M. Vittori Antisari

Solid state reactions and microstructural evolution of AlNi powders during high-energy ball milling

Abstract The solid state reactions and the microstructural evolution during high-energy ball milling of AlNi powder mixtures in the composition range 25–75 at.% Al have been investigated. Experimental observations have shown that the microscopic mechanism underlying the alloying process in this system is the diffusion of Ni atoms in the Al-rich layers and that an important role is played by the oxygen contamination. An amorphous Al-rich phase containing a few at.% oxygen and with a Ni content not exceeding approximately 50 at.% has been detected in the equiatomic and Ni-rich samples milled for a few hours. This phase upon further milling transforms to a Ni-rich fcc solid solution thus allowing one to by-pass the nucleation of Al 3 Ni in these samples. The self-sustaining high-temperature synthesis of the AlNi B2 phase has been found to occur over the 40–60 at.% range of Al concentration after about 3 h of milling. The same reaction has been observed in the Differential Scanning Calorimeter if equiatomic samples premilled for about 3 h (unreacted) are heated at a sufficiently high heating rate.

AC arc discharge synthesis of single-walled nanohorns and highly convoluted graphene sheets

In this paper we report the synthesis of single-walled carbon nanohorns (SWNHs) and highly convoluted graphene sheets by AC powered arc discharge in air. The experimental strategy was based on the design of an experimental set-up able to enhance the synthesis of these compounds at the expense of the usually found cathode crust where multi-walled nanotubes are often found. In this set-up the arc discharge is ignited between pure graphite electrodes experiencing a symmetrical situation as far as the heat generation by the arc and the heat dissipation at the heat sinks is concerned. This is achieved by AC powering two electrodes of the same size and length. A cylindrical steel collector able to control the heat exchange with the surroundings completes the experimental set-up. The yield of the device, in terms of synthesis rate and of fraction of sublimated carbon converted to nanostructures, has been evaluated, and it resulted in being particularly high, indicating that AC powering can represent a key factor in the large-scale synthesis of SWNHs and highly convoluted graphene sheets by arc discharge. The structure of the synthesized powder has been determined by transmission electron microscopy.

Hydrogen sorption in Pd-decorated Mg―MgO core-shell nanoparticles

Mg nanoparticles with metal-oxide core-shell morphology were synthesized by inert-gas condensation and decorated by in situ Pd deposition. Transmission electron microscopy and x-ray diffraction underline the formation of a noncontinuous layer with Pd clusters on top of the MgO shell. Even in the presence of a thick MgO interlayer, a modest (2 at. %) Pd decoration deeply enhances the hydrogen sorption properties: previously inert nanoparticles exhibit metal-hydride transformation with fast kinetics and gravimetric capacity above 5 wt %.

Metal-hydride transformation kinetics in Mg nanoparticles

The hydrogen sorption kinetics of magnesium nanoparticles prepared by inert gas condensation and coated by a magnesium oxide layer were investigated by a volumetric apparatus. The metal-hydride transformation was studied by transmission electron microscopy of the nanoparticles both in the as-prepared state and after hydrogen cycling. In small nanoparticles (≈35 nm) hydride formation proceeds by one-dimensional growth controlled by diffusion through the hydride, while the reverse transformation to metal involves interface-controlled three-dimensional growth of nuclei formed at constant rate. Large nanoparticles (≈450 nm) exhibit very low reactivity attributed to reduced probability of hydrogen dissociation/recombination and nucleation at the particle surface.

Mechanochemical Exfoliation of Graphite and Its Polyvinyl Alcohol Nanocomposites with Enhanced Barrier Properties

Polyvinylalcohol/graphite nanocomposites with graphite nanosheets have been prepared by a mechanical method based on grinding of graphite powder, under low energy pure shearing milling, using water or KOH as lubricant. The use of different lubricant concurs to obtain graphite sheets that differently disperse in hydrophilic polymeric matrix. An improvement of water vapor permeability (up to 12%), compared with homopolymer, has been observed.

Exchange bias and interface structure in the Ni/NiO nanogranular system

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H2, at different temperatures (200 ? Tann ? 300??C), NiO powder previously ball-milled for 20?h. Typically, the samples consist of Ni nanoparticles (mean size of 10?18?nm) embedded in a nanocrystalline NiO matrix. With increasing Tann, the Ni fraction varies from 4% up to 69%. The exchange field depends on the Ni amount, being maximum (~600?Oe), at T = 5?K, in the sample with 15% Ni. In all the samples, the EB effect vanishes at T = 200?K. The structural features of the samples have been investigated by x-ray diffraction, electron microscopy and extended x-ray absorption fine structure and the low-temperature magneto-thermal behaviour has been thoroughly analyzed. The results show the existence of a structurally and magnetically disordered NiO component, which plays the key role in the EB mechanism.

A new method for preparing powders for transmission electron microscopy examination.

In this paper we describe a novel method to prepare powder specimens for transmission electron microscopy examination. The powder samples are embedded in a metallic matrix by a route based on the plastic flow of a soft metal, using a small laboratory type hand driven hydraulic press. The resulting composites are processed with the conventional procedure based on grinding polishing and ion beam milling. The resulting TEM specimens have a self-supporting structure, good thermal and electrical conductivity while showing a well-polished surface resulting from the ion milling process. The method can be applied to a large variety of samples with sufficiently strong mechanical properties; a few examples are reported. The limits, mainly due to the mechanical toughness of the powder, are discussed.

Comparison of spatial resolutions obtained with different signal components in scanning electron microscopy

Comparative studies on the ultimate spatial resolution of the Scanning Electron Microscope, using different components of the electron signal have been performed on specimens providing compositional contrast. By operating the microscope in conventional way as well as with a specifically designed set-up we have ascertained that the delocalized components of the signal provide a spatial resolution of the order of the beam size, even if the practical use can be limited by the noise. To amplify the contribution of the delocalized components of the signal, as backscattered electrons by a bulk specimen or forward scattered electrons by a thin specimen, we used a device consisting of a plate of a material with high secondary yield placed above or below the sample. An important practical implication arises from this study. A detecting system consisting of a standard Everhart-Thornley detector coupled with a converter of backscattered or transmitted electrons represents a high performance detecting device for low voltage observations.

Transmission Electron Microscopy of Self-Annealed Ion Implanted Silicon

Self annealing experiments were performed on thermally insulated (100) Si samples, implanting 100 keV P+ ions at beam power densities of 15 and 25 W/cm2. Cross and plan section characterization by TEM of the specimens lead to two main conclusions about the dynamic annealing mechanism: 1) ion bombardment enhances the rate of amorphous to crystalline transformation; 2) residual crystalline islands are present on the surface. They allow a regrowth of the amorphous region both from the front and the bottom of the layer.

Microstructural and Kinetic Investigation of Hydrogen Sorption Reaction of MgH2/Nb2O5 Nanopowders

MgH2/Nb2O5 composite is one of the most promising candidates for the hydrogen delivery. The performances of these materials are usually improved by mechanical milling because a finer distribution of the catalyst and the induction of defects on the particles accelerate the hydrogen sorption kinetic of the powders. Aiming at elucidating the factors responsible for this improvement, the effect on the reaction kinetics induced by nanometric and micrometric Nb2O5 powders has been investigated by a Sievert type apparatus. Nanometric additive imparts excellent performances in comparison to the micrometric one. The activation of the sample by sorption cycling has been investigated. In order to elucidate the role of the catalyst, a metallographic study of partially desorbed MgH2/Nb2O5 composite has been applied for the first time. The powders have been also characterized by X-ray diffraction and thermal gravimetric analysis.