Effect of Micro- and Nano-Structure of Anodic Aluminum Oxide on the Formation Behavior of Cracks Induced by Heat Treatment

Anodic oxide films on aluminium have been employed in a variety of devices such as solar cell [1], sensors [2] and thin film transistor liquid crystal display (TFT/LCD) [3]. It was reported [3, 4] that anodic aluminium oxide film is a solution to depress hillock formation. Thin film transistors with Al gates and anodic Al2O3 ‡ Si3N4 double layer gate insulators have been successfully fabricated in an 10.4inch diagonal multicolour LCD display panel [3]. Consequently, much progress has been made in understanding the structure [5, 6], composition [7– 10] and electrical properties [6, 7] of anodic aluminium oxide. In previous work it has been shown that an anodic oxide layer formed on pure Al without any prior heat treatment exhibits better dielectric properties, uniformity, and stability than oxide layers formed on Al pre-annealed at 410 8C [6]. Si or Cu doping of Al film was also found to reduce the quality of the anodic oxide layer [5]. For device application, the control of anodic film thickness and uniformity is very important. There are many methods to evaluate the thickness of an anodic oxide film, such as coulometry [11], transmission electron microscopy (TEM), ellipsometry [12], impedance [11], spectrophotometry [13], photoluminescence [14] etc. Coulometry is most convenient for anodic oxide film thickness determination, the thickness being calculated from the charge consumption based on Faraday’s law. The calculated thickness, however, is subjected to uncertainties due to non-ideal current efficiency, roughness of the electrode, film non-stoichiometry, and error in the presumed film density. TEM offers a direct and absolute measurement of film thickness, but is time consuming and destructive. Ellipsometry can determine not only the thickness but also the refractive index of the film [12, 15]. For a top surface layer on a substrate with given optical properties the evaluation of both thickness and refractive index requires the numerical solution of two complex simultaneous equations, and the accuracy of the thickness results depends sensitively on the calculated refractive index [12]. If, however, either the thickness or the refractive index of the top film is known precisely then the numerical solution of the other quantity can be greatly simplified and its accuracy significantly increased. The purpose of this study is to measure the thickness of anodic Al2O3 films precisely by cross-sectional TEM techniques and then use the thickness value to fit the ellipsometry data for the unique solution of refractive index as a function of film thickness. This calculation is made possible by the assumption that the extinction index of the Al2O3 film is zero, or in other words, the film is non-absorbing. Dell’oca [16] has carefully studied anodic Al2O3 films formed on evaporated Al films ellipsometrically and showed that the non-absorbing model fits his experimental data the best. He estimated the extinction index of anodic Al2O3 film to be 0.002. The error in the calculated refractive index is about 0.01 if the extinction index is 0.002 instead of zero. The effect of absorption may be neglected since it affects the refractive index by less than 0.6%. We hope that the refractive index data obtained in the present study can be used in device applications for quick and precise thickness measurement by ellipsometry. Sample preparation is similar to that described previously [5–6]. A 300 nm thick pure Al film was sputtered by DC magnetron on BPSG(borophosphosilicate glass)/SiO2/Si substrates. The substrates were 4-inch diameter, p-type, k1 0 0l Si wafers. Wafers were anodized after the metal deposition without any heat treatment, and the anodization was conducted at room temperature in an AGW electrolyte [6] (AGW electrolyte is a mixture of 3% aqueous solution of taitaric acid and propylene glycol at a volume ratio of 2:8). The wafers were anodized, one at at time, at constant current mode (current density ˆ 0.4 mA/cm2) initially until reaching 100 V, then the anodizing was automatically switched to constant voltage mode until a preset time was reached. To monitor the anodic oxide growth, specimens were anodized for different durations varying from 3.5 to 30 min. TEM samples were prepared for film thickness measurement by ion milling in the usual fashion [17] and examined with a Philips CM20 microscope operating at 200 kV. Using the thickness data obtained from the TEM analysis the refractive index of the anodic oxide film was evaluated by ellipsometry. A Rudolph Research Auto EL-ILL ellipsometer using an He–Ne laser at a wavelength of 632.8 nm was employed for ellipsometry with an incident angle of 708. The refractive index (n) and extinction coefficient (k) for the substrate aluminium are presumed to be 1.3 and 6.5, respectively [16]. Fig. 1a, b, c and d show the morphology of Al2O3

Electrophoretic deposition of PTFE particles on porous anodic aluminum oxide film and its tribological properties

The highly porous titanium granules are currently being used as bone substitute material and for bone tissue augmentation. However, they suffer from weak bone bonding ability. The aim of this study was to create a nanostructured surface oxide layer on irregularly shaped titanium granules to improve their bioactivity. This could be achieved using optimized electrochemical anodic oxidation (anodizing) and heat treatment processes. The anodizing process was done in an ethylene glycol-based electrolyte at an optimized condition of 60 V for 3 hours. The anodized granules were subsequently annealed at 450°C for 1 hour. Scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) were used to characterize the surface structure and morphology of the granules. The in vitro bioactivity of the samples was evaluated by immersion of specimens in simulated body fluid (SBF) for 1, 2, and 3 weeks. The human osteoblastic sarcoma cell line, MG63, was used to evaluate cell viability on the samples using dimethylthiazol-diphenyl tetrazolium bromide (MTT) assay. The results demonstrated the formation of amorphous nanostructured titanium oxide after anodizing, which transformed to crystalline anatase and rutile phases upon heat treatment. After immersion in SBF, spherical aggregates of amorphous calcium phosphate were formed on the surface of the anodized sample, which turned into crystalline hydroxyapatite on the surface of the anodized annealed sample. No cytotoxicity was detected among the samples. It is suggested that anodic oxidation followed by heat treatment could be used as an effective surface treatment procedure to improve bioactivity of titanium granules implemented for bone tissue repair and augmentation.

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

Porous-type anodic aluminum oxide films have attracted considerable attention as a key material in the fabrication of several types of devices owing to their potential technological applications. However, conventional anodic alumina films have low chemical resistance because the anodic oxide films obtained by typical anodizing processes are amorphous. Expanding the application of alumina films necessitates enhancement of the chemical resistance of alumina.Previously, we identified the optimized anodizing conditions and the detachment method for suppressing thermal deformation, such as curving and cracking, during heat treatment of the alumina membrane (1-4). In our previous study, anodizing was conducted under an applied voltage of 25 V–185 V, which resulted in the formation of an anodic porous alumina film with pore diameters tunable over a wide range of approximately 30-350 nm. When oxalic acid was used as an electrolyte, the fabrication of a crack-free and flat α-alumina membrane with a diameter of 25 mm, a thickness of 50 μm and a pore diameter of approximately 60 nm was fabricated using the optimized anodizing conditions in oxalic acid at 40 V, followed by subsequent detachment from the substrate and heat treatment (2). In the case of phosphoric acid, the fabrication of α-alumina membrane with a pore diameter of 350 nm was achieved through optimization of various conditions (3).In the present study, the chemical composition and morphological changes of the alumina membrane formed in phosphoric acid during heat treatment were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry-differential thermal analysis (TG-DTA). The results of analyses clearly showed that the alumina membrane formed in phosphoric acid has unique characteristics such as coexistence of crystalline alumina and aluminum phosphate at high temperature region above around 1100 oC. Therefore, we focused on the effect of heat treatment conditions (e.g., temperature, heating rate and holding time) on the morphological changes of alumina cell wall in comparison with alumina films formed in sulfuric acid and oxalic acid. By controlling of heating conditions, alumina membrane with hierarchically porous structures consisted of main macropores and smaller textural mesopores in the cell walls could be fabricated. Through-hole structure of heated alumina membrane was maintained even at high temperature, and additional mesopores were formed by the selective removal of aluminum phosphate from alumina cell wall.[1] F. Rashidi, T. Masuda, H. Asoh, S. Ono, Surface and Interface Analysis, 45, 1490-1496 (2013)[2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)[3] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)[4] H. Asoh, T. Masuda, S. Ono, ECS Transactions, 69, 225-233 (2015)

Having Low density and being Light weight with better mechanical properties, aluminum is the most significant material and is universally used in highly critical applications like navy, aerospace and particularly automotive activities. This research work is aimed to investigate the effect of micro and nano boron Al2O3 (Alumina Oxide) to aluminium (Al) on the mechanical and wear properties of the Al composites. The micro - nano composites with 1, 2, 3 and 4 % of Al2O3 particulates in Al are fabricated using stircasting processes. It was found that an increase of Al2O3 both as micro and nano particulates content resulted in an improved hardness, enhanced tensile strength and high wear resistance. However, nano Al2O3 reinforced MMCs have better hardness, improved tensile strength and higher wear resistance as compared with micro sized Al2O3 reinforced MMCs. Grain refinement of composite and nano composite materials as compared with pure Al were observed from the microscopic images. Analysis of wornout surface and tensile fracture surface were studied by SEM analysis to examine the nature of wear and tensile fracture mode of composite samples.

Microplastics in indoor environment: Sources, mitigation and fate

Transition metal dichalcogenides (TMDCs), which have a layered thin crystal structure, mechanical strength, and excellent flexibility, have attracted attention as a new material for electronic devices. Molybdenum disulfide (MoS2), a type of transition metal dichalcogenide, is a flexible and chemically and thermally stable material with a band gap of about 1.8 eV in a single layer. MoS2 is also expected to be a flexible and sensitive material for wearable humidity sensor due to its inherent defects and excellent humidity response. To date, several humidity sensors using MoS2 thin films have been reported, but they all suffer from a narrow relative humidity range, long response and recovery times, and low sensitivity. The general working principle of humidity sensors is adsorption/desorption, and one approach to improve the sensitivity is to physically increase the contact surface area for adsorption/desorption by structure. In this study, we attempted to fabricate MoS2 nanostructures to improve the sensitivity of MoS2-based humidity sensors. Furthermore, the humidity response of MoS2 nanotubes was evaluated by comparing the humidity response of the nanotubes in the plane direction and in the perpendicular direction (longitudinal direction of the tube) with the electrodes attached in different ways. An anodic aluminum oxide (AAO) template with a through-hole structure was first prepared by applying 40 V for 3 hours using 0.3 M aqueous oxalic acid solution as the first anode oxidation, followed by AAO selective etching to obtain hole traces. The voltage was then applied for 24 hours under the same conditions as the second anodic oxidation. The hole diameter was increased by widening, and the Al and barrier layers were removed to obtain the through-hole structure. In this study, an AAO through-hole template with a diameter of about 50 nm and a depth of about 40 μm was prepared. In order to coat the MoS2 precursor solution in the AAO template, MoS2 nanotubes were prepared by repeated decompression filtration, followed by a two-step heat treatment. From the scanning electron microscope (SEM) observation of AAO nanoholes before decompression filtration, the average diameter was about 50 nm. The average diameters after 4 and 8 cycles of decompression filtration were about 45 nm and 30 nm, respectively. It is estimated that a MoS2 film of about 2.5 nm was deposited after 4 cycles of decompression filtration, and about 10 nm was deposited after 8 cycles of decompression filtration. The thickness of the MoS2 film also increases with the number of cycles of decompression filtration, indicating that the thickness of the MoS2 layer can be controlled by the number of the cycles. We placed two different pairs of electrodes on the AAO surface and on the back of the AAO surface so that electrical measurements could be made both in the plane direction and in the perpendicular direction (longitudinal direction of the tube). We performed humidity response measurements in each of the two different electrode pairs, respectively. As a result, it was found that the humidity response in the perpendicular direction was about 6.8 times higher than that in the in-plane direction. In addition, compared with the humidity sensor without nanostructure of MoS2 thin film prepared using the same precursor, the humidity response range is wider and the response is about 11 times higher, indicating that the nanostructured sensor is expected to have higher sensitivity. In the presentation, the detailed structure, electrical properties, and humidity response will be discussed. Figure 1

Nanostructured thin-film composites vanadium oxide/porous anodic aluminum oxide (VO/PAO) were investigated. Thin layers of vanadium oxide were formed by sol-gel deposition from the solution of vanadium isopropoxide in isopropanol with subsequent annealing in Ar atmosphere at 500°C. As a substrate, porous anodic aluminum oxide and Si/SiO2 with PAO and without it were used. The orienting effect of PAO on the formation of the vanadium oxide layer has been established. Using the techniques of X-ray diffraction and electron microscopy, the formation of the crystalline vanadium dioxide VO2 phase, the orientation of the grains, which correlates with the morphology of the PAO substrate, has been established. The influence of electrolyte and stresses during the filling of the pores of aluminum oxide on the formation of grains, nanoclusters, and agglomerates of vanadium oxide is analyzed. Electrical properties of the nanostructured vanadium oxide layer were investigated. The characteristic of electrical resistance indicate the reversible insulator-metal phase transition in vanadium dioxide. The capacitances-voltage characteristics of the Al/VO/PAO/Al sandwich structures had an asymmetric character with an increase in capacitance upon increasing the negative bias voltage on vanadium oxide. This effect is determined by the phase transition insulator-metal under the influence of electric field. In addition, injection of electrons from the Al electrode or their extraction from vanadium oxide as well as recharging of the VO/PAO interface states can exert the influence. The mechanism for vanadium dioxide formation on the orienting substrate made of porous anodic aluminum oxide is proposed.

A new fluorescence enhancement technical platform based on the lithographically patterned anodic aluminum oxide (AAO) nanostructures is reported for the first time. The fluorescence images of several types of fluorescent dyes on the AAO and Au-coated AAO substrates have been obtained and examined. Significant fluorescence enhancement has been observed. This type of AAO-nanostructure platform has potential applications, especially its integration with microdevices and microfluidic devices for fluorescence-based biological analysis.

Durable omniphobicity of oil-impregnated anodic aluminum oxide nanostructured surfaces

Nanoporous anodic aluminum oxide (AAO) membranes can be fabricated with highly controllable thickness and porosity, making them ideal for filtration applications. Use of these membranes is currently limited largely due to their size and overall fragility. The objective of this research was to improve mechanical properties of AAO membranes through use of high temperature heat treatment to induce phase transformations in the material. A repeatable two-step anodization process was developed for consistent sample fabrication and heat treatments were performed at 900 °C and 1200 °C in air. The pore morphology and phase composition of the as-anodized and heat-treated membranes were then observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness testing was utilized to evaluate the mechanical behavior of the membranes before and after heat treatment. As-anodized AAO membranes were determined to be amorphous, and membranes heat-treated to 900 °C and 1200 °C were transformed to crystalline phases while retaining their original porous structure. Heat treatment to 900 °C resulted in formation of the γ-alumina transition phase in the skeleton regions of the membrane and nanocrystalline regions of α-alumina throughout the structure, while heat treatment to 1200 °C completely transformed the material to the stable α-alumina structure. The microhardness testing showed an increase in hardness from 2.5 ± 0.4 GPa to 4.7 ± 1.0 GPa in the transformation from amorphous to α-alumina.

Anodic aluminum oxide (AAO) was prepared in three types of aqueous solutions with various applied voltage. The mechanical property of AAO prepared in different electrolyte was investigated and hardness was increased on account of the increase of the thickness between pores. The mechanical property and microstructure change of AAO prepared in oxalic acid at 40V was investigated by heat treatment. AAO prepared in oxalic acid at 40V was transformed from amorphous to crystalline phase by heat treatment above 800oC and hardness was increased about 2.6 times with increase of heat treatment temperature.

Titanium and its alloys have been widely used for orthopedic implants because of their good biocompatibility. We have previously shown that the crystalline titania layers formed on the surface of titanium metal via anodic oxidation can induce apatite formation in simulated body fluid, whereas amorphous titania layers do not possess apatite-forming ability. In this study, hot water and heat treatments were applied to transform the titania layers from an amorphous structure into a crystalline structure after titanium metal had been anodized in acetic acid solution. The apatite-forming ability of titania layers subjected to the above treatments in simulated body fluid was investigated. The XRD and SEM results indicated hot water and/or heat treatment could greatly transform the crystal structure of titania layers from an amorphous structure into anatase, or a mixture of anatase and rutile. The abundance of Ti-OH groups formed by hot water treatment could contribute to apatite formation on the surface of titanium metals, and subsequent heat treatment would enhance the bond strength between the apatite layers and the titanium substrates. Thus, bioactive titanium metals could be prepared via anodic oxidation and subsequent hot water and heat treatment that would be suitable for applications under load-bearing conditions.

Preparation of bioactive titanium metal via anodic oxidation treatment