JPS5999243A - Gas sensitive element - Google Patents

Abstract

PURPOSE:To obtain a gas sensitive element having good responsiveness and excellent durability by forming thin films having specific thicknesses with a carrier consisting of Al2O3, etc. and a catalyst layer contg. a specific amt. of catalyst metal, such as Pt. CONSTITUTION:The surface of an Al2O3 substrate 1 is polished to a specular surface and a pair of comb-shaped electrodes 2 consisting of Au, etc. ate provided thereon by screen printing, etc. A heater 3 consisting of RuO2, etc. is provided on the rear of the substrate 1. A target consisting of SuO2 doped with Sb<5+> is used and a gas sensitive body 4 having 25nm film thickness is formed by a sputtering method between the electrodes 2. A target formed by mixing 1- 80wt% catalyst metal, such as Pt, with Al2O3 power is used and a catalyst layer 5 having 5-1,000nm film thickness is formed by a sputtering method on the body 4. A gas sensitive element having good responsiveness and excellent durability is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a gas-sensitive element, particularly to a gas-sensitive element having a catalyst layer.

[Technical background of the invention and its problems]

2. Description of the Related Art Conventionally, various studies have been conducted on gas-sensitive elements using gas-sensitive materials such as SnOt-based oxide semiconductors whose resistance changes upon contact with various gases. In such a gas-sensitive element, a catalyst is used for the purpose of increasing the detection sensitivity to gas, and one structure of a gas-sensitive element using this catalyst is one in which a catalyst layer is provided on a gas-sensitive member. be.

As such a catalyst layer, a thick film in which a catalytic metal such as Pi is mixed into a carrier such as A-0 or 01 is generally used.

However, since the thick film is formed through a process of applying and sintering a paste-like raw material, there is a problem that the reproducibility is very poor and the characteristics of the gas-sensitive element may vary. Furthermore, since the thickness of the thick film is on the order of 10 .mu.tn, there is a drawback that the response speed during gas detection is relatively slow. In addition, gas-sensitive elements are usually equipped with a heater and detect gas while heating the gas-sensitive element.
When the film is thick like this, a temperature gradient occurs in the catalyst layer, which tends to generate thermal stress, and there is a risk that cracks or the like may occur in the catalyst layer. Furthermore, if the catalyst layer is thick, it is difficult to accurately set the temperature reaction of the gas sensitive body due to heat capacity, etc.
There was also the problem that variations occurred in the characteristics of the gas-sensitive elements.

In order to overcome the drawbacks of using a thick catalyst layer as described above, research has been conducted into using a thin film made of a catalytic metal as the catalyst layer. Thin films are formed by sputtering, vapor deposition, etc. without going through a sintering process, and the film thickness can be reduced to a few nanometers with good reproducibility, so although the above-mentioned drawbacks can be overcome, new problems arise. A dot is produced.

In other words, the catalytic metal aggregates under high temperatures when the gas-sensitive element is used.
The problem is that it recrystallizes and reduces its catalytic ability. This is because the catalyst layer is a porous layer so that the gas comes into contact with the gas sensitive member, but if it aggregates and recrystallizes in this way, it will no longer be able to maintain its porous state.

[Object of the Invention] The present invention has been made in consideration of the above points, and an object of the present invention is to provide a gas-sensitive element having good responsiveness and excellent durability.

[Summary of the invention]

The present invention includes a substrate, a gas sensitive element provided on the substrate and whose resistance value changes upon contact with a gas to be measured, a pair of electrodes provided on the gas sensitive element, and a gas sensitive element provided on the surface of the gas sensitive element. In the gas-sensitive element having a catalyst layer, the catalyst layer includes a carrier made of at least one of A403 and S A02゜Z r02, and 1% by weight based on the catalyst layer.
~80% by weight Pt, Pd.

- A gas-sensitive element comprising a thin film with a thickness of 5 nm to 11,000 nm and containing a catalyst metal consisting of at least one type of Rh. In the present invention, the substrate is A403,
A heat-resistant and insulating substrate such as a ceramic substrate such as 8A3N4, BN, 81o2, etc. is used, and the electrode is made of Au, Pt, etc., and a film is formed by a screen printing method, a sputtering method, a vapor deposition method, or the like. These electrodes are provided facing each other on the gas sensitive member, and may be provided either between the gas sensitive member and the substrate or between the gas sensitive member and the catalyst layer.

The gas to be measured is a reducing gas such as CO or methane, and the gas sensitive material is a commonly used 5nOt system,
An oxide semiconductor, such as ZnO-based or Fe1OB-based, whose resistance value changes when it comes into contact with the gas to be measured is used. This Sn
The main components of the O, ZnO, and Fe103 oxide semiconductors are 5not, ZnO, Fe, and 01, respectively, and Nb'', Sb'', Sb'', A13'', and C as necessary.
It contains subcomponents such as r3+. This gas sensitive body is formed into a film by a sputtering method, a vapor deposition method, a coating sintering method, a thermal decomposition method of an organic compound, or the like.

Next, the composition ratio and film thickness of the catalyst layer in the present invention will be described.

The catalyst layer includes at least one catalyst metal selected from Pd, Pt, and Pt, and 620B, S10. , Z
r□, and at least one type of carrier. This catalytic metal is used to improve gas-sensitive characteristics such as gas responsiveness and gas selectivity, and the carrier prevents deterioration of gas-sensitive characteristics due to agglomeration of the catalytic metal when using a gas-sensitive element. It is used to

If the content of this catalytic metal is less than 1% by weight, the catalytic ability of the catalyst layer will not be fully exhibited, and if it exceeds 80% by weight, the catalyst layer will not be able to maintain its insulation properties. Gas detection is performed by measuring changes in the resistance value of the gas sensitive body, but if the insulation of the catalyst layer provided on the gas sensitive body is not maintained, not only the resistance value of the gas sensitive body itself but also the gas Since the resistance value between the sensitive body and the catalyst layer is measured, the accuracy of gas detection decreases. Furthermore, if the resistance value of the catalyst layer becomes smaller than the resistance value of the gas sensitive body, it becomes difficult to measure the resistance value of the gas sensitive body, and it becomes virtually impossible to detect the gas.

For the above reasons, the weight ratio of the catalyst metal in the catalyst layer is limited to 1% to 80% by weight relative to the catalyst layer.

In the gas-sensitive element of the present invention provided with such a catalyst layer, since the catalyst layer and the gas-sensitive element are manufactured separately, it is possible to set optimal manufacturing conditions for each, and there is freedom in manufacturing the gas-sensitive element. The degree increases. Furthermore, if a catalytic metal is mixed into the gas-sensitive element, its dispersion state may change as the gas-sensitive element is used, and the characteristics such as the resistance value of the gas-sensitive element may change. This fear does not exist in the case of the present invention in which a catalyst layer containing a metal is provided.

Next, the thickness of the catalyst layer will be described.

This is because if the film thickness is less than 5 nm, the catalytic ability of the catalyst layer will not be fully exhibited, and if it exceeds 11,000 nm, the response speed to the gas to be measured will become slow.

This response speed includes a rise speed when it comes into contact with the gas to be measured and a speed to return when the gas to be measured is removed, but both become slow when the film thickness exceeds 11000 nm. Particularly when used in a device that warns of danger such as CO as a gas to be measured, if the response speed is slow, especially when standing up and inhaling quickly, gas detection will be delayed, which is very dangerous. For the above reasons, the thickness of the catalyst layer is limited to 5 nm to 11,000 nm.

Further, for the catalyst layer, a thin film formed by a vapor deposition method, a sputtering method, or the like without a sintering process is used.

When using a thick film in which a paste-like catalyst layer raw material is applied and sintered, a film thickness of only about 102 μm can be obtained, and it is difficult to obtain a catalyst layer with a desired film thickness of 6 to 11,000 nm in the present invention. Can not. Furthermore, when a thick film formed through a sintering process is used, thermal distortion during sintering remains, resulting in poor durability.

In addition, there is a limit to the sintering temperature in order not to deteriorate the characteristics of the gas sensitive material, and sufficient strength cannot be obtained, resulting in poor durability. For the reasons mentioned above, a thin film is used as the catalyst layer.

As a method for forming such a catalyst layer, it is preferable to use a sputtering method, which allows easy control of the composition ratio and allows fine and uniform dispersion of the catalyst metal in the catalyst layer. When forming a catalyst layer by a sputtering method, a material containing a catalyst metal and a carrier in a desired ratio may be used as a target, or a material containing a catalyst metal and a carrier in a desired ratio may be used as a target.
After forming a thin film using a material containing at least one of Zr and Zr, a catalyst layer can be formed by heating and oxidizing it in an atmosphere containing oxygen. It is preferable to use a material containing at least 1--m of +Zr, and after forming a thin film in an oxygen-containing atmosphere, to perform a heat treatment in the air or the like. The same applies when a material containing a catalyst metal and a carrier is used as the target. In this way, the target used in the sputtering method may be a mixture of raw material powders, an alloy, or a target material made of a single material with a part of its surface coated with another material. It can also be used as a target. Furthermore, it may be carried out using a binary sputtering method, in which case various compositions can be realized with the same target.

In addition, gas-sensitive elements are generally equipped with a heater to heat the gas-sensitive element, but by making the catalyst layer thinner, the heat capacity becomes smaller, so the output of the heater can be suppressed, which extends the lifespan of the heater. This will extend the life of the gas-sensitive element.

Furthermore, the catalyst layer is required to be in a porous state so as not to prevent contact between the gas sensitive body and the gas to be measured. When the catalyst layer is sufficiently thin, for example, when the film thickness is 10 nm or less, the thin film will grow in the form of islands even if a normal sputtering method is used, so that a substantially porous state will be realized. Also,
For example, Ar * 02 +N etc. 103M To r r
By performing sputtering under high gas pressure, gas is trapped in the formed catalyst layer, and by removing the gas (by heating), a shibbolus state can be achieved. . Furthermore, it is also possible to mix an organic material such as PVA into the target and perform sputtering to form a catalyst layer containing the organic material, and then remove the organic material by heating to achieve a porous state.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to provide a gas-sensitive element with good responsiveness and excellent durability.

[Embodiments of the invention]

Examples of the present invention will be described below.

FIG. 1 is a diagram showing an embodiment of the present invention, and is a sectional view of a gas-sensitive element.

7 cabinets x 4 fl x 0.3-A Swamp 208 board (1
) was polished to a mirror finish, and screen printing was used to print the standard (i)
ffl (A pair of comb-shaped electrodes made of Au are provided as 21.A heater (3) made of RLI02 is provided on the back surface of the AA20° substrate (1) by screen printing method). The electrode (2) and heater (3) each have a lead (
2)', (3)/ is provided. Further, when the electrode is made into a thin film by vapor deposition or the like, the lead connection part can be made into a thick film to increase the adhesive strength with the substrate at the lead connection part. Next, a gas sensitive material (4) having a thickness of 25 nm is formed between the electrodes (2) by sputtering using a target in which 5no2 is doped with sb5+. Next is AA203
A catalyst layer (5) having a thickness of 5 nm is formed on the gas sensitive body (4) by sputtering using a target formed by mixing 1% by weight of PT with powder. Sputtering is performed, for example, in a mixed atmosphere of Ar, O, and more than 0.01 WHg, and after forming a thin film, the temperature is raised to 500 C in the air, and A is also stabilized by oxidation of 03.

In this example, a target containing a mixture of A, e, O, and Pt was used to form the catalyst layer, but sputtering was performed using a mixture of A4 and Pi as a target to form a thin film in which carbon and pt were finely dispersed. After forming, a is kept in the atmosphere at about 500C for about 1 hour to oxidize a, and AA,
0. A catalyst layer consisting of Pt and Pt may also be formed. Also A
Using a mixture of AA20.4 and Pt as a target, sputtering is performed in an atmosphere of 50% oxygen and 50% argon, for example. A thin film catalyst layer consisting of and Pi may be formed. In general, metals have a faster sputtering speed than oxides, so when a mixture of A, etOs and Pt is used as a target, it is desirable to reduce the amount of Pt to a desired composition ratio. Also, when sputtering is performed in an atmosphere containing oxygen using a mixture of PT and nitrate as a target, if the oxygen concentration is high, AA,
03 has a slower sputtering speed.

The characteristics of the gas-sensitive element of the present invention formed as described above are shown in FIGS. 2 to 4.

FIG. 2 is a diagram showing the gas response characteristics in a 200 ppm CO gas atmosphere, and shows the change in resistance value over time with respect to the saturation value of the resistance value of the gas sensitive body in a 200 ppm CO gas atmosphere. As comparative example-1, Pt-Pd-A120B (Pt/A right 0
3-1/100 (weight ratio), Pt/Pd = 2/1
(atomic ratio)) and a catalyst layer with a film thickness of about several hundred μm is easily provided on the surface of a 5no2 gas sensitive material.
Similar measurements were made. The element temperature was set at about 1000 in both Example and Comparative Example-1.

In the case of the example of the present invention (curve a), the resistance value saturates and becomes a stable value at 1 minute root degree, whereas in the case of comparative example-1 (curve b
), it can be seen that the resistance value does not saturate even after 3 minutes and gradually approaches the saturation value. Therefore, the gas-sensitive element of the present invention has better gas responsiveness.

Furthermore, Fig. 8 shows the change in sensitivity over time.

For the gas-sensitive element, the element temperature at the time of gas detection was set to 1000, and after measuring the resistance value Ra I r in the air atmosphere, the CO gas was set at 20
0 ppm atmosphere 9 series Ra ir/Rga s was calculated as gas sensitivity.

This measurement is performed every 2 hours, and after measuring Rgas
Heat cleaning was performed for C1 minute.

For comparison, the above-mentioned Comparative Example-1 and Comparative Example-2 have the same structure as this example, but the catalyst layer is made of only Pt and has a thickness of 5.
Prepare a thin film by sputtering of nm size,
Similar measurements were made.

As is clear from FIG. 8, the embodiment of the present invention (curve a)
Compared to the comparative example-
1 (curve b) and Comparative Example-2 (curve C), it can be seen that the sensitivity decreases after about several hundred hours.

Thus, it can be seen that the gas-sensitive element of the present invention has extremely excellent durability.

In addition, Fig. 4 shows CO gas 2QOpp due to changes in element temperature.
Similar measurements were also conducted in Comparative Example 2 described above for comparison, showing the change in Rai r/Bug a s in m.

In the case of the present invention, sufficient sensitivity is obtained even at 800 C, whereas in the case of Comparative Example-2, the sensitivity decreases as the element temperature increases.

In general, the gas selectivity of a gas-sensitive element varies greatly depending on the element temperature.At a low temperature of about 100C, it has excellent gas selectivity for CO gas, and as the element temperature increases, it has excellent gas selectivity for methane gas, etc. Become. Therefore, since the gas-sensitive element of the present invention can maintain high sensitivity over a wide range of element temperature, the range in which gas selectivity can be adjusted is wide, and a gas detection device using the gas-sensitive element can be easily designed. In addition, in general, gas-sensitive elements are kept at a high temperature and subjected to heat cleaning in order to prevent a decrease in sensitivity, but in the present invention, there is no decrease in sensitivity even at a high temperature of about AOOC, so there is no need to perform heat cleaning. You can also get

Next, we investigated changes in properties depending on the amount of catalyst metal in the catalyst layer. In the above-mentioned example, Ra ir was measured by changing the amount of Pt in the catalyst layer, and the results are shown in FIG.

As is clear from FIG. 5, when Pt exceeds 80% by weight, the resistance value decreases rapidly. This is P
This is because when i exceeds 80% by weight, the catalyst layer becomes conductive, and if the resistance value decreases in this way, it becomes difficult to measure changes in the resistance value of the gas sensitive element. The sensitivity to this decreases.

Next, changes in the response characteristics of the gas-sensitive element were investigated using the same configuration as in the above-mentioned example, but with only the thickness of the catalyst layer changed. FIG. 6 shows the ratio of the resistance value 80 seconds after introduction of the CO gas to the saturation value of the resistance value in 200 ppm of CO gas.

As is clear from FIG. 6, when the catalyst layer exceeds 11,000 nm, the response characteristics deteriorate rapidly.

Next, CO of gas-sensitive elements using various catalytic metals and carriers.
The sensitivity to gas and H2 gas was measured.

Each gas-sensitive element has a film thickness of 25 mm as a gas-sensitive body.
Using the 81102 nm (Nb doped) system, the contact 1
Layer (7) had a thickness of 5 nm, and both the gas sensitive material and the catalyst layer were provided by sputtering. As a comparative example, measurements were conducted in the same manner on a gas-sensitive element in which the catalyst layer was made of only a catalyst metal and the other conditions were the same. The results are shown in the table below. As is clear from this table below, in the case of the embodiment of the present invention, C
It can be seen that the sensitivity to O gas is large and the sensitivity to Ht gas is small. Therefore, the present invention is particularly effective when CO gas is the gas to be measured. In the comparative example, the CO sensitivity is low and the H2 sensitivity is high, so it is difficult to use it as a gas-sensitive element for measuring CO gas.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas-sensitive element showing an embodiment of the present invention, FIG. 2 is a resistance change rate curve, and FIG. 8 is a sensitivity change rate curve.
Figure 4 is a sensitivity-temperature characteristic curve, and Figure 5 is Ra1r -
FIG. 6 is a diagram showing a characteristic curve with respect to the amount of Pt. FIG. 6 is a diagram showing a resistance change rate curve. 1...Substrate 2...Electrode 4...Gas sensitive body 6...Catalyst layer Agent Patent attorney Noriyuki Chika (and 1 other person) No.
1 Figure 2 Unexamined publication (nt; n) Figure 3 Figure 4 Figure A cliff (°C) Figure 5 PL Outer excavation) Figure 6 臘か (nrn)

Claims (1)

[Claims] A substrate, a gas sensitive element provided on the substrate and whose resistance value changes upon contact with the gas to be measured, a pair of electrodes provided on the gas sensitive element, and a catalyst layer provided on the surface of the gas sensitive element. A gas-sensitive element having a catalyst layer,
A carrier made of at least one of A40s, S102, and Zr0t, and 1 weight to the catalyst layer.
1. A gas-sensitive element comprising a thin film having a thickness of 5 nm to 11,000 nm and containing 80% by weight of a catalyst metal consisting of at least one of Pt, Pd, and Rh.