In situ studies of order–disorder phenomena in the synthesis of mesoporous silica

The initial hydrolysis of a silicon alkoxide in the presence of a suitable structural directing agent (template) so as to form a mesoporous silica powder exhibiting long-range hexagonal ordering was monitored using in situ XRD (X-ray diffraction), SAXS (small angle X-ray scattering) and SANS (small angle neutron scattering). The non-ionic triblock copolymer P123 (EO₂₀PO₆₉EO₂₀) was employed as the organic templating agent and tetramethoxysilane (TMOS) was used as the silica source in the presence of a water/acid catalyst. The synthesis method described herein is based around a high volume concentration ratio of surfactant to TMOS. The formation of a long-range mesoscopically ordered organic–inorganic hybrid that could be subsequently calcined to form a hexagonally structured mesoporous oxide material was monitored over 6 days using the characteristic (1 0 0) reflection. It was seen that during this ‘maturation’ period the reaction is not progressive and SANS and SAXS data together with XRD experiments show that there is an initial kinetically rapid organic ordering process which provides a template for the formation of an ordered metastable organic–inorganic oxide phase which then becomes progressively more disordered before a final kinetically slow stable long-range ordered phase is formed. Discussions of the origin of the unexpected order–disorder phenomena are made.     

In situ crystallization of lithium disilicate glass: Effect of pressure on crystal growth rate

Crystallization of a Li₂O · 2SiO₂ (LS₂) glass subjected to a uniform hydrostatic pressure of 4.5 and 6 GPa was investigated up to a temperature of 750 °C. The density of the compressed glass is ∼2% greater at 4.5 GPa than 1 atm and, depending on the processing temperature, up to 10% greater at 6 GPa. Crystal growth rates investigated as a function of temperature and pressure show that lithium disilicate crystal growth is an order of magnitude slower at 4.5 GPa than 1 atm resulting in a shift of +45 °C (±10 °C) in the growth rate curve at high pressure compared to 1 atm conditions. At 6 GPa lithium disilicate crystallization is suppressed entirely, while a new high pressure lithium metasilicate crystallizes at temperatures 95 °C (±10 °C) higher than those reported for lithium disilicate crystallization at 1 atm. The observed decrease in crystal growth rate with increasing pressure for the lithium disilicate glass up to 750 °C is attributed to an increase in viscosity with pressure associated with fundamental changes in glass structure accommodating densification.     

Elastic anomalous behavior of silica glass under high-pressure: In-situ Raman study

We study the changes in the Raman optical vibrations of pure silica glass under high-pressure up to 4.3 GPa and room temperature, namely in the elastic domain. Several mechanical anomalies, as the decrease of bulk modulus between 0 and 2 GPa, have been revealed many years ago (P.W. Bridgmann, Am. J. Sci 10 (1925) 359), but no physical experiments have explained what happens at the atomic scale. Our experiments show that gradual structural reversible rearrangement from 0 to 2 GPa leads to a more flexible material, in good agreement with molecular dynamics (MD) simulations (L. Huang, J. Kieffer, Phys. Rev. B 69 (2004) 224203). Above 2 GPa, a fast homogenization occurs.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5–25 K), C_p, and low-frequency Raman scattering (<150 cm⁻¹) has been performed in amorphous silica samples synthesized by sol–gel method (xerogels) and thermally densified in a range of densities, from ρ=1250 kgm⁻³ to ρ=2100 kgm⁻³, close to the density of the melt quenched vitreous silica (v-SiO₂). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO₂, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870°C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm⁻¹ and of a peak in C_p(T)/T³, very close to those observed in v-SiO₂; (ii) in silica xerogels treated at temperatures less than about 800°C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.     

Ab initio calculation of the structural and elastic properties,anisotropy, and hardness of nitrogen-doped diamond

The elastic properties of nitrogen-doped diamond are studied with the use of the density functional theory in the plane-wave basis, the electron density gradient approximation, and ultrasoft pseudopotentials. It is established that doping increases the lattice parameter and decreases the elastic constants, elasticity moduli, elastic wave velocities, and the anisotropy of elastic properties and hardness. It is shown that the hardness of the (111) face exceeds that of the (100) face in both pure and nitrogen-doped diamond.     

Sugar bioprotective effects on thermal denaturation of lysozyme: Insights from Raman scattering experiments and molecular dynamics simulation

The bioprotective properties of trehalose, sucrose and maltose on lysozyme protein are studied using Raman spectroscopy and molecular dynamics simulation. The temperature dependencies of the α-helical unfolding processes are determined in presence of different sugars and at various concentrations using the amide I band. The energies of stabilization and acting forces are discussed. The problem of the threshold sugar concentration needed to protect proteins is addressed. The results point out that the three sugars exhibit very similar bioprotective behaviors in the investigated temperature and concentration ranges and suggest that the difference between their respective effects arise mostly from the different denaturated states of lysozyme in the three sugar solutions. Indeed, trehalose gives rise to a significantly more important shift of the denaturation temperature T_m and of the Gibbs net free energy Δ(ΔG_ND) of stabilization of lysozyme with respect to the two other sugars.     

Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel

The Young’s moduli of a set of silica aerogels have been measured by two techniques: 3-point bending and uniaxial compression. The data found by the two methods differ strongly. The uniaxial compression test gives generally underestimated values of Young’s modulus, because of geometrical effects. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and nonuniform stress distribution. The measured compressive moduli were analyzed to correct for machine compliance and possible misalignment under compression of the aerogels. Similarly, moduli obtained by 3-point bending depend on the length/thickness ratio of the sample, reaching equilibrium only for ratios above about 10. The corrected compressive moduli were comparable to those measured by 3-point bending on samples of sufficient length.     

Ultrasonic and brillouin scattering study of the elastic properties of vitreous silica between 10 and 300 K

The velocity and attenuation of longitudinal and transverse waves have been measured for frequencies ranging between 5 MHz and 35 GHz on the same sample of vitreous silica. The main features of the ultrasonic results are described on the basis of the usual model assuming a coupling of phonons with a thermally activated relaxation. In contrast, the frequency dependence of the attenuation peak and velocity minimum are in quantitative disagreement with the calculations based on this model. Above 100 K, at least half of the attenuation at high frequencies has to be assigned to another physical origin, most likely anharmonic three-phonon interactions. Furthermore, the minimum in the hypersonic velocity seems to occur as the result of a combination of dispersion processes more than from the frequency-temperature shift of a relaxational dispersion.     

Defects in a mixed-habit Yakutian diamond: Studies by optical and cathodoluminescence microscopy,infrared absorption,Raman scattering and photoluminescence spectroscopy

Widespread occurrences in the crystallisation history of natural diamonds are epochs of mixed-habit growth in which normal {1 1 1}-faceted growth is accompanied by non-faceted growth on curved surfaces of mean orientation ∼{1 0 0}, termed ‘cuboid’. This paper analyses mixed-habit-related phenomena in a near-central, (1 1 0)-polished slice of an octahedron from the Mir pipe, previously studied principally by SIMS probes analysing N impurity content and C and N isotope composition. In the present work, newly studied features include dislocation content, fine structure in cathodoluminescence (CL) patterns, refined IR absorption data, Raman and photoluminescence (PL) microspectroscopy and microscopy of internal non-diamond bodies. Topographic imaging and spectroscopic techniques traced the specimen's morphological evolution from a cubo-octahedral core containing complex relative development of {1 1 1} and cuboid sectors, both populated by graphite crystallites, diameters up to ∼5 μm, lying on all diamond host {1 1 1}. Coherently overgrowing the core was a zone of widely but smoothly varying relative development of {1 1 1} and cuboid sectors, both on birefringence evidence dislocation-free, emitting strongly from cuboid sectors the PL spectra associated with Ni–N-vacancy complexes. An enclosing octahedral shell of solely {1 1 1} lamellae terminated mixed-habit growth. High-resolution FTIR absorption measurements of I(B′), the integrated absorption due to {1 0 0}-platelet defects, showed from its absence or weakness that total or substantial platelet degradation had taken place in the mixed-habit zones, indicating that these had undergone conditions close to the diamond–graphite phase boundary in their history.     

High frequency acoustic attenuation of vitreous silica: New insight from inelastic x-ray scattering

We present new experimental results on the propagation and damping of the high frequency acoustic-like modes in vitreous silica. The new data are measured by means of the inelastic x-ray scattering technique down to an exchanged wavevector Q ∼ 0.9 nm ^− 1, at the limit of the instrument capabilities. Thanks to the continuous development of the technique, the new spectra are characterized by a very high signal to noise ratio when compared to previous experiments. The higher data quality finally allows for a reliable determination of the position and width of the inelastic excitations. The new data show that the sound damping Γ is marked by a frequency dependence compatible with the Rayleigh law, Γ ∼ ν⁴, for frequencies below the position of the excess vibrational modes at the boson peak. We show that the new data are in good agreement with estimates of the acoustic mean free path from the thermal conductivity, which take into account the peculiar plateau at a few Kelvin. The connection between the boson peak and the Rayleigh law is further confirmed by a comparison of the present data with literature data for the sound attenuation in a permanently densified silica sample.     

Infrared and thermogravimetric study of high pressure consolidation in alkoxide silica gel powders

High density, transparent, crack-free and hard compacts of silica gel were produced by compaction under nearly hydrostatic environment at 4.5 GPa, at room temperature. The starting material was used three days after synthesis by hydrolysis of alkoxides without additional treatment. Fourier transform infrared spectroscopy (FTIR), using the KBr technique and a high vacuum cell at temperatures up to 450°C, and thermogravimetric analysis (TGA) up to 900°C was conducted. A reduction up to 60% in the adsorbed water content of the compacted silica gel was observed. Changes in the 3000 to 3800 cm⁻¹ region indicate that the surface silanol groups became bridged, which promotes condensation reactions of dehydroxylation. Those results suggest that the mechanism for consolidation under high-pressure, is ‘cold sintering’ process, where silanol groups at the surface of the nanoparticles condense to form siloxane bonds between the particles and water, resulting in a stiff body with closed nanopores containing trapped water.     

Orientation of spheroid-shaped silver nanoparticles in mesostructured silica films studied by polarized absorption and low-frequency Raman spectroscopy

Orientation of spheroid-shaped silver (Ag) nanoparticles (NPs) embedded in mesostructured silica sol gel films is studied by polarized absorption measurements and low-frequency Raman scattering, two complementary techniques well-adapted to characterize nanostructures in amorphous matrix. Comparison is made with analysis of mesostructured silica sol gel films containing nanospheres. Polarization-dependent absorption spectra clearly evidence spheroid-shaped NP orientation and their mean aspect ratio is estimated from corresponding polar plots. Mean diameter of spherical NPs is deduced from low-frequency Raman scattering spectra as well as size of elongated spheroid NPs.     

In situ and real-time analysis of TGZM phenomena by synchrotron X-ray radiography

The development of brilliant third-generation synchrotron X-ray sources, together with advances in X-ray optics and detectors, has provided timely efficient tools for in-depth understanding of physical phenomena in a broad spectrum of situations. Synchrotron X-ray radiography enables in situ and real-time observation of microstructure evolution, i.e. a direct access to dynamical phenomena which could not be anticipated from post-mortem analysis. Dedicated experiments are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) in Al-based alloys to study the dynamics of temperature gradient zone melting (TGZM) phenomenon. TGZM occurs when a liquid–solid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. The thorough characterisation of both the initial solid during the thermal stabilisation phase prior to solidification (static TGZM) in Al–3.5 wt% Ni alloy and the dendritic microstructure in the later stage of solidification in Al–7.0 wt% Si alloy is performed. Based on experimental observations, quantitative data (in particular liquid-migration velocity) are measured and a very good agreement is found with theoretical analysis.     

Rapid synthesis of amine cross-linked epoxy and methyl co-modified silica aerogels by ambient pressure drying

The silica aerogels were synthesized by sol–gel method via ambient pressure drying. Tetraethyl orthosilicate (TEOS) was used as a main silica source, methyltriethoxysilane (MTES) as a co-precursor silica source and (3-Glycidoxypropyl)trimethoxysilane (GPTMS) as a silane coupling agent. The silica aerogels obtained were further undergoing cross-linking epoxy from GPTMS with amine from diethylenetriamine (DETA) which played a dual role of base catalyst and reagent. The cumulative volumes for open pores of the cross-linked aerogels were evaluated to be 1.4 cm³/g. The Young's modulus and maximum compression strength were 25.4 MPa and 6.17 MPa, respectively. The addition of MTES accelerated the solvent exchange of alcohol within the pores with n-hexane and reduced the shrinkage of aerogels network during the ambient pressure drying. The formation of organic network enhanced the strength of the cross-linked aerogels to prevent the crack generation and the subsequent failure of the monolith during the ambient drying, therefore, protected the nanoporous structure of aerogels.     

In Situ Raman and Epr Investigations of A Cis-(Ch)X Electrode

Abstract

In situ Raman and EPR experiments have been performed on a cis-(CH)_x electrode in an electrochemical cell {(CH)_x/1M L1C10₄ in THF/Li}. After a n-type doping with Li, the Raman spectra exhibit new features at = 1600 cm^?1 and 1270 cm^?1, comparable to those obtained with [CH)_X films Li doped via a chemical treatment. The evolution of the EPR signal is followed in a doping-dedoping cycle, showing both the reversibility of the process as well as the evidence for a metallic behavior at the maximum doping concentration.     

In situ composites based on blends of PEEK and thermotropic liquid crystalline polymer

A series of blends were prepared by mixing pellets of the thermotropic liquid crystalline polymers (TLCP) with poly (ether ether ketone)(PEEK) in the dry state under injection mouldingmolding conditions. The mechanical properties of blends were measured, and it is proved that the mechanical properties of blends are better than those of their matrix. Compared with the thermoplastic matrix, the tensile modulus and strength of the blends increased obviously, with the addition of only 2 wt% LCP. At the same time, the addition of TLCP decreased the melt viscosity of the blends.     

In situ atomic force microscopy of layer-by-layer crystal growth and key growth concepts

Contradictions that have been found recently between the representations of classical theory and experiments on crystal growth from solutions are considered. Experimental data show that the density of kinks is low in many cases as a result of the low rate of their fluctuation generation, the Gibbs-Thomson law is not always applicable in these cases, and there is inconsistency with the Cabrera-Vermilyea model. The theory of growth of non-Kossel crystals, which is to be developed, is illustrated by the analysis of the experimental dependence of the growth rate on the solution stoichiometry.