AU2006203068B2 - Capacitor powder - Google Patents

Description

Our Ref:12690671 P/00/0 I I Regulation 3:2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): H. C. Starck GmbH Im Schleeke 78-91 38642 Goslar Germany Address for Service: DAVIES COLLISON CAVE Patent & Trade Mark Attorneys 255 Elizabeth Street Sydney, New South Wales, Australia, 2000 Invention Title: Capacitor powder The following statement is a full description of this invention, including the best method of performing it known to me: 5051 I - I Capacitor powder The present invention relates to a powder for the production of electrolytic capacitors, especially a powder for the production of anodes for electrolytic 5 capacitors. In the literature, the acid earth metals niobium and tantalum in particular are described as starting materials for the production of such capacitors. The capacitors are produced by sintering of the finely divided powders to pellets to produce a 10 structure having a large surface area, anodic oxidation of the surface of those sintered bodies to produce a non-conducting insulating layer (dielectric), and application of the counter electrode in the form of a layer of manganese dioxide or of a conductive polymer. The particular suitability of acid earth metal powders is derived from the high relative permittivity of the pentoxides. 15 Hitherto, only tantalum powder has gained industrial importance for the production of capacitors. That is based on the one hand on the reproducible producibility of finely divided tantalum powder and, on the other hand, on the fact that the insulating oxide layer of tantalum pentoxide possesses particularly pronounced stability. That 20 is possibly due to the fact that tantalum, unlike niobium, does not form a stable suboxide. In the course of the rapid development of microelectronics, however, disadvantages of tantalum are also increasingly gaining importance. Tantalum is one of the rare 25 metals (54th position in the natural frequency of the elements in the earth's crust with 2.1 g/t) with few mineable deposits (only hard rock mining) and, moreover, it is found in only very small concentrations in its ores. For example, the tantalum ores typically mined today (e.g. in Australia) often contain less than 0. 1 % Ta 2 0 5 (approx. 300 ppm Ta). 30 Niobium, which is in the same group of the PSE above tantalum and is very similar thereto in terms of its behaviour, occurs from 10 to 12 times more frequently than -2 tantalum and its deposits are more favourably mineable (33rd position in the natural frequency of the elements in the earth's crust with 24 g/t).,The most important deposits in commercial terms are in Brazil (78 % of world reserves), where the ore is mined in opencast pits with over 3 % Nb 2 0 5 . Further deposits are to be found in 5 Canada, Nigeria and Zaire. Accordingly, the raw material prices for niobium ore concentrates are markedly lower than for tantalum ore concentrates and, moreover, are not subject to such pronounced fluctuations. Furthermore, there is a natural growth limit to the achievable specific capacitances 10 for tantalum powder. In order to achieve higher capacitances C in the case of Ta powder, the specific surface area must become larger (C = E, * Aid), which at a particular powder particle geometry is accompanied by a reduction in the size of the particles. If the mean particle size, in the case of an anodically produced dielectric layer in the nanometer range, is likewise in the nanometer range, regions of the metal 15 sintered body become "through-anodised", that is to say there is no metallic conductivity between two particles, particularly in thin areas such as, for example, sinter necks. Parts of the anode thus become inactive. Moreover, the sensitivity of tantalum powders to oxidation increases markedly as the 20 size of the powder particles decreases and the specific surface area increases accordingly. For those reasons, and owing to the markedly higher dielectric constants of niobium (Er ~ 42) as compared with tantalum (Er ~ 27), it has been the aim of many 25 researchers to develop niobium capacitors. However, the use of niobium capacitors has hitherto been reserved for the field of low specific capacitances with a small specific surface area and relatively poor quality. One reason therefor is that pure niobium has two disadvantages in comparison with 30 tantalum with regard to capacitor applications. On the one hand, the tendency of the anodically produced oxide film to field crystallisation is more pronounced than in -3 the case of tantalum. The radial growth rate of crystalline surfaces is, in fact, 1000 times greater than in the case of tantalum under the same conditions of anodisation (N.F. Jackson, J.C. Hendy, Electrocomponent Science & Techn. 1974, 1, 27-37), which can, however, for the most part be suppressed by anodisation at a lower 5 temperature (Y. Pozdeev: "Comparison of tantalum and niobium solid electrolytic capacitors" TIC 1997; films must be amorphous, crystalline areas in the film exhibit increased conductivity). The other disadvantage concerns the greater sensitivity of anodically produced Nb 2 0 5 films to heat treatment. 10 One step in the production of solid electrolytic capacitors is the application of the semiconducting cathode material MnO 2 . That is effected by immersing the anode body in manganese nitrate solutions to produce a thin MnNO 3 layer, which is subsequently decomposed thermally to MnO 2 . In that process, the Ta-Ta20 5 system is exposed to temperatures of from 250 to 450*C for from 10 to 30 minutes. Such 15 heat treatment may, however, lead to an increase in the frequency-, temperature- and BIAS-dependence of the capacitance. The cause thereof is considered to be that, at temperatures above 300"C, the tantalum substrate is able to draw oxygen atoms from the anodically produced tantalum oxide layer, which leads to an exponential gradient of areas in the oxide film that lack oxygen. Such lacking areas bring about a change 20 in the conducting behaviour of the oxide film from a dielectric to an n-type semiconductor or, if the lacking areas are present in a sufficiently high concentration, to a conductor. That is shown diagrammatically in Figure 1. The critical conductivity a-o separates the insulating part of the oxide film from the conducting part. If the temperature is increased, the semiconducting layer in the 25 oxide film widens and the effective insulating layer becomes thinner. That causes an increase in capacitance, independently of the temperature-dependence of the dielectric constant. In such a case, the application of an anodic BIAS voltage causes the electrons to move from the lacking areas into the tantalum metal. This results in the formation of an electric double layer, which is defined on the metal side by 30 electrons at the interface and on the semiconductor side by the positive space charge in a boundary layer low in charge carriers (Schottky-Mott barrier). That effects an -4 increase in the gradient of the conductivity gradient and an increase in the effective thickness of the dielectric, which, however, according to C = * A/d, is associated with a reduction in the capacitance. 5 While anodically produced oxide films on tantalum are dielectric and exhibit semiconducting regions only at elevated temperatures, anodically produced oxide films on niobium behave like n-type semiconductors even at room temperature (A.D. Modestov, A.D. Dadydov, J. Electroanalytical Chem. 1999, 460, pp. 214-225) and exhibit a Schottky- barrier at the Nb 2 0 5 /electrolyte interface (K.E. Heusler, M. 10 Schulze, Electrochim. Acta 1975, 20, p. 237; F. Di Quarto, S. Piazza, C. Sunseri, J. Electroanalytical Chem. 1990, 35, p. 99). The reason therefor may be that niobium, in contrast to tantalum, forms various stable suboxides. For example, it is known from the literature that, in the case of oxide films on niobium, only the outer layer consists of Nb 2 0 5 - (M. Grundner, J. Halbritter, J. AppL. Phys. 1980, 51(1), pp. 397 15 405), which, moreover, is not completely stoichiometric in composition and exhibits an oxygen deficiency x. Between the Nb 2 05., layer and the niobium metal substrate there is a layer of NbO, since that is the thermodynamically stable phase in contact with the oxygen-saturated niobium metal and not, as in the case of tantalum, the pentoxide (K.E. Heusler, P. Schliter, Werkstoffe & Korrosion 1969, 20(3), pp. 195 20 199). The oxygen content of the passive surface layer in the case of niobium is approximately from 3500 to 4500 ppm per m specific surface area. When Nb anodes are sintered, the oxygen of the passive surface layer diffuses into the inside of 25 the metal and is uniformly distributed therein. In that process, the thickness of the NbO layer also increases proportionally to the surface area of the powder used, which can very readily be followed on sintered niobium anodes by means of X-ray diffraction. In an extreme case, with very high specific surface areas and accordingly very high oxygen contents in the powder, the result is that the anode body consists 30 mainly of NbO after sintering and not of niobium metal. In contrast to tantalum, -5 however, that oxygen increase does not manifest itself in a significant rise in the residual current of anodes made of such powders. A further point is that the MnO 2 cathode acting as the solid electrolyte acts as an 5 oxygen donor and is able to compensate the oxygen deficit in the Nb 2 0sx layer. That is not a monotonous process, however, since lower, non-conducting manganese oxide phases (Mn 2

O

3 , Mn 3 0 4 , MnO) form in the vicinity of the MnO 2 /Nb 2

O

5 interface and suppress the further diffusion of oxygen from the MnO 2 cathode to the semiconducting Nb 2 0 5 ., layer. That then leads to an increase in the lacking areas x, 10 an accelerated rise in the residual current and, finally, to the failure of the capacitor (Y. Pozdeev on CARTS-EUROPE '97: 11th European Passive Components Symposium). For that reason, niobium capacitors are said to have a markedly shorter life than tantalum capacitors. 15 That semiconducting behaviour of the anodically produced barrier layer on niobium has the result that, in order to measure correct capacitance values for niobium anodes, which are later achieved also in the finished capacitor, a positive BIAS voltage must be applied thereto, since otherwise a meaningful measurement is not possible and values are simulated that are much too high. 20 By comparative measurements of the capacitance of anodes of niobium metal or niobium(II) oxide and also niobium/tantalum alloys (90:10, 80:20, 70:30) and the capacitors produced therefrom, it has been found that the application of a BIAS voltage of > 1.5 V at the anode is necessary in order to measure for the anodes 25 correct capacitance values, which are also found again later in the finished capacitor, and that capacitances of such anodes measured without an applied BIAS voltage are higher by a factor of from 3 to 4 than those measured with a BIAS voltage of at least 1.5 V, that is to say incorrect values are simulated. Accordingly, values are also obtained for the specific residual current that are lower by a factor of from 3 to 4 30 than the actual specific residual current when reference is made to capacitances measured without BIAS.

-6 A very important parameter for the suitability of a powder as capacitor material is its chemical purity, since both metallic and non-metallic impurities can lead to faults in or to reduced stability of the dielectric layer. The elements Na, K, Fe, Cr, Ni and C 5 in particular are to be regarded as critical for the residual current of tantalum anodes. As a result of continuous improvements to Ta powders, such impurities in powders produced by sodium reduction of K 2 TaF 7 are nowadays in the region of the detection limit. 10 By contrast, the corresponding process via K 2 NbF 7 is not available for the production of highly pure niobium powders because, owing to the high aggressivity of the corresponding heptafluoroniobate salts, the retort material is partly dissolved and the niobium powders so obtained are contaminated with large amounts of Fe, Cr, Ni, etc.. So-called EB powders, which are produced by embrittling with hydrogen a 15 niobium ingot melted by means of an electron beam, grinding it and subsequently dehydrating it, are also not suitable for the production of high-capacitance Nb capacitors. If the above-described grinding is carried out in an attritor under, for example, alcohols, niobiurn flakes are obtained which, however, in most cases contain a very high degree of metallic impurities, such as Fe, Cr, Ni and C, which 20 are trapped in the niobium powder during the grinding operation by mechanical alloying and cannot be washed out later with mineral acids. However, a very high degree of purity is exhibited by the niobium powders obtained by published proposals of the Applicants according to DE 19831280 Al or 25 WO 00/67936 by the two-stage reduction of niobium pentoxide with hydrogen or gaseous magnesium. Such powders contain, for example, metallic impurities such as Fe, Cr, Ni, Al, Na, K in amounts < 25 ppm. In addition to chemical purity, which is of decisive importance for the electrical 30 properties, a capacitor powder must also meet some requirements in respect of physical properties. For example, it must have a certain flowability, so that it can be -7 processed using the capacitor manufacturers' fully automated anode presses. Furthermore, a certain green strength of the pressed anode bodies is necessary so that they do not immediately fall apart again, and a sufficiently high pore distribution is required in order to ensure complete impregnation with manganese nitrate. 5 The present invention seeks to overcome the above-described disadvantages of the known capacitors based on niobium. In particular, the present invention seeks to improve the insulating behaviour and the thermal stability of the niobium pentoxide barrier layer of capacitors based on niobium in such a manner that longer lives 10 associated with higher capacities and lower residual currents can be achieved for such capacitors. In particular, it has been found with the aid of impedance spectroscopic measurements and evaluation of Schottky-Mott diagrams that the concentration of 15 lacking areas in anodically produced oxide layers of such capacitor anodes is markedly reduced and similarly low as in corresponding Ta 2 0 5 layers. Moreover, there are the first signs of long-term stability comparable with that of tantalum anodes, which cannot be achieved with conventional capacitors based on niobium. 20 Capacitors based on niobium within the context of the present invention are capacitors having an anode of sintered finely divided powders based on niobium, "based on niobium" including electrically conductive compounds and alloys whose principal component is niobium, as well as niobium metal. Suitable compounds are, for example, niobium oxides NbOx wherein x = from 0 to 2, niobium nitride, or 25 -8 niobium oxynitrides. Suitable niobium alloys are especially Nb/Ta alloys having a niobium content of at least 50 wt.%. Preference is given according to the invention to niobium metal (having a 5 preparation-dependent oxygen content of from 3000 to 4500 ppm per m 2 specific surface area) and NbO, wherein x = from 0.8 to 1.2. Further preferred capacitors based on niobium have a niobium core, a niobium suboxide intermediate layer and a niobium pentoxide dielectric. 10 The vanadium content of such capacitors based on niobium is preferably from 10 to 100,000 ppm (mass), based on niobium and, optionally, tantalum. The content of vanadium is especially preferably from 200 to 20,000 ppm. 15 The capacitor anodes according to the invention based on vanadium-containing niobium are distinguished by a substantially bias-independent capacitance, that is to say an anodically connected direct voltage on which the alternating voltage is superimposed for measurement of the capacitance. The invention also provides capacitor anodes based on niobium that contain vanadium. The anodes preferably 20 contain from 10 to 100,000 ppm vanadium, based on niobium and, optionally, tantalum. The vanadium content is especially preferably from 200 to 20,000 ppm. The invention also provides powders based on niobium that contain from 10 to 100,000 ppm, preferably more than 200 ppm, especially preferably from 500 to 25 20,000 ppm, vanadium, based on niobium and, optionally, tantalum. Also preferably, the powders based on niobium have impurity contents of Fe, Cr, Ni, Al, Na and K in amounts of less than 25 ppm in each case, especially preferably of less than 100 ppm in total. 30 - 9 The invention also provides a preferred process for the preparation of the powders according to the invention based on niobium and doped/alloyed with vanadium. The process consists in mixing vanadium, a vanadium oxide, or a vanadium compound that can be hydrolysed or decomposed thermally to vanadium oxides, in solid or 5 dissolved form, with Nb, Nb 2 0 5 , NbO 2 or niobium oxide hydrate Nb 2 0 5 * x H 2 0 powder in the appropriate ratio, calcining the mixture, optionally after it has been dried, and then reducing the mixed oxide to the suboxide or metal and, optionally, carrying out nitridation. 10 Suitable vanadium compounds are all vanadium oxides such as V 2 0 5 , V0 2 , V 2 0 3 and VO, with V 2 0 5 being especially preferred. Also suitable are all vanadium compounds that can be hydrolysed or decomposed thermally to oxides, such as, for example, amnmonium metavanadate, vanadium(IV) oxide acetylacetonate, vanadium(IV) oxide sulfate pentahydrate, vanadium halides, etc.. Ammonium 15 metavanadate is especially preferred. There are used as the niobium component preferably finely divided powders of Nb 2 0 5 , especially preferably niobium oxide hydrate Nb 2 0 5 * x H 2 0. 20 Suitable niobium powders are especially highly porous powders which have been obtained by published proposals of the Applicants according to US 6,171,363 B1, DE 19831280 Al, DE 19847012 Al and WO 00/67936 by reduction of niobium pentoxide in liquid or gaseous magnesium, optionally after previous reduction to the suboxide by means of hydrogen. Such niobium metal powders are obtained with 25 extremely low contents of impurities that are harmful for capacitor applications. If the calcination with vanadium oxides is carried out using Nb 2 0 5 or Nb 2 0 5 * x

H

2 0, the reduction for the preparation of vanadium-containing NbO 2 can be carried out by heating in a hydrogen atmosphere at from 950 to 1 500"C. 30 - 10 The reduction of vanadium-containing NbO 2 or Nb 2 0 5 to the vanadium-containing Nb metal powder is preferably carried out according to DE 19 831 280 Al, DE 19 847 012 Al or PCT/US 99/09 772. Preference is given to reduction by means of magnesium vapour under a protecting gas atmosphere in a fixed bed. The reduction 5 takes place especially preferably in a fluidised bed with argon as the carrier gas, the carrier gas being passed, before being introduced into the fluidised-bed reactor, over a magnesium melt at a temperature close to the boiling temperature of the magnesium. 10 The reduction of the calcination product of Nb 2 0 5 or Nb 2 0 5 * x H 2 0 and vanadium oxide or V-containing NbO 2 to vanadium-containing NbO may also advantageously be effected by mixing the vanadium-containing NbO 2 or Nb 2 0 5 powder with Nb metal powder (preferably also containing vanadium) and subsequently heating in a stream of hydrogen or in vacuo at temperatures of from 950 to 1600 0 C. 15 In order to prepare nitridated or oxynitridated vanadium-containing powders based on niobium, the vanadium-containing niobium metal powder or NbO, powder, preferably wherein x = 0.1 ... 0.5, is heated in a nitrogen-containing atmosphere at up to 1000*C. 20 The capacitor powders according to the invention based on niobium preferably have primary particle sizes of from 300 to 1500 nrm, especially preferably from 400 to 600 nm. The powders are preferably used in the form of agglomerates having particle sizes from 40 to 400 tim. In order to adjust the agglomerate size, it is possible to 25 carry out deoxidising agglomeration in a manner known per se by heating at from 800 to 1000 0 C in the presence of a small amount of magnesium, followed by grinding through a sieve having a mesh size of from 250 to 400 Pm. Further processing to capacitor anodes is effected by pressing and sinterirg at from 30 1050 to 1350 0 C to a sintered density of from 50 to 70 % volume ratio.

- 11 Further processing of the anodes to capacitors is effected by "anodisation", that is to say electrochemical generation of the pentoxide layer in an electrolyte, such as dilute phosphoric acid, to the desired anodisation voltage, from 1.5 to 4 times the desired working voltage of the capacitor. 5 - 12 Test conditions The production, forming and measurement of the anodes described in the following Examples were carried out according to the following parameters, unless expressly 5 stated otherwise in the Examples. Anode production: Weight without wire: 0.072 g Diameter: 3.00 mm 10 Length: 3.25 mm Compressed density: 3.14 g/cm 3 Anode sintering: 1250 0 C 20 minutes 15 1450 0 C 20 minutes 1600*C 20 minutes Anodisation: Forming voltage: 40 V or 60 V (see Examples) 20 Forming current: 100 mA/g Complete forming time: 2 h or 4 h (see Examples) Electrolyte: 0.1 % H 3

PO

4 (conductivity 25 0 C: 2.5 mS/cm) Temperature: 80 0 C 25 Measurement of capacitance: Electrolyte: 18 % H 2 SO4 Temperature: 23 0 C Frequency: 120 Hz BIAS: 1.5 V (where applied) 30 - 13 Measurement of residual current: Electrolyte: 18 % H 2 SO4 Temperature: 23 0 C Charging time: 2 minutes 5 Voltage: 70 % of the forming voltage (28 or 42 V) - 14 Example 1 A niobium-vanadium alloy containing 0.894 % vanadium was prepared as follows: 897.9 g of niobium oxide hydrate Nb 2 0 5 * x H 2 0 (Nb 2

O

5 content 63.09 %) were 5 intimately mixed with 9.2 g of ammonium metavanadate NH 4

VO

3 dissolved in 1000 ml of H 2 0, then dried for 24 hours at 1 10 C and then calcined for 3.5 hours at 950'C. The Nb 2 0 5 x H 2 0 had a primary particle size of approximately 600 run. The mixed oxide so prepared was then introduced into molybdenum boats and maintained at 1450*C for 4 hours under a slowly flowing hydrogen atmosphere. The 10 suboxide so obtained had the composition NbO 2 (X-ray diffraction) and contained 0.716 % vanadium. The product was then placed on a fine-mesh grid beneath which there was arranged a crucible containing magnesium in a 1.1 times stoichiometric amount, based on the oxygen content of the suboxide, and heated for 6 hours at 1000'C under argon. During that time, the magnesium evaporated off and reacted 15 with the suboxide located above it. After cooling the oven to room temperature, air was slowly supplied for passivation of the metal surface. The reduction product was subsequently washed with sulfuric acid and then washed neutral with demineralised water and dried. 20 Analysis of the niobium powder gave a content of: V of 8940 ppm 0 of 15,000 ppm (3289 ppm/m 2 ) N of 342 ppm Mg of 190 ppm 25 C of 33 ppm Fe, Cr, Ni, Al, Ta each < 20 ppm The specific surface area of the powder according to BET was 4.56 m 2 /g. In the X ray diffraction, reflexes displaced only slightly at relatively small angles were to be detected for niobium, which indicates a solid solution of vanadium in niobium. 30 - 15 From that Nb-V alloy powder (powder A) and from a niobium powder prepared by published proposals of the Applicants according to DE 19831280 Al (powder B, comparison sample) and having the following contents: V of< I ppm 5 0 of 16,000 ppm (3883 ppm/m 2 N of 180 ppm Mg of 300 ppm , (Fe, Cr, Ni) < 15 ppm C 14 ppm 10 and having a specific surface area according to BET of 4.32 m 2 /g, anodes were produced, sintered at I150*C and formed at 40 V. The specific surface area according to BET (Quantasorb) was determined as 0.83 + 0.2 m 2 /g on a number of anodes prior to forming. 15 Tables I and 2 show the measurement of the specific capacitances for anodes of the two powders in various electrolytes at various BIAS voltages: Table I Measurement in 18 % H 2 SO4 Electrolyte Powder A Powder B BIAS 0 V 2 V 4 V 0 V 2 V 4 V meas. capacitance p.F 241 241 241 881 238 235 spec. capacitance pFV/g 133889 133889 133889 489444 132222 130556 meas. residual current pA 2.97 3.04 spec. residual current 0.31 0.31 0.31 0.09 0.32 0.32 nA/.FV 20 - 16 Table 2 Measurement in 10 % H 3 P0 4 Electrolyte Powder A Powder B BIAS 0 V 2 V 4 V 0 V 2 V 4 V meas. capacitance pF 159 159 159 559 151 149 spec. capacitance pFV/g 88333 88333 88333 310556 83889 82778 meas. residual current pA 2.72 2.81 spec. residual current 0.43 0.43 0.43 0.13 0.47 0.47 nA/pFV Example 2 5 A niobium-tantalum-vanadium alloy containing 1.26 % vanadium was prepared as follows: 1104.3 g of niobium oxide hydrate Nb 2 0 5 * x H 2 0 (Nb 2 0 5 content 67.1 %) were intimately mixed with 322.7 g of tantalum oxide hydrate Ta 2 0 5 * x H 2 0 (Ta 2 OS content 75.4 %) and 28.93 g of ammonium metavanadate NH 4

VO

3 , then 10 dried for 24 hours at I 10*C and then calcined for 12 hours at I 150*C. The mixed oxide so prepared was then introduced into molybdenum boats and maintained at 1500*C for 6 hours under a slowly flowing hydrogen atmosphere. The suboxide so obtained had the composition NbO 2 (X-ray diffraction shows reflexes displaced only at relatively small angles for NbO 2 ) and contained 21.13 % tantalum and 1.05 % 15 vanadium. The product was then placed on a fine-mesh grid beneath which there was arranged a crucible containing magnesium in a 1.2 times stoichiometric amount, based on the oxygen content of the suboxide, and heated for 4 hours at 1050'C under argon. During that time, the magnesium evaporated off and reacted with the suboxide located above it. After cooling the oven to room temperature, air was 20 slowly supplied for passivation of the metal surface. The reduction product was subsequently washed with sulfuric acid and then washed neutral with demineralised water and dried. Analysis of the Nb/Ta/V alloy powder gave a content of: 25 Ta of 24.33 % -17 V of 12,600 ppm 0 of 12,325 ppm (3322 ppm/m2 N of 92 ppm Mg of 45 ppm 5 C of 24 ppm Fe, Cr, Ni, Al each <20 ppm The specific surface area of the powder according to BET was 3.71 m 2 /g. In the X ray diffraction, reflexes displaced only slightly at relatively small angles were to be detected for niobium, which indicates a solid solution of tantalum and vanadium in 10 niobium. From that Nb-Ta-V alloy powder (powder A) and from a niobium-tantalum alloy powder prepared analogously but without the addition of ammonium metavanadate (powder B, comparison sample) and having the following contents: 15 Taof22.14% V of< I ppm 0 of 13,120 ppm (3390 ppm/m 2 ) N of 112 ppm Mg of 67 ppm 20 Z (Fe, Cr, Ni) < 15 ppm C 41 ppm and having a specific surface area according to BET of 3.87 m 2/g, anodes were produced, sintered at 1200*C and formed at 40 V. The specific surface area according to BET (Quantasorb) was determined as 0.91 + 0.4 m 2 /g on a number of 25 anodes prior to forming. Tables 3 and 4 show the measurement of the specific capacitances for anodes of the two powders in various electrolytes at various BIAS voltages: 30 - 18 Table 3 Measurement in 18 % H 2 SO4 Electrolyte Powder A Powder B BIAS 0 V 2 V 4 V 0 V 2 V 4 V meas. capacitance pF 379 379 379 1319 372 367 spec. capacitance pFV/g 210556 210556 210556 732778 206667 203889 meas. residual current pA 7.0 8.4 spec. residual current 0.46 0.46 0.46 0.16 0.56 0.57 nA/pFV Table 4 5 Measurement in 10 % H 3 PO4 Electrolyte Powder A Powder B BIAS 0 V 2 V 4 V 0 V 2 V 4 V meas. capacitance pF 237 237 237 859 231 227 spec. capacitance iFV/g 131667 131667 131667 477222 128333 126111 meas. residual current pA 6.2 6.5 spec. residual current 0.65 0.65 0.65 0.19 0.70 0.72 nA/pFV Example 3 A niobium(II) oxide powder doped with vanadium (powder A) was prepared as 10 follows: 657.3 g of a niobium-vanadium alloy powder prepared according to Example I and having the following purity and physical properties: V of 6047 ppm O of 14,500 ppm Mg of 380 ppm 15 C of 44 ppm E (Fe, Cr, Ni, Al, Ta) < 25 ppm N of 79 ppm - 19 specific surface area according to BET 4.34 m 2 /g, apparent density according to Scott 14.3 g/inch 3 , flowability according to Hall Flow 22 s, particle size determination according to Mastersizer DIO = 65.1, D50 = 170.7, D90 = 292.7 pm were intimately mixed with 566.5 g of niobium pentoxide Nb 2 0 5 < 45 pm having 5 the following contents: E (Al, As, Ca, Co, Cr, Cu, Fe, Ga, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Si, Sn, Ta, Ti, W, V, Zr) < 25 ppm C < 10 ppm S < 10 ppm 10 and placed into a molybdenum boat. The latter was then heated for 6 hours at 1250"C under weakly flowing hydrogen. The resulting product (sample A) had the composition NbO and had the following contents and physical properties: V of 3110 ppm O of 14.71 % 15 Mg of 90 ppm C of 14 ppm Z (Fe, Cr, Ni, Al, Ta) < 15 ppm N of45 ppm specific surface area according to BET 2.31 m 2 /g, apparent density according to 20 Scott 13.9 g/inch , flowability according to Hall Flow 29 s, particle size determination according to Mastersizer D10 = 22.3, D50 = 123.4, D90 = 212.7 pm. In an analogous manner there was prepared, as comparison sample, from a niobium powder prepared by published proposals of the Applicants according to DE 19831280 Al and having the following contents and physical properties: 25 V< l ppm O of 13,200 ppm Mg of 386 ppm C of 47 ppm E (Fe, Cr, Ni, Al, Ta) < 25 ppm 30 N of 84 ppm -20 2 specific surface area according to BET 4.01 m /g, apparent density according to Scott 13.6 g/inch 3 , flowability according to Hall Flow 30 s, particle size determination according to Mastersizer D10 = 44.7, DSO = 156.2, D90 = 283.9 pm and a niobium pentoxide Nb 2 0 5 < 45 pm having the following contents: 5 Z (Al, As, Ca, Co, Cr, Cu, Fe, Ga, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Si, Sn, Ta, Ti, W, V, Zr) < 25 ppm C < 10 ppm S < 10 ppm an undoped niobium(II) oxide NbO (powder B) having the following purity and 10 physical properties: V < I ppm 0 of 14.62 % Mg of 54 ppm C of 14 ppm 15 Z (Fe, Cr, Ni, Al, Ta) < 20 ppm N of 56 ppm specific surface area according to BET 2.47 m 2 /g, apparent density according to Scott 13.6 g/inch 3 , flowability according to Hall Flow 30 s, particle size determination according to Mastersizer D10 = 27.7, D50 = 131.9, D90 = 221.1 pm. 20 Anodes were pressed from the two powders, sintered for 20 minutes at 1350*C and formed at 40 V. Table 5 shows the measurement of the specific capacitances for anodes of the two powders in 18 % H 2

SO

4 at various BIAS voltages: Electrolyte Powder A Powder B BIAS 0 V 2 V 4 V 0 V 2 V 4 V meas. capacitance pF 346 346 346 1261 349 341 spec. capacitance pFV/g 192222 192222 192222 700556 193889 189444 meas. residual current pA 1.1 1.3 spec. residual current 0.08 0.08 0.08 0.03 0.09 0.10 nA/iFV -21 Example 4 Niobium powders having various vanadium contents were prepared according to Example I (see Table below, powders 2 to 6). From those powders and from a 5 niobium powder prepared according to DE 198 31 280 Al (see Table below, powder 1), anodes were produced, sintered at I I50"C and then formed at 40 V. The Table below shows the results of the capacitance measurements for anodes of all six powders, which were carried out without and with 2.0 V applied BIAS voltage. 10 1 2 3 4 5 6 O ppm 13800 12000 15100 14800 15300 13200 N ppm <300 <300 <300 <300 <300 <300 H ppm 225 189 315 237 262 201 C ppm 36 25 29 35 28 31 Z(Fe, Cr, Ni) 9 7 9 6 8 8 Mg ppm 135 195 94 130 160 155 V ppm <1 77 298 371 644 942 BET surface area m 2 /g 4.01 3.39 4.36 4.11 4.21 3.53 meas. capacitance pF 680 400 214 206 209 198 without bias meas. capacitance pF 214 194 205 200 207 198 with bias spec. capacitance pFV/g 119450 107780 113890 111100 115000 110000 meas. residual current pA 4.4 4.2 4.3 4.7 4.1 4.0 spec. residual current 62 58 61 65 57 56 VA/8 -22 Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 5 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia

Claims (14)

1. Capacitor having an anode based on niobium and a barrier layer based on niobium pentoxide with BIAS-independent capacitance, characterised by a content of vanadium at least in the barrier layer. 5

2. Capacitor according to claim I having a vanadium content of from 10 to 100,000 ppm.

3. Capacitor according to claim 2 having a vanadium content of from 500 to 10,000 10 ppm.

4. Capacitor according to any one of claims I to 3, wherein the anode consists of niobium metal, niobium suboxide, niobium nitride, or niobium oxynitride. 15

5. Capacitor according to any one of claims I to 3, wherein the anode consists of niobium-tantalum alloy.

6. Capacitor according to claim 4, wherein the anode consists of a niobium core and a suboxide layer. 20

7. Capacitor according to any one of claims 4 to 6, wherein the anode contains from 10 to 100,000 ppm vanadium.

8. Capacitor according to claim 7, wherein the anode contains from 500 to 10,000 25 ppm vanadium.

9. Capacitor according to any one of claims 4 to 6, wherein the barrier layer contains from 10 to 10,000 ppm vanadium, based on the total anode mass. 30

10. Anode based on niobium with BIAS-independent capacitance. P \WPDOCSkCAB\Specil2691671_H C SwCk_ ISsPA doc.7/4/2E(x9 - 24

11. Anode according to claim 10, containing from 10 to 100,000 ppm vanadium.

12. Anode based on niobium and/or niobium compounds according to claim 10 or 11, containing a surface coating of from 10 to 10,000 ppm vanadium, based on the anode. 5

13. A capacitor substantially as hereinbefore described.

14. An anode substantially as hereinbefore described.