US 3202488 A
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MQZY/7ZEW%Z%W I $2? 3 24,1965 J. E. EHRREICH ETAL 3,202,488
SILVER-PLATED I COPPER POWDER Filed March 4, 1964 3 Sheets-Sheet 1 PROCEDURE I COPPER SHOT 2 ACETIC CHEMICAL CLEANING AC'D 4 3 I -5 6 f WATER RINSE WATER REPLACEMENT 4- PLAT|NG 7 PLATING SOLUTION Aug. 24, 1965 J. E. EHRREICH ETAL SILVER-PLATED COPPER POWDER Filed March 4, 1964 3 Sheets-Sheet 2 PROCEDURE 11' @HOPPED m ACID WATER RINSE PLATI NG REPLACEMENT 28 N0 CN SOLN.
" SOLID Ag CN Y DONALD H AVER ATTORNEYS 1965 J. E. EHRREICH ETAL 3,202,488
SILVER-PLATED COPPER POWDER Filed March 4, 1964 3 Sheets-Sheet 3 PROCEDURE III CLEANED & WASHED COPPER POWDER HEAT ACTIVATE FIG. 3
I/VVENTORS. JOHN E. EHRREICH DONALD H. AVERY ATTORNEYS United States Patent 3,202,488 SILVER-PLATED COPPER POWDER John E. Ehrreich, Arlington, and Donald H. Avery, Na-
hant, Mass., assignors to Chomerics, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Mar. 4, 1964, Ser. No. 349,361 5 Claims. (Cl. 29192) This is a continuation-in-part of application Serial No. 227,756, filed October 2, 1962, now abandoned.
This invention is concerned with a new composition of matter, a silver-plated copper powder having a matte appearance that is particularly useful as a filler for plastic compositions to render the compositions highly electrically conductive. More particularly, it is concerned with an electrically conductive metallic powder consisting of a copper-bearing core replacement plated with silver in such a manner as to have long-term electrical stability, particularly at elevated temperatures and a matte appearance as contrasted to the specular appearance normally desired when plating with silver.
Electrically conductive plastics have been in demand as gasketing or caulking materials for use in the control or suppression of spurious electromagnetic energy radiation, usually referred to as radio frequency or R.F. shielding. In addition, conductive plastics have been used to replace metal solders and as conductive paints, as for example in electrostatic shielding.
Noble metal powders, such as solid silver, have been incorporated in plastics to make them electrically conductive. The volume resistivities of the filled plastics are in the order of 0.1 ohms per centimeter or less. The noble metals must be used because insulating oxide coatings do not form on the particles as is the case with other metal powders, such as copper and aluminum. Conductivity of a plastic mass filled with a conductive metal powder depends upon the particle-to-particle contact between the metal particles. The electric current must be able to flow from particle-to-particle with desirably the lowest amount of contactresistance possible. With a non-noble metal the oxide coating that forms on the particles, while perhaps only a few atoms thick, has a high resistivity and prevents the ready flow of current between contiguous particles.
Non-noble metal particles can be protectively coated with a noble metal to give the necessary noble metal outer surface. Ithas been found, however, that it is difficult to lay down a protective coating that is properly bonded to the core metal so that the coated metal powder is stable and adequately conductive, particularly when using an economical amount of noble metal. It is difficult to secure coated particles that are electrically stable as determined by the heat, oxidation and moisture stability of the coating. If the noble metal coating is not properly electrically and physically attached to the metal core, current fiow into and through the core is hindered or lost.
The present invention is based on the finding that by careful and proper control of processing conditions, a thin silver coating can be replacement plated on a copper powder to form an inexpensive, very stable, highly electrically conductive filler for plastics. The amount of silver deposited by the process of this invention, while effective, can be less than that which can be deposited by "ice normal replacement plating techniques. affects the economics of the process.
In brief compass, this invention is concerned with a new composition of matter: an electrically conductive metal powder comprising copper particles replacement plated with a continuous thin electrically adherent coating of silver from an aqueous solution having a high ratio of silver complexing ions-tosilver ions. The silver is deposited in such a manner to assure that the coated particles will not degrade electrically with time in the presence of an oxidizing atmosphere.
The plated particles produced by this invention, when viewed macroscopically in bulk, have a matte, as opposed to a specular, appearance. When prepared from cyanide plating solutions, the bulk of the powder appears to be dead white. When viewed microscopically, the surface of the particles is uneven with some very fine specular spots or highlights. The reason for this matte appearance being obtained is not known with certainty, but it may be the result of incorporation of some silver oxide in the coating as a result of the plating conditions. In any event, it is known that if a matte appearance is not obtained from the plating procedure and a specular or bright one is obtained instead, the plated powder, while perhaps being initially conductive, will not have longterm stability. The long-term stability of the plated powder can be tested by spreading the powder in loose form in a tray and exposing it at 400 F. for 24 hours to a circulating air atmosphere. The initial conductivity of the powder as determined by probes from a volt-ohm meter or Wheatstone bridge should be under 1 ohm per centimeter and this initial conductivity should not change more than 50% during the test. Shiney or bright-appearing particles have been found not to pass this test. If a conductive powder does pass the test in this loose form, it will usually perform adequately when incorporated into a plastic matrix which further protects the powder.
The plated powders have been prepared in the manner of the present invention by essentially three different replacement (or immersion) plating procedures, two of which are based on the use of an alkali metal cyanidesilver cyanide solution, the other being based on the combination of ammonia hydroxide and silver nitrate. All three procedures have at least three features in common: (1) The type of metal particles which are plated, (2) The type of plating carried out, and (3) The steps taken to prevent deleterious intermittent layers of copper oxide film from forming between the silver coating and the copper substrate.
The base powder plated in all cases is a copper-bearing powder that at its surface consists of at least 50% copper, such as in the case of a brass or a bronze powder, and preferably is substantially pure copper. The powder may have any shape such as rod-like, platelet, irregular and spherical, with the latter being preferred. Because of its intended incorporation in plastic compositions, no dimension of the powder to be plated is larger than 250 mils and is preferably less than mils. The average size of the smallest dimensions of the particles is in the range of 0.25 to 25 mils, preferably 1 to 10 mils. The copper powder is in particulate form before plating and is not comminuted after plating so that all exposed surfaces of the particles receive a coating of the silver. The preferred powders have a surface area in the range of 15 to 750 square feet per pound.
This favorably Replacement or immersion plating from an aqueous solution is used in each procedure as opposed to electrolitic plating or to chemical plating where a reducing agent is used to bring the silver out of solution. In replacement plating, the silver ions are replaced in the solution with copper from the surface being plated. Replacement plating has been found to be essential to securing the desired matte appearance and long-term electrical stability. While not mandatory, it is preferred to control the thickness of the silver deposited or the amount of silver deposited by limiting the total amount of silver ions available in the plating solution, such that at the end of the plating step the plating solution is substantially depleted of silver ions. Stated ditferently, the thickness of the silver deposit is not controlled by the time of the reaction. The amount of silver deposited is preferably in the range of 0.0006 to 0.050 ounce per square foot of surface area, regardless of particle shape.
The copper powder is very carefully cleaned in each case of copper oxide before and/or during the plating step. Copper oxide is an electrically insulating material and forms very readily on cleaned copper surfaces. It is removed from the copper powder prior to the plating step by means of an acid wash. Acetic acid is preferably used since it is known to remove copper oxide while not particularly attacking pure copper. If a good preclean of the powder is not obtained, further cleaning is secured during the plating step by maintaining the amount of silver complexing agent relatively high, much in excess of that required to complex with the silver ions. Glacial acetic acid when used for pre-cleaning is so effective that not too much care has to be taken to rid the copper powder of oxide in the plating step. If diluted acetic acid is used, then it is preferred to maintain a high amount of complexing agent during plating. The ratio of complexing agent to silver ions (e.g., 2 CN/Ag+ in the case of cyanide, i.e. 2 CN form the complexing agent) should be between 2 and 8, preferably between 6 and 7, with cyanide concentrations of at least 20% of saturation, preferably 50%.
The plating solutions do not contain any brighteners as are normally used in silver plating because the brightners may cause an insulating film to form between the silver coating and the copper substrate.
The conductive powder of this invention will normally be incorporated in amounts in the range of to 80, preferably to 60, volume percent in plastic compositions to impart electrical conductivities in the order of 1 ohm centimeter or less and usually as low as 10- ohms centimeters or lower.
In the drawings, FIGS. 1, 2 and 3 schematically illustrate three preparation procedures for making the conductive powder of the present invention.
EXAMPLE I A copper powder (Metal Distintegrating MD 103-A Copper Shot, all through 100 mesh less than 20% through 325 mesh) is admitted by line 1 to cleaning step 2 where it is cleaned with a 10% acetic acid solution at room temperature supplied by line 3. Other cleaning methods can be used but the cleaning solution should not deposit a contaminating film such as a phosphate will do. Spent acid is removed by line 4 and the clean powder is passed to a water rinsing zone by line 5.
Rinse water is supplied by line 7 to water rinse zone 6 and spent water is removed by line 8.
The rinsed powder is passed by line 9 to plating solution 10 which already contains the plating solution supplied by line 11. The powder isplated in a batch-manner and is preferably admitted as rapidly as possible to the plating solution in such a manner as to minimize contact with the air and thus oxidation of the copper surface. Also, the powder is passed as quickly as possible into the plating solution as a whole so that the particles first entering do not acquire a heavier coating than later entering particles, i.e. to prevent uneven coating of the particles which is wasteful of silver. After addition of the copper powder, a black haze may be present initially but will disappear.
A very high level of mechanical agitation as by an impeller is used to keep the solids well dispersed in the solution. Being as heavy as they are they would settle rapidly in the absence of agitation. Quite firm cementation of the particles may occur at this point if the solids are allowed to settle.
The plating solution is based on silver cyanide. An alkali metal cy'aiiide; preferably sodium cyanide, is used to bring the silver cyanide in solution and to provide a high concentration of cyanide ions. The sodium cyanide is first dissolved in water. Then the silver cyanide is added. Dissolution can be accomplished at room temperature.
The amount of cyanide used is high enough to complex the silver cyanide, but not so high as to prevent dissolving of the silver ion because of saturation. The amount of cyanide ion used is preferably in the range of 25 to of that concentration where saturation occurs and silver comes out. If too much cyanide ion is present, silver cyanide is deposited on the copper powder ruining the eifectiveness of the silver coating.
Agitation of the plating reactants is continued until the silver ions are just about depleted. As this point copious evolution of gas starts. It is good practice to time the reaction to determine this point and stop just short of it. The time will depend on various factors, such as the size of the batch, concentration of ingredients, and the like, but it is usually within a few minutes for a batch operation. The reaction is stopped by ceasing agitation. The silver-coated particles drop rapidly. Liquid is then decantered or syphoned off from the particles by line 12. If gas evolution has started, when the solids drop the gas drives the liquid out of the settled mass. This automatically limits the gas evolution. The settled solids tend to agglomerate, although not tightly, and should be promptly removed and washed.
The plated solids are transferred to washing zone 14 by line 13. Wash water is added by line 15 and removed by line 16. The solids are thoroughly washed several times to remove cyanide ions. They are then passed by line 17 to drying zone 18. They can be dried in any suitable and rapid manner such as by an acetone wash followed by air drying. The dried solids are passed by line 19 to packaging and distribution.
Table I gives pertinent conditions for this example Table I Preferred Example Copper powder:
Average particle size, mils 1 to 10 3 Surface area, ftfl/lb 15 to 750 60 Percent copper 80 99 Planting solution (fresh) Silver ions-oz./gal- 4 to 8 G Cyanide i0ns-oz./g 14 to 20 18. 2
GaL/lb. powder to l Electrically conductive powder:
Silver/copper, lb./1b 0. 02 to 1.00 0. 094
Silver/surface, oz./tt. O 006 to 0.050 0. 024
True density, lbs/it. 558 to 606 561 A conductive epoxy adhesive was prepared from the powder identified in the example. Eleven parts by weight of an epoxy (diglycidyl ether of bisphenol A) was mixed with 89 parts of the above conductive powder. Eleven parts by weight of a polyamide (Versamide hardener was mixed with 89 parts of the conductive powder. Equal amounts of the above mixtures were mixed together. This adhesive had the appearance of a heavy non-sagging paste. It cured in 24 hours at room temperature. The heavy paste was used to caulk the seams of an equipment cabinet about 18" wide 2' deep and 4' tall. Two and one-half pounds of the paste were used in the wall seams of the cabinet. \A 900 megacycle source was placed in the cabinet and attenuation of the cabinet was found to be 65 db.
Highly electrically conductive gaskets for sealing microwave flanges have been made from the conductive powder. About 89 parts by weight of the powder were incorporated into 11 parts by weight of polyvinyl chlo ride plastisol having a curing temperature of 330 F. and a viscosity at room temperature in the uncured state of 160,000 c.p.s (Dewey and Almy Chemical Division, W. R. Grace and Co.; Daxene A-60.) The heavy paste obtained was spread into a 30 mesh mil aluminum Wire screen and cured. The reinforced sheet obtained had a thickness about 22 mils and this was rolled to a thickness of 17 mils. This rolling decreased the flange pressures required to seat the gaskets. The sealing pressure required was under 200 lbs. per square inch.
A gasket for an 8.6 kilomegacycle (X-band) RGSl/U waveguide flange was die-cut from the sheet stock. When tested at an internal air pressure of 25 pounds per square inch at a 2.5 megawatt peak load and a 2.5 kilowatt average load the insertion loss from the gasket was 0.005 db. This was considerably better than the performance obtained from a commercial machined metal-molded O- ring composite seal tested in the same apparatus. The peak load obtainable with the commercial gasket was only 1.6 megawatt because of losses in the seal.
EXAMPLE II With reference to FIGURE 2, a chopped copper wire (Metallurgia CH/ 3162) the particles of which are about A" long and 4 mils thick, is added by line 20 to cleaning zone 21 wherein it is cleaned with glacial acetic acid at 200 F. After this cleaning it is passed by line 22 to a second cleaning zone 23 where it is washed with a 10% acetic acid solution at 100 F. Following this, it is passed by line 24 to a rinse zone 25 where it is thoroughly rinsed with water.
A sodium cyanide solution is first made up having a concentration of 0.75 to 2.7 pounds per gallon, e.g., 1. Approximately one pound of sodium cyanide is used for each pound of the copper particles. The sodium cyanide is admitted by line 28 to zone 26. The rinsed powder is then added by line 27 to this solution with continuous agitation. After agitating for a minute or two, solid silver cyanide is added to the slurry by line 29 slowly over a period of about one minute. The amount of silver cyanide added, per pound of copper particles, is approxi mately 0.02 to 0.10 pound, e.g. 0.047 pound. Plating is continued for 10 to minutes after the silver cyanide has been added until the silver ions are depleted. It can be seen that by this method of addition of the silver cyanide the ratio of cyanide to silver ions in the plating solution, at any instant of time, is relatively very high.
The plated solids are removed by line 30 and passed to a water-wash zone 31. They are thoroughly washed with water and passed by line 32 to a drying zone 33 where they are rinsed with acetone and allowed to dry in air. The completed powder is removed by line 33. The plated chopped copper wire so obtained can be used to pre pare an RF. caulking compound useful for sealing the seams of shielded enclosures by incorporating it into an amorphous polyamide solvent solution (Side-Seam Cement 5450, Dewey and Almy Chemical Division, W. R. Grace and Co.) 38 parts by weight of the polyamide are dissolved in 34 parts of toluene and 34 parts of ethanol. After the polyamide is dissolved, 75 parts of the plated chopped wire are added. This mixture is a cohesive, heavy, non-settling paste which sets to a vibration resistant adhesive having moderate tack when the solvent evaporates.
6 EXAMPLE In With reference to FIG. 3, a copper powder having a 3 mil average particle diameter is cleaned and washed as in Example I. The cleaned and washed powder is then added by line 40 to a plating zone 41 which contains a plating and cleaning mixture admitted by line 42 consisting of 0.25 to 0.75 pound per gallon, e.g. 0.47 pound per gallon of silver nitrate with the remainder being concentrated ammonium hydroxide. The copper powder is maintained in plating zone 41 until the silver ions are depleted, which takes about 7 minutes.
After being plated, the particles are removed by line 43 and passed through a water-wash zone 44. After washing, the particles have a matte appearance but are slightly yellowish. It has been found, interestingly enough, that at this point the particles are not electrically conductive but can be made to be electrically conductive by heatactivating them. Accordingly, the washed particles are passed by line 45 to a heat activation zone 46 where they are heated to a temperature in excess of 300 F. for a time of about 30 minutes, at which point they become electrically conductive. The particles are then cooled and removed by line 47. The conductive particles so obtained can then be incorporated in various plastic compositions as previously described.
It might be noted that at this point procedure III has been modified such that the plating solution contains both cyanide and ammonium hydroxide with a satisfactory plated powder being produced.
The term plastic is intended to include resins and elastomers (rubbers) besides the conventionally accepted plastics such as polyethylene and the epoxies. The plastic binder used may be thermosetting, thermoplastic or nonsetting depending upon the use to which the plastic is to be put. Asphalts, polyvinyl chloride, polyurethanes, polyesters, polyamides, acrylates and natural rubber can all be used as the matrix to hold the powder of this invention in particle-to-particle contact. The cured or set form of the conductive plastic can range from soft, flexible rubber-like materials to rock-hard solids. Some formulations, such as an RF. caulking compound, can be non-setting pastes similar to window putty and useable over a wide temperature range.
The term matrix means something holding or capable of holding embedded within it another object to which it gives shape or form. (The Winston Dictionary: College Edition, the John C. Winston Company, Philadelphia, Pa., 1946).
Having described this invention, what is sought to be protected by Letters Patent is succinctly set forth in the following claims.
What is claimed is:
1. A process of preparing an electrically conductive metal powder comprising cleaning a copper-bearing powder to remove copper oxide therefrom, replacement plating the thus cleaned powder while maintaining continuous agitation with a plating solution containing a high ratio of silver complexing ions to silver ions, the amount of silver ions present being less than that which said copper-bearing powder is capable of taking up; continuing plating until said silver ions are substantially depleted; removing the thus plated powder and washing and drying it.
2. The process of claim 1 wherein said silver complexing agent is selecting from the group consisting of sodium cyanide and ammonium hydroxide and said plating solution is substantially free of ions other than those associated with said powder, silver complexing ions and silver ions.
3. A metal powder for rendering plastic compositions electrically conductive comprising a c oppprsbearing powder having a continuous thin electrically adherent stable cqating gfgilver thereon, said powder having a matte appearance inloose form, said coating having been deposited by replacement plating With the plating being terminated by depletion of silver ions in the plating solution and the thickness of said coating being less than that which would be obtainable if an excess of said silver ions were available in said plating solution, and the conductivity of said metal powder changing less than 50% when said powder is exposed to an OXidiZing atmosphere at 400 F. for 24 hours.
4. The process of claim 1 wherein said copper-bearing powder is substantially pure copper and said copper-bearing powder has no dimension larger than 100 rnils, the amount of silver deposited thereon being in the range of 0.006 to 0.050 ounce per square foot.
5. The process of claim 1 wherein said silver ions are derived from a salt selected from the group consisting of silver cyanide and silver nitrate.
References Cited by the Examiner UNITED STATES PATENTS 1,986,197 1/35 Harshaw 750.55 2,504,272 4/50 McCoy 204-46 2,735,809 2/56 Greenspan 20446 10 2,771,380 11/56 Coleman et a1. 117100 DAVID L. RECK, Primary Examiner.
HYLAND BIZOT, Examiner.