Applied Science and Convergence Technology 2019; 28(6): 213-216
Published online November 30, 2019
Copyright © The Korean Vacuum Society.
Kaiqiang Zhanga and Sang-Shik Parkb,*
aDepartment of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea
bSchool of Nano Materials Engineering, Kyungpook National University, Gyeongsangbuk-do 37224, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
The effects of electrolyte temperature during anodization on the microstructure and electrical properties of the ZrO2 coated Al foils were investigated. The specimens were prepared by coating ZrO2 sol on etched Al foils and anodization in MPD-boric acid electrolytes at 30, 60, and 90 °C, respectively. Anodization potential was 700 V. The thickness of oxide layer anodized increased with increasing the electrolyte temperature. In contrast, the crystallization of ZrO2 and Al2O3 decreased at higher anodizing temperature. Compared with the specific capacitance of the sample anodized at 90 °C, the specific capacitances of which anodized at 30 and 60 °C are enhanced about 12.8 % and 8.8 %, respectively. Therefore, the anodization at 30 °C can effectively improve the specific capacitance of the samples, and save the thermal energy during anodization.
Keywords: 2-methyl-1, 3-propanediol, Anodization, ZrO2, Al2O3, Specific capacitance
With the advance of exploitation on new energy resources, energy-storage technology correspondingly acts a significant role in our life. Capacitors as well as batteries known as two kinds of essential energy-storage devices are now researched by numerous experts. In comparison with batteries, capacitors possess an even higher power, and the basic construction of capacitors is a sandwich structure of anode/dielectrics/cathode. The capacitances of the capacitors can be characterized by
The composition and type of the electrolyte used for anodization significantly affects the anodizing efficiency and electrical properties of the Al foils. Although many electrolytes have been extensively researched [5,9–12], the withstanding voltages of anode foils formed by these electrolytes are still relatively low. So, organic electrolyte has received attention to further increase the withstanding voltage. Kinard
ZrO2 shows a much higher permittivity of 22 – 25 than 9.8 of Al2O3 , which could effectively enhance the specific capacitance (
Etched Al foils (99.99 %, thickness: ~125 μm, hole density: ~2.0 × 107 cm−2, hole diameter: 1 – 2 μm, and hole length: 20 – 50 μm) were placed into alcohol (purity: 99.5 %) and ultrasonically cleaned for 20 min to remove surface contaminants. ZrO2 sol of 0.8 M were prepared by mixing zirconium butoxide (Zr(OC4H9)4, 80 wt% in 1-butanol, Sigma-Aldrich, USA), 2-methoxyethanol (CH3OCH2CH2OH, anhydrous, 99.8 %, Sigma-Aldrich, USA), acetic acid (CH3COOH, 99.7 %, Daejung Chemicals & Metals, Korea), and nitric acid (HNO3, 60~62 %, Daejung Chemicals & Metals, Korea). Finally, the transparent precursors were aged for 3 days. The ZrO2 sol was coated on the surface of the etched Al foils by vacuum infiltration, which can be described in detail as a bath with Al foils was placed into a chamber, thereafter, evacuation was carried out. Subsequently, ZrO2 sol was injected into the bath, followed by ventilated and the etched Al foils were lifted from the ZrO2 sol bath. The ZrO2 film on the etched Al foils was dried at 100 °C for 1 h and annealed at 500 °C for 10 min in ambient environment. The same coating process was repeated four times to achieve the optimal thickness. Subsequently, the ZrO2-coated Al foils were firstly anodized at a mode of constant current (0.1 A/cm2) until the settled voltage (700 V), subsequently, the anodization at a fashion of constant voltage was carried out, lasting until the current flowing through the sample is 0.01 A in MPD-boric acid electrolyte of 30, 60, and 90 °C, respectively. The anodization electrolytes were MPD:boric acid electrolyte (100 g of H3BO3/1 L of H2O) with volume % ratios of 3:10. The MPD-boric acid electrolyte must be kept at greater than or equal to 30 °C to avoid the precipitation of boric acid [Fig. 1(a)]. Lastly, the anodized samples were annealed in a furnace of 500 °C for 2 min for characterization.
Crystalline structure of the oxide layer formed on the Al foils was analyzed using X-ray diffraction (XRD, X’pert Pro MRD, PANalytical). The cross-sectional structures of the coated layer and anodized oxide layer were characterized using field emission-scanning electron microscopy (FE-SEM, JSM-6700E, Jeol) and field emission-transmission electron microscopy (FE-TEM, titan G2 ChemiSTEM Cs probe) performed at 200 kV. The SEM samples were prepared by polishing the anodized Al foils and eroding them for 3 min in KOH solution. The TEM samples were thinned with a focused ion beam (FIB, versa 3D LoVac) to a thickness of about 100 nm. The elemental distributions of the samples were examined by TEM coupled to energy-dispersive X-ray spectroscopy (EDS). The cyclic voltammetry (
Figure 1(b) shows the conductivities of MPD-boric acid electrolytes at a series of temperatures. The conductivity of electrolyte rises with increasing the temperature. So, a higher ionic transport rate at higher temperature may be acquired, promoting the efficiency of anodization. Meanwhile, the ionic resistivities of the MPD-boric acid electrolytes correspondingly decrease as the temperature of electrolyte increase. The conductivity of the mixed electrolyte decreases as the ratio of MPD increase, because MPD exhibits high resistivity as shown in previous work . The ionic migration are weakened and a relatively high voltage can be sustained as the conductivity of the solution decrease . Therefore, it is expected that an electrolyte with a lower conductivity can facilitate anodization at a higher voltage but leads to a decrease in the anodization rate.
Figures 2(a)–(c) shows the SEM cross-sectional images of the dielectric oxides formed inside etch pits at different temperature. A compact combination between the coated ZAO layer (inner) and deposited Al2O3 layer (outer) is shown in each sample. The Al2O3 grows at both Al/ZrO2 interface and ZrO2/electrolyte interface by the transport of Al3+ coming from etched Al foils and O2− coming from electrolyte under electric field. The outer Al2O3 layers demonstrates that the formation of Al2O3 is principally depending on the transport of O2− ions under the driving force given by the electric field, further implying that the O2− penetration in ZrO2-coated layer is much easier than Al3+. The total thicknesses of the dielectric oxides are approximately 625, 680, and 750 nm, increasing with enlargement of the conductivity at higher temperature. The electrolyte with a higher ionic resistivity of 1.8 Ω/cm at 30 °C can inhibit the ionic transport even more largely than the electrolyte with ionic resistivity of 1.2 Ω/cm at 90 °C. After anodization at 30, 60, and 90 °C, the thicknesses of the Al2O3 layer were approximately 440, 590, and 625 nm, respectively. The ratio between the thickness of anodized Al2O3 layer and the anodizing voltage, K, has been reported to be approximately 1 nm/V in pure Al . In this work, these ratios were approximately 0.62, 0.84 and 0.89 nm/V after anodizing at 30, 60, and 90 °C, respectively. Lower K value in ZrO2-coated Al compared with Al specimens results from a hindrance in the movement of Al3+ and O2− ions through ZrO2 layer.
Figure 2(d) shows the XRD diffraction patterns of the samples anodized at 30, 60, and 90 °C, respectively. The intensities of Al2O3 peaks are increased gradually, suggesting that the crystallization of Al2O3 is increased by making use of an electrolyte at lower temperature. This result may be explained that an electrolyte with a lower ionic conductivity require a higher applied potential on the ZrO2-coated samples to reach at the constant current density of 0.1 A/cm2 during anodization. That is, a higher electric field is needed on the ZrO2-coated Al foils, and the high electric field can effectively increase the nucleation rate, thereby promoting the crystallization of Al2O3. Also an increase in intensity of ZrO2 peaks after anodization suggests that coating layer is more crystallized during anodization. Since the same ZrO2 coating and annealing processes are applied, this phenomena proves that the anodization process can modify the crystallized ZrO2 by the way of re-crystallization. More detailed information about the re-crystallization of crystallize ZrO2 under the high electric field was presented in our previous work . Herein, the difference of the crystallization of Al2O3 may also be due to the permeation of electrolyte species. Koyama
Figures 3(a) and 3(b) show the TEM cross-sectional images and diffraction patterns of the Al2O3 formed in MPD-boric acid electrolyte at 30 and 90 °C, respectively. The crystallinity of the Al2O3 anodized in the electrolyte of 30 °C is evidently higher than that formed in the electrolyte of 90 °C as corroborated by the diffraction patterns and visible Al2O3 grains in Fig. 3(a). This result also well agrees with the trend in the XRD diffraction patterns [Fig. 2(d)] that the anodization in the electrolyte of low temperature can effectively optimize the crystallization of Al2O3. A well crystallized samples could possibly raise the
Figure 4 shows the HRTEM images of (a) Al2O3 layer and (b) ZrO2 in ZAO composite layer anodized at 30 °C. The resolution of the HRTEM used in this study is 0.08 nm. The measured interplanar spacing of the Al2O3 and separated crystalline ZrO2 is 0.45 and 0.295 nm, respectively. These values are matched with the interplanar spacing of cubic Al2O3 (111) and tetragonal ZrO2 (101). Al2O3 layer is crystallized with clear lattice, but ZAO composite layer is composed of crystalline ZrO2 and amorphous Al2O3 as shown in our previous report .
Figure 5(a) shows the withstanding voltage of the samples anodized in the electrolyte of 30, 60, and 90 °C, respectively. The withstanding voltages are about 760 V and are almost regardless of the electrolyte temperature. Thus, the voltage sustain-abilities of the samples formed at 30, 60, and 90 °C are about 1.21, 1.11, and 1.01 V/nm, respectively. This demonstrates that the oxide with a higher crystallization has a greater voltage-sustainability than that with a lower crystallization. However, a longer time is needed in the potential evolution process for the samples anodized in the electrolyte of lower temperature. Moreover, correspondingly, the leakage currents in Fig. 5(b) also slightly increase in electrolyte of lower temperature. This appearance can be due to the consumption of stored charges, resulted from the micro-cracks generated in anodization process. However, the leakage currents of the samples are less than 0.01 A at applied voltage of 700 V. Furthermore, in consideration of the self-repairing of the Al foil in charging process, over time, the leakage currents of the samples are supposed to diminish. Thus, the samples anodized at 30 °C show better properties in terms of withstanding voltage and leakage current. To further evaluate the electrochemical properties of the samples, we carried out the cyclic voltammetry measurement [Fig. 5(c)]. A better capability is observed in the ZrO2-coated Al foils anodized in the electrolyte of lower temperature. Additionally, there are no evident redox peaks in the
The effects of electrolyte temperature on the microstructure and electrical properties of the dielectrics on etched Al foils are explored. The total thicknesses of the dielectric oxide layers increase with increasing the electrolyte temperatures. Additionally, the crystallizations of ZrO2 and Al2O3 were improved when samples were anodized at lower electrolyte temperature, meanwhile, more micro-cracks were observed in the Al2O3 layer formed at lower temperature. Moreover, the withstanding voltages are independent of the electrolyte temperature and leakage currents slightly enlarge with the decrease in electrolyte temperature. In comparison with the
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2017033541).