Sensor element, gas sensor, and production method for sensor element

A production method for producing a sensor element, the sensor element including

• a detection element portion provided with at least one cell having a solid electrolyte and a pair of electrodes disposed on the solid electrolyte,
• a measurement chamber faced by one electrode of the pair of electrodes, and
• a diffusion resistance portion through which a gas to be measured is introduced from outside the sensor element into the measurement chamber,
• the production method comprising:
• a coating liquid preparation step of preparing a coating liquid in which ceramic particles and a pore-forming material are mixed;
• an application step of applying the coating liquid so as to be in direct contact with the diffusion resistance portion and so as to cover the detection element portion, at an outer surface of the sensor element;
• and
• a porous protection layer formation step which comprises drying and sintering the applied coating liquid and removing the pore-forming material, to form a porous protection layer that contains the ceramic particles which serve as a backbone and has pores formed at removed parts of the pore-forming material, wherein
• in the coating liquid, a diameter ratio R, represented by an average diameter D3 (nm) of the pore-forming material/a particle diameter D2 (nm) at which an accumulated number of the ceramic particles accounts for 50%, is not greater than 100, and D3 is greater than D2.