CA1135790A - Method of making and structure for a solid state ion responsive and reference electrodes - Google Patents

Abstract

ABSTRACT
A solid state ion concentration measuring electrode having the ion concentration measuring electrode structure formed by an outer ion sensitive layer deposited by RF sputtering on a substantially thermally matched supporting solid electrolyte layer. The reference electrode is similarly formed by depositing an outer layer of glass onto a supporting solid electrolyte layer by RF sputtering with the temperature expansion of the glass and supporting solid electrolyte structure being selected to produce a differential expansion causing random cracking of the glass layer during temperature cycling of the reference electrode. A combination structure is provided wherein the ion concentration measuring electrode and the reference electrode are combined in an integrated package. A thermal compensating element may also be included in the integrated package.

Description

BACKGROUND O~ THE INVENTION

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 respon-sive 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 p~ measuring electrode to provide an electrically conductive ion path. An example of a typical prior art pH electrode apparatus is shown in United States Patent No. 3,405,048 of D. J. Soltz issued October 8, 1968. These prior art glass electrodes are expen- -sive mainly because of the extensiva use of highly skilled manual labor in the construction of the glass envelope and the subsequent filling thereof. A some-what 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 the pH electrode shown in United States Patent No. 3,~98,901 of L. T. Metz et al since the response of the glass electrode 7~(~

is faster than other prior art devices with the glass electrode also having the broadest useful pll range. A more recent development using a multilayered solid state structure is shown in United States Patent No. 4,031,606 of E. L. Szonntagh issued June 28, 1977. However, the structure shown therein includes an insulating substrate which adds to the complexity of the manufacturing process and the end product. 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 having a simplified structure and method of manufacturing.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved solid state ion responsive electrode structure and reference electrode structure.
Another object of the present invention is to provide an improved combination ion concentration measuring electrode and reference electrode struc-ture.
In accomplishing this and other objects, there has been provided, in accordance with the present invention, a solid state ion concentration and reference electrode structure and m0thod of manufacturing having an ion measur-ing electrode with an electrical connection attached to a solid electrolyte layer and having an outer ion responsive layer attached to the solid electrolyte layer by RF sputtering. In the reference electrode, an outer glass layer is supported on a solid electrolyte layer having an electrical connection attached thereto.
In the reference electrode, the glass layer has a coefficient of thermal expansion different from the supporting solid electrolyte layer while in the ion measuring electrode, the outer ion responsive layer is substantially thermally matched to the supporting electrolyte layer.
In accordance with the present invention, there is provided a reference electrode comprising: a self-supporting electrolyte pellet, an electrical connec-tion means to said pellet and a cracked glass layer on said electrolyte pellet spaced from said connection means and having a coefficient of thermal expansion substantially mismatched with respect to a coefficient of thermal expansion of said electrolyte pellet to induce cracking of said glass layer.
In accordance with the present invention, there is also provided a method of making a reference electrode comprising the steps of forming a solid electrolyte pellet, depositing a glass layer on said electrolyte pellet with a coefficient of thermal expansion mismatched with respect to a coefficient of thermal expansion of said elec*rolyte pellet to produce cracking of said glass layer during a predetermined temperature cycle and exposing said electrode to said temperature cycle.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the present invention may be had when the following detailed description is read in connection with the accompanying draw- ;
ings, in which:
Figure 1 is a pictorial illustration of a cross-section of an example of a reference electrode embodying the present invention, Figure 2 is a pictorial illustration of a cross-section of an apparatus used in the manufacturing of the reference electrode shown in Figure 1, Figure 3 is a pictorial illustration of a cross-section of an example of another reference electrode structure also emboding the present invention, Figure 4 is a pictorial illustration of a cross-section of an apparatus used in the manufacturing of the reference electrode shown in Figure 3, Figure 5 is a pictorial illustration of a cross-section of an ion con-centration measuring electrode embodying the present invention and using the apparatus shown in Figure 4, Figure 6 is a pictorial illustration of a cross-section of an example 579~3 of a combination electrode having an ion responsive electrode, a reference electrode and thermal compensator embodying the present invention, Figures 7 and 8 are end views of the combination electrode shown in Figure 6 showing different physical configurations, Figure 9 is a pictorial illustration of a cross-section of an example of a combination electrode having a plurality of ion responsive electrode, a reference electrode and a thermal compensator, and Figures 10 and 11 are end view Gf combination electrodes having a plurality of ion responsive electrodes and a reference electrode showing differ-ent physical configurations.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to Figure 1 in more detail, there is shown a pictorial illustration of a cross-section of an example of a reference electrode embodying the present invention. A solid pellet 2 of silver chloride (AgCl) forming an electrolyte layer has a silver strip 4 embedded therein. An outer end 6 o~-the silver strip 4 is arranged to overlie an outer surface 8 of the pellet 2.
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 outar layer 10 is select-ed to have a coafficient of thermal expansion different from the supporting sil-ver chloride pellet ~ 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. ~hese cracks provide ion conduction paths to the silver chlor-ide layer from an aqueous solution in which the reference electrode is immersed during ion concentration measurements. A socket shell 12 is arranged to enclose a single electrically conducting contact pin 14. The contact pin is electrically ~3r~7~3~

connected, e.g., soldered, to the silver strip 4 of the multilayer structure.
An electrically insulating encapsulating, or potting, compound 1~ is subsequently applied to the pellet structure, the socket shell 12, the contact pin 14 to form a moisture-proof barrier, i.e., an hermetic sealJ and to unite the elements into a rigid package. An open window, or hole, 19 is formed, e.g., by masking, through the potting compound 18 to expose the outer glass layer 10.
In Figure 2, there is shown a cross-section of an apparatus suita~le for use in the manufacturing of the electrode shown in Figure 1. A silver chloride powder 2A is located within a p~ellet molding shell 20 having an end plug 22. The internal diameter of the shell 20 is arranged to be the desired diameter of the pellet 2, e.g., one inch. The silver strip 4 in the form of a narrow V is inserted into the powder 2A with the outer end 6 projecting from the powder 2A. A molding plate 24 having a right fit within the shell 20 is forced by a ram 26 against the powder 2A and strip 6 using a suitable pressure, e.g., 5000 to 20,000 psi, to form the pellet 2. The pellet 2 is subsequently RF
sputtered by a conventional technique with the glass layer 10 and temperature cycled to produce cracks in the glass layer 10 as shown in Figure 1. The fur-ther manufacturing operations of attaching the pin 14 and potting the reference electrode and socket shell 12 into a unitary device are operations well-known in the art and no further description thereof is believed to be necessary.
In Figure 3, there is shown a pictorial illustration of a cross-section of another structure of a reference electrode also embodying the present invention. Similar reference numbers have been used for structural elements common with the structure shown in Figure 1. The reference electrode structure shown in Figure 3 has a silver contact pin 30 with a head 32 embedded in the silver chloride pellet 2. This structure eliminates the separate silver strip 4 and contact pin 14 used in the structure shown in Figure 1. The further struc-ture shown in Figure 3 includes the socket shell 12 and potting compound envelope 18 as shown in Figure l.
Figure 4 is a cross-section of an apparatus suitable for use in the manufacturing of the electrode shown in Figure 3. In an apparatus similar to that shown in Figure 2, silver chloride 2A is placed within the molding shell 20. The silver contact pin 30 is inserted into the powder 2A ~ith the head 32 being completely covered by the powder 2A. A molding plate 34 having a tight fit within the shell 20 is provided with a recess 36 closely conforming to the projecting leg of the contact pin 30. The length of the recess 36 is arranged to accomodate the projecting leg of the pin 30 to allow the plate 34 to compTess the powder 2A without disturbing the position of the head 32 in the powder 2A.
The ram 26 is used to force the plate 34 against the powder 2A as described above with respect to Figure 1. The pellet 2 with the pin 30 is subsequently coated with an RF sputtered glass layer, temperature cycled and potted with the socket shell 12 using conventional techniques to achieve the unitary structure shown in Figure 3.
In Figure 5, there is shown a pictorial illustration of a cross-section of an exemplary ion responsive electrode embodying the present invention and using the manufacturing apparatus shown in Figure 4. Similar reference numbers have been used in Figures 3, 4 and 5 to indicate similar structural elements although the structural combination of Figure 5 is directed toward a different device from that shown in Figure 3. A solid pellet 2 of silver chloride ~AgCl) forming an electrolyte layer has a head 32 of a silver contact pin 30 embedded therein using the manufacturing technique previously described with respect to Figure 4. An outer layer 40 of an ion responsive material, e.g., pH sensitive glass, is then RF
sputtered by conventional means on the silver chloride pellet 2. The temperature coefficient of the silver chloride and pH glass layer 40 are matched whereby the pH glass will not produce microscopic cracks as during normal temperature cycling, e.g., 0 to 100C, as in the case of outer glass layer used in the 1~S7~3V

reference elect-rode previously described For example, *Corning 1990 glass has a coefficient of thermal expansion approximately one-half that of silver chlo-ride. Other pll sensitive glasses or materials can be produced to even more closely match the coefficient of thermal expansion of the silver chloride pellet 2 by using glass formulas with the following characteristics: a high co-efficient of expansion can be achieved by using oxides such as ~i20, Na20, K20, Rb20, Cs20, and BaO and SrO while a low coefficient of expansion can be aChieved by using SiO2, B203, A1203, BeO and TiO2. Thus, the coefficient of --thermal expansion of the silver chloride pellet 2 or other solid state electrolyte materials such as CuO, AgI, RbI, RbAg4I5, etc. can be matched to an even closer approximation to maintain the integrity of the responsive layer 40 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 lOK A. The pellet 2 witX the contact pin 2 and the RF
sputtered ion responsive layer 40 is subsequently combined with the socket shell 12 and potting envelope 18 to achieve a unified structure, including the window 19 exposing a portion of the ion responsive layer 40, using conventional techniques. It should be noted that ion responsive electrode can also be manufactured using the apparatus shown in Figure 2 to achieve a structure similar to that shown in Figure 1 with the ion responsiue layer 40 replacing the cracked glass layer 10.
A separate thermal compensator structure ~not shown) may be produced using an insulating substrate with a thermal sensitive element, e.g., resistor, diode, etc., 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 3,405,048 patent.
In order to further utilize the solid state nature of the electrodes * Trade Mark S~ 3 of the present invention, an example of a combination st-ructure having the refer-ence electrode, the ion concentration measuring electrode and the thermal com-pensator integrated therein is shown in Figure 6. A first silver chloride pellet 2C has a cracked glass layer 10 thereon and a silver contact pin 30A embedded therein as previously described with respect to Figures 3 and 4. A second silver chloride pellet 2D has an ion responsive layer 40 thereon and a silver contact pin 30B embedded therein as previously described with respect to Figure 5. A
socket shell 44 which may advantageously be a larger size then the socket shell 12 shown in Figures 1, 3 and 5 to accomodate an additional number of connector pins is provided adjacent to one side of the aforesaid pellet structures and surrounding the pins 30A, 30B.
A pair of additional electrical connector pins 46, 48 are located within the connector shell 24. The additional pins 46 and 48 are connected to a thermal compensator element 50 by separate wires 52 and 54, respectively, whereby the thermal compensator element 50 is electrically connected across the additional pins 46 and 48. The thermal compensator element 34 may be any suitable thermally responsive device, such devices being well-known in the art.
Finally, an outer shell, or covering, of an electrically insulating potting compound 56 is provided to enclose the multi element structure and to space and to secure the pins 46 and 48, the element 50 and the pellets 2C and 2D while engaging the connector shell 24. A first opening, or window, 19A is provided in the covering 56 to expose the cracked glass layer 10 of the reference electrode while a second opening l9B is provided in the covering 40 to expose the ion responsive layer 40 of the ion responsive electrode structure.
In Figures 7 and 8 are end views of combined electTodes using the structure of Figure 6 and showing various configurations for the windows l9A
and l9B to accommodate desired operating and mechanical criteria for the com-bination electrode of Figure 6. Other configurations may be used by those skilled in the art without departing from the scope of the present invention In Figure g, there is shown a pictorial illustration of a cross-sec-tion of an example of a multi-ion sensing electrode structure wherein a plurality of ion responsive electrodes are combined with a common reference electrode and a common thermal compensator. Thus, a first ion responsive electrode 60 using the previously discussed structure of éither Figures 1 or 5 is provided with a first ion responsive outer layer, e.g., pH glass, while a second ion responsive electrode 62 using the previously discussed structure of either Figures 1 or 5 is provided with a second ion responsive outer layer, e.g,, sodium ion selective glass. A shared, or common, reference electrode 64 and a thermal compensator 66 are also provided in the overall electrode structure. A socket shell 68 and thermal compensator connector pins 70, 72 are secured to the electrodes 60, 62, 64 by an electrically insulating and hermetic sealing envelope 74. A first window l9A for the cracked glass layer 40 of the reference électrode 64, a second window l9B for the first ion responsive electrode 60 and a third window l9C for the seeond ion responsive electrode 62 are provided in the hermetic envelope 74 by any suitable conventional technique. Figures 10 and 11 are end view of multi-ion electrode structures using the structure of Figure 9 and show-ing various configurations of the windows l9A, l9B and 19C. It should be noted that the structure shown in Figure 9 may be expanded to accommodate more than two ion responsive electrode, more than one reference electrode and more than one thermal compensator, if desired.
MODE OF OPERATION
Since, in the RF sputtering process operation, the operating tempera-tures are below 200C, the preparation of the ion responsive electrode structure including the ion responsive layer is performed over a much smaller temperature ,f' _ 9 _ .

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~357~

range which further prevents the ion responsive layer from cracking when it is cooled down to room temperature even if the ion responsive layer and the electro-lyte pellet do not have an exact temperature coefficient match. Additionally, the thin, i.e., 10,000 A maximum, ion responsive layer will stretch instead of cracking during temperature cycling to enable the electrode structure to with-stand temperature cycling over a relatively wide temperature range, e.g., -70C
to i200C even with a slight temperature coefficient mismatch. Inherently, the integrated electrode structure has a low impedance due to the thinness of the outer ion responsive layer. Another feature is an extreme ease of replacement whereby the ion responsive 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 product and the reduction of manufacturing rejects enhances the low manufacturing costs of either the separate electrodes shown in Figures 1 and 5 or the combinational electrode structure shown in Figures 5, ~ and 9. Finally, in addition to savings in the amount of materials used for the thin layers of the multilayer structure, addi-tional savings will be effected by the elimination of certain expensive metals which were necessary to previous blown glass electrodes because of the required glass-to-metal seals~ e.g., platinum or other similar thermal property metals.
In order to enable the ion responsive electrode of the present inven-tion either in a separate or combination embodiment to measure other than hydro-gen ions, the ion responsive layer 40 would be changed to an appropriate ion responsive material. For example to measure fluoride ion concentration, the layer 40 would be lanthanum fluoride. Similarly, ~or potassium ions, potassium ion selective glass would be used, while for sodium ions, sodium ion selective glass would be used. The method of depositing the various materials for the ion ~l~S~

responsive layer ~ould be the same as that disclosed above for the pH electr~de and the overall electrode structure ~1culd also be the same as that disclosed for the pll electrode.
Accordingly, it may be seen that there has been provided, in accord-ance with the present inventionJ a solid state ion responsive and reference electrode structure having application in either a separate or a combination electrode construction.

Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A reference electrode comprising: a self-supporting electrolyte pellet, an electrical connection means to said pellet and a cracked glass layer on said electrolyte pellet spaced from said connection means and having a co-efficient of thermal expansion substantially mismatched with respect to a co-efficient of thermal expansion of said electrolyte pellet to induce cracking of said glass layer.

2. A reference electrode as set forth in Claim 1 wherein said connection means includes a silver strip and said electrolyte pellet is silver chloride.

3. A reference electrode as set forth in Claim 2 wherein said electrical connection means is partially embedded in said pellet.

4. A reference electrode as set forth in Claim 3 and further including an encapsulating means covering and hermetically sealing said pellet and a por-tion of said cracked glass layer while exposing said electrical connection means.

5. A method of making a reference electrode comprising the steps of forming a solid electrolyte pellet, depositing a glass layer on said electrolyte pellet with a coefficient of thermal expansion mismatched with respect to a co-efficient of thermal expansion of said electrolyte pellet to produce cracking of said glass layer during a predetermined temperature cycle and exposing said electrode to said temperature cycle.

6. A method of making a reference electrode as set forth in Claim 5 wherein said glass layer is deposited by RF sputtering.

7. A method of making a reference electrode as set forth in Claim 5 and including the further step of providing an electrical connection to said electrolyte pellet.

8. A method of making a reference electrode as set forth in Claim 7 wherein said forming of said electrolyte pellet includes the step of compressing an electrolyte powder into said electrolyte pellet and said step of providing an electrical connection includes the step of supporting an electrical conductor partially embedded in said electrolyte powder during said compression of said electrolyte powder.

9. A method of making a reference electrode as set forth in Claim 7 and including further step of encapsulating said pellet while exposing a portion of said glass and said electrical connection.