US3334031A - High density bipolar electroplating electrolyte - Google Patents

Description

Aug. 1, 1967 E. s. SNAVELY, JR, ETAL 3,33 ,031

HIGH DENSITY BIPOLAR ELECTHOPLATING ELECTROLYTE Filed May 18, 1964 EARL S. SNAVELY, JR.

WALLACE E. HARRELL,JR.

INVENTORS BY -QW United States Patent This invention relates to an electrolyte; more specifically, to an electrolyte containing silver ion and having a high density.

Various silver-silver ion solution combinations have been used in electroplating, primarily in conventional electroplating. Normally, such solutions contain cyanide ion. Such conventional electroplating solutions have a low density and are unsatisfactory, for one reason or another, for bipolar electroplating.

Bipolar electroplating is often regarded as an undesirable side effect of conventional electroplating applications; however, for some purposes, it has been found desirable to utilize bipolar electroplating as the principal reaction in an electroplating system, minimizing as much as possible any other efiects. In some applications it has been found essential that the density of the electrolyte be quite highmuch higher than is available in conventional electroplating solutions.

It is therefore a primary object of this invention to provide a high density silver plating solution having utility as an electrolyte for electroplating applications wherein bipolar action is desired. It is a further object of this invention to provide such an electrolyte that is stable under storage conditions and which is operative, for bipolar applications, to yield a high quality deposit of silver on the bipolar element interposed into the system.

More specifically, it is an object of this invention to provide an electrolyte having almost 100% primary electrode efiiciencies and, at the same time, having high bipolar etficiency.

This invention may have utility for various purposes wherever an electrolyte is required; however, the primary application now known for the invention is in the bipolar electroplating of certain elements. A specific currently known application is in the practice of a certain microbalancing method which is claimed in a copending application, Ser. No. 298,261, filed July 29, 1963, by Donald Luther Ensley, entitled, Ultrasonic Angular Displacement System, now Patent No. 3,293,919.

In the practice of such microbalancing method, an object to be bipolar plated in one case was a silver ring disposed about the periphery of a quartz inertial element in a flotation chamber. The inertial element was to be balanced so that its center of buoyancy and center of mass were essentially at the same location. To practically accomplish such a result, the quartz inertial element assembly, including the silver ring, was placed within an electroplating cell with a plurality of silver electrodes disposed concentrically about the outer periphery of the balance ring. The electrolyte in the cell provided silver ion for plating action on the silver ring. By application of a voltage to selected pairs of electrodes, metallic silver was removed from one portion of the balance ring adjacent cathode while silver was plated onto another portion adjacent an anode. The effective redistribution of material on the inertial ring, as the ring and the element rotate under gravity forces, balanced the inertial element by bringing the center of mass and the center of buoyancy into coincidence.

In the conduct of such a microbalancing method it is important that the inertial element-ring assembly be approximately neutrally buoyant in the electrolyte so that the assembly will not have a net unbalanced force actice ing to cause it to sink in the electrolyte. Since the assembly has a specific gravity of about 2.6 (2.65 being a more precise figure for the preferred assembly which is described in said copending application), it follows that a suitable electrolyte for this specific application need have a specific gravity of at least about 2.6.

More generally, this invention has utility in any bipolar application wherein it is desired to transfer silver to an intermediate silver member between two silver electrodes, and more specifically, wherein it is desired to effectively redistribute the mass of said intermediate silver member by relatively increasing the silver mass of a part of said intermediate member with respect to another part of said member.

The electrolyte of this invention may have utility for various conventional (non-bipolar) applications and it is not intended herein to disclaim such other applications.

In its broadest aspect, the present invention provides a high density electrolyte comprising an acidic solution of silver iodide and a dense complexing agent which is selected from the group consisting of barium iodide and a mixture of indium iodide and lithium iodide. Such a solution has a specific gravity preferably in excess of about 2 at approximately room temperature.

A quite specific preferred embodiment of the instant invention is a high density bipolar electroplating solution consisting essentially of an acidic solution of barium iodide and preferably of no less than about 1 mole liter concentration of silver iodide, about 3 moles per liter being particularly desirable. The solution has a pH no higher than about 2 and a concentration of barium iodide which is complementary with the quantity of silver iodide selected to form a complex therewith and hold it in solution.'The specific gravity of this solution is at least as high as 2 and is preferably about 2.6 or above, not exceeding, of course, the specific gravity possible under optimum conditions in such a silver iodide-barium iodide complex solution.

Another quite specific preferred embodiment of the instant invention is a high density bipolar electroplating solution consisting essentially of an acidic solution of silver iodide containing lithium iodide and indium odide, the latter reagent being added primarily to increase the specific gravity of the solution to a desired high value, and the former to complex the silver iodide. In this system, the pH is preferably no higher than about 2 and the specific gravity is no less than about 2, the latter being desirably above about 2.6. In such system silver iodide is present in a concentration of at least about 0.1 mole per liter and preferably at least as high as 0.25 mole per liter.

In the foregoing recited specific embodiments, lower alcohols, particularly ethanol and methanol, may be utilized as inert additives to increase the stability of the solutions, to adjust specific gravity and to lower freezing point. Moreover, a minor amount, for example, 10 grams per liter, of citric acid may be used in electroplating solutions of this invention to cause the silver deposited to have a smoother texture.

For a more complete understanding of the present invention, reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a schematic side view of a bipolar cell, and

FIGURE 2 is an electrical diagram illustrating the electrical analog of the bipolar plating cell of FIGURE 1.

To more fully appreciate and understand the bipolar plating utility of the electrolyte of the instant invention,

a brief description of the general principles involved in bipolar electroplating is offered.

If a metal electrode is placed in a solution containing Patented Aug. 1, 1967.

ions of that metal, an electrical potential will exist between the electrode and the solution. This potential is determined by several factors, two of which are the concentration of the metal ion surrounding the electrode and the condition of the metal electrode surface. When a second electrode is immersed in the solution and a voltage applied between the two, electrolysis will occur. If the first electrode is made a cathode, a reduction process will occur at the metal-solution interphase in at least one of several ways. Either metal ion will be deposited, the solvent will be reduced, or a combination of both will occur, If more than one type of metal ion is present, one or more may be deposited, depending on the voltage applied to the cell. Likewise, if the metal is made an anode, the metal may undergo dissolution, the solvent may be oxidized, or again both may occur. In order to provide bipolar plating that finds utility in the specific application referred to above, and more specifically described in the copending application mentioned, it is important that the solvent not be oxidized and that metal ions other than those of the electrode material not be codeposited. When a current is passed through a cell, composition changes occur in the solution near the electrodes and a potential change results. In addition, a resistive film may build up on the electrode and produce an additional voltage drop. The total voltage drop across a cell at a given current is therefore made up of the voltage drop at the two electrode-solution interphases plus the LR. drop of the solution. The difference between the voltage across an electrode-electrolyte interphase as a current passes and its equilibrium value is called the overvoltage. It is essential to keep the overvoltage as low as possible in order to prevent side reactions, particularly for the specific application mentioned above. Overvoltage directly affects bipolar plating action.

Referring to FIGURE 1, 11 is a 150 milliliter glass beaker containing an electrolyte 13. Non-conducting plastic cylindrical rod 15 of one inch diameter is suspended concentrically downward from above the beaker with its lower end immersed in electrolyte 13. The lower immersed end of 15 is recessed to receive and support identical semicircular bipolar electrode segments B and B which abut to define a continuous ring. Segments B and B may be formed from 0.025 inch silver foil. Electrodes A and C are adjacent, and identically spaced, for example A inch, from the central opposite portions of B and B, respectively. Electrodes A and C may be of various configurations. Those illustrated in FIGURE 1 are identical curved electrodes, each representing about a 45 segment of a thin annulus. They may be formed from 0.025 inch silver foil about one centimeter square. Silver wires 17 and 19 are connected, as by spot welding, to A and C, respectively. A suitable DC. voltage source (not shown) is provided for 17 and 19, with 17 connected to the anode and 19 to the cathode. The preferred electrolyte for bipolar plating in the cell of FIG- URE 1 is the silver electroplating solution of this invention.

Bipolar plating is a result of a parallel low resistance path between the two main electrodes in an electrolysis cell. For bipolar plating, of high efficiency, it is desired to effectively allow a transfer of mass from the area of B to B when a voltage is applied between A and C (FIG- URE 1), Normally, as in the silver ring for the inertial element described in the copending application and briefly referred to above, areas B and B are opposite areas on a continuous mass of metal, but in the case illustrated the strip has been divided into the opposite abutting bipolar electrodes for convenience of reference. An electrical analog of the cell shown in FIGURE 1 is represented in FIGURE 2. Therein, E and E are the anode and the cathode overvoltages, respectively, and E and E are the overvoltages of the respective portions of the bipolar element. They vary logarithmically with current. In general, E and E are not equal to'E and E since a parallel path is available. E is the LR. drop across the electrolyte between the main anode and cathode and is linear with current. The character F represents a solution resistance that is smaller than E since it includes only the electrolyte between A and B and B and C. Current flow may proceed either directly from A to C via an ionic transfer through the electrolyte 13 or from A to B via ionic transfer, followed by electrochemical reaction at B to convert to electronic conduction, followed by an electronic transfer from B to B followed by another electrochemical reaction at B followed by ionic transfer from B to C. The path chosen, or, more accurately, the ratio of current flowing through one path to the other, will depend on the ratio of the respective voltage drops. The degree of bipolar plating action is enhanced by minimizing E and E by providing an electrolyte of low conductivity to maintain a high E and by maintaining a close electrode spacing to make E substantially smaller than E Since B and E are always smaller than or equal to E and E the problem then is in providing an electrode-electrolyte system of low anode and cathode overvoltage, preferably one which will plate from a low conductivity solution. For the specific application discussed previously herein and explained in more detail in the above-mentioned copending application, it is additionally essential that the density of the solution be of sufficiently high value to suspend a practical inertial element within it, or alternatively, to be slightly heavier than said inertial element, on a per volume basis, in order that the inertial element will be buoyed up somewhat.

The problem of providing a high density bipolar electroplating solution is a difficult one because of various side reactions, undesirable side effects, and low solubilities.

The present invention will now be described, first in relatively general terms as applied to one specific embodiment, followed by three specific examples relating thereto, and then by a general description relating to another specific embodiment, followed by an example relating to that embodiment.

A preferred embodiment of the instant invention is a silver iodide solution, complexed by barium iodide, and adjusted to have the desired specific gravity. Such silver iodide solutions may be obtained, having a high specific gravity, one substantially in excess of 2.0.

Barium iodide is sufiiciently soluble in water to form a solution having a specific gravity of 2.28 at room temperature. A saturated solution of barium iodide dissolves sufificient silver iodide to increase specific gravity to 2.75 at room temperature. Such a solution can, in turn, be utilized to dissolve additional barium iodide. This property of barium iodide to complex silver iodide makes it possible to formulate an electroplating solution having a high specific gravity, for example, one of 2.65. This is most desirable for the specific utility of the instant invention which was described in the preceding portions of this application. In formulating dense solutions of silver iodide, complexed with barium iodide, it is desirable to maintain sufficient acidity to prevent precipitation of the compound BaI -BaO-9H O; however, if the solution is too acid, corrosion results on the electrode material.

The following examples illustrate satisfactory solutions of silver iodine, complex by barium iodine. While these examples are generally for a density of about 2.65, since since this was the area of intense interest for specific application of the instant invention, dense solutions having specific gravities in the range from about 2 to 2.6 may be obtained by proper treatment, as is mentioned in the examples, and these resulting solutions function satisfactorily for bipolar plating at such densities. It is preferred that the specific gravity of the solution obtained be at least as high as 2. It is additionally preferred that the concentration of silver iodine be at least as high as about 1 mole per liter.

Example 1 A solution A is prepared as follows: 110 grams of reagent grade Ba(OH) 8H O' are dissolved in 100 milliliters of purified 48% hydriodic acid. Because of the variations in concentration of commercial 48% hydriodic acid, it may be necessary to add more acid drop-wise to dissolve all the Ba(OH) '8H O or to add more of the latter if too much acid is present. The resulting barium iodine solution is evaporated until a specific gravity of 2.13 is obtained, at which point the solution has a boiling point of approximately 116" C. The pH is adjusted to about 2 by addition of Ba(OH) -8H O, -or hydriodic acid, as required. Small quantities of iodine formed are removed by refluxing the solution with silver metal. The resulting solution obtained is Water-white.

A solution B is prepared as follows: 24 grams of silver iodine are dissolved in 25 millimeters of Solution A to form a solution having a specific gravity of 2.70.

A plating solution of desired specific gravity may be obtained by combining appropriate quantities of Solution A and Solution B.

An electroplating solution obtained by such combination, found to have superior stability and to be excellent for bipolar plating applications, is in ratio of two parts of Solution A to eigth parts of Solution B. It will be observed that this is a silver iodide concentration of about 3.3 moles per liter. The density of the resulting mixture is about 2.65. If a higher density is desired, it may be obtained by adding barium iodide. On the other hand, the density may be decreased by adding water. The addition of a small amount of water causes a yellow precipitate, which may be dissolved by stirring.

The plating solution described above, having a specific gravity of 2.65, was tested by passing current through a simple cell having silver electrodes, the current density being about 2 milliamps per square centimeter. The anode of the cell was found to have lost weight by 15.2 milligrams and the cathode tohave gained weight to the extent of 15.1 milligrams. This is to be compared to a theoretical change of 14.8 milligrams to be transferred from the anode to the cathode.

The texture of the silver deposited from the solution described may be made smoother by utilizing approximately 0.1 gram of citric acid per milliliters of solution as an additive.

On concentrating the solution described aboveto about 2.75 by boiling, then diluting with ethanol to obtain a density of approximately 2.65, a plating solution was obtained, 25 milliliters of which are utilized for a bipolar plating test in the apparatus of FIGURE 1. The resistivity of this solution was on the order of 50 ohm-cm. About 80% bipolar plating efiiciency with 100% electrode efiiciency is obtained.

Current flow of from 1 microamp to 10 milliamps is preferred for bipolar plating with the above solution.

Example 2 Solutions A and B, prepared as described in Example 1 above were mixed in ratio of 1 milliliter of A to 9 milliliters of B. A simple test conducted by passing current through a cell containing this solution resulted in an anode loss of 15.1 milligrams and a cathode gain of this same amount. This is to be contrasted to the theoretical of 14.8 milligrams transferred.

It is to be noted that the density of this solution was adjusted to 2.65, as previously described.

Bipolar plating tests utilizing the mixture of this example give substantially the results of those obtained in Example 1.

Example 3 The various procedures of Example 1 were repeated, but this time using a plating solution having 3 milliliters of Solution A mixed with 7 mlliliters of Solution B. The concentration was adjusted to 2.65 by the method previously described.

It is found that substantially the same results are obtained on a simple test when current density of 2 milliamps flows through the solution from anode to cathode. Bipolar plating tests yielded similar results to those recited in Example 1.

Once again, it is found that the addition of a small amount of citric acid, about 0.1 gram per 10 milliliters of solution, for example, promotes the formation of a smoother deposit of silver.

Silver iodide may likewise form a satisfactory electroplating solution of high density when complexed with lithium iodide to which has been added a compatible agent to increase density, idium iodium being preferred for this purpose. The silver iodide concentration may be low, but it is preferred that it be as high as 0.1 mole per liter, with about 0.25 mole per liter representing a desirable level.

As was the case with the silver iodide-barium iodide solutions, it is necessary to keep the solution acid in order to prevent precipitationprobably of In(OH) (I) The following example describes a specific silver iodidelithium iodide-indium iodide system.

Example 4 Seventy grams of indium metal are dissolved in 300 millimeters of 48% hydriodic acid. The reaction, which is slow at room temperature, is accelerated by refluxing the mixture. If desired, the reaction rate may be increased by refluxing in the presence of spongy indium. Spongy indium may be formed by pouring the molten metal in cold water.

The resulting solution obtained from the dissolution of the indium metal in hydriodic acid has a specific gravity of 1.8. It is evaporated until a specific gravity of 2.7 is obtained, which occurs at a boiling point of about 112 C. Thereafter, lithium iodide (LiI) is added to the solution in concentration of about 2.5 moles per liter (335 grams per liter). Since commercial anhydrous lithium iodide contains up to about 30% water, it is highly desirable to dry the lithium iodide by heating it in a vacuum before the use described above. The dried lithium iodide so obtained is quite hygroscopic and must be protected from the atmosphere.

Finally, silver iodide is dissolved in the indium iodidelithium iodide-hydriodic acid 'water system to a concentration of about 0.25 mole (59 gm.) per liter. The resulting solution is a bright yellow and may be adjusted to a specific gravity of 2.65 by diluting with water, acidified to a pH of about 2.5, or by adding crystalline indium iodide.

In practicing the foregoing procedure, it has been noted that the lithium hydroxide content of commercial lithium iodide varies widely and may neutralize the hydriodic acid excessively. This condition is evidenced by a difliculty in dissolving lithium diodide atthat step where it is added to the solution to a concentration of 2.5 moles per liter. If, for some reason, the ratio of lithium iodide to silver iodide is too low, a second liquid phase will separate from the indium iodide solution. This condition may be remedied by adding more lithium iodide to the solution. It is preferable that the acidity of the solution be maintained below a pH of 2 at all times to prevent precipitation, probably of In(OH) (I) If such precipitation does occur, it may be remedied by refluxing with excess hydriodic acid to redissolve the precipitate. Then excess hydriodic acid must be removed, as by reaction with metallic indium.

Plating experiments, as those described in the examples for the silver iodide-barium iodide complex system (Examples 1-3) were conducted on the solution prepared in accordance with the foregoing procedure. It was found that plating efliciencies of substantially resulted. In contrast to silver iodide-barium iodide solutions, slight corrosion occurs when silver metal is stored in a solution made in accordance with this example; however, the amount of such corrosion is not critical for most purposes. Weight loss through corrosion does not exceed about 1 milligram from a 2 square centimeter area of silver in contact with the solution for hours.

The bipolar plating elliciencies for the solution described above were of the same order tobtained for the silver iodide-barium iodide system.

The specific gravity of the foregoing system may be adjusted upwardly or downwardly by addition of indium iodide or water, respectively.

Silver iodide, complexed either with barium iodide or with a lithium iodide (with indium iodide added to increase density) functions satisfactorily as a plating electrolyte over wide concentration ranges (for example, as low as 0.01 molar AgBr). However, with the specific type application that is now being made of the electrolyte, it is important that it be quite dense, with a specific gravity (room temperature) of at least about 2 and preferably of about 2.6.

Of the two systems described, the barium iodide complexing system appears to be the more desirable. It is quite stable. For example, the solution of Example 1 can be stored for two weeks without any change in its appearance. When chilled, the solution is in equilibrium with crystalline material at 10 C. It is thus seen that wide ranges of operation and high stability are provided by this solution. In addition, the silver iodide-barium iodide system provides a substantially colorless solution, being essentially water-white, and is thus highly transparent. Prolonged exposure to air causes the slightest amount of turbidity, which can be removed quickly by filtration. The translucence of an electrolyte prepared in accordance with silver iodide-barium iodide is an additional important characteristic when the solution is utilized as a bipolar plating solution to practice the microbalancing method described in the above-mentioned copending application.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art and it is intended to cover such modifications as fall within the scope of the appended claims.

What is claimed is:

1. A high density aqueous electrolyte comprising an acidic solution of silver iodide and a dense complexing agent, said complexing agent being selected from the group consisting of barium iodide and a mixture of indium iodide and lithium iodide, said solution having a specific gravity in excess of 2.

2. The electrolyte of claim 1 wherein said dense complexing agent is barium iodide.

3. The electrolyte of claim 1 wherein said dense complexing agent is a mixture of indium iodide and lithium iodide.

4. The electrolyte of claim 1 wherein said solution has a pH of no greater than about 2.

5. The electrolyte of claim 4 wherein said dense complexing agent is barium iodide.

6. The electrolyte of claim 4 wherein said dense complexing agent is a mixture of indium iodide and lithium iodide.

7. The electrolyte of claim 1 wherein said density is in excess of about 2.6.

8. The electrolyte of claim 7 wherein said dense complexing agent is barium iodide.

9. The electrolyte of claim 7 wherein said dense complexing agent is a mixture of indium iodide and lithium iodide.

10. A high density aqueous bipolar electroplating solution consisting essentially of an acidic solution of silver iodide and a dense complexing agent, said complexing agent being selected from the group consisting of barium iodide and a mixture of indium iodide and lithium iodide, said solution having a specific gravity of about 2.6 and a pH of about 2.

11. A high density aqueous bipolar electroplating solution consisting essentially of an acidic solution of barium iodide and silver iodide, the latter in concentration of no less than about one mole per liter, said solution having a pH of no greater than about 2 and a barium iodide concentration complementary to the silver iodide concentration to hold the silver iodide in solution and provide an overall solution specific gravity of at least about 2.

12. The solution of claim 11 wherein a lower alcohol is in the solution.

13. The solution of claim 11 wherein a minor proportion of citric acid is present as a smoothing agent for silver deposit.

14. The solution of claim 11 wherein the silver iodide concentration is about 3 moles per liter.

15. The solution of claim 14 wherein the specific gravity is about 2.6.

16. A high density aqueous bipolar electroplating solution consisting essentially of an acidic solution of silver iodide, indium iodide and lithium iodide, the concentration of silver iodide being at least 0.1 mole per liter of solution and the concentration of indium iodide and lithium iodide being selected to hold the silver iodide in solution while providing an overall solution specific gravity of at least about 2, the pH of said solution being no higher than about 2.

17. The solution of claim 16 wherein a lower alcohol is in the solution.

18. The solution of claim 16 wherein a minor proportion of citric acid is present as a smoothing agent for silver deposit.

19. The solution of claim 16 wherein the silver iodide concentration is about 0.25 mole per liter.

20. The solution of claim 19 wherein the specific gravity is about 2.6.

References Cited UNITED STATES PATENTS 5/1932 Schlotter 204-46 2/1957 Nobel et al 204-46 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,334 ,031 August 1 1967 Earl S. Snavely, Jr., et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 28 after 'mole" insert per line 42 for "odide" read iodide column 3 line 47 for "B" read B column 4 line 3 for "P read E same column 4 lines 64 and 74 and column 5 lines 8 and 17 for "iodine", each occurrence, read iodide column 5, line 17, for "millimeters" read milliliters line 25, for "eigth" read eight line 73, for "mlliliters" read milliliters column 6, line 13, for "idium iodium" read indium iodide line 25 for "millimeters" read milliliters line 53, for "diodide" read iodide column 7, line 4, for "tobtained" read obtained Signed and sealed this 15th day of April 1969.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. EDWARD J. BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. A HIGH DENSITY AQUEOUS ELECTROLYTE COMPRISING AN ACIDIC SOLUTION OF SILVER IODIDE AND A DENSE COMPLEXING AGENT, SAID COMPLEXING AGENT BEING SELECTED FROM THE GROUP CONSISTING OF BARIUM IODIDE AND A MIXTURE OF INDIUM IODIDE AND LITHIUM IODIDE, SAID SOLUTION HAVING A SPECIFIC GRAVITY IN EXCESS OF 2.

Images