Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

The preparation and intra- and intermolecular addition reactions of acyclic N-acylimines: application to the synthesis of (+/-)-sertraline

Intramolecular endo-cyclization reactions of N-acyliminium ions have seen wide application for the synthesis of heterocyclic compounds. The corresponding exocyclic variant, which would provide 1-aminotetralin derivatives, for example, has little precedent. We have discovered that acyclic N-acylcarbamates can be readily reduced to the corresponding N-acylhemiaminal derivatives in high yield using DIBAL as the reducing agent. These intermediates are remarkably stable and, if desired, can be purified and stored. The acyclic N-acylhemiaminals undergo both intra- and intermolecular nucleophilic addition reactions mediated by strong Lewis acids, such as TiCl(4). Diastereoselectivity, induced either by a substituent on the newly formed ring, or by utilizing a chiral ester on the carbamic acid, was disappointingly low. This methodology was successfully applied to the synthesis of the racemic form of the marketed antidepressant sertraline.     

Optimal acoustic fields in compact thermoacoustic refrigerators

Thermoacoustic refrigerators have been developed during the last 15 years, employing quasi-standing resonant acoustic waves inside fluid-filled cavities to transfer heat along a stack region. Because higher efficiency can be reached when a significant travelling wave component exists in the resonator, specific resonant thermoacoustic devices have been designed allowing to adjust more or less the ratio of travelling and standing wave components. However, the acoustic pressure field and the particle velocity field do not appear to be the optimal ones, for the thermal quantities of interest. Thus, it is the aim of the paper to present a new kind of thermoacoustic standing wave-like device which allows to control easily and independently the pressure field and the particle velocity field, after investigating the optimal acoustic field, in the stack region, for the main parameters of interest, i.e. the temperature gradient, the thermoacoustic heat flow and the coefficient of performance.     

Experimental investigation and analytical description of a vibro-impact NES coupled to a single-degree-of-freedom linear oscillator harmonically forced

In this paper, the dynamics of a system composed of a harmonically forced single-degree-of-freedom linear oscillator coupled to a vibro-impact nonlinear energy sink (VI-NES) is experimentally investigated. The mass ratio between the VI-NES and the primary system is about \(1\%\). Depending on the external force’s amplitude and frequency, either a strongly modulated response (SMR) or a constant amplitude response (CAR) is observed. In both cases, an irreversible transfer of energy occurs from the linear oscillator toward the VI-NES: process known in the literature as passive targeted energy transfer. Furthermore, the problem is analytically studied by using the method of multiple scales. The obtained slow invariant manifold shows the existence of a stable and of an unstable branch of solutions, as well as of an energy threshold (a saddle-node bifurcation) for the solutions to appear. Subsequently, the fixed points of the problem are calculated. When a stable fixed point is reached, the system is naturally drawn to it and a CAR is established, whereas when no stable point is attained, the system exhibits a SMR regime. Finally, a good correlation between the experimental and the analytical results is presented.     

Slow electron velocity-map imaging spectroscopy of the C4H- and C4D- anions

High resolution photodetachment spectra of C4H- and C4D- obtained via slow electron velocity-map imaging (SEVI) are presented. The spectra reveal closely spaced transitions to the neutral 2Sigma+ and 2Pi states which can be distinguished based on the corresponding photoelectron angular distributions. The C4H ground state is confirmed as the X2Sigma+ state, with the excited A2Pi state lying only 213 cm(-1) higher (201 cm(-1) for C4D). The electron affinities (EAs) are slightly revised to EA (C4H)=28,497+/-8 cm(-1) and EA (C4D)=28,478+/-10 cm(-1). Progressions in low frequency bending vibrations are observed in both states, yielding experimental frequencies of nu7=179(169) cm(-1) and nu6=408(392) cm(-1) for the X2Sigma+ state of C4H (C4D), and nu7=220(215)cm(-1) and nu6=446(437) cm(-1) for the A2Pi state.     

Brittle Fracture of Aerogels: Stress Corrosion in Alcoholic Environment

The mechanical behaviour of silica aerogels in alcoholic environment has been interpreted in terms of stress corrosion in analogy with silica glass. The chemical susceptibility factor has been determined by the dynamical method and we also measured the Weibull’s modulus which characterizes the strength distribution. These data show a stress corrosion effect which is significant in alcoholic atmosphere. The results could explain a possible fracture of gels during the supercritical drying treatment as already observed.     

Straightforward preparation of polymers based on oligodimethylsiloxane and alkylamine

Novel oligodimethylsiloxane‐based polymers with alkyl side chain were synthesized in bulk by step‐growth polymerization between α,ω‐glycidyl ether oligodimethylsiloxanes and a monoalkylamine in the absence of catalyst and at temperatures ranging between 80 and 180 °C. Matrix assisted laser desorption ionization time of flight results attested for the high reactivity of the amine functions with the glycidyl groups and revealed that the main polymer structure was (A₂B₂)_n type with alkyl moieties as dangling chains. No etherification was observed during the reaction even at high temperatures and the nature of the end groups strongly depended on the molar ratio between glycidyl and amine functions. Polymerization reactions were followed by ¹H NMR and the kinetics of the glycidyl‐amine reaction pointed out the dependence of temperature, molar ratio, and the molar mass of the oligodimethylsiloxane. High conversion rates were obtained, especially with the lowest molecular weight oligodimethylsiloxane. An optimized kinetic model derived from the Horie's model was discussed and permitted to correctly fit the experimental data. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Tetrastrontium dimanganese copper nonaoxide,Sr4Mn2CuO9

Single crystals of Sr₄Mn_2.09Cu_0.91O₉ have been grown by flux synthesis and the structure, closely related to the hexagonal perovskite 2H, was solved from single‐crystal X‐ray data in space group P321. The structure of Sr₄Mn₂CuO₉ is composed of chains of face‐sharing polyhedra with a sequence of two octahedra and one trigonal prism. The octahedra are filled by Mn atoms and the Cu atoms are randomly distributed at the centres of the square faces of the trigonal prism. A stacking fault is observed within one of the two chains, which can be attributed to a shifting of the chain along the c axis.     

Surface modification of several carbon-based materials: comparison between CF4 rf plasma and direct F2-gas fluorination routes

Due to their extreme reactivity, fluorine and fluorinated gases may be used to modify the surface properties of numerous materials. In the following, the surface fluorination of some carbon-based compounds (graphite, graphitised carbon fibres, carbon blacks and elastomers) using CF₄ rf plasma technique and direct F₂-gas fluorination is proposed. From XPS studies, the different types of CF bonding obtained in the materials after treatment have been correlated either to the physico-chemical characteristics of the pristine material or to the experimental parameters of the fluorination. Reaction mechanisms are proposed.