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Numerical and experimental study on keyhole and melt flow dynamics during laser welding of aluminium alloys under subatmospheric pressures
摘要： Porosity defects was highly related to the keyhole and melt flow dynamic during laser welding process. In this paper, a novel 3D numerical model was developed to describe the keyhole dynamic and melt flow behaviors during laser welding of 5A06 aluminium alloy under subatmospheric pressures. The effect of ambient pressure on laser welding process was taken into consideration by optimizing the boiling point of aluminium alloy and recoil pressure of evaporated metallic vapor jets based on vapor–liquid equilibria calculation and Wilson equation. A moving hybrid heat source model was employed to describe the laser energy distribution under subatmospheric pressures. Numerical results indicated that a wider and deeper keyhole with less humps was produced under subatmospheric pressure comparing with that of atmospheric pressure. The vortices in the rear keyhole wall became unapparent or even disappeared with the decrease of ambient pressures. The melt flow velocity on the keyhole wall was larger under a lower pressure. A smaller difference between boiling point and melting point was produced and this led to the formation of a thinner keyhole wall and improved the stability of molten pool. Larger recoil pressure produced under subatmospheric pressure was responsible for the weakened vortices and enhanced melt flow velocity. Bigger keyhole opening size, larger melt flow velocity, thinner keyhole and the weakened vortices all resulted into the reduction of porosity defects during laser welding of aluminium alloys. Based on the simulation results, the plasma distribution, weld formation and porosity defects had been demonstrated. The compared results showed that the simulation results exhibited good agreements with the experimental ones.
关键词： Porosity defects,Keyhole stability,Numerical simulation,Subatmospheric pressure,Melt flow dynamic,Laser welding
更新于2025-11-28 14:24:20
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Effect of Heat Input on Porosity Defects in a Fiber Laser Welded Socket-Joint Made of Powder Metallurgy Molybdenum Alloy
摘要： Porosity defects are still a challenging issue in the fusion welding of molybdenum and its alloys due to the pre-existing interior defects associated with the powder metallurgy process. Fiber laser welding of end plug and cladding tube made of nanostructured high-strength molybdenum (NS-Mo) alloy was performed in this work with an emphasis on the role of welding heat input. The distribution and morphology of porosity defects in the welded joints were examined by computed tomography (CT) and scanning electron microscopy (SEM). Preliminary results showed that laser welding of NS-Mo under low heat input significantly reduced the porosity defects in the fusion zone. The results of computed tomography (CT) showed that when the welding heat input decreased from 3600 J/cm (i.e., 1200 W, 0.2 m/min) to 250 J/cm (i.e., 2500 W, 6 m/min), the porosity ratio of the NS-Mo joints declined from 10.7% to 2.1%. Notable porosity defects under high heat input were related to the instability of the keyhole, expansion and the merging of bubbles in the molten pool, among which the instability of the keyhole played the dominant role. The porous defects at low heat input were generated as bubbles released from the powder metallurgy base metal (BM) did not have enough time to overflow and escape.
关键词： fiber laser welding,molybdenum alloy,porosity defects,heat input
更新于2025-11-28 14:24:20
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Repulsive magnetic field–assisted laser-induced plasma micromachining for high-quality microfabrication
摘要： Surface micro-/nanostructures are widely used in the fabrication of various functional microsystems. Laser-induced plasma micromachining can greatly improve surface quality in terms of recast layers and thermal defects compared with laser direct writing. Magnetic field has the ability to constrain plasma diffusion and can ensure the stability of laser-induced plasma processing. This paper compares the effects of laser direct–writing processing and laser-induced plasma processing of single-crystal silicon at the micro-/nanoscale, and emphatically analyzes the material removal mechanism of repulsive magnetic field–assisted laser-induced plasma micromachining. It is shown that the volume of the laser-induced plasma was constrained under the influence of Lorentz force, a high-quality smooth microgroove without thermal defects was obtained, and its line width was reduced by 30%.
关键词： Laser-induced plasma micromachining,Magnetic confinement,Thermal defects,Micro-/nanofabrication
更新于2025-11-21 11:24:58
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Traps distribution in sol-gel synthesized ZnO nanoparticles
摘要： The distribution of shallow traps within the sol-gel synthesized ZnO nanoparticles was investigated using thermoluminescence (TL) experiments in the 10–300 K temperature range. TL measurements presented two overlapped peaks around 110 and 155 K. The experimental technique based on radiating the nanoparticles at different temperatures (Texc.) between 60 and 125 K was carried out to understand the trap distribution characteristics of peaks. It was observed that peak maximum temperature shifted to higher values and activation energy (Et) increased as irradiating temperature was increased. The Et vs. Texc. presented that ZnO nanoparticles have quasi-continuously distributed traps possessing activation energies increasing from 80 to 171 meV.
关键词： Luminescence,Nanoparticles,Defects
更新于2025-11-21 11:18:25
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Non-radiation creation of complex centers in wide-gap oxide crystals
摘要： It is shown that complex aggregate centers of oxygen divacancy type (F2 centers) and interstitial aluminum type near the anion vacancy (Ali+) are formed in anion-deficient crystals of corundum (α-Al2O3-δ) and beryllium oxide (BeO1-δ) under thermo-optical treatment (TOT). These centers are similar to those created in stoichiometric α-Al2O3 and BeO crystals under neutron irradiation. It is important to note that thermal stability of the TOT-created complex centers is higher than that of similar neutron-induced centers. It is also established that the probability of their formation is related to the temperature of the TOT, the wavelength of the stimulating light, and the initial anionic deficiency manifested as F+ and F centers (anion vacancies with one and two electrons, respectively).
关键词： Formation of complex defects,Wide-gap anion-deficient oxide crystals,Thermoluminescence,Optical absorption,Thermostability of complex defects,Thermo-optical treatment
更新于2025-11-19 16:56:35
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Reversible/Irreversible Photobleaching of Fluorescent Surface Defects of SiC Quantum Dots: Mechanism and Sensing of Solar UV Irradiation
摘要： Knowledge about photobleaching properties of the fluorescent surface defects of the semiconductor quantum dots (QDs) is crucial for their applications. Here, the photobleaching properties of the fluorescent surface defects of the colloidal 3C-SiC QDs are reported. The combined experimental and theoretical study reveals that the observed violet fluorescence at around 392 nm stems from the carboxylic acid group-related surface defects. When the SiC QDs are exposed to the UV irradiation, the 392 nm fluorescent surface defects show both reversible and irreversible photobleaching, whereas the 438 nm fluorescent surface defects show only irreversible photobleaching. The photochemical mechanisms dominating these phenomena are explored. The photobleaching property of the SiC QDs is utilized to detect the solar UV irradiation with high accuracy. The photobleaching of the SiC QDs is highly sensitive to the hydrogen or metal ion concentration in the colloid solution. These findings deepen the understanding of the properties of the fluorescent surface defects of the SiC QDs and pave the way for their applications in sensing.
关键词： photobleaching,silicon carbide quantum dots,surface defects,fluorescence mechanism
更新于2025-11-19 16:46:39
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The Self-Assembly of Cu-In-S Quantum Dots with Aggregation-Induced Emission into 3D Network Triggered by Cation and Its Application as A Novel Metal-Enhanced Fluorescent Nanosensor for Detecting Zn (Ⅱ)
摘要： A novel self-assembly phenomenon triggered by Zn2+ of Cu-In-S quantum dots with aggregation-induced emission effect was presented in this paper. Hydrophilic Cu-In-S quantum dots with aggregation-induced emission effect were successfully prepared. They were monodisperse spherical nanoparticles with the diameter of 2.8 ± 0.4 nm and had weak fluorescence in aqueous solution. However, the solution emitted strong fluorescence after addition of Zn2+. The results of TEM and SEM indicated the monodisperse quantum dots self-assembled into 3D networks of Cu-In-S quantum dots-Zn2+, which hindered the motion of quantum dots. Besides, the Zn2+ in the mixture passivated the surface defects. The phenomenon also proved by florescence lifetime and XPS. Thus the radiation decay decreased and followed by strong fluorescent emission. Interestingly, the degree of aggregation was proportional to the amount of Zn2+ and the fluorescent intensity. Based on this interesting phenomenon, a novel metal enhanced fluorescent nanosensor for detecting Zn2+ was established. The results demonstrated the proposed method had a good selectivity and linearity in the concentration range of 0-800 nmol?L-1 with a limit of detection of 1.99 ppb. These results showed a promising future in the field of metal-enhanced fluorescent probes of the Cu-In-S quantum dots.
关键词： Surface defects,Metal-enhanced sensor,Quantum dots,Restriction of motion,Aggregation-induced emission
更新于2025-11-14 15:23:50
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Computational Study on Interfaces and Interface Defects of Amorphous Silica and Silicon
摘要： The amorphous SiO2/Si interface is arguably the most important part in semiconductor technology, strongly influencing the device reliability. Its electronic structure is affected by the defects, majorly the dangling bonds known as Pb-type defects, which have been studied for decades. These defects are usually passivated by hydrogen atoms in device processing, which eliminates the defect levels in the silicon bandgap and thus removes their electric activity. However, when the interface is exposed to ionization radiation, the passivated defects can be reactivated by the protons generated by radiation, which significantly affects the device performance and causes reliability issues. In this review, computational studies on the amorphous SiO2/Si interface and interface defects are summarized, including the modeling of the interface, the main interface defects, and their depassivation, and compared to experimental results. The hyperfine parameters are emphasized, because they are essential to identify the structures of the interface defects. The defect levels and depassivation of the defects are also emphasized, because the former directly affect the device performance and the latter directly generates the dangling bonds in the interface.
关键词： depassivation,ionizing damage,interface defects,amorphous interfaces,first-principles calculations
更新于2025-09-23 15:23:52
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A generic strategy for preparation of TiO2/BixMyOz (M?=?W, Mo) heterojunctions with enhanced photocatalytic activities
摘要： Employment of nanoscale TiO2/BixMyOz (M = W, Mo) heterojunctions is one of the most promising strategies for improving the photocatalytic efficiency. However, the controllable synthesis and morphology modification of these heterojunctions are still highly challenging. In this work, we developed a generic approach to hydrothermally synthesize TiO2/BixMyOz heterojunctions and tailor their morphologies. The key of this strategy is to intentionally utilize the surface defects of TiO2 as highly active sites to adsorb the intermediate hydrolysis-products, which is in marked contrast to the conventionally direct precipitation methods. In the subsequent hydrothermal reactions, MO4 2- replaced the hydroxyl and nitrate radicals to form stable TiO2/BixMyOz heterojunctions, in which the second phase BixMyOz occupied the defect sites of TiO2 nanobelts. Under visible light irradiation, the photocatalytic reaction rate constant of TiO2/Bi2WO6 heterojunctions was four times higher than that of single phase Bi2WO6 nanosheets, while the photocatalytic reaction rate constant of TiO2/Bi3.64Mo0.36O6.55 heterojunctions exhibited a seven-fold increase compared with Bi3.64Mo0.36O6.55 nanopaticles. The substantial enhancement of photocatalytic activity is primarily ascribed to the matching energy band structure in the TiO2/BixMyOz heterojunctions, which is able to improve the separation efficiency of photo-generated electron-hole pairs and prolong the lifetime of charge carriers in the heterojunctions.
关键词： TiO2/Bi2MoO6 heterojunctions,photocatalytic activities,TiO2/Bi2WO6 heterojunctions,defects induction,TiO2/Bi3.64Mo0.36O6.55 heterojunctions
更新于2025-09-23 15:23:52
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Photocatalytic Activity of ZnO Nanopowders: The role of production techniques in the formation of structural defects
摘要： The effect of the type of structural defect in zinc oxide on its photocatalytic properties was studied for phenol photodegradation under UV-irradiation. It was shown that the use of different types of precursors (zinc oxalate and zinc hydroxide) for the production of zinc oxide leads to the formation of a material with the same phase composition and equal energy of the forbidden band, but different photocatalytic activities. Simultaneously, the peculiarities of the luminescence and electron spin resonance spectra indicate the formation of different types of defects in the structure of the material, namely, oxygen vacancies (Vo) in the anionic and zinc vacancies (VZn) in the cationic sublattices of zinc oxide synthesised from the zinc oxalate and hydroxide, respectively. Also, the different characteristics of the luminescence decays reveal the different recombination paths for the free charge carriers in the systems synthesised from the different precursors. The different times of the luminescence decay also confirmed the different methods of recombination of free charge carriers in systems synthesised from different precursors. It was shown that the appearance of defects in the cationic sublattice leads to a decrease in the photocatalytic activity of the material relative to phenol degradation.
关键词： vacancies,luminescence.,Photocatalytic activity,structure defects,ZnO
更新于2025-09-23 15:23:52