Marco Vittori Antisari

Synthesis of multiwall carbon nanotubes by electric arc discharge in liquid environments

This work reports the experimental results from the production of multiwall carbon nanotubes (MWCN) synthesized by an electric arc discharge performed in liquid environments between pure graphite electrodes. Both liquid nitrogen and deionised water were suitable for a successful synthesis of this form of carbon aggregation. We report a successful synthesis of MWCN by arc discharge submerged in deionised water. Electron microscopy observations of both the reaction products and the surface of the as-synthesized raw material showed the presence of structural degradation of the MWCN, which probably operates after their growth at the cathode. The degradation is tentatively ascribed to a combination of overheating and high current density experienced by the as-synthesized MWNT, which can be caused by the loose structure of the as-deposited material. The damage appeared to be less severe in water environments, probably owing to the better cooling capacity of water relative to liquid nitrogen.

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.

Arc-discharge synthesis of carbon nanohorns and multiwalled carbon nanotubes

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.

Role of Organic Additives in Hydriding Properties of Mg-C Nanocomposites

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 sorptiondesorption 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.

Microstructure and Hydrogen Desorption in Nanostructured MgH2-Fe

Mg-based nanostructured hydrides have been synthesized by ball milling using two alternative approaches. The first is based on the reactive milling of Mg powders in H2 atmosphere, while the second on the milling of commercial MgH2 powders under inert atmosphere. In both cases 10 wt.% of Fe was added to the powder mixture, with the aim of introducing a catalyst agent. The microstructural characterization was carried out by X-ray diffraction, and both scanning and transmission electron microscopy. Hydrogen desorption behavior was evaluated by differential scanning calorimetry. Almost full hydrogenation of pure Mg powders can be achieved by reactive milling. Catalyst addition strongly accelerates the hydride formation. Both reactive milling of Mg powder and inert gas milling of MgH2 induce a nanosized microstructure with similar H-desorption behavior. The role played by Fe becomes particularly evident in H-desorption. In fact, a temperature decrease of about 100°C was found in samples having the same crystallite size and similar powder morphology.

Improvement of DMFC Electrode Kinetics by Using Nanohorns Catalyst Support

One of the factors limiting direct methanol fuel cells (DMFC) performance is the slow kinetics of methanol oxidation at the anode. The importance of the catalyst support for fuel cells has been recognized and different forms of carbon have been suggested. Single wall nanohorns (SWNH) are a new class of carbon with a similar graphitic structure of carbon nanotubes. They are self-assembling materials that produce aggregates of about 100 nm. In the present study, the comparison of the performance of a DMFC equipped with electrocatalysts supported on a commercial carbon black and on SWNH was carried out. The SWNH were synthesized by the arc discharge method in air. The deposition of the Pt and Pt/Ru catalysts on the carbon supports was accomplished by using ethylene glycol as reducing agent. The synthesized catalyst nanoparticles have a very small diameter size (ca. 2.5 nm) and they are uniformly distributed on both carbon supports. The supported electrode catalysts were tested in a DMFC and results indicate that employing SWNH is very promising showing catalytic activities 60 % higher.

Carbon Nanostructures Produced by an AC Arc Discharge

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.

Structural Refinement of Ag-Fe Blends during High Energy Ball Milling

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.

Reaction with Hydrogen of Micro and Nano Composites Based on Mg

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.

Microstructure and Magnetotransport Properties of Nanocrystalline Laser Processed Co-Ag Films

Co-Ag films deposited by Pulsed Laser Deposition have been irradiated by an excimer laser at different energy fluence values. The chemical and structural modification induced by such treatment have been investigated by XPS and by atomic force, scanning and transmission electron microscopies. The irradiation greatly modified the film microstructure. In the as deposited sample a homogeneous Co distribution, in form of isolated 10 nm particles, was observed. After irradiation, a fraction of Co appears to migrate toward the silver grain boundaries. The observed variations of the magnetic and magnetotransport properties were discussed and correlated to the microstructure.