This study investigated crystal structures, microstructures, and electric-field-induced strain (EFIS) properties of Bibased lead-free ferroelectric/relaxor composites. Bi_1/2(Na_0.82K_0.18)_1/2TiO₃ (BNKT) as a ferroelectric material and 0.78Bi_1/2(Na_0.78K_0.22)_1/2TiO₃-0.02LaFeO₃ (BNKT2LF) as a relaxor material were synthesized using a conventional solid-state reaction method, and the resulting BNKT2LF powders were subjected to high-energy ball milling (HEBM) after calcination. As a result, HEBM proved a larger average grain size of sintered samples compared to conventional ball milling (CBM). In addition, the increased sintering time led to grain growth. Furthermore, HEBM treatment and sintering time demonstrated a significant effect on EFIS of BNKT/BNKT2LF composites. At 6 kV/mm, 0.35% of the maximum strain (S_max) was observed in the HEBM sample sintered for 12 h. The unipolar strain curves of CBM samples were almost linear, indicating almost no phase transitions, while HEBM samples displayed phase transitions at 5~6 kV/mm for all sintering time levels, showing the highest S_max/E_max value of 700 pm/V. These results indicated that HEBM treatment with a long sintering time might significantly enhance the electromechanical strain properties of BNT-based ceramics.

n this paper, in order to develop outstanding Pb-free composition ceramics, the Fe2O3-doped(Na0.525K0.443Li0.037)(Nb0.883Sb0.08 Ta0.037)O3 + 0.3 wt% Bi2O3 + x wt% Fe2O3 (x= 0∼1.0 wt%)(abbreviated as NKL-NST) lead-free piezoelectric ceramics have been synthesized using the ordinary solid state reaction method. The effect of Fe2O3-doping on their microstructure and electrical properties were investigated. XRD diffraction pattern studies confirm that Fe2O3 completely diffused into the NKL-NSTlattice to form a new stable soild solution with Fe3+ entering the Nb5+, Sb5+ and Ta5+ of B-site. And, phase structure of all the ceramics exhibited pure perovskite phase and no secondary phase was found in the ceramics. The ceramics doped with 0.6 wt% Fe2O3 have the optimum values of piezoelectric constant(d33), planar piezoelectric coupling coefficient(kp) and mechanical quality factor(Qm) : d33= 233[pC/N], kp= 0.44, Qm= 95. These results indicate that the (Na0.525K0.443Li0.037)(Nb0.883Sb0.08Ta0.037)O3 +0.3 wt%Bi2O3 + 0.6 wt% Fe2O3 ceramic is a promising candidate for lead-free piezoelectric ceramics.

Abstract: (K(0.5)Na(0.5)) (Nb(0.96)Sb(0.04)) O(3)+1.2 mol% K(4)CuNb(8)O(23) ceramics doped with iron oxide (Fe(2)O(3)) were prepared by a conventional mixed oxide method. And then, their piezoelectric and dielectric properties were investigated as a function of Fe(2)O(3) addition. X-ray diffraction studies reveal that Fe(3+) diffuses into the NKN lattices to form a solid solution with a pure perovskite structure at room temperature. At the sintering temperature of 1,060℃, when 0.2 mol% Fe(2)O(3) was doped, the piezoelectric constant (d(33)), electromechanical coupling factor (Kp), and mechanical quality factor (Qm) showed the excellent values of 131.67 pC/N, 0.436, and 696.36, respectively. Results show that Fe(2)O(3) deped (K(0.5)Na(0.5))(Nb(0.96)Sb(0.04))O(3)+1.2 mol% K(4)CuNb(8)O(23) lead-free piezoelectric ceramics are a promising lead free material for piezoelectric transformer applications.