Effect of the anodization voltage on the pore-widening rate of nanoporous anodic alumina

ABSTRACT: A detailed study of the pore-widening rate of nanoporous anodic alumina layers as a function of the anodization voltage was carried out. The study focuses on samples produced under the same electrolyte and concentration but different anodization voltages within the self-ordering regime. By means of ellipsometry-based optical characterization, it is shown that in the pore-widening process, the porosity increases at a faster rate for lower anodization voltages. This opens the possibility of obtaining three-dimensional nanostructured nanoporous anodic alumina with controlled thickness and refractive index of each layer, and with a refractive index difference of up to 0.24 between layers, for samples produced with oxalic acid electrolytes.     

Production of alumina templates suitable for electrodeposition of nanostructures using stepped techniques

In recent years, the possibility of using anodized aluminium as a template to obtain nanowires and nanotubes via electrodeposition has been firmly established. The main problem with this method is the high electrical resistance generated by the barrier layer that isolates the metallic base from the electrodeposition bath. An easy way to decrease this resistance is to perform an additional step before deposition: a barrier layer thinning (BLT) process . Different BLT procedures have been described and although some of them work well, the process is not yet well understood. Here, we present an optimized stepped galvanostatic BLT process and discuss how BLT is achieved. As opposed to the common belief that BLT takes place via dendritic porous growth at the bottom of the porous anodic alumina (PAA), we show that during thinning, a controllable branched-shaped porous structure is generated. Finally, we also analyze the use of stepped techniques to obtain alumina templates with narrower pores than expected in an oxalic acid bath.     

Electrochemical impedance spectroscopic study of barrier layer thinning in nanostructured aluminium

The electrochemical behaviour of electropolished and anodised aluminium was studied by electrochemical impedance spectroscopy (EIS). Freshly electropolished aluminium behaves as a pure capacitor exhibiting Warburg impedance at low frequencies. Storage of the electropolished aluminium, even in an air-tight bottle, results in the reconstruction of a uniform compact barrier layer. The impedance response of a stored electropolished aluminium as well as anodised aluminium after oxide removal, done by chemical etching, exhibits only a capacitive loop in the complex plane. The effect of the oxide layer thickness on the impedance data was investigated for layers formed during anodising at a cell potential of 15 or 23 V. Impedance measurements carried out over a wide range of frequencies gave useful information on the efficiency of the thinning of the barrier layer at the bottom of porous aluminium oxide layers. The rate of thinning of the barrier layer was estimated for samples anodised at different voltage.     

Behavior of alumina barrier layer in the supporting electrolytes for deposition of nanowired materials

In this study, we report the results obtained investigating the behavior of sulfuric and chromic acid alumina templates in typical supporting electrolytes frequently used for alternating current (ac) deposition of various nm-scaled materials. Qualitative analysis of voltammetric profiles taken for as-grown, ac-treated and annealed alumina films in a conventional tetraborate re-anodizing solution revealed dramatical changes in the properties of alumina barrier layer during ac treatment in these supporting electrolytes even at low current density. These changes were related here with the transport of protons through the barrier layer during ac treatment, discharge at the metal/oxide interface and hydrogenation of alumina material by hydrogen atoms in an upward way. This conclusion comes from the behavior of Pt/Hg|alumina|Me^z+ electrode and the valence band X-ray photoelectron spectra taken from the inner part of alumina barrier layer material before and after the ac treatment.     

Kinetic study and model verification of reaction between alumina and charred layer

The abnormal ablation of insulation materials caused by slag deposition in solid rocket motors has gradually attracted considerable attention from researchers. First, an alumina-charred layer system was thermogravimetrically experimented with 1700–1850 °C using ultra-high-temperature thermogravimetric equipment based on the dominant equation of an Al₂O₃–C system obtained from previous studies. The kinetic parameters of the alumina-charred layer reaction system were then determined by the isothermal kinetic mathematical treatment method. Second, an ablation model was established to verify the accuracy of the kinetic parameters. The ablation thickness calculated by the model is nearly similar to that in the experimental results, with an error of 16.01%. Results show that the kinetic parameters of the alumina-charred layer's reaction system obtained in this study are credible and can be used for predicting the insulator's ablation under deposition conditions.     

Fabrication of ultrafine-grained alumina ceramics by two different fast sintering methods

The preparation of ultrafine-grained alumina ceramics by the fast sintering technique Self-propagating High-temperature Synthesis plus Quick Pressing (SHS-QP) method and spark plasma sintering (SPS) technique was reported. The effects of different heating rates (SHS-QP-1600 °C/min, SPS-200 °C/min) on the preparation of ultrafine structure were compared. The densification and grain growth as a function of sintering time and temperature were discussed. Within a short sintering time (<3 min), the full-dense alumina with ultrafine-grained structure was obtained by SHS-QP at 1550 °C under 100 MPa. By SPS, the sintering temperature was lower (1200 °C) than that of SHS-QP. The differences in densification parameters were explained by analyzing the thermodynamics of sintering process.     

Characteristics of gas permeation using two-layered alumina membrane prepared by anodic oxidation

The microporous alumina membrane with asymmetrical structure, having upper layer with pore diameter of 10 nm under and lower layer with pore diameter of 36 nm, was prepared by anodic oxidation using DC power supply of constant current mode in an aqueous H₂SO₄ solution as a electrolyte. The aluminum plate was pretreated with thermal oxidation, chemical polishing and electrochemical polishing before anodic oxidation. The membranes were prepared by controlling the current density such as a very low current density for upper layer and a high current density for lower layer of membrane. By controlling the cumulative charge density, the thickness of upper layer of membranes was about 6 Μm and the total thickness of membranes was about 80–90 Μm. We found from gas permeation experiments with the membranes prepared by above method that the mechanism of gas permeation of the all membranes prepared under each condition complied with model of the Knudsen diffusion.     

铬酸浓度对铝阳极氧化多孔膜阻挡层形成过程的影响

对铝在不同浓度的铬酸中阳极氧化多孔膜阻挡层形成过程进行研究,随着铬酸浓度的增加,阻挡层形成电流密度增加,形成阻挡层所需的时间减少,阻挡层厚主变薄。这些现象可归结为阻挡层形成时应力集中的结果。     

Template-directed synthesis of porous alumina particles with precise wall thickness control via atomic layer deposition

Highly porous alumina particles with precise wall thickness control were synthesized by atomic layer deposition (ALD) of alumina on highly porous poly(styrene-divinylbenzene) (PS-DVB) particle templates. Alumina ALD was carried out using alternating reactions of trimethylaluminum and water at 33 °C. The growth rate of alumina was ∼0.3 nm per coating cycle. The wall thickness can be precisely controlled by adjusting the number of ALD coating cycles. Thermo-gravimetric analysis, X-ray diffraction, nitrogen adsorption, scanning electron microscopy, and transmission electron microscopy were used to characterize the fabricated porous alumina particles. The effect of number of ALD coating cycles and calcination temperature on the mesoporous structure of the alumina particles was investigated. γ-Alumina was formed at temperature above 600 °C. Porous alumina particles with a surface area of 80-100 m²/g were obtained and thermally stable at 800 °C. The pore volume of the porous particles can be as high as 1 cm³/g after calcination at 800 °C. Such porous alumina particles may find wide application in nanotechnology and catalysis.     

Fabrication and structural control of anodic alumina films with inverted cone porous structure using multi-step anodizing

Porous anodic alumina (PAA) film has recently attracted much attention as a key material for the fabrication of various nanostructures. In this study, a multi-step anodization and leaching process was employed to produce three-dimensional nanometer scale structured film. During the leaching process, the porous alumina film was dipped in phosphoric acid solution for pore widening. Each anodization process was followed by this leaching process. This method produced alumina film with multi-step structure. Meanwhile, with five-step film production, the structure showed inverted cone structure. We produced the low aspect ratio pores of this structure, which would be applicable for fabrications of nanomaterials. In addition, the aspect ratio was controlled by changing the anodization duration.     

The interaction of cobalt species with alumina on Co/Al2O3 catalysts prepared by atomic layer deposition

Alumina supported cobalt catalysts were prepared by atomic layer deposition (ALD) of cobalt acetylacetonate precursors (Co(acac)₂ and Co(acac)₃). The main modes of interaction between the acetylacetonate precursors and the support were found to be the exchange reaction between the alumina OH-groups and the acac-ligands of the precursor and dissociative adsorption on coordinatively unsaturated Al³⁺ sites. The amount of precursor that could adsorb on the support was determined by steric hindrance. Samples were prepared using 1–5 reaction cycles, i.e. subsequent precursor addition (Co(acac)₂) and calcination, resulting in catalysts containing ca. 3–10 wt.% Co. Samples were also prepared where the last calcination step was omitted, i.e. uncalcined catalysts. Calcination at 450 °C decreased the reducibility of the Co(acac)₂/Al₂O₃ catalysts due to formation of a cobalt oxide phase strongly interacting with the support and aluminate type surface species. The reducibility increased with metal loading on both calcined and uncalcined catalysts; however the reducibility of the calcined catalysts remained lower than of the uncalcined ones. The dispersion was found to be lower on the calcined catalysts. The cobalt particle sizes on the calcined samples was ca. 8 nm and on the uncalcined 4–5 nm, for cobalt loadings of ca. 6–10 wt.%. Catalytic activity was tested by gas phase hydrogenation of toluene in temperature programmed mode (30–150 °C).     

Transient growth and thinning of the barrier oxide layer on iron measured by real-time spectroscopic ellipsometry

The transient growth and thinning of the passive film on iron was investigated in ethylenediaminetetraacetic acid, disodium salt (EDTA) containing borate buffer solutions of pH 8.4 using real-time spectroscopic ellipsometry under potentiostatic control. EDTA effectively suppressed the formation of the outer layer of the passive film, thereby rendering the barrier layer amenable to direct examination. It was shown that the barrier layer growth was completed in about 10 s upon potential stepping in the anodic direction. On the other hand, the thinning of the barrier oxide layer upon potential stepping in the cathodic direction occurred at a rate that was two orders in magnitude lower than the growth rate. The steady-state barrier layer thickness varied linearly with applied potential, whereas the steady-state current density did not depend on the formation potential. These spectroscopic ellipsometric measurements are qualitatively explained by the point defect model (PDM).     

Densification of fine-grained alumina ceramics doped by magnesia,yttria and zirconia evaluated by two different sintering models

The influence of various dopants (500 ppm MgO and Y₂O_3; 250 ppm ZrO₂) on sintering of fine-grained alumina ceramics was evaluated by high-temperature dilatometry. The apparent activation energy of sintering was estimated with the help of Master Sintering Curve and a model proposed by Wang and Raj. The densification kinetics was controlled by at least two mechanisms operating at low (higher activation energy) and high (lower activation energy) densities. Good agreement between the activation energies calculated with both models was observed for low as well as for high densities. The lowest value of activation energy exhibited undoped alumina; the addition of MgO resulted in slight increase of the activation energy. Y₂O₃ and ZrO₂ significantly inhibited the densification, which was reflected in the higher sintering activation energies. The low activation energies in the final sintering step indicates the importance of proper choice of sintering temperature, namely in the two-step sintering process.     

Atomic layer deposition of Al2O3 thin films on diamond

Atomic layer deposition (ALD) of aluminum oxide thin films on diamond was demonstrated for the first time, and the film properties as a gate insulator for diamond field effect transistor (FET) were examined. The interface between the aluminum oxide and the diamond was abrupt, and the ratio of aluminum to oxygen in the film was confirmed to be stoichiometric by Rutherford back scattering. Even a bumpy surface of polycrystalline diamond film was conformally covered by the Al₂O₃ films. To evaluate the feasibility of the film for FET gate insulator, the electrical characteristics of the Al₂O₃ films deposited by ALD on diamond were measured using metal–insulator–semiconductor structure. It was found that the Al₂O₃ films deposited by ALD were better than those deposited by conventional methods, which indicates that the ALD-Al₂O₃ films are feasible for gate insulators of diamond FETs.     

Tuning nanoporous anodic alumina distributed-Bragg reflectors with the number of anodization cycles and the anodization temperature

The influence of the anodization temperature and of the number of applied voltage cycles on the photonic properties of nanoporous anodic alumina-based distributed-Bragg reflectors obtained by cyclic voltage anodization is analyzed. Furthermore, the possibility of tuning the stop band central wavelength with a pore-widening treatment after anodization and its combined effect with temperature has been studied by means of scanning electron microscopy and spectroscopic transmittance measurements. The spectra for samples measured right after anodization show irregular stop bands, which become better defined with the pore widening process. The results show that with 50 applied voltage cycles, stop bands are obtained and that increasing the number of cycles contributes to enhancing the photonic stop bands (specially for the case of the as-produced samples) but at the expense of increased scattering losses. The anodization temperature is a crucial factor in the tuning of the photonic stop bands, with a linear rate of 42 nm/°C. The pore widening permits further tuning to reach stop bands with central wavelengths as low as 500 nm. Furthermore, the results also show that applying different anodization temperatures does not have a great influence in the pore-widening rate or in the photonic stop band width.     

Modification of interparticle forces for nanoparticles using atomic layer deposition

Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating.     

Nonlinear growth of zinc tin oxide thin films prepared by atomic layer deposition

Zinc tin oxide (ZTO) thin films can be deposited by atomic layer deposition (ALD) with adjustable electrical, optical and structural properties. However, the ternary ALD processes usually suffer from low growth rate and difficulty in controlling film thickness and elemental composition, due to the interaction of ZnO and SnO₂ processes. In this work, ZTO thin films with different Sn levels are prepared by ALD super cycles using diethylzinc, tetrakis(dimethylamido)tin, and water. It is observed that both the film growth rate and atom composition show nonlinear variation versus [Sn]/([Sn]+[Zn]) cycle ratio. The experimental thickness measured by spectroscopic ellipsometry and X-ray reflectivity are much lower than the expected thickness linearly interpolated from pure ZnO and SnO_x films. The [Sn]/([Sn]+[Zn]) atom ratios estimated by X-ray photoelectron spectroscopy have higher values than that expected from the cycle ratios. Hence, to characterize the film growth behavior versus cycle ratio, a numerical method is proposed by simulating the effect of reduced density and reactivity of surface hydroxyls and surface etching reactions. The structure, electrical and optical properties of ZTO with different Sn levels are also examined by X-ray diffraction, atomic force microscope, Hall measurements and ultraviolet–visible–infrared transmittance spectroscopy. The ZTO turns out to be transparent nanocrystalline or amorphous films with smooth surface. With more Sn contents, the film resistivity gets higher (>1 Ω cm) and the optical bandgap rises from 3.47 to 3.83 eV.     

Growing continuous zinc oxide layers with reproducible nanostructures on the seeded alumina substrates using spray pyrolysis

The growth of zinc oxide thin films with controlled nanostructures on the heat resistant dielectric substrates is important for the fabrication of gas sensors, transparent electric heating elements, pyroelectric electron emitters, and many other potential electronic and optoelectronic applications. The preferred substrate for many of these applications is alumina, but the production of uniform ZnO layers on alumina is hindered by the large lattice mismatch between ZnO and Al₂O₃ hexagonal crystal structures. Here, we systematically investigate the growth process of ZnO thin films on alumina substrates using the ultrasonic spray pyrolysis (USP) of zinc chloride solutions in ethanol and, for the first time, demonstrate the deposition of uniform layers on the alumina substrates appropriately seeded using magnetron sputtering prior to USP. On the pristine substrates, random nucleation of the isolated nanocrystallites results in uneven layers, and extending the growth process leads to the hierarchical growth of facetted ZnO nanorods and pyramids with weak physical attachments to the substrate surface. In similar conditions, USP deposition on the seeded substrates reproducibly results in continuous networks of densely packed ZnO crystallites intimately attached to the substrate surface with adjustable thickness and electrical conductance. These results are compared with those obtained for SnO₂ in similar conditions. Regardless of its tetragonal crystal structure, SnO₂ reproducibly forms even layers on the pristine alumina substrates.     

Preferential occupation of molybdenum on the thin alumina film: Characterization by CO titration

A thin alumina film, the alumina model surfaces modified by the metallic molybdenum and by the MoO₃ are titrated by CO chemisorption. Two types of CO adsorption sites, namely octahedrally and tetrahedrally coordinated Al³⁺ sites, are present on the thin alumina film. The thin alumina film prepared under UHV conditions can be used to simulate the conventional high-surface-area alumina supports in real catalysis. The deposited metallic molybdenum preferentially occupies octahedrally coordinated Al³⁺ site and suppresses CO chemisorption on this site, and oxidation of the surface molybdenum species enhances this suppression.     

Flash sintering of yttria-stabilized zirconia powders coated with nanoscale films of alumina by atomic layer deposition

The addition of small quantities of aluminum oxide (Al₂O₃) to 8 mol% yttria-stabilized zirconia (8YSZ) benefits conventional sintering by acting as a sintering aid and altering grain growth behavior. However, it is uncertain if these benefits observed during conventional sintering extend to flash sintering. In this work, nanoscale films of Al₂O₃ are deposited on 8YSZ powders by particle atomic layer deposition (ALD). The ALD-coated powders were flash sintered using voltage-to-current control and current rate experiments. The sintering behavior, microstructural evolution, and ionic conductivities were characterized. The addition of Al₂O₃ films changed the conductivity of the starting powder, effectively moving the flash onset temperature. The grain size of the samples flashed with current rate experiments was ~65% smaller than that of conventionally sintered samples. Measurement of grain size and estimates of sample density as a function of temperature during flash sintering showed that small quantities of Al₂O₃ can enhance grain growth and sintering of 8YSZ. This suggests that Al₂O₃ dissolves into the 8YSZ grain boundaries during flash sintering to form complexions that enhance the diffusion of species controlling these processes.