RU2405621C2 - Method of producing membranes with regular nanopores from barrier-film metal oxides - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to production of membranes with regular nanopores used for micro-nanofiltration, water and gas purification etc. Layer of anode oxide with hexagonal canalised columnar structure is formed on one side of metal foil or thin metal film of high-purity barrier-film metal. Then plasma etching of the film non-anodised side or thin metal film to remove remaining metal and continuous oxide barrier layer that block channels and to produce through nanoholes in oxide film.

EFFECT: reduced process time.

5 cl, 5 dwg, 1 ex

Description

The invention relates to the field of manufacturing membranes with regular nanopores used in medicine, pharmaceuticals, biotechnology, the nuclear industry, analytical chemistry for micro-nanofiltration, for cleaning liquids and gases, in the energy sector in the production of fuel cells and other fields.

It is known that under certain anodizing conditions of valve metals, in particular aluminum, in aqueous solutions of electrolytes based on sulfuric, oxalic or phosphoric acids, anodic oxides consisting of two layers can be obtained on the surface of these metals: a continuous thin layer adjacent to the metal, called a barrier layer , and a porous external highly hydrated layer (EE Averyanov, Anodizing Handbook, Moscow, Mashinostroenie, 1988).

A known method of producing porous oxide by a two-stage process of anodizing an aluminum film with a thickness of 20 μm deposited on a silicon substrate, comprising removing the first layer by dissolving in a mixture of liquid acids HF and H ₃ PO ₄ . An ordered porous oxide structure was obtained with a nanopore diameter of 25 nm with a distance of 90 nm between them.

The disadvantage of this method is the inability to obtain nanomembranes without a substrate, as well as a long cycle of two-stage anodization.

A known method of producing porous nanomaterials based on the use of electrochemical anodic oxidation (patent of the Russian Federation RU No. 224015 C1 IPC C25D 11/06 (2006.01). Published 05/10/2008. Bull. No. 13).

The disadvantage of this method is the long duration of the process and the complexity of its implementation. The formed material - porous oxide is obtained in two stages: first, a “sacrificial layer” porous oxide is created, which is subsequently removed, and then the main layer of porous oxide is formed, while the temperature of the reaction zone is continuously changed with the change in electric current density during anodic oxidation. The porous oxide obtained by the described method is located on a substrate and is not a free membrane.

The closest to the claimed is a method for producing porous films of anodic aluminum oxide in phosphoric acid with a concentration of 5-150 g / l at temperatures of 5-50 ° C and with a continuous change in the voltage on the anode from 40 to 200 V (US patent 4,859,288, 08.22.1989 g .).

In this case, oxide films of a thickness of 5-10 μm with a hole diameter of 100-500 nm were obtained. The disadvantage of this method is that when removing a continuous layer by chemical etching, a simultaneous increase in pore diameter occurs and it is almost impossible to obtain membranes with holes less than 100 nm.

The objective of the invention is to obtain free membranes (without substrate) with regular nanopores with a diameter of 10 to 100 nm from valve metal oxides and reducing the time of the manufacturing process.

This goal is achieved by the fact that on a thin foil or film of a valve metal (aluminum, titanium, tantalum, niobium, magnesium, zirconium), an anode oxide layer with a hexagonal channelized columnar structure is formed by unilateral electrochemical anodization, and then the film is exposed to the ion from the side of the remaining metal - plasma etching to remove the remaining metal layer, as well as a continuous oxide barrier layer that covers the channels, and the formation of through nanoholes in the oxide film, with the anode radiation is carried out in an electrolyte stream, and ion-plasma etching is carried out by a plasma stream of xenon or argon or krypton with an ion energy of 300-400 eV.

An example implementation of the method.

The surface of aluminum foil with a purity of 99.99% and a thickness of 15 μm was cleaned of impurities, smoothed on polished glass by rolling with a glass tube, and the surface was polished with a swab with polishing paste until a mirror shine appeared.

After mechanical polishing, the foil was cleaned of contaminants by washing in HDA acetone, and subsequent etching for 5 min in a 3% KOH solution.

After washing the foil in distilled water and subsequent drying in a stream of filtered hot air, the quality of the workpiece was checked for the presence of micro-holes and deep holes in it. The control was carried out visually and on a metallographic optical microscope at magnifications up to × 800.

The foil samples intended for anodizing were placed in a special mandrel, the design of which provided one-sided access of the electrolyte to the central region of the foil with a diameter of 8 mm. In the manufacture of the mandrel used materials that do not interact with the electrolyte - glass, organic glass, plastic.

In the anodizing process, a solution of oxalic acid in distilled water with a concentration of 0.3 M was used. Unilateral anodization of the foil was carried out in the electrolyte stream in a potentiometric mode at voltages of 20-60 V and electrolyte temperatures of 18-80 ° C.

After the anodizing operation, which, depending on the mode, lasted from 1.5 to 2.5 hours, the foil in the mandrel was washed in running water for 10 minutes and then for 1 minute in a stream of distilled water. After that, the membrane was dried in a mandrel with a stream of filtered hot air (40 ° C) for 5 minutes. To obtain a nanoporous membrane on the surface of the film with the anodized layer on the side opposite to the anodization direction, two continuous barrier layers and aluminum islands between them were removed by ion-plasma etching in an ion-plasma stream from a stationary E-H plasma accelerator operating on xenon.

The anodization operation was carried out in a laboratory cell, a diagram of which is shown in figure 1.

The anodizing laboratory cell consisted of an external bath 1 with distilled water, the temperature of which was maintained with an accuracy of ± 2 ° C using an automatic control system, including a heater 2, a chrome-copel thermocouple 3 and an electronic unit 4. A container with electrolyte 5 was placed in the external bath in which there was anodized foil in the mandrel 6, an aluminum cathode 7 and a glass tube 8 through which the electrolyte was circulated. The electrolyte flow to the anodized foil was created using a centrifugal pump 9 through the filter 10. The voltage to the anode was supplied from a stabilized power source 11 of the TEC 20 type.

A digital milliammeter and resistance, a voltage drop, which was applied to one of the channels of the analog-to-digital converter (ADC) 12 of the E-240 type, and then entered into the computer 13, were sequentially connected to the anode circuit. The ADC was controlled and the data were recorded continuously using the program "Power Graph".

The regularity of the structure of the porous oxide formed during anodization and the main parameters of the structure depend on the temperature in the zone of its formation.

To maintain a constant temperature in the reaction zone, a directed electrolyte flow to the anodized surface was carried out. The electrolyte flow also contributed to the removal of gas bubbles from the anodized surface.

The use of electrolyte circulation makes it possible to obtain regular nanoporous oxide in one stage, which simplifies the process and more than halves the time for manufacturing the membrane.

Figure 2 shows the time dependence of the anode current at a constant voltage of 40 V and an electrolyte temperature of 24 ° C. The above dependence has characteristic regions: initial overflow of current, region of stabilization, and region of incidence of the anode current, each of which reflects the corresponding electrochemical processes that occur during anodization.

At the first stage (current overflow), negative oxygen ions in the solution move to the anode and penetrate through the continuous protective oxide barrier layer, which always exists on the surface of aluminum. At the same time, there is a counter motion of positively charged Al ⁺ ions through the barrier layer to the cathode. At a certain point in time, a dynamic equilibrium of the two flows sets in and a stabilization region that is long over time sets in, in which a regular columnar nanoporous alumina grows above the barrier layer. In this case, the barrier layer together with the porous oxide formed constantly moves downward relative to the upper initial surface of the foil and the roughness of the upper surface of the resulting porous layer increases. The process of complete anodizing of the foil section ends with a drop section of the anode current, on which there are two continuous barrier layers and electrically insulated sections of aluminum located between them on the reverse side of the foil.

After removing the membrane from the mandrel, a visual inspection was performed for the presence of large defects and a subsequent study of the working surface of the membrane using a transmission optical microscope.

The study of the parameters of nanoporous membranes was carried out on a ZEISS EVO-40 scanning electron microscope. To remove the charge, an amorphous carbon film 15–20 nm thick was sprayed onto the surface of the samples.

Figure 3 shows a photograph of the surface of the membrane from the anodizing side. Two zones are visible in the photograph: the initial surface of aluminum and the porous nanostructured oxide layer.

Figure 4 shows the membrane from the side opposite to the direction of anodization, after treatment with a plasma stream of xenon with an energy of 400 eV for 15 minutes The image shows two zones: subjected and not subjected to plasma treatment. It can be seen from the drawing that treating the membrane with a plasma stream removes the barrier layers and opens the nanoporous structure.

Figure 5 shows a typical image of a nanoporous membrane, which was used at the stage of quality control of the final product and in determining its parameters.

As a result of the implementation of the proposed method were obtained membranes with a diameter of 8 mm, a thickness of 8 μm, having a regular hexagonal nanoporous structure with a hole diameter of 25-100 nm. The diameter spread of the holes was ~ 10%.

Claims (5)

1. A method of manufacturing membranes with regular nanopores from valve metal oxides, comprising the formation by anodizing on one side of a metal foil or a thin metal film of high purity valve metal a layer of anode oxide from a hexagonal channelized columnar structure, followed by ion-plasma etching of the non-anodized side of the foil or thin a metal film to remove the remaining metal and a continuous oxide barrier layer covering the channels and form in the oxide film of through nanoholes. 2. The method according to claim 1, characterized in that high-purity metals from the series: aluminum, titanium, tantalum, niobium, magnesium, zirconium are used as the material of the foil or thin film. 3. The method according to claim 1, characterized in that the anodization is carried out in a stream of electrolyte directed to the anodized surface. 4. The method according to claim 1, characterized in that the anodization is carried out in a potentiostatic mode at voltages of 20-60 V and an electrolyte temperature in the range of 18-80 ° C. 5. The method according to claim 1, characterized in that the ion-plasma etching is carried out by a plasma flow of argon, or krypton, or xenon with an ion energy of 300-400 eV.

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