Method of Making a Reference Electrode

1. Field of the Invention

The present invention relates to ion concentration measuring apparatus. More specifically, the present invention is directed to a solid state ion responsive electrode and reference electrode.

2. Description of the Prior Art

Conventional ion concentration measuring electrode structures have usually used a glass measuring electrode, a reference electrode and a thermal compensator. For example, various types of special glasses have been used to measure the pH of aqueous solutions. In making these glass electrodes the pH sensitive glass is usually fused to the end of a less expensive glass tube and is subsequently blown into a small bulb of about two to four mils thick. These "hand-blown" pH glass electrodes are fragile, have very high electrical impedance due to the thickness of the glass and are used for limited temperature ranges mainly because of the internal pressure developed by a liquid electrolyte fill which is subsequently introduced into the interior of the pH measuring electrode to provide an electrically conductive ion source. An example of a typical prior art pH electrode apparatus is shown in U.S. Pat. No. 3,405,048 of D. J. Soltz. These prior art glass electrodes are expensive mainly because of the extensive use of highly skilled manual labor in the construction of the glass envelope and the subsequent filling thereof. A somewhat similar construction is used in the construction of the prior art reference cell which additionally increases the cost of the overall conventional pH measuring system. Despite its disadvantages, the glass electrode has retained its popularity in the field of ion concentration measurement even after attempts to develop a solid state electrode such as that shown in U.S. Pat. No. 3,498,901 of L. T. Metz et al since the response of the glass electrode is faster than other prior art devices with the glass electrode also having the broadest useful pH range. However, in order to provide a low cost and even more useful ion concentration measuring system it is desirable to produce a low impedance, high reliability and relatively unbreakable ion concentration measuring electrode and reference electrode.

An object of the present invention is to provide an improved solid state ion responsive and reference electrode structure.

Another object of the present invention is to provide a method for making a reference electrode structure.

In accomplishing these and other objects, there has been provided, in accordance with the present invention, method for making a combination ion responsive and reference electrode structure having an insulating substrate supporting an electrically conductive structure overlaid with a solid electrolyte layer having a final thin layer of ion responsive glass being attached to the solid electrolyte layer by RF sputtering. In the reference electrode, the outer glass layer has a coefficient of thermal expansion different from the solid electrolyte layer while in the ion responsive electrode, the outer ion responsive glass layer is substantially thermally matched to the supporting electrolyte layer.

A better understanding of the present invention may be had when the following detailed description is read in connection with the accompanying drawings, in which:

FIG. 1 is a pictorial illustration of a cross-section of a reference electrode embodying the present invention,

FIG. 2 is a pictorial illustration of a cross-section of a an ion concentration measuring electrode embodying the present invention, and

FIG. 3 is a pictorial illustration of a cross-section of a combination ion responsive, reference electrode and thermal compensator embodying the present invention.

Referring to FIG. 1 in more detail, there is shown a pictorial illustration of a cross-section of a reference electrode embodying the present invention. An electrically insulating substrate 2 of an electrically insulating material, e.g., ceramic or glass, has a first layer 4 of chromium (Cr) deposited on one side thereof. A second layer 6 of silver (Ag) is subsequently deposited on the chromium and a third layer 8 of silver chloride (AgCl) is deposited on the silver. An outer layer 10 of a suitable glass is RF sputtered on the exposed surface of the silver chloride layer 8. The glass material for the outer layer 10 is selected to have a coefficient of thermal expansion different from the supporting silver chloride structure 8 whereby a subsequent temperature cycling of the multilayer structure is effective to produce microscopic cracks in the outer glass layer. For example, borosilicate glass has a coefficient approximately 1/10 that of silver chloride. These cracks provide ion conduction paths to the silver chloride layer from an aqueous solution in which the reference electrode is immersed during pH measurements. A socket shell 12 is arranged to enclose a single contact pin 14. The contact pin is electrically connected by an electrically conducting wire 16 to the silver layer 6 of the multilayer structure. An encapsulating, or potting, compound 18 is subsequently applied to the multi-layer structure, the socket shell 12, the contact pin 14 and the connecting wire 16 to form a moisture-proof barrier and to unite the elements into a rigid package. An open window, or hole, 19 is formed through the potting compound 18 to expose the outer glass layer 10.

In FIG. 2, there is shown a pictorial illustration of an exemplary cross-section of a pH measuring electrode embodying the present invention. Similar reference numbers have been used in FIGS. 1 and 2 to indicate similar structural elements although the combination of FIG. 2 is directed toward a different device from that shown in FIG. 1. An electrically insulating substrate 2, e.g., glass or ceramic, is used as a support member for a multi-layer structure similar to that used in the reference electrode. Specifically, the glass substrate 2 is first plated with a first layer 4 of chromium which is followed by a second layer 6 of silver and a subsequent third layer 8 of silver chloride. An outer layer 20 of pH sensitive glass is then RF sputtered on the silver chloride layer. The temperature coefficient of the silver chloride and pH glass layer are matched whereby the pH glass will not produce microscopic cracks as during normal temperature cycling, e.g., 0.degree. to 100.degree. C, as in the case of outer glass layer used in the reference electrode previously described. For example, Corning 1990 glass has a coefficient of thermal expansion approximately one-half that of silver chloride. Other pH sensitive glasses can be produced to even more closely match the coefficient of thermal expansion of the silver chloride layer by using glass formulas with the following characteristics: a high coefficient of expansion can be achieved by using oxides such Li.sub.2 O, Na.sub.2 O, K.sub.2 O, Rb.sub.2 O, Cs.sub.2 O, BaO and SrO while a low coefficient of expansion can be achieved by using SiO.sub.2, B.sub.2 O.sub.3, Al.sub.2 O.sub.3, BeO and TiO.sub.2. Thus, the coefficient of thermal expansion of the silver chloride layer or other solid state electrolyte materials such as CuO, AgI, RbI, RbAg.sub.4 I.sub.5, etc. can be matched to an even closer approximation if either the temperature cycling during the measurement operation or the electrolyte material layer imposes a need for such a match. The thickness of the pH glass is approximately 10 to 10,000 A. A thermal compensator structure may be produced using an insulating substrate with a thermal sensitive element mounted therein and having the same overall configuration as that used for the aforesaid reference and pH electrodes whereby the three elements would be used concurrently as shown in the aforesaid Soltz U.S. Pat. No. 3,405,048.

In order to further utilize the solid state nature of the electrodes of the present invention, a combination structure having the reference electrode, the ion concentration measuring electrode and the thermal compensator integrated therein is shown in FIG. 3. As in the case of FIGS. 1 and 2, similar reference numbers have been reused in FIG. 3 for common, or similar, elements of the structure but a capital "A" reference letter has been added to some repeated reference numbers to indicate similar elements in adjacent sections of the integrated cell structure shown in FIG. 3. Thus, in the case of a pH electrode, a first substrate 2 of an electrically insulating material has the first chromium layer 4 followed by the silver layer 6 and the silver chloride layer 8 with an outer layer of the selected mismatched temperature coefficient glass 10 to form the reference electrode portion of the integrated cell. A second electrically insulating substrate 2A has a chromium layer 4A followed by a silver layer 6A and a silver chloride layer 8A with a pH glass outer layer 20. A socket shell 24 which may advantageously be a larger size than the socket shell 12 shown in FIGS. 1 and 2 to accommodate an additional number of connector pins is provided adjacent to one side of the aforesaid multilayer structure.

A plurality of electrical connector pins 26, 28, 30 and 32 are located within the connector shell 24. A first one of the pins 26 is connected to the silver layer 6A in the pH measuring electrode section of the integrated multilayer structure by wire 16A. Similarly, the fourth pin 32 is connected by a wire 16 to the silver layer 6 in the reference electrode portion of the integrated multilayer structure. The second and third pins 28 and 30 are connected to a thermal compensator element 34 by separate wires 36 and 38 whereby the thermal compensator element 34 is electrically connected across the second and third pins 28 and 30. The thermal compensator element 34 may be formed in a recess of the second substrate element 2A by any suitable means which can include the same RF sputtering technique used to provide the layers of the pH and reference electrodes structures. Finally, an outer shell, or convering, of a potting compound 40 is provided to enclose the multilayer structure and to secure the pins 26 to 32 while engaging the connector shell 24. A first hole, or window, 19 is provided in the covering 40 to expose the glass layer 10 of the reference electrode while a second opening 19A is provided in the covering 40 to expose the pH glass layer 20 of the pH electrode structure.

Since, in the RF sputtering process operation, the operating temperatures are below 200.degree. C, the preparation of the ion responsive electrode structure including the ion responsive glass layer is performed over a much smaller temperature range which further prevents the ion responsive glass from cracking when it is cooled down to room temperature even if the ion responsive glass layer and electrolyte layer do not have an exact temperature coefficient match. Additionally, the thin, i.e., 10,000 A maximum, glass layer will stretch instead of cracking during temperature cycling to enable the overall multilayer structure to withstand temperature cycling over a relatively wide temperature range, e.g., -70.degree. C to +200.degree. C. Inherently, the integrated electrode structure has a low impedance due to the thinness of the outer glass layer. Another feature is an extreme ease of replacement whereby the pH electrode, the reference electrode and the thermal compensator can be replaced as a single inexpensive unit. Further, since the delicate glass handling operations required for prior art electrodes have been eliminated, the high manufacturing repeatability of the produce and the reduction of manufacturing rejects enhances the low manufacturing costs of either the separate electrodes shown in FIGS. 1 and 2 or the combinational electrode structure shown in FIG. 3. Finally, in addition to savings in the amounts of materials used for the thin layers of the multilayer structure, additional savings will be effected by the elimination of certain expensive metals which were necessary in previous glass electrodes because of the required glass-to-metal seals, e.g., platinum or other similar thermal property metals.

Accordingly, it may be seen that there has been provided, in accordance with the present invention, a solid state ion responsive and reference electrode structure and method having application in either a separate or a combination electrode construction.