Method for producing a membrane and such membrane

Abstract

The invention relates to a method for producing a membrane and such membrane. The method comprises the steps of: providing a container with electrolyte; placing a structure in the container; and providing at least two electrodes with a potential difference to achieve a plasma electrolytic oxidation on the structure. Preferably, the structure comprises a metallic structure, with the metallic structure chosen from the group of Titanium, Aluminum, Magnesium, Zirconium, Zinc and Niobium, and/or an alloy.

Claims

1. A method for producing a membrane, comprising the steps of: providing a container with electrolyte; placing a structure comprising at least one layer of material in the container, wherein the structure comprises a metal and/or alloy layer; providing at least two electrodes with a potential difference to achieve a plasma electrolytic oxidation on the structure, wherein providing a potential difference comprises applying a voltage of over 100 Volts, such that a plurality of pores are created in a metal oxide layer on the metal and/or alloy layer to form a membrane, wherein at least 90% of the plurality of pores created in the metal oxide layer on the metal and/or alloy layer have a diameter that is within 1 nm to 100 nm at a surface of the metal oxide layer; and removing only a part of the metal and/or alloy layer by an electrochemical etching operation.

2. The method according to claim 1, wherein the metallic structure is chosen from the group of Titanium, Aluminium, Magnesium, Zirconium, Zinc and Niobium, and/or an alloy.

3. The method according to claim 1, wherein the step of providing a container with electrolyte comprising choosing an electrolyte from the group of dilute alkaline fluids or solutions.

4. The method according to claim 1, wherein the structure is placed between the at least two electrodes.

5. The method according to claim 1, wherein the structure is chosen from an existing membrane.

6. The method according to claim 1, wherein providing a potential difference comprises applying a voltage of over 200 Volts.

7. The method according to claim 1, wherein providing a potential difference comprising applying a continuous and/or pulsed direct current, and/or a continuous and/or pulsed alternating current, including negative or bipolar pulses.

8. The method according to claim 7, wherein the pulsed direct and/or alternating current comprising negative or bipolar pulses.

9. The method according to claim 1, wherein achieving a plasma electrolytic oxidation on the structure comprises exceeding an electric break-down potential of an oxide film on the structure thereby causing discharges to occur.

Description

(1) Further advantages, features and details of the invention are elucidated on basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, wherein:

(2) FIG. 1 shows a membrane according to the invention;

(3) FIG. 2 shows a device for producing such membrane;

(4) FIGS. 3A and B show membrane surfaces of an existing method and the method according to the invention, respectively;

(5) FIG. 4A shows an electron microscope image of an untreated titanium structure;

(6) FIG. 4B shows the results of an Energy-Dispersive X-ray (EDX) measurement, indicating the composition of the untreated structure of FIG. 4A;

(7) FIG. 5A shows an electron microscope image of the titanium structure of FIG. 4A after treatment with the method according to the invention; and

(8) FIG. 5B shows the results of an EDX measurement, indicating the composition of the treated structure of FIG. 5A.

(9) A membrane 2 (FIG. 1) comprises a metallic structure 4. In the illustrated embodiment support layer 4 is provided from titanium. On layer or structure 4 is provided a membrane layer 6 that is provided thereon by plasma electrolytic oxidation. Layer 6 comprises TiO.sub.2 that expands in the process. The titanium structure is of a plate or tube-shape. In the illustrated embodiment the structure of a sintered material is made by sintering titanium particles in the desired shape. Optionally, the titanium material is heated very shortly on the outside or treated mechanically to form a dense titanium layer on top of the sintered structure that is subjected in the next step to the plasma electrolytic oxidation process.

(10) In a process 8 (FIG. 2) a container 10 is provided with electrolyte 12. A product 14 is placed between a first electrode 16 and a second electrode 18 in electrolyte 12. Electrodes 16, 18 are connected in a circuit 20. Circuit 20 comprises a voltage source 22 to provide a potential difference to electrodes 16, 18.

(11) When producing a membrane 2, 14 a structure is placed in a container 10 filled with electrolyte 12. Depending on the desired characteristics of membrane 2, 14 the type of material is chosen. Also, the shape of membrane 2, 14 is chosen. For example, tube like membranes can be produced effectively in process 8. It will be understood that it is also possible to produce other three dimensional shapes for the membrane in process 8. After placing the product 2, 14 in electrolyte 12 a potential difference is provided over electrodes 16, 18. The conditions for this potential difference are chosen such that a plasma electrolytic oxidation operation takes place on the product 2, 14 to achieve a membrane 2, 14.

(12) In an alternative embodiment according to the invention, the product 2, 14 acts as one of the electrodes in process 8. This reduces the amount of separate electrodes that are used in process 8. Furthermore, the wall of container 10 may act as one of the electrodes. This enable the use of container 10 and product 2, 14 as electrodes 18, 20, such that no separate electrodes are required.

(13) A membrane 2, comprising a palladium layer, can be used for gas separation processes, for example. Hydrogen dissolves in the metal grid of palladium and diffuses to the permeate side of membrane 2. This takes place at relatively high temperatures, normally above 300 C. By configuring membrane 2, such that only hydrogen passes membrane 2, hydrogen can be separated very effectively.

(14) According to one of the preferred embodiments according to the invention membrane 2 is produced starting with a metal layer comprising palladium and titanium. Such membrane can be used for the gas separation processes mentioned above, for example. Conventionally, the palladium is provided after a titanium layer is made porous. According to the present invention, the palladium is provided in a process that in itself is known to the skilled person on a titanium foil, using chemical vapour deposition, for example. In a next step, the structure is treated, using the method according to the invention, with a plasma electrolytic oxidation operation. The titanium layer gets porous, while at the same time the palladium layer remains substantially non-porous. According to this method a composite membrane is achieved very effectively with a porous titanium layer and a relatively thin palladium layer.

(15) The method according to the invention may use other materials in stead of palladium. For example, copper can be provided. After the plasma oxidation operation the pores in the titanium layer stop at the copper layer as copper does not form an oxide in this operation. By etching a part of the copper support layer a membrane 2 is achieved. Also other metals can be used, including titanium, as support layer.

(16) Experiments have been performed with a process 8 to produce a membrane 2 according to the invention. The resulting membrane 2 is compared to existing membranes that are being produced with a sintering operation. The result of such existing conventional operation (FIG. 3A) shows relatively large pore sizes with a relatively large spread of these sizes. For example, the pore size of the pore in the middle of FIG. 3A of a membrane 24 that is produced in a known manner is about 30 micrometer. The membrane 26 (FIG. 3B) that is produced according to the method according to the invention is illustrated on the same scale and shows a pore size of the order of magnitude of nanometers. At the same time the range of pore sizes in membrane 26 is relatively small so that the characteristics of membrane 26 are controlled more accurately.

(17) In an experiment, an existing porous titanium membrane is used as a structure for treatment according to the invention. The porous titanium membrane has been produced by a conventional method of membrane production, namely by sintering. Electron microscope image 28 (FIG. 4A) shows the structure before the method according to the invention is applied.

(18) The composition of the structure of FIG. 4A is determined by Energy Dispersive X-ray (EDX) spectroscopy. Graph 30 (FIG. 4B) shows the results. The x-axis depicts the energy in keV and the y-axis depicts the number of counts. Table 1 shows the quantitative results for the different peaks of graph 30. The results show that the structure substantially comprises titanium.

(19) TABLE-US-00001 TABLE 1 Quantitative results of energy-dispersive X-ray spectroscopy of a porous titanium structure before applying the method according to the invention. Element line Net Counts Weight % Atom % C K 247 3.18 9.65 O K 210 10.92 24.86 Al K 66 0.26 0.35 P K 22 0.07 0.08 Ti K 11147 85.57 65.06 Ti L 666 Total 100.00 100.00

(20) Subsequently, the porous titanium structure is treated according to the invention. The method is carried out using an electrolyte comprising an aluminium salt. Electron microscope image 32 (FIG. 5A) shows the resulting surface layer of the structure. Clearly, the pores of the treated structure are reduced in size compared to the untreated structure of image 28 (FIG. 4A).

(21) The composition of the treated structure of FIG. 5A is again determined using EDX spectroscopy. Graph 34 (FIG. 5B) shows the results. Table 2 shows the quantitative results for the different peaks of graph 34. These results show that the structure comprises a titanium aluminate. By using aluminium salt in the electrolyte, the surface layer of the untreated titanium structure has been transformed to a titanium aluminate.

(22) TABLE-US-00002 TABLE 2 Quantitative results of energy-dispersive X-ray spectroscopy of the porous titanium structure after applying the method according to the invention. Element line Net counts Weight % Atom % C K 376 3.66 6.83 O K 3737 42.89 60.17 Al K 12698 20.15 16.76 Si K 1038 1.78 1.42 P K 105 0.17 0.12 Ti K 8504 31.35 14.69 Ti L 535 Total 100.00 100.00

(23) The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.