Anodizing of etched aluminum foil coated with modifiedhydrous oxide film for aluminum electrolytic capacitor

Chaolei Ban • Yedong He • Xin Shao •

Liping Wang

Received: 13 September 2013 / Accepted: 17 October 2013 / Published online: 27 October 2013

� Springer Science+Business Media New York 2013

Abstract Prior to galvanostatical anodization in boric

acid solution, aluminum capacitor foil with a tunnel etch

structure is treated in a two-step process in which a non-

dense hydrous oxide film is first formed on foil in neutral

boiling water for 10 min [namely, conventional hydration

(CH)] and the hydrous oxide is then modified in a 80 �C

weakly acidic solution containing trace amount of citric

acid for 3 min [namely, modified conventional hydration

(MCH)]. After modification, the hydrous oxide film

becomes dense and thin. Time variations in the anode

potential during anodizing were monitored, and the struc-

ture and dielectric properties of the anodic oxide films were

examined by transmission electron microscopy, X-ray

diffraction and electrochemical impedance spectroscopy

measurements. It was found that the MCH-induced

hydrous oxide film results in a decreased power con-

sumption during anodization and an increased crystallized

anodic oxide film, which has a high specific capacitance

and a low specific resistance, comparing with the CH-

induced hydrous oxide film.

1 Introduction

Aluminum electrolytic capacitors are extensively used in

the electronics industry. Despite their use for several

decades, research and development efforts to improve the

performance of the capacitor are still underway [1–4].

Among the components of the aluminum electrolytic

capacitor, the aluminum anodized oxide film (dielectric

layer) plays the crucial role during operation. Depending

on anodizing conditions, either amorphous or crystalline

barrier aluminum oxide can be formed. The crystalline

form is always c0-Al2O3, similar to c-Al2O3 but with more

disorder on the cation lattice. Because the crystalline oxide

can sustain a higher voltage, has a higher relative dielectric

constant, and possesses a lower ionic conductivity than the

amorphous one, work on developing an anodized film with

a high degree of crystallinity is of continuous interest to

researchers [5–7]. Chang et al. [8] reported that heat-

treatment of Al foil at 500 �C before or after anodization

could induce the formation of crystalline c0-Al2O3 in the

anodized oxide film. Ono and Alwitt et al. [9] found that

the hydration treatment of Al foil using hot water before

anodization not only could save much electrical power for

the formation of anodized oxide, but also could improve its

crystallinity. In our previous study, we reported that the

crystallinity of the film formed in boric acid -citric acid or

boric acid-tartaric acid mixed solution is higher than that of

the film formed in boric acid only solution [10, 11]. Except

these reports, more research about high crystallized barrier

films can hardly be found. How to obtain such films is

worth further study.

In modern industry, one popular way to make a crystalline

anodic alumina film for high voltage Al electrolytic capacitor

is to electrochemically etch smooth Al foil with d.c. and then

to react the etched foil with boiling water to deposit a hydrous

oxide (pseudoboehmite, PB film), followed by anodization

(namely, formation) in boric acid solution [12]. The growth of

this composite oxide has been extensively studied [13, 14] but

very little on the microstructure of both the hydrous oxide

C. Ban (&) � X. Shao � L. Wang

School of Materials Science and Engineering, Liaocheng

University, Liaocheng 252059, China

e-mail: banchaolei@163.com

C. Ban � Y. He

Beijing Key Laboratory for Corrosion, Erosion and Surface

Technology, University of Science and Technology Beijing,

Beijing 100083, China

123

J Mater Sci: Mater Electron (2014) 25:128–133

DOI 10.1007/s10854-013-1561-z

during the hydration step and the corresponding transformed

anodic oxide film, due to difficulty in sample preparation for

transmission electron microscopy (TEM) micro-analysis. In

this work, after the conventional hydration step, we further

modified the obtained hydrous oxide with weakly acidic

solution containing trace amount of citric acid. A more

crystallized anodic oxide film is formed in the following

anodization step. Meanwhile, an improved ion beam appa-

ratus is used to prepare uniformly thin cross-sections of

etched foils coated with hydrous oxide and composite oxide.

New features of the oxides structure are revealed.

2 Experimental

2.1 Specimen

A high purity ([99.99 %) and cubicity texture ([95 %)

commercial tunnel-etched aluminum foil for high voltage

usage was used as specimen, as illustrated by Fig. 1. For this

foil, 95 % of the area is orientated with the (100) plane

parallel to the surface. Tunnels follow a \100[ direction,

and the metal texture causes the tunnels to be mostly aligned

normal to the surface. The average diameter and length of

tunnels are about 1–2 and 25–30 lm, respectively.

2.2 Formation of anodized alumina films

As anode, foil coupons with 10 cm2 were hydrated in

the neutral boiling deionized water for 10 min, namely

CH pretreatment and then subjected to formation pro-

cess which was composed of primary anodization, stress

relaxation at 500 �C for 2 min and reanodizaiton. Just

before formation, some above hydrated coupons were

further immersed in the 80 �C aqueous solution of citric

acid of pH 5 for 5 min, namely MCH pretreatment. A

304 stainless steel plate was used as the cathode during

anodization. The primary anodization was conducted in

1.29 M H3BO4 solution at 95 �C with a nominal con-

stant current density of ia = 25 mA/cm2 until the

specimens were anodized to 530 V cell voltage(forma-

tion voltage) and then held at this voltage for times up

to 20 min. After the primary anodization, the specimens

were heat-treated in air at 500 �C for 2 min followed by

reanodization in the same solution except with a shorter

holding time of 5 min under the controlled-potential

condition. During the primary anodization, the change

in the cell voltage between the anode and cathode with

time (Vc vs. t curve) was monitored by a digital mul-

timeter connected to a PC.

2.3 TEM examination

For cross-section microstructure analysis, the hydrated or

anodized foils were thinned with a focused beam of Ar ions

radiating parallel to the plane of the specimen face to a

proper thickness and cross sections of the films were

examined under a transmission electron microscope (Hit-

achi H-800H) at 175 kV. A camera length of 80 cm was

adopted as the nanobeam electron diffraction was

performed.

2.4 X-ray diffraction

The crystallinity of the anodic films on Al substrate was

determined by a high power X-ray diffractometer (MAC

Science Co. Ltd M21X).The incident radiation was

obtained from a high power ceramic tube with copper (Cu)

anode operating at 40 kV and 200 mA. 2-Theta scans were

performed on a 2h range of 10�–90�. The samples were

measured in a continuous mode with 0.02� step size and a

scan speed of 10� per minute.

Fig. 1 Surface morphology (a) and cross section morphology (b) of specimen

J Mater Sci: Mater Electron (2014) 25:128–133 129

123

2.5 EIS and capacitance measurement

The electrochemical characteristics of the anodic alumi-

num films were investigated by electrochemical impedance

spectroscopy (EIS) in 80 g/L NH4B5O8�4H2O at 30 �C.

The test cell was a three-electrode system consisting of the

anodized foil, a platinum sheet and a saturated calomel

electrode (SCE) assembled as working electrode, counter

electrode and reference electrode, respectively. The refer-

ence electrode employed a salt bridge with a probe adja-

cent to the working electrode. An EG&G model 273A

potentiostat connected to a Schlumberger 1,255 frequency

response analyzer (FRA) was used to make electrochemi-

cal measurements. The input voltage signal had a root

mean square amplitude of 10 mV at the open circuit

potential and was typically scanned from 100 kHz to

5 mHz.

The capacitance of the anodic aluminum films was

measured using a typical LCR meter in 30 �C 80 g/L

NH4B5O8�4H2O at 100 Hz. A pure aluminum sheet with a

very large area was used as the counter electrode.

3 Results and discussions

3.1 TEM of hydrated film and anodic oxide film

Figure 2a, b show TEM cross section images of the

hydrous oxide (PB film) in a tunnel, formed by CH and

MCH pretreatment, respectively. It can be found that the

PB film formed by CH has a dense inner layer, about

130 nm thick and a fibrous outer layer, about 200 nm thick.

It is difficult to get a precise measure of the dense layer

thickness because of the uncertainty in the position of the

dense/fibrous interface. The fibers are as thin as about

0.8 nm. The gap at the metal/PB interface develops during

TEM observation due to evaporation of H2O from the PB,

especially when examining at high magnification. After

further modified in 80 �C dilute citric acid solution for

5 min, as shown by Fig. 2b, the film fibers dropped dras-

tically in number and the film becomes thin in both dense

inner layer and fibrous outer layer. The reduction in dense

inner layer thickness is less obvious than fibrous outer

layer. The PB film is now mainly composed of dense inner

layer. In the modification step, the dilute citric acid solu-

tion of pH 5 is some aggressive and can dissolve the PB

film. However, the different anti-dissolving ability for both

layers can be responsible for their respective change after

modification.

Figure 3a, b show TEM cross section images and elec-

tron diffraction patterns at different parts of the anodized

alumina film in a tunnel, pretreated with CH and MCH,

respectively. It can be found that both films consist of an

inner layer with high crystallinity, in which crystallites

large in size and rich in number and an outer layer with low

crystallinity, in which crystallites dispersed, small in size

and poor in number. It has been reported that the inner

layer is from crystallization of amorphous anodic oxide and

the outer layer is from the PB transformation [17]. How-

ever, comparing with the CH pretreated film, more and

larger crystallites can be found in the MCH pretreated one,

especially in its inner layer adjacent to the Al substrate.

Moreover, the electron diffraction patterns show that the

outer layer of the CH-pretreated film is rather amorphous

and that of the MCH-pretreated film has been quite

crystallized.

100nm

(b)

100nm

(a)

AlAl

Fig. 2 PB in etch tunnel

formed from CH (a) and MCH

(b)

130 J Mater Sci: Mater Electron (2014) 25:128–133

123

Meanwhile, due to phase transformation and resulted

shrinkage, numerous nanometer white occluded voids,

cavities and slit can be found in both films, especially in

their inner layers. However, comparing with the CH-pre-

treated film, large cavities and slit voids of high concen-

tration can be easily found in the MCH-pretreated film, as

shown by arrows in Fig. 3b. Obviously, the MCH pre-

treatment facilitates transformation of PB and amorphous

anodic oxide to c’-Al2O3, leading to high tensile stress,

severe volume shrinkage and grave defects.

From Fig. 3, the thickness of CH-pretreated film is

550 nm, but that of MCH-pretreated one is only 490 nm.

According to equation E = Vf/d, where Vf is the formation

voltage, and d is the film’s thickness, the field strength of

CH-pretreated and MCH-pretreated film can be valued as

0.96 and 1.08 V/nm, respectively. Obviously, the MCH

pretreatment can contribute to formation of anodic oxide

film with high crystallinity, leading to increase in field

strength.

3.2 XRD of anodic oxide film

Figure 4 shows high power X-ray diffractions of the anode

foils with different pretreatment. The strongest peak corre-

sponds to Al (200) from substrate and the second peak is

c-Al2O3 (400), coming from crystalline oxide in the anodic

alumina film on Al substrate, but overwhelmed by Al (200).

The third peak is the film’s c-Al2O3 (440), further magnified

by Fig. 4’s insert picture, where 2-theta scans is slowly

performed on a 2h range of 64�–70�. Scherrer analysis of the

c-Al2O3 (440) peaks of the film is performed using the

equation: crystallite size = Kk/(B�cosh), where B is half

height width; K is a form factor (shape factor), K = 0.89 for

spheres particles; k = wavelength of the radiation used,

kCu,Ka1 = 1.54056 A. The average crystallite size of the

MCH-pretreated film is 26 nm, larger than 22 nm, that of the

CH-pretreated one.

During anodization, Al cations move to the PB interface,

where they take part in the transformation of PB to

c-alumina. A proposed reaction is AlOOH�H2O ? Al3? ?c-Al2O3 ? 3H?. Amorphous alumina grows under the PB

layer, by anion transport to the metal interface. Crystallites

from the PB transformation serve as seeds for transfor-

mation of the amorphous anodic oxide to c-oxide [14]. As

illustrated by Fig. 2, part of inner dense layer of the CH

formed PB film is more preserved after the modification

step than that of outer fibrous layer. The PB film becomes

thinner and denser. The different composition and structure

of both layers are responsible for their phase transforma-

tion under high electric field. More details are worthy of

further investigation. Maybe the dense layer can be trans-

formed to c-oxide more easily and completely than fibrous

one. As a result, the MCH pretreated anodic film is more

crystallized than that CH pretreated one, well coinciding

with the TEM observation shown in Fig. 3.

110000nnmm

(b)

110000nnmm

(a)

Al

Al

Fig. 3 TEM cross section

images and electron diffraction

patterns of anodized alumina

film pretreated with CH (a) and

with MCH (b)

0 20 40 60 80 100

64 65 66 67 68 69 70

γ−Al2O3(400)

γ−Al2O3(440)

A

γ− Al2O3(440)

2θ / (°)

A: film pretreated with CH B: film pretreated with MCH

Al(200)

A

B

B

γ−Al2O3(440)

Fig. 4 XRD patterns of anode foils formed in different solutions

J Mater Sci: Mater Electron (2014) 25:128–133 131

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3.3 EIS of anodic oxide film

The results of EIS measurements of the anode foils pre-

treated with CH and MCH are shown in Fig. 5. A single

time-constant capacitance-approximate behavior is pre-

dominant over the frequency range in this investigation.

The impedance data presented in Fig. 5 could be simulated

by an equivalent circuit, consisting of a parallel combina-

tion of the film resistance (Rox) and constant phase angle

element (CPE, Qox), connected in series to a solution

resistance (Rs). With this equivalent circuit and the

impedance data obtained, the Rox and Qox of CH-pretreated

and MCH-pretreated film could be evaluated as Table 1

demonstrated. In Table 1, factor n represents the Qox fre-

quence power, which is usually between 0.5 and 1. When

n = 1, a Qox is equivalent to an ideal capacitor (Cox). The

film’s special capacitance(Cox) measured with LCR meter

was also listed in Table 1.

The results in Table 1 show that, although n and Rox of

MCH pretreated film are lower than those of CH pretreated

film, Qox of the former is lager than that of the latter. The

existence of the CPE element (Qox) in the equivalent circuit,

rather than capacitor (Cox) element, is due to the dispersion

effect caused by numerous fine narrow tubes on the foil. The

more tubes there are, the greater dispersion effect. As Fig. 3

shows, the CH pretreated film is 11 % thicker than the MCH

pretreated film. As a result, more fine and narrow tunnels on

etched Al foil will survive after anodization for MCH pre-

treatment than for CH pretreatment, which makes more dis-

persion effect of the MCH pretreated film than CH pretreated

one, leading to decrease in n. Comparing with CH pretreated

film, the reduction in Rox of MCH pretreated film is obviously

attributable to its more defects and higher crystallinity, as

demonstrated by Fig. 3.

As also can be seen in Table 1, the special capaci-

tance(Cox) of MCH pretreated film was higher than that of

CH pretreated one, consistent with film Qox comparision

results indicated by EIS measurement. It is well known that

the capacitance (C) of the barrier film can be expressed by

the equation C = eoerA/d = eoerAE/Vf, where eo represents

the dielectric constant in vacuum (8.854 9 10-12 F/m), er is

the relative dielectric constant of the barrier film, A and d are

the real area and thickness of it, respectively. The film

thickness (d) depends on formation volatage (Vf) and film

growth rate (the inverse of field strength, E-1). A is pro-

portional to the real surface area of Al foil, which has bee

enlarged by electro-etching and must be further guaranteed

by a thin barrier film when Vf is fixed. When the barrier film

sustained fixed Vf becomes more crystallized, its er, A and

E will further increase, therefore a higher C can be obtained.

The phenomenon that the Cox of MCH pretreated film was

higher than that of CH pretreated one is obviously attribut-

able to the fact that the former is more crystallized than the

latter, as demonstrated by Figs. 3, 4.

3.4 Vc versus t curve of hydrated Al foil

during anodization

Figure 6 shows Vc versus ta curve for specimens pretreated

with CH and MCH during primary anodization. Vc is the

reflection of the voltage which the forming film can in situ

0

50

100

150

200

250

300

350(a) film pretreated with CH

film pretreated with MCH

Rs

Qox

Rox

0 100 200 300 400 500 600 -3 -2 -1 0 1 2 3 4 5 6-10

0

10

20

30

40

50

60

70

80

90(b)

film pretreated with MCH

film pretreated with MCH

Fig. 5 EIS for anode foils pretreated with CH and MCH a Nyquist plot; b Bode plot

Table 1 Rox, Qox, Cox and Uw of anodic oxide film pretreated with

CH and MCH

Film types Film

resistance,

Rox/(MX/cm2)

Qox

(lS/cm2

secn)

n Film

capacitance,

Cox/(lF/cm2)

MCH-

pretreated film

6.758 2.163 0.8672 0.6259

CH-

pretreated

film

9.315 1.746 0.9043 0.5827

132 J Mater Sci: Mater Electron (2014) 25:128–133

123

sustain. At the very initial stage, specimens coated with PB

show an about 100 V jump in Vc, due to solution resis-

tance, and this Vc jump is almost independent of MCH and

CH. After the initial stage, Vc increases linearly with ta up

to 530 V. The slope of Vc versus ta curves for specimen

pretreated with MCH is much steeper than that pretreated

with CH, leading to about 15 % decrease in power con-

sumption during the primary anodization.

As illustrated by Fig. 2, the PB film formed by CH

becomes thin after modification in 80 �C dilute citric acid

solution of pH 5. The CH-induced PB film is a poorly

crystallized boehmite (BOE) with a water content of

1.5–2.5 mol [12]. The dense inner layer of PB film maybe

contains less water content than the fibrous outer layer of

it. The modification can densify the PB film by dissolving

the outer layer, leading to a lower water content. We

assume that the densified PB film is more easily trans-

formed to c-oxide by electric field-induced dehydration

than that sparse one. As a result, the reduction in power

consumption during anodization can be achieved by the

use of MCH pretreatment.

4 Conclusion

Prior to galvanostatical anodization in boric acid solution,

aluminum capacitor foil with a tunnel etch structure for

high voltage electrolytic capacitors is treated in a two-step

process in which a non-dense hydrous oxide film is first

formed on foil in neutral boiling water for 10 min and the

hydrous oxide is then modified in a 80 �C weakly acidic

solution containing trace amount of citric acid for 3 min.

The effects of the modification on the microstructure,

capacitance and resistance of anodized aluminum foils are

experimentally investigated here and the important results

can be summarized.

1. After the modification in citric acid solution, the

hydrous oxide film becomes dense and thin. During the

following anodization, the modified hydrous oxide film

is more easily transformed to barrier anodic oxide film,

leading to a much reduction in power consumption.

2. With the modification in citric acid solution, the

crystallinity of the obtained barrier anodic oxide film is

improved, leading to increase in its specific capaci-

tance and decrease in its specific resistance.

Acknowledgments The work is financially supported by the Chinese

National Nature Science Foundation (Grant. 51172102/E020801&

21203085) and promotive research fund for young and middle-aged sci-

entists of Shandong Province (doctor fund) (BS2011CL011).

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0 200 400 600 800 100

100

200

300

400

500

600

BA

A: specimen pretreated with MCHB: specimen pretreated with CH

Anodizing time, ta / s

Cel

l vol

tage

, Ec /

V

Fig. 6 Change in cell voltage, Vc, with time, ta, during anodizing in

1.29 M H3BO4 solution at 95 �C and 25 mA/cm for specimens

pretreated with CH and MCH

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