Formation Behavior of Multi-Layer Oxide Films on Etched Al Foil by Vacuum Inﬁltration and Anodization

A vacuum inﬁltration method was proposed to coat uniform zirconia (ZrO 2 ) ﬁlms on etched aluminum (Al) foil. The formation behavior of the composite oxide ﬁlms on aluminum (Al) foils with etch pits was different from that of ﬂat Al foils. The multi-layer structures were formed on the ZrO 2 -coated Al foil after being anodized. The formation mechanism of the multi-layer ﬁlms on etched Al foils was discussed. The structure and composition of each layer were investigated by scanning electron microscopy and transmission electron microscopy. The multi-layer ﬁlms consisted of an inner aluminum hydrate layer, middle Al 2 O 3 -ZrO 2 (Al-Zr) composite layer, and outer Al 2 O 3 layer. Many Al 3 + ions passed through the coating layer and transferred into the inner layer. A particle-like and layered structure was observed in the inner layer. The formation of multi-layer ﬁlms was mainly affected by the volume expansion of the outer Al 2 O 3 layer and the dense middle Al-Zr composite oxide ﬁlm. The ZrO 2 coated foils exhibited 26% higher speciﬁc capacitance than foils with only Al 2 O 3 layer when being anodized at 600 V. ©

Aluminum (Al) electrolytic capacitors are widely used in electronic devices because of the advantages of large specific capacitance and low costs. 1 The recent development of small-volume electronics requires an increase in the capacitance of Al electrolytic capacitors, 2 which mainly depends on the specific capacitance of Al electrode foil. 3 One of the promising methods to increase the specific capacitance of Al foil is to replace the anodic oxide film, Al 2 O 3 , with a composite oxide with a high dielectric constant. 3,[4][5][6][7] Currently, the techniques used to form composite oxides on Al foils combine sol-gel dip coating, 7 the pore filling method, 8 and electrophoretic coating 9 with anodizing. Generally, tunnels made on Al foil by etching are mostly perpendicular to the Al top surface. The sol-gel method has merits such as its low cost and the ability to coat substrates with complicated surfaces. However, our previous research 10 demonstrated that the existence of air in the etch tunnel may prevent the solution from getting inside the tunnel. To solve this problem, a vacuum infiltration method was proposed in this work. If the air in the etch pits is evacuated, the solution can be infiltrated into etch pits by the liquid pressure. Because almost no air is present in the etch pits, a uniform thin film can be coated. In addition, the shrinkage problem of films coated using the sol-gel method is also inevitable. Therefore, the coating layer is detached from the inner wall of the etch pits as a result of the shrinkage of coating layer. The additional anodizing process in this experiment not only forms the Al-Zr composite film but also fills the empty space caused by shrinkage.
ZrO 2 is one of the promising high-k materials and has a high dielectric constant, k (22-25) and a wide bandgap (5.1-7.8 eV). [11][12] It is expected that the incorporation of ZrO 2 and an anodic oxide film may result in an Al-Zr composite oxide film with a high dielectric constant and improve the capacitance. To date, various researchers have proposed that the dual-layer structures are formed on high-k-materialcoated Al foil after being anodized. 5,8,13 These studies demonstrated that the composite oxide layer and anodic oxide layer were formed by transporting of O 2− and Al 3+ ions and few Al 3+ ions transported to the interface between the coating layer and electrolyte solution. To apply composite oxide dielectrics to high-voltage capacitors with high capacitance, it is necessary to form composite oxide films within Al etch pits of long length (20-50 μm). However, few studies have been conducted on etched Al foil. In this work, Al-Zr composite oxide films were formed on etched Al foil by vacuum infiltration coating and anodizing. The microstructure and composition of the films were investigated, and the formation mechanism of multi-layer films on etched Al foils was discussed.
Coating and anodizing.-The sample preparation processes are presented in Fig. 1. High-purity (99.99%) etched Al foil was used as the substrate for film growth. The thickness, pit diameter, pit length, and pit density of the Al foils were approximately 100 μm, 1-2 μm, 30-40 μm and 2.0 × 10 7 cm −2 , respectively. ZrO 2 thin films were prepared by the vacuum infiltration method. A schematic diagram of the vacuum infiltration process for the coating of ZrO 2 films on Al foils is presented in Fig. 2. The sol was injected into the samples in a  beaker after being pumped for 30 min. During injecting, the solution was infiltrated into the etch pit under the liquid pressure. Afterward, the air valve was opened to recover the air pressure in the chamber. Finally, the samples were withdrawn at a rate of 0.5 mm/s. The coated foils were dried at 100 • C and annealed at 500 • C. To obtain film with proper thickness, the above processes were repeated 4 times. The coated samples were anodized by two-step anodizing process. The samples were anodized at 600 V with 50 mA/cm 2 constant currents in a boric acid (H 3 BO 3 ) solution (100 g H 3 BO 3 /1 L H 2 O) at 85 • C for 50 min and then heated at 500 • C for 2 min. In second step, the samples were anodized at 600 V for 5 min and heated at 500 • C for 2 min. The samples without ZrO 2 were also anodized under the same conditions for comparison.
Characterization.-The cross-sectional morphologies of the samples were examined using field-emission scanning electron microscopy (FESEM, JEOL, JSM-6700F, Japan). The samples to be examined by TEM were thinned using a focused ion beam (FIB, FEI, versa 3D Lovac. USA). The structure and composition of the thin cross-section tunnels were characterized using field-emission transmission electron microscopy and energy dispersive X-ray spectroscopy (FETEM-EDS, FEI, Titan G2 ChemiSTEM Cs Probe, USA).

Results and Discussion
Microstructure of oxide film layers.-The cross-sectional structure of the coated and anodized tunnels was characterized by SEM and TEM. To obtain SEM images of the etch pits, the samples were polished and corroded in potassium hydroxide (KOH) solution for 2 min. Figure 3 shows the cross-sectional structures of coated and/or anodized tunnels. Figure 3a is an SEM image of the tunnel after coating, (b) is an SEM image of the tunnel after anodizing without coating, (c) is an SEM image of the tunnel after coating and anodizing, and (d) is a TEM image of the tunnel after coating and anodizing. As observed in Fig. 3a, a uniform ZrO 2 layer with 110-nm thickness was successfully coated on the etched Al foil. Empty space is observed between the ZrO 2 coating layer and the Al etch pit, which is caused by shrinkage during annealing at 500 • C and corrosion by the KOH solution. The thickness of the anodized Al 2 O 3 layer in Fig. 3b was approximately 610 nm. The ratio of the oxide thickness to the oxide formation voltage, K, is approximately 1.0 nm · V −1 , which is well matched with the reported K values of anodic oxide films of 0.8-1.3 nm · V −1 . 14 The images in Figs. 3c and 3d reveal that a double layer structure was formed after coating and anodizing. In particular, the TEM image in Fig. 3d shows that the inner layer is composed of a particle-like and layered structure. The TEM image reveals that the inner layer is ZrO 2 and the outer layer is Al 2 O 3 . The thicknesses of the outer Al 2 O 3 layers in the SEM and TEM images were approximately 520 nm and 530 nm, respectively. The K value of the Al 2 O 3 layer in the Al foils with the ZrO 2 layer was approximately 0.87 nm · V −1 , which suggests that the ZrO 2 layer prevented O 2− ions from transporting to the interface. The thickness of the inner layer in the TEM image is approximately 290 nm and is thicker than that in the SEM image. It can be expected that part of inner layer was dissolved by the KOH solution used for preparation of the samples for SEM analysis. Although the thickness of the Al 2 O 3 layer in the ZrO 2 -coated foil was thinner, the ZrO 2 -coated foils could withstand a voltage of 640 V because of the inner layer including the ZrO 2 layer. Furthermore, the coating layer could withstand the tensile stress caused by volume expansion during the Al 2 O 3 formation, which most likely contributed to the formation of the Al-Zr composite material. Figure 4 presents the TEM-EDS mapping results for the selected area from Fig. 3d. Figure 4a is a TEM image of the selected area, and Figures 4b, 4c, and 4d show the elements distribution of O, Al, and Zr, respectively. Figs. 4c and 4d reveal that the inner layer is divided into two layers, a layer containing mostly Zr atoms and a layer containing mostly Al atoms. Therefore, the total layer formed by the ZrO 2 coating and anodizing was composed of the inner layer, middle layer, and outer layer marked by dashed lines. Figure 5a presents the EDS line scanning profile obtained from the tunnel of Fig. 4a. The changes in the Al intensity from the inner layer to the outer layer indicated that Al 3+ ions were transported inwards through the coating layer during anodization. Figure 5b presents EDS spectra from points 1, 2, 3 marked in Fig. 4a. Al atoms were observed in both the middle layer and inner layer. The Al-Zr composite oxide was formed in the entire ZrO 2 coating layer, and relatively less Al atoms were present in the middle of the layer. The atomic percentages of the elements Zr, Al, and O in positions 1, 2, 3 marked in Fig. 4a are listed in Table I   ions were transported through the coating layer during anodization, and most Al atoms existed as aluminum hydrate. 15 The atomic ratio of Zr to O at point 2 is 1.06, which is two times higher than the ratio of ZrO 2 . This may be due to the inward transport of Zr 4+ or ZrO 2+ ions under high electric field, as was reported by K. Watanabe, 16 and the loss of O 2− ions to the outer layer may also occur. In addition, 3.52% Al atoms were present even in the middle of the ZrO 2 layer, which can be considered evidence that the Al 3+ ions passed through the coating layer. At point 3, the atomic ratio of Al to O is close to the stoichiometric ratio of Al 2 O 3 (2/3). The lack of O atoms at points 1 and 2 are the most obvious phenomena. Perhaps this is because the dense inner layer and absorbed hydrogen in the inner wall surface prevents the O 2− ions from transporting inside. 17 From EDS analysis results, it was confirmed that the coated and anodized layer could be divided into an inner aluminum hydrate layer, a middle Al-Zr composite layer, and an outer Al 2 O 3 layer. The thicknesses of these layers were approximately 120 nm, 240 nm, and 530 nm, respectively. The results on the etched Al foil were different from those reported on the flat Al foil. It has been observed that the dual-layer structures were formed between the coating layer and flat Al foils. 5,8,13,16 In addition, the coated film on flat Al foils would break off when the anodizing voltage was high. 7 The main reason for these differences was possibly that the coating layer in the etched foil could maintain the round structure under the volume expansion stress of the anodic oxide film during anodizing. Figure 6 presents HR-TEM images of the coated and anodized foil. The image of the middle ZrO 2 layer presented in Fig. 6a reveals that the coating layer was highly densified after being anodized. The inset in Fig. 6a is a magnified lattice image. The measured interplanar spacing, d, is 0.294 nm, which matches with d = 0.29502 nm of the tetragonal ZrO 2 (011) crystal plane indexes according to its JCPDF card (card No. 50-1089). In addition, the XRD result of the ZrO 2 layer not shown here also matched with the JCPDF card. Figure 6b presents an image of the outer Al 2 O 3 layer. The inset in Fig. 6b is the fast Fourier transform (FFT) image of the Al 2 O 3 layer obtained using Gatan Digital Micro-graph program. The electron diffraction pattern indicates that the outer Al 2 O 3 layer has a crystalline structure. The formation of the crystalline structure may be induced by the higher forming voltage 18 and heat-treatment. 19 Formation mechanisms of multi-layer film. -Figures 7a and 7b show the variations of anodizing voltage with anodizing time when  the samples anodized at 600 V with 50 mA/cm 2 constant currents in the first and second step anodizing. At the initial moment, there are relatively high voltage jumps and they are 110.7 V, 120.5 V for non-coated samples and ZrO 2 -coated samples, respectively. The difference of these voltage jumps is caused by the thermal aluminum oxide film. 20 The reasons for the high voltage jumps are not sure, while foils show the lower jumps of voltage when being anodized at lower current density. The different slope of samples with or without coating showed in Fig. 7a is similar to the reported results, 21,22 while the curve of coated samples shows sudden drop and subsequent raise under the forming voltage. The instantaneous variation of voltage may be explained as follows: The cracks in coating layer are filled with newly formed anodic alumina during anodizing. With the growth of outer Al 2 O 3 goes on, the empty space is gradually filled by volume expansion of Al 2 O 3 layer. When the compressive stress by the volume expansion is higher than the value that coating layer can stand, the local breakdown may happen in the coating layer where crack exists. After that, it can be repaired by anodizing. However, the anodic oxide layer become stable in the second step anodizing as shown in Fig. 7b. The ZrO 2 coated samples exhibited 26% higher specific capacitance than samples with pure Al 2 O 3 layer when being anodized at 600 V. Figure 8 presents a schematic model of the multi-layer structure formed in the ZrO 2 -coated sample before and after being anodized. It is assumed that the ZrO 2 layer has a network structure of micro pores and cracks, which may have been formed by the evaporation of organic compounds during drying and/or annealing. When the coated foil is immersed in anodizing solution, water and electrolyte penetrate into the micro pore network and empty space formed by shrinkage. During the initial stage of anodizing, the transport of Al 3+ , O 2− , or OH − through the micro pores was not difficult. The Al 2 O 3 layer was formed between the coating layer and Al substrate by outward transport of O 2− or OH − ions dissociated from water. The Al 3+ ions were transported inward form the aluminum oxide and filled the micro pores in the coating layer. As the Al 2 O 3 layer grew in the empty space, the tensile stress caused by the volume expansion of aluminum oxide increased. Thus, the newly formed aluminum oxide compressed the ZrO 2 coating layer, and the coating layer was gradually densified. With continued anodization, the coating layer became denser, and the outward transport of O 2− ions became more difficult. Mott 23 supposed that the ion transport rates decreased with increasing thickness of the anodic oxide film. The dense middle ZrO 2 layer and volume expansion of the outer Al 2 O 3 layer affect the transport rates of Al 3+ ions.

Conclusions
ZrO 2 thin films were successfully coated on etched foil using the vacuum infiltration method. The thickness of the 4 times coating layer was approximately 110 nm. A multi-layer structure (inner aluminum hydrate layer, middle Al-Zr composite layer, and outer Al 2 O 3 layer) was obtained for the ZrO 2 -coated Al foil after being anodized at 600 V. The thicknesses of these layers were approximately 120 nm, 240 nm, and 530 nm, respectively. This phenomenon differs from the formation behavior of composite oxide films on flat Al foils. Although the thickness of the Al 2 O 3 layer anodized in the ZrO 2coated foil was thinner than that of the non-coated Al foil, the ZrO 2coated foils could withstand a voltage of 640 V because of the inner layer including the ZrO 2 layer. Al atoms existed in the inner layer, and the entire coating layer transformed into the composite film. The volume expansion of the outer layer during anodizing was helpful for transporting the Al atoms and forming the Al-Zr composite layer. The ZrO 2 coated samples revealed 26% higher specific capacitance than samples without coating, which suggests that the vacuum infiltration and anodizing method is an effective way to increase capacitance of anodized aluminum foils.