Sensor element and gas sensor

1. A sensor element comprising:

a laminate having a plurality of laminated oxygen ion conductive solid electrolyte layers and having therein a measurement-object gas flowing portion for introducing and flowing a measurement-object gas;

a measurement electrode disposed on an inner peripheral surface of the measurement-object gas flowing portion;

a measurement-object gas side electrode disposed in a portion of the laminate that is exposed to the measurement-object gas;

a reference electrode disposed in the laminate; and

a porous reference gas introduction layer having an entrance portion and a back portion and being formed of the same porous material throughout for introducing a reference gas as a reference for detecting a specific gas concentration of the measurement-object gas, the porous reference gas introduction layer extending from an entrance of the entrance portion toward a back end at the back portion and allowing the reference gas to flow to the reference electrode disposed at the back portion,

wherein the entrance portion and the back portion of the porous reference gas introduction layer are divided by a partition position defined to be in a range of 50 to 95% of a full length of the porous reference gas introduction layer from the entrance toward the back end at the back portion, the reference electrode is accommodated in the back portion, and the diffusion resistance Rb of the entrance portion is higher than the diffusion resistance Ra of the back portion so that the diffusion resistance of the porous reference gas introduction layer around the reference electrode is less than the diffusion resistance of the porous reference gas introduction layer at the entrance portion to limit outflow of the reference gas from around the reference electrode,

wherein the diffusion resistance Ra is not less than 1000 [/cm] and not more than 5500 [/cm] and the diffusion resistance Rb is not less than 5400 [/cm] and not more than 50000 [/cm], and

wherein the diffusion resistance ratio Ra/Rb is not less than 0.020 and not more than 0.519.

2. The sensor element according to claim 1 , wherein the porous reference gas introduction layer is a layer formed of the porous material having a predetermined thickness, and is formed such that its width increases from the entrance toward the back portion.

3. The sensor element according to claim 2 , wherein both of the back portion and the entrance portion are rectangular in a plan view and a width of a rectangle of the back portion is larger than a width of a rectangle of the entrance portion.

4. A gas sensor comprising the sensor element according to claim 1 .

5. The gas sensor according to claim 4 , comprising

a measurement pump cell that includes the measurement electrode and the measurement-object gas side electrode and a variable power supply generating a first control voltage to maintain a voltage between the measurement electrode and the reference electrode at a constant value;

a reference gas regulation pump cell including a power supply that applies a second control voltage between the reference electrode and the measurement-object gas side electrode to pump in oxygen to around the reference electrode;

the measurement pump cell being configured to detect a value of a pump current,

wherein the specific gas concentration in the measurement-object gas is calculated based on the value of the pump current.