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Microstructure evolution for oxide film of anodic aluminum foil used in high voltage electrolytic capacitor
Journal of Alloys and Compounds 823 (2020) 153795
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure evolution for oxide film of anodic aluminum foil used in high voltage electrolytic capacitor Sining Pan a, b, c, Libo Liang a, *, Baolin Lu a, Huibin Li a a
Postdoctoral Research Center, Guangxi Hezhou Guidong Electronics Technology Co. Ltd. Inc., China College of Mechanical and Electrical Engineering, Hezhou University, China c College of Chemistry and Bioengineering, Guilin University of Technology, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2019 Received in revised form 7 January 2020 Accepted 9 January 2020 Available online 11 January 2020
The oxide film of anodic aluminum foil is the main working medium of aluminum electrolytic capacitor, and its quality directly affects the electrical performance of capacitor. The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, including hydration, formation, heat treatment and phosphoric acid treatment. The microstructure evolution for oxide film of anodic aluminum foil during preparation process is studied quantitatively in this paper. The results show that, the pores area on the foil surface keeps decreasing after hydration and subsequent formation. However, after phosphoric acid treatment, the pores area stops decreasing and increases to a local maximum value. The length distribution of ‘corn-flake’ structures keeps increasing, from the range of 60e120 nm to 100 e140 nm. After phosphoric acid treatment, the petal edges become serrated, and many small corrosion holes with the length of 20e30 nm are left inside. The thickness of the outer amorphous layer in the anodized oxide film keeps decreasing from the initial 390 nm to the final 90 nm. In contrast, the thickness of the inner dense layer keeps increasing from the initial 130 nm to the final 550 nm. The decreasing trend of grain size continues until the end of preparation process, and reaches the final value of 10 nm. The analysis of microstructure of oxide film is helpful for obtaining optimal process parameters and improving performance of aluminum foil. © 2020 Elsevier B.V. All rights reserved.
Keywords: Anodic foil Oxide film Microstructure Aluminum electrolytic capacitor
1. Introduction Aluminum electrolytic capacitors are widely used in electronic devices because of the advantages of large specific capacitance and low costs [1,2]. The capacitor consists of an anodic foil on which an oxide is formed, a suitable electrolyte, and a cathode foil. A high performance capacitor requires high surface area coated with suitable dielectric material [3,4]. For aluminum electrolytic capacitors, the high surface area is obtained by electrochemical etching and subsequent tunnel-widening [5e7]. The dielectric film is grown directly on the surface by hydration treatment followed by anodization (formation). The oxide film of anodic aluminum foil is the main working medium of aluminum electrolytic capacitor, and its quality directly affects the electrical performance of capacitor. The dense gAl2O3 or g’Al2O3 layer with high crystallinity is beneficial to the improvement of specific capacitance and
* Corresponding author. E-mail address: [email protected] (L. Liang). https://doi.org/10.1016/j.jallcom.2020.153795 0925-8388/© 2020 Elsevier B.V. All rights reserved.
withstand voltage. Therefore, it is necessary to study the microstructure evolution of oxide film during preparation process, in order to optimize process parameters and improve performance of aluminum foil. In recent years, the multiple-step anodizing process with hydration and subsequent formation treatment in boric acid solution at different voltages has been widely applied [8e10]. Alwitt et al. [11,12] found that the hydration of aluminum foil in hot water before formation can not only save electrical power in the anodization, but also improve the crystallinity. The obtained oxide film consists of two layers of crystalline barrier g’Al2O3 containing a high density of nanovoids, covered with a layer of residual hydrous oxide [13]. The effects of SiO44, PO3 4 and citric acid anions on the formation of hydrated oxide film in deionized water were studied by Vedder et al. [14,15]. Chi et al. [16] found that hydrous oxide layer was more effective for the transition of amorphous anodic oxides to crystalline aluminum oxides. Ban et al. [17e20] investigated the effect of citric acid, tartaric acid, modified hydration treatment and hydration time on the microstructure and electrochemical characteristics of high voltage anodized alumina film formed on etched
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Al Foils. Since the multiple-step anodizing process has been of technological importance in the manufacture of electrolytic capacitors, however, it has received insufficient attention in the scientific literature. Therefore, the microstructure evolution of the oxide film, which changes at all times during preparation process, deserves further discussion. What’s more, detailed knowledge of preparation process, especially the quantitative analysis of the microstructure of oxide film, is still required, in order for the optimization of process parameters and improvement of performance effectively. The main objective of this paper is to analyze the microstructure evolution for oxide film of anodic aluminum foil during preparation process. A multiple-step anodizing process is used for the fabrication of anodic aluminum foil, including hydration, formation, heat treatment and phosphoric acid treatment. The FE-SEM and XRD technology are applied for the observation and detection of microstructure. The quantitative analysis of microstructure, pores area and length distribution of petal size are conducted based on the surface micrograph, while the thickness of outer layer and inner layer is discussed based on the micrograph of cross section. The evolution of grain size is measured by the utilization of XRD results and Scherrer formula. 2. Experimental procedure 2.1. Sample preparation The high purity (>99.99%) and cubicity texture (>95%) commercial tunnel-ecthed aluminum foil is used as specimens, with a thickness of 115 mm. The surface area is increased about 50 times by electrochemical etching. Tunnels follow a <100> direction, and align mostly normal to the surface which are caused by the metal texture. The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, including hydration, formation, heat treatment and phosphoric acid (H3PO4) treatment. The schematic diagram of preparation process is shown in Fig. 1, and the detailed information of all treatment processes is listed in Table 1. One-sided 50 50 mm2 surface area of specimens are prepared and exposed to electrolytes. The normal current density (the current applied on per unit area of aluminum foil) is 25 mA cm2. Considering the 50 times increase of surface area after electrochemical etching, the specific current density (the current applied on actual surface area of aluminum foil after electrochemical etching) is estimated to be 0.5 mA cm2. The normal formation voltages of different stages are 330 V and 550 V, respectively. 2.2. Microstructure characterization The surface and cross-sectional morphologies of the samples are observed using field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F). In order to obtain the cross-sectional samples, the aluminum foils are dipped into 10 wt% iodinemethanol solution with a constant temperature of 50 C for 8 h. During the procedure, aluminum substrate is dissolved in solution completely, and then the anodic oxide films are retained for observation. Prior to FE-SEM characterization, the as-anodized aluminum foil specimens are gold-sputtered for observing higher resolutions. The pores area and size of petal structure obtained from the FE-SEM are analyzed and calculated by using ImagePro software. The FE-SEM images are binarized firstly, and then the feature areas with pixel difference are filtered out for calculation and statistical analysis. The crystallographic structure of anodic films on Al substrate is determined by a high power X-ray diffactometer (Ultima IV). The incident radiation is obtained from a high power ceramic tube with
Fig. 1. Schematic diagram of preparation process.
copper (Cu) anode operating at 40 kV and 40 mA. 2-Theta scans are performed on a range of 10 ~80 . The samples are measured in a continuous mode with 0.02 step size and a scan speed of 10 per minute. 3. Results and discussion A surface micrograph of the Al substrate used for film growth is presented in Fig. 2 (a). About 1e2 mm radius tunnels along <100> directions can be observed in the Al substrate, which is a result of electrochemical etching and subsequent widening. At the same time, several hundred nm length nucleation sites are produced on the Al surface. These are the early oxide formation sites when exposed to pure deionized water at 95 C for as short as 15s [21]. Fig. 2 (b) shows the hydrous oxide film formed by reaction of Al substrate with deionized water at 95 C for 10 min. As can be seen, fine ‘corn-flake’ structures are formed upon immersion in hot water, which partly block the tunnel. After the stage of ‘Formation I’ (see in Fig. 2 (c)), the oxide with ‘corn-flake’ structures are better
S. Pan et al. / Journal of Alloys and Compounds 823 (2020) 153795
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Table 1 Detailed information of preparation process. Sample No.
Process
Voltage/V
Time/min
Temperature/ C
Solution
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# 12#
Etched foil Hydration Formation I Formation II-1 Heat treatment I Formation II-2 H3PO4 treatment I Formation III-1 Heat treatment II Formation III-2 H3PO4 treatment II Formed foil
/ / 330 550 / 550 / 550 / 550 / /
/ 10 15 30 4 10 10 10 4 10 10 4
/ 95 90 90 500 90 60 90 500 90 35 250
/ deionized water 3% boric acidþ0.1% citric acidþ0.1% ammonium adipate 6% boric acid / 6% boric acid 1.5% phosphoric acid 6% boric acid / 6% boric acid 0.2% phosphoric acid /
Fig. 2. FE-SEM micrograph of surface during preparation process.
refined and flower-like. In addition, the formed oxide increases the blockage of tunnel pore. Experiencing the following formation at higher voltage, heat treatment and phosphoric acid treatment, the surface morphology of Al foil changes significantly. The fine ‘cornflake’ structure is damaged, and the blockage of tunnel pore is relieved to some extent, as shown in Fig. 2 (d). As aluminum electrolytic capacitor is made, the electrolyte enters into the tunnel pore of Al foil, and the working medium is the oxide layer on the foil surface. If the oxide layer blocks the tunnel pore completely, the electrolyte can’t be in full contact with the oxide layer, which affected its performance. Since the tunnel pore shows three dimensional shape, it is difficult to characterize the pore blocking directly, based on the present testing methods. Therefore, it is a common way to simplify a complex threedimensional problem into a simple two-dimensional one. In order to analyze the pore blocking quantitatively, the pores area on the Al foil surface is measured based on the FE-SEM micrograph
such as shown in Fig. 2. As for eliminating artificial error, three different images are selected randomly, to calculate the pores area (expressed in pixels) for each step during the preparation process. The measurement results are listed in Table 2, and the average value is taken for comparison. The pores area of sample 1# is defined as the initial value of normalization, and the normalized area of pores during preparation process is illustrated in Fig. 3. It can be seen from Fig. 3 that, the pores area on the foil surface keeps decreasing after “Hydration” (2#), “Formation I” and “Formation II” (3#, 4# and 6#), including “Heat treatment I” (5#). However, after “H3PO4 treatment I” (7#), the pores area stops decreasing, and becomes increasing to a local maximum value. During subsequent preparation, the pores area shows similar tendency, which performs decreasing previously and increasing later. The second local maximum value appears at the point after “H3PO4 treatment II” (11#). Compared with the “Formation II-2” (6#) and the “Formation III-2” (10#), the pores area slightly increases,
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Table 2 Measurement results of pores area. Sample No.
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# 12#
Pores area (in pixels)
Average
Field 1
Field 2
Field 3
310801 213512 136679 131120 70144 77854 132767 177908 136752 71302 173961 152976
309569 194230 110417 107193 87408 80226 144159 161336 102882 84664 178102 141917
353138 195358 134990 124886 93994 63245 141282 151131 128478 91313 141470 164990
324502 201033 127362 121066 83848 73775 139402 126791 122704 82426 164511 153294
Fig. 3. Evolution of pores area during preparation process.
indicating that the effect of multiple-step anodizing process. The result shows that the multiple-step anodizing process can effectively alleviate the pore blockage on the foil surface which is induced by the formed oxide film. The ‘corn-flake’ structures shown in Fig. 2, is the hydrous oxide films prepared by hydration in hot water, with the structure of crystalline aluminum hydrous oxide [AlO(OH)] films actually [3]. As the preparation process going on, the microstructures of hydrous oxide film change, and a growth of the petal structures is noticed, as shown in Fig. 4. The petal size of 10 min hydration film (Fig. 4 (a-1) and (a-2)) lies mainly from 60e120 nm, however, the length rage of petal size after ‘Formation I’ (Fig. 4 (b-1)and (b-2)) increases to 80e140 nm. During the following process, the petal size keeps increasing, and the final length distribution (Fig. 4 (c-1)and (c-2)) is mainly located between 100e140 nm. As the above analysis of Fig. 3, after phosphoric acid treatment, the pores area increases, and the pores blockage on the foil surface is relieved to some extent. The presence of anion PO3 in the 4 phosphoric acid solution is found to delay the oxide formation [22]. The effect of phosphoric acid treatment on the microstructure of oxide film is presented in Fig. 5. Fig. 5 (a) and Fig. 5 (b) is the micrograph of oxide film before and after the “H3PO4 treatment I”, respectively. Experiencing the “H3PO4 treatment I”, the petal edges become serrated. The inside of petal structure is destroyed slightly, showing small amount of corrosion holes. As shown in Fig. 5 (c), the microstructure of petal structure, including the edge and inside, has
been recovered by the subsequent formation and heat treatment. However, as illustrated in Fig. 5 (d), more serious damage is produced by the “H3PO4 treatment II”. The inserted figure with red border in (d) is the magnified version of petal structure of 11#. More small holes appear inside the petal structure, which are with the length of 20e30 nm. The increase of pores area on the foil surface of aluminum after phosphoric acid treatment, is essentially the same as the damage of petal structure. It is the result of partial dissolution of hydrated oxide film produced by phosphoric acid solution. The dissolution mode of phosphoric acid is perforation-type, rather than planetype, which can be reflected in the comparison of microstructure in Fig. 5. The defects such as gaps and voids in the anodized film are due to the volume shrinkage caused by the crystal transformation [17,18]. At the end of the ‘Formation II’ and ‘Formation III’, the addition of phosphoric acid treatment (7# and 11#) can remove part of the outer layer in the oxide film, open the closed part of the crack within oxide film, and improve the crystallinity of oxide film. In order to better understand the microstructure evolution of oxide film, the samples of cross section are observed by FE-SEM. Samples of cross section after “Hydration” (2#), “Formation I” (3#) and the final formed foil (12#) are displayed in Fig. 6, respectively. Fig. 6 (b), (d) and (f) are the magnified version of the area with orange border in Fig. 6 (a), (c) and (e), respectively. The distance between the two solid lines in Fig. 6 (b), (d), (f) is the thickness of the inner layer. The distance between the middle solid line and the dotted line in Fig. 6 (b), (d), (f) is the thickness of the outer dense layer. As demonstrated in Fig. 6 (a) and (b), the hydrous oxide film of aluminum foil is double-layer structure. The inner layer closed to the metal (between the two solid lines) is a high€ density layer, and the structure is similar to Pseudo BOhmite gAl2O3$2H2O (PB), which is easy to convert into g-Al2O3 during formation process. The outer layer (from the middle solid line to the dotted line) is fibrous, and the root shows concave-convex shape. It is not easy for the outer layer to transform into g - Al2O3 due to the large water content. It can be measured from Fig. 6 (b) that, the thickness of dense inner layer and fibrous outer layer is about 130 nm and 390 nm, respectively, which is basically consistent with the results in reference [13,20]. In the subsequent formation process, a barrier oxide appears between the aluminum surface and the PB film, as presented in Fig. 6 (d) and Fig. 6 (f). The final barrier oxide composes of two layers, the inner layer from crystallization of amorphous anodic oxide, and the outer dense layer from PB transformation which is related to the fibrous structure and petal structure stated before [13]. The thickness of inner layer and outer layer after “Formation I00 is about 400 nm and 180 nm, respectively, while 550 nm and 90 nm for the final sample (12#). The dense gAl2O3 or g’Al2O3 film is the working medium of aluminum electrolytic capacitor, which contributes to the specific capacitance and withstand voltage [2]. During the preparation process, the hydrated oxide film is hoped to completely converted into the dense anodic oxide film. Therefore, it is necessary to analyze the thickness evolution of oxide film during preparation process. The measurement of thickness is based on the micrograph of cross section. Three different positions are selected and detected for each sample, and the average value is considered for the comparation. The thickness evolution of oxide layer is demonstrated in Fig. 7. The thickness of the outer layer composed of petal structure keeps decreasing in the whole process, from the initial value of 390 nm to the final value of 90 nm. In addition, the descent rate in the stage of ‘Formation II’ is faster than in the ‘Formation III’ stage. In contrast, the thickness of the dense layer keeps increasing throughout the whole process, from the initial value of 130 nm to the final value of 550 nm. It is noted that, in the stage of ‘Formation
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Fig. 4. Surface micrograph and distribution of petal size during preparation process.
Fig. 5. FE-SEM micrograph of surface before and after H3PO4 treatment. (a) 6#, (b) 7#, (c) 10#, (d) 11#. The inserted figure with red border in (d) is the magnified version of petal structure of 11#. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
II’ and ‘Formation III’, along with the formation voltage increasing to 550 V, the thickness of dense layer increases rapidly to about 550 nm, which is consistent with the conclusion obtained before
[12]. The oxide thickness obtained in formation is proportional to the voltage used in the anodization. Approximately 1 nm of oxide is obtained per volt. Although the thickness of dense layer seems
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Fig. 6. FE-SEM micrograph of cross section during preparation process.(a) (b) 2#, (c) (d) 3#, (e) (f) 12#. (b), (d) and (f) are the magnified version of the area with orange border in (a), (c) and (e), respectively. The distance between the two solid lines in (b), (d), (f) is the thickness of the inner layer. The distance between the middle solid line and the dotted line in (b), (d), (f) is the thickness of the outer dense layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
unchanged, fluctuating up and down around 550 nm, however, the crystal and microstructure inside are undergoing dramatic changes, which will be analyzed in the following. Fig. 8 shows the XRD patterns of aluminum foil during preparation process. It is indicated that the strongest peak corresponds to Al (200), and other distinct peaks comes from crystalline oxide in barrier films, marked by g’ Al2O3 (400) and g’ Al2O3 (440). g’ Al2O3 is a kind of defect spinel which is similar to g-Al2O3, however, with more disorder on the cation lattice. The Al (200) peak and g’ Al2O3 (400) peak overlap because of the similar diffraction angle, which can be observed in the left insert curves in Fig. 8. In addition, the intensity of diffraction peak g’ Al2O3 (440) increases gradually during the preparation process, as shown in the right insert curves in Fig. 8, indicating the change of grain size. In order to quantify the change of grain size in the preparation process, the grain size is calculated by XRD results. The crystallinity analysis of the anodic oxide films is based on the width of the
g’ Al2O3 (440) peak and the Scherrer formula d ¼ kl/(B$cosq). In that formula, d is the crystallite size, k is a form factor (k ¼ 0.89 for spheres particles), l is the wavelength of the radiation used (lCu,Ka1 ¼ 1.54056 Å), B is half height width of selected peaks. The calculated results of g’ Al2O3 grain size are plotted in Fig. 9, showing a gradual downward trend. The grain size is about 25 nm after hydration treatment. Once the formation process begins, the grain size sharply reduces to below 15 nm. The decreasing trend continues until the end of preparation process, and achieves the final value of 10 nm. The smaller grain size means higher density of oxide film and a lower porosity, which is with better performance under working condition. 4. Conclusion The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, and the microstructure evolution
S. Pan et al. / Journal of Alloys and Compounds 823 (2020) 153795
7
for oxide film of anodic aluminum foil is investigated in this paper. The main conclusions are listed as following:
Fig. 7. Thickness evolution of oxide layer during preparation process.
(1) During the multiple-step anodizing process, the formed oxide increases the blockage of tunnel pore. The pores area on the foil surface keeps decreasing after hydration and the subsequent formation. However, after phosphoric acid treatment, the pores area stops decreasing and increases to a local maximum value. (2) The ‘corn-flake’ structures is the hydrous oxide films with the structure of crystalline aluminum hydrous oxide [AlO(OH)] films. The petal size of 10 min hydration film lies mainly from 60e120 nm, while increasing to 80e140 nm after ‘Formation I’. During the following process, the petal size keeps increasing, and the final length distribution is mainly located between 100e140 nm. (3) Experiencing the phosphoric acid treatment, the petal edges become serrated. The inside of petal structure is destroyed slightly, showing small amount of corrosion holes. Although the petal structure is recovered by the subsequent process, more small holes with the length of 20e30 nm appear inside. (4) The anodized oxide film composes of two layers, the inner layer from crystallization of amorphous anodic oxide, and the outer dense layer from PB transformation. The thickness of the outer layer keeps decreasing from the initial 390 nm to the final 90 nm. In contrast, the thickness of the dense layer keeps increasing from the initial 130 nm to the final 550 nm. (5) Along with the preparation process, the g’ Al2O3 grain size shows a gradual downward trend. The decreasing trend continues until the end of preparation process, and reaches the final value of 10 nm.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Fig. 8. XRD patterns of aluminum foil during preparation process.
Sining Pan: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Libo Liang: Supervision, Writing - original draft, Writing - review & editing. Baolin Lu: Investigation, Data curation. Huibin Li: Resources, Data curation. Acknowledgments This research was supported by the Project fund by China Postdoctoral Science Foundation (2019M663871XB), Postdoctoral Science Foundation of Guangxi Province of China, the Doctor’s Scientific Research Foundation of Hezhou University (HZUBS201806), the Foundation for Research and Development of Science and Technology of Hezhou City (201908007), and the Innovation-driven Development Project of Hezhou City (ZX1907001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153795. References
Fig. 9. Evolution of g’ Al2O3 grain size during preparation process.
[1] F. Chen, S. Park, Formation behavior of multi-layer oxide films on etched Al
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Electrochim. Acta 23 (1978) 355e362. [13] H. Uchi, T. Kanno, R.S. Alwitt, Structural features of crystalline anodic alumina films, J. Electrochem. Soc. 148 (2001) 17e23. [14] W. Vedder, D. Vermiyea, Aluminum water reaction, Trans. Faraday Soc. 65 (1969) 561e568. [15] H. Takahashi, M. Yamaki, R. Furuichi, The role of anions in the formation of hydroxide films on aluminum in hot aqueous solution, Corros. Sci. 31 (1990) 243e248. [16] C. Chi, Y. Jeong, H. Ahn, J. Lee, J. Kim, J. Lee, K. Jang, H. Oh, Transition of hydrated oxide layer for aluminum electrolytic capacitors, Mater. Sci. Eng. A 449e451 (2007) 314e317. [17] C. Ban, Y. He, X. Shao, Effect of citric acid on microstructure and electrochemical characteristics of high voltage anodized alumina film formed on etched Al Foils, Trans. Nonferrous Metals Soc. China 21 (2011) 133e138. [18] C. Ban, Y. He, X. Shao, Effect of trace tartaric acid on anodizing of etched aluminum foil for high voltage electrolytic capacitor, J. Mater. Sci. Mater. Electron. 24 (2013) 3442e3447. [19] C. Ban, Y. He, X. Shao, L. Wang, Anodizing of etched aluminum foil coated with modified hydrous oxide film for aluminum electrolytic capacitor, J. Mater. Sci. Mater. Electron. 25 (2013) 128e133. [20] C. Ban, F. Wang, J. Chen, Z. Liu, Effect of hydration on microstructure and property of anodized oxide film for aluminum electrolytic capacitor, J. Mater. Sci. Mater. Electron. 29 (2018) 16166e16171. [21] A. Geiculescu, A microstructural investigation of low-temperature crystalline alumina films grown on aluminum, Thin Solid Films 426 (2003) 160e171. [22] W. Mao, Y. He, Principles on Fabrication of Aluminum Foils for Electrolytic Capacitors, Higher Education Press, Beijing, 2012.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure evolution for oxide film of anodic aluminum foil used in high voltage electrolytic capacitor Sining Pan a, b, c, Libo Liang a, *, Baolin Lu a, Huibin Li a a
Postdoctoral Research Center, Guangxi Hezhou Guidong Electronics Technology Co. Ltd. Inc., China College of Mechanical and Electrical Engineering, Hezhou University, China c College of Chemistry and Bioengineering, Guilin University of Technology, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2019 Received in revised form 7 January 2020 Accepted 9 January 2020 Available online 11 January 2020
The oxide film of anodic aluminum foil is the main working medium of aluminum electrolytic capacitor, and its quality directly affects the electrical performance of capacitor. The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, including hydration, formation, heat treatment and phosphoric acid treatment. The microstructure evolution for oxide film of anodic aluminum foil during preparation process is studied quantitatively in this paper. The results show that, the pores area on the foil surface keeps decreasing after hydration and subsequent formation. However, after phosphoric acid treatment, the pores area stops decreasing and increases to a local maximum value. The length distribution of ‘corn-flake’ structures keeps increasing, from the range of 60e120 nm to 100 e140 nm. After phosphoric acid treatment, the petal edges become serrated, and many small corrosion holes with the length of 20e30 nm are left inside. The thickness of the outer amorphous layer in the anodized oxide film keeps decreasing from the initial 390 nm to the final 90 nm. In contrast, the thickness of the inner dense layer keeps increasing from the initial 130 nm to the final 550 nm. The decreasing trend of grain size continues until the end of preparation process, and reaches the final value of 10 nm. The analysis of microstructure of oxide film is helpful for obtaining optimal process parameters and improving performance of aluminum foil. © 2020 Elsevier B.V. All rights reserved.
Keywords: Anodic foil Oxide film Microstructure Aluminum electrolytic capacitor
1. Introduction Aluminum electrolytic capacitors are widely used in electronic devices because of the advantages of large specific capacitance and low costs [1,2]. The capacitor consists of an anodic foil on which an oxide is formed, a suitable electrolyte, and a cathode foil. A high performance capacitor requires high surface area coated with suitable dielectric material [3,4]. For aluminum electrolytic capacitors, the high surface area is obtained by electrochemical etching and subsequent tunnel-widening [5e7]. The dielectric film is grown directly on the surface by hydration treatment followed by anodization (formation). The oxide film of anodic aluminum foil is the main working medium of aluminum electrolytic capacitor, and its quality directly affects the electrical performance of capacitor. The dense gAl2O3 or g’Al2O3 layer with high crystallinity is beneficial to the improvement of specific capacitance and
* Corresponding author. E-mail address: [email protected] (L. Liang). https://doi.org/10.1016/j.jallcom.2020.153795 0925-8388/© 2020 Elsevier B.V. All rights reserved.
withstand voltage. Therefore, it is necessary to study the microstructure evolution of oxide film during preparation process, in order to optimize process parameters and improve performance of aluminum foil. In recent years, the multiple-step anodizing process with hydration and subsequent formation treatment in boric acid solution at different voltages has been widely applied [8e10]. Alwitt et al. [11,12] found that the hydration of aluminum foil in hot water before formation can not only save electrical power in the anodization, but also improve the crystallinity. The obtained oxide film consists of two layers of crystalline barrier g’Al2O3 containing a high density of nanovoids, covered with a layer of residual hydrous oxide [13]. The effects of SiO44, PO3 4 and citric acid anions on the formation of hydrated oxide film in deionized water were studied by Vedder et al. [14,15]. Chi et al. [16] found that hydrous oxide layer was more effective for the transition of amorphous anodic oxides to crystalline aluminum oxides. Ban et al. [17e20] investigated the effect of citric acid, tartaric acid, modified hydration treatment and hydration time on the microstructure and electrochemical characteristics of high voltage anodized alumina film formed on etched
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Al Foils. Since the multiple-step anodizing process has been of technological importance in the manufacture of electrolytic capacitors, however, it has received insufficient attention in the scientific literature. Therefore, the microstructure evolution of the oxide film, which changes at all times during preparation process, deserves further discussion. What’s more, detailed knowledge of preparation process, especially the quantitative analysis of the microstructure of oxide film, is still required, in order for the optimization of process parameters and improvement of performance effectively. The main objective of this paper is to analyze the microstructure evolution for oxide film of anodic aluminum foil during preparation process. A multiple-step anodizing process is used for the fabrication of anodic aluminum foil, including hydration, formation, heat treatment and phosphoric acid treatment. The FE-SEM and XRD technology are applied for the observation and detection of microstructure. The quantitative analysis of microstructure, pores area and length distribution of petal size are conducted based on the surface micrograph, while the thickness of outer layer and inner layer is discussed based on the micrograph of cross section. The evolution of grain size is measured by the utilization of XRD results and Scherrer formula. 2. Experimental procedure 2.1. Sample preparation The high purity (>99.99%) and cubicity texture (>95%) commercial tunnel-ecthed aluminum foil is used as specimens, with a thickness of 115 mm. The surface area is increased about 50 times by electrochemical etching. Tunnels follow a <100> direction, and align mostly normal to the surface which are caused by the metal texture. The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, including hydration, formation, heat treatment and phosphoric acid (H3PO4) treatment. The schematic diagram of preparation process is shown in Fig. 1, and the detailed information of all treatment processes is listed in Table 1. One-sided 50 50 mm2 surface area of specimens are prepared and exposed to electrolytes. The normal current density (the current applied on per unit area of aluminum foil) is 25 mA cm2. Considering the 50 times increase of surface area after electrochemical etching, the specific current density (the current applied on actual surface area of aluminum foil after electrochemical etching) is estimated to be 0.5 mA cm2. The normal formation voltages of different stages are 330 V and 550 V, respectively. 2.2. Microstructure characterization The surface and cross-sectional morphologies of the samples are observed using field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F). In order to obtain the cross-sectional samples, the aluminum foils are dipped into 10 wt% iodinemethanol solution with a constant temperature of 50 C for 8 h. During the procedure, aluminum substrate is dissolved in solution completely, and then the anodic oxide films are retained for observation. Prior to FE-SEM characterization, the as-anodized aluminum foil specimens are gold-sputtered for observing higher resolutions. The pores area and size of petal structure obtained from the FE-SEM are analyzed and calculated by using ImagePro software. The FE-SEM images are binarized firstly, and then the feature areas with pixel difference are filtered out for calculation and statistical analysis. The crystallographic structure of anodic films on Al substrate is determined by a high power X-ray diffactometer (Ultima IV). The incident radiation is obtained from a high power ceramic tube with
Fig. 1. Schematic diagram of preparation process.
copper (Cu) anode operating at 40 kV and 40 mA. 2-Theta scans are performed on a range of 10 ~80 . The samples are measured in a continuous mode with 0.02 step size and a scan speed of 10 per minute. 3. Results and discussion A surface micrograph of the Al substrate used for film growth is presented in Fig. 2 (a). About 1e2 mm radius tunnels along <100> directions can be observed in the Al substrate, which is a result of electrochemical etching and subsequent widening. At the same time, several hundred nm length nucleation sites are produced on the Al surface. These are the early oxide formation sites when exposed to pure deionized water at 95 C for as short as 15s [21]. Fig. 2 (b) shows the hydrous oxide film formed by reaction of Al substrate with deionized water at 95 C for 10 min. As can be seen, fine ‘corn-flake’ structures are formed upon immersion in hot water, which partly block the tunnel. After the stage of ‘Formation I’ (see in Fig. 2 (c)), the oxide with ‘corn-flake’ structures are better
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Table 1 Detailed information of preparation process. Sample No.
Process
Voltage/V
Time/min
Temperature/ C
Solution
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# 12#
Etched foil Hydration Formation I Formation II-1 Heat treatment I Formation II-2 H3PO4 treatment I Formation III-1 Heat treatment II Formation III-2 H3PO4 treatment II Formed foil
/ / 330 550 / 550 / 550 / 550 / /
/ 10 15 30 4 10 10 10 4 10 10 4
/ 95 90 90 500 90 60 90 500 90 35 250
/ deionized water 3% boric acidþ0.1% citric acidþ0.1% ammonium adipate 6% boric acid / 6% boric acid 1.5% phosphoric acid 6% boric acid / 6% boric acid 0.2% phosphoric acid /
Fig. 2. FE-SEM micrograph of surface during preparation process.
refined and flower-like. In addition, the formed oxide increases the blockage of tunnel pore. Experiencing the following formation at higher voltage, heat treatment and phosphoric acid treatment, the surface morphology of Al foil changes significantly. The fine ‘cornflake’ structure is damaged, and the blockage of tunnel pore is relieved to some extent, as shown in Fig. 2 (d). As aluminum electrolytic capacitor is made, the electrolyte enters into the tunnel pore of Al foil, and the working medium is the oxide layer on the foil surface. If the oxide layer blocks the tunnel pore completely, the electrolyte can’t be in full contact with the oxide layer, which affected its performance. Since the tunnel pore shows three dimensional shape, it is difficult to characterize the pore blocking directly, based on the present testing methods. Therefore, it is a common way to simplify a complex threedimensional problem into a simple two-dimensional one. In order to analyze the pore blocking quantitatively, the pores area on the Al foil surface is measured based on the FE-SEM micrograph
such as shown in Fig. 2. As for eliminating artificial error, three different images are selected randomly, to calculate the pores area (expressed in pixels) for each step during the preparation process. The measurement results are listed in Table 2, and the average value is taken for comparison. The pores area of sample 1# is defined as the initial value of normalization, and the normalized area of pores during preparation process is illustrated in Fig. 3. It can be seen from Fig. 3 that, the pores area on the foil surface keeps decreasing after “Hydration” (2#), “Formation I” and “Formation II” (3#, 4# and 6#), including “Heat treatment I” (5#). However, after “H3PO4 treatment I” (7#), the pores area stops decreasing, and becomes increasing to a local maximum value. During subsequent preparation, the pores area shows similar tendency, which performs decreasing previously and increasing later. The second local maximum value appears at the point after “H3PO4 treatment II” (11#). Compared with the “Formation II-2” (6#) and the “Formation III-2” (10#), the pores area slightly increases,
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Table 2 Measurement results of pores area. Sample No.
1# 2# 3# 4# 5# 6# 7# 8# 9# 10# 11# 12#
Pores area (in pixels)
Average
Field 1
Field 2
Field 3
310801 213512 136679 131120 70144 77854 132767 177908 136752 71302 173961 152976
309569 194230 110417 107193 87408 80226 144159 161336 102882 84664 178102 141917
353138 195358 134990 124886 93994 63245 141282 151131 128478 91313 141470 164990
324502 201033 127362 121066 83848 73775 139402 126791 122704 82426 164511 153294
Fig. 3. Evolution of pores area during preparation process.
indicating that the effect of multiple-step anodizing process. The result shows that the multiple-step anodizing process can effectively alleviate the pore blockage on the foil surface which is induced by the formed oxide film. The ‘corn-flake’ structures shown in Fig. 2, is the hydrous oxide films prepared by hydration in hot water, with the structure of crystalline aluminum hydrous oxide [AlO(OH)] films actually [3]. As the preparation process going on, the microstructures of hydrous oxide film change, and a growth of the petal structures is noticed, as shown in Fig. 4. The petal size of 10 min hydration film (Fig. 4 (a-1) and (a-2)) lies mainly from 60e120 nm, however, the length rage of petal size after ‘Formation I’ (Fig. 4 (b-1)and (b-2)) increases to 80e140 nm. During the following process, the petal size keeps increasing, and the final length distribution (Fig. 4 (c-1)and (c-2)) is mainly located between 100e140 nm. As the above analysis of Fig. 3, after phosphoric acid treatment, the pores area increases, and the pores blockage on the foil surface is relieved to some extent. The presence of anion PO3 in the 4 phosphoric acid solution is found to delay the oxide formation [22]. The effect of phosphoric acid treatment on the microstructure of oxide film is presented in Fig. 5. Fig. 5 (a) and Fig. 5 (b) is the micrograph of oxide film before and after the “H3PO4 treatment I”, respectively. Experiencing the “H3PO4 treatment I”, the petal edges become serrated. The inside of petal structure is destroyed slightly, showing small amount of corrosion holes. As shown in Fig. 5 (c), the microstructure of petal structure, including the edge and inside, has
been recovered by the subsequent formation and heat treatment. However, as illustrated in Fig. 5 (d), more serious damage is produced by the “H3PO4 treatment II”. The inserted figure with red border in (d) is the magnified version of petal structure of 11#. More small holes appear inside the petal structure, which are with the length of 20e30 nm. The increase of pores area on the foil surface of aluminum after phosphoric acid treatment, is essentially the same as the damage of petal structure. It is the result of partial dissolution of hydrated oxide film produced by phosphoric acid solution. The dissolution mode of phosphoric acid is perforation-type, rather than planetype, which can be reflected in the comparison of microstructure in Fig. 5. The defects such as gaps and voids in the anodized film are due to the volume shrinkage caused by the crystal transformation [17,18]. At the end of the ‘Formation II’ and ‘Formation III’, the addition of phosphoric acid treatment (7# and 11#) can remove part of the outer layer in the oxide film, open the closed part of the crack within oxide film, and improve the crystallinity of oxide film. In order to better understand the microstructure evolution of oxide film, the samples of cross section are observed by FE-SEM. Samples of cross section after “Hydration” (2#), “Formation I” (3#) and the final formed foil (12#) are displayed in Fig. 6, respectively. Fig. 6 (b), (d) and (f) are the magnified version of the area with orange border in Fig. 6 (a), (c) and (e), respectively. The distance between the two solid lines in Fig. 6 (b), (d), (f) is the thickness of the inner layer. The distance between the middle solid line and the dotted line in Fig. 6 (b), (d), (f) is the thickness of the outer dense layer. As demonstrated in Fig. 6 (a) and (b), the hydrous oxide film of aluminum foil is double-layer structure. The inner layer closed to the metal (between the two solid lines) is a high€ density layer, and the structure is similar to Pseudo BOhmite gAl2O3$2H2O (PB), which is easy to convert into g-Al2O3 during formation process. The outer layer (from the middle solid line to the dotted line) is fibrous, and the root shows concave-convex shape. It is not easy for the outer layer to transform into g - Al2O3 due to the large water content. It can be measured from Fig. 6 (b) that, the thickness of dense inner layer and fibrous outer layer is about 130 nm and 390 nm, respectively, which is basically consistent with the results in reference [13,20]. In the subsequent formation process, a barrier oxide appears between the aluminum surface and the PB film, as presented in Fig. 6 (d) and Fig. 6 (f). The final barrier oxide composes of two layers, the inner layer from crystallization of amorphous anodic oxide, and the outer dense layer from PB transformation which is related to the fibrous structure and petal structure stated before [13]. The thickness of inner layer and outer layer after “Formation I00 is about 400 nm and 180 nm, respectively, while 550 nm and 90 nm for the final sample (12#). The dense gAl2O3 or g’Al2O3 film is the working medium of aluminum electrolytic capacitor, which contributes to the specific capacitance and withstand voltage [2]. During the preparation process, the hydrated oxide film is hoped to completely converted into the dense anodic oxide film. Therefore, it is necessary to analyze the thickness evolution of oxide film during preparation process. The measurement of thickness is based on the micrograph of cross section. Three different positions are selected and detected for each sample, and the average value is considered for the comparation. The thickness evolution of oxide layer is demonstrated in Fig. 7. The thickness of the outer layer composed of petal structure keeps decreasing in the whole process, from the initial value of 390 nm to the final value of 90 nm. In addition, the descent rate in the stage of ‘Formation II’ is faster than in the ‘Formation III’ stage. In contrast, the thickness of the dense layer keeps increasing throughout the whole process, from the initial value of 130 nm to the final value of 550 nm. It is noted that, in the stage of ‘Formation
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Fig. 4. Surface micrograph and distribution of petal size during preparation process.
Fig. 5. FE-SEM micrograph of surface before and after H3PO4 treatment. (a) 6#, (b) 7#, (c) 10#, (d) 11#. The inserted figure with red border in (d) is the magnified version of petal structure of 11#. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
II’ and ‘Formation III’, along with the formation voltage increasing to 550 V, the thickness of dense layer increases rapidly to about 550 nm, which is consistent with the conclusion obtained before
[12]. The oxide thickness obtained in formation is proportional to the voltage used in the anodization. Approximately 1 nm of oxide is obtained per volt. Although the thickness of dense layer seems
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Fig. 6. FE-SEM micrograph of cross section during preparation process.(a) (b) 2#, (c) (d) 3#, (e) (f) 12#. (b), (d) and (f) are the magnified version of the area with orange border in (a), (c) and (e), respectively. The distance between the two solid lines in (b), (d), (f) is the thickness of the inner layer. The distance between the middle solid line and the dotted line in (b), (d), (f) is the thickness of the outer dense layer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
unchanged, fluctuating up and down around 550 nm, however, the crystal and microstructure inside are undergoing dramatic changes, which will be analyzed in the following. Fig. 8 shows the XRD patterns of aluminum foil during preparation process. It is indicated that the strongest peak corresponds to Al (200), and other distinct peaks comes from crystalline oxide in barrier films, marked by g’ Al2O3 (400) and g’ Al2O3 (440). g’ Al2O3 is a kind of defect spinel which is similar to g-Al2O3, however, with more disorder on the cation lattice. The Al (200) peak and g’ Al2O3 (400) peak overlap because of the similar diffraction angle, which can be observed in the left insert curves in Fig. 8. In addition, the intensity of diffraction peak g’ Al2O3 (440) increases gradually during the preparation process, as shown in the right insert curves in Fig. 8, indicating the change of grain size. In order to quantify the change of grain size in the preparation process, the grain size is calculated by XRD results. The crystallinity analysis of the anodic oxide films is based on the width of the
g’ Al2O3 (440) peak and the Scherrer formula d ¼ kl/(B$cosq). In that formula, d is the crystallite size, k is a form factor (k ¼ 0.89 for spheres particles), l is the wavelength of the radiation used (lCu,Ka1 ¼ 1.54056 Å), B is half height width of selected peaks. The calculated results of g’ Al2O3 grain size are plotted in Fig. 9, showing a gradual downward trend. The grain size is about 25 nm after hydration treatment. Once the formation process begins, the grain size sharply reduces to below 15 nm. The decreasing trend continues until the end of preparation process, and achieves the final value of 10 nm. The smaller grain size means higher density of oxide film and a lower porosity, which is with better performance under working condition. 4. Conclusion The fabrication of anodic aluminum foil is conducted by a multiple-step anodizing process, and the microstructure evolution
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for oxide film of anodic aluminum foil is investigated in this paper. The main conclusions are listed as following:
Fig. 7. Thickness evolution of oxide layer during preparation process.
(1) During the multiple-step anodizing process, the formed oxide increases the blockage of tunnel pore. The pores area on the foil surface keeps decreasing after hydration and the subsequent formation. However, after phosphoric acid treatment, the pores area stops decreasing and increases to a local maximum value. (2) The ‘corn-flake’ structures is the hydrous oxide films with the structure of crystalline aluminum hydrous oxide [AlO(OH)] films. The petal size of 10 min hydration film lies mainly from 60e120 nm, while increasing to 80e140 nm after ‘Formation I’. During the following process, the petal size keeps increasing, and the final length distribution is mainly located between 100e140 nm. (3) Experiencing the phosphoric acid treatment, the petal edges become serrated. The inside of petal structure is destroyed slightly, showing small amount of corrosion holes. Although the petal structure is recovered by the subsequent process, more small holes with the length of 20e30 nm appear inside. (4) The anodized oxide film composes of two layers, the inner layer from crystallization of amorphous anodic oxide, and the outer dense layer from PB transformation. The thickness of the outer layer keeps decreasing from the initial 390 nm to the final 90 nm. In contrast, the thickness of the dense layer keeps increasing from the initial 130 nm to the final 550 nm. (5) Along with the preparation process, the g’ Al2O3 grain size shows a gradual downward trend. The decreasing trend continues until the end of preparation process, and reaches the final value of 10 nm.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Fig. 8. XRD patterns of aluminum foil during preparation process.
Sining Pan: Conceptualization, Methodology, Software, Writing - original draft, Writing - review & editing. Libo Liang: Supervision, Writing - original draft, Writing - review & editing. Baolin Lu: Investigation, Data curation. Huibin Li: Resources, Data curation. Acknowledgments This research was supported by the Project fund by China Postdoctoral Science Foundation (2019M663871XB), Postdoctoral Science Foundation of Guangxi Province of China, the Doctor’s Scientific Research Foundation of Hezhou University (HZUBS201806), the Foundation for Research and Development of Science and Technology of Hezhou City (201908007), and the Innovation-driven Development Project of Hezhou City (ZX1907001). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153795. References
Fig. 9. Evolution of g’ Al2O3 grain size during preparation process.
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