JPS59107250A - Gas detecting element - Google Patents

Abstract

PURPOSE:To improve a gas sensitivity characteristic and reliability thereof by adding Ge or Th to LiFe5O8 contg. sulfate ion which is a base material of a gas sensitive body. CONSTITUTION:Lithium ferrite (LiFe5O8) is used as a gas sensitive material in a gas detecting element using a composite metallic oxide semiconductor used for detecting combustible gas. Ge or Th is added to the base material. The total amt. of the additive is regulated to 0.1-60mol% respectively expressed in terms of GeO2 and ThO2. Said amt. is most effective in improving the gas sensitive characteristic and the reliability thereof.

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 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 (Sn02) and gamma-type ferric oxide (γ-Fe2O3) have been put into practical use and are being applied to gas leak alarms, etc. . And even if a situation such as a gas leak occurs, L
Before reaching the EL, the presence of propane gas can be quickly detected 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 detect methane gas, the main component of which is highly sensitive, is also increasing.

Of course, gas detection elements sensitive to methane gas and gas have already been developed, but many 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 some issues with this technology, such as low sensitivity to the effects of oxidation, and large changes 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, in order to achieve high sensitivity to methane gas, conventionally, noble metal catalysts have been added to the sensitive material, or
Alternatively, efforts have been made to operate the sensitive body at a temperature much higher than, for example, 450C.

In contrast, the present invention realizes an element with high sensitivity to methane without adding any noble metal catalyst and even with a relatively low operating turbidity of 400p.

Structure of the invention The present invention uses lithium ferrite (L I Fe 60s
) in a gas sensing element using as a gas sensitive body,
This was discovered while studying the effects of various anions contained in this on the gas sensitivity characteristics as well as the effects of additives. In other words, by adding Ge or Th'f to LiEsOs containing sulfate ions, which is the base material of the gas sensitive material, the gas sensitive characteristics and its reliability are dramatically improved. This was achieved by discovering that it was possible to achieve a sufficiently high sensitivity for practical use.

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 LiFe6o8 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 sample of lithium oxide (L 120 ) and a commercially available sample of 3y ferric oxide (Fe203) were collected in sop, and wet-mixed in a stainless steel pot for 6 hours. After drying and crushing the kernel mixture,
Heat treatment was performed at a temperature of C for 2 hours.

This is ground again, and in order to contain sulfate ions, ferric sulfate (Fe2(SO2)3-XH2) is added as an additive.
0) The reagent was added for 30y and mixed for 2 hours using a sieve machine. These mixtures were equally divided into several parts, and commercially available germanium oxide, PM (G e 02 ), and thorium oxide (Th02) reagents were added to each of them individually or in combination. Each of the powders was further rotary mixed in a mill for 3 hours. 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 fired in air at a temperature of 600C for 1 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 produce a sensing element. A current was passed through this heating element, and the current value was adjusted to control the operating temperature of the element. The temperature of the element body was maintained at 400C and its gas sensitivity characteristics were measured.

The resistance value (Ra) in air was measured in a measurement container with a volume of 6oQ in which dry air was stirred slowly to the extent that no turbulence occurred.
cr) contains methane (CH) with a purity of 99% or more in this container.
4) and hydrogen (H2) gas at a volume ratio of 10
ppm was flowed in at a rate of 7 seconds, and measurements were taken when the concentration reached 0.2 volume. The gas concentration to be measured was chosen to be 0.2% because the detection concentration practically required for a gas detection element is the lower explosive limit concentration (LEL) of the gas.
This is because the LEL of each of the above gases is approximately 2 to 6 volumes.

Further, the presence of sulfate ions (SO4-) 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 sensitive bodies was 0.14 to 0.17% by weight.

FIG. 1 and FIG. 2 show the dependence of the gas sensitivity characteristics on the amount added when each additive is added individually. The sensitivity characteristics were evaluated by (i) gas sensitivity (Ra/Rq), and (11n resistance change rate over time ΔR (change rate of resistance value with respect to the initial value when the sensitive body is held at a temperature of 400°C for 2000 hours). Table 1 also shows the gas sensitivity (Ra/Rcy) and resistance change rate over time (Δ
R) is shown. Note that ΔR is written in parentheses ( ) in the table.

As is clear from Figure 1 S-Figure 2 and Table 1, Ge
Alternatively, by adding Thf alone or in combination, gas sensitivity characteristics (gas sensitivity: Ra 7 Rq
) has improved significantly. Also noteworthy is the change in resistance value over time, and the rate of change is greatly reduced by adding these additives. It can thus be seen that the addition of Ge or Tha can dramatically improve the gas sensitivity characteristics and reliability.

In the present invention, the total amount of additives is limited to 0.1 to 56e-% molti, because if it is less than 0.1 molti, as shown in Figures 1 to 2 and Table 1, the gas sensitivity characteristics and reliability will be improved. This is because no effect of improving properties is observed, and on the contrary, if it exceeds 60 molts, 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.

(Margins below) Table 1 Winter Comparative Example By the way, generally speaking, metal oxides in a somewhat amorphous state are more sensitive to physical and chemical properties such as adsorption phenomena for flammable gases than crystallized ones. It is said that the phenomenon is likely to become active. However, L produced using the commercially available reagent used in this example, which was almost completely crystallized,
Even in iF e 608, by containing sulfate ions and further adding Ge or Th, the
．． ! ,,A Once (U 7J・) Moreover, since this is stable over time, it can be seen that extremely high gas sensitivity and high reliability can be realized 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 amounts of 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 amount of sulfate ions contained on the gas sensitivity characteristics will be described.

[Example 2] Created by the same method as Example 1, 7j L i Fe 6
08100y is also a commercially available acid flower germanium (G e
02) Tho and thorium oxide (Tho2) reagents were collected in the proportions shown in Table 2, and each was mixed in a strainer for 2 hours.・Next, each mixed powder was divided into 8 equal parts, and ferric sulfate (Fe2(S04)3-XH20) solutions prepared in advance at various concentrations were added thereto.
Thereafter, each powder was mixed for 1 hour in a clay mixer. 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.

Some of the mixed powders thus obtained were heat treated in air at a temperature of 4oC 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, as a substrate for the gas detection element, an alumina substrate of 0.5 mm in thickness and 5 lines each in length and width was prepared. Gold paste was printed on the surface in a comb shape at intervals of 0.6 mm, and a pair of comb-shaped electrodes were formed by baking. Formed. 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 substrate to a thickness of about 66μ, and after air drying at room temperature, 4o.

The mixture was gradually heated to a temperature of 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 at a temperature of 4oC in the same manner as described above. It was subjected to TCI-DTA and fluorescent X-ray analysis. The presence of the separated 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 sample A-CO gas sensitivity (Ra/Rq) and contained sulfate ions with different oxide compositions, respectively. Further, Table 3 shows, as a representative example of the characteristics over time, the rate of change in resistance over time when samples A to C containing 2 to 6% by weight of sulfate ions were evaluated in the same manner as Example 1χ. 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. Further, 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. 3 to 6, if the amount of sulfate ions is less than 0.005% by weight, there is no effect of adding Ge or Th, and the effects of the present invention cannot be expected. On the other hand, 1
If the weight coefficient exceeds 0.0, 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 0.005 to 10.
The reason why it was limited to C tumor volume was based on the above-mentioned point.

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. This does not limit the starting materials or manufacturing methods 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; Of course, it is also effective.

As described in detail, the gas sensing element of the present invention contains G as an additive to L I Fe s Os containing sulfate ions.
A sintered body or sintered film doped with O or Th is used 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. Even at a relatively low temperature of 400° C., extremely high sensitivity can be achieved for methane gas, which has been considered to be the most sensitive gas. This accurately responds to the social needs that are becoming more 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 and 2 show the amount of additives and sensitivity to methane and hydrogen (Ra/R
q) and resistance time-varying rate (ΔR). Figures 3 to 5-5 show the relationship between the sulfate ion content and the sensitivity to methane and pronochloride ( FIG. 3 is a characteristic diagram showing the relationship with Ra/Rq) for three typical oxide compositions.

Claims (2)

[Claims] (1) Lithium ferrite (LtFe608) containing 0.005 to 10% by weight of sulfate ions is added with at least one of germanium (G e ) and thorium (Th) as additives, GeO2 and T
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 ho2. (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.