Submicron and nano size particle encapsulation by electrochemical process and apparatus
An apparatus and method for coating or treating powdered material, particularly ultra-fine powders in the nanometer or submicron range of mean diameters, by electrolytic processes. A platen is mounted for rotation upon a fixed shaft, and a rotary flow-through electrolytic cell is mounted upon a platen for rotation thereon, the cell's axis of rotation being offset from the platen's axis of rotation. The cells axis of rotation revolves around the platen's axis as the platen rotates. The electrolytic cell accordingly undergoes a planetary rotation, as the cell revolves around the platen's axis of rotation. The planetary rotation of the cell allows the powdered material to be collected by centrifugal force and constantly agitated to promote uniform electroplating. An electrode array and rolling contact system are supplied which allow electric potential to be applied only to those electrodes actually in contact with the powdered material to be treated.
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This application is a divisional application of U.S. patent application Ser. No. 09/872,214, entitled “Submicron And Nano Size Particle Encapsulation By Electrochemical Process And Apparatus”, filed on May 31, 2001, and issuing as U.S. Pat. No. 6,942,765 on Sep. 13, 2005. The specification and claims thereof are incorporated herein by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention (Technical Field)
The present invention relates to apparatuses and methods for electroplating and electrochemically modifying the surface finish of metal and semiconductor powders, particularly by continuous centrifugal means for encapsulation, anodizing, electroetching, electroforming, electrophoretic coating, electrosynthesis, and electrodeposition on powders without limitation on particle size, specifically including submicron- or nano-sized particles.
2. Background Art
Note that the following discussion is given for more complete background of the scientific principles and is not to be construed as an admission that such concepts are prior art for patentability determination purposes.
The technologies for electrochemical enhancement of the surfaces of the particles in bulk powders has previously been limited to two main types: chemical copper and electrolytic nickel auto-catalytic processes; and rotary electroplating devices which require frequent stopping and starting of the electrolytic cell's rotation to tumble the powder to achieve uniform dispersion of the coating upon the particles. A limitation of the previous art using chemical or auto-catalytic processes is the cost of the chemical consumption due to the enormous surface areas of powders. Another limitation of known devices using the rotary techniques is the need to stop the cell to tumble the powder in order to disperse the coating and prevent agglomeration of the particles. Known devices of the latter type known in the art are typified by the disclosure of U.S. Pat. No. 5,879,520, the teachings of which are hereby incorporated by reference.
Previous rotary flow-through devices are capable of centrifugal clarification of the particles in solution and fixing them against the cathode ring for electrical contact. A disadvantage occurs, however, when rotation of the cell must be stopped to tumble the powder particles to foster even electrodeposition upon the individual particles. During this “stop phase,” the particles are re-suspended in the electrolyte solution. If the particles are of sufficient density, continuing the rotation of the cell re-clarifies the solution and again fixes the particles against the electrical contact ring, but the need periodically to stop and re-start cell rotation prolongs total processing times. Further, in the case of submicron-sized, low mass powders, the method of repeatedly stopping and resuming cell rotation is unacceptable from a practical standpoint, because the material particles remain in suspension (rather than in contact with the cathode) for impermissibly, nearly indefinite, lengths of time.
Also, laboratory experimentation and commercial application of the known rotary flow- through devices resulted in a determination that such devices have a powder particle size lower limit of approximately 20 micrometers for most common metals. These devices often have limitations related to the substrate powder's particle density, as well. Because previous rotary flow-through devices use a sintered membrane to allow the electrolyte to flow through the cell, a practical particle size limit occurs when the opening area of the sintered membrane must be smaller than the particle size. For powders below 50 micrometers mean particle diameter, the sintered membrane pores must be reduced to 25 micrometers. For powders below 20 micrometers, the sintered membrane pores must be 10 micrometers. When the sintered membrane pores are reduced below 10 micrometers, the discharge of electrolyte through the membrane is significantly impaired, which in turn depletes the ion species in the electrolyte, dramatically reducing the performance of the device. Because the distribution of size of the particles varies, it is possible to have particles smaller or equal in diameter to the openings in the sintered membrane, which in turn causes clogging or blinding of the membrane—further reducing performance. If the solution flow rate is increased to compensate for the ion depletion, the lightweight particles will overflow the cell, causing unwanted material loss and damage to the system.
Another problem with some previous rotary flow-through devices, such as the device of the U.S. Pat. No. 5,879,520, is that they require a complicated level control sensor to prevent the electrolyte solution from overflowing the top of the cell during the stop phase. This further limits the efficiency of solution flow, which also leads to ion depletion.
Further background in the field of rotary flow-through elecroforming/electrodeposition devices and methods is supplied by U.S. Pat. Nos. 5,487,824 and 5,565,079, the disclosures of which are hereby incorporated by reference.
Moreover, each time the cell rotation is resumed (after stopping to tumble the substrate powder), time is required to clarify the solution and re-fix the particles to the face of the cathode ring; heavier particles are thrown into renewed contact with the cathode first, while finer particles require comparatively more time to move outward under centrifugal force. This results in heavier particles having preferential electrical contact with the cathode, resulting in a wide variance in the uniformity of the thickness distribution. In many cases, ultrafine particles will receive no electrodeposition at all.
Another limitation of known rotary flow-through devices is that the rectifier or power supply must be switched off and on in sync with the stopping and starting of the rotation of the cell. Besides causing extended process time during the off cycle, such intermittent voltage processes risk potential chemical damage to the substrate powder when no voltage potential is present.
Another limitation of known rotary flow-through device is the diameter and overall size of the cell, which had to be optimized to provide adequate stopping and starting performance. If the cell diameter is too large, the distance between the electrodes and the distance of travel of the particles became too great for efficient processing.
Another limitation of known rotary flow-through device is the required stop/start sequence means that the particles are fixed at the cathode during the on time, increasing the possibility of undesirably fusing or electroforming substrate components together. This obligates the high frequency stopping/starting to ameliorate agglomeration.
The foremost requirements for commercial electrodeposition apparatuses are to achieve cathode efficiency (e.g., 60-100 percent efficiency), prevent fusing or agglomeration of the particles, achieve high thickness uniformity, not corrode or damage the substrate powder, perform the electrodeposition in reasonable process time, and contain all particles in the apparatus with reasonable material handling methodologies.
SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)The invention is a continuous rotary flow-through electrodeposition system including a rotating platen supporting a vertical rotating cell on an eccentric axis. The system has a plurality of nozzles and electrodes alignable concentrically to a rotating platen. The eccentric rotating cell is actuated by a planetary gear that allows the cell to orbit around the axis point of the centered platen and electrode.
The present invention also features a rotating cell with sectioned electrical contacts molded into a plastic bowl or vessel, isolating the electrical contact exposed at the inside of the cell and extending to the perimeter of the bowl for sequential current feed from a rotating slip ring device. This innovation promotes catalytic efficiency by bussing current only to the “outermost” contacts that are in contact with the powdered materials.
This invention additionally uses an upper dome to complete the cell that features a helical inner flange or ramp. During clockwise rotation of the cell, the upper dome continuously forces the substrate materials downward to maintain their contact with the cell cathode contacts. Further, by reversing the cell rotation to counterclockwise, material can be augered out of the cell to facilitate unloading the finished powder into the collection drain basin.
The cell is provided with a catch basin and a canopy that catch flow-through electrolyte for return to the solution reservoir.
The present invention can also be used with or without a sintered membrane or laser cut slots to allow solution to flow-through, since the cell is configured to permit overflow of process solution from the top port thereof without discharging therewith the powder material being treated. Further, the present invention operates with continuous rotation, eliminating the need to stop and start the cell to tumble parts.
The present invention has no limitation in diameter of the cell, allowing for increased loading capacities due to the continuous operation of the cell and elimination of the stop/start sequence.
In the present invention, the particles are continuously tumbled in contact with the electrical contacts, thereby improving the dispersion of the coating over the surface of each particle and eliminating potential fusing or agglomeration of particles.
A primary object of the processes of the invention is to provide effective electrolytic microencapsulation of submicron-sized or “nano scale” particles.
A primary object of the apparatus of the present invention is to permit the multi-step electroplating process without physical transfer of the plating fixture or cumbersome manual exchange of solutions.
A primary advantage of the invention is that it can process submicron-sized materials with high efficiency, with or without a sintered membrane or slotted dome.
Another advantage of the present apparatus is that it has virtually no limitation on solution flow rate; thus, the electrolytes ion species can remain at optimum levels during the high mass transfer that is required for the high surface area powdered substrate.
A primary advantage of the process of the invention is that a wide range of useful particles and materials can be made thereby including, but not limited to:
inert micron scale isotope particles for blood trace;
critical stoichiometry alloy composition powders;
reduced cost noble metal catalytic powders;
alloy powders for powder metal forming;
electrophoretic coated iron for soft magnetic powder;
battery and fuel cell negative electrode powders;
micro-ball grid array spheres;
microencapsulation of radioactive fuel rods; and
electrosynthesis of ceramic oxides.
Other objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings, which are incorporated into and form a part of the specification, illustrate several embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. In the drawings:
The present invention offers major improvements to apparatuses and methods in electrolytic cell technologies for microencapsulating or coating powdered materials. The apparatus of the invention incorporates some of the desirable aspects of devices and processes known in the art, such as multiple return drains and multiple selectable feed nozzles, while yet overcoming various disadvantages manifested in previous efforts.
The present invention capitalizes upon the fundamental concept of harnessing centrifugal force to compact bulk materials, particularly submicron- or nano- sized powders, in solution (preferably aqueous) against an electrolytic cathode contact. Throughout this disclosure and in the claims, “substrate material” or “substrate powder” refers to the bulk materials to be treated, and specifically includes but is not limited to super-fine conductive and semi-conductive powders having mean particle diameters in the nanometer or submicron range.
The central component of the apparatus is the cell, which features an upper dome mounted upon a bowl. The substrate material is loaded through a top opening in the cell, and the plating cell is rotated at sufficiently high rpm to centrifugally cast the substrate material against the cathode contact at the outer perimeter of the cell. Electroplating solution is then introduced at the top opening, and flows through the cell, eventually exiting through an osmosis filter disposed between the dome and the top edge of the bowl, or alternatively by overflowing at the top opening of the cell. A key advantage of the present invention is that the cell containing the electroplating solution and the substrate material undergoes planetary rotation, that is, a compound rotary motion wherein the cell rotates about its own axis while simultaneously revolving around a fixed axis offset from the axis of the cell. This planetary rotation of the cell eliminates the counter-productive requirement, common in known devices, that electroplating be accomplished with a cycle of periodic stopping and starting, and/or counter rotation with sequential switching of the DC power supply to the cell. Known devices employ these inefficient stop-start and sequential switching methodologies to circulate the particle position for even coverage and prevention of agglomeration/bridging of the substrate material. Thus, in marked contrast with prior devices, the planetary rotation of the cell of the present invention results in the efficient constant movement and controlled agitation of the substrate material in contact with the cathode, with constant rotation of the cell.
The overall and general configuration of a preferred embodiment of the constant rotary flow-through plating apparatus according to the invention is illustrated in
In this disclosure, reference is made to an “anode” assembly and to “cathode” contact strips. It is immediately understood by one of skill in the art that the electrochemical roles of the electrodes in an electrolytic cell may be reversed according to the type of electrolysis to be performed. Thus, in every cell there is a primary electrode and an opposing electrode, and which of the pair functions as the anode and which serves as the cathode may be selectively determined by the operator to perform the desired electrolytic process within the cell. Thus, while the electrode 50 movable upon an overhead boom in this disclosure is denoted as an “anode,” it may actually serve as an electrode in various alternative embodiments or processes without departing from the scope of the invention. Likewise, the “cathode” contact strips 44 may in alternative applications function as anodic strips. Further, the anode 50 may be either soluble or insoluble according to know principles in the art, depending upon the specific electrolytic process to be performed.
The placement of the anode 50 upon an adjustable boom permits the anode or anode assembly to be controllably disposed into the cell for immersion into the electrolyte, and then controllably withdrawn to a position exterior of the cell. Thus, the anode 50 is positionable outside the bowl assembly 36 so as not to be within the cell during, for example, post- or non-electrolytic processing steps, such as rinsing. Further, a multi-anode assembly may be provided, wherein one type of anode may be withdrawn, and another controllably disposed in its stead, to perform a series of process steps in the cell using different anode types.
A specialized dome 40 is mounted upon and above the bowl assembly 36, with an annular osmosis filter 42 disposed between and in sealed contact with the lower circumferential rim of the dome 40 and the rim of the bowl assembly 36. The bowl assembly 36 and dome 40, together with the anode assembly 50, collectively are the principal elements of the electrolytic cell of the invention. The drain port 26 of the basin 24 is locatable above the inlet of a solution reservoir 80, which may be any one of a plurality of solution reservoirs disposed radially about the exterior of the drain basin 24. Solution from within the reservoir 80 may be pumped into the bowl assembly 36, via one or more feed nozzles 83, by means of a suitable pump 81 and re-circulation conduit 84.
The flow of working solution through the apparatus of the invention during any given treatment cycle is described with reference to
Specific reference is made to
Attention is invited to
The bowl assembly 36 is situated upon the platen 30 for rotation thereupon. Reference is made to
Continued reference is made to
Reference is made to
Combined reference is made to
Continuing reference to
Reference is made to
A key advantage of the present invention thus is presented. The electrolytic cell (mainly including the bowl assembly 36) containing the electroplating solution and the substrate material undergoes planetary rotation, that is, a compound rotary motion, wherein the cell rotates about its own axis B while simultaneously revolving around a fixed axis A offset from the axis of the cell. As the cell orbits around the central axis A of the apparatus, the substrate material is cast by centrifugal force against the “outermost” portion of the interior of the bowl 70. As suggested by
A further advantage of the invention is that while the substrate material M collects at a certain surface within the bowl 70, it nevertheless is in a constant state of agitation. Deliberate agitation of the substrate material fosters uniform electrodeposition upon the individual powder particles. Wherein prior art devices typically repeatedly interrupt and re-start cell rotation to tumble the substrate material, the agitation in the present invention is constant as a result of the continuous rotation of the bowl assembly 36. As seen in
Further understanding of this function is had with reference to
Because the segment of the bowl assembly 36 against which the substrate material collects is predictable and defined, the apparatus advantageously limits to that segment the application of the working electrical potential. The electrical potential required to perform the electrolytic processing of the substrate material M is applied via the anode assembly 50 and the cathode strips 44, 44′, 44″.
The mode of applying the working electrical potential to the substrate—a distinct advantage of the invention—is explained with combined reference to
As previously mentioned, as the bowl assembly 36 rotates, the substrate material is constantly tumbling or rolling along the inside of wall 72. The general location of the substrate remains unchanged in radial relation to the platen's axis of rotation A due to the centrifugal force of the bowl assembly's revolution around first axis A. The substrate tumbles along the wall 72 due to the rotation of the cell bowl assembly 36 around its own axis B, meaning that the wall 72 has a constantly changing radial relation to the first axis A, and thus is always in motion with respect to the substrate material itself.
Combined reference is made to
Because the wire wheel contact 92 is radially collinear with the bowl's axis of rotation B, the wheel contact 92 is at all times situated below the portion of the bowl 70 that is radially outermost from the platen's axis of rotation A. Thus, even thought the bowl 70 is constantly rotating around its own axis B (and thus the portion of the bowl that is maximally distanced from the first axis of rotation A is constantly changing), the wheel contact 92 ever remains below that outermost bowl portion in the vicinity of point P. Significantly, the mass of substrate to be treated also remains in the vicinity of the outermost point P, so that the wheel contact 92 and the tumbling substrate are always in radial alignment with respect to the bowl's axis B.
The constant radial alignment of the tumbling substrate with the wheel contact 92 allows the application of the working voltage to be coordinated with the position of the substrate. As the bowl assembly 36 rotates about the second axis B, the radially arrayed cathode strips 44, 44′, 44″ consecutively contact the wire wheel contact 92, which is in rolling contact with the underside of the rotating bowl 70. As the wheel contact 92 turns, the cathode strips 44, 44′, 44″ come into physical and electrical contact, e.g. one at a time, with the wheel contact 92, permitting a voltage to be applied momentarily to the contacting one of the strips. It will be immediately understood by persons skilled in the art that strips 44, 44′, 44″ need not make electrical contact with the wheel 92 one at a time; alternatively, the contact strips may be interconnected electrically so as to function in groups (e.g., two to five strips per group). In such alternative embodiments, all the strips in a designated group or cluster are electrically active when any one of them is in electrical contact with the wheel 92. Such alternative embodiments may promote better application of current to some types of treated substrate materials. The temporary and abbreviated electrical connection between each strip 44 or 44″ is provided by the rolling contact of the wheel contact 92 with the exposed contact portion 49 on each cathode strip.
At the instant a given one of the cathode strips 44, 44′, 44″ is in contact with the wheel contact 92, that strip (i.e. strip 44″ in
The operation and method of the invention are apparent to one of ordinary skill in the art having reference to the foregoing. The complete apparatus of the invention, in position for use, is depicted in
During the working stage of the process, the auger flange 100 screws about the second axis B of the cell in a manner that urges the cell contents downward into the cell for continued processing, as suggested by the smaller directional label W in
Advantageously, processing continues without the need to stop and start the cell to agitate the substrate. Processing may be staged using different chemicals feed nozzles 83 and solution reservoirs 80 according to known methods and devices.
At the conclusion of the complete processing, the discharge of processing or rinsing liquids into the interior of the cell via the nozzle assembly 83 is discontinued, and the vast bulk of the liquid in the cell interior is spun out through the filter 42 by centrifugal force, leaving the substrate comparatively dry. The directions of rotation of the platen 20 and the bowl assembly may then be reversed to empty the substrate from the bowl assembly. The reversal of the direction of bowl rotation, as indicated by the large directional arrow U in
The inventive apparatus has manifold uses. For example, the following materials constitute but a partial list of the powders, micron scale to sub-micron scale, that may be processed in the invention to satisfy particularized needs:
-
- Inert micron scale isotope particles for blood marker
- Critical or fixed stoichiometry alloy composition powders for powder metal forming and solder paste applications
- Filament platinum-coated nickel catalytic submicron powders
- Microsphere grid array microencapsulation of crushed radioactive fuel rods
- Electrosynthesis of ceramic oxides
- Compactible surface alloy for refractory metal powder forming
- Corrosion protection for nanoscale powders
- Insulating and dielectric coatings for soft magnetic powders
- Electrophorectic/electrophrenic polymer coatings on metallic powders
- Noble metal encapsulated base metal powders
- Capacitive dielectric oxide coatings on tantalum powders
- Ordnance powders
- Armor piercing cores
- Metal encapsulated reactive ignition powders
- Metal encapsulated organic materials for implantable pharmaceutical delivery systems
- Metal encapsulated photocopier toner materials
- Anodized aluminum fines for automotive and industrial paint systems
- Metal encapsulated in conductive metal encapsulated particles for arc welding electrode
- Metal coated diamond and refractory metal powders for cutting tool inserts
- Metal-coated graphite particles and fibers
- Composite metal foils
- Multilayer metal alloy powders
- Metallic flake electro-deposited alloys for pigments and printing ink
- Metallic electro-deposited alloys for brazing and soldering powders
- Metal matrix composites
- Powder metal superconductor material
- Inert submicron magnetic particles marker material for nondestructive testing
- Nickel encapsulated metal hydride battery powders
- Insulating metal powders
- Enhanced compaction surface elements
- Electropolished metal powders
- Low fusing temperature metal coatings over metal powders for rapid prototyping using stereo laser systems
- Anodic coatings of submicron metallic powders
- Low noble weight silver termination paste for multilayer chip capacitors
- Low noble weight micron scale nickel/platinum electrode inks for multilayer chip capacitors
- Dielectric coatings on metallic powders for electronic component inks
- Gold-plated stainless-steel powders for medical and dental implantable components
- Compactable permanent magnetic powder
- Frangible bullets
- Thermal management particles for screenable inks
- Solid rocket fuels
- Metallic powder coating pigments
- Colloidal chemical catalysts
- Critical stoichiometry sintered sputtering targets
- Metal encapsulated mesoscale radioactive powder for waste remediation
- Nickel coated iron powder for magnetic recording media
- Multilayer electrodeposited composition powders for pyrotechnics and explosives
- Zinc encapsulated copper powder for batteries.
Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all patents and publications cited above are hereby incorporated by reference.
Claims
1. A method for depositing a coating on a substrate powder, the method comprising the steps of:
- rotatably mounting a rotary flow-through electrolytic cell on a platen;
- loading a substrate powder comprising a particle size of less than 20 micrometers in the cell;
- rotating the cell about a first axis relative to the platen;
- rotating the platen about a second axis offset from and parallel to the first axis, thereby enabling the cell to undergo planetary revolution;
- flowing a solution through the cell;
- generating sufficient centrifugal force to overcome suspension of the substrate powder in the solution; and
- depositing the coating on at least a portion of the substrate powder.
2. The method of claim 1 wherein the generating step comprises forcing at least a portion of the substrate powder against the inner wall of the cell.
3. The method of claim 2 wherein at least a portion of the substrate powder contacts an electrode strip attached to the inner wall of the cell.
4. The method of claim 2 wherein most of the substrate powder collects along a short arcuate segment of the inner wall.
5. The method of claim 4 further comprising the step of tumbling the substrate powder along the inner wall as the cell rotates.
6. The method of claim 5 wherein the powder contacts one or more electrode strips arrayed on the inner wall.
7. The method of claim 6 wherein only a subset of electrode strips is electrically connected to a power supply at any given time.
8. The method of claim 7 further comprising the step of varying which electrode strips comprise the subset so that most of the tumbling substrate powder is substantially continually in contact with at least one charged electrode strip.
9. The method of claim 1 further comprising the step of reversibly disposing an electrode into the cell.
10. The method of claim 1 further comprising the step of filtering and recirculating the solution.
11. The method of claim 1 further comprising the step of reversing the rotation of the cell.
12. The method of claim 11 further comprising the step of auguring the substrate powder out of the cell.
Type: Application
Filed: Sep 13, 2005
Publication Date: Jan 19, 2006
Applicant: Surfect Technologies, Inc. (Albuquerque, NM)
Inventors: Thomas Griego (Corrales, NM), John Eichman (Grants, NM)
Application Number: 11/225,933
International Classification: C25D 7/00 (20060101); C25D 5/00 (20060101);