Field of the invention.

The invention relates to a wound capacitor and to a method of manufacturing a wound capacitor.

Background of the invention.

Thin film capacitors are used for storing energy in a variety of applications. A capacitor has a pair of conductive electrodes separated by a dielectric medium.

Capacitors can be formed from a pair of metallized polymer films wound together into a roll. Generally, the metallized films are obtained by depositing a thin layer of a conductive material onto a polymer film.

However, this type of capacitors shows a number of drawbacks. The polymer films are characterized by a limited relative dielectric constant Er- Also the thickness of the polymer film (dielectricum) can not be lower than a certain minimum value. The lowest limit is generally accepted to be 0. 7 pm.

As the capacitance of a capacitor is determined as <BR> <BR> <BR> S<BR> <BR> C=sO£r d <BR> <BR> d with S : the area of the capacitor; dd : the thickness of the dielectric material (the separation distance between two metal layers) ; so : the dielectric constant of vacuum; Er : the relative dielectric constant of the dielectric material; only moderate capacitance values can be reached.

Summary of the invention.

It is an object of the present invention to provide a wound capacitor whereby the dielectric material is characterized by a high relative dielectric constant Brand a low thickness.

It is another object of the invention to provide a wound capacitor having a high capacitance per volume.

It is a further object of the invention to provide a method of manufacturing a wound capacitor.

According to a first aspect of the present invention, a wound capacitor is provided. The capacitor comprises at least a first and a second structure. The first structure and the second structure each comprises a metal substrate and a dielectric material.

The dielectric material has preferably a relative dielectric constant er higher than 20.

The thickness of the dielectric material is preferably lower than 1 µm.

More preferably, the thickness of the dielectric material is lower than 0.6 µm.

DIELECTRIC MATERIAL The dielectric material is preferably selected from the group consisting of oxides, titanates, niobates and zirconates.

Some common titanates used for capacitors comprise CaTi03, SrTiO3, BaTiO3 and PbTiO3, (Ba, Sr) TiO3, PbZr(1-x)TixO3, Sr(1-x)BixTiO3, NbxgTiO3, BiBi2NbTiOs, BaBi4ti4O15, Bi4Ti3O12, SrBi4Ti4O15, BaBi4Ti4O15, PbBi4Ti40is or PbBi4Ti4ol6.

Some niobates comprise CaBi2Nb2O9, SrBi2NbzOs, BaBizNb2Os, PbBizNbzO9, (Pb, Sr)Bi2Nb2O9, (Pb, Ba) Bi2Nb209, (Ba, Ca) Bi2Nb209, (Ba, Sr)Bi2Nb2O9, BaBi2Nb2O9, PbBi2Nb2O9, SrBi2Nb2O9, Ba0.75Bi2.25Ti0.25 Nbi. 75O9, Ba0.5Bi2.5Ti0.5Nb1.5O9, Ba0.25Bi2.75Ti0.75Nb1.25O9, Bi3TiNbO9, SrO8Bi22TiO2Nb1 809, Sro. 6Bi2.4Ti0.4Nb1.6O9, Bi3TiNbO9, Pb0.75, Bi2. 25Tio2sNb1 7509, Pbo. 5Bi2.5Ti0.5Nb1.5O9, Pb0.25Bi2.75Ti0.75 Nb1.25O9 or Bi3TiNbOs.

Common oxides comprise Ta2O5 and TiOz.

The dielectric material is preferably deposited on the metal substrate by means of a vacuum deposition technique such as sputtering, for example magnetron sputtering, ion beam sputtering and ion assisted

sputtering, evaporation, laser ablation or chemical vapor deposition such as plasma enhanced chemical vapor deposition.

Dielectric material deposited by means of a vacuum deposition technique have a number of advantages.

First of all, by vacuum deposition dielectric materials having a high relative dielectric constant sr can be obtained. As described above the relative dielectric constant Er of the dielectric material is preferably higher than 20.

However, dielectric materials with a relative dielectric constant srthat is much higher can be obtained.

Typical ranges of dielectric material are from 20 to 100, from 100 to 1000, from 1000 to 10000, from 10000 to 20000 and even higher than 20000.

A second advantage is that thin layers of dielectric material can be deposited.

The thickness of the dielectric material can be much lower than the thickness of the dielectric material (i. e. the thickness of polymer films) in the known metallized film capacitors.

The minimum thickness that can be reached in the known metallized film capacitors is generally accepted to be 0.7 pm.

By vacuum deposition layers of 0.001 pm can be deposited.

Generally, the thickness of a vacuum deposited dielectric layer is between 0.001 and 10 pm, as for example 1 pm, 0.1 pm or 0.01 pm.

Both the increase in the relative dielectric constant Er and the reduction of the thickness of the dielectric material have a positive influence on the capacitance.

The capacitance C is defined as : <BR> <BR> <BR> <BR> <BR> <BR> C = #0 #r S/dd<BR> d S : the area of the capacitor;

dd : the thickness of the dielectric material (the separation distance between two metal layers); so : the dielectric constant of vacuum; 8r the relative dielectric constant Er constant of the dielectric material.

A third advantage of a dielectric material deposited by a vacuum deposition technique is the high quality of the dielectric material that can be obtained and that the ease to control the thickness of the dielectric material.

Furthermore by depositing a dielectric material on a metal substrate higher temperature can be reached compared with metallized polymer films.

METAL SUBSTRATE Preferred metal substrates comprise for example tapes or foils or metallized tapes or foils.

The metal comprises preferably steel, nickel or a nickel alloy, or titanium or a titanium alloy.

The metal substrate preferably has a thickness between 1 and 100 pm, as for example 10 pm.

Metalized tapes or foils comprise preferably a polymer tape or foil coated on both sides with a metal layer.

It is preferred that the first and the second flexible substrate are wound in such a way that the first side of the first flexible structure extends beyond the first side of the second flexible structure and that the second side of the second flexible structure extends beyond the second side of the first flexible structure.

In this way an offset is create between the first and the second flexible structure.

By creating an offset between the first and the second flexible structure, electrical connections to the capacitor can easily be formed.

In a preferred wound capacitor the first and the second structure are bonded to each other by means of metal layer to form a layered structure. This layered structure is then wound into a roll to form a wound capacitor.

The metal coating is preferably applied by applying a metal coating on the first flexible structure and by applying a metal coating on the second flexible structure, by bringing the coated surfaces of said first flexible structure and said second flexible structure together and by pressing the first flexible structure and the second flexible structure together to create a cold welding between said first flexible structure and said second flexible structure.

The coating on the first and the second flexible structure can be applied by any technique known in the art as for example wet chemical deposition techniques or vacuum deposition techniques.

Preferably, the coating on the first and the second flexible structure is applied by means of vacuum deposition techniques such as sputtering, for example magnetron sputtering, ion beam sputtering and ion assisted sputtering, evaporation, laser ablation or chemical vapor deposition such as plasma enhanced chemical vapor deposition.

The metal coating may comprise any metal or metal alloy. Preferred metal layers comprise for example Al, Ti, V, Cr, Co, Ni, Cu, Zn, Rh, Pd, Ag, In, Sn, Ir, Pt, Au, Pb or alloys thereof.

Preferably, the coating applied on the first flexible structure is identical to the coating applied on the second flexible structure.

The coating on the first flexible structure and the coating on the second flexible structure can be applied by one deposition source or by two different deposition sources. The application by one deposition source is preferred.

A cold welding may occur when two clean metal surfaces are brought into intimate contact.

To obtain a cold welding, the metal surfaces have to be free of contamination, such as oxides, nitrides, absorbed gases or organic contaminations. In addition, the metal surfaces have to be brought together under sufficient high mechanical force to bring the atoms at the interface into intimate contact.

The elimination of contamination can be obtained by cleaning the metal surface.

In a preferred embodiment, the application of the metal coating on the first and the second flexible structure and the cold welding of the first and the second flexible structure is performed in a vacuum without breaking the vacuum between the coating step and the cold welding step.

By maintaining the vacuum through the process steps, one prevents the formation of surface oxides and other contaminations.

Furthermore, by performing the different process steps in one process chamber, the need to relocate or otherwise move the flexible structures between different process chambers is eliminated.

A wound capacitor according to the present invention comprising a metal layer to bond the first and the second structure has the advantage that the use of an organic adhesive such as a glue is not necessary.

This is a great advantage as an organic adhesive may damage the dielectric layer.

Brief description of the drawings

The invention will now be described into more detail with reference to the accompanying drawing wherein - Figure 1 shows a first embodiment of a wound capacitor according to the present invention; - Figure 2 shows a second embodiment of a wound capacitor according to the present invention; - Figure 3 shows a method of manufacturing a layered structure of a wound capacitor as shown in Figure 2; - Figure 4 shows a third embodiment of a wound capacitor according to the present invention.

Description of the preferred embodiments of the invention Figure 1 shows a first embodiment of a wound capacitor according to the present invention.

The wound capacitor 10 comprises a first flexible structure 12 and a second flexible structure 14. The first and the second flexible substrate comprises a metal substrate 15 and a dielectric material 16 deposited on the metal substrate.

The metal substrate 15 comprises a metal foil having a thickness of 10 pm.

The dielectric material 16 is deposited on the metal substrate by a sputter process.

The layer of dielectric material has a thickness of 0.15 pm and a relative dielectric constant ar of 500.

The first and the second flexible structures are wound around an axis 18 with an offset 19 between the first and the second flexible structure to form a wound structure. The wound structure is provided with electrical contacts 11 to form the wound capacitor 10.

The electrical contacts 11 are preferably applied by a spraying technique. It can be preferred that the wound structure is embedded in a polymer matrix before the electrical contacts 11 are applied.

Figure 2 shows a second embodiment of a wound capacitor according to the present invention.

The wound capacitor 20 comprises a first flexible structure 22 and a second flexible structure 24. The first and the second flexible substrate comprises a metal substrate 25 and a dielectric material 26 deposited on the metal substrate 25.

The metal substrate 25 comprises a metal foil having a thickness of 10. pm.

The dielectric material 26 is deposited on the metal substrate by a sputter process.

The layer of dielectric material has a thickness of 0.15 pm and a relative relative dielectric constant Er of 500.

The first and second flexible structure are bonded to each other by means of metal layer 27. In this way a layered structure 27'is obtained.

It is preferred to have an offset 29 between the first and the second flexible structure.

The layered structure 27'is then wound around axis 28 to form a wound structure.

The wound structure is provided with electrical contacts 21 to form the capacitor 20.

Figure 3 shows a schematic representation of a preferred method to manufacture a layered structure 27'of the capacitor shown in figure 2.

A first flexible structure 22 and a second flexible structure 24 each comprising a metal foil coated with a dielectric material are provided in a vacuum chamber. The two flexible structures 22 and 24 are coated from a deposition source 32 with a metal coating layer 34. Subsequently, the two coated flexible structures are united by pressing the laminated structure together between two rolls 36.

Between the two coated surface a cold welding is created.

The coating of the flexible structures 22 and 24 and the uniting of the two flexible structures by means of the coating layer 34 is preferably done in the vacuum chamber without breaking the vacuum.

Possibly, the method may be followed by other processing steps such as heating, coating, slitting, another lamination process...

Figure 4 shows a third embodiment of a wound capacitor 40 according to the present invention.

The wound capacitor 40 comprises a first flexible structure 42 and a second flexible structure 44. The first and the second flexible substrate comprises a substrate 45 and a dielectric material 46 deposited on the substrate 45.

The substrate 45 comprises a polymer film metallised on both sides. In Figure 4, the polymer film is indicated with'a', the metallised layers are indicated with'b'.

The dielectric material 46 is deposited on the substrate 45 by a sputter process.

The layer of dielectric material has a thickness of 0.10 pm and a relative relative dielectric constant srof 1000.

The first and the second flexible structures are wound around an axis 48 with an offset 49 between the first and the second flexible structure to form a wound structure. The wound structure is provided with electrical contacts 41 to form the wound capacitor 40.

An advantage of a capacitor as shown in figure 4 is its self-healing properties.

Self-healing is a phenomenon where in the event the electrodes are exposed to each other, the capacitor repairs itself.

If a pinhole or another defect occurs in the dielectric material 46, this acts as a local spot with low dielectric strength is created. If a voltage is applied across the electrodes, the local spot will breakdown.

The energy released in the breakdown channel is sufficient to totally evaporate the thin metal coating in the vicinity of the channel.

After this breakdown the capacitor still functions. This is in contrast with capacitors using thick metal films, whereby a short circuit destroys the capacitive features.

To show the attractiveness of a capacitor according to the present invention, the capacitance per volume of a capacitor according to the present invention is compared with the capacitance per volume of a metallized film capacitor known in the art.

The capacitance per volume is defined as: <BR> <BR> <BR> C 80 8r <BR> <BR> <BR> <BR> V dddoap with so : the dielectric constant of vacuum; Er : the relative dielectric constant Er constant of the dielectric material; dd : the thickness of the dielectric material (the separation distance between two metal layers); dcap : dd = de (with de the thickness of the metal layer (the electrode)).

A metallized film capacitor comprises a metallized polymer film wound into a roll to form a capacitor. The metallized polymer film is formed by depositing a thin layer of a conductive material onto a polymer film.

The metallized film capacitor that is considered as an example comprises a polymer film (dielectricum) having a relative dielectric constant cri of 3.

As thickness of the polymer film ddi, the thinnest thickness known in the art is considered: 0.7 pm.

In case the metal layer on the polymer film is deposited on the polymer film by means of sputtering, the thickness of a metal layer can be considered to be very low. Therefore, in the above calculation dcp is considered to be equal to ddi.

The capacitance per volume of the metalized film capacitor can be calculated as follows : <BR> <BR> <BR> <BR> C1 = #0 #r1<BR> <BR> <BR> <BR> Vl ddlddl As capacitor according to the present invention, a capacitor comprising a first and a second structure each comprising a metal substrate and a dielectric material deposited on this metal substrate is considered.

The dielectric material has a relative dielectric constant #r2 of 500, a thickness of the dielectric material dd2 of 0. 01 pm. The metal substrate (electrode) has a thickness of 10 pm.

The capacitance per volume is: <BR> <BR> <BR> <BR> C2 £o £2r <BR> <BR> <BR> <BR> V2 ddzdcap.

It can be concluded from the above mentioned examples that the capacitance per volume of the second capacitor is about 800 times higher than the capacitance per volume of the first capacitor.

It is clear that the above mentioned calculation may only be considered as an example. As the relative dielectric constant crof the dielectric material of a capacitor according to the present invnetion can be much higher than the one taken in the example and as the thickness of the dielectric material can be lower than the thickness considered in the example, capacitors with a much higher capacitance per volume can be obtained according to the present invention.