Plural-cell gas sensor with heater

1. A plural-cell gas sensor including:

an oxygen pump cell comprising a first oxygen-ion conductive solid electrolyte ceramic layer and internal and external porous electrode layers sandwiching the first solid electrolyte ceramic layer;

an oxygen monitor cell comprising a second oxygen-ion conductive solid electrolyte ceramic layer and inner and reference electrode layers sandwiching the second oxygen-ion conductive solid electrolyte ceramic layer;

an internal space defined between the oxygen pump cell layer and the oxygen monitor cell;

a gas-diffusion inlet through which a gas under measurement enters the internal space;

a heater substrate comprising first and second insulating ceramic layers and a heating resistor embedded in the insulating ceramic layers; and

a thermal conductive porous ceramic layer sandwiched by the oxygen pump cell and the heater substrate, the first insulating ceramic layer arranged closer to the thermal conductive porous ceramic layer than the second insulating ceramic layer;

wherein the thermal conductive porous ceramic layer adjoins the external porous electrode of the oxygen pump cell and the insulating ceramic layer of the heater substrate such that oxygen inside the internal space is pumped out through the first oxygen ion conductive solid electrolyte layer and drained off from the external porous electrode of the pump cell layer via the porous ceramic layer so as to vent to an ambient gas atmosphere outside the gas sensor and such that heat from the heater substrate is directly transferred via the thermal conductive porous ceramic layer to the external electrode of the oxygen pump cell, and

wherein the thermal conductive porous layer covers a part of the external porous electrode layer, the thermal conductive porous layer is only exposed at a lateral side of the plural-cell gas sensor, and having a longitudinally extended rod-like appearance.

2. The plural-cell gas sensor as claimed in claim 1 , wherein the thermal conductive porous ceramic layer which adjoins the porous electrode layer of the oxygen pump cell and the insulating ceramic layer of the heater substrate is greater in porosity than the external porous electrode of the oxygen pump cell.

3. The plural-cell gas sensor as claimed in claim 1 , wherein the thermal conductive porous ceramic layer has a porosity of 20% to 80%.

4. The plural-cell gas sensor as claimed in claim 1 , wherein the porous ceramic layer mainly comprises alumina, zirconia, mullite, spinel or mixtures thereof.

5. The plural-cell gas sensor as claimed in claim 1 , wherein the plural-cell gas sensor is a co-fired plural cell gas sensor made by co-firing a green laminate of layers constituting the oxygen pump cell, the heater substrate and the porous ceramic layer filled between the external electrode of the pump cell and the heater substrate.

6. The plural-cell gas sensor as claimed in claim 1 , wherein the porous ceramic layer adjoining the external electrode of the oxygen pump cell and the heater substrate comprises plural porous ceramic layers adjoining one another, each layer differing from the other(s) in material composition or porosity.

7. The plural-cell gas sensor as claimed in claim 6 , wherein a porous ceramic layer located closer to the external electrode of the oxygen pump cell contains more porous zirconia than a porous ceramic layer located closer to the heater substrate.

8. The plural-cell gas sensor as claimed in claim 6 , wherein the porous ceramic layer located closest to the heater substrate has a porosity that is lower than that of the other porous ceramic layer(s).

9. The plural-cell gas sensor as claimed in claim 1 , wherein the first and second oxygen-ion conductive solid electrolyte ceramic layers each comprises stabilized or partially stabilized zirconia and 10-80% by weight of alumina.

10. The plural-cell gas sensor as claimed in claim 5 , wherein the first and second oxygen-ion conductive solid electrolyte ceramic layers each comprises stabilized or partially stabilized zirconia and 10-80% by weight of alumina.

11. The plural-cell gas sensor as claimed in claim 1 , further including:

a gas detection cell comprising a third oxygen-ion conductive solid electrolyte layer; and

another internal space formed partly by the gas detection cell;

wherein a gas reduced in concentration of oxygen and made rich in a specific gas component in the main internal space, by pumping action of the oxygen pump cell, compared to the gas under measurement entering the main internal space, enters the another internal space.

12. The plural-cell gas sensor as claimed in claim 11 , wherein the specific gas component is NOx.

13. The-plural-cell gas sensor as claimed in claim 1 , wherein the porous ceramic layer mainly comprises alumina.

14. The-plural-cell gas sensor as claimed in claim 1 , wherein the porous ceramic layer mainly comprises zirconia.

15. The-plural-cell gas sensor as claimed in claim 1 , wherein the porous ceramic layer mainly comprises mixtures of alumina and zirconia.