Quantum-chemical modeling of lithium removal from lithium–silicon–silicon carbide composite

The quantum-chemical modeling of the delithiation-induced reorganization of a Li _m Si _n layer applied to the surface of nitrogen-doped silicon carbide is performed by means of non-empirical molecular dynamics in the frame of the gradient-corrected density functional method with the goal for finding promising anode materials for lithium ion batteries. The ratios Li/Si are considered from 8/3 to 1/4. Partial removal of lithium atoms from the surface of the Li _m Si _n layer and annealing at a moderate temperature (400 K) is found to recover rapidly (as soon as within 10 ps) the uniform metal distribution over the layer when the ratio Li/Si is at least 3/4. At lower values of this ratio, the equalization slows down dramatically.     

Synthesis of complex Si–O–N–H derivatives by molecular bombardment of silicon and silicon dioxide

Bombardment of a silicon target in a high vacuum with a molecular beam (mixture of high energy H₂+N₂, obtained by charge exchange) and a thermal beam of O₂ produces on the target a variety of compounds. The target is then bombarded by the same molecular beams which produce, extracted by an electrostatic field at an energy of about 10 keV, molecular ions due to the compounds thus formed. These ions are analysed (electromagnet) to give a primary spectrum of ions according to their mass, which are individually selected and dissociated in a collision cell (same H₂+N₂ mixture). Mass analysis of the dissociation fragments leads to the identification of silicon clusters (Si)_n and of Si–O–N–H derivatives, the fragmentations of which permit a definitive determination of their molecular complexity. Dissociation spectra have thus been obtained for some of the most intense peaks of the primary spectrum, on the one hand, and on the other hand for some peaks of lower intensity but of special interest to us (see below). The composition of the fragments is confirmed by the study of the satellite ions derived from the natural 28, 29 and 30 isotopes of silicon, and by the use of deuterium instead of hydrogen. None of the Si–O–N–H derivatives obtained was apparently known earlier. It is shown that some of these molecules (those ‘of special interest to us’) may be identical with sila-analogues of standard amino acids and of nucleic bases: the fact that their fragmentations are identical with those of the corresponding carbon analogues speaks in favour of a structural identity. However, one cannot yet distinguish between the various possible isomeric arrangements, as none of them has been independently prepared, which excludes a direct comparison with reference samples, and as these isomers might give identical fragments; we hope to be able to resolve this ambiguity later. Anyhow, the substances formed are the most complex molecular silicon derivatives so far produced: e.g. Si₂O₂NH₅, Si₃O₂NH₇, Si₄O₃NH₉, Si₄O₂N₂H₄, Si₄ON₃H₅, Si₅O₃N₂H₁₀, Si₅O₂NH₁₁, corresponding to sila-glycine, sila-alanine, sila-threonine, sila-uracile, sila-cytosine, sila-valine, sila-glutamine, – or isomers. Similar results have been obtained using a silicon dioxide target and high energy molecular beams of hydrogen and of nitrogen, without thermal oxygen, or with a carborundum target. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASmolecular impact / silicon derivatives / amino-acid silicon analogues     

The photometric determination of silicon as α-silicomolybdic acid

A photometric method for the determination of silicon based on the yellow color of α-silicomolybdic acid is described. The pH must be kept within the range 3.0–3.7, and the equilibrium state is established by beating the solution. Since the color of this modification is remarkably stable and reproducible, a high degree of precision can be attained.     

Synthesis of silicon–nitrogen derivatives in quasi-interstellar conditions

At 2–4·10^–5 T, a silicon wafer is the target of a 5–10 keV molecular beam of dinitrogen. The products are extracted by an electric field, and analysed by mass. The ions of this primary spectrum are dissociated in a Kr collision chamber. From the fragments thus obtained, one deduces compositions for the secondary ions, and therefore for the primary products. This is helped by the presence, in silicon, of the isotopes ²⁸Si, ²⁹Si and ³⁰Si. Beside the clusters Si_n (n = 1–7), complex molecular species are thus obtained, such as Si₅N₄⁺, Si₅N₄H⁺, Si₅N₄H₂⁺. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASinterstellar dust / interstellar molecules / atomic impact / silicon / nitrogen derivatives     

1,4-Addition of silicon dienoates to α,β-unsaturated aldehydes catalyzed by in situ-generated silicon Lewis acid

In situ-generated silyl methide species (R(3)Si-CTf(2)R') effectively catalyzed the reaction of β-substituted α,β-unsaturated aldehydes with silicon dienoates such as 3-bromo-2-TESO-furan to give the corresponding γ-adducts with excellent 1,4-selectivity and good anti selectivity.     

Influence of variation in the silicon content on the silicon precipitation in the Al–Si binary system

Ti-based amorphous alloys produced by ultra-rapid melt cooling represent an excellent option as biomaterials because of their mechanical properties and corrosion resistance. However, complete elimination of toxic elements is affecting the glass-forming ability and amorphous structure could be obtained only for thin ribbons or powders that are subsequently processed by powder metallurgy. Amorphous ribbons of special Ti₄₂Zr₄₀Ta₃Si₁₅ alloy, which is completely free of any toxic element, were produced by melt spinning, and the thermostability of resulting material was investigated in order to estimate its ability for further heat processing. Isochronal differential scanning calorimetry (DSC) was used to determine transformation points such as glass transition temperature T _g or crystallization temperature T _x. The activation energy for crystallization of amorphous phase was calculated based on Kissinger method, using heating rates ranging between 5 and 20 °C min^?1. Amorphous structure of resulting ribbon was evidenced by means of X-rays diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). It was determined that amorphous Ti₄₂Zr₄₀Ta₃Si₁₅ alloy has a high activation energy for crystallization, similar to other Ti-based amorphous alloys, which provides good thermal stability for subsequent processing, especially by means of powder metallurgy techniques.     

π-Relaxation of the ring strain: design of polycyclic unsaturated silicon molecules

Introduction of a double bond into cyclic silanes lowers the ring strain by the cyclic delocalization of π-electrons through the hyperconjugation with the σ bonds, which is favored by the high π-orbital energy of the SiSi bond and the low σ*-orbital energy of the Si-H bonds. The π-relaxation of strains significantly occurs in the small rings. Unsaturated small silicon ring molecules are less strained than the saturated ones and the unsaturated carbon congeners. We calculated a series of polycyclic silicon molecules to confirm the π-relaxation and suggested that some unknown molecules could be prepared due to the low strain.     

Direct electrochemistry behavior of Cytochrome c on silicon dioxide nanoparticles-modified electrode

A newfangled direct electrochemistry behavior of Cytochrome c (Cyt c) was found on glassy carbon (GC) electrode modified with the silicon dioxide (SiO2) nanoparticles by physical adsorption. A pair of stable and well-defined redox peaks of Cyt c′ quasi-reversible electrochemical reaction were obtained with a heterogeneous electron transfer rate constant of 1.66×10-3 cm/s and a formal potential of 0.069 V (vs. Ag/AgCl) (0.263 V versus NHE) in 0.1 mol/L pH 6.8 PBS. Both the size and the amount of SiO2 nanoparticles could influence the electron transfer between Cyt c and the electrode. Electrostatic interaction which is between the negative nanoparticle surface and positively charged amino acid residues on the Cyt c surface is of importance for the stability and reproducibility toward the direct electron transfer of Cyt c. It is suggested that the modification of SiO2 nanoparticles proposes a novel approach to realize the direct electrochemistry of proteins.     

Mechanism of the emergence of the photo-EMF upon silicon liquid crystal–single crystal contact

The effect light has on a silicon liquid crystal–single crystal contact at different temperatures of the surface doping of silicon, and when BaTiO₃ nanoparticles are added to the composition of a liquid crystal, is studied. The mechanism of the emergence of the photo-EMF in the liquid crystal–silicon structure is explained.

     

X-ray photoelectron spectroscopy for detection of the different Si–O bonding states of silicon

X-ray photoelectron spectroscopy (XPS) was used to detect the bonding between a silica particle surface and attached silanes. In addition to the commonly recorded Si 2p spectrum, the Si 1 s level is also accessible when monochromatic Ag Lalpha X-rays are applied. Furthermore, the spectrum of the Si 1 s level shows a fine structure. After spectrum deconvolution, we assigned the fitted spectral peaks to Si-C bonds of the silanes and to the Si-O bonds of the silica network. The recorded Si 1 s spectra were deconvoluted into peaks originating from Si-C bonds and the Si-O-Si silica network. To check the results of spectrum deconvolution, several differently functionalized silanes containing stoichiometric amounts of heteroatoms were applied for silica surface modification. We conclude that spectra deconvolution of the Si 1 s signal is an appropriate means for quantification of surface attached silane molecules.     

Synthesis of fluspidine via asymmetric NaBH4 reduction of silicon enolates of β-keto esters

Asymmetric NaBH₄ reduction catalyzed by the Co(II) complex of a chiral diamidine-type sp²N ligand, Naph-diPIM-dioxo-iPr, was successfully applied to 3-silyloxycinnamate substrates without over-reduction, giving quantitatively 3-silyloxy-3-arylpropionates with an enantiomer ratio of up to 99:1. The high utility was confirmed on a 30-g scale using 0.1?mol% catalyst. Both Z and E substrates could be converted to a single enantiomeric product by changing the ligand chirality. The relationship between the Z/E stereochemistry and the absolute configuration of the 1,4-reduction product provided important information about the mechanism underlying enantioface selection. Combination of the asymmetric catalysis with two other key steps, Suzuki coupling with an N-protected tetrahydropyridine boronic acid derivative and intramolecular bromo etherification, realized an efficient synthetic route to both enantiomers of fluspidine. The new strategy permits the introduction of substituents on the two aryl groups and piperidine ring, allowing for structural variations toward the development of higher performance σ1 receptor antagonists.     

Synthesis and pharmacological activity of a silicon—zinc—boron-containing glycerohydrogel

Russian Chemical Bulletin - A new pharmacologically active nanostructured silicon—zinc—boron-containing glycerohydrogel was synthesized by the sol—gel method using silicon, zinc,...     

Magnetron sputtering deposition of silicon nitride on polyimide separator for high-temperature lithium-ion batteries

To date,lithium-ion batteries are becoming increasingly significant in the application of portable devices and electrical vehicles,and revolutionary progress in theoretical research and industrial application has been achieved.However,the commercial polyolefin separators with unsatisfying electrolytes affinity and poor thermal stability have extremely restricted the further application of lithium-ion batteries,especially in the high-temperature fields.In this work,magnetron sputtering deposition technique is employed to modify the commercial polyimide separator by coating silicon nitride on both sides.Magnetron sputtering deposition modified polyimide(MSD-PI)composite separator shows high thermal stability and ionic conductivity.More importantly,compared with the cells using Celgard separator,the cells with MSD-PI separator exhibit superior electrochemical performance,especially long-term cycle performance under high temperature environment,owing to the high thermal conductivity of surface Si3 N4 particles.Hence,lithium-ion batteries with MSD-PI separator are capable of improving thermal safety and capacity retention,which demonstrates that magnetron sputtering deposition technique could be regarded as a promising strategy to develop advanced organic/inorganic composite separators for high-temperature lithium-ion batteries.     

On the possibility of existence of η5-π-complexes of C60 fullerene: π-complexes with silicon

Qualitative estimates of possibility of the formation of fullerene ⁵--complexes have been carried out by the MNDO/PM3 method. It was shown, as exemplified by the C₆₀ cluster, that the introduction of five univalent functional groups R (R = H, Cl, Br) to -positions of a separate pentagon of C₆₀ with the formation of [R₅C₆₀]^– anions results in a pronounced increase in the -electron density on the atoms of this five-membered cycle and more favorable conditions for the formation of -bonds with the ⁵-ligand. The nature of the interaction between the atoms of the separate cycle in [R₅C₆₀]^– anion and ⁵-ligand was analyzed by the example of hypothetical sandwich systems R₅C₆₀SiCp. Half-sandwich complexes R₅C₆₀SiX (X = H, Cl) were also investigated. The local energy minima were found on the potential energy surfaces (PES) of systems R₅C₆₀SiCp and R₅C₆₀SiX with C_5p symmetry. These systems transform barrirlessly into q5-7E-complexes with the angular structure if the symmetry restrictions are removed. The most favorable conditions for ⁵--complexes of R₅C₆₀ to form are realized for R = H. The results obtained were compared to those of semiempirical and nonempirical calculations of bis (cyclopentadienyl) silicon.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2422–2429, October, 1996.     

Correction to: Research progress of nano‑silicon‑based materials and silicon‑carbon composite anode materials for lithium‑ion batteries

Journal of Solid State Electrochemistry - In order to solve the energy crisis, energy storage technology needs to be continuously developed. As an energy storage device, the battery is more widely...     

Enantioselective formal hydration of α,β-unsaturated acceptors: asymmetric conjugate addition of silicon and boron nucleophiles

The direct enantioselective 1,4-addition of water to α,β-unsaturated acceptors is an open challenge in asymmetric catalysis. Enantioselective conjugate addition of either silicon or boron nucleophiles and subsequent enantiospecific oxidative degradation of the carbon-element bond represents, however, an attractive detour. A single extra step thereby enables an indirect enantiocontrolled construction of (in a broader sense) aldols from α,β-unsaturated carbonyl and carboxyl compounds. While that strategy had been obvious for a long time, it was recent stunning progress in transition metal-catalysed activation of interelement linkages that brought about the solution to that long-standing problem. A concise introduction of existing protocols for stereoselective 1,4-addition of oxygen nucleophiles is followed by a comprehensive summary of the recent rapid advances in transition metal-catalysed (and metal-free) asymmetric conjugate transfer of nucleophilic silicon and boron onto α,β-unsaturated acceptors.     

Characterization of SiN and other transient species in a silicon tetrachloride–nitrogen discharge

Infrared diode laser absorption spectroscopy has been used to detect the A–X electronic spectrum of diatomic SiN in a SiCl₄/N₂ plasma. The intensity of the SiN transitions with respect to the ratio of SiCl₄:N₂ flow rate was investigated. A correlation between intense signals of SiN and the optimal growth conditions for a-SiN films reported in literature was found. This observation lends support to the importance of diatomic SiN as a film precursor. The electronic emission spectra of silicon-containing transient species were also recorded in the plasma. A brief discussion of the plasma reaction pathways of SiN is presented.     

Synthesis of hollow silicon nanospheres encapsulated with a carbon shell through sol–gel coating of polystyrene nanoparticles

Journal of Sol-Gel Science and Technology - In this study a templating method to form hollow Si nanospheres encapsulated with a carbon shell (HSi@C) has been investigated. The key synthesis...