Modelling the thermo-mechanical volume change behaviour of compacted expansive clays

Compacted expansive clays are often considered as a possible buffer material in high-level deep radioactive waste disposals. After the installation of waste canisters, the engineered clay barriers are subjected to thermo-hydro-mechanical actions in the form of water infiltration from the geological barrier, heat dissipation from the radioactive waste canisters, and stresses generated by clay swelling under almost confined conditions. The aim of the present work is to develop a constitutive model that is able to describe the behaviour of compacted expansive clays under these coupled thermo-hydro-mechanical actions. The proposed model is based on two existing models: one for the hydro-mechanical behaviour of compacted expansive clays and another for the thermo-mechanical behaviour of saturated clays. The elaborated model has been validated using the thermo-hydro-mechanical test results on the compacted MX80 bentonite. Comparison between the model prediction and the experimental data show that this model is able to reproduce the main features of volume changes: heating at constant suction and pressure induces either expansion or contraction; the mean yield stress changes with variations of suction or temperature

VIBRATIONAL SPECTRA AND n-BODY DECOMPOSITION ANALYSESOF WATER CLUSTERS

The hydrated proton lies at the heart of several key charge transport processes in chemistry and biology, and yet the molecular level description of proton accommodation remains elusive. Both H3O+ (so called Eigen) and (H2O···H···OH2)+ (so called Zundel) have long been thought to play essential roles in the proton transfer process. We characterize the hydrated proton with a "bottom up" approach to monitor the spectral evolution of the proton accommodation motif as water molecules are sequentially added to the H3O+ ion. It is found that a highly symmetrical structure is necessary to observe the Eigen ion. Small asymmetries in the hydration structure around the H3O+ core result in preferential localization of the excess charge on one or two of the hydrogen atoms. This extreme response to symmetry breaking readily explains the lack of a crisp spectral signature of the hydrated proton in the bulk. Density functional theory is used to study the relative stability of various isomers of (H2O)n · H+, n = 4-12, allowing for the influence of vibrational zero point energy and finite temperature effects. Comparison of experimental spectra with and without Ar tagging shows that the inclusion of Ar atoms has little effect on the frequencies.Two low-energy minima of (H2O)21 with very different H-bonding arrangements have been investigated with the B3LYP density functional and RIMP2 methods, as well as with the TIP4P, Dang-Chang, AMOEBA, and TTM2-F force fields. Insight into the role of many-body polarization for establishing the relative stability of the two isomers is provided by an n-body decomposition of the energies calculated using the various theoretical methods

Observation of Quantum Capacitance of individual single walled carbon nanotubes

We report a measurement on quantum capacitance of individual semiconducting and small band gap SWNTs. The observed quantum capacitance is remarkably smaller than that originating from density of states and it implies a strong electron correlation in SWNTs