J. Pelous

Brillouin and Raman scattering study of borate glasses

Abstract We report light scattering results obtained from Brillouin and Raman scattering experiments in a series of B 2 O 3 − x M 2 O glasses (M  Li, Na, K, Rb, Cs and Tl). Raman lines related to the internal vibrations of structural units give information on the composition and concentration dependence of the short range arrangement in the glasses. The “Boson peak” is analysed on the basis of the Martin-Brenig model. The short correlation range is lower than 10 A and increases with the cation radius. From these experiments the very low frequency light scattering (Light Scattering Excess) is extracted. The temperature dependence of the LSE intensity in pure B 2 O 3 glass as well as its variation with composition at room temperature are studied. The Theodorakopoulos-Jackle theory relates this intensity to the acoustic attenuation deduced here from Brillouin linewidth measurements. While a theoretical description of the relation between the temperature variation of both quantities agrees well with experimental results, disagreement is apparent in their composition dependence. Finally, the sound velocity plotted versus concentration gives evidence of the contradictory influence of the hardening due to the change of the boron atom coordination number and the softening related to the distortion of the network by large cations.

Acoustic properties and potential applications of silica aerogels

Abstract Acoustic properties of cylindrical silica aerogels in both ultrasonic and audible range are presented. Velocity measurements for low ultrasonic frequencies show that the low-density aerogels can exhibit unexpected attenuation for well-defined frequency bands. Measurements of the acoustical impedance of samples in the audible range show that the results depend dramatically on the geometry and/or the boundary conditions imposed to the samples. The ‘attenuation’ bands in which the samples present an unexpected high attenuation are related to the aerogel density. These particular results are discussed in two ways; first for application purposes and second in terms of a possible theoretical explanation. Neither the classical theory of propagation in a homogeneous material nor the Biot theory for porous materials can explain the results.

Fractal structure of base catalyzed and densified silica aerogels

Abstract Small angle neutron scattering results obtained on several series of silica aerogels are presented. It is shown that basic catalysis can produce materials that are fractal over at least two orders of magnitude in length. These have bulk densities lower than ∼ 0.02 g/cm3, corresponding to porosities of 99% or larger. In this case, the constituent particles, whose gyration radii are larger than 10 A, have a fractal surface that cannot be smoothened by oxidation. Densification at elevated temperatures mostly increases the particle size. The fractal structure is destroyed when the particle dimensions equal the correlation length at the gel-glass transformation.

Elastic properties of silica aerogels

We have measured the elastic properties of silica aerogels with densities in the range 0.1 to 0.4 g/cm 3 . The static Youngs modulus E was obtained from three-point flexion experiments, while Brillouin scattering allowed the determination of the elastic constants C 11 and C 44 , at hypersonic frequencies. The comparison of these frequency-dependent determinations shows that the samples exhibit large scale homogeneity down to a few μm. The density dependence of the Youngs modulus and elastic constants is shown to obey the same power dependence p α with α = ∼- 3.8. The ratio of the compressibility to the shear modulus is similar to that found in bulk vitreous silica. Percolation models are used for the discussion of these elastic behaviour.

Phonon anomalies in SrTiO3 in the quantum paraelectric regime

New inelastic neutron-scattering and Brillouin-scattering results are reported for pressure-oriented SrTiO, crystals at temperatures below the structural transition. The measurements reveal an unexpectedly large number of anomalies at small frequencies and wave vectors in the quantum paraelectric region. The anomalies include a strong depression of a TA-branch at unusually small wave vectors, an apparent splitting of the same branch, the lifting of a selection rule for the structural mode, and an anomalous temperature dependence of the elastic constant C,, .

Anomalies in the TA-Phonon Branches of SrTiO3 in the Quantum Paraelectric Regime

Brillouin and neutron-scattering measurements of TA-phonon branches are reported for SrTiO3 at small wave vectors and at temperatures well below the 105 K structural transition. Anomalous softening in the former measurements, and dispersion in the latter, are found in the quantum paraelectric regime. Furthermore, an unexpected loss of scattered intensity is observed on the TA[100]-phonon branch at temperatures centred around 30 to 40 K.

Small-angle neutron scattering of aerogels: simulations and experiments

Abstract Numerical simulation of silica aerogels was performed using diffusion-limited cluster-cluster aggregation of spheres inside a cubic box (with periodic boundary conditions). The volume fraction, c , was taken to be sufficiently large to obtain a gel structure at the end of the process. In the case of monodisperse spheres, the wave-vector-dependent scattered intensity, I ( q ), was calculated from the product of the form factor, P ( q ), of a sphere by the structure factor, S ( q ), which is related to the Fourier transform of g ( r )-1, where g ( r ) is the pair correlation function between sphere centers. The structure factor, S ( q ), exhibits large- q damped oscillations characteristics of the short-range (intra-aggregate) correlations between spheres. These oscillations influence the I ( q ) curve in the region of q between the fractal regime and the Porod regime and quantitative comparisons were made with experiments on colloidal aerogels. At small values of q , S ( q ) goes through a maximum characteristic of large-range (inter-aggregate) correlations. Quantitative fits of the maximum in the experimental I ( q ) curves of base-catalyzed aerogels are presented. In the case of polydisperse spheres, I ( q ) was directly calculated from a single aggregate simulation. Increasing polydispersity shifts the location of the cross-over between the fractal and Porod regimes towards lower values of q .

The Density of Vibrational States of Silica Aerogels

Incoherent inelastic neutron spectroscopy combined with Brillouin scattering data are used to estimate the absolute density of states of fractal silica aerogels over the entire frequency range. The results indicate an extended fracton regime, and allow the thermal properties to be calculated in agreement with experiments.

Scaling Nonlinear Elasticity in Silica Aerogels

Brillouin scattering measurements have been performed on a series of fractal silica aerogels under applied uniaxial pressure. The entire nonlinear elastic behaviour scales with the aerogel density using the single scaling exponent that characterizes linear elasticity. Further, the anomalous nonlinear elasticity of the aerogels extrapolate to that of fused silica, strongly suggesting a common origin.

Investigation of Dynamical Scaling in a Mutually Self-Similar Series of Base-Catalyzed Aerogels

The phonon-fracton crossover in a series of fractal aerogels of well-defined microstructure is investigated for the first time. Results of small-angle neutron scattering combined with simulations establish the mutual self-similarity of the series and its fractal dimension D 1.7. Brillouin scattering is used to determine the fracton dimension related to tensorial elasticity, 1.1. Combined with Raman and neutron spectroscopic data, this allows to derive a consistent picture of dynamical scaling in relation with the structure of these materials.