A SAXS study of silica aerogels

Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porod's limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.     

A SAXS study of kinetics of aggregation of silica sols

SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.     

A SAXS study of the gelation process of silicon and titanium alkoxides

The gelation process of silicon ethoxide and titanium iso-propoxide solutions was studied as a function of water content and reaction time by small-angle X-ray scattering (SAXS). Approaching the gelation points, the SAXS intensities for titanium tetra-iso-propoxide solutions start to follow a power-law decay in the Porod region, except for a H₂O/Ti ratio greater than 4. For silicon ethoxide solutions, the fractal dimension, d_f, measured for aggregated clusters increases continually with the H₂O/Si ratio and can be related to the spinnability of the solutions. For solutions of both silicon and titanium alkoxides, a solution of fractal dimension d_f < 1.79 shows spinnability, whereas solutions having d_f > 1.79 and no fractal structures do not show spinnability.     

An EXAFS study of the local structure of Ce---Si---O amorphous thin films

An EXAFS study has been made on the structure of three composition ranges of Ce---Si---O amorphous thin films prepared by RF sputtering. The measurements, carried out on the K edge of silicon and the L₃ edge of cerium, reveal that in the stoichiometric oxygen films of the general formula (Ce, Si) O₂, both cerium and silicon are four-coordinated by oxygen regardless of the O : Si ratio. In the oxygen-deficient films cerium remains four-coordinated by oxygen, but, around silicon, the oxygen atoms are progressively replaced by silicon as the oxygen content of the films is reduced. In silicon-rich films which are very deficient in oxygen, the oxygen atoms prefer to ramain coordinated with cerium, rather than silicon.

A definite decrease in the Si---O distance with increase in Si---O coordination has been found. The effect is attributed to an increase in the charge of silicon with oxygen coordination, and supports a randomly bonded model for the structure.

The total oxygen coordination, derived from a consideration of bond conservation, indicates that the film structures are probably SiO₂-type continuous random networks.     

SAXS and DSC studies on the structural characteristics of siliconized s-triazine glassy hybrid materials

Three hybrid materials composed of planar s-triazine rings and polyhedral silica (SiO₂), phenylsilsesquioxane (PhSiO_1.5) and diphenylsiloxane (Ph₂SiO) building blocks were investigated by differential scanning calorimetry (DSC) and small angle X-ray scattering (SAXS) techniques. These measurements revealed that the geometrically dissimilar components were fully integrated into intact glassy hybrid structures. Their DSC thermograms showed that these hybrids are thermally stable below 350 °C with moderate glass transition temperatures (T_g) of 56–110 °C consistent with the increasing structural connectivity of the silicone component. The SAXS data was analyzed to obtain different structural information using Porod, Guinier and Kratky approximations. The general features of each of the SAXS profiles of these hybrids are very similar to those of polyphenylsilsesquioxane (PPhSQ). The SAXS profiles reveal that these hybrids can be described as nano-scale primary particles that are self-organized in macromolecular ensembles to form extended unfolded textures of varying scattering lengths (91–168 Å). The obtained hybrid particles adopt either 3-D bulk fractals with open structures or 2-D surface fractals with dense cores. The short interfacial thickness (< 3 Å) and the low thermal fluctuation parameters strongly suggest that these particles are held together by substantial cohesion forces.     

An X-ray diffraction study of the amorphous structure of chemically vapor-deposited silicon nitride

X-ray diffraction measurements have been carried out for two kinds of samples of CVD-amorphous silicon nitride with different values of the density. From radial distribution analysis, it became clear that the short range structure in these samples resembles that of the crystalline β-phase. Coloring in CVD-amorphous Si₃N₄ was found to be induced by X-ray irradiation. Based on the deficient numbers of the first nearest neighbours in SiN bondings, it was proposed that CVD-amorphous Si₃N₄ consists of small clusters including vacancies in their matrix.     

SAXS study of the micro-inhomogeneity of industrial soda lime silica glass

The microstructure of a variety of industrial multicomponent soda lime glasses for container and glazing and of a lead alkali silicate glass for pressed tableware articles was analyzed using small angle X-ray scattering (SAXS). No inhomogeneity with size >0.5 nm was found in the industrial samples, while ternary soda lime glass samples with a suitable composition when heat treated developed a phase separation which SAXS detected without difficulty. The results obtained disprove earlier reports which attributed problems of rheology and strength affecting container production (‘bad workability’) to microstructure formation.     

On the structure of amorphous germanium and silicon

The simple microcrystallite model is used to calculate the diffraction and radial distribution functions for a variety of tetrahedrally coordinated crystal structures: diamond, wurtzite, Ge III and Si III — two high-pressure polytypes of Ge and Si — and two clathrate structures based on pentagonal dodecahedral units. Comparison with data for sputtered amorphous Ge suggests that the simple microcrystallite model is inadequate to fit diffraction data. A statistical or combination microcrystallite model may be more promising. Recent electron microscopic investigations of Rudee and Howie are also discussed.     

Diffraction intensities and the structure of amorphous carbon

A model structure of amorphous carbon is investigated incorporating layered domains connected by means of a random network, the relative proportions of the two regions being a variable of the model. It is shown that for a certain relative proportion of the two regions there is good agreement between the predicted and experimentally observed electron and X-ray scattering intensities. The effects of covalent bonding are also studied and shown to be significant for small k.     

On the neighbours in a simulated amorphous structure

We consider a soft sphere model obtained by relaxation techniques to describe the amorphous structure. In a topologically disordered structure like that, it is not easy to define what one could call the neighbours of a given atom: first neighbours, second neighbours, etc. To do it, we first use a geometric criterion. But we show that it coincides with a metric definition in which only interatomic distances are used. In this manner, we can analyze the pair distribution function unambiguously and, in particular, we show that the first neighbour pairs thus defined represent that part of the radial distribution function which extends up to $\text{r = 2}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ (the unit distance corresponds to the position of the first peak). A second relaxed sample was studied. It was very much bigger, obtained with periodic conditions, from another initial state. The conclusions are very similar and confirm the reproducibility of states obtained by relaxation.     

TEM investigation on the structure of amorphous silicon monoxide

Commercially available powder samples of silicon monoxide have been investigated by methods of transmission electron microscopy: electron scattering, electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Pair distribution functions (PDFs) as well as EEL spectra can be shown to be a composition of the PDF and EEL spectra of elemental silicon and amorphous SiO₂. The distribution of the elements silicon and oxygen calculated from ESI images proof the silicon monoxide to be inhomogeneous, i.e. it consists of amorphous silicon and amorphous SiO₂. The phase separated regions measure ≈3–4 nm. One maximum in the PDF at 2.95 Å does not stem from either a-Si or a-SiO₂, and it is assigned to atomic configurations at the interphase boundary layer between Si and SiO₂. The portion of the interphase domain in the total composite material is estimated to be between 20% and 25%.     

The structure of nanovolumes of amorphous materials

In this paper we demonstrate reduced density function (G(r)) analysis from amorphous volumes as small as 2 nm in width. Obtaining the G(r) from a diffraction pattern is a common starting point in the characterisation of amorphous materials. The use of electrons to form the diffraction pattern allows very small volumes of amorphous material to be investigated.     

The network structure of amorphous silicon-carbon alloy

The network structure of amorphous silicon-carbon alloy (a-Si_1−xC_x) has been studied over a wide range of x. The a-Si_1−xC_x thin films were prepared by sputtering silicon and carbon target with argon in radio-frequency magnetron sputtering equipment. The films were characterized by X-ray photoelectron spectroscopy, optical absorption, infrared absorption, and mechanical measurements. The results showed that the network structure could be classified neither as the random covalent network nor as the chemically ordered covalent network. The structure as a whole was close to the random covalent network, but the Si-Si combination at x>0.5 showed a feature of the chemically ordered covalent network. The film at 0.6<x<0.8 was hard and showed a high energy gap, due to the sp³ configuration in Si-C combinations.     

The structure of tetrahedrally coordinated amorphous semiconductors

The diffracted X-ray intensities, F(k)'s, and radial distribution functions of Ge, some III–V compounds, and Ge_0.5Sn_0.5 are presented. It is shown that the radial distribution functions of the III–V compounds are similar to those of Ge. If the continuous random network, as developed for amorphous Ge, is also applicable to the III–V's, then several bonds between like atoms (wrong bonds) must be present. It is argued that the energy of wrong bonds may not be sufficiently high to prohibit their formation.     

The amorphous structure of immiscible Fe---Cu---Ag alloys

By means of facing target dc sputtering, an amorphous phase has been produced for immiscible Fe---Cu---Ag alloys at around the central concentration of the ternary phase diagram. The radial distribution function estimated from X-ray diffraction measurements indicates that a tetrahedron is the dominant structure unit and that the average coordination number is about 11. In the extended X-ray absorption fine structure radial structure function, a clear peak is detected at the nearest neighbor distance, but the peak intensity is weaker than those for binary crystalline Fe---Cu and Fe---Ag alloys and no clear peak is detectable at larger distances. A hard or soft sphere dense random packing model is appropriate for the amorphous structure in these immiscible alloys.