Experimental small-angle X-ray scattering investigation of initial stages of the spinodal decomposition in model sodium silicate glasses

The kinetics of the initial stages of the spinodal decomposition in model glasses of the Na₂O-SiO₂ system has been investigated in situ. It has been demonstrated that there is a quantitative agreement of the experimental results obtained in the framework of the Stephenson theory with the basic principles of modern theories and the data on direct determination of the viscosity, mobility, and diffusion. It has been found that the spatial-temporal evolution of the heterogeneous structure has a multistage character during spinodal decomposition. The characteristic size of the phase regions at each stage varies with time according to the power law. The sequence of stages and the values of exponents for the spinodal decomposition are as follows: 1/20, 1/4, 1/2, and 1/3.     

Characterization of the structure of ultradispersed diamond using X-ray diffractometry and small-angle X-ray scattering

Two samples of detonation-synthesized ultradispersed diamond have been studied using X-ray diffractometry and small-angle X-ray scattering. It has been shown based on the X-ray diffractometry data that two samples contain regions with both the diamond and graphite-like lattices. Grain radii inside both samples are evaluated from the small-angle scattering data as 30–50 nm. The samples also contain low-dimensional components. A broad Bragg peak corresponding to a set of interplanar distances from 5 to 15 nm is revealed in the small-angle scattering curve of sample no. 1. A structural model of ultradispersed diamond particles, which represents the diamond core surrounded by concentric graphite shells similar to the onionskin, is confirmed.     

Time-resolved microfocused small-angle X-ray scattering investigation of the microfluidic concentration of charged nanoparticles

We describe the concentration process of a dispersion of silica nanoparticles undergoing evaporation in a dedicated microfluidic device. Using microfocused small-angle X-ray scattering, we measure in time and space both the concentration field of the dispersion and its structure factor. We show that the electrostatic interactions affect the concentration rate by strongly enhancing the collective diffusion coefficient of the nanoparticle dispersion. En route towards high concentrations, the nanoparticles eventually undergo a liquid-solid phase transition in which we evidence crystallites of micron size.     

Local coordination geometry around Cu and Cu ions in silicate glasses: an X-ray absorption near edge structure investigation

We present an X-ray absorption near edge structure (XANES) study on Cu⁺ and Cu²⁺ ions in silicate glasses at the Cu K-edge, aimed to determine the geometry of the local structure around the metal. This study is based on the comparison between experimental data and theoretical calculations made in the framework of multiple scattering theory. The XANES signals relative to several clusters are simulated on the basis of known crystalline structures involving Cu⁺ and Cu²⁺ ions in silicate matrices. Concerning the Cu²⁺ in glass, the simulations suggest the presence of a square coordination of oxygen atoms around the absorber, with a possible presence of metal ions in the second shell. As for the Cu⁺ ions, the metal clustering is excluded and a linear O-Cu-O coordination is evidenced. Received 30 April 1999     

Critical small-angle X-ray scattering of the liquid sodium-lithium system

Small-angle X-ray scattering of the liquid sodium-37 w/o lithium system in the range $\text{1.034 × 10}{}^{-4}\text{? (T?T}{}_{\mathrm{<mtext>c</mtext>}}\text{)/T}{}_{\mathrm{<mtext>c</mtext>}}\text{?}\text{1.503 × 10}{}^{-2}$ yields the critical exponents v = 0.655 ± 0.030 and λ = 1.296 ± 0.061 for the temperature dependence of the correlation length η and of the coefficient (?c/?μ)_{T, P} respectively.     

小角X射线散射应用研究若干进展

文章从小角X射线散射（small-ande X-ray scattering，SAXS）的理论分析出发，结合国内外SAXS理论和应用研究的最新动态，总结了文章作者所在的研究小组近年来在SAXS技术用于材料微结构表征方面的研究成果。主要包括以下三个方面的内容：（1）有机／无机杂化材料中电子密度波动的研究；（2）弦长度分布函数材料的周期结构的研究；（3）纳米粉末晶化过程的研究。     

小角X射线散射应用研究若干进展

文章从小角X射线散射（small-angle X-ray scattering, SAXS）的理论分析出发,结合国内外SAXS理论和应用研究的最新动态,总结了文章作者所在的研究小组近年来在SAXS技术用于材料微结构表征方面的研究成果。主要包括以下三个方面的内容：（1）有机/无机杂化材料中电子密度波动的研究;（2）弦长度分布函数材料的周期结构的研究;（3）纳米粉末晶化过程的研究。     

The studies of structural aspects of the cluster formation in silicate glasses doped with cerium and titanium oxides by small-angle neutron scattering

The structural aspects of the formation of Ti-Ce-O nanoclusters in silicate glasses doped with oxides TiO₂ and CeO₂ have been studied by means of small-angle neutron scattering. It has been obtained that, in such glasses, complex oxide nanoclusters with sizes of 300–380 Å are formed; their average size increases and the fractal dimension is changed as the concentration of the initial oxides increases. Correlation between the structural characteristics of the nanoclusters and the optical properties of the doped silicate glasses is discussed.     

Determination of structure parameters and size distribution of colloidal particles by means of small-angle X-ray scattering

A dilute silica suspension was used for examining the possibilities of using the small-angle X-ray scattering in the investigation of objects of colloid dimensions. The average radius of gyration, weight, volume and specific surface of the SiO₂ particles were determined. The particle size distribution curve was determined by integral transformation of the scattering curve and by adjusting the parameters of empirical functions.The authors are indebted to Professor P. W.Schmidt for kindly supplying the SiO₂ suspension.     

Study of the microporosity in nanoporous carbon films by small-angle X-ray scattering

The structural difference in the microporous structures of nanoporous carbon films is revealed by small-angle X-ray scattering; it consists in a higher porosity of the layers formed from the titanium carbide. The pore shape is shown to be equiaxed. Pores 20 Å in diameter mainly contribute to the porosity of the nanoporous carbon films. The characteristics of the porous structure of the nanoporous carbon layers synthesized from the titanium or silicon carbide are found using small-angle X-ray scattering. The porous structure is shown to consist of two size fractions of equiaxed pores 10 and 40 Å in radius. The porosity of the films is mainly contributed by the pores of the small size fraction; their fraction is 46 or 10% for the layers synthesized from the titanium or silicon carbide, respectively.     

Investigation of the structure and properties of model membranes of the stratum corneum by small-angle neutron scattering

The structure and properties of a quaternary membrane based on ceramide 6 in unilamellar vesicles have been investigated using small-angle neutron scattering at temperatures of 32 and 60 °C. Calculations performed in the framework of the separated-form-factor method have demonstrated that the nanostructure of a bilayer formed by unilamellar vesicles in an excess of water differs from the nanostructure of a partially hydrated planar bilayer. The calculated neutron scattering spectrum in the range of small scattering angles deviates from the experimental neutron scattering spectrum. This difference between the calculated curve and the experimental spectrum suggests a strong short-range interaction of vesicles with each other and the formation of cluster structures, which confirms the occurrence of chain-flip transitions.     

Evolution of the structure factor in a hyperbolic model of spinodal decomposition

We consider the modification of the Cahn-Hilliard equation when a time delay process through a memory function is taken into account. The memory effects are seen to affect the dynamics of phase transition at short times. The process of fast spinodal decomposition associated with a conserved order parameter - concentration is studied numerically. Details of a semi-implicit numerical scheme used to simulate the kinetics of spinodal decomposition and evolution of the structure factor are discussed. Analysis of the modeled structure factor predicted by a hyperbolic model of spinodal decomposition is presented in comparison with the parabolic model of Cahn and Hilliard. It is shown that during initial periods of decomposition the structure factor exhibits wave behavior. Analytical treatments explain such behavior by existence of damped oscillations in structure factor at earliest stages of phase separation and at large values of the wave-number. These oscillations disappear gradually in time and the hyperbolic evolution approaches the pure dissipative parabolic evolution of spinodal decomposition.     

On the structural features of influenza A nucleoprotein particles from small-angle X-ray scattering data

The structure of ribonucleic particles of influenza A virus of the A/California/07/09pdm strain is investigated by transmission electron microscopy and small-angle X-ray scattering. The small-angle X-ray scattering data obtained at room temperature correspond to previously reported data of ribonucleic particles of this virus. At higher temperatures, noticeable changes in the morphology of ribonucleic complexes are observed.     

Analysis of the structure of aqueous ferrofluids by the small-angle neutron scattering method

The structures of several aqueous magnetic fluids stabilized by different combinations of surfactants have been compared using small-angle neutron scattering. The size distribution functions of colloidal particles in water have been determined. The degree of clustering of magnetic nanoparticles has been obtained from comparison with electron microscopy data. The combinations of surfactants that lead to a minimum clustering have been revealed.     

Lyotropic model membrane structures of hydrated DPPC: DSC and small-angle X-ray scattering studies of phase transitions in the presence of membranotropic agents

Multibilayer structures of hydrated phospholipids, often considered as model biological membranes, are, from the physical viewpoint, lyotropic liquid crystalline systems undergoing temperature-induced mesomorphic phase transitions. Effects of silver nitrate and urocanic acid on lyotropic phase states of hydrated L-α-dipalmitoylphosphatidylcholine (DPPC) have been studied by small-angle X-ray scattering and differential scanning calorimetry (DSC). Both methods show increase of the main phase-transition temperature (T_m) of hydrated DPPC upon introduction of AgNO₃ or urocanic acid, decrease of pre-transition temperature (T_p) in the presence of urocanic acid and its increase in the presence of AgNO₃. Thus, urocanic acid widened the ripple-phase temperature region. Silver nitrate caused the appearance of an additional high-temperature peak on DSC thermograms, evidencing phase separation in the system. Both agents caused minor effects on DPPC lipid bilayer repeat distance (D) in gel phase, but resulted in noticeable increase of D in the liquid crystal phase with temperature as compared to undoped DPPC structures.     

A 125Te and 23Na NMR investigation of the structure and crystallisation of sodium tellurite glasses

125Te static nuclear magnetic resonance (NMR) and 23Na and 125Te magic angle spinning (MAS) NMR have been used, in conjunction with X-ray diffraction, to examine the structure and crystallisation behaviour of glasses of composition xNa2O.(1-x)TeO2 (0.075 x 0.4). The MAS NMR 23Na spectra from the glasses are broad and featureless but shift by approximately +5 ppm with increased x, i.e. as the system becomes more ionic. The static 125Te NMR spectra show an increase in axial symmetry with increasing x, indicating a shift from predominantly [TeO4] to [TeO3] structural units. The 23Na and 125Te spectra from the crystallised samples have been fitted to obtain information on the sites in the metastable crystal phases, which are the first to form on heating and which are therefore more closely related to the glass structure than thermodynamically stable crystal phases. New sodium tellurite phases are reported, including a sodium stabilised, face centred cubic phase related to delta-TeO2; a metastable form of Na2Te4O9 containing 3 sodium and 4 tellurium sites; and a metastable form of Na2Te2O5 containing 2 sodium sites. There is evidence of oxidation of TeIV to TeVI occurring in glasses with high values of x and, at x=0.40 and 0.50 (outside the glass forming range), some sodium metatellurate (Na2TeO4) is formed at the same time as sodium metatellurite (Na2TeO3). The 125Te shift is very sensitive to environment within the sodium tellurite system, covering more than 320 ppm, with anisotropies varying from 640 to 1540 ppm. The lack of features in the 125Te spectra of the glass phases, combined with the large shift range and high but variable anisotropy, means than it is not possible to obtain a unique fit to any presumed species present. Furthermore, the chemical shift anisotropy parameters for three of the four Te sites in the Na2Te4O9 phase are found to lie outside the range used for previous simulations of glass spectra.     

Analysis of the spatial structure of rigid polyphenylene dendrimers by small-angle neutron scattering

The analysis of the spatial structure of a rigid polyphenylene dendrimer G4-M of fourth generation by small-angle neutron scattering (SANS) is presented. This dendrimer is composed of phenyl units and is therefore devoid of any flexible unit. The scattering intensity of dilute solutions of the dendrimer was measured by SANS at different contrast which was adjusted by mixtures of protonated and deuterated toluene. Hence, the method of contrast variation could be applied and the data yield the scattering function extrapolated to infinite contrast. The comparison of this data with simulations demonstrates that the scaffold of the dendrimer is rigid as expected from its chemical structure. The positions of the various units setting up consecutive shells of the dendrimer are relatively well localized and the entire structure cannot be modeled in terms of spherically symmetric models. No backfolding of the terminal groups can occur and the model calculations demonstrate that higher generations of this dendritic scaffold must exhibit a dense shell and a congestion of the terminal groups. This finding is directly corroborated by recent solid-state NMR data. All results show that the rigid dendrimer investigated here presents the first example for a dendritic structure whose segment density does not have its maximum at the center. Rigid scaffolds are therefore the only way to achieve the goal of a “dense-shell” dendrimer whereas flexible scaffolds leads invariably to the “dense-core” case.     

Comparative analysis of the nucleosome structure of cell nuclei by small-angle neutron scattering

The nucleosome structure in native nuclei of normal (chicken erythrocyte and rat leukocyte nuclei) and anomalously proliferating (the human cervical adenocarcinoma cell line HeLa and the Chinese hamster fibroblast cell line A238) cells has been investigated using small-angle neutron scattering. The experimental results obtained allow one to make the inference that the parameters of the nucleosome structure for the chicken erythrocyte and rat leukocyte nuclei (on average over the nucleus) are close to the universally accepted values and that the distance distribution function is bimodal. The bimodality of the distance distribution function reflects a narrow distribution of distances between nucleosomes (on average over the nucleus) at the fibril level of the chromatin organization. The histone core of the nucleosome structure in the nuclei of the HeLa and A238 cells (on average over the nucleus) is considerably less compact than that in the chicken erythrocyte and rat leukocyte nuclei, and the distance distribution function does not exhibit indications of the bimodality.     

A comparison of ion scattering spectra and molecular dynamics simulations at the surface of silicate glasses

Molecular dynamics simulations are correlated with experimental ion scattering spectra to elucidate the surface structure and composition of fused silica and potassium trisilicate glass. The ion scattering spectra and molecular dynamics simulations both show that the oxygen atoms dominate the surface monolayer of fused silica. The ion scattering spectra of fracture surfaces of potassium trisilicate glass show a large potassium signal with little scattering signal from the oxygen or silicon atoms indicating a predominance of potassium in the surface monolayer. This local enrichment of potassium in the surface monolayer is due to their shielding of the charged silicate tetrahedra at the surface. This is also consistent with the simulations.