JPS62217152A - Thick film type gas sensitive body element - Google Patents

Abstract

PURPOSE:To considerably decrease the deterioration of a metallic catalyst in a surface layer by making the grain size of ceramics and grain size of metallic catalyst in the surface layer respectively larger than the grain size of the ceramics and grain size of the metallic catalyst in the remaining layer across the part near electrodes. CONSTITUTION:The calcination temp. of the ceramic grains to be used for the surface layer is adjusted to 1,200 deg.C, more preferably to 1,300 deg.C, by which the grain size thereof is adjusted to 1-5mum to prevent a structural change. The catalyst used in said part is also calcined at >=1,200 deg.C simultaneously with the ceramic grains and therefore, the average grain size thereof is made >=0.5mum to provide the similar effect. On the other hand, the calcination temp. of the ceramic grains to be used in the layer near the electrodes is adjusted to <=1,200 deg.C to make the grain size 0.5-3mum, by which the gas sensitive characteristic is assured and similarly the function of the catalyst grains is maintained. The stable gas sensitive characteristic is thus assured. An Rh catalyst which has extremely high activity and provides good durability is preferably deposited on said part at >=0.1mol% in order to intensity the gas sensitive characteristic.

Description

[Detailed Description of the Invention] (Industrial Application Field) Performance modulation resulting from deterioration of metal catalysts, especially in the surface layer, of thick-film gas sensitive elements useful as oxygen sensors and other gas sensors, such as those used in automobiles. We would like to propose a thick film type gas sensing element that was developed to advantageously avoid the disadvantage of shifting the control air-fuel ratio point to the 1 side after a durability test in feedback control for a three-way catalyst. It is something to do.

(Prior art) Regarding thick film type gas sensitive elements, Japanese Patent Application Laid-Open No. 158346/1983 relates to the dispersion of 5 to 30 mol% of a platinum group element metal catalyst in a titania thick film. This was disclosed in the official bulletin, but due to subsequent research progress,
The metal catalyst near the surface layer affects the control air-fuel ratio point in feedback control for a three-way catalyst for automobiles using this gas sensitive element. It has become clear that the side-shifting defect is caused by the deterioration of the metal catalyst, especially near the surface layer.

By the way, JP-A-56-106147 discloses that the durability of a gas-sensitive element in the form of a pellet can be improved by reducing the amount of catalyst in the surface layer compared to the amount between the electrodes. There is. However, in this case, while it may contribute to suppressing the lean shift mentioned above, it cannot prevent the deterioration itself during use of the catalyst near the surface layer.
Furthermore, this catalyst removes poisonous substances (Pb) from the exhaust gas to the elements.
, P, etc.), a reduction in the amount of catalyst is clearly undesirable.

(Problems to be Solved by the Invention) As a result of subsequent research on the thick film type gas sensing element disclosed in the above-mentioned Japanese Patent Application Laid-open No. 158346/1983, the ceramic particle size and catalyst particle size near the surface layer were found to be It has been found that changes in this gas sensitive element affect the control frequency and control air-fuel ratio in feedback control for a three-way catalyst for automobiles. In other words, conventional elements have the disadvantage that the control frequency increases in the durable layer and the control point shifts to the lean side.This is mainly due to structural changes in the ceramic grains in the surface layer, and at the same time It has also been found that structural changes in the catalyst particles also promote this effect.

In other words, coarsening the ceramic grains in the surface layer increases the initial control frequency and prevents the control air-fuel ratio shift, but the effect depends on the catalyst grains and catalyst species used in this area. However, relatively low activity catalyst species, specifically Rh usage amount is 0 compared to ceramics.
．． It has been discovered that the above effects are maximized when less than 1 mo 1% is used. The reason for this is that by allowing all the changes in ceramic particle size and metal catalyst that will inevitably occur during durability to proceed in advance, the shift before durability can be eliminated.

(Means for Solving the Problems) That is, in this invention, the calcination temperature of the ceramic grains used for the surface layer is set to 1200°C, preferably 1300°C, so that the grain size is 1 to 5 μm, and structural changes are prevented. Preventing. In addition, the catalyst used in this part is fired at 1200℃ or higher at the same time as the ceramic particles, so the average particle size is 0.
．． 5 μm or more, giving the same effect. 130 here
If it is below 0°C, calcination at a temperature higher than this temperature will reduce the gas sensitivity of the ceramic grains, and the catalytic properties will not be affected by grain growth, which will significantly reduce its function. On the other hand, the maximum temperature at which this gas-sensitive body is used is normally 1000°C and it is necessary to be sufficiently higher than this, so it should be 1200°C or higher.

On the other hand, the calcination temperature of the ceramic grains used in the layer near the electrode is 1
At 200° C. or lower, the particle size is set to 0.5 to 3 itm to maintain gas-perturbing properties. Similarly, the function of the catalyst particles is maintained to ensure stable gas-perturbing properties. In addition, 0.0% Rh catalyst is added to this area to give it high activity and good durability. in+o
It is preferable to support the above-mentioned gas sensitivity by carrying 1% or more.

As a result of the above measures, the element has an element structure from the beginning consisting of a highly active layer near the electric wire and a low active surface layer, which would normally form a durable layer, so that there is almost no change in performance before durability, and the gas This results in stable operation of an automotive three-way catalyst feedback control system using a sensitive element.

Moreover, the above-mentioned device formation can provide a multilayer structure very easily due to its thick film structure, which will promote economical industrialization.

(Function) The ceramic substrate forming this thick film element may be a commonly used ceramic substrate (for example, a fired ceramic substrate containing alumina, beryllia, mullite, steatite, etc. as a main component). As long as the material is a conductor that can withstand the firing of the ceramic substrate, a material containing gold or a platinum group element as its main component is usually used, and platinum in particular has electrical resistance and cannot be used as an electrical circuit as it is. It is preferable to use platinum because it can be used.

As the platinum metal element, platinum or a platinum-rhodium alloy is used especially from the viewpoint of cost, catalytic ability, etc.

In addition, as a gas detection element layer, 5nOz, rto,...
Cod.

An oxide semiconductor such as ZnO, Nt)zOs+ Cr2O2 may be used, but from the viewpoint of heat resistance, SnO□, TiO□
is preferable, and it is particularly desirable to use TiO□. The gas detection element layer used has a thickness of 100 to 500 μm.

In order to have a stable gas-sensitive layer, the ratio of the titania layer near the electrode to the layer near the surface must be at least 50% in the lower layer.
, 1jI11 or more, and in order to have a surface layer with little variation in durability, it is desirable that the upper layer be at least 50 μm thick.The thicker the overall thickness, the slower the control speed (but the better the durability. You can choose.

Furthermore, the gas detection element cannot obtain sufficient gas properties unless the temperature is high to a certain extent, so if the ambient temperature is low, it is necessary to heat it using a heater, etc., which reduces the size of the sensor and improves productivity. It is desirable to provide a heater layer on the ceramic substrate in order to improve the lead resistance of the gas detection element.
To be able to heat a gas detection element layer to 500°C or higher.

Taking as an example the case where the gas sensor of the present invention is applied to an oxygen sensor that detects the oxygen concentration in the exhaust gas of an internal combustion engine, its structure and manufacturing procedure will be described as an example of the present invention, and the oxygen sensor will actually be installed in an internal combustion engine. A case in which air-fuel ratio control is performed will be described as an experimental example of the present invention.

Fig. 1 shows a partial cross section of the element sensor, and in the figure, 10 is equipped with a detection element 11 made of a thick gas sensitive film covering a pair of electrodes arranged on a ceramic substrate, and detects the oxygen concentration. 12 is a detection unit for
A metal shell formed in a cylindrical shape for gripping the detection part 10 and attaching the oxygen sensor to the internal combustion engine, and 13 is a metal shell attached to the tip 12a of the metal shell 12 on the internal combustion engine side to protect the detection part 10. 14 is an inner cylinder that holds the detection part 10 together with the metal shell 12.

The detection part IO is held by the metal shell 12 and the inner cylinder 14 via the spacer 15, the filling powder 16, and the glass seal I7.

Also, on the outer periphery of the metal shell 12 are screws 12 for mounting the internal combustion engine.
When the internal combustion engine is mounted against the wall surface of the internal combustion engine, a gasket 18 is provided on the wall to prevent exhaust gas from leaking.

Here, the filling powder 16 is made of a mixed powder of talc and glass in a ratio of 1=1, and fixes the detection section 10 within the inner cylinder 14.

Further, the glass seal 17 is made of low melting point glass, and is designed to prevent leakage of the detection gas and protect the terminals of the detection unit 10 from a part of the substrate of the detection unit 10 and the connection area between the platinum lead wire and the terminal, which will be described later. is covered and filled into the inner cylinder 14.

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 includes lead wires 21 to 23 and a detection member protruding from the glass seal 17 shown in FIG. The connection portions from the portion 10 to the terminals 31 to 33 are insulated by 3W. This lead VA21
As shown in FIG.
21 to 23, and each lead cotton 21 to 23
The caulking metal fittings 24 to 26 are provided at the tips of the terminals, and the caulking metal fittings 24 to 26 are connected to the terminals 31 to 33 to establish electrical continuity.

Next, the detection unit IO is created according to the procedure shown in FIGS. 4 to 8, and is shown in FIGS.
(0) shows the front side of the detection unit 10, and (0) shows the cross section taken along the line A-A.

Here, 40 and 41 in each of the above-mentioned figures 4 to 8 are 8h (h 92% by weight,
5iot 41i”1%, CaO2wt% and MgO2
12 parts by weight of butyral resin and dibutyl phthalate (DBP) per 100 parts by weight of mixed powder consisting of % by weight
6 parts by weight was added, mixed in an organic solvent to form a slurry, and formed using a doctor blade. Sa0, 3 n+
It was created in .

Further, 42 to 47 are patterns printed with a thick film using platinum paste added with 7% of 81□0:l based on pt, and the other 42 and 43 are electrode patterns that will become the electrodes of the detection element 11; Further, 44 is a heating resistor pattern for heating the detection element 11, and 45 to 47 are conductive patterns for applying power to the heating resistor pattern 44 and the detection element it or extracting a detection signal.

As shown in FIG. 4, each pattern 42 to 47 is first thick-film printed with platinum paste on Green Sea 1-40, and then, as shown in FIG. Platinum lead wires 48 to 50 are provided, respectively. Note that when printing the heating resistor pattern 44 as a thick film, it goes without saying that the width of the pattern is adjusted so that the detection element 11 can be heated by applying a predetermined voltage to the heating resistor pattern 44.

Next, as is clear from FIG. 6, the green sheet 4 is
1, an opening 51 is formed by punching so that the tips of the electrode patterns 42 and 43 are exposed, and a green sheet is placed on the green sheet 40 covering all patterns except the tips of the electrode patterns 42 and 43. 41 is laminated and thermocompressed.

In this way, parts of the platinum lead wires 48 to 50 protrude, and the tips of the electrode patterns 42 and 43 are brought into contact with the opening 5I.
A laminate plate is prepared which is exposed to the laminate plate, and subsequently, 80 to 150 mesh spherical granulated particles (secondary particles) 5 made of the same material as the green sheet 40 and 41 are placed on the frontage 51 of the laminate plate.
By dispersing and adhering the particles 52 and leaving them in the atmosphere at 1500° C. for 2 hours, a ceramic substrate having an uneven surface formed by uniformly dispersing each particle 52 is formed as shown in the enlarged view in FIG. 6 (c). and a convex portion 52' made of particles 52 is formed here.
The concave portions 52'' in between widen toward the end, and when a gas-sensing metal oxide paste (described later) is coated and baked, the gas-sensing metal oxide layer sinks into the four parts 52# and is laminated, making it strong against the ceramic substrate. so that it is fixed to the

Next, in order to form a gas detection element, titania raw material powder was first calcined at 1200°C in the atmosphere for 2 hours, then 100 g of butyl calpitol was added to 100 g of powder, mixed and ground in a ball mill for 24 hours, and then 52 g of BM was prepared. Add lhr and mix further. The paste thus prepared is applied to the recesses shown in FIG. At this time, it is possible to select and control the amount of paste depending on the desired thickness. After drying at 100-150°C for 30 minutes, it is fired in the air at 1100°C for 2 hours.

The fired element is impregnated with a metal salt solution. As the catalyst metal, Pt, Pd, Rh, and alloys thereof are preferable, and in particular, the amount of Rh supported must be 0.1 mol % or more. This is because if the amount is less than 0.001 mol 5, good durability will not be exhibited. 250 in the atmosphere after impregnation
It is thermally decomposed at temperatures above 0°C and baked into a tie.

In order to keep the catalyst particle size below 0.5μI,
oo°C is desirable. This completes the element formation of the layer near the electrode.

Next, PL was applied to the TiOz raw powder to form a surface layer.

It is impregnated with a metal salt solution containing mainly Pd. At this time, the Rh content of the titania element is 0.1 mol% or less, pt and Pd
The total amount should be 2 mol% or more.

After the impregnation, it is dried at 200 to 250°C for 24 hours, and then fired in the atmosphere at a high temperature of 1200°C or higher. With this treatment, the catalyst metal grains are 0.5 #m or more, and the titania powder grains are I~5 #m.
It becomes a μ river and has a larger grain size compared to the grains in the layer near the electrode, thus achieving the purpose. Next, this powder was made into a paste by the method described above, applied and dried as shown in FIG.
Bake at 0°C for 2 hours.

With the above steps, the element formation is completed, and the electrode vicinity layer and the surface layer have different catalyst species, particle sizes, and ceramic particle sizes.

The sensor fabricated in this way has a commercially available 21 l! I
'Installed on a three-way catalyst vehicle with I as shown in Figure 10,
The vehicle was driven in PA HOT TRANS4ENT MODE, and the amount of exhaust gas was measured using CvS while driving.

(Example) The sensor was subjected to durability in engine exhaust gas according to the durability pattern shown in Fig. 12, and then allowed to deteriorate.Then, the above emissions were measured again, and the shift in the control air-fuel ratio of the sensor was measured, and the changes between the initial stage and after the durability were evaluated. The amount was measured and evaluated.

As shown in Table 1, the invented product exhibits stable characteristics with less fluctuation in emissions during endurance club activities than the conventional product.

In the above embodiments of the present invention, an example of a two-layer structure has been explained, but the same effect as described above can be obtained not only in the case of three or more layers, but also in the case where an insulating coating layer is further provided on the surface layer. can get.

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

[Brief explanation of drawings]

1 to 9 show an embodiment in which the thick film gas sensing element according to the present invention is applied to an oxygen sensor, FIG. 1 is a cross-sectional view of the main parts showing the overall configuration of the oxygen sensor, FIG. 3 is an exploded view showing the relationship between the outer cylinder 19 and the sealing material 20 stored in the outer cylinder 19 in advance; Figures 4 to 8 show the detection unit 1.
FIG. 9 is an explanatory diagram of the assembly process sequence of No. 0. FIG. 9 is an explanatory diagram of the connection procedure of terminals 31 to 33. Figures 10 and 11 show the oxygen sensor inside P! ,+MIi
Figure 12 is an explanatory diagram of the durability test procedure used for the test. Figure 12 is a diagram of the durability pattern. 40, 41... Ceramic substrate 42, 43... Electrode pattern 11... Detection section (gas sensitive body W-membrane) Patent applicant NGK Spark Plug Co., Ltd. Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 tmW Figure 7 Figure 8 1θ Figure 11

Claims (1)

[Claims] 1. A thick gas sensitive film made of a ceramic semiconductor and a metal catalyst that covers a pair of electrodes disposed on a ceramic substrate; 1. A thick film gas sensitive element, characterized in that the ceramic particle size and the metal catalyst particle size are each larger than those of the ceramic particle size and the metal catalyst particle size in the residual layer extending near the electrode. 2. The device according to 1, wherein the metal catalyst in the remaining layer is different from the metal catalyst in the surface layer. 3. The device according to 1 or 2, wherein the gas sensitive thick film has a laminated structure of ceramic semiconductors in which different types of metal catalysts are dispersed by impregnating a fired ceramic semiconductor paste layer with a metal salt solution and thermally decomposing each layer.