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Pyroelectric materials can harvest energy from naturally occurring ambient temperature changes, as well as from artificial temperature changes, notably from industrial activity. Wurtzite- based materials have the advantage of being cheap, non-toxic, and offering excellent opto-electrical properties. Due to their non-centrosymmetric nature, all wurtzite crystals have both piezoelectric and pyroelectric properties. Nanocrystalline wurtzite ZnS, being a room temperature stable material, by contrast to its bulk counterpart, is interesting due to its still not well-explored potential in piezoelectric and pyroelectric energy harvesting. An easy synthesis method—a co-precipitation technique—was selected and successfully tailored for nanocrystalline wurtzite ZnS production. ZnS nanopowder with nanoparticles of 3 to 5 nm in size was synthesized in ethyl glycol under medium temperature conditions using ZnCl2and thiourea as the sources of Zn and S, respectively. The purified and dried ZnS nanopowder was characterized by conventional methods (XRD, SEM, TEM, TG and FTIR). Finally, a constructed in-house pilot plant that is able to produce substantial amounts of wurtzite ZnS nanopowder in an environmentally friendly and cost-effective way is introduced and described.

The Two-Step Sintering (TSS) process is a useful method to obtain sintered materials of high density and to limit the grain growth associated with the final stage of the sintering process. One of the main advantages of this method is the lowering of the sintering temperature. The development of bulk, dense and small grain size in the wurtzite phase of the ZnS ceramic was investigated by using a micron-sized ZnS powder as a precursor material. The microstructure and morphology of the TSS-fabricated ZnS ceramic pellets were observed by Scanning Electron Microscopy (SEM) and compared to those produced by the conventional sintering process. The ZnS ceramic produced using the TSS method at 1100°C showed comparable density and a much finer microstructure (five times smaller grain size) than the ZnS ceramic produced using conventional sintering at 1250°C. It was demonstrated that the TSS process is a pressureless, simple and cost‐effective sintering method, able to deliver high density bulk, wurtzite phase ZnS ceramics with controlled grain size. &nbsp

The pyroelectric performances of non-ferroelectric pyroelectrics like wurtzite- based materials (e.g. AlN, GaN, CdS or ZnO) make them important, although not widely used, compared to the current state-of-the-art ferroelectrics. Their high chemical and thermal stability allows their use at high temperatures in air, whereas ferroelectrics become ineffective when heated beyond their Curie temperature (TC). Wurtzite based materials have a higher thermal conductivity allowing them to react faster to ambient temperature changes, their raw material costs are lower and many of them are eco-friendly. Current pyroelectrics applications are limited to portable systems or tasks needing only μW–mW power. To be commercially viable, we must improve the current low efficiency of pyroelectric systems and intrinsically enhance the pyroelectric properties of modern materials through suitable doping or material engineering. We chose to study hexagonal wurtzite phase of ZnS, among the structurally simplest of pyroelectrics, as a possible energy harvesting material. An easy synthesis method – a co-precipitation technique, was tailored for nanocrystalline wurtzite ZnS production. This method is easy to scale-up and our next step is to build an in-house pilot plant that will produce substantial amounts of wurtzite ZnS nano-powder in an environmentally friendly and cost-effective manner. We further investigated the development of bulk, dense pyroelectric ceramics by the Two-Step Sintering (TSS) fabrication process, using as the precursor material both a micron-sized commercial powder of the ZnS cubic and hexagonal phases mixture, and an in-house produced wurtzite ZnS nanopowder. The TSS was chosen as being a pressureless, simple and cost‐effective sintering method for obtaining high density materials with controlled grain growth operating at a lower temperature than the conventiona process. Electron Microscopy techniques helped us to study the microstructure and morphology of both the precursor nanopowders and the obtained ceramics. Acknowledgement: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797951.

Pyrolectric materials are able to harvest energy both from naturally and artificially occurring temperature changes. These materials could be the right way to recover some of the enormous amount of energy wasted as heat and help to develop new devices for harvesting thermal ambient energy. In this work it was investigated the development of bulk, dense pyroelectric ceramics, ideally with a highly developed texture and small grain size, using a micron-sized powder of the ZnS wurtzite phase as precursor material. The Two-Step Sintering (TSS) process is a useful method to obtain high sintered density and to limit the grain growth associated with the final stage of the sintering process. One of the main advantages of this method is the lowering of the sintering temperature. The microstructural, morphological and electrical properties of TSS-ZnS were determined and compared to ZnS produced by the conventional sintering process, performed at 1250°C. TSS-ZnS showed comparable density and a finer microstructure than conventional ZnS (five times lower grain size). It was demonstrated that the TSS process is a pressureless, simple and cost‐effective sintering method to obtain high density materials with controlled grain growth, without using a dopant or binder. The TSS produced ZnS ceramic was tested for pyroelectric energy harvesting. It is expected that the efficiency of the ceramic in harvested energy could be further improved by decreasing the grain size down to the nanoscale. Acknowledgement: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 797951.

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage” different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications, The research for the lab-scale compressor is part of the activities of SCCER HaE, which is financially supported by Innosuisse - Swiss Innovation Agency . The authors thank the Alexander von Humboldt Foundation in the frame of the post-doctoral fellowship of Dr. J. Puszkiel (No. 1187279 STP ) as well as the European Union for their funding of projects STORHY (contract Nr. SES6-CT-2004-502667 , FP6-2002-Energy-1, 6.1.3.2.2), NESSHY (contract Nr. 518271 , FP6-2004-Energy-3, 6.1.3.2.2) and the EU Horizon 2020 /RISE project HYDRIDE4MOBILITY. Financial support from the S02 and KP8 S05), the European Union's Seventh Framework Programme ( FP7/2007e2013 ) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement no. 256653 (SSH2S), from the European Fuel Cells and Hydrogen Joint Undertaking in the framework of BOR4STORE (Grant agreement no. 303428 ), from the Australian Research Council for grants LP120101848 , LP150100730 , and LE0989180 , The Innovation Fund Denmark (project HyFill-Fast), DST within Hydrogen South Africa/HySA programme (projects KP3 National Research Foundation/NRF of South Africa , incentive funding grant number 109092 and the Research Council of Norway (project 285147 ) is thankfully acknowledged

Development of technologies which utilize hydrogen as energy vector requires the realization of efficient gas storage systems. Materials suitable for hydrogen storage at the solid state have to fulfill some specific requirements in order to be used inside the reactors. First of all it is necessary to have a material which could be cycled without observing reduction of performances in terms of kinetics and total hydrogen capacity. The reaction of the material with hydrogen must be reversible even after long time cycling, i.e. repeated absorption and desorption runs. For what concerns the efficiency of the reactors, one of the main parameters to be taken into account is the thermal conductivity of the bed of hydride. The use of powder is detrimental for thermal conductivity and also for gas permeability and particle entrainment by the gas flow in the reactor. Moreover powders after long term cycling tend to pack and sinter evidencing the problems reported before. For this reason powders of hydrides are compacted, generally in cylindrical shape, after mixing with agents for enhancing thermal conductivity and mechanical stability (for example Al, Cu, carbon-based materials). It has been reported that compacted systems during cycling tend to disaggregate returning to the form of powder in the loose form. This swelling mechanism causes rapid slowdown of the kinetics and all the problems correlated to the use of the powders. For this reason many efforts have been spent in order to extend the life of the compacted systems. In this work it is reported the study performed to improve the preparation of compacted powder systems by a procedure which includes the deposition of a thin layer of a metal on the surface of the pellet. The pellets prepared with this procedure demonstrated, after 50 cycles, variations of dimension eight time lower in comparison to those obtained on pellets prepared without coating. The quantity of metal deposited is less than 0.1% of the weight of the pellet and it doesn't affect hydrogen capacity and kinetics of reaction. Kinetics of reaction and microstructure have been studied with a volumetric Sievert's type apparatus and with Optical and Scanning Electron Microscopy respectively.

Among hydrogen storage materials, the MmNi5 (Mm = mischmetal, a mixture of rare earth elements) family of alloys has good properties for transportation technologies due to their ability to react reversibly with hydrogen at moderate pressures and temperatures. It is also known that different composition of Mm or partial substitution of Ni by other elements modifies the hydride stability and induces a wide range of plateau pressures, giving them an excellent versatility. This work investigates a set of MmNi5−xAlx materials exploring different kinds of Mm and Al content (x = 0.15,0.20,0.25), in order to determine which composition best satisfies specific working conditions for possible employment in transportation by sea or lagoon where the water of the lagoon is used as a coolant. In the specific case of a public transportation ship (the classical “vaporetto”) on the Venice Lagoon, hard restrictions are required for these conditions since hydrogen sorption pressures are a key factor when considering the lagoon average temperature variations between winter and summer seasons. Experimental results of the present study led to a map of Vant’Hoff that let select the best material according to the required conditions.

Heat management in hydrogen storage tanks is a mandatory issue in order to ensure high level performances both in terms of hydrogen release/uptake kinetics and storage capacity. Previous research has ascertained that powders tend to pack and sinter during charge/discharge cycling giving rise to non-reacting material. Moreover powders, with intrinsically low thermal conductivity, can show problems related to hydrogen permeability, and suffer from hydraulic losses and particle entrainment by gas flow. To overtake these problems and to keep high performances, hydride powders mixed with high thermal conductivity additives are compacted in pellets. Some materials, like Expanded Natural Graphite (ENG), besides improving the pellet mechanical stability during cycling, can impart a thermal conductivity higher by order of magnitudes with respect to a simple powdered bed. On the basis of these considerations, pelleting can be very effective in improving tank performances. However pellets tend to swell and lose compactness during long term operation, returning back to the powder form with a detrimental effect on tank performances. In this work the swelling process caused by cycling of pellets simulating tank exercise has been studied. The pellets, which are constituted by ball milled magnesium hydride (MgH2) with the addition of niobium oxide (Nb2O5) and ENG, have been deeply studied by different microscopic methods including Scanning Electron Microscopy (SEM), Helium Ion Microscopy (HeIM) and high resolution Computed Tomography (CT) with the purpose of ascertaining microstructural modification and degradation mechanism in order to define strategies for material preparation able to reduce the entity of the swelling process.

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.

Mechanochemistry has played an important role in the synthesis of many novel compounds, in particular within the field of materials for solid state hydrogen storage applications. This work investigates reactive milling of ethane 1,2-di-amineborane (EDAB) and sodium hydride which yields the evolution of one equivalent of hydrogen and the formation of a novel compound (named NaEDAB) as evidenced by X-ray diffraction analyses. We postulate for this compound the chemical formula NaB2C2N2H13. The thermolysis of NaEDAB below 400 °C releases about 8 wt.% pure hydrogen, without producing foaming. Moreover, sodium addition significantly modifies hydrogen desorption enthalpies, giving rise to milder exothermic H2 release at moderate temperatures with respect to neat EDAB, as well as an endothermic desorption process at higher temperature. This result opens novel and promising perspectives towards the reversible hydrogenation of these compounds., FL wishes to thank Hydro-Eco Research Center at Sapienza University and Regione Lazio for financial support. MJVP thanks for a FPI PhD fellowship BES 2009-019669. We also thank the COST Action MP1103 ‘Nanostructured materials for solid-state hydrogen storage’, Italian Ministry project Industria 2015 and Spanish Ministry projects CTQ2011-25517 and CTQ2011-27935.

The high H2 storage capacity, the reversibility and the low cost make magnesium hydride a promising material for hydrogen storage at the solid state. The stability of these materials under H2 cycling has been tested in this paper. Moreover, the morphological modifications of MgH 2 particles during hydrogen cycling have been studied. In particular, it is discussed how experimental parameters such as the temperature and the nature of the catalyst could affect the morphological changes occurring after cycling for MgH2 powder ball milled with Fe or Nb2O 5. The cycled sample, presents the formation of hollow particles constituted by MgO shell and the absence of the catalyst in most of the particles has been observed. Ball milled MgH2 with and without catalyst has been prepared and different thermal treatments have been performed in a Sievert's type apparatus. Scanning electron microscope has been used for the observation of the powders. © 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights.

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.

Commercially available Sodium clay (Dellite HPS) and organo-clay (Dellite 72T) are modified via a silylation reaction. These silylated clays are characterized by IR, XRD, thermogravimetric analyses, and their equilibrium contact angles are measured. They are used to prepare nanocomposites at different loading percentage (1, 3, 5% wt) by in situ intercalative polymerization of Methyl methacrylate and morphology and thermal properties of nanocomposites are examined. SEM images of nanocomposites fractured surface show the absence of clays aggregates, confirming a good dispersion and distribution of montmorillonites in the polymer matrix. The effects of modified clays on the thermal properties of nanocomposites are analyzed by differential scanning calorimetry and thermogravimetric analyses showing an increase of glass and decomposition temperatures of all nanocomposites respect to homopolymer ones. The best results are obtained in the presence of silylated montmorillonites, clearly the organosilane improves the compatibility between polymer matrix and clay and as effect the properties of nanocomposites.

Magnesium and its hydride MgH2 are widely regarded as promising candidates for hydrogen storage materials due to its benefits of high gravimetric and volumetric capacity, excellent reversibility, abundance in the earth and a low cost. Much attention has been paid to improve its absorption/desorption kinetics, trying to make it useful for practical applications. To make composite of MgH2 with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties. In this study nano-composite of MgH2 with ZrCrCu alloy was prepared using high energy ball-milling for 5 h under Ar atmosphere. Microstructure and morphology of the composites were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM). XRD patterns show that no alloy formation between MgH2 and elements of the alloys takes place during milling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure–composition isotherms of these composites have been obtained in the pressure range 0.1–10 bar at 275 and 300 °C. The absorption/desorption kinetics data have been analyzed using pressure 0.1–5.0 bar at 275 and 300 °C to understand the mechanism of the hydriding/dehydriding reaction processes. A comparison of these results has been attempted with our previous published results of MgH2–10 wt%ZrCrNi in order to find the better composite for storage applications. It is observed by DSC curves that the onset temperature of hydrogen desorption is decreased for MgH2–10%ZrCrNi in comparison to MgH2 which further decreased for MgH2–10%ZrCrCu. However, little loss in hydrogen absorption/desorption capacity is also observed for ZrCrCu composite in comparison to that of ZrCrNi composite.

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.

Magnesium is one of the most promising materials for hydrogen storage due to its high capacity and low cost. Unfortunately, practical applications are for the moment limited by the slow kinetics and the high operating temperature. Nanostructuring magnesium hydride MgH2, generally by ball milling, introduces plastic deformations and catalysts that highly enhances the H2 absorption and desorption. However a fundamental understanding of the role played by catalysts and interfaces in MgH2 is still lacking. Microscopic characterization of MgH2-Mg system with and without heavy metal catalysts, is achieved by combining accurate SEM observations of samples after partial desorption process and atomic level ab-initio molecular dynamics simulations of MgH2-Mg interfaces. The experimental method is based on low voltage SEM observations of cross sectional powder samples, prepared by a new specific metallographic process. Identification of nucleation sites of the sorption reaction and their correlation with the presence of catalyst particles is achieved by suitable experimental conditions. Moreover ab-initio molecular dynamics clarifies the interplay of interfaces and the deformations induced during desorption by the presence of catalysts that are able to lower binding energies and free hydrogen atoms toward interfaces. Both approaches confirm and characterize the nucleation step in the catalysts driven phase transformation.

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.

MgH2 nanocomposites with ZrCrNi alloy obtained by high energy ball-milling were studied as-milled and after several hydriding-deydriding cycles. The microstructure and morphology of the samples was characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD patterns show that no phase formation between MgH2 and elements of the alloys takes place during milling and after cycling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure-composition isotherms of these composites were obtained in the pressure and temperature range of 0.1–15 bar and 200–300 °C respectively. The maximum reversible storage capacity was found to be 6.2 wt% at 300 °C. Absorption/desorption kinetics data at pressures of 0.1–5.0 bar and temperatures of 275 °C and 300 °C show that an activation process of about 20 cycles at 300 °C is necessary for stabilization of the kinetics and for achievement of the full hydrogen capacity. The thermodynamic parameters, i.e. enthalpy of formation and dissociation calculated using Van't Hoff plots, were found to be 73.53 kJ mol−1 and 87.63 kJ mol−1 respectively, in agreement with MgH2 data reported in literature.

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.

Pt clusters were deposited by an impregnation process on three carbon supports: multi-wall carbon nanotubes (MWNT), single-wall carbon nanohorns (SWNH), and Vulcan XC-72 carbon black to investigate the effect of the carbon support structure on the possibility of reducing Pt loading on electrodes for direct methanol (DMFC) fuel cells without impairing performance. MWNT and SWNH were in-house synthesised by a DC and an AC arc discharge process between pure graphite electrodes, respectively. UV–vis spectrophotometry, scanning and transmission electron microscopy, X-ray diffraction, and cyclic voltammetry measurements were used to characterize the Pt particles deposited on the three carbon supports. A differential yield for Pt deposition, not strictly related to the surface area of the carbon support, was observed. SWNH showed the highest surface chemical activity toward Pt deposition. Pt deposited in different forms depending on the carbon support. Electrochemical characterizations showed that the Pt nanostructures deposited on MWNT are particularly efficient in the methanol oxidation reaction.

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.

The hydrogen desorption from MgH 2 – Mg 2 NiH 4 phase mixtures prepared under different ball milling conditions has been investigated and correlated with the samples microstructure. SEM, XRD and XPS were performed to explore the microstructure in order to ascertain the reciprocal influence of the two phases on the de-hydrogenation reaction, and in particular on the hydrogen desorption temperature. Ball milling imparts different microstructures to the phase mixtures according to the adopted processing parameters. The resulting microstructure influences the hydrogen desorption which appears to occur by a limited number of well-defined channels at characteristic temperatures. A microstructural interpretation of this behavior is proposed on the basis of the characterization results. From the applicative point of view it is possible to notice that this mixture, when properly processed, shows a onset temperature of the desorption of the whole hydrogen content about 150 K lower than MgH 2 , even on samples exposed to the atmosphere.

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.

Mg-graphite composites, suitable for hydrogen storage, have been synthesized by ball milling metallic Mg with different amounts of graphite and benzene. The microstructure and the surface chemical composition have been characterized in order to explain the kinetics of reaction with hydrogen. The presence of benzene in the milled blends induces a finer powder particle size, helps to preserve the structural integrity of the graphite crystals and results to be necessary for a complete transformation of the milled powder to the hydride phase by thermal reaction with hydrogen gas. On the other hand, it induces a more pronounced reaction of the milled Mg-C composites with the air. The transport properties of the resulting surface contamination layer appear to control the kinetics of thermal decomposition of the MgH2 phase, so that the addition of benzene induces a higher reaction temperature owing to a larger thickness of the surface compound. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

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 synthesis, the chemical, morphological, structural characterization and the magnetic properties of amorphous iron oxyhydroxy phosphate nanoparticles are reported. Aggregated spherical particles (diameter: 5-12 nm) are obtained showing a ferrihydrite-like behavior similar to that of the bacterial ferritin iron oxyhydroxy-phosphate core. A superparamagnetic behavior is observed with blocking temperatures ranging from similar to20 to similar to50K depending on the iron and phosphate content.

The sign of the exchange bias in field cooled nanocrystalline antiferromagnetic–ferromagnetic bilayers (Co–O and Ni–O/permalloy) is reversed at temperatures approaching the antiferromagnetic (AFM) blocking temperature. A similar phenomenon is observed after magnetic training processes at similar temperatures. These effects can be explained assuming that the boundaries of nanocrystalline grains in AFM layers exhibit lower transition temperatures than grain cores.

Materials to be used in tanks for hydrogen storage should have suitable thermal properties otherwise their performances in term of kinetics and hydrogen capacity would rapidly decrease during exercise. For these reason in this work ball milled powders of MgH2 with Nb2O5 as catalyst and Expanded Natural Graphite (ENG) have been compacted at different pressures in the range between 100 and 600 MPa. The pellets were tested until 50 cycles after which the material was able to absorb and desorb hydrogen without significant reduction in maximum capacity. The mechanical stability of these compacted systems during repeated cycling under hydrogen pressure is discussed together with the modifications of the microstructure. The compaction pressure revealed to affect largely the distribution of ENG in the pellets and their mechanical stability upon cycling while this parameter have only a slight effect on the sorption kinetics. Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and volumetric Sievert's type apparatus have been used to characterize the samples. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Hybrid structures composed of exfoliated expanded graphite (EG) and iron oxide nanocrystals have been produced by an ex situ process. The iron oxide nanoparticles coated with meso-2,3-dimercaptosuccinic acid (DMSA), or poly(acrylic) acid (PAA) were integrated onto the exfoliated EG flakes by mixing their aqueous suspensions at room temperature under support of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and N-hydroxysuccin-nimide (NHS). EG flakes have been used both, naked and functionalized with branched polyethylenimine (PEI). Complete integration of two constituents has been achieved and mainteined stable for more than 12 months. No preferential spatial distribution of anchoring sites for attachement of iron oxide nanoparticles has been observed, regardless EG flakes have been used naked or functionalized with PEI molecules. The structural and physico-chemical characteristics of the exfoliated expanded graphite and its hybrids nanostructures has been investigated by SEM, TEM, FTIR and Raman techniques. [Projekat Ministarstva nauke Republike Srbije, br. 45015]

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.

Mechanical milling is a technique extensively used to prepare nanocrystalline metallic materials but it has been less frequently applied to semiconductors. The preparation of nanocrystalline silicon by different methods is a subject of increasing interest due to the luminescent properties of this material and its possible application in optoelectronic devices. In this work high-energy ball milling has been used to prepare nanocrystals from single crystalline silicon wafers. The structure of the milled samples has been assessed by X-ray diffraction, transmission electron microscopy (TEM) and Raman spectroscopy, while their 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 nanometers aggregated to form bigger particles of several microns. TEM reveals that the particles consist of nanocrystals with a wide range of sizes including crystallites with dimension of few nanometers. CL spectra of the milled samples show a band at 1.61 eV, attributed to the presence of nanocrystals through a quantum confinement effect. It is suggested that a milling induced infrared emission is related to a high density of extended defects present in the milled samples.

The microstructure and hydrogen sorption behaviour of MgH2 based nano-hydrides with different additives prepared by ball milling and inert gas condensation has been investigated by XRD, Differential Scanning Calorimetry and Mechanical Spectroscopy. Preliminary results on materials with similar composition and different nanostructures show that suitable mechanical processing and additive additions induce in the nanostructured composites tailored channels for improved ab/de-sorption kinetics at reduced temperatures

Flower-like agglomerates with sizes of 200–400 nm of pure and Fe3+-doped TiO2 with rutile crystalline structure were synthesized by the coprecipitation method. The morphology of the agglomerates was determined by electron microscopy (TEM and HRTEM). TiO2 agglomerates consist of nanorods with clearly visible crystalline faces, parallel to the axis of elongation whose direction was along the [101] direction of pure TiO2 and the [111] direction of doped TiO2. Furthermore, nanorods consist of “chains” of spherical particles, most likely interconnected through the so-called oriented attachment or grain-rotation-induced grain coalescence (GRIGC) process. UV/Vis reflection measurements revealed that the absorption of pure TiO2 was significantly shifted from UV toward the visible spectral region upon the incorporation of Fe3+ into the TiO2 host.

The effect of transformations induced by annealing at different temperatures (450 < Ta

The effect of thermal treatments on both the Morin transition and the spin flop field (Hsf) of α-Fe2O3 acicular particles (major axis:(350 ± 50) nm; minor axis: (85 ± 15) nm.) has been investigated by means of magnetization vs temperature-magnetic field measurements. Both the Morin transition and the spin flop process have been observed only after an annealing treatment at 500 °C. Due to the finite size effect, the values of the Morin temperature and spin-flop field are significantly below the bulk value.

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.

This study describes the effect of ball milling and heat treatment on the phases synthesized in a Spex mill starting from mixtures of Al and Fe powders containing 71, 75, 80 and 90 at.% A1. In the 71 and 75 at.% Al mixtures we observed the formation of the Al5Fe2 phase. At the 80 at.% Al concentration we observed the formation of an Al-rich ultrafine phase with a coherent domain size of the order of 1 nm. At the 90 at.% Al concentration, no new phase is observed to nucleate for milling times up to 50 hs. The thermal behaviour of the samples depends on the time of the milling. At all compositions, we observed the nucleation of the metastable A16Fe compound at about 320°C on heating of samples milled for short times (10 h or less) and the formation of the equilibrium phase(s) upon heating to 600°C.

Equimolar solid solutions of Ag[sbnd]Cu and Co[sbnd]Cu were prepared by mechanical milling of the elements. The structure and the thermodynamic properties of the resulting phases were analysed by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. Large mixing enthalpies (ΔH = 6–8kJg-atom−1 and ΔH = 4–8kJg-atom−1 for Ag-Cu and Co-Cu respectively) were measured in the alloyed phases. It is clear that the solid solution forms through a process of grain refinement down to the nanometre scale and is assisted by an atomic-scale dissolution. The possible role played by coherent interfaces in substantially increasing the energy of the solid solutions is discussed.

We have studied the superconducting and structural properties of YBCO thin films and YBCO/STO/YBCO trilayers deposited on (100) SrTiO3 substrates by off-axis PLD technique. The as-obtainedc-axis oriented YBCO films show an excellent surface free of droplets or other irregularities as well as good structural and superconducting properties with Tc (R=0)=89 K, Jc=3×106 A/cm2 (at 77 K and zero magnetic field) and the Full Width at Half Maximum (FWHM) of the (005) YBCO rocking curve of about 0.2°. These structural and electrical properties are very close to those of the YBCO films deposited by the conventional PLD on-axis geometry. The TEM cross-sectional image shows that the YBCO/STO/YBCO heterostructure has smooth interfaces and no short-circuits or pinholes were observed. The electron diffraction analysis has revealed that both YBCO top and bottom layer as well as STO interlayer are epitaxially grown withc-axis perpendicular to the substrate.