Gas sensor

1. A gas sensor comprising:

a detection element, the detection element including a solid electrolyte body having a plate form, and a pair of electrodes formed on opposite sides of the solid electrolyte body, one of the paired electrodes being a sensing electrode which is exposed to a gas-to-be-measured, and the other electrode being a reference electrode which functions as an oxygen reference electrode, wherein:

the reference electrode is connected to a porous reference electrode lead extending along a surface of the solid electrolyte body;

the reference electrode lead contains a noble metal as a main component and also contains a ceramic material;

the reference electrode lead has a specific resistance lower than that of the reference electrode; and

the ceramic material has, in a sintered state, a mean primary grain size greater than that of the noble metal.

2. The gas sensor as claimed in claim 1 , wherein the reference electrode lead has a gas permeation amount which is 66.4% or more than that of the reference electrode.

3. The gas sensor as claimed in claim 1 , which further comprises a heat-generating element for heating the detection element.

4. A gas sensor comprising:

a detection element for detecting a particular gas contained in a gas-to-be-measured, and

a porous heat-generating element for heating the detection element, the heat-generating element being formed on an insulation layer, wherein:

the heat-generating element is connected to a porous heat-generating element lead extending along a surface of the insulation layer;

the heat-generating element lead contains a noble metal as a main component and also contains a ceramic material;

the heat-generating element lead has a specific resistance lower than that of the heat-generating element; and

the ceramic material has, in a sintered state, a mean primary grain size greater than that of the noble metal.

5. The gas sensor as claimed claim 4 , wherein the heat-generating element lead has a gas permeation amount which is 20% or more than that of the heat-generating element.

6. A gas sensor comprising:

a detection element, the detection element including a solid electrolyte body having a plate form, and a pair of electrodes formed on opposite sides of the solid electrolyte body, and

a porous heat-generating element for heating the detection element, the heat-generating element being formed on an insulation layer, one of the paired electrodes being a sensing electrode which is exposed to a gas-to-be-measured, and the other electrode being a reference electrode which functions as an oxygen reference electrode, wherein:

the reference electrode is connected to a porous reference electrode lead extending along a surface of the solid electrolyte body;

the heat-generating element is connected to a porous heat-generating element lead extending along a surface of the insulation layer;

each of the reference electrode lead and the heat-generating element lead contains a noble metal as a main component and also contains a ceramic material;

the reference electrode lead has a specific resistance lower than that of the reference electrode, and the heat-generating element lead has a specific resistance lower than that of the heat-generating element; and

in each of the reference electrode lead and the heat-generating element lead, the ceramic material has, in a sintered state, a mean primary grain size greater than that of the noble metal.

7. The gas sensor as claimed in claim 6 , wherein the reference electrode lead has a gas permeation amount which is 66.4% or more than that of the reference electrode, and the heat-generating element lead has a gas permeation amount which is 20% or more than that of the heat-generating element.

8. The gas sensor as claimed in claim 1 , wherein the noble metal is any one species selected from the group consisting of platinum, palladium, a platinum-palladium alloy and a platinum-gold alloy.

9. The gas sensor as claimed in claim 1 , wherein the noble metal has a mean primary grain size of 1.5 μm or less in a sintered state.

10. The gas sensor as claimed in claim 1 , wherein the ceramic material has a mean primary grain size of 1.5 μm or less in a sintered state.

11. The gas sensor as claimed in claim 1 , wherein the reference electrode is connected at a connection point to the porous reference electrode, the reference electrode overlapping an end of the porous reference electrolyte lead at the connection point.

12. The gas sensor as claimed in claim 1 , wherein the ceramic material has a mean primary grain size that is 1.67 times or more than that of the noble metal in a sintered state.

13. The gas sensor as claimed in claim 4 , wherein the heat-generating element is connected at a connection point to the porous heat-generating element lead, the heat-generating element overlapping an end of the porous heat-generating element lead at the connection point.

14. The gas sensor as claimed in claim 4 , wherein the ceramic material has a mean primary grain size that is 1.6 times or more than that of the noble metal in a sintered state.

15. The gas sensor as claimed in claim 6 , wherein the reference electrode is connected at a connection point to the porous reference electrode lead, the reference electrode overlapping an end of the porous reference electrode lead at the connection point.

16. The gas sensor as claimed in claim 6 , wherein the ceramic material contained in the reference electrode lead has a mean primary grain size that is 1.67 times or more than that of the noble metal in a sintered state.

17. The gas sensor as claimed in claim 6 , wherein the heat-generating element is connected at a connection point to a porous heat-generating element lead, the heat-generating element overlapping an end of the porous heat-generating element lead at the connection point.

18. The gas sensor as claimed in claim 6 , wherein the ceramic material contained in the heat-generating element lead has a mean primary grain size that is 1.6 times or more than that of the noble metal in a sintered state.