Study of Hardening and Softening Characteristics in A-site Acceptor- and Donor-Doped 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 Lead-Free Piezoceramics

The still unclear and inconsistent doping effects, particularly the effects of acceptor (hard) doping in BNT-based lead-free ceramics compared to PZT, are one of the reasons that have limited their usage in commercial applications. In this study, the effects of A-site Ag+ acceptor doping on the structure, dielectric, electrical conductivity and electromechanical properties of lead-free 0.93BNT-0.07BT (BNBT) ceramics (Ag100x, x=0.01-0.04) were systematically investigated. Rietveld refinement analysis revealed the coexistence of rhombohedral (R3c) and tetragonal (P4bm) phases, with an increase in P4bm phase fraction as x increased. An increasing of x value led to a significant decrease in resistivity (~1 order of magnitude) and a change from homogeneous to heterogeneous electrical microstructure. Furthermore, Ag+ doping has been found to exhibit amphoteric effects on the electromechanical properties. At low doping levels (x=0.01-0.02), the materials displayed a constricted P-E loop with a significant decrease in coercive field (Ec), and increases in dielectric loss (tan delta), permittivity (epsilon r), and piezoelectric coefficient (d33), which resemble softening characteristics. In contrast, for higher doping level (x=0.03-0.04), a square P-E loop with a substantial increase Ec and a decrease in Pr, Pm was detected, along with a decrease in tan delta, epsilon r, and d33, which are all the indication of hardening characteristics in PZT. These findings provide a better understanding of the effects of A-site acceptor doping on BNT-based ceramics, which can aid a more reliable modification of these materials for real-world applications.