The mechanical properties of GNP/LDPE nanocomposites (graphite nanoplatelets/low density polyethylene) have been investigated, in order to establish the effect of nanoscale reinforcement within the polymer matrix. Results show that the presence of the filler does not involve a change in the microscopic structure of the polymer. However, on a macroscopic scale, GNPs limit the mobility of the polymer chains, resulting in an increase in stiffness for the final composite. Orientation of GNPs within the LDPE matrix is also an important issue that affects mechanical properties and it has been evaluated by testing nanocomposites made by different manufacturing techniques (compression moulding and blown extrusion). The comparison between the experimental data and the Halpin-Tsai model shows that the orientation of GNPs due to the extrusion process leads to values of tensile modulus higher than that obtained with the randomly oriented disposition resulting from the compression moulding technique.
We report on the optimisation of the growth conditions of manganite La 0.7 Sr 0.3 MnO 3 (LSMO) thin films prepared by Channel Spark Ablation (CSA). CSA belongs to pulsed electron deposition methods and its energetic and deposition parameters are quite similar to those of pulsed laser deposition. The method has been already proven to provide manganite films with good magnetic properties, but the films were generally relatively rough (a few nm coarseness). Here we show that increasing the oxygen deposition pressure with respect to previously used regimes, reduces the surface roughness down to unit cell size while maintaining a robust magnetism. We analyse in detail the effect of other deposition parameters, like accelerating voltage, discharging energy, and temperature and provide on this basis a set of optimal conditions for the growth of atomically flat films. The thicknesses for which atomically flat surface was achieved is as high as about 10-20 nm, corresponding to films with room temperature magnetism. We believe such magnetic layers represent appealing and suitable electrodes for various spintronic devices.
In this work, ZnO thin films were investigated to sense NO2, a gas exhausted by the most common combustion systems polluting the environment. To this end, ZnO thin films were grown by RF sputtering on properly designed and patterned substrates to allow the measurement of the electrical response of the material when exposed to different concentrations of the gas. X-ray diffraction was carried out to correlate the material’s electrical response to the morphological and microstructural features of the sensing materials. Electrical conductivity measurements showed that the transducer fabricated in this work exhibits the optimal performance when heated at 200 °C, and the detection of 0.1 ppm concentration of NO2 was possible. Ab initio modeling allowed the understanding of the sensing mechanism driven by the competitive adsorption of NO2 and atmospheric oxygen mediated by heat. The combined theoretical and experimental study here reported provides insights into the sensing mechanism which will aid the optimization of ZnO transducer design for the quantitative measurement of NO2 exhausted by combustion systems which will be used, ultimately, for the optimized adjustment of combustion resulting into a reduced pollutants and greenhouse gases emission.
Abstract:A density functional theory (DFT) study has been carried out on transition metal phosphates with olivine structure and formula LiMPO 4 (M = Fe, Mn, Co, Ni) to assess their potential as cathode materials in rechargeable Li-ion batteries based on their chemical and structural stability and high theoretical capacity. The investigation focuses on LiMnPO 4 , which could offer an improved cell potential (4.1 V) with respect to the reference LiFePO 4 compound, but it is characterized by poor lithium intercalation/de-intercalation kinetics. Substitution of cations like Co and Ni in the olivine structure of LiMnPO 4 was recently reported in an attempt to improve the electrochemical performances. Here the electronic structure and lithium intercalation potential of Ni-and Co-doped LiMnPO 4 were calculated in the framework of the Hubbard U density functional theory (DFT+U) method for highly correlated materials. Moreover, the diffusion process of lithium in the host structures was simulated, and the activation barriers in the doped and pristine structures were compared. Our calculation predicted that doping increases Li insertion potential while activation barriers for Li diffusion remain similar to the pristine material. Moreover, Ni and Co doping induces the formation of impurity states near the Fermi level and significantly reduces the band gap of LiMnPO 4 .
Single-crystal ZnO nanowires long up to several microns were fabricated by one-step electrochemical deposition. A template-based process employing track-etched polycarbonate (TE-PC) membranes was used for this purpose. The morphology and the structure characteristics of the ZnO nanowires were analyzed by means of Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB), Transmission Electron Microscopy (TEM), and Selected Area Electron Diffraction (SAED). The growth process conditions turned out to have a marked influence on the crystal nature and morphology of the nanowires. Deposition rates ranging from 0.4 nm s −1 and up to 0.6 nm s −1 were recorded for the growth of ZnO nanowires. The obtained results showed that by using carefully controlled deposition conditions single crystalline nanowires and fine-grained structures can be routinely obtained. ZnO has extrusive physical properties such as a direct bandgap in the ultraviolet range (3.37 eV) or a large exciton binding energy (60 meV) which have made it a great application prospect.1, 2 ZnO nano-objects have been widely investigated in the recent years owing to promising applications in nanodevices. They showed high sensitivity to different chemicals, have piezo-and pyro-electric properties, etc. 3,4 As it has been shown that band edge, exciton energies, and UV luminescence of ZnO nanostructures are affected by crystallite size and crystal morphology, 5,6 it is desirable to tailor their electronic properties in order to exploit the broad range of applications.Nowadays, many efforts still focus on synthesizing oriented onedimensional ZnO nanostructures. Up to now, techniques using sophisticated and expensive equipments but also low temperature and cost-effective methods such as electrodeposition (ED) and hydrothermal deposition were employed for the synthesis of nanostructures. 7-9The major advantage of the electrochemical synthesis is given by its application for large area and high throughput productions, therefore suitable for an industrial use. [10][11][12][13] The ED of ZnO nanowires has been focused mostly on the preparation of nanorod/nanowire arrays by the template method and on flat substrates. 14, 15The ZnO electrodeposition is based on the reduction of an oxygen precursor such as dissolved molecular oxygen, nitrate ions or hydrogen peroxide. For example, Cembrero et al. 16 reported on the cathodic electrodeposition of ZnO nanocolumns and nanowires from zinc chloride solutions saturated in molecular oxygen. The general scheme for ED of ZnO employing different oxygen precursors is supposed as follows.The reaction at the cathode surface employing different oxygen precursors: The process evolves itself drifting the pH of the electrolyte to a constant increase and to a local supersaturation of the bath in the vicinity of the electrode, thus provoking the precipitation on the electrode surface of ZnO;18 therefore, if a specific crystal order is desired, the pH of the electrolytic solution has to be precisely controlled and adjusted to a proper value.As...
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