Dielectric properties of segmented polyurethanes for electromechanical applications

cited By 10International audienceThe paper deals with electromechanical and dielectric properties of polyurethanes (PU) block-copolymers. Most of the works published in the literature only consider electrostriction at room temperature at a given frequency. In this work, it is shown that electrostrictive coefficient ME is divided by 3-10 at increasing frequency over 3 decades of frequency, depending on the ratio of hard to soft segments in PU. Thus it is important to analyze the energy conversion efficiency by investigating the dielectric and viscoelastic properties. This work deals with the study of dielectric properties of 3 PU with different fractions of hard segments. Three relaxation phenomena (β, α and conduction) were investigated for each PU in the temperature-frequency range studied here, in order to optimize the copolymer composition in view of their best efficiency as actuators or mechanical energy harvesting devices

From dislocation nucleation to dislocation multiplication in ceramic nanoparticle

Magnesium oxide nanocubes are compressed along the [001] direction in situ in the transmission electron microscope. Incipient plasticity in the smaller samples is characterized by the nucleation of few 1/2{110} dislocations while a larger number of line defects is observed in larger nanocubes. Yield and flow stresses scattered stochastically above a minimum value varying as the inverse of the sample size. The upper bound is given by the reduced number of dislocation sources. Such size-dependent behaviour is justified by a detailed statistical analysis and is fully explained by the deformation mechanism

Mixed percolating network and mechanical properties of polypropylene/talc composites

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Improvement of dispersion and electromechanical properties of polyurethanes nanocomposites by grafting CNT

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Contrast Analysis in Latex/Surfactant Aqueous Suspensions

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Visible luminescence improvement of ZnO/PAA nano-hybrids by silica coating

International audienceThe effect of a silica coating on the improvement of the visible light emission properties of ZnO/polyacrylic acid (PAA) nanohybrids is reported. The synthesized material consists of ZnO nanocrystals incorporated into the PAA mesospheres and then coated with silica. The silica amount can be controlled by the concentration of ammonia used in the sol-gel process as catalyst. The interaction between PAA and ammonia is crucial, the presence of the former tending to inhibit the catalytic action of the latter. We show that there is an optimum in the silica amount around the mesospheres, which leads to a drastic increase in the defect-related visible photoluminescence quantum yield of ZnO nanocrystals. A six-fold increase of the quantum yield can thus be achieved, reaching competitive values higher than 60%. This optimum is a compromise between a complete protective silica layer around the mesospheres and too thick a layer inducing inefficient absorption of excitation light by the coating

Nanostructured silica used in super-insulation materials (SIM), hygrothermal ageing followed by sorption characterizations

cited By 4Several commercial silica powders (precipitated, fumed, hydrophobized) and aerogels were aged in climatic chambers. Four different conditions of temperature [50–70 °C] and relative humidity [70–90%RH] were used to identify the influence of both parameters and the underlying mechanisms. Samples were characterized by sorption measurements (nitrogen and water) giving access to the evolution of specific surface area (BET), pore size distribution (BJH) and hydrophilicity. Structurally, a common trend along with silica ageing is the reduction of the specific surface area and a shift in the pore size distribution (loss of the smallest mesopores). However, the ageing scenario and intensities observed differ according to the type of silica (precipitated, fumed, hydrophobic, aerogels). On the one hand, the hydrophilicity of a precipitated silica decreases with ageing due to the reduction of the specific surface area. On the other hand, the hydrophilicity of a fumed silica first increases then decreases with ageing, as a result of two competing effects: the increase of local hydrophilicity (siloxane hydrolysis) and the decrease of the specific surface area. Surface chemistry appears to be a key parameter governing the extent of these evolutions: a precipitated silica is more sensitive than a fumed silica whereas the hydrophobic silica studied exhibited the strongest resistance. Therefore, the main mechanism of structural evolution, probably implying mass transfer in a dissolution/precipitation process, requires water on the surface of silica and thus hydrophilic groups to adsorb it. Short term ageing (24 days) on precipitated and fumed silicas showed that temperature is the most important parameter in activating a mechanism, the intensity of which is thereafter (96 days) governed by relative humidity. The significant influence of humidity was also observed for aerogels, for which the ageing scenario and intensity appears to be product specific. © 2018 Elsevier B.V