Fully-developed pipe and planar flows of multimode viscoelastic fluids

Two solutions are presented for fully-developed pipe and planar flows of multimode viscoelastic models. The fluids have a Newtonian solvent contribution and the polymer modes are described by the Phan-Thien—Tanner (PTT), the FENE-P or the Giesekus equation. The first solution is exact and can handle any number of modes, but is only semi-analytical. The second solution, which is presented only for the PTT model with a linear stress coefficient and the FENE-P model, can also handle any number of modes. It is based on a truncated series expansion and is completely analytical, but provides only an approximated solution. The complexity of the multimode solutions is investigated first with the exact semi-analytical method and it is shown that at high Deborah number flows the high-order stresses can become as important as the stress of the first mode. It is also under these conditions that the approximated analytical solution deviates from the exact semi-analytical solution. A criterion for the accurate use of the approximated solution is presented. Fortran codes are provided to obtain these solutions at the internet address at the end.     

Time-dependent Green's functions for an anisotropic bimaterial with viscous interface

By virtue of the Stroh formalism, we derive the exact closed-form solutions for the time-dependent two-dimensional Green's functions due to a line force and line dislocation in an anisotropic bimaterial with a viscous interface. We first reduce the boundary value problem to two coupled homogeneous first-order partial differential equations, which can be solved using a decoupling technique. The full-field expressions of the time-dependent displacements and stresses due to the line force and line dislocation interacting with the viscous interface are obtained.     

Influence of the laser parameters on the space and time characteristics of an aluminum laser-induced plasma

In this work, an aluminum laser plasma produced in ambient air at atmospheric pressure by laser pulses at a fluence of 10 J/cm² is characterized by time- and space-resolved measurements of electron density and temperature. Varying the laser pulse duration from 6 ns to 80 fs and the laser wavelength from ultraviolet to infrared only slightly influences the plasma properties. The temperature exhibits a slight decrease both at the plasma edge and close to the target surface. The electron density is found to be spatially homogeneous in the ablation plume during the first microsecond. Finally, the plasma expansion is in good agreement with the Sedov's model during the first 500 ns and it becomes subsonic, with respect to the velocity of sound in air, typically 1 μs after the plasma creation. The physical interpretation of the experimental results is also discussed to the light of a one-dimensional fluid model which provides a good qualitative agreement with measurements.     

Electronic states of chromium carbene ions characterized by high-resolution translational energy loss spectroscopy

The electronic states of the unsaturated organometallic carbene CrCH₂⁺ are investigated using high-resolution translational energy loss spectroscopy. The observed energy loss feature (1.05 ±0.2 eV) is in good agreement with theoretical calculations which predict two higher lying states, ⁶B₁, and ⁶A₁ at 0.78 and 0.82 eV respectively, above the ⁴B₁ ground state of CrCH₂⁺.     

Mott transition for picosecond all-optical nor gate in CdSe

Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical transmission of CdSe platelets induced by strong optical pumping. We investigate particularly the all-optical gate capabilities (resulting from the bandgap shrinkage due to the Mott transition) for laser beams the wavelength of which ranges from 676 to 678 nm. The electron-hole plasma density necessary to enable the optical switching is determined (ϱ ∼ 2 × 10¹⁷cm⁻³ at 20K). The switch-off time (i.e. the transparency recovery time) is also studied at different wavelengths by picosecond spectroscopy.     

On the nonlinear dielectric function of CuCl in pump and probe experiments

We study the dispersion and the absorption changes of CuCl under high excitation conditions in two-beam experiments with parallel and crossed linear polarizations of both beams. The induced optical non linearities are shown to depend sensitively on the presence of interactions between the degenerate transverse exciton states.     

Absorption spectra of NH4MnCl3 and NH4MnF3

The absorption spectra of NH₄MnCl₃ and NH₄MnF₃ crystals have been measured down to 10 K in the 250 to 600 nm region. The observed bands are assigned to electronic transitions from the ⁶A_1g(S) ground state to various excited levels of Mn²⁺ ions in an octahedral crystalline field. The position of the bands have been fitted within the strong crystal field scheme. Resulting parameters at room temperature are B=741, C=2990 and Dq=520 cm⁻¹ for NH₄MnCl₃ and B=800, C=3139 and Dq=694 cm⁻¹ for NH₄MnF₃. At low temperature some bands show a rich fine structure in which some phonon progressions have been identified.     

DC conductivity of ZnOV2O5 glasses

This work presents for the first time results regarding the DC conductivity of glasses belonging to ZnOV₂O₅ system. It is shown that the conduction can be described by a small polaron hopping model. The high temperature activation energy and conductivities are close to the values determined for V₂O₅P₂O₅ glasses of similar composition. The analysis of the pre-exponential factor shows strong evidence for a non-adiabatic conduction regime.This fact agrees with the conclusion drawn for the majority of vanadate glasses. The strong variation of α suggests the presence of fluctuations in the V⁵⁺ surroundings.     

Potential of amorphous materials as catalysts

Amorphous materials have an important role to play in heterogeneous catalysis. They have several properties which make them of interest both as model catalysts and as real catalysts.A very limited amount of work on their catalytic activities has been done. However, some very interesting results have shown either higher activities or selectivities for amorphous catalysts as compared to their crystalline counterparts. Amorphous metals will be most useful in those reactions having reducing environments such as hydrogenation and isomerization. Amorphous oxides and other nonmetals will be valuable in cracking, oxidation and hydrotreating reactions.Modelling catalysts that are ideal for application of modern surface probes is an exciting development in catalysis research. In amorphous model catalysts, it is extremely important to address the local geometric and electronic structure of active sites on these materials. This research area is ripe for exploitation. The route to the solutions of some long-standing catalytic questions may be through the innovative use of the model amorphous catalysts.This article suggests the potential of the amorphous materials as catalysts, reviews the progress being made and attempts to place amorphous catalysis in perspective with the main stream of catalysis research and development.