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1-x)K 0.5 Na 0.5 Nb 1-y Sb y O 3 -zSrZrO 3 -xBi 0.5 Na 0.5 HfO 3 (KNNS-SZ-BNH) lead-free ceramics were developed by the conventional solid-state reaction method. Effects of the additives (Bi 0.5 Na 0.5 HfO 3 , SrZrO 3 and Sb 5+ ) on their phase structure, microstructure, and electrical properties were investigated. The rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary can be established in the ceramics with 0.03≤x≤0.05, 0.04≤y≤0.06, and 0.01≤z≤0.025, and then their piezoelectric properties were improved. The ceramics with x=0.03, y=0.04 and z=0.01 possess the optimum piezoelectric properties (d 33 =470±5 pC/N, k p =0.51±0.02, andT C =244 o C). We believe that R-O-T multiphase coexistence is mainly responsible for the enhancement of piezoelectric properties.

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The Ag-Pd internal electrode of multilayer piezoelectric ceramics needs to be sintered below 1000°C, and lead wires and components need to be welded with leadfree solder at 260°C. PNN-PMW-PZT-xSr piezoelectric ceramics with high Curie temperature (T c > 260°C) were synthesized at a low sintering temperature (960°C) to meet the requirements of multilayer piezoelectric devices. The relationship between structures (phase, domain, and microstructures) and electrical properties (piezo/ferroelectric properties, and dielectric relaxation) in the Sr 2+ substituted ceramics was investigated. Rietveld refinement and Raman spectra show that Sr 2+ substitution can cause the phase change and increase the force constant of [BO 6 ] octahedron. The piezoelectric response increases with increasing the content of the tetragonal phase (CTP) in the rhombohedral-tetragonal (R-T) coexisted ceramics. The ceramics with 0.6 mol% Sr 2+ substitution have minimum activation energy for domain wall movement (E a ) of 0.0362 eV which favors the formation of nanometer-sized domains, and possess excellent electrical properties (d 33 = 623 pC/N, d 33 * =783 pm/V, T c =295°C).The higher the CTP, the lower the E a . The lower E a favors the rotation of polarization direction and extension, and is beneficial to the generation of the nanometer-size domains, resulting in high piezoelectric properties.

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