Papers by Keyword: Passivation

Abstract: Leaching of a pyritic and a pure chalcopyrite concentrate was carried out in stirred tank reactors in the absence and presence of a mixed culture of moderately thermophilic microorganisms at 45°C and pH 1.5. To study the effect of microbial activity on copper dissolution, the abiotic experiments were performed under accurately controlled redox potential conditions to reproduce the same oxidising conditions recorded during the bioleaching experiments. X-ray photoelectron spectrometry (XPS) was used to study the surface of chalcopyrite chips leached for different durations. The results showed that the microorganisms in cases of both concentrates did not have any effect in the copper leaching efficiency other than oxidation of ferrous to ferric ions. Biooxidation of elemental sulphur did not improve the leaching efficiency and bulk and surface jarosite had no negative effect on dissolution. A composite layer composed of mainly elemental sulphur and iron-oxy-hydroxide was found to be responsible for the hindered dissolution.

Abstract: Surface states have hindered and degraded many semiconductor devices since the Bardeen era. Surface states originate from dangling bonds on the surface. This paper discusses a generic solution to surface states, i.e. valence-mending passivation. For the Si (100) surface, a single atomic layer of valence-mending sulfur, selenium or tellurium can terminate ~99% of the dangling bonds, while group VII fluorine or chlorine can terminate the remaining 1%. Valence-mending passivation of Si (100) has been demonstrated using CVD, MBE and solution passivation. The keys to valence-mending passivation include an atomically-clean Si (100) surface for passivation and precisely one monolayer of valence-mending atoms on the surface. The passivated surface exhibits unprecedented properties. Electronically the Schottky barrier height between various metals and valence-mended Si (100) now follows more closely the Mott-Schottky theory. With metals of extreme workfunctions, new records for low and high Schottky barriers are created on Si (100). The highest barrier so far is 1.14 eV, i.e. a larger-than-bandgap barrier, and the lowest barrier is below 0.08 eV and potentially negative. Chemically silicidation between metal and valence-mended Si (100) is suppressed up to 500 °C, and the thermally-stable record Schottky barriers enable their applications in nanoelectronic, optoelectronic and photovoltaic devices. Another application is transition metal dichalcogenides. Valence-mended Si (100) is an ideal starting surface for growth of dichalcogenides, as it provides only van der Waals bonding to the dichalcogenide.

Abstract: Copper-nickel alloys have been used in many applications in marine environments because of excellent corrosion and biofouling resistance. . This investigation is cover a review of many previous studies about the behavior of these alloys in marine environments with different environmental conditions including the effect of sulphur contamination, the effect of CO₂, the effect of chlorine, the effect of flow velocity, the effect of temperature and the effect of adding some alloying elements to the alloy itself. There have been conflicting results in the past about the corrosion rate values and the chemical analysis of the corrosion films in different environmental conditions. More research is needed for studying the behavior of these alloys in marine environments to provide good assessment of the corrosion resisting characteristics.

Abstract: Mg alloy shows higher specific strength and ductility relative to aluminum and also lower machining and casting costs over to steels. However, it also shows limitation to use in many industrial applications due to the relatively poor corrosion resistance in many aqueous solutions.Corrosion and passivation study of magnesium alloy (AZ91D) in various Cl ^̄ ions concentrations and temperatures were carried out using electrochemical impedance spectroscopy (EIS) in NaCl solutions of 0.4, 0.6, 0.8 & 1.0 molar (M) concentrations. The passivation behavior was also observed at temperature of 45°C, 55°C and 65 °C in 3.5 wt% NaCl solutions. The total polarization resistance was observed about 5530, 4030, 2465 and 2000 Ohms.cm² in solutions of 0.4, 0.6, 0.8 & 1.0 M NaCl respectively indicate reduction of film stability at higher chlorides concentration. A similar trend was found on increasing temperatures of 3.5 wt% NaCl solutions at 45°C, 55°C and 65 °C and noticed about 970, 600 and 300 Ohms.cm² respectively showed significant decline of passivity and more pitting tendency.

Abstract: Chromate conversion coating is an important surface finishing process for electroplated zinc coatings that are widely employed in automotive applications. In addition to providing enhanced corrosion protection, the conversion coating offers a shade of colors to the coated products, both for aesthetic and functional benefits. Due to the stringent requirements on environmental issues, the industry is replacing the conventional hexavalent chromate with a more environmentally friendly trivalent chromate for the production of coatings. This effectively poses the requirement of the fundamental understanding on how the keys processing parameters of trivalent chromate conversion coating may relate to coloring of the coating products. In this work, for the first time, a systematic study is carried out to correlate the electroplating parameters, including the current density and electrolyte’s additives, on the formation of the trivalent chromate conversion coating, and hence the color appearance of the top-coats. Focusing on the black conversion coating, the color and optical properties are analyzed using a colorimeter and an optical spectrometer. The results notably show that, while the additives highly influence the observable shade of blackness, current density affects the optical properties in the visual spectrums. The microstructural and chemical characterization techniques, namely FE-SEM, OM, and XRD, are used to shed some light on the underlying mechanism that controls the color appearance. The understanding developed in this study will impact the design and fabrication of the electrogalvanizing products of desired color and esteemed functional performance.

Abstract: The optical and chemical properties of gallium arsenide (GaAs) surfaces treated by ammonium sulfide ((NH₄)₂S) treatments were studied via low-temperature photoluminescence (PL). From the PL mapping and Atomic Force Microscope (AFM) results, the treatment process by (NH₄)₂S is quite effective to remove the oxide layer of GaAs.The PL intensity of (NH₄)₂S-passivated sample was higher than the untreated sample, and the homogeneity of passivated surface was much better. This strategy provides superior promising passivation method for III-V compound semiconductor material in high-speed and optoelectronic device applications.

Abstract: Tuning the visible emission of Si nanomaterials by modifying their size and shape is one of the key issue in optoelectronics. The observed optical gain in Si-nanoclusters (NCs) has given further impulse to nanosilicon research. We develop a phenomenological model by combining the effects of surface passivation, exciton states and quantum confinement (QC). The size and passivation dependent band gap, oscillator strength, radiative lifetime and photoluminescence (PL) intensity for NCs with diameter ranging from 1.0 to 6.0 nm are presented. By controlling a set of fitting parameters, it is possible to tune the optical band gap, PL peak and intensity. In case of pure clusters, the band gap is found to decrease with increasing NC size. Furthermore, the band gap increases on passivating the surface of the cluster with hydrogen and oxygen respectively in which the effect of oxygen is more robust. Both QC and surface passivation in addition to exciton effects determine the optical and electronic properties of silicon NCs. Visible luminescence is due to radiative recombination of electrons and holes in the quantum-confined NCs. The role of surface states on the band gap as well as on the HOMO-LUMO states is also examined and a correlation is established. Our results are in conformity with other observations. The model can be extended to study the light emission from other nanostructures and may contribute towards the development of Si based optoelectronics.

Abstract: The electrodeposited Ni-P coatings are commonly used for their electrocatalytic properties towards hydrogen evolution reaction. However, their corrosion resistance in concentrated alkaline solutions is still less known. In this work, the effect of phosphorus on the resistance of nickel electrodeposits to electrochemical corrosion in 5 M KOH solution, was studied. Open circuit potential and anodic polarization measurements were performed to determine parameters of the corrosion resistance of the crystalline Ni deposit and amorphous Ni-P electrocoating. It was found that alloying of nickel with phosphorus is the effective manner of the improvement of the corrosion resistance of the nickel coatings. The obtained Ni-P electrode material contained 10 wt.% of P and revealed amorphous structure. The increase in the corrosion resistance of the Ni₉₀P₁₀ system as compared to that determined for comparable Ni polycrystalline electrode was due to phosphate formation responsible for broad passivation potential in alkaline medium and amorphous structure.

Abstract: The effect of the output power with different facet passivation methods on 980 nm graded index waveguide structure InGaAs/AlGaAs laser diodes was studied. The output power of the 980 nm laser diodes with Si passivation, and ZnSe passivation at the front and the back facet were compared. The test results show that output power of the ZnSe passivation method is 11% higher than Si passivation method. The laser diode with the Si passivation film is failure when current is 5.1 A, the laser diode with the ZnSe passivation film is not failure until current is 5.6 A And we analyzed the failure reasons for each method. In conclusion, the method of coated ZnSe passivation on the laser diode facet can effectively increase the output power of semiconductor lasers.

Abstract: The pitting potential, E_pit, of the passive layer on the implant alloy can be treated as an accelerated laboratory test to assessment a susceptibility to pitting corrosion of metallic biomaterials in simulated body fluids. This study deals with an evaluation of E_pit of the self-passivated TiO₂ layer formed on the surface of the NiTi implant alloy as a function of the scan rate of polarization. Cyclic potentiodynamic studies were performed in Ringer’s solution at 37°C. It was found out that the more noble value of E_pit in the range of 0.99-2 V was registered at a given polarization scan rate that ranged from 0.16 to 2 mV s^-1, the lower susceptibility of the self-passivated NiTi implant alloy to the initiation of pits was detected.