Sensor for Measuring the Carbon Dioxide Concentration in a Gas Mixture, and Method for Manufacture Thereof

Abstract

A sensor is configured to measure the carbon dioxide concentration in a gas mixture. The sensor has a dielectric layer arranged between a layer-like first electrode and a layer-like second electrode. The second electrode is a composite electrode that has at least one carbonate and/or one phosphate as first material and at least one metal as second material. This sensor can be manufactured by a method comprising applying a layer-like first electrode to a substrate, applying a dielectric layer to the first electrode, and applying a layer-like second electrode to the dielectric layer. The second electrode is applied as a composite electrode that has at least one carbonate and/or one phosphate as first material and has at least one second material that has an electrical conductivity of more than 10-2 S/m.

Claims

1. A sensor for measuring the carbon dioxide concentration in a gas mixture, the sensor comprising: a dielectric layer consisting of a ferroelectric and arranged between a first electrode in the form of a layer and a second electrode in the form of a layer, wherein: the second electrode is a composite electrode which includes a first material and at least one second material, the first material of the composite electrode comprises at least one carbonate, selected from Li.sub.2CO.sub.3, Na.sub.2CO.sub.3, MgCO.sub.3, CaCO.sub.3, SrCO.sub.3, BaCO.sub.3, MnCO.sub.3, CoCO.sub.3, NiCO.sub.3, CuCO.sub.3 or a mixture of a plurality of these carbonates, and/or a phosphate, selected from an apatite and/or a hydroxyapatite which contains at least one of the cations Ca.sup.2+, Sr.sup.2+ or Ba.sup.2+, and the at least one second material of the composite electrode is selected from platinum, gold, silver, copper, indium tin oxide, aluminum-doped zinc oxide, or an alloy or mixture of a plurality of these elements or compounds.

2. The sensor as claimed in claim 1, wherein: a thickness of the dielectric layer is at most 10 μm, a thickness of the first electrode is at most 5 μm, and a thickness of the second electrode is at most 100 μm.

3. The sensor as claimed in claim 1, wherein the first electrode is applied on a membrane of a micro-heating plate as a substrate.

4. The sensor as claimed in claim 3, wherein the first electrode is configured as a heater of the micro-heating plate.

5. The sensor as claimed in claim 1, wherein the first material is arranged between the second material and the dielectric layer.

6. The sensor as claimed in claim 1, wherein the second material is present in the form of particles which are coated with the first material and/or contain the first material in pores of the particles.

7. The sensor as claimed in claim 1, wherein the second electrode comprises a mixture of particles of the first material and particles of the second material.

8. A method for producing a sensor configured to measure the carbon dioxide concentration in a gas mixture, the method comprising: applying a first electrode in the form of a layer onto a substrate; applying a dielectric layer, which consists of a ferroelectric, onto the first electrode; and applying a second electrode in the form of a layer onto the dielectric layer, wherein: the second electrode is applied as a composite electrode which includes a first material and at least one second material, the first material of the composite electrode comprises at least one carbonate, selected from Li.sub.2CO.sub.3, Na.sub.2CO.sub.3, MgCO.sub.3, CaCO.sub.3, SrCO.sub.3, BaCO.sub.3, MnCO.sub.3, CoCO.sub.3, NiCO.sub.3, CuCO.sub.3 or a mixture of a plurality of these carbonates, and/or a phosphate, selected from an apatite and/or a hydroxyapatite which contains at least one of the cations Ca.sup.2+, Sr.sup.2+ or Ba.sup.2+, and the at least one second material of the composite electrode is selected from platinum, gold, silver, copper, indium tin oxide, aluminum-doped zinc oxide, or an alloy or mixture of a plurality of these elements or compounds.

9. The method as claimed in claim 8, wherein: applying the second electrode includes applying at least one oxide, and the at least one oxide is subsequently reacted with carbon dioxide to form the at least one carbonate.

10. The method as claimed in claim 8, wherein: applying the second electrode includes: firstly, depositing the first material on the dielectric layer to form a first sublayer of the second electrode, and secondly, depositing the second material on the first material to form a second sublayer.

11. The method as claimed in claim 8, wherein: applying the second electrode includes: firstly, depositing the second material in the form of particles on the dielectric layer, and secondly, depositing the first material on the surface of the second material and/or in pores of the second material.

12. The method as claimed in claim 8, wherein: applying the second electrode includes simultaneously depositing the first material and the second material on the dielectric layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Exemplary embodiments of the invention are represented in the drawings and will be explained in more detail in the following description.

[0030] FIG. 1 schematically shows the production of a sensor for measuring the carbon dioxide concentration in a gas mixture in a first exemplary embodiment of the invention.

[0031] FIG. 2 schematically shows the production of a sensor for measuring the carbon dioxide concentration in a gas mixture in a second exemplary embodiment of the invention.

[0032] FIG. 3 schematically shows the production of a sensor for measuring the carbon dioxide concentration in a gas mixture in a third exemplary embodiment of the invention.

[0033] FIG. 4 shows the way in which the two electrodes and the dielectric layer of a sensor for measuring the carbon dioxide concentration in a gas mixture are arranged on a micro-heating plate in one embodiment of the invention.

[0034] FIG. 5 shows a diagram of a setpoint temperature cycle during the operation of a sensor according to one embodiment of the invention.

EXEMPLARY EMBODIMENTS OF THE INVENTION

[0035] In a first exemplary embodiment of the invention, which is represented in FIG. 1, a first electrode 2 made of platinum is deposited on a substrate 1, which is an exposed membrane of a micro-heating plate, for example by means of CVD in the middle of the substrate 1. On this electrode, a dielectric layer 3 of lead zirconate titanate is deposited by means of CVD. In order to apply a second electrode 4 onto the dielectric layer 3, in a first step 61 a first porous sublayer 41 of barium carbonate is applied by means of CVD. In a second step 62, a porous second sublayer 42 of platinum is likewise applied onto the first sublayer 41 by means of sputtering. The two sublayers 41, 42 together form the second electrode 4. The first electrode 2 has a thickness d.sub.2 of 100 nm. The dielectric layer 3 has a thickness d.sub.3 of 500 nm. The second electrode 4 has a thickness d.sub.4 of 200 nm. The first electrode 2, the dielectric layer 3 and the second electrode 4 therefore form a thin-film MIM structure.

[0036] A second exemplary embodiment of the invention is represented in FIG. 2. First, in the same way as in the exemplary embodiment, a structure consisting of a substrate 1, a first electrode 2 and a dielectric layer 3 is provided. Then, in the first step 71, platinum is deposited from a colloidal solution onto the dielectric layer 3, so as to produce a layer of porous particles 43 on the dielectric layer 3. Subsequently, in a second step 72, barium oxide is first deposited on the surface and in the pores of the particles 43 by means of CVD and subsequently reacted by means of carbon dioxide to form barium carbonate. In this way, a barium carbonate layer is produced on and in the particles 43, so that coated particles 44 are obtained.

[0037] In a third exemplary embodiment, which is represented in FIG. 3, a structure consisting of the substrate 1 of the first electrode 2 and of the dielectric layer 3 is first provided as in the first and second exemplary embodiments. Subsequently, all the materials of the second electrode 4 are deposited on the dielectric layer 3 in a single step 8. This is done wet-chemically by adding barium chloride to a colloidal platinum solution. During drying in the presence of oxygen and carbon dioxide, barium carbonate 45 is then formed on the surface and in cavities between particles 46 of platinum.

[0038] FIG. 4 represents the way in which the substrate 1 is arranged as an exposed membrane in a micro-heating plate 5. The micro-heating plate 5 forms a cavity 51. The substrate 1 is arranged in such a way that the first electrode 2, the dielectric layer 3 and the second electrode 4 face away from the cavity 51. A heater plane (not represented) is placed centrally in the substrate 1, the first electrode 2 functioning as a heating element. The structure represented allows a power consumption of the sensor of much less than 100 mW even in continuous operation. Furthermore, because of the low thermal mass of the overall structure, rapid modulation at different operating temperatures is possible. A duty cycle of 1:10 may be achieved, and a measurement may be carried out within a very short time at different temperatures. This fact that adsorption and desorption reactions taking place on the second electrode 4 can be accelerated by continuous or pulsed heating is exploited in this way, and the response or regeneration times of the sensor can therefore be shortened.

[0039] FIG. 5 represents an exemplary setpoint temperature cycle of the sensor represented in FIG. 4. To this end, the setpoint temperature T is plotted against time t in a diagram. Between the start of two heating processes, a period t.sub.1 of one second respectively elapses. The maximum setpoint temperature reached during the heating is maintained for a period t.sub.2 of less than 50 ms. Subsequently, the setpoint temperature T is lowered, maintained at the lowered setpoint temperature T again for the period t.sub.2, and lastly increased once more for a period t.sub.2 to a setpoint temperature T which is higher but does not correspond to the setpoint temperature T initially reached, before the heating is turned off for the rest of the period t.sub.1. Readouts 91, 92, 93, 94, 95 of the sensor according to the invention may be carried out at regular time intervals, so that at least one first readout 91 takes place at the maximum setpoint temperature T reached a second readout 92 at the lowered setpoint temperature T and a third readout 93 at the again increased setpoint temperature T.