JP2013007642A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor capable of suppressing generation of deterioration of detection accuracy for detecting components of specific gas, electric leakage between two cells, and blackening of a solid electrolyte by intrusion of soot in a porous protective layer.SOLUTION: A gas sensor has: a gas sensor element, which forms the plate-like shape extending in the axis direction and a detection part of which is provided on the tip side and exposed in gas to be measured, for detecting specific components in the gas to be measured; and a cylindrical housing which holds the gas sensor element inside the housing so that the tip side of the gas sensor element projects from the tip of the housing. The gas sensor element has: an element body on which at least two cells in which a pair of electrodes are arranged are at least laminated on a plate-like solid electrolyte; a laminate on which a heater having a heater element inside is laminated; and a porous protective layer which consists of ceramics for covering at least the detection part of the laminate. Ceria or a ceria compound primarily composed of ceria is contained in the porous protective layer.

Description

  The present invention relates to a gas sensor for detecting the concentration of a specific gas component in a measurement target gas.

  2. Description of the Related Art Conventionally, a gas sensor having a gas sensor element whose electrical characteristics change according to the concentration of a specific gas component in a gas to be measured such as exhaust gas discharged from an automobile is known. This gas sensor element includes a plate-shaped gas sensor element main body that detects the concentration of oxygen or the like in the exhaust gas and outputs it to the outside. The gas sensor element body has, for example, two cells each including a pair of electrodes mainly composed of platinum on a solid electrolyte body mainly composed of zirconia.

  Furthermore, since the cell in such a gas sensor element body cannot detect oxygen concentration (that is, does not activate) when the temperature is low, a plate-like heater is laminated on the gas sensor element body in order to heat it. A stacked gas sensor element is also known. As such a heater, for example, a heater formed of a ceramic substrate (insulating layer) mainly composed of alumina and a heating resistor is used.

  In such a gas sensor element, moisture such as water droplets contained in the exhaust gas or condensed water adhering to the exhaust pipe to which the gas sensor is attached adheres to the tip of the gas sensor element heated to a high temperature by the heater, and cracks are generated. May occur. On the other hand, a configuration in which the tip portion of the gas sensor element is covered with a porous protective layer made of porous ceramic is known. By forming such a porous protective layer, when moisture adheres to the tip portion of the gas sensor element, the moisture can be slowly permeated while being dispersed in the porous protective layer, thereby constituting the gas sensor element. It is possible to suppress the occurrence of cracks in the gas sensor element body and the heater (see, for example, Patent Document 1, Patent Document 2, and Patent Document 3).

JP 2008-216241 A JP 2003-322632 A JP 2009-257817 A

  As described above, in the gas sensor, by providing a porous protective layer that covers the tip portion of the gas sensor element, it is possible to prevent the moisture from adhering to the tip portion of the gas sensor element that has been heated to a high temperature by the heater and causing cracks and the like. ing.

  However, such a gas sensor is used in an atmosphere in which carbon (C) is abundant and soot is likely to be generated in a large amount (for example, in an atmosphere of a diesel engine or a direct injection engine or in an intake side atmosphere) or in an atmosphere in which soot is difficult to burn ( For example, when the measurement is performed under an atmosphere of a rich engine), the following problems occur due to the penetration of soot into the porous protective layer. In a normal atmosphere, there is a sufficient amount of oxygen, and the tip portion of the gas sensor element is heated to a high temperature by the heater, so that most of the soot can be burned by the heat of the heater. Under the two atmospheres described above, there is no oxygen amount that allows most of the present soot to burn, and soot may enter the porous protective layer.

  To block pores in the porous protective layer (that is, clogging occurs due to soot), making it difficult for the gas to be measured to reach the surface of the gas sensor element (cell surface), and to detect components of specific gases such as oxygen There is a risk that the detection accuracy of the lowering. Furthermore, when a soot enters the inside of the porous protective layer, a current path through the soot may be formed between two cells that should be insulated, and an electrical leak may occur. . In addition, a current path is formed between the two cells in the solid electrolyte body in spite of the state in which the gas to be measured is difficult to reach the surface of the gas sensor by closing the pores of the porous protective layer. There is a possibility that the oxygen ions move and the solid electrolyte body is blackened.

  The present invention has been made in response to the above-described conventional circumstances. According to the present invention, when soot penetrates into the porous protective layer, the detection accuracy for detecting the component of the specific gas is lowered, electrical leakage between the two cells, and blackening of the solid electrolyte body. It aims at providing the gas sensor which can suppress generating.

  One aspect of the gas sensor of the present invention has a plate-like shape extending in the axial direction, and a gas sensor element for detecting a specific component in the measurement gas by exposing the detection portion provided on the tip side to the measurement gas. And a cylindrical housing that holds the gas sensor element therein so that the front end side of the gas sensor element protrudes from the front end of the gas sensor element, and the gas sensor element is paired with a plate-shaped solid electrolyte body. An element main body in which at least two cells are arranged, a laminated body in which a heater having a heating element is laminated, and a ceramic that covers at least the detection portion of the laminated body. A gas sensor having a porous protective layer, wherein the porous protective layer contains ceria or a compound containing ceria as a main component.

  In the gas sensor of the present invention, the porous protective layer contains ceria or a compound containing ceria as a main component. Thus, in the porous protective layer, the presence of ceria or a compound containing ceria as a main component exists due to oxygen released from ceria having an oxygen storage function (an action of taking oxygen into itself). Oxygen for burning most of the soot is sufficiently present in the porous protective layer, and soot can be burned even in an atmosphere where a large amount of soot is likely to be generated or in a rich atmosphere. As a result, it is difficult to cause eye clogging in the porous protective layer, and it is possible to suppress a decrease in detection accuracy for detecting a component of the specific gas in the gas to be measured. Moreover, it can suppress that a soot penetrate | invades into the inside of a porous protective layer, and suppresses generation | occurrence | production of an electrical leak by forming the electric current path through the soot between two cells. be able to. Furthermore, clogging of the porous protective layer is less likely to occur, and a current path is less likely to be formed between the two cells, thereby suppressing the occurrence of blackening of the solid electrolyte body.

The ceria compound containing ceria as a main component is a compound containing 50% or more of ceria, and specifically refers to a ceria-zirconia composite compound, a ceria-praseodymium composite compound, and the like.
Moreover, the porous protective layer should just be provided so that a detection part may be covered, and also it may be provided so that the whole front end side of the gas sensor element which protrudes from a housing front end may be covered.
Moreover, the element main body should just have at least two cells laminated | stacked, and an insulating layer etc. may intervene in addition.

  In the gas sensor of the present invention, the porous protective layer contains ceria and a ceria compound containing ceria as a main component so that the content of ceria in the porous protective layer is 0.001% by mass to 5% by mass. It is preferable that When the content of ceria contained in the porous protective layer is less than 0.001% by mass, the oxygen storage effect of ceria is not sufficient, and as a result, the catalytic ability of the porous protective layer (that is, the ability to burn soot) ) May not be obtained. On the other hand, when the content of ceria contained in the porous protective layer exceeds 5% by mass, the detection accuracy error of a specific component may deteriorate due to the influence of oxygen released from ceria. In addition, when a compound containing ceria such as a ceria-zirconia composite compound or a ceria-praseodymium composite compound is used, the “content of ceria in the porous protective layer” is not the content of a compound containing ceria as a main component. It means that the content of ceria in the ceria compound contained in the porous protective layer is within the above-described range.

  Moreover, it is preferable that ceria or a compound containing ceria as a main component is contained more on the outer surface side of the porous protective layer than on the inner surface side of the porous protective layer. Since soot penetrates into the inside from the outer surface side of the porous protective layer, by configuring in this way, combustion of soot is promoted in the part on the outer surface side to which soot adheres, and a specific gas component is detected. It is possible to effectively suppress the deterioration of the detection accuracy and the occurrence of electrical leakage between the two cells and blackening of the solid electrolyte body.

Further, in the case of a configuration in which a measurement chamber for introducing the gas to be measured is provided between the two cells, and a gas diffusion porous is disposed in the gas introduction path for introducing the gas to be measured from the outside into the measurement chamber It is preferable that the porous protective layer contains ceria or a compound containing ceria as a main component so as to be contained more in the porous protective layer provided on the gas diffusion porous than in other parts. The porous protective layer provided on the gas diffusion porous serves as a gas passage route for introducing gas from the outside into the measurement chamber. Therefore, by selectively disposing ceria in this portion, the gas passage route can be reduced. It can be suppressed that the detection accuracy for detecting the component of the specific gas in the gas to be measured is clogged with the soot, and decreases.
The “porous protective layer provided on the gas diffusion porous” refers to a porous protective layer that overlaps with the gas diffusion porous when viewed in the flow direction of the gas to be measured flowing through the gas introduction path. .

  The porous protective layer preferably has an average pore diameter of 0.01 μm to 5.0 μm. When the average pore diameter is less than 0.01 μm, the poisoning resistance is reduced, and when the average pore diameter is more than 5 μm, the pores become too large, so that there are many soot and penetrates into the porous protective layer. This is because the catalytic ability is lowered.

  Further, the porous protective layer may further contain a platinum-based noble metal (for example, Pt, Pd, Rh, Pt—Pd, Pt—Ru, Pt—Rh). Thereby, combustion of soot can be further promoted. In this case, it is preferable that the content of ceria contained in the porous protective layer is larger than the content of platinum-based noble metal. Thereby, the usage-amount of a platinum-type noble metal can be suppressed and the increase in cost can be suppressed.

  A protector having a gas introduction hole for introducing a gas to be measured, which is fixed to the front end side of the housing and is provided so as to cover the front end side of the gas sensor element, is provided on the porous protective layer in the axial direction. It is preferable to provide in the overlapping position. This facilitates the measurement gas introduced into the protector to reach the detection element, and improves the gas detection response. In addition, most of the soot introduced into the protector adheres to the porous protective layer, but the ceria contained in the porous protective layer can promote soot combustion.

  In addition, when the gas sensor element is provided in a gap between the gas sensor element and the housing and includes a holding part for holding the gas sensor element in the housing, the holding part and the porous protective layer are separated from each other in the axial direction. A surface layer in which ceria or a ceria compound containing ceria as a main component is attached to at least a part of the surface of the gas sensor element located in the gap with the porous protective layer may be provided. Thus, by providing the surface layer in the gap between the holding portion where the gas sensor element is exposed and the porous protective layer, combustion of soot adhering to the gap is promoted, and the electrode (lead electrode) and the housing ( It is possible to suppress the occurrence of electrical leakage with the metal shell). More preferably, the surface layer is provided on the entire surface of the gas sensor element located in the gap.

  According to the present invention, when soot penetrates into the porous protective layer, the detection accuracy for detecting the component of the specific gas is lowered, the electric leak between the two cells, and the black of the solid electrolyte body It is possible to provide a gas sensor that can suppress the occurrence of ning.

The sectional view showing the whole gas sensor composition of one embodiment of the present invention. The disassembled perspective view which shows the structure of the gas sensor element of the gas sensor of FIG. The perspective view which shows the structure of the gas sensor element of FIG. Sectional drawing which shows the structure of the gas sensor element of FIG. The figure for demonstrating the manufacturing method of a gas sensor element.

  Embodiments of the present invention will be described below with reference to the drawings.

  Hereinafter, embodiments of a gas sensor of the present invention will be described with reference to the drawings. Hereinafter, as the gas sensor of the present invention, a gas sensor element that is used for air-fuel ratio feedback control in an automobile or various internal combustion engines and detects the concentration of a specific gas (oxygen) in exhaust gas that is a measurement target gas is assembled. A full range air-fuel ratio sensor that is found and mounted on an exhaust pipe of an internal combustion engine will be described as an example.

  FIG. 1 is a cross-sectional view showing the overall configuration of a gas sensor 1 of the present invention. The gas sensor 1 has a cylindrical housing (main metal fitting) 10 having a screw portion 14 formed on the outer surface for fixing to an exhaust pipe, and a plate shape extending in the axial direction (longitudinal direction of the gas sensor 1: vertical direction in the figure). The gas sensor element 20 having a shape, a cylindrical ceramic sleeve 27 arranged so as to surround the circumference of the gas sensor element 20, and the inner wall surface of the contact insertion hole 31 penetrating in the axial direction are the rear end of the gas sensor element 20 Insulating contact member 30 disposed so as to surround the periphery of the portion, and five connection terminals 36 (one is shown in FIG. 1) disposed between gas sensor element 20 and insulating contact member 30. ing.

  The metal shell 10 is configured in a substantially cylindrical shape having a through hole 12 penetrating in the axial direction and a shelf 15 projecting radially inward of the through hole 12. The shelf portion 15 is formed as an inwardly tapered surface having an inclination with respect to a plane perpendicular to the axial direction. In addition, the gas sensor element 20 has a metal shell in which the detection portion 20a is disposed outside the front end 11 of the metal shell 10, and the electrode terminal portions 301 and 302 are disposed outside the rear end portion 13 of the metal shell 10. 10 through holes 12 are inserted.

  In addition, inside the through hole 12 of the metal shell 10, an annular ceramic holder 24, powder filling layers 25, 26 (hereinafter also referred to as talc rings 25, 26) surrounding the radial direction of the gas sensor element 20. And the above-mentioned ceramic sleeve 27 is laminated | stacked from the front end side to the rear end side in this order. A caulking packing 28 is disposed between the ceramic sleeve 27 and the rear end portion 13 of the metal shell 10, and the ceramic holder 24 is disposed between the ceramic holder 24 and the shelf 15 of the metal shell 10. A metal holder 23 for holding the talc ring 25 is disposed. Note that the rear end portion 13 of the metal shell 10 is crimped so as to press the ceramic sleeve 27 against the distal end side via the crimping packing 28.

  On the other hand, on the outer periphery of the front end side (lower side in FIG. 1) of the metal shell 10, a metal (for example, stainless steel, etc.) double cover that covers the protruding portion of the gas sensor element 20 and has a plurality of gas introduction holes 42. The external protector 40 and the internal protector 41 are attached by welding or the like.

  An outer cylinder 50 is fixed to the outer periphery of the rear end side of the metal shell 10. In addition, five lead wires 52 (3 in FIG. 1) electrically connected to the electrode terminal portions 301 and 302 of the gas sensor element 20 are respectively provided in the opening on the rear end side (upper side in FIG. 1) of the outer cylinder 50. A grommet 54 in which a lead wire pushing hole 55 through which this figure is pushed is formed is disposed.

  An insulating contact member 30 is disposed on the rear end side (upper side in FIG. 1) of the gas sensor element 20 protruding from the rear end portion 13 of the metal shell 10. The insulating contact member 30 is disposed around the electrode terminal portions 301 and 302 formed on the rear end surface of the gas sensor element 20. The insulating contact member 30 is formed in a cylindrical shape having a contact insertion hole 31 penetrating in the axial direction, and is provided with a flange portion 32 protruding radially outward from the outer surface. The insulating contact member 30 is disposed inside the outer cylinder 50 by the flange portion 32 contacting the outer cylinder 50 via the holding member 58.

  Next, the gas sensor element 20 will be described in detail. The gas sensor element 20 has a front end side (lower side in the figure) directed toward the gas to be measured as a detection unit, and an axial direction in which electrode terminal portions 301 and 302 are formed on the front and back surfaces on the rear end side (upper side in the figure). And a porous protective layer 22 formed so as to cover at least the detection part 20a of the laminated body 21.

  The connection terminal 36 is electrically connected to the electrode terminal portions 301 and 302 of the gas sensor element 20 by being disposed between the gas sensor element 20 and the insulating contact member 30. The connection terminal 36 is also electrically connected to the lead wire 52 disposed inside the sensor from the outside, and between the external device to which the lead wire 52 is connected and the electrode terminal portions 301 and 302. A current path for the flowing current is formed.

  FIG. 2 is an exploded perspective view of the laminated body 21, and FIG. 3 is a perspective view of the laminated body 21. As shown in FIG. 2, the laminate 21 is a laminate of a gas sensor element body (detection element) 100 and a heater 200, and the gas sensor element body 100 further includes an oxygen concentration detection cell 110 and an oxygen pump cell 130. It is a laminated one.

  The heater 200 includes a first base 201 and a second base 203 mainly composed of alumina, and a heating element 202 mainly composed of platinum sandwiched between the first base 201 and the second base 203. The heat generating body 202 includes a heat generating portion 202a located on the distal end side, and a pair of heater lead portions 202b extending from the heat generating portion 202a along the longitudinal direction of the first base 201. The terminal of the heater lead portion 202b is electrically connected to the electrode terminal portion 302 via the heater side through hole 201a provided in the first base 201.

  The oxygen concentration detection cell 110 includes a first solid electrolyte body 112 and a first electrode 111 and a second electrode 113 formed on both surfaces of the first solid electrolyte body 112. The first electrode 111 includes a first electrode part 111 a and a first lead part 111 b extending from the first electrode part 111 a along the longitudinal direction of the first solid electrolyte body 112. The second electrode 113 includes a second electrode portion 113a and a second lead portion 113b extending from the second electrode portion 113a along the longitudinal direction of the first solid electrolyte body 112.

  Terminals of the first lead portion 111b are provided in the first through hole 112a provided in the first solid electrolyte body 112, the second through hole 120a provided in the insulating layer 120 described later, and the second solid electrolyte body 132. The electrode terminal portion 301 is electrically connected through the fourth through hole 132a and the sixth through hole 140a provided in the protective layer 140. On the other hand, the terminals of the second lead 113b are third through holes 120b provided in an insulating layer 120, which will be described later, fifth through holes 132b provided in the second solid electrolyte body 132, and first terminals provided in the protective layer 140. It is electrically connected to the electrode terminal portion 301 through the 7 through hole 140b.

  On the other hand, the oxygen pump cell 130 includes a second solid electrolyte body 132 and third and fourth electrodes 131 and 133 formed on both surfaces of the second solid electrolyte body 132. The third electrode 131 includes a third electrode portion 131a and a third lead portion 131b extending from the third electrode portion 131a along the longitudinal direction of the second solid electrolyte body 132. The fourth electrode 133 includes a fourth electrode portion 133a and a fourth lead portion 133b extending from the fourth electrode portion 133a along the longitudinal direction of the second solid electrolyte body 132.

  A terminal of the third lead portion 131b is electrically connected to the electrode terminal portion 301 through a fifth through hole 132b provided in the second solid electrolyte body 132 and a seventh through hole 140b provided in the protective layer 140. The On the other hand, the terminal of the fourth lead portion 133 b is electrically connected to the electrode terminal portion 301 through the eighth through hole 140 c provided in the protective layer 140. The second lead portion 113b and the third lead portion 131b are at the same potential through the third through hole 120b.

The first solid electrolyte body 112 and the second solid electrolyte body 132 are partially stabilized zirconia sintered bodies obtained by adding yttria (Y ₂ O ₃ ) or calcia (CaO) as a stabilizer to zirconia (ZrO ₂ ). It is composed of

  The heating element 202, the first electrode 111, the second electrode 113, the third electrode 131, the fourth electrode 133, the electrode terminal portion 301, and the electrode terminal portion 302 can be formed of a platinum group element. Pt, Rh, Pd etc. can be mentioned as a suitable platinum group element which forms these, These can also be used individually by 1 type, and can also use 2 or more types together.

  However, the heating element 202, the first electrode 111, the second electrode 113, the third electrode 131, the fourth electrode 133, the electrode terminal portion 301, and the electrode terminal portion 302 are mainly composed of Pt in consideration of heat resistance and oxidation resistance. It is even more preferable to form them. Furthermore, the heating element 202, the first electrode 111, the second electrode 113, the third electrode 131, the fourth electrode 133, the electrode terminal portion 301, and the electrode terminal portion 302 contain a ceramic component in addition to the main platinum group element. It is preferable to do. This ceramic component is preferably the same component as the main material on the laminated side (for example, the main component of the first solid electrolyte body 112 and the second solid electrolyte body 132) from the viewpoint of fixation. .

  An insulating layer 120 is formed between the oxygen concentration detection cell 110 and the oxygen pump cell 130. The insulating layer 120 includes an insulating part 121 and a gas diffusion porous material 122 that constitutes a diffusion-controlling part. In the insulating portion 121 of the insulating layer 120, a gas detection chamber 120c is formed at a position corresponding to the second electrode portion 113a and the third electrode portion 131a. The gas detection chamber 120c communicates with the outside in the width direction of the insulating layer 120, and the gas diffusion between the outside and the gas detection chamber 120c is realized under a predetermined rate-limiting condition in the communicating portion. Porous 122 is disposed.

  The insulating part 121 is not particularly limited as long as it is an insulating ceramic sintered body, and examples thereof include oxide ceramics such as alumina and mullite. The gas diffusion porous 122 is a porous body made of alumina. This gas diffusion porous 122 performs rate control when the detection gas flows into the gas detection chamber 120c.

  Further, a protective layer 140 is formed on the surface of the second solid electrolyte body 132 so as to sandwich the fourth electrode 133. The protective layer 140 includes a porous electrode protective part 142 that sandwiches the fourth electrode part 133a. The electrode protection part 142 is inserted into a through hole 141a formed in the reinforcement part 141 that sandwiches the fourth lead part 133b.

  FIG. 4 shows a cut surface cut along the line AA ′ in FIG. 3. FIG. 4 shows a porous protective layer 22 in addition to the laminate 21, which is substantially a cut surface of the gas sensor element 20.

  In addition, the site | part in which the gas detection chamber 120c, the 1st electrode 111, the 2nd electrode 113, the 3rd electrode 131, and the 4th electrode 133 are arrange | positioned is the detection part 20a.

  The porous protective layer 22 is made of, for example, a porous ceramic such as spinel, alumina, titania, cordierite, steatite, mullite, zirconia, and covers the entire circumference of the detection unit 20a of the laminate 21. The porous protective layer 22 is formed for the purpose of preventing the gas sensor element 20 from being cracked when the detection unit 20a is flooded.

  In the present embodiment, the porous protective layer 22 contains ceria (single substance). In addition to ceria, for example, a compound containing ceria as a main component such as a ceria-zirconia composite compound or a ceria-praseodymium composite compound may be included. Thus, the presence of ceria or a compound containing ceria as a main component in the porous protective layer 22 causes most of the soot to be burned by oxygen released from ceria having an oxygen storage function. Oxygen is sufficiently present in the porous protective layer, so that the soot can be burned even in an atmosphere where soot is likely to be generated in a large amount or in a rich atmosphere. As a result, clogging due to soot is less likely to occur in the porous protective layer 22, and it is possible to suppress a decrease in detection accuracy for detecting the concentration of the specific gas (oxygen) in the exhaust gas. Moreover, it can suppress that a soot penetrate | invades into the inside of the porous protective layer 22, and suppresses that the electric current path through the soot between two cells is formed, and an electrical leak generate | occur | produces. be able to. Furthermore, clogging of the porous protective layer 22 is less likely to occur, and a current path is less likely to be formed between the two cells, resulting in blackening of the first solid electrolyte body 112 and the second solid electrolyte body 132. This can be suppressed.

  In addition, the said porous protective layer 22 containing the compound (For example, a ceria-zirconia composite compound, a ceria-praseodymium composite compound) which has ceria or a ceria as a main component can be produced with the following methods, for example.

  That is, after forming the porous protective layer 22 with a protective coating material such as spinel, alumina, titania, cordierite, steatite, mullite, zircon, etc., a solution of ceria, ceria-zirconia composite compound, ceria-praseodymium composite compound, etc. And then baking at 600 to 1200 ° C. Alternatively, a solution (for example, sol) in which ceria, a ceria-zirconia composite compound, a ceria-praseodymium composite compound, or the like is added to the protective coating material described above is prepared, and this is applied to the laminate 21 by dipping or spraying. For example, the porous protective layer 22 is formed by baking at 900 to 1200 ° C.

Furthermore, in this embodiment, ceria is contained in the porous protective layer 22 so that the content of ceria in the porous protective layer 22 is 1.0 mass%.
Table 1 shows the results of investigating the catalyst capacity and the O ₂ accuracy error by preparing samples with different amounts of ceria contained in the porous protective layer 22. In the investigation of the catalytic ability, the coated fragments and the carbon solution (carbon powder: ethanol = 1: 1) were mixed at a ratio of 2: 1, and the carbon attenuation was 10% by thermogravimetric analysis (TG (Thermal Gravity)). the temperature at which became T _1, at TG of only carbon without the coating debris (Thermal Gravity), the temperature at which the attenuation amount of the carbon became 10% when the T _2, T ₂ -T ₁ When the temperature was + 50 ° C. or higher, the catalytic ability was good (◯), and when it was lower than + 50 ° C., the catalyst capacity was poor (×).

Further, in the investigation of the O ₂ accuracy error, the stable time (within saturation value ± 5%) of the O ₂ output when the air-fuel ratio of the gas atmosphere is changed from λ = 1 to λ = 0.9 (rich) t when a _{1, t} 1 = of less than 2 seconds _{O 2} accuracy errors as well _(○), and the case of _t 1 = 2 seconds or more and poor (×).

As shown in Table 1, when the amount of ceria contained in the porous protective layer 22 is 0.0005% by mass, which is less than 0.001% by mass, the oxygen storage effect of ceria is not sufficient, and the porous protective layer The catalytic capacity of 22 is not sufficient. On the other hand, when the amount of ceria contained in the porous protective layer 22 is 7% by mass exceeding 5% by mass, an O ₂ accuracy error that detects the concentration of oxygen in the exhaust gas due to the influence of oxygen released from the ceria. Gets worse. Therefore, the amount of ceria contained in the porous protective layer 22 is preferably 0.001% by mass or more and 5% by mass or less.

  In addition, when a compound containing ceria as a main component, such as a ceria-zirconia composite compound or a ceria-praseodymium composite compound, is used, the amount of ceria contained in the compound containing ceria as a main component contained in the porous protective layer 22 is And within the above-mentioned range.

  By the way, in this embodiment, the amount of ceria contained in the porous protective layer 22 is larger on the outer surface side, which is the surface to which soot adheres, than on the inner surface side. This promotes soot combustion at the part of the outer surface where soot adheres, and the detection accuracy for detecting the concentration of oxygen in the exhaust gas is reduced. Electrical leakage through the soot between the two cells , And blackening of the first solid electrolyte body 112 and the second solid electrolyte body 132 can be effectively suppressed.

  Further, in the present embodiment, the amount of ceria in the porous protective layer 22 provided on the gas diffusion porous 122 disposed in the gas introduction path for introducing gas from the outside to the measurement chamber 120c shown in FIG. The structure is larger than other portions of the porous protective layer 22. This is because the porous protective layer 22 located in the outer peripheral portion of the gas diffusion porous 122 serves as a gas passage route for introducing gas from the outside into the measurement chamber 120c, and by selectively disposing ceria in this portion. It can be suppressed that the gas passage is clogged with soot and the detection accuracy for detecting the concentration of oxygen in the exhaust gas is lowered.

  Furthermore, in this embodiment, 20 mass% of Pt as a catalyst is contained in the porous protective layer 22 in addition to the above-described ceria or a compound containing ceria. Note that not only Pt but also platinum-based noble metals (for example, Pd, Rh, Pt—Pd, Pt—Ru, and Pt—Rh) can be contained. Thereby, combustion of soot can be further promoted. In this case, the content of ceria contained in the porous protective layer 22 is greater than the content of platinum-based noble metal. Thereby, the usage-amount of a platinum-type noble metal can be suppressed and the increase in cost can be suppressed.

  By the way, the catalytic ability of ceria or a compound containing ceria varies depending on the average pore diameter of the porous protective layer 22, and it is preferable that the average pore diameter is somewhat small from the viewpoint of catalytic ability. On the other hand, from the viewpoint of poisoning resistance, it is preferable that the average pore diameter is somewhat large in order to prevent clogging. Table 2 shows the results of investigation of the average pore diameter (μm), the above-described catalytic ability and poisoning resistance. In addition, the investigation of poisoning resistance is P poisoning endurance (ZnDTP: 1.3 cc / L, fuel consumption: 3.18 L / hr, exhaust gas temperature 500 ° C., 3000 rpm), Δip (change rate of pump current) around 300 hr. Judged by. When Δip was less than 2%, it was judged as good (◯), and when Δip was 2% or more, it was judged as defective (x).

  As for the average pore diameter (μm), after polishing the cross section of the porous protective layer 22, the pore distribution (horizontal axis: pore diameter, vertical axis: pore cross-sectional area) is calculated by SEM observation, and the peak strength of the pore diameter is calculated. Is defined as the average pore size, and when there are multiple peak strengths, the peak of 1/3 or more of the maximum peak strength is regarded as the peak, and the pore size is the smallest among them. The peak pore size was defined as the average pore size.

  As shown in Table 2, when the average pore diameter is less than 0.01 μm and becomes 0.005 μm, the poisoning resistance decreases. On the other hand, when the average pore diameter exceeds 5 μm and becomes 7 μm, the catalytic ability is lowered. Therefore, the average pore diameter of the porous protective layer 22 is preferably 0.01 μm or more and 5 μm or less. In the present embodiment, the average pore diameter is 0.5 μm.

  Furthermore, in this embodiment, as shown in FIG. 1, the outer side of the porous protective layer 22 is covered with an external protector 40 and an internal protector 41 having a plurality of gas introduction holes 42. Then, when viewed from the radial direction of the gas sensor 1, at least a part of the gas introduction hole 42 is disposed on the porous protective layer 22. That is, the gas introduction hole 42 is positioned so as to overlap the porous protective layer 22 in the axial direction. This facilitates the exhaust gas introduced into the internal protector 41 to reach the gas sensor element 20 (the detection unit 20a), and improves the gas detection response. In addition, most of the soot introduced into the internal protector 41 adheres to the porous protective layer 22, but the ceria contained in the porous protective layer 22 can promote soot combustion.

  In general, the porous protective layer 22 is formed so as to reach the inside of the through hole 12 of the housing 10 from the tip of the laminated body 21 as shown in FIG. However, in order to avoid contact between the ceramic holder 24 and the porous protective layer 22, the porous protective layer 22 cannot be provided to the rear end side of the gas sensor element 20 so as to reach the back of the through hole 12. . For this reason, a gap is formed between the porous protective layer 22 and the ceramic holder 24, and a portion where the laminate 21 is exposed is formed. If wrinkles accumulate here, there is a possibility that electrical leakage may occur between the electrode (lead electrode) in the laminate 21 and the metal shell 10. For this reason, the surface layer 21a made of ceria is provided on at least a part of the exposed portion of the laminated body 21 on the rear end side of the porous protective layer 22. In addition to ceria, the surface layer 21a may be a compound containing ceria as a main component, for example, a ceria-zirconia composite compound or a ceria-praseodymium composite compound. In the example shown in FIG. 1, the surface layer 21 a is formed on the entire exposed portion of the laminate 21 on the rear end side of the porous protective layer 22. In this way, by providing the surface layer 21a, combustion of soot adhering to the gap is promoted, and electrical leakage occurs between the electrode (lead electrode) in the laminate 21 and the metal shell 10. Can be suppressed.

  Next, a method for manufacturing the gas sensor element 20 having the porous protective layer 22 as described above will be described. In the following description, the site before firing and the site after firing are described using the same reference numerals. For example, it is described as an unfired first solid electrolyte body 112 that becomes the first solid electrolyte body 112 after firing.

  First, manufacture of the laminated body 21 in which the gas sensor element main body 100 and the heater 200 are laminated will be described. First, a slurry is prepared by wet-mixing the first raw material powder and the plasticizer. The first raw material powder is composed of, for example, 97% by mass of alumina powder and 3% by mass of silica as a sintering regulator. The plasticizer is made of, for example, butyral resin and dibutyl phthalate (DBP).

  Then, this slurry is formed into a sheet-like material having a thickness of 0.4 mm, for example, by a sheet forming method using a doctor blade device, and then cut into 140 mm × 140 mm, and the unfired reinforcing portion 141 and the first unfired base body 201 are cut. Then, the unfired insulating part 121 of the second unfired substrate 203 and the unfired insulating layer 120 is obtained. And the through-hole 141a is formed in the unbaking reinforcement part 141. FIG. Further, the gas detection chamber 120 c is formed in the unfired insulating portion 121.

  On the other hand, a slurry is prepared by wet-mixing the second raw material powder and the plasticizer. The second raw material powder is composed of, for example, 63% by mass of alumina powder, 3% by mass of silica as a sintering regulator, and 34% by mass of carbon powder. Moreover, a plasticizer consists of butyral resin and DBP, for example. And the unsintered electrode protection part 142 is obtained using this slurry.

Moreover, a slurry is prepared by wet-mixing the third raw material powder and the plasticizer. The third raw material powder is composed of, for example, 97% by mass of zirconia powder and silica (a total of 3% by mass of SiO ₂ powder and alumina powder) as a sintering regulator. Moreover, a plasticizer consists of butyral resin and DBP, for example. Using this slurry, a first unfired solid electrolyte body 112 and a second unfired solid electrolyte body 132 are obtained.

  Furthermore, for example, a slurry is prepared by wet-mixing 100% by mass of alumina powder and a plasticizer. A plasticizer consists of butyral resin and DBP, for example. By using this slurry, the green gas diffusion porous material 122 of the green insulating layer 120 is obtained.

  Then, in order from the bottom, the first unfired substrate 201, the unfired heating element 202, the second unfired substrate 203, the first unfired electrode 111, the first unfired solid electrolyte body 112, the second unfired electrode 113, the unfired The insulating layer 120, the third green electrode 131, the second green solid electrolyte body 132, the fourth green electrode 133, the green protective layer 140, and the like are stacked.

  Specifically, an unfired heating element 202 is formed on the first unfired substrate 201 by screen printing using a paste mainly composed of platinum. Then, the second unfired substrate 203 is laminated so as to sandwich the unfired heat generating portion 102.

  Then, the first green electrode 111 is formed on the first green solid electrolyte body 112. The first unsintered electrode 111 is made of, for example, a platinum paste of 90% by mass of platinum and 10% by mass of zirconia powder. That is, the first green electrode 111 is formed by printing this platinum paste by a screen printing method.

  Further, the first unsintered solid electrolyte body 112 is stacked on the second unsintered substrate 203 so as to sandwich the first unsintered electrode 111, and the second unsintered solid electrolyte body 112 is further laminated on the first unsintered solid electrolyte body 112. The electrode 113 is formed by printing. The second green electrode 113 is made of the same material as the first green electrode 111.

  Then, the green insulating layer 120 is formed on the second green electrode 113. Specifically, the unsintered insulating part 121 and the unsintered gas diffusion porous 122 are formed. It should be noted that a paste mainly composed of carbon is placed between the first solid electrolyte body and the second solid electrolyte body where the porous protective layer 22 is disposed after firing, and the portion that becomes the gas detection chamber 120c after firing. Print.

  Further, the third unfired electrode 131 is printed on the second unfired solid electrolyte body 132 and laminated on the unfired insulating layer 120 so as to sandwich the third unfired electrode 131. Then, the fourth green electrode 133 is printed on the second green solid electrolyte body 132. The third unfired electrode 131 and the fourth unfired electrode 133 are made of the same material as the first unfired electrode 111. Then, the green protective layer 140 is laminated on the fourth green electrode 133. The unsintered protective layer 140 is obtained by previously inserting the unsintered electrode protecting part 142 into the through hole 141 a of the unsintered reinforcing part 141.

  And after pressurizing these by 1 MPa and crimping | bonding, it cut | disconnects to a predetermined magnitude | size and the unbaking laminated body 21 is obtained. Thereafter, the unfired laminate 21 is removed from the resin, and further held at a firing temperature of 1500 ° C. for 1 hour to obtain the laminate 21.

  Next, formation of the porous protective layer 22 on the laminate 21 will be described.

  When the unfired porous protective layer 22 is formed by the dip method, it is necessary to repeat the dip and the drying a plurality of times in order to obtain a necessary thickness. The unfired porous protective layer 22 can be formed, and the manufacturing time can be greatly shortened.

  Hereinafter, the formation of the porous protective layer 22 will be specifically described. First, raw material spinel powder and titania powder are prepared, and ethanol as a volatile solvent is added to obtain a coating solution. Furthermore, ceria, a ceria-zirconia composite compound, a ceria-praseodymium composite compound, or the like is added to the coating solution so that the ceria content is in the range of 0.001 mass% to 5 mass%. And this coating liquid is sprayed on the laminated body 21, it is made to dry, and the unbaking porous protective layer 22 used as the porous protective layer 22 after baking is formed.

  As shown in FIG. 5, spraying is performed by first spraying from the front end side with a spray device 401 using a needle type nozzle in a state where the rear end side of the laminated body 21 is supported by the support member 400. After the coating liquid is applied to the tip part (a), the sprayed body 401 is sprayed from the side while rotating the laminated body 21, and the coating liquid is applied to the side part of the laminated body 21 (b).

  In FIG. 5, reference numeral 402 denotes a suction device for collecting and reusing the ceramic powder (the ethanol from which the volatilized ethanol is volatilized) scattered around. The ceramic powder recovered by the suction device 402 can be easily reused by dispersing it again in ethanol.

  Thus, according to the method of performing the two-stage spraying by changing the direction, the unfired porous protective layer 22 can be uniformly formed on the tip portion and the side portion of the laminate 21. Further, by spraying the tip portion first, the end portion of the unfired porous protection layer 22 formed at the tip portion can be covered with the porous protection layer 22 at the side portion formed thereafter. The porous protective layer 22 formed at the tip portion can be made difficult to peel off.

  Although the method of spraying in two stages from the front end side and the side side has been described above, instead of the method of spraying in two stages, a one-stage spray spraying obliquely from these intermediate positions May be performed.

  In these sprays, a volatile solvent such as ethanol is used for the coating liquid instead of water, and the coating liquid adheres to the surface of the laminate 21 by appropriately selecting the rotation speed, spraying time, and the like. Thus, the volatile solvent is volatilized and the dried ceramic raw material powder can be easily attached to the surface of the laminate 21. Thereby, the unfired porous protective layer 22 having a necessary thickness can be obtained without providing a drying time in the middle.

And the laminated body 21 in which the unfired porous protective layer 22 was formed was heated in an air atmosphere, and after performing a heat treatment that was held at a maximum temperature of 1000 ° C. for 1 hour, it was cooled by air cooling. The laminated gas sensor element 20 in which the porous protective layer 22 is formed can be obtained. In the above example, ceria, a ceria-zirconia composite compound, a ceria-praseodymium composite compound, or the like is added to the coating liquid constituting the porous protective layer 22 to form the porous protective layer 22 containing these. As described above, after forming the porous protective layer 22 without containing ceria, ceria-zirconia composite compound, ceria-praseodymium composite compound in the coating liquid, ceria, ceria-zirconia composite compound, ceria-praseodymium composite compound You may impregnate the solution containing these.
Moreover, you may form the porous protective layer 22 not by the above-mentioned spray method but by the dip method. In this case, the tip of the laminate 21 is immersed in a dip solution containing spinel powder as a raw material, titania powder, ceria, ceria-zirconia composite compound, ceria-praseodymium composite compound, etc., and then heat treatment is performed. Thus, the porous protective layer 22 is formed.

  Thereafter, the gas sensor 1 is manufactured by a known manufacturing method. Specifically, the manufactured gas sensor element 20 is inserted into a metal holder 23 and further fixed with a ceramic holder 24 and a talc ring 25 to produce an assembly. Thereafter, the assembly is fixed to the metal shell 10, the talc ring 26 and the ceramic sleeve 27 are inserted, and the lower assembly is produced by crimping at the rear end portion 13 of the metal shell 10. Note that an external protector 40 and an internal protector 41 are attached to the lower assembly in advance. On the other hand, the outer cylinder 50, the insulating contact member 30, the grommet 54, and the like are assembled to produce an upper assembly. Then, the lower assembly and the upper assembly are joined to obtain the gas sensor 1.

  As mentioned above, although this invention was demonstrated about embodiment, this invention is not limited to the said embodiment, Of course, various deformation | transformation are possible.

  DESCRIPTION OF SYMBOLS 1 ... Gas sensor, 10 ... Housing (main metal fitting), 20 ... Gas sensor element, 20a ... Tip part, 21 ... Laminated body, 21a ... Surface layer, 22 ... Porous protective layer, 40 ... External Protector, 41 ... Internal protector, 42 ... Gas introduction hole.

Claims (9)

It has a plate-like shape extending in the axial direction, and a detection part provided on the tip side is exposed to the gas to be measured, and a gas sensor element for detecting a specific component in the gas to be measured, and a tip side of the gas sensor element A cylindrical housing that holds the gas sensor element inside itself so as to protrude from its tip,
The gas sensor element includes a laminated body in which at least two cells each having a pair of electrodes arranged on a plate-shaped solid electrolyte body are laminated, a laminated body in which a heater having a heating element is laminated, and the laminated body A gas sensor having a porous protective layer made of ceramic and covering at least the detection part of
The gas sensor according to claim 1, wherein the porous protective layer contains ceria or a ceria compound containing ceria as a main component. The gas sensor according to claim 1,
The porous protective layer contains ceria or a ceria compound containing ceria as a main component so that the content of ceria in the porous protective layer is 0.001% by mass to 5% by mass. A gas sensor characterized by that. The gas sensor according to claim 1 or 2,
A gas sensor comprising ceria or a ceria compound containing ceria as a main component on the outer surface side of the porous protective layer rather than the inner surface side of the porous protective layer. The gas sensor according to any one of claims 1 to 3,
Between the two cells, there is a measurement chamber for introducing the measurement gas, and a gas diffusion porous is disposed in the gas introduction path for introducing the measurement gas from the outside into the measurement chamber. The ceria or the ceria compound containing ceria as a main component in the porous protective layer is contained more in the porous protective layer provided on the gas diffusion porous than in other parts. Gas sensor. The gas sensor according to any one of claims 1 to 4,
The porous protective layer has an average pore diameter of 0.01 μm to 5.0 μm. The gas sensor according to any one of claims 1 to 5,
A gas sensor comprising a platinum noble metal further contained in the porous protective layer. The gas sensor according to any one of claims 1 to 6,
A protector having a gas introduction hole for introducing the gas to be measured, which is fixed to the distal end side of the housing and is provided so as to cover the distal end side of the gas sensor element, the gas introduction hole in the axial direction A gas sensor, wherein the gas sensor is provided at a position overlapping the porous protective layer. The gas sensor according to any one of claims 1 to 7,
Provided in a gap between the gas sensor element and the housing, and provided with a holding portion for holding the gas sensor element in the housing, the holding portion and the porous protective layer are spaced apart in an axial direction, A surface layer in which ceria or a ceria compound containing ceria as a main component is attached is provided on at least a part of the surface of the gas sensor element located in the gap between the holding portion and the porous protective layer. Gas sensor. The gas sensor according to claim 8, wherein
The gas sensor according to claim 1, wherein the surface layer is provided on the entire surface of the gas sensor element located in the gap.

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