WO2008007126A1

WO2008007126A1 - Improvements in the production of electrolytic capacitor

Info

Publication number: WO2008007126A1
Application number: PCT/GB2007/002660
Authority: WO
Inventors: Malcolm Ward-Close; Alastair Godrey; Stephen Kyle-Henney; David Hodgson; Stewart Male
Original assignee: Metalysis Limited
Priority date: 2006-07-13
Filing date: 2007-07-13
Publication date: 2008-01-17
Also published as: MX2009000488A; KR20090031462A; GB0613984D0; CN101501798A; RU2009101296A; IL196457A0; EP2052395A1; JP2009543369A

Abstract

A method for the production of a precursor anode for an electrolytic capacitor comprises the steps of: a) forming an anode body from a suitable anode metal or metal alloy; b) subjecting the anode body to electro-deoxidation, whereby it undergoes electrolysis in a molten salt electrolyte under conditions so as to remove some undesired oxygen from the metal or metal alloy; c) washing the anode body to remove the electrolyte; and, d) depositing a dielectric layer on the anode body to form the precursor anode. The method is particularly suitable for producing porous tantalum anodes for solid electrolytic capacitors.

Description

Improvements in the Production of Electrolytic Capacitors

The present invention relates to an improved method for the production of electrolytic capacitors, especially solid electrolytic capacitors, and to the products of such methods. The invention relates, in particular, to the production of tantalum capacitors with improved performance characteristics.

In the initial manufacture of an electrolytic capacitor, a very thin film of oxide is deposited onto the surface of a suitable anode metal, for example aluminium, tantalum or niobium, by passage of an electric current, so as to produce an insulating or dielectric layer. After the dielectric layer has been formed, a conducting layer, cathode layer and packaging layer are added, in turn, to the precursor anode body, to form the end capacitor product. A commonly used conducting layer in solid electrolytic capacitors is manganese dioxide. Alternatively, in wet electrolytics, a liquid conducting layer is used. The cathode typically comprises deposited graphite and silver layers.

The advantage of electrolytic capacitors over other capacitor designs is their high capacitance rating, resulting from the thinness of the dielectric layer and its large surface area, coupled with the anodic oxide having a reasonably high dielectric constant. The dielectric properties of tantalum pentoxide are particularly suitable for use in electrolytic capacitors. Components based on tantalum have significant benefits for circuit design, due to their volumetric efficiency and reliability, and are widely used in the electronics industry in products such as mobile phones, pagers and laptops.

However, the performance of electrolytic capacitors can be degraded by impurities present in the metal anode body. For example, high levels of oxygen can cause defects in the dielectric layer during oxide growth, leading to an undesirably high leakage current in the end capacitor product. This, in turn, leads to a reduction in the capacitance and/or voltage rating of the end capacitor product, and, as a consequence, worse volumetric efficiency.

Conventional processes for the manufacture of tantalum capacitors involve inevitable pickup of undesirable oxygen during some of the processing steps. In the particular case of porous tantalum electrolytics, in which porous anode bodies are formed by sintering tantalum powder, the powder supplier typically goes to great lengths to produce Ta powder with very low bulk oxygen levels so as to ameliorate this problem. However, deoxidised tantalum powder presents a serious ignition hazard during handling, and, to improve safety, a secondary surface layer of tantalum pentoxide (commonly known as a passivation layer) is formed on the powder before shipping. Thus, although the bulk oxygen level in such Ta powders may be low, in some cases around 2800ppm, the total oxygen content of the powder is significantly higher.

Attempts have been made to reduce oxygen concentration levels during the manufacture of Ta capacitors, so as to improve their performance. In JP 11111575 A, for example, a porous anode body of tantalum is sintered in the presence of carbon and magnesium and then subjected to acid-cleaning, to attain a low-oxygen-concentration anode for use in a solid electrolytic capacitor. However, methods proposed to date suffer various problems, notably inherent disadvantages associated with the deoxidation step. Examples of said disadvantages are: residues from leaching processes which affect the dielectric coating, softening of the support wire, and poor anode surface properties.

There is also a particular need for a process that is suitable for scaling up for commercial capacitor production (where production rates can exceed 10⁵ to 10⁶ anodes per week).

In accordance with a first aspect of the present invention, there is provided a method for the production of a precursor anode for an electrolytic capacitor, the method comprising the steps of:

a) forming an anode body from a suitable anode metal or metal alloy;

b) subjecting the anode body to electro-deoxidation, whereby it undergoes electrolysis in a molten salt electrolyte under conditions so as to remove some undesired oxygen from the metal or metal alloy; c) washing the anode body to remove the electrolyte; and,

d) depositing a dielectric layer on the anode body to form the precursor anode.

The present invention provides a convenient way of removing undesired surface and/or bulk oxygen from metallic anode bodies, which process is especially suitable for industrial-scale batch processing.

The method is applicable to all anode metals or metal alloys that form oxide layers with suitable dielectric properties. Only a few metals form dense, stable, tightly adhering, electrically insulative oxides, which metals include aluminium, titanium, zirconium, niobium, tantalum and hafnium (the so-called "valve metals"). Tantalum, in particular, has been found to withstand the present processing conditions without undue damage or contamination. A benefit of the process of the present invention is that the volumetric efficiency of the end capacitor product is increased, therefore it is possible that the invention may render viable the use of more expensive metals or metal alloys that are currently uneconomic. Another benefit of increased volumetric efficiency is that smaller capacitors may be produced, leading to a higher packing density of components.

It has been shown that, under suitable conditions, metals and metal alloys containing dissolved oxygen can be deoxidised by electrolysis in a fused salt electrolyte. Thus, in the present invention, a metal or metal alloy containing dissolved, interstitial oxygen is made into the cathode of an electrolysis cell, or otherwise put in contact with a cathode. The cell contains a fused-salt electrolyte and the cathode and an anode contact the electrolyte. A cell voltage is applied between the cathode and the anode, the voltage being such that the cathode potential is sufficient to remove oxygen from the cathode.

In a preferred embodiment, the applied cell voltage is such that the cathode potential is sufficient to remove oxygen from the cathode but is not sufficient to cause cations in the electrolyte to deposit as metal at the cathode. Advantageously, the applied cell voltage may also be controlled so as to avoid decomposition of the fused salt electrolyte.

This method, sometimes termed electro-deoxidation, may be used to remove oxygen from solid metals or alloys, or to remove oxygen from solid metal compounds or semi-metal compounds. The latter aspect is described for example in WO 99/64638 of Fray et al. Both aspects may be applicable to the deoxidation of anode bodies, for example to remove oxygen dissolved in the metal of an anode body or to remove oxygen from a surface passivation layer of metal oxide.

The present process can be applied to anode bodies made by any of the usual routes, for example, by metal foil, etched foil, sputtered film or powder processing routes. The latter route, involving powder compaction, followed by sintering, leads to the formation of porous anode bodies with high surface areas. Typically, in such a process, tantalum powder is first mixed with a resin binder and mechanically pressed into pellets of a controlled density. Each pellet is then pressed onto a fine tantalum or tantalum alloy wire to form an anode preform or 'slug'. Next, the anode preforms are piled into trays and heated under vacuum, in order to burn out the resin binder. Finally, the 'green' anodes are sintered at high temperature to impart full strength, whilst retaining a porosity of, typically, 50%.

The electro-deoxidation step can be used to deoxidise the anode body at any stage in its manufacturing process prior to anodisation. In order to minimise opportunities for picking up oxygen during subsequent anode processing, however, the electro-deoxidation step is advantageously carried out as the last step prior to forming the dielectric layer.

Particular problems with oxygen contamination arise when the green anodes are formed by a powder processing route, because the sintering step results in oxygen diffusion from the outer protective passivation layer into the bulk tantalum. This results in an increase in the bulk oxygen level, and, in a typical situation, bulk oxygen can increase from around 2800ppm to around 6000ppm. Thus, where a powder processing route is employed for anode manufacture, the anode body is preferably deoxidised after the sintering step so that the bulk oxygen level can be reduced.

We have found that, in addition to removing oxygen present at the surface layer, the process of the present invention is effective in significantly reducing dissolved, interstitial oxygen from the bulk metal or metal alloy, unlike some prior art surface deoxidation methods. This improves the quality of the subsequently formed dielectric layer, leading to a lower risk of short circuits and improved end capacitor performance.

Unexpectedly, we have found that, despite the short diffusion distance for tantalum atoms in a metal matrix, Ta metal containing interstitial oxygen retains its mechanical integrity after electro-deoxidation, permitting further processing of the anode bodies. This not only allows the manufacturer to take advantage of the increased mechanical strength imparted by the sintering process, but enables the bulk oxygen level to be reduced prior to anodisation.

A further advantage of the present invention is that it enables a less hazardous, higher bulk oxygen Ta (or other metal or alloy) powder to be used as a starting material, because electro-deoxidation allows such interstitial oxygen to be removed without damaging the integrity of the anode body. For example, Ta powder with an oxygen level in excess of 6000ppm or IOOOOppm or 14000ppm may be used as a starting material.

In a conventional capacitor manufacturing process, formation of the dielectric layer, by anodisation, is carried out on batches of anode bodies. In a typical process, sintered anode bodies are welded, in batches, to aluminium strips at a tightly controlled pitch and length so that the bottom of all anodes are at the same distance from the aluminium strip. One or more strips are then loaded into a supporting frame, which frame is then transferred to an anodisation vessel for formation of the dielectric layer. The anodisation process is carefully controlled to give a uniform and complete oxide thickness over the external and internal surfaces of the anode. The temperatures used at this stage are below those required for secondary bulk diffusion of oxygen. Suitably, the present invention takes advantage of existing processing methods and a plurality of anode bodies is deoxidised as a batch. In one preferred method, the plurality of anode bodies is fixed to an electrically-conducting support prior to electro-deoxidation, such as a metal strip. The support should be suitable for use in the electro-deoxidation cell. Preferably, the metal strip comprises a corrosion resistant metal or alloy with a melting point greater than the temperature of the electrolysis cell, for example steel. More preferably, a plurality of supports are mounted into a racking system; by processing the anode bodies in supporting racks or frames, handling is made easier. Advantageously, the conducting support is made the cathode of the electrolysis cell, for example by bringing an electrical brush connector into contact with the support or racking system. Conveniently, the frames and/or metal strips are suitable for later use in the anodisation vessel.

It may be desirable to control the depth of immersion of the anodes into the molten salt electrolyte of the electrolysis cell. Preferably, the anode body is fully covered with electrolyte, but the contact wire is only partially immersed. This allows components required for later processing, such as a polymeric insulator for cathode isolation, to be loaded onto the contact wire prior to fixing the anodes to the support, without necessarily introducing those components into the electrolyte. Moreover, possible softening of the contact wire upon electro- deoxidation may be ameliorated.

In another preferred method, a batch of anode bodies is held in a basket during electro-deoxidation and the basket is made the cathode of the electrolysis cell. More preferably, the anode bodies are agitated during electrolysis, in order to make sure that all anode bodies are brought into electrical contact with the cathode.

During the electro-deoxidation stage, the anode bodies are made the cathode of the electrolysis cell, or held in electrical contact with a cathode, and the cell voltage is set at a suitable level for an electro-deoxidation process to take place. This level is sufficient that oxygen is removed from the cathode, but is preferably also below the voltage at which chlorine is evolved from the electrolyte. Suitably, the anode of the electrolysis cell comprises carbon. For best results the temperature of the electrolysis cell should be maintained at 500-1200⁰C, or preferably 700-1000⁰C, throughout the electrolysis, thereby aiding diffusion processes. However, localised cooling or heating of certain regions of the cell, for example the racking system, may be required. For electro-deoxidation using a carbon anode and calcium chloride electrolyte at 500-1200⁰C, the cell voltage for a small-scale cell, such as a laboratory-scale cell, may lie in the range 1.4V to 3.2V. In larger cells higher cell voltages may be required to overcome losses such as IR losses and maintain the cathode potential at a desired level.

The electrolyte should comprise a fused salt, or mixture of salts, which is more stable than the equivalent salt of the metal or metal alloy that is being deoxidised. Furthermore, the salt, or mixture of salts, preferably has a wide difference between its melting and boiling point and a high temperature of operation to improve the diffusion of oxygen and other species in the capacitor anode body and elsewhere. Suitable electrolytes include the chlorides or other salts of barium, calcium, caesium, lithium, strontium and yttrium. CaCI₂ is particularly suitable as an electrolyte. In order to optimise the deoxidation process, a suitable concentration of oxide ions in the electrolyte is also desirable, to ensure continuous oxygen transport from the cathode to the anode.

The electro-deoxidation process is applied until the desired oxygen level has been reached. Typically, this will be after 24 to 48 hours, but depends on the batch size.

Preferably, electrolysis is conducted until the oxygen level is below 14000ppm, IOOOOppm or 6000ppm, or is below 3000ppm, more preferably below 2000ppm and even more preferably below lOOOppm.

Depending on the quality of the starting material, a 5-fold, 10-fold or even 20-fold reduction in oxygen is easily attainable. An anode body manufactured by a powder metallurgy route is particularly suitable for use in an electro-deoxidation process, due to its high porosity.

To avoid secondary problems, if a conventional process is to be used for forming the dielectric on the anode body after electro-deoxidation, the electrolyte must be fully washed from the deoxidised anode bodies, using a suitable solvent such as water, in order to remove any contamination prior to forming the dielectric. Optionally, methods comprising pressure leaching or agitation may be used to improve the penetration of porous anode bodies with the solvent. Alternatively, the electrolyte may be distilled from the anode bodies, or a combined washing and distillation method may be used.

After electro-deoxidation steps are preferably taken to ensure that atmospheric oxygen is not picked up by the anode bodies, particularly while the bodies are at a high temperature. One method of doing this is to control the atmosphere, for example by processing in an inert atmosphere such as argon, or by processing the anodes under vacuum.

The final step in the production of the precursor anodes is to anodise the deoxidised anode bodies to form the dielectric layer. Typically, anodisation is by means of electrolysis in a weak acid.

In a further aspect of the invention, the dielectric layer may advantageously be formed on the anode body surface without exposing the anode body to oxygen after the electro-deoxidation process. This step has the advantage that the electro-deoxidation process may reduce the bulk oxygen concentration in the anode body to a low level, and the low level of oxygen may then be retained in the precursor anode after formation of the dielectric layer and in the final capacitor product, and may lead to improved capacitor performance. Preferably, this step may be achieved by operating the electro-decomposition cell until the bulk oxygen content in the anode body, at the cathode of the cell, is less than a predetermined level, such as 14000ppm, lOOOOppm, ΘOOOppm or even 2000ppm, IOOOppm or 750ppm. At this point, the polarity of the cell is reversed so that the anode body is connected as the anode of the cell. This has the effect of oxidising the anode body under controlled conditions, through control of the anode potential, in order to form the oxide dielectric layer on the anode body surface. Since reversing polarity of the cell is a purely electrical step, the dielectric layer can be formed directly on the low-bulk oxygen-content anode body, for example eliminating any need to wash the electro- decomposition cell electrolyte out of the anode body before the dielectric layer is formed in a separate process. Thus, the low bulk oxygen content may be retained.

Controlling the anode potential and the length of time of application of the reverse polarity in the electro-decomposition cell may advantageously allow precise control of the dielectric layer thickness, and may improve not only the capacitance of the end capacitor product but also leakage currents.

It may be advantageous to vary other parameters of the electro-deoxidation cell in-between the electro-deoxidation step and the dielectric formation step. For example, the temperature of the cell may be changed from a preferred electro- deoxidation temperature to a preferred dielectric formation temperature. Preferably, the electro-decomposition potential at the cathode should be maintained while, for example, the cell temperature is varied, in order to maintain the low bulk oxygen content of the anode body.

After formation of the dielectric layer, the precursor anode may be removed from the electrolyte and washed, as described above, before further processing. Since the dielectric layer has been formed, the anode body is then protected from further oxidation, for example due to exposure to the atmosphere.

In each aspect of the invention, after the precursor anodes have been formed, the end capacitor product is produced by adding, in succession, a conducting layer, a cathode layer and a packaging layer. For a solid electrolytic capacitor, the conducting layer is typically formed by dipping the precursor anodes into manganese nitrate solution, which may then be thermally decomposed to manganese Oxides' until complete coverage of the dielectric is achieved. Next, the outside layer of the manganesed anode may be coated with graphite and then a silver-loaded resin, which forms the cathodic connection to the manganese and an external electrical contact. The anode may be attached to a contact frame and encapsulated in resin before test.

There may be a number of test stages in between each of the above steps and also some processes to repair thermal damage to the dielectric.

Another aspect of the invention provides a precursor anode produced by the process described above.

A further aspect of the invention provides a final capacitor product comprising an anode obtained by the process described above.

Yet another aspect of the invention provides a dielectric capacitor comprising an anode, a dielectric layer, a conducting medium, a cathode and packaging, wherein the oxygen level in the anode is below 3000ppm, preferably below 2000ppm and more preferably below lOOOppm.

The following Examples illustrate the invention:-

Example 1

A batch of sixteen sintered, tantalum anode bodies were welded to a stainless steel strip and electrically connected to the cathode of an electro-deoxidation cell. The tantalum anode bodies were electrolysed at a cell temperature of 950⁰C for 24 hours at 3.0V, using a calcium chloride electrolyte, a carbon anode and an inert atmosphere. When the anode batch was removed from the cell, it was found that the anode bodies had held together mechanically and thus, were capable of further processing to form capacitors.

Subsequent analysis of a sample anode body revealed a desirable reduction in bulk oxygen levels, and a 23-fold reduction in the oxygen concentration of the anodes was measured.

Example 2

A sintered, tantalum anode body was processed under the conditions described in Example 1 by electrolysis in the electro-deoxidation cell. After the electro-deoxidation process, however, before removal of the anode body from the electrolyte, the cell polarity was reversed. During this process the anode body was electrolysed for 20 minutes at 3.0V to form a dielectric oxide layer on its surface. The anode body/anode precursor was then removed from the cell and washed, before further processing to form a capacitor. In a modification of this Example, the temperature of the electro-decomposition electrolyte may be reduced from 95O⁰C for the anodisation step.

Claims

1. A method for the production of a precursor anode for an electrolytic capacitor, the method comprising the steps of:

a) forming an anode body from a suitable anode metal or metal alloy;

b) subjecting the anode body to electro-deoxidation, whereby it undergoes electrolysis in a molten salt electrolyte under conditions so as to remove some undesired oxygen from the metal or metal alloy; and

c) depositing a dielectric layer on the anode body to form the precursor anode.

2. A method according to Claim 1 , wherein the anode body comprises tantalum.

3. A method according to Claim 1 or Claim 2, wherein step a) comprises forming the anode body by a powder metallurgy process including a sintering step.

4. A method according to Claim 3, wherein step b) is conducted immediately after the sintering step of step a).

5. A method according to any preceding claim, wherein, in a subsequent processing step, the outer dielectric layer is coated with a solid conducting layer.

6. A method according to Claim 5, wherein the solid conducting layer comprises manganese dioxide.

7. A method according to any preceding claim, wherein later processing steps comprise cathode deposition and a packaging step, so as to form the end capacitor product.

8. A method according to any preceding claim, wherein a plurality of anode bodies are deoxidised in batches.

5 9. A method according to Claim 8, wherein a plurality of anode bodies are fixed to a conducting support prior to electro-deoxidation.

10. A method according to Claim 9, wherein the support is a metal strip, preferably comprising steel or aluminium. 0

11. A method according to Claim 9 or Claim 10, wherein a plurality of supports are mounted in a racking system.

12. A method according to any one of Claims 9 to 11 , wherein the support iss made the cathode of the electrolysis cell.

13. A method according to Claim 8, wherein the batch of anode bodies is held in a basket during electro-deoxidation. 0

14. A method according to Claim 13, wherein the basket is made the cathode of the electrolysis cell.

15. A method according to Claim 14, wherein the anode bodies are agitated during electrolysis. 5

16. A method according to any preceding claim, wherein electrolysis is conducted until the oxygen concentration in the anode body reaches a desired level. o

17. A method according to Claim 16, wherein the desired oxygen level is less than 3000ppm, preferably less than 2000ppm and more preferably less than lOOOppm.

18. A method according to any preceding claim, wherein between steps b) and c) the atmosphere is controlled to prevent subsequent oxidation of the deoxidised anodes.

19. A method according to Claim 1 , comprising the step of washing the anode body to remove the electrolyte before depositing the dielectric layer.

20. A method according to any of Claims 1 to 18, in which step c) is carried out by anodising, or by applying an anodic potential to the anode body in contact with a molten salt electrolyte.

21. A method according to Claim 20, in which the anodisation step is performed in the same molten salt electrolyte as the electro-deoxidation step b).

22. A precursor anode produced by a method as claimed in any preceding claim.

23. A batch process for the production of a plurality of tantalum anodes, the process comprising the steps of:

a) forming a plurality of anode bodies from a suitable anode metal or metal alloy;

b) subjecting the anode bodies to electro-deoxidation, whereby said bodies undergo electrolysis in a molten salt electrolyte under conditions so as to remove some undesired oxygen from the metal or metal alloy; and

c) depositing a dielectric layer on the anode bodies to form the precursor anodes, and optionally one or more of the following steps:

d) coating the dielectric layer with a solid conducting layer;

e) depositing a cathode layer; and

f) conducting a packaging step so as to form end capacitor products.

and optionally, wherein during at least steps b) to c), and preferably during at least steps b) to e), above the anodes remain in situ on conducting supports.

24. A batch process according to Claim 23, comprising the step of washing the anode bodies to remove the electrolyte before deposition of the dielectric layer.

25. A batch process according to Claim 23, comprising the step of depositing the dielectric layer by anodisation of the anode bodies in a molten salt electrolyte, and preferably in the molten salt electrolyte used for the electro-deoxidation step b).

26. The use of an electro-deoxidation step to deoxidise precursor tantalum anodes as part of a multi-step batch process for producing multiple anode bodies.

27. Any novel feature or combination of novel features hereinbefore described.

28. A method substantially as hereinbefore described.

29. A precursor anode or capacitor product substantially as hereinbefore described.