MULTILAYER ANODIZED ALUMINIUM OXIDE NANO-POROUS MEMBRANE AND METHOD OF MANUFACTURE THEREOF

The present invention relates to a method of producing multilayer anodized aluminium oxide nano-porous membrane and the membrane produced thereof. Further the invention relates to the nano-porous multi-layer membrane for filtration application. The three layered membrane of the present invention avoids sticking of solute components on the surface obviating the problem of coagulation. This membrane imparts anti coagulation capability wherein in-spite of sticking of the solute component on the surface of the membrane appropriate passage is still available for liquid/small solutes to pass beneath the said stuck solute component to enhance effective surface area for filtration obviating the problem associated with coagulation.

DESCRIPTION OF THE INVENTION

In the following description, various embodiments will be disclosed. However, it will be apparent to those skilled in the art that the embodiments may be practiced with only some or shall disclosed subject matter. For purposes of explanation, specific numbers, materials, and/or configuration are set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to one skilled in the art that the embodiments may be practiced without one or more of the specific details, or with other approaches, materials, components etc. In other instances, well-known structures, materials, and/or operations are not shown and/or described in detail to avoid obscuring the embodiments. Accordingly, in some instances, features are omitted and/or simplified in order to not obscure the disclosed embodiments. Furthermore, it is understood that the embodiments shown in the Figures are illustrative representation and are not necessarily drawn to scale.

The nano-porous multi-layer membrane is produced using a combination of hot and mild anodization method. It is surprisingly found that in contrast with the conventional anodization methods wherein voltage is gradually varied in a single step, if the first step anodiation voltage is maintained higher value and further in the second step if voltage is maintained lower than that (in the first step), plurality of pores were generated in the each of the concave surface of the region and further sudden decrease in the pore size was observed (than that of gradual reduction of the pore diameter).

It is further observed that once such pores are generated, there is propagating increase (towards the bottom of the membrane) in the pore diameter and further retaining constant pore diameter while same (as maintained in the second step) anodization voltage is maintained. This phenomenon is depicted in a schematicFIG. 1. One of the concave surfaces1on the membrane is represented for illustration purpose. It can be observed that plurality of pores2are generated on the said concave surface1further leading to channels.

The method of preparing multi-layer nano-porous membrane comprises steps ofelectro-polishing of the substrate,combination of hard and mild anodization,barrier layer removal.

The process of electro polishing of Aluminum substrate (Al) comprises steps of:placing the said Al in the mixture of perchloric acid and ethanol wherein the ratio of respective chemicals is in the range of 1:3 to 1:5 by volume wherein purity of ethanol is in the range of 99%-99.9% and that of Perchloric acid is in the range of 69-72%;applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V;applying potential for 3 to 10 min depending on the surface roughness.

First step hard anodization process comprises steps of:selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;gradual application of voltage from 20 V up to the final voltage that is in the range of 60 to 200 V wherein process time depends on the membrane thickness, it can range from 5 minutes to 20 minutes.

Chemical etching of the anodized aluminum oxide comprises a step of etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein phosphoric acid is in the range of 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%.

Upon etching AAO, hexagonal arrangements of plurality of concave surfaces appear on Al surface. Further plurality of pores appear in the said concave surface.

The Second step mild anodization comprises steps of:selecting electrolyte from either of oxalic acid, phosphoric acid, sulfuric acid and malunic acid wherein the concentration of the said acid depends on the pore size;applying a constant voltage directly to the final value (without gradually increasing it from 20 V as is the case in the said first step) that is lower than that in the first step anodization wherein the said final voltage is in the range of 20 to 195 V depending on the electrolyte and pore size.

Barrier layer (BL) is removed using voltage pulse method comprising steps of;placing the said substrate in perchloric acid and ethanol with volume ratio in the range of 1:3 to 1:5 respectively;applying a voltage pulse from 45 to 50 V for 3 to 5 seconds that causes to detach AAO from Al and remove BL.

The parameter values of the said first step hard anodization and second step mild anodizationas well as electrolyte are varied. It is to be noted that the electro-polishing and barrier removal process is same as described above in all the following embodiments, hence it is not repeated therein. Following are various embodiments of the invention.

In the first embodimentthe said first step anodization is performed at 120 to 130V in oxalic acid as electrolyte for 5 to 10 min depending on the size of the substrate;etching is carried out in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C.; preferably phosphoric acid is in the range 6 wt % to 7 wt % and chromic acid is in the range of 2 wt % to 3 wt % wherein preferably purity of Chromic acid is 99% and purity of phosphoric acid is 85%;upon etching hexagonal arrangement of concave surfaces appeared on Al surface;plurality of small pores initiated in each of the concave surface;the said second step mild anodization is performed at 40 to 45 V using the same electrolyte as used in the first step anodization.

The barrier layer is removed by one of the following two methods:

1) Chemical etching comprising steps of:etching the substrate in saturated mercuric chloride so as to separate anodized aluminum oxide from Al;placing of AAO in 5 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. for etching of BL.
2) Voltage pulse detachment comprising steps of:placing anodized aluminum substrate in Perchloric acid and ethanol with volume ratio from 1:3 to 1:5 respectively,applying a voltage pulse from 45 to 50 V for 3 to 5 s which causes detachment of AAO from Al and remove BL.

The initial diameter of pores in second step of anodization is in the range of 5 nm to 10 nm. The pores diameter increased after covering a distance of about 200 nm. Here average pore diameter is 35 nm. It is to be noted that the surface layer of concave surface is the first layer, the initial small pore diameter as second layer and the extended larger pores as third layer. This is illustrated in theFIG. 2using this non-limiting example. The surface SEM image of multilayer membrane manufactured using this method is depicted inFIG. 2a. This image clearly depicts the first and second layer of membrane21and22respectively. The cross-sectional SEM image of multilayer AAO membrane of this embodiment is depicted inFIG. 2(b). The plurality of concave surfaces22are observed on the membrane surface. The magnified form of the rectangular indicated portion in the image depicts bending of AAO nanochannels of higher diameter23in the second layer followed by substantially parallel and straight nanochannels24in the third layer.

In the second embodiment hard anodization is carried out in both the steps. The electro polishing and barrier layer removal is carried out in accordance with the process already mentioned above. The anodization process comprises steps of:the said first step anodization is performed at voltage in the range of 120 to 130V in oxalic acid as electrolyte for 5 to 10 min;etching is carried out in chromic acid and phosphoric acid;upon etching hexagonal arrangement of concave surfaces appeared on Al surface wherein the depth of this layer is about 100 nm;plurality of small pores initiated in each of the concave surface;the said second step hard anodization is performed at 100 to 110V using same electrolyte as used in the first step anodization wherein average diameter of pore is 25 nm;further, pores diameter is increased as nanochannels proceed by few nanometers wherein in third layer diameter is about 80 nm in one of the preferred variants

The barrier layer is removed by one of the following two methods

1) Chemical etching which comprises ofetching the substrate in saturated mercuric chloride so as to separate anodized aluminum oxide from Al;placing of AAO in 5 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. for etching of BL
2) Voltage pulse detachment comprising steps of:placing anodized aluminum sample in perchloric acid and ethanol with volume ratio from 1:3 to 1:5 respectively;applying a voltage pulse from 115 to 120 V for 3 to 5 s which causes to detach AAO from Al and remove BL.

In the third embodiment:

A combination of hard and mild anodization is used with different electrolytes. The first step hardanodization is carried out at voltage selected in the range of 120 to 130 V in oxalic acid and second step in sulfuric acid wherein voltage is selected in the range of 20 to 25 V. The processes of elector-polishing and barrier layer removal are followed as already described above.

In the fourth embodiment:

Mild anodization is used in both the steps, however the electrolytes used are different in both the steps. The first step mild anodization is performed at a voltage in the range of 40 to 45V in oxalic acid and second step at a voltage selected in the range of 20 to 25 V in sulfuric acid. Three layered membrane is formed with first layer concave surface diameter 100 nm, second layer pore diameter about 5 nm and third layer pores diameter about 15 nm.

The nano-porous mutli-layer membrane is prepared In accordance with the said second embodiment hard anodization is carried out in both the steps. Electro-polishing and barrier layer removal processes were carried out as described above. The two steps of hard anodization comprised steps of:the said first step anodization is performed at a voltage selected in the range of 120 to 130V in oxalic acid as electrolyte for 5 to 10 min wherein etching is carried out in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein preferably 6 wt % phosphoric acid and chromic acid is 2 wt %. wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%;upon etching hexagonal arrangement of concave surfaces appeared on Al surface wherein the depth of this layer is about 100 nm;plurality of small pores initiated in each of the concave surface;the said second step hard anodization is performed at 100 to 110 V using same electrolyte as used in the first step anodization wherein average diameter of pore is 25 nm;further, pores diameter is increased as nanochannels forwarded by few nanometers wherein in third layer diameter is about 80 nm.

FIG. 3provides SEM image of such a nano-porous multi-layer membrane manufactured using two steps of hard anodization as mentioned above.FIG. 3(a) depicts the surface SEM image of this membrane in which first and second layer are clearly observed. The depth of first layer is about 100 nm as shown inFIG. 3(b). InFIG. 3(c) all three layers are visible; the diagonal cut in third layer clearly shows the hexagonal arrangement of pores in this layer.

In accordance with the third embodiment the two step anodization was also carried with different electrolytes. The first step anodization was done at 126V in oxalic acid and second step in sulfuric acid at 20V. Three layered membrane was formed with first layer concave surface diameter 300 nm, second layer pore diameter about 5 nm and third layer pores diameter about 15 nm as shown inFIG. 4. Surface image depicting first and second layer is seen inFIG. 4(a). Further cross-sectional image depicting all the three layers is seen inFIG. 4(b)

In accordance with the fourth embodiment, mild anodization is used in both the steps, however the electrolytes used are different in both the steps. The first step mild anodization is performed at 40 V in oxalic acid and second step at 20 V in sulfuric acid. Three layered membrane is formed with first layer concave surface diameter 100 nm, second layer pore diameter about 5 nm and third layer pores diameter about 15 nm as depicted inFIG. 5.