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Publication numberUS3830650 A
Publication typeGrant
Publication dateAug 20, 1974
Filing dateJun 18, 1973
Priority dateJun 18, 1973
Publication numberUS 3830650 A, US 3830650A, US-A-3830650, US3830650 A, US3830650A
InventorsHenriksen G
Original AssigneeUs Navy
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Non-aqueous silver film formation
US 3830650 A
Abstract
An anhydrous silver plating solution comprising an alcohol such as methanol or ethanol; a silver salt such as silver nitrate; a hydroxide such as NaOH, KOH, etc.; and a complexing agent such as ammonia, ethylene diamine, or pyridine.
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Description  (OCR text may contain errors)

United States Patent [191 Henriksen [45] Aug. 20, 1974 NON-AQUEOUS SILVER FILM FORMATION [75] Inventor: Gary L. Henriksen, Crofton, Md.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: June 18, 1973 [21] Appl.No.:370,907

[52] US. Cl. 106/1, 117/35 S, 117/130 E [51] Int. Cl. C23c 3/02 [58] Field of Search 106/1; 117/35 S, 130 E [56] References Cited UNITED STATES PATENTS 2,879,175 3/1959 Umblia et al. 106/1 X 3,383,247 5/1968 Adlhart et al 106/1 X 3,547,818 12/1970 Wade 106/1 X Primary Examiner-Lewis T. Jacobs Attorney, Agent, or FirmR. S. Sciascia; J. A. Cooke; M. G. Berger [57] ABSTRACT An anhydrous silver plating solution comprising an alcohol such as methanol or ethanol; a silver'salt such as silver nitrate; a hydroxide such as NaOH, KOH, etc.; and a complexing agent such as ammonia, ethylene diamine, or pyridine.

Addition of an anhydrous reducing aldehyde such as dextrose to the solution results in a thin silver plating on objects placed in the solution. The solution is particularly suitable for silver plating electrically nonconductive, water sensitive substances having relatively high vapor pressures.

12 Claims, No Drawings 1 NON-AQUEOUS SILVER FILM FORMATION BACKGROUND OF THE INVENTION This invention relates generally to coating processes and more particularly to the formation of silver films by 5 chemical reduction in solution.

Present methods of silver film formation are chemical reduction aqueous solution, electrochemical plating, and vacuum deposition. Chemical reduction in aqueous solution involves the reduction of silver (l) from a soluble complex [e.g., Ag(NH to silver metal. This method has the obvious disadvantage of causing the deterioration of water sensitive materials. The second method, electrochemical plating, uses an aqueous bath containing salts, including silver salts (e.g., silver cyanide), an anode made of high purity silver, and the object to be plated as the cathode. Obviously, this method will also cause the deterioration of water sensitive materials. In addition, this method has the disadvantage of being limited to materials which are electrically conductive or are coated with electrically conductive materials. The third method, vacuum deposition of silver, involves evacuating a vacuum chamber containing the material to be coated to an absolute pressure of about 10' to 10 mm of mercury while heating the silver to its evaporation temperature. While this method may be used for silver coating electrically nonconductive water sensitive materials, it has the disadvantage of being limited to coating materials with low vapor pressures.

SUMMARY OF THE INVENTION Accordingly one object of this invention is to provide a method for silver plating materials.

Another object of this invention is to provide a method for silver plating water sensitive materials.

Yet another object of this invention is to provide a method for silver plating materials with high vapor pressures.

Still another object of this invention is to provide a method for forming thin silver coatings on water sensitive materials without using a vacuum system.

Yet a further object of this invention is to make water sensitive, electrically nonconductive materials suitable for electrochemical silver plating in nonaqueous solutions.

These and other objects of this invention are accomplished by providing an anhydrous silver plating bath formed by dissolving a silver salt, such as silver nitrate, in an alcohol such as ethanol, methanol, or mixtures thereof, then adding hydroxyl ions to precipitate the silver out as Ag O, and finally adding a complexing agent selected from ammonia, ethylene diamine, pyridine, or mixtures of these compounds to redissolve the silver as a complex ion. Addition of an anhydrous reducing aldehyde, such as dextrose, to this bath reduces the silver, resulting in the silver plating out on all surfaces in contact with the bath.

DETAILED DESCRIPTION OF THE PREFERRED 60 EMBODIMENT The components of the plating bath must be compatible with the materials to be plated. Since the bath is primarily designed for silver plating water sensitive material, the components of the bath must be anhydrous and must not interact to form significant amounts of water. For instance, the solvent used should be either absolute methanol, ethanol, of mixtures thereof. The hydroxyl ion source must be anhydrous, preferably dried LiOH, NaOH, KOH, RbOH, CsOH, or mixtures thereof. Hydroxides which contain water of crystalization (e.g. Ba- (OH) '8H O, Sr(OH) -8H O) should be avoided. Although the alcohol and hydroxyl ions do react to form some water,

ROH OH 3 R0 H O the equilibrium greatly favors the unionized alcohol so that very little water is formed. Finally the remaining ingredients, the silver salts (e.g., AgNO and complexing agent (ammonia, ethylene diamine, or pyridine) must also be anhydrous. The silver ions do react with the hydroxide ions to form some water 2 Ag+ H20.

However, this water is consumed during the complexing step (pyridine) Thus, the water formed will not affect the material to be plated provided that the object to be plated is not placed in the solution until after all the silver has been complexed.

Moreover, the components of the baths should be ested for compatibility with the material to be plated. For example, RbAg I is not compatible with absolute methanol but it is compatible with absolute ethanol. Simple tests for compatibility will be known to one of ordinary skill in the art.

Factors to consider in selecting a silver salt are solubility, ionization, and the possible interference of the anion with the precipitating, complexing, or reducing of the silver. Although any salt which will dissolve in at least trace amounts can be used in the present invention, preferably the salt should be at least slightly soluble in alcohol. The preferred salt is silver nitrate. Besides solubility in alcohol, silver nitrate offers the advantage of having an anion which will not interfere with the precipitating, complexing or plating steps.

Similarly, the hydroxyl ion source should be soluble and ionize in absolute alcohol and have cations which will not interfere with the precipitating, complexing, or

.plating of the silver. Solubility and ionization of the hydroxyl ion source are important because the silver ion complexes are more stable and easily formed at higher pHs. The pH of the solution should be adjusted to at least 8 but preferably more than 11. The preferred hydroxyl ion sources are LiOH, NaOH, KOH, RbOH and CsOH, with NaOH and KOH being the most preferred. Moreover, a saturated solution of the hydroxyl ion source in alcohol is preferable.

The complexing agents are selected on the basis of their relative holding power on silver ions in absolute alcohol. For the three complexing agents used in this invention, the order of increasing holding power is pyridine, ammonia, and ethylene diamine. Pyridine, the weakest, will not form a strong enough complex with silver ion in ethanol to work. However, pyridine does work in methanol. On the other hand, the silver (I) ethylene diamine complex, [Ag(en) wherein en is ethylene diamine and n l, is very stable in ethanol at room temperature. The silver (I) is tied up so well that the redox reaction does not observably proceed when the reducing aldehyde (dextrose) is added until the temperature is raised to C.

The amount of complexing agent used determines the amount of silver complex that will be in solution available for plating. If any amount of complexing agent is added, some silver complex ions will be formed and will be available for at least some plating. Preferably enough complexing agent should be added to dissolve all of the silver as complex ions. Although each silver ion complexes with two molecules of pyridine or ammonia, or with one molecule of ethylene diamine, the complexes are unstable enough to prefer a large excess of the complexing agent.

For instance, a 90 to 1 molar ratio of pyridine to silver was used in example lb. The amount of ethylene diamine preferred was about 1/20 of the pyridine used. It should be noted that although a 90 to 1 ratio was used in example lb to complex the silver, an even larger excess of the complexing agent may be used with the same results. Thus a molar concentration greater than zero will be operable, but a molar concentration of from more than zero to 90 times the molarsaturation concentration of the silver salt used is preferred. In the case of ammonia, or ethylene diamine a molar concentration of from more than zero to ten times the molar saturation concentration of the silver salt used is even more preferred.

Any anhydrous reducing aldehyde should work in this invention. Dextrose was used in the examples. Note that any molar concentration of reducing aldehyde greater than zero will reduce some silver and cause it to plate out; however, a molar amount of at least equal to and preferably twice the molar saturation concentration of the silver salt should be used to maximize the amount of silver reduced.

A number of factors should be considered in determining the amount of reduced silver needed to coat the water sensitive material. First, the silver plates out onto all surfaces in contact with the solution, including the inside walls of the container holding the plating bath. Second, the method provides only a thin coating of silver on the surfaces after which the excess reduced silver forms granules which fall to the bottom of the bath. Thus by adjusting the silver concentration to the point where no or very small amounts of granules are formed, the optimum silver concentration can easily be found. Also, although the silver coating provided by this method is thin, it is protective and electrically conductive. Thus, nonaqueous electrochemical means can be used to thicken the coating of silver. Moreover, since the silver stops coating on a surface after a given thick ness, once the inside walls of the container containing the bath are coated they no longer interfere with the coating process in future batches.

The general nature of the invention having been set forth, the following examples are presented as specific illustrations thereof. It will be understood that the invention is not limited to these specific examples but is susceptible to various modifications that will be recognized by one of ordinary skill in the art.

EXAMPLE 1 Standard solutions of the following concentrations in absolute methanol were prepared:

Silver Nitrate 0.097 X 10 moles/cc Sodium Hydroxide 0.124 X l moles/cc Dextrose 0.l 12 X moles/cc EXAMPLE IA 1.50 cc of the sodium hydroxide solution was added to 5.0 cc of the silver nitrate solution, forming a brown precipitate of Ag O. Then 2.50 cc of the pyridine was added dissolving the Ag O precipitate. Finally, 1.50 cc of the dextrose solution was added, reducing the silver and causing a silver plating to form on the walls of the test tube.

EXAMPLE 1B The experiment of example 1A was repeated using the following proportion of the above solutions: silver nitrate, 5.00 cc; sodium hydroxide, 2.25 cc; pyridine, 3.50 cc; and dextrose, 2.50 cc. A thin silver coating was formed on the sides of the test tube and the excess reduced silver formed granules.

EXAMPLE IC The experiment of example IB was repeated using the following proportions of the above solutions: silver ni trate, 5.00 cc; sodium hydroxide, 2.00 cc; pyridine, 3.00 cc; and dextrose, 4.00 cc. Again a thin silver coating was formed on the sides of the test tube and the excess reduced silver formed granules.

EXAMPLE ll EXAMPLE III The standard solution of silver nitrate, sodium hydroxide, and dextrose used in example I were used in the example.

EXAMPLE IlIA 1.50 cc of the sodium hydroxide solution was added to 5.00 cc of the silver nitrate solution, forming a brown precipitate of Ag O. The solution was stirred while ethylene diamine was added drop by drop until all of the Ag O precipitate had dissolved. Finally, 1.50 cc of the dextrose solution was added, reducing the silver and causing a silver plating to form on the walls of the test tube.

EXAMPLE lIlB 2.25 cc of the sodium hydroxide solution was added to 5.00 cc of the silver nitrate solution, forming a brown precipitate of Ag O. The solution was stirred while ethylene diamine was added drop by drop until all of the Ag O precipitate had dissolved. Finally, 1.50 cc of the dextrose solution was added, reducing the silver and causing a silver plating to form on the walls of the test tube.

EXAMPLE lIIC 2.00 cc of the sodium hydroxide solution was added to 5.00 cc of the silver nitrate solution, forming a brown precipitate of Ag O. The solution was stirred while ethylene diamine was added drop by drop until all of the Ag O precipitate had dissolved..Fina1ly, 4.00 cc of the dextrose solution was added, again causing the silver to reduce and form a thin coating on the walls of the test tube.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An anhydrous silver plating bath comprising:

a. an alcohol selected from the group consisting of methanol, ethanol, and mixtures thereof;

b. a silver salt in a molar concentration greater than zero to a saturated solution;

c. a hydroxyl ion source in an amount sufficient to adjust the pH to a value greater than 8;

d. a complexing agent selected from the group consisting of ammonia, ethylene diamine, pyridine, and mixtures thereof in a molar concentration of greater than zero;

provided that when said complexing agent is pyridine said alcohol is methanol.

2. The anhydrous silver plating bath of claim 1 wherein said silver salt is silver nitrate.

3. The anhydrous silver plating bath of claim 1 wherein the hydroxyl ion source is in an amount great enough to adjust the pH to a value greater than 11.

4. The anhydrous silver plating bath of claim 1 wherein said hydroxyl ion source is selected from the group consisting of LiOH, NaOH, KOH, RbOH, CsOl-l,

and mixtures thereof.

5. The anhydrous silver plating bath of claim 4 wherein said hydroxyl ion source is selected from the group consisting of NaOH, KOH, and mixtures thereof.

6. The anhydrous silver plating bath of claim 1 wherein said complexing agent is in a molar concentration of greater than zero to times the molar saturation concentration of said silver salt.

7. The anhydrous silver plating bath of claim 6 wherein said alcohol is methanol and said complexing agent is pyridine.

8. The anhydrous silver plating bath of claim 6 wherein said complexing agent is selected from the group consisting of ammonia, ethylene diamine, and mixtures thereof.

9. The anhydrous silver plating bath of claim 8 wherein said complexing agent is in a molar concentration of from more than zero to 10 times the molar saturation concentration of said silver salt.

10. A method of plating a surface with silver comprismg:

contacting said surface with the anhydrous silver plating bath of claim 1; and

adding an anhydrous reducing aldehyde to said anhydrous silver plating bath in a molar amount greater than zero.

11. The method of plating a surface with silver of claim 10 wherein said reducing aldehyde is added in a molar amount of greater than zero to twice the saturation concentration of the silver salt.

12. A method as in claim 10 wherein the anhydrous reducing aldehyde is dextrose.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US2879175 *Oct 20, 1954Mar 24, 1959Ericsson Telefon Ab L MMethod for producing a silver coating on a non metallic material
US3383247 *Aug 19, 1965May 14, 1968Engelhard Ind IncProcess for producing a fuel cell electrode
US3547818 *Apr 7, 1969Dec 15, 1970Ventron CorpAlcoholic ionic compositions containing a metal selected from hg (ii),cu (ii),ag (i),cd (ii),ni (ii),zn (ii),and co (ii)
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4965094 *Dec 27, 1988Oct 23, 1990At&T Bell LaboratoriesApplying solutions of silver ammonia complex and reducing agent to the surface of a dielectric material
US6291025Jun 4, 1999Sep 18, 2001Argonide CorporationElectroless coatings formed from organic liquids
WO2000075396A1 *Jun 2, 2000Dec 14, 2000Argonide CorpElectroless coatings formed from organic solvents
Classifications
U.S. Classification106/1.5, 106/1.18
International ClassificationC23C18/31, C23C18/44
Cooperative ClassificationC23C18/44
European ClassificationC23C18/44