Sensor element

A sensor element included in a gas sensor detecting a predetermined gas component in a measurement gas, comprising:

• an element base including:
• an elongated planar ceramic body made of an oxygen-ion conductive solid electrolyte, and having a gas inlet at the one end portion thereof;
• at least one internal chamber located inside the ceramic body, and communicating with the gas inlet under predetermined diffusion resistance;
• at least one electrochemical pump cell including an outer pump electrode located at a location other than the at least one internal chamber in the ceramic body, an inner pump electrode located to face the at least one internal chamber, and a solid electrolyte located between the outer pump electrode and the inner pump electrode, the at least one electrochemical pump cell pumping in and out oxygen between the at least one internal chamber and an outside;
• a heater buried in a predetermined range on a side of the one end portion of the ceramic body;
• and
• an inner leading-end protective layer made up of a porous material having a porosity of 30% or more and 65% or less on at least two main surfaces facing each other on the one end portion;
• an intermediate leading-end protective layer made up of a porous material having a porosity of 25% or more and 80% or less, which is equal to or smaller than the porosity of the inner leading-end protective layer;
• and
• an outer leading-end protective layer surrounding the element base on an outermost periphery on the one end portion of the sensor element, and made up of a porous material having a porosity of 15% or more and 30% or less, which is smaller than the porosity of the intermediate leading-end protective layer, wherein
• the inner leading-end protective layer, the intermediate leading-end protective layer, and the outer leading-end protective layer are laminated in this order on the two main surfaces at least in a range in which the at least one inner chamber is provided in the element base,
• the outer leading-end protective layer is in contact with the inner leading-end protective layer in a range in which the at least one inner chamber is not provided in the element base, and
• a difference of porosity between the inner leading-end protective layer and the outer leading-end protective layer is equal to or larger than 10% and equal to or smaller than 50%.