Search results for " Threshold"
showing 10 items of 497 documents
A methodology for the rapid characterization of Mode II delamination fatigue threshold in FRP composites
2019
Abstract A new methodology to measure the Mode II interlaminar fracture in fatigue for FRP composites is developed. The proposed methodology uses a Modified Transvers Crack Tensile (MTCT) specimen and is able to characterize the near threshold behavior in a robust, easier and significantly faster way than standard procedures. Analytical formulae, able to link the crack growth rate to the load or strain amplitudes, were found and verified, and their importance was explained, in particular, for what concerns the characterisation of the near threshold behavior. Experiments were performed both in load and strain control, while the delamination growth was monitored using different techniques inc…
Microchip Random Laser based on a disordered TiO2-nanomembranes arrangement
2012
International audience; We developed a new scheme for obtaining coherent random lasing based on a chip consisting of a polymer film doped with Rhodamine 6G, having as scatterers butterfly-like TiO2 nanomembranes (TiO2-NM) supported on a glass substrate. The feedback mechanism for laser action is due to the multiple scattering of light by TiO2-NM rather than provided by localized variations of the refractive index in the polymer film. The above-threshold multiple spikes signature indicative of random laser emission with coherent feedback is confirmed. As nanomembranes are foreseen as new MEMS/NEMS building blocks, a new generation of combined active/passive photonic devices can be envisaged.
Electrical conductivity properties of expanded graphite-polycarbonatediol polyurethane composites
2014
Conductive polymer composites of segmented polycarbonatediol polyurethane and expanded graphite (EG) have been synthesized with different amounts of EG conductive filler (from 0 to 50 wt%). SEM, X-ray diffraction measurements, Fourier transform infrared and Raman spectroscopies demonstrated a homogeneous dispersion of the EG filler in the matrix. The dielectric permittivity of the composites showed an insulator to conductor percolation transition with increase in EG content. Significant changes in the dielectric permittivity take place when the weight fraction of EG is in the range 20–30 wt%. Special attention has been paid to the dependence of the conductivity on frequency, temperature and…
Nd:KGW Laser under Flashlamp-pumping at Repetition Rate up to 50 Hz and Average Power of 70 W (free-lasing and Q-switched mode)
1997
The laser performance of Nd:KGd(WO4)2 crystal or Nd:KGW has been studied under flashlamp-pumping in the free-running and Q-switched mode (with active or passive Q-switching) at repetition rate up to 50 Hz. Maximum output powers respectively of 70 and 30 W were achieved at 1.06 pm with a maximum efficiency of 6%. Free-running emission was also obtained at 1.35 pm with a maximum average power of 24 W and a total efficiency of 2.9% at this wavelength. A comparison with a similar Nd:YAG crystal is given showing a noticeable advantage for Nd:KGW.
Analysis of multipactor RF breakdown in a waveguide containing a transversely magnetized ferrite
2016
In this paper, the multipactor RF breakdown in a parallel-plate waveguide partially filled with a ferrite slab magnetized normal to the metallic plates is studied. An external magnetic field is applied along the vertical direction between the plates in order to magnetize the ferrite. Numerical simulations using an in-house 3-D code are carried out to obtain the multipactor RF voltage threshold in this kind of structures. The presented results show that the multipactor RF voltage threshold at certain frequencies becomes considerably lower than for the corresponding classical metallic parallel-plate waveguide with the same vacuum gap
Rheological Percolation Threshold in High-Viscosity Polymer/CNTs Nanocomposites
2017
AbstractPolystyrene (PS)/multi-walled carbon nanotubes (CNT) nanocomposites have been prepared through melt mixing processing aiming at obtaining a uniform and homogeneous dispersion of the used nanoparticles within the polymeric matrix. Optical and scanning electron microscopy has been employed to determine the dispersion and distribution of CNTs at different length scales. Furthermore, the linear viscoelastic behavior of formulated nanocomposites has been deeply investigated. As a result of CNTs added, the nanocomposites experience a transition from liquid-like to solid-like rheological behavior, and a disappearance of relaxation processes at low frequency can be noticed. By plotting G’ v…
Pore structural characteristics, size exclusion properties and column performance of two mesoporous amorphous silicas and their pseudomorphically tra…
2007
Highly ordered mesoporous silicas such as, mobile composition of matter, MCM-41, MCM-48, and the SBA-types of materials have helped to a large extent to understand the formation mechanisms of the pore structure of adsorbents and to improve the methods of pore structural characterization. It still remains an open question whether the high order, the regularity of the pore system, and the narrow pore size distribution of the materials will lead to a substantial benefit when these materials are employed in liquid phase separation processes. MCM-41 type 10 microm beads are synthesized following the route of pseudomorphic transformation of highly porous amorphous silicas. Highly porous silicas a…
Effect of the Content and Ordering of the sp2 Free Carbon Phase on the Charge Carrier Transport in Polymer-Derived Silicon Oxycarbides
2020
The present work elaborates on the correlation between the amount and ordering of the free carbon phase in silicon oxycarbides and their charge carrier transport behavior. Thus, silicon oxycarbides possessing free carbon contents from 0 to ca. 58 vol.% (SiOC/C) were synthesized and exposed to temperatures from 1100 to 1800 °
Distributed feedback lasing in cellulose films
2013
Cellulose derivatives, because of their molecular structure and chirality, can self-assemble to form spatially periodic cholesteric liquid crystal phases. We have synthesized and produced solid cross-linked cholesteric cellulose based films optimized to provide high reflectivity. Since these films are self-assembled photonic bandgap materials, they may be expected to show distributed feedback lasing. By doping samples with fluorescent dyes and optically pumping thin films of these materials, we were able to demonstrate, to the best of our knowledge, for the first time, mirrorless band-edge lasing in cellulose derivatives.
Very low instability threshold in a three-level laser model with incoherent optical pumping
1997
Abstract The stability properties of a laser model based on a closed three-level atomic scheme with incoherent optical pumping are studied. Unexpectedly, the instability threshold can be very low approaching the lasing threshold for large unsaturated gain values.