WO2006027767A1

WO2006027767A1 - Porous anode body for solid electrolyte capacitor and method for manufacturing the same

Info

Publication number: WO2006027767A1
Application number: PCT/IL2004/000865
Authority: WO
Inventors: Assaf Thon; Nissim Cohen
Original assignee: Cerel (Ceramic Technologies) Ltd.
Priority date: 2004-09-09
Filing date: 2004-09-20
Publication date: 2006-03-16
Also published as: IL164017A0

Abstract

The present invention is a porous green anode body, which comprises a multitude of solid particles substantially uniformly dispersed throughout the volume occupied by the anode body and voids that form a network of interconnecting channels interspersed between the particles. The invention also concerns a method for production of the porous green anode body by electrophoretic deposition The invention further encompasses a porous sintered green anode body produced from the porous green anode body, a solid electrolyte capacitor comprising the porous sintered anode body, and methods of producing them.

Description

POROUS ANODE BODY FOR SOLID ELECTROLYTE CAPACITOR AND METHOD FOR MANUFACTURING THE SAME

Field of Invention

The present invention generally relates to solid electrolyte capacitors. More particularly, the present invention relates to solid electrolyte capacitors, featuring complete open pore morphology and made from fine non-

agglomerated electronic grade powders. Also, the present invention relates to a manufacturing process for such electrolytic capacitors.

Background of the Invention

In order to facilitate the reading of the description to follow, a number of terms employed in the art are defined below:

- Primary Particle: The small volume of solid material having the same bulk density (measured in unit mass per unit volume) as said material. "Primary particles" are herein often referred to as "particles" or as "solid particles" herein.

- Agglomerate: A collection of primary particles bonded by surface forces and, when necessary, binder materials.

Capacitor Grade Powder: A collection of pure primary particles and agglomerates having defined chemical and physical properties and commercially suited for the manufacture of solid electrolyte

capacitors.

Capacitor Grade Powder Particles: A collection of pure primary

particles having defined chemical and physical properties and

commercially suited for the manufacture of solid electrolyte

capacitors.

- Green Body: A porous body manufactured from primary particles, granules or agglomerates, packed together and held by surface forces and (optionally) binder additives. Green bodies have low mechanical strength and a mass density appreciably less than the bulk density of

the primary particles.

- Particle Size: An average diameter, usually by volume, of a

collection of particles. Dio denotes the particle diameter below which 10% of the particle volume is to be found, D50 is the median particle diameter by volume. D90 is the particle diameter below which 90% of the particle volume is to be found.

- Sintered Body: A body having appreciable mechanical strength and made by heating a green body in a controlled atmosphere at a

sufficiently high temperature and for long enough time to allow particles to bond. Sintering commonly involves a reduction in pore

volume, accompanied by shrinkage of the green body and an increase

in density. - Volumetric Efficiency of Electrical Capacitance: The electrical

capacitance per unit volume of the sintered anode of a solid

electrolyte capacitor.

- Effective surface area: The total area of the open pore structure created by the outer surface of the particles relative to the total pore

area, which includes the surface of both closed and open pores within

the body.

- The degree of porosity of a green body or a sintered body is defined as the volume fraction occupied by voids in said body relative to the total volume, defined by the external dimensions of said body.

Theoretical density is the density of the material when no voids

are present, i.e. where 100% of the volume of the body is occupied by

material.

Small equipment size, demonstrated by the mobile phone industry, personal computers, cameras, hearing aids and mobile office equipment, drives the market demand for high component packaging density on printed circuit boards. To respond to this challenge, especially for high frequency operation, solid electrolyte chip capacitors are designed to provide high capacitance, low ESR values and low^' height profile cases, enabling the design of more

compact end products for increased convenience and portability. In other

words, the solid electrolyte capacitors market, especially for high frequency

applications, is driven by increasing demands for low manufacturing costs, small size, low height profile, wide range of electrical capacitance, stable

operation and extremely low values of parasitic ESR (equivalent serial

resistance), DCL (direct leakage current) and ESL (equivalent series

inductance).

Solid electrolyte capacitors, available with various forms of encapsulation, feature higher electrical capacitance in package sizes, which are small when

compared to ' other types of capacitors. Due to their high volumetric

efficiency and reliability, miniature 'chip' solid electrolyte capacitors are

especially suitable for surface mounting (SMT) applications and are increasingly being used in current microelectronics and communication

applications.

It is desirable that the geometrical shape of the anode should resemble that of the capacitor package. Since the wire connection and lead frame assembly occupy a large volume fraction of the capacitor package, box shape capacitor

anodes can facilitate efficient stacking of the capacitor anode into the capacitor package, to achieve high capacitance density in the final package.

A solid electrolyte capacitor consists of a high surface area porous sintered pellet, the anode, made from capacitor grade powders, with an embedded conductor wire or foil and a dielectric oxide layer, which is usually formed

by anodizing the surface of the pellet. The anodized porous body is then impregnated with a cathode material, then connected to a cathode lead wire, and the assembly is finally encapsulated in epoxy resin.

Due to the limitations of the state of the art, manufacturing processes for

the smallest solid electrolyte capacitors commercially available have a 0402 case size, which corresponds to a 500 microns profile thickness of the capacitor, including the package. This type of capacitor comprises anodes having a thickness of about 400 microns, including the embedded conducting wire or foil. Large capacitance is achieved with extremely thin

dielectric layers and very high surface areas. The high surface area of the porous pellet structure is the dominant feature that allows the solid

electrolyte capacitor to have better volumetric efficiency than any other type

of capacitor.

In one method known from the art, the powder is first mixed with specific adhesive binders, and pressed to form a 'slug' having a density much lower

than the bulk density of the anode material. The slug is then sintered in vacuum at about 1500⁰C to create a porous rigid pellet with high open

porosity but improved mechanical strength. This semi-sintered pellet acts as the capacitor anode. After sintering, a dielectric oxide layer is formed by

anodizing the entire interconnected open porosity surface of the anode. The thickness of the dielectric layer is controlled by and directly proportional to the formation voltage applied during anodization. Although the dielectric layer follows exactly the contour surface of the open porosity in the sintered anode, defects in the dielectric layer are the main source for the parasitic DCL of the capacitor. Next a cathode is formed on the surface of the dielectric by impregnation of a cathode material, such as manganese oxide

(MnO 2), or a conductive polymer. Good electrical contact for the cathode

requires that the cathode material should conform to the surface microtopology of the oxide layer.

Anodes of solid electrolyte capacitors made of the metals tantalum (Ta), niobium (Nb) and aluminum (Al) dominate today's markets. New types of anodes made from niobium oxide (NbO) have been developed and commercialized in recent years. Solid electrolyte capacitors made of NbO

have excellent electrical properties, high reliability and low manufacturing

costs. The anode morphology largely determines the size and shape of the packaged capacitor and significantly influences its quality and final electrical properties.

The main parameters affecting the ESR value of the final capacitor are the anode open pore structure, the geometrical relationship between the cathode and the anode and the relative electrical conductivities of the anode and

cathode materials. For satisfactory electrical properties, the ratio of the

volume fraction of the anode to that of the cathode is critical, due to the

need to minimize the resistance of both electrodes in the porous structure. The internal surface area of the open pores is the critical factor determining

the capacitance for any given thickness of dielectric having a given dielectric

constant.

The effective capacitive efficiency of any given capacitor powder is indicated by a single lumped parameter, the CV for the powder, which is the product

of the theoretical capacitance and the formation voltage per unit powder

weight. CV has the physical units of Farads multiplied by volts divided by grams. Since the capacitive efficiency increases as the specific surface area of the anode increases, the green capacitor anode is usually made from highly porous agglomerates. An open pore structure is desired, with a pore channel distribution adjusted for maximum impregnation of the cathode material. However, this is usually difficult to achieve. With regard to the anode pore structure, the usual distinction is between closed porosity and open porosity that is the volume fraction of pores that cannot be accessed

from the anode surface and volume fraction that can. The two features that

usually control penetration of an open pore network are the extent to which pores are interconnected and the diameter of the channels.

There are a several factors that inhibit the ability of solid electrolyte capacitors made from particle agglomerates to comply with the demands for miniaturization, reproducible manufacturing and improved electrical

properties: 1. The size of the powder agglomerates, ranging from tens to

hundreds of microns, is too coarse to allow for significant size

reduction and capacitor miniaturization.

2. The broad size distribution of the powder agglomerates

prevents reproducible production of miniature capacitors.

3. The powder agglomerates contain many closed voids and suffer from a non-uniform and entangled pore structure containing pores with a high aspect ratio. (The ratio of the average length of the pore channels to the channel average diameter.) This

results in inefficient impregnation, which in turn leads to

degradation of both the electrical properties and the reliability of the final capacitor and to non-uniformity of capacitor-to- capacitor properties. In particular, this situation leads to high ESR values of the final capacitor, especially at high frequencies. Low and medium CV powder agglomerates are usually processed in an industrial press. However, high CV powders are harder to process, since the fine pore structure can be partially sealed during pressing.

The ESR of a solid electrolyte capacitor comprises two main components. One component is the resistance of connectors and their contacts outside the porous sintered anode but the more significant component is the resistance of the cathode material and interfaces within the porous anode. The dielectric contribution to the ESR is negligible. The morphological structure of the anode elements that are located further beneath the pellet surface, away from the cathode contact, contributes more to the resistance, and therefore to the ESR. This effect is amplified at high frequencies and for

higher aspect ratios of the pore channels in the anode. One way to reduce the ESR is by creating low profile, thinner anodes with wide pores, in order

to shorten the distance between the anode center and the outermost elements of the capacitor.,, and to allow better impregnation and a thicker layer of the cathode material. An alternative is to increase the number of anodes per capacitor and hence to increase the interfacial surface area between the dielectric and the cathode.

In conclusion, a goal of the art is to manufacture low height profile solid

electrolyte chip capacitors made of fine non-agglomerated particles and thus to utilize the open channels and large surface area intra-pore structure to produce capacitors having high capacitance efficiency, low ESR and improved DCL. However, the currently employed methods of manufacturing anodes for solid electrolyte capacitors are not suitable for processing finer

particles effectively. Additional references showing the present state of the art

are: A. Michaelis, C. Schnitter and U. Merker, New Tantalum Metal

Powder Quality for Solid Electrolyte Capacitors, 16^th European

Passive Components Symposium: CARTS-EUROPE 2002.

• I. W. Clelland and R. A. Price, Recent Advances in Capacitor Technology with Application to High Frequency Power Electronics and

Voltage Conversion, Technical Bulletin 3.99A, Illinois Tool Works

Inc., 1999.

• C. Schnitter, A. Michaelis and U. Merker, New Niobium Based

Materials for Solid Electrolyte Capacitors, CARTS 2003: 23^rd

Capacitor and Resistor Technology Symposium

• J. Gill, Basic Tantalum Capacitor Technology, AVX Technical

Paper, www.avx.com

• T. Zednicek, B. Vrana, W.A. Millman, J. Gill, and C. Reynolds,

Tantalum and Niobium Technology Roadmap, AVX Technical Paper,

www.avx.com

• R. W. Franklin, Equivalent Series Resistance of Tantalum

Capacitors, AVX Technical Paper, www.avx.com

• K. Anderson H. Naito and T.B. Tripp, Tantalum Powder

Developments and their Influence on Tantalum Capacitors, CARTS

EUROPE 92 By way of examples only, common aspects of some powders used for the

production of solid electrolyte capacitors are given below. The processing of capacitor grade powders commonly includes agglomeration by spray drying during the production. This is to improve the flow properties of the powders and to allow automatic and reliable green anode production. However, a side effect of powder agglomeration is deterioration of the mechanical and electrical properties of the powders, and especially a reduction in the CV of the powders.

Capacitor Grade Tantalum and Niobium Powders

US Patent No. 5,986,877, dated November 16, 1999 and US Patent No. 5,954,856, dated September 21, 1999, both describe methods of making tantalum metal powder with controlled size distribution and sintered tantalum capacitor anode produced from said tantalum powder, by compressing agglomerates of said tantalum powder. The primary particles of the powder from which the agglomerates are made, have an average particle size ranging between 0.1 micron and about 5 micron. In contradiction, the agglomerated tantalum powder has a size distribution

with an agglomerate size of few tens of microns to a few hundreds of microns.

STA-80 KA (Product number: 01014629) is a 80 KCV capacitor grade tantalum powder made by H. C. Starck. According to the manufacturer's data, the powder contains at least 99% mass fraction of Ta. The reported average size of the primary particles ranges from 1.5 to 2.3 micron with 79% of the particles smaller than 38 microns, 17.9% between 38 microns and 63 microns, 1.7% between 63 microns and 106 microns, 0.8% between 106

microns and 150 microns and 0.1% between 150 microns and 300 microns. This powder has a specific surface area of 1.7 square meter per gram (m²/g).

Capacitor grade tantalum powders, made by Cabot Corporation (www.cabot- corp.com), are available with KCV values ranging from 10 to 70. The powders are agglomerated with a primary mean particle size near 10 micron and an agglomerate mean size of around 400 to 600 microns.

US Patent No. 6,765,786, dated July 20, 2004, discloses a niobium powder

for producing niobium capacitors. The powder has a size distribution from 10 microns to 200 microns.

Capacitor Grade Niobium Monooxide

US Patent No. 6,527,937 B2, dated March 4, 2003 describes a method for producing capacitor grade niobium monoxide powder. Examples of the type of powder that can be used include flaked, angular, nodular and a mixture of these variations, all having a particle size ranging from a few tens of

microns to few hundreds of microns. US Patent Application Publication No. 2003/0170169 Al, published September 11, 2003, describes a niobium monoxide powder for niobium- oxide capacitors, having a non-uniform wide particle size distribution, ranging from 10 microns to 1000 microns and non-uniform pore diameters. As a result, the niobium monoxide sintered body made from the above powder exhibits a non-uniform pore diameter ranging from 0.01 micron to 500 microns.

Typical properties of 80 KCV NbO powder, manufactured by Herman Starck, as published by the manufacturer shows a wide particle size distribution. A maximum of 10% of all diameters are below 38 microns, 5 to 15% are between 38 microns and 63 microns, 10 to 20% are between 63 microns and 100 microns, 25 to 35% are between 100 microns and 150 microns, 25 to 35% are between 150 microns and 200 microns, 5 to 15% are between 200 microns and 300 microns. The powder has a specific surface area of 1.05 to 1.25 m²/g.

It is, therefore, an object of the present invention to make possible a solid electrolyte capacitor with a small package size, said capacitor having high volumetric electrical capacitance, improved electrical properties and in particular low ESR and low DCL. It is another object of the present invention to provide a green body having defined geometry and a controlled degree of porosity.

It is still another object of the present invention to provide a method for controlling the total interconnected surface area of said green body by controlling the amount and interconnectivity of porosity said green body.

It is still another object of the present invention to use said green body to create a sintered body having a defined geometry and controlled degree of

porosity.

Still another object of the current invention is to provide an anode for a solid electrolyte capacitor using said sintered body.

It is still another object of the present invention to provide an anode for a solid electrolyte capacitor, said anode to be made from an oxide film-forming electrical conductor, where the morphology of said anode has an open pore structure, in which said pores have a uniform structure with a narrow size distribution of pore diameters. The anode morphology has a controlled porosity to achieve optimal cathode material impregnation.

It is still another object of the present invention to provide a miniature, low height-profile anode for a solid electrolyte capacitor. The anode has certain specific dimensions of length, width and thickness. In preferred embodiments, the width is less than the length, the anode thickness is less than 100 microns, and the ratio of said length to the thickness is greater than 10.

It is a further object of the present invention to provide a miniature, low height profile solid electrolyte capacitor using said capacitor anode.

It is a still further object of the present invention to provide a non- agglomerated, non-granulated capacitor grade powder, said powder to be made from fine particles of a dielectric oxide film-forming conductor material. The conductor material includes but is not limited to: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide. The powder particles of said conducting materials are characterized by a mean particle diameter not larger than 5 microns. The capacitor grade powder is selected from a group of capacitor grade powders having any KCV values.

It is a still further object of the present invention to provide a method for classifying and sorting said commercial capacitor grade powders, combining primary particles, granules and agglomerates, in order to utilize a segment of said powders appropriate for optimizing the manufacture of solid electrolyte capacitor anodes. It is a still further object of the present invention to achieve high volume efficiency for the electrical capacitance and a low ESR of said solid electrolyte capacitor, said high volume efficiency and low ESR created by said high effective surface area of the open pore structure.

It is a still further object of the present invention to make possible a low-cost solid-electrolyte capacitor manufactured from said anode using any of said capacitor grade powders.

It is a still further object of the present invention to provide a solid electrolyte capacitor cathode made from any suitable cathode material, including but not limited to manganese oxide and a conductive polymer.

It is a still further object of the current invention to demonstrate a manufacturing process for said solid electrolyte capacitor anode using said capacitor grade powders.

Other objects and advantages of the invention will become apparent as the description proceeds. Summary of the Invention

In a first aspect the present invention is directed towards providing a porous green anode body. The green anode body of the invention is characterized in that it comprises a multitude of solid particles substantially uniformly dispersed throughout the volume occupied by the anode body. The voids interspersed between the particles form a network of interconnecting

channels.

In some embodiments of the invention, the volume occupied by the anode body is substantially in the shape of a rectangular box in which the thickness is less than 120 microns. The thickness is at least ten times smaller than the length of the box and the length is longer than the width. In other embodiments the volume occupied by the anode body is substantially in the shape of a cylinder having the length of its radius less than 120 microns and at least ten times smaller than the length of its

height.

In the porous green anode body of the invention, the fraction of the volume of the anode body occupied by voids is approximately 50% to 80% and the particles have a density in the range of 40% to 100% of the theoretical density of the material of which they are comprised. In preferred embodiments of the porous green anode body of the invention, the solid particles are particles of a dielectric oxide film-forming electrical conducting material preferably selected from the group of materials

comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide; wherein, said electrical conducting material may have any CV value. The porous body can be formed on a substrate made from a dielectric oxide film-forming electrical conducting material preferably selected from the group comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide. The substrate can have the shape of a wire or a foil.

In preferred, embodiments of the porous green anode body of the invention, the porous body is produced by electrophoretic deposition; the average diameter of the particles is less than 5 microns; the average diameter of the channels is larger than at least one half of the average diameter of the particles; the channels intersect with at least one adjacent channel; and the particles contact at least three adjacent particles.

In a second aspect the present invention is directed towards providing a

porous sintered anode body comprising a multitude of solid particles substantially uniformly dispersed throughout the volume occupied by the anode body and voids interspersed between the particles. In some preferred embodiments of the porous sintered anode body of the invention, the volume occupied by the anode body is substantially in the shape of a rectangular box. The thickness of the box is less, than 100 microns and is at least ten times smaller than its length and its length is longer than its width. In other preferred embodiments, the volume occupied by the

anode body is substantially in the shape of a cylinder. The cylinder having

the length of its radius less than 100 microns and length of radius at least ten times smaller than the length of its height.

Preferably for the porous sintered anode body of the invention, the fraction of the volume of the anode occupied by the voids is approximately 50% to 80% and the particles have a density in the range of 40% to 100% of the theoretical density of the material of which they are comprised.

In preferred embodiments of the porous sintered anode body of the invention, the particles are particles of a dielectric oxide film-forming electrical conducting material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide. The electrical conducting material may have any CV value. Preferably the average diameter of the particles is

less than 5 microns; the average diameter of the channels is larger than at

least one half of the average diameter of the particles; the channels intersect with at least one adjacent channel; and the particles contact at least three adjacent particles.

In preferred embodiments of the porous sintered anode body of the invention, the porous body is created on a substrate, prior to sintering. The substrate preferably consists of a dielectric oxide film-forming electrical conducting material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide. The substrate can have the shape of a foil

or a wire.

Preferably the porous sintered anode' body of the invention comprises a porous green anode body of the invention, which has been sintered in vacuum at high temperature.

In a third aspect the present invention is directed towards providing a solid

electrolyte capacitor comprising a porous sintered anode body of the invention. In preferred embodiments of the invention the volume occupied by the capacitor is substantially in the shape of a rectangular box in which the thickness is less than 500 microns and is at least ten times smaller than its length and its length is longer than its width. In other embodiments the volume occupied by the capacitor is substantially in the shape of a cylinder. The length of the radius of the cylinder is less than 50 microns and is at least ten times smaller than the length of its height.

In another aspect the present invention is directed towards providing a method for controlling the electrical properties of the solid electrolyte capacitor of the invention. The method comprises the steps of:

Controlling the electrical capacitance of the capacitor by controlling the fraction of volume of the anode body occupied by the voids and the distribution of the voids in the anode body; and

Controlling the amount of electrical serial resistance of the capacitor by controlling the fraction of the volume of the anode body occupied by the voids and the distribution of the voids in the anode body.

In still another aspect the present invention is directed towards providing a method for production of the porous green anode body of the invention by electrophoretic deposition. The method comprises the steps of:

- forming a suspension comprised of polar liquid solvent, capacitor grade powder particles and chemical additives;

applying a voltage across the suspension between an electrode in contact with the suspension and a substrate, such that at least a portion of the particles migrates toward and is deposited on the substrate, forming a particulate deposit layer; and

- washing the anode body to reduce the amount of carbon present on the surfaces of the particulate deposit layer.

In the preferred method for production of the porous green anode body of the invention wherein the values of the following parameters of the electrophoretic deposition process are maintained within the following limits throughout the deposition of the porous green anode body: the zeta potential is higher than — 3OmV and lower than-30mV;

- the electrical conductivity is between 0.5micro-Siemens/cm and 4.0 micro-Siemens/cm; and the pH is between 9 and 11.

In the method for production of the porous green anode body of the invention the solvent can be selected from a group of polar solvents, can be a solution of at least two polar solvents, or can be a solution of at least one polar solvent and at least one non-polar solvent. In preferred embodiments, the solvent is selected from the group of materials comprising: water,

methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, pentyl alcohol, isopropyl alcohol, nitromethane and acetonitrile. Preferably the solvent has

a dielectric constant equal to or greater than 6. In some embodiments of the method for production of the porous green anode body of the invention the concentration of the capacitor grade powder in the liquid solvent is between 1 gram per liter and 1000 grams per liter.

In preferred embodiments of the method of the invention the concentration of the capacitor grade powder in the liquid solvent is between 10 grams per liter and 200 grams per liter. In other embodiments the concentration of the capacitor grade powder in the liquid solvent is between 50 grams per liter and 100 grams per liter.

In the method for production of the porous green anode body of the invention the chemical additives are selected from amongst the following classes of chemical substances: charging agents, steric additives, and electro-steric additives. In different embodiments of the method of the invention, the chemical additives can comprise at least charging agent, at least one steric additive, at least one electro-steric additive, at least one

charging agent, or at least one electrosteric additive. The electrosteric additives are preferably selected from the group comprising: polyethylene imine, quaternary ammonium salts, and phosphate ester compounds. The charging agents are preferably selected from the group comprising: nickel(II) chloride hexahydrate, nickel acetylacetonate, cobalt (II) acetylacetonate, and aluminum chloride hexahydrate. The steric additives are preferably selected from the group comprising: dodecyl sodium sulfate and poly ethylene glycol.

In some embodiments of the method for production of the porous green anode body of the invention the concentration of the chemical additives in the suspension is between 0.1 micro-liters and 1 milliliter of the chemical additive in 100 milliliters of the solvent. In other embodiments the concentration of the chemical additives in the suspension is between 5 micro-liters and 100 micro-liters of the chemical additive in 100 milliliters of the solvent.

In the preferred embodiments of the method for production of the porous green anode body of the invention the electric field in the suspension is held constant and the electrical current through the suspension is held constant during the EPD process. Preferably the electric field across the suspension is between 1 volt per centimeter and 300 volts per centimeter.

In yet another aspect the present invention is directed towards providing a

capacitor grade powder for forming a capacitor anode. The capacitor grade powder consisting of a plurality of non-aggregated particles, wherein the powder particle size is characterized by Dio ranging from 0.4 micron to 2 microns, by D50 ranging from 0.6 micron to 4 microns, and by D90 ranging from 4 microns to 8 microns. In a preferred embodiment of the invention, the capacitor grade powder comprises particles of a dielectric oxide film- forming electrical conductive material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium mono-oxide.

Brief Description of the Drawings

- Fig. 1 is SEM micrograph showing a cross-section of a sintered tantalum anode made by the method of the prior art;

- Fig. 2 to Fig. 4 are micrographs showing cross-sections of sintered tantalum and niobium-oxide anodes made by the process of the invention; and

- Fig. 5 is an optical micrograph showing a cross-section of a green tantalum anode made by the process of the invention.

Detailed Description of the Preferred Embodiments

Fig. 1 is a SEM micrograph showing a cross-section of a sintered tantalum anode made by the prior art technique of pressing tantalum agglomerate around a tantalum wire followed by sintering. The width of the anode pellet shown in the figure is about 1.5 mm. At the center of the pellet is located Ta

wire (130). The pores 120 are seen as black spots distributed unevenly and

intersected by the plane of the section. It should be noted that the pore structure of this anode is non-homogeneous both with respect to pore size and pore spacing. Further relatively large pores are present having lengths of 200 to 250 microns.

Reference is now made to Fig. 2, which is a photographic taken with the aid of an optical microscope showing a cross-section of a green NbO anode made

by the method of the invention. To create the anode shown, NbO particles are deposited by an EPD process on the surface of tantalum wire (230) having a diameter of 250 microns. The resulting green body was sintered in vacuum at 1460 degrees Celsius. The powder used is capacitor grade NbO powder described in Example 1 hereinbelow. The particles of NbO (210) are seen as black spots on the section and the pores (220) are seen between the particles. It should be noted that the pore structure revealed in the section of this anode is homogeneous and the pores are distributed uniformly throughout the anode.

In Fig. 3 is shown a SEM micrograph showing a cross-section of a sintered tantalum anode made by the method of the invention. From the figure it can be seen that a significant portion of the particles (320) are smaller than 2.5 microns and that the anode has an open pore structure featuring large channels between the particles. The powder used to produce the anode shown in Fig. 3 is the capacitor grade tantalum powder described in Example 3 hereinbelow. It should be noted that the pore structure of this anode is also homogeneous and the pores (310) and particles (320) are uniformly distributed throughout the volume of the anode.

Fig. 4 is a region of the anode of Fig. 3 photographed at a higher magnification. The pores (410) are clearly seen interspersed with the tantalum particles (420).

Reference is now made to Fig. 5, which is an optical micrograph showing a

cross-section of a green tantalum anode made according to the method of the invention by depositing Ta particles on the surface of tantalum wire (530) using the EPD process. The powder used to produce the green body shown in Fig. 5 is capacitor grade tantalum powder described in Example 3 hereinbelow. The light spots in the figure are cross-sections of the pores (520) and the dark spots are the tantalum particles (510). It can be seen that the pores and the particles are uniformly interspersed in the green body anode.

A method that uses an EPD process to manufacture metal electrodes that is similar to the method used in the present invention for manufacturing a solid electrolyte capacitor is described in U.S. Patent Application No. 10/480,875 filed on May 18, 2004 by the same applicant, the description of which, including reference cited therein, is incorporated herein by reference in its entirety. In accordance with the present invention, the production of a solid electrolyte capacitor includes the steps of:

1. Selection of capacitor grade niobium mono-exide powder and pre-treatment of said powder to prepare said powder for production of a solid electrolyte capacitor anode, said anode having the desired properties.

2. Preparing a stable suspension of said niobium mono-oxide powder in a polar solvent, said suspension being suitable for processing in a deposition process known in the art as Electrophoretic Deposition and usually abbreviated as EPD.

3. Deposition of a solid electrolyte capacitor green body anode

using EPD.

4. Making a sintered capacitor anode using said green capacitor anode.

5. Making a solid electrolyte capacitor cathode, external electrical connections, including wiring and the capacitor package, by conventional means known in the art.

The capacitor grade powder used in the present invention can be made from

any anodic dielectric oxide film-forming electrical conductive material.

According to preferred embodiments of the present invention, the powder is made from, but is not limited to, particles of one of the following materials: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or

niobium monoxide. According to another embodiment of the present

invention, the capacitor grade powder used in this invention is a made of fine, non-agglomerated particles of said materials.

In some embodiments of the present invention, the powder particle size is

characterized by Dio ranging from 0.4 micron to 2 microns, D50 ranging from

0.6 micron to 4 microns and D90 ranging from 4 microns to 8 microns.

According to other embodiments of the present invention, the powder particle size is characterized by Di₀ ranging from 0.5 micron to 0.8 micron,

D50 ranging from 0.8 micron to 2 microns and D90 ranging from 1.5 microns to 4 microns.

In preferred embodiments of the present invention, the capacitor grade

powder used in this invention has a high chemical purity and contains at

least 99% by mass of a dielectric oxide film-forming electrical conducting

material, preferably selected from, but not limited to the group comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or

niobium monoxide. In other embodiments of the present invention, the

powder contains at least 99.5% by mass of said constituent. The capacitor

grade powder to be used in this invention is selected from a group of

capacitor grade powders that may have any CV value. A stable suspension for EPD is prepared by mixing a certain amount of polar solvent with said capacitor grade powder and certain chemical

additives. The additives act to inhibit agglomeration of the capacitor grade powder particles held in said suspension. For the sake of this invention, a suspension is said to be stable when the rate of EPD significantly exceeds the rate of particle sedimentation under gravity, so that the particle size distribution in the suspension remains effectively unchanged throughout the time of deposition. A stable suspension requires that the zeta potential, which is a measurement of the effective amount of electrical charge on the

particles in the suspension, be sufficiently high (about ±30mV) and that the pH and electrical conductivity remain constant during the time of deposition. In preferred embodiments of the invention the electrical conductivity is between 0.5micro-Siemens/cm and 4.0 micro-Siemens/cm and the pH is between 9 and 11.

The solvent used in this invention may be an unmixed polar solvent,

selected from a group of polar solvents, or a mixture of two or more polar solvents, or a mixture of one polar solvent with one or more non-polar solvents. According to preferred embodiments of the present invention, the solvent used in the present invention should have a dielectric constant having a value equal to or larger than 6 and a chemical purity of 99% or more. The solid loading in suspensions in the present invention, defined as the mass of capacitor grade powder per unit volume of solvent, ranges from 1 gram per liter to 1000 grams per liter. In some preferred embodiments the solid loading ranges from 10 grains per liter to 200 grams per liter and in

more preferred embodiments from 50 grams per liter to 100 grams per liter.

According to a still further embodiment of the present invention, the solvent in the present invention is selected from the group of materials including, but not limited to water, methyl alcohol, ethyl alcohol, n-propyl alcohol, butyl alcohol, pentyl alcohol, isopropyl alcohol, nitromethane and

acetonytrile.

Chemical additives used in the present invention can be selected from three classes of chemical substances. The first of said additives, for the sake of simplicity termed in this invention 'charging agents', have the capacity to inhibit aggregation of said capacitor grade powder particles by creating electrostatic repulsion between said particles in suspension. The second class of said additives, for the sake of simplicity termed in this invention

'steric additives³, has the capacity to inhibit aggregation of said capacitor grade powder particles by creating steric hindrance between said powder

particles. The third class of said additives, for the sake of simplicity termed

in this invention 'electro-steric additives', has the capacity to inhibit aggregation of said capacitor grade powder particles by creating both electrostatic repulsion and steric hindrance between said powder particles.

In different embodiment of the present invention, electrosteric additives, a mixture of steric additives and charging agents, or a mixture of steric additives and electrosteric additives are selected to stabilize the suspension.

Chemical substances in said class of electrosteric additives include, but are not limited to, polyethylene imine, quaternary ammonium salts and phosphate ester compounds. Also, chemical substances in said class of charging agents include, but are not limited to, nickel(II) chloride hexahydrate, nickel acetylacetonate, cobalt(II) acetylacetonate and aluminum chloride hexahydrate. Moreover, chemical substances in said class of steric additives include, but are not limited to, dodecyl sodium sulfate and polyethylene glycol.

According to some embodiments of the present invention, the concentration of said substances, selected from the classes of charging agents, steric additives and electrosteric additives is between 0.1 micro-liters of additive in 100 milliliters of solvent, to 1 milliliter of additive in 100 milliliter of

solvent. In other embodiments, the concentration ranges between 5

microliter of additive in 100 milliliter of solvent, and 500 micro-liters of additive in 100 milliliter of solvent According to the present invention, EPD of the green anode body can be in a

constant electrical field. According to a preferred embodiment of the present invention, the electrical field for EPD of the green anode body ranges from 1 volt per centimeter to 300 volts per centimeter.

According to the present invention, EPD of the green anode body can be at a constant electric current.

According to the present invention, sintering of the green anode body is performed in a vacuum furnace or a furnace having a controlled inert gas atmosphere. In preferred embodiments of the present invention, the temperature in the sintering furnace ranges between 600 degrees Celsius and 2000 degrees Celsius.

The following examples are provided merely to illustrate the invention and are not intended to limit the scope of the invention in any manner.

Example 1: Production of NbO anode and dielectric oxide layer Powder selection

The powder used in this example is 80KCV capacitor grade NbO powder supplied by HC Starck (Germany). The particle size distribution as reported by the manufacturer and before ultrasonic treatment was: Dio (micron) = 4.45, Deo (micron) = 15.50, D90 (micron) = 34.75. After ultrasonic treatment to break down soft powder agglomerates the particle size distribution, as reported by the manufacturer, was: D10 (micron) = 3.34, Dso (micron) = 14.29, D90 (micron) = 33.81. Screen analysis reported by the manufacturer

shows that the size of all powder primary particles and agglomerates falls below 38 micron. Chemical analysis, performed by the manufacturer, shows that the powder used in this example has the following chemical composition, excluding stoichiometric NbO:

Element Abundance Units

C 14 ppm

O 15.03 %

Fe 13 ppm

Cr <3 ppm

Ni 2 ppm

Al 3 ppm

Mg 150 ppm

Mn 1 ppm

Na 0.6 ppm

K <0.5 ppm

Si 15 ppm

Ti <2 ppm

B <3 ppm

Mo 32 ppm

Ta <15 ppm

Cl <3 ppm

F <2 ppm

Ca <3 ppm

Cu <3 ppm

Preparation of NbO suspension

A 50 grams per liter NbO suspension was prepared by mixing 5 grams of NbO powder in 100 milliliter of 2-propanol (Manufacturer: Bio-Lab, Jerusalem, Israel, grade: PEPTIDE synthesis, Cat number: 162633). The suspension was sonicated (Vibracell, output power: 750 Watt, frequency: 20 kHz, 19 mm solid tip probe) for 5 minutes at 80% amplitude, with a 2 seconds on / off pulse rating. The suspension was cooled during sonication and the suspension temperature did not exceed 35 degrees Celsius. 150 microliter of a 17% by weight dilute aqueous solution of polyethylenimine (Sigma-Aldrich, St. Louis, Missouri, USA, Cat. Number 48,259-5) was prepared and added to the suspension. Then, the suspension was again sonicated for 1 minute at 80% amplitude, using a 2 seconds on / off pulse rate. The suspension was cooled during sonication and the suspension temperature did not exceed 35 degrees Celsius. After that, the suspension was magnetically stirred for 30 minutes.

The pH, measured at 21 degrees Celsius after 30 minutes of magnetic stirring, was 9.5 and the electrical conductivity was 0.5 micro-Siemens per centimeter.

Preparation of NbO green anode body by EPD

A 0.2 mm diameter tantalum wire with a length of 2 cm acted as anode. When an external voltage of 80 volts was applied between the anode and a cathode counter^electrode in the presence of the suspension, positively charged NbO particles were deposited on the surface of the tantalum wire.

The deposition time was 40 seconds. The green anode had a length of 1.5

mm and outer diameter of 550 micron. The green anode, not including the wire electrode weighed 10 milligrams and had a green density of about 20%, corresponding to about 80% porosity. The green anode body was then

washed in a water— ethanol mixture to remove traces of carbon left on the surface after EPD. Then_r the sample was dried in open air.

Sintering of the EPD green anode

Sintering was done in a vacuum furnace at a pressure of 5(10"⁵) millibar and a temperature of 1400 degrees Celsius. The sintering cycle included heating from room temperature to 150 degrees Celsius, at a rate of 50 degrees Celsius per minute and then continued heating at a rate of 100 degrees Celsius per minute to the final sintering temperature. The dwell time at the final sintering temperature was 20 minutes, after which the furnace is switched off and filled with helium gas (600 millibar) to speed the cooling rate. As the temperature in the furnace reaches room temperature, the furnace is pumped again to 5(10"⁵) millibar and then filled with 50 millibar of air for an additional -20 minutes. The pressure is again raised to 100

millibar of air for an additional 20 minutes. Finally air is admitted to reach atmospheric pressure and the furnace is opened.

Dielectric formation and measurement thereof on a sintered

Anϋdtectric layer, Nb2θs, was formed by anodization, in this example, using a solution of 85% Orthophosphoric Acid (Frutarom, Israel) in chemically pure (CP) water. The solution had a conductivity of 4.3 mS/cm. The anodic oxidation process for various samples took between 30 minutes and 300 minutes at constant electrical current, which varied between 0.1 mA/mg and

1.5mA/mg. A standard 'wet-check' procedure was performed to validate and

demonstrate the ability to use EPD to manufacture novel solid electrolyte

capacitors, the subject matter of this invention. An electrolytic bath was

filled with 18% H2SO4 acid (manufactured by Frutarom, Israel) with the

sintered capacitor anode body as anode and a cylindrical platinum foil as

cathode.

The following table shows initial results of electrical capacitance and direct

current leakage (DCL) measurements.

Sample Capacitance (micro Farad) DCL (micro Ampere)

1 1 0.02

2 0.6 0.03

3 0.8 0.016

4 0.9 0.016

Example 2: Production of sintered tantalum anode using large

particles

Powder selection

The powder used in this example is IOOKCV capacitor grade tantalum

powder (Lot number: Ha 10302 26Bl) manufactured by HC Starck

(Germany). The particle size distribution before ultrasonic treatment as

reported by the manufacturer was: Di₀ (micron) = 16.25, D50 (micron) =

32.45, D90 (micron) = 173.93. The surface area of the particles measured by BET is 1.63 m²/gram. After ultrasonic treatment, used to break down soft powder agglomerates, the particle size distribution, as reported by the manufacturer, was: Dio (micron) = 11.83, D50 (micron) = 29.05, D90 (micron) = 59.75. Screen analysis reported by the manufacturer shows that the size of all powder primary particles and agglomerates falls above 38 micron and

below 300 micron. Analysis of chemical elements, performed by the

manufacturer, shows that the powder used in this example contains the

following relative amounts of chemical elements in addition to tantalum:

Element Abundan ice Units

C 28 ppm

O 4700 ppm

Fe 5 ppm

Cr <2 ppm

Ni <3 ppm

Al <1 ppm

Mg 250 ppm

Na 1 ppm

K <0.5 ppm

Si <3 ppm

Nb 7 ppm

Preparation of tantalum suspension

A 50 gram per liter suspension was prepared by mixing 5 grams of tantalum

powder in 100 milliliters of 99.9% absolute ethanol (Manufacturer: Bio-Lab, Jerusalem, Israel). The suspension was sonicated (Vibracell, output power:

750 Watt, frequency: 20 kHz, 19 mm solid tip probe) for 5 minutes at 80%

amplitude, with a 2 seconds on / off pulse rating. The suspension was cooled

dxiring sonication and the suspension temperature did not exceed 35 degrees

Celsius. 150 micro-liters of a 17% by weight aqueous solution of

polyethylenimine (PEI, Sigma-Aldrich, St. Louis, Missouri USA (Cat. Number 48,259-5) was prepared and added to the suspension. After that, the suspension was again sonicated for 5 minute at 80% amplitude, using a 2 seconds on / off pulse rate. The suspension was cooled during sonication and the suspension temperature did not exceed 35 degrees Celsius. Then the suspension was magnetically stirred for 30 minutes.

The pH, measured at 21 degrees Celsius after 30 minutes of magnetic stirring, was 9.0 and the electrical conductivity w-as 1.0 micro-Siemens per centimeter.

Preparation of tantalum green anode body by EPD and Sintering

EPD and sintering were performed using the same parameters as in example (1) above, except that the final sintering temperature was: 1300 degrees Celsius.

Example 3: Production of sintered tantalum anode using small particles

Powder selection

The powder in this example is IOOKCV capacitor grade milled tantalum powder (Lot number: Ha 10302 26Bl) manufactured by HC Starck (Germany). The particle size distribution as reported by the manufacturer

before sonication was: Dio (micron) = 1.81, D50 (micron) = 3.02, D90 (micron)

= 4.73. The surface area of the particles measured by BET is 1.69 m²/gτam. After sonication to break down soft powder agglomerates, the particle size distribution, as reported by the manufacturer, was: Dio (micron) = 1.2, Ds₀ (micron) = 2.5, D90 (micron) = 4.53. Chemical analysis, performed by the manufacturer, shows that the powder used in this example contains the

following relative composition of chemical elements in addition to tantalum:

Element Abundance Units

C 31 ppm

O 5902 ppm

Fe 5 ppm

Cr <2 ppm

Ni <3 ppm

Al 2 ppm

Mg 210 ppm

Na 2 ppm

K <0.5 ppm

Preparation of tantalum suspension, green anode and sintered anode

The preparation of the tantalum suspension, the green anode and the sintered anode followed the same procedure as in example 2 of this invention.

Example 4: Change in particle size distribution caused by successive electrophoretic depositions

In this example ten successive EPD runs followed the same procedure as in example 2 of this invention. The particle size distribution performed on samples of the suspension and

determined by 'Mastersizer 2000' particle size distribution analyzer

(Malvern, UK), both prior to the first EPD run and after the 10^th EPD run

showed the following changes in distribution:

Parameter (microns) DlO D50 D90

Initial 0.69 2.08 4.4

After 10^th run 0.54 1.9 4.0

Example 5: Particle size distribution of sintered tantalum anode

made by EPD

In this example the particle size distribution of three sintered tantalum

anodes made by EPD were examined. The deposition followed the same

procedure as in example 2 of this invention.

The particle size distribution was carried out using a standard image

analysis tool on SEM cross sections of sintered anodes.

Results are shown as a percentage of each fraction of the total number of

particles counted in the cross section.

Particle diameter (microns) anode 1 anode 2 anode 3

<0.5 63.2 60.8 59.0

0.5 - 1 25.0 25.6 27.0

1.0 - 2.0 9.4 11.6 11.8

2.0 - 3.0 1.8 1.6 1.7

3.0 - 4.0 0.4 0.4 0.3

4.0 - 5.0 0.2 0.0 0.2

>5 0.0 0.0 0.0

Comparing the results shown in the table above, to the particle size

distribution before ultrasonic treatment recorded in the description of

Example 2 hereinabove it is seen that the particle sizes in the anode of Example 5 are all smaller than Dio of Example 2. Thus it can be seen how

the method of the invention, which comprises the use of EDP, guarantees an essentially uniform distribution of small particles in the anode.

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.

Claims

1. A porous green anode body characterized in that it comprises a

multitude of solid particles substantially uniformly dispersed throughout the volume occupied by said anode body, whereby the voids interspersed between said particles form a network of interconnecting channels.

2. A porous green anode body according to claim 1, wherein the volume occupied by said anode body is substantially in the shape of a rectangular box in which the thickness is less than 120 microns and is at least ten times smaller than the length and the length is longer than the width.

3. A porous green anode body according to claim 1, wherein the volume occupied by said anode body is substantially in the shape of a

cylinder; said cylinder having the length of its radius at least ten times smaller than the length of its height and the length of said radius is less than 120 microns.

4. A porous green anode body according to claim 1, wherein the fraction of the volume of said anode body occupied by voids is approximately 50% to 80%.

5. A porous green anode body according to claim 1, wherein the particles have a density in the range of 40% to 100% of the theoretical density

of the material of which said particles are comprised.

6. A porous green anode body according to claim 1, wherein the solid particles are particles of a dielectric oxide film-forming electrical conducting material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide; wherein, said electrical conducting material may have any CV value.

7. A porous green anode body according to claim 1, wherein said porous body is formed on a substrate; wherein said substrate is made from a dielectric oxide film-forming electrical conducting material preferably

selected from the group comprising: tantalum, aluminum,

magnesium, titanium, niobium, zinc, zirconium or niobium monoxide.

8. A porous green anode body according to claim 7, wherein the substrate has the shape of a wire.

9. A porous green anode body according to claim 7, wherein the substrate has the shape of a foil.

10. A porous green anode body according to claim 1, wherein said porous body is produced by electrophoretic deposition.

11. A porous green anode body according to claim 1, wherein the average diameter of the particles is less than 5 microns.

12. A porous green anode body according to claim 1, wherein the average

diameter of the channels is larger than at least one half of the average diameter of the particles.

13. A porous green anode body according to claim 1, wherein the channels intersect with at least one adjacent channel.

14. A porous green anode body according to claim 1, wherein the

particles contact at least three adjacent particles.

15. A porous sintered anode body comprising a multitude of solid

particles substantially uniformly dispersed throughout the volume occupied by said anode body and voids interspersed between said particles.

16. A porous sintered anode body according to claim 15, wherein the volume occupied by said anode body is substantially in the shape of a rectangular box in which the thickness is less than 100 microns and is at least ten times smaller than the length and the length is longer than the width.

17. A porous sintered anode body according to claim 15, wherein the volume occupied by said anode body is substantially in a shape of a cylinder; said cylinder having the length of its radius at least ten times smaller than the length of its height and the length of said radius is less than 100 microns.

18. A porous sintered anode body according to claim 15, wherein the fraction of the volume of said anode occupied by the voids is approximately 50% to 80%.

19. A porous sintered anode body according to claim 15, wherein the particles have a density in the range of 40% to 100% of the theoretical density of the material of which said particles are comprised.

20. A porous sintered anode body according to claim 15, wherein the particles are particles of a dielectric oxide film-forming electrical conducting material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide; wherein, said electrical conducting material may have any CV value.

21. A porous sintered anode body according to claim 15, wherein the average diameter of the particles is less than 5 microns.

22. A porous sintered anode body according to claim 15, wherein the average diameter of the channels is larger than at least one half of the average diameter of the particles.

23. A porous sintered anode body according to claim 15, wherein the

channels intersect with at least one adjacent channel.

24. A porous sintered anode body according to claim 15, wherein the particles contact at least three adjacent particles.

25. A porous sintered anode body according to claim 15, wherein said porous body is created on a substrate, prior to sintering; wherein said substrate consists of a dielectric oxide film-forming electrical

conducting material preferably selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium monoxide.

26. A porous sintered anode body according to claim 25, wherein the substrate has the shape of a foil.

27. A porous sintered anode body according to claim 25, wherein the substrate has the shape of a wire.

28. A porous sintered anode body according to claim 15, wherein said porous sintered anode body is comprised of a porous green anode body according to claim 1, which has been sintered in vacuum at high temperature.

29. A solid electrolyte capacitor comprising a porous sintered anode body according to claim 15.

30. A solid electrolyte capacitor according to claim 29, wherein the volume occupied by said capacitor is substantially in the shape of a rectangular box in which the thickness is less than 500 microns and is at least ten times smaller than the length and the length is longer than the width.

31. A solid electrolyte capacitor according to claim 29, wherein the volume occupied by said capacitor is substantially in the shape of a cylinder; said cylinder having the length of its radius at least ten times smaller than the length of its height and the length of said radius is less than 50 microns.

32. A method for controlling the electrical properties of a solid electrolyte capacitor according to claim 29, said method comprising the steps of:

Controlling the electrical capacitance of said capacitor by controlling the fraction of volume of the anode body occupied by the voids and the distribution of said voids in said anode body; and: Controlling the amount of electrical serial resistance of said capacitor by controlling the fraction of the volume of said anode body occupied by the voids and the distribution of said voids in said anode body.

33. A method for production of the porous green anode body of claim 1 by electrophoretic deposition, said method comprising the steps of:

forming a suspension comprised of polar liquid solvent, capacitor grade powder particles and chemical additives;

applying a voltage across said suspension between an electrode in contact with said suspension and a substrate, such that at least a portion of said

particles migrates toward and is deposited on said substrate, forming a particulate deposit layer; and

washing said anode body to reduce the amount of carbon present on the surfaces of said particulate deposit layer.

34. A method in accordance with claim 33, wherein the values of the

following parameters of the electrophoretic deposition process are

maintained within the following limits throughout the deposition of the porous green anode body: - the zeta potential is higher than -3OmV and lower than-30mV;

the electrical conductivity is between 0.5micro- Siemens/cm and 4.0 micro-Siemens/cm; and

- the pH is between 9 and 11.

35. A method in accordance with claim 33, wherein the solvent is selected from a group of polar solvents

36. A method in accordance with claim 33, wherein the solvent is a solution of at least two polar solvents.

37. A method in accordance with claim 33, wherein the solvent is a

solution of at least one polar solvent and at least one non-polar solvent.

38. A method in accordance with claim 38, wherein the solvent has a dielectric constant equal to or greater than 6.

39. A method in accordance with claim 33, wherein the solvent is

selected from the group of materials comprising: water, methyl alcohol, ethyl alcohol, n-propyl alcohol, butyl alcohol, pentyl alcohol, isopropyl alcohol, nitromethane and acetonitrile.

40. A method in accordance with claim 33, wherein the concentration of the capacitor grade powder in the liquid solvent is between 1 gram per liter and 1000 grams per liter.

41. A method in accordance with claim 33, wherein the concentration of

the capacitor grade powder in the liquid solvent is between 10 grams per liter and 200 grams per liter.

42. A method in accordance with claim 33, wherein the concentration of the capacitor grade powder in the liquid solvent is between 50 grams per liter and 100 grams per liter.

43. A method in accordance with claim 33, wherein the chemical additives are selected from amongst the following classes of chemical substances:

charging agents;

steric additives; and

electro-steric additives.

44.A method in accordance with claim 43, wherein the chemical additives comprise at least one charging agent.

45. A method in accordance with claim 43, wherein the chemical additives comprise at least one steric additive.

46. A method in accordance with claim 43, wherein the chemical additives comprise at least one electro-steric additive.

47. A method in accordance with claim 43, wherein the chemical

additives comprise at least one steric additive and at least one charging agent.

48. A method in accordance with claim 43, wherein the chemical additives comprise at least one steric additive and at least one

electrosteric additive.

49. A method in accordance with claim 43, wherein the chemical additives comprise electrosteric additives selected from the group comprising: polyethylene imine, quaternary ammonium salts, and phosphate ester compounds.

50. A method in accordance with claim 43, wherein the charging agents are selected from the group comprising: nickel(II) chloride hexahydrate, nickel acetylacetonate, cobalt(II) acetylacetonate, and

aluminum chloride hexahydrate.

51. A method in accordance with claim 43, wherein the steric additives are selected from the group comprising: dodecyl sodium sulfate and poly ethylene glycol.

52. A method in accordance with claim 33, wherein the concentration of the chemical additives in the suspension is between 0.1 micro-liters and 1 milliliter of said - chemical additive in 100 milliliters of_. the solvent.

53. A method in accordance with claim 33, wherein the concentration of the chemical additives in the suspension is between 5 micro-liters and

100 micro-liters of said chemical additive in 100 milliliters of the

solvent.

54. A method in accordance with claim 33, wherein the electric field in

the suspension is held constant

55. A method in accordance with claim 33, wherein the electric field across the suspension is between 1 volt per centimeter and 300 volts per centimeter.

56. A method in accordance with claim 33, wherein the electrical current through the suspension is held constant during the EPD process.

57. A capacitor grade powder for forming a capacitor anode, consisting of a plurality of non-aggregated particles, wherein said powder particle

size is characterized by Dio ranging from 0.4 micron to 2 microns, by Ds₀ ranging from 0.6 micron to 4 microns, and by D90 ranging from 4

microns to 8 microns.

58. The capacitor grade powder of claim 57, wherein the particles are particles of a dielectric oxide film-forming electrical conductive material.

59. The capacitor grade powder of claim 57, wherein the electrical conductive material is selected from the group of materials comprising: tantalum, aluminum, magnesium, titanium, niobium, zinc, zirconium or niobium mono-oxide.