JPS5992339A - Gas detecting element - Google Patents

Abstract

PURPOSE:To improve a gas sensitivity and to detect methane at relatively low temp. by adding Ge or Th to alpha-Fe2O3 containing sulfuric acid ion. CONSTITUTION:A prescribed quantity of ferrous sulfate (FeSO4-7H2O) is added to alpha-Fe2O3 for incorporating the sulfuric acid ion, furthermore germanium oxide (GeO2) and thorium oxide (ThO2) are added individually or combinedly, and then mixed, press formed and calcined in air to manufacture a sintered body. A pair of electrodes is formed to this sintered body to obtain a gas detecting element. The most suitable quantity of sulfuric acid ion to be added and the total quantity of GeO2 and ThO2 are 0.005-10wt% and 0.1-50mol% respectively basing on Fe2O3 in consideration of the adding effect. The gas detecting element constituted in such a way can detect the gaseous methane at relatively low temp. of 400 deg.C, and the change of resistance value of the element in the lapse of time is small.

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.

Conventional Structures and Their Problems In recent years, various research and developments have become 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 ordinary households, at various factories, etc., and this has become a major social problem.

In particular, propane gas has a low lower explosive limit (I, EL).
Moreover, because it has a higher specific gravity than air and tends to stagnate in rooms, accidents continue to occur, resulting in numerous casualties every year.

In recent years, gas detection elements using metal oxides such as stannic oxide (Sn○2) and gamma-type ferric oxide (γ-Fe203) have been put into practical use, and are used in gas leak alarms, etc. It is applied. Even if a gas leak occurs, the presence of propane gas can be quickly detected before reaching the LELK, 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, 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 since most of them use noble metal catalysts as sensitizers in the sensitive material, there are problems with catalyst poisoning by various gases, and the selection of methane gas is difficult. However, there are issues such as low strength 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. , a noble metal catalyst is added to the susceptor material, or the susceptor is heated at, for example, 450'C.
Efforts have been made to operate at considerably higher temperatures. 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 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 relates to a gas sensing element using alpha-type ferric oxide (α-Fe203) as a gas sensitive material, and the effects of various anions contained therein on the gas sensitivity characteristics, as well as additives. This was discovered while studying the effects of

That is, in the gas sensing element of the present invention, sulfate ions are 0.
At least one of Ge and Th as an additive is combined with α-Fe203 containing 0.06 to 10% by weight in an amount of 0.1 to 50 mol in terms of Ge02 and ThO2, respectively. This is the base material of the gas sensitive material, α-Fe203 containing sulfate ions, mixed with Ge or Th.
This was accomplished by discovering 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 examples The present invention will be explained in detail below.

First, in Example 1, a case will be described in which the amount of sulfate ions contained in α-Fe203 is kept constant, and the amount of Ge or Th as an additive and the combination thereof are changed.

[Example 1] Commercially available ferric oxide (Fe203) (confirmed to be all α-Fe2o3 phase by X-ray diffraction) reagent 20
To 0 g, 40% of ferrous sulfate (FeSO4-7H20) reagent was added as an additive for containing sulfate ions, and the mixture was mixed for 2 hours on a rocky shore.

These mixtures were equally divided into several parts, and commercially available germanium oxide (GeO2) and thorium oxide (ThO2) 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 are pressure-molded into a rectangular parallelepiped shape,
It was fired in air at a temperature of 600°C for 1 hour. Next, Au is deposited on the surface of this sintered body to form a pair of comb-shaped electrodes,
A platinum heating element was attached to the back side using an inorganic adhesive to serve as a heater and a sensing element was fabricated. A current was passed through this heating element, and the operating temperature of the element was controlled by adjusting the current value. The element temperature was maintained at 400° C. and its gas sensitivity characteristics were measured.

The resistance value (Ra) in air is measured in a measurement container with a volume of 5oQ in which dry air is slowly stirred to the extent that no turbulence occurs, and the resistance value (Rq
) contains methane (CH4) with a purity of over 99% in this container.
and hydrogen (H2) at a voluminous ratio of 10 ppm
/second, and measured when the concentration reached 0.2 volume.3, the gas concentration to be measured was 0.2.
% was chosen because the detection concentration practically required for a gas detection element is the number 10 of the lower explosion limit temperature (LEL) of the gas.
This is because the LEL of each of the above gases ranges from 1/2 to 1/2 of a volume, and the LEL of each of the above gases ranges from about 2 volumes to a maximum of 5 volumes.

The presence of sulfate ions (SO2--) contained in the oxda 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 sensitive bodies was 0.41 to 0.57% by weight.

Figures 1 and 2 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added individually.

The sensitivity characteristics are (1) gas sensitivity (Ra/Rcr), (i
i) Resistance change rate over time △R (sensor at 40°C temperature)
Evaluation was made based on the rate of change in resistance value from the initial value when held for 000 hours. Table 1 also shows the gas sensitivity (Ha /Rg) and the rate of change in resistance over time (ΔR) when additives are used in combination. Note that ΔR is written within 00 in the table.

As is clear from Figures 1 to 2 and Table 1, Ge
Alternatively, by adding Th alone or in combination, gas sensitivity characteristics (gas sensitivity: Ra/Rq) are greatly improved. Also noteworthy is the change in resistance value over time, and the rate of change is significantly reduced by adding these additives. It can thus be seen that by adding G(a or Th), dramatic improvements in gas sensitivity characteristics and reliability can be realized.

The reason why the total amount of additives is limited to 0.1 to 50 mol% in the present invention is that if it is less than 0.1 mol%, as shown in Figure 1, Figure 2, and Table 1, the gas sensitivity characteristics and reliability will be improved. This is because no effect of improving the properties is observed, and on the contrary, if the molar ratio exceeds 50, the resistance value itself becomes high and the stability of the properties is lacking. Those marked with * in the table correspond to these, and are listed as comparative examples in Table 1.

(See margin 2 Table 1 * Comparative Example By the way, metal oxides in a somewhat amorphous state are generally more sensitive to physicochemical phenomena such as adsorption phenomena for combustible gases than crystallized ones. However, even the commercially available reagent α-Fe2O3 used in this example, which has been almost completely crystallized, contains sulfate ions and further adds Go or Th. As a result, it shows extremely high activity and is stable over time, so it can be seen that extremely high gas sensitivity and high reliability f1 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 combination of five acids as additives is different.

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 contained sulfuric acid ionic acid on the gas sensitivity characteristics will be described.

[Example 2] To 100g of commercially available ferric oxide (Fe203) reagent, commercially available germanium oxide (Ge 02 ) and thorium oxide (The2) reagents were collected in the proportions shown in Table 2, and each The mixture was mixed for 2 hours using a sieve machine. Next, each mixed powder was divided into eight equal parts, and ferrous sulfate (FeSO4-7H2) prepared in advance at various concentrations was added to it.
0) The solution was added, and then each powder was mixed for 1 hour, also in a mill. 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 samples with different amounts of sulfate ions for each oxide composition. Ta.

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 50 to 100 microns, and triethanolamine was added to form a paste. On the other hand, an alumina substrate of 5 mm in length and width and 0.6 mm in thickness was prepared as a substrate for the scum detection element. Gold paste was printed on the surface in a comb shape at 0.5 mm intervals and baked to form a pair of comb shapes. An electrode was formed. A commercially available ruthenium oxide glaze resistor was printed on the g-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 70 μm, air-dried at room temperature, gradually heated to a temperature of 400° C., and kept 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 56μ. 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. Further, 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 the lifting platform of Example 1. FIGS. 3 to 5 show the relationship between sample A-CO gas sensitivity (Ra 71 g) having different oxide compositions and the sulfate ions contained therein.

Table 3 also shows samples A and C as representative examples of aging characteristics.
The rate of change in resistance value over time is shown when evaluated using the same method as in Example 1 for samples containing 2 to 5% of sulfate ions. 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 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 total amount of sulfate ions is less than o, oos, 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 isolate ions contained in the gas sensing element of the present invention is 0.006
The reason why the salary is limited to 10.0 to 10.0 is based on the above-mentioned points.

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 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 effective.

As described in detail, the gas sensing element of the present invention uses a sintered body or a sintered film obtained by adding Ge or Th as an additive to α-Fe203 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 40°C for methane gas, which until now was thought to be difficult to detect in trace amounts without the use of precious metal catalysts. It is something. This is city gas natural gas (main component: methane gas)
It accurately responds to social needs, which are becoming increasingly demanding as the world ages, and its effects are extremely significant. Moreover, one of the effects 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 and 2 show additives and sensitivity to methane and hydrogen (Ra/Rq) in one embodiment of the present invention.
) and resistance change rate over time (ΔR), and Figures 3 to 5 show the relationship between the sulfate ion content and the sensitivity to methane and propane (Ra/Rq) in other examples of the present invention. ) is a characteristic diagram showing the relationship between three typical oxide compositions. Name of agent: Patent attorney Satoshi Nakao and one other person. , /
/ 10 Kurasho 1 Amount of acid ions (wt%) Figure 4 o,oot ,o,Of O,l /
104!4 Ion content (V content% di

Claims (2)

[Claims] (1) Germanium (Ge) and thorium (Th) are added as additives to alpha-type ferric oxide (α-Fe203) containing 0.005 to 10% by weight of sulfate ions.
A gas sensing element characterized in that at least one of them contains 0.1 to 60 mol of additives in total in terms of G e O2 and ThO2, respectively, as a gas sensitive material. (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.