1. Field of the Invention
The present invention relates to a method for producing a tantalum powder suitable for use for a sintered electrolytic capacitor. More particularly, the present invention relates to a tantalum powder which exhibits small shrinkage during sintering, a high specific electrostatic capacity, and improved reliability, e.g., life, over a conventional tantalum powder.
2. Description of the Related Art
It is known, for example, from U.S. Pat. No. 4,356,028, to produce tantalum powder by reducing potassium fluorotantalate or potassium tantalum fluoride salt under the presence of sodium, washing the yielded tantalum powder, heat-treating it in vacuum, then pulverizing it.
Tantalum powder has been recognized as an excellent material for producing sintered electrolytic capacitor. The powder must be of a high purity, i.e., have minimal impurities, since, it is believed, impurities present at the interface between the tantalum-oxide film, i.e., a dielectric material, and the tantalum powder body induce structural defects, increasing the leakage current or causing breakdown of the capacitor. The fact that high purity tantalum powder having minimal impurities is optimal for a capacitor, has continued to be affirmed.
To enhance the specific electrostatic capacity and hence to provide a high capacity tantalum powder, a dopant is currently added to the powder. U.S. Pat. No. 3,825,802 discloses a tantalum powder with a dopant selected from the group consisting of nitrogen silicon, phosphorus, and boron, and a capacitor made of the compressed tantalum powder. U.S. Pat. No. 3,825,802 also discloses mixtures of elements selected from nitrogen, silicon, phosphorus, and boron. A porous body of compressed particles is produced by compressing and sintering particles of a mixture of tantalum and dopant-containing material, e.g., TaN, until the particles are bonded into a rigid porous mass. The dopant material may be combined with a metal e.g., tantalum, capable of electrolytically forming a dielectric film on the surface of the metal.
As mentioned above, U.S. Pat. No. 3,825,802 discloses to use dopants, such as boron and phosphorus. The dopant material(s) disclosed in this patent are used, prior to or during the sintering operation, with a film-forming metal. Sufficient sintering for forming a porous electrode is carried out to diffuse the dopants through the porous mass of the particles.
U.S. Pat. No. 4,009,007 discloses that the electrical capacitance of the tantalum powder is improved by addition of phosphorus to the tantalum powder. It alleges that the electric capacitance is improved by adding phosphorus in an amount smaller than that disclosed in U.S. Pat. No. 3,825,802.
U.S. Pat. No. 4,356,028 states that U.S. Pat. Nos. 3,825,802 and 4,009,007 disclose dopants including phosphorus, but that no significant improvements are observed due to the small phosphorus content. U.S. Pat. No. 4,356,028 discloses a specific capacity of approximately 5000 .mu.FV/g due to the addition of phosphorus to the reaction mix of K.sub.2 TaF.sub.7 and NaCl ("in-situ doping method").
Japanese Unexamined Patent Publication (Kokai) No. 55-113807 discloses to improve the strength of a sintered anode and the specific electrostatic capacity and electrical properties of tantalum powder by means of incorporating one or more members selected from the group consisting of silicon, a calcium salt of silicon compound, and phosphate into the K.sub.2 TaF.sub.7 bearing material. This publication, though not conclusively, describes the reason for the improvement as being that silicon or the like behaves as nuclei during the reduction of K.sub.2 TaF.sub.7 to metallic tantalum. The method disclosed in this publication allegedly attains more uniform distribution of a dopant(s) in the tantalum powder than is attained by the method disclosed in U.S. Pat. No. 3,825,802.
Japanese Unexamined Patent Publication (Kokai) No. 58-71614 discloses a tantalum powder containing up to 0.5% by weight of boron or a boron compound.
Except for U.S. Pat. No. 3,825,802, the prior arts generally disclose an improvement in the relative specific capacity of the tantalum powder and a reduction in the leakage current during a so-called wet, anodic-oxidation process in aqueous electrolyte, for producing dielectric tantalum powder.
The prior patent publications do not recognize that a plurality of dopants, especially those incorporated in the starting material of a reducing operation for forming tantalum powder, leads to an improvement of the dielectric properties of the tantalum powder over a tantalum powder with a single dopant. Especially, the prior publications do not recognize that, when solid electrolyte capacitors produced using their doped tantalum powder are tested for reliability under a life test, their properties greatly deteriorate as compared with those produced using nondoped tantalum powder. In this regard, U.S. Pat. No. 3,825,802 described in Table 1 the life test of capacitors having a relative specific capacity in the range of from approximately 3300 to 3500 .mu.FV/g. Along with recent trends and demands for enhancing the specific capacity of tantalum powder, the life test of powder having a high relative specific capacity has become crucial.