JPS59108948A - Gas detecting element - Google Patents

Abstract

PURPOSE:To obtain a gas detecting element, which has sufficiently high sensitivity for methane gas in actual operation, by adding a specified amount of Ge or Th to MgFe2O4 including sulfuric acid ions, which are a parent material of a gas sensitive body. CONSTITUTION:Ferric sulfate [Fe2(SO4)3-xH2O] reagent is added to magnesium (MgFe2O4), and 0.005-10wt% of oxide ions are included. Germanium oxide (GeO2) and sodium oxide (ThO2) are added to the mixture so that total amount of the added material becomes 0.1-50mol%. Then this powder is compresses and molded in a rectangular parallelopiped shape. Au is evaporated on the surface of a burned sintered body. Thus a comb shaped electrode in a unitary body is formed. A platinum heating body is stuck to the back surface of the electrode with an inorganic bonding agent, and a detecting element is formed as a heater. The reason why the total amount of the added material is limited to 0.1- 50mol% is as follows: at the value less than 0.1mol%, gas responsive characteristic and reliability are not improved; and at the value exceeding 50mol%, resistance value itself becomes high and the characteristics are not stable.

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 (S no 2) and gamma-type ferric oxide (γ-Fe2O3) have been put into practical use, and have been applied to gas leak alarms. There is. In addition, even if a situation such as a gas leak occurs, the presence of gas can be quickly detected before the LEL is reached, 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, there is an increasing demand for gas detection elements that can detect methane gas, which is the main component of LNG, with high sensitivity.

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 very stable, a detection element with sufficient sensitivity needs to be extremely active. Therefore, in order to achieve high sensitivity to methane gas, conventionally, a noble metal catalyst is added to the susceptor material, or the susceptor is operated at a relatively high temperature of, for example, 460°C or higher. Efforts have been made to In contrast, the present invention realizes an element with high sensitivity to methane even at a relatively low operating temperature of 4001: without adding any noble metal catalyst.

Structure of the invention The present invention is based on magnesium ferrite) (Mg Fe 20
This was discovered while studying the effects of various anions contained in the gas detection element using 4) as a gas sensitive material, as well as the effects of additives on the gas sensitivity characteristics. That is, G
This was achieved by discovering that adding e or Th dramatically improves gas sensitivity characteristics and its reliability, and that it is also possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas. .

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

First, in Example 1, a case will be described in which the amount of sulfate ions contained in Mg Fe204 is kept constant, and the amount of Ge or Th 2 added as an additive and the combination thereof are changed.

[Example 1] 20y of magnesium oxide (McrO) and ferric oxide (
Each of Fe203) 2 a o y was 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 1000° C. for 2 hours. This was ground again, and ferric sulfate (Fe) was added as an additive to contain sulfate ions.
2(SO2)3-xH2O) reagent was added for 25y and mixed for 2 hours using a sieve machine. These mixtures were divided equally into several parts, and commercially available germanium oxide (G) was added to each part.
eO2) and thorium oxide (The2) 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 and add 100 to 200μ
The particles were sized to . 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, Au was vapor-deposited on the surface of this sintered body to form an integrated comb-shaped electrode, and a platinum heating element was attached to the back surface with an inorganic adhesive to serve as a heater and to 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 400°C and its gas sensitivity characteristics were measured.The resistance value (Ra) in the air was measured in a volume 504 where dry air was stirred slowly to the extent that no turbulence occurred. The resistance value (R
q) contains methane (CH) with a purity of 99 or higher in this container.
4) and hydrogen (H2) gas at a volume ratio of 1op
It was flowed in at a rate of pm/sec, and each measurement was made when the concentration reached 0.2 volume. The gas concentration to be measured is set to 0.
The 2nd section was selected because the detection concentration practically required for a gas detection element is in the range of several tenths to several tenths of the lower explosive limit concentration (LEL) of the gas. This is because each LEL is approximately 2 to 5 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 0.16 to 0.19% 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 were evaluated using (1) gas sensitivity (Ra/Rq), and (ii) resistance change rate over time ΔR (rate of change in resistance value from the initial value when the sensitive body was held at a temperature of 400°C for 2000 hours). . Table 1 also shows the gas sensitivity (Ra/R (J)) and resistance change rate over time (ΔR) when additives are used in combination. ΔR is indicated in parentheses in the table. .

As is clear from Figures 1, 2, and Table 1, G
By adding e or Th singly or in combination, gas sensitivity characteristics (gas sensitivity f!j: Ra
/Rg) has been greatly improved. Also noteworthy is the change in resistance value over time, and the addition of these additives significantly reduces this change. In this way Ge
Alternatively, it can be seen that the addition of Th 2 can dramatically improve gas sensitivity characteristics and reliability.

In the present invention, the total amount of additives i is limited to 0.1 to 50 mol. If the total amount of additives is less than 0.1 mol, as shown in FIGS. This is because no effect of improving the resistance is observed, and on the contrary, if it exceeds 60 moltiwos, 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 in Table 1, they are listed as comparative examples. It is said that physicochemical phenomena such as adsorption phenomena for flammable gases are more likely to become active in metal oxides in a pure state than in crystallized ones. However, lvigF e204, which was prepared using a commercially available reagent that is almost completely crystallized, contains sulfate ions and further contains Go or Th.
It can be seen that by adding , extremely high activity is exhibited, and since this is stable over time, it is possible to realize extremely high gas sensitivity and high reliability as a result.

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, 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 entirely sintered, and the effect of the amount of sulfate ions contained on the gas sensitivity characteristics will be described.

[Example 2] MgFe2O410 created by the same method as Example 1
0y, commercially available germanium oxide (GeO2) and thorium oxide (The2) reagents were weighed out in the proportions shown in Table 2, and each was mixed for 2 hours using a mokai machine. Next, each mixed powder was divided into eight equal parts, and ferric sulfate (Fe2 (S
04) 3-xH2O) solution was added and then each powder was mixed for 1 hour, also in 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 samples with different amounts of sulfate ions for each oxide composition. Ta.

Table 2 Several of the mixed powders thus obtained were heat treated in air at a temperature of 40° 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, an alumina substrate with a length of 6 mm and a thickness of 0.5 mm was prepared, and gold paste was printed on the surface in a comb shape at intervals of 0.5 mm and baked to form a pair of comb-shaped electrodes. did. 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, it was printed to a thickness of about 65 μm on the surface of the above-mentioned PACE) f substrate, 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 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 fluorescence analysis. In addition, the presence of sulfate ions was confirmed in Example 1.
This was done by analyzing the infrared absorption spectrum in the same way as in .

The scum sensitivity characteristics of each sensing element were measured in the same manner as in the case of Example 1. Figures 3 to 5 show the relationship between the sulfuric acid ions and the sulfuric acid ions contained in samples A to C with different oxide compositions. The following shows the rate of change in resistance value over time when samples A to C containing 2 to 5 weight percent of sulfate ions were evaluated in the same manner as in Example 1. 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.

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, 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 detection element of the present invention
, 005 to 10.0% by weight based on the above-mentioned points.

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 may be within the above-mentioned range, and any It does not limit the starting materials or manufacturing method.

Furthermore, in the examples, methane, hydrogen, or propane were used as the test gases, 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 Ge or Th as an additive to Mg Fe 204 containing sulfate ions as a sensitive body. As a result, the sensitivity of methane gas in particular has been dramatically improved, and even at a relatively low temperature of 400 degrees Celsius, it has achieved extremely high sensitivity for methane gas, which had previously been considered difficult to detect in trace amounts without using a precious metal catalyst. This is something that can be achieved. 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 over time of the resistor 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 drawings]

Figures 1 and 2 show the amount of additives and the sensitivity to methane and hydrogen (Ra /Rg
) and resistance change rate over time (ΔR). Figures 3 to 6 show the relationship between sulfate ion content and sensitivity to methane and propane (Ra/Rcr) in other examples of the present invention. FIG. 3 is a characteristic diagram showing the relationship between three typical oxide compositions. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 3
Figure A: Amount of LM ions C weight: Figure 4: Amount of Nanaate ions (V amount λ)

Claims (2)

[Claims] (1) Magnesium ferrite (MgFe2O4) containing o, oos to 10 sulfate ions by weight, germanium (Ge) and thorium (T
A gas sensing element characterized in that at least one of h) contains 0.1 to 50 mole of additives in total in terms of GeO2 and Th02, 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 thin paste. Gas detection element described in section.