US20060070492A1

US20060070492A1 - High capacitance tantalum flakes and methods of producing the same

Info

Publication number: US20060070492A1
Application number: US11/165,616
Authority: US
Inventors: Yongjian Qiu
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 2004-06-28
Filing date: 2005-06-23
Publication date: 2006-04-06
Also published as: CN101010160A; WO2006012279A3; WO2006012279A2; JP2008504692A; DE112005001499T5

Abstract

Methods of maximizing a tantalum capacitor's capacitance are disclosed, as well as tantalum flake powder and anodes. A two step milling process can be used to mill tantalum particles into tantalum flake powder having flakes of the desired thickness. This flake powder can then be pressed and sintered thereby forming an anode. Other flake capacitance methods and products are also described.

Description

This application claims priority under 35 U.S.C. §119(e) of prior U.S. Provisional Patent Application No. 60/583,498 filed Jun. 28, 2004, which is incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to high capacitance tantalum flakes and a method of producing the same. The present invention also relates to capacitors and anodes.
With the ever increasing demand for capacitor materials, such as tantalum, it has become highly desirable to ensure the maximum amount of capacitance per unit of capacitor material. Tantalum capacitors typically are manufactured by compressing tantalum powder to form a pellet, sintering the pellet to form a porous tantalum body (electrode), and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body.
The amount of electricity that can be stored in a capacitor (CV, hereinafter) is a measure of the capacity of the capacitor multiplied by the voltage. The CV (measured in μFV/g) of a tantalum capacitor is greatly influenced by the characteristics of the tantalum powder from which the anode is formed. Such characteristics can include specific surface area, purity, shrinkage, pressability, and powder particle shape.
First, the powder should provide an adequate electrode surface area when formed into a porous body and sintered. The CV measurement of tantalum capacitors can be related to the specific surface area of the sintered porous body produced by sintering a tantalum powder pellet. The specific surface area of tantalum powder can also be related to the maximum CV attainable in the sintered porous body.
Purity of the powder can also be an important consideration. Metallic and non-metallic contamination tends to degrade the dielectric oxide film in tantalum capacitors. While high sintering temperatures serve to remove some volatile contaminants, high temperatures also tend to shrink the porous body reducing its net specific surface area and thus the capacitance of the resulting capacitor. Minimizing the loss of specific surface area under sintering conditions, i.e., shrinkage, is important in order to produce high CV tantalum capacitor anodes.
As discussed above, the CV of a tantalum pellet can be a function of the specific surface area of the sintered powder. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of powder per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific surface area of tantalum powder.
One proposed method for increasing the specific surface area of tantalum powder is flattening the powder particles into a flake shape. Tantalum flakes are widely used for making high-reliability tantalum capacitors. There is, however, a general lack of guidance in the art as to what thickness of tantalum flake should be used to ensure a particular capacitance at a particular formation voltage. Thus, there is a long felt need in the art for a method of producing the proper thickness of a tantalum flake to produce a maximum capacitance at a particular formation voltage.

SUMMARY OF THE PRESENT INVENTION

It is, therefore, a feature of the present invention to provide a method of forming various thicknesses of a tantalum flake at a given formation voltage to ensure maximum capacitance.
Another feature of the present invention is to provide tantalum flake powders designed to provide a maximum capacitance when formed into the anode of a capacitor for a given formation voltage.
A further feature of the present invention is to provide tantalum flake powders having a uniform flake thickness, with that flake thickness being adapted for providing a maximum capacitance at a given formation voltage.
Additional features and advantages of the present invention will be set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the description and appended claims.
To achieve these and other advantages, and in accordance with the purposes of the present invention as embodied and broadly described herein, the present invention relates to a tantalum flake powder having maximized capacitance capabilities.
The present invention also relates to a method of making an anode having improved capacitance characteristics.
Another feature of the present invention is to provide an anode requiring the smallest amount of tantalum for a given capacitance.
It is to be understood that both the foregoing general description and the following detailed description are exemplary only, and are intended to provide a further explanation of the present invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate some of the embodiments of the present invention and together with the description, serve to explain the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph detailing the Scott densities of tantalum flakes milled for various milling times using different sized milling media.
FIG. 2 is a graph showing the minimum average flake thickness versus maximum capacitance at various formation voltages.
FIG. 3 is a graph showing the maximum CV versus average flake thickness for tantalum powders.
FIG. 4 is a graph showing average flake thicknesses and thickness distribution for various tantalum powders.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention, in part, relates to methods for preparing tantalum flake powder having a maximized capacitance capability for a given formation voltage. The present invention further relates to tantalum flake powder having flake thicknesses that are ideal for desired capacitance needs. Anodes and capacitors are also described.
Tantalum can be extracted from ore and formed into powder by any extraction process. One such process, for example, is chemical reduction by a primary metal processor, as generally described in U.S. Pat. No. 6,348,113, incorporated in its entirety by reference herein. Further metal refining techniques that can typically be performed by a primary metal processor include thermally agglomerating the metal powder, deoxidizing the agglomerated metal powder in the presence of a getter material and then leaching the deoxidized metal powder in an acid-leached solution, as disclosed, for example, in U.S. Pat. No. 6,312,642, incorporated in its entirety by reference herein.
Examples of tantalum powders, including flakes, are described in U.S. Pat. Nos. 5,580,367; 5,448,447; 5,261,942; 5,242,481; 5,211,741; 4,940,490; and 4,441,927, which are incorporated in their entireties by reference herein. The tantalum flake powder can be ingot-derived powder or chemically reduced (e.g. sodium reduced) powder.
Other metal flakes, methods for making metal flakes, and uses for metal flakes are set forth in the following U.S. patents, each of which is incorporated, in its entirety, by reference herein: U.S. Pat. Nos. 4,684,399; 5,261,942; 5,211,741; 4,940,490; 5,448,447; 5,580,516; 5,580,367; 3,779,717; 4,441,927; 4,555,268; 5,217,526; 5,306,462; 5,242,481; and 5,245,514. The tantalum powder can be hydrided or non-hydrided. Also, the tantalum powder can be agglomerated (or granulated) or non-agglomerated. Any of the properties and physical and/or chemical characteristics as described in these patents can be used herein with the present invention.
In one embodiment, the present invention relates to a method of forming a tantalum flake thickness (e.g., an average flake thickness) that maximizes capacitance at particular formation voltage. Inversely, the present invention can form tantalum flake powder using the minimum amount of tantalum required for a given capacitance at a given formation voltage.
In general, tantalum flake powder can be prepared from tantalum particles by any method or device capable of producing tantalum flakes. One possible method of producing tantalum flakes is by the milling of tantalum particles through the use of any known milling device capable of producing tantalum flakes. For example, a ball mill, such as an Attritor mill, may be used to mill tantalum particles into flakes. Flake thickness is generally controlled by milling time and by media size. Larger milling media can mill tantalum particles more forcefully and can be very effective in initial milling for flattening the particles. There can be a drawback to larger media, in that large media also tend to fracture flakes that reach a certain minimum thickness. Smaller media, on the other hand, are effective in further thinning relatively thin flakes without causing significant fragmentation, but generally require more milling time. In addition, smaller media are capable of hitting the flaked particles more often in a given period of time due to their high number count in a given volume, yielding flakes of a more uniform thickness. Staged milling is useful in the present invention. In staged milling, there are at least two stages of milling wherein in the first stage, the diameter of the milling media is larger than the diameter of the milling media used in the second and subsequent stages of milling. This staged milling leads to a variety of beneficial properties including a narrower particle distribution range of the finished material. In addition, the staged milling leads to faster milling times to achieve the targeted particle size of the material.
Any preliminary or intermediate or final milling step can be used in addition to the milling steps described herein. For example, jet milling can be used at any point.
Examples of such preferred starting powders include those having mesh sizes of from about 60/100 to about 100/325 mesh and from about 60/100 to about 200/325 mesh. Another range of size is from 40 mesh to about +325 mesh.
In preparing the material, an ingot can be subjected to a hydriding process in order to embrittle the metal (e.g., tantalum) for purposes of crushing the ingot into powder, which is subsequently subjected to a screen in order to obtain a uniform particle distribution, which is preferably from about 5 to about 300 microns in size. If needed, the powder can be subjected two or more times to the crusher in order to achieve the desired uniform particle distribution. Afterwards, the powder is then preferably subjected to milling in order to obtain the desired average flake thickness, which is from about 0.02 microns to about 2 microns (or more) in size.
In this process, the milling of the metal in order to form the flake powder preferably occurs, as an option, in a mill wherein all of the surfaces that come in contact with the material are tantalum. In other words, preferably all of the contact surfaces of the mill, arms, and grinding media used in the mill have a tantalum surface. The tantalum surface on the contact areas of the mill and grinding media can be accomplished by coating the grinding media and internal surfaces of the mill with tantalum metal or plates of tantalum metal can be placed (e.g., welded) in the mill. The grinding media, such as balls, can be coated with tantalum or can be completely made of tantalum. By having all contact surfaces of the mill and grinding media made of tantalum, the amount of contamination to the material is significantly reduced, and preferably reduced to such a level that acid leaching is not necessary and is preferably avoided. This is especially advantageous since acid leaching can be inconsistent and lead to varying levels of contamination from lot to lot.
Preferably, the amount of tantalum present on the contact surfaces of the mill and grinding media is of a sufficient level such that during the milling process, none of the non-tantalum underlying surfaces come in contact with the tantalum material. Preferably, the thickness (e.g., about 1 mm or less to about 100 mm or more) of the tantalum on the contact surfaces of the mill and grinding media is sufficient such that repeated milling can occur from lot to lot. Preferably, the milling of the tantalum powder occurs in a wet mill, which leads to a more uniform flake thickness of the tantalum. In wet milling, the liquid used can be aqueous or non-aqueous, such as water, alcohol, and the like. Preferably, the milling is sufficient to result in an average flake thickness of from about 0.02 to about 2 microns, and more preferably, from about 0.08 micron to about 1.0 micron.
In a preferred embodiment, the milling occurs in an Attritor mill such as a 1 S mill which is operated at about 250 rpms or other speeds. When the milling is completed, the mixture can then be subjected to the heat treatment.
The milling of the tantalum material can also occur sequentially by milling with varying ball diameters. This use of staged milling can be used as a step in and of itself or can be used in combination with one or more of the above-described steps. This preferred method dramatically reduces the time required to obtain a desired average thickness. The milling of the material, for instance, can occur in stages in different mills or in the same mill. In the preferred embodiment, faster milling is achieved early in the process by using a large ball diameter to product diameter ratio. When the product population increases, the ball diameter should be reduced in order to increase the ratio of balls to product, and thereby increasing the chances of the product being hit and shattered. Preferably, the size of the material (hereinafter feedstock) can be as large as 1/10 of the size of the ball diameter. This feedstock-to-ball ratio can be used until the feedstock-to-ball ratio size is from about 1/1000 to about 1/500, more preferably, until the feedstock-to-ball ratio is about 1/200. The ball diameter can then be changed so that the ratio of feedstock-to-ball diameter is about 1/10. This process can be continued until the original feedstock reaches a thickness size of from about 0.02 to about 2 microns. Rather than using very small ball diameter balls for the second milling step, which would take more time and create a broad distribution, a ball diameter is sequentially selected that takes advantage of being relatively massive while still being numerous, and yet follow the 1/10 ratio of feedstock-to-ball diameter.
In the multi-staged milling embodiment of the present invention, two or more stages of milling using increasingly smaller diameters are used. In other words, in the first initial milling stage, the diameter of the milling media is larger than the diameter of the milling media used in the second stage. Furthermore, if more than two stages are used, preferably each subsequent milling stage uses a milling media that has a diameter smaller than the previous milling stage. More than two milling stages can be used depending upon the desired particle size of the final product. For purposes of the present invention, at least two stages of milling accomplish the desired result, namely a product having an average thickness flake size of from about 0.02 microns to about 2 microns.
Using the stage milling, the overall milling time can be reduced by at least 10% and more preferably can be reduced by at least 15% and even more preferably by at least 50% compared to milling having only one stage of milling using the same milling media.
In the preferred embodiment, in each milling stage, the ball diameter is smaller than the ball diameter of the previous milling. The above process permits a more uniform milling of the feedstock, since smaller diameter balls permit a more uniform milling. This stage milling can be applied to the milling of any of the component(s) used in the present invention. The advantage of using this preferred method of the present invention is that this method reduces the overall milling time to achieve the target flake thickness size of from about 0.02 microns to about 2 microns. Additionally, the reduction of the milling time reduces the production costs and exposure time to contaminants. Moreover, to further reduce contamination, each mill and its grinding ball can be made of tantalum metal or lined with tantalum metal. Preferably, the milling process of this preferred method is a wet milling process. An example of a suitable ratio for a wet milling process is 800 grams of tantalum powder to 300 ml of water. The remaining volume in the mill is taken up by milling media. Dry milling can be used in lieu of wet milling, and generally, the milling process takes place in an inert atmosphere.
For purposes of the present invention, any of the milling steps described in the present application can be conducted under heat, such as described in International Published PCT Patent Application No. WO 00/56486 incorporated in its entirety by reference herein. Also, other additives can be added during any milling step, such as a binder, lubricant, surfactant, dispersant, solvent, and the like.
With the present invention, a narrower flake thickness distribution range (T10, T50, T90) can be achieved. For instance, the thickness distribution range can be such that the T10 and/or T90 is within ±200% of the T50, preferably is within ±100% or ±70% of the T50, more preferably is within 35% of the T50, and still more preferably is within 20% or 10% of the T50. Such a tighter thickness distribution range leads to favorable properties especially with respect to improved electrical properties, such as in the anode. In addition, a tighter thickness distribution range leads to a better quality control of the finished product since each batch of material preferably has more similar physical and electrical properties.
Once the flake is formed, as indicated above, the particles can be mixed with a binder and then optionally compacted. The average particle size after milling is about 60-120 microns. The flakes are then hydrided and crushed into particles of 1-10 microns. Subsequent heat treatment results in the formation of agglomerates, which has a particle size distribution of from about 1 micron to about 1000 microns, and more preferably from about 1 micron to about 500 microns. These particles or agglomerates can then be pressed into anodes and sintered for anode production using conventional techniques known to those skilled in the art.
The type or method of milling is not critical to the present application so long as the selected method used can produce tantalum flakes ranging from about 0.02 micron to about 2.0 microns in thickness.
A preferred method of milling involves using various different sized media, each size media being used separately in one of at least two separate milling stages. For example, a first stage media of 3/16″ could be used for an initial period (e.g., 1 hr., 2 hrs., or more) to obtain flakes of 1-2 μm thick. Smaller size media such as ⅛″ or 1/16″ could then be used for any period of time (e.g., 1 hr., 2 hrs., or more) to achieve a flake thickness of under 1 μm.
FIG. 1 plots thickness distribution (Scott density) of two stage milled tantalum products using the following media and time periods (2 hr. with 3/16″ followed by 1 hr. with ⅛″; 2 hr. with 3/16″ followed by 2 hr. with ⅛″; 2 hr. with 3/16″ followed by 3 2 hr. with ⅛″; and 2 hr. with 3/16″ followed by 4 hr. with 1/16″. Lower Scott density is an indication of thinner flakes at similar process stages. FIG. 1 shows that two stage milling yields the lowest Scott density. Optical quantitative image analysis has confirmed that stage milled flakes are thinner and have a narrower thickness distribution than flakes produced by conventional single stage milling processes such as the conventional single stage use of 3/16″ media, noted as C275 (commercially available tantalum flake from Cabot Corporation) in FIG. 1.
As demonstrated above, there are a variety of methods, which can produce tantalum flakes and thereby tantalum flake powder. In one embodiment, the present invention relates to a tantalum flake powder, wherein the thickness of the flakes is an average flake thickness within ±200 or ±100% or ±10% or ±50% or ±35% of a primary flake thickness, wherein the primary flake thickness equals 2.5KoKta/(Dta*Kf)*1/CV, where CV equals 2.5KoKta/(Dta*2Kf)*1/Vf. Ko=the vacuum permittivity 8.85*10⁻¹²J⁻¹C²m⁻¹, Kta=the dielectric constant 27, Dta=the density of the tantalum material 16.7 g/cm³, Kf=formation constant (dielectric thickness per volt) 2 nmN, CV=μFV/g and Vf=voltage of formation, which is preferably about 4 volts to about 500 volts, and more preferably from 6 volts to 400 volts, and even more preferably from 10 volts to 300 volts. Preferably, the formation temperature is 85° to 90° C. For instance, the CV range can be from 100,000 to 1,500,000 at 6 V. The CV range can be from 25,000 to 400,000 CV/g at 25 V. The CV range can be from 20,000 to 180,000 CV/g at 50 V. The CV range can be from 9,000 to 90,000 CV/g at 100 V. The CV range can be from 6,000 to 60,000 CV/g at 150 V. The CV range can be from 4,000 to 45,000 CV/g at 200 V. The CV range can be from 3,000 to 30,000 CV/g at 300 V.
Various flake thicknesses along with formation voltages and CV/g are set forth below:

Flake Thickness Formation Voltage max CV min CV

0.08 μm 50 V 180K μFV/g 20K μFV/g

0.16 μm 100 V 90K μFV/g 9K μFV/g

0.24 μm 150 V 60K μFV/g 6K μFV/g

0.32 μm 200 V 45K μFV/g 4K μFV/g

0.48 μm 300 V 30K μFV/g 3K μFV/g
The tantalum flake powder of the present invention can have an average thickness ranging from 0.08 micron to 1.0 micron. The desired flake thickness can be obtained by carefully controlling the milling process conditions described in this invention. A narrower thickness distribution can also be achieved. This is highly desirable since flakes that have a broader thickness distribution tend to have a lower CV overall than flakes with a more uniformly distributed thickness profile. With the present invention, a tantalum flake can be tailored to work at specified capacitance ranges wherein the flake thickness of the tantalum powder is maximized for the specified capacitor usage. Thus, the tantalum flake powders of the present invention are designed for specified formation voltages at which almost all of the tantalum metal, but not all, is converted into an oxide and only a mono layer thick of tantalum atoms remains in the middle of the oxide to serve as the electrode. The present invention permits one to achieve, in preferred embodiments, this desired flake design.
The tantalum flake powder of the present invention can have any BET surface area. Preferably, the flake powder of the present invention includes a BET surface area that ranges from about 0.1 to about 6 m²/g or higher. The flake powder can also have any Scott density, such as a Scott density that ranges from about 15 to about 40 g/inch³. Furthermore, the individual flakes of the flake powder of the present invention can have any number of different sizes. Preferably, the flake particles range in overall size from about 0.1 to about 200 microns. The flake powder can have any agglomerated size. Preferably, the flake powder includes an agglomerated size ranging from about 1 to about 500 microns.
FIG. 2 illustrates the above-mentioned relationship between the formation voltage, minimum flake thickness, and maximum CV that is defined through the above-mentioned flake thickness and CV relationships. Namely, when the flake thickness is far greater than the dielectric thickness, CV can be proportional to 1/flake thickness. The dielectric can be an insulating layer of metal oxide formed on the surface of an anode by the action of the formation voltage in the presence of an electrolyte.
The formation voltage can cause the formation of the dielectric through an oxidation reaction between the tantalum metal substrate and an electrolyte, thereby consuming the tantalum metal substrate. The maximum CV (most efficient use of tantalum) can be achieved when the flake is designed to such a thickness that nearly all of the tantalum is converted into oxide. This gives rise to a second advantage which can be taken from the CV flake thickness relationship, namely, the thinner the flake, the higher the CV. However, flakes that are too thin for a given formation voltage can be completely converted into dielectric and will have a CV of 0. FIG. 3 details this relationship, showing a steep drop off in maximum CV at a flake thickness of approximately 0.25 μm for a formation voltage of about 150V.
The tantalum powder of the present invention can be used in combination with non-flake powders. The tantalum flakes of the present invention can be doped with any conventional dopant(s), e.g., nitrogen, boron, phosphorous, and the like. Also, the tantalum flakes can be agglomerated (e.g., thermal or non-thermal) using any conventional techniques, such as water agglomeration as described in U.S. Pat. No. 6,576,038.
The tantalum flake powder of the present invention can be formed into anodes, in the same conventional manner as previous conventional powders. Anode formation generally is performed by pressing and sintering the flake powder for a period of time. The flake anodes can be pressed at press densities ranging from 4 g /cc to 7 g/cc, preferably from 4.5 g /cc to 6.5 g/cc. Various binders and lubricates, such as polyethylene glycol and steric acid, can be used in various proportions to assist anode pressing. These binders or lubricates should be removed from the pressed anodes to eliminate residue contamination. The sintering process can be performed either in vacuum or in inert atmosphere such as Ar that prohibits tantalum metal from oxidation. Sintering can be performed at any temperature at which sintering occurs, but would preferably be performed at a temperature of from about 1000° C. to about 1800° C. The flake powder can be sintered for any length of time sufficient for sintering, and can preferably be sintered from about 10 minutes to about 10 hours. U.S. Pat. Nos. 6,689,187; 6,687,117; 6,679,934; 6,678,144; 6,660,057; 6,696,138; 6,040,975; 5,986,877; 5,699,597; 6,563,695; 6,432,161; and 5,448,447 describe various anodes/capacitors that can be used in the present invention for the tantalum flakes of the present invention. The anode/capacitor can be formed and have the characteristics as described in the above-mentioned patents.
The present invention may be used to produce anodes having a CV of about 1,500,000 CV/g at a formation voltage of about 6 volts to a CV of about 18,000 CV/g at a formation voltage of about 500 volts.
An anode made from the flake powder detailed in this application preferably has low DC leakage, and preferably has a DC leakage of less than 5 nA/CV, such as or 2 nA/CV or less, from about 0.1 to about 2 nA/CV. Furthermore, the anode may have any number of pores of any number of sizes. More preferably, the anode can have pores ranging in size of from about 0.1 micron to about 10 microns.
In the present invention, the thickness of tantalum flake can have, in certain embodiments, a unimodal distribution pattern. For instance, one type of unimodal distribution is shown in FIG. 4. The thickness distribution preferably has a T₅₀of from about 0.08 micron to about 1 micron, a T₁₀of about −100% of T₅₀, and a T₉₀of about +100% of T₅₀. The T₁₀, T₅₀, and T₉₀essentially have the same meaning as D₁₀, D₅₀, and D₉₀in standard particle size measurement except it relates to the particle thickness as opposed to the particle size. The particle thickness can be measured by any technique, such as optical image analysis or scanning electronic microscope (SEM). In the alternative, the flake thickness distribution can be bimodal or multi-modal, depending upon the desired preferences of the capacitor anode manufacturers.
The present invention will be further clarified by the following examples, which are intended to be exemplary of the present invention.

EXAMPLES

Example 1

In order to obtain a tighter flake thickness distribution as well as a lower average flake thickness, various samples were produced using a standard starting material, Kdel tantalum powder, which was a sodium reduced and acid leached basic lot powder used to make C275 Ta flake from Cabot Corporation. 5 lbs of the Kdel tantalum was slowly added into a 1S Attritor mill containing 50 lbs of 3/16″ stainless steel media and 2700 ml of ethyl alcohol. The powder was discharged from the mill after being milled at 250 rpm for 2 hrs. After replacing the 3/16″ media with ⅛″ stainless steel media, the mill was loaded with 2700 ml of fresh ethyl alcohol and 2.5 lbs of the 3/16″ milled powder. After being milled at 250 rpm and 6 hrs, the flake powder was then discharged and rinsed with DI water. The flakes were then acid leached to remove surface contaminants accumulated during the milling process. Flake samples were analyzed for Scott density and chemical composition.
Each sample was then subjected to the milling as described below. Afterwards, the average flake thickness was determined as well as the thickness range. The reference to 75% count is the percent of the flake thicknesses counted which had the thickness range identified in the table below. The thickness measurement was obtained by taking the flakes formed during the milling and adding it to a liquid epoxy and mixed on a vibratory mixer, and an ultrasonic probe was used. The epoxy which cured slowly overnight allowed sufficient time for the flakes to settle thereby separating the flakes and providing plane parallel orientation. The reduction of the epoxy viscosity was achieved by using an epoxy to hardener ratio of 6:1 (weight ratio) and heating the mixture to 60° C. Ultrasonic agitation and vibration of the warm resin with lower viscosity added to the settling and proper orientation of the flakes. Upon curing of the epoxy, the epoxy was cut in half thereby exposing various flake thicknesses for ease of measurement. In preparing the epoxy mount, approximately 0.05 grams of tantalum flake were added to 2 milliliters of mixed epoxy and the mixture was approximately mixed for 5 seconds. The mixture was then added to a mold which was agitated for 30 seconds using a vibrator. The material was then oven cured for 4 hours at 60° C. The epoxy resin used was a Buehler epoxicure resin and the hardener was a Buehler epoxicore hardener.

C275 2 + 0 2 + 1 2 + 2 2 + 3 2 + 4

Ave. thickness 0.8 μm 1.4 μm 0.8 μm 0.8 μm 0.6 μm 0.6 μm

Thickness 0.4-2 μm 0.6-2.2 μm 0.6-1.4 μm 0.4-1.2 μm 0.4-1 μm 0.4-1 μm

range >75% count

Sample Information:



	2 + 0:	1S attritor mill, 2 hr milling using 3/16″ media
	2 + 1:	1S attritor mill, 2 hr milling using 3/16″ media,
		plus 1 hr. mill using ⅛″ media
	2 + 2:	1S attritor mill, 2 hr milling using 3/16″ media,
		plus 2 hr. mill using ⅛″ media
	2 + 3:	1S attritor mill, 2 hr milling using 3/16″ media,
		plus 3 hr. mill using ⅛″ media
	2 + 4:	1S attritor mill, 2 hr milling using 3/16″ media,
		plus 4 hr. mill using ⅛″ media
	C275:	Standard C275 product after Attritor milling stage

As can be seen from the results, the multi-stage milling led to a lower average flake thickness as well as a tighter thickness range distribution.

Example 2

A C275 tantalum flaked powder which is commercially available from Cabot Corporation was compared to tantalum flakes prepared by multi-stage milling. In the example, the flakes of the present invention were prepared using a 2 stage milling process. 5 lbs of Kdel tantalum basic lot powder, a standard starting material for C 275 Cabot flake products, was milled in a IS Attritor mill at 250 rpm for 2 hrs. using 3/16 inch media (1^ststage). 2.5 lbs of the milled material was then milled in the same mill using ⅛ inch media for additional 6 hours (2^ndstage). The 2 stage milled flake sample was then acid leached and dried. The flake sample was then hydrided to reach a 4000 ppm of hydrogen level and milled in a Vortec mill in order to raise its Scott density from about 4-5 g/inch³to about 20-25 g/inch³. The sample was then processed using standard C275 processes with adjusted heat treatment temperatures of 1200° C. and 1300° C. for 1^stand 2^ndheat treatment, respectively. After the heat treatments, the sample was crushed and deoxidized at about 950° C. using Mg metal powder. The samples were analyzed for its electrical properties after screening and acid leached to remove residue Mg metal and oxide. The table below sets forth the data for the flakes of the present invention and the C275 data. As can be seen, the flakes of the present invention had a higher capacitance per gram of material.
The table below sets forth the data for the flakes of the present invention and the C275 data. As can be seen, the flakes of the present invention had a higher capacitance per gram of the material.

Present Present

Invention @ Invention @ C275 @ C275 @

1350° C. 1400° C. 1350° C. 1400° C.

VF = 50 V 39745 CV/g 37559 CV/g 33692 CV/g 32645 CV/g

VF = 100 V 28049 CV/g 28350 CV/g 27404 CV/g 26805 CV/g
Applicants specifically incorporate the entire contents of all cited references in this disclosure. Further, when an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.
Other embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and equivalents thereof.

Claims

1. Tantalum flake powder, wherein the thickness of said flakes is an average flake thickness within ±200% of a primary flake thickness, wherein said primary flake thickness=2.5KoKta/(Dta*Kf)*1/CV, wherein CV=2.5KoKta/(Dta*2Kf)*1Nf and wherein Ko=vacuum permittivity, Kta=dielectric constant, Dta=density of tantalum material, Kf=formation constant (dielectric thickness per volt), CV=μFV/g and Vf=voltage of formation, wherein Vf is from about 6 volts to about 500 volts.

2. The tantalum flake powder of claim 1, wherein said average thickness is within ±50% of said primary flake thickness.

3. The tantalum flake powder of claim 1, wherein said average thickness is within ±10% of said primary flake thickness.

4. The tantalum flake powder of claim 1, wherein said average thickness is within ±100% of said primary flake thickness.

5. An anode comprising the tantalum flake powder of claim 1.

6. An anode comprising the tantalum flake powder of claim 2.

7. A capacitor comprising the anode of claim 5.

8. A capacitor comprising the anode of claim 6.

9. A tantalum flake powder having a unimodal flake thickness distribution is from 0.02 micron to 4 microns.

10. The tantalum flake powder of claim 9, wherein said tantalum flake powder has a flake thickness distribution, wherein T₅₀is from about 0.08 micron to about 1 micron, T₁₀is about −100% of T₅₀, and T₉₀is about ±100% of T₅₀.

11. An anode comprising the tantalum flake powder of claim 9.

12. A capacitor comprising the anode of claim 11.

13. A tantalum flake powder having a flake thickness distribution, wherein T₅₀is from about 0.6 micron, T₁₀is about −100% of T₅₀, and T₉₀is about +100% of T₅₀.

14. The tantalum flake powder of claim 13, wherein said capacitance is from about 37,000 to about 39,000 CV/g when formed at a formation voltage of 50V, and sintered at a temperature of from about 1300° C. to about 1400° C. for ten minutes and at a formation temperature of 90° C.

15. The tantalum flake powder of claim 1, wherein said tantalum flake powder has a flake thickness distribution which is unimodal.

16. The tantalum flake powder of claim 13, wherein said tantalum flake powder has a flake thickness distribution which is multi-modal.

17. The tantalum flake powder of claim 1, wherein said tantalum flake powder has a flake thickness distribution which is multi-modal.

18. A method to maximize capacitance capability in tantalum flake powder of claim 1 for a certain formation voltage, comprising forming said tantalum flakes to an average flake thickness within ±100% of a primary flake thickness, wherein said primary flake thickness=2.5KoKta/(DtaKf)*1/CV, wherein CV=2.5KoKta/(Dta2Kf)*1Vf and wherein Ko=vacuum permittivity, Kta=dielectric constant, Dta=density of tantalum material, Kf=formation constant (dielectric thickness per volt), CV is measured in μFV/g and Vf=formation voltage, wherein Vf is from about 6 volts to about 500 volts.

19. The method of claim 18, wherein said average thickness is from 0.08 to 1 micron.

20. The method of claim 18, wherein said CV range is from 100,000 to 1,500,000 at 6V.

21. The method of claim 18, further comprising forming said tantalum flakes into an anode.

22. The method of claim 18, wherein said forming comprises milling tantalum particles in a multi-staged milling process.

23. The method of claim 22, wherein said multi-staged milling process is a two staged milling process.

24. The method of claim 23, wherein said two staged milling process comprises a first stage milling, wherein said tantalum particles are milled with a 3/16″ media to obtain tantalum flakes; and a second stage milling, wherein said tantalum flakes are further milled with a ⅛″ or 1/16″ media.

25. The method of claim 18, wherein said average thickness is within ±10% of said primary flake thickness.

26. The method of claim 18, wherein said average thickness is within ±50% of said primary flake thickness.

27. The method of claim 18, wherein said average thickness is within ±35% of said primary flake thickness.

28. The method of claim 18, wherein said CV range is from 25,000 to 400,000 at 25V, from 20,000 to 180,000 at 50V, from 9,000 to 90,000 at 100V, from 6,000 to 60,000 at 150V, from 4,000 to 45,000 at 200V, or from 3,000 to 30,000 at 300V.