The present invention relates in general to an electrochemical device and a method of manufacturing the same. More particularly, the invention is concerned with an improved method of producing an electrochemical device having a laminar structure which includes an electrochemical cell using a planar solid electrolyte body, and is also concerned with an electrochemical device produced by such an improved method.
There have been known various electrochemical devices using solid electrolyte, for example, oxygen sensors to detect the oxygen concentration of an exhaust gas emitted from internal combustion engines of automotive vehicles. The 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 sensing and pumping devices for detecting or controling hydrogen, nitrogen, carbon dioxide gas, etc.
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, it has been attempted to replace the tubular solid electrolyte body with a solid electrolyte body of planar shape, as disclosed in U.S. Pat. No. 4,334,974, in view of relatively low productivity 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 a layer or layers of solid electrolyte, and the electrolyte layers and other parts are assembled into a laminar structure constituting an electrochemical cell or cells.
In this type of electrochemical device wherein planar or plate-like layers of solid electrolyte are laminated, an electrically insulating porous protective layer of ceramics may be disposed on one side of a planar laminar electrochemical cell to protect a measuring electrode against direct exposure to an external measurement gas, or an electrically insulating layer may be interposed between the other side of the laminar electrochemical cell and a planar electrical heater to heat the cell (more precisely, its detecting portion adjacent the electrodes), in order to electrically insulate the electrical heater. In the case where such a porous protective or insulating layer is disposed on only one side of the electrochemical cell, however, the generally planar electrochemical device tends to warp or buckle due to a difference in shrinkage during firing between the solid electrolyte material of the cell and the ceramic material of the insulating layer. For example, the electrochemical device using such a protective or insulating layer may warp such that the protective or insulating layer is outwardly convex. Thus, the conventional electrochemical device of the type described above suffers a problem of warpage during manufacture.