Method for producing gas sensor element, gas sensor element, and gas sensor

1. A method for producing a gas sensor element extending in a direction of an axial line thereof and having a detection portion disposed on a front end side thereof, the detection portion being covered by, as a porous protection layer, an inner protection layer that is porous and an outer protection layer that is porous and that has a porosity different from a porosity of the inner protection layer, in this order, the method comprising:

a first dipping step of dipping the front end side of the gas sensor element into a first slurry for the inner protection layer, to form a first coating film made of the first slurry on a front end surface and a peripheral surface of the gas sensor element;

a drying step of drying and solidifying the first coating film;

a second dipping step of dipping, without removing the first coating film, the front end side of the gas sensor element into a second slurry for the outer protection layer, to form a second coating film made of the second slurry on an entirety of a surface of the solidified first coating film; and

a scraping-off step of performing scraping-off on the second coating film so as not to scrape-off the solidified first coating film, to remove a part of the second coating film.

2. The method for producing the gas sensor element as claimed in claim 1 , wherein the scraping-off step is performed such that a surface of the second coating film having been subjected to the scraping-off becomes a flat surface.

3. The method for producing the gas sensor element as claimed in claim 1 , wherein

the scraping-off step includes:

a peripheral surface scraping-off step of scraping-off and removing a peripheral part of the second coating film; and

a front end surface scraping-off step of scraping-off and removing part of a front end side of the second coating film, and

the front end surface scraping-off step being performed after the peripheral surface scraping-off step.

4. The method for producing the gas sensor element as claimed in claim 2 , wherein

the scraping-off step includes:

a peripheral surface scraping-off step of scraping-off and removing a peripheral part of the second coating film; and

a front end surface scraping-off step of scraping-off and removing part of a front end side of the second coating film, and

the front end surface scraping-off step being performed after the peripheral surface scraping-off step.

5. A gas sensor element extending in a direction of an axial line thereof and having a plate shape that has opposing main surfaces, the gas sensor element comprising:

a detection portion at a front end side thereof; and

a porous protection layer configured to cover the detection portion, wherein

the porous protection layer comprises an inner protection layer and an outer protection layer that is porous, the outer protection layer having a porosity different from a porosity of the inner protection layer, and that covers the inner protection layer,

the inner protection layer has a maximum thickness region along the direction of the axial line, and has a reduced thickness from the region of maximum thickness toward a front end side and a rear end side of the inner protection layer,

the outer protection layer has a flat front end surface and flat side surfaces, and

when viewed in a direction perpendicular to the main surfaces, a rear end edge on a radially outermost side of the outer protection layer has a shape in which a center portion of the rear end edge in a width direction of the gas sensor element is recessed to the front end side with respect to a rearmost end.

6. A gas sensor, comprising:

the gas sensor element according to claim 5 ;

a metal shell which holds the gas sensor element; and

a protector mounted to a front end portion of the metal shell.

7. The gas sensor according to claim 6 , wherein

the protector covers a front end portion of the gas sensor element, protruding from a front end of the metal shell, and has a plurality of gas introduction holes.