ELECTROCHEMICAL SENSOR

Abstract

An electrochemical sensor comprising a probe immersible in a measured medium and having at least two electrodes of a first electrically conductive material and at least one probe body of a second, electrically non-conductive material. The electrodes are at least partially embedded in the probe body and insulated from one another by the probe body, wherein the at least two electrodes are embodied of at least one conductive material and the probe body of at least one electrically insulating ceramic, wherein the electrodes are embodied of thin, measuring active layers of a conductive material and sit in an end face of the probe body of a ceramic material, and wherein the electrodes are electrically contacted via connection elements extending through the probe body.

Claims

1. A method of manufacturing an electrochemical probe, comprising: producing a green probe body of an electrically insulating ceramic, the probe body having: a first end and a second end; an axis of symmetry extending from the first end to the second end; at least two cavities disposed in an end face of the second end, wherein each of the at least two cavities is between 10 micrometers (μm) and 3 millimeters (mm) deep; at least two through-holes extending through the probe body from the first end to the second end and communicating with a respective cavity at the second end; providing at least two electrodes embodied of an electrically conducting material, wherein each electrode is between 10 μm and 3 mm thick; providing at least two connection elements embodied of the electrically conducting material, wherein each connection element is embodied to extend from the first end to the second end of the probe body through one of the at least two through-holes, and each connection element is electrically connected with one of the at least two electrodes; pressing each electrode with its connected connection element into the probe body by pressing the electrode into one of the at least two cavities and pressing the connection element into the corresponding through-hole such that the connection element extends through the probe body to the first end and an outer surface of the electrode is flush with the end face of the second end of the probe body; and sintering together the green probe body, the at least two electrodes, and the at least two connection elements, wherein the sintering produces a material bonding between the electrodes and the probe body making a gap-free material transition between each electrode and the probe body, wherein a thermal coefficient of expansion of the electrically insulating ceramic and a thermal coefficient of expansion of the electrically conductive material differ by not more than 2×10.sup.−6 K.sup.−1.

2. The method of claim 1, wherein the electrically insulating ceramic is one of: a zirconium oxide (ZrO.sub.2) ceramic, an aluminum oxide (Al203) ceramic, a chromium oxide (Cr2O3) ceramic, a titanium dioxide (TiO.sub.2) ceramic, and a tialite (Al.sub.2TiO.sub.5) ceramic.

3. The method of claim 1, wherein the electrically conductive material is one of: electrically conductive ceramic, electrically conductive enamel, platinum, titanium, and stainless steel.

4. The method of claim 1, wherein the electrically conductive material is platinum and the electrically insulating ceramic is zirconium oxide ceramic stabilized with magnesium (ZrO.sub.2MgO).

5. The method of claim 1, further comprising: joining a metal process connection to the sintered probe body probe body with an adhesive.

6. The method of claim 1, wherein the probe body includes a cavity accessible from the first end of the probe body, the method further comprising: inserting a temperature sensor into the cavity of the sintered probe body; and joining the temperature sensor to the sintered probe body in the cavity with a thermally conductive adhesive.

7. The method of claim 1, further comprising: machining and polishing the second end face of the sintered probe body and the at least two electrodes to remove any surface roughness.

8. The method of claim 1, wherein the green probe body further has process connection embodied of the same electrically insulating ceramic as the probe body such that the probe body and the process connection form a single molded part.

9. A method of manufacturing an electrochemical probe, comprising: producing a green probe body of an electrically insulating ceramic, the probe body having: a first end and a second end; an axis of symmetry extending from the first end to the second end; at least two cavities disposed in an end face of the second end, wherein each of the at least two cavities is between 10 micrometers (μm) and 3 millimeters (mm) deep; at least two through-holes extending through the probe body from the first end to the second end and communicating with a respective cavity at the second end; providing at least two connection elements embodied of an electrically conducting material, wherein each connection element is embodied to extend from the first end to the second end of the probe body through one of the at least two through-holes; pressing each connection element into the probe body; forming at least two electrodes, wherein each electrode is formed by deposition of a conductive metal into one of the at least two cavities, wherein each electrode makes electrical contact with a respective connection element, wherein each electrode is formed to be between 10 μm and 3 mm thick and to fill the corresponding cavity, and wherein each electrode has an outer surface flush with the end face of the second end of the probe body; and sintering together the green probe body, the at least two electrodes, and the at least two connection elements, wherein the sintering produces a material bonding between the electrodes and the probe body making a gap-free material transition between each electrode and the probe body, wherein a thermal coefficient of expansion of the electrically insulating ceramic and a thermal coefficient of expansion of the electrically conductive metal differ by not more than 2×10.sup.−6 K.sup.−1.

10. The method of claim 9, wherein the deposition of the conductive metal is formed by one of: a vapor deposition of metals, a sputtering of metals, and a screen printing with metal pastes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention will now be explained in greater detail based on the examples of embodiments shown in the drawing, the figures of which show as follows:

[0035] FIG. 1 is a probe of an electrochemical sensor, especially a conductivity sensor, according to a first embodiment of the invention;

[0036] FIG. 2 is a probe of an electrochemical sensor, especially a conductivity sensor, according to a second embodiment of the invention; and

[0037] FIG. 3 is a probe of an electrochemical sensor, especially a conductivity sensor, according to the second embodiment of the invention of FIG. 2 with a diameter expansion of the process connection at the joint.

DETAILED DISCUSSION IN CONJUNTION WITH THE DRAWINGS

[0038] FIG. 1 shows a probe 1 of the invention for an electrochemical sensor, especially a conductivity sensor, with a probe body 3 of an electrically non-conductive, ceramic material and, according to the invention, therein embedded electrodes 5 of a thin, electrically conductive material. The coating thickness of the material of the electrodes 5 of the invention, which are provided in FIG. 1 as concentric rings, respectively sleeves, sintered into the probe body 3, lies in a range of 10 micrometer to 3 millimeter, whereby material for the manufacture of the probe 1 and, thus, costs, are saved. The end faces of the electrodes 5 lie freely exposed on the end face 7 of the probe body 3 and in the case of a measuring of conductivity they are in contact with the measured medium. FIG. 1 shows a perspective view of the probe 1 and shows, concentrically arranged around the rotational symmetry axis Z, the ring elements of the electrodes 5, which in the case of a measuring of conductivity are immersed in the measured medium. Electrodes 5 are embodied as ring elements coaxially arranged around the shared rotational symmetry axis Z and are embedded in the sensor body 3 insulated from one another. Probe 1 is embodied as a measuring probe of a 4-electrode sensor. In the case of this type of sensor, in measurement operation, an alternating voltage is applied to the two electrodes 5 of the electrical current electrodes and the potential difference determined on the other two, remaining electrodes of the voltage electrodes. Using a measurement transmitter (not explicitly shown) connected with the electrodes 5, the impedance of the conductivity measurement cell formed by the probe 3 immersed in the measured medium is ascertained. Taking into consideration the cell constants, the specific resistance, respectively the specific conductivity, of the measured medium can be ascertained therefrom. The ascertained measured values can either be displayed by the measurement transmitter or output to a superordinated control system. A part the functions of the measurement transmitter can be executed by a measuring electronics accommodated in a separate housing outside of the measurement transmitter. This measuring electronics can, at least in part, be accommodated, for example, in a plug head connected with the probe 1, which plug head is available from the applicant under the mark, MEMOSENS®.

[0039] The electrodes 5 are platinum and the probe body 3 a zirconium oxide ceramic stabilized, respectively partially stabilized, by means of magnesium. The platinum of the electrodes 5 and the zirconium oxide ceramic of the probe body 3 stabilized with magnesium possess approximately the same thermal coefficients of expansion, for example, with magnesium stabilized zirconium oxide ZrO.sub.2MgO being at 9.3×10.sup.−6 K.sup.−1 (per degree Kelvin) and platinum Pt at 8.8×10.sup.−6 K.sup.−1. There are, however, other such material combinations for the electrodes 5 and the probe body 3, whose thermal coefficients of expansion differ only little from one another, i.e. preferably deviating from one another by only 1×10.sup.−6 to 2×10.sup.−6 K.sup.−1. Thus, for example, in the case of platinum as material for the electrodes 5, which has a thermal coefficient of expansion of 8.9×10.sup.−6 K.sup.−1, such can be combined with an aluminum oxide ceramic with a coefficient of expansion of 6 to 8×10.sup.−6 K.sup.−1. In the case of titanium with a coefficient of expansion of 10.8×10.sup.−6 K.sup.−1 as electrode material, such can be used with, for example, zirconium oxide ceramic with a coefficient of expansion of 10 to 12×10.sup.−6 K.sup.−1 as material for the probe body 3. A zirconium oxide ceramic for the probe body 3 is likewise suitable for combination with stainless steel as material for electrodes 5, since stainless steel has a thermal coefficient of expansion of about 13×10.sup.−6 K.sup.−1.

[0040] Through the situating of metal in a ceramic shape, e.g. by sintering, the metal of the electrodes 5 is surrounded in a shape-interlocking manner by the ceramic material of the probe body 3 and there arises also, such as earlier described, a material bonding between the two materials. For situating the electrodes 5 in the probe body 3, the electrodes are seated in cavities provided in the probe body 3 or slightly pressed into the green body of the probe body 3. After insertion of the electrodes 5 into the ceramic green body of the probe body 3, the assembly is sintered by means of a predetermined temperature regimen.

[0041] The electrodes 5 can also be produced by deposition of the conductive material into corresponding cavities in the probe body 3. The following methods can be used for the deposition: [0042] vapor deposition of metals [0043] sputtering of metals [0044] screen printing with metal pastes

[0045] In supplementation, also the probe body 3 can be produced by the following deposition methods from a gas phase or liquid phase:

[0046] Chemical vapor deposition (CVD)—In such case, a plurality of gases react with one another at a certain pressure and high temperatures and deposit a ceramic material.

[0047] Physical vapor deposition (PVD)

[0048] Chemical vapor infiltration (CVI)

[0049] Since the coefficients of expansion of ceramics, such as e.g. zirconium oxide and metal, preferably platinum, are almost identical, gap formation can be minimized. Furthermore, such a ceramic is suited due to its poor electrical conductivity as a support material for electrical measurements between the electrodes 5. Furthermore, ceramics are very suitable support material due to their very good chemical durability. Ceramics have the property that they age very much slower than synthetic materials, which leads to a very much longer service life of the sensor. The surface roughness of the end faces 7 of the electrodes and/or of the probe body 3, as well as the joint 8 between probe body 3 and process connection 6, is further reduced by polishing processes after the manufacture, so that possibly arising gaps and openings on the outer surface of the ceramic probe body 3 are removed and, thus, the high hygienic requirements of the probe 1 can be durably fulfilled.

[0050] Used as electrically conductive material can also be an electrically conductive ceramic, respectively enamel, which is cast, injected, respectively introduced into the corresponding cavities in the green body of the probe body 3 and after introduction sintered together with the green body of the probe body 3. This embodiment has the advantage that the used materials and, thus, the coefficients of expansion are very similar.

[0051] Embedded in the probe body 3 and in the process connection 6 are the electrodes 5 of the probe 1, which are electrically contacted via connection elements 2, respectively connection lines. Provided for this, for example, in a region of the sensor body 3 and of the process connection 6 facing away from the process are connection elements 2, via which the electrodes 5 can be connected with a control or measuring electronics.

[0052] Used for measuring the current temperature of the medium can be, furthermore, a temperature sensor 4. Temperature sensor 4 is inserted via a cavity provided in the probe body 3 facing away from the medium, respectively held in place with a thermally conductive adhesive. By means of this temperature sensor 4, the current temperature of the medium on the electrodes 5 can be ascertained and, thus, a thermal correction of the conductivity measurement performed.

[0053] Probe 1 shown in FIG. 2 forms the measuring probe of a so-called 4-electrode sensor immersible in a measured medium. Two electrodes 5, especially two electrodes 5 directly adjoining one another, are operated as so called electrical current electrodes. The two remaining electrodes 5 are operated as voltage electrodes. Applied between the two electrical current electrodes in measurement operation is an alternating voltage, in order to introduce an alternating electrical current into the measured medium. Measured between the voltage electrodes, especially using a currentless measuring, is the resulting potential difference. Using the introduced alternating electrical current and the measured potential difference, the impedance of the conductivity measurement cell formed through immersion of the probe 1 in a measured medium is calculated, and from the impedance while taking into consideration the cell constant, the specific resistance, respectively the conductivity, of the measured medium can be ascertained. Serving for control of the introduced alternating current for measuring the potential difference of the voltage electrodes and converting the measured values into a resistance, respectively conductance or a specific resistance, respectively specific conductivity of the measured medium is a measurement transmitter (not explicitly shown) connected with the probe 1. The measuring electronics can be a component of the measurement transmitter or at least partially accommodated in a separate module, for example, in a plug head connected with the probe 1. The ascertained measured values can either be displayed by the measurement transmitter or output to a superordinated control system.

[0054] As described in WO 2010/072483 A1, the probe 1 can also be produced in a single method step by means of a two component, injection molding method. In the case of this method, preferably an injection molding machine with two injection units is used. In the case of application of one injection unit for the electrode material and an additional injection unit for the material of the sensor body, the two injection units are preferably controlled independently of one another, since, in this way, a larger variety of electrode geometries can be produced. Two component injection molding is a technology established especially for the manufacture of components of different synthetic materials. The injection molding of metals or ceramics, for example, by means of metal powder injection molding (MIM—Metal Injection Molding) or ceramic power injection molding (CIM—Ceramic Injection Molding), is a known and established manufacturing method for technically demanding and complex molded parts. Also, multicomponent injection molding of metals and/or ceramics as individual components is, in principle, known, however, previously not usual in the manufacturing of composites of metal and ceramic.

[0055] In FIGS. 2 and 3 of the probe 1, the probe body 3 is joined with a process connection 6. For this, the probe body 3 is connected mechanically stably and sealingly with the process connection 6, for example, by means of an adhesive. The joint 8 between the sensor body 3 and the process connection 6 can be further worked by means of machining, grinding, and/or polishing. In this way, also adhesive residues are removed. The diameter of the process connection 6 and of the probe body 3 is enlarged at least in this region of the subsequent working of the joint 8. In order that the adhesive gap be as small as possible, thus, as hygienic as possible, the lower end of the process connection 6 as well as the ceramic sensor body 3 are provided with a diameter larger than desired in the target application. Through subsequent grinding or machining of the joint 8 of the connection between sensor body 3 and process connection 6, a region with very much smaller surface roughness is produced. Thus, also highest hygienic requirements can be fulfilled.

[0056] The measuring active layer of the conductive material of the electrodes 5 is embodied in a coating thickness d of, for example, 10 μm-3 mm and so seated in the probe body 3 that its end faces 7 lie in a plane A. The thickness d, respectively height, of the electrodes 5 as well as their diameter D amounts in the embodiment of a four electrode measuring probe 1 of FIG. 2 or FIG. 3 to preferably 1 to 2 millimeter.