Synthesis And Characterization Of Bi₄Ti₃O₁₂ Based Aurivillius Ceramics

Bismuth layered structure materials known as Aurivillius phases has unique crystal structure and wide compositional flexibility,which make the materials exhibit a variety of potential physical and chemical properties.However,there are some critical areas or issues such as its high electrical conductivity which put limitations on their practical applications.To date the microstructure-defect behaviors and compositional-structure-property relation of Aurivillius compounds require due attention for its optimum use to improve its properties and to explore novel compounds of this family.In this research we have investigated the effect of lanthanides such as La,Nd,Gd and Sm,and alkaline such as Na and K substitutions on A-site and transition metals such as Fe,Nb and Ta substitution on Bsite and their co-substitutions effect on its structural and various related properties.We also explored some novel Aurivillius compounds with interesting physical and chemical properties.At first,dielectric relaxation behaviors of Aurivillius Bi₅Ti₃ FeO₁₅ ceramics were investigated in a wide range of frequency and temperature via dielectric and impedance spectroscopies.We distinguished two dielectric relaxations using the combination of impedance and modulus analysis.Resistance of the grain boundary was found to be much larger than grains,whereas capacitance was at the same level.The kinetic analysis of dielectric data was carried out to evaluate the contributions of microstructure and defects to the relaxation and conduction.The possible relaxation-conduction mechanism in the ceramics was discussed.The results enable deep understanding of microstructure-defectrelaxation behaviors in Bi₅Ti₃ FeO₁₅ ceramics.Afterwards,Nd Bi₄Ti₃FeO₁₅ ceramics were prepared for the first time by solid state reaction.The effect of Nd³⁺ on Bi₅Ti₃ FeO₁₅ ceramics in terms of dielectric and electrical properties was investigated in a wide range of frequencies and temperatures by dielectric/impedance spectroscopies.The combination of impedance and electric modulus analysis enabled the distinguishing of two relaxation behaviors that were assigned to originate from grains and grain boundaries.Kinetic analysis of temperature dependent dielectric data was performed to probe the defect-related conduction.The results demonstrated that the Nd³⁺ substitution for Bi³⁺ significantly suppressed the conductivity and the ferroelectric transition temperature was decreased to 250 oC in Nd Bi₄Ti₃FeO₁₅.Subsequently,single-phase Aurivillius Sm_0.5Bi₄.₅Ti₃ FeO₁₅ ceramics was synthesized by the conventional solid-state reaction technique.Dielectric response and electrical properties of the ceramics were investigated in a wide range of temperature and frequency by dielectric and impedance spectroscopies.Two dielectric relaxation behaviors were exhibited in the ceramics,which was distinguished by the combination of impedance and electric modulus analysis to originate from grains and grain boundaries,respectively.The grain boundary resistance was found to be much larger than grain resistance,whereas the capacitances were in the same level.A ferroelectric phase transition was observed to take place at ⁴40 oC.The kinetic analysis was performed to study the corresponding relaxationconduction mechanisms in the material.Above ²70 oC the conduction behavior of the compound was attributed to the motion of ionized oxygen vacancies.Next,novel Aurivillius single-phase Bi₄Nd_0.5Gd_0.5Ti₃ FeO₁₅ ceramics was synthesized via the solid-state reaction technique.Rietveld refinement of the full XRD pattern was carried out to obtain the detailed structure of the compound.Dielectric response and electrical properties of the ceramics were investigated.Two dielectric relaxations were distinguished,which were attributed to grains and grain boundaries respectively.The kinetic analysis of temperature dependent dielectric data was performed to study the corresponding relaxation-conduction behaviors in the material.The thermal magnetization measurement evidenced the paramagnetic characteristic of the material,but localized antiferromagentic coupling would exist below 200 K.In addition,a phase transition was observed at 240 oC.Later on,the dielectric responses and electrical properties of Bi₄Ti₂Fe_0.5Nb_0.5O₁₂ ceramics have been investigated by frequency and temperature dependent dielectric/impedance spectroscopy.Two dielectric relaxations in the ceramics were distinguished,the high-frequency relaxation was evidenced to originate from the grains and the low-frequency relaxation was ascribed to the contribution of grain boundaries.The resistance of grain boundaries was much larger than that of grains,whereas the values of capacitance of grain boundaries and grains were in the same level.Above ³40 oC,the conduction and relaxations of the material were attributed to the motion of ionized oxygen vacancies.As temperature above ⁵40 oC was close to the ferroelectric transition temperature,the smart dielectric response to the external applied ac-field was apparently reflected by the conductance of grain boundaries,the capacitance of grains and the consequent relaxations.Next,Aurivillius single-phase Bi₄Ti_2.5Fe0.25Ta0.25O₁₂ ceramics was prepared via high temperature solid-state reaction method.The detailed structural analysis of the doped Bi₄Ti₃O₁₂ compound was carried out by Rietveld refinement of the full XRD Pattern.Dielectric and electrical properties were studied in a wide range of temperature and frequency by dielectric/impedance spectroscopies.The comprehensive analysis of frequency spectrum reveals the occurrence of two relaxation behaviors in the ceramics at low frequency and high frequency respectively.A phase transition was observed at ⁶50 oC in Bi₄Ti_2.5Fe0.25Ta0.25O₁₂ somewhat lower than the ferroelectric transition temperature of Bi₄Ti₃O₁₂.The possible reason for the decrease of ferroelectric transition temperature is discussed based on the structural analysis.The present results could be useful for designing and/or modifying properties of Bi₄Ti₃O₁₂ related ceramics.Subsequently,single-phase Aurivillius Bi₃.5La_0.5Ti₂Nb_0.5Fe_0.5O₁₂ ceramics was synthesized for the first time by solid state reaction route.The ac data in terms of dielectric and impedance were collected to probe the dielectric relaxation behaviors in the ceramics.Two relaxations were distinguished and determined to originate from grains and grain boundaries,respectively,by the combination of impedance and electric modulus analysis.A phase transition was observed to take place at ⁴40 oC.The possible relaxationconduction mechanisms were discussed on the basis of kinetic analyses of dielectric data.This work elucidated the contributions of microstructures and defects to the relaxations and conduction which is significant for correct understanding of electrical properties of the materials.Next,the ac data in terms of dielectric/impedance spectroscopies were obtained simultaneously to investigate the dielectric responses and electrical properties of Na_0.5Bi_4.5Ti₄O₁₅ and Bi₄Ti₃O₁₂ ceramics.The effect of Na on dielectric relaxation and electrical conductivity was investigated in a wide range of frequency and temperature.The combination of impedance and modulus analyses revealed a single relaxation behavior in NBT ceramics which was attributed to originate from the grains.Kinetic analysis was performed of temperature dependent dielectric data to study the defects-conduction behaviors in the ceramics.Afterwards,Aurivillius K_0.5Gd_0.5Bi₄Ti₄O₁₅ ceramics was prepared by high temperature solid state reaction route for the first time.The dielectric responses and electrical properties were investigated in a wide range of temperature and frequency via dielectric/impedance spectroscopies.A single dielectric relaxation behavior was exhibited in the ceramics,which was assigned to the bulk contribution according to the impedance analysis.A ferroelectric phase transition was observed to take place at ⁵50 oC.Kinetic analysis of ac dielectric data was carried out to probe the possible conduction and relaxation processes.At high temperature the conduction behavior of the compound was assigned to the motion of ionized oxygen vacancies.This work elucidated the contributions of defect behaviors to the conduction and relaxation mechanisms.At the end,we present the synthesis of bismuth layer-structured Na_0.5Gd_0.5Bi₄Ti₄O₁₅ ceramics via high temperature solid state reaction.The samples were annealed under pureargon and pure-oxygen atmospheres respectively.The dielectric relaxation and electrical properties were investigated in a wide range of frequency and temperature by dielectric/impedance spectroscopies.Two anomalies were observed in the temperature dependent dielectric spectra,one occurred in the range of ²00–400 oC and the other was at ⁵90 oC due to the occurrence of a ferroelectric phase transition.The low temperature peak was suppressed significantly by oxygen-annealing and enhanced by argon-annealing,suggesting it is oxygen-vacancy related.The ceramics exhibited a single dielectric relaxation behavior,which was attributed to the bulk contribution according to the combination analysis of impedance and electric modulus frequency spectra.Kinetic analyses of ac dielectric data was carried out to probe the possible conduction-relaxation mechanisms.Above ⁴00 oC,the relaxation and conduction of the compound was assigned to the motion of ionized oxygen vacancies.The experimental results indicated that the Gddoping suppressed the leakage and improve dielectric properties of Na_0.5Bi_4.5Ti₄O₁₅ ceramics.