WO1996001684A1

WO1996001684A1 - Permeable anodic alumina film

Info

Publication number: WO1996001684A1
Application number: PCT/GB1995/001646
Authority: WO
Inventors: Russell Paterson; Peter Petrovitch Mardilovich; Alexander Nikoleivitch Govyadinov; Nadezhda Ivanova Mazurenko
Original assignee: The University Court Of The University Of Glasgow
Priority date: 1994-07-11
Filing date: 1995-07-11
Publication date: 1996-01-25
Also published as: AU2932095A; GB9413918D0

Abstract

A permeable anodic alumina film is described in which amorphous anodic alumina is in the form of a thin layer of film (3) and is initially in communication with the alumina substrate (1) and has an array of blind pores (5) or holes opening into one surface of the thin layer (3). The alumina substrate (1) is removed from the alumina layer by electrochemical treatment which opens the blind pores (5) of the amorphous anodic alumina thereby forming an amorphous permeable film (16) with open pores (6), which are of substantially the same diameter, extending from one side of the film to the other side. In another aspect the electrochemical treatment causes cracks or micropores (9) to form within and extending through the barrier layer (7) and which provide communication between the bottom of the bores and the far side of the barrier layer. This results in the alumina substrate layer becoming detached. The alumina permeable film may be heated to convert the alumina permeable film to the crystalline alumina permeable film.

Description

PERMEABLE ANODIC ALUMINA FILM

The present invention relates to a method of preparing a permeable amorphous anodic alumina film, and a permeable crystalline alumina film and to some of the films thus produced.

It is well known in the art that when aluminium is used as an anode in an electrochemical cell with a suitable electrolyte therein, an amorphous aluminium oxide (alumina) layer comprising an array of pores is formed on the surface of the metal; a process which is referred to as anodising. The pore diameter and density of the pores per unit area can be controlled by the anodising voltage, and by suitable control of the anodising conditions it is possible to achieve uniform pore diameters in the range of about 10 to 200nm. The thickness of the alumina layer is controlled by the anodising time and can be as up to 150μm.

Conventional amorphous anodic alumina film has an impermeable barrier layer between the bottoms of the pores and the metal/oxide interface. Each pore is thus a blind pore which is closed at its bottom end. The thickness of this barrier layer is dependent on the anodising voltage.

In order for a permeable amorphous alumina film to be formed, it is essential to remove the metal and permeabilise the alumina layer to form pores which pass from one surface of the alumina through to the other. This has been achieved by a number of methods.

US 3850762 discloses a process for producing an anodic alumina membrane by anodising aluminium to form a layer of porous alumina and removing the aluminium and barrier layer by chemical etching.

EP-A-178831 describes anodic alumina films having an asymmetric structure. In these films, a system of larger pores extend from one face and a system of smaller pores extend from the other face. The system of larger pores interconnecting the system of smaller pores, such that the inner ends of one or more smaller pores are joined to the inner end of a large pore and there are substantially no blind large pores. The asymmetrical films are made by a controlled slow voltage reduction technique designed to thin the barrier layer by the formation in it of progressively finer pores and eventually to dissolve away any remaining barrier layer at the metal/oxide interface.

US 5061544 describes a membrane similar to EP 178832 which has larger pores connectively attached to smaller pores, but the aluminium and barrier layer is removed by reversing the polarity of the electrodes at the final stage of anodisation.

Amorphous anodic alumina membranes or films have the following advantages over known polymeric membranes: a) They have pores with a narrow pore size distribution. b) Highly porous (from 10 to 65%) leading to high flux when used to filter liquids or gases. c) Substantially defect free leading to high selectivity during filtration. d) Resistant to solvents and radiation, and can tolerate temperatures in excess of 400°C. e) Smooth and rigid, even under pressure.

However, the amorphous membranes or films produced by the previously described techniques suffer from a number of disadvantages. Firstly, the pore size within the oxide film is increased during the dissolving process, which is used to achieve the removal of the barrier layer; thus reducing the wall thickness between adjacent pores and increasing the fragility of the membrane. Secondly, depending on the technique used, the barrier layer may only be removed incompletely which leads to variability of effective pore diameter, or the pores may be opened to a varying extent leading to a distribution of effective pore diameters. Furthermore, some techniques for producing open pores produce pores with a diameter which changes along the length of the pore.

All the amorphous anodic alumina films also suffer from the fact that the oxide reacts with water and aqueous solutions of acids and bases. The anodic alumina reactions with water to produce hydrates within the pores, which are accumulated within the pores, resulting in an effective reduction of the pore diameter. Consequently, the film cannot be used for a long period of time for separating a fluid containing water or moisture. In acid or basic solutions the anodic alumina membranes dissolve. The dissolving process beginning at pH<5.0 and pH>8.2 and if the contact time is sufficient, the membranes can be completely dissolved.

It is known that heat treatment at temperatures greater than the temperature of crystallisation of amorphous anodic alumina, is effective for preventing the pore diameter of the porous anodic alumina from changing over time, and for providing increased chemical resistance to acid and base attack. The crystalline forms of anodic alumina, γ-, Θ - , δ- and α- alumina, and polycrystalline anodic alumina (which is a mixture of crystalline forms) are much more stable to acid and base attack.

However, high temperature heat treatment performed in order to remove the hydroscopic property of the film and to enhance its resistance to acid and base attack, often results in the film being damaged by thermal stresses and may lead to the film curling up. This is due to the difference in porosity on the opposite sides of the film, resulting from asymmetrical structural factors across the film.

US 5061544 and 5087330 describe permeable films which can be heat treated. The films have a large central open pore disposed between a series of smaller branched pores. This creates a membrane with a symmetrical pore structure which does not bend during heating. However, each pore still has a diameter which varies along its length. Furthermore, some of the smaller branched pores do not connect with the larger central pore resulting in a film with decreased porosity.

An object of the present invention is to obviate or mitigate at least one disadvantage of the aforementioned prior art documents.

This is achieved by removing the aluminium metal substrate and opening the blind pores of the amorphous anodic alumina, so as to form an amorphous permeable film with open pores.

The amorphous anodic alumina is in the form of a thin layer or film in communication with the aluminium substrate and has an array of blind pores or holes opening into one surface of the thin layer.

The aluminium substrate is removed from the alumina layer by an electrochemical treatment. The electrochemical treatments causes in one aspect complete removal of the barrier layer and the aluminium, resulting in a film with substantially all pores open and substantially the same diameter, extending from one side of the film to the other side. In another aspect the electrochemical treatment causes cracks or micropores to be formed within and extending through the barrier layer, so as to form communication with the bottoms of the pores and the far side of the barrier layer. The formation of the micropores extending through the barrier layer from one face to the other results in the aluminium substrate layer becoming detached.

The micropores are generally cylindrical and of smaller diameter than the pores. The number of micropores may vary for each pore, but are generally symmetrically distributed radially outwardly from said pores. The amorphous permeable film may then be subjected to a heating regime to convert amorphous permeable film to crystalline permeable alumina film.

According to the present invention there is provided a method of preparing a permeable anodic alumina film, which comprises; in a first stage anodising an aluminium substrate by employing an anodising electrolyte solution and an anodising voltage, such as to produce a layer of impermeable amorphous anodic alumina comprising an array of blind pores and a barrier layer, adjacent said aluminium substrate; and in a second stage subsequently electrochemically treating said anodised substrate with a dissolving electrolyte such as to detach the amorphous anodic alumina layer from the aluminium substrate to provide a permeable film comprising open pores which pass through the film from one fact thereof to the other.

"Anodising" electrolyte is taken to mean an electrolyte in which an amorphous anodic alumina film will form on an aluminium substrate upon anodising. The rate of dissolving the oxide layer is slower than the rate at which it is produced. The anodising electrolyte can be selected from acids such as oxalic, sulphuric, phosphoric and chromic acids, either separately or together.

The "blind pores" are pores which have an impermeable barrier layer disposed between the bottoms of the pores and the aluminium substrate. The barrier layer is impermeable so there is no gas or liquid passage possible across this layer.

The "dissolving" electrolyte is one in which the rate of dissolving of anodic alumina is faster than the rate which the oxide film can be formed. Such electrolyte are typically used for electropolishing of aluminium sheets and can be high concentration of suitable acids which are well known in the art, such as, hydrochloric acid, mixtures of CH₃COOH and H₃P0₄, HC10₄ and (CH₃CO)₂0, and HC10_4/ and the like.

"Open pores" are pores which have been permeabilised by partial or complete removal of the barrier layer.

The size of the pores, the thickness of the alumina and the thickness of a barrier layer are dependent on the voltage and time applied, but preferably the anodising voltage is between 10V and 200V and the time applied is between 30 minutes and 10 hours. Preferably the pores have diameters of lOnm to 0,2 microns, preferably the thickness of the alumina is between lμm and 150μm and preferably the barrier layer is between 15nm and 0,25μm.

The pores are not necessarily truly cylindrical and the terms "cylindrical" and "diameter" are used in a corresponding non-geometrical sense. However, for ease -of understanding, the term diameter is used to refer to the dimension across the pore.

The second stage electrochemical treatment used to detach the anodic alumina from the aluminium substrate is also dependent on voltage and time. If it is required to completely remove the barrier layer and open all the pores, thus forming permeable pores with substantially constant pore diameter throughout the pore length, the electrochemical voltage should be greater than or equal to the anodising voltage, preferably 2 to 30V greater than the anodising voltage.

If, however, the barrier layer is not to be completely removed, but rather micropores are to be formed through the barrier layer, the second stage voltage should be less than the anodising voltage; preferably 1 to 30V less than the anodising voltage. The time for the electrochemical treatment in either case may be relatively short, preferably 0,5-10 seconds and may be one pulse or a series of pulses.

The present invention further provides an anodic alumina permeable film having a plurality of open pores extending therethrough from one face thereof to an oppose face thereof, substantially all the pores having a substantially constant pore diameter throughout their length.

Optionally the permeable anodic alumina film is further subjected to a heat treatment in order to convert the amorphous anodic alumina to polycrystalline or crystalline anodic alumina.

It is important not to allow the film to curl up during heat treatment and so the film is placed between two flat plates, preferably polished ceramic plates. Ceramic material are somewhat porous which allows gases generated during heat treatment to escape avoiding damage of the thin material. The weight on the top plate or a weight applied to the top plate is selected to be sufficient to prevent the film from curling up during heating. Preferably, the weight applied is between 50 to 200g/cm² of anodic alumina film surface.

The temperature of heating will generally exceed 800°C and is preferably between 850°C and 1200°C. The rate of heating and cooling above 650°C is important and should be carefully controlled. Preferably, the rate of heating and cooling should be 1 to 2°C/min at temperatures around the temperature of crystallisation (± 50°C) , particularly in the interval 750°C to 850°C and the interval 1050°C to 1200°C. The rate of heating and cooling may be as high as 5°C/min provided that the rate is constant and does not fluctuate. The temperature range for polycrystalline anodic alumina is between 800°C and 1550°C and the a- alumina crystallisation temperature is approximately 1150°C. If an inappropriate heating regime is used the membrane may be damaged or destroyed.

The maximum temperature should be maintained for a length of time before cooling is carried out. Preferably, this time is greater than 5 minutes and more preferably greater than 20 minutes.

The present invention further provides a flat crystalline permeable anodic alumina film.

Embodiments of the present invention will now be described with reference to the drawings and examples presented herein,

Figure 1 shows a schematical representation in cross section of an impermeable porous anodic alumina layer attached to an aluminium substrate.

Figure 2 shows a schematical representation of a film according to the present invention.

Figure 3 shows a schematical representation of a further film according to the present invention.

Figure 4 is a graph showing the amounts of aluminium released into solution from amorphous anodic alumina and polycrystalline alumina membranes with time of treatment, in a 0.1M HC1 solutation; and

Figure 5 is a graph showing the amount of aluminium released into solution from amorphous anodic alumina and polycrystalline alumina films of the present invention, at various pH values.

A representation of impermeable anodic alumina comprising blind pores is shown in Figure 1. A layer of anodic alumina 3 comprising an array of regularly spaced blind pores 5 is formed on an aluminium substrate 1. An impermeable barrier layer 7 of anodic alumina is disposed between the pores 5 and the aluminium substrate 1.

In order to permeabilise the anodic alumina layer, the .aluminium substrate 1 must be removed and the barrier layer 7 either completely removed (Figure 2) forming completely open pores 6, forming an open pore of permeable anodic alumina film 10. Or, in another embodiment, the barrier layer is not removed, but, rather and micropores 9 formed within and the barrier layer 7 (Figure 3) forming a micro-porous permeable anodic alumina 11. Example 1

A 99.99% aluminium panel (5cm x 5 cm) was degreased in boiling octane, ultrasonically cleaned in isopropanol and washed in distilled water. The panel was then electrochemically polished (glacial acetic acid/88% H₃P0₄; 1:1 by volume) and washed once more in distilled water.

The panel was then anodised in a mixture of 0.35% oxalic acid (H₂C₂0₄) and 1% citric acid (H₈C₆0₇) at 12°C and at 160V for 2 hours. The panel was then washed in distilled water, followed by isopropanol, before being allowed to dry.

The panel was then transferred to a dissolving electrolyte comprising a mixture of a HC10₄ (70ml; 72% w/w) and (CH₃CO)₂0 (130ml pure) and subjected to 165V for approximately 1 second. This separated the anodic alumina film from the aluminium panel and the film was washed in distilled water and allowed to dry. Figure 2 shows a schematic representation of the permeable anodic alumina as prepared by this example. Example 2

An aluminium plate was initially prepared as described in Example 1 and then anodised in 3% oxalic acid (H₂C₂0₄) at i = 30 mA/cm², at 12°C for 1 hour. The final voltage was 115V. The panel was then washed as before and dried, before being transferred to a dissolving electrolyte comprising a solution of HCl (30%) and subjected to five, one second pulses at 100V. The anodic alumina separated from the aluminium and the film was washed in distilled water and allowed to try.

Figure 3 shows a schematic representation of the permeable anodic alumina as prepared by this example. Example 3

An aluminium plate was initially prepared as described in Example 1 before being anodised in 3% oxalic acid at 10°C and 70V for 4 hours. The resulting anodic alumina layer was about 50 microns thick. The panel was then washed and dried as before and transferred to a dissolving electrolyte comprising a mixture of HC10₄ (70ml; 72% w/w) and (CH₃CO)₂0 (130ml; pure) and subjected to 72V for approximately 2 seconds. The separate film was then washed in distilled water and dried. The pores were completely open in the manner shown in Figure 2. The film was then placed between two polished ceramic plates; the weight of the top plate was lOOg/cm² of film surface area and placed in an oven and heated. The heating regime was as follows; 10°C/min between room temperature and 650°C, 2°C/min between 650°C and 850°C and then the film was calcined at 850°C for 30 minutes before cooling to 650°C at a rate of 2°C/min and down to room temperature from 650°C at a rate of approximately 10°C/min.

The crystalline structure of the film was examined by X-ray analysis and observed to be approximately 95% γ-alumina and approximately 5% δ-alumina. Example 4

A permeable anodic alumina film was prepared as described in Example 3, but treated to a different heating regime. The heating regime was as follows; 10°C/min between room temperature and 650°C, 2°C/min between 650°C and 1160°C and then the film was calcined at 1160°C for 30 minutes before cooling down to 650°C at a rate of 2°C/min and down to room temperature at a rate of approximately 10°C/min.

The crystalline structure of the film was examined by x-ray analysis and observed to be pure γ-alumina. Example 5

The polycrystalline anodic alumina film from Example 3 was tested to observe its resistance to chemical attack.

Figure 4 shows the amount of aluminium ions released into solution over time, when the polycrystalline film is placed in a 0.1M HCl solution. This is shown in comparison to an amorphous anodic alumina film as prepared by Example 1.

Figure 5 shows results of a similar experiment, in which the polycrys.talline film was placed in solutions of various pH for periods of 2 hours or 16 hours. Again, this is shown in comparison to amorphous anodic alumina.

The data shown in Figures 4 and 5 show that the polycrystalline anodic alumina films are most resistant to acid and base attack than the amorphous films.

It will be understood that various modifications may be made to the embodiments herein before described without departing from the scope of the invention. For example, the anodising voltage may be varied to provide barrier layers of a desired thickness to facilitate control of film permeability. It will be appreciated that the film is planar but may be of any other suitable shape, for example tubular or curved. It will also be understood that secondary pores may not be formed in each primary pore; it is sufficient that enough secondary pores be formed to allow the film to function as permeable, but in practice it is believed that secondary micropores are formed for each primary pore.

The invention has application in a variety of fields; for example as membrane filters, for gas and/or liquid separation, as microtemplates in the semiconductor industry, as the support substrate for catalytic elements such as platinum, vanadium etc. to create catalytic membrane reactors. The inorganic anodic alumina substrate can also be used in the manufacture of micromechanical components, such as gear wheel and the like. The film may also have application in the film of optical filters and sensors.

Advantages of the invention are that it provides a thermally and chemically stable film or membrane with a variety of applications.

Claims

1. A method of preparing a permeable anodic alumina film, which comprises; in a first stage anodising an aluminium substrate by employing an anodising electrolyte solution and an anodising voltage, such as to produce a layer of impermeable amorphous anodic alumina comprising an array of blind pores and a barrier layer, adjacent said aluminium substrate; and in a second stage subsequently electrochemically treating said anodised substrate with a dissolving electrolyte such as to detach the amorphous anodic alumina layer from the aluminium substrate to provide a permeable film comprising open pores which pass through the film from one face thereof to the other.

2. A method as claimed in claim 1 wherein the rate of dissolving the layer is slower than the rate at which it is produced.

3. A method as claimed in claim 1 and claim 2 including the step of selecting the anodising electrolyte from acids such as oxalic, sulphuric, phosphoric and chromic acids, either separately or together.

4. A method as claimed in any preceding claim wherein the permeable anodic alumina film is further subjected to a heat treatment in order to convert the amorphous anodic alumina to polycrystalline or crystalline anodic alumina.

5. A method as claimed in any preceding claim wherein the film is placed between two flat plates to prevent the film from curling up during heat treatment.

6. A method as claimed in claim 5 including the step of allowing gases generated during heat treatment to escape to avoid damage of the thin material.

7. A method as claimed in claim 5 or claim 6 wherein the weight of the top plate or a weight applied to the top plate is selected to be sufficient to prevent the film from curling up during heating.

8. A method as claimed in any one of claims 4 to 7 wherein the temperature of heating is greater than 800°C.

9. A method as claimed in claim 8 when the temperature of heating is between 850°C and 1200°C.

10. A method as claimed in any one of claims 4 to 9 wherein the rate of heating and cooling of the film is 1- 2°C/min at a temperature around the temperature of crystallisation (± 50°C) , particularly in the interval 750°C to 850°C and the interval 1050°C to 1200°C.

11. A method as claimed in any one of claims 4 to 10 wherein the maximum temperature is maintained for a predetermined length of time greater than 5 minutes before cooling is carried out.

12. A method as claimed in claim 11 wherein the maximum temperature is maintained for a period of at east 20 minutes.

13. An anodic alumina permeable film having a plurality of open pores extending therethrough from one face thereof to an opposite face thereof, substantially all the pores having a substantially constant pore diameter throughout their length.

14. A film as claimed in claim 13 wherein a film has an array of blind pores or holes opening into one surface of the film.

15. A film as claimed in claim 13 or claim 14 wherein a plurality of micropores are formed between the bottoms of the pores and the surface of the opposite face therof, said micropores being generally cylindrical and of smaller diameters than the pores.

16. A film as claimed in any one of claims 13 to 15 wherein the micropores are generally symmetrically distributed radially outwardly from said pores.