SENSOR ELEMENT FOR A POTENTIOMETRIC SENSOR AND PRODUCTION METHOD

Abstract

The present disclosure relates to a sensor element for a potentiometric sensor, including a substrate and an ion-selective enamel layer arranged on the substrate. The substrate has at least one region which is electroconductively connected to the ion-selective enamel layer. The region of the substrate, which is electroconductively connected to the sensor layer, is made of a copper-based alloy having a mass fraction of at least 60% of copper.

Claims

1-21. (canceled)

22. A sensor element for a potentiometric sensor, comprising: a substrate, and an ion-selective enamel layer arranged on the substrate, wherein the substrate has at least one region electrically conductively connected to the ion-selective enamel layer, such that the region of the substrate electrically conductively connected to the ion-selective enamel layer is made of a copper-based alloy with a mass fraction of at least 60% copper.

23. The sensor element of claim 22, wherein the region of copper or the copper-based alloy electrically conductively connected to the ion-selective enamel layer is in contact with the ion-selective enamel layer via a transition zone comprising copper(I)-oxide.

24. The sensor element of claim 22, wherein the transition zone has a layer comprising copper (I) oxide with a thickness of less than 5 μm.

25. The sensor element of claim 22, wherein the substrate is a body formed from the copper-based alloy having a mass fraction of at least 80% copper.

26. The sensor element of claim 22, wherein the substrate is formed by at least one layer arranged on a base body, wherein the at least one layer consists of the metal alloy.

27. The sensor element of claim 22, wherein the copper-based alloy is Cu1−xSnx or Cu1−xZnx with x≤0.1.

28. The sensor of claim 22, wherein the ion-selective enamel layer is formed from an ion-selective glass.

29. The sensor element 22, wherein the ion-selective enamel layer is configured as a single- or multi-layer coating applied to the region of the substrate formed from the copper-based alloy or to an oxide layer comprising copper(I) arranged on the surface of the region consisting of the copper-based alloy.

30. A potentiometric sensor comprising: a sensor element, a substrate, an ion-selective enamel layer arranged on the substrate, wherein the substrate has at least one region electrically conductively connected to the ion-selective enamel layer, such that the region of the substrate electrically conductively connected to the ion-selective enamel layer is made of a copper-based alloy with a mass fraction of at least 60% copper, a reference electrode, and a sensor circuit electrically conductively connected to the sensor element and to the reference electrode, wherein the sensor circuit is configured to detect a potential difference between the sensor element and the reference electrode.

31. A method for manufacturing a sensor element for a potentiometric sensor, wherein the potentiometric sensor includes a sensor element, a substrate, and an ion-selective enamel layer, comprising a step of: applying the ion-selective enamel layer directly to a region of the substrate consisting at least of a copper-based alloy having a mass fraction of at least 60% copper.

32. The method of claim 31, wherein when the enamel layer is applied to an interface between the region of the substrate and the enamel layer that forms, a transition zone results which comprises copper oxide and via which the region of the substrate is in electrically conductive contact with the enamel layer.

33. The method of claim 31, wherein the application of the ion-selective enamel layer to the region of the substrate comprises: applying an enamel preparation of an ion-selective glass, to the region of the substrate; and thermally treating the enamel preparation applied to the substrate to form the ion-selective enamel layer.

34. The method of claim 31, wherein the thermal treatment of the enamel preparation is performed at least intermittently at a temperature between 400° C. and 1085° C.

35. The method of claim 31, wherein the enamel preparation is manufactured as a powder comprising at least glass particles from the ion-selective glass, or as a liquid or paste-like preparation comprising at least glass particles from the ion-selective glass, especially, the pH-glass.

36. The method of claim 31, wherein the application of the ion-selective enamel layer to the region of the substrate comprises: applying a glass body made of an ion-selective glass to the region of the substrate and fusing the glass body to the substrate to form the ion-selective enamel layer.

37. The method of claim 31, wherein the application of the ion-selective enamel layer to the region of the substrate comprises: applying a melt of an ion-selective glass to the region of the substrate and allowing the melt to solidify to form the ion-selective enamel layer.

38. The method of claim 31, wherein the substrate is a body formed from the copper-based alloy having a mass fraction of at least 80% copper.

39. The method of claim 31, wherein the substrate is formed by at least one layer arranged on a base body, wherein the at least one layer consists of the metal alloy.

40. The method of claim 31, wherein the substrate is conditioned before the enamel layer is applied.

41. The method of claim 31, furthermore comprising: sheathing a unit comprising at least the enamel layer and the substrate with an electrically insulating material such that a sheath formed in this way leaves open, in a region of the sensor element intended for contact with a measuring medium, only a surface of the enamel layer facing away from the substrate.

42. The method according to claim 31, wherein an electrical conductor contacting the substrate is passed through the sheath in order to contact the substrate from outside the sheath.

Description

[0072] The invention is explained in further detail below on the basis of the exemplary embodiments shown in the figures.

[0073] The following are shown:

[0074] FIG. 1 a schematic longitudinal sectional view of a sensor element for a potentiometric sensor according to a first exemplary embodiment;

[0075] FIG. 2 a schematic longitudinal sectional view of a potentiometric sensor with the sensor element according to the first exemplary embodiment; and

[0076] FIG. 3 a schematic longitudinal sectional view of a sensor element for a potentiometric sensor according to a second exemplary embodiment.

[0077] FIG. 1 schematically shows a sensor element 1 for a potentiometric sensor according to a first exemplary embodiment in a longitudinal section. The sensor element 1 has a substrate 3 in the form of a rod-shaped body made of a copper-based alloy having a mass fraction of at least 60% and an ion-selective enamel layer 7 serving as a sensor layer that is arranged directly on the substrate 3 in a front region intended for contact with a liquid, especially an aqueous, measuring medium 5. In the present example, the enamel layer 7 consists of a sodium- or pH-selective glass, e.g., MacInnes glass, Corning 015 glass, or any of the glasses known from U.S. Pat. No. 3,458,422. The glasses mentioned in U.S. Pat. No. 3,458,422 contain a portion of Li.sub.2O and are free of Na.sub.2O so that the cross-sensitivity of pH measurements with a sensor layer of one of these glasses is reduced. Alternatively, however, pH glasses containing sodium can also be used.

[0078] The enamel layer 7 can be applied to the substrate 3 in one or more layers arranged one above the other. An electrically conductive, i.e., electron- and/or ion-conducting, transition zone 9 is formed between the substrate 3 and the enamel layer 7. The thicknesses and thickness ratios of the transition zone 9 and the enamel layer 7 are shown greatly exaggerated in FIGS. 1-3. While the enamel layer 7 is being applied, the transition zone 9 is formed at least in part by enameling as a result of redox reactions (e.g., corrosion) and transport processes (e.g., diffusion) at the temperatures occurring in the process. In the transition zone, copper(I) oxide is present both in the region of the substrate 3 covered by the enamel layer 7 and in the glass forming the enamel layer 7. Chemical bonding of the Cu.sub.2O present in the transition zone 9, the enamel layer 7, and the substrate 3 therefore results in good adhesion of the enamel layer 7 to the substrate 3. This transition zone 9 remains stable even after enameling, i.e., after formation and cooling of the enamel layer 7. It produces a good electrically conductive connection between the enamel layer 7 serving as an ion-selective sensor layer and the substrate 3 serving as potential terminal lead.

[0079] The substrate 3 and the transition zone 9 serve as a solid terminal lead for an electrode potential forming on the ion-selective enamel layer 7 in contact with the measuring medium 5. On the rear side, i.e., on the side facing away from the measuring medium 5, the substrate 3 has a contact point 10 at which an electrical conductor 11, e.g., a metallic wire or a conductor path, is electrically contacted to the substrate 3. This conductor 11 can be connected to a sensor circuit of a potentiometric sensor.

[0080] The sensor element 1 moreover has a sheath 12, which is formed by an insulation enamel layer in the present example. Alternatively, the sheath can be formed from a polymer instead of an enamel layer of glass. It surrounds the body forming the substrate 3 and an edge region of the layers 7 and 9 intimately so that no liquid, especially, not the measuring medium 5, reaches the substrate 3.

[0081] The sheath 12 can be produced using methods that are known in connection with enameling metal substrates. Suitable materials for the sheath 12 and suitable methods for applying the sheath 12 to the unit formed from the substrate 3 and the ion-selective enamel layer 7 with the transition zone 9 arranged between them can be taken from, for example, EP 1 231 189 A1. The sheath 12 can be produced by applying particles of a glass composition to the substrate 3 and the enamel layer 7 and subsequent thermal treatment.

[0082] FIG. 2 shows a schematic longitudinal sectional view of a potentiometric sensor 100 for measuring an activity or concentration of an analyte ion or a measured variable dependent thereon, e.g., a pH value, with a sensor element 1 as measuring electrode and a reference electrode 13.

[0083] The sensor element 1 substantially structurally corresponds to the sensor element 1 shown in FIG. 1. It has, as a sensor layer, an ion-selective enamel layer 7 that is applied to a cylindrical body made of a copper-based alloy serving as substrate 3, wherein a transition zone 9 comprising copper(I)-oxide is formed between the enamel layer 7 and the substrate. The sensor element 1 furthermore comprises a sheath 12 which surrounds the substrate 3 and leaves free only a surface of the enamel layer 7 intended for contact with the measuring medium, said sheath being made of an insulation enamel or of a polymer which insulates the substrate 3 and an edge region of the enamel layer 7 or the transition zone 9 from the liquid medium.

[0084] The reference electrode 13 can be designed as a conventional electrode of the second type, e.g., as a silver/silver chloride electrode. In the example shown here, it comprises a tubular housing 19 which surrounds a section of the sheath 12 of the substrate 3 and which is closed at its front end facing the measuring medium by an annular diaphragm 21. The diaphragm 21 can be formed, for example, from a plastic, e.g., PTFE, or from a porous ceramic, e.g., a ZrO.sub.2 ceramic. The annular chamber formed between the sheath 12 and the housing 19 contains a reference electrolyte, e.g. a KCl solution, in which a reference element 23, e.g. a silver electrode coated with silver chloride, is immersed. Instead of a diaphragm 21, the reference electrode 13 can also have another bridge that establishes an ion-conducting and/or an electrolytic contact between the reference electrolyte and the measuring medium. The annular chamber containing the reference electrolyte is closed on its rear side, e.g., by casting or adhesive bonding.

[0085] The substrate 3 is connected via a first electrical line 11 to a sensor circuit 25 and thus forms the measuring electrode of the potentiometric sensor 100. The sensor circuit 25 is accommodated in an electronics housing 27 connected to the reference and measuring electrode of the sensor 100. The reference element 23 is passed out of the annular chamber through the casting or bonding and is also connected to the sensor circuit 25. The sensor circuit 25 is configured to detect a voltage arising in contact of the diaphragm 21 and of the ion-selective enamel layer 7 with the measuring medium between the measuring electrode 1 and the reference electrode 13. This voltage is a function of the activity of the analyte ion present on the ion-selective enamel layer 7. The sensor circuit 25 can be configured to generate a measurement signal representing the detected voltage and to output it, e.g., to a measuring transducer that is connected to the sensor circuit 25 and processes the measurement signal, and to determine therefrom, using a predetermined calibration function, a measured value of the ion concentration of the analyte ion or, if the potentiometric sensor 100 is designed as a pH sensor, of the pH value. Alternatively, the sensor circuit 25 can also be configured to determine the measured value and to output it via an interface 29 to a measuring transducer or another operating or display device.

[0086] FIG. 3 schematically shows another exemplary embodiment of a sensor element 1 for a potentiometric sensor. This sensor element 1 has a base body 31 made of a ceramic or glass ceramic, on which a substrate 3 formed from a metal alloy is arranged in the form of a layer. The layer can be formed by a small plate bonded, adhesively bonded, or otherwise fastened to the base body or by a foil of a copper-based alloy. The ceramic of the base body can, for example, be a zirconium oxide ceramic or an aluminum oxide ceramic.

[0087] An ion-selective enamel layer 7 serving as a sensor layer is applied to the substrate 3. A transition zone 9 containing copper(I) oxide is arranged between the enamel layer 7 and the substrate 3. Enameling of the substrate 3 with simultaneous formation of the transition zone 9 can be carried out in the same way as disclosed above with reference to the exemplary embodiment described in FIG. 1.

[0088] The transition zone 9 is electrically conductive, e.g., ion- and/or electron-conducting, and together with the substrate 3 forms the solid terminal lead of the sensor element 1. At a contact point 10, the substrate 3 is contacted on its rear side by an electrical conductor 11 which is passed through the base body 31 and which can connect the sensor element 1 to a sensor circuit of a potentiometric sensor. The unit formed by the base body 31, the substrate 3, the transition zone 9, and the ion-selective enamel layer is embedded in a glass sheath 12 which leaves free only a surface region of the ion-selective enamel layer 7 and which insulates the interfaces between the substrate 3 and the base body 31 and between the substrate 3 and the overlying layers from a measuring medium.

[0089] Optionally, the sensor element can comprise a preamplifier and/or an impedance transformer (not shown here) which serves to increase the signal-to-noise ratio of the measurement signal of the sensor element or of a potentiometric sensor with the sensor element. The integration of a preamplifier in the signal path close to the ion-selective enamel layer is especially advantageous if the ion-selective enamel layer has a high impedance.

[0090] A potentiometric sensor comprising the sensor element 1 shown here as a measuring electrode can have a reference electrode that is also completely formed by a layer stack and whose potential terminal lead is designed as a solid terminal lead. Both electrodes may be arranged on a common base body, e.g., a circuit board or a non-conductive ceramic, and be connected to a sensor circuit via electrical lines, e.g., conductor paths extending on the base body. In this way, a very compact potentiometric sensor may be realized.

[0091] To produce the sensor elements 1 shown in FIGS. 1 to 3, the enameling of the substrate 3 can be carried out in the following way:

[0092] In a first method variant, an enamel preparation, e.g., a powder formed from glass particles of the ion-selective glass or a suspension or paste containing glass particles of the ion-selective glass, can be applied to the surface of the substrate 3 and heated to a temperature between 800 and 850° C., depending on the composition of the ion-selective glass. The thermal treatment forms the enamel layer 7 and at the same time, at least in part, the transition zone 9. This method has been shown to be highly suitable for the application of an enamel layer 7 containing lithium and/or sodium. Since the glass forming the enamel layer 7 wets a CuO-containing surface significantly more poorly than a Cu.sub.2O-containing or copper-containing surface, it is advantageous in this embodiment to suppress the formation of CuO on the substrate surface. For this purpose, the enamel preparation can advantageously contain constituents that form low-melting salts, such as boric acid hydrates, carbonates, or nitrates. Advantageously, the thermal treatment takes a few minutes, e.g., less than 15 min.

[0093] In a second method variant, a glass body, e.g., a glass plate, of the ion-selective glass can be placed and fused onto the substrate 3. Temperatures in the range of 400° C. and 1085° C. should also be reached here in order to ensure that, in addition to the enamel layer being produced, the ion-selective enamel layer 7 and the transition zone 9 containing Cu(I) are formed.

[0094] This method has proven to be especially suitable for lithium-free, sodium-containing pH-selective glasses and sodium-selective glasses. It is advantageous in this method that CuO formation at the surface of the substrate 3 during the fusing of the glass body is not observed.

[0095] In a further method variant, the substrate 3 can be conditioned, for example passivated, before the enameling, in order to produce a copper(I)-oxide-containing, i.e. monovalent copper-containing oxide layer on the substrate surface, which oxide layer can form part of the transition zone 9 after the enamel layer 7 has been applied. Depending on the conditions prevailing during the enameling, the oxide layer can, however, also be at least partially or even completely dissolved in the transition zone 9 during the application of the enamel layer.

[0096] The oxide layer having copper(I) can be produced by a thermal treatment of the surface of the substrate 3, for example in a flame, by means of a laser or in an oven, in air or under an oxygen-lean or oxygen-free protective gas atmosphere. Likewise, the oxide layer can be produced by treatment in an oxygen plasma or by coating methods such as sputtering or vapor deposition. The ratio in which copper(I) and copper(II) are present in the oxide layer can be controlled by adjusting the process conditions and the amount of oxygen provided. For example, the substrate can be heated for passivation to a temperature of 400 to 500° C. in an atmosphere of protective gas, for example nitrogen, having a low oxygen content. This can be performed, for example, in a furnace chamber purged continuously with nitrogen in which an oxygen partial pressure around 0.8 hPa prevails. The thermal treatment produces an oxide layer that contains a high proportion of monovalent copper Cu(I). The layer has a thickness of less than 5 μm or even less than 1 μm. In addition to monovalent copper, the oxide layer can also contain Cu(II) and oxides of further alloying constituents. The Cu(I) portion in the oxide layer can be controlled or selectively adjusted via the conditions prevailing during the passivation (e.g., temperature program, gas atmosphere, especially the oxygen content thereof). This allows, for example, reproducible conditioning of the substrates for a subsequent enameling to produce a plurality of sensor elements having similar properties.

[0097] The ion-selective glass layer can be applied to the passivated surface of the substrate by enameling, for example by the application of the enamel preparation described further above, also referred to as enamel slip, and subsequent thermal treatment of the preparation to form an enamel layer, or by fusing a glass platelet. Preferred layer thicknesses of the oxide layer after passivation are between 0.05 and 2 μm, preferably less than 1 μm. Substantially thicker oxide layers produced on a copper-based alloy with copper as the main component, e.g. those having a thickness of more than 20 μm, can very easily detach as scale from the metallic substrate. Layers having the indicated layer thickness by contrast adhere firmly and also bring about a good adhesion of the enamel layer also after the subsequent application of the enamel layer. It also appears that the preceding passivation of the metal surface results in uniform wetting of the metal or alloy surface during the enameling step so that the enamel layer formed has significantly fewer enamel defects, such as cracks, irregularities or pores, than an enamel layer applied to a non-passivated substrate. Accordingly, the enamel layer on the passivated substrate can be applied relatively thinly and nevertheless cover the substrate in a liquid-tight manner. This makes it possible to provide a comparatively low-impedance sensor layer for the sensor element. Ion-selective enamel layers of conventional enamel electrodes according to the prior art are usually thicker than ion-selective membranes of conventional glass electrodes with liquid discharge, in order to ensure that the enamel layer covers the potential discharge of the enamel electrode in a liquid-tight manner. Accordingly, the ion-selective enamel layers of conventional enamel electrodes have a higher impedance than the glass membranes of conventional glass electrodes. In order to compensate for this, the surface of the ion-selective enamel layers is chosen to be relatively large for conventional enamel electrodes used in the process industry so that a conventional enamel electrode requires significantly more installation space than a conventional glass electrode with liquid discharge. By contrast, the ion-selective enamel layer of a sensor element which is produced according to the method described here can have a thickness of less than 500 μm, or of less than 300 or 200 μm, or even of less than 100 μm. The surface of the ion-selective enamel layer can thus likewise be kept small in order to provide a sensor element with little installation space required.

[0098] As already mentioned at the outset, the application of the ion-selective enamel layer can be carried out by enameling in a conventional manner in air or in an oxygen-free or low-oxygen protective gas atmosphere, in order to influence the proportions of Cu(I) and Cu(II) present in each case in the transition zone produced here between substrate and the glass layer that forms.