Claims
- 1. A flaked tantalum powder produced from a tantalum powder prepared by a chemical reduction process comprised of flakes and having a Scott density greater than about 18 g/in.sup.3.
- 2. A flaked tantalum powder produced from a tantalum powder prepared by a chemical reduction process comprised of flakes and having a Scott density in the range of about 18 g/in.sup.3 to about 60 g/in.sup.3.
- 3. A flaked tantalum powder produced from a tantalum powder prepared by a chemical reduction process comprised of flakes and having a Scott density in the range of about 20 g/in.sup.3 to about 35 g/in.sup.3.
- 4. A flaked tantalum powder produced from a tantalum powder prepared by a chemical reduction process comprised of flakes and having a Scott density of about 21 g/in.sup.3.
- 5. The flaked tantalum powder of claim, 1, 2, 3 or 4, wherein at least about 90% of the flakes have no dimension greater than about 55 micrometers.
- 6. The flaked tantalum powder of claim 1, 2, 3 or 4, wherein at least about 90% of the flakes have no dimension greater than about 45 micrometers.
- 7. The flaked tantalum powder of claim 1, 2, 3 or 4, wherein at least about 90% of the flakes have no dimension greater than about 25 micrometers.
- 8. A flaked tantalum powder comprised of flakes, having a Scott density greater than about 18 g/in.sup.3 and a BET surface area value in the range of about 0.4 m.sup.2 /g to 0.6 m.sup.2 /g.
- 9. The flaked tantalum powder of claim 8, wherein the Scott density is in the range of about 18 g/in.sup.3 to about 60 g/in.sup.3.
- 10. The flaked tantalum powder of claim 8, wherein the Scott density is in the range of about 20 g/in.sup.3 to about 35 g/in.sup.3.
- 11. The flaked tantalum powder of claim 8, wherein the Scott density is about 21 g/in.sup.3 and the BET surface area value is about 0.5 m.sup.2 /g.
- 12. The flaked tantalum powder of claim 8, 9, 10 or 11, wherein at least about 90% of the flakes have no dimension greater than about 55 micrometers.
- 13. The flaked tantalum powder of claim 8, 9, 10 or 11, wherein at least about 90% of the flakes have no dimension greater than about 45 micrometers.
- 14. The flaked tantalum powder of claim 8, 9, 10 or 11, wherein at least about 90% of the flakes have no dimension greater than about 25 micrometers.
- 15. The flaked tantalum powder of claim 12, wherein the tantalum flake is ingot-derived.
- 16. The flaked tantalum powder of claim 12, wherein the tantalum flake is derived from a tantalum powder produced by a chemical reduction process.
- 17. An agglomerate prepared from the tantalum flake powder of claim 1, 2, 3, 4, 8, 9, 10 or 11.
- 18. An agglomerate prepared from the tantalum flake powder of claim 5.
- 19. An agglomerate prepared from the tantalum flake powder of claim 12.
- 20. A pellet prepared from the tantalum flake powder of claim 1, 2, 3, 4, 8, 9, 10 or 11.
- 21. A pellet prepared from the agglomerated flaked tantalum powder of claim 17.
- 22. A capacitor prepared from the flaked tantalum powder of claim 1, 2, 3, 4, 8, 9, 10 or 11.
- 23. A capacitor prepared from the agglomerated flaked tantalum powder of claim 17.
- 24. A capacitor prepared from the pellets of claim 20.
- 25. A pellet prepared from the flake of claim 5.
- 26. A capacitor prepared from the flake of claim 5.
- 27. A pellet prepared from the flake of claim 16.
- 28. A capacitor prepared from the flake of claim 16.
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of our pending U.S. patent application Ser. No. 126,706, filed Nov. 30, 1987, now abandoned.
The invention relates to flaked tantalum powders often used for electrical capacitors and methods for making such powders. More particularly, the invention relates to flaked tantalum powders which when agglomerated provide the required electrical properties and good processing properties, e.g., flowability, high green strength and pressability.
Tantalum capacitors, made from tantalum powder, have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Tantalum capacitors typically are manactured by compressing agglomerated tantalum powder to form a pellet, sintering the pellet in a furnace 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.
Development of powders suitable for making tantalum capacitors has resulted from efforts by both capacitor producers and tantalum processors to delineate the characteristics required for tantalum powder in order for it to best serve in the production of quality capacitors. Such characteristics include surface area, purity, shrinkage, pressability, green strength, and flowability.
First of all, the powder should provide an adequate surface area when formed into a porous body and sintered. The .mu.fV/g of tantalum capacitors is proportional to the specific surface area of the sintered porous body produced by sintering a tantalum powder pellet; the greater the specific surface area after sintering, the greater the .mu.fV/g. The specific surface area of tantalum powder is related to the maximum .mu.fV/g attainable in the sintered porous body.
Purity of the powder is 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 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 necessary in order to produce high .mu.fV/g tantalum capacitors.
Flowability of the tantalum powder and green strength (mechanical strength of pressed unsintered powder pellets) are also important characteristics for the capacitor producer in order to provide efficient production. The flowability of the agglomerated tantalum powder is essential to proper operation of automatic pellet presses. Sufficient green strength permits handling and transport of a pressed product, e.g., pellet, without excessive breakage.
A `pellet`, as the term is used herein, is a porous mass or body comprised of tantalum particles. Green strength is a measure of a pellet's mechanical strength. The term `pressability` describes the ability of a tantalum powder to be pressed into a pellet. Tantalum powder that forms pellets that retain their shape and have sufficient green strength to withstand ordinary processing/manufacturing conditions without significant breakage have good pressability.
Currently, tantalum powders suitable for use in high performance capacitors are produced by several methods. One powder production method involves chemical reduction, e.g., sodium reduction of potassium fluorotantalate, K.sub.2 TaF.sub.7. In another method, powder is produced by hydriding a melted (typically arc melted or electron beam melted) tantalum ingot, milling the hydrided chips, and dehydriding.
As discussed above, the .mu.fV/g of a tantalum pellet is 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 of the methods proposed for increasing the specific surface area of tantalum powder is flattening the powder particles into a flake shape.
Efforts to further increase specific surface area by making thinner tantalum flakes have been hindered by concomitant loss of processing characteristics. For example, several of the major deficiencies of very thin tantalum flake are poor flow characteristics, poor pressability and low green strength and low forming voltages.
It is an object of this invention to provide a method for making flaked tantalum powder having a Scott density greater than about 18 g/in.sup.3.
It is another object of this invention to provide a flaked tantalum powder wherein at least about 90% of the flakes have no dimension greater than about 55 micrometers and the individual flakes have a substantially uniform cross section.
It is another object of this invention to provide a flaked tantalum powder having a Scott density greater than about 18 g/in.sup.3 suitable for use in economical high speed processes for the manufacture of tantalum capacitors.
It is another object of this invention to provide an agglomerate of flaked tantalum powder that has good flowability and pressability characteristics.
It is another object of this invention to provide flaked tantalum pellets having high green strength.
It is another object of this invention to provide flaked tantalum pellets having reduced sensitivity to sintering temperatures, i.e., pellets that can be sintered over a wide range of temperatures, relative to the prior art, to form an electrode useful in a tantalum capacitor.
It is another object of this invention to provide a tantalum electrode having reduced sensitivity to forming voltages, i.e., dielectric oxides can be formed on the electrodes over a range of voltages.
The present invention provides a method for making flaked tantalum powder that in agglomerated form, is flowable and pressable comprising the steps of preparing tantalum flakes and reducing the flake size so that the resulting tantalum flake Scott density is greater than about 18 g/in.sup.3. In one embodiment at least about 90% of the flakes have no dimension greater than about 55 micrometers, and in another embodiment no greater than about 25 micrometers. Preferably, at least about 90% of the flakes have no dimension greater than about 45 micrometers. The flake may be embrittled, e.g., by hydriding, oxidizing, cooling to low temperatures or like methods to facilitate the flake size reduction step.
The present invention provides a flaked tantalum powder produced from a tantalum flake prepared from tantalum powder produced by chemical reduction methods. The flaked tantalum powders of this invention have improved processing properties, when agglomerated, including flow properties suitable for high speed manufacturing operations and also have good pressability for forming high green strength pellets. The flaked tantalum powder of the present invention has a Scott density greater than about 18 g/in.sup.3, preferably, in the range of about 18 g/in.sup.3 to about 60 g/in.sup.3. A still more preferred range for the Scott density of the flaked tantalum powder is about 20 g/in.sup.3 to about 35 g/in.sup.3. A Scott density of about 21 g/in.sup.3 is most preferred. Preferably, at least about 90% of the individual tantalum flakes have no dimension greater than about 45 micrometers and as such will pass through a 325 mesh screen.
The present invention also provides an agglomerate of the above-described flaked tantalum powder having improved flowability and pressability characteristics. The agglomerated flaked tantalum powder of this invention may be prepared by any conventional method for preparing agglomerates such as, for example, by heating the tantalum flake, described in the preceding paragraphs, to a temperature of about 1300.degree. to 1600.degree. C. in an inert atmosphere or under vacuum for a period of about 30 to 60 minutes and crushing the resulting product to a size of about 40 mesh (0.015 inch screen opening).
The present invention also provides pellets prepared from the flaked tantalum powder described in the preceding paragraph.
The present invention also provides pellets prepared from the above described agglomerate of flaked tantalum powder.
The present invention also provides a capacitor electrode formed from the pellets described in the preceding paragraphs. In general the capacitors are prepared by sintering the pellets described above and anodizing the sintered pellets.
Other details, objects and advantages of the invention and methods for making and using the same will become apparent from the following detailed description and accompanying Figures. A legend at the bottom of the Scanning Election Micrograph (SEM) Figures gives the voltage, magnification e.g., 400X, and a reference scale in micrometers.
FIG. 1 is a SEM at a magnification 1000X, of a prior art ingot-derived flaked tantalum powder having a Scott Density of 13.4 g/in.sup.3 ;
FIG. 2 is a SEM at a magnification of 1000X, of an ingot-derived flaked tantalum powder having a Scott Density of 59.8 g/in.sup.3 prepared in accordance with the present invention;
FIG. 3 is a SEM at a magnification of 400X, of a prior art flaked tantalum powder produced in accordance with Example II, specimen H of U.S. Pat. No. 3,647,415 Yano, et al., and which is an example of a tantalum flake that is not produced in accordance with the teachings of that patent;
FIG. 4 is a SEM at a magnification of 400X, of a flaked tantalum powder produced by subjecting the flake of FIG. 3 to the process of the present invention;
FIG. 5 is a SEM at a magnification of 400X, of a prior art flaked tantalum powder produced in accordance with Example I, specimen C, of the U.S. Pat. No. 3,647,415 and which is representative of tantalum flake included within the teaching of that patent;
FIG. 6 is a SEM at a magnification of 500X, of a flaked tantalum powder produced by subjecting the flake of FIG. 5 to the process of the invention;
FIG. 7 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 3;
FIG. 8 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 4;
FIG. 9 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 5;
FIG. 10 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 6;
FIG. 11 is a graph showing the particle size distributions of the flaked tantalum powders shown in FIGS. 3 to 10;
FIG. 12 is a SEM at a magnification of 400X, of the flaked tantalum powder shown in FIG. 3 after agglomeration;
FIG. 13 is a SEM at a magnification of 400X, of the flaked tantalum powder shown in FIG. 4 after agglomeration;
FIG. 14 is a SEM at a magnification of 400X, of the flaked tantalum powder shown in FIG. 5 after agglomeration;
FIG. 15 is a SEM at a magnification of 400X, of the flaked tantalum powder shown in FIG. 6 after agglomeration;
FIG. 16 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 3 after agglomeration;
FIG. 17 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 4 after agglomeration;
FIG. 18 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 5 after agglomeration;
FIG. 19 is a SEM at a magnification of 1000X, of the flaked tantalum powder shown in FIG. 6;
FIG. 20 is a photograph of pellets pressed from the agglomerated flaked tantalum powder shown in FIGS. 12, 13, 16 and 17; and
FIG. 21 is a photograph of pellets pressed from the agglomerated flaked tantalum powder shown in FIGS. 14, 15, 18 and 19.
US Referenced Citations (4)
Continuation in Parts (1)
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Number |
Date |
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126706 |
Nov 1987 |
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Patent Grant
4940490
