HK1218434B - Method for producing low-oxygen valve-metal sintered bodies having a large surface area - Google Patents

Description

Method for producing a low oxygen valve metal sintered body having a large surface area

The present invention relates to a method for producing sintered bodies from valve metals which, despite their large surface area, have a low oxygen content and good anodizability. The invention also relates to sintered bodies obtainable by this method and their use in electronic components, in particular capacitors. The invention also relates to valve metal powders particularly suitable for use in the method.

Solid electrolytic capacitors with a very large active capacitor area and a compact design suitable for mobile communications electronics are primarily capacitor modules with a tantalum pentoxide barrier layer applied to a suitable conductive tantalum metal support. This utilizes the stability of this metal ("valve metal"), its high dielectric constant, and the fact that the insulating pentoxide layer can be produced electrochemically with extremely uniform layer thickness. The metal support, which also serves as the capacitor anode, consists of a highly porous sponge-like structure produced by pressing and sintering an ultrafinely dispersed primary structure or an already sponge-like secondary structure. The mechanical stability of the pressed body is crucial for further processing into a sintered body, the actual support structure for the capacitor, or the anode. The surface of the support structure is electrolytically oxidized to form an amorphous pentoxide (anodic oxidation; "forming"). The thickness of the pentoxide layer depends on the maximum voltage of the electrolytic oxidation (the "forming voltage"). The counter electrode is produced by impregnating the sponge-like structure with manganese nitrate (which is thermally converted to manganese dioxide) or with a liquid precursor of a polymer electrolyte and polymerizing it.

On the cathode side, the contacts to the electrodes are made by means of a layer structure of graphite and conductive silver on a ribbon or wire-shaped outgoing conductor, and on the anode side, by means of a tantalum or niobium wire (usually inserted into the pressing mold and led out of the capacitor before the sintering operation). The strength of the wire sintered to the anode structure (pull-out strength or wire tensile strength) is essential for further processing to form the capacitor.

A common problem in the production of such sintered bodies is controlling the oxygen content, which has a significant impact on the properties of capacitors made from them, particularly when using tantalum. Numerous studies have demonstrated the adverse effects of oxygen on the properties of produced tantalum capacitors. Elevated oxygen content can lead to undesirable crystallization of amorphous tantalum oxide, which accumulates during the formation of the tantalum sintered body. While amorphous tantalum oxide is an excellent insulator, crystalline tantalum oxide has at least low electrical conductivity, which can lead to capacitor failure due to increased leakage current or breakdown. Because tantalum possesses a natural tantalum oxide passivation layer that prevents further oxidation of the metal, oxygen cannot be completely eliminated; at best, its content can be minimized. Passivation requires an oxygen content of approximately 3000 ppm*g/m²; otherwise, the tantalum powder becomes pyrophoric and ignites upon contact with ambient air. The natural tantalum oxide layer has a thickness of approximately 1 to 2 nanometers, which corresponds to an oxygen content of approximately 3000 ppm*g/m², and thus constitutes, for example, approximately 0.6% in a powder with a specific surface area of 2 m2/g. Therefore, even tantalum powders described in the literature as "oxygen-free" always have at least this oxygen content (see, for example, Y. Freeman et al., J. Electrochem. Soc. 2010, vol. 157, no. 7, G161; JD Sloppy, Pennsylvania State University, Thesis 2009, p. 180; Q. Lu, S. Mato, P. Skeldon, GE Thompson, D. Masheder, Thin Solids Films 2003, 429 (1-2), 238; G. Battistig, G. Amsel, E. D'artemare, Nuclear Instruments & Methods in Physics Research B , 1991, 61, 369-376; L. Young, Trans. Faraday Soc. 1954, 50 , 153-159; V. Macagno, JW Schultze, J. Electroanal. Chem. 1984, 180 , 157-170; O. Kerrec in Transfert Electronique pour les systems de type MOE modification des electrodes par constitution de structures de type MOM , Chimie 1992, Paris). This problem is more serious for sintered bodies with a large surface area, since the oxygen content is proportional to the surface area of the tantalum substrate.

WO 02/45109 A1 describes a method for producing tantalum or niobium capacitors which, in addition to sintering and deoxidation, also includes doping the sintered body with nitrogen. These sintered bodies are produced in an oxygen-free atmosphere.

US Pat. No. 4,722,756 discloses a method for reducing the oxygen content in tantalum or niobium sintered compacts. These compacts are sintered in a hydrogen atmosphere in the presence of a reducing material. The reducing material can be beryllium, calcium, cerium, hafnium, lanthanum, lithium, praseodymium, scandium, thorium, titanium, uranium, vanadium, yttrium, or zirconium, or alloys or mixtures thereof.

DE 33 09 891 C2 describes a two-stage process for producing valve metal sintered anodes for electrolytic capacitors, in which a sintered tantalum sintered body is deoxidized in the presence of a reducing metal, such as magnesium. This involves introducing the metal together with the sintered body into a reaction chamber and heating them together to a temperature of 650°C to 1150°C.

WO 2009/140274 A2 describes an anode in which the inner portion has a higher density than the outer portion. This is intended to improve wire bonding. To deoxidize the compact, magnesium is supplied to it, and the process temperature is high enough to evaporate the magnesium.

DE 31 30 392 A1 describes a method for producing pure agglomerated valve metal powder, wherein thermal agglomeration is carried out at relatively low temperatures in the presence of a reducing agent such as aluminum, beryllium, calcium or magnesium. To this end, the valve metal powder is intensively mixed with, for example, magnesium powder, and the mixture is pressed into cylindrical shaped bodies, which are then sintered at temperatures of 1200° C. to 1400° C.

Although the oxygen content of sintered bodies made of tantalum can be reduced by thermal post-treatment under reducing conditions or by sintering the green body under reducing conditions, these methods result in a loss of, or at least a significant deterioration of, the bond between the sintered body and the intercalated wire. The resulting deterioration in the electrical properties of the anode body can subsequently lead to complete failure of the capacitor or at least a significant loss of yield.

Therefore, one object of the present invention is to provide a method for manufacturing a valve metal sintered body having a low oxygen content and simultaneously exhibiting good wire bonding. Consequently, a capacitor having low leakage current and simultaneously high capacitance can be manufactured. Another object of the present invention is to provide a method for manufacturing a valve metal sintered body having a shorter process duration than conventional methods for manufacturing low-oxygen valve metal sintered bodies.

A further object of the present invention is to provide valve metal powders which are particularly suitable for the method according to the invention and which result in improved sintering properties and increased bonding of the wires to the sintered body.

This object is achieved according to the present invention by providing a method for producing a sintered body, the method comprising the following steps:

a) pressing of powders containing or consisting of valve metals,

b) providing the green compact obtained in step a) together with a reducing agent, wherein the green compact is not or cannot be in direct contact with a solid or liquid reducing agent,

c) heating to sinter the powder into a sintered body and simultaneously reduce the oxygen content of the valve metal in the sintered body; and

d) Removal of oxidized reducing agents with the aid of mineral acids.

Preferred valve metal powders have a high purity, particularly with respect to the content of impurities that could adversely affect leakage current. The total sodium and potassium content is preferably less than 5 ppm, more preferably less than 2 ppm. The total iron, chromium, and nickel content is preferably less than 25 ppm, more preferably less than 15 ppm. The ppm values are each based on parts by mass.

One of the main goals in mobile communication electronics is to satisfy the market for components that achieve high performance but have only small dimensions.Preferred are therefore embodiments of the method of the invention in which the powder is pressed into small shaped bodies.

To ensure efficient operation of the method, the compacts are preferably produced using an automated press. Since the proportion of fine particles in the powder also increases with the surface area of the powder, the risk of clogging or even damage to the pressing die increases. Therefore, in a preferred embodiment of the method, the powder contains a pressing agent. These pressing agents act as a binder. In a preferred embodiment, when the valve metal powder contains a pressing agent, an additional degreasing step is performed between steps a) and b) of the method according to the invention.

The compression aid is preferably selected from polyacrylates, polyethylene glycol, camphor, polyethylene carbonate and stearic acid. The nature and amount of the compression aid are preferably selected so that it can be effectively incorporated into the powder to be compressed and can be removed again with little difficulty after the compression operation.

If a pressing aid is used, preferred embodiments of the method of the present invention are those in which the pressing aid is removed from the green compact after the pressing operation. This degreasing operation can be carried out, for example, thermally (e.g., by evaporation or pyrolysis of the pressing aid) or by alkaline hydrolysis. The degreasing method is preferably selected so that the carbon content remaining in the powder is as low as possible to avoid damage to the subsequent capacitor.

Preferably, the carbon content of the sintered body is less than 200 ppm (parts per million), more preferably less than 100 ppm, in particular less than 50 ppm, especially less than 40 ppm and greater than 1 ppm, in each case based on parts by mass.

In a preferred embodiment of the process according to the invention, the powder is pressed to a green density of 4.5 to 9 g/cm3, preferably 5 to 8 g/cm3, more preferably 5.5 to 7.5 g/cm3, in particular 5.5 to 6.5 g/cm3.

An important component of a capacitor is the electrical contact with the electrodes. On the cathode side, the contact is formed by a layer structure composed of graphite and conductive silver on an output conductor in the form of a ribbon or wire, and on the anode side, by a wire made of tantalum or niobium (this wire is typically inserted into a pressing mold and led out of the capacitor before the sintering operation). The sintering strength of the wire within the anode structure and, therefore, within the sintered body (i.e., the bond between the wire and the anode body) is essential for the further processing of the sintered body to form a capacitor. Therefore, preferred embodiments of the method of the present invention are those in which the powder is pressed around a wire, preferably a valve metal wire.

In a further preferred embodiment of the method according to the invention, the wires are bonded to the sintered body by soldering.

The valve metal of the wire is preferably selected from tantalum and niobium. The valve metal powder pressed around the wire also preferably comprises a valve metal selected from tantalum and niobium. More preferably, both the wire and the powder comprise a valve metal selected from tantalum and niobium.

In a preferred embodiment of the method of the present invention, the valve metal powder is composed of agglomerated primary particles having a minimum size of 0.05 to 0.4 microns. The primary particles preferably have a specific surface area in the range of 1.5 to 20 m2/g, as determined according to ASTM D3663. More preferably, the valve metal powder has a particle size distribution with a D10, as determined according to ASTM B822, of 2 to 80 microns, preferably 2 to 30 microns. The valve metal powder preferably has a D50 of 10 to 200 microns, preferably 15 to 175 microns. The valve metal powder also preferably has a D90 of 30 to 400 microns, preferably 40 to 300 microns. Both D50 and D90 can be determined, for example, according to ASTM B822.

In a preferred embodiment of the method according to the invention, the valve metal powder has a BET surface area of 1.5 to 20 m2/g, preferably 2.0 to 15 m2/g, particularly 3.0 to 10 m2/g, and especially 4.0 to 8.0 m2/g. The BET surface area herein refers to the specific surface area determined by the method of Brunauer, Emmett, and Teller (DIN ISO 9277).

To achieve the maximum active surface area, it is advantageous to use a powder with a maximum open porosity. Pore clogging or the formation of closed pores (e.g., due to oversintering) leads to a loss of active capacitor surface area. This loss can be prevented by anti-sintering doping with nitrogen and/or phosphorus, and previously also with boron, silicon, sulfur, or arsenic. However, excessively high concentrations of anti-sintering doping sometimes lead to a significant reduction in sintering activity. Therefore, preferred embodiments of the method according to the invention are those in which the valve metal powder has a phosphorus content of less than 20 ppm, preferably 0.1 to less than 20 ppm, in each case based on parts by mass.

In a preferred embodiment of the method of the invention, the valve metal powder has a minimum content of substances known for their anti-sintering effect. In a particularly preferred embodiment, the valve metal powder does not have any effective content of anti-sintering agents.

Preferably, the valve metal powder has a nitrogen content of less than 300 ppm, in particular less than 300 ppm and greater than 0.1 ppm, in each case based on parts by mass.

It is also preferred that the valve metal powder has a boron content of less than 10 ppm, in particular less than 10 ppm and greater than 0.01 ppm, in each case based on parts by mass.

Likewise preferably, the valve metal powder has a sulfur content of less than 20 ppm, in particular less than 10 ppm and greater than 0.1 ppm, in each case based on parts by mass.

It is also preferred that the valve metal powder has a silicon content of less than 20 ppm, in particular less than 20 ppm and greater than 0.01 ppm, in each case based on parts by mass.

It is also preferred that the valve metal powder has an arsenic content of less than 10 ppm, in particular less than 10 ppm and greater than 0.01 ppm, in each case based on parts by mass.

In a preferred embodiment, the valve metal powder has a phosphorus content of less than 20 ppm and more than 0.1 ppm, a nitrogen content of less than 300 ppm and more than 0.1 ppm, a boron content of less than 10 ppm and more than 0.01 ppm, a sulfur content of less than 20 ppm and more than 0.1 ppm, a silicon content of less than 20 ppm and more than 0.01 ppm and an arsenic content of less than 10 ppm and more than 0.01 ppm, in each case based on parts by mass.

Valve metals are characterized by their oxides blocking current in one direction but allowing it to pass in the other. These valve metals include, for example, tantalum, niobium, or aluminum. Valve metals may also be alloys in the present invention. Another characteristic of valve metals is that they have a passivation layer of valve metal oxide that prevents further oxidation and thus prevents the metal from igniting. The oxygen content is based on the specific surface area of the powder, i.e., the quotient of the oxygen content in ppm by mass and the specific surface area measured according to BET.

Typical examples of valve metals are selected from Al, Bi, Hf, Nb, Sb, Ta, W, and Zr. Alloys of these valve metals with one another are also possible. In another embodiment, the valve metal in the present invention may also be an alloy of the above-mentioned valve metals with another metal preferably selected from Be, Ge, Mg, Si, Sn, Ti, and V.

Preference is given to alloys of valve metals with other metals than valve metals, wherein the proportion of valve metal is at least 50% by weight, more preferably at least 70% by weight, in particular at least 90% by weight, especially at least 95% by weight or at least 98.5% by weight or at least 99.5% by weight of the entire alloy.

Preferred valve metals for use in the present invention are tantalum and niobium.

In a preferred embodiment of the method according to the invention, the valve metal powder has an oxygen content of more than 3000 ppm*g/m², in particular more than 3500 ppm*g/m², more preferably from 4100 ppm*g/m² to 8000 ppm*g/m², in each case based on parts by mass. The oxygen content is determined by means of carrier gas reactive fusion using a Nitrogen/Oxygen Determinator Model TCH 600 from Leco Instrumente GmbH.

Also preferred are embodiments of the method of the present invention in which the valve metal powder has a median particle size D50 of 15 to 175 micrometers, preferably 20 to 100 micrometers, wherein the median particle size is determined according to ASTM B822.

It has surprisingly been found that the use of valve metal powders with an elevated oxygen content and a low and defined amount of sintering inhibitors significantly improves the bonding and sintering capabilities of the wire to the sintered body.

The present invention therefore also provides a valve metal powder comprising the following components:

i) oxygen in an amount greater than 4100 ppm∙g/m ² , preferably in an amount from 4100 ppm∙g/m ² to 8000 ppm∙g/m ² ,

ii) nitrogen, optionally in an amount of less than 300 ppm, preferably in an amount of 0.1 ppm to 300 ppm,

iii) optionally boron in an amount of less than 10 ppm, preferably from 0.01 ppm to 10 ppm,

iv) optionally sulfur in an amount of less than 20 ppm, preferably from 0.1 ppm to 10 ppm,

v) silicon, optionally in an amount below 20 ppm, preferably in an amount from 0.01 ppm to 20 ppm, and

vi) arsenic, optionally in an amount below 10 ppm, preferably in an amount between 0.01 ppm and 10 ppm,

vii) phosphorus, optionally in an amount of less than 20 ppm, preferably in an amount of 0.1 ppm to 20 ppm, wherein the ppm values are each based on parts by mass.

A particularly preferred valve metal powder of the present invention comprises the following components:

i) oxygen in an amount greater than 4100 ppm∙g/m ² , preferably in an amount from 4100 ppm∙g/m ² to 8000 ppm∙g/m ² ,

ii) nitrogen in an amount below 300 ppm, preferably in an amount from 0.1 ppm to 300 ppm,

iii) boron in an amount below 10 ppm, preferably from 0.01 ppm to 10 ppm,

iv) sulfur in an amount lower than 20 ppm, preferably in an amount between 0.1 ppm and 10 ppm,

v) silicon in an amount below 20 ppm, preferably from 0.01 ppm to 20 ppm, and

vi) arsenic in an amount below 10 ppm, preferably from 0.01 ppm to 10 ppm,

vii) Phosphorus in an amount of less than 20 ppm, preferably in an amount of 0.1 ppm to 20 ppm, wherein the ppm values are each based on parts by mass.

The oxygen content of the valve metal powder of the present invention is higher than the oxygen content of valve metal powders of the prior art and, therefore, higher than the normal (i.e., natural) oxygen content that results in the formation of an oxide layer on the metal surface upon contact of the metal with ambient air. The elevated oxygen content of the valve metal powder of the present invention can be established in a specific manner, for example, by treating the metal powder with oxygen.

Surprisingly, it has been found that a powder having this composition has improved sintering properties and, therefore, improved bonding of the wire to the sintered body. Due to the increased oxygen content in the valve metal powder compared to the prior art, higher temperatures are achieved in the compact during the sintering operation, thereby achieving a better bonding of the wire to the sintered body.

In a preferred embodiment, the valve metal powder has a BET surface area of 1.5 to 20 m2/g, preferably 2.0 to 15 m2/g, particularly 3.0 to 10 m2/g, especially 4 to 8 m2/g.

Also preferred is a valve metal powder composed of agglomerated primary particles having a size of 0.05 to 0.4 micrometers. These primary particles preferably have a specific surface area in the range of 1.5 to 20 m2/g, wherein the specific surface area is determined according to DIN ISO 9277. Furthermore, the valve metal powder preferably has a particle size distribution with a D10, measured according to ASTM B822, of 2 to 80 micrometers, preferably 2 to 30 micrometers. The valve metal powder preferably has a D50 of 10 to 200 micrometers, preferably 15 to 175 micrometers. The valve metal powder also preferably has a D90 of 30 to 400 micrometers, preferably 40 to 300 micrometers. Both D50 and D90 can be determined, for example, according to ASTM B822.

The valve metal powder of the present invention is preferably selected from niobium and/or tantalum.

In a particularly preferred embodiment, the valve metal powder according to the invention is used in step a) of the process according to the invention, ie is pressed.

Preferred valve metal powders have a high purity, particularly with respect to the content of impurities that negatively influence leakage current. The total sodium and potassium content is preferably less than 5 ppm, more preferably less than 2 ppm. The total iron, chromium, and nickel content is preferably less than 25 ppm, more preferably less than 15 ppm. The ppm values are each based on parts by mass.

An increased oxygen content in a capacitor can lead to poorer electrical properties. For example, an increased oxygen content can cause the previously amorphous oxide of the valve metal to transform into a crystalline form, which, however, has a relatively high electrical conductivity. This reduces the insulating effect of the dielectric and makes the capacitor more susceptible to so-called leakage currents. In addition to the natural oxygen content resulting from metal passivation, further oxygen is incorporated during the sintering of the compact under non-reducing conditions. Therefore, it is important to reduce the oxygen content in the compact during the sintering process. This is particularly important because the compact shrinks during the sintering operation, reducing its surface area, which in turn leads to excess oxygen in the porous metal composite. Due to the high temperatures during the sintering process, much more oxygen can be incorporated into the metal lattice, up to the saturation limit, compared to the situation at room temperature. This incorporation causes the metal lattice to expand. Once a critical value is exceeded, crystalline valve metal oxides precipitate, which leads to the aforementioned adverse effects, such as increased leakage currents. Therefore, preferred are embodiments of the method of the present invention in which the sintering of the compact is combined with simultaneous deoxidation.

Deoxidation in the context of the present invention is the removal of excess oxygen from the reduced metal, for example from the metal lattice.

In a preferred embodiment of the method according to the invention, the compact is sintered in a reducing atmosphere. Methods known in the prior art involve mixing the powder with a reducing agent or heating the reducing agent simultaneously with the sintered body. However, these methods have the disadvantage that the resulting deoxidation results in poor wire bonding, to the extent that it can become detached from the sintered body during further processing operations, and makes it impossible to produce an anodic oxide layer on the sintered body or subsequently measure the electrical properties of the anode obtained therefrom. Therefore, methods are preferred that reduce the oxygen content while simultaneously achieving a sufficiently strong wire bond to the sintered body.

In a preferred embodiment of the method according to the invention, the reducing agent, in solid or liquid form, is spatially separated from the valve metal. Preference is given to embodiments in which the reducing agent is evaporated. As soon as the desired temperature is reached, the compact, preferably in a basket made of porous niobium or tantalum sheets, is immersed in the vapor, and the oxygen in the compact can react with the reducing agent. The preferred immersion operation of the method according to the invention enables higher temperatures to be achieved in the compact, which leads to higher shrinkage and therefore higher densification of the compact. Surprisingly, it has been found that this effect occurs in particular when using valve metal powders according to the invention having an oxygen content of more than 4100 ppm*g/m². The final shrinkage of the compact depends on the temperature and duration of the deoxidation.

In another preferred embodiment of the method of the present invention, magnesium is placed on a flat plate. A porous sheet of tantalum or niobium is suspended above it, with a green compact placed on top. In this case, the distance between the flat plate carrying magnesium and the porous niobium sheet can be, for example, 4 to 8 centimeters. The flat plate with magnesium on it is heated. The oxygen in the green compact reacts upon contact with magnesium vapor.

Also preferred is an embodiment in which the reducing agent and the green compact are in the same process chamber but spatially separated. In this case, the reducing agent is preferably evaporated first, and then the green compact is contacted with the vapor, for example by suspending the green compact in the vapor. This avoids the laborious transfer or advancement of the vapor from one process chamber to another, thereby saving time. Furthermore, immersing the green compact in the vapor prevents the green compact from losing too much specific surface area.

Here too, it has surprisingly been found that the use of a valve metal powder according to the invention having an oxygen content of more than 4100 ppm*g/m² promotes the bonding of the wire to the sintered body.

In a preferred embodiment of the method according to the invention, the deoxidation of the green compact is carried out under an inert carrier gas, preferably argon, and the reduction is carried out at a vapor partial pressure of the reducing metal of 5 to 650 hPa, preferably greater than 40 hPa, in particular 100 to 400 hPa. More preferably, the inert gas pressure is 50 to 800 hPa, preferably less than 600 hPa, in particular 100 to 500 hPa.

Furthermore preferred are embodiments of the method according to the invention in which the reduction of the oxygen content of the valve metal in the sintered body is achieved at a pressure below atmospheric pressure, preferably at a pressure of 50 to 800 hPa, preferably below 600 hPa, in particular 100 to 500 hPa.

Preferred are embodiments of the method of the present invention in which heating is performed at temperatures between 800°C and 1400°C, preferably between 900°C and 1200°C, and especially between 900°C and 1100°C (which results in sintering and a reduction in the oxygen content). This temperature range is preferably selected to reduce the oxygen content of the sintered body to the desired level. Oxygen is generally known to inhibit the sintering of compacts. This reduction in oxygen content results in more efficient sintering of the powder particles with each other and with the embedded wires at lower temperatures than conventionally possible.

In a preferred embodiment of the process according to the invention, the reducing agent is selected from lithium and alkaline earth metals, preferably magnesium or calcium, in particular magnesium.

The oxidation products of the reducing agent formed during the deoxidation process—if they have not already evaporated during the sintering process—are washed out of the sintered body with the aid of a dilute mineral acid. To avoid additional mechanical stress on the sintered bodies, they are preferably placed on a porous niobium or stainless steel sheet, which is introduced into the scrubber along with the washing liquid. The solution is stirred, preferably at a speed adjusted so that the sintered body does not begin to move during the washing operation. Preferably, the oxidation product of the reducing agent is magnesium oxide having the composition MgO. Inorganic acids in the present invention are carbon-free acids, such as hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid.

Deoxidation and the subsequent washing process reduce the sintering strength (wire tensile strength) of the embedded wires within the sintered body. The wire bond to the sintered body can be reduced to the point where the wires can escape from the sintered body or break during further processing. However, it has surprisingly been discovered that the use of the valve metal powder of the present invention, with an oxygen content exceeding 4100 ppm*g/m², can further enhance the wire bond to the sintered body. In other cases, this bond is so low that any electrical measurements are impossible, and the sintered body becomes unusable as a capacitor. As known to those skilled in the art, this problem is overcome according to the prior art by re-sintering the deoxidized and washed sintered body. This does improve the wire bond to the anode body, but at the same time, the oxygen content in the sintered body increases again.

Preferred are embodiments of the method according to the invention in which the oxygen content of the deoxidized sintered body is from 2400 to 3600 ppm*g/m², where the ppm values are each based on parts by mass. The oxygen content is determined using a Nitrogen/Oxygen Determinator Model TCH 600 from Leco Instrumente GmbH using carrier gas reactive fusion.

Studies have shown that the hydrogen content of deoxidized and washed sintered bodies is significantly increased compared to bodies that have been sintered alone. This increased hydrogen content not only increases the brittleness of the embedded wires, but also reduces the strength of the entire sintered body. Therefore, in a similarly preferred embodiment of the method according to the present invention, the deoxidized and washed sintered body is reheated. The conditions are preferably selected to evaporate the hydrogen. Surprisingly, it has been found that sintered bodies that have undergone this additional degassing step exhibit improved bonding of the embedded wires.

The bond between the wire and the sintered body, i.e., the "wire tensile strength," was determined as follows: the anode wire was inserted through a hole with a diameter of 0.25 mm in the support plate, the free end was clamped in the fixing fixture of a dynamometer (Chatillon, model: DFGS-50, with LTCM-6 drive unit), and a load was applied until the wire was pulled out of the anode structure, i.e., the sintered body.

Surprisingly, it has been found that the wire bonding of the anode wire can be improved by chemically forming the sintered body. Therefore, preferred embodiments of the method according to the invention are those in which chemically forming the sintered body in step e) is performed after removing the oxidized reducing agent in step d). Chemical forming is a process familiar to those skilled in the art.

As already mentioned, the wire bond of the anode wire to the sintered body is an important criterion that ultimately determines the suitability of the sintered body for subsequent use in capacitors. Surprisingly, it has been found that the wire bond is very high after deoxidation but significantly decreases after washing. Furthermore, it has been found that after forming the sintered body, the wire bond returns to a level comparable to that after deoxidation and before washing. This is crucial because mechanical stresses are present on the anode wire before forming, for example, due to soldering to the forming frame ("lead frame"; the anode is incorporated for immersion in the electrolyte and for connecting the anode to the contacts). These mechanical stresses can lead to wire detachment, rendering the sintered body unsuitable for further processing, as subsequent fixing of the anode wire is impossible. Furthermore, the washing operation after deoxidation (in which the oxidized reducing agent is removed using mineral acid) increases the number of required production steps, resulting in higher economic costs and operational complexity.

Therefore, preferred embodiments of the method of the present invention are those in which the oxidized reducing agent is removed from the sintered body while being simultaneously formed. This can avoid a significant reduction in wire bonding while optimizing the process. In a preferred embodiment, the oxidized reducing agent is removed in step d) during simultaneous formation.

Also preferred are embodiments of the method according to the invention in which the formation in step d) is carried out in the presence of a liquid electrolyte. In this case, the liquid electrolyte is preferably selected to ensure both efficient removal of the oxidized reducing agent from the sintered body and satisfactory formation. Surprisingly, it has been found that the oxidized reducing agent can be effectively removed, in particular, with the aid of mineral acids. Furthermore, the presence of an oxidizing agent, such as hydrogen peroxide, has a favorable effect on the washing results. This, in particular, reduces hydrogen absorption during the washing process. The addition of hydrogen peroxide also allows the formation temperature to be lowered to below 80°C.

Therefore, preferred are embodiments in which the liquid electrolyte comprises hydrogen peroxide (H ₂ O ₂ ) and at least one inorganic acid.

In a preferred embodiment, the inorganic acid is selected from the group consisting of sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, and mixtures thereof.

Particularly preferred are embodiments of the process according to the invention in which the liquid electrolyte comprises one or more inorganic acids in an amount of 0.1 to 10% by weight, preferably 1 to 8% by weight, more preferably 3 to 6% by weight, based in each case on the total weight of the liquid electrolyte.

Also preferred are embodiments in which the liquid electrolyte comprises hydrogen peroxide in an amount of 0.1 to 0.9% by weight, preferably 0.3 to 0.7% by weight, based in each case on the total weight of the liquid electrolyte.

In a preferred embodiment of the method of the present invention, the sintering step is followed by a nitridation step at a temperature below 500°C, preferably between 200°C and 400°C. During this process, nitrogen is supplied to the sintered body, for example during cooling. Since the surface of the sintered body contains only a small amount of oxygen, nitrogen can occupy a portion of the surface of the sintered body, thereby reducing the oxygen coverage of the surface. The nitrogen concentration in the sintered body is preferably selected to ensure the lowest leakage current and highest reliability of the capacitor.

In a preferred embodiment, after the compact has been sintered, the sintered body is passivated by oxidation of the body surface. To this end, the sintered body remains in the reactor after it has cooled to below 100° C. The surface of the sintered body is then passivated, preferably by controlled and gradual introduction of oxygen into the reactor. Preferably, the passivation is carried out after sintering and after nitriding the sintered body.

The oxides formed by the reducing agent are preferably subsequently washed out with the aid of a dilute mineral acid.

The present invention also provides a sintered body obtainable by the process of the present invention.

In a preferred embodiment, the sintered body obtainable by the process according to the invention has a BET surface area of 1.5 to 10 m2/g, preferably 2 to 8 m2/g, in particular 3 to 6 m2/g, the BET surface area being determined in accordance with DIN ISO 9277.

In another preferred embodiment, the sintered body according to the invention has a wire bonded to the sintered body, in particular pressed together with it, which preferably consists of or contains a valve metal. The valve metal is preferably selected from tantalum and niobium.

In another preferred embodiment, the sintered body according to the invention has a wire, preferably made of a valve metal, welded to the sintered body. The valve metal is preferably selected from tantalum and niobium.

Furthermore, preferred are embodiments in which the sintered bodies obtainable by the process according to the invention have an oxygen content of 2000 ppm*g/m² to 4000 ppm*g/m², preferably 2500 ppm*g/m² to 3500 ppm*g/m², and in particular 2700 ppm*g/m² to 3500 ppm*g/m². The ppm values are each based on parts by mass. The oxygen content is determined using a Nitrogen/Oxygen Determinator Model TCH 600 from Leco Instrumente GmbH using carrier gas reactive fusion.

In a preferred embodiment, the sintered body comprises a sintering inhibitor, which is preferably selected from

i) nitrogen in an amount below 300 ppm, preferably in an amount from 0.1 ppm to 300 ppm,

ii) boron in an amount below 10 ppm, preferably from 0.01 ppm to 10 ppm,

iii) sulfur in an amount lower than 20 ppm, preferably in an amount between 0.1 ppm and 10 ppm,

iv) silicon in an amount below 20 ppm, preferably from 0.01 ppm to 20 ppm,

v) arsenic in an amount below 10 ppm, preferably from 0.01 ppm to 10 ppm, and

vi) Phosphorus in an amount of less than 20 ppm, preferably in an amount of 0.1 ppm to 20 ppm, wherein the ppm values are each based on parts by mass.

The sintered bodies of the present invention are particularly suitable for use in electronic components, especially those in the mobile communications industry.

The present invention therefore also provides the use of the sintered body according to the invention in electronic components, in particular capacitors.

The process of the present invention is illustrated by the following examples, which are not to be understood as limiting the concept of the present invention.

General Description:

Tantalum powder was pressed together with stearic acid and tantalum wire as a pressing aid to produce a compact with embedded wires and a compact density of 6.0 g/cm³. The stearic acid was removed by alkaline hydrolysis (NaOH) and subsequent washing of the compact with water. This was followed by washing with dilute acid. This resulted in a compact with a carbon content of less than 50 ppm and a sodium content of less than 20 ppm. Magnesium was heated in a reaction chamber. Once the desired temperature was reached, the compact, contained within a basket made of porous niobium sheets, was introduced into magnesium vapor. The exact temperatures and durations for each experiment are shown in Table 2. The deoxidized and sintered compact was passivated using standard methods known to those skilled in the art. MgO formed during the deoxidation process was washed out of the sintered body with dilute sulfuric acid. To this end, the sintered body was placed on a porous niobium sheet and introduced into a scrubber containing dilute sulfuric acid. The mixture was carefully stirred to prevent the sintered body from moving.

The exact composition of the tantalum powder used is summarized in Table 1 and Table 1a.

Table 1 (Powders used in Examples 1 to 9):

¹ Oxygen content based on specific surface area

² Prepared according to the teaching of WO 2006/039999 A1

^* The ppm values given are each based on parts by mass.

The following values were also measured for the powder:

Table 1a*:

*The ppm values given are each based on parts by mass.

The tantalum powder used in Examples 1 to 4 had an oxygen content of 2992 ppm*g/m², the powder used in Examples 5 and 6 had an oxygen content of 4155 ppm*g/m² and the powder used in Examples 7 and 9 had an oxygen content of 3044 ppm*g/ ^m² .

Table 2 shows the sintering conditions for producing the sintered bodies. Inventive Examples 1 to 7 and 9 were produced by the method of the present invention as described above.

The sintered bodies according to Comparative Examples 1 to 4 were sintered by a standard method under specified conditions.

Table 2

Example Temperature/℃ Time/minute 950 15 1050 15 1160 10 1120 15 Example 1 950 15 Example 2 1000 15 Example 3 1050 15 Example 4 1050 30 Example 5 950 15 Example 6 1000 15 Example 7 950 15 Example 9 950 15

^* Sintering was performed under reduced pressure.

The sintered bodies thus obtained exhibited the compositions summarized in Table 3.

Table 3:

Furthermore, the sintered bodies obtained by the process of the present invention have the properties summarized in Table 4:

Table 4:

¹ Oxygen content based on specific surface area

² PD: green compact density

³ SD: sintered density

⁴ WPS: Wire Tensile Strength (Pull-out strength of the wire embedded in the anode body)

*The ppm values given are each based on parts by mass.

The bond between the wire and the sintered body, i.e. the "wire tensile strength" (WPS), was determined as follows: the anode wire was inserted through an opening with a diameter of 0.25 mm in the support plate and the free end was clamped in the fixed fixture of a dynamometer (Chatillon, model: DFGS-50 with LTCM-6 drive) and a load was applied until the wire was pulled out of the anode structure, i.e. the sintered body.

The sintered body was immersed in 0.1% phosphoric acid and formed at a current limited to 150 mA/g to a forming voltage of 10 V (Comparative Examples 1 to 4 and Examples 1 to 6) or 17.5 V (Comparative Example 3a and Examples 8 and 10). For the formation of the sintered body proposed in Example 10, an aqueous electrolyte to which 5% by weight of sulfuric acid and 0.5% by weight of hydrogen peroxide were added, the weight values being based on the total weight of the aqueous electrolyte, was used, and the forming voltage was 17.5 V. After the current was reduced, this voltage was maintained for 3 hours.

The results of the wire bonding analysis in the various process steps are summarized in Table 5. This table shows that sintered bodies produced from the powder of Example 3 according to Table 1 have very low wire bonding after washing out the oxidized reducing agent (Example 7), indicating that the sintered bodies are highly susceptible to mechanical stress. After formation at 17.5 V, the wire bonding increases again (Example 8). In contrast, the sintered bodies directly after deoxidation have relatively high wire bonding (Example 9). Example 10 shows that high wire bonding can be achieved when the sintered bodies are subjected to a combined washing and formation step directly after deoxidation, in which 5 wt% sulfuric acid and 0.5 wt% hydrogen peroxide are added to the aqueous electrolyte. This avoids a decrease in wire bonding during this process. Consequently, a process step can be omitted, making the method for producing sintered bodies less time-consuming and more cost-effective. Furthermore, the sintered bodies have high wire bonding throughout the entire process, thus preventing any damage caused by mechanical stress, such as wire breakout.

Table 5

WPS Example 7 2.9 Example 8 16.5 Example 9 13.8 Example 10 18.6

As can be inferred from Table 7, the combined washing and forming steps do not have any adverse effects on the electrical properties of the sintered body (Table 7, Example 10). Rather, the reported data are within a comparable range to those for sintered bodies in which the removal of the oxidized reducing agent and the forming were performed in two separate process steps (Table 7, Example 8). The sintered body, whose electrical properties are reported as Comparative Example 3a in Table 7, was produced similarly to Comparative Example 3, but using a forming voltage of 17.5 V.

Capacitance measurements were performed using a cathode composed of 18% sulfuric acid and applying a bias voltage of 1.5 V at 20 Hz and 120 Hz using an AC voltage.

For the sintered body thus obtained, the properties summarized in Table 6 were obtained.

Table 6:

Table 7

The data presented indicate that the oxygen content in the resulting sintered bodies is significantly lower than that obtainable using existing standard methods. The sintered bodies produced using the method of the present invention also exhibit the common disadvantage of reduced pullout strength of wires embedded in anode bodies. The combination of washing the sintered bodies after deoxidation and chemically forming the sintered bodies according to the present invention further prevents temporary degradation of wire bonding during this process. The sintered bodies also exhibit low leakage current.

From the values in Table 4, it can be inferred that wire bonding is superior for sintered bodies made using tantalum powder with a relatively high oxygen content (4155 ppm*g/m²) compared to sintered bodies made using tantalum powder with an oxygen content conventional in the art (2992 ppm*g/m²). As known to those skilled in the art, sintering conditions significantly influence wire bonding. Therefore, only sintered bodies sintered under the same conditions can be compared. Therefore, a comparison of Example 1 with Example 5 and a comparison of Example 2 with Example 6 demonstrate the improved wire bonding achieved according to the present invention.

It was also found that the comparative example had a much higher leakage current than the comparable sintered body according to the present invention. Furthermore, it was found that the bonding of the sintered body produced at low temperature according to Comparative Example 1 was insufficient for measuring electrical properties. Regarding Comparative Sintered Body 2, only 2 out of 10 sintered bodies were found to be suitable for measurement.

FIG. 1 shows a secondary electron microscope image of the sintered body described in Example 5 listed above.

FIG. 2 shows a secondary electron microscope image of the sintered body used as Comparative Example 3 in Table 4. FIG.

The microscope images shown in Figures 1 and 2 show the pores of the sintered body in black, the tantalum oxide deposits in grey, and the tantalum in white by elemental contrast.

As is evident in Figure 2, the proportion of grey areas is high, indicating a significant proportion of tantalum oxide. In contrast, these grey areas are completely absent in the image according to Figure 1, indicating that the sintered body according to the invention has a much lower proportion of tantalum oxide.

Claims (63)

1. A method for manufacturing a sintered body, comprising the following steps: a) A pressure valve metal powder, said powder containing oxygen in an amount of 4100 ppm∙g/ ^m² to 8000 ppm∙g/ ^m² . b) Provide the compact obtained in step a) together with a reducing agent, wherein the compact does not or cannot come into direct contact with a solid or liquid reducing agent. c) Heating to sinter the powder into a sintered body and simultaneously reduce the oxygen content of the valve metal within the sintered body; and d) Removing the reducing agent from the oxidizing agent using inorganic acids. Wherein, the oxygen content of the sintered body described in step c) is 2400 to 3600 ppm∙g/ ^m2 . 2. The method according to claim 1, characterized in that the heating in step c) is performed at a temperature of 800°C to 1400°C. 3. The method according to claim 2, characterized in that the heating in step c) is performed at a temperature of 900°C to 1200°C. 4. The method according to claim 3, characterized in that the heating in step c) is performed at a temperature of 900°C to 1100°C. 5. The method according to any one of claims 1 to 4, wherein the reducing agent is selected from lithium and alkaline earth metals. 6. The method according to claim 5, wherein the reducing agent is selected from magnesium or calcium. 7. The method according to claim 6, wherein the reducing agent is selected from magnesium. 8. The method according to any one of claims 1 to 4, characterized in that the valve metal powder has a BET surface area of 1.5 m²/g to 20 m²/g. 9. The method according to claim 8, wherein the valve metal powder has a BET surface area of 2.0 m²/g to 15 m²/g. 10. The method according to claim 9, wherein the valve metal powder has a BET surface area of 3.0 m²/g to 10 m²/g. 11. The method according to claim 10, wherein the valve metal powder has a BET surface area of 4 to 8 square meters per gram. 12. The method according to any one of claims 1 to 4, characterized in that the powder is pressed around a wire. 13. The method according to claim 12, characterized in that the powder is pressed around a wire made of valve metal. 14. The method according to any one of claims 1 to 4, characterized in that the reducing agent in solid or liquid form is spatially separated from the valve metal. 15. The method according to any one of claims 1 to 4, characterized in that the powder is pressed to a green density of 4.5 g/cm³ to 9 g/cm³. 16. The method according to claim 15, characterized in that the powder is pressed to a green density of 5 g/cm³ to 8 g/cm³. 17. The method according to claim 16, characterized in that the powder is pressed to a green density of 5.5 g/cm³ to 7.5 g/cm³. 18. The method according to claim 17, characterized in that the powder is pressed to a green density of 5.5 g/cm³ to 6.5 g/cm³. 19. The method according to any one of claims 1 to 4, wherein the powder comprises a pressure-enhancing agent. 20. The method of claim 19, wherein the powder comprises a pressure-boosting agent, and when the powder comprises a pressure-boosting agent, a degreasing step is included between step a) and step b). 21. The method according to claim 19, wherein the pressure-boosting agent is selected from polyacrylic acid, polyethylene glycol, camphor, polyethylene carbonate and stearic acid. 22. The method according to any one of claims 1 to 4, wherein the valve metal powder has a phosphorus content of < 20 ppm. 23. The method according to claim 22, wherein the valve metal powder has a phosphorus content of 0.1 ppm to less than 20 ppm. 24. The method according to any one of claims 1 to 4, characterized in that a nitriding step at a temperature below 500°C follows the sintering step c). 25. The method according to claim 24, characterized in that a nitriding step at 200°C to 400°C follows the sintering step c). 26. The method according to any one of claims 1 to 4, wherein the valve metal powder is selected from tantalum and niobium. 27. The method according to any one of claims 1 to 4, characterized in that the valve metal powder has a median particle size D50 of 10 to 200 micrometers. 28. The method according to claim 27, wherein the valve metal powder has a median particle size D50 of 15 to 175 micrometers. 29. The method according to any one of claims 1 to 4, characterized in that the oxygen content of the valve metal in the sintered body is reduced under a pressure below atmospheric pressure. 30. The method according to claim 29, characterized in that the oxygen content of the valve metal in the sintered body is reduced under a gas pressure of 50 to 800 hPa. 31. The method according to claim 30, characterized in that the oxygen content of the valve metal in the sintered body is reduced at a gas pressure below 600 hPa. 32. The method according to claim 31, characterized in that the oxygen content of the valve metal in the sintered body is reduced under a gas pressure of 100 to 500 hPa. 33. The method according to any one of claims 1 to 4, characterized in that step d) is followed by step e). 34. The method according to any one of claims 1 to 4, characterized in that the removal of the oxidizing reducing agent in step d) is carried out simultaneously with the formation of the sintered body. 35. The method according to any one of claims 1 to 4, characterized in that the formation is carried out in the presence of a liquid electrolyte in step d). 36. The method according to claim 35, wherein the liquid electrolyte comprises hydrogen _peroxide ( _H₂O₂ ) and at least one inorganic acid. 37. The method according to claim 35, wherein the inorganic acid is selected from sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, and mixtures thereof. 38. A sintered body that can be obtained by any one of the methods of claims 1 to 37. 39. The sintered body according to claim 38, characterized in that it has a BET surface area of 1.5 to 10 square meters per gram. 40. The sintered body according to claim 39, characterized in that it has a BET surface area of 2 to 8 square meters per gram. 41. The sintered body according to claim 40, characterized in that it has a BET surface area of 3 to 6 square meters per gram. 42. The sintered body according to any one of claims 38 to 41, characterized in that the sintered body comprises a sintering inhibitor. 43. The sintered body according to claim 42, characterized in that the sintering inhibitor is selected from... i) Nitrogen levels below 300 ppm, ii) Boron in amounts below 10 ppm, iii) Sulfur at levels below 20 ppm, iv) Silicon in amounts below 20 ppm, v) Arsenic levels below 10 ppm, and vi) Phosphorus in amounts less than 20 ppm, where each ppm value is based on parts by mass. 44. The sintered body according to claim 43, characterized in that the amount of nitrogen is from 0.1 ppm to 300 ppm. 45. The sintered body according to claim 43, characterized in that the amount of boron is from 0.01 ppm to 10 ppm. 46. The sintered body according to claim 43, characterized in that the amount of sulfur is from 0.1 ppm to 10 ppm. 47. The sintered body according to claim 43, wherein the amount of silicon is from 0.01 ppm to 20 ppm. 48. The sintered body according to claim 43, characterized in that the amount of arsenic is from 0.01 ppm to 10 ppm. 49. The sintered body according to claim 43, characterized in that the amount of phosphorus is from 0.1 ppm to 20 ppm. 50. Use of the sintered body according to any one of claims 38 to 49 for electronic components. 51. The use according to claim 50, wherein the electronic component is a capacitor. 52. Valve metal powder, comprising: i) Oxygen ^{concentrations} greater than 4100 ppm∙g/m² ii) Nitrogen at levels below 300 ppm, iii) Boron in amounts below 10 ppm, iv) Sulfur at levels below 20 ppm, v) Silicon at levels below 20 ppm, vi) Arsenic in amounts below 10 ppm, and vii) Phosphorus in amounts less than 20 ppm, where each ppm value is based on parts by mass. 53. The valve metal powder of claim 52, wherein the amount of oxygen is from 4100 ppm∙g/ ^m² to 8000 ppm∙g/ ^m² . 54. The valve metal powder of claim 52, wherein the amount of nitrogen is from 0.1 ppm to 300 ppm. 55. The valve metal powder of claim 52, wherein the amount of boron is from 0.01 ppm to 10 ppm. 56. The valve metal powder of claim 52, wherein the amount of sulfur is from 0.1 ppm to 10 ppm. 57. The valve metal powder of claim 52, wherein the amount of silicon is from 0.01 ppm to 20 ppm. 58. The valve metal powder of claim 52, wherein the amount of arsenic is from 0.01 ppm to 10 ppm. 59. The valve metal powder of claim 52, wherein the amount of phosphorus is from 0.1 ppm to 20 ppm. 60. The valve metal powder according to any one of claims 52 to 59, characterized in that the valve metal powder has a BET surface area of 1.5 m²/g to 20 m²/g. 61. The valve metal powder according to claim 60, characterized in that the valve metal powder has a BET surface area of 2.0 m²/g to 15 m²/g. 62. The valve metal powder according to claim 61, characterized in that the valve metal powder has a BET surface area of 3.0 m²/g to 10 m²/g. 63. The valve metal powder according to claim 62, characterized in that the valve metal powder has a BET surface area of 4.0 m²/g to 8.0 m²/g.