Capacitor grade tantalum powder

Abstract

Tantalum powders of capacitor grade are provided, containing interacting silicon and phosphorous dopants to effect low D.C. leakage of electrolytic capacitors having anodes made from such powders, with anodic formation at low temperatures (40.degree.-60.degree. C.), consistent with high capacitance.

Description

BACKGROUND OF THE INVENTION

The present invention relates to capacitors and tantalum powders of capacitor grade, used in production of 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. leadage, particularly in anodes formed (oxidized) at lower temperatures, e.g. 60.degree. c.

It is a principal object of the present invention to provide an improved capacitor grade tantalum powder affording practical high capacitance at low leadage and to produce anodes therefrom, formable at over 40.degree. C. and throughout the range 40.degree.-90.degree. C.

That is, the anode is to be formable at any temperature in such range consistent with high capacitance and low leakage (whereas prior art items have unacceptably high leakage at the lower portion of such range). The terms "form", "forming" "formable" and the like refer to the well known wet process of anodic oxidation to establish a tantalum oxide surface film as a capacitor dielectric element at the surface of the tantalum powder particles.

SUMMARY OF THE INVENTION

The object of the invention is achieved in a multi-dopant 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 sq. cm./gm.), tantalum powder. The phosphorous enhances capacitance and the silicon suppresses leakage normally associated with high phosphorous doping levels and also enhances capacitance. The silicon can be added at various stages of tantalum production but is preferrably added during reduction of a tantalum precursor (e.g., the standard method of K.sub.2 TaF.sub.7 reduction by Na reducing agent). Phosphorous content is more flexibly established at various stages, but preferrably after reduction.

The preferred ranges are 50-1,000 ppm of silicon in relation to tantalum, and 10-300 ppm phosphorous, but more preferrably 100-500 ppm Si and 20-80 ppm P.

The resultant tantalum can be agglomerated, deoxidized, sintered at or over 1,400.degree. C. and anodized at or over 40.degree. c. to form a high specific capacitance, low leakage anode of an electrolyte capacitor (wet or solid electrolytic).

Other objects, features, and advantages will be apparent from the following detailed description of preferred embodiments thereof illustrative of practice thereof and of various aspects of discovery leading to the invention, including certain graphs of performance data shown in the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1 and 2 are curves of leakage vs. time results for powders made in accordance with preferred embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A stirring reduction reactor can be charged with K.sub.2 TaF.sub.7 or Na.sub.2 TaF.sub.7 double salt (tantalum precursor), preferrably 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 preferrably added to the charge prior to or during moltent state reduction. The silicon dopant is preferrably provided in the form of compounds thereof, such as Si.sub.3 N.sub.4 and K.sub.2 SiF.sub.6. The phosphorous can be a component of the K.sub.2 TaF.sub.7 or Na.sub.2 TaF.sub.7 charge or diluent, or added to tantalum after the reduction, as viable alternates or supplements to addition during reduction. Preferrably the phosphorous is added after the reduction stage.

The reduction processing, 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.

Preferrably, the primary powders are agglomerated by presintering to produce sponge-like secondary powders, and deoxidized, both procedures being also well known in the art.

The following non-limiting examples are presented.

EXAMPLE I

Tantalum primary/secondary powders were produced as described above with these specifics of silicon and phosphorous 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.sub.3 N.sub.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.degree. C. in 0.01 (vol. %) phosphoric acid under an electrical schedule of 60 ma/gm to 70 V with a four hour hold at 70 V.

(3) The anodized pellets were tested for capacitance, leakage and breakdown voltages in wet cells.

These results were obtained:

(a) capacitance: 10 (vol.%) H.sub.3 PO.sub.4 /22.degree. C. bath, 120 cycles, 0.5 test voltage, GenRad 1658 RLC bridge ("Digibridge") instrument; (b) D.C. leakage: 10 (vol.%) H.sub.3 PO.sub.4 /22.degree. C. bath, 49 V D.C., value at five minutes; (c) breakdown: 0.1 (vol.%) H.sub.3 PO.sub.4 /60.degree. C. bath, 60 ma/gm, average of five pellets' breakdowns. TABLE I__________________________________________________________________________Sinter T/Sample Temp of Presinter P level Si CV/gm DCL VBD__________________________________________________________________________1,500.degree. C. 1 1,375.degree. C. 20 500 22,100 .193 1341,500.degree. C. 2 1,375 40 500 23,100 .273 1331,500.degree. C. 3 1,375 40 0 25,200 .599 1301,500.degree. C. 4 1,300 40 500 23,900 .318 1321,500.degree. C. 5 1,375 40 500 24,700 .376 1361,500.degree. C. 6 1,375 20 0 22,900 .442 1341,500.degree. C. 7 1,300 20 0 26,800 .722 1231,500.degree. C. 8 1,300 40 0 26,800 .732 1351,500.degree. C. 9 1,300 40 0 25,700 .667 1341,500.degree. C. 10 1,300 20 500 27,800 .640 1251,500.degree. C. 11 1,300 20 500 26,400 .796 1241,500.degree. C. 12 1,375 20 0 24,200 .416 1371,600.degree. C. 13 1,375 20 500 13,300 .169 1811,600.degree. C. 14 1,375 40 500 13,800 .211 1851,600.degree. C. 15 1,375 40 0 14,900 1.69 1721,600.degree. C. 16 1,300 40 500 13,700 .231 1911,600.degree. C. 17 1,375 20 500 14,700 .262 1831,600.degree. C. 18 1,375 20 0 12,700 .147 1781,600.degree. C. 19 1,300 20 0 15,100 .211 1761,600.degree. C. 20 1,300 40 0 15,100 2.17 1811,600.degree. C. 21 1,300 40 0 13,900 2.60 1881,600.degree. C. 22 1,300 20 500 16,000 .191 1601,600.degree. C. 23 1,300 20 500 15,200 .144 1681,600.degree. C. 24 1,375 20 0 14,400 .125 185__________________________________________________________________________

The capacitance and leakage units are microfarad volts per gram and nanoamperes per microfarad volt. Each expression of capacitance and leakage is an average of results for four pellets.

EXAMPLE II

Similar experimental processing, compared to Example I, 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.degree. C.:

TABLE II______________________________________Leakage 500* ppm Si/Sample 0 Si**/Sample______________________________________20 ppm P .144 25 .125 31 .191 26 .261 32 .169 27 .147 3340 ppm P .262 28 .260 34 .231 29 .217 35 .211 30 .169 36______________________________________ **except for incidental impurities, usually 10-30 ppm; i.e., no silicon *500 added, at least half of which is retained in the secondary (agglomerated and deoxidized) pcwder

EXAMPLE III

The Example II work on 40 ppm P samples was extended with modifications of silicon addition schedule with the results shown in Table III.

TABLE III______________________________________Sinter T/Sample Si DCL______________________________________1,500.degree. C. 37 500 .1871,600.degree. C. 38 500 .2261,500.degree. C. 39 500 .4141,600.degree. C. 40 500 .4691,500.degree. C. 41 500 [*] .3371,000.degree. C. 42 500 [*] .5901,500.degree. C. 43 500 .2661,600.degree. C. 44 500 .3211,500.degree. C. 45 500 [*] .2661,600.degree. C. 46 500 [*] .5011,500.degree. C. 47 250 .3591,000.degree. C. 48 250 .7161,500.degree. C. 49 250 .3131,600.degree. C. 50 250 .6891,500.degree. C. 51 125 .2831,600.degree. C. 52 125 .4761,500.degree. C. 53 125 2.911,600.degree. C. 54 125 8.711,500.degree. C. 55 0 4.891,600.degree. C. 56 0 20.92______________________________________

EXAMPLE IV

The work of the foregoing examples 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 V

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.degree. C. and in a second series (B) at 80.degree. C., capacitance being expressed in specific capacitance units of microfarad-volts per gram.

______________________________________A. 60.degree. C. Formations CapacitanceSample Si Conc. 0 ppm P 20 ppm P# PPM 1,500 1,600 1,500 1,600______________________________________57 500 24,800 12,500 26,500 1,60058 500 22,100 11,800 25,600 15,80059 500 21,700 11,500 25,800 15,50060 500 23,500 11,800 25,200 14,50061 500 22,700 11,600 23,600 14,20062 250 21,600 11,000 26,000 14,90063 125 20,500 11,300 24,600 14,40064 0 15,900 9,720 22,200 12,900______________________________________ ______________________________________B. 80.degree. C. Formations CapacitanceSample Si Conc. 0 ppm P 40 ppm P# PPM 1,500 1,600 1,500 1,600______________________________________65 500 19,800 10,400 21,700 13,40066 500 19,300 10,500 21,400 14,00067 500 18,400 10,100 20,700 13,60068 500 10,700 9,450 21,700 13,80069 500 17,800 10,100 20,400 13,30070 250 18,300 9,720 22,900 14,30071 125 17,500 9,780 22,100 13,800______________________________________

The import of the data of the foregoing 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 usuage 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.degree. C.+) 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.degree.-90.degree. C., as a workable range.

(4) The silicon itself functions as a powerful sinter retardant and it is stably maintained (non-volatile) at the preferred industrial conditions of 1,400.degree. C.-1,600.degree. 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 tantalum powder properties.

The leakage reducing silicon dopant can be combined effectively with capacitance enhancing dopants other than phosphorous. However, it is believed that optimum results are realized in the silicon/phosphorous 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.

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

Claims

1. Improved capacitor grade tantalum powder which can be anodically formed to a low leakage product at formation temperatures selected substantially throughout the range 40.degree.-90.degree. C., comprising a content of silicon and phosphorous in combined concentration effecting a specific D.C. leakage (L/CV) of such powder of less than 0.5 nanoamperes per microfarad-volt and high specific capacitance (CV/g) in excess of 15,000 cv/gm in anodized pellets as made therefrom and sintered at 1,400.degree. C. or higher.

2. Tantalum powder in accordance with claim 1 wherein the silicon content is 50-1,000 ppm relative to tantalum and the phosphorous content is 10-300 ppm relative to tantalum.

3. The tantalum powder of claim 1 as formed into an agglomerated powder.

4. Tantalum powder in accordance with claim 2 wherein the silicon content is 100-500 ppm and the phosphorous content is 20-80 ppm relative to tantalum.

5. An anodic pellet made of a sintered mass of the agglomerated powder of claim 3.

6. The pellet of claim 5 as anodically formed at a temperature in the range 40.degree.-60.degree. C.

7. The pellet of claim 6 as formed in a dilute mineral acid.