Pore Depth Research Articles

Numerical study of surface texturing for improving tribological properties of ultra-high molecular weight polyethylene

Abstract Ultra-high molecular weight polyethylene (UHMWPE) has been used in total joint arthroplasty for over 50 years. Conventionally, smooth UHMWPE surfaces are used for total joint replacements; however, smooth surface contacts have been shown to be inadequate in friction reduction and/or anti-wear. More recently, micro-textured surfaces have been investigated for reduction of the friction and wear of two contact interfaces. Unfortunately, the tribological behavior of textured UHMWPE surfaces requires further research to understand its tribological behavior. A numerical model is presented to understand the potential use of specially textured surfaces to improve the tribological properties of UHMWPE. A two dimensional, transient form of Reynolds equation was used to model the lubrication condition of the textured surfaces. The effects of area densities and pore depths over varying diameters were examined for several textured geometries including circle, rectangle, square and triangle. The simulation results show that the surface texturing can effectively be used to enhance hydrodynamic effects. More specifically, it was shown that the rectangular surface texture displayed superior characteristics over the other geometries investigated. The results provide a theoretical reference for the tribological design of surface texture on UHMWPE.

Synthesis of porous silicon with silver nanoparticles by low-energy ion implantation

In this paper, a new technique is proposed for synthesis of porous silicon (PSi) layers with silver nanoparticles based on the method of low-energy high-dose metal ion implantation into Si. In order to demonstrate this technique, the implantation at room temperature of a polished Si wafer by Ag+ ions with the ion energy of 30 keV, ion dose of 1.5 × 1017 ion/cm2, and ion current density of 8 µA/cm2 is carried out. Using methods of high resolution scanning electron and atomic-force microscopy, electron probe microanalysis, and Raman scattering, it is shown that ion implantation results in the formation, on the surface of irradiated Si, of a thin amorphous layer of PSi with the average pore size of 150–180 nm, a pore depth of about 100 nm, and wall thickness between pores of about 30–60 nm. Moreover, the PSi structure contains Ag nanoparticles 5–15 nm in size. It is established that, during the ion implantation, the sputtering of the Si surface by Ag+ ions takes place, which was not observed previously. Based on the data obtained, it is concluded that, in contrast to chemical techniques, the proposed physical technique for PSi formation can be integrated into the modern advanced process of fabricating and improving electronic circuits based on industrial ion implantation.

Fabrication of porous boron-doped diamond electrodes by catalytic etching under hydrogen–argon plasma

Porous boron-doped diamond (BDD) was prepared by hydrogen–argon plasma etching using electrodeposited Ni nanoparticles as a catalyst. The etching process and formation mechanism of porous BDD were investigated by changing the etching time from 30s to 300s. Pores were produced due to the C atoms around Ni nanoparticles are easy to react with hydrogen plasma and form methane. With the increase of etching time, the pore size increased, the pore density decreased, and the pore depth first increased and then maintained unchanged. The sp2-bonded graphitic carbons existing on the surface of BDD increase with increasing etching time due to the increase of surface area. No preferential etching was observed due to the high energy of argon plasma. The electrochemical behaviors of the pristine and porous BDD electrodes were characterized by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The results showed that the porous BDD electrode exhibited high specific capacitance, which is attributed to its high electrical conductivity and large specific surface area. The highest specific capacitance of porous BDD electrode is 9.55mFcm−2, which is 22 times higher than that of pristine BDD electrode. The specific capacitance retention of the porous BDD electrode reduced to 98.2% of the initial capacitance after 500 cycles and then increased to 120.0% after 10,000 cycles. For the first 500 cycles, the reduction of capacitance can be attributed to the dissolution of Ni nanoparticles that attached on the porous BDD surface or buried in the shallow layer. The capacitance increase after 10,000 cycles is due to the better contact of the electrolytic solution with the residual Ni with the increase of cycle number.

Structural and Electrical Properties of an Electrolyte-insulator-metal Device with Variations in the Surface Area of the Anodic Aluminum Oxide Template for pH Sensors

In this study, we fabricated an electrolyte-insulator-metal (EIM) device incorporating a high-k Al₂O₃ sensing membrane using a porous anodic aluminum oxide (AAO) through a two-step anodizing process for pH detection. The structural properties were observed by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction patterns (XRD). Electrochemical measurements taken consisted of capacitance-voltage (C-V), hysteresis voltage and drift rates. The average pore diameter and depth of the AAO membrane with a pore-widening time of 20 min were 123nm and 273.5nm, respectively. At a pore-widening time of 20 min, the EIM device using anodic aluminum oxide exhibited a high sensitivity (56mV/pH), hysteresis voltage (6.2mV) and drift rate (0.25mV/pH).

Metabolism-Induced CaCO3 Biomineralization During Reactive Transport in a Micromodel: Implications for Porosity Alteration.

The ability of Pseudomonas stutzeri strain DCP-Ps1 to drive CaCO3 biomineralization has been investigated in a microfluidic flowcell (i.e., micromodel) that simulates subsurface porous media. Results indicate that CaCO3 precipitation occurs during NO3(-) reduction with a maximum saturation index (SIcalcite) of ∼1.56, but not when NO3(-) was removed, inactive biomass remained, and pH and alkalinity were adjusted to SIcalcite ∼ 1.56. CaCO3 precipitation was promoted by metabolically active cultures of strain DCP-Ps1, which at similar values of SIcalcite, have a more negative surface charge than inactive strain DCP-Ps1. A two-stage NO3(-) reduction (NO3(-) → NO2(-) → N2) pore-scale reactive transport model was used to evaluate denitrification kinetics, which was observed in the micromodel as upper (NO3(-) reduction) and lower (NO2(-) reduction) horizontal zones of biomass growth with CaCO3 precipitation exclusively in the lower zone. Model results are consistent with two biomass growth regions and indicate that precipitation occurred in the lower zone because the largest increase in pH and alkalinity is associated with NO2(-) reduction. CaCO3 precipitates typically occupied the entire vertical depth of pores and impacted porosity, permeability, and flow. This study provides a framework for incorporating microbial activity in biogeochemistry models, which often base biomineralization only on SI (caused by biotic or abiotic reactions) and, thereby, underpredict the extent of this complex process. These results have wide-ranging implications for understanding reactive transport in relevance to groundwater remediation, CO2 sequestration, and enhanced oil recovery.

Highly Resolved Nanostructured PEDOT on Large Areas by Nanosphere Lithography and Electrodeposition.

Poly(ethylenedioxythiophene) (PEDOT) films were electrodeposited galvanostatically from an EDOT/sodium dodecyl sulfate solution in water, through a carboxylated polystyrene template monolayer self-assembled on ITO, after which the template was dissolved away in tetrahydrofuran. Analysis of the films by scanning electron microscopy and atomic force microscopy reveals large-area PEDOT honeycomb structures. The morphology of these structures was varied electrochemically, as the effective thickness and, surprisingly, the shape of the honeycomb arrangement depend on the polymerization time. Using nanospheres of 1 μm diameter and charge densities between 12 and 30 mC cm(-2) for electrodeposition generates PEDOT hexagons with very thin rectilinear walls 30-35 nm-thick and 800 nm-long, whereas at higher charge densities, circular bowls are created with 60 nm walls separating adjacent bowls; triangular areas as small as 0.02 μm(2) develop at the intersection of three nanospheres. These morphologies are specific to the use of carboxylated PS spheres and a water-based solution with a surfactant in the galvanostatic electrodeposition mode. Using smaller nanospheres, i.e. 500 nm in diameter, makes it possible to reach PEDOT hexagons with rectilinear walls as small as 11-17 nm-thick and 300 nm-long; circular bowls with 25-35 nm walls separating adjacent bowls and triangular areas as small as 0.003 μm(2) can also be generated. The wettabilities of the surfaces depend markedly on the pore depth of the PEDOT nanostructure, with contact angles going from 82° to 130° with increasing pore size. Finally these nanostructured PEDOT electrodes were used in Grätzel-type dye-sensitized solar cells (DSSCs) as Pt-free counter-electrodes, with an increase in the yield from 7.0 (bulk PEDOT) to 8.1%.

Self-powered electrochemical anodic oxidation: A new method for preparation of mesoporous Al2O3 without applying electricity

Anodic oxidization (AO) is one of the most important methods available for fabricating mesoporous Al2O3, which can be conducted at either high potential or low potential; however, the need for an external electricity power source limits its applications. In this work, a novel self-powered electrochemical anodic oxidization (SPAO) system was introduced for preparing mesoporous Al2O3, by using newly-invented triboelectric nanogenerator (TENG) arrays driven by wind power. Using the controllable voltage output of the TENG arrays, the SPAO system was shown to regulate the pore depth and pore size of the mesoporous Al2O3. In contrast to traditional AO systems, this technique takes advantage of the high output voltage of TENG arrays without any additional energy costs. In addition, the SPAO system can be used for the preparation of other mesoporous materials.

Morphological evolution of the porous silicon surface for different etching time and current density in hf-ethanol solution

Porous silicon samples were obtained by the anodization etching process of a n-type silicon wafer. The pores formation was investigated taking into account the anodization time. Scanning Electron Microscopy and Optical Profilometry were used to characterize the morphology, pore size, pore depth, thickness, roughness, surface area and also the morphological evolution of porous silicon layer. The formed porous silicon layer showed a tendency to increase the pore size, pore depth, roughness and surface area as a function of the etching time. The porous silicon was obtained at different etching times and at a fixed current density. The experimental results showed that the porosity of the porous silicon prepared under different anodization times can be controlled.

The Synthesis of Nano-Porous Alumina by Anodization Process

The porous alumina templates were synthesized by anodization process. For preparation, the aluminum foils were cleaned by acetone acid and ethanol. After that, cleaned foils were firstly anodized by oxalic acid with direct current (DC) voltage source. When the first anodization process was complete, the aluminum foils were etched by chromic and phosphoric acid for an hour and then they were secondarily anodized for 15 min to increase the pore depth. For characterization, the surface morphology of porous alumina on aluminum surface was evaluated by scanning electron microscope. The results show that the pore diameter increases with increasing DC voltage and concentration of oxalic acid. The lowest pore diameter is 57.19 nm at DC voltage of 20 V for concentration of 0.2 M that it has highest pores density of 157 Gpores/in2.

Real-time sensing of epithelial cell-cell and cell-substrate interactions by impedance spectroscopy on porous substrates

Oxidized porous silicon (PS) is a common topographical biocompatible substrate that potentially provides a distinct in vitro environment for better understanding of in vivo behavior. But in the reported studies on oxidized PS, cell-cell and cell-substrate interactions have been detected only by fluorescent labeling. This paper is the first attempt to investigate real-time sensing of these interactions on HaCaT cells by label-free impedance spectroscopy on oxidized PS of two pore diameters (50 and 500 nm). One of the major requirements for successful impedance spectroscopy measurement is to restrict the channeling of electric field lines through the pores. To satisfy this criterion, we have designed the pore depths after analyzing the penetration of the medium by using computational fluid dynamics simulation. A distributed electrical model was also developed for estimating the various cellular attributes by considering a pseudorandom distribution of pores. It is observed from the impedance measurements and...

The effect of sodium chloride on microstructure, water migration, and texture of rice noodle

It is believed that salt can reduce the hardness and increase the elasticity of dried rice noodle. However, there is no scientific evidence on the role of salt on the quality of the dried rice noodle. This work aimed to investigate the effect of NaCl on the microstructure of dried rice noodle, water migration in rice noodle during cooking, and the texture of cooked rice noodle. The level of NaCl used in this study was 0, 3, and 5 g NaCl/100 g rice flour. Confocal Laser Scanning micrographs revealed that NaCl enhanced the development of the protein network but it reduced the packing of starch lumps in the dried rice noodle. Water distribution inside the rice noodle during rehydration was revealed for the first time using MRI. Water migrated more slowly into the rice noodle with NaCl than the control (without NaCl) because the amount, size, and the depth of pores of the dried rice noodle with NaCl were reduced due to the formation of the protein network. It was found that NaCl increased the extensibility and decreased the tensile strength of the cooked rice noodle. The cooking loss increased with the increase in the NaCl concentration.

Engineering optical properties of metal/porous anodic alumina films for refractometric sensing

We show that ultrathin metallic films deposited on porous anodic alumina (PAA) templates can enhance the reflection at certain wavelengths in the visible range, due to Fabry–Perot optical interference. The reflectance spectra can be tuned in a broad range by tuning the structural parameters of PAA including its pore depth, size and porosity. This results in tunable, iridescent colors of the films that can be spatially patterned. A refractometric chemical sensor is designed to convert the change in effective refractive index into the change in the reflectance spectra, which leads to color change discernible by naked eyes. This opens up the possibility for easy-to-operate chemical sensors with extremely low cost.

Catalytic Activity of Ru for Metal-Assisted Etching of Si

Metal-assisted chemical etching of Silicon (Si) in hydrofluoric acid (HF) solution is an electroless method to produce porous Si (1, 2). Such etching generally uses not only metal-modified Si but also an oxidizing agent as hydrogen peroxide, metal ions and oxygen. We reported that etching rate of platinum (Pt), rhodium (Rh), gold (Au) and silver (Ag)-particle-modified Si depended on catalytic activity of metal for oxygen reduction (3), and palladium (Pd) exhibits high catalytic activity for the etching of Si without an oxidizing agent even under dissolved oxygen-free and dark conditions (4). This etching reaction using Pd is explained by electron injection into the conduction band of Si due to Pd assisted anodic oxidization of Si (Fig. 1). In this study, we have investigated the catalytic activity of ruthenium (Ru), which is in the same platinum group as Pd, for etching of Si in a HF solution. Single-crystalline n-Si wafers (CZ, (100), ca. 10 Ω cm) were used as substrates. Si wafers were immersed in 1 mM (M = mol dm-3) ruthenium chloride (RuCl3) solution including 0.15 M HF at 313 K for electroless displacement metal deposition. Metal deposited Si wafers were immersed in a 7.3 M HF solution at 298 K under dark conditions for 24 h. Argon or oxygen gas bubbling was applied to the HF solution before and during etching to remove or saturate the dissolved oxygen in the solution, respectively. The etching rate of Si was measured by a gravimetric procedure. No metal deposition occurred on mirror-polished Si wafers even by immersing in the deposition solution for even long time of 24 h. On the other hand, on Si wafers roughed with sand paper (#1200), Ru particles were deposited by immersing in the solution for 5 min. The etching rates of such Ru-particle-deposited Si wafers in 7.3 M HF solution were 1.1×10-5 and 2.0×10-4 g cm-2 h-1 under oxygen-free and -saturated conditions, respectively. The etch rate in the oxygen-free case is much higher than 1.2×10-6 g cm-2 h-1 of Pt-particle-modified case and one order lower than 3.4×10-4 g cm-2 h-1 the of Pd case. Figure 2 shows the SEM images of Ru-particle-deposited Si wafers before and after etching with the oxygen free HF solution. Ru particles were localized on scratches before etching (fig. 2a). After etching, a porous layer covered whole surface of Si (Fig. 2b and c). Pores, which have Ru particle at the bottom, were formed. The depth of pores, ca. 0.2 mm, is much shorter than 1 mm of depth calculated using the gravimetric procedure. These results indicate that Ru has much lower activity for electroless displacement deposition than that of other noble metals and high catalytic activity for metal assisted etching of Si similar to Pd. ACKNOWLEDGEMENTS The present work was partly supported by JSPS KAKENHI (26289276). REFERENCES Z. Huang, N. Geyer, P. Werner, J. Boor, U Gösele, Adv. Mater., 23,285 (2011).S. Yae, Y. Kawamoto, H. Tanaka, N. Fukumuro, and H. Matsuda, Electrochem.Commun., 5, 632 (2003).S. Yae, Y. Morii, N. Fukumuro, H. Matsuda, Nanoscale Res. Lett., 7, 352 (2012).S. Yae, M. Tashiro, M. Abe, N. Fukumuro, and H. Matsuda, J. Electrochem. Soc., 157, D90 (2010). Figure 1

(Invited) Inhomogeneity of Barrier Layer Inducing Irregularity of Porous Anodic Oxide Film on Aluminum

Porous anodic oxide films formed on various metals, especially anodic alumina, have attracted huge attention as a starting material in the fabrication of nano-devices because of the high controllability of porous structures in nanometer scale such as pore size, pore interval, as well as pore depth by changing electrolysis condition in addition to their high regularity of pore arrangement. In recent years, mechanism of self-ordering of anodic alumina films has been intensely studied. We proposed anodizing under high electric field strength as a key factor for self-ordering of pore arrangement (1). However, only a few reports are found on the mechanism of irregularity or defects formation of porous anodic oxide films. For example, an regular deposition of metals into all anodic pores are often difficult because of irregularity of the barrier layer. In the case of anodic films formed on aluminum in chromic acid solution, for example, sever pore blanching appears especially at the higher voltage. Anodic alumina films formed in phosphoric acid and Ematal bath also indicates numerous pore branching and caved pores in the cell wall (2, 3) where anodizing conditions such as high formation voltage and relatively low dissolution ability of electrolytes to anodic oxides are used. These irregularities were caused by defects/holes induced by electric breakdown between pore bottom and protrusion at triple points of cell boundary. Impurity elements in metals and alloys also bring defects and irregularity into porous structures. Furthermore, anodic films formed at some specific condition, especially on titanium, yielded tubular structure having interspaces at cell boundaries. All of these irregularities should be created only at the barrier layer grown under the anodic field by specific reactions other than stable anodic oxide growth. This inhomogeneity of barrier layer such as unevenness of the thickness and voids that induce irregularity in porous oxide seems to be playing an important role at an application for industry both in a positive and negative way. In this paper, we report the specific futures of irregularity appeared in porous anodic oxides films formed on aluminum and evaluated to get unified formation mechanism of such imperfections to be induced in the oxides grown on various substrates. [1] S. Ono, M. Saito, M. Ishiguro and H. Asoh, J.Electrochem. Soc., 151, B473 (2004). [2] S. Ono, H.Ichinose and N. Masuko, J.Electrochem.Soc., 138, 3705 (1991) [3] S. Ono and N. Masuko, J. Jpn Inst. Light Metal, 43, 453 (1993).

Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm

Porous anodic oxide film, which is formed by anodizing of aluminum, is a typical self-ordered nanoporous material. Over the past several decades, studies have been carried out on the protection or design of aluminum surfaces for commercial applications. Recently, anodic alumina films have attracted considerable attention as a key material due to their potential technological applications in area such as filters, catalyst supports, biological applications, electronic, magnetic and optical devices. However, commercially available anodic porous alumina membranes have insufficient pore arrangement and low chemical resistance because the porous alumina films obtained by typical anodizing processes are amorphous. To expand the application of anodic porous alumina with ordered pore arrays, it is necessary to control the cell dimensions (e.g., pore interval, pore diameter, and pore depth) and to enhance the chemical resistance of alumina membranes in extreme environments such as high temperature, high vapor pressure, and high-concentration acid/base solutions. Several fundamental studies have been reported on the fabrication of crystalline anodic porous alumina aimed at improving the chemical properties of α-alumina membranes using a combination of anodizing and heat treatment processes. Since the 2000s, the crystallization of free-standing alumina membranes to the α-phase has also been attempted; however, curling and cracking as a result of thermal deformation caused by the change in density and decomposition and desorption of electrolyte anions from the outer anion-incorporated layer in the cell wall were unavoidable during the transition to crystalline alumina in addition to the desorption of bound water at lower temperatures. In this study, we investigated the optimum conditions for the fabrication of α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm by the anodizing of aluminum and subsequent heat treatment. The morphological and structural change of the porous alumina membrane during heat treatment was also evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and thermogravimetry-mass spectrometry (TG-MS). Because the pore diameter could be adjusted by changing the anodizing conditions including the electrolyte species and formation voltage, anodizing of high-purity aluminum sheets was conducted in sulfuric acid, oxalic acid and phosphoric acid at each specific voltage. Here, we applied two-step anodizing process to improve the regularity of the pore arrangement of the anodic film. As a result, the deformation of alumina membrane such as warp and cracking during heat treatment for crystallization was effectively suppressed [1, 2]. By optimizing the conditions for anodizing, subsequent detachment, and heat treatment, nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated. Even in the case of anodic porous film with short pore interval of approximately 60 nm, which was formed at 25 V in sulfuric acid, α-alumina membrane that maintained straight channels was obtained. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure [2]. These results open a new route to technological and scientific applications.[1] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)[2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)

Concept and Nanostructure Control of Plasmonic Porous Silicon Solar Cells

[Introduction]A multi-junction solar cell can generate high open current voltage by each layer absorbing a different wavelength light, although lattice mismatch between materials is one of its problems. A possible new type of solar cell is an all-Si tandem solar cell, which will have no mismatch. Our laboratory noticed that porous silicon is a light-absorbing material for short wavelengths. Its nano-scale holes on its surface increases its band gap due to the quantum size effect. Because this porous structure, however, leads to high resistivity and low absorption coefficient, the porous layer must be made thinner by enhancing light absorption. To improve light absorption, we propose using localized surface plasmon of metal nanoparticles [1, 2]. By such introduction of these particles into the holes of porous silicon, a thinner porous layer can be also achieved. Here, we proposed this plasmonic porous silicon solar cell (or PPSSC), and aimed to control the structure of porous silicon. To determine guidelines for fabricating PPSSCs, we created solar cells consisting of porous silicon, and then measured their I-Vcharacteristics. By simulation, we estimated the enhancement of light absorption of porous silicon with metal nanoparticles. [Experimental]1. Preparation of porous silicon: Here, p-type monocrystalline silicon wafers with three different resistivities were anodized for different times and current density. The surfaces and cross-sections were then observed by field emission scanning electron microscopy (FE-SEM) and evaluated using UV-vis spectrum. Several wafers were also treated with Ar dry etching, and its effect on their surface structure observed. 2. Fabrication of solar cells: An n-type silicon layer was deposited on top of the porous silicon by chemical vapor deposition (CVD) using SiH4 and PH3 in H2. Silver electrodes were fabricated using RF sputtering, and then their rectification property and quantum efficiency were measured. 3. FDTD simulation: With the Finite-Different-Time-Domain (FDTD) method, we created a simulation model in which silver nanoparticles exist in the holes on the silicon surface. The simulation showed how the light absorption in this combined material is enhanced by adding silver nanoparticles to the porous silicon. Then, the necessary layer thickness of porous layer was estimated by the model, and the influence of localized surface plasmon was verified by investigating the strength of the electrical field near the interface between the silicon and silver nanoparticles. [Results and Discussion] 1.Structure control of porous silicon: By changing the anodization time and current density, we successfully fabricated porous silicon of various pore size, depth, and porosity. Figure 1 shows the relationship between current density I and pore diameter. In addition, results reveal that dry etching expands the pore size of a porous silicon surface (Fig. 2), indicating that dry etching might facilitate both the introduction of metal nanoparticles and the control of band gaps. 2.Evaluation of porous silicon solar cells: Figure 3 shows that a thin n-type silicon layer was successfully deposited on the porous silicon by CVD. Figure 4 shows that p-n junctions were also formed by this deposition. However, compared with a monocrystalline silicon solar cell (Fig.5), porous silicon solar cells tended to have a smaller increase rate of I than that of V. One factor in this trend is the high resistivity of porous silicon, indicating that making the porous silicon layer thinner is important to increase the efficiency of PPSSCs. Figure 6 compares the quantum efficiency of porous silicon and monocrystalline silicon solar cells, revealing that porous silicon solar cells have lower quantum efficiency. One explanation is that insufficient deposition of n-type silicon onto the porous silicon accelerates surface recombination. This surface recombination can be improved by performing passivation. 3.Evaluation of combined materials by simulation: FDTD simulation indicated that the light absorption of the combined materials can be improved when the silver nanoparticles exist in the holes of porous silicon, and that a strong electrical field is injected into the silicon around the interface between silicon and the silver nanoparticles. These results indicate that localized surface plasmon might improve light absorption and enhance thinner porous silicon as a light absorption layer. [Conclusion] Current density, resistivity of wafers, and dry etching can be factors in controlling the structure of porous silicon. We successfully formed p-n junctions with porous silicon, and based on the measured I-Vcharacteristics and quantum efficiency of porous silicon solar cells, the porous silicon needs to be thinner and needs passivation. The simulations suggest that metal nanoparticles enhance the light absorption.

Opaque White Anodic Oxide Film Formed on Aluminum

Anodic porous alumina film, which is formed by anodizing of aluminum in an appropriate acidic solution, has been studied on the protection or design of aluminum surfaces owing to commercial applications. The appearance and surface quality of aluminum products (e.g., architectural materials, electronic devices) are key points as with surface properties such as a corrosion resistance, hardness and wear resistance. Although dyeing, painting and electrolytic coloring are well known as industrial methods to provide color variation for aluminum surface, integral coloring, which produces color by anodizing step itself without use any pigment or metallic salt, is also applied in terms of the simplicity and production cost. In our previous work, it was reported that alumina films with irregular porous structure formed in Ematal bath or chromic acid showed opaque white appearance [1, 2]. Irregular porous structure in the direction of pore depth, which contained branching, curving and distortion of pores in anodic alumina, produced the roughening of the metal-oxide interface and made the appearance of the film more opaque [1, 2]. However, Ematal bath containing expensive potassium titanium oxalate deteriorates easily. In terms of reduction of environmental burdens, it is required to avoid using harmful chemical compound such as chromic acid. Under such background, we studied a low-cost alternative anodizing method to form opaque white oxide film. In addition, we also investigated in detail the relation between the appearance and porous structure of oxide film. Anodizing was conducted using high purity (99.99 %) aluminum sheets in various electrolytes (phosphoric acid, sulfuric acid, critic acid, and Ematal bath). Whiteness of the obtained samples was evaluated by spectrophotometry (KONICA MINOLTA, CM-5) based on standard of American Society for Testing Materials (ASTM E313-73). When anodizing was carried out in phosphoric acid at 60 °C at a constant current density of 200 Am−2, anodized specimen indicated whiteness value of 39. Surface appearance was gray in color. The aluminum specimen anodized in Ematal bath showed whitish appearance compared with that treated in phosphoric acid. Whiteness value of aluminum specimens anodized in Ematal bath increased from 35 to 47 with increasing final arrival voltage from 150 V to 350 V during constant current anodizing. On the other hand, whiteness value increased effectively using two-step anodizing; long first anodizing and short second anodizing were conducted in phosphoric acid and subsequently Ematal bath. Under optimized conditions, whiteness value increased to 58. The more increase of the thickness of total oxide film and barrier layer and the degree of branching, the more white appearance indicated. Thus, it is confirmed that the appearance of anodized specimen was strongly affected by these factors. [1] S. Ono, S. Chiaki, T. Sato; J. Met. Finish. Soc. Jpn., 26, (10), 456-460 (1975). [2] S. Ono, T. Sato; J. Met. Finish. Soc. Jpn., 31, (3), 134-139 (1980).

Effect of Displacement Deposition on Platinum Deposition within Nanoporous Silicon

In the use of nanoporous electrode, the control of mass transfer within nanopores is crucially important to elicit their best performance. However, mass-transfer rate for ionic species through electrolyte solution within nanopores tends to be very low, so that only a limited proportion of the whole specific surface area contributes to electrochemical reactions. In the case of the metal filling within the porous silicon by electrodeposition, poor supply of metal precursor into the nanopores also causes a problem to induce a plugging at the pore opening. We have investigated the pore filling with platinum in an aqueous solution with tuning the hydration property of the surface of pore wall and platinum complex anions [1-3]. The important insight we have found is the surface-induced phase transition (SIFT: the concentration of the hydrophobic solutes near the hydrophobic surface becomes orders of magnitude higher than that in the bulk) in the nanopores plays an important role in electrodeposition of platinum within the nanoporous silicon. Our previous reports, however, focused only on the presence of platinum deposits in the porous layer corresponding to the various experimental conditions and didn’t elucidate the dynamic aspects of the penetration of platinum complex anions into the nanopores. We adopted the time developments of the mixed-potential measured during the displacement deposition of platinum within the porous silicon to find the peculiar penetration dynamics that could not be understood within the framework of simple diffusion. One of the typical observations shows the penetration is faster when the pore diameter is smaller and the anion size is larger. In this paper, we also discuss the effect of the pore depth on the penetration dynamics. [1] K. Fukami, R. Koda, T. Sakka, T. Urata, K. Amano, H. Takaya, M. Nakamura, Y. Ogata, and M. Kinoshita, Chem. Phys. Lett., 2012, 542, 99-105. [2] K. Fukami, R. Koda, T. Sakka, Y. Ogata, and M. Kinoshita, J. Chem. Phys., 2013, 138, 094702. [3] R. Koda, A. Koyama, K. Fukami, N. Nishi, T. Sakka, T. Abe, A. Kitada, K. Murase, and M. Kinoshita, J. Chem. Phys., 2014, 141, 074701.

A Controllable and Widely Applicable Electrochemical Anodization Process to Fabricate Porous Anodic Aluminum Oxide Membrane

Inspired by the versatile applications of porous alumina nanostructures on solar cells, supercapacitors, biosensors, and so forth, research on fabrication of anodized aluminum oxide (AAO) with nanopore structure has achieved substantial advancements. Porous AAO membrane with nanostructures has become one of the most popular template materials for synthesis of a broad range of functional nanostructures such as nanowires, nanopillars, nanotubes, etc. While fabrication of highly ordered nanoporous AAO with tunable geometric features has been widely reported, its growth rate can be easily affected by the fluctuation of parameters such as acid concentration and temperature during the electrochemical anodization process. In order to meet the demand of fabricating AAO with various surface parameters and precise target pore depth control for scientific research and commercial applications, a controllable fabrication process is essential. Here, we developed a method which can implement precise pore depth control of AAO layer during the electrochemical anodization process, via monitoring the integrated charge. This method is applicable in a large voltage range and can be widely applied in fabrication of nanostructure with material such as TiO2and so on.

Effect of Aluminum Coating on the Surface Properties of Ti-(~49 at. pct) Ni Alloy

Stable porous layer of mixed Al2O3 and TiO2 has been formed on the Ti-(~49 at. pct) Ni alloy surface with an aim to suppress leaching of Ni from the alloy surface in contact with bio-fluid and to enhance the process of osseointegration. Aluminum coating on the Ni-Ti alloy surface prior to the anodization treatment has resulted in enhancement of depth and uniformity of pores. Thermal oxidation of the anodized aluminum-coated Ni-Ti samples has exhibited the formation of Al2O3 and TiO2 phases with dense porous structure. The nanoindentation and nanoscratch measurements have indicated a remarkable improvement in the hardness, wear resistance, and adhesiveness of the porous aluminum-coated Ni-Ti sample after thermal oxidation.