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The mechanical behavior of carbon aerogels is not very well understood, presenting a bottle-neck in synthesizing aerogels for specific applications where mechanical loads play a role. Accordingly, in this paper, three different types of carbon aerogels with varying mechanical properties and flexibility are synthesized and analyzed. The morphology is characterized by using a scanning electron microscope (SEM) and nitrogen adsorption-desorption isotherms. The mechanical behavior is investigated under uniaxial quasistatic compression as well as in-situ compression under a SEM. While in-situ tests reveal microstructural evolution under deformation, the macroscopic deformation is described by the digital image correlation. Based on cyclic compression tests with step-wise increasing strain amplitude, novel empirical relations are proposed to describe the damage characteristics such as energy dissipation and residual deformation. Furthermore, while testing carbon aerogels under tension is not very feasible, three-point bending tests are conducted and the resulting flexural properties of carbon aerogels are identified.

International audience; Silicon clathrates are unusual open-framework solids formed by tetrahedrally bonded silicon that show remarkable electronic and thermal properties. The type I structure has a primitive cubic unit cell containing cages occupied by metal atoms to give compositions such as Na(8)Si(46) and Na(2)Ba(6)Si(46). Although their structure and properties are well described, there is little understanding of the formation mechanism. Na(8)Si(46) is typically produced by metastable thermal decomposition under vacuum conditions from NaSi, itself an unusual structure containing Si(4)(4-) polyanions.,In this study, we used in situ synchrotron X-ray diffraction combined with rapid X-ray detection on samples taken through a controlled temperature ramp (25-500 degrees C at 8 degrees C/min) under vacuum conditions (10(-4) bar) to study the clathrate formation reaction. We also carried out complementary in situ high-temperature solid-state (23)Na NMR experiments using a sealed tube loaded under inert-gas-atmosphere conditions. We find no evidence for an intermediate amorphous phase during clathrate formation. Instead, we observe an unexpectedly high degree of structural coherency between the Na8Si46 clathrate and its NaSi precursor, evidenced by a smooth passage of several X-ray reflections from one structure into the other. The results indicate the possibility of an unusual, epitaxial-like, growth of the dathrate phase as Na atoms are removed from the NaSi precursor into the vacuum.

cited By 3; International audience; The in situ sintering of a powder of Pb3(VO4) 1.6(PO4)0.4 composition was performed in an Environmental Scanning Electron Microscope. The electric current induced by the electron beam was found to reduce the effective temperature of sintering as well as to accelerate the kinetics of shrinkage of a cluster composed of sub-micrometric grains of material. The presence of the residual current flow in the cluster during observation for in situ experiments helps to reduce the apparent sintering temperatures from 50 to 150 °C compared to conventional heating conditions without current. © 2011 Elsevier B.V. All rights reserved.

The polarization screening mechanism and ferroelectric phase stability of ultrathin BaTiO3 films exposed to water molecules is determined by first principles theory and in situ experiment. Surface crystallography data from electron diffraction combined with density functional theory calculations demonstrate that small water vapor exposures do not affect surface structure or polarization. Large exposures result in surface hydroxylation and rippling, formation of surface oxygen vacancies, and reversal of the polarization direction. Understanding interplay between ferroelectric phase stability, screening, and atomistic processes at surfaces is a key to control low-dimensional ferroelectricity. © 2009 American Chemical Society.

We have designed and constructed a temperature-controllable shear flow cell for in-situ study on flow alignable systems. The device has been tested in the neutron diffraction and has the potential to be applied in the small angle neutron scattering configuration to characterize the nanostructures of the materials under flow. The required sample amount is as small as 1 ml. The shear rate on the sample is controlled by the flow rate produced by an external pump and can potentially vary from 0.11 to 3.8 × 10(5) s(-1). Both unidirectional and oscillational flows are achievable by the setting of the pump. The instrument is validated by using a lipid bicellar mixture, which yields non-alignable nanodisc-like bicelles at low T and shear-alignable membranes at high T. Using the shear cell, the bicellar membranes can be aligned at 31 °C under the flow with a shear rate of 11.11 s(-1). Multiple high-order Bragg peaks are observed and the full width at half maximum of the "rocking curve" around the Bragg's condition is found to be 3.5°-4.1°. It is noteworthy that a portion of the membranes remains aligned even after the flow stops. Detailed and comprehensive intensity correction for the rocking curve has been derived based on the finite rectangular sample geometry and the absorption of the neutrons as a function of sample angle [See supplementary material at http://dx.doi.org/10.1063/1.4908165 for the detailed derivation of the absorption correction]. The device offers a new capability to study the conformational or orientational anisotropy of the solvated macromolecules or aggregates induced by the hydrodynamic interaction in a flow field.