Pin-Yi Chen

Polar nanoregions and dielectric properties in high-strain lead-free 0.93(Bi1/2Na1/2)TiO3-0.07BaTiO3 piezoelectric single crystals

A structural coexistence of rhombohedral (R) and tetragonal (T) phases has been revealed in the (001)c-cut lead-free 0.93(Bi1/2Na1/2)TiO3–0.07BaTiO3 (BNB7T) piezoelectric crystals, which grown by the self-flux method, in the lower temperatures by high-resolution synchrotron X-ray diffraction, reciprocal space mapping, and transmission electron microscopy. The dielectric permittivity exhibits a thermal hysteresis in the region of 120–260 °C, implying a first-order-like phase transition from R+T to T. The real part (e′) of dielectric permittivity begins to deviates from the Curie-Weiss equation, e′ = C/(T − To), from the Burns temperature TB = 460 °C, below which the polar nanoregions (or nanoclusters) develop and attenuate dielectric responses. The polar nanoregions of 5–10 nm were revealed by high-resolution transmission electron microscope. The normal piezoelectric coefficient d33 exhibits a rapid increase at E = 15–20 kV/cm and reaches a maximum of d33 ∼450 pC/N. The high piezoelectric response and E-fi...

Structural stability and depolarization of manganese-doped (Bi0.5Na0.5)1−xBaxTiO3 relaxor ferroelectrics

This work reveals that 0.5 mol. % manganese (Mn) doping in (Bi0.5Na0.5)1−xBaxTiO3 (x = 0 and 0.075) solid solutions can increase structural thermal stability, depolarization temperature (Td), piezoelectric coefficient (d33), and electromechanical coupling factor (kt). High-resolution X-ray diffraction and transmission electron microscopy reveal coexistence of rhombohedral (R) R3c and tetragonal (T) P4bm phases in (Bi0.5Na0.5)0.925Ba0.075TiO3 (BN7.5BT) and 0.5 mol. % Mn-doped BN7.5BT (BN7.5BT-0.5Mn). (Bi0.5Na0.5)TiO3 (BNT) and BN7.5BT show an R − R + T phase transition, which does not occur in 0.5 mol. % Mn-doped BNT (BNT-0.5Mn) and BN7.5BT-0.5Mn. Dielectric permittivity (e′) follows the Curie-Weiss equation, e′ = C/(T − To), above the Burns temperature (TB), below which polar nanoregions begin to develop. The direct piezoelectric coefficient (d33) and electromechanical coupling factor (kt) of BN7.5BT-0.5Mn reach 190 pC/N and 47%.

Magnetic and Photovoltaic Properties of Calcium-Doped

The magnetization hysteresis loop, structure, and grain morphology of (Bi_0.85Ca_0.15)FeO_2.925 (15%Ca-BFO) ceramic, prepared by the solid state reaction, have been studied at room temperature and were compared with BiFeO₃ (BFO) ceramic. The 15%Ca-BFO ceramic exhibits a pseudo-cubic structure and a linear antiferromagnetic magnetization loop with magnetic susceptibility of 4.5×10^-6 emu/gOe, which is slightly less than 6.7×10^-6 emu/gOe observed in BFO due to the heterovalent substitution from Bi³⁺ to Ca²⁺. The 15%Ca-BFO with electrodes of indium tin oxide (ITO) and Au films shows significant photovoltaic effects under near-ultraviolet light of λ = 405 nm. The open-circuit voltage (V_oc) and short-circuit current density (J_sc) under illumination can be described by the exponential equations V_oc = V_b[1-exp(-I/α)] and J_sc = J_b[1-exp(-I/β)] as a function of light intensity (I). V_b and J_b are the balance open-circuit voltage and short-circuit current density, respectively. α and β are characteristic constants. The maximal power-conversion efficiency (η) can reach η ~ 0.0034%, which is larger than η ~ 0.0025% found in graphene/ polycrystalline BFO/ Pt films. The characteristic curve of current versus bias voltage at dark suggests a p-n junction-like depletion region in the interface between ITO film and 15%Ca-BFO ceramic.