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Single wall carbon nanohorns as well as their thermally oxidized derivatives were decorated with Pd nanoparticles and their H2 sorption performance was examined at 298 K up to 20 bar. The specific surface area of the nanohorns was increased through air oxidation, while both the thermal treatment and the metal doping led to the enhancement of the H2 uptake. The higher uptake of the hybrid materials could not be attributed only to the additive effect of the carbon support and Pd content suggesting the existence of a cooperative mechanism between the metal particles and the carbon surface. This weak chemisorption process was found to be fully reversible after mild heating; still, its contribution to the overall H2 uptake was not found to be of great significance. © 2014 Elsevier B.V. All rights reserved.

The effect of hydrogen pressure and of reaction temperature on the phase transformation from Mg to MgH2 by exposure of Fe doped metallic Mg to hydrogen gas has been studied with the purpose of evaluating the importance of operative parameters on the hydrogen storage characteristics of this system. Two temperatures have been investigated and, for each temperature, two values of the hydrogen gas pressure. The gas pressure has been adjusted in order to induce the same thermodynamic driving force in samples processed at different temperatures, with the aim of separating the effect of temperature from the one of gas pressure. Experimental results show that the reaction mechanism is independent from the value of the operative parameters which, instead, influence strongly the absolute value of the kinetics constant. Sample microstructure during phase transformation is affected in a major way by the reaction temperature, with the gas pressure playing a minor role.

The rate-limiting step of the hydriding of catalysed Mg has been ascertained through a key experiment in which the density of nuclei of the product phase was artificially modified. The various microstructures observed allow accurate identification of the reaction rate-limiting step for different hypotheses. The experimental work was based on kinetics analysis coupled with scanning electron microscopy metallographic observation. It is found that the reaction barrier is located at the external surface of the powder particles, suggesting decoration with catalyst as a possible solution.

Carbon nanostructures are under deep investigation due their peculiar properties and possible applications. In particular, development of new methods for the synthesis of these materials and their mechanism of formation represent interesting research fields. Arc discharge allows to produce different forms of carbon nanostructures. The parameters involved in the process, voltage, current density, type and pressure of the surrounding gas can be controlled especially for achieving high quantity of material with enhanced characteristics in terms of purity while the use of transition metal-graphite mixtures has been used to produce single wall structures. Moreover direct current (DC) and alternating current (AC) are suitable for producing carbon nano-materials, but different results can be obtained. In this work the effect of the power frequency in an AC arc discharge technique on the synthesis of carbon nanostructures is reported. Pure graphite electrodes have been arched in air in an homemade apparatus where the material can be collected directly on a cylindrical collector fixed near the arc. In order to avoid the formation of deposits under the arc a symmetrical configuration of the electrodes has been set. The production of carbon soot containing Single Wall Nanohorns (SWNH) and highly convoluted graphene sheets is optimized. The range of power frequencies 32-1000Hz has been investigated and the arcs have been ignited fixing the voltage at 28 V. The materials has been analyzed by field emission scanning electron microscope and high resolution transmission electron microscope. The microstructure of the material synthesized by this apparatus is affected by the power frequency, as the experimental results demonstrate. The samples produced at low frequency presented high amounts of single wall structures, SWNH-type. More compact structures, similar to large onion-like structures, have been found in samples synthesized at high frequency values.

Scanning electron microscopy was used to support kinetic analysis in order to ascertain the physical mechanisms involved in the Mg–MgH2 phase transformation. Kinetic asymmetry in hydrogen absorption and desorption was explained by the different requirements for fast nucleation in the two reactions. In particular, MgH2 nucleation in Mg is assisted by crystal defects, while free surface is the only nucleation site in the reverse reaction. This explains, for the first time, the different role of the metal catalyst in the two reactions.

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.

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.

Mechanical spectroscopy is employed, in correlation with differential scanning calorimetry and hydrogen sorption analysis, to investigate the hydrogen desorption reaction in nanostructured magnesium hydride prepared by ball milling. The hydrogen desorption process is marked by a dramatic drop of the specimen's resonance frequency and by an internal friction peak that occurs – under high vacuum – at about 580 K. After full hydrogen desorption, a broad anelastic relaxation peak similar to commercial Mg is observed at 470 K, with activation energy close to the one for self-diffusion in Mg.

The sorption behavior of MgH2 is the subject of numerous investigations concerning the safe hydrogen storage in metallic hydrides. With the purpose of integrating kinetic studies on the MgH2–Mg phase transformation with the analysis of the local microstructure, we have developed an experimental method for the metallographic examination by Scanning Electron Microscopy of partially desorbed MgH2 powders. The setting of the microscope and the sample preparation procedure have been optimized for highest contrast between Mg and MgH2, based on different electron emissions. In these experimental conditions the two phases can be clearly detected even in critical condition when strongly scattering catalyst particles are present. The method has been tested on a set of ball milled MgH2 samples either pure or containing Fe as catalyzing agent. A complete information on the spatial distribution of all the phases constituting the sample can be obtained by integrating observations carried out at different primary beam energies with different signals. As far as the desorption behavior is concerned, SEM analysis shows that the MgH2–Mg phase transition is strongly affected by catalyst particles which, in the present case, appear to support the nucleation step in the phase transformation sequence.

Potentiodynamic polarization of a mechanically polished titanium electrode in a diluted solution of Rhodium(III) chloride in 0.1 M perchloric acid was performed, resulting in simultaneous formation of both Rh and TiO2 films. The morphology of obtained Rh/TiO2 composite film followed the morphology of titanium support, as evidenced by SEM technique. This composite surface was examined by cyclic voltammetry in both acidic and alkaline solutions, in the potential region of both hydrogen and oxygen underpotential deposition. The charge related to hydrogen underpotential deposition corresponded to a surface roughness of 43. As a consequence of high surface roughness, the diffusion current of oxygen reduction in an oxygen saturated 0.1 M NaOH solution, measured by voltammetry on rotating disc electrode, was found to be comparable to the current of hydrogen underpotential deposition.

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.

Carbon nanohorns and multiwalled carbon nanotubes have been synthesized by DC arcdischarge carried out at room pressure in air and Ar-enriched environment, by a specially designed experimental device. The resulting nanostructured material, characterized by electron microscopy and X-ray diffraction, shows different structures according to the condensation channels through which the sublimated carbon atoms are re-condensed in the solid state. Multi-Walled Carbon Nano- Tubes are mainly found in the hard crust formed at the cathode, while nano-horned particles can be recovered from a cylindrical collector surrounding the discharge. Further material, rag-like shaped and with an amorphous structure, can be collected in the reaction area. When the discharge occurs under Ar atmosphere, a larger quantity of this latter phase is synthesized. This suggests that the atmospheric oxygen could play an active role by burning the most reactive among the synthesized phases, like amorphous carbon contributing so to an “in situ” purification of the raw material.

The hydrogen storage performances of Mg-C nanocomposites have been studied on materials synthesized by ball milling with and without addition of an organic additive. The main purpose of this work is to study in more detail the cooperative effect observed when both graphite and benzene are added to the milled blend. In fact, when both components are added to Mg in the ball milling process, good catalyzing properties of the composite surface allow improved sorption-desorption behavior of the synthesized material. The processed materials have been characterized by XRD to assess the details of the phase structure by Rietveld analysis, while surface features have been studied by XPS, which evidences structural modifications of both the surface Mg oxide and the graphite particles. The hydrogen desorption behavior has been correlated with the surface structure which appears to depend on the carbon to benzene ratio. Thermal stability and hydrogen desorption properties were investigated by DSC. Experimental results on nanocomposites with the same Mg to C weight ratio (70:30) show improved performances for a proper choice of carbon to benzene ratio weight (1/3), even after manipulation of the milled material in air. Current Research in Advanced Materials and Processes, 6th Conference of the Yugoslav-Materials-Research-Society, Sep 13-17, 2004, Herceg Novi, Montenegro

The characterization of self-diffusion in MgO grain boundaries is a materials science problem of general interest, being relevant to the stability and reactivity of MgO layers in artificial nanostructures as well as to the understanding of mass transport and morphological evolution in polycrystalline metal oxides which are employed in many technological applications. In addition, atomic transport in MgO is a key factor to describe the rheology of the Earth’s lower mantle. In this work, we tackle the problem using a classical molecular dynamics model and finite-temperature simulations. To this purpose, we first design a stable grain boundary structure, which is meant to be representative of general internal interfaces in nanocrystalline MgO. The Mg and O self-diffusion coefficients along this grain boundary are then determined as a function of temperature by calculating the mean-square ionic displacement in the boundary region. Two different diffusion regimes at low and high temperature are identified, allowing to obtain the relevant activation enthalpies for migration from the temperature dependance of the diffusion coefficients. Our results prove that Mg diffusion along MgO grain boundaries is sufficiently fast to explain the recently reported development of MgO hollow structures during repeated hydrogen sorption cycles in Mg/MgO nanoparticles.

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. © 1998 Chapman & Hall

The magnetic properties of α-Fe 2 O 3 nanoparticles, prepared as powders by chemical route and as granular films by sputtering, have been investigated for different size (3 - 700 nm) and shape (rhombohedral, acicular and spherical). The particle size and shape effect on the Morin transition (T M = 263 K in the bulk system) has been investigated by analyzing the temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) magnetization. For rhombohedral (30 - 350 nm) and spherical (10 - 50 nm) particles, the Morin temperature was found to decrease with decreasing particle size and increasing magnetic field. On the other hand, acicular particles (350 - 700 nm) do not show the Morin transition, unless annealed. In samples characterized by a large distribution of particle size, the ZFC and FC measurements show the superimposed effects of the superparamagnetic behaviour of very small particles and the Morin transition for the larger ones. Only the superparamagnetic component is observed in granular films consisting of very small spherical particles (3 nm), whose average blocking temperature is about 140 K.

The effect of high energy ball milling on the structural refinement of the binary system Ag-Fe has been studied by Transmission and Scanning Electron Microscopy, Small Angle Neutron Scattering and Mossbauer spectroscopy. Samples containng 10 wt% of Fe were submitted to high energy ball milling for times up to 50 hours. The process produces a fine dispersion of Fe into the Ag matrix; the size distribution function is quite broad with an average radius of the order of a few nm. The minimum average particle size is obtained after about 10 hours of milling, longer term processing leads to a detectable particle coarsening. TEM analyses give evidence of a well defined orientation relationships between the Fe particles and the Ag matrix. Mossbauer spectroscopy reveals a modification of the hyperfine field experienced by the Fe atoms only for short term milling. On the basis of the experimental results one can assume that the fine dispersion of Fe into the Ag matrix occurs by mutual solubilisation probably induced by plastic shear, followed by fast decomposition by precipitation. After this step, further milling can induce only coarsening of the microstructure.

We report the preparation of amorphous Fe80−xCrxB20 nanoparticles by chemical reduction, their morphological and structural characterization and the study of their magnetic properties by means of Mossbauer and magnetization measurements. The results show that the mean particle size is composition dependent, decreasing with increasing x. The magnetic properties, determined by the chemical composition and by the particle size distribution, change with increasing x from a ferromagnetic character to a state characterized by the coexistence of Fe-Cr-B ferromagnetic particles with a growing fraction of antiferromagnetic (partially oxidized) and paramagnetic (oxidized) particles.

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 L12 structure has been analyzed by means of x-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry. Atomistic simulations on an Ni3Al 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.

The electrical conductivity of composites obtained by mixing epoxy resin and expanded graphite (EG) nanosheets has been investigated. Conductivity changes of up to I I orders of magnitude were observed as the filler load increased from 0 to 8 wt.%, while the percolation threshold was around 3 wt.%. It was also shown that epoxy/EG composites with intermediate filler contents exhibit a phase transition from predominantly contact conductivity to tunneling conductivity during heating from room temperature to 120 degrees C. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Graphite nano-sheets, having a thickness of the order of 10 nm, high shape asymmetry, and a well preserved crystalline order, have been prepared by a mechanical method based on the grinding of graphite powder under low energy pure shear milling using water as a lubricant. The small applied stress activates the plastic deformation process mainly on the easy gliding (00 1) hexagonal planes so that planar flakes bounded by the basal planes are obtained by a purely mechanical process. (c) 2006 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Extensive electronic structure calculations, based on the density-functional theory, are used to characterize the equilibrium properties and the behaviour under pressure of MgH2. Moreover, substitutional solid solutions for several 3d metals (Fe, Ni, Ti, Al, Pd, Co, etc.) in MgH2 are characterized by computing both relaxed structures and solution energies. Theoretical results are combined with experimental results on samples synthesized by ball milling. MgH2 was milled with 10 wt.% of 3d metals followed by microstructural characterization and hydrogen desorption behaviour. Good correlation between theoretical solution energies and experimental desorption temperatures reveals a general trend in these systems. A few notable exceptions can be detected, which can be explained in terms of peculiarities in the experimental composite microstructure.

Hydrogen, being a regenerative and environmentally harmless fuel, can play a crucial role in the energetic scenario of the near future. In recent years several systems for solid-state hydrogen storage have been investigated, among which a few metals and metal alloys show the most promising properties. Mg and Mg-based micro and nanocomposites are widely studied for this application owing to the high gravimetric storage capacity, even if a proper microstructure, mainly at the material surface, has to be setup in order to overtake kinetic constraints often related to the high surface reactivity. In this paper, we present our approach to this problem and report the main findings. We have examined the influence on hydrogenation/dehydrogenation characteristics of different modifications of the bulk and surface microstructure of MgH2 powders processed by ball milling, which appears to be one the most useful methods for preparing powders suitable for hydrogen storage. Research Trends in Contemporary Materials Science, 8th Conference of the Yugoslav-Materials-Research-Society (Yu-MRS), Sep 04-08, 2006, Herceg Novi, Montenegro

Magnesium based composites MgH2 + X (X=Ti, Co) were synthesized by ball milling in an argon atmosphere using stainless steel vial and balls. The crystallographic behavior of the resulting powders was examined by XRD. Thermal stability and hydrogen desorption properties were investigated by thermal analysis methods. In order to obtain a deeper insight into bonding mechanisms of the transition metal in MgH2 relaxed structure, ab initio electronic structure calculation of MgH2 + X (X=Ti, Co) was performed using Full Potential Linearized Augmented Plane Wave method, implemented in WIEN2K code. DOS analysis, confirmed by DTA measurements, resulted in the conclusion that, in the composite, in comparison to MgH2, the bonding Mg-H was weakend, on account of the shortening of interatomic distances hydrogen-transition metal. Research Trends in Contemporary Materials Science, 8th Conference of the Yugoslav-Materials-Research-Society (Yu-MRS), Sep 04-08, 2006, Herceg Novi, Montenegro