JPS59107251A - Gas detecting element - Google Patents

Abstract

PURPOSE:To improve gaseous methane sensitivity by using a sintered body or sintered film formed by adding Ge or Th as an additive to ZnFe2O4 contg. sulfate ion as a sensitive body. CONSTITUTION:ZnO and Fe2O3 are wet mixed and is subjected to drying, grinding and heat treatment. The mixture is again ground and Fe2(SO4)3-XH2O is added as an additive for incorporating sulfate ion to the powder and both are mixed. GeO2 or ThO2 is added to the mixture thereof and is dry mixed. An org. binder is added to the mixture and is then granulated. The powder is press molded to a prism shape and is calcined in air. Au is deposited by evaporation on the surface of the sintered body, whereby a pair of comb-shaped electrodes are formed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a gas detection element using a composite metal oxide semiconductor for use in detecting combustible gases.

Conventional Structures and Their Problems In recent years, various research and developments have become active regarding materials for detecting flammable gases. This is strongly attributable to the fact that explosion accidents caused by flammable gases and poisoning accidents caused by harmful gases occur frequently, mainly in households and in various factories, and have become a major social problem.

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

In recent years, gas detection elements using metal oxides such as stannic oxide (S no 2 ) and gamma-type ferric oxide (γ-Fe2O3) have been put into practical use, and are being applied to gas leak alarms. ing. Even if a situation such as a gas leak occurs, 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 is gradually becoming popular for general household use. Therefore, this LNG
The demand for gas detection elements that can detect methane gas, the main component of which is highly sensitive, is also increasing.

Of course, gas detection elements that are sensitive to methane gas have already been developed, but many of them use noble metal catalysts as sensitizers in the sensitizer material, so there are problems with catalyst poisoning by various gases and sensitivity to methane gas. However, there are issues such as a small amount of change in characteristics over time, or a large change in characteristics over time.

Therefore, 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 it is an object of the present invention to provide a gas detection element having a sensitivity sufficiently high for practical use even to methane gas. Since methane gas itself is a very stable gas, a detection element that has sufficient sensitivity for methane gas needs to be extremely active. Therefore, to achieve greater sensitivity to methane gas, 1
Conventionally, efforts have been made to use a noble metal catalyst by adding it to the sensitive material, or to operate the sensitive body at a temperature much higher than, for example, 460°C.In contrast, the present invention uses a noble metal catalyst. 4 without adding any
Structure of the Invention The present invention realizes an element with high sensitivity to methane even at a relatively low operating temperature of 0°C. This was discovered while studying the effects of various anions contained in this material on the gas-sensitive characteristics, as well as the effects of additives.In other words, the base material of the gas-sensitive material, We have discovered that by adding Go or Th to ZnFe2O4 containing sulfate ions, the gas sensitivity characteristics and its reliability are dramatically improved, and it is also possible to achieve a sensitivity that is sufficiently high for practical use even to the aforementioned methane gas. This was done by

Description of examples The details of the present invention will be explained below using examples.

First, in Example 1, the amount of sulfate ions contained in Z n Fe 204 was kept constant, and the amount of G as an additive was kept constant.

Alternatively, a case will be described in which the amount of Th added and the combination thereof are changed.

[Example 1] A commercially available reagent of zinc oxide (ZnO) was converted into 41y1 ferric oxide (
80y of commercially available reagents of Fe203) were each weighed out and wet-mixed in a stainless steel pot for 5 hours. This mixture was dried, ground and then heat treated at a temperature of 1300° C. for 2 hours. This was ground again, and ferric sulfate (Fe2(S04)3-xH2O) reagent was added at 35F 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 germanium oxide ((jeo2) and thorium oxide (Th2) reagents were respectively 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, and the particles were sized into particles having a size of 100 to 200 μm. Next, these powders were pressure-molded into a rectangular parallelepiped shape and heated in air for 60 minutes.
It was baked for 1 hour at a temperature of 0°C. 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 adhered to the back surface with an inorganic adhesive to produce a sensing element as a heater. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. Set the element temperature to 40
The gas sensitivity characteristics were measured while maintaining the temperature at zero.

The resistance value ()la) in air was measured in a measuring container with a volume of 50 tons in which dry air was stirred slowly to the extent that no turbulence occurred, and the resistance value (R
q) contains methane (CH4) with a purity of 99% or more 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 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 to 5 volumes, ranging from 1/0 to several fractions.

Further, the presence of sulfate ions (SO4) contained in the gas sensitive material was confirmed by infrared absorption spectroscopy, 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 sensitive bodies was 0.23 to 0.26% 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 (1) gas sensitivity (1(a/Rq)), (ii) resistance change rate over time ΔR (the sensitive body is heated to 2000 at a temperature of 400'C
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/f(q)) and resistance change rate over time (ΔH) when additives are used in combination. did.

As is clear from Figures 1, 2, and Table 1, G
By adding e or Th alone or in combination, gas sensitivity characteristics (gas sensitivity: Ra /Rq
) has improved significantly. Also worth noting 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 Ge or Th can dramatically improve the gas sensitivity characteristics and reliability.

The reason why the total amount of additives is limited to 0.1 to 60 molty in the present invention is that if it is less than 0.1 molty, as shown in Figures 1 to 2 and Table 1, the gas sensitivity characteristics and This is because no effect of improving the resistance is observed, and on the other hand, if it exceeds 50 molts, the resistance value itself becomes high and the stability of the characteristics is lacking. Those marked with a red seal in the table correspond to these, and are listed as comparative examples in Table 1.

(Left below) Table 1 Comparative Examples By the way, in general, metal oxides in a somewhat amorphous state are better than crystallized ones due to their physical and chemical properties, such as adsorption phenomena for combustible gases. It is said that the phenomenon tends to become active. However, even ZnFe2o4 prepared using a commercially available reagent that is almost completely crystallized can achieve extremely high activity by containing sulfate ions and further adding Go or Th. Because of its stability, it can be seen that extremely high gas sensitivity and high reliability can be achieved as a result.

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. In other words, in Example 2, it was 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 was investigated. state

[Example 2] Z n Fe 2 prepared in the same manner as Example 1
04100F Niyahashi commercially available germanium oxide (G e
O2) and thorium oxide ('rho2) reagent in the second
The proportions shown in the table were weighed out, and each was mixed in a sieve machine for 2 hours. Next, add 8 pieces of each mixed powder.
Ferric sulfate (Fe2(SO2)-3-xH2O) solutions prepared in advance at various concentrations were added to the powders, and then the respective powders were mixed for 1 hour using a miller. In this way, a total of 24 types of samples were obtained, including 3 types of representative oxide compositions (samples A-C) and 8 types of oxide compositions with different amounts of sulfate ions. .

(Hereinafter referred to as Kinpaku) 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 iooμ, and triethanolamine was added to form a paste. On the other hand, as a substrate for the gas detection element, an alumina substrate with 6 cylinders in both the vertical and horizontal directions and a thickness of 0.5 mm was prepared. Gold paste was printed on the surface in a comb shape at intervals of 0.6 cm, and then baked to form a pair of comb shapes. An electrode was formed. and,
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-mentioned paste was printed on the surface of the substrate to a thickness of about 65 μm, air-dried at a constant 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 and became a sintered film of each composite oxide composition containing sulfate ions. The thickness of this gas sensitive member was approximately 66μ. 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. were subjected to TG-DTA and 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. FIGS. 3 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. Also, in Table 3,
As a representative example of the aging characteristics, Example 1 is shown for samples A to C containing 2 to 5% by weight 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. 3 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.

Furthermore, as can be seen from Figures 3 to 5, if the amount of sulfate ions is less than 0.005% by weight, Ge or Th
Since there is no effect of addition of , the effect 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 from 0.005 to
10.0 The reason for limiting it to the weight category is based on the point mentioned above.

(Hereinafter referred to as "Kinpaku") Table 3 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 was within the above-mentioned range. Any starting material or manufacturing method is not limited in any way.

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 also be applied to heatable gases such as ethane, imbutane, and alcohol. Of course, it is effective.

As described in detail, the gas sensing element of the present invention contains G as an additive in Znke 204 containing sulfate ions.
This method uses a sintered body or a sintered film to which O or Th is added as a sensitive body, and this dramatically improves the sensitivity of methane gas, which was previously difficult to detect in trace amounts without using a precious metal catalyst. It has achieved extremely high sensitivity even at a relatively low temperature of 400°C for methane gas, which has been considered to be a natural gas (main component: methane gas). It accurately responds to certain social needs,
The effect is terrible and great. Another effect of the present invention is a significant reduction in the life characteristics, especially the change over time in the resistance value due to energization. In other words, this brings an extremely large advantage to improving the reliability of the element, which is the most important element of any sensing element.

[Brief explanation of the drawing]

Figures 1 and 2 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 (△H) in one embodiment of the present invention, and Figure 3 - Figure 5 shows the sensitivity to methane and propane (H
a/Rq) for three typical oxide compositions.

Claims (2)

[Claims] (1) Zinc ferrite (znFe204) containing o, oos to 10% by weight of sulfate ions, and at least one of germanium (Ge) and thorium (Th) as additives are added to Cs e O2 and TE01, respectively. A gas sensing element characterized in that a gas sensitive material containing 0.1 to 60 mole of additives in total 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.