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Ordered langasites La3Ga5MO14:Eu3+ (M = Zr, Hf, Sn) as red-emitting LED phosphors
摘要： The temperature-dependent luminescence behavior of three ordered langasites La3Ga5ZrO14 (LGZr), La3Ga5HfO14 (LGHf) and La3Ga5SnO14 (LGSn) doped with up to 20% Eu3+ were investigated. Band gap energies of 4.32 eV (LGZr), 4.43 eV (LGHf), and 4.46 eV (LGSn) were determined by diffuse reflection measurements of the undoped compounds. The influence of the structural differences on the luminescence properties of the three langasites were investigated by means of excitation, emission (77-500 K) and luminescence lifetime measurements (77-300 K). The quenching temperatures of the three langasites do vary. They were attained using different excitation wavelengths in the charge transfer band (300 nm) and in the 4f levels (394 nm). LGSn:Eu 20% Eu3+ showed the best temperature performance with intensities at 500 K of 85% (300 nm) and 67% of the intensity measured at 77 K. At room temperature and with an excitation wavelength of 394 nm, maximum quantum efficiencies of 41% for LGZr:Eu 20% Eu3+, 58% for LGHf:Eu 20% Eu3+ and 68% for LGSn:Eu 20% Eu3+ were reached. The quenching behavior observed is explained by semi-quantitative configurational coordinate diagrams. Based on these results and the high red color purity, the langasites were built into LEDs with (In,Ga)N chips emitting at 394 nm, to test the applicability of the materials as red LED phosphors.
关键词： inorganic materials,optical properties,rare earth alloys and compounds,phosphors,oxide materials,luminescence
更新于2025-09-11 14:15:04
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Pore Closure Effect of Laser Shock Peening of Additively Manufactured AlSi10Mg
摘要： This article reports on an exceptional insight provided by nondestructive X-ray tomography of the same samples before and after laser shock peening (LSP). The porosity in two additively manufactured aluminum alloy (AlSi10Mg) tensile samples before and after LSP was imaged using identical X-ray tomography settings and overlap of the data was performed for direct comparison. The results indicate clearly that near-surface pores are closed by the process, while internal pores remain unaffected. LSP has become well known as a method to improve the fatigue properties of materials, including those of additively manufactured aluminum alloys. This improvement is usually attributed to the compressive residual stress induced by the process. The additional effect of closure of near-surface pores that is illustrated in this work is of interest for additive manufacturing because additive manufacturing is not yet able to produce completely pore-free components. Since the critical pore initiating fatigue cracks are always attributed to surface or subsurface pores, the closure of these pores may play an additional role in improving the fatigue properties. While more work remains to unravel the relative importance of near-surface porosity compared to the compressive residual stress effect, this work clearly shows the effect of LSP—closing of pores near the surface. For the processing conditions demonstrated here, all pores up to 0.7 mm from the surface are closed without damaging the surface, while higher peening power results in surface damage.
关键词： additive manufacturing,laser shock peening,X-ray tomography,aluminum alloys,laser powder bed fusion
更新于2025-09-11 14:15:04
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Laser cladding process of Cobalt and Nickel based hard-micron-layers on 316L-stainless-steel-substrate
摘要： In the present study, two different types of coatings such as Cobalt (Co-Cr-W-C) and Nickel (Ni-Cr-B-Si-C) based layers have produced using a high power of Ytterbium-doped yttrium aluminum garnet (Yb:YAG) disk laser on 316L stainless steel substrate. Then the coatings were analyzed to expose their phase characterization, microstructure, and hardness using X-ray diffraction analysis, electron microscopes and microhardness tester. Further, tribo-test has performed through a ball-on-plate machine to analyze the wear properties of coatings. Besides, worn-surfaces of cladding and surface roughness have examined. The results showed that both Cobalt and Nickel based cladding have exhibited a dendrite homogeneous structure with some changes in shape and grain size due to higher cooling rates. XRD results indicate that the mixing of a γ-Co peak with other metastable phases such as Cr7C3 and Cr23C6 were noticed on Cobalt coating and γ-Ni rich matrix was the most dominant peak in Nickel-based coating. Microhardness of the cladding layers has remarkably increased than the substrate. The wear performance of Nickel cladding has increased than Cobalt cladding and substrate sample, which reveals that Nickel-based laser cladding plays a role in wear resistance. The analyzed wear track indicates that adhesion and abrasion were the two major wear mechanisms.
关键词： cladding,316L,alloys,wear,microstructure,nickel,steel,performance,roughness,stainless,cobalt,surface
更新于2025-09-11 14:15:04
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Metal ablation study with a 10 picosecond laser under low and median fluence
摘要： Single shot 10 ps laser ablation on aluminium and gold at a wavelength of 1064 nm under low and median laser fluence was studied theoretically and experimentally. Simulations have been performed by using an optimized two temperature model. It is revealed that electron-phonon coupling ability of the target metal affected both the ablation depth and target metal’s morphology. Under low fluence, melting could well be the relevant ablation mechanism. With laser fluence rising, vaporization or phase explosion happens thereafter, depending on material’s electron-phonon coupling ability.
关键词： Metals and alloys,Picosecond laser processing,Modeling of laser ablation
更新于2025-09-11 14:15:04
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Influence of laser surface melting on the properties of MB26 and AZ80 magnesium alloys
摘要： In the present investigation, laser surface melting (LSM) was performed on MB26 and AZ80 magnesium alloys and their surface properties were evaluated. The finite element method (FEM) was used to calculate the resulting temperature during the laser surface melting process. The melted layer average thickness values were about 296 and 190 μm for MB26 and AZ80 alloys, respectively and the variation in the thickness is mainly attributed to the absorption in laser energy. The average grain size of the LSMed MB26 alloy was decreased about 14.5 times and AZ80 had very finer grains than the MB26 along with the fine distribution of second phases. Further, the X-ray diffraction results revealed that the second phase intensities were decreased in both the alloys due to their dispersion. The laser absorption efficiency of MB26 was higher due to the higher plasma attachment during LSM process, thereby has higher melt layer thickness compared to AZ80. The refined microstructure of the melted layer resulted in an increase of micro-hardness up to 110 Hv. Potentiodynamic polarization test results revealed that the corrosion current density (icorr) values of the LSMed MB26 and AZ80 alloys were decreased about 1.8 and 2.5 times, respectively compared to the as-received alloys. A variation in solidification rates of the melt pool due to the alloying elements were attributed to an improvement in the surface and electrochemical properties.
关键词： Corrosion,Polarization,Laser surface melting,Hardness,Magnesium alloys
更新于2025-09-11 14:15:04
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Computer simulation for formation of critical spaces in II–VI solid solutions
摘要： Predicting of optimal properties of II–VI materials is an important task in solving the problems of micro- and nanoelectronics. Multicomponent solid solutions based on such semiconductors are promising for modern optoelectronics, for they allow creating devices that operate in a wide spectral range. However, under certain conditions multicomponent solid solutions lose their thermodynamic stability, and thus the properties of the devices based on such materials degrade. Modern methods of computer simulation are used to analyze processes of formation of self-organizing ordered structures. The authors use di?erential topological approach to obtaining multicomponent phase diagrams taking into account the possibility of existence of bifurcation spaces, critical spaces, and spaces of phase coexistence of di?erent orders.
关键词： Solid solutions,Computer simulation,Semiconducting II–VI materials,Phase diagrams,Alloys
更新于2025-09-11 14:15:04
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Homoepitaxy of Ge on ozone-treated Ge (1?0?0) substrate by ultra-high vacuum chemical vapor deposition
摘要： Germanium (Ge) homoepitaxy is still a big challenge due to the immature surface cleaning of Ge, which often results in imperfect Ge epitaxial growth in terms of surface roughness, pits and islands. In this work, high crystal quality Ge and GeSi with very smooth surface were epitaxially grown on Ge (0 0 1) substrates, which were thermally cleaned at 650 °C in an ultrahigh vacuum chemical vapor deposition system after ozone oxidation. The ozone oxidation of Ge substrates at optimal duration could strongly diminish Ge suboxides which should be responsible for the formation of square concave or islands in Ge epilayers. The complete removal of fully stoichiometric GeO2 on Ge substrates by thermal annealing provided a clean surface for almost perfect Ge homoepitaxy or GeSi heteroepitaxy with a low temperature Ge epilayer. This technology would solve the dilemma of epitaxy on Ge for micro- and opto-electronic device applications.
关键词： A3. Vapor phase epitaxy,A1. Surface processes,B2. Semiconducting germanium,B1. Germanium silicon alloys
更新于2025-09-11 14:15:04
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Investigation on the effect of indium composition on ultrafast carrier dynamics in InGaN alloys
摘要： In this letter, we investigated the effect of indium composition on carrier relaxation mechanisms in InGaN alloys. Four high quality alloys with indium composition from 25% to 75% were fabricated using energetic neutral atomic-beam lithography/epitaxy molecular beam epitaxy. Using sub-picosecond resolved photoluminescence at high carrier density, it was found the effective carrier lifetime extends with increasing indium composition. Moreover, the calculated initial carrier temperature also rises with higher indium composition. These results are consistent with the theoretical prediction that a greater phonon bandgap could reduce the carrier cooling rate to a certain extent via hot carrier bottleneck effect.
关键词： InGaN alloys,indium composition,carrier dynamics,photoluminescence,molecular beam epitaxy
更新于2025-09-10 09:29:36
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Density of Liquid Ni-Ti and a New Optical Method for its Determination
摘要： Liquid Ni-Ti alloys were processed in a containerless way using the technique of electromagnetic levitation in order to determine their densities. An improved optical method was utilized where, in addition to recording shadowgraph images from the side, a second camera recorded images of the sample from the top. A correction factor for the density was calculated from the top-view images. This method yields measurements insensitive to droplet rotation and static deformation which removes the need to assume axial symmetry. The measured densities are discussed in terms of the molar volume. A negative molar excess volume was obtained, indicating that Ni-Ti is a highly non-ideal system. These measurements were then used to test a recently proposed relationship between the molar excess volume, the excess free energy, and the isothermal compressibility. For the first time, the excess volume of a binary alloy, i.e., Ni-Ti, is adequately predicted by a thermodynamic model.
关键词： molar volume,excess volume,density,electromagnetic levitation,Ni-Ti alloys
更新于2025-09-10 09:29:36
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Form-stable benzoxazine-urethane alloys for thermally reversible light scattering materials
摘要： Thermally reversible light scattering (TRLS) materials based solely on benzoxazine-urethane (BA-a/PU) alloys were successfully fabricated and demonstrated in this work. The alloys displayed the opaque state below 40 °C. The alloys were transformed to the transparent state upon exposing to the transition temperature of 60–130?°C, depending on the molecular weights and mass concentrations of urethane prepolymers in the BA-a/PU alloys. The optical state transitions were reversible with small hystereses. BA-a/PU alloys exhibited a good optical contrast with 0%T at the light scattering state and almost 100%T at the transparent state. The alloys were glassy and form-stable up to 250?°C, due to the synergistic behavior in the glass transition temperatures. The reaction-induced phase separation effectuated by the incorporation of urethane prepolymer into thermosetting polybenzoxazine, the sizes and local concentrations of the phase-separated urethane microdomains in the supporting polybenzoxazine matrix, and the reversible dissolution and demixing of urethane microdomains and polybenzoxazine phase played crucial roles on TRLS properties of the developed benzoxazine-urethane alloys.
关键词： smart polymers,urethane,polymer blends and alloys,polybenzoxazine,thermally reversible light scattering
更新于2025-09-10 09:29:36