Modélisation des propriétés des matériaux ferroélectriques displacifs monocristallins

Abstract : The ferroelectric displacive materials present an increasing interest in the industrial field, in particular for telecommunications, because of the tunability of the permittivity or of the piezoelectric coefficient. The objective of this PhD thesis is a better understanding of the properties of these materials by carrying out a theoretical study. At the nanometric scale, the effects of temperature, of a mechanical stress or of an applied electric field are examined in particular in terms of the deformation of the unit cell and the change of the dipole moment. It is then possible to obtain the permittivity or the piezoelectric coefficient according to these variables, in the both polar and non-polar phases. To describe the properties of a ferroelectric displacive material at microscopic scale, we must add to the previous description the notion of domain of polarization. When the material is submitted to an external solicitation, the walls separating the different domains move. The study of this motion provides the variation of the polarization, of the deformation, of the permittivity or of the piezoelectric coefficient as a function of applied electric field or mechanical stress (hysteresis and butterfly loops). The role of defects in the material is taken into account by considering pinning and depinning of the walls by these defects. A distinction is carried out between minor loops, for which the density of domain walls is constant, and the major loops where one must take into account the variation of the density of walls.
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Mostafa Ragheb. Modélisation des propriétés des matériaux ferroélectriques displacifs monocristallins. Physique [physics]. UNIVERSITE DE NANTES, 2013. Français. ⟨tel-01107308⟩



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