Anodizing of Aluminum Coated with Silicon Oxide by a Sol-Gel Method

Article abstract of DOI:10.1149/1.1408633

Aluminum specimens were covered with SiO₂ film by a sol-gel coating and then anodized galvanostatically in a neutral borate solution. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric prope

Full text of DOI:10.1149/1.1408633

Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
B473
0
013-4651/2001/148͑11͒/B473/9/$7.00 © The Electrochemical Society, Inc.
Anodizing of Aluminum Coated with Silicon Oxide by a
Sol-Gel Method
a
a,
a, ,z
Keiji Watanabe, Masatoshi Sakairi, * Hideaki Takahashi, *
b
b
c
Katsumi Takahiro, Shinji Nagata, and Shinji Hirai
a
Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan
Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, Japan
b
c
Department of Materials Science and Engineering, Muroran Institute of Technology,
Mizumoto-cho, Muroran-shi, Hokkaido 050-8585, Japan
Aluminum specimens were covered with SiO film by a sol-gel coating and then anodized galvanostatically in a neutral borate
2
solution. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the
anodic oxide films were examined by transmission electron microscopy, Rutherford backscattering spectroscopy, and electro-
chemical impedance measurements. It was found that anodizing of aluminum coated with SiO films leads to the formation of
2
anodic oxide films, which consist of an outer Al-Si composite oxide layer and an inner Al O layer, at the interface between the
2
3
SiO film and the metal substrate. The capacitance of anodic oxide films formed on specimens with a SiO coating was about 20%
2
2
larger than without a SiO coating. In the film formation mechanism, the conversion of Al O to Al-Si composite oxide at the
2
2
3
interface between the inner and outer layers is discussed in terms of inward transport of Si-bearing anions across the outer layer.
2001 The Electrochemical Society. ͓DOI: 10.1149/1.1408633͔ All rights reserved.
©
Manuscript submitted January 9, 2001; revised manuscript received June 11, 2001. Available electronically October 9, 2001.
Barrier-type anodic oxide films formed on aluminum play an
important role as dielectric films in aluminum electrolytic capaci-
tors, and their physical and chemical properties determine the per-
formance of the electrolytic capacitor. Recent development of mo-
bile electronic devices, such as notebook computers and portable
telephones, requires very small electrolytic capacitors with high
electric capacitance. The electric capacitance, C, of the aluminum
electrolytic capacitor is expressed by the following equation
Another way to increase Qmax is by increments in ␧ and decre-
ments in K. These values, however, cannot be changed indepen-
dently, since they are closely related to each other. Generally, mate-
rials with high ␧ have a relatively loose lattice-bonding and easily
produce dipoles even in low electric fields. This strongly suggests
that high-␧ materials are able to sustain only a low electric field,
leading to a large K value. Table I indicates the value of ␧, K, and
4
␧
/K for anodic oxide films formed on valve metals. It is obvious
from Table I that oxide films with higher ␧ values have higher K
values. The value of ␧/K is not constant, however, and differs from
oxide film to oxide film. Titanium anodic oxide film has the largest
␧/K value, and aluminum oxide film has the smallest one. Anodiz-
C ϭ ␧ ␧S/␦
͓1͔
0
where ␧0 is the vacuum permittivity, ␧ the specific dielectric con-
stant of the anodic oxide film, S the surface area, and ␦ the film
thickness. As the charge accumulated in the capacitor, Q, is ex-
pressed by the product of C and the applied voltage, Vappl , the
following equation can be derived
5
6
ing aluminum after hydrothermal or thermal treatment causes the
formation of an anodic oxide film containing ␥-Al O and leads to
2
3
an increase in ␧/K. The increase in ␧/K is mainly due to a decrease
in K, since the ␥-Al O -containing oxide film is able to sustain a
2
3
high electric field. The formation of composite oxide films incorpo-
rating valve metal oxides in aluminum anodic oxide films would
appear to be useful to increase ␧/K because of the high ␧/K values
of valve metal oxides ͑see Table I͒.
Q ϭ ͑␧ ␧S/␦͒V
͓2͔
0
appl
Vappl should be smaller than the formation potential of anodic
oxide film, E , since the application of Vappl beyond E leads to a
breakdown of the anodic oxide film. Hence, the maximum accumu-
lated charge, Qmax , in an aluminum electrolytic capacitor is ex-
pressed by the following equation
a
a
In previous investigations, the authors formed Al-Ti composite
1
oxide films using a pore-filling method and metal-oxide chemical
7
vapor deposition ͑MOCVD͒/anodizing and found that the parallel
capacitance of the Al-Ti composite oxide film is 40-60% higher than
that of barrier-type anodic oxide films on aluminum. Mochitsuki
et al. found an 80% increase in ␧/K by forming Al-Ti composite
Qmax ϭ ͑␧ ␧S/␦͒E
͓3͔
0
a
8
oxide films using ion plating/anodizing. With MOCVD and ion
The film thickness, ␦, is proportional to E , giving Eq. 4
a
plating it is relatively difficult, however, to form a uniform dielectric
film on the etched surface of aluminum foil, which is disadvanta-
E ϭ ␦/K
a
͓4͔
where K is the film thickness per unit film-formation potential. Sub-
1
stitution of E with ␦/K into Eq. 3 gives Eq. 5
a
Table I. Dielectric constant, ␧, K value, and the ␧ÕK ratio for
anodic oxide films on various metals.
Qmax ϭ ␧␧ S/K
͓5͔
0
Oxide
SiO2
K
␧
␧/K
It is obvious from Eq. 5 that larger values of S and ␧0 and
smaller values of K give larger Qmax values. Increases in S are
achieved by electrochemical etching of the aluminum substrate be-
fore industrial anodization,2 and many authors have investigated dc
and ac etchings.
0.4
1.3
3.5
9.8
22-25
27.6
41.4
90.0
9.0
7.7
11-12.5
17.0
18.4
30.0
Al O3
2
2.0
,3
ZrO2
Ta O5
1.6
2.3
3.0
2
Nb O5
2
TiO2
*
Electrochemical Society Active Member.
E-mail: takahasi@elechem1-mc.eng.hokudai.ac.jp
K is the film thickness per unit potential ͑nm/V͒.
␧ is the dielectric constant of oxide film.
z
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B474
Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
geous in industrial mass production. Sol-gel coatings have the ad-
vantage of providing a uniform coating on rough surfaces with high
industrial productivity.9,10
Previously, the authors investigated the formation of Al-Zr com-
posite oxides by sol-gel coating/anodizing and found that the ␧/K
value of composite oxide films is 20% higher than that of barrier-
type anodic oxide films on aluminum.1
1,12
Lai reported a 50% in-
crease in capacitance by forming anodic oxide films with sol-gel
Nb O coating/anodizing.13 Here, the increase in the apparent ␧
2
5
value was responsible for the increase in ␧/K. Silicon oxide films
have smaller values of ␧ and K than aluminum oxide films, but they
have higher ␧/K values ͑see Table I͒. The formation of Al-Si com-
posite oxide films, therefore, may cause the increase in ␧/K, mainly
due to the decrease in K.
In the present investigation, the formation of Al-Si composite
oxide films was attempted by sol-gel coating/anodizing, and the
structure and the dielectric properties of the composite oxide films
were examined as functions of the heating temperature of the SiO2
coating. Mechanisms of the formation of the composite oxide films
on SiO -coated specimens during anodizing are discussed.
2
Experimental
Figure 1. Flow chart for the preparation of the coated film from sol solution.
Specimen.—Highly pure aluminum ͑99.99%͒ foil was used as
specimens ͑2 ϫ 2 cm with handle͒. The specimens were elec-
tropolished in a perchloric acid/acetic acid mixture ͑78:22 v/v͒ at 28
V and 285 Ϯ 1 K for 1 min after ultrasonic degreasing in ethanol.
The electropolished specimens were rinsed in distilled water and
acetone before storage in a silica gel desiccator. The handle of speci-
mens was shielded by a Shin-Etsu silicone coating to leave only the
square part exposed.
Film characterization.—Encapsulated specimens were first
trimmed and then sliced into 30 nm thick sections, using an ultra-
microtome ͑Reichert-Nissei Ultra Cuts E͒ with a diamond knife ͑Di-
atome, Ltd., Diatome Ultra͒. Slices of the vertical section of the
specimens were then examined under a transmission electron micro-
scope ͑Hitachi H-700H͒ at 200 kV and analyzed with an energy-
dispersive X-ray analyzer ͑Noruman 623M-3SST͒ equipped with
another TEM ͑JEOL JEM-2000ES͒.
Sol-gel coating.—Electropolished specimens were coated by
SiO films via a sol-gel dip-coating method. The sol for dip-coating
2
Anodic oxide films were also analyzed by Rutherford back-
scattering spectroscopy ͑High Voltage Engineering 1.7 MV Tande-
was prepared in a N atmosphere glove box, using the following
2
9
,10
2
ϩ
procedure.
Reagent-grade pure tetraethoxysilane ͑TCL Co.͒ was
tron͒ using 2.0 MeV of a He ion beam supplied by a Van de Graff
first diluted with anhydrous ethanol while stirring. Then, nitric acid
and distilled water were slowly added to the tetraethoxysilane solu-
tion while stirring at room temperature. The mixing molar ratio of
tetraethoxysilane, anhydrous ethanol, distilled water, and nitric acid
2ϩ
accelerator. The He ion beam angle was normal to the specimen
surface, and the detector angle was 170° to the incident direction.
The chemical composition of the anodic oxide films was determined
15
using the RUMP program.
d
was 1:10:5.75:0.07. The electropolished specimen was immersed in
the sol and withdrawn from the sol at 1 mm/s in the glove box. After
being taken out from the glove box, the specimen was dried in air
for 300 s at room temperature. The dried specimens were heated in
a furnace at 573 K for 1.8 ks under a flow of pure oxygen. This
procedure was repeated two times. As the final step in the dip-
coating ͑the second time͒ heating was carried out in pure oxygen for
Results
SiO film coated by the sol-gel method.—Figure 2 shows the
TEM images of vertical cross sections of specimens covered with
2
SiO films by a sol-gel method at T ϭ ͑a͒ 573, ͑b͒ 673, and ͑c͒
2
h
7
73 K. All the films have uniform thickness, and the thickness of the
SiO films is 430, 260, and 230 nm at T ϭ 573, 673, and 773 K,
3
0 min at T ϭ 573, 673, or 773 K. Flow charts of the dip-coating
2
h
h
are given in Fig. 1.
decreasing with increasing Th . The cracks observed in the SiO2
films in Fig. 2b and c are due to stress originating during slicing of
the sample with a microtome. Figure 3a shows the Rutherford back-
scattering ͑RBS͒ spectrum obtained for the specimen coated with
SiO at T ϭ 773 K ͑see Fig. 2c, and Fig. 3b͒ shows the concen-
Anodizing.—The specimen coated with SiO film was anodized
2
in 0.5 M H BO /0.05 M Na B O solution ͑pH 7.4͒ at Ta
3
3
2
4
7
2
ϭ 293 K with a constant current of i ϭ 10 A/m , and the change
in the anode potential ͑vs. Ag/AgCl sat. KCl͒ with time ͑E vs. ta
curve͒ was monitored by a digital multimeter connected to a PC
system. Electropolished specimens without a SiO layer were also
a
2
h
a
tration depth profile of Si, Al, and O simulated from the RBS spec-
trum. It can be seen from Fig. 3b that the coated film has about 200
nm thickness, consisting of an outer SiO layer and an inner Al O
3
2
2
2
anodized under the same conditions to elucidate the effect of the
or Al-Si mixed oxide layer. The inner oxide layer may be formed by
4
ϩ
SiO films on anodic oxide film formation.
thermal oxidation of the metal substrate and/or interdiffusion of Si
2
3
ϩ
and Al during heating. The film thickness estimated by RBS
Electrochemical impedance spectroscopy.—Specimens anodized
to different Ea values were immersed in 0.5 M H BO /
analysis is lower than that obtained from ͑TEM, see Fig. 2c͒, and
3
3
this may be due to a porous structure of the SiO film.
2
0
.05 M Na B O solution ͑pH 7.4͒ at 293 K, and impedance mea-
2 4 7
surements were carried out by applying 10 mV of sinusoidal alter-
nating voltage in the 0.01 Hz to 50 kHz range with a frequency-
Anodic oxide films formed on aluminum with SiO2 coat-
ing.—Figure 4 shows E vs. t curves for specimens coated with
a
a
response analyzer ͑FRA͒ ͑NF S-5720B͒. Details of the procedure
have been described elsewhere.14
SiO2-coating at Th ϭ 573, 673, and 773 K. In Fig. 4, the Ea vs. ta
curve for electropolished specimens without SiO is also indicated
2
as a chained curve, a straight line starting from zero. Specimens
d
A clear and stable sol, with a viscosity of 2.0 mPa s and no turbidity, was prepared.
coated with SiO show a 2-3 V jump in E at the very initial stage,
The viscosity of the sol was almost unchanged, even after aging at room temperature for
2
a
1
week.
and the E jump is almost independent of T . After the initial stage,
a
h
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Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
B475
Figure 2. TEMs of vertical cross sec-
tions of specimens coated with SiO by
2
the sol-gel method at T ϭ 573-
h
773 K. Procedure for the coating as in
Fig. 1.
strate: an outer dark layer and an inner somewhat darker layer. There
is no SiO layer on the specimens anodized up to E ϭ 400 V.
E increases linearly with t up to E ϭ 300 V. The slope of E vs.
a
a
a
a
t curves for specimens with SiO coating is much steeper than that
2
a
a
2
Both the inner and outer layers increase with t ͑or E ͒, while the
without SiO coating, and is slightly steeper with higher T . All the
a
a
2
h
specimens with SiO coating show flatter slopes on the E vs. t
SiO2 layer decreases with Ea . The disappearance of the SiO2 layer
at Ea ϭ 400 V is a result of decrease in its thickness with Ea . The
outer dark layer in Fig. 5c shows a fine texture, the size of which is
several nanometers. This could be due to mixing of fine particles of
SiO2 with Al2O3. The texture can be observed in the outer layer of
all specimens, and the clear texture is possible only for thin slices of
specimens with good focusing at TEM observation.
2
a
a
curves above E ϭ 300 V, where gas evolution was observed on
a
the surface of the specimen.
Figure 5 shows TEM images of the vertical cross sections of
specimens anodized up to E ϭ ͑a͒ 100, ͑b͒ 200, ͑c͒ 300, and ͑d͒
a
4
00 V after SiO coating at T ϭ 773 K. All the films show two
2 h
layers at the interface between the SiO layer and the metal sub-
2
Figure 4. Change in anode potential, E , with time, t , during anodizing in
a
a
2
Figure 3. ͑a͒ RBS spectrum and ͑b͒ the concentration depth profiles of Si,
Al, and O for SiO -coated specimens with T ϭ 773 K.
0.5 MH BO /0.05 M Na B O solution at 293 K and 10 A/m for speci-
3 3 2 4 7
mens coated with SiO at different T .
2
h
2
h
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B476
Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
Figure 5. TEMs of vertical cross sections of specimens anodized up to E ϭ ͑a͒ 100, ͑b͒ 200, ͑c͒ 300, and ͑d͒ 400 V after SiO coating at T ϭ 773 K.
a
2
h
Conditions of anodizing as in Fig. 4.
Figure 6 shows the TEM images of the vertical cross sections of
parts, whereas the Si concentration decreases from the outer layer
inward. The chemical composition of the outer layer at positions 1
and 3 can be estimated as AlSi3.3O8.1 and AlSi0.4O2.3, assuming that
the outer layer consists of Al-Si composite oxide. It is considered
from Table II that the inner layer consists of alumina.
E ϭ 400 V specimens with SiO coating at ͑a͒ 573, ͑b͒ 673, and
a
2
͑
c͒ 773 K. There are outer and inner layers on all the specimens, and
the thickness of both layers decreases only slightly with increasing
T . The T ϭ 573 K specimen shows a SiO layer on the outer
h
h
2
layer, but the T ϭ 673 K specimen shows no SiO layer like the
Figure 7a shows the RBS spectrum obtained for the Ea
h
2
T ϭ 773 K specimen. In Fig. 6c (T ϭ 773 K), the positions ex-
ϭ 400 V specimen with SiO coating at T ϭ 773 K ͑Fig. 5d and
h
h
2
h
amined by energy-dispersive X-ray ͑EDX͒ are indicated by the num-
bers 1-4, and the mole concentration of Si and Al at each position is
shown in Table II. The values are similar to those at the other two
positions of the oxide films. Both aluminum and Si are observed in
the outer layer, whereas only Al is observed in the inner layer. The
concentration of Al in the outer layer is lower at the more outer
6c͒, and Fig. 7b shows the concentration depth-profile of Si, Al, and
O, derived from the spectrum. Figure 7b shows that the Si concen-
tration in the outer layer decreases with increasing the distance from
the film surface, X, while the concentration of Al in this layer in-
creases with X. This is consistent with the results of EDX ͑see Table
II͒.
Figure 6. ͑a͒ TEMs of cross sections of
specimens anodized up to E ϭ 400 V
a
after SiO2 coating at T ϭ ͑a͒ 573, ͑b͒
h
673, and ͑c͒ 773 K. Conditions of coat-
ing as in Fig. 1, and anodizing as in Fig.
4.
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Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
B477
Table II. The Si and Al content and the SiÕAl mole ratios from
the EDX analysis at the numbered positions indicated in Fig. 6c.
Figure 8 shows time variation in the thickness of ͑a͒ the SiO2
layer, ␦ , ͑b͒ the outer layer, ␦ , and ͑c͒ the inner layer, ␦ , during
Si
c
a
anodizing after SiO coating with different T . The ␦ of all the
2
h
Si
specimens decreases linearly with t , and the slope of the ␦ vs. ta
a
Si
curves decreases with T . The thickness of the outer layer, ␦ , on
h
c
all the specimens coated with SiO increases linearly with t at t
2
a
a
Ͻ 300 s, and beyond this period the rate of increase in ␦c de-
creases on the specimens with T ϭ 673 and 773 K. The slope of
h
the ␦ vs. t curves at the initial stage increases slightly with increas-
c
a
ing T . The ␦ of all SiO -coated specimens increases with t , and
h
a
2
a
the rate of increase in ␦ decreases with t . The slope of the curve of
a
a
the SiO -coated specimens is much flatter than that of the specimen
2
without SiO coating ͑see chained curve in Fig. 8c͒, and it becomes
2
slightly flatter with higher Th .
Figure 8. Changes in the thickness of the SiO -coated layer, ␦ ͑Fig. 9a͒,
2
Si
the outer Al-Si composite oxide layer, ␦c ͑Fig. 9b͒, and of the inner Al O
2
3
layer, ␦ ͑Fig. 8b͒, with t during anodizing after SiO coating with different
a
a
2
T . Conditions of anodizing as in Fig. 4.
h
Here, it must be noted that on all the SiO -coated specimens, the
2
rate of increase in ␦ during anodizing is higher than that of the
c
decrease in ␦Si .
Dielectric properties of anodic oxide films formed after SiO2
coating.—Figure 9 shows the impedance Bode diagrams for speci-
mens anodized up to E ϭ 100, 200, 300, and 400 V after SiO2
a
coating at T ϭ 773 K. E ϭ 0 V represents the specimen without
h
a
anodizing after SiO coating. The log ͉Z͉ vs. log f curve for all
2
specimens is divided into three regions: a horizontal straight line in
the low-frequency region, a straight line with a slope of Ϫ1 in the
middle-frequency region, and a horizontal straight line in the high-
frequency region. The impedance,
͉Z͉, in the low and high fre-
3
Ϫ1
Ϫ2
quency does not depend on E and is about 10 and 10 ⍀ m
.
a
The log ͉Z͉ vs. log f curve for SiO -coated specimens (Ea
2
Figure 7. ͑a͒ RBS spectrum and ͑b͒ concentration depth profiles of Si, Al,
and O in depth for specimens anodized up to E ϭ 400 V after SiO coating
ϭ 0 V) is located in a much lower frequency region than that for
electropolished specimens with heating at 823 K for 3 h.6 The
straight line in the middle-frequency region shifts to the high-
a
2
at T ϭ 573 K. Conditions of anodizing as in Fig. 4.
h
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B478
Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
Figure 10. Change in the reciprocal capacitance, 1/C , with anode poten-
p
Figure 9. Bode diagram obtained for specimens anodized up to E ϭ 0,
a
tial, E , obtained for SiO -coated specimens with different T . Conditions
a
2
h
1
00, 200, 300, and 400 V after SiO coating with T ϭ 773 K. Conditions
2
h
of anodizing as in Fig. 4.
of coating as in Fig. 1, and anodizing as in Fig. 4.
Discussion
frequency direction with increasing E . The phase shift, ␪, for all
specimens changes from 0° through Ϫ90 to 0° with increasing f, and
a
Dielectric properties of anodic oxide films after SiO2 coat-
ing.—Figure 11 shows the E dependence of the thickness of the
a
the curve shifts to the higher frequency direction as E increases.
a
outer Al-Si composite oxide layer, ␦ , and the inner Al O layer,
c
2
3
The impedance data in Fig. 9 may correspond to the equivalent
␦a , obtained from specimens with SiO coating at T ϭ ͑a͒ 573,
e
2
h
electric circuit inserted in Fig. 9. The symbols C , R , and R
in
p
p
soln
͑b͒ 673, and ͑c͒ 773 K ͑see Fig. 8͒. The chained curves in Fig. 11
the insert represent parallel capacitance of the film, the parallel re-
sistance of the film, and the solution resistance, respectively. Figure
represent the E dependence of the thickness of anodic oxide films
a
formed on aluminum without SiO film. Both ␦ and ␦ of all the
2
c
a
9
implies that C decreases with increasing E , and that R and R
p a p soln
specimens are proportional to E . The slope of (␦ ϩ ␦ ) vs. E
a
c
c
a
do not depend on E . The specimens with T ϭ 573 and 673 K
a
h
curve for the specimens with SiO coating is much flatter than with-
2
showed Bode diagrams similar to Fig. 9, suggesting similar equiva-
out SiO coating and decreases only slightly with increasing T .
2
h
lent electric circuits to that in Fig. 9. The values of R and R did
p
soln
The anode potential is sustained by both the inner and the outer
layer of the anodic oxide films formed on the specimen with SiO2
coating. Hence, the average electric field across the anodic oxide
not depend on T and E , and only the C value changed with the
h
a
p
f
parameters.
The relationship between the reciprocal of the parallel capaci-
films (␦ ϩ ␦c) can be calculated to be 0.93, 1.03, and 1.08
c
tance, 1/C , and anode potential, E , for specimens with and
p
a
9
ϫ 10 V/m at T ϭ 573, 673, and 773 K. These values are con-
h
without SiO coating is shown in Fig. 10. The 1/C for the specimen
2
p
siderably larger than the value for anodic oxide films formed on
is proportional to E . The specimens coated by SiO show a flatter
9
a
2
aluminum without SiO2 coating, 0.67 ϫ 10 V/m ͑see chained
slope than those without SiO coating, and this is more pronounced
2
curves in Fig. 11͒. This is because the outer Al-Si composite oxide
layer is able to sustain a high potential. Assuming that the K value
with increasing T . Consequently, the capacitance of anodic oxide
h
film formed after SiO coating is at most about 20% higher than
2
across the inner layer is K ϭ 1.5 nm/V (ϭ1/(0.67
a
without SiO coating.
9
2
ϫ 10 ) m/V), the K value of the outer layer, K , can be estimated
c
using the following equation
e
Here, the authors adopted the simplest equivalent circuit. As shown in Fig. 5 or 6,
the anodic oxide films formed after SiO2 coating generally consist of three layers: an
SiO2 layer, an Al-Si composite oxide layer, and an Al2O3 layer, and this could lead to
more complicated equivalent circuits. The authors cannot explain the impedance re-
K ϭ ␦ /͓E Ϫ ͑␦ /K ͔͒
͓6͔
c
c
a
a
a
The K of all the SiO -coated specimens is in the range of 0.7-0.9
sponse in Fig. 9, but this behavior has been also obtained in
a
previous
c
2
investigation.1
1,12
nm/V, decreasing with E and much smaller than the 1.5 nm/V for
a
f
The authors are not able to explain the independence of Rp with Ea . However, this
anodic oxide films on aluminum without SiO coating, as shown in
2
has also been observed for specimens anodized in a neutral borate solution after thermal
Fig. 12. The decrease in K with T may be correlated with the fact
c
h
treatment and for specimens anodized in a neutral boric acid solution up to high anodic
potentials.16 Defects in the structure of the oxide films could be an explanation for the
that the thickness of the coated SiO film before anodizing decreases
2
phenomenon.
with T . The outer layer of the SiO -coated specimens is formed by
h
2
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Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
B479
Figure 12. Relationship between Kc and the anode potential, E , obtained
a
for sol-gel coated/anodized specimens with different T . Conditions of an-
h
odizing as in Fig. 4. Chained line indicates the K -E relationship for speci-
c
a
men without SiO coating.
2
Bode diagram for the specimen anodized after the SiO coating cor-
2
responds to an equivalent circuit of a uniform layer structure ͑see
Fig. 9͒.
Consequently, the appreciable increase in parallel capacitance of
anodic oxide films by coating with SiO before anodizing is due to
2
the formation of the Al-Si composite oxide layer with a low Kc
value.
Formation mechanism of anodic oxide films after SiO2 coat-
ing.—The important experimental facts on the film-formation
mechanism during anodizing are the following
1
. For SiO -coated specimens the anode potential, E , increases
2
a
Figure 11. Change in the thickness of the outer Al-Si composite oxide layer,
␦c , and the inner Al O layer, ␦ , with E , obtained for SiO -coated speci-
linearly with time, t , after a jump of 2-3 V at the very initial stage
of anodizing, and the slope of the E vs. t curves is much steeper
than for the specimen without SiO coating ͑see Fig. 4͒.
a
2
3
a
a
2
a
a
mens with different T . Condition of anodizing as in Fig. 4, and the chained
h
2
curve for electropolished specimen without SiO coating.
2
2. For anodic oxide films with an outer Al-Si composite oxide
layer and the metal substrate during anodizing ͑see Fig. 5͒, the con-
centration of Si decreases inward through the outer layer, while the
Al concentration increases ͑see Fig. 7 and Table II͒.
3
. During anodizing, the thickness of the inner (␦ ) and outer
a
the conversion of SiO film during anodizing, as discussed in the
layers (␦ ) increase, while the thickness of the SiO2 layer, ␦Si ,
2
c
following section. Lower heating temperature in the sol-gel coating
decreases. The rate of increase in ␦ is appreciably larger than the
c
allows more organic compounds to remain in the SiO film, and this
rate of decrease in ␦Si ͑see Fig. 8͒.
2
may lead to the formation of the outer layer with a higher K value.
Figure 13 shows a schematic model of the formation of the an-
odic oxide film during anodizing after SiO2 coating. It is assumed
that the SiO2 layer has a network structure of micropores and cracks
which may have been formed by the evaporation of organic com-
pounds during heating of the sol precursor film. During the heat-
treatment, a thin thermal aluminum oxide film is formed at the in-
c
As shown in Fig. 10 and 11, both the 1/Cp and (␦ ϩ ␦ ) values
c
a
are proportional to E . Hence, the apparent dielectric constant, ␧
,
app
a
of anodic oxide films formed on SiO -coated specimens can be cal-
2
culated using Eq. 1. Table III shows the change in the apparent
dielectric constant, ␧app , with heating temperature, T , in SiO coat-
h
2
terface between the SiO layer and metal substrate. The thermal
2
ing. The dielectric constant of anodic oxide films formed on speci-
mens without SiO coating is also plotted. The ␧ of the anodic
2
app
oxide films formed after SiO coating is much smaller than that
Table III. Change in the apparent dielectric constant, ␧app, with
2
heat-treatment temperature, T , for specimens anodized up to
without SiO coating, due to the formation of the composite oxide
h
2
E ϭ 400 V after SiO coating.
a
2
layer with a small dielectric constant. The slight decrease in ␧app
with increasing T can be explained in terms of a larger ␦ to ␦ ratio
h
c
a
for the anodic oxide films formed after the SiO coating at higher
2
Th . The dielectric constant of the outer composite oxide layer itself
cannot be obtained from the impedance measurements, since the
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B480
Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
These reactions occur in a molten slag at high temperatures with no
2
4
electric field. The network structure of silicon dioxide and P O5
2
tends to break up to form anions when Al O , MgO, and CaO are
2
3
mixed with SiO and P O . This is due to the covalent bonding
2
2
5
characteristics of SiO and P O , which cause a strong affinity with
2
2
5
2
Ϫ
O
.
The steeper slope of E vs. t curves for the SiO -coated speci-
a
a
2
men can be explained by the following. The growth of the inner and
2
Ϫ
3ϩ
outer layers is a result of the transport of O and Al ions. As-
3
ϩ
suming that the transport number of Al ions remains around 0.4
2
5
during anodizing, the growth rate of the outer layer would be
much higher than that of the inner layer. This is because the forma-
tion of the outer layer consumes electric charge only for repairing
cracks and voids in the SiO film. Hence, the total growth rate of the
2
Figure 13. Schematic illustration of the formation of the anodic oxide film
inner and outer layer is much higher than the growth rate of anodic
during anodizing after SiO coating.
oxide films on specimens without SiO in galvanostatic anodizing.
2
2
This leads to the steeper slope of the E vs. t curve on specimens
a
a
with SiO . The ability of the outer layer to sustain a higher potential
2
also plays a role in this phenomenon.
oxide film may convert partly to an Al-Si mixed oxide by the inter-
diffusion of Al3ϩ and Si4ϩ during heating.
Only slight dependence of the growth of anodic oxide films on
specimens with SiO on heating temperature, T , at the final stage
Water and electrolyte penetrates into the micropore network in
2
h
of the sol-gel method is difficult to explain. One could expect an
appreciable difference in the structure of SiO films coated at differ-
SiO film when dipping the SiO -coated specimen in anodizing so-
2
2
2
lution prior to anodizing. The small jump in E at the very initial
a
ent T because of the difference in the SiO film thickness ͑see Fig.
stage of anodizing is due to the thermal oxide layer formed during
h
2
6
2͒. Differences in the micropore network structure and chemical
anodizing. The increase in E after the initial jump corresponds to
a
composition of SiO may also give rise to a compensatory effect on
the growth of an anodic oxide film with double-layer structure, an
2
the conversion of the inner Al O layer to the outer Al-Si composite
inner Al O layer, and an outer Al-Si composite oxide layer. During
2
3
2
3
2Ϫ
layer, leading to the only slight dependence of the anodic oxide film
growth.
anodizing after SiO coating, O ions dissociated from water at the
2
bottom of the SiO layer transport inward across the anodic oxide
2
film to form Al O at the interface between the inner Al O layer
and metal substrate. Aluminum ions transport outward to form the
2
3
2
3
Conclusion
composite oxide layer by filling the micropores with Al O at the
Aluminum was anodized in a neutral borate solution after SiO2
coating by sol-gel processing to examine the dielectric properties
and formation mechanism of the anodic oxide films. The following
conclusions may be drawn.
. Anodizing of aluminum coated with SiO2 films by sol-gel
dip-coating leads to the formation of oxide films which consist of an
outer Al-Si composite oxide layer and an inner Al O layer at the
interface between the SiO layer and the metal substrate.
2
3
interface between the outer composite oxide layer and the SiO2
layer.
In addition to the formation of oxide at the two interfaces, a
1
conversion of Al O to composite oxide may occur at the interface
2
3
between the outer and inner layers due to the inward transport of
4
4
Ϫ
2Ϫ
SiO or SiO3 ions under the electric field across the composite
2
3
oxide layer. This causes the lower concentrations of Si and the
higher concentrations of Al deeper in the outer layer. The inward
transport of Si-bearing anions may also result in the rate of increase
in the outer layer thickness being higher than the rate of decrease in
2
2
. The outer and inner layers grow during anodizing, while the
SiO layer becomes thinner. The rate of increase in the outer layer
2
thickness is higher than that of the decrease in the SiO2 film. This
behavior can be explained by the conversion of Al2O3 to Al-Si com-
posite oxide at the interface between the inner and outer layers due
to the inward transport of Si-bearing anions across the outer layer.
3. The capacitance of anodic oxide films formed on specimens
with SiO2 coating is about 20% larger than that without SiO2 layer.
This is due to the formation of the Al-Si composite oxide layer,
which can sustain a higher electric field than Al2O3.
the SiO layer.
2
Considering the formation of the Si-bearing anions, it is well
known that anodic oxide films on aluminum contain electrolyte an-
ions from anodizing solutions, and that the electrolyte anions trans-
port inward or outward across anodic oxide films during anodizing.
Phosphate ions transport inward, and chromate and molybdate ions
transport outward.1
7-20
Borate ions move little in either
2
1-23
direction.
The outward transport is due to the formation of cat-
ions, like CrO , under a high electric field across the oxide, and
the inward transport is due to the formation of anions, like PO .
2
ϩ
Acknowledgments
2
Ϫ
3
The authors thank S. Toda, K. Nishinaka, and Y. Ohisa, Muroran
Institute of Technology, for assistance with the experiments on the
sol-gel coating, and Professor Emeritus S. Yamaguchi at Tohoku
University for the RBS measurement. The work was financially sup-
ported by the Light Metal Education Foundation of Japan, and the
Ministry of Education, Science, Sports and Culture, Japan, with a
Grant-in-Aid for Scientific Research. A part of this work was carried
out under the Visiting Researcher’s Program of the Institute for Ma-
terials Research, Tohoku University.
The formation of cations or anions under the electric field depends
on whether the affinity of metal ions with oxygen ions is stronger or
weaker than that of aluminum ions with oxygen ions. P͑V͒ with a
2
Ϫ
strong affinity to O forms anions, and Cr͑VI͒ with a weak affinity
2
Ϫ
to O forms cations. B͑III͒ does not form either cations or anions,
since its affinity is similar to that of Al͑III͒.
In the outer Al-Si composite oxide layer, Si forms a network
structure with oxygen ions, and under an electric field the incorpo-
ration of Al O may break the network structure locally by the fol-
lowing reaction
2
3
Hokkaido University assisted in meeting the publication costs of this
article.
2
Ϫ
ϩ
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SiO ϩ Al O ϭ SiO ϩ 2AlO
͓7͔
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Journal of The Electrochemical Society, 148 ͑11͒ B473-B481 ͑2001͒
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