Studies On Doping Modification Of Sodium Potassium Niobate-based Ceramics And Fabrication Of The Single Crystal

At present, the environmental and friendly lead-free piezoceramics become a research hotspot and have attracted a wide spread attention. In this paper, (K,Na)NbO3-based (abbreviated KNN-based) lead-free piezoceramics were modified with doping second composite. Microstructure, piezoelectric and dielectric were investigated. The small KNN single crystal was fabricated by high-temperature melting method.Crystal structure, thermal stability, and dielectrical property were studied.(1) (1–x)K_0.5Na_0.5NbO₃–xBaCu_0.5W_0.5O₃ ceramics were fabricated by the traditional solid state reaction. The crystal structure and electrical properties of K0.5Na0.5NbO3(KNN) ceramics were investigated with different BaCu_0.5W_0.5O₃(abbreviated BCW) content. The results show that KNN ceramics have the orthorhombic perovskite structure after doping a bit of BCW. The sintering temperature of KNN ceramics decreases and the relative density increases evidently comparing with the pure KNN. 5 kV/mm and 140℃are the optimum polarize condition. At the composition of x = 0.25%, it obtains optimum properties: d₃₃ = 128 pC/N, k_p = 0.40, and Qm was enhanced. The trend was represented that Tc and To-t increased at first and then decreased. The reason is that A-site and B-site of KNN crystal are substituted respectively by Ba²⁺,Cu²⁺ and W³⁺.(2)Li_0.04Na_0.52K_0.44Nb_0.86Ta_0.10Sb_0.04O₃(abbreviated LF4) ceramics have the excellent piezoelectrical property and were modified by doping BCW. The microstructure and electrical properties of LF4 ceramics are investigated with different BCW content. The results show that BCW has the hard modified effect on the electrical properties of LF4. Piezoelectric constant d33, planar electromechanical coupling factor kp, dielectric loss tanδand dielectric constantεr decreased with increasing the content BCW, while the mechanical quality factor Qm was enhanced. Furthermore, the sintering temperature was decreased and the density of ceramics was enhanced with doping BCW. The structures of the samples have changed from tetragonal to orthorhombic when x≥1%, To-t can be detected, and Tc shifts to lower temperature.(3)The KNN ceramics was modified by doping BaCu₀₃₃Nb_0.67O₃ (abbreviated BCN). The results show that the samples have the orthorhombic perovskite structure after doping BCN. BCN could dissolve into the KNN structure, but the conten was limited.The sintering temperature was decreased and the density of ceramics was improved remarkablely. BCN was considered to be the sintering aid. At the composition of x = 0.2%, it obtains optimum properties: d₃₃ =125 pC/N,k_p=0.53,tanδ= 0.011,ε_r = 389,Qm = 337,ρ= 4.41 g/cm3.(4)Moreover, [001] oriented-KNN crystal was grown by high-temperature melting method, and the size is small. The room temperature crystal structure of orthorhombic perovskite-type lattice was determined from XRD measurment. SEM observation has revealed the growth steps aligning approximately along [001] direction. Base on the model of negative ion coordination polyhedrons, it is explained that the layer growth mechanism was dominating for the [001] face. Two phase transition temperatures of orthorhombic-to-tetragonal (O-to-T) and tetragonal-to-cubic (T-to-C) are around 240℃and 405℃for KNN crystals according to the dielectric measurements, respectively. A linear fitting of the modified Curie–Weiss law to experimental data shows that the normal ferroelectric property is predominant for KNN crystal.