Au vu des mesures de confinement actuelles, la défense aura lieu par vidéo-conférence via le lien suivant (Teams)

Promoteur : Prof. Maurice GONON

Résumé :

Most piezoelectric sensors and actuators used nowadays are based on ferroelectric polycrystalline ceramics. Because of initially randomly distributed polar domains within the ceramic grains, polarization under a high electric field is required to confer macroscopic piezoelectric properties. These ceramics exhibit high piezoelectric performance, but their main drawback is the depolarization occurring over time and with increasing temperature.
For high temperature applications, pyroelectric non-ferroelectric phases can be used. In the case of polycrystalline ceramics, a preferential orientation of the crystals polar direction needs to be induced during the elaboration process to obtain a macroscopic piezoelectric material.
In the present PhD thesis, the glass-ceramic route was used to produce highly textured glass-ceramics containing Strontium-Fresnoite Sr2TiSi2O8 crystals, a piezoelectric and non-ferroelectric phase. According to the parent glass composition and the crystallization conditions, surface crystallization mechanism can be enhanced, leading to preferential orientation of the (00l) lattice planes parallel to the specimens’ free surfaces. Usually, a tilt from (002) to (201) planes occurs when penetrating the bulk. The samples obtained exhibit d33 values ranging from 10 pC/N to 14 pC/N. The stability of the material at high temperature has been proved by HT-XRD and thermomechanical analyses. Kinetic studies and determination of preferential orientation in depth were demonstrated by XRD analyses, pole figures, and TEM. Detailed microstructure observations and explanation of the environment influence on the characteristics and the evolution of the preferential orientation have been investigated and a new proposition of crystallization mechanism was proposed. At the same time, the potential of this substrate to generate Surface Acoustic Waves was evidenced, and so its potential use as sensor device operating at high temperature and in harsh environment.