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°-60° 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° 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° C. and throughout the range 40°-90° 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 &#34;form&#34;, &#34;forming&#34; &#34;formable&#34; 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 2  TaF 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° 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). 
     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 2  TaF 7  or Na 2  TaF 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 3  N 4  and K 2  SiF 6 . 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 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 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 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 3  PO 4  /22° C. bath, 120 cycles, 0.5 test voltage, GenRad 1658 RLC bridge (&#34;Digibridge&#34;) 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  PO 4  /60° C. bath, 60 ma/gm, average of five pellets&#39; breakdowns. 
     
                                           TABLE I__________________________________________________________________________SinterT/Sample      Temp of Presinter               P level                   Si  CV/gm                            DCL VBD__________________________________________________________________________1,500° C.1        1,375° C.               20  500 22,100                            .193                                1341,500° C.2     1,375    40  500 23,100                            .273                                1331,500° C.3     1,375    40  0   25,200                            .599                                1301,500° C.4     1,300    40  500 23,900                            .318                                1321,500° C.5     1,375    40  500 24,700                            .376                                1361,500° C.6     1,375    20  0   22,900                            .442                                1341,500° C.7     1,300    20  0   26,800                            .722                                1231,500° C.8     1,300    40  0   26,800                            .732                                1351,500° C.9     1,300    40  0   25,700                            .667                                1341,500° C.10    1,300    20  500 27,800                            .640                                1251,500° C.11    1,300    20  500 26,400                            .796                                1241,500° C.12    1,375    20  0   24,200                            .416                                1371,600° C.13    1,375    20  500 13,300                            .169                                1811,600° C.14    1,375    40  500 13,800                            .211                                1851,600° C.15    1,375    40  0   14,900                            1.69                                1721,600° C.16    1,300    40  500 13,700                            .231                                1911,600° C.17    1,375    20  500 14,700                            .262                                1831,600° C.18    1,375    20  0   12,700                            .147                                1781,600° C.19    1,300    20  0   15,100                            .211                                1761,600° C.20    1,300    40  0   15,100                            2.17                                1811,600° C.21    1,300    40  0   13,900                            2.60                                1881,600° C.22    1,300    20  500 16,000                            .191                                1601,600° C.23    1,300    20  500 15,200                            .144                                1681,600° 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° 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° C.      37          500     .1871,600° C.      38          500     .2261,500° C.      39          500     .4141,600° C.      40          500     .4691,500° C.      41          500 [*] .3371,000° C.      42          500 [*] .5901,500° C.      43          500     .2661,600° C.      44          500     .3211,500° C.      45          500 [*] .2661,600° C.      46          500 [*] .5011,500° C.      47          250     .3591,000° C.      48          250     .7161,500° C.      49          250     .3131,600° C.      50          250     .6891,500° C.      51          125     .2831,600° C.      52          125     .4761,500° C.      53          125     2.911,600° C.      54          125     8.711,500° C.      55          0       4.891,600° 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° 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      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° 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 &#34;primary&#34; capacitance enhancing dopant, the word &#34;primary&#34; 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.+) 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 (non-volatile) 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 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.