This application is based upon and claims the benefit of priority from earlier Japanese Patent Application No. 2004-120683 filed on Apr. 15, 2004 so that the description of which is incorporated herein by reference.
The present invention relates to a multilayered gas sensing element including a sensor cell detecting the concentration of a specific gas in an exhaust gas and a ceramic heater integrally laminated with this sensor cell.
It is conventionally known that a multilayered gas sensing element includes a sensor cell detecting the concentration of a specific gas in an exhaust gas and a ceramic heater integrally laminated with this sensor cell. The sensor cell consists of a measured gas side electrode and a reference gas side electrode provided on both surfaces of a solid electrolytic substrate containing zirconia or the like as a main component. On the other hand, the ceramic heater includes a heater pattern embedded in a heater substrate containing alumina or comparable insulating ceramic as a main component.
In short, the solid electrolytic substrate and the heater substrate laminated with each other to arrange the multilayered gas sensing element are made of different materials. Thus, there is the possibility that warpage or exfoliation (or separation) may occur in a multilayered gas sensing element during the sintering operation due to the difference of shrinkage factors of these different materials. To solve this drawback, the Japanese Patent Application Laid-open No. 2002-71629 proposes adding alumina or comparable insulating ceramic to the solid electrolytic substrate. As the alumina or comparable insulating ceramic is the main component of the heater substrate, it is expected that the difference of heat shrinkage factors of the solid electrolytic substrate and the heater substrate can be reduced.
However, adding the insulating ceramic into the solid electrolytic substrate will lessen the ionic conductivity (i.e. electrolytic conductivity) of the solid electrolytic substrate and accordingly will reduce an output current of the sensor cell (refer to a later-described relationship shown in
In view of the above-described problems of the prior art, the present invention has an object to provide a multilayered gas sensing element capable of suppressing warpage or exfoliation (or separation) and also securing satisfactory sensor output.
In order to accomplish the above and other related object, the present invention provides a first multilayered gas sensing element including a sensor cell and a ceramic heater which are laminated integrally. The sensor cell has a solid electrolytic substrate containing an electrolytic component serving as a main component of an ionic conductive solid electrolyte. The ceramic heater has a heater substrate containing an insulating ceramic as a main component. Furthermore, the solid electrolytic substrate of the first multilayered gas sensing element includes a first electrolytic layer provided at a position closest to the ceramic heater and a second electrolytic layer laminated with the first electrolytic layer. The first electrolytic layer contains the insulating ceramic. And, the second electrolytic layer has an insulating ceramic content smaller than that of the first electrolytic layer.
The first multilayered gas sensing element of the present invention brings the following functions and effects.
The solid electrolytic substrate of the present invention has the first electrolytic layer at the position closest to the ceramic heater. The first electrolytic layer contains insulating ceramic. Accordingly, the solid electrolytic substrate can reduce, at the portion near the ceramic heater, the difference of heat shrinkage factors of the solid electrolytic substrate and the ceramic heater. According to this arrangement, it becomes possible to suppress the warpage occurring in the multilayered gas sensing element or the exfoliation (or separation) occurring between the solid electrolytic substrate and the heater substrate during the sintering operation.
Furthermore, the solid electrolytic substrate has the second solid electrolytic layer whose insulating ceramic content is smaller than the insulating ceramic content of the first electrolytic layer. Accordingly, the solid electrolytic substrate can reduce the insulating ceramic content as a whole and can secure satisfactory ionic conductivity. According to this arrangement, the sensor cell can produce a sufficient sensor output.
As described above, the present invention can provide an excellent multilayered gas sensing element capable of suppressing warpage or exfoliation and also securing satisfactory sensor output.
Furthermore, to accomplish the above and other related object, the present invention provides a second multilayered gas sensing element including a sensor cell and a ceramic heater which are laminated integrally. The sensor cell has a solid electrolytic substrate containing an electrolytic component serving as a main component of an ionic conductive solid electrolyte. And, the ceramic heater has a heater substrate containing an insulating ceramic as a main component. The heater substrate of the second multilayered gas sensing element includes an electrolytic component containing layer at a position closest to the solid electrolytic substrate. And, the electrolytic component containing layer contains the electrolytic component serving as a main component of an ionic conductive solid electrolyte. According to second multilayered gas sensing element of the present invention, it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element due to the difference of heat shrinkage factors of the solid electrolytic substrate and the heater substrate. For example, the electrolytic component containing layer has a thickness of 3 to 600 μm.
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description which is to be read in conjunction with the accompanying drawings, in which:
As a best mode for embodying the present invention, the inventors of this application provide a first multilayered gas sensing element including a sensor cell and a ceramic heater which are laminated integrally. The sensor cell has a solid electrolytic substrate containing an electrolytic component serving as a main component of an ionic conductive solid electrolyte. The ceramic heater has a heater substrate containing an insulating ceramic as a main component. Furthermore, the solid electrolytic substrate of the first multilayered gas sensing element includes a first electrolytic layer provided at a position closest to the ceramic heater and a second electrolytic layer laminated with the first electrolytic layer. The first electrolytic layer contains the insulating ceramic. And, the second electrolytic layer has an insulating ceramic content smaller than that of the first electrolytic layer.
In the first multilayered gas sensing element of the present invention, the first electrolytic layer needs not be explicitly discriminated from other layer via a boundary surface facing to this different layer of the solid electrolytic substrate. For example, it is possible to define a predetermined region of the solid electrolytic substrate (e.g. a region corresponding to ⅓ of the entire thickness) as the first electrolytic layer.
Furthermore, the electrolytic component of the solid electrolytic substrate is a main component of the ionic conductive solid electrolyte, such as zirconia, barium oxide, and lanthanum oxide. Furthermore, the insulating ceramic is the ceramic having an electric conductivity equal to or less than 10−18Ω1 cm−1 at a room temperature (25° C.), such as alumina, mullite, spinel, and steatite. Furthermore, it is preferable to provide (i.e. laminate) a gas-permeable diffusion layer on a measured gas surface the sensor cell. In this case, it is possible to provide the diffusion layer on a surface opposed (i.e. not facing) to the ceramic heater. Furthermore, it is possible to provide the diffusion layer between the sensor cell and the ceramic heater.
Furthermore, it is preferable that the second electrolytic layer occupies at least 10% of an entire volume of the solid electrolytic substrate. According to this arrangement, it is possible to greatly reduce the insulating ceramic content of the solid electrolytic substrate and accordingly it is possible to obtain satisfactory sensor output.
Furthermore, it is preferable that the insulating ceramic content of the first electrolytic layer is larger than an entire insulating ceramic content of the solid electrolytic substrate. According to this arrangement, the entire insulating ceramic content of the solid electrolytic substrate is smaller than the insulating ceramic content of the first electrolytic layer. Accordingly, the insulating ceramic content of the solid electrolytic substrate can be further reduced as a whole and accordingly it becomes possible to secure excellent ionic conductivity (i.e. electrolytic conductivity). According to this arrangement, the sensor cell can produce a sufficient sensor output.
Furthermore, it is preferable that the first electrolytic layer has the thickness in the range from 3 to 300 μm. According to this arrangement, it becomes possible to suppress warpage or exfoliation (or separation) occurring in the multilayered gas sensing element and also possible to secure a sufficient sensor output. When the thickness of the first electrolytic layer is less than 3 μm, it is difficult to sufficiently suppress warpage or exfoliation occurring in the multilayered gas sensing element. On the other hand, when the thickness of the first electrolytic layer is larger than 300 μm, it is difficult to obtain satisfactory sensor output.
Furthermore, it is preferable that the solid electrolytic substrate includes a third electrolytic layer at a position farthest from the ceramic heater, and the insulating ceramic content of the third electrolytic layer is smaller than the insulating ceramic content of the solid electrolytic substrate other than the first electrolytic layer. According to this arrangement, the thermal stress acting during the sintering operation can be decentralized, and accordingly it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element. Furthermore, the insulating ceramic content of the solid electrolytic substrate can be reduced as a whole, and accordingly satisfactory sensor output can be obtained.
Furthermore, it is preferable that the insulating ceramic content of the third electrolytic layer is equal to or less than 50 wt. %. According to this arrangement, it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element during the sintering operation, and accordingly it becomes possible to obtain satisfactory sensor output. When the insulating ceramic content of the third electrolytic layer exceeds 50 wt. %, the ionic conductive of the solid electrolytic substrate is lessened and accordingly it is difficult to obtain satisfactory sensor output.
Furthermore, it is preferable that the insulating ceramic content of the solid electrolytic substrate decreases with increasing distance from the ceramic heater. According to this arrangement, the thermal stress acting during the sintering operation can be decentralized, and accordingly it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element. Furthermore, the insulating ceramic content of the solid electrolytic substrate can be reduced as a whole, and accordingly it becomes possible to obtain satisfactory sensor output.
Furthermore, it is preferable that the insulating ceramic content of the first electrolytic layer is in the range from 10 to 80 wt. %. According to this arrangement, it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element, and accordingly it becomes possible to obtain satisfactory sensor output. When the insulating ceramic content of the first electrolytic layer is less than 10 wt. %, it is difficult to sufficiently reduce the difference of heat shrinkage factors of the solid electrolytic substrate and the heater substrate. Accordingly, it is difficult to sufficiently suppress warpage or exfoliation occurring in the multilayered gas sensing element. On the other hand, when the insulating ceramic content of the first electrolytic layer exceeds 80 wt. %, the ionic conductive of the solid electrolytic substrate is lessened and accordingly it is difficult to obtain satisfactory sensor output of the multilayered gas sensing element.
Furthermore, as a best mode for embodying the present invention, the inventors of this application provide a second multilayered gas sensing element including a sensor cell and a ceramic heater which are laminated integrally. The sensor cell has a solid electrolytic substrate containing an electrolytic component serving as a main component of an ionic conductive solid electrolyte. And, the ceramic heater has a heater substrate containing an insulating ceramic as a main component. The heater substrate of the second multilayered gas sensing element includes an electrolytic component containing layer at a position closest to the solid electrolytic substrate. And, the electrolytic component containing layer contains the electrolytic component serving as a main component of an ionic conductive solid electrolyte.
Moreover, according to the second multilayered gas sensing element of the present invention, it is preferable that the content of the electrolytic component in the electrolytic component containing layer is in a range from 2 to 40 wt. %. According to this arrangement, it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element while sufficiently securing the insulation properties of the heater substrate. When the content of the electrolytic component in the electrolytic component containing layer is less than 2 wt. %, it is difficult to sufficiently suppress warpage or exfoliation occurring in the multilayered gas sensing element. On the other hand, when the content of the electrolytic component in the electrolytic component containing layer exceeds 40 wt. %, it is difficult to sufficiently secure the insulation properties of the heater substrate. It will be difficult to obtain an accurate sensor output due to adverse influence of the current flowing in the ceramic heater.
Hereinafter, preferred embodiments of the present invention will be explained with reference to attached drawings.
A multilayered gas sensing element in accordance with a first embodiment of the present invention will be explained with reference to
The solid electrolytic substrate 21 includes a first electrolytic layer 211 and a second electrolytic layer 212. The first electrolytic layer 211, containing alumina, is disposed at a position closest to the ceramic heater 3. The alumina content of the second electrolytic layer 212 is smaller than the alumina content of the first electrolytic layer 211. The first electrolytic layer 211 has a thickness of 3 to 300 μm. The solid electrolytic substrate 21 has a thickness of 10 to 500 μm. Furthermore, the alumina content of the first electrolytic layer 211 is in the range from 10 to 80 wt. %. The alumina content of the second electrolytic layer 212 is less than 50 wt. % and is, as described above, smaller than the alumina content of the first electrolytic layer 211.
The alumina content can be measured by using an EPMA analyzing apparatus in the following manner.
First of all, preliminary measurement is performed to obtain characteristic X-ray intensities of standard samples (e.g., samples differentiated in the contents of alumina and zirconia) whose contents are already known.
Next, a measuring object sample (i.e. the multilayered gas sensing element 1) is subjected to the measurement of characteristic X-ray intensity.
More specifically, the multilayered gas sensing element 1 is cut along a surface normal to the longitudinal direction of the element to expose a cross-sectional surface as shown in
Hereinafter, the arrangement of the multilayered gas sensing element 1 in accordance with this embodiment will be explained in more detail.
As shown in
Furthermore, a heater pattern 32 having a heat-generating portion is formed in the heater substrate 31. The heater pattern 32 and the heater substrate 31 cooperatively arrange the ceramic heater 3. Furthermore, a gas-permeable porous diffusion layer 11 is formed on the measured gas side surface of the solid electrolytic substrate 21 so as to cover the measured gas side electrode 23. The porous diffusion layer 11 is a porous member containing zirconia as a main component. The ceramic heater 3, the sensor cell 2, and the porous diffusion layer 11 are integrally laminated in the order.
As shown in
Next, the functions and effects of this embodiment will be explained.
As shown in
Furthermore, the alumina content of the second electrolytic layer 212 is smaller than the alumina content of the first electrolytic layer 211. Accordingly, the solid electrolytic substrate 21 can reduce the alumina content as a whole and can secure excellent ionic conductivity (i.e. electrolytic conductivity). For example, as shown in
Furthermore, the first electrolytic layer 211 has the thickness of 3 to 300 μm. Accordingly, it becomes possible to suppress warpage or exfoliation occurring in the multilayered gas sensing element 1 and secure satisfactory sensor output. Furthermore, the alumina content of the first electrolytic layer 211 is in the range from 10 to 80 wt. %. Accordingly, it becomes possible to sufficiently reduce the difference of heat shrinkage factors of the solid electrolytic substrate 21 and the heater substrate 31. Thus, it becomes possible to sufficiently suppress warpage or exfoliation occurring in the multilayered gas sensing element 1 and also secure excellent ionic conductivity of the solid electrolytic substrate 21 to obtain satisfactory sensor output (refer to
As described above, this embodiment can provide an excellent multilayered gas sensing element capable of suppressing warpage or exfoliation and securing satisfactory sensor output.
The second embodiment of the present invention, as shown in
The third embodiment of the present invention, as shown in
According to the arrangement of the third embodiment, it becomes possible to decentralize the thermal stress acting during the sintering operation. Thus, the multilayered gas sensing element 1b of the third embodiment can suppress warpage or exfoliation. Furthermore, the multilayered gas sensing element 1b of the third embodiment can reduce the insulating ceramic content of the solid electrolytic substrate 21 as a whole, and accordingly can obtain satisfactory sensor output. The rest of the multilayered gas sensing element 1b is structurally identical with the multilayered gas sensing element 1 explained in the first embodiment.
The fourth embodiment of the present invention, as shown in
According to the arrangement of this embodiment, it becomes possible to reduce the difference of heat shrinkage factors of the solid electrolytic substrate 21 and the heater substrate 31. Thus, the multilayered gas sensing element 1c of the fourth embodiment can suppress warpage or exfoliation. Furthermore, when the zirconia content of the electrolytic component containing layer 311 is in the range from 2 to 40 wt. %, it becomes possible to suppress warpage or exfoliation of the multilayered gas sensing element 1c while sufficiently securing insulation ability of the heater substrate 31. Furthermore, this embodiment can bring the same functions and effects as those of the first embodiment.
The fifth embodiment of the present invention, as shown in
According to this arrangement, as shown in
The sixth embodiment of the present invention, as shown in
Furthermore, the second electrolytic layer 212 is disposed between the first electrolytic layer 211 and the third electrolytic layer 213. The first electrolytic layer 211, the second electrolytic layer 212, and the third electrolytic layer 213 have the same thickness equivalent to ⅓ of the entire thickness of the solid electrolytic substrate 21.
Furthermore, the alumina content of the second electrolytic layer 212 is equal to or less than 50 wt. %. Regarding the practical alumina content of the multilayered gas sensing element 1e according to this embodiment, the alumina content of the first electrolytic layer 211 can be set to 50 wt. %, the alumina content of the second electrolytic layer 212 can be set to 10 wt. %, and the alumina content of the third electrolytic layer 213 can be set to 2 wt. %. The rest of the multilayered gas sensing element 1e is structurally identical with the multilayered gas sensing element 1 explained in the first embodiment.
According to the arrangement of this embodiment, the thermal stress acting during the sintering operation can be decentralized. Thus, the multilayered gas sensing element 1e of the sixth embodiment can suppress warpage or exfoliation. Furthermore, the multilayered gas sensing element 1e of the sixth embodiment can reduce the alumina content of the solid electrolytic substrate 21 as a whole, and accordingly can obtain satisfactory sensor output. Furthermore, this embodiment can bring the same functions and effects as those of the first embodiment.
The seventh embodiment of the present invention, as shown in
The eighth embodiment of the present invention, as shown in
The solid electrolytic substrate 41 of the pump cell 4 includes a fourth electrolytic layer 411 containing alumina at the position closest to the ceramic heater 3. Furthermore, the solid electrolytic substrate 41 includes a fifth electrolytic layer 412 whose alumina content is smaller than that of the fourth electrolytic layer 411. For example, the thickness of the fourth electrolytic layer 411 is in the range from 3 to 300 μm. Alternatively, it is possible that the alumina content is uniform everywhere in the solid electrolytic substrate 41. The rest of the multilayered gas sensing element 1g is structurally identical with the multilayered gas sensing element 1 explained in the first embodiment.
According to the arrangement of this embodiment, the solid electrolytic substrate 41 of the pump cell 4 can possess the function of reducing the thermal stress and can sufficiently secure the pumping ability of the pump cell 4. Furthermore, this embodiment can bring the same functions and effects as those of the first embodiment.
FIGS. 12 to 19 show experimental data obtained in the evaluation tests for checking various characteristics of the multilayered gas sensing element in accordance with the present invention, according to which the alumina content was variously changed in three layers of the solid electrolytic substrate dissected in its thickness direction.
The solid electrolytic substrate 21 of the experimental multilayered gas sensing element 10 includes a first electrolytic layer 211, an intermediate electrolytic layer 214, and an external electrolytic layer 215 which are disposed or laminated in this order from a boundary facing to the ceramic heater 3. Each of these layers 211, 214, and 215 has a thickness equivalent to ⅓ of the entire thickness of the solid electrolytic substrate 21. More specifically, in the thickness direction of the solid electrolytic substrate 21, a region equivalent to ⅓ of the solid electrolytic substrate 21 from the boundary facing to the ceramic heater 3 is defined as the first electrolytic layer 211. A region equivalent to ⅓ of the solid electrolytic substrate 21 from the boundary facing to the porous diffusion layer 11 is defined as the external electrolytic layer 215. The remaining region of the solid electrolytic substrate 21, intervening between the first electrolytic layer 211 and the external electrolytic layer 215, is defined as the intermediate electrolytic layer 214.
Table 1 shows alumina contents in the first electrolytic layer 211, the intermediate electrolytic layer 214, and the external electrolytic layer 215 of respective samples 1 to 12 used in the evaluation tests. The samples 2-5 and 7-12 are experimental multilayered gas sensing elements according to the present invention. The samples 1 and 6 are experimental multilayered gas sensing elements according to the prior art.
The measured items in these evaluation tests include the amount of warpage, the probability of crack generation, and the sensor resistance of the multilayered gas sensing element 10 (refer to
More specifically, after finishing the sintering operation, the thickness of each tested element was measured at a portion where the thickness is largest. As shown in
Furthermore, the probability of crack generation in the multilayered gas sensing element 10 during the sintering operation was evaluated. To evaluate the crack generation probability, an insulation resistance between the measured gas side electrode 23 and the reference gas side electrode 24 of the sintered multilayered gas sensing element 10 was measured. When the insulation resistance is equal to or less than 500MΩ, it was regarded as indicating the presence of any crack in the sintered multilayered gas sensing element 10. A total of 100 samples were prepared for each test condition. And, the crack generation probability was obtained by counting the number of samples having caused any cracks among 100 samples.
As understood from
As apparent from the test data, the samples 7 to 12 according to the present invention have extremely small values in both the amount of warpage and the probability of crack generation. The samples 2 to 5 according to the present invention have smaller values in both the amount of warpage and the probability of crack generation, compared with the sample 1 according to the prior art.
Next, the sensor resistance of each sample was measured.
Regarding the measuring method, a constant voltage (e.g. 0.5 V) was applied between the measured gas side electrode 23 and the reference gas side electrode 24 of the multilayered gas sensing element 10 shown in
As understood from
Furthermore, the following analysis can be made based on the obtained test data.
As understood from
Next,
As understood from
Next,
As understood from
Number | Date | Country | Kind |
---|---|---|---|
2004-120683 | Apr 2004 | JP | national |