Oxygen sensor element holding lamination type ceramic heater

Abstract: An oxygen sensor element (1) has a cup-type solid electrolyte body (10) defining an air chamber (100) therein, and a plate-like lamination-type ceramic heater (2, 2a-2j) is disposed in the air chamber (100) with a front end portion contacting the bottom portion (109) of the air chamber (100). Accordingly, the lamination type ceramic heater (2, 2a-2j) is prevented form being swung and hit against the electrolyte body (10).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an oxygen sensor element that holds a lamination type ceramic heater, which is applied to an exhaust system of an internal combustion engine to control an air fuel ratio of the engine. 2. Related Arts: Conventionally, an air fuel ratio sensor is installed in an exhaust system of an automotive engine to detect an air fuel ratio, and the combustion of the engine is controlled based on the detected air fuel ratio. An oxygen sensor containing an oxygen sensor element is widely employed as the air fuel ratio sensor. The oxygen sensor element is composed of a cup-like solid electrolyte body having oxygen ion conductivity and defining an air chamber therein, an outer electrode disposed on an outer surface of the solid electrolyte body, and an inner electrode disposed on an inner surface of the solid electrolyte body. The inner surface of the solid electrolyte body is exposed to the air chamber. Further, a ceramic heater is disposed in the air chamber to heat the oxygen sensor element to an activation temperature. A pole-like ceramic heater is typically employed as the ceramic heater. In recent years, to reduce manufacturing cost, a plate-like lamination type ceramic heater has been suggested as a replacement for the pole-like ceramic heater. For example, JP-A-7-35723, JP-B2-5-2101, and WO 94/14057 disclose this kind of ceramic heater. The lamination type ceramic heater is composed of a plurality of laminated heater substrates, one of which has a heater layer for developing heat, and a lead layer for supplying electricity to the heater layer. Examples of a lamination type ceramic heater are shown in figures 1-4E. However, the mechanical strength of the lamination type ceramic heater is smaller than that of the pole-like ceramic heater. As mentioned above, the ceramic heater is disposed within the air chamber of the oxygen sensor element. Within the air chamber, the ceramic heater may be swung like a pendulum into contact with the solid electrolyte body due to impact, vibrations, and the like applied externally to the oxygen sensor element. A particular problem arises of the front end portion of the ceramic heater is sized incorrectly. In such a case, the lamination type ceramic heater is easily damaged. The damage to the ceramic heater can involve not only the heater substrates but the heater and lead layers to cause electrical disconnection. In addition, the heater and lead layers can be oxidized by air coming through the damaged portion, resulting in increase in resistance of the heater and lead layers. SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide an oxygen sensor element securely holding a lamination type ceramic heater therein. Another object of the present invention is to prevent damage to a lamination type ceramic heater installed in an air chamber of an oxygen sensor element. These objects are attained with the oxygen sensor element of claim 1. The features of the preamble to claim 1 are known from US-A-5451748. DE-A-19702096 also describes a plate-like ceramic heater in contact with the solid electrolyte body. It is recommended to round off the end of the heater to prevent damage. Also, in JP-A-04157358, the shape of a rod-type ceramic heater is adapted to improve heat transfer efficiency. Briefly, an oxygen sensor element according to the present invention has a solid electrolyte body defining an air chamber therein and a plate-like lamination type ceramic heater disposed in the air chamber. The lamination ceramic heater has an end portion contacting an inner surface of the electrolyte body at a bottom portion of the air chamber. Accordingly, the lamination type ceramic heater is prevented from being swung against the solid electrolyte body. Consequently, the ceramic heater is prevented from being damaged. In addition, because the ceramic heater contacts the solid electrolyte body, heat transfer efficiency from the ceramic heater to the solid electrolyte body is improved. The end portion of the ceramic heater has two faces with radiuses of curvature different from one another. Accordingly, the ceramic heater contacts the inner surface of the electrolyte body at the bottom portion of the air chamber. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become more readily apparent from a better understanding of preferred embodiments described below with reference to the following drawings. FIG. 1 is a cross-sectional view partially showing an oxygen sensor element holding a ceramic heater; FIG. 2 is a perspective view showing the ceramic heater shown in figure 1; FIG. 3 is a cross-sectional view showing the oxygen sensor of figure 1; FIGS. 4A - 4E are schematic views for explaining a method of manufacturing this type of ceramic heater; FIG. 5 is a cross-sectional view showing a front end portion of a ceramic heater installed within an air chamber of the oxygen sensor element according to the invention; FIG. 6A is a perspective view showing a front end portion of a ceramic heater in a second embodiment; FIG. 6B is a cross-sectional view showing the front end portion of the ceramic heater installed within the air chamber of the oxygen sensor element in the sixth embodiment; FIG. 7 is a cross-sectional view showing a front end portion of a ceramic heater installed within the air chamber of the oxygen sensor element in a seventh embodiment; FIG. 8A is a schematic plan view showing a bottom face of the front end portion of the ceramic heater of the third embodiment; FIGS. 8B, 8C are schematic views showing contact portions between the front end portion of the oxygen sensor element and the ceramic heater of the third embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A conventional oxygen sensor element 1 will be explained with reference to FIGS. 1-4E. As shown in FIG. 1, the oxygen sensor element 1 is composed of a cup-type solid electrolyte body 10 having an air chamber 100 therein, an outer electrode 11 provided on an outer surface 101 of the solid electrolyte body 10, and an inner electrode 12 provided on an inner surface 102 of the solid electrolyte body 10. The inner surface 102 of the solid electrolyte body 10 is exposed to the air chamber 100. The outer surface 101 of the solid electrolyte body 10 is covered with a protection layer 191 with the outer electrode 11 interposed therebetween, and the protection layer 191 is further covered with a trap layer 192. A plate-like lamination type ceramic heater 2 is held within the air chamber 100. The bottom portion 109 of the air chamber 100 has an arc-like concave shape relative to the ceramic heater 2, and a front end face 20 of the ceramic heater 2 having a rectangular shape abuts the bottom portion 109 of the air chamber 100 at four corner points thereof. As shown in FIG. 2, the ceramic heater 2 is composed of a heater substrate 201 holding a heater layer for developing heat, a lead layer for supplying electricity to the heater layer thereon, and a cover substrate 202 disposed on the heater substrate 201 to cover the heater layer and the lead layer. The heater substrate 201 and the cover substrate 202 are made of alumina (Al 2 O 3 ). The heater and lead layers are made of a conductive material containing tungsten (W). Parts of the lead layer are exposed on both side surfaces 28 of the ceramic heater 2, and conductive members 290 are disposed on the exposed lead layer on the side surfaces 28. Further, lead wires 29 are connected to the conductive members 290. The above-mentioned oxygen sensor element 1 is disposed in an oxygen sensor 3 as shown in FIG. 3. The oxygen sensor 3 is composed of a housing 30 for holding the oxygen sensor element 2, measurement gas side covers 301, 302 covering the lower portion of the housing 30 and defining a measurement gas chamber 300 therein, and an air side cover 331 covering the upper portion of the housing 30. A heater holder 31 is inserted into the air chamber 100 of the oxygen sensor element 1 so that the ceramic heater 2 is fixed to the oxygen sensor element 1 within the air chamber 100. The inner and outer electrodes 11, 12 of the oxygen sensor element 1 are connected to lead wires 391, 392 for taking out an output signal from the oxygen sensor element 1. The lead wires 29 of the ceramic heater 2 are connected to lead wires 393 for supplying electricity to the ceramic heater 2, through terminals 383. In the oxygen sensor element 1, the ceramic heater 2 abuts the bottom portion 109 of the air chamber 100 at four points. When the oxygen sensor element 1 is installed in the oxygen sensor 3, the upper portion of the ceramic heater 2 is supported by the heater holder 31, while the lower portion of the ceramic heater 2 is supported by the bottom portion 109 of the air chamber 100 at the four points. As a result, the ceramic heater 2 is securely fixed to the solid electrolyte body 10 of the oxygen sensor element 1 within the air chamber 100 and does not swing and vibrate within the air chamber 100, thereby preventing damage to the ceramic heater 2. Next, a method of manufacturing the ceramic heater 2a will be explained. First, slurry is formed from source powders composed of 92 wt% Al 2 O 3 and 8 wt% mixture of silicon dioxide (SiO 2 ) and magnesium oxide (MgO). The slurry is formed into a sheet with 1.2 mm in thickness by a doctor blade method. Then, two square green sheets 41, 42 sized to 120 mm × 120 mm are stamped out from the sheet by a punching press method. The green sheet 41 is for the heater substrate 201 and the green sheet 42 is for the cover substrate 202. The green sheets 41, 42 can be formed by other methods, such as an extrusion method. Next, conductive paste containing metallic materials such as W and molybdenum (Mo) is printed on the green sheet 41 for the heater substrate 201 to form five heater patterns 40 for the heat and lead layers as shown in FIG. 4A. Then, the green sheet 42 is laminated with the green sheet 41 such that the heater patterns 40 are interposed between the green sheets 41, 42, thereby forming a lamination body 43 as shown in FIG. 5B. The lamination body 43 is dried. After it is dried, the lamination body 43 is cut along dashed lines shown in FIG. 4B so that it is divided into five intermediate bodies 44 holding a corresponding one of the heater patterns 40. At that time, edge portions 449 including no heater pattern therein are removed. One of the intermediate bodies 44 is shown in FIG.4C. Herebelow, descriptions will be done with respect to the one intermediate body 44. Successively, conductive paste is printed on side surfaces 48 of the intermediate body 44 where parts of the heater pattern 40 are exposed, thereby forming printed portions 490. Then, the intermediate body 44 is baked at a temperature in a range of 1400 °C - 1600 °C. The baked intermediate body is indicated with reference numeral 45 in FIG. 4D. Simultaneously, the printed portions 490 are baked to serve as the conductive members 290. Next, the front end portion 209a of the baked intermediate body 45 is polished by a polishing machine to have the same radius of curvature as that of the bottom portion 109 of the air chamber 100 as shown in FIG. 4D. After brazing filler metal containing copper (Cu) and the like is disposed on the conductive members 290, the lead wires 29 are brazed to the conductive members 290 through the brazing filler metal at 1100 °C - 1150 °C within a furnace. As a result, the ceramic heater 2a shown in FIG. 4E is provided. Thus, the ceramic heater 2a can be more stably disposed within the air chamber 100 of the oxygen sensor element 1. In addition, because the front end face 20a of the ceramic heater 2a contacts the entire bottom portion 109 of the air chamber 100, heating efficiency of the ceramic heater 2 with respect to the oxygen sensor element 1 is enhanced. (First Embodiment) A ceramic heater 2d in a first preferred embodiment has a front end portion 209d with a front end curved face which is composed of two portions 501, 502. The portions 501, 502 have different radiuses of curvature from one another. FIG. 5 shows the ceramic heater 2d which is installed within the air chamber 100 of the oxygen sensor element 1. The portion 501 has a radius of curvature smaller than that of the bottom portion 109 of the air chamber 100, while the portion 502 has a radius of curvature larger than that of the portion 501. In this case, even if the front end portion 209d of the ceramic heater 2d is sized incorrectly, the front end portion 209d of the ceramic heater 2d contacts the bottom portion 109 of the air chamber 100 at the portion 501. The mechanical strength of a typical ceramic heater is generally not so great that it cannot be inserted into the air chamber 100 with large force until the front end portion of the ceramic heater abuts the bottom portion 109 of the air chamber 100. The ceramic heater 2d can solve this problem. In addition, even if the axis of the ceramic heater 2d deviates from the axis of the air chamber 100, the ceramic heater 2d contacts the bottom portion 109 of the air chamber 100 at the portion 501, thereby retaining the advantages of the conventional oxygen sensor. (Second Embodiment) As shown in FIG. 9A, a ceramic heater 2e in a second preferred embodiment has a front end portion 209e with a front end curved face. The front end curved face is composed of a first front end face (tapered face) 51 and a second front end face (top face) 52 which is provided at the top portion of the front end curved face. The first front end face has a portion 501 with a radius of curvature that is smaller than that of the bottom portion 109 of the air chamber 100. Therefore, the ceramic heater 2e contacts the bottom portion 109 of the air chamber 100 at the portion 501 of the first front end face 51 as shown in FIG. 6B. The other features and effects are the same as those in the first embodiment. (Third Embodiment) As shown in FIG. 7, a front end portion of a ceramic heater 2f in a third preferred embodiment has a first front end curved face 51 and a second front end curved face 52 which is provided at the top portion of the front end portion. The radius of curvature of the second front end curved face 52 is substantially equal to or larger than that of the bottom portion 109 of the air chamber 100. In this case, a clearance 525 between the bottom portion 109 and the second front end curved face 52 can be reduced. Therefore, even if the front end portion of the ceramic heater 2f does not entirely contact the bottom portion 109 of the air chamber, heat-transfer efficiency from the ceramic heater 2f to the oxygen sensor element 1 can be sufficiently increased. The other features and effects are the same as those in the first embodiment. In the ceramic heater 2f of the third embodiment, the bottom face of the front end portion, which is composed of the first and second front end faces 51, 52, is shown in FIG. 8A. The ceramic heater 2f or 2g can contact the bottom portion 109 of the air chamber 100 at two lines as shown in FIG. 8B or at two faces as shown in FIG. 8C. In FIGS. 8B, 8C, reference numerals 58, 59 indicate contact portions between the ceramic heater 2f and the bottom portion 109 of the air chamber 100.

Claims: An oxygen sensor element (1) comprising: a solid electrolyte body (10) defining an air chamber (100) therein; an outer electrode (11) disposed on an outer surface (101) of the electrolyte body (10); an inner electrode (12) disposed on an inner surface (102) of the electrolyte body (10) to be exposed to the air chamber (100); and a plate-like lamination type ceramic heater (2, 2a-2j) disposed within the air chamber (100), characterized in that the plate-like lamination type ceramic heater (2, 2a-2j) has a curved end portion (209, 209a-209j) contacting the inner surface (102) of the electrolyte body (10) at a bottom portion (109) of the air chamber (100), wherein the end portion (209, 209a-209j) has two faces (20, 21; 51, 52) with radii of curvature, wherein the radii are made different from one another so that, even if the curved end portion (209, 209a-209i) of the ceramic heater (2, 2a-2i) is sized incorrectly, the curved end portion (209, 209a-209i) contacts the bottom portion (109) of the air chamber (100). The oxygen sensor element according to claim 1, wherein: the second face (52, 20) is disposed at a position closer to a top of the end portion than the first face (51, 21); and the radius of curvature of the first face (51, 21) is smaller than that of the bottom portion of the air chamber (100). The oxygen sensor element according to claim 1 or 2, wherein: the second face (52, 20) is disposed at a position closer to a top of the end portion than the first face (51, 21); and the radius of curvature of the second face (52, 20) is larger than that of the first face (51, 21). The oxygen sensor element according to any one of claims 1 to 3, wherein the radius of curvature of the second face (52, 20) is larger than that of the bottom portion of the air chamber (100). The oxygen sensor element according to claim 1, wherein the radius of curvature of the first face (51, 21) is smaller than that of the bottom portion of the air chamber (100).

Inventors: KOBAYASHI MASAYUKI, SHIRAI MAKOTO, KOBAYASHI, MASAYUKI, SHIRAI, MAKOTO
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1998
Main classification: G01N27
IPC codes: G01N27