1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor adapted for detecting the concentration of a particular gas contained in, for example, combustion gas or exhaust gas of a combustor, an internal combustion engine, etc.
2. Description of the Related Art
Conventionally, a gas sensor is used for detecting the concentration of a particular component (oxygen, etc.) in exhaust gas of an internal combustion engine. The gas sensor has a gas sensor element provided therein. The gas sensor element comprises a laminate of a detection element and a heater. The detection element has at least one cell composed of a solid electrolyte body and a pair of electrodes disposed on the solid electrolyte body. The heater has an insulating ceramic body and a heat-generating element provided in the insulating ceramic body and is adapted to generate heat by passing electric current therethrough. Meanwhile, the gas sensor element may be exposed to poisoning substances, such as silicon and phosphorus, contained in exhaust gas and may be subjected to adhesion of water droplets contained in exhaust gas. Thus, in order to trap poisoning substances and to prevent direct contact of water droplets with the gas sensor element, the outer surface of the gas sensor element is covered with a porous protection layer. That is, in the laminate, a forward end portion that is exposed to a gas-to-be-measured is entirely covered with the porous protection layer.
Also, the following technique has been developed: the porous protection layer consists of two layers; namely, an upper layer and a lower layer. By means of the lower layer having a porosity higher than that of the upper layer, an anchor effect brings the upper layer into close contact with the lower layer, whose surface is roughened by pores (refer to Patent Documents 1 and 2).
The above-mentioned laminate has a measuring chamber which one of the above-mentioned pair of electrodes faces. The gas-to-be-measured is introduced into the measuring chamber. A diffusion resistor intervenes between the measuring chamber and the ambient atmosphere and is adapted to adjust the diffusion rate of the gas-to-be-measured into the measuring chamber. Thus, the lower layer is in direct contact with the diffusion resistor.