Cu-catalyzed Si-NWS grown on “carbon paper” as anodes for Li-ion cells

The very high theoretical capacity of the silicon (4200mAh/g more than 10 times larger than graphite), environmental-friendly, abundant and low-cost, makes it a potential candidate to replace graphite in high energy density Li-ion batteries. As a drawback, silicon suffers from huge volume changes (300%) on alloying and dealloying with Li, leading a structural deformation that induces disruption. The use of nanostructured silicon materials has been shown to be an effective way to avoid this mechanical degradation of the active material. In this paper the synthesis of silicon nanowires, grown on a highly porous 3D-like carbon paper substrate by CVD using Cu as the catalyst, is presented. The use of carbon paper allows to achieve remarkable loadings of active material (2-5 mg/cm2) and, consequently, high capacity densities. The silicon electrode was investigated both morphologically and electrochemically. To improve the electrochemical performance various strategies have been carried out. It was observed that a very slow first cycle (C/40), which helps the formation of a stable solid electrolyte interphase on the silicon surface, improves the performance of the cells; nevertheless, their cycle life has been found not fully satisfactory. Morphological analysis of the Si-NWs electrodes before and after cycling showed the presence of a dense silicon layer below the nanowires which could reduce the electrical contact between the active material and the substrate

Dust Mobilization Experiments in the Context of the Fusion Plants - STARDUST Facility

This paper deals with the dust mobilization in fusion facilities, in which the plasma disruptions induce an erosion of small particles causing external releases if accidents occur. In the ITER safety guidelines the administrative limits of 200 kg carbon, 100 kg beryllium and 100 kg tungsten inside the VV have been fixed to avoid the population evacuation in case of accident. The conservative assumption to mobilize all these dusts is adopted in the accident analyses. To support a less conservative hypothesis some experiments have been performed inside the STARDUST facility (ENEA Frascati laboratories, Italy). The ECART code has been used for blind simulations to validate the dust transport model implemented. The results match satisfactorily the experiments. The dusts used were carbon, stainless steel, tungsten and a mixed dust (C, SS, W). The experiments represent a LOVA due to a small or a large air leak through two different VV ports. The measured mobilization rate ranges from 0.03% to 100% of the total amount of dust. That means the mobilization is strongly dependent on the relative position between air inlet and dust location and that the dust mobilization assumptions in the accident analyses shall be reduced, in some cases of several factors

Dust mobilization experiments in the context of the fusion plants—STARDUST facility

This paper deals with the dust mobilization in fusion facilities, in which the plasma disruptions induce an erosion of small particles causing external releases if accidents occur. In the ITER safety guidelines the administrative limits of 200 kg carbon, 100 kg beryllium and 100 kg tungsten inside the VV have been fixed to avoid the population evacuation in case of accident. The conservative assumption to mobilize all these dusts is adopted in the accident analyses. To support a less conservative hypothesis some experiments have been performed inside the STARDUST facility (ENEA Frascati laboratories, Italy). The ECART code has been used for blind simulations to validate the dust transport model implemented. The results match satisfactorily the experiments. The dusts used were carbon, stainless steel, tungsten and a mixed dust (C, SS, W). The experiments represent a LOVA due to a small or a large air leak through two different VV ports. The measured mobilization rate ranges from 0.03% to 100% of the total amount of dust. That means the mobilization is strongly dependent on the relative position between air inlet and dust location and that the dust mobilization assumptions in the accident analyses shall be reduced, in some cases of several factors

6Li-enriched LiF films grown by thermal evaporation for neutron detection

The new generation of neutron detectors based on 6LiF thin films has been widely investigated in the last decades. The structure of polycristalline thin films has a significant impact upon their performance in neutron detectors, however the relationship between the physical properties of LiF-based thin films and their performance is still far from being completely understood. A systematic investigation of the role played by the growth conditions on the optical and morphological properties of thermally evaporated 6LiF thin films has been carried out by combining spectrophotometry, stylus profilometry and atomic force microscopy. A clear picture of the influence of film thickness and deposition temperature on transparency, porosity, roughness and grain size of polycristalline 6LiF thin films thermally evaporated on amorphous substrates has been obtained

Modelling approach of a devolatilization-combustion process in a well stirred reactor

Object of this work is the definition of the optimal operative parameters of an experimental industrial burner working in flameless conditions. To reach this objective the main steps are described in the following. First, a parametrical optimization of the carbon oxy-combustion process in pressurized environment was performed. When carbon is injected in the combustor in slurry form, the particles break due to the combined effect of the shear stress induced by the injected air swirled and of the particle devolatilization process. In this phase, the carbon volatile species evolves and, successively, burns. The optimization was carried out by analyzing the chemical species generated after the devolatilization and their dependency on the main operative parameters, like temperature and pressure, which rule the process. The analysis needs acquisition of chemical and physical knowledge about the phenomenon of devolatilization and was performed with a dedicated scientific software. The kinetic parameters of the devolatilization process were obtained by assuming a single step kinetic model and by using the Arrhenius equation to correlate the data. The simulated composition of the volatile species and the definition of the fundamental parameters of the combustor, necessary to study the feasibility of a plant at the industrial size, allowed to perform a sensitivity analysis for the evaluation of the most efficient kinetic configuration of the combustion. The analysis was carried out with a commercial CFD software appropriately tailored and the results of the simulations were validated against the analysis of solid and gaseous emissions. This work allows the definition of an acceptable configuration of the combustor and provides a new starting point for the development of the mild technology applied to coal combustion

SERS and Raman studies of UV-Induced Modification of Graphene/DNA Interface

The peculiar properties of the graphene/DNA interface show interesting trends under irradiation, which is promising for the realization of sensors with superior performances for a wide range of applications. In this frame, hybrid nanomaterials based on the assembly of DNA and graphene nanoplatelets (GNP) have been used for the real time detection of UV radiation, and are expected to be useful to monitor the biological damage induced by UV in Earth and space environment

Electrochemical Characterization of Cu-Catalysed Si Nanowires as an Anode for Lithium-Ion Cells

Silicon (Si) nanowires (NWs) grown on stainless-steel substrates by Cu-catalysed Chemical Vapour Deposition (CVD) have been prepared to be used as anodes in lithium-ion batteries. The use of NWs can overcome the problems related to the Si volume changes occurring during lithium alloying by reducing stress relaxation and preventing material fragmentation. Moreover, since the SiNWs are grown directly on the substrate, which also acts as a current collector, an excellent electrical contact is generated between the two materials without the necessity to use additional binders or conducting additives. The electrochemical performance of the SiNWs was tested in cells using lithium metal as the anode. A large irreversible capacity was observed during the first cycle and, to a lesser extent, during the second cycle. All the subsequent cycles showed good reversibility even if the coulombic efficiency did not exceed 95%, suggesting the formation of an unstable SEI film and a continuous decomposition of the electrolyte on the silicon surface. The absence of a stable SEI film was assumed responsible for a linear capacity fade observed upon cycling. On the other hand, the electrochemical characterization performed at different values of the charging current showed that SiNWs possess an exceptionally high rate capability

Electrochemical Characterization of Cu-Catalysed Si Nanowires as an Anode for Lithium-Ion Cells

Silicon (Si) nanowires (NWs) grown on stainless-steel substrates by Cu-catalysed Chemical Vapour Deposition (CVD) have been prepared to be used as anodes in lithium-ion batteries. The use of NWs can overcome the problems related to the Si volume changes occurring during lithium alloying by reducing stress relaxation and preventing material fragmentation. Moreover, since the SiNWs are grown directly on the substrate, which also acts as a current collector, an excellent electrical contact is generated between the two materials without the necessity to use additional binders or conducting additives. The electrochemical performance of the SiNWs was tested in cells using lithium metal as the anode. A large irreversible capacity was observed during the first cycle and, to a lesser extent, during the second cycle. All the subsequent cycles showed good reversibility even if the coulombic efficiency did not exceed 95%, suggesting the formation of an unstable SEI film and a continuous decomposition of the electrolyte on the silicon surface. The absence of a stable SEI film was assumed responsible for a linear capacity fade observed upon cycling. On the other hand, the electrochemical characterization performed at different values of the charging current showed that SiNWs possess an exceptionally high rate capability

Si NWs as anodes in Li-ions batteries: electrochemical properties of different morphologies

Silicon is one of the most interesting candidates to replace graphite as anode in Li-ion batteries for its high theoretical capacity (4200mAh/g, more than 10 times larger than graphite) and because it is an abundant, low-cost and environmental-friendly material. Despite these promising aspects, Si structure still suffers from huge volume changes (300%) on alloying with Li, which determines a structural deformation and a rapid degradation of the electrode [1]. Nanostructures and particularly nanowires can better sustain the volume variation avoiding material degradation [2,3]. We have grown Si-NWs by Cu-catalysed CVD (Chemical Vapor Deposition) [4,5]. Despite the promising electrochemical performances obtained using SiNWs based anodes, the stability of the cell was not satisfying. By an accurate morphological characterization, we have observed [6] the presence of a thick silicon layer below the nanowires which could to be responsible of the lack of the electrical contact between the active material and the current collector upon cycling. In this work the role of the growing parameters is investigated with the aim to reduce the silicon thick layer under the nanowires and tune the dimension of the nanowires. How the morphology and the NWs dimensions affect the electrochemical performances was also investigated