Relation entre synthèse, structure et propriétés de céramiques piézoélectiques à base de plomb, de hafnium et de titane (PHT)dopées au strontium

par Cédric Bedoya

Thèse de doctorat en Science des matériaux

Sous la direction de Marc Roubin.

Soutenue en 2000

à Toulon , en partenariat avec Université du Sud Toulon-Var. UFR de Sciences et Techniques (autre partenaire) .

mots clés

• Céramiques piézoélectriques
• Cristaux ferroélectriques
• Rayons X -- Diffraction
• Solutions solides

Résumé

The oxides, formulated Pbo^Sro. O&HfuxTixOs (noted PHT), having in a perovskite-like structure, represent an alternative to the PZT oxides industrially used in transducers and piezoelectric sensors. These oxides crystallise, at room-temperature, in a ferroelectric structure whose the symmetry is tetragonal (space group P-tmm) for x > 0. 58 and rhombohedral (space group R3c or R3m) for x < 0. 42- For the intermediate compositions, the oxides crystallise in the morphotropic phase boundary in which both structures coexist. The oxides have been elaborated using a chime douce process and a solid state route. The first method consisted to pyrolyse an organometallic oxalate-type complex whose the synthesis conditions have been optimised. The tetragonal and rhombohedral oxides have been obtained from a pyrolysis at 1070 K. The solid state reaction of a stoichiometric mixture of Pb3O4, HfO2l TiO; and SrCCb fired at U70 K, has allowed to obtain ail the oxides of the PHT series. DTA-TGA and X-ray diffraction studies have exhibited the different steps of their formation. The PHT solid solution presents a large variety of phase transitions between various electrical states, involving cationic displacements and deformations of the oxygen octahedra. Neutron and X-ray diffraction experiments have been performed to follow the compositional and thermal evolution of the crystallographic structures and to correlate the structural parameters to the modification of the electrical properties. The effect of an electrical field on the macroscopic polarisation has been studied on Sr-doped PZT ceramic to apprehend the reorientation mechanisms of the ferroelectric domains occurring in the two coexisting phases. Finally, the ceramics PHT and PZT, crystallising in the morphotropic phase boundar>', have been characterised electrically in order to correlate the composition to the changes of the dielectric and piezoelectric properties.

Titre traduit

Relation between synthesis, structure and properties of Sr-doped lead, hafnium and titanium based piezoelectric ceramics (PHT)

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