Prismatic ceramic heater for heating gas sensor element, prismatic gas sensor element in multilayered structure including the prismatic ceramic heater, and method for manufacturing the prismatic ceramic heater and prismatic gas sensor element

1. A laminated prismatic gas sensor element having a substantially rectangular cross section and comprising a ceramic heater which includes a heating resistor embedded in ceramic, and at least one solid electrolyte ceramic layer partially constituting a sensor cell having two electrodes, said gas sensor element being characterized in that at least part of a longitudinally extending edge portion having an angle of about 90 degrees of said gas sensor element, said part being located in the vicinity of said heating resistor, is coated with a porous protective layer having a porosity of 15%-65% and a thickness not less than 20 μm and is adapted to prevent cracking induced by contact with water.

2. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said electrolyte ceramic layer is made of an oxygen-ion-conductive zirconia ceramic.

3. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said porous protective layer is made of a porous ceramic having a porosity of 30%-60% and has a thickness of 20-500 μm.

4. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said porous protective layer is formed by firmly fixing a ceramic powder by firing to a surface, including an edge in the vicinity of said heating resistor, of said longitudinally extending edge portion of said gas sensor element, and the porous protective layer has a curved surface of a curvature radius not less than 10 μm.

5. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said porous protective layer comprises two or more porous layers.

6. The laminated prismatic gas sensor element as claimed in claim 5 , wherein said porous protective layer comprises two or more porous layers of different porosities.

7. The laminated prismatic gas sensor element as claimed in claim 6 , wherein said porous protective layer comprises a bottom layer directly formed on said gas sensor element and a surface layer exposed to an ambient atmosphere, and the porosity of said surface layer is lower than that of said bottom layer.

8. The laminated prismatic gas sensor element as claimed in claim 7 , wherein the porosity of said bottom layer of said porous protective layer is at least twice as great as that of a layer located on or above said bottom layer.

9. The laminated prismatic gas sensor element as claimed in claim 5 , wherein an average pore diameter of said bottom layer of said porous protective layer is at least twice as great as that of a layer located on or above said bottom layer.

10. The laminated prismatic gas sensor element as claimed in claim 1 , wherein at least a portion of said porous protective layer is firmly fixed to an edge portion of said gas sensor element through simultaneous firing and another portion of said porous protective layer is fired at a temperature lower than that of said simultaneous firing.

11. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said ceramic heater is configured such that a heating resistor formed from a mixture of a noble metal and a ceramic is embedded in a ceramic laminate formed predominantly from alumina.

12. The laminated prismatic gas sensor element as claimed in claim 1 , wherein an ion-migration-preventing electrode for preventing deterioration of said heating resistor is embedded in said ceramic, an electric potential of said ion-migration-preventing electrode is equal to or lower than that of said heating resistor, and said ion-migration-preventing electrode is disposed between said heating resistor and said porous protective layer.

13. The laminated prismatic gas sensor element as claimed in claim 1 , wherein said porous protective layer is formed on an end portion of said prismatic gas sensor element so as to be exposed to exhaust gas, in the vicinity of said heating resistor, and said porous protective layer is not formed on at least the other end portion of said prismatic gas sensor element.

14. A gas sensor comprising a laminated prismatic gas sensor element as claimed in claim 13 and a sensor housing accommodating said prismatic gas sensor element, wherein said prismatic gas sensor element is fixedly supported by said sensor housing at a position where said porous protective layer is not formed.