Patent Application
20230384256
The present invention relates to a sensor element and a gas sensor.
Sensor elements that detect the concentration of a specific gas, such as NOx, in the measurement-object gas, such as an automotive exhaust gas, are known (e.g., PTL 1). The sensor element described in PTL 1 includes a long-length element body; an outer electrode, an outer lead portion, and connector electrodes that are disposed on the upper surface of the element body; and a porous layer that covers the outer electrode and the outer lead portion. The outer electrode, the outer lead portion, and the connector electrodes are connected to and in electrical conduction with one another in this order. The connector electrodes are electrically connected to the outside. The sensor element described in PTL 1 also includes a dense layer arranged to divide the porous layer in the longitudinal direction of the element body. The dense layer covers the outer lead portion. Since moisture is unlikely to pass through the dense layer, in the case where moisture included in the measurement-object gas moves inside the porous layer by capillarity, the presence of the dense layer reduces the likelihood of the moisture reaching the connector electrodes. This reduces the rusting and corrosion of the connector electrodes and the short circuit between the connector electrodes.
There has been a demand for a further reduction in the likelihood of the moisture reaching the connector electrodes of a sensor element including a dense layer as in PTL 1.
The present invention was made in order to address the above issues. An object of the present invention is to prevent the moisture from reaching the connector electrodes.
The present invention employs the following structures in order to achieve the primary object.
A sensor element according to the present invention is a sensor element including: a long-length element body including front and rear ends and one or more side surfaces, the front and rear ends being ends of the element body in a longitudinal direction of the element body, the one or more side surfaces being surfaces extending in the longitudinal direction; a detection unit including a plurality of electrodes disposed in the front end-side part of the element body, the detection unit detecting a specific gas concentration in a measurement-object gas; one or more connector electrodes disposed on the rear end-side part of any of the one or more side surfaces, the one or more connector electrodes used for electrical conduction with an outside; a porous layer that covers at least the front end-side part of the side surface on which the one or more connector electrodes are disposed, the porous layer having a porosity of 10% or more; and a dense layer disposed on the side surface so as to divide the porous layer in the longitudinal direction or to be located closer to the rear end than the porous layer, the dense layer being located closer to the front end than the one or more connector electrodes, the dense layer covering the side surface and having a porosity of less than 10%, wherein the dense layer includes an overlap portion that is a front end portion of the dense layer, the overlap portion covering an outer surface of a part of the porous layer.
In the above-described sensor element, the connector electrodes are disposed on a rear end-side part of any of the one or more side surfaces of the element body, and the porous layer is arranged to cover at least the front end-side part of the side surface. Furthermore, the sensor element includes the dense layer disposed on the side surface so as to divide the porous layer in the longitudinal direction or to be located closer to the rear end than the porous layer. The dense layer is located closer to the front end than the connector electrodes. Therefore, when the front end-part of the element body, in which a plurality of electrodes constituting the detection unit are present, is exposed to a measurement-object gas, even if the moisture contained in the measurement-object gas moves inside the porous layer toward the rear end of the element body by capillarity, the moisture reaches the dense layer before reaching the connector electrodes. Since the dense layer has a porosity of less than 10% and the capillarity of water through the dense layer is unlikely to occur, the likelihood of the moisture passing through the dense layer is small. Furthermore, the dense layer includes an overlap portion that is the front end portion of the dense layer and covers the outer surface of a part of the porous layer. This reduces the likelihood of water that has moved toward the rear end of the element body through the porous layer moving backward of the dense layer along the outer surface of the dense layer. Thus, in the sensor element, since water is unlikely to pass through the inside and outer surface of the dense layer, the likelihood of the moisture reaching the connector electrodes can be reduced.
In the sensor element according to the present invention, the length of the overlap portion in the longitudinal direction, that is, the overlap length Lov, may be 40 μm or more.
In the sensor element according to the present invention, the length of the overlap portion in the longitudinal direction, that is, the overlap length Lov, may be 10000 μm or less.
The sensor element according to the present invention may further include an outer lead portion disposed on the side surface on which the one or more connector electrodes are disposed, the outer lead portion providing conduction between any of the electrodes and the one or more connector electrodes. The porous layer and the dense layer may cover the outer lead portion. For reducing the likelihood of the moisture moving along the outer surface of the dense layer, for example, a gap region may be interposed between the porous layer and the dense layer, instead of forming the above-described overlap portion. However, if such a gap region is formed when the outer lead portion is present, the outer lead portion is disadvantageously exposed to the outside of the sensor element at the gap region. In contrast, interposing an overlap portion, instead of a gap region, between the porous layer and the dense layer protects the outer lead portion while reducing the likelihood of the moisture reaching the connector electrodes.
In the above case, the porous layer may cover the entirety of the part of the outer lead portion which is not covered with the dense layer. The sensor element according to the present invention may include an outer electrode that is one of the electrodes included in the detection unit, the outer electrode being in conduction with the connector electrodes via the outer lead portion and disposed on the side surface on which the connector electrodes are disposed. In such a case, the porous layer may cover the outer electrode.
The gas sensor according to the present invention includes the sensor element according to any one of the above-described aspects. Therefore, the gas sensor has the same advantageous effects as the above-described sensor element according to the present invention. That is, for example, the gas sensor is capable of reducing the likelihood of the moisture reaching the connector electrodes.
Embodiments of the present invention are described below with reference to the attached drawings.
As illustrated in
The protective cover 30 includes, as illustrated in
The element-sealing member 40 is a member with which the sensor element 20 is sealed and fixed. The element-sealing member 40 includes a cylindrical body 41 including a main fitting 42 and an inner cylinder 43, insulators 44a to 44c, compacts 45a and 45b, and a metal ring 46. The sensor element 20 is located on the central axis of the element-sealing member 40 and penetrates the element-sealing member 40 in the top-to-bottom direction.
The main fitting 42 is a hollow cylindrical member made of a metal. The front-side part of the main fitting 42 is a thick-wall portion 42a having a smaller inside diameter than the rear-side part of the main fitting 42. The protective cover 30 is attached to a part of the main fitting 42 which is on the same side as the front end-side of the sensor element 20 (front-side part of the main fitting 42). The rear end of the main fitting 42 is welded to a flange portion 43a of the inner cylinder 43. A part of the inner peripheral surface of the thick-wall portion 42a serves as a bottom surface 42b, which is a stepped surface. The bottom surface 42b holds the insulator 44a such that the insulator 44a does not protrude forward.
The inner cylinder 43 is a hollow cylindrical member made of a metal and includes the flange portion 43a formed at the front end of the inner cylinder 43. The inner cylinder 43 and the main fitting 42 are coaxially fixed to each other by welding. The inner cylinder 43 includes a diameter reduction portion 43c that presses the compact 45b toward the central axis of the inner cylinder 43 and a diameter reduction portion 43d that presses the insulators 44a to 44c and the compacts 45a and 45b in the downward direction in
The insulators 44a to 44c and the compacts 45a and are interposed between the inner peripheral surface of the cylindrical body 41 and the sensor element 20. The insulators 44a to 44c serve as a support for the compacts and 45b. Examples of the material for the insulators 44a to 44c include ceramics, such as alumina, steatite, zirconia, spinel, cordierite, and mullite, and glass. The compacts 45a and 45b are formed by, for example, molding a powder and serve as a sealing medium. Examples of the material for the compacts 45a and 45b include talc and ceramic powders, such as an alumina powder and boron nitride. The compacts 45a and 45b may include at least one of the above materials. The compact 45a is filled between the insulators 44a and 44b and pressed by the insulators 44a and 44b as a result of both (front and rear) ends of the compact 45a in the axial direction being sandwiched therebetween. The compact 45b is filled between the insulators 44b and 44c and pressed by the insulators 44b and 44c as a result of both (front and rear) ends of the compact 45b in the axial direction being sandwiched therebetween. The insulators 44a to 44c and the compacts 45a and 45b are sandwiched between the diameter reduction portion 43d and the metal ring 46, and the bottom surface 42b of the thick-wall portion 42a of the main fitting 42 and thereby pressed in the front-to-rear direction. As a result of the compacts 45a and 45b being compressed between the cylindrical body 41 and the sensor element 20 by the pressing force applied by the diameter reduction portions 43c and 43d, the compacts 45a and 45b seal the communication between the element chamber 33 formed inside the protective cover 30 and a space 49 created inside the external cylinder 48 and fix the sensor element 20.
The bolt 47 is fixed to the outer surface of the main fitting 42 coaxially with the main fitting 42. The bolt 47 includes a male thread portion formed in the outer peripheral surface of the bolt 47. The male thread portion is inserted into a fixing member 59, which is welded to the pipe 58 and includes a female thread portion formed in the inner peripheral surface of the fixing member 59. This enables the gas sensor 10 to be fixed to the pipe 58 while the front end-side part of the sensor element 20 of the gas sensor 10 and the protective cover 30 of the gas sensor 10 are protruded toward the inside of the pipe 58.
The external cylinder 48 is a hollow cylindrical member made of a metal and covers the inner cylinder 43, the rear end-side part of the sensor element 20, and the connector 50. The upper part of the main fitting 42 is inserted into the external cylinder 48. The lower end of the external cylinder 48 is welded to the main fitting 42. A plurality of the lead wires 55, which are connected to the connector 50, are drawn from the upper end of the external cylinder 48 to the outside. The connector 50 is in contact with upper and lower connector electrodes 71 and 72 disposed on the rear end-side parts of the surfaces of the sensor element 20 and electrically connected to the sensor element 20. The lead wires 55 are in electrical conduction with electrodes 64 to 68 and a heater 69 disposed inside the sensor element 20 via the connector 50. The gap between the external cylinder 48 and the lead wires 55 is sealed with the rubber stopper 57. The space 49 inside the external cylinder 48 is filled with a reference gas. A sixth surface (rear end-side surface) of the sensor element 20 is located inside the space 49.
The sensor element 20 includes an element main body 60, a detection unit 63, a heater 69, an upper connector electrode 71, a lower connector electrode 72, a porous layer 80, and a water-penetration reduction portion 90 as illustrated in
The detection unit 63 detects the specific gas concentration in the measurement-object gas. The detection unit 63 includes a plurality of electrodes disposed in the front end-side part of the element main body 60. In this embodiment, the detection unit 63 includes an outer electrode 64 disposed on the first surface 60a and an inner main pump electrode 65, an inner auxiliary pump electrode 66, a measurement electrode 67, and a reference electrode 68 that are disposed inside the element main body 60. The inner main pump electrode 65 and the inner auxiliary pump electrode 66 are disposed on the inner peripheral surface of a cavity formed inside the element main body 60 and have a tunnel-like structure.
Since the principle on which the detection unit 63 detects the specific gas concentration in the measurement-object gas is publicly known, detailed description is omitted herein. The detection unit 63 detects the particular gas concentration, for example, in the following manner. The detection unit 63 draws oxygen included in the measurement-object gas which is in the vicinity of the inner main pump electrode 65 to or from the outside (the element chamber 33) on the basis of the voltage applied between the outer electrode 64 and the inner main pump electrode 65. The detection unit 63 also draws oxygen included in the measurement-object gas which is in the vicinity of the inner auxiliary pump electrode 66 to or from the outside (the element chamber 33) on the basis of the voltage applied between the outer electrode 64 and the inner auxiliary pump electrode 66. This enables the measurement-object gas to reach a space around the measurement electrode 67 after the oxygen concentration in the gas has been adjusted to be a predetermined value. The measurement electrode 67 serves as a NOx-reducing catalyst and reduces the particular gas (NOx) included in the measurement-object gas. The detection unit 63 converts an electromotive force generated between the measurement electrode 67 and the reference electrode 68 in accordance with the oxygen concentration in the reduced gas or a current that flows between the measurement electrode 67 and the outer electrode 64 on the basis of the electromotive force into an electrical signal. The electrical signal generated by the detection unit 63 indicates the value reflective of the particular gas concentration in the measurement-object gas (the value from which the particular gas concentration can be derived) and corresponds to the value detected by the detection unit 63.
The heater 69 is an electric resistor disposed inside the element main body 60. Upon the heater 69 being fed with power from the outside, the heater 69 generates heat and heats the element main body 60. The heater 69 is capable of heating the solid-electrolyte layers constituting the element main body 60 and conserving the heat such that the temperature is adjusted to be the temperature (e.g., 800° C.) at which the solid-electrolyte layers become active.
The upper connector electrode 71 and the lower connector electrode 72 are each disposed on the rear end-side part of any of the side surfaces of the element main body 60. The upper connector electrode 71 and the lower connector electrode 72 are electrodes that enable electrical conduction between the element main body 60 and the outside. The upper and lower connector electrodes 71 and 72 are not covered with the porous layer 80 and exposed to the outside. In this embodiment, four upper connector electrodes 71a to 71d, which serve as an upper connector electrode 71, are arranged in the left-to-right direction and disposed on the rear end-side part of the first surface 60a, and four lower connector electrodes 72a to 72d, which serve as a lower connector electrode 72, are arranged in the left-to-right direction and disposed on the rear end-side part of the second surface 60b (lower surface), which is opposite to the first surface 60a (upper surface). Each of the connector electrodes 71a to 71d and 72a to 72d is in electrical conduction with any of the electrodes 64 to 68 and the heater 69 included in the detection unit 63. In this embodiment, the upper connector electrode 71a is in conduction with the measurement electrode 67; the upper connector electrode 71b is in conduction with the outer electrode 64; the upper connector electrode 71c is in conduction with the inner auxiliary pump electrode 66; the upper connector electrode 71d is in conduction with the inner main pump electrode 65; the lower connector electrodes 72a to 72c are each in conduction with the heater 69; and the lower connector electrode 72d is in conduction with the reference electrode 68. The upper connector electrode 71b and the outer electrode 64 are in conduction with each other via an outer lead wire 75 disposed on the first surface 60a (see
The outer lead wire 75 is a conductive material including a noble metal, such as platinum (Pt), or a high-melting point metal, such as tungsten (W) or molybdenum (Mo). The outer lead wire 75 is preferably a cermet conductive material that includes the noble metal or high-melting point metal and the oxygen-ion-conductive solid electrolyte (in this embodiment, zirconia) included in the element body 60. In this embodiment, the outer lead wire 75 is a cermet conductive material that includes platinum and zirconia. The porosity of the outer lead wire 75 may be, for example, 5% or more and 40% or less. The line width (thickness, i.e., width in the left-to-right direction) of the outer lead wire 75 is, for example, 0.1 mm or more and 1.0 mm or less. An insulating layer, which is not illustrated in the drawings, may be interposed between the outer lead wire 75 and the first surface 60a of the element body 60 in order to provide electrical insulation between the outer lead wire 75 and the solid electrolyte layer of the element body 60.
The porous layer 80 is a porous body that covers at least the front end-side parts of the side surfaces of the element main body 60 on which the upper and lower connector electrodes 71 and 72 are disposed, that is, the first and second surfaces 60a and 60b. In this embodiment, the porous layer 80 includes an inner porous layer 81 that covers the first and second surfaces 60a and 60b and an outer porous layer 85 disposed on the outer surface of the inner porous layer 81.
The inner porous layer 81 includes a first inner porous layer 83 that covers the first surface 60a and a second inner porous layer 84 that covers the second surface 60b. The first inner porous layer 83 covers the entirety of the region extending from the front end to the rear end of the first surface 60a on which the upper connector electrodes 71a to 71d are disposed, except the regions in which a first water-penetration reduction portion 91 and the upper connector electrode 71 are present (see
The second inner porous layer 84 covers the entirety of the region extending from the front end to the rear end of the second surface 60b on which the lower connector electrodes 72a to 72d are disposed, except the regions in which a second water-penetration reduction portion 94 and the lower connector electrode 72 are present (see
The outer porous layer 85 covers the first to fifth surfaces 60a to 60e. The outer porous layer 85 covers the first surface 60a and the second surface 60b as a result of covering the inner porous layer 81. The length of the outer porous layer 85 in the front-to-rear direction is smaller than the length of the inner porous layer 81 in the front-to-rear direction. The outer porous layer 85 covers only the front end of the element main body 60 and a region of the element main body 60 around the front end, unlike the inner porous layer 81. Thus, the outer porous layer 85 covers a part of the element main body 60 which surrounds the electrodes 64 to 68 included in the detection unit 63. In other words, the outer porous layer 85 covers a part of the element main body 60 which is disposed inside the element chamber 33 and exposed to the measurement-object gas. Thereby, the outer porous layer 85 serves as, for example, a protection layer that reduces the likelihood of moisture and the like included in the measurement-object gas adhering to the element main body 60 and causing cracking of the element main body 60.
The porous layer 80 is composed of, for example, a ceramic porous body, such as an alumina porous body, a zirconia porous body, a spinel porous body, a cordierite porous body, a titania porous body, or a magnesia porous body. In this embodiment, the porous layer 80 is composed of an alumina porous body. The thicknesses of the first and second inner porous layers 83 and 84 may be, for example, 5 μm or more or 14 μm or more. The thicknesses of the first and second inner porous layers 83 and 84 may be 40 μm or less or 23 μm or less. The thickness of the outer porous layer 85 may be, for example, 40 μm or more and 800 μm or less. The porosity of the porous layer 80 is 10% or more. Although the porous layer 80 covers the outer electrode 64 and the measurement-object gas introduction port 61, the measurement-object gas can pass through the porous layer 80 when the porosity of the porous layer 80 is 10% or more. The porosity of the inner porous layer 81 may be 10% or more and 50% or less. The porosity of the outer porous layer 85 may be 10% or more and 85% or less. The outer porous layer may have a higher porosity than the inner porous layer 81.
The porosity of the inner porous layer 81 is determined by the following method using an image (SEM image) obtained by inspecting the inner porous layer 81 with a scanning electron microscope (SEM). First, the sensor element 20 is cut in the thickness direction of the inner porous layer 81 such that a cross section of the inner porous layer 81 can be inspected. The cross section is buried in a resin and ground in order to prepare an observation sample. An image of the observation cross section of the observation sample is taken with a SEM at a 1000 to 10000-fold magnification in order to obtain an SEM image of the inner porous layer 81. Subsequently, the image is subjected to image analysis. A threshold value is determined on the basis of the brightness distribution included in brightness data of pixels of the image by a discriminant analysis method (Otsu's binarization). On the basis of the threshold value, the pixels of the image are binarized into an object portion and a pore portion. The areas of the object portions and the pore portions are calculated. The ratio of the area of the pore portions to the total area (the total area of the object portions and the pore portions) is calculated as a porosity (unit: %). The porosity of the outer porous layer 85 and the porosities of the first and second dense layers 92 and 95, which are described below, are also calculated by the same method as described above.
The water-penetration reduction portion 90 reduces the capillarity of water through the element main body 60 in the longitudinal direction. In this embodiment, the water-penetration reduction portion 90 includes a first water-penetration reduction portion 91 and a second water-penetration reduction portion 94. The first water-penetration reduction portion 91 is disposed on the first surface 60a, on which the upper connector electrode 71 and the first inner porous layer 83 are disposed. As described above, the first water-penetration reduction portion 91 is disposed on the first surface 60a so as to divide the first inner porous layer 83 into front and rear parts in the longitudinal direction. The first water-penetration reduction portion 91 is arranged closer to the front end of the element main body 60 than the upper connector electrode 71, that is, disposed forward of the upper connector electrode 71. The first water-penetration reduction portion 91 is disposed backward of the outer electrode 64. The first water-penetration reduction portion 91 is disposed backward of any of the electrodes 64 to 68 included in the detection unit 63, in addition to the outer electrode 64 (see
The second water-penetration reduction portion 94 is disposed on the second surface 60b, on which the lower connector electrode 72 and the second inner porous layer 84 are disposed. As described above, the second water-penetration reduction portion 94 is disposed on the second surface 60b so as to divide the second inner porous layer 84 into front and rear parts in the longitudinal direction. The second water-penetration reduction portion 94 is arranged closer to the front end of the element main body 60 than the lower connector electrode 72, that is, disposed forward of the lower connector electrode 72. The second water-penetration reduction portion 94 is disposed backward of the outer electrode 64. The second water-penetration reduction portion 94 is disposed backward of any of the electrodes 64 to 68 included in the detection unit 63, in addition to the outer electrode 64 (see
The length L of the first and second water-penetration reduction portions 91 and 94 in the longitudinal direction (see
The first and second dense layers 92 and 95 may be composed of any of the ceramics described above as examples of the material for the porous layer 80, although the first and second dense layers 92 and 95 are different from the porous layer 80 in that the porosity of the first and second dense layers 92 and 95 is less than 10%. In this embodiment, the first and second dense layers 92 and 95 are composed of an alumina ceramic. The porosity of the first and second dense layers 92 and 95 is preferably 8% or less and is more preferably 5% or less. The smaller the porosity of the first and second dense layers 92 and 95, the higher the degree of reduction in the capillarity of water in the longitudinal direction of the element body 60 which is achieved by the first and second dense layers 92 and 95.
The length Le of the first and second dense layers 92 and 95 in the longitudinal direction (see
The length Lg of the first gap region 93 and the second gap region 96 in the longitudinal direction is preferably 1 mm or less. When the length Lg is relatively small, the area of the parts of the side surfaces (in this embodiment, the first and second surfaces 60a and 60b) of the element main body 60 which are exposed to the outside, that is, the parts of the side surfaces which are not covered with any of the porous layer 80, the first dense layer 92, and the second dense layer 95, can be reduced. In particular, in this embodiment, the outer lead wire 75 is disposed on the first surface 60a, and the outer lead wire 75 is disadvantageously exposed to the outside in the region in which the first gap region 93 is present. Setting the length Lg of the first gap region 93 to be small reduces the area of the part of the outer lead wire 75 which is not covered with any of the porous layer 80 and the first dense layer 92.
The first dense layer 92 includes an overlap portion 92a that is the front end portion of the first dense layer 92 and covers the outer surface of a part of the porous layer 80.
The length of the overlap portion 92a in the longitudinal direction (in this case, the front-to-rear direction) of the element body 60, that is, the overlap length Lov, may be 40 μm or more. The overlap length Lov may be 100 μm or more or 150 μm or more. The overlap length Lov may be 10000 μm (i.e., 10 mm) or less. The overlap length Lov may be less than 0.2 times the length Le of the first dense layer 92.
The overlap length Lov is the value measured using an image (SEM image) obtained by observation with a scanning electron microscope (SEM) by the following method. First, as illustrated in
As described above, the overlap length Lov is determined on the basis of the lengths of the overlap portion 92a in the two cross sections (A1 and A2). It is preferable that the length of the overlap portion 92a in another cross section be substantially equal to the overlap length Lov. For example, even in the case where the overlap portion 92a is observed in any cross section parallel to the top-to-bottom or front-to-rear direction of the sensor element 20, the length of the overlap portion 92a in the cross section in the front-to-rear direction is preferably 0.6 times or more and 1.4 times or less and is more preferably 0.84 times or more and 1.16 times or less the overlap length Lov. In other words, it is preferable that the minimum length of the overlap portion 92a in the front-to-rear direction be 0.6 times or more the overlap length Lov and the maximum length of the overlap portion 92a in the front-to-rear direction be 1.4 times or less the overlap length Lov, and it is more preferable that the minimum length of the overlap portion 92a in the front-to-rear direction be 0.84 times or more the overlap length Lov and the maximum length of the overlap portion 92a in the front-to-rear direction be 1.16 times or less the overlap length Lov. In still other words, the quotient of the maximum length of the overlap portion 92a in the front-to-rear direction divided by the minimum length of the overlap portion 92a in the front-to-rear direction is preferably 2.33 times (=1.4/0.6) or less and is more preferably 1.38 times (=1.16/0.84) or less.
The method for producing the gas sensor 10 is described below. First, the method for producing the sensor element 20 is described. In the production of the sensor element 20, first, a plurality of (in this embodiment, six) unbaked ceramic green sheets that correspond to the element body 60 are prepared. In each of the green sheets, as needed, notches, through-holes, grooves, and the like are formed by punching or the like, and electrodes and wire patterns are formed by screen printing. The wire patterns include a pattern of an unbaked lead wire that is to be formed into an outer lead wire 75 after baking. In addition, unbaked porous layers that are to be formed into the first and second inner porous layers 83 and 84 after baking and unbaked dense layers that are to be formed into the first and second dense layers 92 and 95 after baking are formed on the surfaces of the green sheets which correspond to the first and second surfaces 60a and 60b by screen printing. Subsequently, the green sheets are stacked on top of one another. The green sheets stacked on top of one another are an unbaked element body that is to be formed into the element body after baking and include unbaked porous layers and unbaked dense layers. The unbaked element body is baked to form the element body 60 including the outer lead wire 75, the first inner porous layer 83, the second inner porous layer 84, the first dense layer 92, and the second dense layer 95. Subsequently, the outer porous layer 85 is formed by plasma spraying. Hereby, the sensor element 20 is prepared. For producing the porous layer 80, the first dense layer 92, and the second dense layer 95, gel casting, dipping, and the like can be used in addition to screen printing and plasma spraying. After the unbaked porous layer that is to be formed into a first inner porous layer 83 has been formed, the unbaked dense layer that is to be formed into a first dense layer 92 is formed so as to partially overlap the above unbaked porous layer in order to produce a first dense layer 92 including an overlap portion 92a. The overlap length Lov can be adjusted by changing the shapes of the unbaked porous layer and the unbaked dense layer and the positions at which the unbaked porous layer and the unbaked dense layer are formed.
The gas sensor 10 that includes the sensor element 20 is produced. First, the sensor element 20 is inserted into the cylindrical body 41 so as to penetrate the cylindrical body 41 in the axial direction. Subsequently, the insulator 44a, the compact 45a, the insulator 44b, the compact 45b, the insulator 44c, and the metal ring 46 are disposed in the gap between the inner peripheral surface of the cylindrical body 41 and the sensor element 20 in this order. Then, the metal ring 46 is pressed in order to compress the compacts 45a and 45b. While the compacts 45a and 45b are compressed, the diameter reduction portions 43c and 43d are formed. Hereby, the element-sealing member 40 is produced, and the gap between the inner peripheral surface of the cylindrical body 41 and the sensor element 20 is sealed. The protective cover 30 is welded to the element-sealing member 40, and the bolt 47 is attached to the element-sealing member 40. Hereby, the assembly 15 is produced. Lead wires 55 attached to a rubber stopper 57 so as to penetrate the rubber stopper 57 and a connector 50 connected to the lead wires 55 are prepared. The connector is connected to the rear end-side part of the sensor element 20. Subsequently, the external cylinder 48 is fixed to the main fitting 42 by welding. Hereby, the gas sensor is produced.
An example of the application of the gas sensor 10 is described below. When the measurement-object gas flows inside the pipe 58 while the gas sensor 10 is attached to the pipe 58 as illustrated in
The measurement-object gas may contain moisture, which may move inside the porous layer 80 by capillarity. If the moisture reaches the upper and lower connector electrodes 71 and 72, which are exposed to the outside, the water and the components dissolved in the water, such as sulfuric acid, may cause rusting and corrosion of the upper and lower connector electrodes 71 and 72 and a short circuit between some of the upper and lower connector electrodes 71 and 72 which are adjacent to one another. However, in this embodiment, even when the moisture contained in the measurement-object gas moves inside the porous layer 80 (in particular, inside the first and second inner porous layers 83 and 84) toward the rear end-side part of the element body 60 by capillarity, the moisture reaches the first water-penetration reduction portion 91 or the second water-penetration reduction portion 94 before reaching the upper and lower connector electrodes 71 and 72. The first water-penetration reduction portion 91 includes the first dense layer 92 having a porosity of less than 10% and the first gap region 93 that is a space in which the porous layer 80 is absent, and both of them reduce the capillarity of water in the longitudinal direction of the element body 60. By the above mechanisms, the first water-penetration reduction portion 91 reduces the likelihood of the moisture passing through the first water-penetration reduction portion 91 from the front end-side portion 83a-side and reaching the upper connector electrode 71 (the upper connector electrodes 71a to 71d). Therefore, in the sensor element 20, the above-described trouble caused by the water adhering to the upper connector electrode 71 may be reduced. In the similar manner as described above, the second water-penetration reduction portion 94, which includes the second dense layer 95 and the second gap region 96, reduces the likelihood of the moisture passing through the second water-penetration reduction portion 94 from the front end-side portion 84a-side and reaching the lower connector electrode 72 (the lower connector electrodes 72a to 72d). Therefore, in the sensor element 20, the above-described trouble caused by the water adhering to the lower connector electrode 72 may be reduced. The length L of the first water-penetration reduction portion 91 in the longitudinal direction is preferably 0.5 mm or more in order to reduce the likelihood of the moisture passing through the first water-penetration reduction portion 91 to a sufficient degree. Similarly, the length L of the second water-penetration reduction portion 94 is preferably 0.5 mm or more.
As described above, since the porosity of the first dense layer 92 is less than 10%, the moisture is unlikely to pass through the inside of the first dense layer 92. Moreover, in this embodiment, the first dense layer 92 includes the overlap portion 92a. Consequently, the moisture that has moved inside the porous layer 80 toward the rear end of the element body 60 moves into the rear end portion 83c of the front end-side portion 83a of the first inner porous layer 83, that is, a portion of the front end-side portion 83a which is below the overlap portion 92a, so as to sink below the overlap portion 92a (see the hollow arrows in
The thickness of the first dense layer 92 may be, for example, 3 μm or more. The thickness of the first dense layer 92 may be, for example, 40 μm or less, 32 μm or less, 10 μm or less, 6 μm or less, 5 μm or less, or less than 5 μm. The thickness of the first dense layer 92 is the thickness of a portion of the first dense layer 92 which is other than the overlap portion 92a. The above numerical ranges also apply to the thickness of the second dense layer 95. The thickness of the second dense layer 95 may be equal to or different from that of the first dense layer 92.
The correspondences between the elements constituting this embodiment and the elements constituting the present invention are explicitly described below: the element body 60 in this embodiment corresponds to the element body in the present invention; the detection unit 63 corresponds to the detection unit; the upper connector electrodes 71a to 71d correspond to the connector electrodes; the first surface 60a corresponds to the side surface on which the connector electrodes are disposed; the porous layer 80 corresponds to the porous layer; the first dense layer 92 corresponds to the dense layer; the overlap portion 92a corresponds to the overlap portion; the outer lead wire 75 corresponds to the outer lead portion; and the outer electrode 64 corresponds to the outer electrode.
In the sensor element 20 according to this embodiment which is described in detail above, the first dense layer 92 having a porosity of less than 10% is disposed on the first surface 60a so as to divide the porous layer 80 in the longitudinal direction of the sensor element Therefore, even when the moisture included in a measurement-object gas moves inside the porous layer 80 toward the rear end of the element body 60 by capillarity, the moisture is unlikely to pass through the inside of the first dense layer 92 because the capillarity of water is unlikely to occur inside the first dense layer 92. Furthermore, since the first dense layer 92 includes the overlap portion 92a that is the front end portion of the first dense layer 92 and that covers the outer surface of the rear end portion 83c, which is a part of the porous layer 80, the likelihood of the water that has moved inside the porous layer 80 toward the rear end of the element body moving backward of the first dense layer 92 along the outer surface of the first dense layer 92 can also be reduced. As described above, in the sensor element 20 according to this embodiment, water is unlikely to pass through the inside and outer surface of the first dense layer 92. This reduces the likelihood of the moisture reaching the upper connector electrode 71.
The sensor element 20 further includes an outer lead wire 75 disposed on the first surface 60a, on which the upper connector electrode 71 is disposed, and that connects the outer electrode 64, which is one of the plurality of electrodes included in the detection unit 63, and the upper connector electrode 71b. The porous layer 80 and the first dense layer 92 cover the outer lead wire 75. For reducing the likelihood of the moisture moving along the outer surface of the first dense layer 92, for example, a gap region may be interposed between the front end-side portion 83a of the first inner porous layer 83 included in the porous layer 80 and the front end of the first dense layer 92, instead of forming the overlap portion 92a. However, if such a gap region is formed when the outer lead wire 75 is present, the outer lead wire 75 is disadvantageously exposed to the outside of the sensor element 20 at the gap region. If the outer lead wire 75 is exposed to the outside of the sensor element 20, for example, when a gas sensor 10 that includes the sensor element 20 is produced, the outer lead wire 75 may become worn. In contrast, forming the overlap portion 92a instead of the gap region reduces the exposure of the outer lead wire 75 and thereby protects the outer lead wire 75 while reducing the likelihood of the moisture reaching the upper connector electrode 71.
It is to be understand that the present invention is not limited to the above-described embodiment at all, but intended to include a variety of forms within the technical scope of the present invention.
For example, although the first water-penetration reduction portion 91 includes the first dense layer 92 and the first gap region 93 in the above-described embodiment, the first water-penetration reduction portion 91 includes at least the first dense layer 92. That is, the first water-penetration reduction portion 91 does not necessarily include the first gap region 93. In other words, the length Lg in the first water-penetration reduction portion 91 may be 0 mm.
Although the first water-penetration reduction portion 91 divides the first inner porous layer 83 into the front end-side portion 83a and the rear end-side portion 83b in the longitudinal direction in the above-described embodiment, the present invention is not limited to this. The first water-penetration reduction portion 91 may be arranged closer to the rear end than the porous layer 80. For example, in the above-described embodiment, the first inner porous layer 83 does not necessarily include the rear end-side portion 83b. In such a case, the region in which the rear end-side portion 83b is disposed in
Although the second dense layer 95 is disposed forward of the second gap region 96 so as to be adjacent to the second gap region 96 in the above-described embodiment, the second dense layer 95 may be disposed backward of the second gap region 96 so as to be adjacent to the second gap region 96. In another case, the second gap region 96 may be formed both forward and backward of the second dense layer 95 so as to be adjacent to the second dense layer 95.
Although the first and second water-penetration reduction portions 91 and 94 are arranged to overlap the insulator 44b in the front-to-rear direction in the above-described embodiment, the present invention is not limited to this. For example, the first and second water-penetration reduction portions 91 and 94 may be arranged to overlap the insulator 44a or the insulator 44c in the front-to-rear direction or may be disposed backward of the metal ring 46. The first and second water-penetration reduction portions 91 and 94 are preferably disposed so as not to be exposed to the inside of the element chamber 33.
In the above-described embodiment, the sensor element 20 does not necessarily include the second inner porous layer 84 and the second surface 60b is not necessarily covered with the porous layer 80. In such a case, the sensor element 20 does not necessarily include the second water-penetration reduction portion 94. The water-penetration reduction portion may be disposed on at least one of the side surfaces of the element main body (in the above-described embodiment, the first to fourth surfaces 60a to 60d) on which the connector electrodes and the porous protection layer are disposed (in the above-described embodiment, the first or second surface 60a or 60b). This reduces the likelihood of the moisture reaching the connector electrodes at least on the side surface on which the water-penetration reduction portion is disposed.
Although the first inner porous layer 83 covers the region that extends from the front to rear ends of the first surface 60a except the region in which the first water-penetration reduction portion 91 and the upper connector electrode 71 are present in the above-described embodiment, the present invention is not limited to this. For example, the first inner porous layer 83 may cover a region that extends from the front end of the first surface 60a to the front end-side ends of the upper connector electrodes 71a to 71d except the region in which the first water-penetration reduction portion 91 is present. Alternatively, the first inner porous layer 83 may cover at least a region that extends from the front end of the first surface 60a to the rear of the first water-penetration reduction portion 91 except the region in which the first water-penetration reduction portion 91 is present. The same applies to the second inner porous layer 84.
Although the element main body 60 has a rectangular cuboid shape in the above-described embodiment, the present invention is not limited to this. For example, the element main body 60 may have a hollow cylindrical shape or a solid cylindrical shape. In such a case, the element main body 60 has one side surface.
In the above-described embodiment, the first dense layer 92 includes the overlap portion 92a. Similarly to this, the second dense layer 95 may include an overlap portion. Specifically, the second dense layer 95 may include an overlap portion that is the front end portion of the second dense layer 95 and that covers the outer surface (in this case, the bottom surface) of the rear end portion of the front end-side portion 84a of the second inner porous layer 84 that is a part of the porous layer 80.
Although the gas sensor 10 detects NOx concentration as a specific gas concentration in the above-described embodiment, the present invention is not limited to this. The concentration of another oxide may be detected as a specific gas concentration. In the case where the specific gas is an oxide, oxygen is generated when the specific gas is reduced in the vicinity of the measurement electrode 67 as in the above-described embodiment, and the concentration of the specific gas can be detected on the basis of the value detected by the detection unit 63 which corresponds to the oxygen. The specific gas may be a non-oxide, such as ammonia. In the case where the specific gas is a non-oxide, the specific gas is converted to an oxide in the vicinity of, for example, the inner main pump electrode 65 (e.g., ammonia is oxidized to NO) and oxygen is generated when the oxide is reduced in the vicinity of the measurement electrode 67. Thus, in such a case, the concentration of the specific gas can be detected on the basis of the value detected by the detection unit 63 which corresponds to the oxygen. As described above, regardless of whether the specific gas is an oxide or a non-oxide, the gas sensor 10 is capable of detecting the concentration of the specific gas on the basis of the oxygen that is derived from the specific gas and generated in the vicinity of the measurement electrode 67.
Example cases where a specific sensor element was prepared are described below as Examples. Note that the present invention is not limited by Examples below.
In Example 1, a sensor element similar to the sensor elements 20 illustrated in
In Comparative Example 1, a sensor element 20 was prepared as in Example 1, except that the first inner porous layer 83 and a first dense layer 192 were formed such that the rear end portion 83c was located at a position closer to the outside than the first dense layer 192 as illustrated in
[Liquid Penetration Test]
The sensor elements 20 prepared in Example 1 and Comparative Example 1 were subjected to a liquid penetration test, in which whether the first dense layer was capable of blocking the penetration of a liquid into the rear end-side part of the element body 60 by capillarity when the front end-side part of the element body 60 was immersed in the liquid was determined. First, while the sensor element 20 was held such that the longitudinal direction of the sensor element 20 was parallel to the vertical direction, a part of the sensor element 20 which extended from the front end (fifth surface 60e) of the element body 60 to a position (hereinafter, “immersion position”) 25 mm from the front end toward the rear end was immersed into a red-check solution. While the sensor element was immersed in the red-check solution, the sensor element was left to stand for 24 hours. Subsequently, whether the red-check solution penetrated a region closer to the rear end than the first dense layer was visually determined. An evaluation of “Good” was given in the case where the red-check solution did not penetrate the above region, while an evaluation of “Poor” was given in the case where the red-check solution penetrated the above region. For each of Example 1 and Comparative Example 1, five sensor elements 20 were subjected to the liquid penetration test. In both of Example 1 and Comparative Example 1, the front end of the first dense layer was located at a position 26 mm from the front end of the element body 60. The red-check solution used was a stamp ink produced by Shachihata Inc. (for sol stamp stand) (Model No.: S-1, Color: Red). The red-check solution included water: 50 to 60 wt %, glycerin: 30 to 40 wt %, and dye: 5 to 15 wt %. The components and composition of the red-check solution are described in a safety data sheet (SDS) produced by Shachihata Inc.
Table 1 summarizes the positional relationship between the first dense layer and the first inner porous layer, the overlap length Lov, and the results of the liquid penetration test obtained in each of Example 1 and Comparative Example 1.
As is understood from Table 1, in Example 1 where the first dense layer 92 included the overlap portion 92a, that is, the front end portion of the first dense layer 92 covered the outer surface of the rear end portion 83c of the first inner porous layer 83, all of the five sensor elements were evaluated as “Good” in the liquid penetration test. On the other hand, in Comparative Example 1 where the first dense layer 192 was located below the first inner porous layer 83, all of the five sensor elements were evaluated as “Poor” in the liquid penetration test. This confirms that, when the first dense layer 92 includes the overlap portion 92a, the likelihood of water moving backward of the first dense layer 92 toward the rear end can be reduced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-057628 | Mar 2021 | JP | national |
This application is a continuation application of PCT/JP2022/006509, filed on Feb. 18, 2022, which claims the benefit of priority of Japanese Patent Application No. 2021-057628 filed on Mar. 30, 2021, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/006509 | Feb 2022 | US |
| Child | 18359847 | US |
