Papers by Author: Bong Yeon Lee

Abstract: Lead-free ferroelectric (Bi_0.5Na_0.5)TiO₃ (BNT) thin films were prepared by chemical solution deposition. BNT and Mn-doped BNT precursor thin films crystallized in the perovskite single phase at 700 °C on Pt/TiO_x/SiO₂/Si substrates. The leakage current density of the perovskite BNT films, especially in the high applied field region, was reduced by doping with a small amount of Mn. Also, Mn doping markedly improved the ferroelectric properties of the films. 0.5 and 1.0 mol% Mn-doped BNT thin films exhibited well-shaped ferroelectric polarization (P) electric field (E) hysteresis loops at room temperature. Furthermore, the 1 mol% Mn-doped BNT films showed a typical field-induced strain loop, and the effective d₃₃ values were estimated to be about 60 pm/V.

Abstract: Barium titanate (BaTiO3, BT) – bismuth titanate magnesium oxide (Bi(Mg0.5Ti0.5)O3, BMT) solid solution system ceramics were prepared by conventional sintering method in pursuit of the enhancement of the BT Curie temperature (TC, 132 °C). Normal ferroelectric polarization vs. electric-field (P-E) hysteresis loops were observed for BT-BMT ceramics with BMT contents below 20 and above 60 molar%. On the other hand, broad P-E double hysteresis loops were observed for BMT contents from 30 to 50 molar%. The origin was investigated using synchrotron XRD measurement and Rietveld analysis. The crystal structure was assigned to ferroelectric phase with domain-pinning by certain defect structures. A modified phase diagram was proposed on the basis of the temperature dependence of the crystal structure.

Abstract: Barium titanate (BaTiO3, BT) – bismuth lanthanum zinc titanium oxide [(BiyLa1-y)(Zn0.5Ti0.5)O3, ByL1-yZT] solid solution ceramics were prepared to enhance Curie temperature (TC) of BT, 132 °C, to 200 °C. As the raw materials, nanoparticles were used to make sintering temperatures lower. Optimization of calcination and sintering conditions resulted in a formation of a perovskite single-phase, and their densities were always greater than 94 %. The synchrotron XRD measurement revealed that the crystal structures were assigned to rhombohedral 3m. Temperature dependence of dielectric property revealed that for the 0.5BT-0.5BZT ceramics, TC was 230 °C. Moreover, La-doping into BT-BZT ceramics resulted in decrease of TC significantly. Finally, their piezoelectric properties were measured by electric-field dependence of strain at room temperature, and for the 0.6BT-0.4BZT ceramics, the apparent d33 was measured at 250 pC/N.