Topological insulators, such as the Bi2Se3 material, exhibit significant optical nonlinearities. This work investigates the impact of the pulse duration on the nonlinear optical responses of Bi2Se3 ...layers. Scanning electron microscopy studies have been performed to reveal the crystalline structure of the samples. The nonlinear optical performance has been investigated for a wide range of pulse durations, from 400 fs to 10 ps, using 1030 nm laser excitation. The nonlinear absorption coefficients recorded in this study range from-1.45 x10 −7 m/W to-4.86 x10 −7 m/W. The influence of two different mechanisms on optical nonlinearities was observed and discussed. Identical experimental conditions have been employed throughout the studies allowing a direct comparison of the results.
In this work, we studied the effect of annealing temperature on the kinetics of hydrogen absorption/desorption in nanocrystalline Pr2Co7 alloy. The pressure-composition (P–C) isotherm measurements ...revealed that the plateau pressure of Pr2Co7 alloy after annealing becomes comparatively flat. The maximum hydrogen absorption content of as-milled alloy and the annealed one reached 1.01 and 1.2 H/M (H/M: hydrogen per metal), respectively. The derived formation enthalpy ΔH and Gibbs free energy ΔG of the hydride in the annealed alloy are higher than those of the as-milled compound, indicating that the hydrides stability increases after annealing. The mechanism of hydrogen absorption-desorption process in the as-milled and annealed alloys was further investigated according to the Johnson-Mehl-Avrami formula. The hydrogenation of the as-milled and annealed Pr2Co7 alloys was found to be controlled by the three-dimensional-interface process. The activation energies Ea were determined and indicate that annealing is a very beneficial way to improve hydrogen absorption-desorption kinetics of the nanocrystalline Pr2Co7 alloy. The latter has a very fast absorption kinetic at 298 K. Throughout the study, it has been shown that Pr2Co7 alloy exhibits excellent hydrogenation properties which open a new perspective to be a new candidate for important uses in the field of hydrogen storage such as negative electrodes for nickel metal hydride (Ni-MH) batteries, mobile and short range vehicular applications.
•The annealing can improve the hydrogen absorption content of Pr2Co7 alloy.•The Pr2Co7 showed excellent absorption-desorption stability.•The Pr2Co7 presented a very rapid absorption kinetics at room temperature.•The Pr2Co7 alloy exhibits important hydrogenation properties.
Geopolymer, a nanoporous aluminosilicate filled with water and ions, has been selected as a potential matrix to encapsulate MgZr alloy fuel cladding. In this study, we investigate the evolution of ...the corrosion products formed during the corrosion of MgZr in poral solutions extracted from geopolymers with and without NaF as corrosion inhibitor. Using various characterization techniques such as Scanning Electron and Scanning Transmission Electron Microscopies coupled to Energy Dispersive X-ray spectroscopy and Grazing Incidence X-ray Diffraction, we show that the amounts of dissolved silica and fluoride species in solution are the key parameters driving the nature of corrosion products and probably their passivating properties regarding MgZr corrosion.
MCVD germanosilica glass embedded with YbPO4 crystals were for the first time drawn into optical fibers. Solution doping was used to embed the crystals in the silica soot prior to the collapsing ...step. We demonstrate, using scanning/transmission electron microscopes and confocal Raman microscope, that YbPO4 crystals survive not only the MCVD process but also the drawing process despite the high temperature involved (up to 2100 °C) during fabrication processes. The fiber contains 100 nm-crystals with the same composition and structure as the as-prepared crystals. During the drawing process, these crystals tend to have a preferential orientation of their c-axis along the drawing direction. These results open a new route to fabricate glass-based composite fibers containing crystalline particles without additional post heat-treatment.
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•YbPO4 crystals are prepared by precipitation form aqueous solution.•YbPO4 crystals survive all along the MCVD process and the fiber drawing step.•YbPO4 crystals are aligned during the fiber drawing.
The thickness-dependent saturable absorption behaviour of atomically thin bismuth selenide films has been optimized using 515 nm and 1030 nm laser excitation wavelengths. The studies have been ...performed by the Z-scan technique employing 400 fs laser pulses emitted by a fiber laser at a repetition rate of 100 Hz. The obtained results allowed the determination of the nonlinear optical parameters as a function of the film thickness. Additional studies have been carried out in order to determine the third-order optical nonlinearities as a function of the annealing temperature. The samples have also been studied by spectrophotometry, electron microscopy and X-ray diffraction, which allowed a deeper knowledge of the optical properties, as well as the crystalline structure of the Bi2Se3 material.
•Giant ultrafast nonlinear optical responses have been recorded and compared with the literature.•Precise optimization of both the 2D material and excitation parameters for maximizing the optical nonlinearities.•The 2D materials are nonlinear optically active in the visible (515 nm) and infrared (1030 nm) parts of the spectrum.•The topological insulators were characterized by X-ray diffraction, scanning electron microscopy and spectrophotometry.
Segregation of Ni was observed by atom probe tomography at the edges of a pseudo-hexagonal dislocation loop within As
+-implanted (0
0
1) Si wafers, after Ni deposition but before heat treatment. ...Thanks to crystallographic information retained within the atom probe tomography data, the orientation of the loop was determined to be within the {1
1
1} plane and elongated along the
<
1
1
¯
0
>
direction. The presence of pseudo-hexagonal dislocation loops was confirmed by transmission electron microscopy. Concentrations of more than 10
at.% in Ni were measured at the edges of the loop.
•Geothermal alteration of K-spar at acid pH results in Al-rich coating formation.•Weak impact of coatings on K-spar reactivity observed for week-long experiments.•Modeling suggests an order of ...magnitude decrease in rate for month-long durations.•Acid stimulation on site suggested to remain effective despite enhanced K-spar reactivity.
Heat transfer fluid mining represents a thermodynamic perturbation for geothermal reservoirs: The pumping of hot water coupled with the re-injection of colder water at depth favors the dissolution of some rock-forming minerals of the deep reservoir (e.g. feldspars), while promoting the precipitation of secondary phases, resulting in a possible change in the permeability and porosity of the reservoir. Such an impact is even greater when one considers the acid stimulations aimed at increasing the injectivity of the geothermal system. In that respect, no consensus exists in the literature regarding the impact of secondary phases on the dissolution rate of primary phases and therefore, on the sustained modification of pore structure. The present study aimed at shedding new light on these questions. Hydrothermal experiments of K-feldspar alteration were conducted at conditions relevant for the geothermal reservoir of Soultz-sous-Forêts (T = 180 °C, acidic pH domain). Measurements of cation release rates were combined with characterizations of secondary coatings (mineralogy, extent of coverage, thickness and porosity) to determine the reactivity of submillimeter K-feldspar powders with and without secondary precipitates. The formation of µm-thick boehmite coatings on K-feldspar grains was found to result in a modest decrease in its reactivity, which might be better explained by the presence of dissolved Al in the bulk solution. This result was independently confirmed by reactive transport simulations, which revealed that the impact of secondary coatings may become significant only when their thickness exceeds a few tens of microns, or if the dissolution rate of the primary phase is significantly greater (106 times) than that of orthoclase. Taken together, this study offers new constraints on the intricate interplay between dissolution and precipitation reactions, of prime importance for modeling more accurately the impact of mass transfer and porosity generation resulting from fluid circulation in geothermal reservoirs.
Global food security concerns have spurred increasing demand for locally sourced and produced K-fertilizers. Various processes have been explored for more than a century; one promising solution is ...based on the alkaline aqueous alteration of feldspar-rich rocks at elevated temperatures. However, knowledge of the overall physico-chemical reactions comprising dissolution of feldspar and precipitation of secondary phases is still rudimentary, in particular how the feldspar structure evolves at the nm-scale during hydrolysis at alkaline conditions. Here we report on the results of a study aimed at converting potassium feldspars to K-rich fertilizer based on the alteration of sanidine and microcline samples at 190 °C in pH 12 Ca(OH)2 solutions for 24 h. Based on X-ray diffraction and Rietveld refinement, the secondary authigenic minerals that precipitated are primarily composed of Ca-carbonate (calcite, vaterite), and Ca-(Al)-silicates, such as tobermorite and hydrogrossular. Short-term bench top leaching experiments in water prove that the hydrothermal product releases up to two orders of magnitude more K than the unaltered K-feldspar starting material, pointing to its application as a ready-to-use fertilizer for K-deficient soils. Detailed chemical mapping and energy dispersive X-ray spectroscopy (FESEM- and TEM-EDXS) analyses of the precipitates at the μm to nm-scale show that the distribution of K associated with the secondary phases is very heterogeneous, both spatially and in terms of concentrations. Using various analytical transmission electron microscopy (TEM) techniques, e.g., HRTEM, TEM-EDXS, EFTEM, to investigate the structure and chemistry of the feldspar interface, we find no evidence for a change in chemistry or structure at the nm-scale, even though dissolution continuously decreases the volume of each grain. Our observations also show the existence of an amorphous surface altered layer (SAL) of variable thickness (10–~100 nm) forming at the feldspar interface. Nanometer-scale chemical measurements show that this amorphous SAL is rich in K, and therefore may also be an important reservoir of easily leachable K. We hypothesize that it forms continuously and in situ at the expense of the feldspar by a coupled interfacial dissolution-reprecipitation process (CIDR).
•Alkaline hydrothermal alteration of K-feldspars yields K-fertilizer for soils.•SEM and TEM techniques used to measure K in secondary phases and interface.•Labile K in hydrothermal product associated with soluble secondary phases.•The K-feldspar structure dissolves stoichiometrically, with no change in chemistry.•Coupled interfacial dissolution-reprecipitation (CIDR) controls dissolution process.
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