We report picosecond acoustic measurements of longitudinal sound dispersion and attenuation in an amorphous SiO2 layer at temperatures from 20 to 300 K over frequencies ranging from about 40 to 200 GHz. The sample is a radio frequency cathodic sputtered silica layer grown on a sapphire substrate with an aluminum filmtransducer deposited on top. Acoustic attenuation is evaluated from the simultaneous analysis of three successive echoes using transfer matrix calculation. Results are found to follow rather well a model combining coupling to thermally activated relaxations of structural defects and interactions with thermal vibrations. This leads to a nontrivial variation of the attenuation coefficient with frequency and temperature. The number density of relaxing defects in the SiO2 layer is found to be slightly higher than that in bulk v-SiO2. In contrast, similar anharmonic contribution to acoustic absorption is observed in both systems. The velocity variations are also measured and are compared to the dynamical velocity changes deduced from the sound attenuation.

Wheat grains can be considered as a natural cemented granular material. They are milled under highforces to produce food products such as flour. The major part of the grain is the so-called starchy endosperm.It contains stiff starch granules, which show a multi-modal size distribution, and a softer protein matrix thatsurrounds the granules. Experimental milling studies and numerical simulations are going hand in hand tobetter understand the fragmentation behavior of this biological material and to improve milling performance.We present a numerical study of the effect of granule size distribution on the strength of such a cementedgranular material. Samples of bi-modal starch granule size distribution were created and submitted to uniaxialtension, using a peridynamics method. We show that, when compared to the effects of starch-protein interfaceadhesion and voids, the granule size distribution has a limited effect on the samples’ yield stress.

Polarized Raman scattering is performed in a series of sodo-silicate glasses. The Si–O–Si inter-tetrahedral angle and its distribution are extracted using a model developed previously for densified silica, and the result are compared to ab initio atomistic calculations. The two techniques reveal a reduction of the most probable angle of about 0.30°/mol% of Na2O, but the shape of the angular distributions are different. The results suggest that Raman scattering enhances a specific angular distribution of Si–O–Si bridges, likely those close to sodium atoms, highlighting local angular heterogeneities.

Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.