Piezoelectric Properties And Domain Structure Of The Tetragonal KNN-based Lead-free Single Crystals

Piezoelectric materials is one of the important functional materials,most of applied piezoelectric materials are Pb-based materials,these Pb-based materials are harmful to the environment and our physical health due to the large content of Pb.In order to maintain the sustainable development of ecological environment,lead-free piezoelectric materials is a hot topic.Meanwhile Piezoelectric crystals are expected to exhibit even better properties due to their crystallographic anisotropy and simple configuration to study easily the physical mechanisms.However,KNN based crystals have not been developed and little attention in tetragonal phase.So it will have a promising study to growth process electric properties and domain structure of KNN-based crystals.It is a systematic investigation about crystal growth process and structure of KNN based crystals.Different crystal growth methods is analyzed and compared,top-seeded solution growth method is better choice to observe and growth KNN based crystals.The large and high-quality tetragonal KNLNT and KNLNTS crystals have been grown by the top-seeded solution growth method by selecting appropriate growth technical paraments.The structure and compositional uniformity is investigated by XRD and EDS methods,the results show: the grown crystals has pure tetragonal structure,and symmetrical component.There is particulary research about electric properties of KNN based crystals.There is a study about leakage current mechanism of KNN based crystals.In process of crystal growth,the Oxygen vacancy produces a nagative effect on the KNLNT single crystal,and the KNLNT crystal has a bad ferroelectric hysteresis loop due to the leak current.It is effective to reduce crystal defects with Sb doped and have a better hysteresis loop.It is effective way to improve piezoelectric property to build ‘2T’ engineering domain poled along [011] C direction.It is important to explore extrinsic effect of KNN based crystals.The results of Rayleigh analysis of KNLNT and KNLNTS crystals shows the dielectric improvement is due to extrinsic effect increasing.It is significative to improve the electric property through increasing extrinisic of KNN-based crystals.Because the lattice coefficient(c/a)has a change with temperature,it reduce the piezoelectric temperature stability along [011] C direction for KNLNTS as-grown crystal.So if increasing extrinsic effect,the temperature stability will have a improvement.It is a systematic investigation about domain structure for as-grown crystal.Domain configuration changes with temperature and electric field applied along [001] C,[011] C directions in tetragonal KNLNTS crystal were studied by means of polarizing light microscopy(PLM)and piezoresponse force microscope(PFM).It is difficult to get single domain date by poled along [001] C direction because high elastic stiffness and internal stresses of KNLNTS crystal.After being poled along [001] C direction,the volume fraction of domains with polarization is still about 25.2% by calculating unpoled and poled dielectric constants.When poling along [011] C direction,‘2T’ engineered domain was formed.The size of domains increase with the electric field intensity and it has a stable domain structure when the electric field remove out.The macro performance is closely related with domain,so it is important factor for KNN based crystal.