| Sodium bismuth titanate (BNT) is an important A-site ion substituted complexes perovskite ferroelectric. It has rhombohedral structure and excellent ferroelectric properties (remanent polarization of 38μC/cm2, coercive field of 7.9kv/cm) at room temperature. But BNT based relaxor ferroelectrics have considerable controversy in many places, such as depolarization hehavior appears near the peak of dielectric anomalies, deformation of the hysteresis loop and if ferroelectric-antiferroelectric phase transition exit during the heating process. Research and interpretation of these basic questions is the premise of full use of relaxation ferroelectrics in BNT group, so study the structure and relaxation properties of BNT-based material is particularly important.(1) Using conventional solid state reaction to prepare strontium titanate (referred as ST) doped BNT relaxation ferroelectric (referred as BNT-xST). It was found that when x＜0.45, the samples show two dielectric anomalies between 25 and 375℃,high temperature dielectric anomalie Tm and low temperature dielectric anomalie Tf. The dielectric curve near Tf has a strong diffuse behavior while the dielectric curve near Tm is frequency independent. With the increase of ST content, both Tf and Tm are moving toward lower temperature. When x=0.30, the sample has a good temperature stability (at 1kHz, within the temperature range of 80～220℃, capacitance change rate TCC＜5%, loss tanδ＜0.02). When x＞0.45, the samples only have one dielectric peak at Tm and the dielectric constant increases with the decreament of ST content.(2) When the ST content 0.4＜x＜0.6, it can obtain a single cubic perovskite structure. All samples have a good relaxation. With the increasment of ST, the relaxation peak dropped from 50℃ to -15℃. When SrTiO3 content is 0.45, the sample exhibits excellent relaxation, dielectric properties and electrostriction performance: diffusion coefficient y=1.82; relative permittivity ε= 3800, dielectric loss tanδ=0.01 at room temperature of 1kHz; a maximum electrostrictive of 0.3% under electric field 4kv/mm,Smax/Emax= 754ppm/v.(3) Deduced dielectric relaxation spectroscopy/hysteresis loop of second order and relaxation phase transition by Devonshire thermodynamic theory. Adjusted the parameters to fit the experimental data. Combined with experimental data and Devonshire theory to explaine dielectric relaxation and ferroelectric hysteresis loop distortion phenomenon. The results show that the dielectric spectroscopy and hysteresis loop fit the theoretical curve well. We guess that the causes of hysteresis loops distortion in BNT-based ferroelectric is due to the coexistence of rhombohedral and tetragonal phase, as well as result of the interaction.(4) 0.7BNT-0.3ST and 0.5BNT-0.5ST doped Er2O3 or Nb2O5 respectively, to improve its temperature stability. Experimental results show that the proper amount of rare earth in 0.7BNT-0.3ST can broaden the temperature stability range. When the amount of Er2O3 is 0.75mol%, the sample has a temperature stability of the widest range of 50～230℃ (ε2412±61) loss tanδ＜0.02 (70～295 ℃). The peoper amount of Nb2O5 doping in 0.5BNT-0.5ST can reduce its Curie temperature and significant pressure peak effect. When the amount of Nb2O5 is 8mol%, the sample has the best medium-temperature stability, ε25℃=1530, tanδ=0.015, in the range of -55～125 ℃, TCC＜15%, which satisfy the EIAⅡ standard of X7R. |
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