Title:
Deoxidation of Valve Metal Powders
Kind Code:
A1


Abstract:
Deoxidation of valve metal powders, in particular of niobium powders, tantalum powders or their alloys, by treating the valve metal powder with calcium, barium, lanthanum, yttrium or cerium as deoxidising agent, and valve metal powders that are distinguished by a ratio of the sum of the contents of sodium, potassium and magnesium to the capacitance of less than 3 ppm/10,000 μFV/g.



Inventors:
Loffelholz, Josua (Langelsheim, DE)
Bartmann, Ulrich (Goslar, DE)
Application Number:
11/574675
Publication Date:
01/17/2008
Filing Date:
08/26/2002
Assignee:
H.C. Starck GmbH & Co. KG (Goslar, DE)
Primary Class:
Other Classes:
75/343
International Classes:
B22F9/22; B22F1/00
View Patent Images:



Primary Examiner:
MAI, NGOCLAN THI
Attorney, Agent or Firm:
POLSINELLI PC (HOUSTON, TX, US)
Claims:
1. 1-10. (canceled)

11. A process for the deoxidation of valve metal powders which comprises deoxidizing valve metal powders with a deoxidizing agent, wherein the deoxidizing agent is calcium, barium, lanthanum, yttrium or cerium.

12. The process according to claim 11, wherein the valve metal powder is a niobium powder, a tantalum powder or a niobium-tantalum alloy powder.

13. The process according to claim 11, wherein the deoxidizing agent is calcium or lanthanum.

14. The process according to claim 11, wherein the deoxidizing agent is calcium and the deoxidation is carried out at a temperature of from 880 to 1050° C.

15. The process according to claim 11, wherein the deoxidizing agent is lanthanum and the deoxidation is carried out at a temperature of from 940 to 1150° C.

16. The process according to claim 12, wherein the deoxidizing agent is calcium and the deoxidation is carried out at a temperature of from 880 to 1050° C.

17. The process according to claim 12, wherein the deoxidizing agent is lanthanum and the deoxidation is carried out at a temperature of from 940 to 1150° C.

18. The process according to claim 11, wherein the deoxidation is carried out in two steps.

19. The process according to claim 11, wherein the valve metal powder obtained by reduction of a valve metal oxide with gaseous calcium, barium, lanthanum, yttrium or cerium is deoxidised.

20. A valve metal powder which comprises the ratio of the sum of the impurities sodium, potassium and magnesium to the capacitance of the valve metal powder is less than 3 ppm/10,000 μFV/g.

21. The valve metal powder according to claim 20, wherein the ratio of the sum of the impurities sodium, potassium and magnesium to the capacitance of the valve metal powder is less than 1 ppm/10,000 μFV/g.

22. The valve metal powder according to claim 20, wherein the valve metal powder is a niobium powder.

23. The valve metal powder according to claim 20, wherein the valve metal powder is a tantalum powder.

24. The valve metal powder according to claim 21, wherein the valve metal powder is a niobium powder.

25. The valve metal powder according to claim 21, wherein the valve metal powder is a tantalum powder.

Description:

The invention relates to a process for the deoxidation of valve metal powders, in particular of niobium powders, tantalum powders or their alloys, by treating the valve metal powder with a deoxidising agent from the group calcium, barium, lanthanum, yttrium and cerium, and to valve metal powders distinguished by a low content of sodium, potassium and magnesium.

Valve metals, which are to be understood as being especially niobium and its alloys, tantalum and its alloys, as well as the further metals of groups IVb (Ti, Zr, Hf, Vb (V, Nb, Ta) and VIb (Cr, Mo, W) of the periodic system of the elements, and their alloys, are widely used in the manufacture of components.

Particular mention is to be made of the use of niobium or tantalum in the manufacture of capacitors, especially of solid electrolyte capacitors. In the manufacture of niobium or tantalum capacitors there are conventionally used as starting material corresponding metal powders, which are first compressed and then sintered in order to obtain a porous body. This body is anodised in a suitable electrolyte, whereby a dielectric oxide film forms on the sintered body. The physical and chemical properties of the metal powders used have a critical influence on the properties of the capacitor. Critical characteristics are, for example, the specific surface, the content of impurities and, as the most important electrical parameters the specific capacitance at a given forming voltage Uf. The specific capacitance is generally given in the unit microfarad*volt per gram (μFV/g).

General trends in circuit design in the electronics industry are towards ever higher clock frequencies at ever lower operating voltages with minimal electric losses. For the solid electrolyte capacitors used in such applications this means that ever lower forming voltages are used and at the same time ever lower leakage currents are required.

Valve metal powders which are to be used in the manufacture of capacitors must therefore meet ever higher demands, with the content of impurities being of great importance. This applies, for example, to the content of oxygen in the valve metal powder, which must not be too high, but also to metallic impurities, which have a decisive influence on the leakage current properties of the capacitor. Such impurities are especially Na, K, Mg, but also C, Fe, Cr, Ni.

However, the impurities Na, K and Mg in particular are introduced during preparation of the valve metal powders owing to the process that is used. Thus, for example, the preparation of tantalum powder is generally still carried out today according to the reduction, known from U.S. Pat. No. 2,950,185, of K2TaF7 with sodium or potassium, which results in high contents of sodium and potassium in the product.

According to U.S. Pat. No. 4,141,720, tantalum powders having a high oxygen and sodium content can be worked up by adding K2TaF7 and alkali halides and heating the reaction mixture. The contents of oxygen, sodium and potassium can be reduced in that manner. However, even the powders so treated have a sodium content of from 10 to 87 ppm and a potassium content of from 112 to 289 ppm.

For the preparation of tantalum powder having a high specific surface and a minimal content of sodium and potassium, U.S. Pat. No. 5,442,978 proposes reducing highly diluted K2TaF7 by the stepwise addition of sodium, the addition being carried out at a high rate. According to Example 1 it is possible in this manner to obtain a tantalum powder having a sodium content ≦3 ppm and a potassium content <10 ppm. However, a deoxidation step is necessary to adjust the oxygen content. To that end, the tantalum powder is mixed with magnesium and then heated, as a result of which magnesium is introduced into the tantalum powder.

In addition to the reduction of fluoride salts of the valve metals with alkali metals, oxides of the valve metals are increasingly being used as starting material recently, which oxides, as described in U.S. Pat. No. 6,558,447 B1, are reduced with gaseous magnesium to form the corresponding valve metal. The content of alkali metal can be kept low in this manner. However, there is an increased introduction of magnesium. In addition, this procedure generally requires a deoxidation step to reduce the oxygen content after the reduction, whereby the magnesium content in the valve metal powder increases further.

Owing to their high ionic conductivity and the formation of crystalline phases with the dielectric layer of amorphous valve metal oxide produced during capacitor manufacture, the impurities sodium, potassium and magnesium cause an increased leakage current in the electric field or on thermal loading during the processing process of the capacitor manufacturer. This is particularly pronounced in the case of the ever thinner valve metal oxide layers of <100 nm which capacitors have today. (1 V forming voltage corresponds, for example, to about 2 nm tantalum oxide film thickness).

The object of the present invention is accordingly to provide an economical process for the preparation of valve metal powders which makes available valve metal powders that are distinguished by a low content of the elements sodium, potassium and magnesium, which are critical for the residual current of a capacitor. During capacitor manufacture, such valve metal powders form very uniform amorphous oxide layers at a high specific charge (>35,000 CV/g).

The object is achieved by subjecting the valve metal powder to a deoxidation step in which a deoxidising agent having low ionic mobility is used.

The invention accordingly provides a process for the deoxidation of valve metal powders, wherein calcium, barium, lanthanum, yttrium or cerium is used as the deoxidising agent.

The process according to the invention permits the preparation of valve metal powders that have a very low content of impurities having high ionic conductivity.

As a result, no crystalline phases form with the resulting valve metal oxide during further processing of such valve metal powders to capacitors, so that defects in the oxide lattice and high residual currents are avoided.

The process according to the invention is suitable for the deoxidation of a wide variety of valve metal powders. Preference is given, however, to the deoxidation of niobium powder, tantalum powder or niobium-tantalum alloy powder, particularly preferably tantalum powder.

Accordingly, the valve metal is preferably tantalum.

According to the invention, calcium, barium, lanthanum, yttrium or cerium is used as the deoxidising agent. Calcium or lanthanum is preferably employed, particularly preferably calcium. The valve metal powder to be deoxidised is mixed with the deoxidising agent.

This mixture of the valve metal powder with the deoxidising agent is heated to a temperature above the melting point of the deoxidising agent. It is preferably heated to a temperature that is at least 20° C. above the melting point of the deoxidising agent used.

If calcium is used as the deoxidising agent, the deoxidation is preferably carried out at a temperature of from 880 to 1050° C., particularly preferably at a temperature of from 920 to 1000° C. When lanthanum is used, the preferred deoxidation temperature is from 940 to 1150° C., particularly preferably from 980 to 1100° C.

The deoxidation is preferably carried out at normal pressure. However, it is also possible to work at a lower pressure. The presence of hydrogen is not necessary in the process according to the invention. The process can be carried out, for example, in vacuo or under inert gas, such as neon, argon or xenon. Nor does the process require a solvent or agent for suspending the solids in a liquid phase, such as, for example, a salt melt, as is conventionally used in the reduction of valve metal compounds to valve metals.

The amount of deoxidising agent added and the treatment time may vary within wide limits and depend especially on the oxygen content of the valve metal powder to be deoxidised and on the deoxidation temperature.

A deoxidation time of from 2 to 6 hours is generally sufficient. Preferably, deoxidation is carried out for from 2 to 4 hours.

There is preferably used a 1.1- to 3-fold stoichiometric excess of deoxidising agent, based on the amount that is theoretically required to reduce the oxygen content to 0. It has been shown that it is generally sufficient to use the deoxidising agent Ca in an amount of from 3 to 6 wt. % and the deoxidising agent La in an amount of from 6 to 14 wt. %, based on the amount of valve metal powder to be deoxidised, in order to achieve the desired lowering of the oxygen content and of the elements sodium, potassium and magnesium. There are preferably used from 3.5 to 5.9 wt. % of deoxidising agent Ca or from 9 to 11.5 wt. % La, based on the amount of valve metal powder to be deoxidised, particularly preferably from 4 to 4.7 wt. % Ca or from 10 to 115 wt. % La.

After the deoxidation, the oxides of the deoxidising agent used that form during the deoxidation are preferably extracted with an acid. The acid used is preferably nitric acid or hydrochloric acid. It is to be noted that the use of sulfuric acid is to be avoided when calcium is used as the deoxidising agent.

The deoxidation according to the invention is preferably carried out in two steps. In this case, further deoxidising agent is added to the valve metal powder after the above-described deoxidation and acid extraction, and the mixture is subjected to the described heat treatment again. The amount of deoxidising agent is chosen to be lower in the second deoxidation step than in the first deoxidation step and preferably corresponds to a stoichiometric excess of from 1.3 to 2.0, based on the amount of oxygen in the valve metal powder. The deoxidising agent is used in the second deoxidation step preferably in an amount of from 1 to 3 wt. % when Ca is used as the deoxidising agent and in an amount of from 1.5 to 7 wt. % when La is used, based on the amount of valve metal powder to be deoxidised. Preferably, from 1 to 1.3 wt. % Ca or from 3 to 6.1 wt. % La are used as the deoxidising agent, based on the amount of valve metal powder to be deoxidised.

The process according to the invention is suitable for the deoxidation of valve metal powders prepared by any method. For example, it is possible to deoxidise niobium and tantalum powders that are prepared by reduction of a fluoride salt of the valve metal by means of sodium in the presence of a diluting salt. Such a procedure is known from U.S. Pat. No. 5,442,978, for example.

In the deoxidation of tantalum powders, particularly advantageous results are achieved when the tantalum powder used as starting material is obtained by reaction of K2TaF7 with sodium in the presence of potassium chloride and potassium fluoride under the following reaction conditions: The salt mixture of K2TaF7, potassium chloride and potassium fluoride is placed in a test retort and heated preferably for 6 hours at 400° C. in order to remove residual moisture from the salts. The test retort is then heated to a temperature of from 850° C. to 950° C., preferably from 850° C. to 920° C., particularly preferably to a temperature of 900° C., whereby the salt mixture liquefies. The liquid melt is stirred under an argon atmosphere (1050 hPa) for the purpose of homogenisation. When the reduction temperature is reached, liquid sodium is added in portions. The total amount of sodium corresponds to a 3 to 6 wt. % excess, based on the amount of potassium heptafluorotantalate used. During the addition it must be ensured that the temperature in the test retort always remains in the range of the reduction temperature (T+/−20° C.). In order to adjust the surface of the precipitated tantalum powder, an additive that influences the surface tension of the salt melt, for example anhydrous sodium sulfate, is added to the mixture before the first addition of sodium. When the reduction is complete, stirring is continued for a further 0.5 to 3 hours in the range from 800° C. to the reduction temperature. Preferably, stirring is continued for about 3 hours while simultaneously cooling from the reduction temperature to 800° C. The reaction material is cooled to room temperature and steam is passed through the test retort in order to passivate excess sodium. The retort is then opened and the reaction material is removed and pre-comminuted by means of jaw breakers (<5 cm, preferably <2 cm). The inert salts are then removed by washing, and the resulting tantalum powder is dried. A step of doping with phosphorus can optionally be inserted here, in which the tantalum metal powder is treated with a (NH4)H2PO4 solution in order to adjust the P content of the finished tantalum metal powder. The powder is then exposed to a high temperature in vacuo. For example, heating is carried out for 30 minutes at from 1250° C. to 1500° C., preferably from 1280° C. to 1450° C., particularly preferably from 1280° C. to 1360° C. The tantalum powder so prepared is then subjected to the deoxidation according to the invention.

If is, of course, also possible to use as starting materials valve metal powders which are obtained by reduction of the valve metal oxides using gaseous magnesium, as described in U.S. Pat. No. 6,558,447 B1.

It has been shown that it is particularly advantageous to use calcium, barium, lanthanum, yttrium or cerium as the reducing agent in this case instead of magnesium.

In a particularly preferred embodiment of the process according to the invention, therefore, there is used as the valve metal powder to be deoxidised a valve metal powder that is obtained by reduction of a valve metal oxide using gaseous calcium, barium, lanthanum, yttrium or cerium.

The procedure for the preparation of the corresponding valve metal powder is according to U.S. Pat. No. 6,558,447 B1, but calcium, barium, lanthanum, yttrium or cerium is used as the reducing agent.

For the preparation of a tantalum powder, which is preferably used, tantalum oxide (Ta2O5) is, for example, placed on a tantalum gauze in a tantalum dish. A 1.1-fold stoichiometric amount, based on the oxygen content in the tantalum oxide, of calcium, barium, lanthanum, yttrium or cerium is placed beneath the tantalum gauze. The reduction is carried out at a temperature that is sufficiently high to convert the reducing agent to the gaseous state. In order to increase the vapour pressure of the reducing agent at a given reduction temperature, it is possible to work at a reduced overall pressure in the reactor. Accordingly, the process is generally carried out at an overall pressure in the reactor of less than or equal to 1000 mbar, preferably at an overall pressure in the reactor of less than or equal to 500 mbar. The reduction temperature is then preferably from 950 to 1100° C., particularly preferably from 980 to 1050° C. In general, reducing times of up to 8 hours are sufficient. When the reduction is complete, the reaction material is removed and the resulting oxide of the reducing agent is extracted with nitric acid or hydrochloric acid. Analogously to the above-described procedure, a P-doping step may also optionally be inserted here. Finally, the valve metal powder so obtained is subjected to a deoxidation according to the invention.

Valve metal powders that are distinguished by a content of Na, K and Mg of less than 3 ppm, based on a capacitance of 10,000 μFV/g, are accessible for the first time by means of the deoxidation process according to the invention.

The invention accordingly further provides valve metal powders that have a ratio of the sum of the impurities sodium, potassium and magnesium to the capacitance of the valve metal powder of less than 3 ppm/10,000 μFV/g.

The ratio of the sum of the impurities sodium, potassium and magnesium to the capacitance of the valve metal powder is preferably less than 2 ppm/10,000 μFV/g, particularly preferably less than 1 ppm/10,000 μFV/g.

The content of the impurities K, Na, Mg is determined after acid decomposition of the valve metal sample by means of HNO3/HF. K and Na are determined by the method of flame atom adsorption spectroscopy (FAAS) in an acetylene/air mixture, and magnesium is determined by the ICP-OES method (inductive coupled plasma-optical emission spectroscopy). For the acid decomposition, 2 ml of 65 wt. % HNO3 and 10 ml of 40 wt. % HF are added to 1.0 g of the valve metal sample to be tested, and stirring is carried out for 10 hours at a temperature of 105° C. under normal pressure. After cooling, 5 ml of 30 wt. % HCl are added, and the volume of the sample is made up to 100 ml with H2O. The solution so obtained is then tested by means of FAAS or ICP-OES. The contents that are determined are indicated in ppm (parts per million).

The capacitance of the valve metal powder is determined by the following procedure: Cylindrical compressed bodies having a diameter of 4.1 mm and a length of 4.26 mm and having a compressed density of 4.8 g/cm3 are each prepared from 0.296 g of a deoxidised valve metal powder, a tantalum wire of 0.2 mm diameter being inserted axially into the compression mould as contact wire before the valve metal powders are introduced. The compressed bodies are sintered at a sintering temperature of from 1330° C. to 1430° C. for 10 minutes under a high vacuum (<10−5 mbar) to form anodes. The anode bodies are immersed in 0.1 wt. % phosphoric acid and formed at a current intensity limited to 150 mA to a forming voltage of 30 V. After the current intensity has diminished, the voltage is maintained for a further 100 minutes. In order to measure the capacitor properties, a cathode of 18 wt. % sulfuric acid is used. Measurement is carried out at a frequency of 120 Hz. The residual current is then measured in phosphoric acid of conductivity 4300 μS. The resulting values of the capacitance of the individual anode and the residual current of the individual anode are standardised to μFV/g, where μF=capacitance, V=forming voltage, g=anode mass, or μA/g, where μA=measured residual current and g=anode mass used, or μA/μFV.

The valve metal powders according to the invention preferably have a capacitance of at least 35,000 μFV/g, particularly preferably of at least 40,000 μFV/g.

The valve metal powders according to the invention are preferably niobium or tantalum powders, which are optionally doped with one another and/or with one or more of the metals Ti, Mo, V, W, Hf and Zr. Further doping elements, such as, for example, phosphorus, are possible.

The valve metal powders according to the invention can be used for a wide variety of applications and are suitable in particular for the manufacture of solid electrolyte capacitors.

The examples which follow serve to illustrate the invention in greater detail, the examples being intended to facilitate comprehension of the principle according to the invention and not to limit it.

EXAMPLES

Unless indicated otherwise, percentages are by weight (wt. %).

Example 1

A tantalum primary powder was prepared at a reduction temperature of 900° C. starting from a mixture of 150 kg of K2TaF7, 136 kg of KCl, 150 kg of KF, 4 kg of a superfine tantalum powder and 300 g of Na2SO4 in a nickel-coated INCONEL retort by the increment-wise addition of sodium, analogously to U.S. Pat. No. 5,442,978. The tantalum powder was isolated from the cooled and comminuted reaction mixture by washing with weakly acidified water, a cleaning treatment with a washing solution comprising sulfuric acid and hydrogen peroxide subsequently also being carried out. The material was doped with 20 ppm of phosphorus using a sodium dihydrogen phosphate solution containing 1 mg of P per ml of solution. After drying, heat treatment was carried out under a high vacuum at 1430° C. Following this, the phosphorus content of the tantalum powder was adjusted to 60 ppm by means of the sodium dihydrogen phosphate solution (1 mg of P per ml). The powder exhibited the following impurities (in ppm):

Mg: <1 ppm

Na: 0.7 ppm

K: 7 ppm

2 kg of this powder (starting powder) were mixed with 90 g (4.5 wt. %) of calcium powder and heated at 980° C. for 3 hours in a covered tantalum crucible in a retort tinder an argon atmosphere. After cooling and the controlled introduction of air for passivation, the reaction material was removed and calcium oxide that had formed was removed with a washing solution of dilute nitric acid and hydrogen peroxide solution. The washing solution was decanted oft and the powder on the suction filter was washed with demineralised water until free of acid. The dried powder had an oxygen content of 2831 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step. To that end, 19.2 g of calcium powder (based on the oxygen content, the 1.5-fold stoichiometric amount) were mixed into the powder and the mixture was likewise heated at 980° C. for 3 hours. After cooling and passivation, the CaO that had formed was again removed by acid washing, and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 1 ppm

K: 8 ppm

The electric test gave a capacitance of 37,419 μFV/g at a sintering temperature of 1400° C.

Example 2

Comparison Example

2 kg of the starting powder from Example 1 were mixed with 50 g of magnesium turnings (2.5 wt. %) and heated at 980° C. for 3 hours in a covered tantalum crucible in a retort under an argon atmosphere. After cooling and the controlled introduction of air for passivation, the reaction material was removed and magnesium oxide that had formed was removed with a washing solution of dilute sulfuric acid and hydrogen peroxide solution. The washing solution was decanted off, and the powder on the suction filter was washed with demineralised water until free of acid. The dried powder had an oxygen content of 2781 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step. To that end, 11.4 g of magnesium turnings (based on the oxygen content, the 1.5-fold stoichiometric amount) were mixed into the powder and the mixture was likewise heated at 980° C. for 3 hours. After cooling and passivation, the MgO that had formed was again removed by acid washing, and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: 8 ppm

Na: 1 ppm

K: 6 ppm

The electric test gave a capacitance of 38,261 μFV/g at a sintering temperature of 1400° C.

Example 3

200 g of the starting powder from Example 1 were mixed with 22 g of lanthanum powder (11 wt. %) and heated at 980° C. for 3 hours in a covered tantalum crucible in a retort under an argon atmosphere. After cooling and the controlled introduction of air for passivation, the reaction material was removed and lanthanum oxide that had formed was removed with a washing solution of dilute nitric acid and hydrogen peroxide solution. The washing solution was decanted off, and the powder on the suction filter was washed with demineralised water until free of acid. The dried powder had an oxygen content of 3045 ppm.

180 g of this powder were then subjected to a second deoxidation step. To that end, 6.5 g of lanthanum powder (based on the oxygen content, the 1.5-fold stoichiometric amount) were mixed into the powder and the mixture was likewise heated at 980° C. for 3 hours. After cooling and passivation, the La2O3 that had formed was again removed by acid washing, and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 0.7 ppm

K: 8 ppm

The electric test gave a capacitance of 38,093 μFV/g at a sintering temperature of 1400° C.

Example 4

A tantalum primary powder was prepared at a reduction temperature of 920° C. starting from a mixture of 75 kg of K2TaF7, 125 kg of KCl, 225 kg of KF, 5 kg of a superfine tantalum powder and 500 g of Na2SO4 in a nickel-coated INCONEL retort by the increment-wise addition of sodium, analogously to U.S. Pat. No. 5,442,978. The tantalum powder was isolated from the cooled and comminuted reaction mixture by washing with weakly acidified water, a cleaning treatment with a washing solution comprising sulfuric acid and hydrogen peroxide subsequently also being carried out. The material was doped with 100 ppm of phosphorus using a sodium dihydrogen phosphate solution containing 1 mg of P per ml of solution. After drying, heat treatment was carried out under a high vacuum at 1280° C. The powder exhibited the following impurities (in ppm):

Mg: <1 ppm

Na: 1 ppm

K: 49 ppm

2 kg of this powder were mixed with 90 g (4.5 wt. %) of calcium powder and heated at 960° C. for 3 hours in a covered tantalum crucible in a retort under an argon atmosphere. After cooling and the controlled introduction of air for passivation, the reaction material was removed and calcium oxide that had formed was removed with a washing solution of dilute nitric acid and hydrogen peroxide solution. The washing solution was decanted off, and the powder on the suction filter was washed with demineralised water until free of acid. The dried powder had an oxygen content of 3700 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step. To that end, 25 g of calcium powder (based on the oxygen content, the 1.5-fold stoichiometric amount) were mixed into the powder and the mixture was likewise heated at 960° C. for 3 hours. After cooling and passivation, the CaO that had formed was again removed by acid washing, and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 1 ppm

K: 12 ppm

The electric test gave a capacitance of 59,764 μFV/g at a sintering temperature of 1400° C.

Example 5

500 g of tantalum pentoxide (Ta2O5) having a particle size <400 μm are placed on a tantalum gauze in a tantalum crucible. The 1.1-fold stoichiometric amount, based on the oxide content in the tantalum pentoxide, of calcium (249.4 g) is placed beneath the tantalum gauze. The tantalum dish is introduced into a scalable retort.

The reduction is carried out for 8 hours under an argon atmosphere at 980° C. and at a reaction pressure of 600 mbar. The reaction material is removed, and the resulting calcium oxide is extracted with nitric acid. The tantalum powder, which has been washed until free of acid, is doped with 100 ppm of P on the suction filter using a sodium dihydrogen phosphate solution containing 1 mg of P per ml of solution, and then dried. The tantalum powder so prepared has an oxygen content of 7143 ppm.

400 g of this powder are mixed with 18 g (4.5 wt. %) of calcium powder and heated at 960° C. for 3 hours in a covered tantalum crucible in a retort under an argon atmosphere. After cooling and the controlled introduction of air for passivation, the reaction material is removed and calcium oxide that has formed is removed with a washing solution of dilute nitric acid and hydrogen peroxide solution. The washing solution is decanted off, and the powder on the suction filter is washed with demineralised water until free of acid. The dried powder has an oxygen content of 4953 ppm.

300 g of this powder are then subjected to a second deoxidation step. To that end) 5.6 g of calcium powder (based on the oxygen content, the 1.5-fold stoichiometric amount) are mixed into the powder and the mixture is likewise heated at 960° C. for 3 hours. After cooling and passivation, the CaO that has formed is again removed by acid washing, and the powder is washed until free of acid.

The powder so prepared exhibits the following impurities:

Mg: <1 ppm

Na: <1 ppm

K: 2 ppm

The electric test gave a capacitance of 70,391 CV/g at a sintering temperature of 1400° C.