JP2012173146A - Gas sensor element and gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor element capable of preventing a crack caused by freezing of water permeated into a porosity protective layer when forming the porous protective layer into a plurality of layers, and to provide a gas sensor using the gas sensor element.SOLUTION: A gas sensor element 100 comprises: a laminate formed by laminating a detection element part 300 having solid electrolytes 105 and 109 and a pair of electrodes 104, 106 and a pair of electrodes 108, 110 disposed at the solid electrolytes respectively, and a heater part 200, and having an approximately rectangular cross section in a direction intersecting perpendicularly with a longitudinal direction; and a porous protective layer 20 formed by covering the entire circumference of a tip part exposed to a measuring object gas in the laminate. The porous protective layer includes an inner side porous layer 21 provided on the side of the detection element, and an outer side porous layer 23 formed on the inner side porous layer. The porosity of the inner side porous layer is higher than that of the outer porous layer, and also a difference of porosities of four portions in the inner side porous layer positioned on two short sides and two long sides of the laminate in a cross section is 10% or less.

Description

  The present invention relates to a gas sensor element and a gas sensor that are suitably used for detecting the gas concentration of a specific gas contained in combustion gas or exhaust gas of, for example, a combustor or an internal combustion engine.

Conventionally, a gas sensor for detecting the concentration of a specific component (oxygen or the like) in exhaust gas of an internal combustion engine has been used. This gas sensor has a gas sensor element inside itself, and the gas sensor element includes a detection element portion composed of a solid electrolyte body and a pair of electrodes arranged on the solid electrolyte body, and a heater that generates heat by energizing the insulating ceramic body. It has a laminated body formed by laminating heater sections. Here, the gas sensor element may be exposed to poisonous substances such as silicon and phosphorus contained in the exhaust gas, or water droplets in the exhaust gas may adhere to the gas sensor element. A porous protective layer is coated to prevent trapping or direct contact of water droplets with the gas sensor element. That is, the entire periphery of the tip exposed to the measurement target gas (exhaust gas) in the above laminate is covered with the porous protective layer.
In addition, a technology has been developed in which the porous protective layer is made into two layers, and the porosity of the lower layer is made larger than the porosity of the upper layer, whereby the upper layer is adhered to the lower layer roughened by the pores by the anchor effect. (See Patent Documents 1 and 2).

JP 2003-322632 A (Claim 15) JP 2007-206082 A (Claim 15)

Thus, when the porous protective layer is made into two layers and the porosity of the lower layer is made larger than the porosity of the upper layer, the total volume of voids (spaces) contained in the lower layer is increased and heat insulation is imparted. Even if the upper layer side is submerged and cooled, the inner gas sensor element is hardly cooled rapidly, and the gas sensor element can be effectively prevented from being damaged by the submersion even when the detection element unit is heated by the heater unit. .
However, as described above, the phenomenon of water exposure of the gas sensor element includes not only water droplets (for example, condensed water condensed in the exhaust pipe) scattered and adhering to the gas sensor element, but also water is immersed in the porous protective layer. May freeze. For example, when an internal combustion engine is left in a cold district or the like, condensation may occur in the gas sensor element mounted on the exhaust pipe, and water may soak and freeze in the porous protective layer. In this case, if the porosity of the lower layer of the porous protective layer is larger than that of the upper layer, water tends to easily penetrate into the lower layer (inner porous layer). At this time, in the inner porous layer covering the circumferential direction of the laminate, if there is a portion where the porosity is locally excessive, the excessive portion is frozen in a state where a large amount of water has entered, and then the gas sensor element It has been found by the present inventors that when the heater is heated, the moisture in that portion may locally expand and cause cracking of the porous protective layer or the element. That is, if the porosity varies greatly in each part of the inner porous layer, water resistance may be reduced due to freezing of the gas sensor element.
Accordingly, the present invention provides a gas sensor element and a gas sensor that prevent cracking of the porous protective layer and the gas sensor element caused by freezing of water immersed in the porous protective layer, which occurs when the porous protective layer is formed in a plurality of layers. The purpose is to provide.

In order to solve the above problems, a gas sensor element of the present invention includes a detection element unit having a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body, and a heater that generates heat by energizing the insulating ceramic body. A laminated body in which the heater section is laminated and the transverse section extending in the longitudinal direction and orthogonal to the longitudinal direction forms a substantially rectangular shape, and all of the tip parts of the laminated body that are exposed to the measurement target gas. A porous protective layer covering a circumference, wherein the porous protective layer includes an inner porous layer provided on the detection element side, and an outer formed on the inner porous layer. A porosity of the inner porous layer is higher than a porosity of the outer porous layer, and further, on the cross section, located on two short sides and two long sides of the laminate. In the inner porous layer The difference in porosity of 4 sites is 10% or less.
Thus, when the porosity of the inner porous layer is higher than the porosity of the outer porous layer, the total volume of voids in the inner porous layer on the laminate side becomes larger than that of the outer porous layer, thereby increasing the heat insulation. Even if the outer porous layer side is covered with water and cooled, the gas sensor element is hardly cooled rapidly. On the other hand, by reducing the porosity of the outer porous layer relative to the inner protective layer, poisonous substances and water droplets are effectively captured by the outer porous layer with a reduced porosity. Hard to reach.
Further, in the gas sensor element of the present invention, the relationship between the porosity of the inner porous layer having a large porosity with respect to the outer porous layer is such that the two short sides and two long sides of the laminate having a substantially rectangular cross section. When the porosities of the four regions on the side are compared, the difference between the four porosities is set to satisfy within 10%. Thus, the difference between the porosity of the four parts of the inner porous layer located on the two short sides and the two long sides, which can be said to be the four main parts of the inner porous layer (the maximum value of the porosity of the four parts) By setting the difference from the minimum value to 10% or less, even if the inner porous layer absorbs water, it prevents or prevents the local infiltration of water and freezing at that site. Since the thermal stress generated when the moisture expands when the heater is heated is uniformly approached to the entire inner protective layer, the porous protective layer and the gas sensor element are hardly broken.

  Here, the laminated body having a substantially rectangular cross section is a rectangle in which edges are formed at four corners in the cross section, and a configuration in which slight C chamfering or R chamfering is performed at the four corners. Is also included. Further, when determining the porosity of the four portions of the inner porous layer located on the two short sides and the two long sides, for example, at any position where the porous protective layer is formed (gas sensor element in the longitudinal direction) Take the cross section of the gas sensor element at the center of the porous protective layer when viewed, and in the cross section, representatively the part of the inner porous layer located on the central part of each side of the laminate After taking the SEM image, the porosity of the four regions may be obtained through image analysis. In the present invention, the inner porous layer constituting the porous protective layer refers to a layer provided on the surface of the laminate, and the outer porous layer refers to a layer provided on the inner porous layer. . Therefore, the porous protective layer may be formed of three or more layers in addition to the form formed of two layers of the inner porous layer and the outer porous layer.

  The gas sensor of the present invention is a gas sensor comprising a sensor element that detects the concentration of a specific gas component in a gas to be measured and a housing that holds the sensor element, wherein the sensor element uses the gas sensor element. To do.

  According to the present invention, it is possible to prevent cracking of the porous protective layer and the gas sensor element caused by freezing of water immersed in the porous protective layer, which is generated when the porous protective layer is formed into a plurality of layers. It is possible to obtain a gas sensor element and a gas sensor which improve both water resistance and achieve both protection from poisonous substances.

It is sectional drawing which follows the longitudinal direction of the gas sensor (oxygen sensor) which concerns on embodiment of this invention. It is a model exploded perspective view of a detection element and a heater. FIG. 2 is a partial enlarged cross-sectional view of a front end side of the detection element in FIG. 1. It is a schematic cross section orthogonal to the axial direction of a gas sensor element. It is a figure which shows the actual cross-section photograph of the porous protective layer of an Example. It is a figure which shows the actual cross-section photograph corresponding to the area | regions ad of FIG. It is process drawing which shows typically an example of the manufacturing method of a porous protective layer. In an Example and a comparative example, it is a figure which shows the cumulative particle size distribution of the particle | grains (alumina powder) in the slurry used as an inner porous layer.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is a cross-sectional view taken along the longitudinal direction (axis L direction) of a gas sensor (oxygen sensor) 1 according to an embodiment of the present invention, FIG. 2 is a schematic exploded perspective view of a detection element 300 and a heater 200, and FIG. It is sectional drawing orthogonal to the axis line L direction.

  As shown in FIG. 1, the gas sensor 1 includes a gas sensor element 100 including a detection element unit 300 and a heater unit 200 stacked on the detection element unit 300, a metal shell that holds the gas sensor element 100, etc. (Corresponding to a “housing” in the range) 30, a protector 24 attached to the tip of the metal shell 30, and the like. The gas sensor element 100 is arranged so as to extend in the direction of the axis L.

  As shown in FIG. 2, the heater unit 200 includes a first base 101 and a second base 103 mainly composed of alumina, and a heating element 102 mainly composed of platinum sandwiched between the first base 101 and the second base 103. have. The heating element 102 has a heating part 102a located on the tip side and a pair of heater lead parts 102b extending from the heating part 102a along the longitudinal direction of the first base 101. The terminal of the heater lead portion 102b is electrically connected to the heater side pad 120 via a conductor formed in the heater side through hole 101a provided in the first base 101. A laminated body of the first base 101 and the second base 102 corresponds to the insulating ceramic body.

  The detection element unit 300 includes an oxygen concentration detection cell 130 and an oxygen pump cell 140. The oxygen concentration detection cell 130 is formed of a first solid electrolyte body 105 and a first electrode 104 and a second electrode 106 formed on both surfaces of the first solid electrolyte 105. The first electrode 104 is formed of a first electrode portion 104 a and a first lead portion 104 b extending from the first electrode portion 104 a along the longitudinal direction of the first solid electrolyte body 105. The second electrode 106 is formed of a second electrode portion 106 a and a second lead portion 106 b extending from the second electrode portion 106 a along the longitudinal direction of the first solid electrolyte body 105.

  The terminals of the first lead portion 104b are first through holes 105a provided in the first solid electrolyte body 105, second through holes 107a provided in an insulating layer 107 described later, and second terminals provided in the second solid electrolyte body 109. The detection element side pad 121 is electrically connected through a conductor formed in each of the fourth through hole 109a and the sixth through hole 111a provided in the protective layer 111. On the other hand, the terminal of the second lead portion 106b is a third through hole 107b provided in the insulating layer 107 described later, a fifth through hole 109b provided in the second solid electrolyte body 109, and a seventh through hole provided in the protective layer 111. It electrically connects with the detection element side pad 121 through the conductor formed in each of 111b.

  On the other hand, the oxygen pump cell 140 is formed of the second solid electrolyte body 109 and the third electrode 108 and the fourth electrode 110 formed on both surfaces of the second solid electrolyte body 109. The third electrode 108 is formed of a third electrode portion 108 a and a third lead portion 108 b extending from the third electrode portion 108 a along the longitudinal direction of the second solid electrolyte body 109. The fourth electrode 110 is formed of a fourth electrode portion 110 a and a fourth lead portion 110 b extending from the fourth electrode portion 110 a along the longitudinal direction of the second solid electrolyte body 109.

  And the terminal of the 3rd lead part 108b is the detection element side via the conductor formed in each of the 5th through hole 109b provided in the 2nd solid electrolyte body 109, and the 7th through hole 111b provided in protective layer 111. It is electrically connected to the pad 121. On the other hand, the terminal of the fourth lead portion 110b is electrically connected to the detection element side pad 121 via a conductor formed in an eighth through hole 111c provided in the protective layer 111 described later. The second lead portion 106b and the third lead portion 108b are at the same potential.

The first solid electrolyte body 105 and the second solid electrolyte body 109 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 102, the first electrode 104, the second electrode 106, the third electrode 108, the fourth electrode 110, the heater side pad 120, and the detection element side pad 121 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 102, the first electrode 104, the second electrode 106, the third electrode 108, the fourth electrode 110, the heater side pad 120, and the detection element side pad 121 are mainly composed of Pt in consideration of heat resistance and oxidation resistance. It is even more preferable to form it. Furthermore, the heating element 102, the first electrode 104, the second electrode 106, the third electrode 108, the fourth electrode 110, the heater side pad 120, and the detection element side pad 121 include ceramic components in addition to the main platinum group element. It is preferable to contain. This ceramic component is preferably the same component as the main material on the side to be laminated (for example, the main component of the first solid electrolyte body 105 and the second solid electrolyte body 109) from the viewpoint of fixation. .

  An insulating layer 107 is formed between the oxygen pump cell 140 and the oxygen concentration detection cell 130. The insulating layer 107 includes an insulating portion 114 and a diffusion rate controlling portion 115. In the insulating portion 114 of the insulating layer 107, a hollow gas detection chamber 107c is formed at a position corresponding to the second electrode portion 106a and the third electrode portion 108a. The gas detection chamber 107c communicates with the outside in the width direction of the insulating layer 107, and the communication portion has a diffusion rate-determining method that realizes gas diffusion between the outside and the gas detection chamber 107c under a predetermined rate-limiting condition. Part 115 is arranged.

  The insulating part 114 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 diffusion control part 115 is a porous body made of alumina. The diffusion rate-determining unit 115 performs rate-limiting when the detection gas flows into the gas detection chamber 107c.

  A protective layer 111 is formed on the surface of the second solid electrolyte body 109 so as to sandwich the fourth electrode 110. The protective layer 111 sandwiches the fourth electrode portion 110a, sandwiches the porous electrode protection portion 113a for protecting the fourth electrode portion 110a from poisoning, and the fourth lead portion 110b. It comprises a reinforcing part 112 for protecting the second solid electrolyte body 109. In the gas sensor element 100 of the present embodiment, the current flowing between the electrodes of the oxygen pump cell 140 so that the voltage (electromotive force) generated between the electrodes of the oxygen concentration detection cell 130 becomes a predetermined value (for example, 450 mV). This corresponds to an oxygen sensor element that linearly detects the oxygen concentration in the gas to be measured corresponding to the current flowing through the oxygen pump cell 140.

  Returning to FIG. 1, the metal shell 30 is made of SUS430, and has a male screw portion 31 for attaching the gas sensor to the exhaust pipe, and a hexagonal portion 32 to which an attachment tool is applied at the time of attachment. Further, the metal shell 30 is provided with a metal side step portion 33 protruding radially inward, and this metal side step portion 33 supports a metal holder 34 for holding the gas sensor element 100. . Inside the metal holder 34, a ceramic holder 35 and a talc 36 are arranged in this order from the tip side. The talc 36 includes a first talc 37 disposed in the metal holder 34 and a second talc 38 disposed over the rear end of the metal holder 34. The gas sensor element 100 is fixed to the metal holder 34 by compressing and filling the first talc 37 in the metal holder 34. Further, the second talc 38 is compressed and filled in the metal shell 30, so that a sealing property between the outer surface of the gas sensor element 100 and the inner surface of the metal shell 30 is ensured. An alumina sleeve 39 is disposed on the rear end side of the second talc 38. The sleeve 39 is formed in a multi-stage cylindrical shape, is provided with a shaft hole 39a along the axis, and the gas sensor element 100 is inserted through the shaft hole 39a. The caulking portion 30 a on the rear end side of the metal shell 30 is bent inward, and the sleeve 39 is pressed to the front end side of the metal shell 30 through the stainless steel ring member 40.

  Further, a metal protector 24 having a plurality of gas intake holes 24a is attached to the outer periphery on the front end side of the metallic shell 30 by covering the distal end portion of the gas sensor element 100 protruding from the distal end of the metallic shell 30 by welding. . This protector 24 has a double structure, a cylindrical outer protector 41 having a uniform outer diameter on the outer side, and an outer diameter of the rear end part 42a on the inner side from the outer diameter of the front end part 42b. An inner protector 42 having a bottomed cylindrical shape that is formed to be larger is also arranged.

  On the other hand, on the rear end side of the metal shell 30, the front end side of the outer tube 25 made of SUS430 is inserted. The outer cylinder 25 has a distal end portion 25a whose diameter is enlarged on the distal end side fixed to the metal shell 30 by laser welding or the like. A separator 50 is disposed inside the rear end side of the outer cylinder 25, and a holding member 51 is interposed in a gap between the separator 50 and the outer cylinder 25. The holding member 51 is fixed by the outer cylinder 25 and the separator 50 by engaging a protrusion 50 a of the separator 50 described later and caulking the outer cylinder 25.

  The separator 50 is provided with through holes 50b for inserting the lead wires 11 to 15 for the detection element unit 300 and the heater unit 200 from the front end side to the rear end side (note that the lead wires 14, 15 is not shown). The connection holes 16 that connect the lead wires 11 to 15 to the detection element side pads 121 of the detection element unit 300 and the heater side pads 120 of the heater unit 200 are accommodated in the through holes 50b. Each lead wire 11-15 is connected to a connector (not shown) outside. Electric signals are input and output between the external devices such as the ECU and the lead wires 11 to 15 through this connector. Moreover, although not shown in detail in each lead wire 11-15, it has the structure which showed the conducting wire with the insulating film which consists of resin.

  Further, a substantially cylindrical rubber cap 52 for closing the opening 25 b on the rear end side of the outer cylinder 25 is disposed on the rear end side of the separator 50. The rubber cap 52 is fixed to the outer cylinder 25 by caulking the outer periphery of the outer cylinder 25 toward the radially inner side in a state where the rubber cap 52 is mounted in the rear end of the outer cylinder 25. The rubber cap 52 is also provided with through holes 52a for inserting the lead wires 11 to 15 from the front end side to the rear end side.

Next, the porous protective layer 20 that is a characteristic part of the present invention will be described. As shown in FIG. 1, the porous protective layer 20 is provided so as to cover the entire circumference on the distal end side of the gas sensor element 100.
FIG. 3 is a partial enlarged cross-sectional view of the gas sensor element 100 of FIG. 1 on the front end side, and the porous protective layer 20 is an inner porous layer provided immediately above the surface of the laminate of the detection element unit 300 and the heater unit 200. 21 and an outer porous layer 23 formed so as to cover the outer surface of the inner porous layer 21.

  The porous protective layer 20 includes the front end surface of the gas sensor element 100, is formed to extend to the rear end side along the direction of the axis L, and 4 on the front and back surfaces and both side surfaces of the gas sensor element 100 (laminated body). The surface is completely enclosed. Further, as viewed in the direction of the axis L, the porous protective layer 20 extends from the region where at least the first electrode portion 104a, the second electrode portion 106a, the third electrode portion 108a, and the fourth electrode portion 110a of the detection element 300 overlap to the rear end. It is good to be formed.

FIG. 4 is a schematic cross-sectional view orthogonal to the axis L direction of the gas sensor element 100 including the porous protective layer 20.
As described above, the porous protective layer 20 includes the inner porous layer 21 and the outer porous layer 23 formed so as to cover the inner porous layer 21, and the porosity of the inner porous layer 21 is high. It is higher than the porosity of the outer porous layer 23. The pores formed in the porous protective layer 20 have a three-dimensional network structure so as to allow gas permeation. Furthermore, in the present embodiment, the relationship between the porosity of the inner porous layer 21 having a large porosity with respect to the outer porous layer 23 is such that the two short sides and two of the laminated body having a substantially rectangular cross section are formed. When the porosity of the four regions on the long side is compared, the difference between the maximum value and the minimum value of the four porosity is set to satisfy within 10%. This point will be described later.
As described above, when the porosity of the inner porous layer 21 is higher than the porosity of the outer porous layer 23, the total volume of voids in the inner porous layer 21 on the laminate side is increased, and the heat insulating property is increased, thereby increasing the outer porous property. Even if the layer 23 side is covered with water and cooled, the gas sensor element 100 is hardly cooled rapidly. On the other hand, since the porosity of the outer porous layer 23 can be made smaller than that of the inner porous layer, poisonous substances and moisture are captured by the outer porous layer 23 having a reduced porosity, and thus reach the gas sensor element 100. It is hard to do.

In addition, as the moisture of the gas sensor element 100, not only water droplets scatter and adhere to the gas sensor element 100 but also the inside of the porous protective layer 20 when the gas sensor element 100 mounted on a vehicle or the like is left in a cold region. Water may soak and freeze. In such a case, the inner porous layer 21 has a higher porosity than the outer porous layer 23, so that water tends to easily penetrate. At this time, in the inner porous layer 21, if a portion having a locally excessive porosity is present, the portion is frozen in a state where a large amount of water has entered the portion, and then the driving of the gas sensor element 100 is started to start the gas sensor element. When the heater 100 is heated with heater, the moisture in the portion may locally expand and cause the porous protective layer 20 or the gas sensor element 100 to crack.
Accordingly, when the porosity of the inner porous layer 21 is compared with the porosity of four portions on the two short sides and two long sides of the laminate having a substantially rectangular cross section, the four pores are compared. By making the difference in rate within 10% (more preferably within 5%), even if the inner porous layer 21 absorbs water, it is possible to suppress the occurrence of a portion where water is locally infiltrated. In addition, since the thermal stress generated when the moisture expands when the heater is heated becomes substantially uniform throughout the inner porous layer 23, the porous protective layer 20 and the gas sensor element 100 are difficult to break.

The inner porous layer 21 can be formed by, for example, bonding one or more ceramic particles selected from the group of alumina, spinel, zirconia, mullite, zircon, and cordierite by firing or the like. By sintering the slurry containing these particles, pores can be formed in the skeleton of the coating. However, when the slurry containing the above particles is added with a burnable pore former, the pore former is sintered. Since the burned-out portion becomes pores, the inner porous layer 21 can have a high porosity as described below, which is preferable. As the pore former, for example, carbon, resin beads, organic or inorganic binder particles can be used.
Moreover, it is preferable that the porosity of the inner porous layer 21 determined by image analysis described later is 35 to 70%, because the inner porous layer 21 can have good heat insulation. When the porosity of the inner porous layer 21 is less than 35%, the total volume of the voids is reduced, the effect as a heat insulating layer is lowered, and it may be difficult to produce a film exceeding 70%.
The inner porous layer 21 preferably has a thickness of 100 to 800 μm.

The outer porous layer 23 can be formed by bonding, for example, one or more ceramic particles selected from the group of alumina, spinel, zirconia, mullite, zircon, and cordierite by firing or the like. By sintering the slurry containing these particles, pores are formed in the skeleton of the coating when the gaps between the ceramic particles and the organic or inorganic binder in the slurry are burned away.
In particular, when the outer porous layer 23 is formed from coarse particles and fine particles having a smaller particle diameter than the coarse particles, the fine particles move to the inner porous layer 21 side, as will be described later, and adherence to the inner porous layer 21. Can be further improved.
Further, it is preferable that the porosity of the outer porous layer 23 obtained by image analysis described later is 10 to 50% because gas permeability is not lowered while ensuring barrier properties of poisonous substances and water droplets. If the porosity of the outer porous layer 23 is less than 10%, it is likely to be clogged with poisonous substances, and if it exceeds 50%, water may enter the outer porous layer 23 and the water resistance may decrease. .
The thickness of the outer porous layer 23 is preferably 100 to 800 μm.

Here, when the inner porous layer 21 is formed from the above-described slurry, if the variation in the particle size distribution of the ceramic particles contained in the slurry increases, the porosity of the inner porous layer 21 also varies among the parts. growing. Further, if the stirring time for preparing the slurry is not managed for a predetermined time or more, the degree of dispersion of the particles in the slurry changes, so that the variation in porosity between the respective portions of the inner porous layer 21 also increases.
For this reason, it is preferable that D90 / D10 (90% cumulative frequency distribution particle size / 10% cumulative frequency distribution particle size) is 6 or less as variation in the particle size distribution of the ceramic particles contained in the slurry.
The stirring time for preparing the slurry varies depending on the composition and viscosity of the slurry, but is preferably set to 3 hours or more (preferably 5 hours or more).

The porosity of the inner porous layer 21 and the outer porous layer 23 is determined as follows.
First, based on a cross-sectional photograph (SEM image, see FIG. 5) of the porous protective layer 20 of Example 1 described later, binarization in the thickness direction of the porous protective layer 20 is performed using commercially available image analysis software, The ratio of the black part of the cross-sectional photograph is obtained. In the cross-sectional photograph of the porous protective layer 20, first, the gas sensor element 100 is taken in a cross-section (transverse cross-section) in a direction orthogonal to the longitudinal direction, and in the cross-section, Among them, a cross-sectional photograph (SEM image) was taken representatively of the part located on the central part of each side of the laminate, and the porosity of the four parts was determined through image analysis. The black portions (corresponding to arrows C _A and C _B in FIG. 5) of the cross-sectional photograph correspond to the pores, and the white portions correspond to the skeleton of the film. Therefore, the more black portions, the larger the porosity.

Then, the porosity of the outer porous layer 23 was obtained by averaging the porosity obtained by performing image analysis at the plurality of positions (four sites) of the outer porous layer 23. Similarly, the porosity of the inner porous layer 21 was obtained by averaging the porosity obtained by image analysis at the plurality of positions (four sites) of the inner porous layer 21.
In the example of FIG. 5, since the porous protective layer 20 is manufactured in detail as shown in FIG. 7 described later, an intermediate region 22 is formed on the inner porous layer 21 side of the outer porous layer 23. There is no problem.

Further, in order to measure the variation (difference) of the porosity of the inner porous layer 21, in the present embodiment, as shown in FIG. Cross-sectional SEM images were taken at positions a to d and image analysis was performed to determine the variation in porosity at each position a to d.
In the example of FIG. 4, in the gas sensor element 100 having a rectangular cross section (transverse cross section), a region a including a portion of the inner porous layer 21 positioned on the central portion of each side (four surfaces) of the laminate. The four cross-sectional SEM images of .about.d are taken and image analysis is performed, and the difference in the porosity of the areas a to d (that is, the difference between the areas) is obtained. If the particle size distribution of the slurry in the slurry and the stirring time of the slurry are controlled, the porosity of the coating obtained by baking after applying the slurry is almost constant at each part, If a cross-sectional SEM image of a representative position of the porous layer 21 (for example, the central part of each of the four surfaces of the detection element 10) is taken, the difference in the entire porosity of the inner porous layer 21 is 10% or less. This is because it may be considered. Therefore, the photographing position of the cross-sectional SEM image in the inner porous layer 21 is not limited. For example, the cross-sectional SEM images may be photographed at a plurality of positions at regular intervals in the circumferential direction of the inner porous layer 21. A cross-sectional SEM image may be taken at each of the four surfaces at the center and both ends.

  6 shows cross-sectional SEM images corresponding to regions a to d (100 μm square) in FIG. 4 in Example 1 to be described later. As a result of obtaining the porosity from the cross-sectional SEM images of the regions a to d, the variation (difference) in the porosity of the regions a to d is 10% or less (3% in the present embodiment) as described later. There was found.

Next, an example of a method for manufacturing the porous protective layer 20 will be described with reference to the schematic diagram of FIG.
First, the inner porous layer 21 is formed over the entire surface of the tip of the gas sensor element 100 (FIG. 7A). At this time, a slurry obtained by adding a burnable pore former to the slurry for forming the inner porous layer 23 so that the inner porous layer 21 has a higher porosity than the outer porous layer 23 is baked. well when sintered, parts pore former is burned to form a relatively large pore C _a three-dimensional network structure. The slurry for forming the inner porous layer 23 can be applied to the gas sensor element 100 by a dipping method.
Next, slurry 23x to be the outer porous layer 23 is applied to the surface of the inner porous layer 21 by a dipping method (FIG. 7B). The slurry 23 x includes coarse particles 231 and fine particles 232 having a smaller particle diameter than the coarse particles 231. At this time, fine particles 232 in the slurry 23x is enters the portion of the pores C _A in the vicinity of the interface between the inner porous layer 21. In this state, the outer porous layer 23 is formed by sintering (FIG. 7C).

In this way, a region that has entered the fine particles 232 in a part of the pores C _A on the surface of the inner porous layer 21, a dense intermediate region 22. Since the intermediate region 22 includes the fine particles 232, adhesion between the intermediate region 22 and the outer porous layer 23 is improved through the fine particles 232.
In the outer porous layer 23, the fine particles 232 decrease in the portion on the intermediate region 22 side, but the coarse particles 231 remain to form a skeleton of the film, so that the outer porous layer 23 is stably formed. The Clearance coarse particles 231 and particles 232 to form pores C _B which forms a three-dimensional network structure of the outer porous layer 23.

The method for producing the porous protective layer 20 is not limited to the above-described method, and the slurry to be the inner porous layer 21 and the outer porous layer 23 may be applied and sintered in order. In this case, the slurry to be the inner porous layer 21 may be applied and sintered, and then the slurry to be the outer porous layer 23 may be applied and sintered. Alternatively, the slurry for forming the inner porous layer 21 and the outer porous layer 23 may be sequentially applied and sintered at a time.
Needless to say, it is not essential that the slurry 2 to be the outer porous layer 23 contains both coarse particles and fine particles.

  The present invention is not limited to the above embodiment, and can be applied to any gas sensor (gas sensor element) having a detection element portion and a heater portion having a solid electrolyte body and a pair of electrodes. However, the present invention is not limited to these uses, and it goes without saying that the present invention covers various modifications and equivalents included in the spirit and scope of the present invention. For example, the present invention may be applied to a NOx sensor (NOx sensor element) that detects the NOx concentration in the gas to be measured, an HC sensor (HC sensor element) that detects the HC concentration, and the like. In the above embodiment, the porous protective layer 20 is formed of ceramic particles. However, the porous protective layer 20 may be formed by containing ceramic fibers in the ceramic particles.

The following slurry A, which will be the inner porous layer 21, on the front surface (front and back surfaces and both side surfaces) of the plate-like gas sensor element (oxygen sensor element) 100 shown in FIGS. Then, the film was applied by a dip (immersion) method so as to have a thickness of 300 μm, and then dried in a dryer set at 200 ° C. for several hours in order to volatilize excess organic solvent in the slurry A. The inner porous layer 21 was baked under the conditions of 3 hours at 0 ° C. The slurry A was stirred for 10 hours in Example 1, and the slurry in Example 3 was used in Example 2.
Slurry A: Alumina powder (D10: 0.24 μm, D50: 0.40 μm, D90: 0.60 μm particle size distribution) 40 vol%, carbon powder (D10: 10.5 μm, D50: 20.6 μm, D90: 42.2 μm) Particle size distribution) of 60 vol% and alumina sol (external blend) of 10 wt% were weighed and further ethanol was added and stirred.

Next, the following slurry B to be the outer porous layer 23 was adjusted to have an appropriate viscosity on the surface of the inner porous layer 21 and applied to a thickness of 250 μm by a dip (immersion) method. Then, in order to volatilize the excess organic solvent in the slurry B, it dried for several hours with the dryer set to 200 degreeC, and the outer porous layer 23 was baked on the conditions for 3 hours at 1100 degreeC in air | atmosphere.
Slurry B: Spinel powder (D10: 24.6 μm, D50: 44 μm, D90: 88 μm) 60 vol%, alumina powder (D10: 0.24 μm, D50: 0.40 μm, D90: 0.60 μm) 40 vol%, alumina sol (outside) Formulation) 10 wt% was weighed and further ethanol was added and stirred for 10 hours.
The particle size distribution of each powder constituting the slurry A and the slurry B is 10% cumulative frequency distribution particle diameter from the fine particle side of the cumulative particle size distribution measured by laser diffraction scattering method for each ceramic particle (powder) and carbon powder is D10. The 50% cumulative frequency distribution particle diameter was expressed as D50, and the 90% cumulative frequency distribution particle diameter was expressed as D90.

The obtained gas sensor element 100 including the porous protective layer 20 is cut in a direction perpendicular to the longitudinal direction, and the cross section of the porous protective layer 20 is photographed with a scanning electron microscope (SEM) under the cross section (transverse cross section). A cross-sectional photograph was obtained.
Based on the obtained cross-sectional photograph, image analysis of the outer porous layer 23 side and the inner porous layer 21 was performed, and the ratio of the black portion in the cross-sectional photograph was obtained. Individual image analysis was performed on a 100 × 100 μm area. In addition, the acquisition position of a cross-sectional photograph and how to obtain the porosity are as described above.
In this way, the porosity (average porosity) of the outer porous layer 23 and the inner porous layer 21 was determined. Next, with respect to the inner porous layer 21, based on the cross-sectional SEM images of the respective regions a to d shown in FIG. 6, the variation in the porosity of the regions a to d (that is, the difference between the maximum value and the minimum value of the porosity). ) Was calculated.

For comparison, a gas sensor element having a porous protective layer was produced in the same manner as in the above example except that the inner porous layer 21 was fired using the following slurry C instead of the slurry A.
Slurry C: Alumina powder (D10: 0.48 μm, D50: 1.26 μm, D90: 3.78 μm particle size distribution) 40 vol%, carbon powder (D10: 10.5 μm, D50: 20.6 μm, D90: 42.2 μm) Particle size distribution) 60 vol%, 10 wt% of alumina sol (external blend) was weighed, ethanol was added, and the mixture was stirred for 10 hours.
In the same manner as in the example, a cross-sectional SEM of the porous protective layer of the comparative example was photographed, and the porosity of the outer porous layer 23 and the inner porous layer 21 was determined. Moreover, the variation in the porosity of each region a to d of the inner porous layer 21 was calculated.

Using the gas sensor element including the porous protective layer obtained as described above, a moisture test was performed.
First, the porous protective layer of the gas sensor element at room temperature was immersed in water and left under reduced pressure for 30 minutes. Thereafter, the gas sensor element was put in an environmental test machine at −20 ° C. and left until the water was completely frozen. After confirming that the water was frozen, a voltage of 12 V was applied to the heater portion of the gas sensor element, and when the sensor element temperature reached 800 ° C., energization control of the heater portion was continued and held for 5 minutes. The sensor element temperature reached 800 ° C. was determined based on the impedance value of the oxygen concentration detection cell.
After completion of the test, the appearance of the porous protective layer was observed with a magnifier, and the presence or absence of damage (peeling or chipping) of the porous protective layer was visually determined.
The obtained results are shown in Table 1. In Table 1, the result of the water test indicates the number of porous protective layers damaged in the test of five gas sensor elements.

As is clear from Table 1, in each of the examples in which the porosity variation of the inner porous layer was 10% or less, the porous protective layer was not damaged and the water resistance was excellent. In addition, compared with Example 2, in Example 1 in which the stirring time of the slurry to be the inner porous layer was increased, the variation in the porosity of the inner porous layer was further reduced.
On the other hand, in the comparative example in which the porosity variation of the inner porous layer exceeded 10%, more than half of the porous protective layers were damaged and the water resistance was poor. As shown in FIG. 8, in the case of the comparative example, this is probably because the cumulative particle size distribution of the particles (alumina powder) in the slurry serving as the inner porous layer was broader than in Examples 1 and 2.

DESCRIPTION OF SYMBOLS 1 Gas sensor 20 Porous protective layer 21 Inner porous layer 23 Outer porous layer 30 Housing 104,105,108,110 A pair of electrode 105,109 Solid electrolyte body 100 Gas sensor element 200 Heater part 300 Detection element part L Axial direction a ~ d Four sites in the inner porous layer

Claims (2)

A detection element portion having a solid electrolyte body and a pair of electrodes arranged on the solid electrolyte body, and a heater portion provided with a heater that generates heat when energized to the insulating ceramic body are laminated and extend in the longitudinal direction. A laminate having a substantially rectangular cross section in a direction perpendicular to the longitudinal direction, and a porous protective layer covering the entire circumference of the tip of the laminate exposed to the gas to be measured. A gas sensor element,
The porous protective layer includes an inner porous layer provided on the detection element side, and an outer porous layer formed on the inner porous layer,
The porosity of the inner porous layer is higher than the porosity of the outer porous layer;
Furthermore, in the said cross section, the gas sensor element whose difference of the porosity of 4 site | parts in the said inner porous layer located on two short sides and two long sides of the said laminated body is 10% or less. In a gas sensor comprising a sensor element that detects the concentration of a specific gas component in a gas to be measured, and a housing that holds the sensor element,
The gas sensor according to claim 1, wherein the sensor element is the gas sensor.