Piezoelectricity And Domain Structures Of ?K,Na?NbO₃-Based Lead-free Materials

With growing concern over world environmental problems,lead adopton in the production of piezoelectric materials is subjected to a stricter restriction due to laws and regulations.And there is a trend that lead-contained piezoelectrics would be replaced by the lead-free ones.?K,Na?NbO₃?KNN?-based lead-free piezoceramics possess comprehensive piezoelectric properties among all the lead-free candidates and are expcted to substitute lead-based piezoceramics in certain scenarios.To further expand the scope where lead-free piezoelectrics are qualified,it is essential to conduct a more syetematic and in-depth research.In this thesis,by virtue of composition optimization,we investigated the electrical properties as well as their temperature stability of KNN piezoceramics.Besides,the domain structures of KNN single crystals were characterized by piezoresponse force microscopy?PFM?.Firstly,the electromechanical properties of KNN piezoceramics were engineered by means of manganese doping and the phase structures as well as the electrical properties of the materials at room temperature were investigated.It was found that appropriate incorporation of manganese into KNN could significantly improve the piezoelectric properties.Piezoelectric constant d₃₃ was raised from 245 to 340 pC/N and normalized piezoelectric strain d^*₃₃3 was enhanced from 355 to 475 pm/V,showing a significant growth of 40%and 35%,respectively.Further study revealed enhanced domain activities in manganese-modified ceramics,where the domains respond more easily to electric stimulus,contributing to the enhanced piezoelectricity.Secondly,the temperature-dependent piezoelectric properties of the manganese-modified ceramics were investigated.It was found that ceramics doped with manganese exhibited outstanding temperature stability while maintaining conspicuous performance enhancement.A normalized piezoelectric strain d^*₃₃ of 475 pm/V and 375 pm/V was obtained at room temperature and 170 ^oC,respectively,showing only a 20%drop in this whole temperature range.On the basis of phase structure refinement,temperature-dependent Raman analysis as well as temperature-dependent permittivity test,a coexistence of rhombohedral and tetragonal phase was found to exist in a wide temperature range from room temperature to higher temperature.This difussed two phase coexistence resulted in the enhanced piezoelectricity as well as the temperature stability.Finally,the ferroelectric domains of KNN single crystals were characterized by PFM.The crystal structures were inferred based on a complete polarization component of the domains.Aimed at the problem that the direction of the domain polarization could not be directely determined during the process of PFM charactrization,a method in terms of domain switching spectroscopy was proposed,making the direction determination possible.