Anodic Aluminum Oxide Layer Research Articles

Annealing temperature dependence of structural, optical, and thermal properties of CdSe thin films grown on porous anodic alumina

In this study, cadmium selenide (CdSe) were synthesizd by a chemical route process using l-cysteine (l-cyst) as a ligand, cadmium acetate as a Cd source and Na2SeO3 as a Se precurseur. CdSe nanocrystals were embedded in porous anodic alumina (PAA) layer by the chemical bath deposition (CBD) method. The influence of annealing temperature in air-atmosphere from 150 to 350 °C on the microstructural, optical properties, and thermal conductivity of CdSe in PAA template has been studied by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), Raman spectroscopy, UV–Vis and photothermal deflection (PTD) techniques. The annealing temperature at 300 and 350 °C resulted in a considerable change in the surface morphology of CdSe film on the PAA layer, from a nonhomgeneous CdSe clusters into microrods array with a pyramidal ends. Raman spectroscopy measurements show an enhancement in the crystallinity of CdSe film on PAA layer and an increase of the average radius size of the nanocrystallites from 2.41 to 2.78 nm with the increase of air-annealing temperature. The optical reflectivity result of CdSe on PAA template showed that the crystal quality was significantly improved after the annealing step, bringing about an enhancement in optical absorption and a decrease in bandgap energy. The thermal conductivity Kth was studied via PTD technique, and the Kth increase was depicted with thermal treatement from 4.40 to 5.22 W M−1 K−1.

Water droplet evaporation from sticky superhydrophobic surfaces

The evaporation dynamics of water from sticky superhydrophobic surfaces was investigated using a quartz crystal microresonator and an optical microscope. Anodic aluminum oxide (AAO) layers with different pore sizes were directly fabricated onto quartz crystal substrates and hydrophobized via chemical modification. The resulting AAO layers exhibited hydrophobic or superhydrophobic characteristics with strong adhesion to water due to the presence of sealed air pockets inside the nanopores. After placing a water droplet on the AAO membranes, variations in the resonance frequency and Q-factor were measured throughout the evaporation process, which were related to changes in mass and viscous damping, respectively. It was found that droplet evaporation from a sticky superhydrophobic surface followed a constant contact radius (CCR) mode in the early stage of evaporation and a combination of CCR and constant contact angle modes without a Cassie–Wenzel transition in the final stage. Furthermore, AAO membranes with larger pore sizes exhibited longer evaporation times, which were attributed to evaporative cooling at the droplet interface.

Nano-patterned aluminum surface with oil-impregnation for improved antibacterial performance

Extensive research has been reported to improve the performance of existing antibacterial surfaces by reducing the extent of bacterial adhesion and biofilm formation. This study aimed to validate oil-impregnated slippery nanoporous anodic aluminum oxide (AAO) layers for the prevention of bacterial attachment on aluminum substrate. The oil-impregnation into the nanoporous structures of Teflon-coated hydrophobic AAO enhanced the repellency to the water with a low sliding angle as low as 2°, showing the omniphobicity, which is effective to reduce the contact of bacterial culture medium onto the surface of AAO. Bacterial attachment tests using Escherichia coli K-12 and Salmonella Typhimurium were conducted to evaluate the effect of antibacterial performance on the developed surfaces. Teflon-coated large pore AAO with oil-impregnation showed the highest bacterial reductions for both E. coli K-12 and S. Typhimurium among the treated samples. These results can lead to further optimization of nano-patterned AAO surfaces for antibacterial attachment.

Enhanced light extraction of LYSO scintillator by photonic crystal structures from a modified porous anodized aluminum oxide layer

Although porous anodized aluminum oxide layer can be used to extract scintillation light from a LYSO scintillator, the low refractive index contrast of porous AAO layer obtains a moderate enhancement. In this investigation, we have designed and fabricated a modified porous anodized aluminum oxide layer with conformal deposition layer of high refractive index material of TiO2 on the surface of LYSO scintillator, achieving a significant enhancement by 60% with wavelength- and angle-integrated emission intensity. The fabrication method of the present study is simple and low-cost for the large area applications in the field of radiation detection.

Influence of the heat treatment condition of alloy AlCu4Mg1 on the microstructure and properties of anodic oxide layers

Due to their outstanding specific mechanical properties, high-strength, age-hardenable aluminum alloys offer a high potential for lightweight security-related applications. However, the use of copper-alloyed aluminum is limited because of their susceptibility to selective corrosion and their low wear resistance. These restrictions can be overcome and new applications can be opened up by the generation of protective anodic aluminum oxide layers. In contrast to the anodic oxidation of unalloyed aluminum, oxide layers produced on copper-rich alloys exhibit a significantly more complex pore structure. It is the aim of the investigation to identify the influence of microstructural parameters such as size and distribution of the strengthening precipitations on the coating microstructure. The aluminum alloy EN AW-2024 (AlCu4Mg1) in different heat treatment conditions serves as substrate material. The influence of the strengthening precipitations’ size and distribution on the development of the pore structure is investigated by the use of high-resolution scanning electron microscopy. Integral coating properties are characterized by non-destructive and light-microscopic thickness measurements and instrumented indentation tests.

Oil‐Impregnated Nanoporous Oxide Layer for Corrosion Protection with Self‐Healing

The major drawback of current passivation techniques for preventing corrosion is the lack of ability to withstand any external damages or local defects. In this study, oil‐impregnated nanoporous anodic aluminum oxide (AAO) layers are investigated to overcome such limitations and thus advance corrosion protection. By completely filling hydrophobized nanopores with oil via a solvent exchange method, a highly water‐repellent surface that prevents the penetration of corrosive media into the AAO layer and hence the corrosion of aluminum is achieved. The impregnation of oil into the hydrophobic nanoporous AAO layer enhances the corrosion resistance of an AAO layer by two and four orders of magnitude compared to that of a hydrophobic (i.e., air‐entrained) and a bare (hydrophilic) AAO, respectively. In the presence of local defects, the oil impregnated within the hydrophobic nanoporous AAO layer naturally permeates into the defects and ultimately inhibits the exposure of the aluminum surface to corrosive media. Whereas the corrosion current density of the air‐entrained hydrophobic AAO layer increases by more than 30 times after cracks, that of the oil‐impregnated AAO layer increases by no more than 4 times, showing superior anticorrosion property even after there are cracks, owing to the effective self‐healing capability.

Nanostructure analysis of InGaN/GaN quantum wells based on semi-polar-faced GaN nanorods

We demonstrate a series of InGaN/GaN double quantum well nanostructure elements. We grow a layer of 2 μm undoped GaN template on top of a (0001)-direction sapphire substrate. A 100 nm SiO2 thin film is deposited on top as a masking pattern layer. This layer is then covered with a 300 nm aluminum layer as the anodic aluminum oxide (AAO) hole pattern layer. After oxalic acid etching, we transfer the hole pattern from the AAO layer to the SiO2 layer by reactive ion etching. Lastly, we utilize metal-organic chemical vapor deposition to grow GaN nanorods approximately 1.5 μm in size. We then grow two layers of InGaN/GaN double quantum wells on the semi-polar face of the GaN nanorod substrate under different temperatures. We then study the characteristics of the InGaN/GaN quantum wells formed on the semi-polar faces of GaN nanorods. We report the following findings from our study: first, using SiO2 with repeating hole pattern, we are able to grow high-quality GaN nanorods with diameters of approximately 80-120 nm; second, photoluminescence (PL) measurements enable us to identify Fabry-Perot effect from InGaN/GaN quantum wells on the semi-polar face. We calculate the quantum wells’ cavity thickness with obtained PL measurements. Lastly, high resolution TEM images allow us to study the lattice structure characteristics of InGaN/GaN quantum wells on GaN nanorod and identify the existence of threading dislocations in the lattice structure that affects the GaN nanorod’s growth mechanism.

Analysis of peel strength of consisting of an aluminum sheet, anodic aluminum oxide and a copper foil laminate composite

Laminate composites consisting of an aluminum sheet, anodic aluminum oxide, and copper foil have been used as heat-spreader materials for high-power light-emitting diodes (LEDs). These composites are comparable to the conventional structure comprising an aluminum sheet, epoxy adhesives, and copper foil. The peel strength between the copper foil and anodic aluminum oxide should be more than 1.0 kgf/cm in order to be applied in high-power LED products. We investigated the effect of the anodic aluminum oxide morphology and heat-treatment conditions on the peel strength of the composites. We formed an anodic aluminum oxide layer on a 99.999% pure aluminum sheet using electrochemical anodization. A Ti/Cu seed layer was formed using the sputtering direct bonding copper process in order to form a copper circuit layer on the anodic aluminum oxide layer by electroplating. The developed heat spreader, composed of an aluminum layer, anodic aluminum oxide, and a copper circuit layer, showed peel strengths ranging from 1.05 to 3.45 kgf/cm, which is very suitable for high-power LED applications.

A template/electrochemical deposition method for fabricating silver nanorod arrays based on porous anodic alumina

A template/electrochemical deposition method for fabricating silver nanorod arrays based on porous anodic alumina was presented. The barrier layer of porous anodic alumina templates was thinned by step-by-step voltage decrement method. Subsequently, silver ions were reduced into the channels of porous anodic alumina templates by electrochemical deposition method. Electrochemical impedance spectroscopy was utilized for analyzing the thickness of barrier layer of porous anodic alumina templates; the elementary composition and the size of silver nanorod arrays were characterized by X-ray diffraction and field-emission scanning electron microscope, respectively. Experimental results showed that the thickness of barrier layer of porous anodic alumina was suitable for alternating current electrochemical deposition, when anodizing potential was decreased to 70 V and widening time of porous anodic alumina in H3PO4 solution is 80 min. And the silver particles could be deposited into the channels of porous anodic alumina templates at 11 V, 13 V, and 15 V. Different sizes of silver nanorod arrays were obtained by controlling the deposition time. The average diameter of silver nanorod is in the range from 346 nm to 351 nm which is almost consistent with the pore diameter of porous anodic alumina templates (367 nm). The uniform silver nanorod arrays have a considerable potential in the flexible and wearable electronic devices, optics, solar cell, the catalytical electrode, and so on.

Nanoflower structures of ZnO–SnO2 arranged on nanoporous Al2O3 for sensing ethanol and methanol gases

A compound sensing structure based on nanoflower-like ZnO---SnO2 coated on the top of nanoporous anodic aluminum oxide (AAO) layer to form ZnO---SnO2---AAO composite as a gas sensing structure is developed for detecting ethanol and methanol simultaneously. The ZnO---SnO2 powders were synthesized by hydrothermal synthesis method. The SnO2 was grown densely and uniformly that has unique loose and porous nano structures to increase the accessible surface area for improving gas diffusion and mass transport. A solenoid-type heater was designed to surround the sensing column for controlling operation temperature to promote gas sensitivity. The ethanol and methanol responses of ZnO---SnO2---AAO are high because the porous AAO greatly improves the specific adsorption surface area, and the ZnO produces more electron donor states or oxygen vacancies to enhance oxygen adsorption as well as the heterojunction of SnO2 and ZnO grains offers access to facile electronic interaction to enhance the surface reaction between adsorbed oxygen and current-carrying electrons. By analyzing a series of temperature-varied cyclic tests, the optimal operation temperatures of 180 °C for sensing ethanol and 260 °C for sensing methanol were obtained to have the strongest output responses. The developed ZnO---SnO2---AAO gas sensing composites include the advantages: simple fabrication process, low cost, high specific surface area and porosity, rapid detection, high sensitivity, stable stability, and good repeatability.

Photo- and electroluminescence properties of lanthanide tungstate-doped porous anodic aluminum oxide

A new cathode material for the potential use in light-emitting devices, based on porous anodic alumina (PAA), aluminum and ITO layers has been synthesized. Porous alumina samples with ordered pore arrays were prepared electrochemically from high purity Al sheet in H2SO4 and H3PO4. To be able to apply the matrix obtained in the electroluminescence cell, the thickness of the barrier layer of aluminum oxide was decreased by slow reduction of the anodization voltage to zero. The luminescence and electroluminescence (EL) properties of the Al2O3 matrix admixtured with Eu3+ and Tb3+ ions as well as europium and terbium tungstates, were determined. The particles of inorganic luminophore were synthesized on the walls of the matrix cylindrical nanopores in the two-step process of immersion in solutions of TbCl3 or EuCl3 and Na2WO4. The effect of the nanopores diameter and the thickness of the porous Al2O3 layer on the intensity and relative yield of electroluminescence was analyzed, the best results were obtained for 80–90 µm PAA layers with 140 nm nanopores.

Ionic transport and barrier effect of anodic oxide layer in a solid-state Al2O3 capacitor under high electric field

Dry anodic oxidation as an effective electrical self-healing method in a solid-state Al2O3 capacitor was investigated in terms of ionic transport study. Sol-gel-derived amorphous aluminum oxide (AmAO) film as dielectric and aluminum (Al) film as top electrode of the capacitor were prepared on platinized silicon substrates. When a negative voltage was applied to the Al electrode pad, the ionic transport would induce the failure of the Al electrode. Nevertheless, such failure could be inhibited by the barrier effect of an anodic aluminum oxide (AAO) layer formed at the Al/AmAO interface when positive voltage was applied to the Al electrode pad previously. The barrier effect of the AAO layer was controlled by the maximum applied positive voltage. An ionic transport model was proposed to explore the dry anodic oxidation in the solid-state Al2O3 capacitor.

Nanoporous anodic alumina oxide layer and its sealing for the enhancement of radiative heat dissipation of aluminum alloy

Various types of nanoporous anodic aluminum oxide layers and their sealings were studied to enhance the thermal emissivity and hence improve the heat dissipation of aluminum alloy for energy application. Dissipating heat fluxes from the anodized aluminum surfaces were measured using a modified steady-state method and investigated with respect to the various nanoporous morphologies obtained with different anodizing conditions and sealing methods. Results show that the anodized nanoporous oxide layers significantly enhance the thermal emissivity and heat dissipation of aluminum alloy, compared to bare aluminum alloy, and such enhancement is further improved with sealings. A thicker nanoporous oxide layer anodized in oxalic acid results in higher thermal emissivity and better heat dissipation than that in sulfuric acid, showing a darker color which is attributed to the more irregular and disordered pore size and pattern of the nanoporous oxide layer. The nanoporous oxide layer with cold NiF2 or black sealing shows further enhancement in thermal emissivity and heat dissipation, demonstrating the highest enhancement in emissivity up to 0.906 in case of the nanoporous oxide layer anodized in oxalic acid with black sealing, which is seven times greater than that of bare aluminum. The nanoporous oxide layer with black sealing also results in the significant improvement of the cooling efficiency of a heat exchanger system of aluminum alloy by 36.4%, suggesting great energy saving for real energy application.

Characterization of nanopores arrangement of anodic alumina layers synthesized on low-(AA1050) and high-purity aluminum by two-step anodizing in sulfuric acid with addition of ethylene glycol at low temperature

In this work the anodic aluminum oxide (AAO) was produced by two-step self-organized anodization of two classes purity aluminum in 0.3 M sulfuric acid aqueous solution with addition of ethylene glycol as the solution modifier. The arrangement analysis method based on fast Fourier transforms was applied to characterize the porous arrays ordering. The quality, arrangement, circularity and regularity of nanoporous AAO formed on the low-purity (AA1050) and high-purity aluminum were investigated in details. It was found that the regularity of the nanopores ordering obtained on low-purity aluminum at applied experimental conditions was extraordinarily high while compared to the data published previously. The relation between the structural features of nanopores as well as aluminum concaves ordering obtained in two-step anodization process depending on the step of reaction, was analyzed. It was proven that purity of anodizing aluminum influence the interpore distance, wherein the purer aluminum the smaller interpore distance. The observed phenomenon was discussed in details in accordance with intrinsic mechanism. Furthermore, there are no simple relations between regularity ratio of obtained nanostructures and the step of anodizing.

SIMULATION OF TECHNOLOGICAL PROCESS BY USAGE NEURAL NETWORKS AND FACTORIAL DESIGN OF EXPERIMENTS

The possibilities of simulation of technological process ofaluminium anodic oxidation using the methodology of Design of Experiments (DOE) and theory of neural networks in order to monitor the anodizing process under various operating conditions are presented in this paper. The influence of chemical and physical input factors on the resulting AAO (anodic aluminium oxide) layer thickness at applied current density of 1 A x dm-2 and 6 A x dm-2 has been investigated. Based on the evaluation of experimentally obtained data, the computational predictive model describing the effect of individual input factors and their mutual interactions on the AAO layer thickness was developed in the form of cubic function. This model indicates which factors are important and how they combine to influence the response, it will enable us to optimize operating conditions. The most significant benefit of our research work in this field is the fact that all relevant factors were varied simultaneously.

Self-Ordering Regimes of Porous Anodic Alumina Layers Formed in Highly Diluted Sulfuric Acid Electrolytes

Nanoporous anodic alumina films with long-range hexagonal order have been obtained from a series of highly diluted sulfuric acid electrolytes. A simple linear relationship was established between the self-ordering voltages and acid concentrations (28, 29, and 30 V for 0.2, 0.1, and 0.05 M electrolytes, respectively). Besides establishing new self-ordering regimes, our experimental work sheds new light on some fundamental principles of honeycomb anodic alumina formation. It suggests that the spontaneous self-organization of a stable nanoscale structure originates from the electrohydrodynamic (EHD) convection rather than from Marangoni-type instability at the anode surface. Theoretical analysis displays a decreasing exponential functional relationship between electrolyte concentration and the critical values of the earlier found electrochemical analogue of Rayleigh number, which can be used for prediction of hexagonal cell pattern in currently unexplored anodizing electrolytes.

The effect of electrolyte change on the morphology and degree of nanopore order of porous alumina formed by two-step anodization

The aim of this work was to verify if the quality, regularity and spacing of nanoconcaves formed on the Al surface during the first step of anodization (pre-texturing) have a significant effect on the morphology of AAO layers grown during the second anodization continued at the same potential but in different electrolytes. High purity (99.999%) aluminum substrates were anodized at 25V and 45V in sulfuric acid and oxalic acid, respectively. After chemical etching of initially formed oxide in a mixture of chromic and phosphoric acid, the samples were re-anodized under the same potentials, but in different electrolytes (sulfuric, oxalic, phosphoric acids). It was confirmed that the regular nanoporous structure formed on the Al surface during the first anodizing step is initially replicated at the beginning of the second anodization, independently of the electrolyte used. However, these initially formed oxide layers exhibit significantly different pore diameters but the same pore spacing, which is determined by the cell spacing on the Al surface. When phosphoric acid is used as an electrolyte, the prolonged anodization resulted in the formation of typical serrated nanochannels. However, we successfully obtained the almost ideally-ordered porous anodic alumina layer with a thickness of about 700nm, and the average diameter of nanochannels of ∼50nm after the second anodization carried out for 20min at 45V in 0.3M H3PO4, outside from the self-ordering regime for this electrolyte.

Porous anodic alumina layers with modulated pore diameters formed by sequential anodizing in different electrolytes

A high purity (99.999%) Al foil was pre-textured by 1h of anodic oxidation in 0.3M oxalic acid at 45V and room temperature, followed by the chemical etching of as formed irregular oxide layer. As prepared samples were used for fabrication of porous anodic aluminum oxide (AAO) layers with modulated pore diameters by a simple anodization carried out at the same anodizing conditions (potential and temperature) but in a sequentially changed electrolyte (0.3M H2C2O4 and 0.3MH3PO4). It was proved that anodization carried out in the oxalic acid result in a relatively fast (~250nmmin−1) formation of porous Al2O3 structure with an average pore diameter of about 30nm. On the other hand, anodic oxidation in the phosphoric acid electrolyte leads to a much slower (~40nmmin−1) formation of AAO layer with widen nanopores – about 45nm in diameter. As a result, nanoporous Al2O3 layers with distinguishable nanochannel segments having different pore diameters were obtained. Moreover, no loss of hexagonal pore order was observed even when the anodization was carried out in phosphoric acid at the potential of 45V which is far away from the self-ordering regime for this electrolyte (~195V).

The Influence of Input Factors of Aluminium Anodizing Process on Resulting Thickness and Quality of Aluminium Oxide Layer

In order to optimize the technological process of aluminium anodic oxidation, the possibilities of usage of sodium chloride in the electrolyte has been studied, since very small concentration of sodium chloride allows us to reduce concentration of other components of the electrolyte. Also the influence of sodium chloride concentration in the electrolyte on the final thickness and quality of the formed anodic aluminium oxide (AAO) layer has been investigated in this paper. In contrast to common anodizing experiments, in which the influence of only one separate factor at a time is considered, in our research all relevant factors (four chemical factors) were varied simultaneously according to the methodology of statistical experimental design, i.e. design of experiments (DOE). Based on the evaluation of experimentally obtained data by application of mathematical-statistical methods and theory of neural networks, the relationship between the concentration of sodium chloride in the electrolyte and final thickness of the AAO layer was experimentally determined. Thanks to that it was possible to obtain the predictive model which can determine the final thickness of AAO layer. Moreover, the results of this research allows us to reduce the concentration of other components of the electrolyte up to the level of 25% of commonly used concentration of these electrolyte components designed for the process of aluminium anodic oxidation.

Manipulation of stored charge in anodic aluminium oxide/SiO2 dielectric stacks by the use of pulsed anodisation

A method of fabricating anodic aluminium oxide (AAO) with the capability of manipulating its stored charge is reported. This method involves the use of a pulsed current source to anodise aluminium layers instead of the typically used constant current/voltage source, with the test structures experiencing positive and negative cycles periodically. By tuning the positive cycle percentage, it is demonstrated that the effective stored charge density can be manipulated in a range from −5.2×1011 to 2.5×1012q/cm2 when the AAO is formed over a 12nm SiO2 layer. An investigation of the stored charge distribution in the dielectric stacks indicates a positive fixed charge at the SiO2/Si interface, a negative fixed charge at the AAO/SiO2 interface and a positive bulk charge within the AAO layer. The effective stored charge density and interface states were found to be affected by annealing conditions and it is suggested that oxygen annealing can reduce the bulk positive charge while post-metallisation anneal is most effective in reducing silicon interface defects. Charge manipulation using pulsed anodisation is shown to reduce carrier recombination on boron-diffused silicon surfaces highlighting the potential of the process to be used to tune the electrical properties of dielectric layers so that they can reduce surface recombination on silicon surfaces having different dopant polarity and concentrations.