JPH0444691B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to improvements in gas detection elements. [Technical background of the invention and its problems] Conventionally, mainly for detecting reducing gases in the atmosphere,
BACKGROUND OF THE INVENTION Gas detection elements whose main constituents are metal oxide semiconductors belonging to n-type semiconductors such as SnO ₂ , Zno, and Fe ₂ O ₃ are used. When the metal oxide semiconductor of such a gas sensing element comes into contact with a reducing gas, positive ions are adsorbed, so its electrical conductivity increases and its resistance value decreases. Further, when an oxidizing gas comes into contact with the metal oxide semiconductor, negative ions are adsorbed, so that the electrical conductivity of the metal oxide semiconductor decreases and the resistance value increases. However, metal oxide semiconductors alone generally have low sensitivity and are not always fully satisfactory as gas detection elements. For these reasons, recently, efforts have been made to add noble metal catalysts such as Pt and Pd to metal oxide semiconductors, and to cover the semiconductors with catalyst layers that support noble metals on carriers such as Al ₂ O ₃ . Therefore, attempts have been made to improve the sensitivity. However, these gas sensing elements have had problems in terms of moisture resistance. In particular, in high humidity of 90% or more, a phenomenon of low resistance may occur, or if exposed to high humidity for a long period of time, gas sensitivity characteristics may deteriorate significantly. There were problems when using it under high humidity. [Object of the Invention] It is an object of the present invention to provide a gas sensing element that has not only high sensitivity but also excellent moisture resistance. [Summary of the invention] The present invention provides a gas sensitive body made of a metal oxide semiconductor on an insulating substrate or an insulating tube provided with a pair of opposing electrodes and a heater, and on the surface of the gas sensitive body, Pt, Pd, Rh, Ir,
providing a first catalyst layer made of at least one insulating material selected from Al ₂ O ₃ , SiO ₂ , ZrO ₂ containing at least one noble metal selected from Ru;
Further, by providing a second catalyst layer made of the same insulating material containing Ag on the first catalyst layer, gas sensitivity is improved by the first catalyst layer, and moisture due to Ag in the second catalyst layer is improved. The reaction with the catalyst prevents moisture from diffusing into the first catalyst layer below,
The goal is to obtain a gas sensing element that has improved moisture resistance while maintaining high sensitivity. The content of noble metal in the first catalyst is 0.05 to 40 ^wt.
It is desirable to set it in the range of %. The reason for this is that if the precious metal content is less than ^{0.05 wt} %, sufficient sensitivity improvement cannot be expected;
If it exceeds %, it does not contribute to improving the sensitivity of the element, and the initial change over time increases. The Ag content in the second catalyst is preferably 0.05 to 20 ^wt %. The reason for this is that the Ag content
If the amount is less than 0.05 ^wt %, the moisture resistance effect will not be sufficiently achieved, while if the amount exceeds 20 ^wt %, the added effect will not be apparent and the gas sensitivity will rather decrease. In addition, the thickness of the above gas sensitive body is 0.2 μm to 1 μm,
The thickness of each catalyst layer is preferably about 10 to 50 μm. [Embodiments of the Invention] Next, embodiments of the present invention will be described with reference to the drawings. Example 1 First, a predetermined amount of tin octylate was weighed and dissolved in n-butanol to prepare a 10 ^wt % tin octylate solution. Next, this solution was applied to an insulating tube 2 in which a pair of opposing electrodes 1 ₁ and 1 ₂ were formed as shown in FIG.
It was applied to the outer circumferential surface of the container, dried in air for about 30 minutes, and then dried at about 120° C. for 30 minutes to evaporate the n-butanol. Subsequently, it was fired at 400 to 700°C for 30 minutes to produce a 0.5 μm thick cylindrical gas sensitive body 3 made of a SnO ₂ thin film. In this case, rapid heating and cooling is preferred. Further, in order to control the film thickness, it is desirable to repeat the coating and baking steps 1 to 4 times. Next, a predetermined amount of Al ₂ O ₃ powder was added to the chloroplatinic acid aqueous solution and thoroughly mixed, followed by drying under reduced pressure for 1 to 2 hours, and then thoroughly drying at about 100°C. Continuing,
This dried material is powdered and used in a quartz crucible etc.
A catalyst containing 1 ^wt % Pt was prepared by calcining at 400-800°C. Subsequently, this catalyst is added to the gas sensitive body 3.
Coat it on the outer circumferential surface of the
A 20 μm cylindrical first catalyst layer 4 was produced. Next, a predetermined amount of Al ₂ O ₃ powder was added to the silver nitrate aqueous solution and thoroughly mixed, and then a catalyst containing 10 ^wt % Ag was prepared in the same manner as the above catalyst. Continuing,
After applying this catalyst to the first catalyst layer 4, approximately
A cylindrical second catalyst layer 5 having a thickness of 30 μm was prepared by firing at 300 to 400° C., and an element body 6 was manufactured (as shown in FIG. 1). Next, the element main body 6 shown in FIG. 1 is placed on the insulating plate 8 through which the six terminals 7 are supported, and the heater coil 9 is inserted into the insulating tube 2 of the main body 6 .
Both ends of the coil 9 were connected to two of the terminals 7, respectively. Next, two lead wires 10 are connected to the pair of opposing electrodes 1 ₁ and 1 ₂ of the element body 6 , respectively.
and connected their other ends to the remaining terminals 7... to assemble the gas detection element (as shown in Figure 2). Examples 2 to 12 Pd, Rh, Ir, Pt-Pd, Pd- as the first catalyst layer
Rh, Ir−Pt, Pt−Pd−Rh, Pd−Rh−Ir, Ir−
Al ₂ O ₃ containing 1 ^wt % each of Pt-Pd and Pt-Pd-Rh-Ir
Eleven types of gas sensing elements were assembled using the same steps as in Example 1, except that the elements consisting of the following were used. Therefore, for the gas detection elements of Examples 1 to 12, the initial resistance value in 200 ppm CO gas and 40
The resistance value was examined after being left in the same CO gas at 90% RH for 1000 hours. The results are shown in Table 1 below.
In addition, as a reference example, the initial resistance value of a gas detection element consisting only of the first catalyst layer and the temperature at 40℃ and 90%RH
The resistance values after being left for 1000 hours are also listed in Table 1 below.

【table】

[Table] As is clear from Table 1 above, the gas sensing element of the present invention has excellent moisture resistance, with almost no change in resistance from the initial resistance even after being left in high humidity for a long period of time. Examples 13 to 24 A gas sensitive body similar to that of Example 1 was provided on a 7 ^L × 4 ^W × 0.3 ^t mm Al ₂ O ₃ substrate having a pair of electrodes, and Examples 1 to 12 were placed on this gas sensitive body. A first catalyst layer having the same composition as that of Al ₂ O 2 is formed by a sputtering method, and a second catalyst layer made of Al ₂ O ₃ containing 1 ^wt % of Ag is provided on the first catalyst layer by a sputtering method _. Twelve types of gas detection elements were assembled by forming heaters on the back side of the substrate. Therefore, for the gas detection elements of Examples 13 to 24, the initial resistance value and 40
The resistance value was examined after being left in the same CO gas at 90% RH for 1000 hours. The results are shown in Table 2 below.
As reference examples, gas sensing elements of Examples 13 to 24 in which only the first catalyst layer was formed by sputtering are also listed in Table 2.

【table】

〔Effect of the invention〕

As described in detail above, according to the present invention, it is possible to provide a gas detection element that has high sensitivity and moisture resistance and can perform stable gas detection over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing an element body used in a gas sensing element, and FIG. 2 is a perspective view of a gas sensing element showing an embodiment of the present invention. 1 ₁ , 1 ₂ ... Counter electrode, 2 ... Insulation tube, 3 ...
Gas sensitive body, 4...first catalyst layer, 5...second catalyst layer, 6 ...element body, 8...insulating plate, 9...heater coil.

Claims (1)

[Claims] 1. A gas sensitive body made of a metal oxide semiconductor whose resistance value changes when it comes into contact with gas on an insulating substrate, a pair of electrodes for detecting the resistance of the gas sensitive body, and a gas sensitive body that mainly heats the gas sensitive body. Al ₂ O ₃ containing at least one noble metal selected from Pt, Pd, Rh, Ir, and Ru in a gas detection element having a structure in which a heater is provided and a catalyst layer is provided on the gas sensitive body;
A first catalyst layer made of at least one insulating material selected from SiO ₂ , ZrO ₂ is provided on the gas sensitive body, and Al ₂ O ₃ containing Ag,
A gas sensing element comprising a second catalyst layer made of at least one insulating material selected from SiO ₂ and ZrO ₂ .