The relevant literature describes the earth-acid metal (acidic earth metal) tantalum as a starting material for the production of electrolytic capacitors. To produce anodes for such capacitors, tantalum powders are pressed around a semifinished tantalum part, usually a wire or ribbon (band/strip) or a metal sheet, and sintered at high temperatures of typically 1000° C. to 2000° C. in a high vacuum to give a porous sintered body having about 30% to 50% of the theoretical density of tantalum and a high internal surface area. An insulation layer, known as the dielectric, is subsequently produced on the internal surface area of the porous sintered body by anodic oxidation in a usually aqueous electrolyte, e.g., a dilute phosphoric acid, usually at temperatures in the range 50° C. to 90° C. The counterelectrode (cathode) is subsequently applied in the form of a layer of manganese dioxide or a conductive polymer, such as polypyrrole or polythiophene, on the tantalum anode produced in this way. After application of a graphite layer and contacting of the cathode by means of a conductive silver layer, the capacitor is finally embedded in epoxy resin.
A heat treatment (tempering) at 200° C. to 400° C. is usually carried out between the anodic oxidation and the application of the cathode layer in order to relieve stresses and heal defects in the dielectric layer. The application of the manganese dioxide layer is effected by impregnation with manganese nitrate solutions and subsequent pyrolysis at ≧300° C. with elimination of nitrogen oxides, with these steps being repeated a number of times until ideally the entire internal surface area of the anode body is covered. In these steps, the tantalum anodes are subjected to thermomechanical stress which usually leads to a certain reject rate.
Modern tantalum capacitors require not only a high capacity per unit volume, a low equivalent series resistance (ESR), and a low leakage current, but also a high stability towards external stresses. High mechanical stresses occur particularly during the production process during the preparation for the anodic oxidation (e.g., welding of the anode wires onto metal frames) or during the later encapsulation in epoxy resin, and these can lead to failure of capacitors and reduce the process yield.
The tantalum wire present in the tantalum anodes has the sole purpose of providing an electric contact for the capacitor anode. For manufacturing reasons, e.g., to be able to weld the sintered tantalum bodies to the treatment struts for the anodic oxidation, the wire usually has to project at least 10 mm from the sintered tantalum body. Owing to the increasing miniaturization of electronic components and thus also of tantalum electrolytic capacitors, the mass ratio of tantalum powder to tantalum wire is increasingly shifted in the direction of the wire, i.e., the production of relatively small types of capacitors having dimensions of only about 2×1×1 mm requires only a few milligrams of powder but, depending on the wire diameter, a mass of tantalum wire which is a number of times thereof, i.e., the tantalum wire is increasingly becoming the cost-determining factor. To save costs, the manufacturers of such capacitors are therefore continuously making efforts to use ever thinner wires.
This increasingly results, however, in the problem that the tantalum wires lose a great deal of mechanical stability due to embrittlement during anode production or during the further manufacturing steps to produce the capacitor, i.e., they can no longer be bent without breaking. This bending strength is important, however, since the anodes are subjected to mechanical stresses as described above at a number of points in the capacitor production process. This embrittlement of the wires therefore has a tremendous influence on the costs of capacitor production since in the case of unsatisfactory mechanical stability of the wire, i.e., a bending strength which is too low, the entire batch must be downgraded. The importance of this is also indicated by the presence of standards such as the Japanese EIAJ RC-2361A in which a test method for metering the bending strength of the wire is described.
In the production of tantalum capacitors, it has been observed that the extent of wire embrittlement tends to increase when the specific surface area of the tantalum powders used is gradually increased in order to increase the capacity per unit volume of the capacitors. The embrittlement of the tantalum wires in anodes thus increases dramatically, for example, when, instead of powders having a specific surface area of 2 m2/g, powders having a surface area of 3 m2/g and above are used for production of the wires. It has furthermore been observed that relatively thin tantalum wires having diameters of less than 0.3 mm become embrittled more quickly than thicker wires. It has also been observed that the embrittlement of the tantalum wires immediately after sintering is low (i.e., the wires have a high bending strength), but increases greatly after only a short time (some hours) after sintering of the sintered tantalum bodies, especially when the sintered bodies are stored under high atmospheric humidity. It has also been observed that, as a result of the abovementioned heat treatments during capacitor production, e.g., during the anodic oxidation or during heat treatment, the embrittlement of the tantalum wires increases even more or the bending strengths of the wires decrease. The embrittlement can be to such an extent that the tantalum wires of the sintered bodies or of the anodes can break off under the slightest shock even a short time after sintering or anodic oxidation. The effect of embrittlement is worst at the place where the wire enters the sintered body or the anode since the greatest forces act here when bending occurs.
It is known that the earth-acid metals tantalum and niobium can be embrittled by reaction with various gases and vapors at elevated temperature (e.g., 300° C.) even after only a short time. It is also known that embrittlement of the wires can occur as a result of oxygen diffusion during the sintering of relatively fine tantalum powders having oxygen contents of greater than 1600 ppm and when using high sintering temperatures. In order to prevent this, the use of tantalum wires containing dopants, e.g., doping with 10-1000 ppm of rare earths (U.S. Pat. No. 3,268,328), with 10-1000 ppm of yttrium (U.S. Pat. No. 3,497,402), with 50-700 ppm of silicon (U.S. Pat. No. 4,235,629), or with a combination of 50-1000 ppm of silicon and 50-1000 ppm of finely divided metal oxides, has been proposed. The use of these wires brings no advantage, however, over undoped wire in the present case.
It is also known that tantalum can be significantly embrittled by hydrogen in very low concentrations of, for example, a few hundred ppm, even at room temperature since the diffusion rate of hydrogen atoms is significantly higher compared to oxygen or nitrogen. To increase the resistance of tantalum to hydrogen embrittlement, it has been proposed that tantalum be alloyed with elements of the platinum group (WO 2008/134439). These alloys have not, however, become established in the production of tantalum capacitors for cost reasons.
The precise mechanism of the above-described embrittlement of the tantalum wires after production of the sintered bodies or of the anodes or the further processing steps to the capacitor is unknown. Without wishing to be tied to a theory, it is assumed that the embrittlement in these cases occurs as a result of hydrogen which is formed by a reaction of the sintered tantalum bodies or anodes with atmospheric moisture (during storage) or water (e.g., when the sintered bodies are dipped into the electrolyte for anodic oxidation) according to the equation 2 Ta+5H2O→Ta2O5+5 H2.