In components built by selective laser melting (SLM), possible anisotropy in microstructure significantly affects mechanical properties (i.e. service performance) and there is a constant effort to ...solve this problem. This work carried out a comparative study using fully dense AlSi10Mg alloy and in-situ nano-TiB2 decorated AlSi10Mg composite samples processed by SLM. The aim was at clarifying the effect of integrated nanoparticles on the anisotropy of the as-built component. Microstructure and texture evolution were investigated by scanning and transmission electron microscopy, electron backscatter diffraction, X-ray and neutron diffraction. The SLMed AlSi10Mg sample shows a coarse columnar grain structure with and fiber orientation texture and obvious anisotropy in mechanical properties. However, the nano-TiB2 modified AlSi10Mg sample exhibits fine equiaxed grains, no preferred crystallographic texture and remarkably reduced anisotropy. Besides, both tensile strength and ductility have been improved in the SLMed TiB2 decorated AlSi10Mg sample. The preliminary mechanism upon grain refinement effect due to nano-TiB2 particles on columnar-to-equiaxed transition and reduction of anisotropy was discussed.
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•A competition growth of and fiber texture is found in SLMed Al alloy.•The introduction of TiB2 eliminates texture and anisotropic mechanical behavior.•Random orientation of TiB2 nucleation agents determines random orientation of Al.•Both strength and ductility have been improved in SLMed TiB2/AlSi10Mg alloy.
The crystallization kinetics and the resulting structure and morphology of polylactide (PLA) were investigated in the presence of carbon nanotubes (CNTs). Nanocomposite samples prepared by solution ...and melt mixing present homogeneous filler dispersion, as observed by scanning electron microscopy. Calorimetric characterization of the nonisothermal and isothermal crystallization behavior analyzed according to Avrami’s theory provides evidence of the significant impact of CNTs on the crystallization kinetics of the PLA matrix. The nucleating effect of the nanofillers is confirmed by Raman spectroscopy experiments. Indeed, during isothermal crystallization, the nanotube characteristic vibrations are strongly affected by the development of polymer crystalline phase. Additionally, CNTs increase the number of nucleation sites and thereby decrease the average spherolite size as observed by optical microscopy. The PLA crystal structure is not modified by the presence of CNTs, as probed by X-ray diffraction.
This work focuses, for the first time, on the structural evolution of Polylactide (PLA) and PLA-Talc nanocomposites upon biaxial stretching. Biaxial stretching is a widely used technique to improve ...the end-use properties of polymer films. Encountered in a large number of elaboration processes it involves important structural changes into the material which directly govern the properties gain. In this work the influence of both stretching conditions and clay content were addressed. Besides it is observed that while relatively low clay content, i.e. below 10 wt%, has a limited impact on the mechanical behavior, higher contents dramatically modify the latter and particularly decreases the stretchability of the material. It was also evidenced that both the stretching conditions and clay content influence the strain-induced structure. Particularly the presence of talc favors the formation of a crystalline structure upon stretching due to its nucleating ability.
The mechanical behavior of the biaxially stretched samples has also been investigated. As a key point it was observed that while the as elaborated materials are brittle when uniaxially stretched at room temperature, the biaxially stretched ones exhibit a ductile behavior with achieved strains at break up to 100%. The origin of this brittle to ductile transition, assessed by means of in situ SAXS experiments, was found in the inhibition of the crazing mechanism for samples biaxially oriented under appropriate conditions.
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•Influence of talc content on the mechanical behavior of PLA upon biaxial stretching.•Nucleating effect of talc on the strain-induced crystallization.•Strain-induced structural evolution upon biaxial stretching.•Brittle to ductile transition induced by texturation•Macromolecular orientation involves crazing inhibition.
At the boundary between mechanochemistry, supramolecular chemistry and catalysis, the present study explores the role of cyclodextrins (CDs) and other saccharide additives in the mechanosynthesis of ...gold nanoparticles (AuNPs) and their use as catalysts in the reduction of substituted nitrobenzene derivatives into their corresponding aniline products. CDs not only allow for the stabilization of the AuNPs but also help diffuse a substrate within a solid mixture via supramolecular means, and orient the chemical reaction to the selective formation of aniline derivatives. Parameters influencing both the formation of AuNPs and the synthesis of aniline derivatives have been investigated. We show that the catalytic performance strongly depends upon the nature of the saccharide additive, the nature and location of the substituent on the benzene, and the ball-milling conditions. Water also plays a key role in both the reduction mechanism of the nitro groups and the supramolecular interactions with the substrate. Very interestingly, the amount of reductive agent (NaBH4) was drastically reduced compared to reductions performed in solution. Additionally, the catalytic system could be recycled over three consecutive runs without significant loss in activity, thus highlighting the efficacy of the combination of mechanochemistry, supramolecular chemistry, and catalysis.
The mechanisms and the sequence of crystallization of a Fe64B24Y4Nb6Al0.4 metallic glass were investigated experimentally. To this end, the microstructure of the metallic glass after interrupted ...isothermal treatments at a temperature between glass transition and primary crystallization was studied by complementary techniques, namely atom probe tomography, transmission electron microscopy and X-ray diffractometry. During the early stages of crystallization of the glass, some nanometric (Fe,Nb)B crystals nucleate first by rejecting Y at their interface, which hinders their growth. The Fe62B14Y3 dendritic phase starts growing in a second step. Longer annealings lead to the development of chemical gradients in the amorphous matrix surrounding Fe62B14Y3 crystals, which favors the nucleation of additional nanometric (Fe,Nb)2.4B crystals.
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•Fe64B14Y4Nb6Al0.4 glass crystallization is studied by APT, HRTEM and XRD.•Crystallization is studied during annealings at 667 °C in-between Tg and Tx.•Equiaxed (Fe,Nb)B crystals form during the early stages of crystallization.•Then dendritic Fe62B14Y3 crystals nucleate generating chemical gradients.•These chemical gradients finally promote the nucleation of (Fe,Nb)2.4B phase.
We have studied electronic transport in undoped GaAs/SrTiO3 core-shell nanowires standing on their Si substrate with two-tip scanning tunneling microscopy in ultrahigh vacuum. The resistance profile ...along the nanowires is proportional to the tip separation with resistances per unit length of a few GΩ/um. Examination of the different transport pathways parallel to the nanowire growth axis reveals that the measured resistance is consistent with a conduction along the interfacial states at the GaAs{110} sidewalls, the 2 nm-thick SrTiO3 shell being as much as resistive, despite oxygen deficient growth conditions. The origin of the shell resistivity is discussed in light of the nanowire analysis with transmission electron microscopy and Raman spectroscopy, providing good grounds for the use of SrTiO3 shells as gate insulators.
The confinement of catalytically active metallic nanoparticles within discrete and robust microenvironments was successfully achieved by using a water-compatible three-dimensional beta ...-cyclodextrin-based polymer. The strategy was examined using ruthenium through an aqueous colloidal approach involving the chemical reduction of ruthenium nitrosyl nitrate by sodium borohydride in the presence of a water-soluble beta -CD polymer crosslinked with citric acid (poly(CTR- beta -CD)). The advantage of this polymer for nanoparticle synthesis is that (i) additional stabilizing effects are exerted through steric interactions (crosslinked chains and beta -cyclodextrin entities) and electrostatic interactions (ionisable-COOH groups) and (ii) accessible nanopockets are provided between the stable junctions of the polymer skeleton. The poly(CTR- beta -CD) Ru(0) system was characterized at different stages of the synthesis by combining proton nuclear magnetic resonance spectroscopy, dynamic light scattering and transmission electron microscopy measurements. The results highlighted that, in contrast with a series of control colloidal ruthenium catalysts, the specific use of poly(CTR- beta -CD) allowed not only the stabilization of smaller size-controlled ruthenium nanoparticles (approximately 1.8 nm) but also their confinement in individual superstructures having sizes mostly in the range of 50 to 100 nm. These polymer-encapsulated ruthenium nanoparticles were applied as catalysts for the aqueous phase hydrogenation of biomass-derived 2-furaldehyde and 3-(2-furyl)acrolein under mild reaction conditions, i.e.303 K and 1 MPa. The high reactivity was related to the presence of individual globular objects acting as catalytic "microreactors", in which the consecutive hydrogenation reactions and product/substrate diffusional exchanges can occur efficiently in the confined spaces. The robustness of the system was demonstrated through recycling experiments and TEM characterizations after catalytic tests.
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