Ab initio optical and energy loss spectra of transition metal monopnictides TaAs, TaP, NbAs, and NbP

Transition metal monopnictides represent a new class of topological semimetals with low-energy excitations, namely, Weyl fermions. We report optical properties across a wide spectral energy range for TaAs, TaP, NbAs and NbP, calculated within density functional theory. Spectra are found to be somewhat independent of the anion and the light polarization. Their features are explained in terms of the upper $s$, $p$, $d$, and $f$ electrons. Characteristic absorption features are related to the frequency dependence of the Fresnel reflectivity. While the lower part of the energy loss spectra is dominated by plasmonic features, the high-energy structures are explained by interband transitions.Comment: Added reference 21 to "S.-Y. Xu, et al, Science 349, 613 (2015)

Nanostructured nickel film deposition on carbon fibers for improving reinforcement-matrix interface in metal matrix composites

The issues in dispersing any form of carbon in metal matrix is the major problem in the field of metal matrix composites with carbon reinforcement (MMCcr). The low wettability of carbon in molten metals and the difference in density are some of the difficulties to obtain a good dispersion of carbon fibers in the matrix and, as a consequence, an improvement of some critical properties for metals in a wide range of application (mechanical properties, electrical properties, optical properties). For this reason, the aim of this work is to obtain a metallic coated carbon fiber to enhance the interaction between the reinforcement and the matrix. Moreover, also the density of carbon fibers could be adjusted depending on the thickness of the coating. Electroless Nickel-Phosphorus Plating (ENP) is one of the candidate to be a coating technique to improve the interaction between the carbon fibers and the metal matrix. Despite of its versatility in terms of complex geometry of the substrate and homogeneity and adhesion of the coating, the presence of the phosphorus in the alloy could create some problems with the metal matrix such as the formation of metal-phosphorus products that can drastically decrease the mechanical properties of the composite. For this reason, in this work, is presented a new way of Electroless Pure Nickel Plating (EPP) without any introduction of phosphorus in the nickel coating. The dependence of the coating thickness and the density of the coated fibers were studied under different plating parameters (temperature of the plating solution, deposition rate and plating solution composition). All the samples were characterized with SEM and XRD and the thickness, density and homogeneity were compared for all the samples obtained

Al2O3/ZrO2/Y3Al5O12 composites. A high-temperature mechanical characterization

An Al2O3/5 vol%·ZrO2/5 vol%·Y3Al5O12 (YAG) tri-phase composite was manufactured by surface modification of an alumina powder with inorganic precursors of the second phases. The bulk materials were produced by die-pressing and pressureless sintering at 1500 °C, obtaining fully dense, homogenous samples, with ultra-fine ZrO2 and YAG grains dispersed in a sub-micronic alumina matrix. The high temperature mechanical properties were investigated by four-point bending tests up to 1500 °C, and the grain size stability was assessed by observing the microstructural evolution of the samples heat treated up to 1700 °C. Dynamic indentation measures were performed on as-sintered and heat-treated Al2O3/ZrO2/YAG samples in order to evaluate the micro-hardness and elastic modulus as a function of re-heating temperature. The high temperature bending tests highlighted a transition from brittle to plastic behavior comprised between 1350 and 1400 °C and a considerable flexural strength reduction at temperatures higher than 1400 °C; moreover, the microstructural investigations carried out on the re-heated samples showed a very limited grain growth up to 1650 °C

Mechanical behaviour with temperatures of aluminum matrix composites with CNTs

Aluminum is a very useful structural metal employed in different industrial sectors, in particular it is used in large quantities in automotive, aeronautic and nautical industries. The main reasons of its wide use are: a very good oxidation resistance, excellent ductility, low melting temperature (660 °C) and low density (2.71 g/cm3). However, in order to reduce the emissions and fuel consumption is necessary to reduce the overall weight of vehicles by increasing mechanical properties of the structural material. The improvement of mechanical properties is normally achieved through use of reinforcement in materials, used like matrix, in order to improve some specific characteristics. In this work composites of carbon nanotubes (CNTs) dispersed in aluminum were made. The most difficulties in the preparation of this type of composite are represented by the low wettability between metallic matrix and fillers and the possibility of the oxidation of metal during melting with consequent decreasing of mechanical proprieties. The composite was obtained by three consecutive step: the first one is the functionalization of fillers surface to improve the fillers dispersion, the second one is the dispersion of fillers in the matrix by powder mixing and the third one is the melting and casting of the mix prepared. In particular, fillers used are multi walled carbon nanotubes (MWCNTs) with functionalized surface by treatment with a solfonitric solution. Melting and casting are carried out with the aid of an induction furnace with a controlled atmosphere system and centrifugal casting. Argon is the inert gas used to prevent the oxidation of aluminium during fusion. Young’s modulus was evaluated at different temperature and correlated with the different CNTs percentage. The dispersion rate of fillers and the microstructure of the sample were evaluated by FESEM micrograph

Local-field effects in silicon nanoclusters

The effect of the local fields on the absorption spectra of silicon nanoclusters (NCs), freestanding or embedded in SiO2, is investigated in the DFT-RPA framework for different size and amorphization of the samples. We show that local field effects have a great influence on the optical absorption of the NCs. Their effect can be described by two separate contributions, both arising from polarization effects at the NC interface. First, local fields produce a reduction of the absorption that is stronger in the low energy limit. This contribution is a direct consequence of the screening induced by polarization effects on the incoming field. Secondly, local fields cause a blue shift on the main absorption peak that has been explained in terms of perturbation of the absorption resonance conditions. Both contributions do not depend either on the NC diameter nor on its amorphization degree, while showing a high sensitivity to the environment enclosing the NCs

Lightweight metallic matrix composites. Development of new composites material reinforced with carbon structures

Carbon nano/micro-structures used as fillers in metallic lightweight alloys matrix composites are receiving considerable attention in scientific research and industrial applications. Aluminum and magnesium are the most studied light metals used as matrices in metal composites materials principally for their low density (respectively 2.7 g/cm3 and 1.7 g/cm3) and low melting temperature (around 660 °C for both metals). A good interaction between matrix and fillers is the first step to obtain an increase in bulk properties; furthermore, the manufacturing procedure of the composite is fundamental in terms of quality of fillers dispersion. In this work the influence of surface modifications for three classes of carbon fillers for aluminum and magnesium alloy (AZ63) as matrices is studied. In particular, the selected fillers are short carbon micro fibres (SCMFs), carbon woven fabrics (CWF) and unidirectional yarn carbon fibres (UYFs). The surface modification was carried out by a direct coating of pure nickel on fibres. The electroless pure nickel plating was chosen as coating technique and the use of hydrazine as reducing agent has prevented the co-deposition of other elements (such as P or B). SEM and EDS analyses were performed to study the effect of surface modifications. The mechanical properties of manufactured composites were evaluated by four point flexural tests according to ASTM C1161 (room temperature). Results confirm improved interactions between matrix and fillers, and the specific interaction was studied for any chosen reinforcement

First-principles calculations and bias-dependent STM measurements at the alpha-Sn/Ge(111) surface: a clear indication for the 1U2D configuration

The nature of the alpha-Sn/Ge(111) surface is still a matter of debate. In particular, two possible configurations have been proposed for the 3x3 ground state of this surface: one with two Sn adatoms in a lower position with respect to the third one (1U2D) and the other with opposite configuration (2U1D). By means of first-principles quasiparticle calculations we could simulate STM images as a function of bias voltage and compare them with STM experimental results at 78K, obtaining an unambiguous indication that the stable configuration for the alpha-Sn/Ge(111) surface is the 1U2D. The possible inequivalence of the two down Sn adatoms is also discussed.Comment: Submitted to PR

Giant excitonic absorption and emission in two-dimensional group-III nitrides

Absorption and emission of pristine-like semiconducting monolayers of BN, AlN, GaN, and InN are here systematically studied by ab-initio methods. We calculate the absorption spectra for in-plane and out-of-plane light polarization including quasiparticle and excitonic effects. Chemical trends with the cation of the absorption edge and the exciton binding are discussed in terms of the band structures. Exciton binding energies and localization radii are explained within the Keldysh model for excitons in two dimensions. The strong excitonic effects are due to the interplay of low dimensionality, confinement effects, and reduced screening. We find exciton radiative lifetimes ranging from tenths of picoseconds (BN) to tenths of nanoseconds (InN) at room temperature, thus making 2D nitrides, especially InN, promising materials for light-emitting diodes and high-performance solar cells

Static and dynamic weighing of rolling stocks by mean of a customized FBG-Sensorized-Patch

The structural health monitoring (SHM) of an infrastructure is of fundamental importance for the structure and people safety. Fiber Bragg Grating (FBG) sensors allow to design for each application, a tailored array of quasi-distributed sensors integrated to the infrastructure. To ensure the structural integrity of the railways is crucial to verify that the infrastructures comply with safety requirements to carry out their task. Railways rolling stock must comply with speed limits, the maximum number of wagons, maximum weight limit distributed on each axis of the wagons and the allowed number of trains on specific routes. The identification of the vertical load acting on each wheel is fundamental for the safety of a rolling-stock moving on a railway line. This paper presents the results of a test campaign on sensitive smart patches for static and dynamic weighing of trains. The system aims to generate a gripping system based on the magnetic force of a plastoferrite patch, taking advantage of the peculiarity that the rails are made of ferritic steel. This solution has the benefit of simplifying and speeding up the installation process and enabling a fast and easy removal or change in the configuration of the sensors array on the rail