Formation of Al 2 O 3 BaTiO 3 nanocomposite oxide films on etched aluminum foil by sol gel coating and anodizing

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1 J Sol-Gel Sci Technol (2) 45:57 61 DOI.7/s ORIGINAL PAPER Formation of 2 O 3 BaTiO 3 nanocomposite oxide films on etched aluminum foil by sol gel coating and anodizing Xianfeng Du Æ Youlong Xu Received: 26 June 27 / Accepted: 13 August 27 / Published online: 1 December 27 Ó Springer Science+Business Media, LLC 27 Abstract Barium titanate BaTiO 3 (BT) sol was dipcoated on the etched aluminum foils by a sol gel process. After annealed at 6 C in air, the foils were anodized in ammonium adipate solution. The voltage time variations during anodizing were monitored. The structure, composition, and electric properties of the anodic foils were investigated. The obtained foils were fabricated into aluminum electrolytic capacitors and the load and shelf life test were measured. It was found that the slope of the voltage time curve of aluminum foil covered with BT films became steeper. A triple layer of / 2 O 3 /BT was formed after anodizing /BT foil. The specific capacitance and the product of specific capacitance and withstanding voltage of anodic foil with a BT coating were about 46.36% and 3.9% larger than that without a BT coating. After the load life and shelf life of 25 h at 5 C, two kinds of capacitors have similar behaviores and meet with Nichicon standard of Japan. From the results, BaTiO 3 is promising to be used as the dielectric of aluminum electrolytic capacitor to increase the specific capacitance. Keywords uminum electrolytic capacitor Sol gel Barium titanate Electrical properties Load and shelf life X. Du Y. Xu (&) Electronic Material Research Laboratory, Key Laboratory of the Ministry of Education, Xi an Jiaotong University, Xi an 749, P.R. China ylxu@mail.xjtu.edu.cn X. Du loonydxf@3.com 1 Introduction With the characteristics of the large capacitance and low cost, aluminum electrolytic capacitors are widely used in home appliances, computer motherboards, peripherals, power supplies, industrial electronics, communication products, and automobiles [1, 2]. They can mainly provide filtering, bypassing, rectifying, coupling, blocking, energy storing, and transforming functions, which play an important role in the electric and electronic products [2, 3]. The aluminum electrolytic capacitor basically consists of two aluminum electrodes separated by thin dielectric layers for accumulating electrical charges. The barriertype anodic oxide layers on aluminum processed by formation treatment are used as dielectric layers in aluminum electrolytic capacitors, and the capacitance of this dielectric layers mainly determine the capacitance of capacitors. Recent developments in mobile electronic devices strongly require small capacitors with high capacitance. Thus, in order to meet the requirement, it is necessary to increase the specific capacitance of dielectric. The basic equation relating to capacitor can be expressed by: C ¼ e e r S d ; ð1þ where C is electric capacitance, e is the dielectric constant of vacuum atmosphere, e r the relative dielectric constant of the anodic oxide films, S the effective surface area of the dielectric (electrode), and d the thickness of the dielectric. From Eq. 1, we know that there are three ways to increase the C-value: increases in S, decreases in d, and increases in e r [4, 5]. The increases in S can be generally obtained by electrochemical etching a high purity aluminum foil before anodization, but now it faces with the limit [6, 7]. It is also

2 5 J Sol-Gel Sci Technol (2) 45:57 61 very difficult to decrease d because d is related to the working voltage of the aluminum electrolytic capacitor. While the working voltage is decided, the d-value is largely decided [2, 6]. Increases in e r may be possible by incorporating relatively large e r -values compounds into anodic oxide film to form composite dielectric layers. Many groups have tried to increase the electric capacitance by incorporating value metal oxides such as Ta 2 O 5,Nb 2 O 5, TiO 2, ZrO 2, and others [4 9]. Normally, the ferroelectric materials have the relative dielectric constant more than s, even 1,s, and can be easily prepared by sol gel method. BaTiO 3 (BT) is a typical ferroelectric material, which was widely studied [ ]. In this paper, we tried to introduce BT into dielectric layer of aluminum electrolytic capacitor to promote its capacitance, and the electric properties of capacitor incorporated with BT were discussed. The structure of the composite oxide film was also investigated. 2 Experimental In this experiment, BT sol was prepared as follows: the required amount of barium acetate was dissolved in 36% acetic acid and stoichiometric tetra-n-butyl titanate was dissolved in lactic acid, respectively. Then the two kinds of solution were mixed. Thereafter, adding deionized water and 36% acetic acid, the solution was diluted to.3 M and ph was controlled between 1 and 2. A high purity commercially low-voltage etched aluminum foil with spongy pores was used as specimen. The specimen was first dipped in the BT sol for 5 min. After drying, the specimen was heat-treated at 6 C for 3 min. The specimens with or without BT coating were anodized (respectively marked as /BT and ) in 15 wt.% ammonium adipate with a constant current of 25 ma/cm 2. They were held under 21 V until the residual current density reached 1.25 ma/cm 2. The changes in the anode voltage and the current with time during anodizing were monitored by a PC system. Then the specimens were annealed at 5 C for 2 min, followed by re-anodization. At last, the specimens were dried at C for measurement. The flow chart is illustrated in Fig. 1. The obtained foil was manufactured into capacitors with WV, 1.4 AV, 1, lf, 9 mm. Electric properties test of anode foil including specific capacitance (C), dissipation factor (tan d), withstanding voltage (V), and leakage current (I) were measured according to the method of Japan Capacitor Industrial CO., Ltd. (JCC). Load/shelf life test of capacitor were measured according to the method of Nichicon standard of Japan [13]. The structure and composition of composite oxide films were investigated by field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX). 3 Results and discussion 3.1 Anodization behavior Figure 2 shows the changes of the anode voltage and current with anodizing time. At the initial moment, there are jumps of about 1.6 V for and an about.6 V for /BT. The former may be due to the natural aluminum oxide film the latter presumably results from thermal aluminum oxide film and BT thin films [, 15]. Then, the anode voltage of two specimens linearly increases with anodizing time. The slope of the anode voltage versus anodizing time curve for /BT is much steeper than that for. It can be explained that the presence of thin thermal aluminum oxide film on the surface of aluminum promotes the growth of additional crystalline c - 2 O 3 during anodizing [, 17], which also results in the decrease in anodizing time. 3.2 FE-SEM micrograph and EDX pattern FE-SEM microscopy shows the changes in the film structure by dip-coating and the subsequent anodizing. Figure 3a and b show the surface morphology of and /BT (The sample has been calcined at 6 C for 3 min), respectively. The surface morphology of is rough and there are many small grains on the surface. After dip-coating BT and heat-treatment, its surface Fig. 1 Flow chart for the preparation of /BT specimens: (1) BT sol, (2) ammonium adipate solution Heat-treatment Annealing Drying 1 2 Dip-coating Anodization Re-anodization

3 J Sol-Gel Sci Technol (2) 45: Anodizing Voltage /V /BT Anodizing Current /ma cps O Ti(Ba) Ba(Ti) Ba Ba Anodizing Time/s Fig. 2 Anodizing curves of and /BT E/keV becomes smooth and the grains can hardly be seen, which is maybe caused by the covering of BT thin film. Figure 3c shows back-scattering electron image of cross section of anodized /BT. It clearly indicates that the anodized /BT consists of three layers. They are identified as the substrate, the anodized oxide 2 O 3, and the covering BT, respectively. The BT layer has a thickness of about nm and the 2 O 3 layer has a thickness of about 3 nm. The EDX pattern of /BT is performed in Fig. 4. Ti and Ba can be observed on the surface of coated BT. The pattern exhibits the presence of Ti and Ba peaks with an average atomic percentage ratio of about 1:1 which suggests that BT exists as BaTiO 3 on the surface of lowvoltage etched aluminum foil. 3.3 Electric properties test In order to test the electric properties of the anodic foils, the following parameters are measured: specific capacitance (C), dissipation factor (tan d), withstanding voltage (V), and leakage current (I). The data are listed in Table 1. Fig. 4 EDX pattern of /BT We also give the data of the product of C and V (CV), increase rate of C (DC), and increase rate of CV (DCV) in Table 1. The DC and DCV are defined as follows: DC ¼ C =BT C C % ð2þ DCV ¼ CV =BT CV % ð3þ CV From Table 1, wecanfindthatthec and CV of /BT increase sharply (from 65.36, 1,54 to 95.66, and 2,139, respectively) when the BT is incorporated into the anode foil. This is due to the formation of the 2 O 3 BaTiO 3 composite oxide layer with a high dielectric constant. The DC and DCV amount to 46.36% and 3.9%, respectively. The tand of /BT is higher than that of, which is partly caused by the higher C according to the Eq. 4 [1]. Fig. 3 FE-SEM micrograph of (a) surface of, (b) surface of /BT, and (c) cross section of anodized /BT

4 6 J Sol-Gel Sci Technol (2) 45:57 61 Table 1 Electrical properties of the foil coated with or without BT thin film Specimen C (lf/cm 2 ) tan d (%) V (V) I (la) CV (lfv/cm 2 ) DC (%) DCV (%) /BT Fig. 5 Load life test at 5 Cof capacitor: (a) capacitance reduction, (b) dissipation factor, and (c) leakage current (a) CR (%) (b) I(µA) µ tnaδ δ (%) (c) Time (hrs) /BT tan d ¼ xcr ð4þ where x is the measured frequency, and r is the equivalent series resistance. The increases in I and the decreases in V are resulting possibly from the defects in 2 O 3 BaTiO 3 composite oxide film and thermal aluminum oxide film [9, 19, 2]. 3.4 Load and shelf life test The anodic foils were fabricated into aluminum electrolytic capacitors and their load and shelf life test were measured. The results are illustrated in Figs. 5 and 6. According to the standard of Nichicon Co., LTD [13], the judging criteria of failure are defined as follows: (1) leakage current increases more than.1p (P, the product of initial capacitance and working voltage); (2) capacitance changes greater than ±25% of initial value; (3) tan d goes beyond 3%. In this paper, if one of these three criteria happens, the capacitor was treated as failure. From Fig. 5, it is found that there is little difference between and /BT during the load life test. After application of the rated voltage for 2,5 h, three parameters of electric properties still meet the requirement. During the shelf life test, changes of /BT are similar to that of except leakage current (see Fig. 6). The leakage current of /BT is always greater than that of ; it may be due to the defects such as crack and hole in composite oxide film. However, the leakage current increases less than.1p after 2,5 h. From the results, we can see that the capacitor life time of /BT is more than 2,5 h, which indicates that the method reported in this paper can be applied to the industrial production. 4 Conclusion BT thin film was coated on high purity low-voltage etched aluminum foil by sol gel method for application as dielectrics in aluminum electrolytic capacitor. The /BT has a higher V-jump, a steeper slope, and a shorter anodizing time during the anodization, which is due to the thermal aluminum oxide film. The composite oxide film on the substrate consists of an outer BT coating and an inner 2 O 3 layer. The specific capacitance and the product of specific capacitance and withstanding voltage of anodic oxide film formed on specimen with a BT coating are about 46.36% and 3.9% larger than that without a BT coating.

5 J Sol-Gel Sci Technol (2) 45: Fig. 6 Shelf life test at 5 C of capacitor: (a) capacitance reduction (CR), (b) dissipation factor, and (c) leakage current (a) CR (%) (b) I (µa) µ tanδ δ (%) (c) /BT Time (hrs) The behavior of capacitor for /BT is similar to that for during the load and shelf life test. The capacitor life time is larger than 2,5 h. BaTiO 3 is promising to be used as the dielectric of aluminum electrolytic capacitor to increase the specific capacitance. Acknowledgments The authors wish to thank Shenzhen City DongYangGuang Industrial Development Co., LTD for experimental assistance. The authors are grateful to Dr. Jingping Wang for his help with drawing of Fig. 1. This work was supported financially by the National 63 plan Foundation of China (Granted No. 23AA3253) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Granted No. 2469). References 1. Xu Y (24) Ceram Int 37(7): Wang K-L, Chang R-F (26) J Power Sources 2:55 3. Nishino A (1996) J Power Sources 6: Watanabe K, Sakairi M, Takahashi H, Hirai S, Yamaguchi S (1999) J Electroanal Chem 473:25 5. Shikanai M, Sakairi M, Takahashi H, Seo M (1997) J Electrochem Soc 4(): Park S-S, Lee B-T (24) J Electroceram 13: Kangping Y, Xingsu S (1997) Electron Comp Mater (1):13. Watanabe K, Sakairi M, Takahashi H, Hirai S, Yamaguchi S (21) Electrochemistry 69(6):47 9. Lai G-C (1999) J Ceramic Soc Jpn 7():21. Choi KJ, Biegalski M, Li YL, Sharan A, Schubert J, Uecker R, Reiche P, Chen YB, Pan XQ, Gopalan V, Chen L-Q, Schlom DG, Eom CB (24) Science 36(569):5 11. Kamalasanan MN, Deepak Kumar N, Subhas C (1993) J Appl Phys 74(9):5679. Li A, Ge C, Lu P, Wu D, Xiong S, Ming N (1997) Appl Phys Lett 7(): 13. Technical note CAT1D, Nichicon Co., Japan. Hirai S, Aizawa S, Shimakage K, Wada K (1995) J Jp Inst Metals 59(5): Kobayashi K, Shimizu K (19) J Electrochem Soc 135(4): 9. Crevecoeur C, de Wit HJ (1976) The influence of crystalline alumina on the anodization of aluminum. Paper 132, presented at the 27th Meeting of the International Society of Electrochemistry, Extended abstract, Zurich, Sept 6 (1976) 17. Bernard WJ, Florio SM (195) J Electrochem Soc 132(): Chen G, Cao W (1993) Electrolytic capacitor. Xi an Jiaotong University Press, 13pp 19. Raymond MV, Smyth DM (1996) J Phys Chem Solids 57(): Staiti P, Lufrano F (25) J Electrochem Soc 152(3):617