Patent Application
20070131555
The present invention relates to apparatus, methods and systems for applying electrophoretic deposition (EPD) to produce small components and coatings. More specifically, this invention describes a system for manufacturing anodes for solid electrolytic capacitors by electrophoretic deposition (EPD).
The present invention relates to apparatus, systems and methods for electrophoretic deposition (EPD). Precisely shaped, small metal and ceramic bodies are used in many applications including as anodes for electrolytic capacitors, as pitch bonding capillaries in microelectronics, as high temperature nozzles, as ferrules for connecting optical fibers, as high temperature engine components, as dental crowns, and as bearing parts. To achieve the precise shaping required for some of these applications, like electrolytic capacitor anodes and bonding capillaries, it has been necessary to use the process of pressing to fabricate the components. Pressing is generally a discrete operation, producing one part at a time where agglomerated powders are used.
More recently, electrophoretic deposition (EPD) processes have become available for producing small bodies, coatings and components. EPD uses non-agglomerated powders and provides a way for production in batch mode or discrete operation.
Electrophoresis is well known in the art as a process to form green bodies. EPD has been used by Sarkar, Haung and Nicholson [Electrophoretic deposition and its use to synthesize A1.sub.2 O.sub.3 /YSZ micro-laminate ceramic composites, Ceram. Eng. Sci. Proc., vol. 14, pp. 707-716 (1993)] to deposit laminated composites of alumina and yttria-stabilized zirconia (YSZ). Electrophoretic deposition processes are described in U.S. Pat. No. 5,919,347 “Method of Electrophoretic Deposition of Laminated Green Bodies”, in Published International Patent Application WO 02/103728 “Process for Manufacturing a Metal Electrode”, in Israeli Patent application IL168397 “Solid Electrolyte Capacitor with Controlled Properties and a Method for Manufacturing the Same”, and in International Patent Application WO/IL2005/000763 “Fabrication of Electrical Components and Circuits by Selective Electrophoretic Deposition (S-EPD) and Transfer” all by the applicant of the present application, the descriptions of which, including references cited therein, are incorporated herein by reference in their entirety.
The prior art extensively covers dispersion composition and process parameter requirements for producing components by EPD; however, little innovation is known for the EPD apparatus and deposition system. A typical design of prior art apparatus 100, which includes a power supply 130, dispersion cell 120, holding fixtures 125129, and electrodes 132150, is shown in
Prior art apparatus, such as that shown in
Use of prior art EPD technology for manufacturing monolithic bodies like electrolytic capacitor anodes produces shapes of limited dimensional tolerances. Furthermore, deposition weight changes significantly for each deposition cycle for a given EPD dispersion. For laboratory or research purposes, a fresh dispersion can be used for each deposition cycle; however, this is not practical for large scale manufacturing.
The limitations that restrict the use of EPD in mass production of monolithic bodies like capacitor anodes can be further highlighted by comparison with the process of electrolytic deposition. Typical industrial solutions used for electro-deposit of metal exhibit only minor side reactions. The ratio between deposition mass and transferred coulombs is well known and fixed, providing easy control of total deposited mass. By comparison, charge transfer to the deposition electrode cannot be used as a control parameter for mass production by EPD, since most of the electrical charge transferred to the electrode is not carried by the deposited powder particles, but rather by ions from the EPD dispersions. In EPD, preexisting particles are charged and transferred to an electrode under an electric field. Changes in the EPD dispersion chemistry, particle mass load and particle size distribution occur during each deposition cycle. These changes modify the amount of mass deposited per coulomb of charge collected at the electrode.
Prior art EPD technology does not provide methods or apparatus to tightly control deposited mass in subsequent depositions from a given dispersion.
There exists a need for reconsideration of and improvement over the prior art in several aspects of any mass production EPD process and apparatus. Some of these aspects include:
Only careful and full consideration of all of these aspects can yield a cost effective EPD mass production that consistently meets predetermined specifications.
It is, therefore, an object of the present invention to make EPD a feasible industrial manufacturing process for small components; a process which achieves the desired shapes having tight dimensional tolerances and material properties that meet the requirements of high volume assembly lines.
It is a still further object of the present invention to manufacture a batch constituting a plurality of discrete components by EPD wherein a large number of components are deposited simultaneously.
It is a still further object of the present invention to manufacture by EPD a batch constituting a plurality of components; wherein a large number of components are deposited on a single substrate.
Yet another object of the present invention is to manufacture by EPD small components and coatings from non-agglomerated powders having small particle size and a narrow particle size distribution.
It is another object of the present invention to provide an EPD manufacturing apparatus and system capable of operating in an ‘open loop’ mode with pre-determined work parameters, or alternatively in a ‘closed loop’ mode with real time process monitoring and control.
It is still another object of the present invention to design the local electrophoretic electric field around the substrate undergoing deposition in order to shape the component to a desired form.
It is still another object of the present invention to shape the component being formed by EPD by use of physical barriers which restrict the deposited material in one or more dimensions with tight dimensional tolerance and further providing a flat or otherwise shaped surface to the component.
It is still another object of the present invention to exploit EPD dispersions used to near depletion while maintaining tight control on deposit mass in sequential production runs.
It is another object of the present invention to provide cost effective EPD manufacturing apparatuses and systems.
It is still another object of the current invention to correct EPD dispersion chemistry and particle content by means of recycling EPD dispersion by drawing fluid from the EPD cell to an external tank and transferring the corrected dispersion to the EPD cell.
It is still another object of the present invention to manufacture components with EPD apparatus that incorporates capillary structures or other structures of different cross sections, yet having small inside dimensions for containing the substrates undergoing deposition.
It is still another object of the present invention to manufacture by EPD components as discrete units, in batches, or in a semi-continuous manner.
It is a further object of the present invention to manufacture by EPD components using many kinds of substrates including wire, foil, mesh, perforated substrates, 3D shapes and substrates with masking for selective deposition on designated areas of the substrate.
It is a still further object of the present invention to manufacture components by EPD with minimum precipitation of particle contamination on surfaces within the EPD apparatus.
It is a still further object of the current invention to provide an EPD manufacturing process and apparatus for efficient production of solid electrolytic capacitor anodes.
Other objects and advantages of the invention will become apparent in the following description of this invention.
The invention described herein provides for a high yield and low cost EPD industrial manufacturing apparatus with efficient materials utilization for the manufacture of small metal and ceramic components, such as anodes of electrolytic capacitors. The invention further provides component shapes and dimensional tolerances not obtained by prior art EPD apparatus.
The EPD apparatus of the invention incorporates one or more design features to achieve the desired components and dimensions.
In another aspect of the invention, control systems are incorporated into the EPD apparatus in order to perform one or more of the following functions.
The apparatus and methods of the invention may be used with a variety of substrates including but not limited to foil substrates, wire, substrates that are masked for selective deposition in designated areas, 3D shapes and substrates which are perforated or in the shape of sieves.
The invention may be applied to sequential production of discrete components or to batch production. The invention may also be applied to semi-continuous production of components as may be carried out by attaching substrates to a strip and transferring the substrates through the EPD cell using a reel to reel or similar process. The invention may further be applied to semi-continuous operation in a carousel type process, where substrates are inserted into the carousel, transferred through the EPD cell and finished green components are extracted from the carousel.
In a first aspect the invention is an apparatus for the production of small components having defined pre-set dimensions and properties. The method of the invention is based on electrophoretic deposition (EPD) on electrically conducting or semi-conducting substrates. The apparatus comprises:
The apparatus of the invention can comprise components for monitoring the chemical and electrical properties of the EPD dispersion, for controlling the shape and strength of the electrical field in the EPD cell, or for controlling the electrical current flowing through the external electrical circuit and the voltage across the circuit.
The substrate can be wire-shaped, flat-shaped, sieve-shaped, or have a 3D shape. The substrate can be comprised of conductive and non-conductive areas.
The elements for shaping the component or providing tight dimensional tolerances can be one of the following:
In some embodiments of the invention, the substrate fixture covers a part of the substrate, thereby protecting selective areas on the conductive substrates from being exposed to the EPD dispersion. In other embodiments the substrate is contained is a capillary or a tube or a slit which is slightly wider and thicker than the substrate.
In embodiments of the apparatus of the invention the structure in which part of the substrate is contained is filled with clean solvent compatible with the dispersion fluid, thereby further reducing diffusion of particles into the gap between the substrate the walls of the structure. In these embodiments pressure can be applied to the top end of the structure for example by means of a fluid reservoir at the upper end of the structure. The applied pressure will create a small amount of flow of clean solvent from the structure into the dispersion fluid, thereby further reducing diffusion of particles into the gap between the substrate and the walls of the structure. In embodiments in which the structure is a capillary, preferably the applied pressure is made equal to the pressure exerted by the capillary force, thereby preventing capillary flow of dispersion fluid into the structure. If the clean solvent is immiscible with the dispersion fluid, it will create a sharp barrier between the end of the structure and the dispersion fluid, thereby further reducing diffusion of particles into the gap between the substrate and the walls of the structure.
In preferred embodiments of the apparatus the chemical and particle composition of the dispersion is maintained uniform throughout the deposition cell and a uniform electric field is created within the deposition cell around the components undergoing electrophoretic deposition.
The structures in the EPD cell, which are necessary for structural support or other functions, are preferably made of conductive material and charged to the potential of the counter electrode, thereby reducing deposition of dispersion particles on the structures.
A preferred embodiment of the apparatus of the invention comprises a system for recycling the EPD dispersion. The system removes the dispersion from the EPD cell, either continuously or between deposition cycles, and causes it to flow to a recycling cell, wherein the properties of the dispersion are changed before returning it to the EPD cell. The properties of the dispersion that are changed before returning it to the EPD cell can include the particle concentration, the pH, the Z-potential, and/or the electrical conductivity. The concentration of the dispersion transferred to the recycling cell can be corrected to its original concentration before returning it to the EPD cell.
In some embodiments the recycling cell comprises ultrasonic agitation means to break down agglomerates to their original size distribution. In other embodiments the dispersion in the recycling cell is allowed to undergo sedimentation for a predetermined time in order to remove large agglomerates not compatible with the original particle size distribution and/or excessively fine particles that are not in compliance with the original particle size distribution are removed from the dispersion in the recycling cell. The chemistry of the dispersion fluid in the recycling cell can be corrected by the addition of tiny amounts of chemical additives.
The apparatus of the invention can be configured to form the small components by electrophoretic deposition as discrete components or in batches. The small components can be formed by electrophoretic deposition in a semi-continuous operation on a substrate in the form of a continuous strip or ribbon or on substrates that are arranged on a carousel.
The apparatus of the invention can be used to form small components having many types and uses, for example those selected from the group comprising: porous anodes of electrolytic capacitors, pitch bonding capillaries in microelectronics, high temperature nozzles, ferrules for connecting optical fibers, high temperature engine components, dental crowns, and bearing parts.
In another aspect, the invention is a method for controlling the use of the apparatus of the invention for the preparation of a component by electrophoretic deposition. The method comprises making use of one of the following:
The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only. They are presented to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description of the invention below taken together with the drawings makes apparent to those skilled in the art how the several forms of the invention may be embodied in practice.
As is well known, EPD is carried out by applying a voltage between two electrodes immersed in a suitable dispersion. Suitable EPD dispersions are composed of a powder of the material that will be deposited as a green film or body and which has been dispersed within a solvent of relatively high dielectric constant, typically above 6. Particle size used for stable dispersions will typically be in the submicron range up to about 10 microns.
Various additives may be added to the dispersion to adjust pH, Z-potential, conductivity and other dispersion characteristics. Processes are known in the art for producing from such dispersions various components in green form ready for sintering, such as porous anodes of electrolytic capacitors. However, lacking in the art are apparatus and methods for maximizing utilization of the powder in the dispersion or for achieving dimensional tolerances and shapes in subsequent manufacturing runs as required for mass production of such components. This invention provides efficient and precise mass manufacturing of components by EPD, such as anodes of electrolytic capacitors, by achieving high utilization of expensive raw materials like tantalum powder and by consistently achieving tight dimensional tolerances of defined component shapes in repeated depositions. This includes production sequences of discrete components, batches of components, and production by semi-continuous operation.
The apparatus of this invention is designed so that the substrates undergoing electrophoretic deposition are submerged a significant depth below the free surface of the dispersion with minimal structural elements in their vicinity.
In another embodiment of the invention, the apparatus substrate fixtures and the support structures used for placement of EPD cell elements such as substrate fixtures, counter electrodes, etc. are designed with minimal surface area exposed to the dispersion liquid. Submersed insulated structures tend to become contaminated by dispersion particles and to disturb the electrical field in the EPD cell and the quality of the EPD deposits. Therefore a preferred embodiment of the invention is based on producing symmetrical support structures of conducting material and charging these to the counter electrode potential, thereby reducing the sticking of dispersion particles to the structure. Similarly, the support structure and the counter electrode may be the same entity. Furthermore, a support structure that is distant from the substrates undergoing deposition is preferred, thereby minimizing interference with dispersion uniformity and particle transport in the vicinity of the substrates.
In other embodiments of this invention described herein, secondary structures such as physical barriers or electrodes for shaping the local EPD cell electric field are supplied for the purpose of achieving shaped or anisotropic deposition. These embodiments of the invention are designed so that restriction of the dispersion volume around the substrates undergoing deposition is minimized.
In another embodiment of the invention, EPD cell counter electrodes are arranged symmetrically around the substrates undergoing deposition in order to provide a uniform and equal global electric field within the EPD cell around all of the substrates undergoing deposition. Preferred embodiments include cylindrical counter electrodes surrounding the substrates undergoing deposition or arranging the substrates symmetrically between inner and outer cylindrical electrodes.
The surface area of the counter electrode is chosen to be much larger than the substrate area in order to minimize electrical polarization at the counter electrode, as is known to occur in the art. According to the invention, the preferred counter electrode area is at least five times the substrate area. Although the global EPD cell electric field is designed to be uniform, this invention provides means for shaping the local electric field around the substrates in order to shape the deposit in accordance to specifications of the final part.
Particles deposit on a substrate in an isotropic manner when placed in a uniform dispersion and a uniform electric field is applied. However, many components, such as anodes of electrolytic capacitors, require specific non-symmetrical shapes, extremely tight dimensions, and/or flat surfaces. Prior art restricts the EPD deposit to specified parts of the substrate by application of a thin insulating mask to certain areas of the substrate so that deposition takes place selectively on the surface of the substrate; however, thick deposits, as required for monolithic parts like anodes of electrolytic capacitors, creep over the top of the thin insulating mask due to the isotropic nature of the deposition process. Furthermore, for applications like capacitor anodes the mask layer must be separated from the green body after completion of the EPD process, which may be problematic.
The present invention shapes the electrophoretic deposit by use of engineered counter electrodes, secondary counter electrodes, current thieves, and non-conductive inserts to modify the local electric field around the substrate; thereby resulting in a defined anisotropic deposition. Anisotropic deposition through use of specifically engineered, multiple counter electrodes are demonstrated in the photographs reproduced in
Anisotropic deposition through use of insulated inserts in proximity to the substrate undergoing deposition is demonstrated in the photographs reproduced in
Electric field simulations may be utilized in design of such secondary counter electrodes and insulated inserts that shape the component. Typical examples of electrophoretic field simulation are presented in
In cases in which a very flat surface is required at the end or edge of the green body formed by EPD, a physical barrier in very close proximity to the end or edge of the substrate is used. The EPD deposit grows until it fills the gap between the substrate and the barrier. The end or edge of the green body attains the shape of the barrier surface. In order to minimize interference with dispersion convection and uniformity in the vicinity of the substrates undergoing deposition, the barrier is designed as an element having the minimal size and support structure required to achieve shaping of the EPD deposit. Preferably, the surface of the barrier is smooth or coated with a material that reduces adhesion between the green body and the barrier, so that the green body is readily separated from the barrier surface after deposition. A typical example of a product that can be advantageously produced using barriers in the EPD cell according to the invention is the anode of an electrolytic capacitor.
Anodes of electrolytic capacitors require tight dimensional tolerances for high yield mass assembly and tight tolerances on anode weight for exact and repeatable capacitance values. One or more flat surfaces are often required to facilitate accurate, high speed assembly of the anode into the capacitor package. An example of a preferred anode shape is an anode deposited about a wire substrate, having an abrupt end for the EPD deposit at a designated point on the wire with tight dimensional tolerance.
In the case of a wire immersed in an EPD dispersion in a prior art system, particles would deposit well beyond the desired endpoint. This would occur even if the wire were masked with a thin insulator to prevent deposition beyond the desired endpoint on the wire since typical EPD processes are isotropic. Therefore the deposit will creep up on top of the insulating mask to a length similar to the deposit thickness.
In one embodiment of the present invention, the substrate or wire is held within a capillary or capillary like structure, with only the part of the substrate designated for deposition extending beyond the end of the capillary or capillary like structure. For a wire substrate, the capillary is a tube. For a flat substrate, the capillary structure has in internal cross section that is only slightly longer and wider than the substrate dimensions. A preferred size of the capillary structure is <0.1 mm larger than the dimensions of the substrate thickness and width. The small capillary gap greatly reduces diffusion of dispersion particles to parts of the substrate where material deposition is not desired. Furthermore, a flat deposit surface is formed at a well defined point on the substrate or wire with tight dimensional tolerance.
Apparatus 200 comprises a container 202 for holding the EPD suspension. In container 202 are suspended counter electrodes 204, which are held in position by insulated holder 206 that is seated on seating ring 208 on the inner surface of the outer cylindrical counter electrode. Inserted into the insulated holder 206 are capillary tubes 214. Each capillary tube has a substrate 212 passing through its interior with the portion of the substrate designated for deposition extending beyond the end of the capillary. In this example, the substrates are wires, having circular cross-sections. External contact ring 210 is in electrical contact with the upper end of each of the wire substrates 212.
In a preferred embodiment, the apparatus 200 comprises a base 216 and spacer 218, an example of which is schematically shown in
In one preferred embodiment, a barrier, which is present in the bottom of the cell during deposition, may be designed having a cone shaped structure 220 as shown schematically in
In a preferred embodiment of the invention, the capillary extends a significant depth into the dispersion, where uniformity and diffusion of dispersion components is optimal.
In a preferred embodiment of this invention, the end of the capillary structure is polished or coated with a material to reduce adhesion of the green body formed by the EPD process, thereby facilitating easy separation of the green body from the capillary structure at the conclusion of deposition.
In a preferred embodiment of this invention, contamination of the substrate within the capillary structure is further reduced by filling the capillary structure with liquid before its immersion into the EPD dispersion. The liquid used to fill the capillary structure may be a clean solvent that is compatible with the EPD dispersion composition. For example, for a dispersion based upon ethanol, the clean solvent used to fill the capillary structure will be ethanol. The presence of clean solvent in the capillary gap reduces diffusion of particles to parts of the substrate within the capillary.
The liquid used to fill the capillary structure may be immiscible with the EPD dispersion. This immiscible liquid creates a robust and impenetrable physical barrier to the diffusion of particles to that part of the substrate within the capillary structure.
A liquid reservoir may further be supplied at the upper end of the capillary structure above the EPD cell, or other means to create hydrostatic pressure within the capillary structure. For a capillary filling solvent that is compatible with the EPD dispersion, the generated pressure creates a very low fluid flow within the capillary, opposing diffusion of particles into the capillary gap. For a capillary filling liquid that is immiscible with the dispersion, location of the meniscus demarcating the capillary filling liquid and the dispersion fluid can be more accurately maintained at the very end of the capillary structure when a slight overpressure is applied.
As shown in
The resulting tantalum anodes 316 are shown in
In some embodiments of the invention, innovative control systems are an integral part of the deposition system. For efficient and low cost manufacturing, dispersions used for manufacturing components by electrophoretic deposition must be utilized to near exhaustion or alternatively, the dispersion chemistry, particle mass load, and particle size distribution should be corrected during electrophoretic deposition or between batch depositions. This is particularly true for dispersions of high cost powders such as tantalum for anodes of electrolytic capacitors. In one embodiment of the invention, consistent deposit weight is achieved in subsequent depositions from an original dispersion through a model based system that modifies current, voltage and time for subsequent depositions. Deposition weight achieved from dispersion is a complex function of electrophoretic voltage, current, and deposition time. Each deposition cycle alters the particle mass load, particle size distribution, and chemistry of the dispersion fluid. Deposited mass is sensitive to these parameters. In order to achieve the consistency of EPD mass deposit which is essential for manufacturing, this embodiment incorporates model based parameter adjustment into the EPD apparatus for multiple deposition cycles from partially depleted dispersion.
This embodiment is realized for a specific EPD apparatus by performing test runs with various control parameters. Knowing the relationship between the weight W and the deposition number N over different values of the control variables, a control algorithm is developed for that particular apparatus to compensate for depletion of the dispersion from run to run, thereby achieving consistent mass deposit for all runs.
A preferred control variable is fVdt (I=constant), where V is the applied electrophoretic voltage and I is the current in the EPD cell. The control model is constructed by performing linear regression as is known in the art using exponential models of the form W=αN−β(W=deposit weight; N=sequential deposition run number using the depleted dispersion) and determining the α and β coefficients for each fVdt. An example of data used for building such a control model is presented in
The data in the above table are used to calculate the control model. In this particular case, the exponent βis roughly constant at 0.09524. On the other hand, for the above example, αis approximately a quadratic function of fVdt as can be seen in
fVdt=3.00×106α2−2.18×105α+6.93×10 3.
Using this quadratic fit for αand a constant value of 0.0952 for β, the control model equation for this particular example is:
where W/Nβhas been substituted for α in the above equation in accordance with the curve fit for deposit weight W shown in
A typical example of a control model built from such data using a target deposit weight of 0.05 gram is shown in
Other preferred control variables that can be used are fIdt (V=constant) or simply t (time). According to the invention, the power supply of the EPD apparatus is programmed to apply the specified value of the control parameter for subsequent depositions in accordance with the experimentally established model, which is specific to that particular EPD apparatus, dispersion composition and desired deposit weight.
Another embodiment of a control method for partially depleted dispersion in EPD apparatus is based on monitoring of the electrophoretic current under constant voltage operation in order to determine the endpoint for usage of the depleted dispersion, i.e. the time at which replacement with fresh dispersion is necessary.
In another embodiment, which is shown schematically in
The method and apparatus 400 includes low flow rate transfer of dispersion fluid from the EPD cell 402 to an external recycling tank 404 using a circulation pump 406 through a short piping system. The fluid transfer may occur in small batches between deposition cycles or may be a continuous and constant flow. Chemical and emulsion and particle property monitoring detectors (not shown in the figure) are incorporated within the recycling tank 404. Chemistry monitoring includes pH, Z-potential, conductivity, additives concentration, concentration of contaminants and more. Particle monitoring includes mass per cc of dispersion and particle size distribution. Small quantities of chemicals are added in accordance with monitor results in order to correct the dispersion chemistry and counteract contaminants. Particle mass load is corrected by addition of powder while performing ultrasonic agitation 408 in the recycling tank 404. Particles that have coalesced into large agglomerates settle to the bottom of the recycling tank 404 by gravity and are removed from the recycling bath, thereby maintaining the original dispersion particle size distribution. Also methods are applied to remove excessively fine particles in the dispersion in the recycling cell that are not in compliance with the original particle size distribution.
Although the above description of the different embodiments of the invention generally refers to the manufacture of batches of components, where a batch may consist of a single component or a large number of components, it will be understood that the invention is also applicable to manufacturing by semi-continuous operation. Such semi-continuous production is achieved by systems such as reel to reel continuous or stepwise transfer through the EPD cell of components attached to a tape or strip. Such semi-continuous production may also be achieved by systems utilizing a carousel continuously or stepwise moving through the EPD cell, where completed green bodies are extracted from the carousel and new substrates are inserted for electrophoretic deposition of the green component body.
For all of the embodiments described herein, it will be understood that specific mention of wire or flat substrates does not limit the invention. The invention may be used with any kind of substrate including but not limited to 3D shapes and substrates which are perforated or in the shape of sieves.
Furthermore, although some of the embodiments herein specifically reference mass production of capacitor anodes using the invention, it will be understood that this is by way of example and does not limit applicability of the invention to manufacture of many kinds of small components by EPD.
The following examples are provided merely to illustrate the invention and are not intended to limit the scope of the invention in any manner.
Niobium Oxide Capacitor Anode Manufactured by Fixturing in a Capillary Tube and Immersing in an EPD Cell
The dispersion was prepared by dispersing 5 g capacitor grade niobium oxide powder in 100 ml of 2-Propanol. The suspension was subjected to ultrasonic treatment in a Fisher Scientific Sonic Dismembrator 550 at 20 KHz and a power level of 550 Watts for 5 minutes in pulse regime, 2 sec pulse on and 2 sec pulse off. 150 μl of Polyethylenimine 17 wt % aqueous solution was added to the suspension and pulsed sonification repeated for 1 minute. The dispersion was further stirred for 20 minutes. A dispersion pH in the range of 9 to 10 and conductivity of 1 μS/cm were obtained.
The 0.142 mm diameter tantalum wire was inserted into a capillary tube of 0.152 mm ID and 3 mm OD and extended beyond the end of the capillary tube by 0.7 mm. The end of the capillary tube was then immersed to a depth of 1 cm into the dispersion. Cathodic voltage of 100 volts was applied to the tantalum wire for 50 seconds. The counter electrode was a large diameter cylindrical platinum foil surrounding the capillary tube and wire. A green niobium oxide anode of 0.9 mm in length and 0.5 mm in diameter was deposited on the tantalum wire.
The sharply defined upper end of this anode is to be compared to
Restoring Particle Size Distribution by Agglomerate Sedimentation
A narrow particle size distribution was produced in niobium oxide dispersion, where the dispersion included large agglomerates. This process applies to both improvement of fresh dispersion by reducing dispersion particle size and narrowing particle distribution, and to restoring particle size distribution of used dispersion where particle size distribution has degraded due to particle agglomeration.
The niobium oxide dispersion was composed of the following material:
The equipment used for preparation of the dispersion and its characterization and measurements is as follows:
Preparation of dispersion:
The pH of the above dispersion was 8.37 and its conductivity was 0.7 μS/cm at 25.2° C.
Particle size distribution was analyzed for the above dispersion. A one hour sedimentation process was then performed, removing particle agglomerates from the dispersion. The particle size distribution measurement was then repeated. The results are as shown in the following table, where 10% of the dispersion particles have particle size larger than D [0.1], 50% of the dispersion particles have particle size larger than D [0.5], and 90% of the dispersion particles have particle size larger than D [0.9]:
The sedimentation process successfully removed particle agglomerates, thereby narrowing the dispersion particle size distribution and reducing average particle size.
While some embodiments of the invention have been described by way of illustration, it will be apparent that the invention can be applied in practice with many modifications, variations and adaptations, and with the use of numerous equivalent or alternative solutions that are within the capacity of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 172478 | Dec 2005 | IL | national |
