1. Field of the Invention
The present invention relates to a laminated gas sensor element, a gas sensor equipped with the laminated gas sensor element, and a method for manufacturing the laminated gas sensor element.
2. Description of the Related Art
Conventionally, known gas sensors include those for detecting a particular gas component contained in exhaust gas or for measuring the concentration of the particular gas component. Some of these gas sensors employ a laminated gas sensor element configured such that a pair of electrodes are provided on a solid electrolyte body. The solid electrolyte body of zirconia or the like used in the gas sensor element becomes active at a high temperature of 300° C. or higher. Usually, the gas sensor element is used in an activated state upon heating by a heater laminated on the solid electrolyte body. In this case, adhesion, to the gas sensor element, of a water or oil droplet contained in a gas to be measured (hereinafter, also referred to as adhesion of water) may generate a crack in the gas sensor element due to thermal shock. According to a known technique for solving this problem, a distal end portion (detecting portion) of the gas sensor element which is exposed to a gas to be measured is protected with a porous protection layer. For example, Patent Document 1 discloses a technique for restraining the generation of a crack at corner portions of a gas sensor element by increasing the thickness of a protection layer at the corner portions, which are susceptible to the generation of a crack.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2003-322632
3. Problems to be Solved by the Invention
However, even in the case of a laminated gas sensor element whose detecting portion is protected with a porous protection layer as in the case of Patent Document 1, the protection layer may fail to provide sufficient protection. Namely, adhesion thereto of a larger amount of water droplets or oil droplets contained in a gas to be measured (adhesion of water), potentially results in the generation of a crack in the laminated gas sensor element due to thermal shock.