JPS6170449A - Gas detecting element - Google Patents

Abstract

PURPOSE:To detect reducing gas with high sensitivity and good selectivity by providing a thin In-Sn-O film on an insulating substrate and providing further a catalyst layer such as Pd thereon thereby constituting a gas detecting element. CONSTITUTION:A pair of electrodes 2 are provided on an insulating substrate 1 such as alumina. An org. soln. contg. indium octylate, etc. and tin octylate, etc. is coated thereon and is thermally decomposed to form a thin In-Sn-O film 3. The catalyst layer 4 deposited with at least one kind among Pt, Pd and Rh on Al2O3 is laminated thereon, by which the gas detecting element is obtd. The element is brought into contact with the reducing gas and the gas is detected from the change in the electric resistance. Since the thin In-Sn-O film is provided, the detection of the CO, CH4, C3H8, etc. which are the reducing gas with high sensitivity is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a gas detection element, and particularly to a gas detection element that detects reducing gases in the atmosphere with high sensitivity and selectivity.

[Technical background of the invention and its problems]

Conventionally, gas detection elements using sintered bodies of metal oxide semiconductors such as sno zno and 2'Fe2O3, which exhibit N-type semiconductor characteristics, have been known as elements for detecting reducing gases in the atmosphere. When these metal oxide semiconductors come into contact with a reducing gas, their electrical conductivity increases, and the gas is detected by measuring the change in electrical resistance at that time.

By the way, in recent years, in response to the demand for smaller devices and multi-functionality, research has been progressing on thin film type elements instead of the above-mentioned sintered type gas sensing elements. Such a thin film type element is made into a thin film WI structure by depositing a metal oxide semiconductor having the gas-sensitive ability as described above using a thin film forming method such as a sputtering method, a vapor deposition method, or a CVD method.

Regardless of whether the gas sensing element is a sintered type or a thin film type,
In general, metal oxide semiconductors alone have low sensitivity and insufficient selectivity as gas detection elements, so negative metals such as platinum (Pt) and palladium (Pd) are usually used as catalysts to detect gas detection elements. Attempts are being made to increase sensitivity. However, methods have been adopted in which Pt or Pd is directly added to a metal oxide semiconductor, or a catalyst layer supporting Pt or Pd is formed on the metal oxide semiconductor.

Although such treatment improves sensitivity compared to the case without catalyst, it still does not exhibit sufficient sensitivity to reducing gases at low concentrations. Moreover, when various reducing gases are mixed, it is extremely difficult to selectively detect only one reducing gas with high sensitivity because the influence of other reducing gases may induce malfunction of the element. . In particular, it has been extremely difficult to detect a gas such as CO, which has an adverse effect on the human body even at low concentrations, while excluding malfunctions caused by other reducing gases. Generally speaking, when it is assumed that the gas detection element will be used in a general household, miscellaneous gas,
In particular, eliminating malfunctions caused by alcohol vapor is an important issue.

From this point of view, as a conventionally proposed gas detection element, an element in which a thin film of stannic oxide is formed (Japanese Patent Application No. 1983) has been proposed.
38641) or an element formed with an indium oxide thin film (Japanese Patent Application No. 59-38642), but further improvements in sensitivity and selectivity are desired.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned points, and detects low concentration reducing gases with high sensitivity. Ti
i! The present invention aims to provide a thin film type gas detection element that selectively and sensitively detects methane (CI-1> and propane (C3H8) at 350 to 450°C).

[Summary of the invention]

In order to achieve the above object, the gas detection element according to the present invention includes:
A 1n-Sn-0 thin film created by thermally decomposing an organic compound containing in and Sn is provided on the surface of an insulating substrate provided with a pair of electrodes so as to cover the electrodes, and further the i1 It is characterized in that a catalyst layer in which Al2O3 supports at least one member selected from the group consisting of Pt, Pd, and Ril is laminated on the crotch.

[Detailed description of the invention]

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

FIGS. 1 and 2 show a specific example of the present invention, with FIG. 1 being a cross-sectional view of a cylindrical element, and FIG. 2 being a perspective view of the element in use.

First, in FIG. 1, reference numeral 1 denotes a cylindrical base made of an insulating material such as alumina or mullite, and a pair of electrodes 2 are provided on the outer peripheral surface of the base 1. An In-Sn-0 thin film 3 is provided covering the substrate 1 and the electrodes 2, and a catalyst layer 4 is laminated thereon to cover the entire structure, thereby forming the element of the present invention.

Here, the thickness of the In-Sn-0-based thin film is 10o8 to 1
It is preferable that the thin film is in the range of 1 μm; if the thickness of the thin film exceeds 1 μm, its sensitivity to reducing gases will decrease;
Even when the number of people is smaller than 000, sufficient sensitivity cannot be obtained.

Further, the thickness of the catalyst layer 4 is preferably in the range of 10 to 50 μmrL, and if it is outside this range, the catalyst effects such as sensitivity and selectivity will deteriorate.

As shown in FIG. 2, the element of the present invention thus constructed is mounted and held on top of the bin 6 erected on the insulating plate 5 without contacting other parts.

In the figure, 7 represents a lead wire for the electrode, and 8 represents a heater, which is provided to adjust the surface temperature (operating temperature) of the element.

Next, a method for creating an In-Sn-0 thin film will be described.

The In-Sn-0 thin film according to the present invention has in and Sn
It is formed by thermally decomposing an organic compound containing . First, In or Sn metal soap (for example, indium octylate, tin octylate) or Sn or i
Resin salts and alkoxides containing n, as well as Sn and [
A predetermined amount of an organic compound containing ln and Sn, such as an organometallic compound containing n, is added to toluene, benzene, n
- Prepare a raw material solution for forming a thin film with a desired concentration by dissolving it using an appropriate solvent such as butyl alcohol.

Next, this solution is applied to the outer peripheral surface of the insulating substrate 1 having a pair of electrodes 2, left in the air for a predetermined time (usually 30 minutes to 1 hour), and then heated to an appropriate temperature (usually 120°C). to vaporize the solvent used. After that, the whole thing was heated in air for 30 minutes to 1 hour at 400 to 700'.
When fired at a temperature of C, the organic compound containing In and Sn is thermally decomposed to form an In-Sn-0 thin film.

This coating-baking process is repeated about 1 to 4 times to form a thin film with a predetermined thickness, although it varies depending on the concentration of the raw material solution used and is not generally determined.

In the composition of the thin film, the content ratio of Sn to in is 1sn/In)xlOO), and if it is less than about 10%, Sn acts as an impurity of indium oxide, and if it is more than 10%, it becomes indium oxide and tin oxide1,) polycrystalline. It is thought that .

Next, a catalyst layer 4 is layered 8 on the membrane 3 thus provided by the following method.

The catalyst layer 4 according to the present invention is made of aluminum oxide (A120
3) Palladium (Pd), platinum (Pt), rhodium (
Rh) or palladium-platinum (Pt-
Pt), palladium-rhodium (Pd-Rh), platinum-
It is composed of a catalyst supporting any one of rhodium (Pt-Ril).

This catalyst is manufactured as follows.

First, for example,
Chloride 2° such as dCl RhCl -3H20
3 or (NH4)2Pt016°(NH)PdC16(NH4)3RhC16 to prepare an aqueous solution of pdlRh at a predetermined temperature. When Pd, Pt, and R11 are individually supported on Al2O3, a predetermined amount of A I 203 is immersed in each aqueous solution, and Pd-Pt, Pd-
When Rh, Pt-Rh is supported on Al2O3,
Aqueous solutions of Pd, Pt, and Rh are mixed at a predetermined ratio to obtain a mixed solution, into which a predetermined m of Al2O3 is immersed.

After sufficiently stirring and mixing the two, the mixture is dried under reduced pressure, for example, for 1 to 2 hours, and then heated and dried at about 100°C. For example, crush this in a mortar, make a powder, and put it in a quartz crucible for 40 minutes.
Calcinate at a temperature of 0 to 800°C. Thus, A I 2
Pd of predetermined m in 03.

Catalysts supporting Pt, Rh, Pt-Pt, Pd-Rh, and Pt-R11 are obtained.

At this time, when each of P(j, Pt, and Rh is supported on AI 203 alone, m is preferably in the range of 0.05 to 20.0% by weight based on the weight of AI 203,
Outside this range, it will not contribute to improving the sensitivity of the element. In addition, the A I 20 of Pd-Pt1Pd-Rh1pt-Rh
Regarding the loading ω on 3, in the case of Pd-Pt and Pd-Rh, Pd is 0.05 to 20.
0% by weight, and the atomic ratio of Pt and Rh to Pd (Pt
/Pd or Rh/Pd) is preferably in the range of 0.05 to 1.0, and in the case of Pt-Rh, Pt is A 1203(7) Iffik: vs. 1.0. ．． ,0.05～
20.0tffi% and the atomic ratio of Rh to Pt (
Ph/Pt)rO is preferably in the range of 05 to 1.0.

The catalyst thus prepared is then made into a slurry using, for example, an aqueous solution of aluminum hydroxychloride as a binder, and this slurry is applied onto a thin film to a predetermined thickness and dried, followed by heating at 300 to 400°C.
The catalyst layer according to the present invention is formed by firing at a temperature of .

(Embodiments of the invention) Example 1 Indium octylate and tin octylate were used as raw materials for a 1n-Sn-0 thin film, and metal P, metal (7)
Content m, content rate ((Sn/In) Xi 00
) to prepare a raw material solution by dissolving it in toluene to a concentration of 50%.

Next, a pair of electrodes 2 as shown in FIG. 1 were coated on the outer surface of the cylinder of the insulating base 1, which was left in the air for one hour, and then heated to 120'C to vaporize the toluene. Ta.

The whole was then fired at 500° C. for 1 hour in air. This coating-baking process was repeated three times to form a thin film with a thickness of about 3,000 layers.

Subsequently, a catalyst layer was formed on this thin film. First, PdC
Pd1 by dissolving Pd1 in water. An aqueous solution containing O type % was prepared. Here, A I 2 with a surface area of about 100 TIL/g
After removing the 03 fine powder, it was evaporated to dryness. Next, the powder was crushed in a mortar and placed in a quartz crucible for 40 minutes.
It was fired at 0°C.

This catalyst powder was put into water and an aqueous aluminum hydroxychloride solution (A12031%) to form a slurry. This slurry was applied onto the In-Sn-0-based R film, dried, and the entire film was fired at 400°C. In this way, 1.0% by weight of Pd in the Pd supporting port with a thickness of 20μ
A d-AI 203 catalyst layer was formed.

Similarly, Pt-Al ORh-23・A I 203 catalyst and Pd-Pt-Al
OPd-Rh-At OP'll knee 2 3.
2 3'Rh -A I
A device using a 20 s catalyst was created.

In addition, in the catalyst, Pd, Pt, and Rh supported individually on Al2O3 were all 1.0% by weight based on the weight of A I 203. Also, Pd-
In the case of Pt and Pd-Rh, the Pd supported on Al2O3 was 1.0% by weight, and the atomic ratio of Pt and Rh to Pd was 0.5. Regarding Pt-Rh,
Pt is 1.0% by weight based on A I 203, Rh is Pt
The atomic ratio was 0.5.

6 produced in this way, each with a different type of catalyst layer.
A device as shown in Fig. 2 was assembled from different types of gas detection elements, and this was used to detect Go, )-I at a concentration of 200 ppm.
The sensitivity to each gas of CHCH and 02 H5011 at concentrations of 2° 4° 38 1000 ppm was measured as R·/R. operating temperature air
Gas was conducted at 100°C and 400°C. Here, R is the resistance value exhibited by the element R in air that does not contain the measurement gas, and R is the resistance value exhibited by the element in air containing each of the above gases at a concentration of aS. Therefore, air g with large R,/R
As means higher sensitivity.

The measurement results are shown in the table below.

As is clear from the table, no matter which catalyst is used, at an operating temperature of 100°C, CO2001l
l) It can be seen that the III sensitivity is higher than that of C2H3oH11000u, showing extremely high sensitivity to CO gas. On the other hand, at 400℃, C3H8, CH4200p
It can be seen that the pm@degree is greater than the C2H5oH1000ppm sensitivity.

Example 2 Elements were prepared in the same manner as in Example 1 using In-Sn-0-based thin films with varying amounts of In and Sn, and the sensitivity to the Sn content relative to In was determined. , was measured under the same conditions as in Example 1.

CH4 (200
FIG. 3 shows an example of the measurement results for the DDffl) gas. As is clear from Fig. 3, In203 (In
-0 series) alone or S n 02 (S n -0
system) than when forming a thin film alone, In-Sn-0
The sensitivity is higher when system 11Q is used, especially when i
It can be seen that excellent sensitivity is exhibited in a region where the Sn content relative to n is approximately 50%. Such a tendency was similarly observed even when the type of catalyst layer or the gas to be measured was changed.

〔Effect of the invention〕

As is clear from the results of the above examples, the gas sensing element of the present invention exhibits high sensitivity to low concentration reducing gases,
Furthermore, it has excellent sensitivity characteristics to CO gas in the low temperature range from room temperature to about 120°C, and CH4 in the high temperature range of about 350 to 450°C.

It shows high sensitivity to C3H8 and has excellent selectivity. Therefore, by changing the operating temperature, 02H5
Each) 1 reducing gas can be detected with high sensitivity while eliminating malfunctions caused by miscellaneous gases such as 0H.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a gas detection element according to the present invention, FIG. 2 is a perspective view of a gas detection element to which the element is applied, and FIG.
3 is a graph showing changes in sensitivity characteristics with respect to the composition ratio of Sn and Sn. DESCRIPTION OF SYMBOLS 1... Insulating base, 2... Electrode, 3... Thin film, 4...
...Catalyst layer, 5...Insulating plate, 6...Bin, 7...
Lead wire, 8...heater. Applicant's agent Kiyoshi Inomata Figure 1 Figure 2

Claims (1)

[Claims] An In-Sn-O thin film created by thermally decomposing an organic compound containing In and Sn is provided on the surface of an insulating substrate provided with a pair of electrodes so as to cover the electrodes, and further the thin film is A gas sensing element characterized in that a catalyst layer formed by supporting Al_2O_3 on at least one member selected from the group consisting of Pt, Pd, and Rh is laminated thereon.