JPS5957153A - Gas detecting element - Google Patents

Abstract

PURPOSE:To obtain a gas detecting element having gas responsiveness and high sensitivity, especially, for CH4 at a relatively low operation temp., by using a gas sensor obtained by baking a composition prepared by adding one or more of Sn, Zr, Ti to alpha -Fe2O3 containing SO4<--> in a ratio in a specific range in a mole ratio within a specific range. CONSTITUTION:Fe2SO4-7H2O is added to alpha-Fe2O3 (which may be commercial ferric oxide) to prepare a powder mixture containing 0.005-10wt% SO4<-->. One or more of Sn, Zr and Ti and added to are mixed in the powder mixture so as to contain said elements in an amount of 0.5-50mol% as a total addition amount of the basis of SnO2, ZrO2 and TiO2 and an org. binder is added to th obtained composition. The resulting mixture is screened to prepare a granule which is, in turn, molded under heating and the molded one is sintered to prepare a sintered body as a gas sensor or the paste formed from the powder mixture is applied to a substrate by printing and the printed substrate is baked to obtain a sintered film to be used as the gas sensor. A comb shaped electrode is provided to the surface of the gas sensor while a heater to the back surface thereof to obtain a gas detecting element which is operated at about 400 deg.C and has high sensitivity, especially, for CH4 without adding a noble metal to the sensor.

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 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-Fe203) have been put into practical use, and have been applied to gas leak alarms. There is. 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.

By the way, liquefied natural gas (LN (1)), whose main component is methane gas, has come to be used for general household use in Japan and is gradually becoming popular.
Demand for gas detection elements that can detect methane gas, the main component of G, with high sensitivity is also increasing.

Of course, gas detection elements 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 problems with methane gas. They have problems such as low selectivity 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 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 with high methane sensitivity even at a relatively low operating temperature 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-Fe2es) 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. This was discovered while considering the following.

That is, in the gas detection element of the present invention, sulfate ions are 0. o
(Z-Fe203 contains at least one of Sn, Zr, and Ti as an additive in a total amount of 0.1 to 6Q moles of additives, calculated as Zr02 and TiO2, respectively) This was used as a gas sensitive material, and this is made of α-Fe203 containing sulfate ions, which is the base material of the gas sensitive material, and Sn
This was achieved by discovering that by adding Zr or Ti, the gas sensitivity characteristics and its reliability were dramatically improved, and that it was possible to achieve a sufficiently high sensitivity for practical use even to the aforementioned methane gas. It is.

Description of examples The present invention will be explained in detail below.

First, in Example 1, the amount of sulfate ions contained in α-Fe203 was kept constant, and the additives Sn and Z
A case will be described in which the amount of r or Ti added and the combination thereof are changed.

[Example 1] Commercially available ferric oxide (Fe203) (all of which was confirmed to be α-Fe203 phase by X-ray diffraction) reagent 20
To the oy-, 4Q of ferrous sulfate (FeSO4-7H20) reagent was added as an additive to contain sulfate ions, and the mixture was mixed for 2 hours using a sieve machine. Divide these mixtures into several equal parts, and add commercially available stannic oxide (
Sn02), zirconium oxide (ZrO2), and titanium oxide (Ti02) reagents were added 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 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 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 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 601 in which dry air is slowly stirred to the extent that no turbulence occurs.
) contains methane (C)I4 with a purity of 99% or more in this container.
) and hydrogen (H2) in a volume ratio of 1o p
It was allowed to flow in at a rate of pm/sec, and measurements were taken when the concentration reached 0.2 volume. Set the gas concentration to be measured to 0.
．． 2% was chosen because the detection concentration practically required for a gas detection element is the lower explosive limit concentration 1i (LEL) of the gas.
This is because the LEL of each of the above gases is approximately 2 to 6 volumes.

In addition, the presence of sulfate ions (304-→) 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. The amount of sulfate ions contained in the receptor was 0.46 to 0.58% by weight.

Figures 1 to 3 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 (ua/Rg), (11)
Resistance change rate over time ΔR (resistance change rate at 400°C
Evaluation was made based on the rate of change in resistance value from the initial value when held for 00 hours. Table 1 also shows the gas sensitivity (Ra/Rg) and the rate of change in resistance over time (ΔR) when additives are used in combination. Note that ΔR is indicated within 0 in the table.

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/
Rg) has been greatly improved. What is still noteworthy is the change in resistance value over time, and the rate of change is significantly reduced by adding these additives. In this way Sn
, it can be seen that by adding Zr or Ti, dramatic improvements in gas sensitivity characteristics and reliability can be realized.

In the present invention, the total amount of additives is set at 0.1 to 60 mol.If the total amount of additives is less than 0.1 mol, as shown in FIGS. 1 to 3 and Table 1, gas sensitivity characteristics and reliability are This is because no effect of improving properties is observed, and on the contrary, if it exceeds 60m, the resistance value itself becomes high and stability of properties is still lacking. Those marked with an X in the table correspond to these, and are listed as comparative examples in Table 1.

(Left below) Table 1 By the way, in general, metal oxides in a somewhat amorphous state are more active in physicochemical phenomena such as adsorption phenomena for combustible gases than crystallized ones. It is said that it is easy to become However, even the commercially available reagent α-Fe203 used in this example, which was almost completely crystallized, showed extremely high activity by containing sulfate ions and further adding Sn, Zr, or Ti. Moreover, since this is stable over time, 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 combination of additive amounts 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] 100 commercially available ferric oxide reagents were mixed with commercially available stannic oxide (SnOz), zirconium oxide (ZrO2), and titanium oxide (Ti02) reagents in proportions as shown in Table 2. The mixture was weighed and mixed for 2 hours using a sieve machine. Next, each mixed powder was divided into eight equal parts, and ferrous sulfate (F6SO4-7H) prepared in advance at various concentrations was added to it.
20) The solutions were added and then each powder was mixed for 1 hour, also in a mill. In this way, there are three types of oxide compositions (samples A-C) as representative examples, and samples with different amounts of sulfate ions are prepared for each oxide composition.
A total of 24 types of samples were obtained.

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, the length and width of the substrate for the gas detection element were 5 mm each.

Prepare an alumina substrate with a thickness of 0.6 mm, and coat the surface with 0.
．． A pair of comb-shaped electrodes was formed by printing gold paste in a comb shape at 6 mm intervals and baking it. Then, on the back of the alumina substrate, a commercially available ruthenium oxide glaze resistor was printed between the gold electrodes and baked to serve as a heater.

Next, the above-mentioned paste was printed on the surface of the substrate to a thickness of about 7 μm, air-dried at room temperature, and then gradually heated to a temperature of 40Q° 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. 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 Example 1. FIG. 4 to FIG. 6 show the relationship between the sample A-CO gas sensitivity (Ra/Rg) and the sulfuric acid ion contained in the samples A having different oxide compositions. Table 3 also shows the rate of change in resistance value over time when samples A to C containing 2 to 6 weight percent of sulfate ions were evaluated using the same method as in Example 1. shows. In Example 2, methane and propane were used as the test gases.

As is clear from FIGS. 4 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.

Further, as can be seen from FIGS. 4 to 6, when the amount of sulfate ion is 0.006 weight/weight groove, there is no effect of adding Sn, Zr or T1, and 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 0.0
The reason for limiting the weight to 05 to 10.0 is based on the above-mentioned reason.

Table 3 Incidentally, 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; It does not limit the starting materials or manufacturing method.

In addition, in the examples, methane, hydrogen, or propane was used as the test gas, but the effects of the present invention are by no means limited to these gases, and the effect of the present invention is not limited to these gases. Of course, it is also effective.

As described in detail, the gas sensing element of the present invention contains α-Fe203 containing sulfate ions with Sn,
This uses a sintered body or a sintered body doped with Zr or Ti as a sensitive body, and this dramatically improves gas sensitivity, making it difficult to detect trace amounts without using a precious metal catalyst. 4oo for the methane gas that came
Extremely high sensitivity can be achieved even at a relatively low temperature of °C. This is city gas natural gas (main component:
It precisely responds to social needs, which are becoming increasingly demanding as a result of the shift to methane gas, and its effects are extremely impressive. Still, 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 show the relationship between the amount of additives, the sensitivity to methane and hydrogen (Ra/1g), and the rate of change in resistance over time (ΔR) in one embodiment of the present invention. Figure 6 shows the sulfate ion content and sensitivity to methane and propane (Ra/
1g) for three typical oxide compositions. Name of agent: Patent attorney Toshio Nakao and one other person: +N Ryo
Law bK

Claims (2)

[Claims] (1) Alpha-type ferric oxide (α-Fe 203 ) containing 0.005 to 10 sulfate ions by weight is added with tin (sn'), zirconium (Zr), and titanium (Ti) as additives. At least one is S
A gas detection element characterized in that it contains 0.1 to 60 mole of carbon additives in terms of nO2, ZrO2, TiO2, and TiO2 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. Gas detection element described in section.