JPH0713602B2 - Thick film gas sensor element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) Modulation of performance resulting from deterioration of a metal catalyst of a thick film type gas sensitive element, particularly in a surface layer, useful as an oxygen sensor and other gas sensors, for example, for automobiles. The purpose of this invention is to propose a thick film type gas sensitive element, which was developed aiming at an advantageous avoidance of the drawback of shifting the control air-fuel ratio point to the lean side after the endurance test in the feedback control for three-way catalyst. is there.

(Prior Art) Japanese Patent Application Laid-Open No. 60-158346 relates to a thick film type gas sensitive element in relation to the presence of 5 to 30 mol% of a platinum group metal catalyst dispersed in a titania thick film. As disclosed in the official gazette, the metal catalyst near the surface layer has an influence on the control air-fuel ratio point in the feedback control for the three-way catalyst for automobiles using this gas sensitive element, due to the progress of subsequent research. That is, it has been clarified that the defect that the controlled air-fuel ratio point shifts to the lean side after the durability test is due to the deterioration of the metal catalyst particularly near the surface layer.

By the way, JP-A-53-11226, 53-130093, and 54-485
No. 96 and No. 56-106147, in particular, in the last-mentioned publication relating to a pellet-shaped gas-sensitive element, by comparing the amount of catalyst in the surface layer between electrodes, by making it smaller,
It is disclosed to improve the durability. However, in this case, while it can contribute to the suppression of the lean shift, it cannot prevent deterioration itself of the catalyst in the vicinity of the surface layer during use. , P, etc.), the reduction of the amount of catalyst is obviously not desirable for the function of trapping.

(Problems to be Solved by the Invention) With respect to the thick film type gas sensitive element disclosed in Japanese Patent Laid-Open No. 60-158346, a shift occurs during durable service of emission due to alteration of metal catalyst. Moreover, since the alteration mainly depends on the adsorption of a specific component in the exhaust gas atmosphere, the influence of this atmosphere is minimized, and thus, the improvement of the durability of the thick film type gas sensitive element is attempted. Is the object of this invention.

(Means for Solving the Problems) With respect to the gas sensitive film of the thick film type gas sensitive element described above, the present invention utilizes a fact that it can be formed in multiple layers. A thick film of a gas sensitive material consisting of a ceramic semiconductor and a metal catalyst covering the electrode was formed, and the thick film was multilayered so that at least the layer near the electrode contained Rh and the concentration of Rh in the layer near the electrode was higher than other layers. A thick film type gas sensitive element.

The metal catalyst in the surface layer of the gas-sensitized thick film multilayered here consists of only Pt or a dispersion of Pt and Rh, and the metal catalyst in the layer near the electrode is a dispersion of Rh and Pt or a dispersion of Rh and Pd. Also, similarly, the metal catalyst occupying the surface layer does not contain Rh or contains Rh in an amount of 0.2 mol% or less with respect to the ceramic semiconductor even if it contains Rh in the metal catalyst present in the layer near the electrode.
Containing 2 mol% or more and 3 mol% or less, and that the metal catalyst occupying the surface layer has a larger particle size than the metal catalyst existing in the layer near the electrode, and the metal occupying the surface layer The catalyst is Pt with a particle size of 0.5 μm or less, and the metal catalyst existing in the layer near the electrode has a particle size of 0.5.
If it is less than μm, it exceeds 0.2 mol% to the ceramic semiconductor. 3
Containing less than or equal to mol% of Rh or its alloy, and the gas sensitive material thick film is a ceramic semiconductor laminated structure in which a metal salt solution is impregnated in a ceramic semiconductor paste firing layer and the metal catalyst is dispersed by thermal decomposition. In addition, the gas sensitive material thick film has a laminated structure of ceramic semiconductors in which the metal catalyst is dispersed by mixing the metal catalyst powder into the ceramic semiconductor paste and firing, and the gas sensitive material thick film is A ceramic semiconductor in which a metal salt solution is dispersed in a ceramic semiconductor paste by impregnation of a ceramic semiconductor paste firing layer and thermal decomposition, and a ceramic semiconductor in which a metal catalyst powder is mixed and burned in a ceramic semiconductor paste to be dispersed in a metal catalyst. A laminated structure is recommended as an embodiment.

Examples of the green ceramic substrate include ceramic materials that are commonly used, such as alumina, beryllia, mullite, and steatite as main components, and can be fired as a thin plate.

The electrodes may be any conductive material that can sufficiently withstand the firing of the ceramic substrate, but usually, platinum, which contains gold or a platinum group element as a main component, can be directly used as an electric circuit. preferable.

Next, the thick film of the gas sensitizer was SnO ₂ , TiO ₂ , CoO, ZnO, Nb ₂ O ₅ , Cr ₂ O ₃
May be used ceramic semiconductor chosen from metal oxides such as but, SnO _2, TiO ₂ is preferred from the viewpoint of heat resistance, in particular it is desirable to use TiO _2.

The gas sensitive thick film contains 0.2 to 30 mol% of platinum group element,
Suitable for a thickness of 00 to 500 μm.

When typical Pt is used as a catalyst here, it exhibits excellent gas-sensing properties, but when used for a long time in high-temperature exhaust gas, Pt evaporates and durability deterioration easily occurs.

If Rh or Rh-Pt, Rh-Pd alloy is used instead of Pt, R
Due to the heat resistance of h, durability deterioration could be greatly improved.

However, when Rh is used in the entire element area, Rh is easily oxidized in an oxidizing atmosphere.Therefore, if it is left in a certain atmosphere for a long time, the properties of Rh change, and the rate of oxidation-reduction of Rh is gas-sensitive. This will affect the response of the device and shift the control point of the exhaust gas control system using this device.

However, in the case of a thick film element, the inventors have found that the center of gas sensitivity is not the element surface, but rather the electrode vicinity, so the thick film should be multilayered to increase the Rh concentration only in the electrode vicinity. To minimize the instability of Rh atmosphere,
The heat resistance of the element was improved, and a gas-sensitive element with good durability could be manufactured. The Rh concentration in the surface layer of the multi-layered gas sensitive material film is 0.2 mol% or less (including 0 mol%), and if it exceeds 0.2 mol%, the gas-sensitive element characteristics greatly depend on the atmosphere. In addition, the response of the device is deteriorated.

On the other hand, in the layer near the electrode, the Rh concentration exceeds 0.2 mol% and exceeds 3 mol%
The following is desirable, and if it is 0.2 mol% or less, the durability when used at high temperature is insufficient, and if it exceeds 3 mol%, the responsiveness of the gas sensitive element is deteriorated.

In catalysts other than Rh, Pt and Pd that are usually used are used alone or as an alloy with Rh as a co-catalyst, but it is necessary that the total amount is 30 mol% or less, and if it is more than that, the gas sensitive element Responsiveness deteriorates.

The thickness of the surface layer and the layer near the electrode should be selected according to the actual application, but the stability of the controlled air-fuel ratio during durability,
100-500μ in total over 50μm due to gas-sensitive stability
m is desirable, the durability is insufficient when it is less than 100 μm, and the response of the device is deteriorated when it exceeds 500 μm, and the film is easily peeled off due to the thermal stress due to the difference in thermal expansion between the substrate and the device film depending on the use conditions. Become.

On the other hand, the catalyst of the surface layer has a property of trapping poisoning substances in the exhaust gas, so that Pt may be added to the surface layer to some extent for the purpose of potentially poisoning. At this time, the Pt of the surface
If it deteriorates during use, it will affect the gas sensitivity characteristics and shift the exhaust gas control point to the lean side (lean side) .To prevent this, in the surface layer, the particle size is smaller than that in the layer near the electrode. This shift can be minimized by making Pt large, preferably 0.5 μm or more, as a main component. 0.5 μm
This is because the following catalysts change their catalytic properties because they are easily vaporized and condensed during use, and this acts as a shift of the gas-sensitive properties. By using 0.5 μm or more,
The characteristics were shifted from the beginning to the conditions after the above use, and a stable product with little change was able to be obtained in the subsequent use.

As a method of adding a metal catalyst, a method of adding a metal catalyst powder,
There is an impregnation method in which a metal salt solution is impregnated and then thermally decomposed to precipitate a metal, but a fine catalyst is desirable for the metal catalyst in the vicinity of the electrode in order to enhance gas sensitivity, and the impregnation method is more suitable for this purpose. ing.

The thermal decomposition can be carried out in a burner or an electric furnace, but in the case of the present invention, it is desirable to carry out the thermal decomposition in a reducing furnace in order to deposit near the electrodes.

In order to reduce the particle size of the metal catalyst in the vicinity of the electrodes, it is possible to make the starting materials finer by the catalyst powder mixing method, but in this case, it is quite difficult to uniformly disperse them, and in practice, those with stable characteristics should be used. Since it is difficult to manufacture, a metal salt solution that becomes a metal catalyst by thermal decomposition is used especially in that solution state, and it is impregnated into the firing layer of the ceramic semiconductor that has been previously coated and fired on the ceramic substrate, and then heat is applied at a relatively low temperature. A method of decomposing is more preferable, while a powdery method is advantageous because it is easier to prepare a coarse metal catalyst, but an impregnation method may also be used. In this case, the particle size of the metal catalyst can be adjusted by changing the thermal decomposition temperature. In the case of the present invention, these features can be combined to form a thick film type gas sensitive element.

Since the thick film type gas sensitive element cannot obtain sufficient gas sensitive characteristics unless the temperature is high to some extent, it may be necessary to heat using a heater or the like at a low ambient temperature. It is desirable to provide a ceramic substrate with a heater layer in order to reduce the size and improve the productivity. As this heater layer, the gas detection element layer is capable of being heated to 500 ° C. or higher so as not to deteriorate the corrosion resistance of the gas detection element.

Now, the structure of an example in which the thick film type sensitive element of the present invention is applied to an oxygen sensor for detecting the oxygen concentration in the exhaust gas of an internal combustion engine will be specifically described.

Fig. 1 shows a partial cross section of the element sensor.
Reference numeral 10 is a detector for detecting oxygen concentration, which is provided with a detector element 11 made of a gas sensitive material thick film covering a pair of electrodes arranged on a ceramic substrate, and 12 is a detector.
A metal shell formed into a tubular shape for holding the oxygen sensor 10 to attach the oxygen sensor to the internal combustion engine, and a protector 13 attached to the tip 12a of the metal shell 12 on the internal combustion engine side, which protects the detection unit 10. , And 14 together with the metal shell 12 are the detection unit 10
It is an inner cylinder for holding.

The detection unit 10 includes a spacer 15, a filling powder 16 and a glass seal 17.
The metal shell 12 and the inner cylinder 14 are gripped via the.

Further, a screw 12b for mounting the internal combustion engine is carved on the outer periphery of the metal shell 12, and a gasket 18 is provided on the mounting wall that is in contact with the wall surface of the internal combustion engine so that exhaust gas does not leak.

Here, the filling powder 16 is made of a 1: 1 mixed powder of talc and glass, and the detection unit 10 is fixed in the inner cylinder 14.

Further, the glass seal 17 is made of a low melting point glass, so as to prevent the detection gas from leaking and protect the terminals of the detection unit 10, a part of the substrate of the detection unit 10 and a connecting portion between the platinum lead wire and the terminal described later. And the inside of the inner cylinder 14 is filled.

19 is an outer cylinder attached to the metal shell 12 so as to cover the inner cylinder 14,
Reference numeral 20 is a sealing material made of silicone rubber, which insulates and protects the connecting portions between the lead wires 21 to 23 and the terminals 31 to 33 from the detecting portion 10 protruding from the glass seal 17 shown in FIG. As shown in FIG. 3, the lead wires 21 to 23 and the terminals 31 to 33 have the seal material 20 and the lead wires 21 to 23 housed in the outer cylinder 19 in advance, and at the tips of the lead wires 21 to 23. The caulking metal fittings 24 to 26 are provided, and the caulking metal fittings 24 to 26 are connected to the terminals 31 to 33 for electrical conduction.

Next, the detection unit 10 is prepared in accordance with the procedure shown in FIGS. 4 to 8. Here, (a) shown in FIGS. 4 to 8 is the front of the detection unit 10, and (b) is the AA line. The cross section is shown.

Here, in each of FIGS. 4 to 8 above, 40 and 41
Is Al ₂ O ₃ 92% by weight, SiO ₂ 4% by weight, C having an average particle size of 1.5 μm.
12 parts by weight of butyral resin and 6 parts by weight of dibutyl phthalate (DBP) were added to 100 parts by weight of mixed powder consisting of aO2% by weight and MgO2% by weight, and mixed in an organic solvent to form a slurry, which was then prepared using a doctor plate. The formed ceramic substrate is a green sheet, and the green sheet 40 has a thickness of 1 mm and the green sheet 41 has a thickness of 0.3 mm.

42 to 47 are patterns of thick film printed with platinum paste containing 7% Al ₂ O ₃ added to Pt.
42 and 43 are electrode patterns serving as electrodes of the detection element 11, 44 is a heating resistor pattern for heating the detection element 11, and 45 to 47 are power sources applied to the heating resistor pattern 44 and the detection element 11. It is a conductive pattern for extracting a detection signal.

As shown in FIG. 4, first, each pattern 42 to 47 is thick-film printed with a platinum paste on the green sheet 40, and then, as shown in FIG. 5, a platinum lead wire having a diameter of 0.2 mm is formed on the electrode patterns 45 to 47. Place 48 to 50 respectively. Needless to say, when the heating resistor pattern 44 is thick-film printed, the pattern width is adjusted so that the detection element 11 can be heated by applying a predetermined voltage to the heating resistor pattern 44.

Next, as apparent from FIG. 6, the green sheet 41
In this, an opening 51 is formed by punching so that the tips of the electrode patterns 42 and 43 are exposed.
And 43, all the patterns except the tip end portions of the green sheets are covered, and the green sheet 41 is laminated and thermocompression-bonded on the green sheet 40.

Thus, a part of the platinum lead wires 48 to 50 is projected, and a laminated plate in which the tips of the electrode patterns 42 and 43 are exposed in the opening 51 is formed, and subsequently, the green sheet 40 is formed on the opening 51 of the laminated plate. Spherical granulated particles (secondary particles) 52 of 80-150 mesh made of the same material as 1, 41 are dispersed and attached at 1500 ° C.
6 (C) by leaving it in the atmosphere for 2 hours.
As shown in the enlarged view, a ceramic substrate having a concavo-convex surface formed by uniformly dispersing each particle 52 is formed.
When the gas-detectable metal oxide paste described below is applied and baked, the gas-detectable metal oxide layer has the above-mentioned recesses 5 ″.
It is 2 "thick and stacked so that it is firmly fixed to the ceramic substrate.

Next, as shown in FIG. 7, the detection element 11 is laminated in the opening 51 of the ceramic substrate.

The detection element 11 was manufactured as follows.

(1) Preparation of TiO ₂ element paste (I) 500 g of TiO ₂ raw material powder is calcined in the air at 1200 ° C. for 1 hour to obtain element powder. Next, 100 g of this powder was added to 100 g of this powder, and pulverized and mixed in a ball mill for 24 hours, then ₂ g of butyral was added to 100 g of TiO ₂ as a binder for 1 h.
r Mix and paste is completed.

(2) Formation of TiO ₂ element (I) The paste of (1) was applied to the recesses on the substrate and dried to form 1100
Bake at ℃ for 2 hours.

(3) Support of metal catalyst (I) The TiO ₂ device (I) obtained in (2) is impregnated with a metal salt solution. The type and concentration of this solution depend on the desired catalyst type and amount. For example, in the case of supporting Pt 5 mol% and Rh 0.5 mol%, Pt 200 g / and Rh 20 g / chloroplatinic acid / rhodic acid aqueous solution is used in an amount of 1 μ per 100 μm of TiO ₂ film thickness. After impregnating in this way, it is dried at 100 to 150 ° C. for 1 hour, and then thermally decomposed at 800 ° C. in an electric furnace to burn Pt of about 0.1 μm to TiO ₂ .

With the above procedure, the formation of elements near the electrodes is completed.

(4) Preparation of TiO ₂ element paste (II) In order to support 10 mol% of a catalyst metal of a different type from (I) on the TiO ₂ raw material powder, 10 mol% of 1.2 μm Pt black is kneaded. Or (powder method) or impregnate 10% by mol of Pt with a chloroplatinic acid solution at 200 to 250 ° C for 24 hours
After drying, bake in an electric furnace at 1200 ℃ for 1 hour to obtain element powder.
Baking 1.3 μm Pt (impregnation method). Then 100g of this powder
Butyl carbidol 100g was added to the ball mill.
After crushing and mixing for 24 hours, add 2 g of butyral resin and mix for 1 hour to complete the paste.

(5) Formation of TiO ₂ element (II) The paste of (4) is applied and dried on the element (I) as shown in FIG. 8 and baked in an electric furnace at 1100 ° C. for 2 hours to complete the element formation.

By the above treatments (1) to (5), the TiO ₂ element film has different catalysts in the surface layer and the layer in the vicinity of the electrodes, and the kind and amount of the catalyst can be arbitrarily selected by selecting the metal powder and the metal salt solution. The thickness of the TiO ₂ layer having the catalyst can also be controlled by the amount of the applied paste. Incidentally method for supporting a metal catalyst impregnation methods described above, if a well-supported amount either powder mixing method is the same to the same effect, a powder mixing method using a TiO ₂ element (I) TiO ₂ elements (II) Alternatively, an impregnation method may be used.

In the detector 10 thus produced, the platinum lead wires 48 to 50 protruding outside are connected to the terminal 31 as shown in FIG.
Thru 33 connected. In the figure, (a) is the front,
(B) indicates the right side surface.

The terminals 31 to 33 shown in FIG. 9 are integrally formed in advance on a nickel plate having a thickness of about 0.5 mm by etching, and platinum lead wires 48 to 50 are mounted on the respective terminals, and the portions are spotted. After the terminals are joined by welding, the detecting portion 10 is fixed in the metal shell 12 and the inner cylinder 14, and then cut into a predetermined length as shown by a chain line for easy handling.

After that, connect the lead wires 24, 25, 26 shown in Fig. 3 to terminals 31, 32, 33.
, And the sealing material 20 and the outer cylinder 19 are fitted together and welded to assemble the sensor as shown in FIG.

The sensor was attached to a commercially available two-way three-way catalyst vehicle with EFI as shown in Fig. 10, and the US EPA HOT TRANSIENT MODE was run, and the amount of exhaust gas during running was measured by CVS.

In FIG. 10, 60 is a test engine, 61 is an exhaust pipe, 61a is a sensor mounting portion, S is a sensor, 65 is a control unit, and 67 is a three-way catalyst. FIG. 11 shows the circuit configuration of the control unit 65, where 70 is a power source, 72 is a heater, 74 is a gas sensitive element, and 76 is a comparative resistor.

(Example) 300 HR in engine exhaust gas with the durability pattern shown in FIG.
Was subjected to durable service to cause deterioration, and then the emission amount was measured again, the shift of the control air-fuel ratio of the sensor was measured, and the change amount before and after the durable service was observed and evaluated.

As shown in Table 1, the results of the conventional products are NO before and after durable operation.
This indicates that the control point has shifted to the lean side due to the large fluctuations in the x emission amount, especially the NOx emission after endurance.

On the other hand, the sensor according to the present invention can form a stable exhaust gas system with little variation between the initial stage and the end stage.

In the embodiment of the present invention, the example of the two-layer structure has been described above. However, the same effect as described above can be obtained not only when the number of layers is three or more but also when the insulating coating layer is further provided on the surface layer. can get.

(Effect of the Invention) According to the present invention, the deterioration of the metal catalyst of the thick film type gas sensitive element, especially in the surface layer, is drastically reduced, and the disadvantage caused by the catalyst deterioration is eliminated.

[Brief description of drawings]

1 to 9 show an embodiment in which the thick film type gas sensitive element according to the present invention is applied to an oxygen sensor, and FIG. 1 is a sectional view of a main part showing the overall configuration of the oxygen sensor, and FIG. Terminal 31 protruding from the inner cylinder 14 and the glass seal 17
Fig. 3 is an exploded view with a section of 33 to 33, and Fig. 3 shows the outer cylinder 19 and the sealing material 20 previously stored in the outer cylinder 19.
4 to 8 are explanatory views of the assembling process sequence of the detection unit 10, FIG. 9 is an explanatory view of the connecting procedure of the terminals 31 to 33, and FIG. 10 and FIG. FIG. 12 is an endurance test procedure explanatory diagram using an oxygen sensor in an internal combustion engine, and FIG. 12 is an endurance pattern diagram. 40,41 …… Ceramic substrate 42,43 …… Electrode pattern 11 …… Detector (gas sensitive film)

Front Page Continuation (72) Inventor Akio Takami 14-18 Takatsuji-cho, Mizuho-ku, Nagoya, Aichi Nihon Special Ceramics Co., Ltd. (56) Reference JP-A-56-106147 (JP, A) JP-A-56 -112638 (JP, A)

Claims (8)

[Claims] 1. A gas sensitive material thick film composed of a ceramic semiconductor and a metal catalyst covering a pair of electrodes arranged on a ceramic substrate, wherein the thick film is multi-layered and at least the layer near the electrode contains Rh, and A thick film type gas sensitive element characterized by having a higher Rh concentration in the layer near the electrode than in other layers. 2. The metal catalyst in the surface layer of a multi-layered gas-sensitizer thick film is composed of Pt alone or a dispersion of Pt and Rh, and the metal catalyst in a layer near the electrode is a dispersion of Rh and Pt or Rh and Pd. A device according to claim 1, which comprises a dispersion of 3. A metal catalyst which occupies the surface layer of a multi-layered gas-sensing element thick film, contains no Rh or contains Rh in an amount of 0.2 mol% or less with respect to the ceramic semiconductor and existing in a layer near the electrode. The catalyst contains more than 0.2 mol% Rh and 3 mol%
The device of claim 1 comprising: 4. The metal catalyst occupying the surface layer has a larger particle size than the metal catalyst existing in the layer near the electrode.
The device according to claim 1, 2 or 3. 5. The metal catalyst occupying the surface layer has a particle size of 0.5 μm.
Claim 3 wherein the Pt is less than or equal to m, and the metal catalyst present in the layer near the electrode has a particle size of less than 0.5 μm and more than 0.2 mol% and 3 mol% or less of Rh or its alloy with respect to the ceramic semiconductor. The element described in 1. 6. The gas-sensitizer thick film has a laminated structure of ceramic semiconductors in which a calcined layer of a ceramic semiconductor paste is impregnated with a metal salt solution and a metal catalyst is dispersed by thermal decomposition. The element according to 3, 4, or 5. 7. The gas sensitive material thick film has a laminated structure of ceramic semiconductors in which the metal catalyst powder is mixed into the ceramic semiconductor paste and the metal catalyst is dispersed by firing, and the laminated structure of ceramic semiconductors. The element according to 4 or 5. 8. A gas-sensitizer thick film is a ceramic semiconductor in which a metal salt solution is impregnated into a ceramic semiconductor paste firing layer and a metal catalyst is dispersed by thermal decomposition, and a metal catalyst powder is mixed and fired in the ceramic semiconductor paste. The element according to claim 1, 2, 3, 4, or 5, which has a laminated structure with a ceramic semiconductor in which the metal catalyst is dispersed according to (3).