ELECTROCHEMICAL DEVICE

Abstract: An electrochemical device e.g. an oxygen sensor, has a plurality of stacked solid electrolyte bodies (56,72,86) and a pair of electrodes (58,64) in contact with said solid electrolyte bodies. A heating element (78) and conductors (80) connected to said heating element are sandwiched by first and second insulation layers (82,84). To improve accuracy of the device, the insulation layers (82,84) are porous and are sandwiched by said solid electrolyte bodies (72,86) so that said heating element (78) and conductors (80) are isolated from a gaseous fluid, such as a gas to be measured by said device.

Description: The present invention relates generally to an electrochemical device, and more particularly to a device which comprises a laminated electrochemical cell having a plurality of stacked solid electrolyte bodies and a heating element. There are known various electrochemical devices using solid electrolytes, for example oxygen sensors to detect the oxygen concentration of an exhaust gas from internal combustion engines of automotive vehicles. Typical examples of such oxygen sensors include: an oxygen sensor which comprises a tubular body of oxygen-ion conductive solid electrolyte such as zirconia ceramics, and electrodes of platinum or the like provided on both inner and outer surfaces of the tubular solid electrolyte body, and which operates to determine the oxygen concentration according to the principle of an oxygen concentration cell; an oxygen sensor similar to the above, which incorporates a heater which enables the sensor to operate at a relatively low exhaust gas temperature; and a so-called "lean-­ burn" sensor suitable for detecting the oxygen concentration of an exhaust gas which is produced at lean air-fuel ratios, i.e. an exhaust gas having a larger oxygen content. Also known in the art are electrochemical devices such as sensing and pumping elements for detecting hydrogen, nitrogen, carbon dioxide gas, etc. These sensing and pumping elements operate according to the principle of a concentration cell, like the oxygen sensor indicated above. In such electrochemical devices, solid electrolyte materials have been generally used in the form of a tubular body which has an elongate bore closed at its one end. In recent years, however, attempts have been made to replace the tubular solid electrolyte body with a solid electrolyte body of planar shape, as disclosed in US Patent No. US-A-4,334,974, in view of the relative difficulty of production and high cost of manufacture of the solid electrolyte bodies of tubular shape, and from the standpoint of easy assembling of parts with the solid electrolyte body. When such planar solid electrolyte bodies are employed, suitable electrodes are disposed in contact with the surfaces of the planar body of solid electrolyte, and the electrolyte bodies and other parts are assembled into a laminar structure constituting an electrochemical cell. It is known to provide a heater element in the electrochemical device, to heat the device rapidly to operating temperature. Usually the heater is at one side of the laminated stack and may be electrically isolated from the solid electrolyte bodies by an insulating layer of the stack (US-A-­ 4,450,065). US-A-4,505,807 shows a heater element embedded in a laminated stack between two electrochemical cells and electrically isolated from the solid electrolyte layers of the cells by respective insulating layers. It is an object of the present invention to provide an electrochemical device of laminar or integrally laminated type, which suffers low noise and measurement errors caused by the conductors embedded in the solid electrolyte, and which is thus improved in response. The present invention is set out in claim 1. Preferably, the electrical insulation layers consist essentially of alumina or spinel An embodiment of the present invention will be given in the following description to be read in connection with the accompanying drawing in which: Fig. 1 is an exploded perspective view of a sensing element forming one embodiment of the electrochemical device of the invention, in the form of an oxygen sensor; and Fig. 2 is a schematic view in cross section taken along line 2-2 of Fig. 1. The oxygen sensor of Figs. 1 and 2, which is an embodiment of the electrochemical device of the invention, is a sensor to measure an exhaust gas which is produced in combustion of an air-fuel mixture at the stoichiometric ratio and which therefore has a low oxygen content. This oxygen sensor includes a planar solid electrolyte body 56 which is provided on its outer surface with a measuring electrode 58. The electrode 58 is connected via a lead 60 to an external device and exposed to the measurement gas through a porous protective layer 62 which covers the electrode 58. A reference electrode 64 with a lead 66 is disposed on the inner surface of the planar solid electrolyte body 56. A substantive portion of the lead 66, that is, the portion of the lead 66 which is exposed to an elevated temperature, is sandwiched between two preferably porous electrical insulation layers 68 and 70. On the side of the planar solid electrolyte body 56 on which the reference electrode 64 is disposed, a U-shaped solid electrolyte body 72 and a planar solid electrolyte body 86 are superposed such that the U-shaped solid electrolyte body 72 is sandwiched between the two planar solid electroyte bodies 56 and 86. The three solid electrolyte bodies 56,72 and 86 cooperate to define a cavity 76 which is open at its one end to the ambient atmosphere. The reference electrode 64 is positioned so as to be exposed to a reference gas such as an ambient atmosphere introduced into the cavity 76. In the above arrangement, the measuring electrode 58 exposed to the measurement gas through the protective layer 62, and the reference electrode 64 exposed to the reference gas, form an electrochemical cell which operates as an oxygen concentration cell in response to a difference in oxygen concentration between the measurement and reference gases. On the side of the planar solid electrolyte body 86 facing the U-shaped solid electrolyte body 72, there is provided a heating element 78 with leads 80 sandwiched by a pair of porous ceramic electrical insulation layers 82,84 which protect and electrically insulate the heating element 78. The heater heats a portion of the planar solid electrolyte body 56 adjacent to the electrodes 58,64 to an elevated temperature so as to maintain a sufficient oxygen-ion conductivity of that portion of the solid electrolyte body 56. The end portions of the leads 80 remote from the heating element 78 are exposed for electric connection to an external power source. The insulation layers 82,84 sandwiching the heating element 78 and the leads 80 are themselves sandwiched by the solid electrolyte body 86 and the previously indicated U-shaped solid electrolyte body 72 having the cavity 76. As the drawing shows, the layers 82,84 are narrower in width than the corresponding portions of the sandwiching layers 72,86 and thus do not extend to the edges of the sandwiching layers 72,86. If the heating element 78 were directly embedded between the solid electrolyte bodies 72 and 86, a portion of the electric current flowing in the heating element 78 and the leads 80 would leak through the solid electrolyte bodies 72,86 when the heating element 78 is energized to heat the oxygen sensor, more precisely a portion of the planar solid electrolyte body 56 adjacent to the measuring and reference electrodes 58 and 64. In the present embodiment, however, not only the heating element 78 but also the leads 80 are electrically insulated relative to the solid electrolyte bodies 72 and 86 by the upper and lower electrical insulation layers 82 and 84, respectively. In other words, the electrolyte bodies 72,86 are separated or spaced away from the heating element 78 and the leads 80 by the insulation layers 82,84 and thus kept free from electrolysis due to energization of the heating element 78. Consequently, otherwise possible deterioration and cracking troubles with the solid electrolyte bodies 72,86 due to the electrolysis will be effectively prevented. Further, the insulation layer 82 serves to hinder a leakage flow of current from reaching the reference electrode 64 through the solid electrolyte body 72, thus preventing an unfavourable influence of such a leakage current upon the reference electrode 64. Although the electrochemical devices according to the invention have been illustrated in the form of a sensor having an oxygen concentration cell, various other forms of the electrochemical device may be constructed according to the invention. Further, the invention may be applied to an electrochemical pump, polarographic electrochemical cell, and the like.

Claims: 1. An electrochemical device comprising a plurality of stacked solid electrolyte bodies (56,72,86); a pair of electrodes (58,64) in contact with said solid electrolyte bodies; a heating element (78) and conductors (80) connected to said heating element; first and second insulation layers (82,84) sandwiching said heating element and conductors therebetween; characterized in that said first and second insulation layers (82,84) are porous and are sandwiched by said solid electrolyte bodies (72,86) so that said heating element (78) and conductors (80) are isolated from a gaseous fluid to which the device is exposed in use. 2. An electrochemical device according to claim 1, wherein said porous first and second insulation layers (82,84) consist essentially of alumina or spinel. 3. An electrochemical device according to claim 1 or claim 2 wherein said porous first and second insulation layers (82,84) are embedded within the solid electrolyte bodies (72,86) spaced from the lateral edges of the solid electrolyte bodies (72,86). 4. An electrochemical device according to any one of claims 1 to 3 which is an oxygen sensing device.

Inventors: MASE, SYUNZO, SOEJIMA, SHIGEO
Applicants: NGK INSULATORS, LTD.
Application year: 1984
Main classification: G01N27
IPC codes: G01N27, C04B35, H01M8