Electrochemical sensor element with a porous reference gas accumulator

The present invention relates to an electrochemical sensor element, in particular for determining the oxygen level in gas mixtures.

Sensor elements are known. They are designed as planar sensor elements, which have, on a solid electrolyte designed as a support, a first electrode exposed to the measured gas and a second gas exposed to a reference gas. Furthermore, an electrical resistance heater is embedded in the support. A reference gas, which is in most cases made up of atmospheric air, is supplied to the reference electrode via a reference gas channel integrated in the support. At the same time, the reference channel forms a gas chamber having a bottom surface matching the reference electrode in the reference electrode area, so that sufficient oxygen may reach the reference electrode.

It is known from European Patent No. 125069 that the width of the reference gas channel can be adapted to match the width of the electrode over its entire length for this purpose, or two reference gas channels, with one electrode arranged in each, can run in a layer plane parallel to one another, with the two electrodes connected together, forming the reference electrode. The disadvantage of a wide reference gas channel or a reference gas channel made up of two adjacent parts is that one part of the heating coil of the resistance heater element is always in the area of the perpendicular projection of the reference gas channel. This results in overheating of the solid electrolyte in the area of the reference gas channel. In addition, a wide reference gas channel provides poor heat transfer between the resistance heating element and the electrodes.

The method described in German Patent Application No. 19609323 in which the reference gas channel is branched in the area of the heating device, offers a possible remedy. However, in this case the reference electrodes must also be branched.

The sensor element according to the present invention has the advantage that it allows improved heat transfer between the electrodes and the resistance heating element, resulting in uniform heat distribution. The porous layer also helps relieve mechanical stresses that occur at the edges where the reference gas channel and the adjacent solid electrolyte film meet, and which may result in stress cracks in the ceramic support. In bridging a wide reference gas channel, the solid electrolyte film is bent, which results in additional mechanical stresses. Using the narrow reference gas channel, excessive bending of the adjacent solid electrolyte film is avoided. Furthermore, due to the large-surface contact of the reference electrode with the adjacent porous layer, better adhesion of the latter is achieved, since the reference electrode remains pressed between the adjacent films during lamination. This is also true for the lead to the reference electrode, with its resistance also being thereby reduced.

It should be emphasized that the reference gas channel may have a slightly widened handgrip shape in the area of the reference electrode. This allows oxygen exchange to be improved, in particular in the case of low pore volumes. The effect of the reference atmosphere can be intensified by adding an oxygen-storing material, for example, CeO 2 , to the porous layer. This can be achieved by impregnating the porous layer or the porous electrode.

FIG. 1 shows a cross section through the sensitive part of a sensor element according to the present invention.

FIG. 2 shows a longitudinal section through the sensor element along line II—II of FIG. 1 according to a first embodiment.

FIG. 3 shows a longitudinal section through the sensor element along line II—II of FIG. 1 according to a second embodiment.

FIG. 1 shows a cross section through the measured gas side of a sensor element. The sensor element is a component of a gas sensor (not shown) and is secured in a housing of the sensor and its sensitive section is exposed to a gas to be measured. The sensor element is made of a ceramic support 10 having a planar layer structure with a first solid electrolyte film 11 , a second solid electrolyte film 12 , and a third solid electrolyte film 13 superimposed on one another. An outer large surface of first solid electrolyte film 11 has a measuring electrode 15 and its inner large surface has a reference electrode 17 . Measuring electrode 15 is covered with a porous protective layer 19 . Adjacent to first solid electrolyte film 11 is second solid electrolyte film 12 , which has a narrow hollow cavity forming reference gas channel 21 in the middle. Between second solid electrolyte film 12 and third solid electrolyte film 13 an electrical resistance heating element 23 is arranged between two electrical insulating layers 25 . Since electrical insulating layers 25 are porous so they can absorb mechanical stresses due to the different heat expansion coefficients of the materials used, a gas-tight solid electrolyte frame 27 is placed around insulating layer 25 . Electrical resistance heating element 23 is designed as a heating coil on the sensitive section of the sensor element.

On one narrow side of ceramic support 10 , reference channel 21 has a reference gas opening 29 and runs approximately to the opposite end face of ceramic support 10 , where reference gas channel 21 is closed. In the embodiment of FIG. 2, reference gas channel 21 has a uniform, for example, rectangular, cross section over its entire length. For example, the width of unsintered reference gas channel 21 is 0.4 mm to 0.8 mm, preferably 0.6 mm. The height of reference gas channel 21 is equal to the thickness of sintered solid electrolyte film 12 , for example, 0.4 mm. In the embodiment of FIG. 3, reference gas channel 21 has a slightly widened section 31 in the area of reference electrode 17 , so that reference gas channel 21 has a handgrip shape overall when viewed from above. Reference gas channel 21 may also branch off in the area of reference electrode 17 .

Reference electrode 17 , which has a flat shape in the plane of ceramic support 10 , is covered with a porous layer 33 according to a first embodiment. Porous layer 33 , which is represented by a dotted surface in FIGS. 2 and 3, is embedded between reference electrode 17 and the adjacent second solid electrolyte film 12 . First solid electrolyte film 11 has a depression, for example, on whose bottom reference electrode 17 is placed with the porous layer filling the depression over reference electrode 17 . Thus porous layer 33 spans reference gas channel 21 in this area after films 11 , 12 , 13 have been laminated together. The reference gas penetrating via reference gas opening 29 then diffuses via porous layer 33 to reference electrode 17 positioned thereon. The thickness of the porous layer is 5 μm to 200 μm, preferably 20 μm to 50 μm.

In another embodiment for performing gas exchange with the reference gas, reference electrode 17 itself has a porous design. Furthermore, an embodiment may use one porous layer and one porous reference electrode. A suitable pore volume is required in order to form an appropriate reference gas chamber. This is achieved through the thickness of porous layer 33 and/or of porous reference electrode 17 .

Good oxygen exchange can be achieved at reference electrode 17 by adding an oxygen-storing material, for example, CeO 2 , to porous layer 33 and/or porous reference electrode 17 . The oxygen-storing material can be added by impregnating porous layer 33 and/or porous reference electrode 17 .

If the sensor element is operated as a concentration cell, reference electrode 17 can be supplied with sufficient oxygen by applying an electric voltage to measuring electrode 15 and reference electrode 17 . Thus an oxygen pumping effect is achieved in that oxygen is pumped from measuring electrode 15 to reference electrode 17 . An additional pumped internal oxygen reference is thus formed on reference electrode 17 .