The Study Of Phase Transition Dynamics In BNT-based Materials Based On Raman Spectroscopy

Ferroelectric materials have complex phase transition behavior and coupling effect,and are widely used in new technology,civil and other fields.Pb（Zr,Ti）O₃-based ferroelectric ceramic system has been dominant in the application field for a long time,but the use of lead is not friendly to the environment,lead free is the current research hotspot and development trend.The A site of Bismuth sodium titanate(Bi_0.5Na_0.5Ti O₃,BNT)is composed of Bi³⁺and Na⁺,which has excellent electrical properties and is regarded as one of the most potential systems to replace lead based materials.In recent decades,most of the studies on BNT have focused on the optimization of properties and analysis of structure.However,the phase transition process and mechanism of BNT are quite complex,and the phase transition mechanism of BNT remains to be further studied.Raman spectra can be sensitive to symmetry breaking induced by intrinsic structure,external environment and other factors,and the phonon vibration mode displayed by the Raman spectra can provide highly accurate and rich information,which can be used as an effective method for the study of phase transition dynamics and provide characteristics of phase transition that cannot be tested by other technologies.Raman spectroscopy system is used to study the phase transition dynamics of BNT-based ceramic materials and regulate the phase transition of BNT-based ceramic materials by hetero-valent ion doping,which provides an important research basis for the development of related devices.The specific research is as follows:1.Bi_0.5Na_0.5TiO₃ ceramic material was prepared by homogeneous solid phase method,and the phase transition of BNT ceramic material under temperature field and electric field was studied.X-ray diffraction and room-temperature Raman results show that Bi_0.5Na_0.5Ti O₃ceramic material has been successfully prepared.The sample is pure phase,and no second phase is produced.The rise and fall dielectric test was carried out.Through the temperature rise dielectric test result diagram,it can be clearly observed that the depolarization phase transition point at T_m=200℃and the Curie transition point at T_C=320℃of the sample.In addition,there is a non-coincidence phenomenon and thermal hysteresis phenomenon in the temperature range of the cooling dielectric test.In the variable temperature Raman test,with the gradual increase of temperature,the two Raman vibration modes ofν₂andν₃ appear obvious widening phenomenon,and the intensity ratio of the vibration modes ofν₃ andν₄also changes gradually.All these phenomena mean that the disorder in the system is gradually enhanced,and the Raman vibration mode of BNT above 520℃does not disappear completely.This indicates that the tetragonal-cubic phase transition here is not sudden,but there is a wide range of polyphase coexistence temperature.In the in-situ electric field Raman test,different temperatures are selected to perform the boost test until the sample is broken down.According to the test results,it can be inferred that the vibration of A-site ions can bring strong polarity,while Ti-O bond and Ti-O₆ lattice will still have A certain degree of distortion under the action of electric field,thus contributing weak electric polarization value.2.Bismuth sodium titanate ceramics were doped with isovalent ions,and BNT-based ceramics were synthesized by traditional solid phase method to further regulate their electrical properties.The doping of heterovalent ions can be divided into doping of different valence ions and doping of ions with different band-gap widths.Through the incorporation of heterovalent ions,based on solid phase reaction,through the exploration of preparation technology,the preparation of polyferric ceramic materials with pure phase.X-ray diffraction results showed that all samples showed a typical single perovskite（ABO₃）structure without the formation of a second phase,proving that the introduction of heterovalent ions did not change the structure of BNT-based ceramics at room temperature.The room-temperature Raman results show that with the inclusion of heterovalent ions,the peak position of the Raman peak will shift regularly.After the inclusion of ions with different valence states and different band-gap widths,the peak ofν₂ will shift to the left gradually with the increase of valence states and band-gap,which proves that heterovalent ions are successfully mixed into the B-site of BNT-based ceramic materials.3.Test the properties of bismuth sodium titanate ceramic materials doped by isovalent ions and explore the action mechanism of the ceramic materials doped by isovalent ions,and test the properties of the prepared ceramic materials,such as:Measurement of ferroelectric properties,dielectric properties and Raman,adjustment of the preparation process,to obtain better ceramic materials,through the comparative analysis of the electrical properties of materials,analysis of the physical mechanism.Studies on ferroelectric and dielectric properties show that the electric polarization behavior of samples can be significantly regulated by doping different valence ions.In the dielectric test,the doping of heterovalent ions has an obvious regulation effect on Curie temperature Tm,which makes Tm gradually move towards high temperature.Ferroelectric test shows that multistage ferroelectric phase transition occurs in different valence band ion doping.In situ Raman test of the sample shows that the variation is consistent with the dielectric.In the rise and fall test,the rise and fall tests of BNT do not coincide.Doping of different valence ions can effectively regulate the thermal hysteresis effect of BNT-based ceramic materials.