JPS6216376B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a porous gas component detection element and a method for manufacturing the same. Metal oxide sintered bodies such as TiO ₂ are used as gas component detection elements because their electrical resistance changes depending on the gas component. When this is used in an automobile exhaust gas air-fuel ratio detector, it is necessary to show a sudden change in electrical resistance at the stoichiometric mixture ratio (λ = 1) in order to quickly feed back changes in the exhaust gas air-fuel ratio to operating conditions. For this reason, the thickness of the sintered body constituting the element was reduced to facilitate the penetration of the detection gas, but when the thickness was thin, there was a problem of insufficient strength. In addition, by calcining the raw material for the element sintered body, it is possible to coarsen the powder particles and increase the pore size between the particles, increasing the gas exchange ability and increasing the response speed. At the same time, durability deteriorates and mechanical strength also decreases. Furthermore, although it is possible to improve the response by adding Cr ₂ O ₃ or Al ₂ O ₃ to the raw material of the element sintered body, there is a problem in that the gas sensitivity of the element is deteriorated. Furthermore, in JP-A No. 55-82045 "Oxygen concentration detection element", it is made of transition metal oxide and produces an electrical resistance value depending on the oxygen concentration in the detection gas.
Among oxygen concentration sensing elements activated by group elements, an oxygen concentration sensing element characterized by forming a thin film-like coating layer of a transition metal oxide having high porosity on the surface of the element has been proposed. An even faster response speed has become necessary. The present invention solves the above-mentioned difficulties and provides a gas component detection element that does not deteriorate gas sensitivity, has excellent mechanical strength and durability, and has a fast response speed. It is characterized by having a pore diameter larger than the internal pore diameter, and for this purpose, one or more of Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , and Cr ₂ O ₃ is added near the surface to prevent sintering. The present invention also provides a manufacturing method in which a green sheet made of raw material powder with a particularly large particle size is pressed onto the surface of the green sheet. The metal oxide that forms the element has an electrical resistance value that changes depending on the gas component, such as _TiO2 .
CoO, Cr ₂ O ₃ , SnO ₂ and ZnO can be used.
The TiO ₂ raw material powder needs to have a specific surface area of 0.5 m ² /g or more, preferably 1 m ² /g or more from the viewpoint of gas sensitivity, and a sintered body of such powder can be used near the electrode. A gas-sensitive layer is formed. However, when such fine raw materials are used, the pore diameter after firing inevitably becomes smaller, and when the pore diameter becomes 0.2μ or less, it becomes equivalent to the mean free path of gas, and when used to detect automobile exhaust gas, the pore diameter becomes smaller. The diffusion rate of is rapidly reduced, slowing down the response rate. Therefore, it is a feature of the present invention that a material having a larger pore diameter than the inside is disposed outside the electrode part, which has good exhaust gas diffusion ability even if the gas sensitivity is poor. Such a sintered body with a large pore size is produced by pre-calcining and sintering the raw material to give a specific surface area of 1 m ² /g or less, preferably 0.5 m ² /g or less, so that the pore size on the element surface after firing can be reduced. is 0.3μ or more, and gas sensitivity is low, but gas diffusion is increased and a detection element with a fast response speed can be obtained. There are other ways to obtain sintered bodies with such large pore diameters, such as Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ ,
When one or more of Cr ₂ O ₃ is added, these act as anti-sintering agents and can maintain a large pore size.
The content may be 20% or less, and 1 to 10% is more preferable. The reason for this is that if it exceeds 10%, the sintering temperature of the protective layer becomes too high and the degree of sintering does not match that of the gas sensitive layer. Moreover, if it is less than 1%, the effect of preventing sintering and enlarging the pore diameter is poor. Since such a surface has the effect of protecting the vicinity of the electrode without reducing the responsiveness of the gas component detection element, it is hereinafter referred to as a protective layer. A more specific explanation will be given below with reference to examples.
The present invention is not restricted by the following examples unless the gist of the invention is exceeded. Example 1 Rutile type TiO ₂ with a specific surface area of 3.0 m ² /g and 9 m ² /g
2 mol% of platinum black was added thereto, and a butyral resin and ethyl alcohol were added and mixed to form a slurry, which was then formed into a sheet by a doctor blade method. The thickness of the dried sheet (hereinafter referred to as "green tape") was adjusted to 0.5 mm. This was cut into 4 mm x 3 mm, Pt paste was printed on the top and bottom surfaces of this, and Pt electric wires were placed to form electrodes. Its shape is shown in FIG. In the figure, 1 is a green tape, 2 is an electrode, and 3 is a Pt wire. 1 is a portion that becomes a gas sensitive layer with good gas sensitivity.
On the other hand, a green tape with a thickness of 0.3 mm was produced using coarse-grained TiO ₂ with a specific surface area of 0.5 m ² /g by calcining the TiO ₂ powder at 1300°C in the same manner as the gas-sensitive layer. It was cut into pieces of 4 mm x 3 mm, crimped onto the outside of electrode 2, and baked for 1 hour at 1200°C to obtain gas component detection element product A of the present invention. That state is the second
As shown in the figure. In the figure, reference numeral 11 indicates a gas sensitive layer with a small pore diameter, and 14 indicates a protective layer with a larger pore diameter. This is attached to the exhaust pipe of a 6-cylinder, 2000c.c., EFI-controlled car, and the engine speed is 2700r.p.
The feedback frequency was measured when racing at an intake pressure of 370 mmHg. Separately, as a comparative example, Comparative Product 1 was manufactured in the same manner as Inventive Product A, except that the protective layer was also made of a green sheet manufactured in the same manner as the gas-sensitive layer 11 in the central part, and the gas-sensitive layer in the central part was also protected. Table 1 shows the measured values of comparative product 2, which was manufactured in the same manner as product A of the present invention except that a powder calcined in the same manner as layer 14 was used.

【table】

[Table] Example 2 The additives listed in Table 2 were added to the protective layer of comparative product (1) using TiO ₂
Manufactured in the same manner as comparative product (1) except that it was added to
When tested in the same manner as in Example 1, the feedback frequency was sufficiently high and the resistance value of the element was low enough to withstand use. Furthermore, as a protective layer for comparative product 1, the specific surface area is 6.0 m ² /
AAl ₂ O ₃ with an average particle size of 0.5μ is added to 5 g of rutile TiO ₂ .
When the microstructures of the gas-sensitive layer and protective layer were investigated for samples BR ₂ and BR ₃ , which were manufactured in the same manner as comparative product 1 except for using a base material containing % and 7%, the porosity of the gas-sensitive layer was 25%. In contrast, the porosity of the protective layer was 35% and 49%, but the pore diameter was approximately the same in both cases, with an average of 0.20μ. These properties are shown in Table 2. From this, the feedback frequencies, that is, the response speeds in Table 2 are Al ₂ O ₃ , Y ₂ O ₃ ,
Sample No.B with 5% Cr ₂ O ₃ and ZrO ₂ added _Al , B _Y , B
_C and B _Z have a high feedback frequency, that is, good response, and B _A2 with 1.0% Al ₂ O ₃ also has good response, whereas B with 0.5% Al ₂ O ₃ added has a good response.
The responsiveness of _R was unsatisfactory. Furthermore, while the porosity of the gas-sensitive layer is 25%, the porosity of the protective layer is
35% and 49%, but BR ₂ and BR ₃ , both of which have pore diameters of 0.20μ, had an unsatisfactory response at a feedback frequency of 2.0Hz.

[Table] Example 3 Rutile type TiO ₂ with a specific surface area of 3.0 m ² /g was added and mixed with butyral resin and ethyl alcohol to form a slurry, and then formed into a 1 mm thick sheet using a doctor blade to form a 3 mm x 4 mm sheet. A green tape 21 is cut into a rectangular shape and becomes a gas sensitive layer as shown in Fig. 3.
And so. Next, a green tape serving as a protective layer is formed in the same manner as in Example 1, first a protective layer 24 is placed, a gas sensitive layer 21 is placed on top of the protective layer 24, and a Pt wire 23 is placed on top of the protective layer 24.
placed in parallel with a spacing of mm, and a gas sensitive layer 21
Further, the protective layer 24 was placed on top of the protective layer 24, and the protective layer 24 was bonded with a press and baked at 1200°C for one hour to obtain product C of the present invention.The performance was measured in the same manner as in Example 1, and the feedback frequency was 2.6Hz. Element resistance is 3.1KΩ
As described above, the product of the present invention, which has a gas sensitive layer with a fine pore size and a protective layer with a pore size larger than that of the gas sensitive layer, has a good feedback frequency and It exhibited electrical resistance and good mechanical strength, making it highly useful as a gas component detection element. In the above embodiments, a metal oxide, which is a semiconductor, was molded into a green sheet, but the present invention is not limited to this, and pressing, slurry casting, rolling molding, etc. can be used, and especially for forming the protective layer. You can also use slurry application, spraying, or dry powder spraying. In this case, the protective layer becomes particularly porous, so the raw material powder for the protective layer should have a particularly large particle size, or be made of Al ₂ O ₃ , ZrO ₂ ,
The pore diameter can be increased without adding Y ₂ O ₃ , Cr ₂ O ₃ or the like.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a component in which an electrode and an electric wire are attached to a gas sensitive layer according to the first embodiment, and FIG. 2 is a perspective view of a product A of the present invention in which a protective layer is attached and sintered.
FIG. 3 is a perspective view of the product C of the present invention according to Example 3.

Claims (1)

[Scope of Claims] 1. A gas component detection element made of a metal oxide sintered body that exhibits an electric signal according to the gas component of a detected gas, which has a pore diameter of 0.3 μm or more near the surface, and has a pore diameter of 0.3 μm or more in the interior. A gas component detection element characterized by being larger than the pore diameter. 2. The gas component detection element according to claim 1, characterized in that a sintering inhibitor is contained near the surface of the gas component detection element. 3 In claim 2, the sintering inhibitor contains 1 to 10% by weight of one or more of Al ₂ O ₃ , ZrO ₂ , Y ₂ O ₃ , and Cr ₂ O ₃ based on TiO ₂ near the surface. A gas component detection element characterized by: 4. Electrodes are provided at regular intervals on the upper or lower surfaces of a green sheet made of metal oxide powder and synthetic resin, and electrodes with a particle size larger than the metal oxide powder in the green sheet are provided on the outside of the electrodes. A method for producing a gas component detection element, which comprises bonding and firing green sheets made of metal oxide powder and synthetic resin.