Study On The Composition Modulation And Properties Of Sodium Niobate Basd Lead-free Antiferroelectric Ceramics

Antiferroelectric ceramic materials with high strains have attract much attention in the application of large displacement actuators and sensors.As an important member of antiferroelectric material family,NaNbO₃?NN?has also gradually become popular due to the field induced strain.However,there are still many problems in the study of NN-based antiferroelectric ceramics in terms of electrostrain.For example,many researches are mainly focused on the phase transitions of pure NN,and less researches on the NN-based antiferroelectric ceramic materials and their strain characteristics were carried out.The strain values obtained in these studies are small and the strain mechanism of NN-based antiferroelectric materials is not clear enough.In this thesis,NN was used to be the matrix.Through a reasonable composition design,the relationship among the composition,structure,and electromechanical properties of NN-based ceramics were systematically studied.Also,the strain evolution behavior and the origin of the improved strain in these systems were especially investigated by using different analysis methods.The main contents of this thesis are as follows:?1?In chapter 1,NN was chosen as the matrix,the substitution of BZ for NN stabilizes the ferroelectricity of NN.A series of phase transitions were observed from antiferroelectric orthorhombic P phase to ferroelectric orthorhombic Q phase and finally into ferroelectric rhombohedral phase with increasing the BZ content.An electric field induced irreversible phase transition was found out in different composition irrespective of their virgin phase structure and property.Particularly,an antiferroelectric orthorhombic phase was irreversibly transformed into a ferroelectric monoclinic phase under a high external field in a wide composition range of 0.02?x?0.05,leading to giant poling strains as large as 0.43%⁰.58%.This is much larger than those observed in ferroelectric orthorhombic?0.06?x?0.07?and rhombohedral phase?0.08?x?0.11?zone suffering from an irreversible ferroelectric-ferroelectric?monoclinic?phase transition.The synchrotron x-ray diffraction and the measurement of in-situ longitudinal and transverse strains together suggested that the electric-field induced irreversible phase transition should involve not only a distinct volume expansion,but also an obvious lattice elongation along the field direction.By comparison,the volume contribution of strains seems to be negligible for compositions in both ferroelectric orthorhombic and rhombohedral phase zone.The present study demonstrated a unique nature of the composition and field dependent phase stability in a typical lead-free NN-BZ system and an underlying mechanism of giant poling strain revealing a large potential of NN-based antiferroelectric ceramics for high-displacement actuator.?2?Based on the NN-BZ binary system,an appropriate amount of CaZrO₃?CZ?was substituted for NN to construct a new NN-BZ-CZ ternary system.The results show that the ferroelectric orthorhombic phase in the matrix gradually changes to the antiferroelectric orthorhombic phase with the increase of CZ content.Similar to NN-BZ system,both the antiferroelectric phase and the ferroelectric phase are irreversibly transformed to ferroelectric phase upon the application of electric field as 0?x?27?0.01.Importantly,a partial reversible antiferroelectric-ferroelectric phase transformation occurs at high CZ content?x?0.01?and a recoverable strain of⁰.35%is obtained in x=0.03.Strain measurement results show that the volume strain in the composition range 0?x?27?0.01 almost unchanged,while gradually increased with x increases for composition with x?0.01.Synchrotron radiation X-ray diffraction results show that in the composition range of 0?x?27?0.01,both the ferroelectric and antiferroelectric orthorhombic phases are irreversibly transformed to ferroelectric monoclinic phase under the application of an electric field.In x?0.01,the antiferroelectric orthorhombic-ferroelectric monoclinic phase transition occurred in the first electric field cycle,and the field-induced-ferroelectric monoclinic reverted to antiferroelectric monoclinic phase upon removal of the electric field.In addition,an irreversible preferred orientation of the domains was developed both in ferroelectric and antiferroelectric phases.The results show that the large reversible strain in this system is closely related to the phase transition and the reversible degree of the antiferroelectric domain,which provides a new idea for designing a new lead-free antiferroelectric ceramic actuator.?3?The effect of CeO₂ doping on the strain behavior of 0.92NN-0.06BZ-0.02CZ antiferroelectric ceramics was studied systematically.An improved large reversible strain of⁰.28%was obtained in x=0.005 when E=20 kV/mm.At the same time,a symmetric strain curve with negligible remnant strain was obtained in the poled sample for composition with x=0.005.The comparison of strain curves for quenched and poled samples shows that the origin of large strain are related to the pinning effect on the domains due to the presence of defects in this system.The results show that the strain in the 0.92NN-0.06BZ-0.02CZ antiferroelectric ceramics can be adjusted by introducing defects,which provides a new way for improving the strain characteristics of antiferroelectric ceramics.