The Research On Modification And Texture Of Sodium Potassium Niobate-based Lead-free Piezoelectric Ceramics

Lead-free piezoelectric ceramics is a kind of green materials which does no harm to environment. Lead-based PZT piezoelectric ceramic material has been taken a dominant position of the research and application in the piezoelectric ceramic area.Lead is highly toxic, however, as the requirements of human beings for the improvement of living environment quality. Lead-free materials as alkaline niobate piezoelectric materials of doped and texture KNN ceramics are comparable to the traditional PZT piezoelectric ceramics. So technique of preparing high properties lead-free piezoelectric ceramics becomes an important research topic.In chapter 3, CaTiO3 doped (K0.5Na0.5)NbO3 (KNN) lead-free piezoelectric ceramics were prepared by normal solid-phase synthesis method The results show that the KNN-CT ceramics own a perovskite structure. With the increase of CaTiO3, the ceramics changed from orthorhombic phase to tetragonal phase gradually. A morphotropic phase boundary was found when x is around 4%. The sintering ability of the ceramics was improved by CaTiO3 enormously, and the maximum density of the ceramics has reached to 99% of the theoretical density by optimizing, with d33=94pC/N, and Kp=33.2%.In chapter 4, through control the precursor's structure with layer morphology, the plate-like particles with different edge length and thickness can be synthiszed by the two step molten salt (MSS) method. Plate-like layered-perovskite Bi1.5Nax-1.5NbxO3X+3 (X=2-6) as precursors was prepared in the first instance. Under opt sinter point and dwelling time,by changing the proportion of BiNNx and other oxide, pure platelet-like NaNbO3 are obtained. The results of SEM showed that the NaNbO3 particles were characterized by platelet-like morphology with an edge length of 8-20μm and a thickness of 0.5-3μm. The results of XRD indicated the NaNbO3 particles synthesized had a strong orientation in (001) crystal planes. The plate-like NaNbO3 particles we synthesized are a suitable template candidate to fabricate textured ceramics by TGG or RTGG method, especially to the textured potassium sodium niobate lead-free piezoelectric ceramics. In chapter 5, lead-free textured K0.5Na0.5NbO3 (KNN) piezoelectric ceramics have been prepared by reactive-templated grain growth method (RTGG) by using NaNbO3 with different morphologies as templates, and discuss the relationship between NaNbO3 template particles's ratio of edge length to thickness and the textured degree of ceramics systematically. The X-ray diffraction shows that all the KNN ceramics are orthorhombic perovskite structure with (001) orientation degree 20-40%. With the increasement of x, the ratio of edge length to thickness become smaller gradually and textured degree decrease slowly. However, the increase of density contribution to the enhancement of electrical performance. When the the ratio of edge length to thickness is 6-12, the textured ceramics with optimized piezoelectric properties was prepared. The improvement of piezoelectric constant d33 is from 80 pC /N to 135pC/N and electromechanical coupling coefficient kp is from 28% to 36%.The results indicate that the enhanced properties of textured ceramics are concerned with the template morphology.