"Etude des Propriétés Mécaniques et de L’atténuation des Ondes de Surface dans le Si Poreux en Utilisant un Capteur à Lentilles Annulaires"

Abstract

"The investigation of the characteristics of acoustic wave surface propagation in homogeneous materials, by application of the acoustic microscopy, is subjected to a big request in scientific research. This request orientated research to the characterization of the porous materials. Our objective consists in studying structure and calculating the acoustical parameters of porous silicon (PSi) by using the quantitative acoustic microscopy. For it we simulate the curves of the output signal of the sign of exit cited acoustical material signature, V (z), to study the evolution of different acoustical parameters as well as microstructure of PSi according to the porosity. At first, we represented all tools necessary for the application of the quantitative acoustic microscopy and cited the different application domains. Subsequently we represented the porous materials in general and porous silicon particularly. In this section the quantitative investigation, in dark field, allowed us to found that the decrease of the energy distribution at the transducer strongly affects both the amplitude of the acoustic signatures that their periods tend to infinite values when the occultation reaches a threshold. This angle threshold is generally right before the critical angle of excitation of the mode of Rayleigh, θR. A slip of this last towards the great values, excited by the presence of air in the pores, appears when porosity increases. After this study, we have been interested to the attenuation of the surface wave propagation in PSi by using three different methods. The first method is the analytical method, which is obtained by solving the Viktorov equation. The second method is that of the spectral analysis, where the fast Fourier transform (FFT) is applied to the curves V (Z). The last method uses the dark field. The results obtained by simulation in good agreement with those experimental found in the literature. Another area of interest is to study the influence of crack on the propagation acoustic waves. The presence of the crack to the material surface disturbs the wave propagation as it modifies the amplitude of the reflection coefficient. To investigate the latter we used the model Schoch. Our study shows that there is a reduction of 27%. Thus by taking in consideration the approximation of Rayleigh and modeling the acoustic signature V (Z), we find that the output signal decreases by 29.3% in presence of the crack. In the last part of this thesis we applied our calculations to porous alumina (PAl2O3). We showed that the acoustic technique is a very powerful nondestructive method to characterize this type of biomaterials. We noted that as a given porosity, when the frequency increases the attenuation increases, and at a given frequency, when porosity increases, attenuation becomes stronger. The curves of V (z) are a key for the microstructural investigations of PAl2O3 with an aim of establishing empirical relationships between the acoustic parameters and compare them to those reported in the literature."