International audienceThis paper presents a method for ultrasonic characterization of porous silicon in which a genetic algorithm based optimization is used to solve the inverse problem. A one dimensional model describing wave propagation through a water immersed sample is used in order to compute transmission spectra. Then, a water immersion wide bandwidth measurement is performed using insertion/substitution method and the spectrum of signals transmitted through the sample is calculated using Fast Fourier Transform. In order to obtain parameters such as thickness, longitudinal wave velocity or density, a genetic algorithm based optimization is used.A validation of the method is performed using aluminum plates with two different thicknesses as references: a good agreement on acoustical parameters can be observed, even in the case where ultrasonic signals overlap.Finally, two samples, i.e. a bulk silicon wafer and a porous silicon layer etched on silicon wafer, are evaluated. A good agreement between retrieved values and theoretical ones is observed. Hypothesis to explain slight discrepancies are proposed

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International audienceThe floating zone method (FZM) is used to grow large crystal ingots of lead-free (K0.5Na0.5)NbO3 (KNN), (K0.5Na0.5)(Nb0.95Mn0.05)O3 (KNN-Mn), and (K0.5Na0.5)(Nb0.95Ta0.05)O3 (KNN-Ta). FZM allows high quality crystal to be obtained without using a crucible. However, the KNN molten zone is instable at high temperature (∼1200 °C) due to its incongruent melting and alkali segregation. The challenge of this work is to control the stability and the viscosity of the molten zone which allows long growth in order to achieve large volume single crystals. Our work reveals that the use of a slight overpressure of gas (O2 or N2) and doping KNN with Mn and Ta allows the molten zone to be maintained over 50 h. In such conditions, the as-grown crystals are preferentially oriented along the [011] direction and display piezoelectric properties such as kt of 0.48, d33 of 65–70 pC/N, ε33,rS of 123–148, δe of 0.03–0.07, and δm of 0.11–0.21. The ferroelectric measurements show saturated hysteresis loops with a decrease in EC and Pr when attaining single crystal after several hours of growth. High resolution transmission electron microscopy (HRTEM) investigations show complex domain structures that are mainly attributed to the accommodation of the stresses occurring during the crystal growth in the high thermal gradient and in the phase transition from tetragonal to orthorhombic phases during cooling