Controlled Synthesis of β-SiC Nanopowders with Variable Stoichiometry Using Inductively Coupled Plasma

In the growing field of nanomaterials, SiC nanoparticles arouse interest for numerous applications. The inductively coupled plasma (ICP) technique allows obtaining large amount of SiC nanopowders from cheap coarse SiC powders. In this paper, the effects on the SiC structure of the process pressure, the plasma gas composition, and the precursor nature are addressed. The powders were characterized by X-ray diffraction (XRD), Raman and fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high resolution electron microscopy (HREM), chemical analyses, BET and photon correlation spectroscopy (PCS) measurements. Whatever the precursor (α- or β-SiC), the nanoparticles were crystallised in the cubic β-SiC phase, with average sizes in the 20-40nm range. Few residual grains of precursor were observed, and the decarburization due to the reductive Ar-H2 plasma lead to the appearance of Si nanograins. The stoichiometry of the final product was found to be controllable by the process pressure and the addition of methan

Si1-xGex alloys as negative electrode for Li-ion batteries: Impact of morphology in the Li + diffusion, performance and mechanism

International audienceSi1-xGex alloys present interesting cycling performance as Li-ion anodes, owing to the synergetic effect from silicon's high capacity and germanium's electrical conductivity and Li+ diffusion. Various morphologies of Si1-xGex powders were obtained, micron-sized by ball-Milling (BM) and nano-sized by laser pyrolysis (LP) to study the effect of the morphology and composition on the electrochemical behavior. The electrical conductivity was measured and shows an increase with the Ge content, from 4.58E-03 for Si to 0.11 S m−1 for Si0.5Ge0.5. Half-cells were cycled at C/5, LP samples showed a better capacity retention than their BM counterparts (88% vs 72 % at the 35th cycle for Si0.5Ge0.5). To rationalize these trends, Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance Spectroscopy (EIS) were used to determine the apparent Li+Diffusion in the Si1-xGex series. It was shown that the apparent Li+Diffusion is dependent to the state of charge. Moreover, it gave insight about phase transformations during cycling. During lithiation, similar values (10−11 cm2 s−1) were obtained for BM and LP Si0.5Ge0.5. However, a big variation (10−13 – 10−10 cm2 s−1) was found for the BM sample delithiation, which is attributed to the delithiation of c-Li15(Si0.5Ge0.5)4 phase into amorphous Lix(Si0.5Ge0.5) (x<3.75). Asymmetric C-rate tests evidenced lithiation as the limiting mechanism for the Si1-xGex negative electrodes

Photoluminescence decay dynamics of noninteracting silicon nanocrystals

Time-resolved photoluminescence measurements on size-selected silicon nanocrystals have been carried out in order to elucidate the nonexponential behavior of the photoluminescence decay kinetics. The nanoparticles are gas-phase synthesized, extracted as a supersonic beam, size selected, and deposited downstream as films of variable densities. The nanoparticle number densities were determined by atomic force microscopy. The photoluminescence properties appear totally independent of the film density. Even in the very low density film where nanoparticles are completely isolated from each other, the decay kinetics corresponds to a stretched exponential law. This means that the stretched exponential kinetics does not originate from the interaction between nanoparticles, but is actually a characteristic of the silicon nanocrystals

Mécanismes et impact de nanoparticcules de TiO2 sur la réparation de l'ADN dans des cellules pulmoniares

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Analysis of Si-based Anode in Li-ion Batteries Combining Electrochemical Characterization and Multi-Scale Modeling

International audienceDue to its high capacity compared to graphite, silicon has attracted attention among li-ion batteries technology as a promising material for negative electrodes. It is abundant, not toxic. However, this material is well known to be subject to large volume changes upon lithation and delithation (up to 320 %). This phenomenon causes particles cracking, instability of the passivation layer appearing at the interface of solid Si and liquid electrolytes (SEI) and finally leads to electrode delamination, affecting the cycle performance of such batteries in particular long time performances. To overcome these problems, several strategies have been proposed such as using submicronic particles (i.e.< 150 nm) to mitigate the volume changes and/or protection of the silicon material with a carbon layer [reference]. Combining both strategies Si@C core-shell nanoparticles synthesized in one step process have recently been proposed as a promising anode material. More specifically, the Si-based nanoparticles are synthesized in a two stages laser pyrolysis reactor, which allows obtaining carbon coated silicon nanoparticles in one single step, mitigating oxidation because there is no air exposure between the synthesis of the core and shell as well as particle degradation due to nanometric size. The synthesis technique also allows controlling organization of the core, both crystalline and amorphous silicon cores have been synthesized. Moreover, the shell thickness can be varied by changing the flow of carbon precursor. To optimize the design of composite electrodes based on such active materials, an in-depth understanding of their performance and chemical/mechanical degradation processes remains critical. In this work, multi-scale modeling is developed based on the analysis of the electrochemical performances of electrodes composed of Si-based nanoparticles (with and without shell). Several electrochemical techniques such as GITT (Galvanostatic Intermittent Titration Technique), galvanostatic cycling and Electrochemical Impedance Spectroscopy (EIS) were used to characterize the battery performance. These data provide physical parameters to feed a mathematical model (Newman) of the Si electrode based on the porous-electrode theory. This Newman model combines description at the nanoparticle scale up to the composite electrode scale. Besides, the EIS study, carried out at various cell states of lithiation of Si material, allows tracking the evolution of several critical parameters (for example SEI resistance, charge transfer resistance at the interface) of the mathematical model (figure 1). Figure 1b shows the evolution of the exchange current density with SOC for coated and non-coated materials together with the expected theoretical evolution with constant kinetic rate evidencing the need for an accurate modelling. This presentation will be devoted to our first results concerning such a modelling and application to the multiscale behavior of coated as well as non-coated silicon as active anode materials

Interface Analysis of Si-based Anode in Li-ion Batteries through Electrochemical Impedance Spectroscopy and equivalent electrical circuit analysis

International audienceDue to its high capacity compared to graphite, silicon has attracted attention for li-ion battery technology as a promising material for negative electrodes. It is abundant and non-toxic. However: this material is weil known to undergo large volume changes upon lithation and delithation (up to 320 %). This phenomenon causes particle cracking,jnstability of the solid electrolyte interphase (SEI) at the interface between solid Si and liquid electrolyte, and finally leads to electrode delamination and battery performance loss. To overcome these problems, several strategies have been proposed such as using submicronic particles « 150 nm) to mitigate the volume changes and protection of the silicon material with a carbon layer to stabilize the active surface in contact with electrolyte. Combining both strategies, Si@C. core-shell nanoparticles synthesized in one step process have recently been proposed as a promising anode material [1]. Specifically, the Si-based nanoparticles are synthesized in a two stage laser pyrolysis reactor, which yields carbon coated silicon nanoparticles in a si~gle step. This approach mitigates material oxidation because there is no air exposure between the synthesis of the core and shell. Additionally, the nanometric size of the particles prevents material pulverization upon cycling. The synthesis technique also allows control of the core crystallinity, and both highly crystalline and amorphous silicon cores have been synthesized. Moreover, the shell thickness can be tuned by changing the flow of carbon precursor. To optimize the design of composite electrodes based on such active materials, an in-depth understanding of their performance and chemical/mechanical degradation processes remains critical. In this work, the analysis of the electrochemical performances of su ch Si-based electrodes was performed using several electrochemical techniques to compare crystalline or amorphous Si nanoparticles without shell as weil as crystalline Si nanoparticles coated by a thin or a thick layer of carbon (Si@C). The EIS study, carried out at various states of lithiation of Si material, allows tracking the evolution of several critical parameters (for example SEI resistance and charge transfer resistance) of the equivalent electrical circuit describing the elec.trode electrical behavior. Figure la shows typical impedance spectra while Figure lb shows the evolution of the of charge transfer resistance for the different coated and non-coated materials. A very different behavior is observed as a function of the interface material and carbon thickness. The modificatfon of the interface between e1ectrolyte and Si or Si@C materials is also visible on measured equilibrium potentials and power capabilities. This presentation will be devoted to the analysis of the impact of interfaces between electrolyte and Si or Si@C materials on electrochemical performances

Toxicity of TiO2 nanoparticles upon chronic exposure of pulmonary cells in vitro

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