Abstract:
A system and method for preparation of nano-porous membrane using anodized aluminium oxide and the membrane/film/thin lamina produced thereof. The system comprises a template forming device that comprises of two rolls provided with one or plurality of projections wherein the Al sheet is passed through the said rolls that are rotatable in opposite direction with respect to each other wherein in operation as the Al sheet is passed through the said rolls, the said projections of the rolls punch depressions to the predetermined depth in the said sheet wherein the depth of the depression is governed by the height of the projections. A method for preparation of anodized aluminum oxide nano-porous membrane comprising electro polishing of Al substrate; first step anodization; chemical etching of alumina; second stage anodization; etching Al for separation of alumina and barrier layer removal or voltage pulse detachment for barrier layer removal and detachment of membrane from Al substrate.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a system and a method for preparation of nano-porous membrane using anodized aluminium oxide and the membrane/film/thin lamina produced thereof. 
       BACKGROUND OF THE INVENTION 
       [0002]    Anodized Aluminum Oxide (AAO) membranes are gaining popularity in the field of sensors, hemodialysis, protein separation, photovoltaics, high density memories, control molecular release, drug release, nano dots, nano rods, nano tubes, and making masks for semiconductors industry. The vital aspects to characterize AAO membrane include pore size, pore depth, inter-pore distance, thickness of membrane and pore geometry. 
         [0003]    The method of fabrication of the AAO membrane is reported in the prior art. 
         [0004]    U.S. Pat. No. 3,850,762 discloses fabrication of AAO free standing membrane wherein steps include anodizing one side of Al sheet, etching Al substrate for exposing back side of alumina, and then etching of barrier layer (BL) for making AAO membrane. The O-ring setup is used for anodizing one side and then etching Al and BL from other side. 
         [0005]    Another reported method includes anodizing Al sheet from both sides and then using O-ring setup for etching alumina from one side, then Al etching and etching BL of other side AAO so that free standing AAO can be achieved. 
         [0006]    However, these methods suffer from the drawback that the area of AAO free standing membrane actually produced is substantially lower than that of Al sheet used for fabrication leading to wastage of material. Depending on application low grad Al or high purity Al can be used in the process. If 99.999% pure Al is used in the fabrication of AAO membrane then wastage of material cause vital problem. The 99.999% pure Al is much expensive leading to increasing the cost of the membrane. 
         [0007]    There is another method in which Al sheet is painted from one side by insulting material and anodization is carried on other side, after anodization the insulting material on back side is etched chemically, then etching Al and removing BL for making free standing AAO membrane. However, the limitation is that two extra steps of depositing insulting layer and then again removing this layer are required. 
         [0008]    PCT patent GB95/01646 discloses electrochemical etching and detachment of AAO from Al substrate. Accordingly the anodized aluminum is subjected to dissolving electrolyte and applying some potential. The dissolving electrolyte opens BL and detaches AAO from Al substrate. This method was further modified by Yuan et al. and named as one step pulse voltage detachment. In this process anodized Al is subjected to electro polishing solution and a voltage pulse is applied for few seconds and it causes to etch BL and detach AAO membrane from Al substrate. This method has a capacity to use for detachment of AAO from both sides of Al sheet in few seconds. Technology has to develop so that such a method can be use for producing more AAO with less amount of Al. 
         [0009]    However, it is to be noted that many of applications demand small size of AAO membrane. The limitation of the above mentioned methods result in fabrication of AAO plane membrane in one piece at a time. It is necessary to have plurality of setups for fabrication of number of membrane pieces leading to substantially increasing production and process cost. There is need to develop a method to fabricated plurality of membranes at the same time out of a single Al sheet or block. Technology also has to be developed to produce number of small AAO pieces from a single Al sheet or block. 
         [0010]    The tubular membrane is characterized as a three dimensional type of AAO membrane. Tubular membrane is one of 3D membrane which has been used in the field of drug delivery, hemodialysis and gas separation (Gong, 2003; Attaluri, 2009; Li, 2008). N. Itoh et al (Itoh, 1996), fabricated AAO nonporous tubular membrane using one step MA. Later the tubular AAO membranes were fabricated using two step anodization (Gong, 2003; Belwalkar, 2008; Li, 2008; Zhou, 2009). Various attempts have been made to fabricate AAO tubular membrane (Gong, 2003; Belwalkar, 2008; Itoh, 1996; Li, 2008; Zhou, 2009). 
         [0011]    The conventional method includes steps to insulate the Al tube from outside, followed by anodization at inner side, and etching Al chemically from outside after removing the insulating cover. Further, the barrier layer is removed using chemical etching. This process can only be used for the manufacture of centimeter and/or millimeter size tubular membrane. However, this process is not appropriate for the manufacture of tubular membranes in micrometer and below size. It is a challenge to make Al tube in small size, covering and uncovering the outer surface for different steps, blocking the two ends to protect AAO from inside during etching process and again opening the two ends to give final shape. 
         [0012]    Bong-Young Yoo et. al report fabrication of the micrometer size AAO tubular membrane by drilling in Al block. But most of the steps were same as in previous reported methods (Gong, 2003; Belwalkar, 2008; Itoh, 1996; Li, 2008; Zhou, 2009). In the method proposed by Bong-Young Yoo et al (Yoo, 2006), alumina was removed mechanically from one side of Al block. However this imposes serious limitation in case of nanostructure membranes. 
         [0013]    One of the challenges in the development of aluminum oxide membrane is the issue of cracks that occur during the conversion of alumina from aluminum by virtue of ion exchange process. Cracks occur inside AAO and/or at the edges of AAO. Cracks inside AAO normally occur due to rapid growth of AAO as in hard anodization. Woo Lee et al. introduced modified form of hard anodization in which cracks were minimized. To overcome crack problem Woo Lee et al also introduced pulse anodization. In mild anodization crack inside AAO do not occur if appropriate processing conditions are applied. Another type of cracks occurs at the edges wherein aluminum (Al) surfaces are substantially perpendicular to each other. These cracks are due to volume expansion when Al is converted into AAO. This type of cracks made obstacles in preparation of AAO 3D membranes. However attempts are available to utilize cracks as well. 
         [0014]    The aspect of utilization of cracks to fabricate anodic aluminum oxide nanoporous tubular and rectangular membrane is reported by Kasi et. al. However, this method suffers disadvantage due to the lack of development of the shaping aspect of Al and relevant production mass production process. 
         [0015]    The methods provided in prior art suffer from following limitations:
       Substantially reduced area of AAO free standing membrane produced from available Al sheet used for fabrication leading to wastage of material and reduced productivity;   Need to insulate Al from any side in the electrolyte solution or cutting any surface mechanically;   Necessity of two extra steps of depositing insulting layer and then again removing this layer;   It is necessary to have plurality of setups for fabrication of number of membrane pieces leading to substantially increasing production and process cost.   There is need to develop a method to fabricated plurality of membranes at the same time out of a single Al sheet or block   Only single surface anodization of the substrate (rectangular or square) and removal of entire substrate leading to material waste, increase in specific energy consumption, substantially reduced productivity, posing limitation on scaling of the process/production   Lack of utilization of all the surfaces of the substrate to form the membrane;   Need of Al solid cylinder to manufacture tubular membrane,   Lack of flexibility and possibility to manufacture tubular member from rectangular/square, circular geometry substrate;   Lack of flexibility and possibility to manufacture tubular as well as laminar membrane from the same rectangular/square substrate.       
 
         [0026]    There is a need in the market place to provide a method to enable:
       Fabricating/producing plurality of millimeter or micrometer size membranes at the same time out of a single Al substrate (sheet or block);   relatively simple method obviating number of complex steps;   substantially reducing Al substrate wastage;   obviate need of separate set up and method for two dimensional as well as three dimensional membrane manufacture/production;   Flexibility of producing diverse shapes of the membrane such as tubular, rectangular, conical, fish mouth shaped etc.   effective utilization of cracks to produce plurality of membranes from faces of the Al block/substrate at the same time in a single process step to obviate material wastage, enhance productivity and substantially reduce process cost;   effective utilization of cracks to produce three dimensional tubular membrane from Al block;   effective utilization of mild anodization (MA), hard anodization (HA) and pulse anodization;   flexibility of detachment of AAO using chemical etching or one step voltage pulse detachment;   fabrication of plurality of millimeter or micrometer size membranes at the same time out of a single Al sheet or block obviating the problems of the conventional method.       
 
       SUMMARY OF THE INVENTION 
       [0037]    The main object of the invention is to provide a system and a method for preparation of nano-porous membrane using anodized aluminium oxide. 
         [0038]    Another object of the invention is to provide a method for effective utilization of cracks to produce plurality of membranes from faces of the Al block/substrate at the same time in a single process step to obviate material wastage, enhance productivity and substantially reduce process cost; 
         [0039]    Another object of the invention is to provide a method for producing plurality of membranes at the same time out of a single Al substrate (sheet or block) in a single process step. 
         [0040]    Yet another object of the invention is to provide a method for producing millimeter or micrometer size membranes at the same time out of a single Al substrate (sheet or block) in a single process step. 
         [0041]    Yet another object of the invention is to provide a simple method, preferably single stage method obviating number of complex steps; 
         [0042]    Yet another object of the invention is to provide a method to substantially reduce Al substrate wastage. 
         [0043]    Yet another object of the invention is to provide a method to obviate need of a separate set up for the manufacture of two dimensional as well as three dimensional membranes. 
         [0044]    Yet another object of the invention is to provide a method to enable flexibility of producing diverse shapes of the membrane such as tubular, rectangular, conical, fish mouth etc. 
         [0045]    Yet another object of the invention is to provide a method for effective utilization of cracks to produce three dimensional tubular membranes from Al block. 
         [0046]    Yet another object of the invention is to obviate problem of cracks inside AAO. 
         [0047]    Yet another object of the invention is to provide a method for effectively utilizing a combination of mild anodization (MA), hard anodization (HA) and pulse anodization. 
         [0048]    Yet another object of the invention is to provide a method to enable flexibility of detachment of AAO using one step voltage pulse detachment. 
         [0049]    Thus in accordance with the invention the system comprises of a template forming device that comprises of two rolls provided with one or plurality of projections wherein the Al sheet is passed through the said rolls that are rotatable in opposite direction with respect to each other where in operation as the Al sheet is passed through the said rolls, the said projections of the rolls punch/create depressions to the desired depth in the said sheet wherein the depth of the depression is governed by the height of the projection 
         [0000]    wherein the method comprises of
       electro polishing of Aluminum substrate (Al) comprising steps of:
           placing the said Al sheet with the punched depressions in the mixture/solution of perchloric acid and ethanol respectively wherein the ratio 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 anodization comprising 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;   using oxalic acid as electrolyte in the range of 0.2M to 0.3M;   applying a potential in the range of 35 to 45V wherein the process time is in the range of 1 to 6 h.   
           chemical etching of the anodized aluminum oxide comprising steps 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%;   second step anodization comprising steps of:
           Repeating the process in the first step anodization wherein hexagonally arranged nanoporous structures are formed with one end blocked with barrier layer wherein process time depends on the membrane thickness, it can range from 1 h to 48 h.   
           barrier layer (BL) removal either by chemical etching or voltage pulse method.   wherein chemical etching of barrier layer comprises of steps
           etching Al in saturated mercuric chloride so as to separate anodized aluminum oxide from Al;   placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. for etching of BL.   
           alternately the barrier layer 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.   
               
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0069]    Features and advantages of this invention will become apparent in the following detailed description and the preferred embodiments with reference to the accompanying drawings. The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings. 
           [0070]      FIG. 1  illustrates elevation and top view of the system of the present invention. 
           [0071]      FIG. 2  illustrates the schematic of the punched Al sheet as one of the embodiments. 
           [0072]      FIG. 3(A)  to (C) illustrates schematically the formation of Anodized Aluminum Oxide (AAO) and the aspect of occurrence of cracks. 
           [0073]      FIG. 4  illustrates one of the embodiments of punching circular outline in the Al sheet  FIG. 5(A)  to (B) illustrates one of the embodiments relating to two and three dimensional membrane preparation from Al block. 
           [0074]      FIG. 6(A)  to (F) illustrates the formation of the one dimensional membranes using cracks from the Al block  FIG. 7(A)  to (D) illustrates SEM images during the process of preparation of three dimensional membrane  FIG. 8(A)  to (H) depicts membrane characterization during each of the stage of membrane preparation. 
           [0075]      FIG. 9  depicts the SEM image illustrating hexagonal order of the pores 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0076]    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. Further more, it is understood that the embodiments shown in the Figures are illustrative representation and are not necessarily drawn to scale. 
         [0077]    The system of the present invention comprises of a template forming system depicted in  FIG. 1   that  illustrates elevation and top view of the system. It comprises of first roll  1  and second roll  2 . Each of the said roll is provided with at least one projection indicated as  1   a ,  1   b  and  2   a ,  2   b  as illustrated in the  FIG. 1 . Al sheet  3  is passed through the said rolls in the direction indicated by the arrow  5 . The said first and second rolls are rotatable in opposite directions so as to punch the depression  4  in the said Al sheet from top and bottom surface of the sheet as indicated in  FIG. 1 . 
         [0078]    One of the embodiments of such punched Al sheet is depicted in  FIG. 2 . The punched lines with depression can be created in horizontal and vertical manner indicated by lines with numeral  21  and  22  respectively in the  FIG. 2 . Part of the Al sheet  23  is extended to be used for connection with the electrode in the anodization process. The pattern of the punched lines with horizontal and vertical combination can be designed according to the dimension of the membrane required. 
         [0079]    In one of the embodiments the said depressions can also be created using a sharp tool. In one of the embodiments the Al sheet is a block or piece of a block etc. The person trained in the art can contemplate various forms of Al geometry. The  FIGS. 1 and 2  are represent one of the preferred embodiments of the invention, however the process step is not restricted to merely Al sheet. 
         [0080]    The nano-porous membrane is produced from such an Al sheet provided with depressions as illustrated in  FIG. 3 . The method comprises of: 
         [0000]    (i) electro polishing of AI substrate
 
(ii) first step anodization;
 
(iii) chemical etching of the alumina;
 
(iv) second stage anodization;
 
(v) etching Al for separation of alumina and barrier layer removal
       or voltage pulse detachment for barrier layer removal and detachment of membrane from Al substrate       
 
         [0082]    The process of electro polishing of Al sheet comprises steps of:
   (i) placing the said Al sheet with the punched depressions in the mixture of perchloric acid and ethanol respectively wherein the ratio of the perchloric acid and ethanol 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%;   (ii) Applying potential at a temperature less than 10° C. wherein the potential is in the range of 10 to 20 V;   (iii) Applying potential for 3 to 10 min depending on the surface roughness.   
 
         [0086]    The electro polished sheet of Al as depicted in  FIG. 3(   a ) is further anodized. The process of first stage anodization comprises steps of:
   (i) 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;   (ii) using 0.3M oxalic acid as electrolyte;   (iii) applying a potential in the range of 35 to 45V wherein process time depends on the membrane thickness, it can range from 1 h to 48 h.   
 
         [0090]    The process of the chemical etching of the anodized aluminum oxide substrate comprises steps of:
       performing chemical etching in chromic acid and phosphoric acid wherein the temperature is in the range of 65-80° C. wherein preferably 6 wt % phosphoric acid is 6 wt % and chromic acid is 2 wt % wherein purity of Chromic acid is 99% and purity of phosphoric acid is 85%;       
 
         [0092]    The second stage anodization process comprises steps of as elaborated in the first stage anodization. This process results in the formation of hexagonally arranged nanoporous structures with one end blocked with barrier layer. 
         [0093]    As depicted in  FIG. 3   b , the process of anodization results in formation of Anodized Aluminum Oxide (AAO) layer  32  on the Al substrate  31 . It is to be noted that during the anodization process and formation of AAO there is expansion of the substrate resulting in displacement of the surface of the AAO due to increase in volume. This results in occurrence of cracks at edges wherein aluminum (Al) surfaces are substantially perpendicular to each other. This phenomenon is illustrated in  FIG. 3 . The cracks  33  occur at the edges as depicted therein. However it is to be noted that as depicted in  FIG. 4  it is possible to punch circular outline wherein there is as well occurrence of crack at the edges wherein the surfaces are substantially perpendicular. 
         [0094]    It is surprisingly found that these cracks can be utilized effectively as a passage/conduit to reach the chemical at the Al substrate surface for etching purpose. There is occurrence of cracks at every edge of the substrate. The present invention utilizes these cracks as a means to pass on the chemical upto the each of the surface of the substrate so as to etch the AAO from that surface. It is to be noted that the substrate is entirely immersed in the chemical (saturated mercuric chloride) during chemical etching process and NOT that only one surface is etched as practiced in the conventional method. The immersion of the entire surface in the chemical for chemical etching results in the AAO layer detachment from each of the faces of the substrate. Thus in the present depiction the substrate is in the form of rectangular/cubical geometry having six surfaces wherein each of the surface would then yield an AAO layer after chemical etching. However since one surface is used as electrode, practically there would be five such AAO layer formation from each substrate of rectangular or cubical geometry. This obviates problems associated with insulating the substrate and enables effective utilization of all the surfaces resulting in substantial reduction in material wastage. 
         [0095]    The Al sheet/substrate with AAO layer and cracks is immersed in saturated mercuric chloride for chemical etching so as to separate anodized aluminum oxide (AAO) from Al. Thus each of the surface on which AAO is formed is utilized to produce an AAO membrane with barrier layer (BL). 
         [0096]    The barrier layer is removed either by chemical etching or voltage pulse method. The method of chemical etching comprises steps of:
       etching Al in saturated mercuric chloride so as to separate anodized aluminum oxide from Al;   placing of AAO in 5 wt % to 6 wt % Phosphoric acid for about 35 to 40 min at 31° C. to 32° C. for etching of BL.       
 
         [0099]    Alternately the barrier layer is removed using voltage pulse method that comprises 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.       
 
         [0102]    The present invention enables manufacture/preparation of two dimensional as well as three-dimensional membrane simultaneously in the same process step using a single substrate. One of the embodiments is depicted in  FIG. 5 . The  FIG. 5  ( a ) illustrates the Al substrate  50  in the form of a block provided with electrode end  51 . One or plurality of holes  52  are drilled in the said block  50  as indicated in the  FIG. 5(   a ). There are five surfaces available in the said substrate  50 . The front and back surface  53  and  54  respectively wherein holes are drilled; top and bottom surface  55  and  56  respectively and the side surface  57  as depicted in the  FIG. 5(   a ). 
         [0103]    The said substrate (drilled block  50 ) is processed using the method of electro polishing, first step anodization, chemical etching of the anodized Al, second stage anodization, etching for separation of membrane and barrier layer removal. The details of the method are already mentioned above. It is be noted that each of the said surfaces of the said substrate  50  result in the production of the membrane as a result of the method of the present invention. As indicated in  FIG. 5(   b ), the front surface  53  and back surface  54  yield the membranes  53   a  and  54   a  respectively. The top surface  55  and bottom surface  56  yields membranes  55   a  and  56   a . These are all two dimensional membranes. It is to be underlined that the drilled holes  52  in the said block/substrate  50  yield the three dimensional membranes in the form of a tube/s  52   a . The same substrate/block is thus utilized to produce three dimensional as well as two dimensional membranes simultaneously. 
         [0104]    In one of the variants of this embodiment, based on the requirement, one can create other than cylindrical forms as well from the said block. 
         [0105]    The nano-porous membrane prepared by the method of present invention result in majority of pores with a constant diameter, and inter-pore distance. In one of the embodiments the pore diameter is 37 nm, and inter-pore distance 95 nm which is in great agreement with 10% porosity rule. 
         [0106]    In one of the embodiments the AAO membrane is prepared using 99.56% pure Al. In another variant of this embodiment the AAO membrane is prepared using 99.999% pure Al. 
         [0107]    The technical effect of this inventive aspect of the present invention is tangible, concrete and measurable. This is quantified and elaborated in the following non-limiting examples 
       Example 1 
       [0108]    As illustrated in  FIG. 6(   a ), Al block  60  is used as a substrate to establish the aspect of the invention of using cracks at the edges to produce membranes from the five faces of the said substrate. One of the ends  61  is used as electrode in the anodization process. The said substrate  60  is treated using method of the present invention comprising steps of electro polishing of Al substrate, first step anodization, chemical etching of the alumina, second stage anodization and etching Al for separation of alumina and barrier layer as elaborated in the description of the invention. 
         [0109]      FIG. 6(   b ) represents schematically the aspect of Al substrate  62  and formation of AAO and development of the crack  64  at the edge.  FIG. 6(   c ) depicts SEM image of the said substrate  60  after anodization wherein cracks  64  at the edges are observed. Further AAO  63  on the surfaces of the said substrate are observed in this SEM image.  FIG. 6(   d ) depicts the magnified image of the AAO surface  63  (exploded view of the rectangular mark indicated on  FIG. 6(   c ), depicting the hexagonal pores structure on the surface of AAO. 
         [0110]      FIG. 6(   e ) illustrates the schematic of the AAO that could be derived from the faces of the said substrate  60 . Numerals  65  to  69  respectively depict the AAO from back surface, front surface, top, bottom and side surface of the substrate.  FIG. 6(   f ) depicts SEM image of the AAO separated from one of the said surfaces. 
       Example 2 
       [0111]    Using the method of the present invention, three dimensional nano-porous tubular membranes are obtained. 
         [0112]      FIG. 7(   a ) depicts SEM image of the nano-porous tubular membrane. The  FIG. 7(   b ) depicts image of the top surface plane membrane separated from tubular membrane. The rectangle in the image indicates the cut edge of the top plane membrane magnified image of which is depicted in  FIG. 7(   c ). Further the  FIG. 7  ( d ) indicates the blown up image of the portion indicated by rectangle in  FIG. 7(   c ). The pores  70  of AAO as well as the barrier layer  71  are seen in  FIG. 7(   d ). 
       Example 3 
       [0113]    Using the method of the present invention, nano-porous membranes are obtained. The Al substrate used in this non-limiting example is 99.999% pure. The characterization at each of the process stages is depicted in  FIG. 8 .
       The surface SEM result of 99.999% pure electro-polished Al is depicted in  FIG. 8(   a ). The process of electro polishing results in the removal of micro size roughness from the said Al substrate surface. However nano size random pits can be seen in this image. These nano size pits causes the initiation of pores in first step of anodization.   In first step of anodization the AAO forms with random pores as shown in  FIG. 8(   b ). These pores initiates in nano pits or concave surfaces on the Al substrate surface. As these pores propagate inside (in the form of a nano-channel) the substrate some of the pores proceed and some are blocked. Pores are further arranged in hexagonal order with substantially equal distance from each other.       
 
         [0116]    The AAO formed in first step of anodization result in random pore structure at top surface but at bottom of each nano-channel there is substantial periodic configuration/distribution. The AAO formed in the first step of anodization is etched to access the periodic structure on the Al surface.  FIG. 8(   c ) depicts the surface SEM image after etching of the AAO formed in the said first step anodization process. The hexagonal structure can be seen on the Al surface.
       The second step anodization is carried on the said etched Al surface having periodic hexagonal order of nano pits/concave surfaces. The membrane formed in the second step of anodization is of substantially hexagonal order as depicted in  FIG. 8(   d ). It can be observed that majority of pores is with constant diameter and have constant inter-pore distance. In this embodiment the pore diameter is 37 nm, and inter-pore distance 95 nm which is in good agreement with 10% porosity rule.   The bottom portion of the formed nano-channel of the AAO is covered by the barrier layer (BL). It is depicted in the  FIG. 8(   e ). It is observed that cells are in hexagonal shape and seven cells combine to make hexagonal order.     FIG. 8(   f ) depicts the cross-section SEM image of AAO formed in the second step of anodization. Substantially straight and parallel nano-channels are observed in this image. The diameter of the said nano-channel is substantially uniform. Further the distance between the said nano-channels is substantially uniform.   The BL needs to be etched to prepare a nano-porous AAO membrane.  FIG. 8(   g ) depicts the SEM image of AAO membrane wherein the BL is etched. It is to be noted that the said process of chemical etching is used for BL detachment.     FIG. 8(   h ) depicts the SEM image of AAO which was detached from Al using the said voltage pulse detachment. Here pore diameter is 37 nm and inter-pore distance 95 nm.       
 
       Example 4 
       [0122]    In this representative non-limiting example, AAO is prepared using 99.56% pure Al. The AAO is prepared using method of the invention described above.  FIG. 9  depicts the hexagonal order of pores.