Gas sensor

Abstract: An oxygen sensor comprises a planar sensor element (12) which generates electromotive forces or varies an electric resistance between electrodes formed on opposite surfaces of the planar sensor element depending upon concentrations of oxygen in exhaust gases, a metallic housing member (10) for housing the sensor element therein, a ceramic powder (30) placed around the planar sensor element in the metallic housing member and adapted to fix the planar sensor element and form a gas-seal section and ceramic supporters (28) for compacting the powder under a given pressure therebetween. In order to reduce loosening of the powder (30) by vibration, a portion of at least one of the ceramic supports (28) which contacts the ceramic powder is provided with a projection (16).

Description: GAS SENSOR The present invention relates to a gas sensor, such as an oxygen sensor, which comprises a planar sensor element, used for example generating electromotive forces between electrodes or varying an electric resistance between electrodes depending upon the concentration of oxygen in exhaust gases, a metallic housing member for housing the sensor element, ceramic powder in the metallic housing member and to fix the planar oxygen sensor element and forming a gas-seal, and ceramic supporters for compacting the ceramic powder under a given pressure. A known oxygen sensor is shown in Fig. 5. As shown in Fig. 5a, a ceramic powder 30 such as talc is filled between a metallic cap 10 and a sensor element 12, the sensor element 12 is fixed by compacting the ceramic powder under pressure, and air is used as a reference oxygen atmosphere. In such an oxygen sensor, the air as the reference oxygen atmosphere is isolated, by means of the compacted ceramic powder 30, from a gas to be measured. Ceramic supporters 28: 28a, 28b and 28c are used for forming the gas-seal by compacting the ceramic powder such as talc under pressure, and as shown in a perspective view of Fig. 5b by way of example, that portion of each of the ceramic supporters which contacts the ceramic powder 30 such as talc is flat. However, in the oxygen sensor of this construction, when the sensor undergoes heating, a gap is formed between the lower end surface of the ceramic supporter 28a and the cap 10 and/or between the upper end surface of the ceramic supporter 28a and the talc 30a, due to difference in thermal expansion between the cap 10 and the ceramic supporter 28a. At that time, when vibrations are applied to the sensor, the ceramic supporter 28a vibrates to vibrate the sensor element 12. As a result, the sensor element is vibrated, and fixing forces and the gastightness between the sensor element and the talc are deteriorated. US-A-4818363 shows a sensor in which powder is held between two ceramic sealing elements having opposed conical recesses, through which the sensor element passes. The sealing elements are pressed together by a spring. It is an object of the present invention to solve the above-mentioned problems, and to provide a gas sensor, such as an oxygen sensor, in which loosening of the powder will not occur even during a long time of use, that is, in which fixing forces of the sensor element and gastightness of the gas-seal are maintained. The gas sensor according to the present invention is as set out in claim 1. In the construction of the invention, since the projection is provided on that portion of the ceramic supporter which contacts the ceramic powder, the projection is radially pressed with the ceramic powder. Therefore even when vibrations and/or heat cycling are applied to the sensor, the ceramic supporter will not vibrate relative to the sensor, so that the sensor element is not vibrated. Accordingly, almost no loosening of the ceramic powder occurs. That is, deterioration in the fixing forces of the sensor element and the gastightness of the gas-seal is decreased. Although a plurality of the ceramic supporters are provided, the above function of the invention can be exhibited by providing at least one of the ceramic supporters with the projection. The greater the number of the ceramic supporters on which the projection is provided, the smaller the deterioration in the fixing forces and the gastightness. Needless to say, the maximum effect can be obtained when all the ceramic supporters are provided with such projections. These and other optional features and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that some modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Figs. 1a and 1b are a partially sectional view of a structural example of an oxygen sensor according to the present invention and a perspective view of a ceramic supporter thereof, respectively; Fig. 2 is a graph showing the relationship between gastightness and testing time under vibrating-heating cycle conditions; Fig. 3 is a partially sectional view of another structural example according to the present invention; Figs. 4a and 4b are perspective views of other ceramic supporters useful in the present invention; and Fig. 5 is a structural example of a known oxygen sensor. Figs. 1a and 1b are a partially sectional view of one structural example according to the present invention and a perspective view of the ceramic supporter. In the oxygen sensor 20 according to the present invention shown in Figs. 1a and 1b, a planar sensor element 12 is fixed in a cylindrical metallic cap 10 through talc 30: 30a and 30b filled between ceramic supporters 28: 28a, 28b and 28c. The talc 30: 30a and 30b forms the gas-seal. In this oxygen sensor, as shown in Fig. 1b, a projection 16 is formed on that portion of the ceramic supporter 28a which contacts talc, and the sensor element is passed through a central portion of the projection. The sensor element 12 is fixed with talc 30 through compacting the talc 30 by means of the ceramic supporters 28 under given pressure. On the other hand, a rubber plug 14 is fixed, by caulking the cap 10, to an upper open end side opposed to the side on which the cap 10 is fixed to the housing 4 through a gastight ring 11. Lead wires 13 are inserted through the rubber plug 14. Thereby, the cap 10 is sealed. Ends of the lead wires 13 are electrically connected to terminal electrodes of the oxygen sensor 12. When the thus constituted oxygen sensor 20 undergoes heat, a gap is formed between the lower end face of the ceramic supporter 28a and the cap 10 due to difference in thermal expansion between the cap 10 and the ceramic supporter 28a. However, since the projection 16 of the ceramic supporter 28a is fixed by the talc 30a while being pressed radially by the talc 30a, the ceramic supporter 28a will not vibrate relative to the oxygen sensor 20 and the sensor element 12 is not vibrated, even if the oxygen sensor receives vibrations. Accordingly, almost no deterioration in the fixing forces and gastightness between the sensor element and the talc due to vibrations of the sensor element 12 will occur. With respect to the known oxygen sensor of Fig. 5 and the oxygen sensor according to the present invention, deterioration in the gastightness of talc was examined in vibration-heat cycling test, and results are shown in Fig. 2. The vibration-heat cycling conditions were that a vibration frequency and a vibration acceleration were 50-250 Hz (swept from 50 to 250 Hz in 30 minutes) and 30-50 G, respectively, while the heat cycling was repeatedly effected between 900°C for 30 minutes and room temperature for 25 minutes. The gastightness of talc was examined by measuring an amount of air compressed at 4 kg/cm² passing through both the talc sections 30a and 30b. As is clear from Fig. 2, deterioration in the gastightness of talc due to vibrations and heat cycling of the oxygen sensor can remarkably be decreased by employing the structure in which the ceramic supporter is provided with the projection as in the present invention. Fig. 3 is a partially sectional view of another embodiment according to the present invention. A planar sensor element 12 is fixed in a metallic housing member 24 and a cylindrical metallic inner cylinder 26 with talc 30: 30a and 30b filled between ceramic supporters 28: 28a, 28b and 28c. The talc 30: 30a and 30b forms the gas-seal. The metallic inner cylinder is fixed to the housing by welding. Similarly to the first embodiment, a projection 16 is provided on the ceramic supporter 28a. In order to protect the sensor element 12 from the exterior environment, a metallic outer cylinder 32 is fitted to the outer peripheral portion of an upper annular projection 34 of the housing member 24, and the former is gastightly fixed to the entire outer periphery of the latter by welding. On the other hand, a rubber plug 38 is sealingly fixed to an upper open end side of the outer cylinder 32 which is opposed to the side of the outer cylinder 32 fitted to the housing member 24. Lead wires 36 are inserted through the rubber plug 38. The lead wires 36 are electrically connected to terminal electrodes of the sensor element 12 at their ends, respectively. Where the embodiment in Fig. 3 differs from that in Fig. 1 is that the lower end face of the ceramic supporter 28a is bevelled and contacts a corresponding surface of the metallic housing member 24. The present invention is not limited to the above-mentioned embodiments, but various modifications and variations can be made. For example, although only that portion of the ceramic supporter 28a which contacts the talc 30a is provided with the projection 16, it goes without saying that that portion of the other ceramic supporter or supporters which contacts the talc may be provided with such a projection. The projection provided on the ceramic supporter is not limited to that shown in Fig. 1b, and may be of the shapes shown in Figs. 4a and 4b. That is, it is possible to use a projection 16 concentric with the ceramic supporter 28 as shown in Fig. 4a, or opposed projections 16 formed by providing a recess to surround the central sensor element as shown in Fig. 4b. Further, in the above embodiments, although the planar sensor element is used in the above embodiments, needless to say, a cylindrical sensor element may be used. As is clear from the above-mentioned explanation, in the sensor of the present invention, since at least one of the ceramic supporters defining the gastightly sealed section is provided with the projection, the projection can radially be pressed through the ceramic powder, so that even when vibrations and/or heat cyclings are applied to the sensor, almost no loosening of the ceramic powder occurs. Consequently, a sensor free from deterioration in the fixing forces of the oxygen sensor element and the gastightness of the gas-sealed section even for a long time use can be obtained. The invention is not limited to oxygen sensors, but may be applied to other gas sensors.

Claims: A gas sensor comprising a planar sensor element (12) which has electrodes contacted by gases to sense oxygen concentration, a metallic housing member (10;24,26) for the sensor element extending in an axial direction parallel to the plane of the sensor, ceramic powder (30) placed around the sensor element in the metallic housing member to fix the planar sensor element and form a gas-seal, and ceramic supporters (28) for compacting the ceramic powder between them, wherein at least one of the ceramic supporters (28) is provided with at least one projection (16) at its end which contacts the ceramic powder, characterized in that said projection (16) has a surface which is parallel to said axial direction so that said surface is radially pressed by said ceramic powder. A gas sensor according to claim 1, wherein the ceramic supporters (28) each have an annular shape, a through hole is provided at a central portion of each of the ceramic supporters through which the sensor element (12) extends, and the projection (16) is formed around the through hole to surround the sensor element, said surface radially pressed by the ceramic powder being the outer surface of the projection (16).

Inventors: KATO NOBUHIDE, KATSU MASANORI NGK KITAKAZOKU, KATO, NOBUHIDE, KATSU, MASANORI, 45, NGK KITAKAZOKU APARTMENTS, 9,
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1990
Main classification: G01N27
IPC codes: G01N27