Abstract:
Method and apparatus are disclosed for electrolytically producing very uniform coatings of a desired material on discrete microsized particles. Agglomeration or bridging of the particles during the deposition process is prevented by imparting a sufficiently random motion to the particles that they are not in contact with a powered cathode for a time sufficient for such to occur.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for forming uniformly thick coatings on discrete microsized particles and more particularly to an electrolytic method and apparatus for applying such coatings on discrete microsized particles. 
     A mixture of deuterium (D) and tritium (T) is a preferred fuel for laser fusion, primarily because the least energy is required to cause these two isotopes to undergo thermonuclear reaction. It is desirable that the DT mixture acted upon by the laser radiation be as dense as possible. The optimum density is achieved by cooling the mixture sufficiently that it becomes a solid. This, however, requires temperatures below 20 K which imposes very severe constraints not only on the manufacture, but also on the handling of laser fusion targets. 
     Alternatively, hollow, spherical, DT-gas-filled targets with diameters ranging from 30 to greater than 200 μm and with contained fuel pressures varying from 10 to 1000 atm (at 298 K) are of interest for laser fusion. The primary gas-containment vessels of these targets are hollow microspheres called microcapsules The targets are filled by diffusing DT fuel gas through the walls at elevated temperatures, taking advantage of the exponential temperature dependence of the permeability to allow the gas to be retained for useful times at room temperature. Thus, when the microcapsules are placed in a deuterium and tritium gas mixture of a desired ratio at high pressure and elevated temperature, the deuterium and tritium readily enter the microcapsules and equilibrate to the surrounding gas pressure. When the microcapsules are cooled to room temperature, the diffusion rate through their walls is greatly reduced, so that the DT mixture within the microcapsules remains at high pressure for times which permit useful storage before the targets are irradiated by the laser. 
     To most effectively produce the compression of the DT fuel necessary for thermonuclear reaction, it is desirable that the fuel be surrounded by a pusher shell of high-Z metal. Presently available microcapsules are not composed of materials having the desired high-Z characteristics, so that it is necessary to coat them with a thin layer of high-Z metal. This metal shell of the laser fusion target must be fully dense and of very uniform thickness, with aspect ratios in the range of 10 to 1. It should have a minimum tensile strength of 690 MPa. 
     It is well known in the art that electrolytic techniques are well suited to the deposition or plating of metals on various surfaces and to the precise control of the thickness of the resultant coatings. Unfortunately, conventional electrolytic plating apparatus and methods have been found to be largely unsuccessful in producing metal coatings of the requisite thickness and uniformity requirements on microcapsules. A primary problem has been the tendency of the microcapsules to agglomerate and/or bridge during the coating process. By bridging is meant the joinder of the coatings of two or more microcapsules to form one common structure. 
     SUMMARY OF THE INVENTION 
     Agglomeration or bridging of discrete microsized particles subjected to electrolytic coating techniques is avoided and coatings of very uniform thickness are achieved by imparting a sufficiently random motion to the particles in a solution of a desired electrolyte which regularly brings them into contact with the powered cathode of the electrolytic coating apparatus for a time insufficient for agglomeration or bridging to occur between the particles. In a preferred embodiment, the particles to be coated are placed in an elongated container having porous walls with opposed screened openings therein. The interstices of the screens permit a substantially free flow of the electrolytic solution therethrough but do not allow the particles to pass therethrough. Two cathodes are disposed within this container, with one being adjacent one screened opening and the other adjacent to the second screened opening. Also disposed within the container are a plurality of inert beads which have diameters substantially larger than those of the particles to be coated. The container is submerged within the electrolytic solution and means are provided for cyclically flowing the solution through the container in alternating opposite directions at a flow rate sufficient to bring the particles alternately in contact with first one cathode and then the other. Anodes are disposed within the electrolytic solution around the outside of the particle container. The anodes and the cathodes are attached to a power supply and switching means are provided for switching power from one cathode to the other in synchronization with the change in the direction of flow of the solution through the screened openings. By flowing the electrolytic solution first in one direction through the screened openings and then in the other direction in a cyclic fashion, a sufficiently random motion is imparted to the particles to be coated to effectively prevent agglomeration or bridging of the particles during the coating run. This motion permits the deposition of coatings of very uniform thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of apparatus useful in the practice of the invention. 
     FIG. 2 is a photomicrograph of a cross section of a Solacell electrolytically plated with nickel in accordance with the process of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Apparatus useful for electrolytic plating of metals on discrete microsized particles is shown schematically in FIG. 1. The individual particles 8 to be coated are confined in a cylindrical plating column 1, the bases of which are screens 2, 3 with interstices smaller than the smallest diameter particle to be coated. Screens 2, 3 should be formed of a nonconducting material and nylon mesh is well suited for use in these screens. The wall 4 of plating column 1 is formed of a porous material which is an electrical insulator. A preferred material for wall 4 is a porous alumina ceramic. Cathode wires 5, 6 are mounted adjacent to screens 2, 3 on the inside of plating column 1. Plating column 1 has at least one-fourth and preferably about three-fourths of its volume filled with loose packing plastic, glass, or ceramic beads 7. The size of these beads is not critical, but they should be substantially larger than the size of the particles 8 being coated. Each screened base of plating column 1 is connected to the discharge of a separate filter pump. Thus, in the schematic of FIG. 1, screen base 2 is connected by discharge line 9 to filter pump 10 while screen base 3 is connected to filter pump 12 by discharge line 11. Pumps 10, 12 are connected to plating tank 13 by return lines 14, 15 respectively. Plating column 1 is submerged in plating solution 16 in tank 13. Anodes 17 (only one of which is shown in FIG. 1), connected to power supply 27 are placed concentrically about plating column 1 within tank 13. Cathode wires 5, 6 are routed by means of leads 18, 19, respectively, to terminals 20, 21, respectively, of an electronically controlled switching device 22. Leads 18, 19 are provided with insulation 23, 24 wherever they are submerged in plating solution 16. Switching device 22 is provided with timers 25, 26 which respectively control the operation of filter pumps 10 and 12, and, simultaneously in synchronization flip the cathodic current from power supply 27 between the cathodes 5 and 6 of plating column 1. When pump 10 is ON, the individual particles 8 to be plated are forced against screen 3 and make contact with negatively charged cathode 6 and electrolytic deposition takes place. During this portion of the operating cycle, cathode wire 5 is electrically neutral and pump 12 is in the OFF position. After the elapsed time of this half cycle, pump 10 is turned OFF while pump 12 is simultaneously turned ON and the cathodic current is flipped to cathode 5. The reversal in direction of the solution flowing through plating column 1 drives the individual particles 8 to negatively active cathode wire 5 where metal deposition again occurs. The time of each half cycle is kept relatively short to reduce particle agglomeration by bridging of the deposited material. Preferably, each half cycle lasts for 5 to 30 seconds. The inert, column-packing beads 7 impose a random motion on the individual particles 8 as they move between retaining screens 2 and 3. This random, forced motion provides the necessary action to achieve a uniformly thick deposit and to provide solution turbulence within plating column 1 which separates in-motion particles 8 and which in turn serves to effectively prevent agglomeration. Wall 4 of plating column 1 is saturated with electrolyte 16 which provides the electrical path between anodes 17 and cathodes 2 and 3. 
     EXAMPLE I 
     In the apparatus of FIG. 1, a copper pyrophosphate electrolyte is used to electroplate copper onto nickel-manganese microcapsules in the size range of 180 to 210 μm in diameter. These microcapsules are commercially available under the trade name Solacells from the Solar Division of International Harvester Corporation. The electrolyte is prepared from commercially available plating salts (M&amp;T Chemical Co. &#34;UNICHROME&#34; Pyrophosphate Copper Plating Process), high purity water, and reagent grade ammonium hydroxide. The major solution constituents and their concentrations are: 
     
         ______________________________________Copper                 30 g/lPyrophosphate (as P.sub.2 O.sub.7.sup.-.sup.4)                 225 g/lAmmonia                2 g/l______________________________________ 
    
     The operating conditions are: 
     
         ______________________________________Bath temperature      55°CpH                    8 to 8.5Run duration          45 minCathode current density (avg)                 1.5 A/dm.sup.2Half cycle            10 secFlow rate             6 l/minAmount of Solacells   0.3 cm.sup.3______________________________________ 
    
     The average deposit thickness obtained under these conditions is 14 μm. 
     Example II 
     With the apparatus of FIG. 1, a nickel plating solution based on the Watts formulation is used to electroplate nickel onto Solacells in the 190 to 210 μm diameter size range. Two proprietary addition agents (Brightener No. 610 and Brightener No. 63, manufactured by the Udylite Corporation) are added to the bath in order to increase the brightening and leveling properties of the solution and to obtain ductile deposits. The solution is prepared from reagent grade chemicals to obtain the following composition: 
     
         ______________________________________Nickel sulfate (NiSO.sub.4.6H.sub.2 O)                 300 g/lNickel chloride (NiCl.sub.2.6H.sub.2 O)                  60 g/lBoric acid (H.sub.3 BO.sub.3)                  45 g/lBrightener No. 610     5 ml/lBrightener No. 63      10 ml/l______________________________________ 
    
     The operating conditions are: 
     
         ______________________________________Bath temperature      70°CpH                    3.5 to 4.5Run duration          150 minCathode current density (avg)                 1 A/dm.sup.2Half Cycle            10 secFlow rate             6 l/minAmount of Solacells   0.5 cm.sup.3______________________________________ 
    
     Fig. 2 is a photomicrograph of a cross section of a Solacell 30 with a nickel coating 31 plated in accordance with the foregoing conditions. The average deposit thickness of coating 31 is 12.5 μm. 
     EXAMPLE III 
     With the aparatus of FIG. 1, a sulfite based electrolyte (BDT-100, marketed by the Sel-Rex Company) is used to electroplate a cadmium hardened gold alloy of approximately 99.0% purity onto Solacells and metallized glass microcapsules having diameters of 50 to 210 μm. The operating conditions are: 
     
         ______________________________________Metallic gold content in plating solution                    12 g/lpH                       9.5Specific gravity         16° BaumeTemperature              55° CRun duration             360 minCathode current density  0.1 A/dm.sup.2______________________________________ 
    
     The average deposit thickness obtained under these conditions is 20 μm. 
     The foregoing example serve merely to exemplify the invention and are in no way intended to limit its scope. The apparatus and method of the invention may be used with any metal capable of being electroplated. The density of the microsized particles being coated is not critical and may be less than, equal to, or more than that of the plating solution. Although the microcapsules coated in the foregoing examples are spherical, the method and apparatus of the invention may also readily be used to coat discrete, microsized particles which are irregular in shape.