Method and sensor element for producing a sensor element of a potential sensor

Abstract

The present invention relates to a method for producing a sensor element (1) for a potentiometric sensor (100), comprising: conditioning at least one region of a substrate (3) composed of copper or a copper-based alloy having a mass fraction of at least 60% of copper to produce an oxide layer comprising cuprous copper (Cu(I)), and applying an ion-selective, in particular pH-selective, enamel layer (7) at least onto the region of the substrate (3).

Description

Technical Field

[0001] The present invention relates to a method for producing a sensor element for a potentiometric sensor and a sensor element. Background Art

[0002] Potentiometric sensors are used in laboratory and process measurement technologies in many fields of chemistry, biochemistry, pharmacy, biotechnology, food technology, water management, and environmental measurement technology for analyzing measurement media, especially measurement liquids. A potentiometric sensor allows the detection of the activity of chemical substances, such as ionic activity, and related measurement variables in a liquid. The substance whose activity or concentration is to be measured is also referred to as an analyte. The measurement medium can be a measurement liquid, such as an aqueous solution, an emulsion, or a suspension.

[0003] A potentiometric sensor typically includes a measuring electrode, a reference electrode, and a sensor circuit for detecting a measured value and for signal processing. The measuring electrode and the reference electrode can be combined in a measuring probe that can be immersed in the measurement liquid. The measuring probe can also include the sensor circuit or at least a part of the sensor circuit. The measuring probe can be connected via a cable or wirelessly to a higher-level unit, such as a measuring transducer, an electronic operating device, a computer, or a controller for communication. The higher-level unit can be used for further processing the measurement signal or the measured value detected by means of the measuring probe and for operating the measuring probe.

[0004] When in contact with the measurement medium, the measuring electrode forms a potential as a function of the activity of the analyte in the measurement medium, while the reference electrode provides a stable reference potential independent of the analyte concentration. The sensor circuit generates an analog or digital measurement signal that represents the voltage (potential difference) between the measuring electrode and the reference electrode and thus represents the activity of the analyte in the measurement medium. The measurement signal can be output from the sensor circuit to the higher-level unit, which further processes the measurement signal. It is also possible to further process part or all of the measurement signal in the sensor circuit in the measuring probe.

[0005] The reference electrode of a conventional potentiometric sensor is typically designed as a second-type electrode, such as a silver / silver chloride reference electrode, and is conductively electrically connected to the sensor circuit. It can include a housing and a reference element, such as a silver wire coated with silver chloride, the reference element being arranged in the housing and, during the measurement operation, being in electrolytic conductive contact and / or ionic conductive contact with the measurement liquid via a reference electrolyte and an electrochemical bridge, such as a diaphragm, contained in the housing.

[0006] The measuring electrode comprises a sensor element that forms a potential, and the sensor element that forms a potential includes an ion-selective membrane depending on the type of potential sensor. An example of such a measuring electrode is an ion-selective electrode (ISE). A conventional ion-selective electrode has a housing that is enclosed by an ion-selective membrane and contains an internal electrolyte in contact with the membrane. The ion-selective electrode also includes a terminal lead in contact with the internal electrolyte. The terminal lead is conductively electrically connected to a sensor circuit. If the ion-selective membrane used for measurement contacts the measurement medium, the membrane will selectively interact with a specific ion species contained in the measurement medium, i.e., with the analyte. Changing the activity or concentration of the ions in the measurement medium causes a relative change in the equilibrium current voltage (galvanic voltage) between the measurement medium and the terminal lead that contacts the ion-selective membrane via the internal electrolyte. A special case of such an ion-selective electrode, i.e., an electrode that selectively detects the activity of hydronium ions in a measurement liquid, is the well-known pH glass electrode, which includes a glass membrane as the sensor element that forms a potential. The terms "ion-selective layer", "membrane", or "electrode" used herein and hereinafter refer to an ion-sensitive layer, membrane, or electrode whose potential is preferably mainly affected by an analyte, such as a specific ion type or pH value, where cross-sensitivity of the layer, membrane, or electrode to other types of ions is not excluded, but is preferably low. The term "ion-selective glass" refers to a glass suitable for forming such an ion-selective layer, membrane, or electrode.

[0007] There has long been an attempt to improve the design of the measuring electrode of a potential sensor with the goals of cost savings, simplified manufacturing, and higher robustness and longer service life. A method that has been repeatedly adopted is to use a solid terminal lead that does not require contact with the internal electrolyte of the ion-selective membrane.

[0008] An ion-selective electrode with a solid terminal lead (also called a fixed-contact electrode) is described in WO 2018 / 069491 A1. The electrode comprises a measuring element having an ion-selective layer that contacts the measurement medium during operation and is conductive for lithium ions. In addition, the measuring element has a conductive layer that contains metallic lithium, a lithium(0) alloy, or a conductive lithium compound. The measuring element also includes a solid electrolyte layer that is disposed between the ion-selective layer and the conductive layer. The measuring element includes a series of other layers for protecting the lithium-containing conductive layer from oxygen or moisture and for contacting the ion-selective layer. Thus, the manufacture of this sensor element requires a series of individual steps for constructing the layer stack and is accordingly complex.

[0009] A pH sensor is known from DE 10-2016-202083A1, which has a terminal electrode with a metal reference electrode as a solid metal terminal lead and a pH-sensitive glass membrane made of lithium silicate glass applied to the reference electrode, and which further has a contact area made of a contact metal such as copper, on the outer side of the metal reference electrode, a monovalent metal cation is formed. The glass membrane is doped with monovalent metal cations from the contact area of the reference electrode, so that a defined sensor potential is formed.

[0010] DE 1291139 B describes a glass electrode which has a terminal electrode made of metal - preferably copper - which has an oxide skin on its surface to which a glass layer is fused on its upper surface.

[0011] DE 2220841 A describes a method for producing an ion-selective electrode, in which a palladium wire plated with copper is coated with a pH-sensitive glass, where the wire is heated in a flowing oxygen atmosphere at 850 °C and then the hot wire is dipped into the crushed glass. The wire is then heated again in order to fuse the glass particles onto it.

[0012] In Belford, R.E., Owen, A.E., Kelly, R.G., Thick-Film Hybrid pH Sensors, Sensors and Actuators, Vol. 11, 1987, pp. 387-398, a method for producing a pH sensor for thick-film technology is described, where a pH glass membrane is applied to a printed metal conductor track. The conductor traces are printed with a diluted Au / Pt paste. Between the conductor track and the glass membrane applied to it, a metal oxide intermediate layer is produced by vapor-depositing copper or iron onto the printed conductor track and heating it in air.

[0013] US 4,133,735 A describes a glass electrode with solid terminal leads, which has a conductor with a surface layer made of a chemically active material such as copper. The surface layer has a first coating of a mixture of glass and a halogen of the active metal. The ion-sensitive glass coating is applied to this first coating by dipping the conductor into the melt of the ion-sensitive glass.

[0014] In the textbook H. Galster, “pH-Messung—Grundlagen, Methoden, Anwendungen, "pH Measurement - Basic Principles, Methods, Applications, Instruments", VCH Verlagsgesellschaft mbH, Weinheim, 1990, pp. 135–136 describes some other methods of providing electrodes with solid terminal leads. One of these methods is formed by the so-called enamel electrodes, which are usually constructed of multiple layers of different compositions, with the top layer formed by a pH-selective enamel layer.

[0015] Regarding the terms "enamel electrode" or "ionic or pH-selective enamel layer", the following should be noted. According to the definition / tag standard, the vitreous material made by completely or partially melting substantially oxidized raw materials is called enamel in the RAL registration RAL-RG 529A2 of the RAL Deutsches Institut für Gütesicherung und Kennzeichnung e.V. (RAL German Institute for Quality Assurance and Certification, registered association) in July 2007. The inorganic preparation thus obtained is applied together with additives to one or more layers of a workpiece made of metal or glass and fused at a temperature above 480 °C. The basic components of the (ion-selective) enamel layer are one or more of oxides such as silicon oxide, sodium oxide, potassium oxide, calcium oxide, magnesium oxide, and aluminum oxide. In addition to the RAL definition, the definitions of Adolf Dietzel and Hans Kyri are also common. According to this definition, enamel is a preferably glassy solidified composition produced by using inorganic, mainly oxidized compositions, by melting or sintering, and the composition is partially fused or has been fused to one or more layers of a workpiece of metal or glass by using aggregates.

[0016] Therefore, among these types of definitions, in the following, the ion-selective glass such as pH glass that is applied to a metal substrate by the method used in the RAL definition or by fusing to or onto the substrate will be called an ion-selective enamel layer, or in the case of an enamel layer specifically selective for hydronium ions, it will be called a pH enamel layer, and the corresponding electrode will be called an enamel electrode.

[0017] Enamel electrodes are characterized by high mechanical stability and can be hygienically designed by providing an enamel coating that covers all parts of the probe in contact with the process. Therefore, they can be particularly advantageously used in processes in the food industry and in chemical processes that require frequent purification.

[0018] Two examples of enamel electrodes are provided in the above-mentioned textbook by H. Galster. In the first example, an insulating enamel layer is arranged on an iron substrate, a silver layer is arranged on the insulating enamel layer, and a pH-selective enamel layer is arranged on the silver layer. In this example, the silver layer serves as an electrical terminal lead. In the second example, a Pt / Pd layer is applied to a ceramic substrate of forsterite, a glued oxide layer of CuO / FeO is applied thereon, and a pH-selective enamel layer of MacInnes glass is applied thereon. The respective layers are applied by means of thick-film technology. The final ion-selective layer is applied according to the screen-printing process. For this purpose, a paste of ground MacInnes glass with a suitable binder is used and then calcined at 850 °C.

[0019] The manufacture of sensor elements for potentiometric sensors known from the prior art is relatively complex and requires some or even several separate manufacturing steps. A reproducible and stable quality of the sensor elements is also desired. Summary of the Invention

[0020] The object of the present invention is to illustrate an improved manufacturing method for a sensor element with a solid terminal lead and a corresponding sensor element with a solid terminal lead.

[0021] This object is achieved by the method according to the invention and a sensor element produced according to the method. Advantageous embodiments are listed in the further description.

[0022] The method according to the invention for producing a sensor element for a potentiometric sensor comprises:

[0023] Adjusting at least one region of a substrate composed of copper or a copper-based alloy having a mass fraction of at least 60% copper to produce an oxide layer comprising monovalent copper (hereinafter referred to as Cu(I) or copper(I)), and

[0024] Applying an ion-selective enamel layer, in particular a pH-selective enamel layer, at least to the region of the substrate.

[0025] The sensor element produced in this way has a very simple structure, because the enamel layer as the sensor layer and the substrate region conductively connected thereto have already formed a fully functional solid-contact electrode. Therefore, compared with the solid-contact electrodes known in the prior art, the manufacture of the sensor element is also very simple and only requires a few method steps. The ion-selective enamel layer can be a pH enamel layer.

[0026] It has been found that sensor elements produced by the method according to the invention are unexpectedly robust even with respect to mechanical and thermal loads. According to technical knowledge, a basic requirement for the mechanical stability of solid contact electrodes exposed to mechanical or thermal loads is that the coefficients of thermal expansion of the materials involved are as consistent as possible but at least do not differ by more than 10%. The coefficients of thermal expansion of known ion-selective glasses - for example, pH glasses such as Macinnes glass, Corning 015 glass or, for example, the glass known from US 3,458,422 - are of the order of 10·10 -6 K -1 . For example, the expansion coefficient of Corning 015 glass is 11·10 -6 K -1 , or the expansion coefficient of the glass known from US 3,458,422 is 9.3 and 10.4·10 -6 K -1 . On the other hand, copper has a coefficient of thermal expansion of 16.5·10 -6 K -1 . It has been found that, compared to, for example, typical steels, the high ductility of copper still enables it to be very stably connected to ion-selective glass and ordinary enamels for steel, even under thermal or mechanical loads. Here, the compressive stress in the enamel layer in the cooled state only remains high in order to resist flaking and cracking.

[0027] Copper-based alloys that contain, in addition to copper, for example, zinc or tin have a lower ductility than pure copper and impart higher mechanical stability with respect to the mechanical stress of the sensor element, but still have sufficient ductility in the case of a copper content of at least 60% (by mass) in order to keep the compressive stress acting on the enamel layer low in a similar manner as in the case of a pure copper substrate.

[0028] Suitable copper-based alloys are respectively formed or produced with a defined copper component as the main component with a mass fraction of at least 60% and other alloy components. The copper-based alloys can advantageously include zinc or tin as additional alloy components. Suitable copper-based alloys are, for example, red brass alloys used in the jewelry industry or in the manufacture of special equipment, such as, for example, CuZn5 or CuZn10.

[0029] Subsequently, at least an ion-selective enamel layer can be applied to the substrate area in a single enameling step or several enameling steps. In this way, the enamel layer is applied to the oxide layer having Cu(I) and previously produced by conditioning. Advantageously, the enamel layer is applied in a one-stage enameling step for producing a single ion-selective enamel layer. However, applying the enamel layer in several layers is not excluded. If the enamel layer is applied in a one-stage enameling step, this enables the production of a particularly thin ion-selective enamel layer. The oxide layer having defined Cu(I) and produced by conditioning exhibits a defined wetting behavior during enameling, such that enameling defects are minimized, and the enamel layer can be applied in one layer and / or relatively thinly, preferably having a thickness of less than 500 μm.

[0030] In the above method, a fully functional sensor element suitable for measuring ion concentration or pH value can be produced in one enameling step. In the sense of the definition given at the beginning, the enameling step can include applying an enamel article, followed by heat treatment to form an enamel coating on the substrate, or melting or fusing the glass forming the enamel layer onto the area of the substrate. Thus, the method can be used in a very simple manner for producing a sensor element having a solid terminal lead for a potentiometric sensor.

[0031] When an enamel layer is applied to the interface between the substrate area and the formed enamel layer, a transition zone is produced, which includes copper(I) oxide, and the substrate area is in conductive contact with the enamel layer via the transition zone. The transition zone can be electron- and / or ion-conductive. The transition zone can be formed in a temperature range between 400 and 1085 °C.

[0032] In a first embodiment, applying an ion-selective enamel layer to the substrate area can include the following steps:

[0033] Applying an enamel article of an ion-selective glass, especially a pH glass, to the substrate area; and

[0034] Heat-treating the enamel article applied to the substrate to form an ion-selective enamel layer.

[0035] The heat treatment in the enamel article can occur at least sometimes at a temperature between 400 °C and 1085 °C.

[0036] The enamel article can be manufactured to comprise at least a powder of glass particles from an ion-selective glass, especially a pH glass, or be manufactured as a liquid or pasty article, such as an enamel slip, comprising at least glass particles from an ion-selective glass, especially a pH glass. The enamel article can contain other additives, as described in the introduction.

[0037] In an advantageous embodiment of the method, the formation of the more highly oxidized copper(II) oxide CuO is blocked. The CuO formed on the substrate surface is wetted less well by the ion-selective glass and can lead to defects in the glass coating or the resulting enamel layer. Therefore, advantageously, the temperature sequence of the heat treatment is adjusted in a manner that counteracts the formation of CuO, for example, by exposing the substrate to which the enamel article is applied to a high temperature for the shortest possible period of time. Advantageously, the enamel article can contain substances that form low-melting salts such as boric acid hydrates, nitrates, or carbonates. These can form a protective film on the substrate during heating.

[0038] In a second embodiment, applying an ion-selective enamel layer to a region of a substrate can include:

[0039] Applying a vitreous body made of ion-selective glass to the region of the substrate and fusing the vitreous body to the substrate to form an ion-selective enamel layer. The fusing can be carried out, for example, by heat treatment in a furnace or by heating using a gas flame or using a laser. In an advantageous method embodiment, the temperature occurring in this process is at least sometimes in the range between 400 °C and 1085 °C.

[0040] The advantage of this method variant is that during the melting or fusing of the glass body, the formation of CuO does not occur or occurs only to a small extent at the substrate surface, and the formation of the desired Cu 2 O is promoted.

[0041] In a third embodiment, applying an ion-selective enamel layer to a region of a substrate can include:

[0042] Applying a melt of ion-selective glass to the region of the substrate and solidifying the melt to form an ion-selective enamel layer. The solidification of the melt can optionally be achieved by defined cooling, for example, actively controlled or regulated cooling.

[0043] In all method embodiments described herein, applying an ion-selective enamel layer to a substrate region can be done in air in a classical manner. Alternatively, in all method designs described herein, the application of the enamel layer can be carried out completely or partially under an oxygen-free or low-oxygen atmosphere or under a protective gas or inert gas. For example, nitrogen or an inert gas such as argon is suitable as a protective gas or inert gas. By controlling the oxygen content present in the atmosphere during the application of the ion-selective enamel layer, the corresponding proportion of Cu(I) oxide present in the transition zone, especially the proportion relative to Cu(II) oxide also present in the transition zone, can be influenced and / or specifically adjusted. Enameling under a protective gas or in a lean-oxygen atmosphere counteracts the formation of CuO.

[0044] The ion-selective glass used in all method embodiments disclosed herein can be pH glass or sodium-, potassium-, or lithium-selective glass. Such ion-selective glass can be formed of at least the following components: silica (SiO 2 ), at least one alkali metal oxide (R 2 O, where R = Li, Na, K, Rb, or Cs), and at least one alkaline earth metal oxide (RO, where R = Mg, Ca, Sr, Ba). Optionally, the components forming the ion-selective glass can include multiple alkali metal oxides and / or multiple alkaline earth metal oxides. Optionally, the ion-selective glass can contain other additives, such as boron oxide (B 2 O 3 ). If the sensor element is intended for potentiometric measurement of pH, lithium- and / or sodium-containing pH glass is preferred. Lithium-containing glass exhibits low or negligible cross-sensitivity to sodium ions in the measurement medium, while sodium-containing pH glass is generally easier to apply to a substrate by enameling. For example, lithium-containing, sodium-free glass or alternatively, sodium-containing, lithium-free glass is suitable as pH glass.

[0045] The ion-selective enamel layer can be formed, for example, by continuously applying one or more layers one after another on the area of the substrate to be conductively connected to the enamel layer, so as to ensure its complete coverage of the substrate. At least one base layer in the multi-layer enamel layer directly applied on the substrate can be formed of electronically conductive and / or ionically conductive enamel, the composition of which is different from the ion-selective glass on which one or more layers are formed. At least one base layer provides a conductive contact between the substrate and the covering layer of ion-selective glass. At least one substrate layer can also improve the adhesion of the ion-selective enamel layer to the substrate.

[0046] The substrate can be a body formed of copper or a copper-based alloy having a mass fraction of at least 80% copper. Alternatively, the substrate can be formed of at least one layer disposed on a matrix - especially a metal or ceramic matrix - the at least one layer being composed of copper or a copper-based alloy.

[0047] Insofar as the substrate includes a body or layer made of a copper-based alloy, in addition to copper, as additional components, it can also contain, for example, tin or zinc. The copper-based alloy can be, for example, Cu1-xSnx or Cu1-xZnx, where, in each case, x ≤ 0.1. As further described above, these alloys having a mass fraction of, for example, greater than 90% copper have the advantage of higher mechanical stability while still having sufficient ductility.

[0048] The conditioning of at least this area of the substrate can include passivation performed before applying the enamel layer, especially by heat treatment, plasma treatment, electrochemical or chemical reactions in solution, or by applying an oxide layer by means of a coating process according to the gas phase.

[0049] Due to the prevailing conditions during conditioning, such as the temperature and / or oxygen content of the environment, the proportion of monovalent copper in the oxide layer produced by conditioning can be adjusted in a controlled or targeted manner. The conditioning of the substrate can include, for example, passivation by thermal pretreatment or plasma pretreatment. The conditioning can include passivation by heat treatment at a temperature between 300 °C and 600 °C, advantageously between 400 °C and 500 °C. The passivation can be carried out in an oxygen-deficient atmosphere, for example in an atmosphere essentially having one or more protective or inert gases - such as nitrogen or noble gases - and oxygen, at a partial pressure of less than 10 hPa, for example between 0.001 and 10 hPa. This low oxygen content is sufficient to produce a thin copper(I)-containing oxide layer on the substrate surface, for example having a thickness between 0.05 and 2 μm.

[0050] It has been found that this conditioning or pretreatment and the presence of the copper(I)-containing oxide layer produced on the substrate surface result in the formation of a uniform and low-defect enamel layer during subsequent enameling. Compared to untreated copper or copper-based alloy substrates for enameling, this is accompanied by improved stability with respect to mechanical loading and improved sensor behavior, in particular an increase in the sensor gradient of sensor elements or potential sensors including sensor elements. Advantageously, the copper(I)-containing oxide layer formed during conditioning or passivation has a thickness of less than 5 μm, preferably 2 μm. Obviously, the thicker the oxide layer is basically, the less stable it is and it can partially detach.

[0051] In addition to copper(I), the oxide layer produced in this way can also contain a preferably small proportion of copper(II), and especially if the substrate area is formed from a copper-based alloy, it can contain other alloy components in oxidized form. For the production of sensor elements with reproducible mechanical and / or sensory quality, it is advantageous that by controlling the oxygen content of the atmosphere in which the substrate is treated, a controllable, especially defined, copper(I) fraction can be produced in the oxide layer having copper(I).

[0052] As a result of the conditions selected during conditioning, an oxide layer can be produced on the substrate in a reproducible manner, especially with a defined Cu(I) fraction, and thus also having defined, especially reproducible, wetting properties during enameling. These properties of conditioning the substrate surface are particularly also independent of the composition of the enamel to be applied or the glass to be fused. For example, this advantage is achieved by performing the conditioning in a separate step upstream of enameling.

[0053] The adjustment of the substrate area can include, for example, thermal pretreatment in an oven, by means of a flame or by laser. Alternatively, the adjustment can include, for example, plasma treatment of the surface in an oxygen plasma. The adjustment can also be carried out, for example, by (reactive) sputtering, CVD (chemical vapor deposition) or ALD (atomic layer deposition), by applying an oxide layer by means of a coating process from the gas phase.

[0054] After applying the ion-selective enamel layer, the oxide layer containing monovalent copper formed during the adjustment can at least partially remain as a component of the transition zone, but can also be completely dissolved in the transition zone during the application of the enamel layer.

[0055] The method can additionally include the following steps: coating at least the unit including the enamel layer and the substrate with an electrically insulating material such that the sheath formed in this way leaves only the surface of the enamel layer facing away from the substrate open in the region of the sensor element intended to be in contact with the measurement medium. An electrical conductor in contact with the substrate can pass through the sheath to contact the substrate from outside the sheath.

[0056] The method steps for coating the unit including the enamel layer and the substrate can include:

[0057] applying a powder including glass particles or a suspension or paste including glass particles to the unit; and

[0058] heat-treating the applied powder or suspension or paste to form a glass layer constituting the sheath.

[0059] Alternatively, the method steps for coating can also include applying a glass melt to the unit and cooling the glass melt or coating the unit with a plastic or ceramic.

[0060] The sensor element for a potentiometric sensor according to the invention is produced by means of the above method.

[0061] The sensor element thus obtained has a substrate and an ion-selective enamel layer arranged on the substrate, wherein the substrate has at least one region conductively joined to the ion-selective enamel layer, and a substrate region of copper or a copper-based alloy with a mass fraction of at least 60% copper is conductively connected to the ion-selective enamel layer.

[0062] The ion-selective enamel layer applied to a substrate region composed of copper or a copper-based alloy having a mass fraction of at least 60% copper can have a thickness of less than 500 μm, preferably less than 300 μm, or even 200 μm or even less than 100 μm.

[0063] The region of the substrate made of copper or a copper-based alloy that is conductively connected to the enamel layer can be in contact with the enamel layer via a transition region comprising cuprous oxide (I). Due to corrosion, dissolution, and the firing process, this transition region can be formed at least partially during enameling, i.e., during the application of the enamel layer. Copper from the region of the substrate covered by the formed enamel layer is oxidized to Cu(I), which occurs in the region near the surface in the metallic copper or copper-based alloy of the substrate and in the glass forming the enamel layer, and acts as a very good binder via chemical bonding between the region of the substrate composed of copper or a copper-based alloy and the glass of the enamel layer. Monovalent copper present in the oxide layer formed by conditioning the substrate region can also be involved in the formation of the transition region.

[0064] In a possible embodiment, the transition region can be an oxide layer comprising monovalent copper or cuprous oxide (I), with a thickness less than 5 μm, preferably less than 2 μm, more preferably less than 1 μm. The minimum thickness of this layer can be 0.05 μm or less. In addition to cuprous oxide (I), the layer comprising cuprous oxide (I) can also contain a portion of cupric oxide (II), and in some cases, if the substrate is formed from a copper-based alloy, it can also contain oxides of other alloying components.

[0065] In a possible embodiment, the substrate can be a body formed of copper or a copper-based alloy having a mass fraction of at least 80% copper. Alternatively, the substrate can be formed by at least one layer disposed on a matrix—especially a metallic or ceramic matrix—wherein the at least one layer is composed of copper or a copper-based alloy. The layer can be formed, for example, as a small plate placed on the matrix and bonded to its material, or as a foil of copper or a copper-based alloy placed on the matrix and bonded to its material.

[0066] The copper-based alloy can be, for example, Cu1-xSnx or Cu1-xZnx, where in each case x ≤ 0.1.

[0067] The ion-selective enamel layer can be formed from an ion-selective glass, especially pH membrane glass. If the ion-selective enamel layer is intended for pH measurement, it can contain sodium and / or lithium. The glass further specified above in the description of the method can be used for the ion-selective enamel layer.

[0068] The ion-selective enamel layer can be designed as a single-layer or multi-layer coating. The multi-layer application of the coating reduces the risk that the enamel layer is not completely closed, thereby reducing the danger of the measuring medium reaching the surface of the substrate during the measuring operation of the sensor element.

[0069] The single-layer or multi-layer coating can be applied to the region of the substrate composed of copper or a copper-based alloy, or to an oxide layer comprising copper(I) disposed on the surface of the region composed of copper or a copper-based alloy.

[0070] In an advantageous embodiment, the sensor element comprises a preamplifier and / or an impedance converter which can be connected to a potential sensor circuit. One input of the preamplifier can be connected to a conductive potential terminal lead of the substrate which serves as the sensor element and is conductively connected to a region of the ion-selective enamel layer, and the second input can be at the housing potential or at a virtual ground of the sensor circuit as a reference potential. For example, if the sensor element is a component of a potential sensor, wherein it serves as a measuring electrode and also has a reference electrode and a sensor circuit, and the component is configured to detect the voltage between the measuring electrode and the reference electrode and generate a measurement signal based on this voltage, then the preamplifier or the impedance converter can be used to increase the signal-to-noise ratio of the measurement signal. This is particularly advantageous if the ion-selective layer has a high impedance.

[0071] The preamplifier can be arranged in a cavity in the aforementioned substrate, or in a housing which at least partially surrounds the sensor element, or in a sheath of electrically insulating material which surrounds a unit comprising at least the enamel layer and the substrate.

[0072] The invention also includes a potential sensor having at least one sensor element according to one of the above embodiments,

[0073] a reference electrode, and

[0074] a sensor circuit which is conductively electrically connected to the sensor element and the reference electrode, wherein the sensor circuit is configured to detect the potential difference between the sensor element and the reference electrode. Description of the Drawings

[0075] Hereinafter, the invention will be explained in further detail based on the exemplary embodiments shown in the drawings.

[0076] The following are shown:

[0077] Figure 1 is a schematic longitudinal sectional view of a sensor element for a potential sensor according to a first exemplary embodiment;

[0078] Figure 2 is a schematic longitudinal sectional view of a potential sensor having a sensor element according to a first exemplary embodiment;

[0079] Figure 3 is a schematic longitudinal sectional view of a sensor element for a potential sensor according to a second exemplary embodiment; and

[0080] Figure 4 is a graph of measurement values obtained with various potential sensors. Detailed Description of the Invention

[0081] Figure 1The sensor element 1 for a potential sensor according to a first exemplary embodiment is schematically shown in a longitudinal section. The sensor element 1 has a base material 3 in the form of a rod-shaped body made of copper or a copper-based alloy having a mass fraction of at least 60%, and an ion-selective enamel layer 7 serving as a sensor layer, which is directly arranged on the front region of the base material 3 intended to be in contact with a liquid - especially an aqueous - measurement medium 5. In the present example, the enamel layer 7 consists of sodium or pH-selective glass - such as Macinnes glass, Corning 015 glass or any glass known from US 3,458,422. The glass mentioned in US 3,458,422 contains a portion of Li 2 O and does not contain Na 2 O, thereby reducing the cross-sensitivity of the pH measurement of the sensor layer with one of these glasses. However, alternatively, pH glass containing sodium can also be used.

[0082] The enamel layer 7 can be applied to the base material 3 in one or more layers arranged layer by layer. A conductive - i.e., electron- and / or ion-conductive - transition region 9 is formed between the base material 3 and the enamel layer 7. The thickness and thickness ratio of the transition region 9 and the enamel layer 7 are greatly exaggerated in Figures 1-3 . When the enamel layer 7 is applied, the transition region 9 is formed at least partially by redox reactions (e.g., corrosion) and transport processes (e.g., diffusion) occurring during the process at the temperature at which the enamel is formed. In the transition region, cuprous oxide (I) is present in the region of the base material 3 covered by the enamel layer 7 and in the glass forming the enamel layer 7. Therefore, the chemical bonding of Cu 2 O present in the transition region 9, the enamel layer 7, and the base material 3 results in good adhesion of the enamel layer 7 to the base material 3. Even after enameling, i.e., after forming and cooling the enamel layer 7, the transition region 9 remains stable. It produces a good conductive connection between the enamel layer 7 serving as an ion-selective sensor layer and the base material 3 serving as a potential terminal lead.

[0083] The base material 3 and the transition region 9 serve as solid terminal leads for the electrode potential formed on the ion-selective enamel layer 7 in contact with the measurement medium 5. On the back side, i.e., on the side facing away from the measurement medium 5, the base material 3 has a contact point 10 at which an electrical conductor 11 - e.g., a metal wire or a conductor path - is in electrical contact with the base material 3. The conductor 11 can be connected to the sensor circuit of the potential sensor.

[0084] Furthermore, the sensor element 1 has a sheath 12 which, in the present example, is formed by an insulating enamel layer. Alternatively, the sheath can be formed by a polymer instead of a glass enamel layer. It closely surrounds the body forming the base material 3 and the edge regions of the layers 7 and 9 such that no liquid, especially no measurement medium 5, reaches the base material 3.

[0085] The sheath 12 can be produced using known methods related to enamelled metal substrates. Suitable materials for the sheath 12 and suitable methods for applying the sheath 12 to the unit formed by the substrate 3 and the ion-selective enamel layer 7 with a transition zone 9 arranged therebetween can be taken from, for example, EP1231189A1. The sheath 12 can be produced by applying particles of a glass composition to the substrate 3 and the enamel layer 7 and subsequently subjecting them to a heat treatment.

[0086] Figure 2 A schematic longitudinal sectional view of a potentiometric sensor 100 for measuring the activity or concentration of analyte ions or a measurement variable dependent thereon, such as the pH value, using a sensor element 1 as a measuring electrode and a reference electrode 13 is shown.

[0087] The sensor element 1 basically structurally corresponds to Figure 1 the sensor element 1 shown in. It has an ion-selective enamel layer 7 as a sensor layer, which is applied to a cylinder made of copper or a copper-based alloy serving as the substrate 3, wherein a transition zone 9 including cuprous oxide (I) is formed between the enamel layer 7 and the substrate. In addition, the sensor element 1 includes a sheath 12, which surrounds the substrate 3 and leaves only the surface of the enamel layer 7 intended to come into contact with the measurement medium, and the sheath is made of insulating enamel or a polymer, which insulates the edge region of the substrate 3 and the enamel layer 7 or the transition zone 9 from the liquid medium.

[0088] The reference electrode 13 can be designed as a conventional electrode of the second type, for example, designed as a silver / silver chloride electrode. In the example shown here, the reference electrode 13 includes a tubular housing 19, which surrounds a section of the sheath 12 of the substrate 3 and is closed at its front end facing the measurement medium by an annular diaphragm 21. The diaphragm 21 can be formed, for example, from a plastic such as PTFE or a porous ceramic such as ZrO 2 ceramics. The annular chamber formed between the sheath 12 and the housing 19 contains a reference electrolyte, such as a KCl solution, in which a reference element 23, such as a silver electrode coated with silver chloride, is impregnated. Instead of the diaphragm 21, the reference electrode 13 can also have other bridges, which establish an ion-conductive contact and / or an electrolyte contact between the reference electrolyte and the measurement medium. The annular chamber containing the reference electrolyte is closed, for example, by casting or adhesive bonding on its back side.

[0089] The substrate 3 is connected via a first electrical wire 11 to the sensor circuit 25 and thus forms the measuring electrode of the potentiometric sensor 100. The sensor circuit 25 is accommodated in an electronic housing 27 that is connected to the reference electrode and the measuring electrode of the sensor 100. The reference element 23 projects out of the annular chamber by casting or bonding and is also connected to the sensor circuit 25. The sensor circuit 25 is configured to detect the voltage generated by contacting the measuring medium between the measuring electrode 1 and the reference electrode 13 with the diaphragm 21 and the ion-selective enamel layer 7. This voltage is a function of the activity of the analyte ions present on the ion-selective enamel layer 7. The sensor circuit 25 can be configured to generate a measurement signal representative of the detected voltage and output it to, for example, a measuring transducer that is connected to the sensor circuit 25, processes the measurement signal, and uses a predetermined calibration function to determine therefrom the measured value of the ion concentration of the analyte ions or, if the potentiometric sensor 100 is designed as a pH sensor, the measured value of the pH. Alternatively, the sensor circuit 25 can also be configured to determine the measured value and output it via an interface 29 to a measuring transducer or another operating or display device.

[0090] Figure 3 Another exemplary embodiment of a sensor element 1 for a potentiometric sensor is schematically shown. The sensor element 1 has a substrate 31 made of ceramic or glass-ceramic, and a substrate 3 formed of a metal alloy is arranged on the substrate 31 in the form of a layer. The layer can be formed of a copper foil or a foil of a copper-based alloy or a small plate bonded, adhered, or otherwise fastened to the substrate. The ceramic of the substrate can be, for example, a zirconia ceramic or an alumina ceramic.

[0091] An ion-selective enamel layer 7 that will serve as the sensor layer is applied to the substrate 3. A transition zone 9 containing cuprous oxide (I) is arranged between the enamel layer 7 and the substrate 3. The enameling of the substrate 3 and the simultaneous formation of the transition zone 9 can be carried out in the same manner as disclosed in the exemplary embodiment described above with reference to Figure 1 The same manner as described in the exemplary embodiment.

[0092] The transition zone 9 is conductive, for example, ionically and / or electronically conductive, and together with the substrate 3 forms a solid terminal lead of the sensor element 1. At the contact point 10, the substrate 3 is in contact on the back side with an electrical conductor 11 that passes through the substrate 31 and can connect the sensor element 1 to the sensor circuit of the potentiometric sensor. The unit formed by the substrate 31, the substrate 3, the transition zone 9, and the ion-selective enamel layer is embedded in a glass sheath 12 that leaves only the surface area of the ion-selective enamel layer 7 and insulates the interfaces between the substrate 3 and the substrate 31 and between the substrate 3 and the overlying layer from the measuring medium.

[0093] Optionally, the sensor element may include a preamplifier and / or an impedance converter (not shown here), which is used to increase the signal-to-noise ratio of the measurement signal of the sensor element or the potential sensor having the sensor element. If the ion-selective enamel layer has a high impedance, it is particularly advantageous to integrate a preamplifier in the signal path close to the ion-selective enamel layer.

[0094] The potential sensor including the sensor element 1 shown here as the measuring electrode may have a reference electrode, which is also entirely formed by a layer stack, and the potential terminal lead thereof is designed as a solid terminal lead. The two electrodes may be arranged on a common substrate - for example, a circuit board or a non-conductive ceramic - and connected to the sensor circuit via wires - for example, conductor paths extending on the substrate. In this way, a very small potential sensor can be achieved.

[0095] In order to produce Figures 1 to 3 the sensor element 1 shown in, the enameling of the substrate 3 can be performed in the following manner:

[0096] In the first method variant, an enamel product, such as a powder formed by glass particles of an ion-selective glass or a suspension or paste containing glass particles of an ion-selective glass, can be applied to the pre-conditioned surface of the substrate 3 and heated to a temperature between 800 - 850 °C, which depends on the composition of the ion-selective glass. The heat treatment forms the enamel layer 7 and at least partially forms the transition zone 9. This method has been shown to be very suitable for applying an enamel layer 7 containing lithium and / or sodium. Since the glass forming the enamel layer 7 wets the CuO-containing surface significantly worse than the Cu 2 O-containing or copper-containing surface, it is advantageous to suppress the formation of CuO on the substrate surface in this embodiment. For this purpose, the enamel product may advantageously contain components that form low-melting salts - such as boric acid hydrates, carbonates or nitrates. Advantageously, the heat treatment takes a few minutes, for example less than 15 minutes.

[0097] In the second method variant, a vitreous body of an ion-selective glass - such as a glass plate - can be placed and fused on the conditioned substrate 3. Here, a temperature in the range of 400 °C to 1085 °C should also be reached to ensure that in addition to the resulting enamel layer, an ion-selective enamel layer 7 and a transition zone 9 containing Cu(I) are formed.

[0098] This method has proven to be particularly suitable for lithium-free, sodium-containing pH-selective glasses and sodium-selective glasses. In this method, it is advantageous that no CuO formation is observed at the surface of the substrate 3 during the fusion of the vitreous body.

[0099] In an exemplary embodiment, a carrier body of Macor (Corning, dimensions 15×15×3 mm) is covered with a cleaned and conditioned copper foil (99.99%, 5×5×0.03 mm). The copper foil is covered with a pH-sensitive planar glass, for example made of Corning 015 glass with dimensions 10×10×0.5 mm. Subsequently, the carrier body together with the structure is heat-treated at 800 °C for 4 minutes. Then, the copper foil is in contact with the glass-ceramic on the back side, and the copper foil consisting of the carrier body, the copper foil, and the fused enamel layer is coated by casting. The sensor element manufactured in this way can be connected to a potentiometric sensor circuit and used to measure the pH-related voltage between the sensor element and a reference electrode at a stable potential.

[0100] In all these method variants, in particular before enameling, the substrate 3 or at least the substrate area to be enamelled is conditioned, in particular passivated, in order to produce a copper(I)-containing (i.e., monovalent) copper oxide layer on the substrate surface, which oxide layer can form part of the transition zone 9 after application of the enamel layer 7. However, depending on the main conditions during enameling, during application of the enamel layer, the oxide layer can also at least partially or even completely dissolve in the transition zone 9.

[0101] The oxide layer can be produced, for example, by heat-treating the surface of the substrate 3 in a flame, by means of a laser or in an oven, in air or in an oxygen-free or oxygen-protected gas atmosphere. Similarly, the oxide layer can be produced by treatment in an oxygen plasma or by coating methods such as sputtering or chemical vapor deposition. The ratio of copper(I) and copper(II) 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 to a temperature of 400 to 500 °C for passivation in an atmosphere of a protective gas with a low oxygen content, such as nitrogen. This can be carried out, for example, in a furnace chamber continuously purged with nitrogen, where the oxygen partial pressure is about 0.8 hPa. Heat treatment of the surface area of the substrate consisting of copper produces a mixed-valence CuO layer containing a high proportion of monovalent copper Cu(I). x layer. If the substrate area consists of a copper-based alloy, an oxide layer is also formed, which contains a high proportion of monovalent copper and, in some cases, can contain a part of Cu(II) and other oxidized alloy parts. This layer has a thickness of less than 5 µm or even less than 1 µm. The Cu(I) portion in the CuO x layer can be controlled or selectively adjusted via the conditions prevailing during passivation (e.g., temperature program, gas atmosphere, especially its oxygen content). This allows, for example, the substrate to be reproducibly adjusted for subsequent enameling to produce a plurality of sensor elements with similar properties.

[0102] The glass layer may be applied to the passivated surface of the substrate by enameling, for example by application of the enamel preparation described further above or by fusing to a glass plate. x The preferred layer thickness of the layer is between 0.05 μm and 2 μm, preferably less than 1 μm. x Layers, such as CuOx layers with a thickness of more than 20 μm, can be separated from the metal substrate very easily as scale. In contrast, layers with the layer thickness shown adhere firmly and also lead to good adhesion of the enamel layer after the subsequent application of the enamel layer. It can also be seen that the aforementioned passivation of the metal surface leads to uniform wetting of the metal or alloy surface during the enameling step, so that the formed enamel layer has significantly fewer enamel defects - such as cracks, irregularities or holes - than the enamel layer applied to the non-passivated substrate. Therefore, the enamel layer on the passivated substrate can be applied relatively thinly and still cover the substrate in a liquid-tight manner. This makes it possible to provide a sensor layer with a relatively low impedance for the sensor element. The ion-selective enamel layer of a conventional enamel electrode according to the prior art is usually thicker than the ion-selective membrane of a conventional glass electrode with a liquid discharge to ensure that the enamel layer covers the potential discharge of the enamel electrode in a liquid-tight manner. Therefore, the ion-selective enamel layer of a conventional enamel electrode has a higher impedance than the glass membrane of a conventional glass electrode. To compensate for this, the surface of the ion-selective enamel layer is selected to be relatively large for conventional enameled electrodes used in the process industry, so that conventional enameled electrodes require significantly more installation space than conventional glass electrodes with liquid discharge. In contrast, the ion-selective enamel layer of the sensor element produced according to the method described herein can have a thickness of less than 500 μm, or less than 300 or 200 μm, or even less than 100 μm. As a result, the surface of the ion-selective enamel layer can also be kept small in order to provide a sensor element that requires less installation space.

[0103] As already mentioned, the application of the ion-selective enamel layer can be carried out in a conventional manner by over-enamelling in air or in an oxygen-free or low-oxygen protective atmosphere in order to influence the ratio of Cu(I) and Cu(II) present in each case in the transition zone produced here between the substrate and the formed glass layer.

[0104] Figure 4Shows the measurement results of the relative electromotive force (EMF) in mV detected between a potentiostatically stable reference electrode and various pH-selective sensor elements 1 acting as measuring electrodes as a function of the pH of the sample solution. Two of the measuring electrodes used utilize a copper substrate enameled with pH glass as the sensor element. The measured values of the electromotive force detected using different sensor elements are used for comparison with offset correction in the figure and are thus referred to as relative EMF values. For comparison, the measured values (dots) obtained using a conventional pH glass electrode as the measuring electrode relative to a potentiostatically stable reference electrode are also shown. To obtain each of the series of measurements, the respective measuring electrode and the reference electrode are brought into contact with sample solutions having pH values of 4, 6, and 7 as the measurement chain, and the voltage between the measuring electrode and the reference electrode is captured in each case.

[0105] The first measurement series (squares) was obtained using the first sensor element 1, which was produced by enameling a copper substrate with a pH enamel layer without prior conditioning. The second measurement series (triangles) was obtained using the second sensor element 1, which was produced by enameling a copper substrate with a pH enamel layer according to one of the above method variants, but with prior conditioning of the surface of the copper substrate to be enameled while generating a Cu(I) oxide layer. The conditioning was carried out by passivation as described above by heat treatment in a low-oxygen protective atmosphere in an oven. The sensor elements used in both series were additionally produced in the same way by enameling, where the chemical composition of the enamel used was the same.

[0106] The measured values of the comparative measurement series obtained using a conventional pH glass electrode are shown as dots.

[0107] It is evident from the measured values of the first measurement series that pH measurement is in principle possible even in the case of sensor elements 1 produced without a prior passivation step. However, a significant improvement in the sensing characteristics of the sensor elements was achieved, since the second measurement series, shown by additional passivation, has a course that approaches the course of the comparative measurement series with a conventional glass electrode.

Claims

1. A method of manufacturing a sensor element (1) for a potential sensor (100), comprising: conditioning at least one region of a substrate (3) composed of copper or a copper-based alloy having a mass fraction of at least 60% copper to produce an oxide layer having cuprous copper, and applying an ion-selective enamel layer (7) at least to the region of the substrate (3); wherein the oxide layer having cuprous copper has a thickness of less than 5 μm after conditioning; Wherein, when applying the ion-selective enamel layer (7) to the interface between the region of the substrate and the formed ion-selective enamel layer, a transition region (9) is generated, and the transition region (9) includes Cu 2 O, and the region of the substrate is in conductive contact with the ion-selective enamel layer via the transition region (9); wherein applying the ion-selective enamel layer (7) to the region of the substrate (3) comprises: applying an enamel article of an ion-selective glass to the region of the substrate; and heat-treating the enamel article applied to the substrate to form the ion-selective enamel layer; and wherein the conditioning of at least the region of the substrate is performed in a protective gas atmosphere.

2. The method according to claim 1, wherein, the ion-selective enamel layer is a pH-selective enamel layer.

3. The method according to claim 1, wherein, the ion-selective glass is a pH glass.

4. The method according to claim 3, wherein, the heat-treatment of the enamel article is performed at least intermittently at a temperature between 400 °C and 1085 °C.

5. The method according to claim 3, wherein, the enamel article is manufactured to comprise at least a powder of glass particles from the ion-selective glass, or is manufactured as a liquid or paste article comprising at least glass particles from the ion-selective glass.

6. The method according to claim 1, wherein, applying the ion-selective enamel layer (7) to the region of the substrate (3) comprises: applying a vitreous body made of an ion-selective glass to the region of the substrate (3), and fusing the vitreous body to the substrate to form the ion-selective enamel layer (7).

7. The method according to claim 1, wherein, applying the ion-selective enamel layer (7) to the region of the substrate (3) comprises: applying a melt of an ion-selective glass to the region of the substrate (3), and solidifying the melt by defined cooling to form the ion-selective enamel layer (7).

8. The method according to any one of claims 1 to 7, wherein, the substrate (3) is a body formed of copper or a copper-based alloy having a mass fraction of at least 80% copper, or the substrate (3) is formed of at least one substrate body (31), wherein at least one layer is composed of copper or a copper-based alloy.

9. The method according to claim 8, wherein, the substrate body is a metal or ceramic substrate body.

10. The method according to any one of claims 1 to 7, wherein, the conditioning of at least the region of the substrate (3) includes passivation performed before applying the ion-selective enamel layer (7), by thermal pre-treatment, plasma pre-treatment, electrochemical or chemical reaction in solution, or by applying an oxide layer by means of a coating process according to the gas phase.

11. The method according to claim 1, wherein, The passivation of at least the region of the substrate is carried out in a protective gas atmosphere.

12. The method according to any one of claims 1 to 7, further comprising: Coating at least the unit including the ion-selective enamel layer (7) and the substrate (3) with an electrically insulating material such that the sheath (12) formed in this way leaves only the surface of the ion-selective enamel layer (7) facing away from the substrate (3) open in the region of the sensor element (1) intended to be in contact with the measurement medium (5).

13. A sensor element (1) for a potentiometric sensor (100) manufactured by the method according to any one of claims 1 to 12.

14. The sensor element according to claim 13, wherein, the thickness of the ion-selective enamel layer (7) applied to the region of the substrate (3) composed of copper or a copper-based alloy having a mass fraction of at least 60% copper is less than 500 μm.

15. The sensor element (1) according to claim 14, wherein, The region of the substrate (3) on which the ion-selective enamel layer (7) is applied is conductively joined to the ion-selective enamel layer (7) by contacting the ion-selective enamel layer (7) via a transition region (9) comprising Cu 2 O.

16. The sensor element (1) according to claim 15, wherein, The transition region (9) has a layer including Cu 2 O, and the thickness of the layer including Cu 2 O is less than 5 μm.

17. The sensor element (1) according to claim 16, wherein, The layer including Cu 2 O has a thickness of less than 2 μm.

18. The sensor element (1) according to claim 17, wherein, The layer including Cu 2 O has a thickness of less than 1 μm.

19. The sensor element (1) according to any one of claims 13 to 18, wherein, the copper-based alloy is Cu1-xSnx or Cu1-xZnx, where x ≤ 0.

1.

20. The sensor element (1) according to any one of claims 13 to 18, wherein, the ion-selective enamel layer (7) is formed of ion-selective glass.

21. The sensor element (1) according to claim 20, wherein, the ion-selective glass is pH membrane glass.

22. The sensor element (1) according to any one of claims 13 to 18, wherein, the ion-selective enamel layer (7) is designed as a single-layer or multi-layer coating directly applied to the region of the substrate formed of copper or a copper-based alloy.

23. A potentiometric sensor (100), comprising: at least one sensor element (1) according to any one of claims 13 to 22, a reference electrode (13), and a sensor circuit (25) electrically connected to the sensor element (1) and the reference electrode (13) conductively, wherein the sensor circuit (25) is configured to detect the potential difference between the sensor element (1) and the reference electrode (13).

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