JPS63231255A - Manufacture of thin film type gas sensing body element - Google Patents

Abstract

PURPOSE:To improve the stability and durability of an element by impregnating a ceramic substrate where a thick porous gas sensing body film is formed by coating with a solution of a platinum compound, and carrying out a heat treatment in a gaseous hydrogen furnace and depositing platinum between an electrode and the thick film on the substrate. CONSTITUTION:Patterns 44 and 49 of platinum paste are printed in thick films on a green sheet 40 and platinum lead wires 51 and 53 are further connected. Then a green sheet 41 with an opening 55 is laminated and other green sheets 42 and 43 are also laminated in steps. Then this laminate plate is calcined in an atmosphere, TiO2 paste is charged in the opening 55, and the plate is calcined again to form a detecting element 11 of a porous gas sensing body. Then a chloroplatinic acid solution is dripped on the detecting element 11, which is calcined in a hydrogen atmosphere. Consequently, a conductive material formed by the condensation deposition of platinum is interposed in the TiO2- electrode interface to improve the stability of the operation and the durability of the gas sensing body element.

Description

[Detailed Description of the Invention] (Industrial Application Field) This specification describes the method for manufacturing a thick film gas sensitive element.
Results of research and development on increasing durability by preventing operational stability and performance deterioration of thick-film gas-sensor elements, which consist of an electrode provided on a ceramic substrate and a porous gas-sensor thick film covering the electrode surface. This will be discussed below.

(Prior Art) This type of thick film type gas sensitive element has a thick film on a flat substrate, and therefore tends to easily peel off due to the difference in thermal expansion between the two.

To address this problem, we have previously disclosed that the adhesion can be improved by providing artificial irregularities on the substrate (Japanese Patent Application No. 58-203222), but the adhesion between the microscopic substrate and the thick film is still insufficient. (Also, impurities migrate during use and condense on these interfaces, further increasing internal resistance and deteriorating gas performance.) there were.

In other words, it was found that because the electrode surface and the thick film element are in plane contact, the stress deteriorates the adhesion between the two and increases the internal resistance.

Therefore, attempts were made to stabilize the contact resistance by changing the bonding interface between the two from two-dimensional to three-dimensional (see Japanese Patent Laid-Open No. 62-5156), but sometimes the furnace conditions It has been found that variations occur in the condensation and precipitation of platinum.

(Problems to be Solved by the Invention) It is an object of the present invention to provide a new method for stably depositing platinum, paying attention to the fact that the conditions for condensation and precipitation of platinum vary depending on the furnace conditions.

(Means for Solving the Problems) This invention involves coating a ceramic substrate equipped with electrodes with a porous gas sensitive thick film by firing, and then impregnating it with a compound solution containing platinum as a main component. 60-180
This is a method for producing a thick film type gas sensitive element, which involves performing heat treatment in a hydrogen gas furnace of 'C' and interposing a conductive material by condensing and depositing platinum at the interface between the thick film and the electrode.

In the present invention, the ceramic substrate provided with the electrodes is made of alumina, mullite, steatite, forsterite, or the like, and has a flat plate shape with high heat resistance, and the electrodes are printed on this ceramic substrate.

The electrode is a thick film made of at least one pair of metal materials,
For high-temperature applications, electrode films containing PT as the main component (hereinafter referred to as PT) are used.
metallized electrodes) are used.

On the other hand, a porous gas-sensitive thick film (hereinafter referred to as a gas-sensitive film) is
Mainly composed of transition metal oxides whose electrical resistance changes according to changes in the surrounding gas concentration, such as SnO2, ZnO, and Fe.
zO, etc. are used as propane gas sensors and humidity sensors, and TiOz+Coo and the like are used as oxygen sensors.

The conductive material to be filled at the interface between the electrode and the gas-sensitive membrane according to the present invention may be any material as long as it can be filled at the interface without adversely affecting the gas-sensitive membrane. It is necessary to have good electrical conductivity between the two, and a material similar to the electrode material is desirable.For PT metalized electrodes, PT and PT group metals such as Pt/Rh, Pt/Pa Alloy is most suitable.

To form this interfacial layer, after baking the gas-sensitive film onto the substrate, a solution containing the conductive material is allowed to seep through the porous gas layer, followed by heating in a hydrogen gas oven, especially at 60-180"C. This allows the conductive material to be stably separated and deposited around the electrode.

(Function) The three-dimensional bonding in which the conductive material is dispersed near the electrode interface of the porous gas sensitive body particularly stabilizes the contact resistance, and can appropriately avoid variations and fluctuations in the internal resistance. .

(Example) Hereinafter, a case where the present invention is applied to an oxygen sensor for detecting the oxygen concentration in the exhaust gas of an internal combustion engine as a gas detector will be described with reference to the drawings.

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 body on a ceramic substrate and detects oxygen concentration;
12 is a cylindrical metal shell 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; 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.

The outer periphery of the metal shell 12 is provided with a threaded portion 12b for attaching the internal combustion engine, and a gasket 18 is provided at the portion that contacts the wall of the internal combustion engine to prevent exhaust gas from leaking.

Here, the filling powder 16 is made of a 1:1 mixed powder of talc and glass, and fixes the gas detector 10 within the inner cylinder 14. 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.

Note that 19 is an outer cylinder attached to the metal shell 12 so as to cover the inner cylinder 14, and 20 is a sealing material made of silicone rubber, which connects the leads 21 to 23 and the terminals of the gas detector 10 protruding from the glass seal 17. Insulate and protect the connections. 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 23 to 23, and then connect them to the caulking metal terminals by caulking.

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 its assembly process, and (b) shows its sectional view along the line A-A or end view, and each figure shows the manufacturing process of the gas detector 10. For ease of understanding (for the sake of explanation, the dimensions of each part do not necessarily correspond to those of the gas detector shown in FIG. 1 (this point also applies to FIGS. 8 and 9, which will be described later).

Here, in each of the above-mentioned figures 2 to 7, 40 to
43 is A1zO+ 92% by weight with an average particle size of 1.5 pm, 5iOz4 heavy view'1. , 12 parts by weight of butyral resin and 6 parts by weight of dibutyl phthalate (BDP) were added to 100 parts by weight of a mixed powder consisting of , Ca02 heavy 4JX and MgO2 heavy Ix, and mixed in an organic solvent to form a slurry with a doctor blade. This is a green sheet formed using green sheet 40.
The green sheet 41 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 49 in the figure indicate 7% A1□0 in the PT powder.
This is a thick film printed pattern using a platinum paste containing 3 powders, which corresponds to the conductive part, of which 44 and 4 powders are added.
5 is an electrode pattern serving as an electrode 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 used to apply power to the heating resistor pattern 46 and the detection element 11. Or it is a terminal pattern for extracting a detection signal.

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 rrtm are connected to the wires 51 to 49, respectively.

Next, as shown in FIG. 4, an opening 55 is punched out in advance in the green sheet 41 so that the tips of the electrode patterns 44 and 45 are exposed. A green sheet 41 is laminated and thermocompressed onto the green sheet 40, covering the pattern. This laminate 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 which corresponds to the element layer is laminated inside the opening 55.

Subsequently, as shown in FIG. 5, a green sheet 42 is laminated and thermocompressed onto the green sheet 41, and further, a green sheet 43 is laminated and thermocompressed onto the green sheet 42 in a stepped manner as shown in FIG.

Here, the green sheet 42 is the first ceramic layer r.
The green sheet 43 corresponds to the second ceramic layer S.

After that, the particle size of about 100 becomes the same material as the green sheet.
um ceramic balls are applied to the surface of the green sheet 41 within the openings 55 of the green sheet 41 to provide an uneven layer.

In this way, a step-like laminated plate is created in which a portion of the platinum lead wires 51 to 53 protrude and the tips of the electrode patterns 44 and 45 are exposed within the opening 55 of the ceramic substrate B.

Next, this laminate is left in an atmospheric 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.
．． 3% by weight based on 100 mol parts of 2 μm Tie2 powder
of ethyl cellulose and butyl carpitol (2
- (trade name of (2-butoxyethoxy)ethanol)) and the viscosity was adjusted to 300 poise.
After printing a thick film to cover the tip of No. 5, print again at 1200
The sample was left in an air atmosphere firing furnace at ℃ for 1 hour and baked to form a porous gas sensitive material.

Samples 1 to 12 shown in Table 1 were manufactured using the detection element 11 fired in this manner.

First, chloroplatinic acid (200 g/l) and rhodium chloride were added thereto as needed, and 2 μl of each was added dropwise from above the element to perform various heat treatments.

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 are made in step 9. As shown in FIG. 8, terminals 31 to 33 with one runner, which were integrally formed by etching on a 0.3 mm thick nickel plate, were adapted to platinum lead wires 51 to 53 and welded to them. Note that this terminal 31 to 3
The gas detector 10 is fixed to the metal shell 12, and then a part of the substrate of the gas detector 10, the platinum lead wires 51 to 53, and the terminals 31 to 3 are connected to the nickel plate on which the gas detector 10 is integrally formed.
3 is protected by a glass seal 17,
After being fixed in the inner cylinder 14, the runner is cut to a predetermined length. In FIG. 8, (a) is a plan view of the gas detector 10, and (b) is a right side view thereof.

The oxygen concentration can be detected by heating the heating resistor pattern 46, activating the detection element 11, 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. Needless to say.

The internal resistance of this oxygen sensor was measured in a propane burner with λ = 0.9 and a gas temperature of 350°C, and then heated to 900°C.
Table 2 shows the results of measuring the internal resistance under the same conditions as above after performing 500 ON-OFF cycles of 5 minutes of heating and 5 minutes of cooling in a Bunsen burner at O gas temperature.

In sample 1, in which the heat treatment temperature was too low, pt was precipitated in the shape of large droplets in the element, so even though pt was well-precipitated, the electrode had no activity and did not precipitate around the electrode.

Sample 10 where the heat treatment temperature exceeded the upper limit. '11 pt
It is presumed that this is because the pt was finely dispersed in the element and was decomposed faster than the pt moved to the electrode and was deposited, but in any case, there was little precipitation at the interface between the electrode and the element.

Table 2 During measurements and durability tests, +12V was connected to the lead wire 51, ground was connected to the same <53, and a fixed resistance of 50 kΩ was connected between 52 and 53.

The detection element according to the present invention has pt at the titania-electrode interface.
It can be seen that the conductive material is effectively filled, filling the space between the titania and the electrode, and maintaining a three-dimensional electrical connection between the two.

(Effects of the Invention) According to the present invention, it is possible to stably and efficiently produce this type of high-performance detection element while significantly improving the operational stability and durability of the thick-film gas sensing element.

[Brief explanation of drawings]

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. Figure 1? ? Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. A thick porous gas sensitive film is formed on a ceramic substrate equipped with electrodes by firing, and then impregnated with a compound solution containing platinum as a main component, and heated in a hydrogen gas furnace at 60 to 180°C. It consists of heat treatment and the interposition of a conductive material by condensation and precipitation of platinum at the interface between the thick film and the electrode.
Manufacturing method for thick film gas sensitive element.