Sensor element of NOx sensor

A sensor element of a limiting current type NOx sensor, the sensor element comprising:

• a base part containing an oxygen-ion conductive solid electrolyte as a constituent material;
• a measurement internal space into which a measurement gas having an oxygen concentration adjusted in advance is introduced;
• and
• at least one pump cell including a measurement electrode disposed in the measurement internal space, an out-of-space pump electrode disposed at a location other than the measurement internal space, and a portion of the base part located between the measurement electrode and the out-of-space pump electrode, wherein
• the measurement electrode includes a noble metal, the oxygen-ion conductive solid electrolyte material, and pores, and includes:
• an upper layer consisting of the noble metal, the oxygen-ion conductive solid electrolyte material, and the pores;
• and
• a lower layer consisting of the noble metal and the oxygen-ion conductive solid electrolyte material,
• in the measurement electrode, a region boundary variation degree being a degree of variation of a boundary between a first region and a second region in a thickness direction of the sensor element is 0.5 μm or more and 6.5 μm or less, the first region being formed of the base part and the oxygen-ion conductive solid electrolyte material contiguous with the base part, the second region being occupied by the noble metal and the pores,
• a volume ratio of the oxygen-ion conductive solid electrolyte material in the upper layer is 20% to 40%, and
• a volume ratio of the oxygen-ion conductive solid electrolyte material in the lower layer is 50% to 60%.