US20100258445A1

US20100258445A1 - Method for the production of an ordered porous structure from an aluminium substrate

Info

Publication number: US20100258445A1
Application number: US12/739,785
Authority: US
Inventors: Laurent Arurault; Francois Le Coz; Rene Bes
Original assignee: Universite Toulouse III Paul Sabatier
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Toulouse III Paul Sabatier
Priority date: 2007-10-26
Filing date: 2008-10-23
Publication date: 2010-10-14
Also published as: ATE522640T1; EP2207915A2; JP2011500969A; JP5199376B2; WO2009056744A3; ES2372790T3; FR2922899A1; WO2009056744A2; FR2922899B1; EP2207915B1

Abstract

A method for making a porous structure, includes producing, by anodization of an aluminum substrate, an outer surface layer (3), part of the thickness of which is formed by an ordered porous structure (7), characterized in that it includes an anodization step on a smooth aluminum substrate for a duration sufficient for obtaining a thickness of ordered porous structure (7). The method further includes removing by mechanical machining a portion of the thickness of the layer (3) formed by anodization, the thickness portion extending from the outer surface of the layer (3) formed by anodization, while maintaining an ordered porous structure (7) with a non-zero thickness, so that the ordered porous structure defines the free outer surface of the residual layer.

Description

The invention relates to a method for the production of an ordered porous structure from an aluminium substrate.
Throughout the entire text, a porous structure is called “ordered” if it has pores in the form of rectilinear channels of the same transverse cross-section (shape and dimensions) which are parallel and adjacent in a radial plane and uniformly distributed in the radial plane.
In addition, throughout the entire text the aluminium piece and the anodic structures resulting from anodization of the said aluminium piece are orientated according to their two opposite faces, a first face, the so-called outer face, in contact with the electrolyte solution and a second face, the so-called substrate face, which is not in contact with the electrolytic solution.
In addition, throughout the entire text alumina is understood as meaning the general term covering oxidized forms of aluminium, that is to say aluminium oxides, aluminium hydroxides and also aluminium oxyhydroxides.
Many electronic, mechanical, biotechnological or chemical systems are tending towards extreme miniaturization, opening up a vast field of applications in domains as varied as medicine, aeronautics, space, electronics, information technology or photonics. For this purpose, control of the structure of materials, of dimensions and of the regularity of their ultrastructures is becoming essential in order to reduce the dimensions of these systems, to increase the ratio between the specific surface area and the total volume of the sample and/or to obtain specific physical phenomena.
With this aim, it is known to realize, by anodization of aluminium metal substrates, ordered porous structures based on the chemical element aluminium, the surface of which extends over several μm². These porous structures, also called porous anodic films, can be used as a support or as a matrix for original applications, such as nanofiltration, or also the realization of functional elements of nanometre dimension, such as nanocontacts, nanofilaments and nanotubes. The improvement in the technical performances of these materials, the ultrastructures of which are of meso- or nanometre dimension, is attributed directly to technological advances allowing the realization of porous anodic films of large dimension and controlled thickness.
The growth of a porous structure in the course of anodization of an aluminium substrate is guided by a complex method involving an equilibrium between on the one hand an oxidation reaction of the aluminium into oxidized, hydroxylated or also oxyhydroxylated aluminium derivatives and on the other hand a dissolving reaction of this alumina formed. It is thus known that the formation of a porous structure results from the equilibrium, depending on the anodization operating conditions, between the respective contributions of these two antagonistic chemical reactions. The publication Masuda H., Yada K. et Osaka A., (1998), Jpn. J. Appl. Phys., 37, 1340-1342 “Self-ordering of cell configuration of anodic porous alumina with large-size pores in phosphoric acid solution” describes a method for obtaining a porous structure comprising several treatments, that is to say an anodization treatment for a duration varying from 0.5 h to 16 h on a non-pretexturized aluminium substrate in a phosphoric acid solution at a concentration of 0.3 mol/l under a voltage of 195 V, a chemical dissolution treatment of the residual aluminium substrate by means of a saturated HgCl₂solution, a chemical dissolution treatment of the barrier layer, also called compact layer, by means of a phosphoric acid solution of 10% strength by weight, and finally a chemical treatment for enlarging the diameter of the pores of the porous structure by means of a phosphoric acid solution.
This document thus describes a reference method for the production of a porous structure based on aluminium by means of several successive treatments, all of which are of a chemical or electrochemical nature. This document demonstrates the state of the surface of the substrate face of the porous structure after removal of the residual substrate and of the barrier layer. It does not describe the state of the porous structure within its thickness, in particular at the outer face.
It is furthermore known that it is possible to obtain an ordered porous structure by anodization of an aluminium substrate having on its outer surface a plurality of concavities of the same shape and regularly distributed. Such an impression can be obtained by nanoindentation of the aluminium substrate, for example by applying to and pressing on the aluminium substrate a hard matrix, in particular of silicon carbide, having a plurality of convexities. However, this nanoindentation stage is technologically very difficult to carry out because of the technical difficulties in realizing the silicon carbide matrix having a plurality of convexities on meso- and nanometre scales. This stage of realization of a matrix is consequently an expensive stage.
Another known method allowing a plurality of concavities of the same shape and regularly distributed to be obtained on the surface of the aluminium or aluminium alloy substrate is called “double anodization”. In this method of “double anodization”, a first anodization stage allows the formation of a plurality of concavities at the interface of the initially smooth aluminium substrate and the porous structure resulting from this anodization. Complete dissolution, by a chemical route, of the porous structure resulting from the anodization then reveals the plurality of underlying concavities. These concave impressions then serve to guide the growth of an ordered porous structure during a second anodization stage. This method of “double anodization” takes a long time to carry out because of the duplication of the anodization stage. This method moreover necessitates a stage of chemical dissolution of the porous structure resulting from the first anodization, which is difficult to carry out and consequently is not very, if at all, compatible with use on an industrial scale. This method furthermore necessitates the use of toxic chemical products during the dissolution stage, such as derivatives of chromium, in particular of chromium(VI). Finally, the thickness of the porous structure produced at the end of the initial anodization treatment on the aluminium substrate and then dissolved by the chemical treatment is not upgraded.
Other methods have been proposed to perfect the development of an ordered porous structure, but without succeeding in avoiding carrying out a “double anodization” with intermediate removal of a structure formed by a first anodization. EP 1715085 thus proposes a method in which the chemical dissolution treatment is replaced by an electrochemical treatment, leading to separation of the residual aluminium substrate and the entirety of the structure resulting from the first anodization. Here also, this method takes a long time to carry out, and is relatively complex, expensive and not very compatible with use on an industrial scale.
Thus, to date, in order to obtain an ordered porous structure having pores in the form of rectilinear channels of the same transverse cross-section (shape and dimensions) which are parallel and adjacent in a radial plane and uniformly distributed in the radial plane, it was always considered necessary to carry out a pretexturization of the aluminium substrate, either by nanoindentation of the said aluminium substrate or by a first anodization, followed by a chemical dissolution/electrochemical separation.
In this context, the object of the invention is to lessen the effect of these disadvantages by proposing a production method for an ordered porous structure by anodization of a smooth aluminium or aluminium alloy substrate which avoids resorting to a double anodization and which no longer necessitates carrying out a prior stage of mechanical nanoindentation of the aluminium or aluminium alloy substrate.
The object of the invention more particularly is to propose a method for the production of an ordered porous structure by anodization which is simple, rapid, inexpensive and environment-friendly, and which is compatible with use on an industrial scale.
The object of the invention more particularly is to propose a method allowing an ordered porous structure to be obtained which is of high quality and homogeneous throughout its entire thickness and in which the shape, the diameter of the pores and the organization of the pores are perfectly controlled.
The object of the invention more particularly is to propose a method allowing an ordered porous structure to be obtained, which can have a high thickness—in particular greater than 50 μm.
The object of the invention is also to propose a method for the production of an ordered porous structure which does not necessitate the use of toxic chemical compounds, such as derivatives of chromium, in particular of chromium(VI).
The invention thus relates to a method for the production of a porous structure in which an outer surface layer comprising an ordered porous structure is produced by anodization of an aluminium substrate, characterized in that:

- an anodization treatment is carried out on a smooth aluminium substrate with a duration sufficient to allow at least a thickness of ordered porous structure to be obtained,
- a part of the thickness of the said layer formed by anodization is then removed by mechanical machining, this part of the thickness extending from the outer surface of the said layer formed by anodization, while maintaining a non-zero thickness of the ordered porous structure and in a manner such that this ordered porous structure forms the free outer surface of the residual layer.

The porous structure obtained by a simple anodization of a smooth aluminium substrate has on the side of its outer face a thickness of imperfectly ordered porous structure—that is to say which does not have pores in the form of rectilinear channels of the same transverse cross-section (shape and dimensions) which are parallel and adjacent in a radial plane and uniformly distributed in the radial plane. However, if the duration of the anodization is sufficiently long, the porous structure also has, underlying this imperfectly ordered porous structure, a perfectly ordered porous structure, that is to say having pores in the form of rectilinear channels of the same transverse cross-section (shape and dimensions) which are parallel and adjacent in a radial plane and uniformly distributed in the radial plane.
Thus, merely the act of carrying out directly an anodization on a smooth aluminium substrate, that is to say having a arithmetic roughness of less than 5 nm and consequently not having a plurality of concavities as the result either of a prior anodization—double anodization method—or of a mechanical nanoindentation stage, allows in reality, if the duration of the anodization is sufficiently long, a thickness of ordered porous structure to be obtained underneath an imperfectly ordered porous layer extending into the surface. According to the invention, the thickness corresponding to this imperfectly ordered layer is removed by mechanical machining in a manner such that the pores of the ordered porous structure emerge in the surface.
In particular, in a method according to the invention, an anodization treatment is carried out from a substrate formed by an aluminium alloy of series 1XXX, for example aluminium alloy 1050A, or also refined aluminium, in particular chosen from the group formed by aluminium 4N and aluminium 5N.
In a method according to the invention, the outer surface layer comprising at least a thickness of porous structure is obtained after a duration of the anodization which depends on the speed of growth of the said outer surface layer. The speed of growth of the outer surface layer depends here on the operating conditions chosen for realizing the physical properties of the porous structure.
The method according to the invention thus allows a porous structure having a non-zero thickness of an ordered porous structure to be realized rapidly and easily in a single anodization and without necessitating either a subsequent chemical treatment of selective dissolution or an electrochemical separation. This ordered porous structure has an open porosity at least on one of its faces, the so-called outer face, and a controlled thickness of ordered porous structure on the micrometre scale.
That being so, according to a possible variant of the production method according to the invention, nothing prevents the outer surface layer from being realized such that it comprises a plurality of superimposed thicknesses of ordered porous structures. These various thicknesses are obtained in particular by an anodization treatment comprising a plurality of successive anodization stages, none of the said anodization treatment stages being followed by a treatment by selective chemical dissolution or electrochemical separation of a part of the thickness of the layer formed by anodization. In this variant, the various anodization treatment stages are carried out under anodization conditions in which at least one of the anodization parameters chosen from the group formed by the anodization voltage, the temperature of the anodization solution, the chemical composition of the anodization solution and the anodization current density is modified between two successive anodization stages.
In a method according to the invention, any known method for removal of material by mechanical machining can be used to carry out the removal of a part of the thickness of the said layer formed by anodization, this part of the thickness extending from the outer surface of the said layer, while maintaining at least a non-zero thickness of ordered porous structure and in a manner such that this ordered porous structure forms the free outer surface of the residual layer.
In a method according to the invention, mechanical machining is understood as meaning any method suitable for superficial removal of particles of material. In a method according to the invention, these particle of material removed by mechanical machining are particles in the solid state. However, in a method according to the invention, the particles of material removed by mechanical machining can be in the gaseous state.
In a method according to the invention, any known method for removal of material by mechanical machining can be used, excluding treatments by chemical attack, in particular chemical treatments on the layer formed by anodization with a solution capable of penetration by capillarity into the pores of the said porous layer and of modification of the shape and size of the pores of the porous structure.
Such a mechanical machining can be carried out in a single stage, or on the other hand by a plurality of successive stages. Such a mechanical machining can also be carried out with a single mechanical machining technique used during the various machining stages, or on the other hand by using a plurality of mechanical machining techniques during successive mechanical machining stages.
Such a mechanical machining according to the invention can be carried out in particular by ionic polishing using an ion flux, in particular a precision ion polishing system (PIPS), or also using the wide and high-energy primary beam of a secondary ion mass spectrometer (SIMS).
In particular, in a method for mechanical machining by removal of material by PIPS, the anodized outer surface layer is subjected for several hours, in particular between 1 h and 20 h, especially between 3 and 6 h, to at least a beam of accelerated argon ions under a voltage of between 1 keV and 6 keV, in particular of the order of 5 keV, under a secondary vacuum of the order of 1.31·10⁻³Pa.
Advantageously and according to the invention, the said part of the thickness is removed by mechanical abrasion, that is to say by dynamic solid/solid friction, by means of a movable abrasive solid tool, which is applied to the outer surface of the porous layer formed by anodization, a pressure being exerted on the said movable abrasive solid tool.
Advantageously and according to the invention, this mechanical abrasion is carried out in a manner such that an ordered porous structure of which the outer surface is flat is obtained.
More particularly, the said part of the thickness is removed by a mechanical machining treatment, in particular by mechanical abrasion, which affects only the outer surface of the porous layer formed by anodization, and which does not affect, in the thickness of the porous structure, either the diameter of the pores of the ordered porous structure or the shape of the said pores revealed in the course of the abrasive treatment. This treatment by mechanical abrasion differs from a treatment by chemical dissolution, which necessarily affects not only the thickness of the porous layer formed by anodization, but also the shape and the diameter of the pores of the said layer.
Advantageously and according to the invention, the mechanical abrasion is carried out by means of a piece of fabric—in particular a piece of felt—impregnated with a suspension, so-called abrasive suspension, of a powder in an aqueous phase, the said powder comprising at least a mineral chosen from the group of abrasive minerals consisting of diamond and ceramics—in particular corundum.
The use of a piece of fabric impregnated with an abrasive suspension according to the invention allows a regular abrasion and a high fineness to be obtained. Furthermore, it also allows permanent wetting of the surface of the porous layer obtained by anodization and maintaining of the temperature thereof, even in the course of mechanical abrasion. It thus avoids deterioration of the porous anodic structure in the course of the said abrasion.
The choice of abrasive mineral also allows the hardness of the said mineral to be selected in a manner such that the speed of abrasion of the porous structure is controlled. At all events, the hardness of the abrasive mineral contained in the abrasive suspension is greater than the hardness of the porous layer, the composition of which is based on alumina, in particular oxidized, hydroxylated and/or oxyhydroxylated aluminium derivatives.
Advantageously and according to the invention, the mechanical abrasion is carried out in a single stage, or on the other hand by a plurality of successive abrasion stages, each of the said successive abrasion stages being carried out by means of an abrasive suspension, the abrasive suspensions of each of the successive abrasion stages being chosen in a manner such that a granulometry which decreases from one stage to the other is achieved.
Advantageously and according to the invention, the mechanical abrasion is carried out by a succession of abrasion stages from a less fine and more rapid abrasion to a finer and slower abrasion. The choice of the size of the particles of the mineral powder allows both the speed of the abrasion of the porous structure and the quality of the finish of the surface of the porous structure to be controlled.
The succession of abrasion stages carried out by means of abrasive suspensions of decreasing granulometry also allows the abrasion time to be reduced while imparting to the surface of the porous structure a low roughness and an excellent finish.
Advantageously and according to the invention, each abrasion stage of the plurality of successive abrasion stages is carried out by means of a piece of fabric impregnated with an abrasive suspension, the said piece of fabric being applied to the surface of a rigid support chosen from the group formed by a vibrating support and a rotating support.
In particular, each abrasion stage of the plurality of successive abrasion stages is carried out by means of a piece of fabric impregnated with an abrasive suspension, the said piece of fabric being applied to the surface of a rigid support chosen from the group formed by a vibrating support and a rotating support, the smallest dimension of the piece of fabric and of the rigid support being greater than the largest dimension of the outer surface layer.
Advantageously and according to the invention, each abrasion stage of the plurality of successive abrasion stages is carried out by means of a rotating support having a speed of rotation of less than 30 rad/s, in particular between 2 rad/s and 20 rad/s. The pressure applied to the surface of the porous layer in the course of the mechanical abrasion is in particular between 1 kPa and 50 kPa.
Advantageously and according to the invention, the mechanical abrasion is carried out by a first abrasion stage by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is between 0.8 μm and 1.5 μm, in particular of the order of 1 μm, and by a second abrasion stage by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is between 0.2 μm and 0.4 μm, in particular of the order of 0.25 μm.
Advantageously and according to the invention, the total duration of the mechanical abrasion is less than 30 min, in particular between 10 min and 20 min. This duration in practice allows the total thickness of the non-ordered porous layer formed in the outer surface during the anodization to be removed.
In a variant of a method according to the invention, it is possible to carry out a single mechanical abrasion stage of a duration of the order of 20 min by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is close to 0.25 μm.
According to another variant of a method according to the invention, it is possible to carry out three successive mechanical abrasion stages. Three mechanical abrasion stages are carried out successively, each of these three stages being of a duration of the order of 10 min. The first abrasion stage is carried out by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is close to 1 μm, the second stage is then carried out by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is close to 0.25 μm, and finally the third stage is carried out by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is close to 0.10 μm.
There is nothing to prevent the mechanical abrasion from being carried out by more than three successive stages. There is moreover nothing to prevent the mechanical machining stages from being carried out with distinct machining techniques, for example chosen from the PIPS, SIMS and abovementioned abrasion techniques.
Advantageously and according to the invention, a thickness of the outer surface layer of between 15 μm and 25 μm—in particular of the order of from 17 μm to 20 μm—is removed. This thickness represents at least the thickness of the imperfectly ordered porous structure extending from the outer surface of the layer formed by anodization.
Advantageously and according to the invention, an anodization treatment is carried out on a smooth aluminium substrate with a duration suitable for obtaining an outer surface layer formed by anodization having a total thickness of between 25 μm and 300 μm, in particular between 100 μm and 200 μm.
Advantageously and according to the invention, a single anodization treatment is carried out on a smooth aluminium substrate, the said treatment having a duration of between 1 h and 12 h, in particular of the order of 4 h.
A method according to the invention thus comprises carrying out a single anodization treatment comprising at least one anodization stage and then a stage of removal of the part of the thickness of the outer surface layer of which the porous structure is imperfectly ordered. According to a method according to the invention, the anodization stage marking the end of the anodization treatment is immediately followed by a treatment by mechanical machining, in particular by mechanical abrasion.
Advantageously and according to the invention, a single anodization treatment is carried out on a smooth aluminium substrate over a duration suitable for the thickness of the ordered porous structure formed by anodization to be between 1 μm and 150 μm, in particular between 50 μm and 150 μm.
Advantageously and according to the invention, the anodization is carried out in an aqueous solution of electrolyte chosen from the group formed by aqueous solutions of acids—in particular sulfuric acid, a mixture of sulfuric acid and boric acid, oxalic acid, phosphoric acid, malonic acid, tartaric acid and citric acid.
Furthermore, advantageously and according to the invention, the anodization is carried out in an aqueous solution of electrolyte, the composition of which is suitable for providing an ordered porous structure, the pores of which have a diameter of between 10 nm and 500 nm, in particular between 100 nm and 200 nm.
In addition, advantageously and according to the invention, the anodization is carried out at a temperature of between −2° C. and +2° C.—in particular of the order of −1.5° C.
Advantageously and according to the invention, the anodization is carried out under a voltage of between 19 V and 240 V—in particular between 125 V and 195 V, with an aqueous solution containing phosphoric acid as the electrolyte.
In particular, in a method according to the invention, at least one anodization is carried out in a single stage, or in a combination of immediately successive anodization stages, and then a mechanical abrasion treatment is carried out. A porous structure comprising at least one thickness of ordered porous structure is obtained. Thus, in a method according to the invention, it is not necessary to carry out other anodization treatment after having removed, in particular by mechanical abrasion, the part of the thickness of non-ordered structure of the layer formed by the first anodization. Consequently, in a method according to the invention, a single treatment by anodization of the aluminium substrate is carried out. After the stage of removal by mechanical machining of the non-ordered structure, in particular by mechanical abrasion, other subsequent treatments can possibly be carried out, but it is not necessary to carry out either chemical or electrochemical dissolution or a new treatment by anodization.
Advantageously and according to the invention, immediately after having removed the said part of the thickness, the non-oxidized aluminium substrate and a part of the non-porous thickness of the said layer are removed to preserve only the ordered porous structure.
Advantageously and according to the invention, a chemical treatment is then carried out on the ordered porous structure which is suitable for increasing the diameter of the pores of the said porous structure.
Such a chemical treatment is particularly suitable for partly dissolving the dividing wall of the pores from the face of the said dividing wall which is facing the pore and in the direction of the internal part of the dividing wall. The inventor has found that the chemical composition of the layer of material constituting the said dividing wall varies according to the radial axis of the pores. The chemical composition of the face of the layer of material constituting the dividing wall facing the pore is a mixture based on oxidized, hydroxylated and/or oxyhydroxylated aluminium and comprising up to 20% of compounds resulting from the electrolytic solution used for the anodization. In return, the internal part of the said dividing wall is composed essentially of oxides, hydroxides and/or oxyhydroxides of aluminium.
The choice of the duration of this treatment of opening the pores and also the nature of the chemical agent chosen for this treatment allows the thickness of the layer of material dissolved inside the pores to be controlled and, consequently, the final diameter of the pores of the ordered porous structure to be determined.
The invention also relates to a method for the production of a porous structure, characterized by a combination of all or some of the characteristics mentioned above or below.

Other objects, characteristics and advantages of the invention will emerge from reading the following description, which refers to the attached figures showing preferred embodiments of the invention given merely by way of non-limiting examples. In these figures:

FIGS. 1 a to 1 e are illustrative diagrams in section on which the thickness and width are not to actual scale, illustrating successive stages of a method according to the invention.

FIG. 2 is a diagrammatic flow chart of a method according to the invention.

FIG. 3 shows a field effect scanning electron microscopy (FE-SEM) photograph of a section along the growth axis of an ordered porous structure according to the invention.

FIG. 4 shows a field effect scanning electron microscopy (FE-SEM) photograph of an ordered porous structure according to the invention without the aluminium substrate but with the barrier layer, viewed from the side of the barrier layer.

FIG. 5 shows a field effect scanning electron microscopy (FE-SEM) photograph of the outer face of an outer surface layer according to the invention after anodization and before mechanical abrasion.

FIG. 6 shows a field effect scanning electron microscopy (FE-SEM) photograph of the outer face of an ordered porous structure according to the invention after mechanical abrasion, the said porous structure being angled with respect to the anodization direction.

FIG. 7 shows a field effect scanning electron microscopy (FE-SEM) photograph of the outer face of an ordered porous structure according to the invention, the said porous structure being angled with respect to the anodization direction and comprising neither the aluminium substrate nor the barrier layer, the said porous structure being characteristic of nanostructuring of the “honeycomb” type.

FIG. 8 shows a field effect scanning electron microscopy (FE-SEM) photograph of the outer face of an ordered porous structure according to the invention without the aluminium substrate and without the barrier layer, characteristic of a nanostructuring of the “wasps' nest” type.

FIG. 1 a shows a piece 1 of aluminium or aluminium alloy serving as the substrate for treatment 24 by anodization and allowing an ordered porous structure 7 according to the invention to be obtained. This aluminium piece 1 has at least one face, the so-called outer face 2, which is subjected to a combination of physical or chemical treatments on the piece 1 as indicated below. The aluminium substrate used can be formed, for example, from an aluminium alloy of the series 1XXX, for example the alloy 1050A, or from refined aluminium of the 4N type (pure to 99.99%) or also of the 5N type (pure to 99.999%).

Pretreatment

18 of the Substrate

A pretreatment 18 is carried out on the piece 1 to prepare it for its anodization 24. The purpose of this pretreatment 18 is to promote the obtaining of a thickness of ordered porous structure 7. It allows on the one hand an increase in the wettability of the piece 1 in aqueous solution, and on the other hand a reduction in or the removal of pre-existing defects in the surface of the piece 1. The pretreatment 18 contributes towards establishing a regular contact between the piece 1 and the solution for the anodization 24. By removing defects in the structure of the piece 1, a substrate of which the outer face 2 is smooth and of which the arithmetic roughness is in particular less than 5 nm is obtained. This pretreatment 18 of the piece 1 comprises a succession of four treatments 19, 20, 21, 22.
The first treatment 19 is a degreasing of the piece 1 by means of organic or aqueous chemical solvents. This first treatment can be carried out by steeping the piece 1 in an aqueous alcoholic solution, allowing contaminants, greases, oils or lubricants originating from the previous methods of forming the said piece 1, for example lamination, to be dissolved and then removed by rinsing. The piece 1 is then rinsed with distilled water.
The second treatment 20 is a mechanical polishing allowing the roughness of the surface of the piece 1 to be reduced and thus a smooth substrate to be obtained. In contrast to the state of the art, in which it is generally considered that pretexturization of the surface of the substrate is favourable for obtaining an ordered porous structure, the inventor has demonstrated that on the contrary it is preferable to carry out the anodization from an outer surface which is as smooth and regular as possible. In fact, defects in the structure of the substrate, which are known to be distributed in an irregular manner over the outer face 2 of the substrate, are the cause of the formation of irregular pores and of the growth of imperfectly ordered porous structures. To adjust the arithmetic roughness of the aluminium to a value of less than 5 nm, finer and finer rotating or vibrating abrasive discs are used sequentially, and then pieces of fabric, in particular felt, impregnated with abrasive suspensions. Typically, a cloth impregnated with a suspension of diamond powder, the mean dimension of the diamond grains of which is of the order of 1 μm, allows a finish suitable for carrying out the method according to the invention to be obtained. At the end of the mechanical polishing 20, the piece 1 is rinsed with distilled water.
The third treatment 21 comprises a heat treatment on the piece 1 with the aim of releasing internal stresses and of increasing the size of the aluminium grains. In order to avoid oxidation of the piece 1 in the course of the heat treatment 21 and taking account of the speed of the kinetics of the oxidation of aluminium at a high temperature, this heat treatment 21 is preferably carried out under a non-oxidizing atmosphere, typically under a neutral or even reducing atmosphere, that is to say under an inert gas atmosphere, typically under a nitrogen atmosphere, or also under a partial vacuum. The piece 1 is heated in an oven at a temperature of between 350° C. and 600° C., preferably at 450° C. The heat treatment lasts between 0.1 h and 8 h, in particular between 0.5 h and 5 h, preferably for 1 h at an effective temperature of 450° C. under a nitrogen atmosphere.
The fourth treatment is an electropolishing 22 of the piece 1. The object of this is to improve the state of the surface of the outer face 2 of the piece 1 which, as indicated above, must be as smooth as possible. To this effect, the piece 1 is subjected to an electrolysis under a voltage of between 25 V and 26 V for a duration of between 1 min and 1 h in a cell containing a bath regulated at a temperature of between 20° C. and 30° C. The said bath can be an alkaline bath or an acid bath. It is, for example, a Jacquet bath. In particular, the Jacquet bath is made up of a mixture of 33% by volume perchloric acid and 66% by volume glacial acetic acid, the piece 1 constituting the anode of the electrolysis. Typically, an electropolishing 22 according to the invention is obtained by treating the piece 1 by electrolysis under 25 V for 2 min in a Jacquet bath thermoregulated at 20° C. The piece 1 is then rinsed with distilled water and subjected to the treatment 24 by anodization immediately after rinsing.
At the end of the pretreatment 18 of the substrate, a piece 1 of which the outer face 2 has a low and regular arithmetic roughness, in particular an arithmetic roughness of less than 5 nm, is obtained.
This piece 1 is used to prepare an ordered porous structure 7 by a treatment 23 comprising an anodization 24 followed by an abrasion 25.

Anodization

24

The piece 1 is subjected to a single anodization 24, in which the piece 1 constitutes the anode. A single anodization 24 is understood as meaning a treatment comprising either a single anodization stage or successive anodization stages, without an intermediate stage of chemical or electrochemical treatment of the porous structure. The anodization conditions are preferably of the “hard anodization” type as described, for example, in the publication Lee W., Ji, R., Gösele, U. and Nielsch K., (2006), Nature Mat., 5; 9, 741-747 “Fast fabrication of long-range ordered porous alumina membranes by hard anodization”.
Under these operating conditions, the speed of oxidation of the aluminium is advantageously greater than the speed of dissolution, by the electrolyte, of the alumina formed. The anodization 24 leads to the formation of an anodic structure 35 comprising an outer surface layer 3 supported by a residual aluminium layer 4.
The anodization 24 can be carried out in an electrolyte chosen from sulfuric acid, a mixture of sulfuric acid and boric acid, oxalic acid, phosphoric acid, malonic acid, tartaric acid or also citric acid.
Typically, the use of a mixture of sulfuric acid and boric acid as the electrolyte allows a thickness of the structure 35 of up to 300 μm to be obtained. Such a thickness of the anodic structure 35, however, does not have an ordered porous structure 7 over its entire thickness.
To promote the formation of a high thickness of ordered porous structure 7, for example, an aqueous solution of phosphoric acid at a concentration of between 1% and 8%, preferably 8% by weight is employed in a cell of which the temperature is regulated at between −2° C. and +2° C., preferably at −1.5° C. In order to promote a homogeneous and regular growth of the porous structure, the solution is continuously homogenized by agitation. The voltage applied to the aluminium piece 1 is typically between 125 V and 195V.
The anodization treatment 24 is carried out for a duration sufficient for the outer surface layer 3 to have a sufficient thickness, and for the outer surface layer 3 to have a thickness of ordered porous structure 7 over a part of its thickness. Under the preferred operating conditions mentioned above, for example, an outer surface layer 3 of 130 μm thickness is obtained for an anodization duration of 4 h.
The anodic structure 35 is shown in diagram form on FIG. 1 b. FIG. 1 b is merely a diagram and for illustration, and is not to scale. It comprises a residual non-oxidized aluminium layer 4 supporting an outer surface layer 3. The outer surface layer 3 is made up of a non-porous barrier layer 5, also called compact layer, defining on its inner face 6 the interface between the residual aluminium layer 4 and the outer surface layer 3, and on its outer face 10 the non-emerging extremity of the pores 8. In addition, the outer surface layer 3 comprises on its outer face a non-ordered porous layer 11 extending from the outer face of the outer surface layer 3 to the ordered/non-ordered interface 14 with the ordered porous structure 7. The ordered porous structure 7 has a regular juxtaposition of pores 8 empty of material in the form of linear tubular channels of constant diameter extending axially along a main direction corresponding to the direction of anodization, orthogonal to the outer face 2 of the anodic structure 35, and dividing walls 9 separating the pores 8. The dividing walls 9 additionally have a constant thickness over the entire thickness of the porous structure 7.
Depending on the conditions of the anodization 24, the mean distance joining the centres of two adjacent pores varies from 50 nm to 600 nm and the mean diameter of the said pores varies from 10 nm to 500 nm. The non-ordered porous layer 11 is formed from an irregular juxtaposition of pores empty of material and of variable shapes, orientations and dimensions, separated by dividing walls also of variable shapes, orientations and thickness dimensions, over the whole non-ordered porous layer 11.
In practice, it is found that the non-ordered porous layer 11 superimposed on the ordered porous structure 7 partly masks and obstructs the external surface of the said ordered porous structure 7.

Abrasion

25

According to the invention, the non-ordered porous layer 11 is then removed from the anodic structure 35 in a manner such that at least a thickness of ordered porous structure 7 is revealed.
According to a preferred embodiment of the invention, the non-ordered porous layer 11 is removed by removal of material, in particular by at least one mechanical abrasion treatment 25. For this, a solid tool 12, such as a rotating, discoid, rigid, flat device, to the surface of which is attached a piece 13 of fabric, in particular felt, impregnated beforehand with an abrasive suspension, is applied to the outer face of the outer surface layer 3.
The abrasive suspension is made up of an aqueous dispersion of particles which are insoluble in water and are characterized by their hardness as well as by their size.
The solid particles of the abrasive suspensions are chosen from the group consisting of solid and abrasive materials, for example diamond and ceramics—in particular corundum.
A first part of the non-ordered porous layer 11 is removed by abrasion from the outer face of the outer surface layer 3 for some minutes, for example 10 min, with an abrasive suspension formed from a suspension of diamond particles, the mean diameter of the said particles being close to 1 μm. After this first abrasion stage, the surface of the porous layer is rinsed with distilled water. In a second subsequent stage, a second part of the non-ordered porous layer 11 is removed by fine abrasion from the outer face of the anodic structure 35 for some minutes, for example 10 min, with an abrasive suspension formed from an aqueous suspension of diamond particles, the mean diameter of the said particles being close to 0.25 μm.
A thickness of the outer surface layer 3 is thus removed by mechanical abrasion 25, the said thickness being between 15 μm and 25 μm, in particular of the order of from 17 μm to 20 μm, corresponding to the thickness of the non-ordered porous layer 11, revealing at the flat and non-rough outer surface 16 of the piece 15 a thickness of ordered porous structure 7.
By increasing the duration of the abrasion 25 with diamond particles having an mean size of 1 μm, it is possible to extend the abrasion 25 of the outer face of the outer surface layer 3 in a manner such that at least a non-zero thickness of the ordered porous structure 7 is preserved.
The piece 15 resulting from the abrasion 25 of the outer face of the outer surface layer 3 by removal of the non-ordered porous layer 11 is shown in diagram form on FIG. 1 c. This piece 15 comprises an anodized layer 36 supported on a residual aluminium layer 4, the said layer 36 having a porosity which traverses it but does not emerge because of the presence of a barrier layer 5 and of the aluminium layer 4.
The removal of the non-ordered porous layer 11 by mechanical abrasion 25 of the surface allows the radial distribution of the pores 8 on the outer surface 16 of the ordered porous structure 7 to be preserved intact. In particular, the removal of the non-ordered porous layer 11 by mechanical abrasion 25 of the surface allows the value of the diameter of the pores 8 to be preserved unchanged at a value equal to that which it had at the end of the anodization 24.

Adjustment

26, 30 of the Structural Properties

The piece 15 shown in diagram form on FIG. 1 c has on its outer surface 16 a uniform distribution of tubular pores 8 of circular transverse cross-section organized according to a hexagonal network, that is to say according to a “honeycomb” configuration. The pores 8 have a circular transverse cross-section and have, for example, a diameter of the order of 250 nm.
In certain applications, this piece 15 can be used without other modification, with the barrier layer 5 and the residual aluminium layer 4. In other application, this piece 15 is subjected to at least one of the subsequent treatments 26, 30 allowing the functional properties of the piece 15 to be adjusted.
In a first variant of the subsequent treatment 30, the residual aluminium layer 4 is removed by electrochemical separation 31 of the anodized layer 36 and the residual aluminium layer 4. This separation 31 is carried out in an agitated solution of phosphoric acid at a concentration of between 5% and 20%, typically 16% by weight and at a temperature of between 25° C. and 35° C., typically 30° C., under an alternative voltage of 30 volts for 30 min. This treatment 30 moreover leads simultaneously to the removal of the barrier layer 5 and to opening of the pores 8, in particular on the inside face of the porous structure 33. The piece 34 obtained has a porosity which traverses it and emerges on the two faces—outer surface 16 and inner surface 17—of the ordered porous structure 7, and is shown in diagram form on FIG. 1 e.
In a first variant of the subsequent treatment 30, a treatment 32 can then also be carried out by chemical dissolution, leading to widening of the pores 8 of the ordered porous structure 7. For this, the piece 34 is immersed in a solution of phosphoric acid at a concentration of between 5% and 16%, typically 16% by weight. The duration of the treatment 32 and the concentration by weight of the phosphoric acid are chosen to increase the diameter of the pores 8 until a value of the diameter which is, for example, of the same order of size as the distance separating the centre of two adjacent pores in the ordered porous structure 7 is reached.
In a second variant of the subsequent treatment 26, on the piece 15 obtained at the end of the mechanical abrasion 25 a succession of three treatments 27, 28, 29 is carried out by selective dissolution of constituents of the piece 15: a first treatment 27 of controlled opening of the pores 8, a second treatment 28 of chemical/redox dissolution of the residual aluminium layer 4, and then a third treatment 29 of chemical dissolution of the barrier layer 5.
The first treatment 27 comprises partial chemical dissolution of the dividing walls 9 and allows the diameter of the pores 8 to be increased up to a value which depends on the duration of the reaction and on the concentration by weight of the acid used. This first treatment 27 allows perfect control not only of the diameter but also of the geometry of the transverse cross-section of the pores 8, from a circular section up to a hexagonal section. This first treatment 27 additionally allows the diameter of the pores 8 to be modified without, however, affecting the barrier layer 5 or the residual aluminium layer 4.
This first treatment 27 is carried out by immersing the piece 15 in a solution of phosphoric acid at a concentration of between 5 and 16% by weight at a regulated temperature, in particular between 25 and 35° C. Typically, the concentration of the phosphoric acid solution is 16% and the temperature is 30° C. The duration of the treatment varies according to the desired geometry in the surface 16 of the anodized layer 36. A duration of the treatment of 65 min leads to an ordered porous structure 7 in which the pores 8 are ordered hexagonally and have a hexagonal transverse section and a diameter of the order of 400 nm, according to a “wasps' nest” configuration. Intermediate durations of the treatment lead to intermediate configurations between the “honeycomb” configuration and the “wasps' nest” configuration, in which the diameter of the pores varies between 250 nm and 400 nm.
The second treatment 28 by chemical or redox dissolution of the aluminium layer 4 allows the residual aluminium layer 4 to be removed specifically. The piece 15 is immersed in an oxidizing solution at ambient temperature. This oxidizing solution can be a mixture of CuCl or also of CuCl₂at a concentration of 0.1 mol/l and hydrochloric acid at a concentration of 18% by weight. This immersion simultaneously causes oxidation of the metallic aluminium and reduction of the copper cations. Other redox pairs having a large difference in redox potential with the pair Al³⁺/Al can advantageously be used, in particular the pair Hg²⁺/Hg.
In a variant of the second treatment 28, an amalgam of a metal which is liquid at ambient temperature, in particular gallium or mercury, with the aluminium of the residual aluminium layer 4 is realized. Extraction of the amalgam allows the aluminium of the support to be removed in this way.
This second treatment 28 leads to a piece 33 having a porosity which traverses it, without the aluminium substrate, but which does not emerge because of the presence of the barrier layer 5.
The third treatment 29 comprises chemical dissolution of the barrier layer 5 by immersion of the piece 33 in a solution of phosphoric acid at a concentration of between 5% and 20%, for example of the order of 16% by weight, the temperature of the said solution being regulated between 25° C. and 35° C., in particular at 30° C.
A piece 34 formed from an ordered porous structure 7 of a porosity which traverses it and emerges on the two faces of the piece 34 is obtained in this way.
The piece 34, the hardness of which is low, in particular of the order of 150 Hv, is then subjected to heat treatment in order to increase its hardness, in particular up to a value of 2,000 Hv.

EXAMPLE 1

A piece 1 of refined aluminium of 4N quality, of discoid shape, of 10⁻²m diameter and of 10⁻³m thickness is subjected to a mechanical polishing 20 by means of a polisher, abrasive discs and a fabric impregnated with a suspension of diamond particles, the mean size of which decreases down to 1 μm. The total duration of the abrasion is approximately from 20 min to 30 min. The aluminium piece 1 is then rinsed with distilled water and placed in an oven under a nitrogen atmosphere at 450° C. for 2 h. After cooling, the aluminium piece 1 is subjected to a treatment 22 by electropolishing for 2 min in a Jacquet bath, the composition of which is 33% by volume perchloric acid and 66% by volume glacial acetic acid, regulated at 20° C. under a voltage of 25 V.
Immediately after the end of the treatment 22 by electropolishing, the aluminium piece 1 is placed in an anodization cell containing an aqueous bath of 8% (by weight) phosphoric acid homogenized by rotary agitation at a speed of 37 rad/s and regulated at a temperature of −1.5° C. The voltage is fixed at 180 V and the duration of the anodization is 4 h.
The analysis by field effect electron microscopy of the outer face 2 of the outer surface layer 3 obtained after anodization 24 and before polishing 25 is shown on FIG. 5. This photograph shows a plurality of pores irregularly distributed over the entire surface with a transverse cross-section heterogeneous in size and in shape. It is furthermore noted that a minority of these pores have an emerging porosity.

EXAMPLE 2

An aluminium piece 1 is prepared as described in Example 1 and subjected to an anodization under a voltage of 185 V for 4 h.
After the anodization, the outer surface of the anodic structure 35 is removed by abrasion 25 by means of a piece of felt impregnated with a suspension of diamond particles, the mean diameter of which is 1 μm, for 10 min and then by means of a piece of felt impregnated with a suspension of diamond particles, the mean diameter of which is 0.25 μm, for another 10 min.
The aluminium piece 1 supporting the porous structure is then treated for 1 h with a solution of phosphoric acid at a concentration of 16% by weight, regulated at a temperature of 30° C. and homogenized by rotary agitation at a speed of 37 rad/s.
After rinsing, the aluminium piece 1 supporting the porous structure is treated with a solution of CuCl and HCl at a temperature of 20° C. until the thickness of residual aluminium is totally dissolved.
The analysis by field effect electron microscopy of the longitudinal section of the ordered porous structure 7 obtained is shown on FIG. 3. A juxtaposition of sections of linear tubes elongated along the direction of growth of the porous structure, the average width of which is 360 nm, can be seen.

EXAMPLE 3

An aluminium piece 1 is prepared as described in Example 1, is then anodized under a voltage of 185 V for 4 h and finally is subjected to a mechanical abrasion as described in Example 2.
After rinsing, the ordered porous structure is immersed in a solution of CuCl and HCl, regulated at a temperature of 20° C., until the thickness of residual aluminium is totally dissolved. A piece 33 without a residual metallic aluminium layer 4, of a porosity which traverses it and does not emerge, and with a barrier layer 5 is obtained
The analysis by field effect electron microscopy of the surface of the barrier layer 5 of the piece 33 is shown on FIG. 4. A juxtaposition of non-emerging hexagons of hexagonal cross-section regularly ordered according to a centred hexagonal arrangement, and of which the mean diameter of the circle describing this hexagon is 460 nm, can be seen.
On the other hand, the analysis by field effect electron microscopy of the outer polished face 2 of the piece 33 is shown on FIG. 6. A hexagonal arrangement of regularly ordered pores 8 of circular section, the mean diameter of which is 300 nm, can be seen.

EXAMPLE 4

An aluminium piece 1 is prepared as described in Example 1, is then anodized under a voltage of 180 V for 4 h and finally is subjected to a mechanical abrasion as described in Example 2.
After rinsing, the porous structure 15 is treated by electrochemistry for 45 min under a voltage of 30 V/50 Hz in a solution of phosphoric acid at a concentration of 16% by weight, regulated at a temperature of 30° C. and homogenized by rotary agitation at a speed of 37 rad/s. A piece 33 without a residual aluminium layer 4, of a porosity which traverses it and emerges, without a barrier layer 5 and of which the diameter of the pores 8 has been widened is obtained.
The analysis by field effect electron microscopy of the outer surface of the ordered porous structure 7 obtained in this way is shown on FIG. 7. A juxtaposition of pores of circular section, regularly ordered, and of which the mean diameter is 240 nm, of the “honeycomb” type can be seen.

EXAMPLE 5

An aluminium piece 1 is prepared as described in Example 1, is then anodized under a voltage of 210 V for 15 h and finally is subjected to a mechanical abrasion as described in Example 2.
After rinsing, the structure 15 is treated by electrochemistry as described in Example 4 under a voltage of 35 V/50 Hz for 65 min. A piece 33 without a metallic aluminium layer 4, of a porosity which traverses it, without a barrier layer 5, and which emerges on the two faces of the ordered porous structure 7 is obtained.
The analysis by field effect electron microscopy of the outer surface of the ordered porous structure 7 obtained in this way is shown on FIG. 8. A juxtaposition of pores 8 of hexagonal section, regularly ordered according to a centred hexagonal arrangement, of the “wasps' nest” type, of which the mean diameter of the pores is 240 nm can be seen.

Claims

1-16. (canceled)

17. Method for the production of a porous structure in which an outer surface layer comprising an ordered porous structure is produced by anodization of an aluminium substrate, whereby:

an anodization treatment is carried out on a smooth aluminium substrate with a duration sufficient to allow at least a thickness of ordered porous structure to be obtained,

a part of the thickness of the said layer formed by anodization is then removed by mechanical machining, this part of the thickness extending from the outer surface of the said layer formed by anodization, while maintaining at least a non-zero thickness of the ordered porous structure and in a manner such that this ordered porous structure forms the free outer surface of the residual layer.

18. Method as claimed in claim 17, whereby the said part of the thickness is removed by mechanical abrasion.

19. Method as claimed in claim 18, whereby the mechanical abrasion is carried out by means of a piece of fabric impregnated with a suspension, a so-called abrasive suspension, of a powder in an aqueous phase, the said powder comprising at least a mineral chosen from the group of abrasive minerals.

20. Method as claimed in claim 18, whereby the mechanical abrasion is carried out by a plurality of successive abrasion stages, each of the said successive abrasion stages being carried out by means of an abrasive suspension, the abrasive suspensions of each of the successive abrasion stages being chosen in a manner such that a granulometry which decreases from one stage to the other is achieved.

21. Method as claimed in claim 20, whereby each abrasion stage of the plurality of successive abrasion stages is carried out by means of a piece of fabric impregnated with an abrasive suspension, the said piece of fabric being applied to the surface of a rigid support chosen from the group formed by a vibrating support and a rotating support.

22. Method as claimed in claim 18, whereby the mechanical abrasion is carried out by a first abrasion stage by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is between 0.8 μm and 1.5 μm, in particular of the order of 1 μm, and then by a second abrasion stage by means of a piece of felt impregnated with a suspension of diamond, the mean granulometry of which is between 0.2 μm and 0.4 μm, in particular of the order of 0.25 μm.

23. Method as claimed in claim 17, whereby a part of the thickness of the outer surface layer (3) is removed, the thickness of the said part of the thickness of the outer surface layer being between 15 μm and 25 μm—in particular of the order of from 17 μm to 20 μm.

24. Method as claimed in claim 17, whereby an anodization treatment is carried out on a smooth aluminium substrate with a duration suitable for obtaining an outer surface layer having a thickness of between 25 μm and 300 μm, in particular between 100 μm and 200 μm.

25. Method as claimed in claim 17, whereby a single anodization treatment is carried out on a smooth aluminium substrate, the said treatment having a duration of between 1 h and 12 h, in particular of the order of 4 h.

26. Method as claimed in claim 17, whereby a single anodization treatment is carried out on a smooth aluminium substrate for a duration suitable for the thickness of the ordered porous structure formed by anodization to be between 1 μm and 150 μm.

27. Method as claimed in claim 17, whereby the anodization is carried out in an aqueous solution of electrolyte chosen from the group formed by aqueous solutions of oxidizing acids—in particular sulfuric acid, a mixture of sulfuric acid and boric acid, oxalic acid, phosphoric acid, malonic acid, tartaric acid and citric acid.

28. Method as claimed in claim 17, whereby the anodization is carried out in an aqueous solution of electrolyte, the composition of which is suitable for providing an ordered porous structure, the pores of which have a diameter of between 10 nm and 500 nm, in particular between 100 nm and 200 nm.

29. Method as claimed in claim 17, whereby the anodization is carried out at a temperature of between −2° C. and +2° C.—in particular of the order of −1.5° C.

30. Method as claimed in claim 17, whereby the anodization is carried out under a voltage of between 19 V and 240 V—in particular between 125 V and 195 V, with an aqueous solution containing phosphoric acid as the electrolyte.

31. Method as claimed in claim 17, whereby immediately after having removed the said part of the thickness, the non-oxidized aluminium substrate and a part of the non-porous thickness of the said layer are removed to preserve only the ordered porous structure.

32. Method as claimed in claim 17, whereby a chemical treatment is then carried out on the ordered porous structure which is suitable for increasing the diameter of the pores of the said porous structure.

33. Method as claimed in claim 19, whereby the mechanical abrasion is carried out by a plurality of successive abrasion stages, each of the said successive abrasion stages being carried out by means of an abrasive suspension, the abrasive suspensions of each of the successive abrasion stages being chosen in a manner such that a granulometry which decreases from one stage to the other is achieved.