Preparation And Electrical Properties Of The （Ba,Ca）（Zr,Ti）O₃ And （K,Na）NbO₃ Lead-free Piezoelectric Ceramics

Piezoelectric ceramics have been used worldwide in sensors, actuators, transducers, resonators and filters, buzzers, piezoelectric ignition devices and other electronic devices. For half a century, the PZT（lead zirconate titanate） family has been the icon of a large class of technologically important materials-piezoelectric ceramics. Despite its outstanding piezoelectric property, PZT is currently facing global restrictions due to its Pb toxicity, there is an urgent need to develop a environment-friendly Pb-free substitute that can compete with PZT.In this thesis, the effect of processing and solid solution on the microstructure and electrical properties of the perovskite structure(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃（BCZT）- and(K_0.5Na_0.5)_1-x Li_x Nb O₃（NKLN）-based lead-free piezoelectric ceramics were investigated, attempting to develop a high-performance lead-free piezoelectric ceramics.Firstly, including tape casting and templated grain growth were employed to develop textured(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ ceramices, the microstructure and electrical property of BCZT ceramics were investigated. The results are shown that Ba_1-x Cax Ti O₃（x≤ 0.25） platelike particles with high aspect ratio（edge length to thickness） were synthesized via a topochemical microcrystal conversion reaction between the platelet Bi₄Ti₃O₁₂ and the powdered Ba CO₃-Ca CO₃ mixture in KCl flux. The(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃-5mol%(Ba_0.85Ca_0.15)Ti O₃（BCZT-5mol%BCT） ceramics were prepared by tape casting and templated grain growth method using Ba_0.85Ca_0.15 Ti O₃ platelike particles as a template, the results show that no textured structure were developed in BCZT-5mol%BCT sintered at 1500°C with 4¹6h by XRD and SEM, and the reason lies in template and sintering schedule was analyzed. The piezoelectric property of the BCZT-5mol%BCT ceramics was influenced significantly by grain size and internal stress, the optimum piezoelectric property was achieved in BCZT-5mol%BCT with the largest grain size and the minimum internal stress sintered at 1500°C with 8h.Secondly, the nanocrystalline(Ba_0.85Ca_0.15)（Ti0.90Zr0.10）O₃ powders with grain size about 20 nm was synthesized by sol-gel technique calcined at 700°C for 2 h, the sintering temperature was reduced markedly from 1540°C to 1300°C with a high relative density（> 97%） and d33²80 p C/N. Then the(Ba_0.85Ca_0.15)（Ti0.90Zr0.10）O₃ ceramics from nanocrystalline powders sintered by the two-step sintering technique（sintered at T1 of 1300°C for 1 min and T2 of 1150°C for 20 h） exhibited the finest average grain size of 0.7¹μm and a high relative density of 98% with d33¹00 p C/N, and Tc⁹9°C. The electrical properties of the(Ba_0.85Ca_0.15)（Ti0.90Zr0.10）O₃ ceramics, made from nanocrystalline powders, show an enhancing trend with grain size increased.Thirdly, the electrical property of(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ ceramics prepared by spark plasma sintering technique were degraded by internal stress from the elastically clamped grain in the high dense ceramics with a relative density of 99%.Finally, the effect of adding(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ and Ca Sn O₃ compound on the microstructure and electrical properties of the（K0.5Na0.5）1-x Lix Nb O₃（NKLN）-based ceramic by conventional sintering technique were investigated. The results are shown that the orthorhombic and tetragonal phases coexist in the（Na0.5K0.44Li0.06）Nb O₃-x(Ba_0.85Ca_0.15)（Zr0.10Ti0.90）O₃ ceramics with 0.04 ≤ x ≤ 0.06 at room temperature, and the enhanced piezoelectric properties are achieved in the two phase’s coexisting region. The ceramics in the mixed phase region exhibits the optimum electrical properties: d33=130 p C/N¹47 p C/N, kp=22%²5%, and Tc=350°C³84°C. The significantly enhanced piezoelectric properties are achieved in（Na0.5K0.5）0.94Li0.06 Nb O_3-x Ca Sn O₃ ceramics with the concentration of Ca Sn O₃ was 0.02⁰.025mol%, the optimum electrical properties of the ceramics with the grain size was 2⁵μm as flows: d33=205 p C/N ²17 p C/N, kp=35%³8.3%, and Tc=387.5°C ³95.5°C.