Detailed angle and energy resolved measurements of positrons ejected from the back of a gold target that was irradiated with an intense picosecond duration laser pulse reveal that the positrons are ejected in a collimated relativistic jet. The laser-positron energy conversion efficiency is ∼2×10{-4}. The jets have ∼20 degree angular divergence and the energy distributions are quasimonoenergetic with energy of 4 to 20 MeV and a beam temperature of ∼1  MeV. The sheath electric field on the surface of the target is shown to determine the positron energy. The positron angular and energy distribution is controlled by varying the sheath field, through the laser conditions and target geometry.

The properties of the eigenmodes of a capillary tube are examined in the context of ultrashort intense laser pulse guiding. The dispersion relation for the eigenmodes of a cylindrical hollow waveguide is derived and the family of eigenmodes EH(nus) is shown to be a solution of the wave equation up to the first order under the condition k(0)a >>1, where k(0) is the light wave number and a the capillary tube radius. The expressions of the fields for the eigenmodes are given at zero and first order of a small parameter equal to the ratio of the perpendicular to longitudinal wave number and the absorbed intensity at the wall is estimated.

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Poly(vinylidene difluoride) (PVDF) displays attractive ferroelectric/piezoelectric properties and its polar β-crystals are specifically targeted for achieving electroactive applications. However, their direct crystallization from the melt state represents a challenging task that has never been addressed using melt-state processes. The use of poly(methyl methacrylate) (PMMA) is herein investigated to promote the PVDF polar β-phase using melt-blending and extrusion-calendering technologies. The presence of the β-phase is here confirmed by ATR-FTIR and WAXS experiments with blends at a PMMA content as low as 5 wt%. The key role of PMMA for the β-phase crystallization from the melt state was unambiguously highlighted with the help of Flash DSC experiments in non-isothermal cooling mode from the melt state. PMMA is able to efficiently shift the α-to-β crystal transition to lower cooling rates (>100-200 °C s-1), making the achievement of the PVDF polar β-phase for these blends compatible with conventional processing tools. A crystal phase diagram is proposed for the PVDF/PMMA blends to highlight the dual effects of both PMMA and cooling rate on the PVDF crystallization during melt-processing. Ferroelectric properties were even observed for the blends containing PMMA up to 10 wt% with the highest remanent polarization obtained at 5 wt% PMMA. After 10 wt% PMMA, a progressive transition from ferroelectric to pseudo-linear dielectric behavior is observed more likely due to the presence of PMMA in the interlamellar amorphous phase of the polar PVDF spherulites as shown by SAXS experiments. In this work, we successfully demonstrated that PMMA plays a key role in the crystallization of PVDF polar crystals from the melt state, enabling large-scale and continuous extrusion processing of PVDF-based materials with attractive dielectric properties for sensing and harvesting applications.

Nanometer-sized BaTiO 3 powders have been synthesized hydrothermally from Ba(OH) 2 and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH) 2 prevents full incorporation of barium, and bariumdeficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K s (BaTiO 3 ,-25°C) = 7 × 10 −8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO 3 are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Roomtemperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.

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Metal-organic frameworks represent a class of porous materials which developed considerably over the past few years. Their porous structure makes them outperforming conventional adsorbents in hot topics such as dihydrogen...

Measurement of magnetic fields generated by a high-energy, high-intensity laser-driven millimeter-scale Helmholtz coil target is reported. The magnetic field is derived from a reverse current that passes through two coils of 1.25-mm radius. A peak field of 7T with a time decay of 17ns is inferred from a series of induction coil measurements when the hot-electron temperature driving the reverse current is approximately ∼15keV. A simple model of the laser-driven Helmholtz coil target suggests how the target should be optimized to produce high-peak fields and indicates that peak magnetic fields above 100T are accessible. This could lead to exciting experiments in laser-plasma physics, in particular, experiments related to laboratory astrophysics.