Capacitor grade tantalum powder

Abstract: Tantalum powders of capacitor grade are provided, containing interacting silicon and phosphorous dopants and are used to make anodes for ectrolytic capacitors.

Description: BACKGROUND OF THE INVENTION The present invention relates to capacitors and tantalum powders of capacitor grade, used in production and miniature, high-specific-capacitance, low-leakage, solid state electrolytic and wet electrolytic capacitors with anode pellets or slabs made of sintered valve metal powders, e.g., tantalum. It is known in the art that phosphorous doping retards sinter closure of tantalum powders used in anode production to preserve a high capacitance of the tantalum powder. However, increasing levels of phosphorous doping also produce unacceptable increases in D.C. leakage, particularly in anodes formed (oxidized) at lower temperatures, e.g. 60°C. Chemical Abstracts, vol. 94, no. 4, 26 th January 1981, page 202, abstract no. 18876r, Columbus, Ohio, USA discloses Ta powder useful for electrolytic capacitors, said powder comprising Si (7000 ppm) and P (due to phosphates) and being sintered at 1200°C. In US-A-3,825,802 it is proposed to provide an electrode for a capacitor with improved voltage breakdown and storage capacity, said electrode comprising Ta powder with N, Si, P, B or a mixture thereof as a dopant. It is a principal object of the present invention to provide an improved capacitor grade tantalum powder affording practical high capacitance at low leakage and to produce anodes therefrom. SUMMARY OF THE INVENTION The object of the invention is achieved in a multidopant tantalum powder comprising uniquely balanced silicon and phosphorous dopants or equivalents in an otherwise high purity (capacitor grade), high surface area (in excess of 4 000 cm 2 /gm), tantalum powder. This invention specifically relates to a capacitor grade tantalum powder being of such size, configuration and composition so that the sintered at 1.400°C or higher and formed anodes can achieve a high specific apacitance (CV/gm) in excess of 15.000 CV/gm, wherein the tantalum powder contains dopant of silicon and phosphorous in proportions of silicon: 50 - 1000 ppm relative to tantalum, and phosphorous: 10 - 300 ppm relative to tantalum the powder having such combined concentration of silicon and phosphorous therein that it is electrolytically formable at any formation temperature substantially throughout the range of 40-90°C, consistent with affording said high specific capacitance, and a D.C. leakage (L/CV) of less than 0.5 nanoampere per microfarad-volt. The phosphorous enhances capacitance and the silicon suppresses leakage normally associated with high phosphorous doping levels after low temperature formation and also enhances capacitance. The preferred range are 100-500 ppm Si and 20-80 ppm P. The resultant tantalum can be agglomerated, deoxidized, sintered at or over 1 400°C and anodized at or over 40°C to form a high specific capacitance, low leakage anode of an electrolyte capacitor (wet or solid electrolytic). This invention thus also particularly relates to the powder of this invention as formed into an agglomerated powder, an anodic pellet formed of that powder and an anode formed of that pellet. In a preferred embodiment of this invention the pellet is formed at a temperature in the range 40°C - 60°C to form an anode, according to one preferred variation being formed in a dilute mineral acid. This invention also relates to a method of producing high capacitance, low leakage capacitor grade valve metal powder comprising the step of providing phosphorous as a primary capacitance enhancing dopant in an amount from 10 to 30 ppm relative to tantalum and, as an interacting further dopant silicon in an amount from 50 to 1000 ppm relative to tantalum, the silicon and phosphorous being provided in such combined concentration to effect a specific D.C. leakage (L/CV) of such powder of less than 0.5 nano-amperes per microfarad-volt and high specific capacitance (CV/g) in excess of 15 000 in anodized pellets as made therefrom and sintered at 1 400°C or higher, after anodic formation at temperatures selected in the range 40-60°C. The silicon can be added at various stages of tantalum production but is preferably added during reduction of a tantalum precursor (e.g., the standard method of K 2 TaF 7 reduction by Na reducing agent). Phosphorus content is more flexibly established at various stages, but preferable after reduction. In a preferred embodiment of this invention a stirring reduction reactor can be charged with K 2 TaF 7 or Na 2 TaF 7 double salt (tantalum precursor), preferably diluted in NaCl or other practical halide salts of sodium or potassium, which is melted and stirred and subject to reduction by molten sodium added to the charge after melting (or pre-mixed therewith). The dilution can vary from 0:1 to 1:1 weight ratio of diluent to precursor. Silicon and dopants are preferably added to the charge prior to or during molten state reduction. The silicon dopant is preferably provided in the form of compounds thereof, such as Si 3 N 4 . The phosphorous can be a component of the K 2 TaF 7 or Na 2 TaF 7 charge or diluent, or added to tantalum after the reduction, as viable alternatives or supplements to addition during reduction. Preferably the phosphorous is added after the reduction stage. The reduction procesing, per se, can be in accordance with any of the well established industry procedures for effecting the same in batch reactors, or in continuous processing extensions of such procedures. All such procedures involve production of an end product mass containing elemental tantalum and salt by-products. The tantalum may be isolated from the by-products by chemical (acid leaching) methodology and/or other separation techniques to produce primary powder particles. Product crushing, sieving and other physical handling means incidental to such separation and subsequent size sorting of the primary powders are well known per se. Preferably, the primary powders are agglomerated by pre-sintering to produce sponge-like secondary powders, and deoxized, both procedures being also well known in the art. A preferred method of producing a high capacitance low leakage anode comprises electrolytic formation at 40-60°C of a sintered pellet of powders of tantalum in a dilute mineral acid, the powder comprising mixed dopants in the ranges: (a) Silicon - 50 to 1 000 ppm, and (b) Phosphorus - 10 to 300 ppm, relative to tantalum. The import of the data of the following examples, and other aspects of the invention, includes at least the following: (1) The dilemma of known benefit of high P content (e.g., 50 ppm) on capacitance and known drawback of high P content to leakage is resolved. Increasing Si content allows usage of higher P contents with high capacitance and low leakage. (2) Silicon doping alone can provide enhanced capacitance of the tantalum powder. Capacitance of the P/Si doped tantalum powders is enhanced compared to P doping per se. P doping or equivalent is referred to herein as "primary" capacitance enhancing dopant, the word "primary" being arbitrary and not a measure of relative volume inclusion or relative benefit. (3) The limited window of opportunity of the state of the prior art of P dopant alone (high temperature (80°C or higher) anodization enabling high P content/high capacitance-with-acceptable-leakage) is expanded to allow anodization at lower, as well as high, temperatures on the order of 40-90°C, as a workable range. (4) The silicon itself functions as a powerful sinter retardant and it is stably maintained (nonvolatile) at the preferred industrial conditions of 1 400°C - 1 600°C sinter temperatures. Surface area and intrinsic capacitance are maintained more effectively. This is related to point (2) above. In turn, this enables a reduction of P content, with the P/Si combined dopant system providing a more effective control of tantalum powder properties. The leakage reducing silicon dopant can be combined effectively with capacitance enhancing dopants other than phosphorus. However, it is believed that optimum results are realized in the silicon/phosphorus combination. The invention can also be applied to other valve metal powders of capacitor grade including niobium, titanium, zirconium and alloys thereof with each other and/or tantalum. The following non-limiting examples are presented. It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the foregoing disclosure and within the scope of this invention as defined by the appended claims. FIGS. 1 and 2 are curves of leakage vs. time results for powders made in accordance with preferred embodiments of the invention and comparative examples. The tables shown in the following contain data for both powders made in accordance with the invention and comparative examples. Tantalum primary/secondary powders were produced as described above with these specifics of silicon and phosphorus content: (1) Several samples had 20 ppm P and others had 40 ppm P, these levels being established in both instances, primarily after reduction and leaching and before agglomeration (to secondary powder form) and deoxidation. The presintering conditions were as set forth in Table I below. Silicon introduction was made by addition of Si 3 N 4 to the reduction charge in amounts shown in Table I (either 500 or zero) where 500 ppm is added and the resultant tantalum powder has over 250 ppm of retained silicon after processing losses. (2) Sintered pellets (1 gram, pressed to green density of 4.5 gm/cc and sintered for thirty minutes at sinter temperatures of Table 1) were produced from the secondary powders and anodized at 60°C in 0.01 (vol.%) phosphoric acid under an electrical schedule of 60 ma/gm to 70V with a four hour hold at 70V. (3) The anodized pellets were tested for capacitance, leakage and breakdown voltages in wet cells. These results were obtained: (a) capacitance: 10 (vol.%) H 3 PO 4 /22°C bath, 120 cycles, 0.5 test voltage, GenRad 1658 RLC bridge ("Digibridge") instrument; (b) D.C. leakage: 10 (vol.%) H 3 PO 4 /22°C bath, 49 V D.C., value at five minutes; (c) breakdown: 0.1 (vol.%) H 3 P 4 /60°C bath, 60 ma/gm, average of five pellets' breakdowns. Sinter [°C] T/Sample T[°C] of Presinter P level [ppm] Si [ppm] CV/gm DCL VBD 1 500 1 1 375 20 500 22 100 0,193 134 1 500 2 1 375 40 500 23 100 0,273 133 1 500 3 1 375 40 0 25 200 0,599 130 1 500 4 1 300 40 500 23 900 0,318 132 1 500 5 1 375 40 500 24 700 0,376 136 1 500 6 1 375 20 0 22 900 0,442 134 1 500 7 1 300 20 0 26 800 0,722 123 1 500 8 1 300 40 0 26 800 0,732 135 1 500 9 1 300 40 0 25 700 0,667 134 1 500 10 1 300 20 500 27 800 0,640 125 1 500 11 1 300 20 500 26 400 0,796 124 1 500 12 1 375 20 0 24 200 0,416 137 1 600 13 1 375 20 500 13 300 0,169 181 1 600 14 1 375 40 500 13 800 0,211 185 1 600 15 1 375 40 0 14 900 1,69 172 1 600 16 1 300 40 500 13 700 0,231 191 1 600 17 1 375 20 500 14 700 0,262 183 1 600 18 1 375 20 0 12 700 0,147 178 1 600 19 1 300 20 0 15 100 0,211 176 1 600 20 1 300 40 0 15 100 2,17 181 1 600 21 1 300 40 0 13 900 2,60 188 1 600 22 1 300 20 500 16 000 0,191 160 1 600 23 1 300 20 500 15 200 0,144 168 1 600 24 1 375 20 0 14 400 0,125 185 The capacitors and leakage unit are microfarad volts per gram and nanoamperes per microfarad volt. Each expression of capacitors and leakage is an average of results for four pellets. EXAMPLE 2 Similar experimental processing, compared to Example 1, above, with varied parameters were conducted to evaluate leakage effects further for tantalum powders with and without (500 ppm) silicon in tantalum powders sintered at 1 600°C: Leakage 500 ppm Si/Sample 0 Si /Sample 20 ppm P 0,144 25 0,125 31 0,191 26 0,261 32 0,169 27 0,147 33 40 ppm P 0,262 28 2,60 34 0,231 29 2,17 35 0,211 30 1,69 36 EXAMPLE 3 The Example 2 work on 40 ppm F samples was extended with modifications of silicon addition schedule with the results shown in Table III. Sinter T/Sample Si DCL [%] [ppm] 1 500 37 500 0,187 1 600 38 500 0,226 1 500 39 500 0,414 1 600 40 500 0,469 1 500 41 500 [*] 0,337 1 000 42 500 [*] 0,590 1 500 43 500 0,266 1 600 44 500 0,321 1 500 45 500 [*] 0,266 1 600 46 500 [*] 0,501 1 500 47 250 0,359 1 000 48 250 0,716 1 500 49 250 0,313 1 600 50 250 0,689 1 500 51 125 0,283 1 600 52 125 0,476 1 500 53 125 2,91 1 600 54 125 8,71 1 500 55 0 4,89 1 600 56 0 20,92 EXAMPLE 4 The work of the foregoing examles was extended at 60 ppm P and Si additions of 0, 125, 250 and 500 ppm to produce the leakage data shown in FIGS. 1 and 2. EXAMPLE 5 The capacitance-related effects were established for two series of reduction produced tantalum powders in various concentrations of Si and P, formed in a first series (A) at 60°C and in a second series (B) at 80°C, capacitance being expressed in specific capacitance units of microfarad-volts per gram. A. 60°C Formations Sample Si Conc. Capacitance [ppm] 0 ppm P 20 ppm P 1 500 1 600 1 500 1 600 57 500 24 800 12 500 26 500 1 600 58 500 22 100 11 800 25 600 15 800 59 500 21 700 11 500 25 800 15 500 60 500 23 500 11 800 25 200 14 500 61 500 22 700 11 600 25 600 14 200 62 250 21 600 11 000 26 000 14 900 63 125 20 500 11 300 24 600 14 400 64 0 15 900 9 720 22 200 12 900 B. 80°C Formations Sample Si Conc. Capacitance [ppm] 0 ppm P 40 ppm P 65 500 19 800 10 400 21 700 13 400 66 500 19 300 10 500 21 400 14 000 67 500 18 400 10 100 20 700 13 600 68 500 10 700 9 450 21 700 13 800 69 500 17 800 10 100 20 400 13 300 70 250 18 300 9 720 22 900 14 300 71 125 17 500 9 780 22 100 13 800

Claims: Capacitor grade tantalum powder which is to be sintered into a porous anode and electrolytically formed, the powder being of such size, configuration and composition so that the sintered at 1.400° C or higher and formed anodes can achieve a high specific capacitance (CV/gm) in excess of 15.000 CV/gm, wherein the tantalum powder contains dopant of silicon and phosphorous in proportions of silicon: 50 - 1.000 ppm relative to tantalum, and phosphorous: 10 - 300 ppm relative to tantalum the powder having such a combined concentration of silicon and phosphorous therein that the powder is electrolytically formable at any formation temperature substantially throughout the range of 40° C - 90° C, consistent with affording said high specific capacitance, and a D.C. leakage (UCV) of less than 0.5 nanoampere per microfarad-volt. Tantalum powder in accordance with claim 1 wherein the silicon content is 100 - 500 ppm and the phosphorous content is 20 - 80 ppm relative to tantalum. The powder of claim 1 as formed into an agglomerated powder. An anodic pellet formed of the powder of claim 3. The pellet of claim 4 as formed at a temperature in the range 40° C - 60° C to form an anode. The anode of claim 5 as formed in a dilute mineral acid. Method of producing high capacitance, low leakage capacitor grade tantalum powder comprising the step of providing phosphorous as a primary capacitance enhancing dopant in an amount from 10 to 30 ppm relative to tantalum and, as an interacting further dopant silicon in an amount from 50 to 1.000 ppm relative to tantalum, the silicon and phosphorous being provided in such combined concentration to effect a specific D.C. leakage (L/CV) of such powder of less than 0.5 nano-amperes per microfarad-volt and high specific capacitance (CV/g) in excess of 15.000 in anodized pellets as made therefrom and sintered at 1.400° C or higher, after anodic formation at temperatures selected in the range 40° C - 60° C. Method in accordance with claim 7 wherein the valve metal powder is produced from a salt source by reduction of a melt of such salt and the silicon is provided at the reduction stage. Method in accordance with claim 8 wherein the primary capacitance enhancing agent is added to the powder at a stage after reduction. Method of producting a high capacitance low leakage anode comprising electrolytic formation at 40° C - 60° C of a sintered pellet of powders of tantalum in a dilute mineral acid, the powder comprising mixed dopants in the ranges: (a) Silicon - 50 to 1.000 ppm, and (b) Phosphorous - 10 to 300 ppm, relative to tantalum.

Inventors: HILDRETH RICHARD W, TRIPP TERRANCE B, HILDRETH, RICHARD W., TRIPP, TERRANCE B.
Applicants: STARCK H C INC, H.C. STARCK INC.
Application year: 1989
Main classification: H01G9
IPC codes: B22F9, H01G9, C22C1