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nanoparticles
摘要： We report changes on the magnetic behavior of the Heavy-Fermion ferromagnetic YbNi2 alloy when reducing the size of the particles to the nanometer scale by high-energy ball milling. The milling process induces a reduction on the particle size of the bulk alloy down to 10 (2) nm and a lattice strain of 1.8 (2)%, for 30 h of milling time, as calculated from the x-ray diffraction data. The ferromagnetic transition at TC =10.5 K in the bulk alloy tends to disappear whereas other magnetic transitions arise at lower temperatures with the milling process. The AC magnetic susceptibility of the 30 h milled sample shows a frequency independent peak at 3 K, in a wide range of frequencies, consistent neither with superparamagnetic nor simple freezing of the magnetic moments, and as expected for a ferromagnetic behavior. However, from the specific heat measurements in the 30 h milled sample, a peak at 3.6 K is shifted respect to that observed in the AC magnetic susceptibility, as found in spin glasses. Additionally, the specific heat on nanometric samples show reduced magnetic contributions respect to the bulk alloy with ?cmax around 0.45 J/molK. The results are explained by the influence of magnetic inhomogeneities and disorder of the alloys enhanced by the milling process in the ensemble of nanoparticles.
关键词： Specific heat,Ferromagnetic,Spin glass,Magnetic properties,Yb alloys
更新于2025-09-23 15:21:21
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DC and RF characteristics optimization of AlGaN/GaN/BGaN/GaN/Si HEMT for microwave-power and high temperature application
摘要： AlGaN/GaN/Si high electron mobility transistors (HEMTs) developed by molecular beam epitaxy (MBE) are studied with several methods for characterization, the most utilized are direct-current and radio-frequency measurements, to see power and microwave performance of components. The increase in these parameters is not based just with on improvement technological for example, decrease of length gate (Lg) and passivation. For sure, another very important point is to reduce the thickness of barrier while keeping the reduction in the length of gate, in order to reduce the transit time (τ), and consequently access to highest cut-o? frequency (FT). For this situation, it’s appears a harmful phenomenon of type “punch-through”, because of the weak con?nement of electrons in the channel. In this paper, the main objective is to show how to reduce this e?ect.
关键词： HEMT GaN,Radio-frequency,Current-voltage characteristics,RF characteristics,Alloys BGaN
更新于2025-09-23 15:21:21
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Interface and Strength of Laser Impact Welding of Fe-Based Nanocrystalline Alloys to Aluminum
摘要： Fe-based nanocrystalline alloys have the characteristics of low power consumption, corrosion resistance and good magnetic stability, while pure Al is characterized by good electrical conductivity, high reflectivity and non-magnetism. To combine the respective performance advantages of the two materials, the welding of Fe-based nanocrystalline alloy foil and Al foil was realized by laser impact spot welding. In addition, the microstructure and mechanical properties of welded joints with different standoff distance were observed and tested. The results show that with the increase in standoff distance, the plastic deformation of the flyer surface will be more serious. When the standoff distance increases gradually (0.1 mm–0.2 mm–0.3 mm), the welding interface will transit from straight interface to wavy interface and finally to wavy interface with local melting zone. The maximum tensile force of the welded sample will increase with the increase in the thickness of the flyer.
关键词： Tensile force,Laser impact spot welding,Wave interface,Fe-based nanocrystalline alloys
更新于2025-09-23 15:21:01
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Laser Operating Windows Prediction in Selective Laser-Melting Processing of Metallic Powders: Development and Validation of a Computational Fluid Dynamics-Based Model
摘要： The rapidly ascending trend of additive manufacturing techniques requires a tailoring of existing solidification models and the development of new numerical tools. User-friendly numerical models can be a valid aid in order to optimize operating parameter ranges with the scope to extend the modelling tools to already existing or innovative alloys. In this paper a modelling approach is described simulating the generation of single tracks on a powder bed system in a selective laser melting process. The approach we report attains track geometry as a function of: alloy thermo-physical properties, laser speed and power, powder bed thickness. Aim of the research is to generate a numerical tool able to predict laser power and speed ranges in manufacturing porosity-free printed parts without lack of fusion and keyhole pores. The approach is based on a simplified description of the physical aspects. Main simplifications concern: the laser energy input, the formation of the pool cavity, and the powder bed thermo-physical properties. The model has been adjusted based on literature data providing the track’s geometry (width and depth) and relative density. Such data refer to different alloys. In particular, Ti6Al4V, Inconel625, Al7050, 316L and pure copper are considered. We show that the printing process presents features common to all alloys. This allows the model to predict the printing behavior of an alloy from its physical properties, avoiding the need to perform specific experimental activities.
关键词： metallic alloys,numerical model,selective laser melting,additive manufacturing,laser operating window
更新于2025-09-23 15:21:01
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Surface engineering of a titanium alloy for tribological applications by nanosecond-pulsed laser
摘要： In many applications, surface engineering is needed in order to overcome the poor wear properties of titanium alloys. A fiber laser was used in different operation modes for remelting a Ti6Al4V surface and subsequently smoothing it again, resulting in a smooth, glossy and crack-free surface. The laser treated surface was characterized by tribological experiments, nanoindentation and (transmission) electron microscopy and compared to samples treated by plasma nitridation and thermal oxidation. All surface treatments improved the tribological behavior of Ti6Al4V to different extents. Nanoindentation measurements showed no strict correlation of mechanical properties and tribological behavior. It is hypothesized that apart from mechanical properties, binding of titanium electrons by interstitials plays a role in the occurrence or absence of adhesive wear.
关键词： Titanium alloys,Surface engineering,Adhesive wear,Ti6Al4V
更新于2025-09-23 15:21:01
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Comparison of quantitative analyses using SIMS, atom probe tomography, and femtosecond laser ablation inductively coupled plasma mass spectrometry with Si <sub/>1a??X</sub> Ge <sub/>X</sub> and Fe <sub/>1a??X</sub> Ni <sub/>X</sub> binary alloys
摘要： Due to their electrical and physical properties, Si1?XGeX materials are widely used in microelectronic devices. In particular, the Ge component found within Si1?XGeX compounds is important for enhancing carrier mobility and altering the lattice constant of metal-oxide-semiconductor field-effect transistors. In this study, magnetic sector secondary ion mass spectrometry (magnetic sector SIMS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to determine the accurate concentrations of major compositions present within binary alloy samples. However, quantitative SIMS analysis is limited by the matrix effect, which influences the sputter yield of an element in a compound and alters the secondary ionization yields. Quantitative deviations that were due to the matrix effect were reduced by using Cs cluster ions (MCs+ and MCs2+) instead of elemental ions; the SIMS results using the elements were, therefore, compared with those using MCs+ and MCs2+ cluster ions. In the case of Fe1?XNiX alloys that have a less matrix effect compared to Si1?XGeX alloys, both the Cs primary ion beam (Cs+) and an oxygen primary ion beam (O2+) were used to measure the Fe1?XNiX compositions. The quantitative results from the two different primary ion beams were then compared to understand the ionization process. Deviations in the quantitative values gained with the O2+ beam were lower than those obtained using the Cs+ primary ions, meaning that using oxygen as the primary ion improves the accuracy in quantifying Fe1?XNiX compounds. Other reliable tools for analysis such as atom probe tomography and femtosecond laser ablation inductively coupled plasma mass spectrometry were also used in the quantitative analysis, with results that were consistent with the most accurate results obtained using magnetic sector SIMS and ToF-SIMS.
关键词： Si1?XGeX,femtosecond laser ablation inductively coupled plasma mass spectrometry,atom probe tomography,SIMS,Fe1?XNiX,binary alloys
更新于2025-09-23 15:21:01
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Brazing, Laser, and Electron-Beam Welding of Additively Manufactured GRCop-84 Copper for Phased Array Lower Hybrid Launchers
摘要： Recent advances in selective laser melting 3-D printing technology allow additive manufacturing of lower hybrid current drive (LHCD) RF launchers from a new material, Glenn Research Copper 84 (GRCop-84), a Cr2Nb (8 at. % Cr, 4 at. % Nb) precipitation hardened alloy, in configurations unachievable by conventional machining. Cr2Nb crystals pin grain boundaries within the copper matrix resulting in high tensile strength and resistance to annealing at elevated temperatures. Brazing, laser, and electron-beam welding (EBW) techniques are explored for joining a thin-walled GRCop-84 waveguide with zero porosity and minimal internal surface roughness. GRCop-84 wets well with the silver solder, CuSil, and Cusil-ABA brazes, once the durable surface oxide is mechanically removed. GRCop-84 melt pool size and flow during EBW is reduced compared to oxygen-free copper (OFC). Pulsed laser and e-beam welding maintains the Cr2Nb precipitate size; precipitate coarsening occurs in conduction mode e-beam welding.
关键词： joining processes,fusion reactor design,magnetic confinement,Annealing,tokamaks,copper alloys,melt processing,materials processing,phased arrays,waveguide
更新于2025-09-23 15:21:01
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Influence of Focused Pulsed Laser Radiation on the Properties of Amorphous FeSi6B16 Metal Alloy
摘要： The effect of focused pulsed laser radiation (number of pulses and power density) on an amorphous FeSi6B16 metal film is studied. The states of the alloy before and after laser irradiation are analyzed using methods of atomic force microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and microhardness measurements. Focused laser radiation in any exposure mode is established to cause high-temperature heating of the surface of an amorphous alloy in the irradiation zone. The result of such heating is melting of the material surface and propagation of the heat wave front from the strongly heated surface to its inner layers, which induces the redistribution of elements in the Fe–Si–B system, a change in the structure, and an increase in the microhardness.
关键词： microhardness,nonequilibrium state,X-ray photoelectron spectroscopy,atomic-force microscopy,surface,laser radiation,amorphous alloys
更新于2025-09-23 15:21:01
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Enhancing Galvanic Replacement in Plasmonic Hollow Nanoparticles: Understanding the Role of the Speciation of Metal Ion Precursors
摘要： Hollow nanostructures offer great potential for plasmonic applications due to their strong and highly tunable localized surface plasmon resonance. The relationship between the plasmonic properties and geometry of hollow nanoparticles, such as core-shell size ratio, concentricity of the cavity and porosity of the wall, is well documented. Nanoscale galvanic replacement provides a simple, versatile and powerful route for the preparation of such hollow structures. Here we demonstrate how the efficiency of reductant-assisted galvanic replacement processes can be enhanced by controlling the degree of hydration and hydrolysis of the metal ion precursor using pH and pL as key control parameters (by analogy to pH, the letter p in the expression pL is used to indicate the decimal cologarithm associated with the concentration of the ligand L). Adjusting precursor speciation prior to the sacrificial template’s hollowing process offers a new strategy to tune the morphology and optical properties of plasmonic hollow nanostructures.
关键词： co-reduction,localized surface plasmon resonance,nanoscale galvanic replacement,hollow nanoparticle synthesis,Ag-Au, Ag-Pd and Ag-Pt alloys,aqueous metal ion speciation
更新于2025-09-23 15:21:01
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Investigation the effect of laser ablation parameters in a liquid in order to reduce the pulse energy during laser shock peening
摘要： The features of laser shock peening technology was studied in relation to the physical processes occurring during and after laser treatment. The effect of laser parameters on the results of the process was investigated. It was shown that the reduce of the diameter of the laser spot while maintaining the intensity leads to a decrease in the effect of LSP. This phenomenon was studied from the point of view of the features of plastic deformation at various sizes of the treatment zone using the standard model for the LSP process. Besides the features of the development of a plasma plume during laser ablation in a liquid are considered. It was shown that the liquid determines the geometry of the plume evolution and the shock wave caused by it. The process parameters used and the results obtained are important for the development of low-energy LSP technology.
关键词： Surface treatment,Laser shock peening,Laser-produced plasmas,Laser ablation in liquid,Nickel alloys,Laser pulses,Lasers
更新于2025-09-23 15:21:01