Numbers Of Si Research Articles

Network structure and thermal properties of bioactive (SiO2\u2013CaO\u2013Na2O\u2013P2O5) glasses

Ca- and P-based bioactive glasses are excellent candidates for design and manufacture of biomaterials. Understanding the structure and physico-chemical–thermal behaviour of bioactive glasses is a fundamental step towards the design of a new generation of biocompatible materials. In this study, the structure of SiO2–CaO–Na2O glasses and its derivatives, obtained by substituting Na2O with P2O5 and prepared by melt–quench technique, was studied with neutron and electron diffraction techniques combined with thermal analysis, high-resolution electron microscopy and X-ray photoelectron spectroscopy. Neutron and electron diffraction data were analysed with reverse Monte Carlo simulation and pair distribution function analysis, respectively. Bioactivity of P2O5 substituted glasses was also investigated and proven in vitro using simulated body fluid. Based on the structural analysis, it was found that Si and P atoms are in well-defined tetrahedral units with a bond distance of 1.60 Å for both Si–O and P–O bonds, although P exhibits a higher average coordination number than Si. With increasing phosphate content, tendentious changes in the glass behaviour were observed. Linear increase in Tg, supported by the changes in the average coordination numbers of Si and P, indicates strengthening of network structure with increasing P content and formation of P–O–Ca atomic linkages, which lead to Ca–P-rich atomic environments in the silicate network. These Ca–P-rich environments trap volatile elements and thus decrease the total weight loss during heating at higher P concentrations. In the case of the highest investigated P2O5 content (5 mol%), nanoscale structural inhomogeneity and the formation of Ca–P-rich clusters were also revealed by electron diffraction and atomic resolution imaging. This type of Ca–(Na)–P clustering has a key role in the behaviour of phosphate-substituted silicate glasses under physiological conditions.

Molecular Dynamics Analysis of the Microstructure of the CaO-P<sub>2</sub>O<sub>5</sub>-SiO<sub>2</sub> Slag System with Varying P<sub>2</sub>O<sub>5</sub>/SiO<sub>2</sub> Ratios

The structures of the CaO-P2O5-SiO2 slag system with varying P2O5/SiO2 ratios at a fixed CaO content have been investigated by molecular dynamics simulation using the pairwise potential model. The results showed that the average bond lengths of Si-O and P-O were 1.610 « 0.001i and 1.531 « 0.005i in the examined range of the P2O5/SiO2 ratio. More than 95% Si and P ions were 4-coordinated and formed tetrahedral structures. Average coordination numbers of P-O and Si-O decreased slightly while average coordination number of Ca-O revealed a rising trend with the addition of P2O5. The non-bridging oxygens (Si-O-Ca and P-O-Ca) were the overwhelming majority and further increased with the substitution of SiO2 by P2O5, which results in the decrease of the polymerization constant Kp. The numbers of Si and P groups linked to a tagged Si or P tetrahedron (Qn speciation) characterized the polymerization degree of the slag system. The Q0 of both P and Si declined remarkably with the increase of P2O5. Correspondingly, (Q1 + Q2 + Q3 + Q4) of Si and P increased observably, implying that the polymerization degree of the slag system was enhanced. [doi:10.2320/matertrans.M2014363]

Monitoring of the Degree of Condensation in Alkoxysiloxane Layers by NIR Reflection Spectroscopy

This paper introduces a novel analytical approach for monitoring the degree of condensation of thin siloxane films, which is potentially suitable for in-line process control during the deposition of such layers, e.g., to polymer films. Near-infrared (NIR) reflection spectroscopy in combination with chemometric methods was used as a process monitoring tool. The state of the formation of the inorganic Si–O–Si network in partially condensed 3-methacryloxypropyltrimethoxysilane batches was analyzed by inverse gated 29Si NMR spectroscopy. Results were expressed in terms of different relative ratios of the Ti species (i.e., structures with different numbers of Si–O–Si units per Si atom). These data were used for calibration of the NIR method, which was applied to thin layers printed on a polymer foil with a thickness of ∼2.2 g m–2. The root-mean-square error of prediction (RMSEP) for the determination of the ratio of the Ti species from the NIR spectra was found to be less than 3%. The error of the reference data from 29Si NMR spectroscopy is 4%, which results in an overall error of 5%. Moreover, the thickness of siloxane layers was determined by this method in a range from 2.5 to 5.5 g m–2 using gravimetry for calibration (prediction error ∼0.3 g m–2).

Kinetic Modeling of Ethyl Benzoylformate Enantioselective Hydrogenation over Pt/Al2O3

A kinetic model was developed for the enantioselective hydrogenation of ethyl benzoylformate (EBF) on a modified Pt/Al2O3 catalyst. This model was based on the assumption of different numbers of si...

Heart rate, central hemodynamics and physical performance in middle-distance runners

Aim. To determine the vegetative ensuring of central hemodynamics and working capacity of runners Methods and results. Results of the survey of the 46 athletes, who specialized in run on middle distance qualification of II–III discharge to IMS showed some features in the ECG. Conclusion. The prevalence of parasympathetic VNS significantly correlated with numbers of SI corresponding to hypokinetic TC, which was accompanied by high values of physical performance and IFS.

Structures and electronic properties of SimN8-m(0 < m < 8) clusters: a density functional theory study

The geometries, electronic structures and related properties of SimN8-m(0 < m < 8) clusters are studied using density functional theory (DFT) with hybrid functional B3LYP. The calculated results reveal several trends. For any stoichiometric clusters, the lowest energy isomers with an alteration of N and Si atoms are favourable in energy if the numbers of Si and N atoms are large enough to form … Si–N–Si–N… alternative chains. The bond lengths of single Si–N bonds are very close to the corresponding values of the bulk and other Si–N clusters. The geometries for N-rich and Si4N4 clusters are planar structures, but three-dimensional structures are favourable in energy for Si-rich clusters. With the increase of m, the isotropic polarizability and average polarizability increase, the total binding energies generally decrease, the HOMO-LUMO gap and vertical ionization potential oscillate with increasing number of valence electrons, and their values with even valence electrons are larger than those with odd valence electrons. The atomic charges, IR and Raman properties are also reported.

Ab initio investigation on the nonlinear optical properties of silicon clusters Sin (n=3–8)

First and second hyperpolarizabilities of small silicon clusters have been calculated using conventional ab initio methods systematically increasing the amount of electron correlation. Besides Si$_{5}$, upon successive addition of electron correlation in the MP2, MP3, MP4, CCSD, and CCSD(T) series, all clusters display the same behavior: i) the HF $γ _{//}$ values are the smallest, ii) the MP2 $γ _{//}$ values the largest, and iii) the latter values are good approximate to the reference CCSD(T) results because the overestimation is smaller than 10%. Contrary to the polarizability per Si atom, which decreases with the cluster size until reaching the bulk limit, the average second hyperpolarizability per Si atom presents a sawtooth behavior with maxima in $γ _{//}$ associated with even numbers of Si and minima with odd numbers of Si atoms.

Growth kinetics and physical characterisation of Si 1− xGe xO 2 films obtained by plasma assisted oxidation

Oxides of Si 1− x Ge x alloys have been grown using the method of plasma assisted, anodic oxidation at temperatures <100°C. The growth kinetics have been analysed as a function of Ge concentration to obtain information on the O − ion mobility, μ, in the oxide. We observe μ(SiO 2)< μ(GeO 2)< μ(Si 1− x Ge x O 2) for 0.0< x<1.0. Infrared absorption spectroscopy has been used to examine the network bonding, it shows that the numbers of Si O Si, Si O Ge and GeOGe bonds do not follow those expected on the basis of simple statistics related to the relative concentrations of Si and Ge. Auger electron spectroscopy and secondary ion mass spectroscopy reveal that the anodic oxides are spatially inhomogeneous being composed of mixtures of SiO 2, GeO 2 and Si 1− x Ge x O 2, consistent with the results of infrared analysis.

Excitation energy dependence of luminescent sol-gel organically modified silicates

The light emission features of a new class of intensely luminescent organic/inorganic materials are reported. These sol-gel-derived hybrids contain short highly solvating oxyethylene units (OCH 2CH 2) grafted onto a silica network by means of urea cross-links. Three xerogels series designated as ureasils U(600), U(900) and U(2000), have been synthesized incorporating different units of oligopolyoxyethylene chains, approximately 8.5, 15.5 and 40.5, respectively. The emission spectra of these ureasils are characterized by a large broad band between 1.82 and 3.44 eV, with a blue ( 2.6 eV) and a purplish-blue ( 2.8–3.0 eV) component. These bands are associated with planar oxygen-based rich-silicon domains with different numbers of Si O structural units. The emission peak position, E P , of the three ureasils shift to higher energies with the increasing of the excitation energy, E X . For excitation energies between 3.10 and 3.76 eV a linear relation with a slope approximately equal to 0.5 has been found between E P and E X .

Formation and evolution of weathering products in rhyolitic pyroclastic flow deposit, southern Kyushu, Japan.

Weathering reactions of a rhyolitic pyroclastic flow deposit in southern Kyushu, Japan were investigated in order to understand the behavior of elements and the formation and evolution of reaction products. The weathering profile developed in the pyroclastics contains variable amounts of erionite, halloysite and noncrystalline Al-Si-Fe-rich materials with or without noncrystalline Mn-rich material as weathering products. The formation of these products appears to reach depth of more than 10 m below the surface. The mobility of cations in this profile is Na>Ca>K>Si>Fe>Al>Ti>Mn, in which Al, Fe, Mn, and Ti accumulate mostly in the surface horizon during the weathering. The accumulation of these elements is caused by dissolution of volcanic glass, feldspar, and pyroxene, and precipitation of noncrystalline clays enriched in Al, Fe, Mn, and Ti. Aluminum and Si released into solutions from volcanic glass and feldspar tend to combine with each other to produce Al-Si complexes with high reactivity with other metal cations at the initial weathering stages. Fe2+ and Mn2+ derived mainly from pyroxene are incorporated with the Al-Si complexes and precipitate as Al-Si materials containing Fe and Mn exhibiting irregular aggregates of randomly elongated very fine noncrystalline fibers less than 0.1μm in length. These noncrystalline fibers tend to crystallize to halloysite by partial development of a domain structure and elimination of Fe and Mn during crystallization processes. This weathering profile characteristically contains erionite crystals exhibiting hexagonal rods and thin needles normally less than 5μm in length as a major weathering product of the rhyolitic pyroclastic flow deposit. The chemical compositions of erionite crystals show a wide range with negative correlation between numbers of Si and those of Al+Fe3++Ti in which Si content increases continuously with depth. These erionite crystals may have been formed in a micro-environment where high pH and high cation activity conditions have been achieved by heterogeneous dissolution of volcanic glass and feldspar.

Metal Lines Associated with Lyα Absorbers: A Comparison of Theory and Observations

We study the metal-line absorption of C IV, C II, Si IV, and N V at redshifts z = 3.5 to z = 2 within the framework of a cosmological model for the Lyα forest, comparing the results of numerical simulations to recent observations by Songaila & Cowie (SC). We find that the observed mean value of the C IV/H I ratio at z 3 is reproduced if a uniform metallicity of [C/H] ~ -2.5 is assumed in our model, but that the observed scatter in this ratio is larger than predicted, implying a scatter of roughly an order of magnitude in the metallicity of the absorbing systems. The enrichment pattern that is required for our model to match SC's C IV observations is very similar to that predicted by other simulations of reionization and early metal enrichment in this type of cosmological scenario. Our model predicts no significant evolution in the mean values of metal-line column densities between z = 3.5 and z = 2. Comparison of the predicted and observed numbers of Si IV and N V systems suggests that the photoionizing background radiation field at z ~ 3 is somewhat softer than that proposed earlier. Our model does not account for the increase in the Si IV/C IV ratio at z 3.2 found by SC. While the latter study suggested that the increase could be explained by a softening of the radiation field caused by He II absorption at z 3, such a modification does not raise the mean value of Si IV/C IV significantly in our simulation, because it shifts numerous weak Si IV systems to just above the detection limit, thus keeping the mean column density of observable Si IV systems low.

The Dispersion of Silicate Glasses as a Function of Composition II*

The volume per gram atom of oxygen in a well-annealed silicate glass is given quite accurately by the equation VO=k+bSi+cSiNSi+∑cMNM, where k is a very small constant characteristic of the annealing technique, NSi and NM are the numbers of Si or other “metal” atoms per atom of oxygen, bSi and cSi are constants which are different for different ranges of NSi values, and cM are constants valid (insofar as can be determined from present data) for all values of NM. For boron-containing glasses separate terms are required for tetrahedrally surrounded and for triangularly surrounded atoms, the relative numbers of which depend on the relative amounts of Si, B and O. With the aid of the above equation, one can calculate densities from compositions simply and quite accurately. The empirical results obtained are interpreted theoretically.

The Density of Silicate Glasses as a Function of Composition*

The volume per gram atom of oxygen in a well-annealed silicate glass is given quite accurately by the equation VO=k+bSi+cSiNSi+∑cMNM, where k is a very small constant characteristic of the annealing technique, NSi and NM are the numbers of Si or other “metal” atoms per atom of oxygen, bSi and cSi are constants which are different for different ranges of NSi values, and cM are constants valid (insofar as can be determined from present data) for all values of NM. For boron-containing glasses separate terms are required for tetrahedrally surrounded and for triangularly surrounded atoms, the relative numbers of which depend on the relative amounts of Si, B and O. With the aid of the above equation, one can calculate densities from compositions simply and quite accurately. The empirical results obtained are interpreted theoretically.