Dielectric and Piezoelectric Properties of Textured Lead-Free Na0.5Bi0.5TiO3-Based Ceramics

Properties of Na0.5Bi0.5TiO3 Ceramics Modified with Fe and Mn

Na_0.5Bi_0.5TiO₃ (NBT) and Fe- and Mn-modified NBT (0.5 and 1 mol%) ceramics were synthesized by the solid-state reaction method. The crystal structure, dielectric and thermal properties of these ceramics were measured in both unpoled and poled states. Neither the addition of iron/manganese to NBT nor poling changed the average crystal structure of the material; however, changes were observed in the short-range scale. The changes in shapes of the Bragg peaks and in their 2Θ-position and changes in the Raman spectra indicated a temperature-driven structural evolution similar to that in pure NBT. It was found that both substitutions led to a decrease in the depolarization temperature T_d and an increase in the piezoelectric coefficient d₃₃. In addition, applying an electric field reactivated and extended the ferroelectric state to higher temperatures (T_d increased). These effects could be the result of: crystal structure disturbance; changes in the density of defects; the appearance of (Fe_Ti'-), (Mn'_Ti-V^••_O) and (Mn″_Tii-V^••_O )-microdipoles; improved domain reorientation conditions and instability of the local polarization state due to the introduction of Fe and Mn into the NBT; reinforced polarization/domain ordering; and partial transformation of the rhombohedral regions into tetragonal ones by the electric field, which supports a long-range ferroelectric state. The possible occupancy of A- and/or B-sites by Fe and Mn ions is discussed based on ionic radius/valence/electronegativity principles. The doping of Fe/Mn and E-poling offers an effective way to modify the properties of NBT.

Effect of Different Ca2+ and Zr4+ Contents on Microstructure and Electrical Properties of (Ba,Ca)(Zr,Ti)O3 Lead-Free Piezoelectric Ceramics

In the preparation of (Ba,Ca)(Zr,Ti)O3 lead-free piezoelectric ceramics, different Ca2+ and Zr4+ contents will greatly affect the phase structure, microstructure, and electrical properties of the ceramics. XRD shows that all samples have pure perovskite phase structure, and the (Ba0.85Ca0.15)(ZryTi1−y)O3 ceramics morphotropic phase boundary region from tetragonal phase to rhombohedral phase near 0.08 ≤ y ≤ 0.1. From the dielectric temperature curve, the phase transition temperature (TO-T) was found near room temperature at 0.12 ≤ x ≤ 0.18 for the (Ba1−xCax)(Zr0.1Ti0.9)O3 ceramics. Both Ca2+ and Zr4+ increase have a significant decrease on the Curie temperature Tc. All samples were revealed as relaxers with diffusivities in the range 1.29 ≤ γ ≤ 1.82. Different from the undoped ceramics, ceramics doped with Ca and Zr ions exhibit saturated P–E hysteresis loops, and their ferroelectric properties are significantly optimized. In particular, the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic demonstrated optimal properties, namely d33 = 330 pC/N, kp = 0.41, εr = 4069, Pr = 4.8 μC/cm2, and Ec = 3.1 kV/cm, indicating that it is a viable lead-free piezoelectric contender. Variations in Ca and Zr content have a significant effect on the crystal grain sizes and densities of ceramics, which is strongly associated with their piezoelectricity.