JPH055305B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) Gas sensitive elements are used for various purposes as gas detectors, and are generally composed of titania-based ceramics and supported catalysts. This specification describes the results of research and development on improving the heat resistance of catalysts and advantageously suppressing their coarsening due to sintering, including gas sensitive elements under high temperatures, including so-called thick-film gas sensitive elements. It is described below. (Prior Art) As mentioned above, a gas sensitive element is composed of titania ceramics and a supported catalyst, and for example, the following patent publication JP-A-55-136949 is referred to. (Problems to be Solved by the Invention) When this type of gas sensitive element is exposed to high temperatures, the catalyst first begins to sinter and become coarse, leading to deterioration. Therefore, in order to provide a gas sensitive element with high heat resistance, it is necessary to increase the heat resistance of the catalyst and prevent sintering. (Means for Solving the Problem) The present invention provides a gas sensitive element which is a gas sensitive material mainly composed of titania and which supports a platinum metal element or an alloy thereof as a catalyst. corresponding to 10 to 250 mol% of
The gas sensitive element is characterized by containing at least one of Zro ₂ and CeO ₂ and a titania thick film covering an electrode provided on a ceramic substrate. The titania thick film has an interface layer in which a platinum metal element or its alloy is precipitated at a high concentration at the interface between the electrode and the titania electrode, and the titania thick film contains at least one of Zro ₂ and CeO ₂ of 10 to 250 mol%. The present invention is a thick film type gas sensitive element characterized in that it supports a catalyst containing seeds and having a composition of a platinum metal element or an alloy thereof. That is, the present invention was completed based on the knowledge that the heat resistance of the catalyst can be advantageously increased by adding Zro ₂ and CeO ₂ to the supported catalyst. As a catalyst, a platinum metal element or its alloy is used, especially R _h as an alloy with a Pt content of 1 to 20 mol%.
is preferably 10% or less of Pt. The amount of catalyst can be selected depending on the application of the element: Generally, for applications with unleaded gasoline and low pollution, it is 1 to 5 mol%, and Pb in gasoline is
For applications where pollutants such as P are mixed, it is appropriate to add a larger amount of 5 to 20 mol%.
The greater the amount added, the worse the response of the element becomes. Particularly in the case of thick-film type gas sensitive elements, even if the heat resistance of the catalyst is high, the contact at the interface between the element and the electrode is unstable, and even when the above-mentioned catalyst is used, the contact at the interface is unstable. It was found that it was not very effective in improving. At this time, it has been found that it is better to fill the interface between the electrode and the titania thick film in advance with a high concentration of platinum metal element, and then to uniformly support the catalyst in the titania film. (Function) Regarding the amount of Zro ₂ and CeO ₂ added in the catalyst, it is desirable to add 10 to 250 mol % with respect to the catalyst.
If it is less than 250%, there is no effect, and if it is more than 250%, the manufacturing process becomes unstable and uniform catalyst support becomes impossible. (Example) Hereinafter, a case will be described with reference to the drawings, taking as an example a case where the gas detector is adapted to an oxygen sensor for detecting the oxygen concentration in the exhaust gas of an internal combustion engine. FIG. 1 is a partial cross-sectional view of the oxygen sensor. In the figure, 10 is a gas detector that includes a detection element 11 as a porous gas sensitive material on a ceramic substrate and detects oxygen concentration, and 12 is a cylindrical shape that holds the gas detector 10 and attaches the oxygen sensor to the internal combustion engine. 13 is a protector that is attached to the internal combustion engine side end portion 12a of the metal shell 12 and protects the gas detector 10; and 14 is an inner cylinder that holds the gas detector 10 together with the metal shell 12. The gas detector 10 is housed and fixed within the metal shell 12 and the inner cylinder 14 via a holding spacer 15 and a filling powder 16. 17 is a glass seal. A threaded portion 12b for attaching an internal combustion engine is formed on the outer periphery of the metal shell 12, and a gasket 18 is provided at a portion where the internal combustion engine comes into contact with the wall surface to prevent exhaust gas from leaking. Here, the filling powder 16 is made of talc and glass.
1, and the gas detector 10 is connected to the inner cylinder 1.
Fixed within 4. Further, the glass seal 17 is made of low melting point glass, and prevents leakage of the detection gas and protects the terminals of the gas detector 10. In addition, 19 is a metal shell 1 so as to cover the inner cylinder 14.
The outer cylinder 20 attached to the glass seal 2 is a sealing material made of silicone rubber, and insulates and protects the connection portion between the leads 21 to 23 and the terminal of the gas detector 10 that protrudes from the glass seal 17. In addition, to connect the lead wires 21 to 23 and the terminals 31 to 33, the sealing material 20 and the lead wires 21 to 23 are placed in the outer cylinder 19 in advance, and each lead wire 21 to 23 is connected to the terminals 31 to 33.
It is preferable to connect the caulking metal fittings to the tips of the terminals 1 to 23, and then connect the caulking metal fittings with the caulking metal terminals. The gas detector 10 is manufactured according to the procedure shown in FIGS. 2 to 7. In the figures, A shows a plan view of the gas detector 10 in the assembly process, B shows a cross-sectional view taken along the line A-A, or an end view, and each figure shows the gas detector 10.
In order to simply explain the manufacturing process of 0 in an easy-to-understand manner,
The dimensions of each part do not necessarily correspond to the gas detector shown in Figure 1.
The same applies to figures. Here, in each of the above figures 2 to 7, 40 to 43 are 92% by weight of Al ₂ O ₃ with an average particle size of 1.5 μm, 4% by weight of SiO _{2 ,} % by weight of CaO2, and MgO2
To 100 parts by weight of mixed powder having a composition of The green sheet 40 was prepared in advance to have a thickness of 1 mm, the green sheet 41 to have a thickness of 0.2 mm, and the green sheets 42 and 43 to have a thickness of 0.8 mm. Furthermore, 44 to 4 in the figure
9 is a pattern printed with a thick film using a platinum paste containing 7% Al ₂ O ₃ powder added to Pt powder, and corresponds to a conductive part, of which 44 and 45 are electrodes that become electrodes of the detection element 11 46 is a heating resistor pattern serving as a heater for heating the detection element 11, and 47 to 49 are terminals for applying power to the heating resistor pattern 46 and the detection element 11 or for extracting a detection signal. It's a pattern. As shown in FIG. 2, the manufacturing of the gas detector 10 begins by thickly printing each of the patterns 44 to 49 on a green sheet 40 using platinum paste, and then, as shown in FIG. 3, a terminal pattern 47 is printed. Platinum lead wires 51 to 53 having a wire diameter of 0.2 mm are connected to wires 51 to 49, respectively. Next, as shown in FIG. 4, openings 5 are formed in advance in the green sheet 41 by punching so that the tips of the electrode patterns 44 and 45 are exposed.
5 are provided, and electrode patterns 44 and 45 are provided.
A green sheet 41 is laminated and thermocompressed onto the green sheet 40, covering all the patterns except for the leading edge of the green sheet 40. The stacked body of the green sheets 40 and 41 which are laminated and bonded by thermocompression corresponds to the ceramic substrate B, and later the detection element 11 corresponding to the element layer is laminated inside the opening 55. Next, as shown in FIG. 5, a green sheet 41 is
A green sheet 42 is laminated and thermocompression bonded thereon, and further, a green sheet 43 is laminated and thermocompression bonded in a stepped manner on the green sheet 42 as shown in FIG. Here, the green sheet 42 corresponds to the first ceramic layer f, and the green sheet 43 corresponds to the second ceramic layer f.
This corresponds to the ceramic layer s of . Thereafter, ceramic balls made of the same material as the green sheet and having a particle diameter of approximately 100 μm are applied to the surface of the green sheet 41 within the openings 55 on the green sheet 41 to form an uneven layer. In this way, the platinum lead wires 51 to 53
A part of the electrode patterns 44 and 4 protrude.
A step-shaped laminated plate in which the tips of the ceramic substrates 5 and 5 are exposed in the openings 55 of the ceramic substrate B is prepared. Next, this laminate is left in an air atmosphere firing furnace at 1500° C. for 2 hours to fire the first ceramic layer f and the second ceramic layer s together with the ceramic substrate B. Thereafter, as shown in FIG. 7, a detection element 11 is provided in the opening 55, and this detection element 11 is made by adding, for example, 3% by weight of ethyl cellulose to 100 mol parts of TiO ₂ powder with an average particle size of 1.2 μm. TiO ₂ paste mixed in butyl carbitol (trade name of 2-(2-butoxyethoxy) ethanol) and adjusted to a viscosity of 300 poise is filled in the opening 55,
After printing a thick film so as to cover the tips of the electrode patterns 44 and 45, it is baked again by leaving it in an air atmosphere baking oven at 1200° C. for 1 hour to form a porous gas sensitive body. The thus fired detector element 11 was electroplated in a chloroplatinic acid (200 g/) solution at 2V for 10 minutes using the platinum lead wires 51, 52, and 53 as cathodes and the platinum electrode as an anode. Afterwards, 2μ of an aqueous solution containing the specified ZrO ₂ and Ce(NO ₃ ) ₂ dissolved in the platinum chloride solution was added dropwise, and the solution was rapidly thermally decomposed at 950°C in a propane burner to uniformly support the Pt catalyst in the element. I let it happen. Next, the connections between the platinum lead wires 51 to 53 protruding to the outside of each of the obtained gas detectors 10 and the terminals 31 to 33 have a thickness of 0.3 as shown in FIG.
Terminals 31 to 33 with runners integrally formed by etching on a nickel plate having a diameter of 2 mm were adapted to platinum lead wires 51 to 53 and welded to them, respectively. The nickel plate on which the terminals 31 to 33 are integrally formed has the gas detector 10 connected to the metal shell 1.
2, and then a part of the substrate of the gas detector 10 and the joint portion between the platinum lead wires 51 to 53 and the terminals 31 to 33 are protected by a glass seal 17, and after being fixed in the inner cylinder 14, a predetermined Cut it to length and discard the runner. Note that E in FIG. 8 is a plan view of the gas detector 10, and B is a right side view thereof. The heating resistor pattern 46 is heated and the detection element 1
The oxygen concentration can be detected by activating the sensor 1 and detecting a change in the resistance value of the detection element 11 between the lead wires 22 and 23 depending on the oxygen concentration. This sensor was placed in gas at 950℃ for 100 hours at λ=1.1.
The sensor characteristics were then examined using a 350°C propane burner measuring device. The measuring device switches between λ = 0.9 and 1.1 every second, and the sensor outputs approximately 1V and 0V accordingly. The characteristics of the catalyst are correlated with the time to increase the pressure from 300 mv to 600 mv (denoted as Tlr), so the amount added and
Table 1 shows the relationship with Tlr. During the durability measurement, the lead wire 51 was connected to +12V, the lead wire 53 was connected to ground, and a fixed resistor of 50 kΩ was connected between 52 and 53.

【table】

[Table] (Effects of the Invention) According to the present invention, the heat resistance of the catalyst is improved, sintering is prevented, and the durable life of the gas sensitive element is extended.

[Brief explanation of the drawing]

FIG. 1 is a side view showing a cross section of the left half, FIGS. 2 to 7 are process explanatory diagrams of an embodiment applied to an oxygen sensor, and FIG. 8 is a front view and a sectional view of a gas detector.

Claims (1)

[Scope of Claims] 1. A gas sensitive element comprising titania as a main component and supporting a platinum metal element or an alloy thereof as a catalyst, wherein the amount of the catalyst is 10 to 250 mol% based on the amount thereof. corresponds to,
A gas sensitive element characterized in that it contains at least one of Zro ₂ and CeO ₂ . 2. An interface consisting of a titania thick film covering an electrode provided on a ceramic substrate, with a platinum metal element or its alloy precipitated at a high concentration at the interface between the titania thick film and the electrode on the ceramic substrate. has a layer,
and the titania thick film contains 10 to 250 mol% Zro ₂ ,
1. A thick film type gas sensitive element, characterized in that it supports a catalyst having a composition of at least one of CeO ₂ and the remainder being a platinum metal element or an alloy thereof.