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Defect engineering of lead-free ferroelectric ceramics BaTiO3 through acceptor doping

Abstract : Ferroelectric materials are famous for their multiple properties, including large permittivity, high piezoelectric response, pyroelectric, and optical properties, and so on. To make ferroelectric materials better applied in commercial applications and meet miniaturization requirements and needs of an environment-friendly society, lead-free ferroelectric BaTiO3 has been intensively developed through a continuous worldwide research efforts. Massive microstructural engineering and chemical modifications have been investigated to enhance the properties of lead-free ferroelectric BaTiO3. In this PhD work, we shall introduce acceptors and oxygen vacancies to form defect dipoles, the ferroelectric domain rotations are controlled, which enables tuning the properties of lead-free ferroelectric BaTiO3 ceramics. The acceptors Cu2+ (rCu2+ = 87 pm) and Fe3+ (rFe3+ = 78.5 (69) pm for high (low) spin) are selected to substitute for the Ti ions (rTi4+ = 74.5 pm) on the B site of the perovskite structure of BaTiO3 through traditional solid-state synthesis method. The Fe and Cu substitutions in BaTiO3 are demonstrated by the EPR spectra. Combined with the SEM images, Rietveld-refined XRD patterns, and Raman spectra, the perovskite structure of doped BaTiO3 ceramics with a single tetragonal phase (P4mm) at room temperature are determined. The homogeneous Cu distributions are observed on the EDX maps. The oxygen vacancies, either trapped by dopants or accumulated at grain boundaries, create an internal electric field, playing a crucial role to harden acceptor-doped ferroelectrics. According to these two trapping positions for the oxygen vancancies, the hardening mechanism is generally explained by the volume effect or the surface effect, respectively. Resiscope measured the resistance reduction of the interior of the grains (ΔRG) and grain boundary when hardened 0.4 at%Cu-coped BaTiO3 became the relaxed. The higher reduction of the interior of the grains resistance (ΔRG) than the reduction of grain boundary resistance (ΔRGB)n demonstrates that the volume effect is the principal hardening mechanism in polycrystalline acceptor-doped ferroelectrics. The movements of ferroelectric domain walls are restricted by the defect dipoles that are created as oxygen vacancies reach positions nearest-neighbor to dopants. Inspired by this defect dipole-domain interaction, we design several strategies including thermal and field excitation to control the oxygen vacancies migration, and manipulate the orientation of defect dipoles. Consequently, the domain walls movements are controlled, which makes the acceptordoped BaTiO3 present different hysteresis loops, including de-aging process, re-aging process, shifting process. Through DFT calculations, the lowest energy barrier of oxygen vacancies movement following the spontaneous polarization of rhombohedral acceptor doped BaTiO3 ceramics is established. This result is consistent with the symmetry-conforming principle of point defects. In addition, the lower energy barriers of oxygen vacancies diffusion in Fe-doped BaTiO3 than in Cu-doped BaTiO3 are calculated, which indicates the higher mobility of oxygen vacancies in Fe-doped BaTiO3. These DFT calculation results are backed up by two experiments : field cooling measurement and fatigue measurement.
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Submitted on : Thursday, November 17, 2022 - 6:56:13 PM
Last modification on : Saturday, November 19, 2022 - 3:34:24 AM

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  • HAL Id : tel-03858744, version 1

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Zechao Li. Defect engineering of lead-free ferroelectric ceramics BaTiO3 through acceptor doping. Mechanics of materials [physics.class-ph]. Université Paris-Saclay, 2022. English. ⟨NNT : 2022UPAST098⟩. ⟨tel-03858744⟩

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