Seeing is Believing. - Nanostructure of Anodic Alumina Film - The International Hard Anodizing Association 15th Technical Symposium

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1 Seeing is Believing - Nanostructure of Anodic Alumina Film - The International Hard Anodizing Association 15th Technical Symposium September 24-26, 2014 Sheraton Lincoln Harbor Hotel, Weehawken, NJ Sachiko Ono Department of Applied Chemistry Kogakuin University Tokyo, Japan 1

2 Typical honeycomb structure of anodic porous alumina amorphous Al 2 O 3 + electrolyte anion Cell dimension is proportional to the formation voltage ( 5V to 300V ) Cell diameter: 2.5 nm/v, up to 1mm Pore diameter: 1 nm/v, up to 1mm Barrier layer thickness: 1 nm/v Film thickness: up to several hundred mm Keller- model <Application> Corrosion resistance, Decoration Filter, Membrane Template for micro / nano-devices 200 nm Cell arrangement is highly ordered at some adequate anodization conditions 2

3 Two-step anodization for self-ordered cell configuration (in oxalic acid at 40 V) 1 st Anodization Al One step anodization Two step anodization 6 wt% Phosphoric acid- 2 wt% Chromic acid 2 nd Anodization 500 nm

4 FE-SEM images of anodic films formed at steady state 0.3M Oxalic acid, 40V 0.05M Oxalic acid, 80V 0.25M Phosphoric acid, 80V 600nm

5 Imprinting process to fabricate ideally ordered anodic alumina SiC Al Anodizing Imprinting Al 2 O 3 Pores initiate at concaves by preferential dissolution H. Masuda et al., Appl. Phys. Lett., 71, 2770 (1997). TEM images S. Ono, H. Asoh, T. Hirose, T. Yamaguchi, M. Nakao and H. Masuda, 53 rd I S E, (Dusseldorf,, 2002) Abstracts p.192

6 Substrate concave served as pore nucleation site Current density / A m Imprinted Al 150 at 80V Electropolished Al Anodizing time / sec Al TEM: After imprinting Current density / ma cm V 30V 40V 40V 40V Electropolished Al Anodizing time / s TEM: Two step anodizing 40V 40V Purpose: Clarification of effect of surface topography on pore initiation process

7 Clarification of effect of surface topography and crystal orientation on pore initiation process: Experimental 99.99% Al sheet (polycrystalline) Heat treatment: < 500, 1h Electropolishing: in perchloric acid-ethanol, < 10 Electropolished Al Planarization process Planarized Al Al Formation of barrier type oxide Removal of barrier oxide Anodization in 0.3 mol dm -3 oxalic acid at 100 Am -2, 30 Observation by AFM (Digital Instruments Nano Scope IIIa) Film surface Al surface Removal of porous alumina

8 AFM images of pretreated Al substrate Alkaline degreasing Width 600 nm Depth 50 nm Electropolishing Width 75 nm Depth 5 nm 400 nm 200 nm μm μm In the case of alkaline degreasing, surface roughness is ten times larger

9 Effect of electropolishing voltage (as-rolled Al) Before polishing 15V 20V 25V 30V 35V A regularly aligned striped structure appeared after electropolishing of as-rolled aluminum and the width of stripe was dependent on voltage X, Y; 200nm/div Z; 50nm/div

10 EBSD analysis of electropolished Al

11 XRD: Dependence of crystal orientation on heating temperature Intensity / a.u. (111) (200) (220) (311) (400) (420) (422) K. Asahina, H. Ishihara, H. Asoh, S.Ono J. Jpn. Inst. Light Metals, 58, 375 (2008) As-received θ / degree Preferential crystal orientation of as-rolled substrate was (220) and (311). On the other hand, the preferential orientation changed to (100) for the substrate heated at above 300 ºC.

12 Effect of electropolishing voltage (heated Al at 300ºC) Before polishing 15V 20V 25V 30V 35V After heating at 300 ºC and electropolishing, the substrate with (100) orientation induced isotropic array of hexagonal cell structure, in contrast to that for as-rolled substrate with (110) orientation with stripe structure X, Y; 200nm/div Z; 50nm/div

13 SEM images of pore configuration of anodic porous alumina 1.5M Sulfuric acid, 20V, 10 Stripe structure on (110) Cellular structure on (100) 99.99%Al, Electropolished

14 Dependences of electropolishing voltage and heat treatment on Al topography Width and depth of stripes and cells / nm width of stripes (As-received) width of cells (300,1hour) 20 0 depth of cells (300,1hour) depth of stripes (As-received) Voltage / V The size of cell or stripe as well as their depth increased linearly with increasing electropolishing voltage

15 nm Line analysis of electropolished and planarized Al nm nm Heating Planarize Electropolished Al (110) Electropolished Al (100) Planarized Al mm Width of cell : 51 nm Depth : 2.8 nm mm Width of cell : 50 nm Depth : 3 nm mm Width of cell : 20 nm Depth : <1 nm X, Y; 200nm/div Z; 50nm/div Aluminum substrate was re-anodized up to 80 V to give a dense and flat oxide layer as a result of leveling effect of anodization. Then, the oxide layer was dissolved out. Highly planarized surface was successfully obtained.

16 Constant current anodization: differently treated Al Voltage / V III 40 II IV V I Planarized Al Electropolished Al (110) Electropolished Al (100) Anodizing time / s 0.3mol dm -3 oxalic acid (30 ) at constant current of 100Am -2 AFM observation was carried out at different stages : I~V

17 AFM: as-received and electropolished Al (striped structure) Film surface Electropolished Al (110) Al surface stage I (8.0s) stage II (17.7s) stage V (120s) Initial film growth proceeds at metal ridges Thompson, G.E., et al., Trans Inst Met Finish, 56, 159 (1978) X, Y; 200nm/div Z; 50nm/div

18 AFM: heated and electropolished Al (cellular structure) Film surface X, Y: 200nm/div Z: 50nm/div Electropolished Al (100) Al surface stage I (8.3s) stage II (17.8s) stage V (120s) Although the surface topography didn t change much, but smaller cells are indicated on substrate at the stage I, showing that pores

19 AFM: Heated, electropolished and planarized Al Film surface Flattened Al stage I (6.6s) Al surface stage II (14.8s) stage V (120s) Using planarized aluminum with less than 0.5 nm asperities, a large number of fine pores initiated at the very early stage with the formation of porous cell protruded to the substrate.

20 Size and depth of cells / nm Voltage / V Size and depth of cells / nm Voltage / V Change in cell structure with anodizing time Electropolished Al (100) Planarized Al I II III IV Size of cells V I II III IV Size of cells V V-t curve Depth of cells V-t curve Depth of cells Anodizing time / s (0.3mol dm -3 oxalic acid, 30, 100Am -2 ) Anodizing time / s Electropolished Al Stage Ⅰ Stage II Stage III Stage IV Stage V

21 Sulfuric acid at 20V Oxalic acid at 20V Chromic acid at 20V Phosphoric acid at 20V Difference in pore size

22 Voltage dependence of cell dimensions of various anodic films Pore and cell diameters, barrier layer thickness and porosity 0.3M Oxalic acid S.Ono and N. Masuko: Surf. Coat. Technol., 169, 139 (2003) S. Ono, K. Takeda and N. Masuko, Proc. 2nd Int. Symp. Alum. Surf. Sci. Tech., 398 (2000) Cell dimensions of anodic films formed in various electrolytes are quite different 22 because of the difference in current density, i.e., electric field strength

23 TEM cross section (ion-milling) of anodic film formed in sulfuric acid at a hard anodizing condition: high current density, high voltage, low temperature H. Masuda, F. Hasegawa and S. Ono Self-Ordering of Cell Arrangement of Anodic Porous Alumina Formed in Sulfuric Acid Solution J. Electrochem. Soc., 144, L127 (1997) 23

24 Cross section (ion-milling) of self-ordered anodic film formed in sulfuric acid 24

25 Cross section of anodic film formed in oxalic acid at 40 V prepared by ion-milling 25

26 Difference in cell boundary structure observed by TEM: Films on Al When we compare the both films after electron beam irradiation, the difference originated from the difference in anion incorporation behavior is obvious Phosphoric acid Sulfuric acid Before electron beam irradiation After electron beam irradiation 26

27 What does proceed by sealing? TEM images of anodic films formed in oxalic acid at 40V after 3 min sealing in a boiling water a) 0 min b, c) 3 min Change in pore structure proceeds by dissolution of cell wall and precipitation of hydrated alumina in pore 27

28 Structural changes during boiling water sealing (11 min) Oxalic acid film at 40 V Sealing rate is highly dependent of anodizing electrolyte used Sulfuric acid film at 20 V Phosphoric acid film at 80 V 28

29 Sulfuric acid film: Comparison after sealing Ni sealing at 95 Boiling water for 10 min Before 20min sealing Li-Sealing for 1 min at RT a b c Y Y 100 nm Size of Platelet-like hydroxide was bigger than that formed in boiling water Platelet-like hydroxide was deposited more deeply in the film up to 1-2 mm 29

30 Cross section of sulfuric acid film before and after sealing TEM images