GB2056503A - Porous metal films - Google Patents

Abstract

A process for preparing porous high surface over metal films e.g. for electrolytic capacitor manufacture. A valve metal, e.g. aluminium, is evaporated in a vacuum chamber from a vapour source 25, the vapour being directed towards a substrate foil 21, drawn past the source. The deposition angle is chosen such that the evaporated metal condenses as a porous surface layer. The coated foil may be used in the manufacture of electrolytic capacitors by anodising the coating, applying contacts, and winding up the foil. <IMAGE>

Description

SPECIFICATION Porous metal films This invention relates to the manufacture of electrolytic capacitors from porous metal films or coatings. This invention also relates to the preparation and treatment of metal films and coatings, and in particular to a process for the deposition of a metal in a porous form.

Porous metal films are employed in a variety of industrial applications. For example, wrapped foil electrolytic capacitors are fabricated conventionally from aluminium foil anodes and cathodes that have been chemically or electrochemically etched to produce a high surface area. Typically a strip of aluminium foil is etched to provide a microporous surface which is then anodised to- produce a uniform dielectric layer.

There are a number of problems involved in the etching process, which nevertheless is the standard commercial process for the production of sufficiently high capacitance foils. Etching requires the use of large scale aqueous solution treatment and coating baths, and presents maintenance and effluent disposal problems. Also etching solutions typically contain ions such as chloride (CU) which inhibitanodization and must therefore be thoroughly removed before anodization can take place. Furthermore the nature of the etched pits in the metal foil is such that high capacitance foils have narrow tunnels which results in a rapid fall in capacitance and a consequent deterioration in high frequency performance at higher anodising voltages. This effect is caused by complete filling of the etched pore by the anodic oxide formed in the anodising process.

The conventional etching process is an extremely complex electrochemical process which is subject to continual change so that the surface area so produced is rather variable and not well controlled.

Additions of trace elements alloying constituents to the aluminium foil are required to give uniform etch characteristics. Metals such as tantalum are difficult to etch to yield high surface areas and all etching processes degrade the removed metal into aqueous solutions which are costly to recover. Alloy foils which may have advantageous properties for anodising are also difficult to etch.

The object of the invention is to minimise or to overcome these disadvantages, and to provide a general method of preparing metal films or foils in a high area form.

The object of the etching process is of course to produce the highest surface area consistent with subsequent anodising and handling and it is the feature of the described invention that the etching process is replaced with a more uniform controllable process capable of producing higher usable surface areas than conventional etching and-hence produce more compact electrolytic capacitors with an improved capability for automatic manufacture. A further feature of this invention is that anodisable bodies with high specific area can be made in wide variety of forms including foil.

According to one aspect of the invention there is provided a process for preparing a porous metal film or coating on a substrate surface, including directing a stream of the metal vapour on to the surface in vacuum, the deposition being performed under such condition as to provide a porous metal deposit.

According to another aspect of the invention there is provided a process for preparing a porous metal film of coating on a substrate surface, including traversing the substrate surface adjacent a source of the meta! vapour, wherein the deposition is performed in vacuum, wherein the metal vapour source is so constructed as to generate the vapour primarily in one direction, and wherein the deposition is performed within such a sample of angles of incidence as to provide a porous metal deposit.

It is weli known to produce films of metals or other materials in a porous state by vacuum deposition.

Metal powders have been prepared by evaporation into inert gas atmosphere and the powders so produced have been sintered or compacted to make coherent porous bodies.

It is also known that metallic films can be grown with a columnar structure under certain conditions of substrate temperature, gas ambient and angle of incident of the condensing atom beam on the substrate. The dependence of structure on substrate temperature has been described by B. A. Movchan and A. V. Demchishin in Fizika Metallov i Metallovedenie28 (4) April 1969 654-660 and extended by J. A. Thornton in J. Vac. Sci. Technol. 11(4)1974666 to include gas pressure. The effect of gas pressure and vapour incidence angle has been described by N. G. Nahodkin and A. I. Shaldervan in Thin Solid Films 101972 a109-122.

In this invention porous films of valve metal or alloy are prepared by condensation under such conditions given by the choice of substrate temperature, gas pressure and condensation angle that subsequent anodisation can be carried out, the requirements are described below.

In order to produce an anodic dielectric film it is necessary to convert the valve metal into oxide by the steps of (1) placing the valve metal in a suitable electrolyte with an inert cathode; (2) passing a current with the valve metal mode anodic between the valve metal and the cathode until the valve metal oxide of the desired thickness is formed; (3) treating the oxide to certain regimes of voltage and temperature in electrolytes to yield a good dielectric as known by those skilled in the art.

The voltage to which the valve metal is subjected in this anodising process determines the working voltage of the capacitor by virtue of controlling the thickness of the dielectric oxide layer. For any valve metal there is a relation between anodising voltage an oxide thickness such that to = a,V or tm = amV where to is the oxide thickness ab the anodising constant (oxide) Vthe Voltage.

tm is metal thickness consumed am anodising constant (metal) V the Voltage For aluminium Am = 11A"V' which is equivalentto 11A"V' of aiuminium metal consumed. It is necessarytherefore for a porous material to possess a minimum thickness of metal in the structure such that, on anodising, the oxide does not completely consume the finely divided metal. Hence for a columnar structure the column diameter d must be d > 2amV.

For aluminium therefore which is to be anodised to say 1 00V the column diameter must be greater than 2200 A. Columnsfinerthan this size will tend to completely converted to oxide and thus not active in the capacitor.

For any given working voltage of capacitorthere will be a preferred column size, as columns smaller than those given by the relation above will be anodised right through, while those substantially larger will be inefficient in the use of valve metal since the maximum surface area is achieved by the smallest column site.

We have found that, by evaporating a metal on to a substrate surface under appropriate conditions, a porous dentritic coating is obtained. The coating has the appearance of an array of bristles and provides a large surface area for subsequent anodisation.

Embodiments ofthe invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of a metal evaporation coating apparatus showing a simple embodiment.

Fig. 2 is a schematic diagram of a coating apparatus for continuous processing; Fig. 3 shows an alternative coating apparatus-for continuous processing; Fig. 4 illustrates the typical relationship between anodising voltage and specific capacitance for anodised metal coatings prepared via the apparatus of Fig. 1; and Fig. 5 illustrates the effect of deposition temperature on the foil properties.

Referring to Fig. 1, a valve metal or a valve metal alloy is vacuum deposited on to a plane conductive or insulating substrate surface, the deposition being performed at such an angle to the surface that the metal is deposited as an array of spaced columnar crystals of dentrites. A resistor body 11 of boron nitride/titanium diboride is maintained at a temperature of about 1 600'C in a vacuum chamber 12 by the passage of an electric current, typically 100-150 amps, and is supplied at a constant rate with aluminum wire 13 from a reel (not shown). The aluminium fuses on the resistor body 11 and evaporates therefrom, the evaporation taking place substantially normal to the resistor surface. In other applications this thermal evaporation technique may be replaced e.g. with an electron beam evaporation process.

The aluminium vapour stream thus produced impinges on a substrate 14, typically an aluminium foil, carried on a support 15. Preferably the support 15 is water cooled. The deposition of aluminium on the substrate 14 is performed at such an angle of incidence 0 with respect to that surface that the metal is deposited as a columnar array of metal crystals thus providing the substrate with a high surface area. it is preferred, that this angle 0 is less than 60". In a particular advantageous embodiment the deposition angle lies between 5 and 10 . When observed under a microscope the deposited aluminium has the appearance of an array of bristles or whiskers.

In some applications an even higher surface area may be obtained by admitting a trace of oxygen into the vacuum chamber in which deposition is performed. We have found that a partial oxygen pressure up to an including 10-4 torr has the effect of reducing the deposited crystal dimensions and causing a certain amount of crystal branching.

Fig. 2 shows a metal coating apparatus, which as before is mounted in a vacuum chamber, and which provides continuous coating of a substrate foil. The foil 21, typically aluminium, to be coated is supplied from a reel 22 and is drawn around the periphery of the drum 23 it passes a source 25 of a valve metal vapour This source may be similar to that described with reference to Fig. 1 of the accompanying drawings.

The emission of vapour from the source will normally be strongest in a direction normal to the liquid surface of the source metal but inevitably there will be some spread in the angle of emission. The vapour source is positioned so that the-strongest emission strikes the foil at the required acute angle with respect to the foil surface. If the movement of the foil is away from the source at this point, as illustrated in Fig. 2, the last part of the emission received by any part ofthe foil is at a still more acute angle providing the useful characteristic of increasing porosity towards the surface of the deposit.

Fig. 3 shows a continuous deposition apparatus in which no substrate foil is required. A metal drum 31 rotates past a metal vapour source 32, the drum surface previously having been coated with a release agent from a source 33. The drum 31 and source 32 are so arranged that initial deposition of metal is in solid form followed by an upper porous coating so as to form a self-supporting foil. This foil is the drawn off the drum on to the storage reel 34.

In a further application a plurality of metal vapour sources 25 may be employed to provide a thicker coating and/orto provide sequential coatings of different metals.

The processes described herein are not of course limited to the deposition of aluminium. Thus, other valve metals, and in particular tantalum, may be deposited in a porous form for subsequent anodisation. For many such metals it is of course preferred to employ some form of electron beam evaporation or sputtering technique in place of thermal evaporation. Also alloys oftwo or more metals may be deposited.

The substrate may be made ofthe same metal as that which is being deposited or of a different metal.

Moreover, in some applications, deposition may be affected on an insulating substrate such as plastic foil or a ceramic body.

The deposited porous metal layers described herein have particular application, although they are not so limited, to the manufacture of electrolytic capacitors, their high surface area being of particular advantage for this purpose. For such use the metal layer is first anodised in a conventional anodising electrolyte and to a voltage exceeding, usually by 30%, that of the intended working voltage of the finished capacitor for wet electrolysis, and to 4 to 5 times the working voltage for solid capacitors with manganese dioxide electrodes. The capacitance yield of an anodised metal layer will of course depend on the forming voltage applied, the relationship being illustrated in Fig. 4 which shows typical capacitance yields for a vacuum deposited aluminium films compared with a conventional etched aluminium film.

The properties of the deposited metal layers have been found to depend on the temperature at which the metal is deposited. The optimum temperature will depend on the anodisation voltage to which the layer is subsequently subjected. The effect is illustrated in Fig. 5.

The following example illustrates the invention: EXAMPLE: Using the apparatus shown in Fig. 1, aluminium was evaporated on to a 10 micron thick aluminium foil at a deposition angle of 10". After 10 minutes of evaporation the vacuum system was vented and the foii removed. It was found that a film 6 microns thick and having a highly porous columnar structure had been grown. This film was anodised in a 4% by weight boric acid solution to a forming voltage of 200 V. The measured capacitance of this anodised foil was found to be 0.96 F/cm2which corresponds to a capacitance yield of 6.4x 102 ,t4FV/cc.

It is thought that the porous columnar deposits produced by the methods described herein are dendritic in nature. However, it is merely necessary to produce a porous material.

Further modification to the metal surface may be achieved by the inclusion in the vacuum chamber of small quantities of inert gases, e.g. argon, which scatter the metal vapour and cause some vapour phase nucleation of fine metal particles.

The extension to a continuous process is possible using techniques of large scale vacuum evaporation in which electron-beam sources impinge the vapour onto a large roll of aluminium sheet processed from reel-to-reel within a vacuum system.

Alternatively the technique of directly depositing a film onto a thermally stabilised roller of suitable material can be used in which the valve metal is caused to build first a thin cohesive film then a thick porous film onto the roller. The composite film is then stripped and rolled up.

In a further example, a piece of foil 10 cm x 1 cm, prepared as above, with an aluminium tag cold weight to one end, was formed in 1% potassium biphthalate solution to 33 V. This foil was wrapped with a similar piece of foil with interleaved paper such that the vacuum deposited sides were facing.

After reforming at 85"C in a propriety working electrolyte and cooling to room temperature the assembled device had the following characteristics: Cap = 80,uF tan rP ;= 10% leakage = < 1.OCLAat25V Since the thickness of the foil was 50,am total this represented a reduction in anode and cathode volume of some 50%.

A thicker foil prepared in the same way gave a similar device with the characteristics: Cap = 195 tan8 = 10% 12 = 2.OCLAat25V.

EXAMPLE ll: Using the apparatus shown in Fig. 3 with a source as described with reference to Fig. 1, aluminium foils were prepared using the following deposition parameters: Wheel rotation rate 1 rev in 20 minutes Wheel diameter 30 cm Wire feed 0.14grms/minute Current to source bar 120 amps at 12V Wheel temperature 30"C to 300"C as per Fig. 5 For wheel temperatures above 50"C it was found useful to coat the wheel with a release agent to aid parting of the foil this could be either"Teepol" or other wetting agents applied as a thin smear or evaporated oxide film or a thin layer of aluminium deposited with the wheel cold.

The chamber in each case was evacuated to 10-5 Torr and deposition continued for one revolution of the wheel. After venting the system the foil was stripped from the wheel and then anodised in 3% ammonium tartrate solution at room temperature.

The capacitance per unit volume of foil was determined and converted to cap yield, i.e. the product of capacitance and forming voltage per unit volume.

Fig. 5 compares foils so prepared with conventional high gain etched foil and it can be seen that in the temperature range 20 to 300"C the foil has a higher capacitance yield at 200 V than etched foil and from 20"C to 160"C the foil has a higher capacitance yield at 30 V than etched foil. In accordance with the ideas set out in the preamble there is an optimum substrate temperature of 1 20"C for 200 V formed foil, while for foil formed to 30 V the optimum temperature appears to be below 20"C.

It is believed that a useful property ofthe foil prepared in this rotating cylinder system is that by virtue of the change of angle of incidence of vapour beam from point A two point B in Fig. 4 there is deposited a more dense foil in the initial stages and a more porous foil in the later stages (B) of the deposition. By this means foils which are selfsupporting and capable of being wound can be pre pared in which the strength is achieved by the more dense foil and the high capacitance gain by the less dense foil.

EXAMPLE /1/: Using the apparatus shown in Fig. 3 an aluminium foil was prepared with the following parameters: Wheel rotation rate 1 rev in 20 minutes Wheel diameter 30 cm Current to source bar 120 amps at 12 V Wheel temperature 30"C Oxygen pressure 1 x 10-4 Torr during deposition.

The foil was stripped from the wheel and after anodising in ammonium tartrate, had the following characteristics: Capacitance yield Forming voltage (,aFVcm-3) 30 220,000 100 141,000 200 89,000 The capacitance yield was thus comparable at 30 V with foils deposited in vacuo at 30 C but higher at 200 V than foils deposited at the same temperature in the absence of oxygen.

Further embodiments will be apparent to those skilled in the art. For example thick porous layers of a valve metal can be grown on a wire of the same material for the fabrication of compact small valve electrolytes. This invention is also applicable to the manufacture of continuous foils for instance by suitable modification of the methods described by H.

R. Smith in U.S. Patents Nos. 3,270,381,3,183,563 and 3,181,209.

Claims (28)

1. A process for preparing a porous metal film or coating on a substrate surface, including directing a stream of the metal vapour on to the surface in vacuum, the deposition being performed at such an angle of incidence to the surface as to provide a porous metal deposit.

2. A process for preparing a porous metal film or coating on a substrate surface, including traversing the substrate surface adjacent a source of the metal vapour, wherein the deposition is performed in vacuum, wherein the metal vapour source is so constructed as to generate the vapour primarily in one direction, and wherein the deposition is performed within such a sample of angles of incidence and substrate temperature as to provide a porous metal deposit.

3. A process for preparing a metal film as claimed in claim 1 or2, and in which the metal is a valve metal or an alloy of two or more valve metals.

4. A process as claimed in claim 1,2 or 3, and in which the substrate is aluminium.

5. A process as claimed in claim 1,2 or 3, and in which the substrate is a plastic foil.

6. A process as claimed in claim 4 or 5, and in which the metal is aluminium.

7. A process as claimed in claim 6, and wherein the aluminium vapour is obtained by thermal evaporation of aluminium.

8. A process as claimed in claims 4 or 5, and in which the metal is tantalum.

9. A process as claimed in claim 6 or7, and wherein the aluminium is deposited in the presence of oxygen art a partial pressure not exceeding 10-4 torr.

10. A process as claimed in any one of claims 1 to 8, and wherein the porous metal coating is subsequently anodized.

11. A metal deposition process substantially as described herein with reference to Figs 1 and 3 orto Figs 2 and 3 of the accompanying drawings.

12. A metal film or coating produced buy a process as claimed in any one of the preceding claims.

13. An electrolyticcapacitorcontaining one or more metal coatings as claimed in claim 12.

14. A method of preparing electrolytic capacitor electrodes including the steps of fabricating a porous valve metal body in bulk or foil form by depositing metal in a vacuum system under conditions where columnar growth occurs and anodizing said porous metal.

15. A method of preparing electrolytic capacitor electrodes, including depositing on a solid surface in vacuum a porous layer of a valve metal so as to form a foil having a porous surface, and anodizing said solid porous surface.

16. A method as claimed in claim 15 and wherein said porous layer is deposited on a substrate foil.

17. A method as claimed in claim 15 or 16 wherein used metal is aluminium.

18. A method as claimed in claim 16 or 17 wherein used substrate foil is an aluminium foil.

19. A method of preparing a capacitor electrode substantially as described herein with reference to Figs 1,2 or 3 of the accompanying drawings.

20. A capacitor electrode produced by a method as claimed in anyone of claims 14to 19.

21. An electrolytic capacitor having one or more electrodes prepared by a method as claimed in any one of claims 14to 19.

22. An apparatus for preparing a porous metal foil, including a rotatable drum, and a metal vapour source disposed adjacent the drum so as to direct a stream of metal vapour on the surface thereof, the source and drum being so disposed that the deposited metal foil is porous and is in anodizable form.

23. An apparatus as claimed in claim 22, and which includes means for feeding a substrate foil on to the surface of the drum.

24. An apparatus as claimed in claim 22 or 23 and which includes a plurality of metal vapour sources.

25. A coating apparatus substantially is described herein with reference to Fig. 2 or Fig. 3 of the accompanying drawings.

26. A method of making a capacitor, including depositing on a solid surface in vacuum a porous layer of a valve metal so as to form a foil having a porous surface, anodizing said surface, providing electrical contact to the foil, and winding the anodized foil into a capacitor.

27. A method of making a capacitor substantially is described herein with reference to the accompanying drawings.

28. A capacitor made by a method as claimed in claims 26 or 27.