F. Cardellini

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 in the Al-Fe system induced by ball milling of elemental powders

Elemental aluminium and iron powders have been mechanically alloyed in the atomic Al∶Fe ratios of 1∶1 and 1∶3. The structural evolution of the samples was followed by X-ray diffraction and differential calorimetry. Extended milling caused the formation of an almost completely disordered b c c solid solution instead of the equilibrium B2 and DO3 compounds. Upon heating to 700 °C the formation of the B2 phase was observed in the equiatomic samples and no significant variation of the long-range state of order in the iron-rich samples. Upon heating of samples pre-milled for short or intermediate times, at about 400 °C, the nucleation and growth of Al5Fe2 was observed which, upon further heating to 700 °C, transformed to the B2 phase or to the b c c solid solution, depending on the sample composition. These results suggest that the solid-state reactions proceed through diffusion of iron atoms in the aluminium layers, and that for an iron concentration in these layers below about 50 at%, the Al(Fe) solid solution could be in a metastable phase.

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.

Study on the formation of Pt/C catalysts by non-oxidized active carbon support and a sulfur-based reducing agent

The formation mechanism of Pt/C catalysts using non-oxidized active carbon support and the weak reducing agent Na2S2O4 was investigated. Platinum on carbon catalysts were fabricated by an impregnation/reduction process of the Pt-precursor H2PtCl6 on carbon support. The effect of thermal treatment in argon up to 700°C on the structural characteristics of these catalysts was studied by XRD and TEM analyses. The importance of carbon support properties on Pt/C formation was recognized. Before thermal treatment a very weak internal organization (a very small particle size and amorphous structure) in the metal was obtained. Thermal treatment at relatively low temperatures leads to the growth and then to the crystallization of platinum particles in the well-known face centered cubic structure. The sintering of Pt particles occurs through the migration of Pt atoms on the carbon support, likely by a bridge-bonding mechanism on sulfur atoms. A fast growth of Pt particles occurred in the temperature range 300—400°C. Thermal crystallization, instead, occurred mostly going from 400 to 550°C. Following annealing at 550°C, the formation of platinum sulfide was revealed. The sample thermally treated at 700°C showed an anomalous XRD pattern with Pt reflexions shifted towards high angles and an increase of Pt[111]/Pt[220] peak intensity ratio.

Ordering kinetics of disordered Ni3Al synthesized by mechanical alloying

The Ni[sub 3]Al compound belongs to that class of intermetallics whose virtual critical temperature for the order-disordered transformation is higher than the melting temperature. Chemically disordered Ni[sub 3]Al can be prepared by several methods including high energy ball milling (BM) of the ordered compound. Another possibility is offered by mechanical alloying (MA) of elemental Ni and Al powders. Aluminum and nickel powders in the 3:1 atomic ratio were mechanically alloyed in an air-cooled SPEX mill for times up to 40 hours. EDS microanalysis after MA showed an Fe contamination of about 2 and 4 at.% in the powders milled for 20 and 40 hours respectively. The structural characterization of the samples was performed by x-ray diffraction and their thermal stability was tested in a Perkin-Elmer DSC7 calorimeter. The diffraction patterns of the samples milled for at least 10 hours were very similar and showed the presence of a supersaturated fcc Ni(Al) solid solution whose diffraction peaks were considerably broadened with respect to those of the starting Ni powder.

Visible cathodoluminescence from mechanically milled germanium

There has been interest in the past years in the visible luminescence properties of germanium nanocrystals. Most of the previous works refer to Ge nanocrystals embedded in an oxide matrix. In this work nanocrystalline germanium has been prepared by high-energy ball milling of Ge single crystals and Ge powder. The structure of the milled samples has been assessed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) while the luminescence has been investigated by cathodoluminescence (CL) in the scanning electron microscope (SEM). The samples consist of a powder of particles with sizes of hundreds of nanometres, normally aggregated to form larger particles. TEM reveals that the particles consist of nanocrystals with a wide range of sizes including crystallites of some nanometres. Milled Ge shows two CL bands at about 2.4–2.5 eV and 3.1 eV, respectively. Both emissions appear to be related to the presence of Ge nanocrystals. The CL of untreated and milled GeO2 powder has been also investigated for comparison.

Solid-state reactions in mechanically alloyed Al-V powders

Abstract High-energy ball milling has been performed on Al x V1−x powder mixtures in the Al-rich range of composition (x = 0.61, 0.70, 0.75, 0.85 and 0.90). The structural evolution and the thermal stability of the samples was followed by X-ray diffraction and differential scanning calorimetry. We have observed that, similar to other Al-transition metal binary systems, the reaction rate decreases with increasing Al content and that milling promotes diffusion of both elements into each other; in fact for short times of milling Al(V) and V(Al) solid solutions have been observed at all investigated compositions. With the exception of the Al-richest composition, several metastable phases have been observed to nucleate in the early stages of milling. In a broad range of composition around the stoichiometric ratio extended milling promotes the formation of the Al3V intermetallic compound in a heavily strained nanocrystalline state. Upon heating of pre-milled samples to 360°C the disappearance of the metastable ...