Influence of Heating Modes of Compacted Samples from Nickel Powders with Nanosized Particles on Their Interaction with Air

In this paper, we study compact samples of pyrophoric nickel powders with the average particle size of 85 nm, obtained by the chemical-metallurgical method. For the first time, it is experimentally shown that it is possible to passivate compact samples with a diameter of 3 mm from pyrophoric nickel powders with nanosized particles in air. For a relative density of 0.4 to 0.5, the passivation time is only 3–5 s. According to the X-ray phase analysis data, only the Ni phase is observed in passivated samples. It is found that passivated samples retain their thermal stability in air upon slow (<10 deg/s) heating to ~200°C, which is an important parameter for fire safety when handling nanopowders. The electron microscopic analysis of the passivated samples did not reveal traces of sintering of nickel nanoparticles, including after checking for thermal stability. The uniform distribution of oxygen over the passivated samples according to the data of energy dispersive analysis (the standard deviation is 0.9 at %) indicates the volumetric nature of the interaction of the samples with air during passivation. For the obtained passivated samples, the critical heating conditions were determined, under which self-ignition occurs, which is in agreement with N.N. Semyonov’s classical theory of thermal explosion.

Nanostructure of Porous Si and Anodic SiO2 Surface Passivation for Improved Efficiency Porous Si Solar Cells

The photovoltaic effect in the anodic formation of silicon dioxide (SiO2) on porous silicon (PS) surfaces was investigated toward developing a potential passivation technique to achieve high efficiency nanostructured Si solar cells. The PS layers were prepared by electrochemical anodization in hydrofluoric acid (HF) containing electrolyte. An anodic SiO2 layer was formed on the PS surface via a bottom-up anodization mechanism in HCl/H2O solution at room temperature. The thickness of the oxide layer for surface passivation was precisely controlled by adjusting the anodizing current density and the passivation time, for optimal oxidation on the PS layer while maintaining its original nanostructure. HRTEM characterization of the microstructure of the PS layer confirms an atomic lattice matching at the PS/Si interface. The dependence of photovoltaic performance, series resistance, and shunt resistance on passivation time was examined. Due to sufficient passivation on the PS surface, a sample with anodization duration of 30 s achieved the best conversion efficiency of 10.7%. The external quantum efficiency (EQE) and internal quantum efficiency (IQE) indicate a significant decrease in reflectivity due to the PS anti-reflection property and indicate superior performance due to SiO2 surface passivation. In conclusion, the surface of PS solar cells could be successfully passivated by electrochemical anodization.

A Novel Testing Method for Examining Corrosion Behavior of Reinforcing Steel in Simulated Concrete Pore Solutions

In this paper, a new mechanical-based experimental method is proposed to determine the corrosion initiation and subsequent corrosion behavior of steel in simulated concrete pore solutions. The proposed experiment is used to investigate the corrosion of the steel wire under various different conditions and to examine the effects of pre-stress level in steel wire, passivation time of steel wire, composition and concentration of simulated concrete pore solution on the corrosion initiation, and subsequent corrosion development in the steel wire. The experimental results show that the reduction rate of the cross-section area of the steel wire increases with the increase of chloride concentration or decrease of pH value in the solution. However, for the case where the chloride concentration is high and the pH value is low, there is a slight decrease in the corrosion rate due to the coating function of the corrosion products surrounding the wire.

Preconditioning of Bioactive Glasses before Introduction to Static Cell Culture: What Is Really Necessary?

Due to their high bioreactivity, the in-vitro analysis of bioactive glasses (BGs) can be challenging when it comes to maintaining a physiological pH. To improve BG biocompatibility, a heterogenic spectrum of preconditioning approaches, such as “passivation” of the BGs by incubation in cell culture medium, are used but have never been directly compared. In this study, the effect of passivation periods of up to 72 h on pH alkalization and viability of human bone marrow-derived mesenchymal stromal cells was evaluated to determine a time-efficient passivation protocol using granules based on the 45S5-BG composition (in wt%: 45.0 SiO2, 24.5 Na2O, 24.5 CaO, 6.0 P2O5) in different concentrations. pH alkalization was most reduced after passivation of 24 h. Cell viability continuously improved with increasing passivation time being significantly higher after passivation of at least 24 h compared to non-passivated 45S5-BG and the necessary passivation time increased with increasing BG concentrations. In this setting, a passivation period of 24 h presented as an effective approach to provide a biocompatible cell culture setting. In conclusion, before introduction of BGs in cell culture, different passivation periods should be evaluated in order to meet the respective experimental settings, e.g., by following the experimental protocols used in this study.

Development of Superhydrophobic Coatings on AA 7075 by Surface Modification Using Fatty Acids

The present work is aimed at developing superhydrophobic coatings on AA 7075 using two different fatty acids namely Stearic acid and Myristic acid. The coating was developed by initially etching the substrate in 1 M NaOH solution at a temperature of 70 °C for 10 hours, followed by passivating in 0.01 M ethanolic stearic acid and 0.01 M ethanolic myristic acid for different durations of passivation time. The coated samples were further investigated for their surface morphology, wettability and corrosion resistance. The contact angle and surface morphology of the developed coatings were assessed by contact angle goniometer and scanning electron microscopy (SEM) respectively. The corrosion behavior of the coatings was studied in 3.5 Wt. % aqueous NaCl solution by potentiodynamic polarization test (PDP). Passivation using fatty acids resulted in alteration of the surface morphology, which resulted in the increase of the contact angle exhibiting superhydrophobic nature, which also enhanced the corrosion resistance of the alloy.

Property Improvement of GaAs Surface by 1-Octadecanethiol Passivation

In this study the effects of 1-Octadecanethiol (ODT, 1-CH3 [CH2]17SH) passivation on GaAs (100) surface and GaAs/Al2O3 MOS capacitors are investigated. The results measured by X-ray photoelectric spectroscopy (XPS), Raman spectroscopy and scan electron microscopy (SEM) show that the ODT passivation can obviously suppress the formation of As-O bonds and Ga-O bonds on the GaAs surface and produce good surface morphology at the same time, and especially provide better protection against environmental degradation for at least 24 h. The passivation time is optimized by photoluminescence (PL), and the maximum enhancement of PL intensity was 116%. Finally, electrical property of a lower leakage current was measured using the metal-oxide-semiconductor capacitor (MOSCAP) method. The results confirm the effectiveness of ODT passivation on GaAs (100) surface.

Effect of passivation on chloride concentration threshold of steel reinforcement corrosion

Purpose When concrete is manufactured, it can be instantaneously contaminated by chloride (Cl−) ions or later by their intrusion from the environment. This work aims to study the electrochemical behavior of the passive layer formed on the reinforcing steel surface in the presence of the same Cl− ion concentration, with and without passivation time. This will, undoubtedly, affect the corrosion threshold values thereafter. Design/methodology/approach Electrochemical polarizations were carried out in two concrete pore solutions. The surfaces of samples immersed for 255 days in saturated Ca(OH)2 solution were examined with optical and scanning electron microscopy and Raman microspectroscopy. Findings Cl− ion origins in reinforced concrete lead to different values of corrosion thresholds. The passive layer behaves like a physicochemical barrier, and corrosion occurs at higher NaCl concentration thresholds. The formed passive film on the steel surface shows differences in the chemical composition and the morphology. The results show a rich presence of hematite. Maghemite, lepidocrocite, akaganeite and goethite are also present in much lower concentrations. The Cl− ion presence in fresh concrete at the beginning of the manufacture harms the good formation and the good stability of these oxides, leading to corrosion initiation. Originality/value This study contributes to a better understanding of the passive layer role, not only in reducing the corrosion rate value but also in reconsidering new Cl− ion corrosion threshold values.

Electrochemical Characteristics of a Cobalt Alloy with a Protective Passive Layer

The aim of this study was to assess the quality of the passive layer formed on the surface of a CoCr alloy used on dentures to see the effect on resistance to pitting corrosion and electrochemical parameters. Studies were performed using on potentiodynamic tests and electrochemical impedance spectroscopy. The passive layer was formed spontaneously and by electrochemical method on samples after casting and after simulation porcelain firing on metal. Samples of the passivation electrochemical characterized by the improvement in corrosion resistance. Prolonging the passivation time from one to 24 hours and increasing the potential value from 0 to 200 mV improved the quality of the passive layer and all characteristics of electrochemical parameters.

The Control Method of Surface Morphology and Etch Rates for Silicon Etch Process With Extremely Deep and High Aspect Ratio

The manufacture method based on the silicon etching process is one of the most important methods to fabricate micro mechanical structure, e.g. micro-engine. In the processing, the high aspect ratio silicon etch process (HARSE process) is very important to improve the efficiency of structure. At the same time, the surface morphology should be controlled exactly to keep the performance of structure. In this paper, the feasibilities of controlling the surface morphology and Si etch rates were experimentally investigated. In the experiments, the width of structure changes from 15um to 1500um and the depth changes from 50um to 500um. The parameters of surface morphology including sidewall angle, surface roughness, and so on were measured and compared. The influence mechanisms of etching parameters were analyzed. The etching process were completed in a surface technology system (STS) multiplex advanced silicon etcher inductively coupled plasma (ICP) system with SF6/O2 plasma as etching plasma and C4F8 as passivation plasma. In the experiments, the etching experiments were conducted in a low pressure (5–50mTorr), high density, inductively coupled plasma etching reactor (ICP) with a planar coil. The Si etches rates and sidewall angle were investigated as a function of chamber pressure, cathode RF-power, and gas flow. The results indicated that the increasing of total etching time results in an acceleration in etch rate as well as the decrease in sidewall angle (the top width of trench is narrow than the bottom width). Meanwhile, the total passivation time has an opposite effect in the influence of etch rate and sidewall angle. All the experiments indicate that the quick shift between etch and passivation period leads to a smoother surface. An interesting phenomenon were discovered that the etch rate will not change with the changing of width parameter in most of the high aspect ratio silicon etch recipes when the width-depth ratio is upper than 0.34. An experiential function formula were fitted based on four parameters, including width and depth of the structure, and total etching and passivation time.