.
It shows the rate of change in resistance value over time when evaluated using the same method as .

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. 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 d1, 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 amount of sulfate ions contained in the gas sensing element of the present invention is 0.00
The reason why the content is limited to 5 to 10.0% by weight is based on the above-mentioned reason.

Table 3 By the way, in Examples 1 and 2, a commercially available oxide reagent was used as the starting material, 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 the detailed examples, methane, hydrogen, or propane were used as the gases to be tested, but the effects of the present invention are by no means limited to these gases; ethane, isobutane,
Of course, it is also effective against flammable gases such as alcohol.

As described in detail, 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. As a result, gas sensitivity has been dramatically improved, and extremely high sensitivity has been achieved even at a relatively low temperature of 400℃ for methane gas, which until now has been 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 impressive. 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/Rq), and rate of change in resistance over time (△R) in one embodiment of the present invention, and Figure 4 〜FIG. 6 shows fAt in another embodiment of the present invention
The relationship between the H ion content and the sensitivity (Ha/Rq) to methane and propane was determined using three representative oxides.
It is a characteristic diagram shown about the case of a composition. Name of agent: Patent attorney Toshio Nakao and one other person Figure central bi

Claims (2)

[Claims] (1) Zinc oxide (ZnO) containing 0.005 to 10% by weight of sulfate ions is added with at least one of tin (Sn), zirconium (Zr), and titanium (Ti) as additives, 5n02 and ZrO2, respectively. and T
A gas sensing element characterized in that a gas sensing element containing Q, 1 to 50 molti in total amount of additives in terms of iO2, is used as a gas sensing element. (2) Claim (1) characterized in that 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. The gas detection element described.