The application of the pore-filling method to the study of the structure of porous anodic films on aluminium

The process in which aluminium covered with a porous layer is re-anodized to obtain a compact layer is known as the pore-filling method. The purpose of this work was to study the relationship between anodizing conditions and the structure of the porous anodic film on aluminium by application of the pore-filling method.In a first series of experiments the influence of the anodization voltage on the porosity was examined. It was found that the inverse of the porosity is a function of the square root of the anodization voltage. The second part of our investigations consisted in studying the dependence of the thickness of the barrier layer on the anodization voltage. At each stage of the anodization process this barrier layer thickness is a function of the anodization voltage. Moreover, there is some evidence of a variation in the thickness of the barrier layer during an anodization process performed at a constant voltage.     

用低纯度铝制备AAO模板及其结构研究

在0.3mol/dm3草酸溶液中,通过不同纯度铝的恒电位二次阳极氧化制备了纳米孔氧化铝模板,并用场发射扫描电子显微镜(FE-SEM)和原子力显微镜(AFM)观察模板结构.实验结果表明,一次氧化除膜后低纯度铝基体表面呈现较为规则的六边形结构,这种蜂巢结构有利于二次氧化过程中获得有序度更高的纳米孔模板.低纯度铝制备的模板表面被晶界分隔为微小的区域,只是在较窄区域内才出现六边形规则排列的纳米孔.恒电位40V时所得模板经扩孔处理后,孔径由35nm增大到100nm左右,且孔径大小几乎一致.从纳米孔的有序度来看,由低纯度铝制备模板还需要进一步优化阳极氧化参数.     

Anodizing of etched aluminum foil coated with modified hydrous oxide film for aluminum electrolytic capacitor

Prior to galvanostatical anodization in boric acid solution, aluminum capacitor foil with a tunnel etch structure is treated in a two-step process in which a non-dense hydrous oxide film is first formed on foil in neutral boiling water for 10 min [namely, conventional hydration (CH)] and the hydrous oxide is then modified in a 80 °C weakly acidic solution containing trace amount of citric acid for 3 min [namely, modified conventional hydration (MCH)]. After modification, the hydrous oxide film becomes dense and thin. Time variations in the anode potential during anodizing were monitored, and the structure and dielectric properties of the anodic oxide films were examined by transmission electron microscopy, X-ray diffraction and electrochemical impedance spectroscopy measurements. It was found that the MCH-induced hydrous oxide film results in a decreased power consumption during anodization and an increased crystallized anodic oxide film, which has a high specific capacitance and a low specific resistance, comparing with the CH-induced hydrous oxide film.     

铝阳极氧化膜纳米孔阵列的微细结构

用电化学阳极氧化法制备了纳米多孔铝阳极氧化膜(AAO模板).采用原子力显微镜(AFM)测试AAO膜,研究了纳米孔阵列的形成机制.结果表明,在AAO膜的表面,除存在六方形的纳米孔阵列外,在孔端还存在六个微小的隆起,相邻的隆起之间彼此相连,看上去酷似一朵盛开的梅花,花的中心就是六方形纳米孔.二维AFM图像显示,以往用扫描电镜表征的纳米孔阵列,实际上是一幅排列整齐、并呈周期性变化的梅花阵列图案.膜背面阻挡层的AFM二维图像表明,膜胞呈六方形,且排列高度有序.膜胞密度为4.3×109/cm2,与孔密度基本一致.阻挡层的三维照片显示,膜胞的底部存在半球状突起,也呈现出规整的阵列图案.     

高度有序多孔阳极氧化铝模板的制备

为了得到纳米孔排列高度有序的多孔阳极氧化铝模板,以0.3 mol·L-1的草酸为电解液研究了模板的制备工艺.采用场发射扫描电子显微镜(FE-SEM)对多孔氧化铝模板的表面形貌进行表征,X射线衍射分析高纯铝及氧化膜的结构.实验结果表明,铝基体不经过高温退火处理,同样能够得到高度有序的氧化铝膜,简化了多孔氧化铝膜的制备工艺.分别讨论了阳极氧化电压和电解液温度对多孔阳极氧化铝膜的形貌及孔径的影响,并对一步法和两步法制得的多孔氧化铝膜进行比较,结果表明,两步阳极氧化法制备的多孔氧化铝模板的有序性优于一步氧化法.XRD分析证实,多孔氧化铝膜由非晶态的Al2O3组成.     

Ion-beam pore opening of porous anodic alumina: The formation of single nanopore and nanopore arrays

Anodized aluminium oxide (AAO) fabricated by electrochemical anodization of aluminium is used as the substrate for focused ion-beam (FIB) fabrication of single nanopore and nanopore arrays. The method is based on the controlled and selective removal of AAO barrier oxide film by FIB milling. FIB process with different milling conditions is applied to open single nanopores (diameter of 60 nm) or circular nanopore arrays with a different number of opened pores on AAO substrate. This process combines a low cost AAO substrate and FIB milling, and offers an economically viable alternative for the fabrication of nanopore devices with controlled dimensions for a range of applications in nanofluidics, molecular separations and biosensing.     

The nanoporous structure of anodic aluminum oxide fabricated on the Au/Nb/Si substrate

We have studied the anodization behavior of an Al film evaporated on the Au/Nb/Si substrate and demonstrated an effective approach to fabricate the through-hole anodic aluminum oxide (AAO) template on the conducting substrate. The smoothness of the initial metal films and an appropriate wet etching of the oxide film anodized in the first step were found to be critical factors for successfully anodizing the Al film on Au surface. The barrier layer of the obtained AAO structure presented a convex and thinner characteristic, and the underlying Au surface became porous after the anodization. This phenomenon was similar to the case of anodizing the Al film on an ITO glass substrate and could be explained reasonably by the effect of high pressure O₂ gas impelling and H⁺ etching at the interface of the barrier oxide and the Au layer.     

Improvement in corrosion resistance of NiTi by anodization in acetic acid

NiTi was galvanostatically anodized in acetic acid to increase the oxide film thickness for improving corrosion resistance. The galvanostatic anodization behavior of NiTi in acetic acid differed markedly from that of Ti. In particular, the anode potential reached was much lower for NiTi, and thus only thin oxide films could be obtained. With a suitable choice of anodizing conditions, the anodic oxide film formed had a thickness of 20–25 nm, as determined by profilometric measurement. Atomic force microscopy (AFM) revealed that the surface roughness was increased after anodization. Analysis by X-ray photoelectron spectroscopy (XPS) showed a low Ni/Ti ratio of 0.04 at the anodic oxide surface versus a value of 0.30 for bare NiTi. Electrochemical impedance measurements of the anodized sample in Hanks' solution at 37 °C recorded a 9-fold increase in polarization resistance, and cyclic polarization tests also recorded a matching reduction in the passive current density. These observations indicate that anodization of NiTi can serve as a simple low-temperature method to enhance the corrosion resistance of NiTi when used as an implant material.     

Nanopore gradients on porous aluminum oxide generated by nonuniform anodization of aluminum

A method for surface engineering of structural gradients with nanopore topography using the self-ordering process based on electrochemical anodization of aluminum is described. A distinct anodization condition with an asymmetrically distributed electric field at the electrolyte/aluminum interface is created by nonparallel arrangement between electrodes (tilted by 45°) in an electrochemical cell. The anodic aluminum oxide (AAO) porous surfaces with ordered nanopore structures with gradual and continuous change of pore diameters from 80 to 300 nm across an area of 0.5-1 cm were fabricated by this anodization using two common electrolytes, oxalic acid (0.3 M) and phosphoric acid (0.3 M). The formation of pore gradients of AAO is explained by asymmetric and gradual distribution of the current density and temperature variation generated on the surface of Al during the anodization process. Optical and wetting gradients of prepared pore structures were confirmed by reflective interferometric spectroscopy and contact angle measurements showing the ability of this method to generate porous surfaces with multifunctional gradients (structural, optical, wetting). The study of influence of pore structures on cell growth using the culture of neuroblastoma cells reveals biological relevance of nanopore gradients and the potential to be applied as the platform for spatially controllable cell growth and cell differentiation.     

One-step anodization fabrication and morphology characterization of porous AAO with ideal nanopore arrays

A fabrication method for one-step anodization of an anodic aluminum oxide (AAO) template with nanopore arrays using pretreated high purity aluminum foil is reported in this article. Morphology of the AAO was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results showed that porous AAO with ideal nanopore arrays can be fabricated by one-step anodization fabrication technology on high purity aluminum foil which had been anodized at 45?V direct current (DC), in 0°C, 0.5?M H₂C₂O₄ solution for 48 hours. The average pore diameter and the interpore distance were 80?nm and 120?nm, respectively. Nanopores in porous AAO had very narrow size distribution and were arranged into hexagonal array. The formation mechanism of nanopore arrays in porous AAO is discussed. Porous AAO with ideal nanopore arrays provide an ideal template for preparation of many one-dimensional nanomaterials. One-step anodization of AAO is a simpler procedure and more applicable in industrial application than the previous two-step anodization technology.     

Structural engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium

Nanoporous anodic aluminium oxide has traditionally been made in one of two ways: mild anodization or hard anodization. The first method produces self-ordered pore structures, but it is slow and only works for a narrow range of processing conditions; the second method, which is widely used in the aluminium industry, is faster, but it produces films with disordered pore structures. Here we report a novel approach termed "pulse anodization" that combines the advantages of the mild and hard anodization processes. By designing the pulse sequences it is possible to control both the composition and pore structure of the anodic aluminium oxide films while maintaining high throughput. We use pulse anodization to delaminate a single as-prepared anodic film into a stack of well-defined nanoporous alumina membrane sheets, and also to fabricate novel three-dimensional nanostructures.     

In-situ fabrication of AAO template without oxide barrier layer and its applications

Porous anodic aluminum oxide (AAO) film is the most widely used template in combination with electrodeposition (ED) method to fabricate one-dimensional nanostructures such as nanowires, nanotubes and nanorods. However, the existing oxide barrier layer after the anodization blocks the application of AAO template in synthesis of nanostructures via direct electrodeposition. In this paper, AAO template without oxide barrier layer was successfully fabricated by stepwise voltage decrement; influence of two types of stepwise voltage decrement on the removal of oxide barrier layer was introduced. Field Emission Scanning Electron Microscopy (FESEM) images indicated that stepwise voltage decrement could make the oxide layer thin effectively. Meanwhile, highly ordered gold nanowire arrays were fabricated by using direct electrodeposition method based on AAO template with the second anodization process with stepwise voltage decrement of 1 V/min, FESEM image showed that as-prepared gold nanowires are uniform in diameter and the diameter is in accordance with the diameter of AAO template pores. XRD pattern revealed that gold nanowires were indexed as face-centered cubic phase.     

Variation of nanopore diameter along porous anodic alumina channels by multi-step anodization

In order to form tapered nanocapillaries, we investigated a method to vary the nanopore diameter along the porous anodic alumina (PAA) channels using multi-step anodization. By anodizing the aluminum in either single acid (H3PO4) or multi-acid (H2SO4, oxalic acid and H3PO4) with increasing or decreasing voltage, the diameter of the nanopore along the PAA channel can be varied systematically corresponding to the applied voltages. The pore size along the channel can be enlarged or shrunken in the range of 20 nm to 200 nm. Structural engineering of the template along the film growth direction can be achieved by deliberately designing a suitable voltage and electrolyte together with anodization time.     

Effect of hydration on microstructure and property of anodized oxide film for aluminum electrolytic capacitor

Etched aluminum foil for aluminum electrolytic capacitor was first boiled in water for different time to form hydrous film on Al foil and then anodized in H₃BO₄ solution at 530 V to form anodic oxide barrier film as insulating dielectric layer. The obtained films were characterized by field-emission scanning electron microscopy, transmission electron microscope and X-ray diffraction for surface morphology, microstructure and crystallinity examination. Small-current charging, LCR meter and electrochemical impedance spectroscopy were exploited to measure the propertied of the anodized oxide film such as withstanding voltage (U_w), specific resistance (R_ox) and specific capacitance (C_s and C_ox) for its electrochemical performance. The results show that the hydrous film is pseudoboehmite (PB) with a dense inner layer and a fibrous outer layer. The crystallinity of the PB film increases with hydration time. During anodization, the PB film was transformed into anodic oxide (γ′-Al₂O₃) barrier film. Prolonging hydration time promotes transformating PB into γ′-Al₂O₃ and improves the crystallinity of the barrier film, leading to increase in C_s and C_ox and decrease in R_ox and U_w.     

Electrical conduction phenomena in thin tetracene films

Thin film aluminium oxide capacitors using anodic Al₂O₃ as the dielectric are described. The dependence of oxide thickness and dielectric loss on anodization voltage was studied. Variation of capacitance with temperature and frequency was also investigated. The capacitors were used in conjunction with tantalum resistors to fabricate an astable multivibrator circuit and the waveforms were recorded. The combination of aluminium oxide capacitors and tantalum resistors has some advantages over all-tantalum RC networks.     

包铝层和氧化时间对2E12铝合金硫酸阳极氧化及膜层性能的影响

对保留表面包铝和去除包铝的2E12-T3铝合金采用硫酸阳极氧化处理工艺,研究了包铝层和氧化时间对铝合金阳极氧化行为及膜层耐蚀性的影响。采用扫描电子显微镜观察氧化膜的表面以及截面形貌,应用动电位扫描极化曲线和电化学阻抗谱对膜层的电化学性能进行分析。结果表明:两种铝合金表面均能形成具有防护性能的阳极氧化膜,膜层随氧化时间延长而增厚。富铜的第二相颗粒会使得不带包铝的2E12铝合金氧化膜具有更多孔洞缺陷,甚至出现微裂纹。保留包铝的2E12铝合金表面氧化膜更厚,孔洞缺陷少,耐蚀性更好。阳极氧化30min和45min的2E12铝合金阳极氧化膜具有较低的腐蚀电流和较高的多孔层阻抗,耐蚀性好。     

How structure changes in fabrication of large size ordered anodic alumina film?

Large size anodic alumina film has not been used in the industry due to that the fabrication parameters are very difficult to control, but the fabrication of large size anodic alumina is exigent as a template in the fabrication of diverse nano-devices oriented to the industrialization. In this paper, large size (width length = 80 mm × 80 mm) porous ordered anodic alumina film was fabricated by using two-step anodization process as compared to the small size (diameter = 40 mm) anodic alumina film in the structures. Pore size and film thickness of anodic alumina film are strongly related to the size of the anodization film. The large size anodic alumina film has an ideally ordered pattern by applying low voltage. However, with the increase of voltage, the ordered pattern of the PAA films was gradually disrupted, especially in the 70 V due to the local thermal imbalance.     

Nanopore domain growth behavior by nanopore changes near domain boundaries in porous anodic alumina

We investigate the nanopore changes near domain boundaries during the nanopore growth in porous anodic alumina (PAA) to understand the domain growth behavior with the anodization time. In order to observe the pore changes with the time, we analyze cleavage planes of PAA according to the nanopore length using a field emission scanning electron microscopy. The domain growth can be explained with three kinds of nanopore changes observed near domain boundaries: a change of pore diameter, a pore-branching, and a pore-movement.     

Investigations on the self-organized growth of TiO2 nanotube arrays by anodic oxidization

The formation mechanism of a thin film of self organized TiO₂ nanotube arrays prepared by anodic oxidization of a pure titanium sheet in electrolyte solutions containing potassium fluoride and sulfate was investigated through near-real time monitoring the anode mass, the current density, and the surface topography during the anodization. Energy dispersive X-ray spectrometry was used to monitor the surface composition change during the anodization. The titanium surface was first electrochemically oxidized to form a layer of dense oxide under which nanotubes were originated. With the protection of the oxide layer, long nanotubes could be formed in electrolyte solutions with relatively high pH. The surface composition analysis indicates that the nanotubes were not totally oxidized to TiO₂. However, no other elements but Ti and O were found in the oxide film. This work reveals a way to fabricate long nanotubes with defined sizes.     

Synergistic effect of rare earth salt and organic acid in the anodization of aluminum in phosphoric acid

The effect of rare-earth element (cerium salt) and organic acid (citric acid) on the anodic oxide film obtained in phosphoric acid and their synergistic mechanism in the anodizing process were studied. The results show that the synergistic effect of cerium salt and citric acid in anodization of phosphoric acid can reduce surface defects, improve its microstructure and properties of the anodic oxide film. With the analysis of EDAX and XPS, the hydroxide of cerium salt was deposited on the film surface. It is deduced that cerium salt takes part in the formation of oxide film directly on the synergistic effect of citric acid.