JP5747801B2 - Multilayer ceramic exhaust gas sensor element, exhaust gas sensor using the same, and method of manufacturing multilayer ceramic exhaust gas sensor element - Google Patents

Description

  The present invention relates to a multilayer ceramic exhaust gas sensor element for measuring a specific gas component contained in exhaust gas from an internal combustion engine or the like, and more particularly, a highly reliable multilayer ceramic exhaust suitable for an air-fuel ratio sensor for a multi-cylinder internal combustion engine. The present invention relates to a gas sensor element, a manufacturing method thereof, and an exhaust gas sensor using the same.

  In general, an exhaust gas sensor for an internal combustion engine includes a sensor unit in which a pair of electrodes are formed with an oxygen ion conductive solid electrolyte layer interposed therebetween, and a porous diffusion resistance layer is formed to cover a detection electrode on the exhaust gas side, It is comprised by the laminated ceramic exhaust gas sensor element provided with the heater part which heats a sensor part to predetermined temperature. A multilayer ceramic exhaust gas sensor element is manufactured by stacking unfired ceramic sheets constituting each part and firing them integrally (for example, Patent Document 1).

  Such a multilayer ceramic exhaust gas sensor element can be used for air-fuel ratio control of an internal combustion engine, for example, as an air-fuel ratio sensor. In the sensor unit, exhaust gas is introduced into the detection electrode on the exhaust gas side via a porous diffusion resistance layer, and the reference oxygen concentration gas ( Air) is introduced, and the air-fuel ratio can be detected from the relationship between the sensor current value flowing through the sensor element and the air-fuel ratio when a predetermined voltage is applied between the electrodes.

  In order to improve the detection accuracy, Patent Document 1 adopts a configuration in which a dense layer is provided on the surface in the thickness direction of the porous layer, and the diffusion distance of exhaust gas reaching the detection electrode through the diffusion resistance layer is made constant. ing. In this case, the exhaust gas is introduced only from the side surface side, and intrusion from the thickness direction is blocked. Therefore, the porosity of the dense layer is less than 5%, and the porosity of the coarse layer serving as the diffusion resistance layer is 5 To 20%.

  In addition, in order to improve the water resistance and poisoning resistance of the multilayer ceramic exhaust gas sensor element, there is one in which a porous sensor protective layer is provided to cover the outer periphery of the element. For example, Patent Document 2 discloses a multilayer gas sensor element having an element body in which a ceramic substrate for a heater, a solid electrolyte body in which a detection electrode and a reference electrode are formed on the front and back surfaces, and a porous electrode protective layer are stacked. It has been proposed that a porous protective layer is provided so as to cover at least the outer peripheral corner of the element tip side, and the thickness from the corner is 20 μm or more.

  The porous protective layer is composed of a porous adhesive layer and a porous surface layer, and the exhaust gas to be measured passes through the porous protective layer and further passes through the porous electrode protective layer formed on the element surface. And reach the sensing electrode. The porous protective layer is formed, for example, so that the porosity is within a range of 15% to 65%, and water droplets adhering to the element are dispersed in the porous protective layer to crack the ceramic substrate for the heater. Is prevented from occurring. The electrode protective layer and the porous protective layer are formed using a sheet or paste obtained by adding carbon powder as a pore forming agent to ceramic raw material powder.

  In Patent Document 3, the entire surface of a gas sensor element in which a diffusion resistance layer and a shielding layer are stacked on one side of a sensor substrate having a pair of electrodes provided on a solid electrolyte body and a heater substrate is stacked on the other side. Is covered with a porous protective layer and the maximum thickness of the surface in the stacking direction and the thickness of the corners are defined. By optimizing the formation state of the porous protective layer, it can be sufficiently protected from cracks and cracks.

  On the other hand, regulations on exhaust emissions of internal combustion engines have been strengthened, and accordingly, an exhaust gas sensor capable of monitoring a difference between exhaust gas cylinders is required. In order to monitor the difference between the cylinders, an exhaust gas sensor may be provided for each cylinder. However, since this increases the cost, Patent Document 4 discloses that a single air-fuel ratio sensor is provided in an exhaust pipe assembly portion of a multi-cylinder engine. There is disclosed a method of performing air-fuel ratio control for each cylinder by calculating the individual air-fuel ratio of each cylinder based on the signal of the air-fuel ratio sensor in the arranged engine control device.

JP 2007-248351 A JP 2006-171013 A JP 2009-80110 A JP 2000-220489 A

  When air-fuel ratio control is performed by arranging an air-fuel ratio sensor in the exhaust pipe downstream of the exhaust manifold of an internal combustion engine, generally the output of the air-fuel ratio sensor is corrected to suppress fluctuations in the air-fuel ratio due to cylinder-to-cylinder variations. Based on the obtained value, feedback control of the air-fuel ratio is performed. However, uniform correction of the fuel injection amount and the air amount to each cylinder may result in a large variation among the cylinders and may deteriorate exhaust emission. Therefore, as in Patent Document 4, it is important to detect an imbalance (imbalance) of the air-fuel ratio for each cylinder.

  However, in the air-fuel ratio sensor using the gas sensor elements having the configurations of Patent Documents 2 and 3, by forming a sensor protective layer of about 10 μm, for example, the water resistance and poisoning resistance are improved. It was found that it was difficult to accurately detect. This is because in the multilayer ceramic exhaust gas sensor element in which the sensor protective layer is formed, the exhaust gas passes through both the porous sensor protective layer and the diffusion resistance layer and is introduced into the sensor unit. It is. In the gas sensor element of Patent Document 2, since the exhaust gas is introduced from the thickness direction and the side surface direction, the diffusion distance is not constant, and application to imbalance detection is difficult. Although the gas sensor element of Patent Document 3 is configured to block intrusion from the thickness direction, exhaust gas from each cylinder is mixed while reaching the electrode of the sensor unit, and sensor response is considered to be reduced. It is done.

  Therefore, the present invention relates to a multilayer ceramic exhaust gas sensor element provided with a sensor protective layer, and improves the sensor response without impairing the water resistance and poisoning resistance of the sensor protective layer, thereby providing a high performance and highly reliable multilayer. An object of the present invention is to provide a ceramic exhaust gas sensor element and a manufacturing method thereof, and to provide an exhaust gas sensor that can be suitably used as an air-fuel ratio sensor for a multi-cylinder internal combustion engine and detect a difference between cylinders.

According to the first aspect of the present invention, an exhaust gas side electrode into which exhaust gas is introduced is provided on one surface of an oxygen ion conductive solid electrolyte layer, and a reference gas side electrode into which a reference gas is introduced is provided on the other surface. A sensor unit, a porous gas diffusion resistance layer laminated on at least one surface side of the solid electrolyte layer so as to cover at least the exhaust gas side electrode, and laminated on the other surface side of the solid electrolyte layer for insulation An element body having a heater portion in which a heating element is disposed on a heater base layer made of a ceramic material;
The element body has a shielding layer that does not transmit exhaust gas on the surface opposite to the solid electrolyte layer of the gas diffusion resistance layer, covers at least the outer surface of the element body exposed to the exhaust gas, and is porous A multilayer ceramic exhaust gas sensor element provided with a sensor protective layer of
The gas diffusion resistance layer has a porous skeleton made of a ceramic sintered body, the porosity of the porous sensor protective layer is 45 to 60%, and the porosity of the gas diffusion resistance layer is 70 to 80% .

  In the invention of claim 2 of the present application, the gas diffusion resistance layer is formed by firing ceramic particles forming a skeleton of a ceramic sintered body and a non-fired layer containing pore-forming particles that are burned off during firing. The particles are resin beads.

  In the invention of claim 3, the pore-forming particles are nylon beads.

In the invention of claim 4 of the present application, the gas diffusion resistance layer is made of a material in which a base material of a ceramic sintered body forming a skeleton is sintered at the firing temperature of the element body.

In the invention of claim 5 of the present application, the solid electrolyte layer is formed of a zirconia solid electrolyte, and the gas diffusion resistance layer has a ceramic sintered body forming a skeleton having an average particle size of 0.2 to 0. Formed using 4 μm alumina.

The invention of claim 6 of the present application is an exhaust gas sensor using the multilayer ceramic exhaust gas sensor element, and is installed in a collective portion of an exhaust passage of a multi-cylinder internal combustion engine.

The invention of claim 7 is a method of manufacturing the laminated ceramic exhaust gas sensor element,
The gas diffusion resistance layer is laminated with ceramic particles forming a skeleton, and a non-fired layer containing resin beads as pore-forming particles that are burned off during firing, with other unfired ceramic sheets forming the element body. By integrally firing at ˜1500 ° C., a ceramic sintered body having a porous skeleton with a porosity of 70-80 % is obtained.

  The present inventors have intensively studied the sensor characteristics of a multilayer ceramic exhaust gas sensor element having a sensor protective layer, and the porosity of the gas diffusion resistance layer is larger than that of the sensor protective layer. It has been found that the gas permeability is such that the difference between cylinders can be monitored, and the invention of claim 1 of the present application has been achieved. That is, the formation of the sensor protective layer improves the water resistance and the poisoning resistance. However, when the porosity of the gas diffusion resistance layer is significantly lower than the porosity of the sensor protective layer, the difference between the cylinders of the exhaust gas cannot be detected. The reason is that the exhaust gas from each cylinder is mixed because both the sensor protective layer and the gas resistance diffusion layer are made of a porous body.

Therefore, in the present invention, a gas diffusion resistance layer that allows permeation of exhaust gas is combined with a sensor protective layer that protects the sensor element from being exposed to water and poisoning (Si, S, etc.), and gas discharged from each cylinder. To the sensor unit without mixing. At this time, since the penetration from the thickness direction is blocked by the shielding layer provided on the surface of the diffusion resistance layer, the exhaust gas can be introduced into the sensor portion at a predetermined diffusion rate with the diffusion distance being substantially constant. Thereby, the element body can be protected from being exposed to water and poisoning, and the sensor response can be improved. For example, a cylinder difference of a multi-cylinder engine can be detected. Furthermore, if the porosity of the porous sensor protective layer is 45 to 60%, the porous structure of the sensor protective layer is maintained, and both water resistance and poisoning resistance effects and gas permeability are compatible. it can. Moreover, since the porosity of the gas diffusion resistance layer is 70 to 80% and the porous body skeleton is composed of a ceramic sintered body , the detection performance is improved and the reliability is improved while suppressing a decrease in strength. be able to.

  As in the second aspect of the present invention, the gas diffusion resistance layer can be formed by firing an unfired layer obtained by adding pore-forming particles to ceramic particles forming a skeleton, and burning out the pore-forming particles. Preferably, when resin beads are used as the pore-forming particles of the gas diffusion resistance layer, the resin beads can be melted and decomposed in the degreasing step of the organic binder contained in the unfired layer. And, since the removal of the binder is promoted through the formed communication holes, it is possible to prevent problems due to residual cracked gas, etc., facilitating process management, and good sintering of the ceramic particles forming the skeleton Can proceed to. In general, resin beads are spherical particles, which facilitates the formation of a diffusion path in which pores communicate with each other and the adjustment of the porosity. Therefore, the strength of the ceramic sintered body having a large porosity can be ensured, and a high-quality gas diffusion resistance layer can be obtained.

  According to the invention of claim 3 of the present application, as the resin beads that are pore-forming particles, nylon beads having excellent solvent resistance are used, so that dissolution by the solvent contained in the sheet or paste that becomes the unfired layer is prevented, A stable porous body can be formed. Therefore, it is easy to handle, and a higher quality gas diffusion resistance layer can be formed.

According to the invention of claim 4 of the present application, the gas diffusion resistance layer can sinter the base material of the ceramic sintered body forming the skeleton at the firing temperature of the element body. As a result, while suppressing an increase in the firing temperature, the gas diffusion resistance layer is sintered to improve the strength, and the above effects can be easily obtained.

According to the invention of claim 5 of the present application, by using alumina having an average particle size of 0.2 to 0.4 μm for the skeleton of the gas diffusion resistance layer, the firing temperature of the solid electrolyte layer made of a zirconia solid electrolyte (usually 1450). The ceramic sintered body can be obtained at a temperature of about 0.degree. Therefore, integral sintering is possible at a relatively low temperature, and a high-performance sensor element can be realized at low cost.

As in the invention of claim 6 of the present application, the multilayer ceramic exhaust gas sensor element of the present invention is useful as an exhaust gas sensor such as an air-fuel ratio sensor and is excellent in sensor response. It is installed and the cylinder difference can be detected. Therefore, it is possible to accurately detect the air-fuel ratio for each cylinder and perform optimal control.

As in the invention of claim 7 of the present application, an element main body is obtained by laminating and firing unfired green sheets constituting each part. At this time, the gas diffusion resistance layer uses a paste or the like in which pore-forming particles are added to ceramic particles forming a skeleton, and fires the unfired layer disposed so as to cover the exhaust gas side electrode, thereby burning the pore-forming particles. Can be formed. Preferably, the use of resin beads facilitates the formation of pores that communicate with each other and serve as diffusion channels, and ensures strength by sintering at a relatively low temperature to provide a porous structure having a desired porosity. it can. By forming a porous sensor protective layer on the outer periphery of the element body, the multilayer ceramic exhaust gas sensor element of the present invention can be manufactured.

(A) is an expanded sectional view which shows the structure of the multilayer ceramic exhaust gas sensor element in the 1st Embodiment of this invention, and is AA sectional view taken on the line of FIG. 2, (b) is a multilayer ceramic exhaust gas sensor element. It is a disassembled perspective view which shows this laminated structure. It is a figure which shows the whole structure of the exhaust-gas sensor to which a multilayer ceramic exhaust-gas sensor element is applied. It is a figure which shows the whole schematic structure of the control apparatus of an internal combustion engine provided with the air fuel ratio sensor to which this invention is applied.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a multilayer ceramic exhaust gas sensor element 1 according to a first embodiment of the present invention, and FIG. 2 shows an overall configuration of an exhaust gas sensor S including the multilayer gas sensor element 1 of FIG. FIG. The exhaust gas sensor S is installed, for example, in an exhaust pipe of an automobile engine as an internal combustion engine, and detects a specific gas component concentration, for example, an oxygen concentration, an air-fuel ratio, a NOx concentration, etc. contained in the exhaust gas that is a measured gas. It can be used as a sensor. In particular, the control device for a multi-cylinder engine is suitable for an air-fuel ratio sensor (A / F sensor) used for air-fuel ratio control, and can detect the air-fuel ratio for each cylinder with high responsiveness. This will be described later.

  In FIG. 2, the exhaust gas sensor S has a cylindrical housing H1 attached to an exhaust pipe wall (not shown), and a multilayer ceramic exhaust gas sensor element 1 insulated and held in the housing H1. The multilayer ceramic exhaust gas sensor element 1 has a long and narrow plate shape, and a central portion is held in a cylindrical insulator H2 disposed in the housing H1, and a front end portion (lower end portion in the figure) of the multilayer ceramic exhaust gas sensor element 1 is a housing. It is housed in an element cover H3 fixed to the lower end of H1. An atmospheric cover H4 is fixed to the upper end of the housing H1, and a lead wire S1 connected to an external control circuit (not shown) extends into the atmospheric cover H4.

  The metal terminal provided at the extending end of the lead wire S1 holds the base end portion (upper end portion in the figure) of the multilayer ceramic exhaust gas sensor element 1 so as to sandwich it from both sides. Thereby, it conducts with terminal part S2 provided in the base end part of multilayer ceramic exhaust gas sensor element 1, and enables input and output of a sensor signal. A cylindrical insulator H5 is filled between the air cover H4 and the base end portion of the multilayer ceramic exhaust gas sensor element 1.

  The element cover H3 projecting into the exhaust pipe has an inner / outer double bottomed cylinder structure, and exhaust ports H61 and H71 are provided on the side wall and the bottom wall of the inner cylinder H6 and the outer cylinder H7, respectively. Thereby, the exhaust gas containing the specific gas flowing through the exhaust pipe can be taken into the element cover H3 where the tip of the multilayer ceramic exhaust gas sensor element 1 is located. On the other hand, an air inlet H8 is formed in the side wall at the upper end of the cylindrical member H4 exposed to the outside of the exhaust pipe, so that air can be introduced into the inside from the base end side of the multilayer ceramic exhaust gas sensor element 1. .

  FIG. 1A is an enlarged view (cross-sectional view taken along the line AA in FIG. 2) of the tip portion of the multilayer ceramic exhaust gas sensor element 1, and FIG. The structure is shown. As shown in the figure, the element body 1 ′ of the multilayer ceramic exhaust gas sensor element 1 includes an oxygen ion conductive solid electrolyte layer 11, a sensor part 2 having a pair of electrodes, a heater part 3 for heating the sensor part 2, A gas diffusion resistance layer 4 for controlling the amount of exhaust gas introduced into the sensor unit 2.

  The sensor unit 2 forms the detection electrode 12 that is an exhaust gas side electrode on the surface of one surface side (upper side in the figure) of the oxygen ion conductive solid electrolyte layer 11 and the other surface side (lower side in the figure). A reference electrode 13 that is a reference gas side electrode is formed on the surface to form a pair of opposed electrodes. The diffusion resistance layer 4 is laminated on the upper surface side of the sensor unit 2 into which exhaust gas is introduced, and the heater unit 3 is laminated on the lower surface side of the sensor unit 2. Further, the entire outer peripheral surface of the element body 1 ′ is covered with the sensor protective layer 5.

The oxygen ion conductive solid electrolyte layer 11 is a flat plate of a zirconia-based solid electrolyte mainly composed of partially stabilized zirconia, and a detection electrode 12 and a reference electrode 13 serving as a pair of electrodes are provided at opposite positions on both sides. Is formed. As the partially stabilized zirconia, for example, yttria partially stabilized zirconia obtained by adding a stabilizer such as yttria (Y ₂ O ₃ ) to zirconia (ZrO ₂ ) is preferably used. Further, by adding other ceramic components such as alumina, it is possible to improve the bondability with other layers to be laminated.

On the lower surface of the oxygen ion conductive solid electrolyte layer 11, a ceramic base 21 made of an insulating ceramic material mainly composed of alumina (Al ₂ O ₃ ) is laminated. The ceramic base 21 has a groove for forming a reference gas chamber 22 at the center of the upper surface, and the reference electrode 13 is disposed facing the reference gas chamber 22. The reference gas chamber 22 communicates with the space in the atmosphere cover H4 in FIG. 2, and the atmosphere as the reference gas is introduced from the outside.

  On the other hand, a chamber forming layer 14 is laminated on the upper surface of the oxygen ion conductive solid electrolyte layer 11. An opening serving as an exhaust gas chamber 42 is formed in the chamber forming layer 14, and the detection electrode 12 is disposed at a position facing the exhaust gas chamber 42 and facing the reference electrode 13. The chamber forming layer 14 is made of an insulating ceramic material mainly composed of alumina. Further, a gas diffusion resistance layer 4 made of a porous ceramic sintered body is formed on the upper surface side of the detection electrode 12. The gas diffusion resistance layer 4 is laminated on the chamber forming layer 14 so as to close the opening serving as the exhaust gas chamber 42, covers the upper side of the detection electrode 12, and the shielding layer 41 is laminated on the upper side.

  The heater unit 3 is configured by forming a heater 32 made of a heating resistor in a predetermined pattern on a heater base layer 31 made of an insulating ceramic material mainly composed of alumina. The heater unit 3 is laminated on the lower surface side of the ceramic base 21 that forms the reference gas chamber 22, whereby the heater 32 is embedded and held between the ceramic base 21 and the heater base layer 31. The heater 32 generates heat by energization from the outside, and heats the sensor unit 3 to a predetermined temperature and activates it.

  These oxygen ion conductive solid electrolyte layer 11, ceramic substrate 21, chamber forming layer 14 and heater substrate layer 31 can be formed into a sheet shape by a known method such as a doctor blade method or an extrusion method. Further, lead portions 12a, 13a, and 32a are connected to the detection electrode 12, the reference electrode 13, and the heater 32, respectively, and through the through holes 15 formed in the oxygen ion conductive solid electrolyte layer 11 and the heater base layer 31, respectively. It is connected to the terminal portion 16. These detection electrode 12, reference electrode 13, heater 32, connected lead portions 12a, 13a, 32a, and terminal portion 15 can be formed by screen printing or the like.

The gas diffusion resistance layer 4 has a porous skeletal structure including a large number of pores communicating with each other, and is configured so that exhaust gas, which is a gas to be measured, can permeate through diffusion. In the present invention, particularly, the gas diffusion resistance layer 4 is composed of a porous ceramic sintered body, and the required strength and gas permeability are both achieved by adjusting the porosity to be 70 to 80%. Let When the porosity of the gas diffusion resistance layer 4 is less than 70%, exhaust gas passes through the sensor protective layer 5 is introduced into the gas diffusion resistance layer 4 is mixed before reaching the detection electrode want precautions may cause There is a decrease in sensor responsiveness. If it exceeds 80%, it is difficult to sufficiently maintain the strength of the porous skeleton.

  The shielding layer 41 is a dense ceramic layer, and is laminated on the upper surface of the chamber forming layer 14 with the gas diffusion resistance layer 4 interposed therebetween to block the introduction of exhaust gas from the upper surface of the gas diffusion resistance layer 4. Thereby, the introduction of exhaust gas is limited only from the side surface of the gas diffusion resistance layer 4, the diffusion distance can be made substantially constant, and the amount of exhaust gas introduced can be adjusted appropriately. The shielding layer 41 is made of an insulating ceramic material mainly composed of alumina, and is formed into a sheet shape by a known method such as a doctor blade method.

  The gas diffusion resistance layer 4 is formed by firing an unfired layer containing aggregate particles that constitute a porous skeleton and pore-forming particles that are burned off by firing. Specifically, the unsintered layer is formed using a paste or sheet in which pore-forming particles are mixed with aggregate particles and an organic binder or solvent is added. In the case of using a paste, the unfired layer can be integrally formed by printing the paste at a predetermined position of the ceramic sheet to be the shielding layer 41, and handling and thickness adjustment are easy. By firing this, the pore-forming particles in the unfired layer are burned out, and a porous ceramic sintered body in which aggregate particles are bonded together can be obtained. Since the aggregate particles come into contact with the exhaust gas introduced into the gas diffusion resistance layer 4, ceramic particles having excellent heat resistance and chemical resistance such as alumina are used. As the pore-forming particles, resin beads such as nylon beads are preferably used, and the amount of the pore-forming particles added is adjusted so that a predetermined porosity is obtained.

  Preferably, the base material of the gas diffusion resistance layer 4 is made of a material that can be sintered at the firing temperature of the element body 1 ′. Here, the firing temperature of the element body 1 ′ is the firing temperature (for example, 1450 ° C.) of the oxygen ion conductive solid electrolyte layer 11 constituting the main part of the sensor unit 2. An alumina powder having an average particle size of 0.2 to 0.4 μm is particularly preferable for the aggregate particles constituting the base material, that is, the skeleton. Generally, alumina has a high sintering temperature (for example, 1600 ° C. or higher), and a sintering aid such as calcia (CaO) is added to sinter at a lower temperature. When the base material of the diffusion resistance layer 4 contains other components, the base material of the diffusion resistance layer 4 may be diffused to other layers at the time of firing. On the other hand, by using alumina powder having an average particle size of 0.2 to 0.4 μm, low-temperature sintering (for example, 1400 ° C. to 1500 ° C.) can be performed without using a sintering aid. A porous ceramic sintered body with high material strength can be obtained. When the average particle size is less than 0.2 μm, handling is difficult, and when it exceeds 0.4 μm, sintering at 1500 ° C. or less becomes difficult.

  In the present embodiment, the laminated gas sensor element 1 has a structure in which a zirconia sheet as the conductive solid electrolyte layer 11 and an alumina sheet constituting another layer are integrated by simultaneous firing, and the zirconia sheet and the alumina are combined. The sheet is designed to match the firing shrinkage behavior so that there is no peeling during simultaneous firing. For this reason, in order to make the gas diffusion resistance layer 4 to be a porous body having high strength, it is preferable that the element base material is a combination of a zirconia sheet and an alumina sheet, which are fired shrinkage behaviors. It is preferable to use the same material (alumina having the same particle diameter) as the used alumina sheet.

  The resin beads serving as the pore-forming particles are usually spherical particles having an average particle size larger than that of the aggregate particles, and are preferably formed into a true sphere. In particular, when nylon beads with excellent solvent resistance are used, the shape is maintained without dissolving in the solvent used at the time of preparing the paste, so that the porosity can be easily adjusted, and the pores that are burned down and communicate with each other Easy to form. If the average particle diameter of the resin beads is small, closed holes that do not contribute to the diffusion of exhaust gas are likely to be formed. If the average particle diameter is large, the strength of the gas diffusion resistance layer 4 may be reduced. According to the thickness of 4 etc., it sets suitably. In general, the thickness of the gas diffusion resistance layer 4 is 10 to 40 μm, preferably about 15 to 30 μm. On the other hand, the average particle diameter of the resin beads is 1/2 or less, preferably 1/3. For example, the average particle size may be set in the range of 1 to 20 μm.

  By using the pore-forming particles as resin beads, it becomes possible to melt and decompose at a temperature (for example, about 400 ° C.) equivalent to that of the resin binder added to the unfired layer. Therefore, the decomposition gas of the resin binder can be easily removed through the pores formed by burning out the resin beads. On the other hand, carbon particles, which are general pore-forming particles, have a higher temperature for melting and decomposition (for example, about 600 ° C.), the decomposition gas of the resin binder is not quickly discharged, the strength of the porous skeleton, May be affected. Moreover, since the carbon particles have high solvent absorbability, the amount of solvent added to the slurry increases when the green particles are formed into a shape. As a result, problems such as time-consuming solvent drying and an increase in the amount of attack by the solvent on the unsintered base sheet may occur.

  The element body 1 ′ is obtained by integrally firing an unfired laminated body in which unfired layers to be the gas diffusion resistance layers 4 are laminated together with unfired sheets of the respective layers constituting the sensor unit 2 and the heater unit 3. . At this time, the gas diffusion resistance layer 4 becomes a porous ceramic sintered body having a predetermined porosity. The firing temperature is preferably selected in the range of 1400 to 1500 ° C. For this reason, when the ceramic base 21, the chamber forming layer 14, and the heater base 31 constituting the sensor part 2 and the heater part 3 are made of alumina, like the gas diffusion resistance layer 4, an average particle size of 0.2 to By forming an unsintered sheet using 0.4 μm alumina powder, low-temperature sintering (for example, 1450 ° C.) is possible without using a sintering aid.

  The entire outer peripheral surface of the tip of the element body 1 ′ is covered with a porous sensor protective layer 5 at least in a region heated by the heater unit 3. For this reason, in the cross section of FIG. 1A, the element body 1 ′ has a notch surface in which all corners are notched in a tapered shape, and the thickness of the sensor protective layer 5 including the corners is a predetermined thickness. That's it. Further, the upper notch surface on which the gas diffusion resistance layer 4 is formed is formed wider than the lower notch surface on which the heater unit 3 is formed, and the gas diffusion resistance layer 4 serving as an exhaust gas introduction port. The side opening area is ensured, and the exhaust gas can be appropriately controlled to be introduced into the detection electrode.

The sensor protective layer 5 is composed of a mixture containing heat-resistant ceramic particles and an inorganic binder, and traps moisture and poisonous substances (Si, S, etc.) in the exhaust gas in pores formed between the particles, The element body 1 ′ is protected. The porosity of the sensor protection layer 5 is preferably set to a range of small have from 45 to 60% than the porosity of the gas diffusion resistance layer 4. If the porosity of the sensor protective layer 5 is less than 45%, sufficient gas permeability may not be obtained, and if it exceeds 60%, it is difficult to maintain the porous structure of the sensor protective layer and is resistant to moisture. This is not preferable because the effect of poisoning resistance is reduced and the strength is lowered.

  In the configuration of the present invention, gas diffusion from the upper surface of the gas diffusion resistance layer 4 is blocked by the shielding layer 41. For this reason, exhaust gas is introduced into the sensor protection layer 5 from the entire outer periphery, whereas exhaust gas is introduced into the gas diffusion resistance layer 4 only from the side opening without the shielding layer 41. . Therefore, even if the porosity of the gas diffusion resistance layer 4 is larger than that of the sensor protective layer 5, the rate limiting of the gas introduction amount greatly depends on the sensor protective layer 5 in the diffusion path of the exhaust gas introduced into the sensor unit 2. Absent. In the present invention, by appropriately setting the porosity of the gas diffusion resistance layer 4 and the sensor protective layer 5 as described above, it is possible to achieve both water resistance, poisoning resistance and sensor response.

The heat-resistant ceramic particles constituting the sensor protective layer 5 include hydrophilic heat-resistant particles such as alumina (Al ₂ O ₃ ), spinel (MgO · Al ₂ O ₃ ), silica (SiO ₂ ), titania (TiO ₂ ). ) Is used. Thereby, even if water droplets adhere to the multilayer ceramic exhaust gas sensor element 1, the sensor protective layer 5 can quickly diffuse into the pores formed between the hydrophilic heat-resistant particles. Therefore, moisture does not reach the sensor unit 2 and the heater unit 3, and water cracking is prevented.

  The heat-resistant ceramic particles constituting the sensor protective layer 5 include hydrophilic heat-resistant particles, hydrophobic heat-resistant particles such as silicon carbide (SiC), tantalum carbide (TaC), tungsten carbide (WC), and titanium carbide (TiC). It is also possible to mix metal carbides such as. Alternatively, the sensor protective layer 5 can have a multilayer structure of a layer of hydrophilic heat-resistant particles and a layer of hydrophobic heat-resistant particles.

  As for the thickness of the sensor protective layer 5, the maximum thickness on the upper and lower surface sides in the stacking direction of the element body 1 ′ is in the range of 30 to 2000 μm and the thickness in the corners is 10 to 50 μm in the cross section of FIG. Is good. By ensuring that the thickness of the sensor protective layer 5 is 10 μm or more in all the parts, it is possible to ensure water resistance and poisoning resistance. Moreover, in the flat element body 1 ′, by increasing the thickness in the stacking direction, the thickness of each part can be made appropriate and cracking against impact can be prevented.

  The sensor protective layer 5 is obtained after firing the unfired laminated body obtained by laminating the unfired sheets of the respective parts to be the sensor part 2, the heater part 3, the diffusion resistance layer 4, and the shielding layer 41 to obtain the element body 1 ′. The obtained element body 1 ′ is obtained by dipping in a mixture slurry containing heat-resistant ceramic particles and an inorganic binder, drying, and then heat-treating. At this time, it can form in desired thickness by adjusting the viscosity of a mixture slurry and repeating an immersion and a drying process. Further, before the sensor protective layer 5 is formed, the base layer 51 may be formed on the outer periphery of the element body 1 ′ and baked to improve the bondability with the element body 1 ′.

  The multilayer ceramic exhaust gas sensor element 1 thus obtained can be used as the exhaust gas sensor S in FIG. 2 to detect a specific component in the exhaust gas flowing through the exhaust pipe with high responsiveness. FIG. 3 is a configuration example of a control device for a multi-cylinder engine E (here, four cylinders) using the exhaust gas sensor S as an air-fuel ratio sensor. In the drawing, the exhaust gas sensor S is fixed to the wall of the exhaust pipe 6 near the exhaust manifold E1 of the multi-cylinder engine E, and the tip part projects into the exhaust pipe 6. A catalyst C is disposed downstream of the exhaust gas sensor S.

  An injector I is attached to each cylinder of the multi-cylinder engine E, and the fuel injection amount is controlled by the electronic control unit ECU. The output signal of the exhaust gas sensor S is input to the electronic control unit ECU, and feedback control is performed so that the air-fuel ratio detected by the exhaust gas sensor S becomes a desired value.

  Here, in the multi-cylinder engine E, the air-fuel ratio in each cylinder may vary due to variations in the injection amount of the injector I and the like. However, the conventional multilayer ceramic exhaust gas sensor element has a sensor protective layer especially on the outer periphery, so that exhaust gas from each cylinder is mixed before reaching the sensor section. For this reason, the detected air-fuel ratio is an average of the air-fuel ratio between the cylinders. When the fuel injection amount of each cylinder is determined based on this, the actual air-fuel ratio can be controlled to the target value. It becomes difficult.

On the other hand, in the multilayer ceramic exhaust gas sensor element 1 of the present invention, the porosity of the gas diffusion resistance layer 4 is 70 to 80%, which is sufficiently larger than the sensor protection layer 5, and the exhaust gas that has passed through the sensor protection layer 5 Since the detection electrode 12 is quickly reached at a predetermined diffusion rate, the sensor response is improved and the air-fuel ratio for each cylinder can be detected. Moreover, the gas diffusion resistance layer 4 is made of a ceramic sintered body, and can suppress the strength reduction and improve the reliability.

  In order to confirm the effect of the multilayer ceramic exhaust gas sensor element 1 of the present invention, imbalance detection when various test elements are produced by the following method and the porosity and firing conditions of the gas diffusion resistance layer 4 are changed. And the influence on the strength of the gas diffusion resistance layer 4 was examined.

1) Production of Multilayer Ceramic Exhaust Gas Sensor Element 1 First, in order to obtain a zirconia-based green sheet to be an oxygen ion conductive solid electrolyte layer 11, an yttria-added zirconia powder (yttria addition amount 6 mol%) is mixed with alcohol-based and An ester solvent was added and mixed and ground by a ball mill. Further, a binder such as polyvinyl butyral (7.2% by weight) and a plasticizer (4.8% by weight) were added and mixed, and a slurry adjusted to a predetermined viscosity was formed into a sheet by a doctor blade method. Thus, a zirconia green sheet was obtained.

  Next, in order to obtain an alumina-based green sheet that becomes the ceramic substrate 21, the heater substrate layer 31, the chamber forming layer 14, and the shielding layer 41, a raw material alumina powder (average particle size 0.2 to 0.4 μm) is mixed with an alcohol-based material. Then, an ester solvent was added and mixed and ground by a ball mill. Further, a binder such as polyvinyl butyral (12% by weight) and a plasticizer (8% by weight) are added and mixed, and a slurry adjusted to a predetermined viscosity is formed into a sheet shape by a doctor blade method. A sheet was obtained.

  On the zirconia-based green sheet to be the oxygen ion conductive solid electrolyte layer 11, the detection electrode 12, the reference electrode 13, and the electrode paste to be the lead portions 12 a and 13 a were printed at opposite positions on both sides. Further, the alumina green sheet for the ceramic substrate 21 is formed with a reference gas chamber 22 in a groove extending in the longitudinal direction on the upper surface, and the heater substrate layer 31 has a conductor 32 serving as a heater 32 and a lead portion 32a on the upper surface. A functional paste was printed.

A green sheet obtained by printing a slurry (paste) to be the diffusion resistance layer 4 on the green sheet to be the shielding layer 41 was laminated on these green sheets, and pressure-bonded to obtain an unfired laminate. The gas diffusion resistance layer 4 is composed of alumina particles (average particle size 0.2 to
0.3 μm), nylon beads (average particle size 5 μm) are added as pore-forming particles, and an ester solvent and a binder such as ethyl cellulose (26.0 wt% when the aggregate alumina addition amount is 100 wt%) An unsintered layer was formed using a paste-like material added with (thickness 20 μm). At this time, the blending ratio of the nylon beads to the alumina particles was changed within a range of 35 to 85%, so that sintered bodies having various porosities were obtained. The green laminate was degreased and then fired at a predetermined firing temperature (1450 ° C., 2 hours) to obtain an element body 1 ′ (Test Example 1-5).

  A sensor protective layer 5 was formed on the obtained element body 1 ′ by the following method. A slurry in which alumina particles (average particle size 22 μm) and an inorganic binder (5% by weight) are dispersed in a dispersion medium is formed, applied to the element body 1 ′ to a predetermined thickness (thickness 300 μm), and then heat-treated. (Processing temperature 900 ° C.) to form the sensor protective layer 5 that covers the outer peripheral surface of the tip of the element body 1 ′. At this time, two types of exhaust gas sensor elements 1 in which the porosity of the sensor protective layer 5 was 45% and 65% with respect to each element body 1 ′ of Test Example 1-5 were produced.

  For comparison, the case where the diffusion resistance layer 4 was an unsintered ceramic body was examined. When preparing the slurry (paste) to be the gas diffusion resistance layer 4, a green sheet to be the shielding layer 41 is obtained in the same manner except that alumina particles having an average particle diameter of 1.5 μm are used as aggregate particles to be used. An unfired layer was formed. This green sheet was laminated on the other green sheets to be the sensor unit 2 and the heater unit 3, and pressure-bonded to form an unfired laminate, which was fired at 1450 ° C. to obtain an element body 1 ′ (Test Example 6). 9). In the same manner, a sensor protective layer was formed on the obtained element body 1 ′, and two types of exhaust gas sensor elements 1 having a porosity of 45% and 65% were produced. When alumina particles having an average particle size of 1.5 μm are used, the sintering temperature is 1600 ° C. or higher, and sintering is not performed at this firing temperature.

2) Evaluation of monolithic ceramic exhaust gas sensor element 1 The monolithic ceramic exhaust gas sensor element 1 (Test Example 1-9) thus obtained was subjected to an imbalance detection evaluation test, and the porosity of the gas diffusion resistance layer 4 was evaluated. The effect was investigated. Specifically, each of the exhaust gas sensor elements 1 of Test Examples 1-9 shown in Table 1 is installed in an actual engine, and the sensor output is detected under the same conditions. evaluated.
The evaluation criteria for imbalance detectability were as follows.
A: Sensitivity at least 1.2 times the target imbalance value B: Sensitivity at least 1.1 times the target imbalance value Δ: Sensitivity at least 1.0 times the target imbalance value x: Those that do not meet the target imbalance value

Moreover, the test piece of the gas diffusion resistance layer 4 was subjected to an evaluation test of the gas diffusion resistance layer strength, and these results are shown in Table 1. Specifically, for each of Test Examples 1-9, the strength of the gas diffusion resistance layer 4 alone was evaluated by a four-point bending test. Moreover, after producing the exhaust gas sensor element 1 of Test Example 1-9 with the porosity changed, it was observed whether the pore structure was established.
The evaluation criteria for the gas diffusion resistance layer strength were as follows.
A: Strength lower than 25% with respect to the strength of alumina as a base material when the sintered density is 95% B: Less than 1/2 strength drop with respect to A B: Decrease strength with respect to A to 1/2 to 1/3 X: Collapse occurs in the pore structure by firing

As is apparent from Table 1, when the porosity of the gas diffusion resistance layer 4 is 35%, open pores that allow gas to pass through cannot be formed, and even 50% is not sufficient. Imbalance detection is possible at 60% or more, and is best at 70 to 80%. However, if the gas diffusion resistance layer 4 is green, the decrease in strength of 60% or more, collapse of 85% for pore shape, cracks occur in the gas diffusion resistance layer. For this reason, when the gas diffusion resistance layer 4 is made of a ceramic sintered body and the porosity is 60% or more, it is possible to achieve both imbalance detectability and strength, and it is particularly preferable that it is 70 to 80%. .

In addition, in the multilayer ceramic exhaust gas sensor element 1 of Test Example 1-9, the same result was obtained when the porosity of the sensor protective layer 5 was 45% or 60%. Therefore, when the porosity of the sensor protective layer 5 is in the range of 45% to 60%, there is no effect on imbalance detectability and strength, and the gas diffusion resistance layer 4 is ceramic-fired with a porosity of 70 to 80%. By using a combined body, a high-performance element capable of detecting a difference between cylinders can be obtained as an air-fuel ratio sensor.

  The multilayer ceramic exhaust gas sensor element of the present invention is not limited to the structure shown in FIG. 1, but can be changed as appropriate within the scope of the present invention. For example, a buffer layer for absorbing irregularities of the substrate (sheet) can be formed between the layers constituting the multilayer ceramic exhaust gas sensor element. Moreover, in order to improve the bondability of a sensor protective layer, a base layer can also be formed.

  The multilayer ceramic exhaust gas sensor element of the present invention can be used as an exhaust gas sensor for an internal combustion engine, for example, to detect oxygen concentration, air-fuel ratio, NOx concentration, etc. in exhaust gas. Of course, the present invention can be applied to an exhaust gas sensor for detecting a specific gas component in exhaust gas discharged in various fields, not limited to an internal combustion engine.

C Catalyst ECU Electronic controller E Multi-cylinder engine (internal combustion engine)
E1 Exhaust manifold H1 Tubular housing H2 Tubular insulator H3 Element cover H4 Atmosphere cover H5 Tubular insulator H6 Inner cylinder H7 Outer cylinder H61, H71 Exhaust port H8 Atmospheric port I Injector S Exhaust gas sensor S1 Lead wire S2 Terminal 1 ' Element body 1 Multilayer ceramic exhaust gas sensor element 11 Zirconia-based solid electrolyte layer 12 Detection electrode (exhaust gas side electrode)
13 Reference electrode (reference gas side electrode)
14 Chamber forming layer 2 Sensor portion 21 Ceramic base body 22 Reference gas chamber 3 Heater portion 31 Heater base layer 32 Heater electrode 4 Gas diffusion resistance layer 41 Shielding layer 42 Exhaust gas chamber 5 Sensor protective layer 6 Exhaust pipe

Claims (7)

A sensor unit having an exhaust gas side electrode into which exhaust gas is introduced on one surface of the oxygen ion conductive solid electrolyte layer, and a reference gas side electrode into which reference gas is introduced on the other surface, and the solid electrolyte layer A porous gas diffusion resistance layer laminated so as to cover at least the exhaust gas side electrode on one surface side, and a heater base layer made of an insulating ceramic material, laminated on the other surface side of the solid electrolyte layer. An element body having a heater portion provided with a heating element;
A multilayer ceramic exhaust gas sensor element that covers at least the outer surface of the element body exposed to the exhaust gas and is provided with a porous sensor protective layer,
The element body has a shielding layer that does not transmit exhaust gas on the surface of the gas diffusion resistance layer opposite to the solid electrolyte layer, and the gas diffusion resistance layer has a porous skeleton made of a ceramic sintered body. And a porosity of the porous sensor protective layer is 45 to 60%, and a porosity of the gas diffusion resistance layer is 70 to 80% .   The gas diffusion resistance layer is formed by firing ceramic particles forming a skeleton of a ceramic sintered body and an unfired layer containing pore-forming particles that are burned off during firing, and the pore-forming particles are resin beads. 2. The multilayer ceramic exhaust gas sensor element according to 1. Upper Symbol pore forming particles laminated ceramic exhaust gas sensor element according to claim 2 wherein the nylon beads. The laminate according to any one of claims 1 to 3, wherein the gas diffusion resistance layer is formed of a material in which a base material of a ceramic sintered body forming a skeleton is sintered at a firing temperature of the element body. Ceramic exhaust gas sensor element. The solid electrolyte layer is formed of a zirconia-based solid electrolyte, and the gas diffusion resistance layer is formed of a ceramic sintered body forming a skeleton using alumina having an average particle diameter of 0.2 to 0.4 μm. laminated ceramic exhaust gas sensor device according to any one of claims 1 to 4 is. An exhaust gas sensor using the multilayer ceramic exhaust gas sensor element according to any one of claims 1 to 5, wherein the exhaust gas sensor is installed in a collecting portion of an exhaust passage of a multi-cylinder internal combustion engine. A method of manufacturing the multilayer ceramic exhaust gas sensor element according to claim 1 ,
The gas diffusion resistance layer is laminated with ceramic particles forming a skeleton, and a non-fired layer containing resin beads as pore-forming particles that are burned off during firing, with other unfired ceramic sheets forming the element body. A method for producing a multilayer ceramic exhaust gas sensor element, characterized by forming a ceramic sintered body having a porous skeleton with a porosity of 70 to 80% by integrally firing at ~ 1500C.

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