Study On Preparation And Electric Properties Of(Na_0.5Bi_0.5)TiO₃-based Lead-free Piezoelectric Ceramics And Crystals

(1) LiSbO3-modified0.8(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3(abbreviated as NBT-KBT-LS) lead-free piezoelectric ceramics were prepared by the solid state reaction method. The effect of LS content (0<x<1.50%) on the microstructure and electrical properties of the ceramics were studied carefully. It is found that all the samples exhibit a pure perovskite phase. The grain size of those materials increases after doping LS. Some large and plate-like second phase appears in such ceramic systems because some Bi3+and Na+volatilize due to the overheating at high temperature. The piezoelectric constant d33, electromechanical planar coupling factor kp, mechanical quality factor Qm, the remanent polarization Pr, the coercive field Ec were increased by doping appropriate amount of LS. This means that the dopant of LS have "soft and hard double properties dopant" effect. The optimum electric properties (d33=154pC/N, kp=0.268,Qm=107) could be found in the composition of x=0.75%.(2)(0.8-x)(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3-xBaMnO3(abbreviated as NBT-KBT-BM) lead-free piezoelectric ceramics were synthesized by the solid state reaction. The phase composition, microstructure and electrical properties for NBT-KBT-BM samples with different BM content (x=0,0.25%,0.50%,0.75%,1.00%,1.25%in mole) were investigated. It is found that all the NBT-KBT-BM samples exhibit a pure perovskite phase. Sintering temperature of NBT-KBT ceramics after doping BM decreases and relative density increases compared with pure NBT-KBT. The piezoelectric constant d33, electromechanical planar coupling factor kp and mechanical quality factor Qm increase first and then decrease, while dielectric constant tanδ decrease with the increase of BM content. BM doping can reduce the coercive field Ec and increase the strength of the remanent polarization Pr, thus improving the ferroelectric properties. NBT-KBT-BM system shows relaxor characteristics. The dispersion factor y values for NBT-KBT-BM ceramic with x=1.25%was found to be1.969, which is close to ideal relaxor ferroelectric. This system ceramics with the best comprehensive properties can be obtained at x=0.75%: d33, Qm, kp, εr and tamδ is167pC/N,0.269,133,774and2.93%, respectively.(3) α(Na0.5Bi0.5)TiO3-b(K0.5Bi0.5)TiO3-cBaTiO3-dSrTiO3(abbreviated as NBT-KBT-BT-ST100a/b/c/d) lead-free piezoelectric ceramics with compositions of (1-2x)(0.8NBT-0.2KBT)-x(0.94NBT-0.06BT)-x(0.74NBT-0.26ST)(x=0.10,0.20,0.25,0.30,0.35,0.40,0.45) have been fabricated using the solid state reaction. The morphotropic phase boundary (MPB) between the rhombohedral and tetragonal phases was identified in all the samples. The results demonstrate that such ceramic systems near MPB exhibit superior electrical properties. The piezoelectric constant d33, electromechanical planar coupling factor kp and remanent polarization Pr increase first and then decrease with the increase of x. The best electric properties (d33=210pC/N, kp=0.319, Pr=39.3μC/cm2,=20.2kV/cm) could be obtained for a composition with x=0.35. It is suggested that NBT-KBT-BT-ST with the composition (x=0.35) is a candidate material for lead-free applications. Therefore, the MPB linear combination rule is applicable to search for the composition near MPB with excellent properties of NBT-KBT-BT-ST ceramics.(4) Lead-free ferroelectric{001}-face NBT-KBT crystals with dimension of6×4.5x1.2mm have been obtained by flux method. X-ray diffraction patterns indicate that the room temperature crystal structure is tetragonal. It is found that the NBT-KBT crystals exhibit ordered domain structure. And the width of it is about4nm. The temperature dependence of dielectric properties suggests that NBT-KBT crystal have diffuse-type phase transition characteristics. The dielectric constant decreases with increasing frequency while the dielectric loss decreases first and then increases. The piezoelectric properties of d33=78pC/N, kp=0.164are obtained for NBT-KBT crystal.