Formation and Property of Al₂O₃-TiO₂ Composite Anodic Oxide Film on DC-Etched Al Foil for Al Electrolytic Capacitors

Abstract

This research article aims to improve the specific capacitance of DC-etched Al foil for Al electrolytic capacitors by forming an Al₂O₃-TiO₂ composite anodic oxide film. DC-etched Al foils for aluminum electrolytic capacitors were immersed in a TiO₂ precursor sol, followed by calcination and anodizing to manufacture a TiO₂-Al₂O₃ composite anodic oxide film. TiO₂ precursor sol–gel particles after calcination were analyzed by XRD. During anodization, the anode potential with time was measured by a digital meter. A scanning electron microscope, electrochemical impedance measurements, and a general digital LCR meter were adopted to explore the microstructure and property of the anodic oxide films. The specific capacitance for the TiO₂-Al₂O₃ composite anodic oxide film and a pure Al anodic one is 3.013 μF/cm² and 2.435 μF/cm² at C_60V, respectively. The thickness is 87.26 nm for the former and 177.65 nm for the latter. The results show that the TiO₂-Al₂O₃ composite anodic oxide film is about 51% thinner than the single Al anodic film, accounting for a large improvement in specific capacitance. The formation efficiency of the pretreated sample is much higher than that of the blank sample, owing to the pre-deposited TiO₂ layer and thermal Al oxide layer. However, the composite anodic oxide film’s specific resistance was reduced and its dielectric loss was also aggravated, resulting from the doping-introduced structural defects.

1. Introduction

Al electrolytic capacitors are indispensable, essential, and key components in electric and electronic devices including computers, mobile phones, and automobiles because they can provide filtering, energy storing and transformation functions, rectifying, blocking, and coupling due their small size and self-healing property, which are associated with high specific capacitance. The aluminum electrolytic capacitor primarily consists of anode and cathode foil electrodes separated by a thin barrier-type aluminum anodic oxide film (Al₂O₃) and acting as dielectric layers on anode Al foil for accumulating electrical charges. The capacitance and property of an anodic oxide film determines the size and performance of capacitors. The developing trend of the miniaturization and integration of electronic and electron products necessitates increasing the specific capacitance of capacitors.

The specific capacitance of capacitors may be expressed with basic Formula (1)

C = ε_rε_oS/d = ε_rε_oS/(E_a·K)

(1)

where C is the capacitance, ε_r the relative dielectric constant of dielectric layer, ε_o the vacuum permittivity, S the surface area of the dielectric (electrode), and d the thickness of the dielectric layers [1]. The d value depends on the formation potential (E_a) and formation constant (K). Equation (1) shows that increasing the C value can be achieved by increases in S, decreases in d, and increases in ε_r. Increases in S may usually be realized by the D.C or A.C electrochemical etching of Al foil before anodization to prepare high-density holes and tunnels on the Al foil, but now, the development of these methods has come to a standstill and a limit has been encountered. It is also very difficult to decrease d because d is related to the working voltage (E_a) of the aluminum electrolytic capacitor. While E_a is decided, the d value is largely decided because K is almost a fixed value when other anodizing conditions remain unchanged. Increases in ε_r may be possible by incorporating relatively large ε_r value compounds into the aluminum anodic oxide film to form composite dielectric layers. The ε_r value of Al₂O₃ is about 8~10, below other valve metal oxides, such as Ta₂O₅, ZrO₂ [1,2,3], Nb₂O₅ [4], and TiO₂ [5,6,7] and far less than the ferroelectric materials such as BaTiO₃ [8,9], Bi₄Ti₃O₁₂ [10], and (Ba_0.5Sr_0.5)TiO₃ [11]. The ε_r values of these oxides are summarized in

Table 1. Relative dielectric constant (ε_r) of some oxides.

Oxide	Al2O3	ZrO2	Ta2O5	Nb2O5	TiO2	BaTiO3	SrTiO3
εr	9.8	22~25	27.6	41.4	90.0	500~6000	>200,000

. Introducing these high-ε_r-value oxides into a single Al₂O₃ film is becoming a prospective and effective way to enhance the specific capacitance.

By virtue of electrodeposition, the sol–gel method, the pore-filling method, and hydrolysis/precipitation, many groups have successfully prepared Ta₂O₅-Al₂O₃, Nb₂O₅-Al₂O₃, TiO₂-Al₂O₃, ZrO₂-Al₂O₃, BaTiO₃-Al₂O₃, and (Ba_0.5Sr_0.5) TiO₃-Al₂O₃ composite anodic oxide films [12,13,14,15,16,17,18].

These studies focus on specific capacitance and flat Al foil. However, the other dielectric properties, apart from the capacitance of composite oxide film on DC-etched Al foil with high-density tunnels, have not been given sufficient attention in the past. Meanwhile, compared with electrodeposition, the pore-filling method, and hydrolysis/precipitation, the sol–gel process possesses noticeable advantages such as homogeneity, controllability, and low cost, promising industry application. In our previous work, a TiO₂-Al₂O₃ composite oxide film was successfully fabricated on an AC-etched Al foil via the sol–gel process [19].

Therefore, in this study, a 60 V formation voltage TiO₂-Al₂O₃ composite oxide film was produced on DC-etched Al foil with high-density tunnels by sol–gel, calcination, and anodization. Its composition and microstructure are studied by SEM and EDS. Its properties and performance are evaluated by EIS, specific capacitance measurement, loss tangent, and anode voltage–time curves.

2. Experimental

2.1. Samples

The 120 μm thick commercial DC-etched Al foil with multitudinous tiny tunnels, 99.99% high purity, and >98% cubic texture was tailored into samples in a size of 1 cm × 5 cm with a handle.

2.2. Sol–Gel Preparation and Coating

First, 10 mL tetrabutyl titanate was slowly added to the 35 mL dehydrated ethanol under intense magnetic stirring, which lasted for 10 min to obtain yellow clear solution A. Second, 2 mL glacial acetic acid and 10 mL distilled water were slowly dripped to the 35 mL dehydrated ethanol under intense magnetic stirring to obtain solution B, the pH of which was adjusted to about 3 by trace amount of hydrochloric acid. Third, solution A was slowly dripped into solution B under room-temperature water bath and intense stirring to obtain light-yellow TiO₂ precursor sol.

The specimen was immersed in the sol for 2min and pulled out from the sol at 1 mm/s. After being taken out from the sol, the specimen was heated in drying oven for 5 min at 80 °C. The dried specimens were calcinated in a furnace at 500 °C for 5 min. This procedure was repeated two times.

2.3. XRD Characterizing

TiO₂ gel was prepared by stirring and heating the sol at 40 °C in a water bath for 1 h and then were calcined at 500 °C for 5 min. The obtained powders were characterized by X-ray diffraction (XRD, D8ADVANCE). During characterization process, the scan rate was set as 10°/min and step size was set as 0.02° in the 2 θ range of 10–80° for phase identification.

2.4. Anodizing

According to the EIAJ criterion, the samples were first anodized to 60 V in 13% C₆H₁₃NO₄ electrolyte at 85 °C with a constant current of 25 mA·cm⁻². Once the selected voltage was obtained, the anodizing continued for 10 min, while the voltage was held constant and the current was permitted to drop. The time variation in the anode voltage were monitored and recorded by a multi-meter combined with PC system during anodizing process. After anodization, the samples were taken out from the solution, rinsed with deionized water, blown dry, and annealed at 500 °C for 2 min. After annealing, the samples were given re-anodization at 60 V for 2 min. The blank specimen without any pretreatment suffered the same anodizing process for comparison.

2.5. Micromorphology and Composition of Anodized Samples

After electro-polishing in acetic acid and perchloric acid solutions, scanning electron microscope (SEM, ZESSEVo18, Carl Zeiss GmbH, Oberkochen, Germany) and energy-dispersed X-ray analyzer (EDX, JEOL JEM-2000ES, Tokyo, Japan) were adopted to examine and analyze the micromorphology and composition of anodized samples.

2.6. Property of Anodized Specimens

The specific capacitance (C_60V) and loss tangent (tan δ) of the anodized samples was measured in 13% C₆H₁₃NO₄ electrolyte at 30 °C by a digital LCR meter (YY2810A, Tianjin Anfutai Electronic Technology Co., Ltd., Tianjin, China). A pure Al sheet with a very large area was adopted as the counter electrode. The testing frequency was adjusted to 100 Hz. Three parallel samples are used for mean value and reliability as one group.

Dielectric properties of the anodic oxide film were further examined by EIS measurement (Chenhua, CHI660, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). In EIS measurement, after C_60V and tan δ measurements for each group, only one sample was immersed in 13% C₆H₁₃NO₄ electrolyte at 30 °C, and 100 mV of sinusoidal voltage in 10⁻²–10⁵ Hz range was exerted. The software ZSimpWin 3.20 was used to analyze the EIS results.

3. Results and Discussion

3.1. XRD Patterns

Figure 1 demonstrates the XRD pattern of TiO₂ precursor gel calcined at 500 °C for 5 min. The diffraction peaks correspond to an anatase structure, a manner well consistent with JCPDS card No. 21-1272. It is conspicuous that calcination at 500 °C for 5 min is enough for the crystallization of TiO₂ gel.

During the sol–gel process, the following hydrolysis and polycondensation reactions can occur.

Ti(OR)_n + H₂O → Ti(OH)(OR)_n−1 + ROH

(2)

Ti(OH)(OR)_n−1 + H₂O → Ti(OH)₂(OR)_n−2 + ROH

(3)

Ti(OH)_n−1(OR) + H₂O → Ti(OH)_n + ROH

(4)

-Ti-OH + HO-Ti- → -Ti-O-Ti + H₂O

(5)

-Ti-OR + HO-Ti- → -Ti-O-Ti + ROH

(6)

In the following calcination step, the above -Ti-O-Ti gel structure was further transformed into an anatase structure. It has been reported that the calcination temperature dominated over calcination time during the process of crystallization of TiO₂ films, because when the annealing temperature is increased, the TiO₂ films show a higher crystallinity with anatase structures. However, at 580 °C and higher temperatures, the highly pure Al foil substrate is estimated to recrystallize among the tunnel and even melt, which will probably reduce the specific capacitance and effective surface area of the Al foils. Therefore, it is reasonable to select 500 °C as the annealing temperature for crystalizing TiO₂ precursor gel into an anatase structure.

3.2. Anodizing Behavior of Samples

Figure 2 shows the variations in the anode voltage with anodization time for the TiO₂-coated sample and a blank one. The voltage oscillation caused by the breakdown and reformation of an anodic film can be observed in the two curves for specimens, in agreement with others reports [18]. However, the initial anode voltage for the TiO₂-coated sample is about 3V higher than the blank one. At the same time, the curve slope for the TiO₂-coated sample is steeper than the blank one. The thermal aluminum oxide film and TiO₂ layer resulting from calcination may account for differences in initial anode voltage and V-t slope. It is generally believed that amorphous or crystalline γ-Al₂O₃ (for anodizing pure Al foil) can form depending on electrolyte temperature and anodizing voltage, and a high electrolyte temperature and high anodizing voltage tend to form a crystalline one. The reports form H. Uchi, T. Kanno and R. S. Alwitt suggested that the formation constant, K value, for amorphous e Al₂O₃ anodic films is somewhat larger than that for crystalline ones. The thermal Al₂O₃ film is beneficial for the growth of an additional crystalline γ-Al₂O₃ film, and a TiO₂ layer can promote the formation of a composite dielectric oxide film with a low K, during anodization, contributing to power conservation for anodizing.

3.3. Surface Microstructure and EDS of Specimens

Figure 3 shows the surface SEM images of anodized samples (a,b) with a pure Al₂O₃ aluminum anodic oxide film and (c,d) with an Al₂O₃-TiO₂ composite anodic oxide film. Cubic tunnels characteristic of DC-etching can be easily discerned for both samples. After anodization, barrier-type anodic oxide films have formed on the walls of these tunnels. Some films were stripped off the tunnel walls and even fall off due to electropolishing in CH₃COOH, HClO₄, and C₂H₆O mixed solution.

The thickness of the pure Al₂O₃ barrier-type film is about 177.65 nm, much larger than the Al₂O₃-TiO₂ composite one at only 87.26 nm. Therefore, the composite anodic film will maintain the effective surface area from DC-etching, beneficial for increasing the specific capacitance of the anode Al foils.

Figure 4 illustrates the area EDS of anodized specimens with a single Al₂O₃ aluminum anodic oxide film and an Al₂O₃-TiO₂ composite one. For the blank sample, Al, O, and C elements can be detected, characteristic of a pure Al₂O₃ aluminum anodic oxide film. Trace amounts of C elements can be attributed to the pollution of samples, impurities in the vacuum chamber, and ammonium adipate solution during anodization. For samples pretreated by TiO₂ precursor sol immersion and calcination, Al, O, Ti, and a trace of C elements can be discerned, corresponding to the Al₂O₃-TiO₂ composite anodic oxide film.

Figure 5 illustrates a schematic model about the growing of the Al₂O₃-TiO₂ composite anodic oxide film.

After immersion in the TiO₂ precursor sol and then drying, a lay of sol–gel was coated on the surface of DC-etched Al foil. After calcination, the above sol–gel was transformed into a TiO₂ coating and a thermal layer was also formed. During the following anodization, the thermal layer was transformed into barrier-type Al₂O₃ anodic films under the electric field. At the same time, O²⁻ anions were separated from the water at the base of the TiO₂ layer, while TiO₄⁴⁻ or TiO₃²⁻ ions were transported inward and Al³⁺ ions transported outward. The movement of these ions led to the formation of an Al₂O₃-TiO₂ composite anodic oxide film.

3.4. Property of Anodized Specimens

The specific capacitance (C_60V) and loss tangent (tan δ) of blank samples and ones with an Al-Ti composite oxide film are listed in

Table 2. C_60V and tan δ values of blank samples and Al₂O₃-TiO₂ composite oxide film.

Samples		C60V (μF/cm2)		tan δ
Blank	1	2.435	Mean value 2.435	0.033	Mean value 0.034
	2	2.440		0.034
	3	2.430		0.034
Composite	1	2.956	Mean value 3.013	0.039	Mean value 0.041
	2	3.040		0.042
	3	3.042		0.040

and further illustrated in Figure 6. It can be found that pretreatment via TiO₂ precursor sol immersion followed by calcination can increase C_60V from 2.435 μF·cm⁻² to 3.013 μF·cm⁻², about a 23.7% growth rate. This can be attributed to the high relative dielectric constant, ε_r, and the low formation constant, K, of an Al-Ti composite oxide film, compared to a single Al₂O₃ film, leading to improvements in the effective surface area of a DC-etched Al foil electrode and its specific capacitance. The change in C_60V for both specimens is consistent with SEM observations and EDS and XRD analysis.

However, the tan δ for the Ti-Al₂O₃ composite oxide film is 0.041, 20.6% higher than that of the blank one. During the anodization process after TiO₂ deposition, O²⁻ ions detached from the water and TiO₄⁴⁻ ions in the coating were transported inward and Al³⁺ ions in the substrate transported outward to form Al-Ti composite anodic oxide film under an electric field. More structural defects due to the doping effect by Ti⁴⁺ anions in the anodic oxide film may account for the increase in the tan δ value.

3.5. EIS of the Anodized Specimens

The EIS of a pure aluminum anodic oxide film and the Ti-Al₂O₃ composite one is shown in Figure 7. The Nyquist plots in Figure 7a indicate that a single time-constant capacitance behavior was predominant throughout the 10⁻²–10⁵ Hz range in this study.

In Bode plots in Figure 7b, the |Z| values with f curves decline sharply with a −∞ slope around 10⁻²~10⁰ Hz region and suddenly change to a horizontal line with f enhancement. The phase angle θ with f displays a similar variation tendency to that of the |Z|-f curves. These patterns are characteristic of dielectric materials. Both samples display the similar performance of |Z|-f and θ-f, suggesting that they possess the similar dielectric properties. However, the |Z| value in the Bode plot and the arc radius in the Nyquist plot for pure aluminum anodic oxide film are larger than those the for composite one.

As inserted in the diagram in Figure 7a, the EIS can be well simulated and interpreted by an equivalent electric circuit, consisting of a parallel combination C_ox and R_ox, connected in series to R_s, where C_ox and R_ox represent the specific capacitance and resistance of the film, and R_s is the solution resistance. The measured data and calculated data show good coincidence. According to the equivalent circuit and spectra data, the C_ox and R_ox values of the film can be calculated and listed in

Table 3. Dielectric property of samples with single Al anodic oxide film and composite one.

Specimen	Rox (MΩ·cm−2)	Cox (μF·cm−2)	C60V (µF·cm−2)	tan δ
Blank film	6.534	0.3785	2.435	0.034
composite film	0.7924	1.188	3.013	0.041

, which also includes film’s other electrical properties, such as C_60V and tan δ.

As shown by Table 3, the C_ox and C_60V values for the Al-Ti composite anodic film are higher than those for the blank one. According to the equation C = ε_rε_oS/d = ε_rε_oS/(E_a·K), the C value depends on ε_r/K under the same formation voltage, E_a. It has been reported that the ε_r/K values of TiO₂ and are Al₂O_3, 30.0 and 7.7, respectively. Therefore, the C_ox and C values of the specimen with the Al-Ti composite oxide film was higher than the blank Al₂O₃ film in this study.

However, the R_ox of the Al-Ti composite anodic film is lower than that for the blank one. The decreases are possibly due to the higher number of defects in Al-Ti composite oxide film due to doping, which simultaneously accounts for the increase in tan δ.

4. Conclusions

DC-etched Aluminum foil was anodized to 60 V in an ammonium adipate solution after TiO₂ coating by sol–gel processing and calcination to examine the dielectric properties and performance of the anodic oxide films. The following conclusions may be drawn.

• The anodizing of aluminum foil coated with TiO₂ films by sol–gel dip-coating and calcination leads to the formation of Ti-Al composite anodic oxide films on the metal substrate.
• Under the same formation voltage, the thickness of the Al-Ti composite anodic oxide film is only 87.26 nm, much less than 177.65 nm, i.e., that of the pure Al one, which is beneficial for maintaining the effective surface area from DC-etching and improvements in capacitance.
• The specific capacitance and formation efficiency of the pretreated sample are much higher than that of the blank sample due to the higher dielectric constant of TiO₂ and thermal aluminum oxide. But the specific resistance of composite anodic oxide film is decreased and its dielectric loss is also aggravated, due to doping-introduced structural defects.