Development of active phononic crystals for ultrasound applications

Oct 1, 2013 - study new tunable phononic crystals to meet these requirements. ... Initial training in university or engineering school including a master level research experience, you have good basic training in physics and ideally skills in ...
88KB taille 1 téléchargements 271 vues
Development of active phononic crystals for ultrasound applications Institutions : -

Fédération de Recherche – FANO FR CNRS 3110 ; GREMAN (groupe de recherche en matériaux, microélectronique, acoustique et nanotechnologies) – UMR CNRS 7347 - Université François Rabelais de Tours ; LOMC (Laboratoire Ondes et Milieux Complexes) – UMR CNRS 6294 – Université du Havre

Keywords : Ultrasound, Piezoelectricity, Instrumentation.

Abstract : Phononic crystals (PC) are periodic structures made up of several constitutive materials. The aim of this periodic structure, that can either be in one, two or three dimensions, is to strongly influence the properties of acoustic wave propagation. More specifically, the dispersion curves of acoustic waves can, under certain conditions of geometry and composition, have bandgaps of frequencies for which the elastic wave propagation is forbidden, whatever the direction of propagation. The existence of bandgaps can be exploited for many applications such as the achievement of selective or semi-transparent mirrors, frequency filters by coupling defects or resonators, as well as low frequency sound barriers. In addition, bands whose curvature can be negative correspond to a the wave vector and the Poynting vector which are opposed. This property leads to the phenomenon of negative refractive and may allow focusing the acoustic waves with a resolution better than the diffraction limit (i.e. half the wavelength). This "super focusing" effect may have several applications, such as medical imaging or therapy. However, for some applications the current phononic crystals have several limitations, in particular because of their lack of tunability. Indeed, control of wave propagation is obtained in a limited frequency range that is completely defined by the geometry and physical properties of constitutive materials of the phononic crystal. Today, the development of tunable and reconfigurable devices seems essential to meet the growing demands of industry especially for frequency filters (RF MEMS). In this context, the objective of this thesis is to study new tunable phononic crystals to meet these requirements. For this, the introduction of piezoelectric materials in the structure that can be controlled by an external voltage will change the mechanical properties of these materials and thus change the properties of acoustic wave propagation. First, a simple model for 1D behavior inetgrating the piezoelectric effect will be implemented. It will be described in the frequency domain using a transfer matrix to model a structure

corresponding to stacks of active and / or passive layers. Acoustic and electrical terminations will be taken into account. In parallel, the first simple experimental set-up will be developed to quantify, according to the changes of the electrical and mechanical conditions, the range of possible variations, i.e. typically the frequency positions of bandgaps. Comparisons between theory and experiments will then be performed to validate the approach and quantify the limits of tunability. In a second phase, efforts will be devoted to the design of 1D and 2D demonstrators. Typically for a 2D device, piezoelectric rods will be aligned and distributed periodically in an inert solid matrix. To fabricate these demonstrators, the establishment of a multi-channel instrumentation control will be required. Preliminary studies will be performed in order to minimize the number of independent commands needed to perform the functions described above.

Framework : This work will take place between two laboratories within FANO research federation: GREMAN (located in Blois) and LOMC (Located in Le Havre). Regular exchanges will made between the two laboratories. The first part of the thesis will be mainly held at GREMAN laboratory and fabrication of 2D demonstrators and their characterization will be performed within the LOMC. In addition, this thesis is in the framework of a French National Reserch Agency (ANR) research project and collaborations with other partners are planned.

Contract : -

Beginning of the PhD Thesis : 1st october 2013 Duration : 3 years Funds : CNRS and « Région Centre ».

Contact : For further information or to apply (CV and cover letter), please contact: Franck Levassort – GREMAN - Université de Tours – [email protected] (0247366194) Lionel Haumesser – GREMAN - Université de Tours – [email protected] Bruno Morvan – LOMC - Université du Havre - [email protected]

Candidat profile : Initial training in university or engineering school including a master level research experience, you have good basic training in physics and ideally skills in acoustics, materials science, scientific computing and/or electronics.