The Structure And Properties Of KNN-based Lead-free Piezoelectric Ceramics Near The Polymorphic Phase Boundary

In this dissertation,the?K,Na?Nb O₃ lead-free piezoelectric ceramics modified by Li,Sb and Ta?KNN-LTS?were investigated.Firstly,linear combination rule was used to prepare the KNN-LTS ceramics with high piezoelectric properties.Secondly,nano powders were synthesized via sol-gel to lower the sintering temperature.Finally,the correlation between domain structrue? atomic structure and piezoelectric properties were investigated in detail in order to elucidate the origin of high piezoelectric activity near the polymorphic phase transition?PPT?boundary in?K,Na?Nb O₃–based ceramics.The optimized composition of?K,Na?Nb O₃–based ceramics near PPT region could be theoretically predicted according to the linear combination rule.The PPT composition of n-components phase system located in the connecting line between the two PPT compositions of?n-1?-components phase systems.The PPT composition of?Na0.52K0.4415Li0.0385??Nb0.8735Ta0.064Sb0.0625?O₃,predicted by the linear combination rule,exhibited the excellent piezoelectric coefficient of 359 p C/N.Changing with the composition in the connecting line between the two PPT compositions of?n-1?-components phase systems,the trend of remanent polarization was opposite to that of permittivity.Therefore,the PPT composition with the highest piezoelectric coefficient located in the connecting line.Compared with the submicron-scale powder prepared by the solid state reaction route,the KNN-LTS nano powder could be sintered at a lower sintering temperature and be densified in a broader temperature window.The?Na0.52K0.4425Li0.0375??Nb0.86Ta0.06Sb0.08?O₃ ceramics prepared from nano powder exhibited the excellent piezoelectric coefficient of 424 p C/N and converse piezoelectric coefficient of 680 pm/V at the electric field of 2 k V/mm.By contrast,the ceramics with the same compositon prepared from submicron-scale powder exhibited piezoelectric coefficient of 342 p C/N and converse piezoelectric coefficient of 490 pm/V at the electric field of 2 k V/mm.Nanodomains with the width of 20⁴0 nm were found by TEM observation in the ceramics prepared from nano powder.The nanodomains could easily respond to the external electric field,leading to the enhanced piezoelectric activity.The atomic structures of the?K,Na?Nb O₃–based ceramics were inverstigated by extended X-ray absorption fine structure analysis in synchrotron radiation.The single peak,representing the B-O bond,would split into three different peaks in the orthorhombic or tetragonal phase region.More new peaks were verified to appear near PPT region.The fickle B-O bonds resulted from the various spontaneous polarization directions,which made domains rotate easily and response more actively to the external electric or force field.The extrinsic contribution,which derived from domain wall translation or domain switching,to the piezoelecctric properties was analyzed by the electro-mechanical properties of the?K,Na?Nb O₃–based ceramics.The extrinsic contribution derived from domain wall translation minimized in the PPT region.However,the domain of the poled ceramics were much easier to switch near the PPT region.Therefore,the enhanced piezoelectric activity near the PPT region originated mainly from the domain switching.