EP4453289A1

EP4453289A1 - Alumina particles, process of forming high purity aluminum or alumina particles, and use of a foil as a raw material

Info

Publication number: EP4453289A1
Application number: EP22822348.3A
Authority: EP
Inventors: Cristiano BACCO; Erkan Küzeci
Original assignee: TDK Electronics AG
Current assignee: TDK Electronics AG
Priority date: 2021-12-23
Filing date: 2022-11-24
Publication date: 2024-10-30
Also published as: WO2023117289A1; DE102021134542A1

Abstract

The application relates to a process of forming high purity aluminum or alumina particles, microparticles and nanoparticles formed by the process, a process of recovering an electrolyte, use of nanoparticles and the use of an etched foil. In particular a process of forming high purity aluminum and/or alumina particles from etched and/or anodized foils is provided.

Description

ALUMINA PARTICLES , PROCESS OF FORMING HIGH PURITY ALUMINUM OR ALUMINA PARTICLES , AND USE OF A FOIL AS A RAW MATERIAL

The application relates to a process of forming high purity aluminum or alumina particles , microparticles and nanoparticles formed by the process , a process of recovering an electrolyte , use of nanoparticles and the use of an etched f oil .

Among particle materials , aluminum and alumina materials play an important role in several technical fields such as catalysis , ceramics or several others .

In this context it is advantageous to have new means of access to aluminum or alumina materials .

This aim is at least partly achieved by a process according to claim 1 . Additional ef fects are achieved by embodiments addressed in further independent or dependent claims .

According to a first aspect , a process of forming high purity aluminum and/or alumina particles from etched and/or anodi zed foils is provided .

Etched and/or anodi zed foils , in particular i f comprising aluminum or containing aluminum as a main component , can be etched and/or anodi zed to be surface-enhanced in an anodic reaction . Several types of surface enhancements are available by electrochemical anodic etching of aluminum . In particular tubular structures may be formed . The tubular structures may have si zes in the micrometer or nanometer si ze range . In this case , micrometer or nanometer si zed structures , which may be called microstructures or nanostructures , can be electrochemically formed on the surface .

According to the present invention it is possible to use the etched and/or anodi zed foil as a raw material for forming high purity aluminum and/or alumina particles .

Therefore the inventive process enables a new way of access to aluminum and/or alumina particles , in particular to nanoparticles or microparticles comprising aluminum or alumina .

The inventive process may also be applied as a means for recycling . In particular i f a surface-enhanced aluminum foil has been formed in an earlier process but for some reason is defective and thus not completely suitable for its originally intended use , the defective foil may still serve as a raw material to produce nanoparticles or microparticles from it .

The aluminum or alumina particles formed by the process can either be a product itsel f for several applications or serve as an intermediate material to be applied as a raw material for another process or sub-process .

According to an aspect , the etched and/or anodi zed foil has been microstructured and/or nanostructured by an etching or anodi zation process .

Etching and/or anodi zing are highly ef ficient ways to microstructure or nanostructure an aluminum foil . In other words , a process is proposed in which an aluminum containing foil is first etched and/or anodi zed electrochemically and is subsequently used to produce aluminum and/or alumina particles from it .

In this process it is possible to form structures on the surface of the foil and subsequently particles are formed from the foil , which may preserve or maintain structural motives of the foil . In other words , the foil may be surface- structured in two or three dimensions by well controllable electrochemical processes , and subsequently micro- or nanoparticles are formed from the foil , which maintain certain features of the structure on the foil . This means the process allows for defining a part of the particle structure in a two or three dimensional surface process before the particles are formed .

The etching and/or anodi zation process may be performed either in a static process or in a roll-to-roll process .

A static process may be any process in which the foil is fully immersed in an electrolyte during etching . During the etching the foil remains in the electrolyte and is not moved out of the electrolyte . Following the etching, the entirety of the foil may be taken out of the electrolyte .

In a roll-to-roll process the foil is coiled up on a first roll . In the roll-to-roll process the first roll is unwound and the foil is rewound on a second roll . An unwound fraction of the foil is suspended between the two rolls . A part of this unwound fraction passes through an electrolyte bath while a voltage or a current is applied, which allows the foil to be continuously etched while it is passing through the electrolyte bath .

The roll-to-roll process is a highly ef ficient method for etching an aluminum foil to produce high quantities of etched or microstructured or nanostructured aluminum foil .

According to another aspect , the etched and/or anodi zed foil is either suitable for use as an electrode foil in an electrolytic capacitor or is a foil which, has reduced usability as an electrode in a capacitor . The capacitor may be , for example , a wound capacitor .

According to this aspect the foil which is used in the process may be a foil which has been used, or is capable of being used or has been intended for use , in an electrolytic capacitor . In this case , the inventive process allows for an alternative use of the foil as a raw material for aluminum and/or alumina particle formation from it .

This approach can be particularly advantageous i f , for some reason, the foil has a reduced usability in an electrolytic capacitor . In this case it may be more beneficial to use the foil for the inventive process than to apply it in a capacitor . I f a foil with the reduced usability were used in a capacitor, a capacitor of reduced performance , such as reduced capacity or reduced stability leading to reduced li fetime , would be created . Therefore , such a foil often has been disposed previous to the present invention . However, the inventors have identi fied such foils as a valuable raw material . The inventive process allows for formation of valuable highly pure aluminum and/or alumina particles , by which means the value of the highly pure aluminum foil may be maintained or even partly enhanced . This means that foils which do not ful fil high-end technical requirements may still be used beneficially in the inventive process .

In an alternative scenario , the foil may be a used foil . The foil may be a foil , which has been used or applied in an electrolytic capacitor . For example , in case the capacitor became damaged or has deteriorated in performance , still the foil may serve as raw material for the inventive process .

In this context , as highlighted by the above scenarios , the inventive process allows for reuse or recycling of material by which waste may be avoided .

According to another aspect , the foil may comprise macroscopic defects which reduce its usability as a capacitor f oil .

Macroscopic defects may, for example , occur during winding in a roll-to roll-process . Defects may for example be the folding or ripping of the foil . Aluminum foils may be processed by winding in fast-running machines and are thereby subj ect to mechanical stress . As the foil may be quite thin it may easily rip or further disintegrate , which may be considered as formation of macroscopic defects .

Macroscopic defects may reduce the usability of a foil in a capacitor compared to a foil which has no such defect . The inventors of the present invention have found that macroscopic defects hardly influence the microscopic or nanoscopic structure of the foils for which also foils with macroscopic defects may serve as a suitable raw material for aluminum and/or alumina particle formation . Accordingly, the general inventive process may be beneficially applied to macroscopically defective foils . Thus the material may maintain economic value and waste may be avoided .

According to another aspect , the forming of high purity aluminum and/or alumina particles includes grinding of the f oil .

In other words , the means for forming the high purity aluminum and/or alumina particles from the etched and/or anodi zed foils may include grinding .

The inventors of the present invention have found that grinding is an ef ficient way to form particles from the etched and/or anodi zed foil .

Alternatively chemical means may be applied to form particles from the foil .

Both chemical means and grinding may be used to form the particles .

According to another aspect , the grinding involves that shear forces or impact forces are applied .

The inventors have found that , by shear or impact forces it is possible to form particles from the foil , preferably of micrometer si ze or nanometer si ze . Grinding including shear or impact forces may also allow for maintaining at least partly the structures which have been formed by the etching and/or anodi zation of the foil . Accordingly these structures can be trans ferred from the foil to the aluminum and/or alumina particles .

According to another aspect the grinding is a milling process . It has been found out that milling processes are technically easy to conduct and allow for preservation of structural motives of the two- or three-dimensionally formed nanostructures in the nanoparticles or microparticles .

Furthermore , it may be preferred for certain applications of the particles , i f structural motives of the structures on the foil are preserved in the particles during forming of the particles . This is particularly preferred for grinding as means for particle formation .

Preservation of structural motives allows for determination of a structure on the foil surface in two or three dimensions by the etching and/or anodi zation of which at least part of structural motives may be trans ferred to the aluminum and/or alumina particles .

According to a further aspect , the high purity aluminum and/or alumina particles formed by the above process are subj ect to a thermal plasma process to form spherical aluminum or alumina nanoparticles . In other words , after the forming of the high purity aluminum and/or alumina particles from the foil , these may be applied as a raw material to feed a thermal plasma process by which means spherical aluminum or alumina particles are formed .

Due to the fact that the foils used for the etching process have high purity requirements , the particles formed from it directly have high purity as well . As thermal plasma processes have high purity requirements for the raw material introduced into the process , the inventors found out that the particles formed by the above processes may advantageously be applied as a raw material for the thermal plasma process by which means spherical aluminum or alumina particles are formed .

The thermal plasma process may use a plasma torch in which gas is ioni zed between an electrode and a noz zle between which a voltage is applied . Alternatively, inductively coupled plasma technology may be applied in which a radio frequency alternating current is imposed on a spiral coil which generates heat leading to the plasma formation .

According to another aspect , the microparticles and/or nanoparticles which may be formed by any of the processes , are described .

Microparticles may be understood as particles which have a si ze in the micrometer range . Analogously, nanoparticles may be understood as particles which have a si ze in the nanometer range .

Particle si zes which may be achieved directly from the etched and/or anodi zed foil , for example by grinding, preferentially have micrometer si ze range on average . Particle si zes can for example range from 3 pm to 300 pm . An average particle diameter may for example be 100 pm .

Particles formed directly from the etched and/or anodi zed foil may be a mixture of microparticles and nanoparticles . Concerning particles which are produced by the thermal plasma process , mostly spherical-shaped nanoparticles are formed . Accordingly the si ze range may be below 1 pm . However it is possible that at least a fraction of larger particles is also produced, such as particles having si zes below 10 pm .

Most preferably from the thermal plasma process , particle si zes between 0 . 1 to 1 pm are achieved . Average particle diameters achieved from the thermal plasma process may be 200 nm .

It is noted that the particles formed directly from the foil may serve as a raw material for forming the nanoparticles of the thermal plasma process .

The thermal plasma process also gives access to much smaller particles . For example , particle si zes below 50 nm may be achieved, such as in the range between 20 to 30 nm .

According to another aspect the microparticles and/or nanoparticles may have rod-like , tubular or spherical shape .

In particular for particles formed from the foil by grinding a rod-like or tubular shape may be achieved by having tubular or rod-like structures present on the foil previous to the grinding . These structural features are preserved in the particles . A spherical shape can be achieved by milling or most preferably by the thermal plasma process .

According to an aspect , the microparticles and/or nanoparticles may comprise pores . These may be preserved structures from the etched and/or anodi zed foil . The pores may be one order of magnitude smaller than the average si ze of the particle. The smaller pores in the larger particles form a hierarchical structure. Particle sizes in this case may, for example, 200 pm on average. Pores may have the size scale of the pores which are created by anodization and/or etching. The sizes of the pores here may range below 10 pm. More preferably the sizes are smaller than 2 pm. The sizes may be in the range of 0.5 to 0.7 pm.

By having an hierarchical structure a high surface area material may be formed which can be versatile for several applications, in which high surface area materials are required, such as carrier substrate material in catalysis or a catalyst material.

According to another aspect, the microparticles and/or nanoparticles comprise aluminum oxide, i.e. alumina, or metallic aluminum or mixtures of said substances.

Aluminum or alumina particles are highly versatile materials with great technical importance in several fields such as ceramics or catalysis.

According to another aspect a process is provided in which an aluminum-containing electrolyte solution, which is formed by the etching and/or anodization process, is recovered. This principle may be extended for any etching of aluminum foils independent of the particle formation process described above. However it can be beneficially applied for the process .

As has been stated above, the etched aluminum foils are of high purity and accordingly are a valuable resource in general. During etching, up to 30 wt% of the etched foil is dissolved into the electrolyte . The inventors have found that by the etching and/or anodi zation process high purity aluminum-ion containing compounds are formed in the electrolyte . Accordingly the inventors of the present invention have identi fied the electrolyte comprising these compounds to be a highly valuable resource for high purity aluminum . According to this aspect , the electrolyte from the etching or anodi zation process is used as a resource or raw material for further processing .

According to the state of the art , recovery of the solution had not been performed or only very impure materials have been recovered, i . e . not maintaining the high purity of the aluminum compounds in the electrolyte .

According to an aspect of this process , the process includes formation of high purity alumina particles from the recovered solution . In particular the particles formed may be of nanoparticle shape .

The inventors have identi fied the electrolyte solution as a highly valuable source to precipitate , deposit or otherwise form particles from . According to an aspect , the recovery of aluminum-containing electrolyte can be conducted such that alumina particles are formed by precipitation, hydrothermal synthesis , flame spray pyrolysis or thermal plasma synthesis from the recovered solution . The inventors have found out that high purity particles can be formed with high ef ficiency formed by these methods .

Furthermore , the electrolyte solution may comprise aluminum chlorohydrates as an aluminum compound . The aluminum chlorohydrates can be represented by the formula

( (Al_n ( OH) mCl ) 3n-m) x, wherein n, m and x are natural numbers .

In the electrochemical etching reaction such aluminum chlorohydrate complexes are formed and the inventors have identi fied that these may be applied to serve as precursor for particle formation . They may be introduced, for example into the above-mentioned particle formation techniques .

When compared to other precursor salts , like aluminum chlorides or aluminum nitrides typically used in aluminum nanoparticle formation from solution, the aluminum chlorohydrates formed or used in an etching and/or anodi zation process , contain on average higher quantities of aluminum with a high ratio of aluminum ions to anions . The aluminum amount may be above twice of the conventionally applied precursor molecules . Accordingly, such solutions have been identi fied by the inventors as an ideal precursor for the production of alumina particles or alumina nanoparticles from solution . An example for the aluminum chlorohydrates is Al₂ ( OH) ₅C1 .

According to another aspect the use of the particles as named above includes use as pigments , explosives or precursors for surface treatments or use as a substrate in catalyst applications .

In the following the invention is described in more detail by means of examples of embodiments and with reference to figures . The figures include both embodiments and process- related information . It is noted that the components are not shown to scale in schematic drawings . In these , components may be shown distorted in their si zes , lengths or length ratios . Accordingly si zes or ratios may not be taken from the schematic drawings .

Figure 1 shows a section of a cross-section image of an electrolytic capacitor .

Figure 2 shows a schematic representation of an electrochemical setup .

Figure 3 shows an electron microscopy cross-section image of a high voltage capacitor foil after first etching .

Figure 4 shows an electron microscopy cross-section image of a high voltage capacitor foil after a widening process .

Figure 5 shows the current wave profile of a current wave pulse etching process .

Figure 6 shows an electron microscopy image of a pulsed etched tunnel structure .

Figure 7 shows an electron microscopy image of a pulsed etched cauli flower structure .

Figure 8 shows an electron microscopy image of the resulting etched structure of a sinusoidal alternating current etching of aluminum foil .

Figure 9 shows a transmission electron microscopy image of a cross-section of a single tunnel of a high voltage foil during the forming process and after the exposure to water . Figure 10 shows a transmission electron microscopy image of a cross-section of a single tunnel of a high voltage capacitor foil after complete conversion at high voltage .

Figure 11 shows an electron microscopy image of high purity gamma alumina particles obtained from high voltage formed foils .

Figure 12 shows an electron microscopy image of one tube of the gamma alumina structures shown in Figure 11 .

Figure 13 shows the particle si ze distribution of particles from etched foils .

Figure 14 shows the X-ray di f fraction pattern of particles formed from etched foils .

Figure 15 shows a scanning electron microscopy image of an aluminum powder formed from an etched foil .

Figure 16 shows a zoomed-in scanning electron microscopy image of one aluminum particle .

Figure 17 also shows a in a scanning electron microscopy image a surface detail of another aluminum particle .

Figure 18 shows an electron microscopy image of an alumina composite coating formed by a high velocity oxygen fuel process .

Figure 19 shows an electron microscopy image of an aluminum alumina composite coating formed by controlled atmosphere plasma spray . Figure 20 shows a schematic cross-sectional representation of a plasma torch set up for coating substrate .

Figure 21 shows a formation schematic of nanoparticle formation from inductively coupled plasma technology with an on-axis temperature graph .

Figure 22 shows a scanning electron microscopy image of nano alumina particles formed by inductively coupled plasma technology .

Figure 23 shows a graph of the si ze distribution of nanoparticles formed by inductively coupled plasma technology .

Figure 1 shows a cross-section image of an electrolytic capacitor 7 comprising an anode 9 , a cathode 8 and an dielectric material with an electrolyte 10 separating anode 9 and cathode 8 .

Electrolytic capacitors are passive devices used, for example , in electric circuits for charge storage purposes . The basic construction as shown in Figure 1 is composed of two conductive electrodes , the anode 9 and the cathode 8 and a dielectric material with electrolyte 10 between them . In the present example , the electrodes comprise aluminum as main component . Accordingly, the present capacitor 7 is an aluminum capacitor . One of the electrodes , in the present case the anode 9 , is covered with aluminum oxide which provides a dielectric function . Both of the electrodes present a high speci fic surface area in order to store as much charge as possible according to the capacitor equation : C = E x A/D . Here C is the capacitance ; E is the electric constant ; A is the surface area of the capacitor ; D is the thickness of the dielectric material .

The formation of high surface area foils is addressed in the following .

Figure 2 shows a schematic representation of an electrochemical setup 1 . In the electrochemical setup 1 an anode 2 and a cathode 3 are connected to a voltage supply 6 .

Please note that the anode 2 and the cathode of the electrochemical setup are not necessarily identical to the anode and cathode of the capacitor and may not be mixed .

Furthermore , the anode 2 and the cathode 3 are immersed in an electrolyte 4 . The anode 2 comprises or consists of an aluminum material . Preferably the anode 2 consists of pure aluminum with certain additives and few impurities in it . For example additives may be present in an amount of 100 to 150 mg of hetero atoms per kg of aluminum material . The purity of the aluminum used for the foil may be in the range of 99 . 99% or above . Depending on the etching process applied, the aluminum foil is preferred to be highly crystalline and a high cubic texture orientation may be desired . A foil with high cubic texture orientation may for example have a 98 % or higher alignment of ( 100 ) planes of the aluminum crystal unit cells with the surface of the foil , i . e . the [ 100 ] direction of all unit cells has an average alignment of 98 % or above with the surface normal of the film .

In the electrochemical setup 1 a voltage and a current is applied by a voltage and current supply 6 between the anode 2 and the cathode 3 . Optionally, in addition a further electrode , which is not shown in the example of Figure 2 , may also be used in an electrochemical setup, creating a typical three-electrode electrochemical setup, in which the voltage of the anode 2 is defined versus a reference electrode and the cathode 3 has the role of a counter electrode providing the current .

Upon application of a positive voltage to the anode 2 , the foil may be partially dissolved and may be etched and/or anodi zed, whereby a surface-enhancement can archived . In the process aluminum ions are introduced into the electrolyte 4 . The aluminum ions are represented here by Al³⁺ . However, please note that complex ions may also form, for example including ligands which come from the electrolyte 4 .

The following simpli fied reactions may take place at the anode 2 and the cathode 3 , respectively :

Anodic reaction : 2A1 2A1³⁺ + 6e~

Cathodic reaction : 6H+ + 6e~ 3H2

The electrolyte 4 may be any suitable electrolyte , preferably an aqueous electrolyte . In the aqueous electrolyte aluminum chlorohydrates are formed by the etching and/or anodi zation . The aluminum chlorohydrates may have the formula ( (Al_n ( OH) mCl ) 3n-m) x . In particular one of the aluminum chlorohydrates formed may be A12 ( OH) 5C1 . These substances may serve as a raw material for particle production .

The process associated with the depiction of Figure 2 is a static process in which the electrodes ( anode 2 and cathode 3 ) are fully immersed in the electrolyte 4 during the etching . The electrodes are taken out of the electrolyte 4 subsequent to completing the etching .

Alternative to a static process , a roll-to-roll process may be applied . In a roll-to-roll process a band-like foil is dragged through the electrolyte with a fraction of the foil always in contact with the electrolyte , which becomes etched as long as it is in contact with the electrolyte . The voltage is accordingly applied to the foil passing through the electrolyte . In the roll-to-roll process it is preferred that the foil is suspended on a first roll and unwound from the first roll and rewound on a second roll . The unwound fraction in between the rolls is passing through the electrolyte with a part of the unwound fraction being in contact with the electrolyte , while passing through it .

By the above-described process an etched foil is formed at the anode side . The formed foil may be called etched and/or anodi zed foil . The etched and/or anodi zed foil is preferably surface-enhanced, this means it has a higher ef fective surface area than previous to the etching . For example , the surface may be microstructured and/or nanostructured . This means that the foil may have structures in the micrometer range or in the nanometer range or both . For example , the structures may be tubes .

In the following possible structures on aluminum foils are addressed in more detail . Figures 3 and 4 show electron microscopy images of foils after an intermediate step and a final step of an etching process , respectively . The morphology created in the aluminum foil is generally a result of several process parameters and foil properties . Di f ferent etching structures in the nanometer or micrometer si ze range can be produced, depending on the process parameters . Of particular interest may be the crystal structure of the aluminum foil , the composition of the etching solution or the voltage and current shape of the anodic polari zation .

For use in a capacitor as shown in Figure 1 , for example , growth of an oxide layer may be required . As the oxide adds a dielectric function to the film . The voltage used for creating the oxide increases with the oxide thickness . For example , a voltage increase of 1 V per nanometer of oxide growth is observed . Furthermore for the surface structure and the dimensions of the structure on the surface it must be kept in mind that the surface structures are produced in a si ze in which it is still possible to form a thick enough oxide to provide suf ficient dielectric function without the pores being blocked by the oxide .

Figure 3 shows an electron microscopy cross-section image of a high voltage anodic foil after the creation of the etch structure in a first etching . In the electron microscopy cross-section image of Figure 4 the same foil is shown after a widening step . In the first step, which is the etching step, tunnels with a pore diameter of 0 . 5 to 0 . 7 pm are achieved with a tunnel density of around 20 million per cm² and a stochastic tunnel distribution . In the widening step a tunnel diameter in the order of 1 to 2 pm is achieved .

It is desired that during all the etching procedures , a core of the foil remains unetched at the center of the foil . This means that as the etching typically takes place on both sides , the depth of the tunnels formed is less than hal f of the foil used, for example . The foil thickness may be in the order of 10 to 150 pm . For example the thickness may be 120 pm . The depth of the tunnels etched into the foil may be 30 to 40 pm for a foil of a thickness above 90 pm .

Furthermore , for the example of Figures 3 and 4 , a foil with a high cubic texture orientation of 98 % has been used . Such a foil may be formed by certain solidi fication rolling and annealing procedures .

The etching is followed by surface cleaning which removes the remains of etching reagents and the electrolyte from the foil . A final step of the etching is an inline heat treatment which serves to dry and to passivate the surface of the etched foil .

In Figure 5 , the current wave of a pulse etching process is shown . The pulse etching process is suitable to create nanometer si zed morphology .

A pulsed current is applied in the pulse etching process . Basic parameters defining the result are the pulse frequency, the duty cycle and the anodic and cathodic current densities . By varying these and other parameters together with morphologies of the raw material di f ferent structures , such as micrometer-si zed tunnel structures or nanometer scale structures similar to cauli flowers may be formed . Figure 6 shows an electron microscopy image of micrometer-si zed tunnels formed by certain pulse etching techniques . Figure 7 shows an electron microscopy image of a cauli flower structure with nanometer range cauli flower structures . In particular for this process , the crystal morphology of the raw foil may play an important role . The dimensions of the structures formed are influenced by two factors . This is , on the one hand, the anodic pulse length and, on the other hand, the crystallites of the foil . For example , by using cooled deformed foils for the etching it is possible to create three-dimensional networks of small etch units . I f such a foil is instead annealed moderately and etched using a sinusoidal alternating current , structures can be obtained which present exactly the geometry of the cubic crystalli zed units . Accordingly, cube-like nanostructures may be formed as can be seen in the electron microscopy image of Figure 8 .

Furthermore , the surfaces of the surface-enhanced, i . e . etched and/or anodi zed foils , may be functionali zed chemically . Possible functionali zation includes phosphate or silicate functionali zation . These layers can be formed by dipping in a solution with the corresponding chemicals or by high temperature reactions with water to create a boehmitetype coating .

In Figure 9 the cross-section a transmission electron microscopy image of a single tunnel created by high voltage etching of a foil is shown as an intermediate step of a electrode foil anodi zation process . The etched foil has been brought into reaction with boiling water to form a boehmite layer 12 on the surface . The boehmite layer 12 creates a fibrous structure in the inside of the tunnel . There is a hole 13 in the middle of the tunnel 11 and the boehmite layer 12 . The boehmite layer 12 can be seen between the hole 13 and the tunnel wall 11 of alumina . The tube wall has a thickness of 0 . 9 pm . In Figure 10 the same tube is shown in a transmission electron microscopy image after conversion of the boehmite 12 of Figure 9 into gamma alumina . Accordingly, the formed tube comprises a tube wall 11 with the gamma alumina and a hole 13 in the middle . The formation of the gamma alumina takes place at a forming voltage of around 560 V . In the present case the achieved alumina layer thickness is 0 . 56 pm . This roughly equals the sum of the previous tube wall 11 plus the boehmite layer 12 .

Figure 11 and Figure 12 show electron microscopy images of high purity gamma alumina particles obtained from high voltage formed foils , i . e . from etched and/or anodi zed foils . In particular a tube 14 can be identi fied .

By means of post treatment of the etched and/or anodi zed foils after the etching procedure , it is possible to separate the metallic aluminum matrix from formed structures . The post treatment may include any process , which allows for separating surface structures comprising alumina from aluminum . More preferably the alumina is fully separated from all remains of aluminum . The post treatment may include chemical means or grinding by shear or impact forces or a combination of both .

The nanoparticles of Figures 11 and 12 contain or comprise high purity alumina of di f ferent crystallographic phases , i . e . gamma alumina, amorphous alumina or others .

The geometrical form of such structures can be varied based on the etching procedure . By appropriate post treatment motives of the tubular structures formed by etching and/or anodization are preserved in the alumina nanostructures.

In Figures 13 to 17 examples and data for particles formed via milling of etched and/or anodized foils are shown.

Figure 13 shows a particle-size distribution of particles formed from etched and/or anodized foils by milling, applying shear and impact forces to the foil. Two graphs are shown in Figure 13. The distribution-shaped curve having a maximum at above 100 pm is associated with the right side axis "/channel". The other curve having an integral-like shape is associated with the left side axis "/passing".

As can be seen from the curves, an average particle size is about 100 pm. The distribution indicates a particle size distribution of between 3 to 300 pm. The maximum of the particle size distribution is found at above 100 pm.

In Figure 14 an X-ray diffraction image is shown. It differentiates between the peaks shown for the aluminum component (top spectrum) and for the aluminum oxide component (bottom spectrum) formed from the foil. As can be seen in Figure 14, sharp X-ray diffraction peaks may be observed indicating high crystallinity of the materials.

Please note that the foil comprises both aluminum oxide and aluminum. The aluminum oxide may be located at the surface of the foil. The aluminum may be located at the center of the foil. Accordingly, a mixed particle material can be formed from the foil. This mixed material may, in other steps, be converted to purely aluminum oxide or to pure aluminum if necessary . In Figures 15 , 16 and 17 electron microscopy images of particles formed via milling using shear and impact forces are shown . The particles shown in Figure 15 are associated with the particles of the distribution of Figure 14 . Figures 16 and 17 show detailed images of single particles from the distribution .

As can be seen in Figures 16 and 17 , the surfaces of the particles are porous . This porosity results from the porosity of the foil which has been etched and subsequently milled, by which the surface structural motives of the foil have been at least partially maintained in the particles .

In Figure 18 , a secondary electron microscopy image of an aluminum-alumina surface coating using the aluminum-alumina powder associated with Figures 15 to 17 is depicted . Here the particle powder has been applied to a surface by high velocity oxygen fuel processing (HVOF) . This means that the ground material may serve as an ideal precursor for preparation of surface coatings .

In Figure 18 circular shapes may be identi fied . They indicate that structures from the particles are maintained in the coating . In particular, the porous or tubular structures in the particles is maintained at least partially in the coating . The circular shapes originates from alumina tubes and thus indicates the presence of alumina material distinguishable in the coating .

In Figure 19 , an aluminum-alumina particle composite formed in a similar or identical manner as the particles of Figures

15 to 17 , was applied to a resin surface by controlled atmosphere plasma spray ( CAPS ) . With this technique a more homogeneous and fine mixing for aluminum and alumina from the particles is achieved with no phases or structures to be clearly distinguishable in the electron image .

Furthermore , depending on the deposition technique a pure alumina or a pure aluminum coating may be achieved . Also several mixed material coatings are available as shown .

In summary, it can be seen that from the powders obtained from etched and/or anodi zed foils , a wide range of aluminum and/or alumina materials for surface coating may be achieved .

In Figure 20 , a schematic representation of a plasma torch 15 for coating applications is depicted .

The plasma torch comprises a noz zle 18 to which a gas for plasma formation is conducted to via the plasma gas channel

16 . The gas passes by an electrode 17 . Voltage is applied via a voltage supply 20 between the noz zle 18 and the electrode

17 . Thereby the gas becomes ioni zed to form a plasma .

Furthermore , in order to cool the system, a cooling water supply 21 is part of the plasma torch 15 . The cooling water circulates around the noz zle and is entered and let out via openings above and below the plasma gas channel 16 .

The plasma torch 15 comprises an insulator 19 which insulates the electrode 17 from the noz zle 18 .

The shape of the noz zle 18 and the arrangement of the electrode 17 with respect to the noz zle and together with the inlet pressure of the gas leads to a stream of the plasma flowing out of the noz zle 18 . A powder, together with a carrier gas , is led via a precursor feed 22 to pass by the outlet of the noz zle 18 . The plasma takes a part of the powder and the carrier gas with it and chemical reactions may occur therein . A spray stream 23 can be established by these processes .

In an application the spray stream 23 is directed towards a substrate 25 and creates a spray deposit 24 on the substrate 25 .

The plasma spray may be operated in three modes . There can be controlled atmosphere plasma spray ( CAPS ) , vacuum plasma spray (VPS ) and atmosphere plasma spray (APS ) .

The following gases may be used for both carrier gas and plasma gas : argon, hydrogen, nitrogen and helium . Also mixtures of these gases may be applied .

Alternatively to the plasma torch shown in Figure 20 , an inductively coupled plasma technology approach may be chosen . In this technology a radio frequency with alternating current is imposed on a spiral coil 31 . The conductor placed in the center of the spiral coil 31 is heated up by the alternating electromagnetic field . The advantage of this method is that these reactors may work without electrodes and can be fed with solid liquid or gas precursors . The reactor design permits the use of plasma generating gas , sheath gas and reactive gas together . The precursors are evaporated or vapori zed by the high temperature created inside the reactor . They are transported into the quenching zone at the bottom of the plasma reactor . A depiction of this approach is shown in Figure 21 . The brightness represents the temperature , with brighter parts indicating hotter regions of the device or of zones near the device . The temperature of the reactive mixture comprising the precursor 26 is shown in an on axis temperature diagram .

The precursor 26 enters the spiral coil 31 and is vapori zed in a precursor vapori zation zone 27 . In a condensation zone 28 in a distance from the precursor vapori zation zone 27 , the previously vapori zed precursor condensates to form particles . The particles are formed in the quenching zone 29 . Thereby nanoparticle nucleation and growth takes place . In particular, a radial quench 30 occurs . On the right-hand side of Figure 21 , the on-axis temperature of the reactive mixture is shown . It can be seen that a nearly linear temperature drop with a decline of around 10⁵ K/ s leads to the formation of the particles .

Figure 22 shows a secondary electron microscopy image of nanoalumina ( alumina nanoparticles ) produced by inductively coupled plasma technology which was fed with precursor particles formed from an etched and/or anodi zed aluminum f oil .

The alumina particles shown in Figure 22 are spherical and have si zes in the nanometer range . In Figure 23 the particle si ze distribution of the spherical nanoparticles formed by the inductively coupled plasma process is shown . The distribution curve having a maximum at about 0 . 2 pm is associated with the right-hand side axis labelled " /channel" . The integral style curve is associated with the left-hand side axis " /passing" . Both graphs together show that a good particle si ze homogeneity is produced with an average particle si ze in the order of 100 to 200 nm. A few particles in si zes of 2 to 10 pm may be formed as a minority species . In particular, it can be shown that the maj ority of around 95% of the particles have a si ze below 1 pm and therefore fall into the nanometer range .

Alternatively the method is also capable of producing si zes in the range of 20 to 30 nm .

Reference signs :

1 electrochemical setup

2 anode

3 cathode

4 electrolyte

5 reaction vessel

6 voltage supply

7 electrolytic capacitor

8 cathode of the electrolytic capacitor

9 anode of the electrolytic capacitor

10 separator and electrolyte

11 tube wall

12 boehmite layer

13 hole

14 tube

15 plasma torch

16 plasma gas channel

17 electrode

18 noz zle

19 insulator

20 voltage supply

21 cooling water supply

22 precursor feed

23 spray stream

24 spray deposit

25 substrate

26 precursor

27 vapori zation zone

28 condensation zone

29 quenching zone

30 radial quench

31 spiral coil

Claims

1. Process of forming high purity aluminum and/or alumina particles from etched and/or anodized foils.

2. Process according to claim 1, wherein the etched and/or anodized foil has been micro- and/or nanostructured by an etching or anodization process.

3. Process according to claim 1 or 2, wherein the foil is structured by an electrochemical etching process.

4. Process according to claim 3, wherein the foil is formed in a static process or a roll-to-roll process.

5. Process according to any of claims 1 to 4, wherein the foil is a foil suitable for use as an electrode in an electrolytic capacitor or with reduced usability as an electrode in a capacitor.

6. Process according to claim 5, wherein the foil can comprise macroscopic defects which reduces its usability as a capacitor foil.

7. Process according to any of claims 1 to 6, wherein the forming of high purity aluminum or alumina particles includes grinding of the foil.

8. Process according to claim 7, wherein the grinding is performed by using sheer forces or impact forces.

9. Process according to claim 7 or 8, wherein the grinding is a milling process.

10 . Process according to any of claims 1 to 9 , wherein structural motives of the structures on the foil are preserved when performing the grinding process .

11 . Process according to any of claims 1 to 9 , wherein the high purity aluminum or alumina particles are subj ected to a thermal plasma process to form spherical aluminum or alumina nanoparticles .

12 . Micro- and/or nanoparticles formed by the process of at least one of claims 1 to 10 .

13 . Micro- and/or nanoparticles according to claim 12 , which comprise particles of rod-like , tubular, or spherical shape .

14 . Micro- and/or nanoparticles according to claim 12 or 13 , wherein the particles comprise pores as preserved structures from the micro- and/or nanostructured foil .

15 . Micro- and/or nanoparticles according to claim 14 , wherein the pores are one order of magnitude smaller than the average si ze of the original particles .

16 . Micro- and/or nanoparticles according to any of claims 12 to 15 , which comprise aluminum oxide or metallic aluminum or mixtures of said substances .

17 . Process of recovering an aluminum-containing electrolyte solution which is formed by the electrochemical etching process according to claims 2 or 3 .

18. Process according to claim 17, which includes formation of high purity alumina particles from the recovered solution.

19. Process according to claim 18, wherein the alumina particles are formed by precipitation, hydrothermal synthesis, flame spray pyrolysis or thermal plasma synthesis.

20. Process according to claims 18 or 19, wherein the solution comprises an aluminum compound represented by the formula ( (Al_n (OH) _mCl ) 3n-m) x .

21. Use of the micro- and/or nanoparticles according to any of claims 12 to 16, as pigments, explosives, precursors for surface treatments.

22. Use of an etched and/or anodized foil as a raw material for production of micro- and/ or nanoparticles.

23. Use of micro- and/ or nanoparticles according to any of claims 1 to 10 as a raw material to be used in a thermal plasma process to produce spherical aluminum or alumina nanoparticles .