Amelia Montone

Microstructural evolution of Al–Fe powder mixtures during high-energy ball milling

High-energy ball milling has been performed on FexAl1-x powder mixtures with x=0.75, 0.50, 0.25 and 0.20. X-ray diffraction, Mossbauer spectroscopy and electron microscopy have been used to characterize the samples milled for different times and annealed in a differential scanning calorimeter. It is found that, during milling, there is diffusion of both elements into each other, with a prevalence of iron diffusion into aluminium, at least in the early stages of the process. This behaviour is more pronounced in the aluminium-rich samples. The growth of the Fe(Al) and Al(Fe) solid solutions has been observed for x≥0.5, different from the lower iron concentrations where the Fe(Al) phase has not been detected. The annealing of pre-milled samples favours the formation, depending on the sample composition and on the annealing temperature, of intermetallic phases such as Fe3Al, FeAl, Fe2Al5 and FeAl3.

Solid state reactions between Ni and Al powders induced by plastic deformation

Abstract The solid state reactions induced by high-energy ball milling of Al/Ni elemental mixtures at 25, 33, 50, 67 and 75 Ni at.% have been studied by X-ray diffraction and electron microscopy. The thermal stability of the reaction products at different stages of milling has been investigated by differential scanning calorimetry. The sequence of reactions observed in each sample depends even in its early steps on the initial composition of the mixture. The end-product of the milling process is, in some cases, a metastable phase. For long milling times, in fact, we have observed the formation of a metastable f.c.c. solid solution at the two Ni-rich compositions, of the AlNi phase at 50 and 33% Ni and of the Al3Ni compound at 25% Ni. Thermal treatment of samples at an early stage of milling shows, contrary to the behaviour induced by plastic deformation, that at all compositions the first thermally driven reaction is the formation of Al3Ni. The experimental observation of an enhancement of Ni diffusivity in Al during ball milling, suggests that this high diffusivity is the main cause of the observed differences.

Effects of thermal treatments on structural and magnetic properties of acicular α-Fe2O3 nanoparticles

The effect of transformations induced by annealing at different temperatures (450 < Ta <1000 °C) on the structure and magnetic properties of acicular α-Fe2O3 particles (major axis: 330 ± 50 nm; minor axis: 70 ± 10 nm) have been investigated by X-ray diffraction, TEM, SEM and magnetization measurements. The particles were prepared by hydrolysis and polimerization in an aqueous solution of FeCl3. In the as-prepared sample, particles are constituted by small units (3–5 nm) separated by a less dense interlayer, while keeping the same crystallographic orientation. With increasing Ta, the sample microstructure evolves towards a collapse of the small units and then to a coalescence of particles. Coherently, the spin reorietation Morin temperature TM increases with Ta, the highest value (TM = 259 K) being comparable to that of the bulk sample.

Experimental and theoretical investigation of the order-disorder transformation in Ni3Al

The crystalline disordered phase obtained by mechanical alloying of elemental 75 at. % Ni and 25 at. % Al powders has been investigated. The stability of this phase with respect to the thermal reordering process leading to the L1 2 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni 3 Al model, reproduced via molecular dynamics using a many-body potential, have been used to interpret experimental data. The ordering transformation takes place in an extended range of temperature (from 320 to 600 °C) and occurs simultaneously with the release of internal strain. Numerical simulations performed under different conditions show that the activation energy of the Ni-vacancy migration mechanism responsible for the ordering process depends on the local state of strain, thus suggesting an explanation for the considerable lowering of this energy in samples obtained by ball milling.

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.

Structural and cathodoluminescence study of mechanically milled silicon

The structural and luminescent properties of nanocrystalline silicon produced by high-energy ball milling of Si single crystals have been investigated using transmission electron microscopy (TEM), x-ray diffraction (XRD) and cathodoluminescence (CL) in a scanning electron microscope. XRD measurements show that the average size of the nanocrystals in the milled samples is about 30 nm but TEM reveals a wide range of size distribution including crystallites with the dimension of few nanometres. Ball milling causes the appearance of a visible luminescence band at 1.61 eV, attributed to the presence of nanocrystals, and a near-infrared band peaked at about 0.79 eV which is suggested to be related to the high density of extended defects formed during the mechanical treatment. These bands, attributed to processes in Si, are not observed in the cathodoluminescent spectra of untreated and ball-milled SiO2 powder.

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.

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.

Desorption behaviour in nanostructured MgH2-Co

The influence of the Co addition and synthesis route on desorption properties of MgH2 were investigated. Ball milling of MgH2-Co blends was performed under Ar using different milling intensities and different weight ratios. Microstructural and morphological characterization, performed by XRD and SEM, show a huge correlation with thermal stability and hydrogen desorption properties investigated by DSC. A complex desorption behaviour is correlated with the dispersion of the catalytic particles that appears to play a main role in desorption performances. The optimum catalyst concentration was found to be around 10 wt.%, while the optimum value of the ball to powder ratio was 10:1.