On the structure of glassy carbon

Glassy carbons made by heating a phenolic resin to temperature ranging from 500 to 1000°C were studied by X-ray diffraction and small angle X-ray scattering. Crystallite size, interlayer spacing and submicroscopic void size show rapid variation for heat treatment temperatures near 700°C. The variations in these structural parameters may be explained with the model proposed by Jenkins et al., i.e. an initial amorphous structure collapsing at higher temperatures to tangled graphitic ribbons.     

Ge-As-Te玻璃结构的X射线衍射和RMC模拟研究

本文通过Ｘ射线衍射分析和逆蒙特卡洛计算机模拟,得到Ｇｅ１０Ａｓ１８Ｔｅ７２玻璃中,Ｇｅ,Ａｓ原子的平均本位数分别为３．５５,３,平均每个Ｔｅ与０．８个Ｔｅ原子配位键;玻璃网络结构为变形的「ＧｅＴｅ４」四面体和「ＡｓＴｅ３」,「ＧｅＴｅ３」三角锥通过－Ａｓ（Ｇｅ）－Ｔｅ－Ａｓ（Ｇｅ）－链连接起来的     

Medium range order in high surface area amorphous silicas

Medium-range order (MRO) in amorphous (a-) SiO₂ can be defined as the degree of orientational correlation between neighbouring SiO_4/2 tetrahedra. Analysis of X-ray diffraction data on high surface area a-silicas shows that the MRO can be characterized quantitatively by means of (a) the first peak position in the interference function and (b) the ratio between O-O(II) and Si-O(II) coordination heights in the pair distribution function. These two quantities are physically related.     

Structural characterization of the ac conductivity in Ag ion conducting glasses

The material trends in various silver ion conducting glasses have been studied recently by focusing on the relationship between the first sharp diffraction peak (FSDP) wave number Q, and the fitting parameters of the ac conductivity, precisely the ratio (log A)/n, where A and n represent the pre-exponential factor and the power law exponent of Jonscher’s law, respectively. In the present paper, a model for the FSDP wave number dependence of the ratio (log A)/n has been proposed and a good agreement has been found with the experiments. By using the concept of bond fluctuation in superionic conductors, the results have been successfully explained, leading to the conclusion that the universal aspect of the power law reflects the universal pattern of the potential barrier at intermediate length scales. The result reconfirms that the ion transport in glasses is intimately related with the FSDP wave number.     

Structural changes studies of a-Si:H films deposited by PECVD under different hydrogen dilutions using various experimental techniques

Plasma enhanced chemical vapour deposition (PECVD) has been used to prepare hydrogenated amorphous silicon (a-Si:H) thin films at different hydrogen dilution of silane source gas. The films were deposited on Corning glass 1737 substrate and on (100) oriented c-Si wafers and characterized by XRD diffraction, micro-Raman and FTIR spectrometry. Experimental data show evolution from amorphous to nanocrystalline silicon and contain the medium-range order (MRO) with varying hydrogen dilution during deposition. From X-ray diffraction and Raman analysis, it is found that the presence of crystalline phase depends on the kind of substrate and on the dilution scale.     

A new approach to understanding the mechanisms of diffraction imaging of dislocations in X-ray topography

Regularities in the formation of diffraction imaging of dislocations are studied by numerical modeling and experimental section X-ray topography. The study of X-ray scattering by irregularities of the crystal lattice is of interest for several reasons. First, the contrast of defects is connected with the fundamental problem of development of a dynamic theory of X-ray scattering in real crystals. Second, knowledge of the special features of diffraction makes possible a qualitative and, in some cases, quantitative analysis of the X-ray diffraction contrast of defects in a crystal lattice (measurements of the deformation, determination of the sign and parameters of the Burgers vector, etc.).     

Effect of the vertical correlation of quantum dots on diffuse X-ray scattering

The effect of the vertical correlation of quantum dots (QDs) on diffuse X-ray scattering from multilayer structures has been studied in the framework of a statistical theory of X-ray diffraction. A model of the long-range structural order for the spatial distribution of vertically stacked QDs is considered. Diffuse X-ray scattering in superlattices with QDs has been numerically simulated. The obtained results are applied to a quantitative analysis of heterostructures with QDs using experimental data of high-resolution X-ray diffractometry.     

Microstructural study of as-extruded and heat-treated ribbons of poly(p-phenylene benzobisthiazole)

Ribbons of poly(p-phenylene benzobisthiazole) (PBT) prepared by extrusion from methane sulphonic acid solution into acid-water coagulation baths, were examined by electron microscopy, wide-angle X-ray scattering and small angle X-ray scattering. Electron diffraction patterns and corresponding dark-field images of as-extruded ribbons suggest that the chains are well oriented in the extrusion direction, with laterally-ordered regions averaging 2 nm in width. Upon heat treatment, the equatorial reflections sharpen and dark-field imaging indicates an increase in lateral order to about 15 nm. Small-angle X-ray scattering patterns show very little scattered intensity except for strong equatorial scattering attributed to voids elongated parallel to the extrusion direction. The absence of distinct crystalline images in 0 0l dark-field is supporting evidence for axial shift of the molecules along the chain axis, as suggested by previous diffraction studies.     

X-Ray scattering on a superlattice with quantum dots

A statistical theory of X-ray diffraction on a semiconductor superlattice (SL) with quantum dots (QDs) is developed within the framework of the kinematic approximation. The proposed theory describes in a common approach the coherent and diffuse scattering with allowance for the spatial (vertical and lateral) correlations of QDs. The X-ray diffraction on an SL is numerically modeled taking into account fluctuations in the QD size. It is established that the lateral (short-range) correlations of QDs influence the angular distribution of the scattering intensity. Calculated reciprocal-space maps of the scattering intensity distribution are compared to the experimental patterns.     

Characterization of microvoids in polyacrylonitrile-based carbon fibres

The microvoids in PAN-based carbon fibres covering a wide range of crystallite size were measured by the method using small-angle X-ray scattering on fibre bundles, and the fractional content of voids and the parameters representing the cross-sectional size of voids perpendicular to the fibre axis were determined. The variation in the shape and size distribution of voids with crystallite thickness was considered by introducing an elliptical crosssection model and a cross-sectional size distribution model. The electron density distribution in the inside of a void was also considered. It is concluded that the electron density difference between a void and the solid surrounding the void is relatively larger in the periphery than in the inner part of a void, and that the electron density in the inner part of a void decreases with increasing crystallite thickness.     

Medium-range order dictates local hardness in bulk metallic glasses

Bulk metallic glasses (BMGs) are materials with outstanding strength and elastic properties that make them tantalizing for engineering applications, yet our poor understanding of how their amorphous atomic arrangements control their broader mechanical properties (hardness, wear, fracture, etc.) impedes our ability to apply materials science principles in their design. In this work, we uncover the hierarchical structure that exists in BMGs across the nano- to microscale by using nanobeam electron diffraction experiments. Our findings reveal that local hardness of microscale domains decreases with increasing size and volume fraction of atomic clusters with higher local medium range order (MRO). Furthermore, we propose a model of ductile phase softening that will enable the future design of BMGs by tuning the MRO size and distribution in the nanostructure.     

In-situ synchrotron X-ray scattering study of thin film growth by atomic layer deposition

We report an atomic layer deposition chamber for in-situ synchrotron X-ray scattering study of thin film growth. The chamber was designed for combined synchrotron X-ray reflectivity and two-dimensional grazing-incidence X-ray diffraction measurement to do a in-situ monitoring of ALD growth. We demonstrate ruthenium thermal ALD growth for the performance of the chamber. 10, 20, 30, 50, 70, 100, 150 and 250-cycled states are measured by X-ray scattering methods during ALD growth process. Growth rate is calculated from thickness values and the surface roughness of each state is estimated by X-ray reflectivity analysis. The crystal structure of initial growth state is observed by Grazing-incidence X-ray diffraction. These results indicate that in-situ X-ray scattering method is a promising analysis technique to investigate the initial physical morphology of ALD films.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Cubic CdS thin films studied by spectroscopic ellipsometry

CdS polycrystalline films were grown onto glass substrates by chemical bath deposition (CBD) and characterized by spectroellipsometry, X-ray diffraction and transmission electron microscopy. The X-ray diffraction patterns of the samples showed the presence of a CdS cubic phase (β-CdS) and of Cd2SiO4 as interfacial material. Using electron diffraction it was possible to index the films as cubic CdS. From effective dielectric function measurements and from reported optical data for the dielectric function of cubic CdS crystals, grown by vapour phase epitaxy, it was possible to fit the experimental data to an effective medium approximation, and to deduce the film thickness, the void fraction and the field screening. This revised version was published online in August 2006 with corrections to the Cover Date.     

Model of the acoustic microscope response to scattering by a near-surface void

The response of the scanning acoustic microscope to scattering by a near-surface void is formulated rigorously. The transmission and reception responses of the acoustic microscope transducer-lens assembly are represented as Fourier spatial frequency spectra. Likewise, the interaction of the focused wavefield with the specimen fluid-solid interface is formulated via Fourier integrals. The scattering of the focused beam by a void positioned within the solid near the fluid-solid interface is formulated as a boundary integral equation employing the Green function for contacting fluid-solid half-spaces. The problem is evaluated numerically to demonstrate the scattering response dependence on void depth, transducer defocus, and void diameter.     

SAXS experiments on voids in gel-spun polyethylene fibres

The morphology and properties of extracted gel-spun polyethylene fibres depend on the spinning conditions. The main structures in the extracted fibre are shish-kebabs and lamellae. Equatorial small-angle X-ray scattering (SAXS) experiments show that the former structure is very porous due to the presence of lamellar overgrowth preventing a close package of the backbone fibrils, whereas the latter structure is relatively dense. After hot-drawing, due to melting/recrystallization, both structures are transformed on a 100 nm scale into a dense structure consisting of shish-kebabs or fibrils containing a void volume fraction of about 1%, as revealed by the scattering power of equatorial SAXS experiments. Moreover, a slight decrease of the equatorial intensity especially at the smallest angles after treating the hot-drawn fibres with paraffin oil, points to a small contribution of multiple scattering to the equatorial scattering. This implies the presence of a superstructure of not too closely packed macrofibrils. SAXS measurements of strained ultra-high-strength polyethylene fibres show that no or very little void formation is involved in the fracture mechanism. Most probably this is due to the (partly) fibrillar structure.     

The effect of periodic bending on the X-ray beam channeling

The X-ray radiation passage through randomly arranged narrow (submicron) channels in a solid was studied by the method of numerical modeling taking into account the diffraction and reduced coherency effects. It was found that the X-ray transmission is markedly deteriorated in the channels subject to periodic deformation. The randomization effects are interpreted within the framework of the statistical theory of X-ray scattering in a rough transitional layer. An anomaly in the energy dependence of the X-ray transmission through a thin (1620 Å) Cr/C/Cr channel observed for a 17 keV mode is explained by small periodic bending perturbations and related interferences in the structure.     

The effect of fluctuations in the lateral period of a multilayer grating on the scattering of hard synchrotron radiation

The effect of fluctuations in the lateral period of a nonideal multilayer grating on the scattering of hard synchrotron radiation (X-ray frequency range) was studied. Maps of the angular distribution of the coherent and diffuse scattering intensity in the reciprocal space are constructed. Theoretical diffraction curves calculated for a Ni/C multilayer grating are compared to the experimental data obtained by high-resolution triple-axis diffractometry. It is shown that good agreement between theory and experiment is observed for a fluctuation dispersion within 3% of the average lateral period of the grating.