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Sachiko Ono Department of Applied Chemistry

Kogakuin University

Tokyo, Japan

“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

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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

＜Application＞

Corrosion resistance, Decoration

Filter, Membrane

Template for micro / nano-devices

Typical honeycomb structure of anodic porous alumina

Keller- model

◆ Cell arrangement is highly ordered at some adequate anodization conditions

200 nm

amorphous Al2O3 + electrolyte anion

One step anodization

Two-step anodization for self-ordered cell configuration (in oxalic acid at 40 V)

500 nm

Two step anodization

Al

6 wt% Phosphoric acid-

2 wt% Chromic acid

1st Anodization

2nd Anodization

600nm

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

Imprinting process

to fabricate ideally ordered

anodic alumina

SiC

Al2O3

Al

TEM images

Pores initiate at concaves

by preferential dissolution

Imprinting

Anodizing

H. Masuda et al., Appl. Phys. Lett., 71, 2770 (1997). S. Ono, H. Asoh, T. Hirose, T. Yamaguchi, M. Nakao and H.

Masuda, 53rd I S E, (Dusseldorf,, 2002) Abstracts p.192

0

2

4

6

8

10

0 30 60 90 120

40V 30V→40V

40V→40V

Cu

rren

t d

ensi

ty /

mA

cm

-2

Anodizing time / s

TEM: Two step anodizing

40V→40V

TEM: After imprinting

Substrate concave served as pore nucleation site

0

50

100

150

200

250

0 200 400 600

Anodizing time / sec

Cu

rren

t d

en

sity

/ A

m-2 Imprinted Al

Electropolished Al

Electropolished Al

Purpose: Clarification of effect of surface topography on pore initiation process

at 80V

Al

Electropolished Al

Formation of

barrier type oxide

Anodization in 0.3

mol dm-3 oxalic acid

at 100 Am-2, 30 ℃

Electropolishing: in perchloric

acid-ethanol, < 10℃

Planarized Al

99.99% Al sheet

(polycrystalline) Heat treatment:

< 500 ℃, 1h

Removal of

barrier oxide

Removal of

porous alumina

Film surface

Al surface

Observation by AFM (Digital Instruments Nano Scope IIIa)

Planarization process

Al

Clarification of effect of surface topography and crystal

orientation on pore initiation process: Experimental

1

3 4

μｍ

400 nm

2 0.2

0.6 0.8

μｍ

200 nm

0.4

Width 600 ㎚

Depth 50 ㎚

Width 75 ㎚

Depth 5 ㎚

AFM images of pretreated Al substrate

Alkaline degreasing Electropolishing

In the case of alkaline degreasing, surface roughness is ten times larger

20V 15V Before polishing

30V 25V 35V

X, Y; 200nm/div

Z; 50nm/div

Effect of electropolishing voltage (as-rolled Al)

A regularly aligned striped structure appeared after electropolishing of

as-rolled aluminum and the width of stripe was dependent on voltage

EBSD analysis of electropolished Al

30 50 70 90 110 130 150

As-received

100℃

200℃

300℃

400℃

150℃

250℃

350℃

450℃

500℃

2θ / degree

Inte

nsit

y /

a.u

.

(20

0)

(311

)

(22

0)

(40

0)

(111

)

(42

0)

(42

2)

XRD: Dependence of crystal orientation on heating

temperature

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.

K. Asahina, H. Ishihara, H. Asoh, S.Ono

J. Jpn. Inst. Light Metals, 58, 375 (2008)

20V 15V Before polishing

30V 25V 35V

X, Y; 200nm/div

Z; 50nm/div

Effect of electropolishing voltage (heated Al at 300ºC)

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

1.5M Sulfuric acid, 20V, 10℃

SEM images of pore configuration of anodic porous alumina

99.99%Al, Electropolished

Cellular structure on (100) Stripe structure on (110)

0

20

40

60

80

0 10 20 30 40

Wid

th a

nd

dep

th o

f st

rip

es

an

d c

ell

s /

nm

Voltage / V

width of stripes

(As-received)

depth of cells

(300℃，1hour)

width of cells

(300℃，1hour)

depth of stripes

(As-received)

Dependences of electropolishing voltage and heat

treatment on Al topography

The size of cell or stripe as well as their depth increased linearly with increasing electropolishing voltage

Planarize Heating

X, Y; 200nm/div

Z; 50nm/div

Electropolished Al (110)

Width of cell : 51 nm

Depth : 2.8 nm

0 2.5 5.0 7.5 1mm

-5

0

5 n

m

Electropolished Al (100)

Width of cell : 50 nm

Depth : 3 nm

-5

0

5 n

m

0 2.5 5.0 7.5 1mm

Planarized Al

Width of cell : 20 nm

Depth : <1 nm

-5

0

5 n

m

0 2.5 5.0 7.5 1mm

Line analysis of electropolished and planarized Al

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.

0.3mol dm-3 oxalic acid (30℃) at constant current of 100Am-2

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Anodizing time / s

Vo

lta

ge

/ V

I

II V

III

IV

Planarized Al

Electropolished Al (110)

Electropolished Al (100)

AFM observation was carried out at different stages : I~V

Constant current anodization: differently treated Al

Film surface

Al surface

stage I (8.0s) stage II (17.7s)

X, Y; 200nm/div

Z; 50nm/div

AFM: as-received and electropolished Al (striped structure)

Initial film growth proceeds at metal ridges Thompson, G.E., et al., Trans Inst Met Finish, 56, 159 (1978)

stage V (120s)

Electropolished Al (110)

stage I (8.3s) stage II (17.8s)

Film surface

Al surface

AFM: heated and electropolished Al (cellular structure)

Although the surface topography didn’t change much, but smaller

cells are indicated on substrate at the stage I, showing that pores

initiated at this very early stage.

stage V (120s)

X, Y: 200nm/div

Z: 50nm/div

Electropolished Al (100)

stage I (6.6s) stage II (14.8s)

Film surface

Al surface

AFM: Heated, electropolished and planarized Al

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.

stage V (120s)

Flattened Al

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

0

10

20

30

40

50

60

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120

0

10

20

30

40

50

60

Electropolished Al (100) Planarized Al

Anodizing time / s Anodizing time / s (0.3mol dm-3 oxalic acid, 30℃, 100Am-2)

Siz

e a

nd

dep

th o

f ce

lls

/ n

m

Siz

e a

nd

dep

th o

f ce

lls

/ n

m

V-t curve

Vo

lta

ge

/ V

V-t curve

Vo

lta

ge

/ V

V IV III II I V IV III II I

Size of cells Size of cells

Depth of cells Depth of cells

Change in cell structure with anodizing time

Electropolished Al Stage Ⅰ Stage II

Stage V Stage III

Stage IV

Sulfuric acid

at 20V

Chromic acid at 20V

Phosphoric

acid at 20V

Oxalic acid

at 20V

Difference in pore size

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Voltage dependence of cell dimensions of various anodic films

Pore and cell diameters, barrier layer thickness and porosity

◆ Cell dimensions of anodic films formed in various electrolytes are quite different because of the difference in current density, i.e., electric field strength

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)

0.3M Oxalic acid

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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)

TEM cross section (ion-milling) of anodic film formed in sulfuric acid at a hard

anodizing condition: high current density, high voltage, low temperature

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

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

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Phosphoric acid Sulfuric acid

Difference in cell boundary structure observed by TEM: Films on Al

Before

electron beam

irradiation

After

electron beam

irradiation

When we compare the both films after electron beam irradiation, the difference originated

from the difference in anion incorporation behavior is obvious

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What does proceed by sealing？

Change in pore structure proceeds by

dissolution of cell wall and precipitation of hydrated alumina in pore

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

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Oxalic acid film at 40 V

Sulfuric acid film

at 20 V

Phosphoric acid film

at 80 V

Structural changes during boiling water sealing (11 min)

Sealing rate is highly dependent of

anodizing electrolyte used

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Ni sealing at 95 ℃

for 20min

Y

Y

a b c

Sulfuric acid film: Comparison after sealing

Boiling water for 10 min Li-Sealing for 1 min at RT

100 nm

Before sealing

☆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

TEM images

Cross section of sulfuric acid film before and after sealing