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Studies of novel lead-free piezoelectric and relaxor ferroelectric materials

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(Thesis) Ph.D.
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Author: Lei, Chao
The (1-x)BaTiO3–xBaSnO3 perovskite solid solution ceramics were prepared by solid state reaction and studied by dielectric spectroscopy. The temperature-composition phase diagram of this pseudo-binary system has been established, which delimits the paraelectric, ergodic polar cluster, nonergodic ferroeletric and relaxor phases, and indicates a crossover from ferroelectric to relaxor behavior at xc= 0.19. The mechanisms of the dielectric response in different parts of the phase diagram are discussed. In particular, the crossover from a diffuse ferroelectric phase transition to relaxor behavior is attributed to the appearance at x > xc of the additional dielectric contribution arising from the flipping of the local polarization of the polar clusters. The structure and the dielectric and ferroelectric properties of the (1-x)BaTiO3-xAgNbO3 (BT-AN) solid solution ceramics were investigated. The most interesting feature of this system is that it exhibits re-entrant-like phenomenon which means that the relaxor state occurs after the para- to ferro-electric phase transition upon cooling, i.e., inside the ferroelectric phase. The temperature-composition phase diagram of BT-AN system has been established. This anomalous relaxation behaviour can be attributed to compositional segregation into mesoscopic ferroelectric regions (with x = 0) and AN-substituted regions (x > 0). This unusual phase transition sequence has never been reported in canonical lead-based ferroelectrics. In the search for new lead-free materials with high piezoelectric performance, the ceramics of (1-x)(K0.5Na0.5)NbO3–xAgNbO3 (KNN-AN) solid solution have been designed and prepared. It is shown that they exhibit piezoelectric properties which are comparable or superior to the hot-pressed KNN ceramics and other KNN-based systems. In particular, the piezoelectric properties exhibit very good thermal stability up to high temperatures, presenting a significant improvement from most KNN-based piezoceramics. Novel lead-free solid solutions of (1-x)(Na0.5Bi0.5)TiO3–xAgNbO3 (NBT-AN) were prepared and characterized. The dielectric constant was found to be stable in a wide temperature range, indicating that this system is promising for such application as ceramic capacitors within a wide temperature range, especially at high temperatures. In addition, the transparency of the NBT-AN ceramics obtained by sintering in O2 opens a road towards new applications in the field of electro-optics.
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