Blog

Oct 17, 2024

Influence of micro-arc oxidation on the microstructure and dielectric properties of anodic aluminum oxide | Scientific Reports

Scientific Reports volume 14, Article number: 23673 (2024) Cite this article 390 Accesses Metrics details To improve the dielectric performance of the anodic alumina film used in aluminum electrolytic

Scientific Reports volume 14, Article number: 23673 (2024) Cite this article

390 Accesses

Metrics details

To improve the dielectric performance of the anodic alumina film used in aluminum electrolytic capacitors, this study comparatively investigated the microstructure and dielectric properties of anodic aluminum oxide obtained through micro-arc oxidation (MAO) and conventional anodic oxidation (CAO). It is found that from the perspective of microstructure, the internal structure of the MAO treated oxide film has more and larger pores than that of CAO. This was attributed to the generation and overflow of numerous oxygen bubbles from within the oxide film at the locations where plasma sparks occurred during the process, thus forming larger pores. Regarding dielectric properties, the leakage current of the oxide film after MAO treatment was significantly reduced compared to CAO, with reductions of 58%, 56%, 64%, and 74% for the tested electrolytes Y1–Y4, respectively.

The utilization of electrochemical anodization for the preparation of metal anodic oxide films has garnered increasing attention in academic and industrial realms alike1,2,3,4,5. Dense film of anodic aluminum exhibit exceptional dielectric properties, rendering aluminum foils coated with such films suitable for utilization as anodes in electrolytic capacitors, relying on this oxide layer as an energy storage medium6,7. Electrolytic capacitors made of alumina film with exceptional dielectric properties are widely used in the field of electronic power8,9. However, after the oxide film obtained by conventional anodic oxidation (CAO) is made into a capacitor, the internal leakage current is large. Therefore, reducing the leakage current of the oxide film is a key factor affecting the life of the capacitor. Researchers usually improve the leakage current of the oxide film by changing the composition of the electrolyte10 and high temperature heat treatment11. In this study, a new oxidation method will be used to improve the leakage current of the oxide film.

Micro-arc oxidation (MAO) is to apply a higher voltage based on CAO, which is usually higher than the dielectric breakdown voltage to promote the generation of plasma sparks at the metal-electrolyte interface. Oxide films produced via MAO exhibit larger pore sizes and greater thickness compared to those generated through CAO, consequently showcasing superior mechanical and electrical properties12. An insulating layer with exceptional dielectric properties can be grown on the surface of aluminum wire by MAO13. At the same time, the insulating layer also has good corrosion resistance14,15,16,17.

Yerokhin et al. clearly pointed out in the artical that the MAO process is a continuation of the CAO process18, but this view has been seriously ignored. In this study, we examined the impact of MAO on the microstructure and dielectric properties of anodic aluminum oxide under such constant current conditions by regulating the duration of oxidation after reaching the sparking voltage of the electrolyte. Comparing the anodic alumina films obtained by MAO and CAO, it was found that the dielectric properties of anodic alumina films obtained by MAO were significantly improved.

Figure 1 illustrates the voltage-time curves of aluminum strips undergoing MAO in four electrolytes (Y1–Y4), after the polishing treatment. Since the current density of oxidation and other conditions are the same, the curve of CAO is the time from 0 s of the curve in Fig. 1 to the moment when the curve reaches the sparking. Through the observation of the voltage-time curve in Fig. 1, it is found that under the action of constant current density, the voltage begins to rise gradually from about 10 V, and it is stage I when it reaches 480–500 V. At this time, the total current of anodic oxidation is mainly ionic current, which is completely used for the growth of oxides19. Now, the oxide divides into barrier layer oxide and anionic contamination layer. O2− are not only present on the surface but also migrate to the interior, reacting to release electrons and generate O2 under the influence of the electric field. However, because of electrolyte and atmospheric pressure, oxygen generated within the oxide film does not diffuse immediately to the surface. This oxygen generation reaction results in the production of electronic current. Nevertheless, as the total current density remains constant, the ionic current starts to diminish, reducing the efficiency of oxide film generation and decelerating the voltage rise rate20,21,22,23. The stage II commences after reaching 480–500 V, characterized by voltage oscillation, indicating the sparking voltage. At this stage, the ionic current diminishes to zero, and the overall anodic oxidation current shifts to electronic current. Consequently, oxide film growth ceases, and oxygen bubbles continuously emanate from the surface of the aluminum strip24. The voltage-time curves for electrolytes Y1–Y4 each display distinct sparking voltages, namely 522 V, 539 V, 457 V, and 446 V, respectively. The determination method of sparking voltage refers to this article25.

Voltage-time curves of aluminum strips undergoing anodic oxidation in four different electrolytes. (a–d) electrolyte Y1–Y4.

Figure 2 depicts digital photographs of aluminum strips following the attainment of sparking during anodic oxidation. From the digital photographs in Fig. 2, it is evident that upon sparking, the surfaces of the aluminum strips in all four electrolytes display plasma sparks, with some sparks emitting oxygen bubbles. The regions where sparks occur signify the initiation of MAO. In the stage I, only electrochemical reactions take place. However, in the stage II, owing to micro-arc discharge and the localized high temperature generated by the discharge, intricate side reactions occur26,27,28,29. Figure 3 displays FESEM images and EDS spectra of the surface and cross-section of the oxide film after MAO in electrolyte Y1. From Fig. 3, it can be observed that the content of element C is higher near the pores on the oxide surface. This phenomenon may be attributed to the partial decomposition of organic electrolytes caused by micro-arc discharge, leading to an increase in the surface element C content. Additionally, it is noteworthy that the plasma sparks in electrolytes Y1 and Y2 occur across the entire surface of the aluminum strip, whereas in electrolytes Y3 and Y4, the sparks mainly occur at the interface between the electrolyte and air or at the edges near the interface as shown in Fig. 2. This disparity could be ascribed to variations in the electrolyte composition.

Digital photographs of aluminum strips (all with a width of 10 mm) after reaching sparking during anodic oxidation in four different electrolytes. (a–d) electrolyte Y1–Y4.

FESEM images and EDS spectra of the surface and inner layer of the oxide film on aluminum strips after MAO in electrolyte Y1. (a) surface of the oxide film; (b) cross-section of the oxide film obtained from the man-made fracture.

To investigate the effects of MAO on the microstructure and dielectric properties of the oxide film, information regarding surface morphology and the interior of the oxide film was obtained through FESEM imaging. Information regarding dielectric properties was obtained through leakage current testing.

The FESEM section below illustrates the impact of MAO on the surface and internal structure of the oxide film from two perspectives: (1) a comparison of FESEM images between CAO and MAO under the same electrolyte; (2) a comparison of FESEM images in electrolyte Y4 during MAO, distinguishing between regions with and without sparks at the interface.

Figure 4 presents the surface and cross-sectional FESEM images of aluminum strips subjected to CAO and MAO in electrolyte Y3. From Fig. 4, it can be observed that there is almost no difference in surface morphology between aluminum strips obtained from CAO and MAO. However, the internal structure of the oxide film obtained from CAO exhibits few or small pores, while the oxide film obtained from MAO shows larger pores internally.

Figure 5 presents FESEM images of aluminum strips obtained from MAO in electrolyte Y4, where sparks occur only at the interface, distinguishing between regions with and without plasma sparks. From Fig. 5, it can be observed that there is slight difference in surface morphology between the two regions. However, the cross-sectional FESEM image of the oxide film at the area with plasma sparks reveals larger pores, while the region without plasma sparks exhibits few or small pores internally. Combining this observation with the digital photographs captured during sparking in Fig. 2, it is suggested that oxygen bubbles released at the plasma sparks during MAO may leave behind voids in the interior of the oxide film, thus acting as a mold effect30.

Surface and cross-sectional FESEM images of aluminum strips after CAO and MAO in electrolyte Y3. (a, b) CAO; (c, d) MAO.

FESEM images of an aluminum strip after MAO in electrolyte Y4 are provided, depicting regions with and without plasma sparks. (a,b) regions with plasma sparks; (c, d) regions without plasma sparks.

Leakage current serves as a pivotal parameter in assessing the dielectric properties of anodic aluminum oxide films. These films exhibit imperfections characterized by small cracks, pores, and other defects, where oxidation reactions occur upon the application of voltage. New oxide grows to mend the existing oxide film, resulting in a gradual decline in current over time. This decrease in current signifies the repair reaction of the dielectric layer induced by oxidation-reduction. Eventually, the current stabilizes at a certain value, referred to as the leakage current of the capacitor31,32. Consequently, a higher number of defects on the oxide film typically corresponds to a higher measured leakage current value over the same period.

Figure 6 displays the leakage current data of aluminum strips subjected to CAO and MAO in different electrolytes, tested in electrolyte Y3 for 2 min at a voltage of 450 V, slightly below the sparking voltage of the original electrolyte. It is evident from Fig. 6 that irrespective of the electrolyte utilized for anodic oxidation, the leakage current values obtained after MAO treatment consistently demonstrate lower values. Specifically, the leakage current decreased by 58%, 56%, 64%, and 74% for electrolytes Y1–Y4, respectively. However, considering the FESEM images of the cross-sections depicted in Figs. 4 and 5, it is anticipated that the oxide film subjected to MAO, exhibiting more defects, would display a higher leakage current. However, the fact that the leakage current of the MAO film is lower indicates that after 200s of micro-arc oxidation, the electronic current pathway within the MAO films that causes the leakage current to increase is repaired. Similar results have also been obtained when the test electrolyte for leakage current was changed to electrolyte Y1 and Y2. This proves that the decrease of leakage current shown in Fig. 6 is not related to the test electrolyte.

The leakage current of anodic alumina films formed through CAO and MAO in different electrolytes at 20 °C.

The anodic oxide films obtained by conventional oxidation and micro-arc oxidation in four working electrolytes were compared for the first time. Microscopically, the oxide film produced by MAO exhibited larger pores compared to CAO. This occurrence is ascribed to the generation and overflow of numerous oxygen bubbles from the interior of the oxide film at the sites where plasma sparks occurred during MAO, resulting in the formation of larger pores. Regarding dielectric properties, the leakage current of the oxide film tested after MAO markedly decreased compared to CAO. Specifically, the leakage current reduced by 58%, 56%, 64%, and 74% for electrolytes Y1–Y4, respectively. These interesting findings have important guiding significance for the performance of modified aluminum electrolytic capacitors.

The aluminum foil (200 μm, purity, 99.99%), underwent cutting into aluminum strips measuring 9 cm×1 cm. These strips underwent chemical polishing in a 5 wt% H3PO4 aqueous solution at 60 °C for 30 min to eliminate the natural oxide layer from the surface. Subsequently, they were rinsed with deionized water and dried at room temperature.

The electrolyte Y1 was prepared according to the composition in Table 1. Based on electrolyte Y1, the content of p-nitrobenzoic acid was increased to 2 wt% to prepare electrolyte Y2. The electrolyte Y3 was prepared by replacing ammonium hypophosphite in electrolyte Y1 with ammonium hydrogen azelate, and the content of p-nitrobenzoic acid in electrolyte Y3 was increased to 2wt % to prepare electrolyte Y4. The composition of these electrolytes is close to that in aluminum electrolytic capacitors. And the conductivity of electrolyte Y1–Y4 are 1323 µS cm− 1, 1303 µS cm− 1, 1593 µS cm− 1 and 1639 µS cm− 1, respectively. The pH of electrolyte Y1–Y4 are 6.31, 6.35, 6.32 and 6.02, respectively.

Graphite served as the cathode, and a constant current density of 5 mA/cm2 was applied using a direct current power supply (H-1–3-6TV-(A/B/C), Huahang Engineer Software) at room temperature (20 °C) for anodization. The exposed surface area of the aluminum strips in the electrolyte was approximately 4 cm2 (both sides of the strip). The oxidation process was segregated into two groups: the first group underwent CAO, where oxidation was ceased immediately upon reaching the sparking voltage; the second group underwent MAO, with oxidation prolonged for an additional 200s after attaining the sparking voltage.

Following the rinsing of the oxidized aluminum strips with deionized water and their subsequent drying at room temperature, they underwent leakage current testing using a leakage current tester (TH2686N, Tonghui Electronic Co. Ltd). The testing voltage was set at 450 V, slightly lower than the sparking voltage of the four electrolytes, with electrolyte Y3 utilized for the test. During testing, the positive terminal of the instrument was connected to the anodized aluminum strip, while the negative terminal was connected to the cathode graphite. The leakage current was recorded after applying the voltage for 2 min.

Field-emission scanning electron microscopy (FESEM, Zeiss, SUPRA 55) was utilized to examine the surface and cross-sectional morphology of the aluminum oxide film on the aluminum substrate at both the interface and non-interface regions with the electrolyte. Additionally, energy-dispersive X-ray spectroscopy (EDS, Zeiss, SUPRA 55) was employed to analyze the chemical composition of the anodic oxide film. During FESEM testing, the samples were bent into an Ω shape to from man-made fracture in the oxide film, facilitating the observation of the morphology on the cross-Sect33.

The data used to support the findings of this study are included within the article.

Liao, L., Huang, W., Cai, F. & Zhang, Q. Preparation and mechanism of honeycomb-like nanoporous SnO2 by anodization. J. Mater. Sci. Mater. Electron. 32, 9540–9550 (2021).

Article CAS Google Scholar

Zhang, W. et al. Anodic growth of TiO2 nanotube arrays: effects of substrate curvature and residual stress. Surf. Coat. Technol. 469, 129783 (2023).

Article CAS Google Scholar

Kondo, T. Formation of porous Ga oxide with high-aspect-ratio nanoholes by anodizing single Ga crystal. Sci. Rep. 13, 12408 (2023).

Article ADS PubMed PubMed Central CAS Google Scholar

Iwai, M., Kikuchi, T. & Suzuki, R. O. Self-ordered nanospike porous alumina fabricated under a new regime by an anodizing process in alkaline media. Sci. Rep. 11, 7240 (2021).

Article ADS PubMed PubMed Central CAS Google Scholar

Kikuchi, T. et al. Ultra-high density single nanometer-scale anodic alumina nanofibers fabricated by pyrophosphoric acid anodizing. Sci. Rep. 4, 7411 (2014).

Article PubMed PubMed Central CAS Google Scholar

Pan, S., Liang, L., Lu, B. & Li, H. Microstructure evolution for oxide film of anodic aluminum foil used in high voltage electrolytic capacitor. J. Alloys Compd. 823, 153795 (2020).

Article CAS Google Scholar

Du, X. et al. Fabrication and characterization of the hierarchical AAO film and AAO-MnO2 composite as the anode foil of aluminum electrolytic capacitor. Surf. Coat. Technol. 419, 127286 (2021).

Article CAS Google Scholar

Ali, M., Hossain, M. I., Al-Ismail, F. S., Abido, M. A. & Khalid, M. Capacitor ripple reduction in T-type multilevel inverter operation for solar PV-application. Alex Eng. J. 77, 613–624 (2023).

Article Google Scholar

Souri, S., Shourkaei, H. M., Soleymani, S. & Mozafari, B. Flexible reactive power management using PV inverter overrating capabilities and fixed capacitor. Electr. Power Syst. Res. 209, 107927 (2022).

Article Google Scholar

Zhu, Y. et al. Voltage oscillations during anodizing process of aluminum and their suppression. J. Mater. Sci. Mater. Electron. 35, 563 (2024).

Article CAS Google Scholar

Wang, S. et al. Influence of heat treatment process on leakage current of anodic aluminum oxide films. J. Mater. Sci. Mater. Electron. 34, 1017 (2023).

Article CAS Google Scholar

Correa, D. R. N. et al. Growth mechanisms of Ca- and P-rich MAO films in Ti-15Zr-xMo alloys for osseointegrative implants. Surf. Coat. Technol. 344, 373–382 (2018).

Article CAS Google Scholar

Li, H., Zhang, M., Wu, X., Geng, Y. & Lei, L. Effect of electrical modes on dielectric and insulation properties of plasma electrolytic oxidation ceramic films on Al. Surf. Coat. Technol. 130881 (2024).

Kosaba, T., Muto, I. & Sugawara, Y. Effect of anodizing on galvanic corrosion resistance of Al coupled to Fe or type 430 stainless steel in diluted synthetic seawater. Corros. Sci. 179, 109145 (2021).

Article CAS Google Scholar

Kaseem, M., Choi, K. & Ko, Y. G. A highly compact coating responsible for enhancing corrosion properties of Al-Mg-Si alloy. Mater. Lett. 196, 316–319 (2017).

Article ADS CAS Google Scholar

Kaseem, M., Hussain, T. & Ko, Y. G. Tailored alumina coatings for corrosion inhibition considering the synergism between phosphate ions and benzotriazole. J. Alloys Compd. 822, 153566 (2020).

Article CAS Google Scholar

Hussain, T., Kaseem, M. & Ko, Y. G. Hard acid–hard base interactions responsible for densification of alumina layer for superior electrochemical performance. Corros. Sci. 170, 108663 (2020).

Article CAS Google Scholar

Yerokhin, A. L., Nie, X., Leyland, A., Matthews, A. & Dowey, S. J. Plasma electrolysis for surface engineering. Surf. Coat. Technol. 122, 73–93 (1999).

Article CAS Google Scholar

Zhou, Q. et al. Dense films formed during Ti anodization in NH4F electrolyte: evidence against the field-assisted dissolution reactions of fluoride ions. Electrochem. Commun. 111, 106663 (2020).

Article CAS Google Scholar

Gong, T. et al. Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides. Nanoscale Adv. 3, 4659–4668 (2021).

Article ADS PubMed PubMed Central CAS Google Scholar

Li, C. et al. Simulation of anodic current and optimization of the fitting equation and the fitting algorithm during constant voltage anodization. J. Phys. Chem. C. 127, 9707–9716 (2023).

Article ADS CAS Google Scholar

Chen, W. H., Lai, M. Y., Tsai, K. T., Liu, C. Y. & Wang, Y. L. Spontaneous formation of ordered nanobubbles in anodic tungsten oxide during anodization. J. Phys. Chem. C. 115, 18406–18411 (2011).

Article CAS Google Scholar

Yu, M. et al. Studies of oxide growth location on anodization of Al and Ti provide evidence against the field-assisted dissolution and field-assisted ejection theories. Electrochem. Commun. 87, 76–80 (2018).

Article ADS CAS Google Scholar

Zhang, Y. et al. Effect of chloride ions on the sparking voltage of working electrolytes and its restraint method. J. Phys. Chem. C. 127, 16148–16155 (2023).

Article CAS Google Scholar

Wang, S. et al. Determining the sparking voltage of working electrolytes. J. Electrochem. Soc. 170, 093504 (2023).

Article ADS CAS Google Scholar

Wang, Y., Yu, H., Chen, C. & Zhao, Z. Review of the biocompatibility of micro-arc oxidation coated titanium alloys. Mater. Des. 85, 640–652 (2015).

Article CAS Google Scholar

Stojadinović, S., Vasilić, R., Petković, M. & Zeković, L. Plasma electrolytic oxidation of titanium in heteropolytungstate acids. Surf. Coat. Technol. 206, 575–581 (2011).

Article Google Scholar

Kaseem, M., Min, J. H. & Ko, Y. G. Corrosion behavior of Al-1wt% Mg-0.85wt%Si alloy coated by micro-arc-oxidation using TiO2 and Na2MoO4 additives: role of current density. J. Alloys Compd. 723, 448–455 (2017).

Article CAS Google Scholar

Kaseem, M., Repycha Safira, A. & Fattah-alhosseini, A. Novel interface engineering of LDH-based materials on mg alloy for efficient photocatalytic systems considering the geometrical linearity of condensed phosphates. J. Magnes Alloys. 12, 267–280 (2024).

Article CAS Google Scholar

Lv, R. et al. The holes produced by field-assisted dissolution and the pores produced by oxygen bubble mould effect. J. Electrochem. Soc. 168, 103501 (2021).

Article ADS CAS Google Scholar

Torki, J., Joubert, C. & Sari, A. Electrolytic capacitor: properties and operation. J. Energy Storage. 58, 106330 (2023).

Article Google Scholar

Nordstrand, J., Zuili, L., Toledo-Carrillo, E. A. & Dutta, J. Predicting capacitive deionization processes using an electrolytic-capacitor (ELC) model: 2D dynamics, leakages, and multi-ion solutions. Desalination. 525, 115493 (2022).

Article CAS Google Scholar

Li, P. et al. The role of fluoride and phosphate anions in the formation of anodic titanium dioxide nanotubes. Electrochem. Commun. 158, 107641 (2024).

Article CAS Google Scholar

Download references

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51577093, 61171043) and the National Undergraduate Training Program for Innovation and Entrepreneurship (202410288026Z).

Key Laboratory of Soft Chemistry and Functional Materials of Education Ministry, Nanjing University of Science and Technology, Nanjing, 210094, China

Liyang Qin, Zhongyou Fu, Xiuhao Han, Bowen Li & Xufei Zhu

School of Environmental and Chemical Engineering, Jiangsu Ocean University, Lianyungang, 222005, China

Lin Liu & Juanjuan Ma

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

You can also search for this author in PubMed Google Scholar

LQ: Methodology, Writing – original draft, Investigation. ZF: Methodology, Investigation. LL: Conceptualization, project administration. XH: Investigation. JM: Review & editing. BL: Formal analysis. XZ: Supervision. All authors reviewed the manuscript.

Correspondence to Lin Liu or Xufei Zhu.

The authors declare no competing interests.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.

Reprints and permissions

Qin, L., Fu, Z., Liu, L. et al. Influence of micro-arc oxidation on the microstructure and dielectric properties of anodic aluminum oxide. Sci Rep 14, 23673 (2024). https://doi.org/10.1038/s41598-024-74827-1

Download citation

Received: 27 June 2024

Accepted: 30 September 2024

Published: 10 October 2024

DOI: https://doi.org/10.1038/s41598-024-74827-1

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative