Preparation And Performance Of Sr₂Bi₄Ti₅O₁₈ Based Ferroelectric Ceramics

Ferroelectric materials have a wide range of applications in various aspects of electronic devices such as sensors, actuators, ultrasonic transducers, resonators, non-volatile memory and capacitors and so on. With the development of technology, higher requirements are put forward to the working conditions of materials. More and more electronic devices are required to work under high temperature. Due to the low application temperature and high driving voltage of traditional lead-based materials, as well as environmental pollution, research about ferroelectric and piezoelectric materials mainly focus on the bismuth layered perovskite structure materials.Compared to other bismuth-layered perovskite structure materials, Sr2Bi4Ti5O18 （SBT） has relatively superior performances, including low coercive field, low aging rate, high resistivity and good temperature stability. However, its properties of piezoelectric activity, Curie temperature and remnant polarization intensity are poor to replace the widely used lead-based materials in industrial field. At present, studies about SBT intensively focus on the ferroelectricity and piezoelectricity, which can not reveal the mechanisms of conductive and relaxation directly. It is of important significance for guiding and optimize ferroelectric properties to reveal the inherent contact between defects with conductance, relaxation and ferroelectric property, through research on its dielectric properties.In this paper, SBT is taken as the research object to improve its performance. We prepared CaxSr2-xBi4ti5O18 ferroelectric ceramics by Ca substitution. Through systematical study on CaxSr2-xBi4Ti5O18 ferroelectric ceramic in different sintering atmosphere, we found an effective way to improve its performance, and confirm its relaxation, conduction and leakage mechanism. There is a large leakage current in BiFeO3（BFO） while （Bi2O2）2+ layer in SBT having a dual role of insulation and library space charge. We also developed single phase multiferroic materials with magnetic group BiFeO3 inserted Sr2Bi4Ti5O18. It is of great theoretical significance for guidance of multiferroic material design with related research of its properties and discussion on its conductive and leakage current mechanism. Besides it will provide novel ideas for the development of multiferroic devices. The main contents are as follows:（1） Nanoscale, well-distributed SBT and CBT powder was prepared with self-propagating sintering method, and the optimal sintering temperature of SBT and CBT ferroelectric ceramics was confirmed. SBT ferroelectric ceramics exhibit higher remnant polarization and lower Curie temperature, while CBT ferroelectric ceramics have smaller remnant polarization and higher Curie temperature. The SBT sample sintered in oxygen atmosphere has higher conductivity activation energy due to the fewer defects in sample.（2） Ca²⁺ substitution on the properties of SBT ferroelectric ceramic was studied, and the optimal substitution amount was identified. Raman spectroscopy confirmed that Ca substitution makes no difference to （Bi2O2）2+ layer. When the substitution amount of Ca²⁺ is 0.15, CSBT ferroelectric performs optimally in ferroelectrics. The remnant polarization and coercive field were 2Pr=17.7μC/cm2 and 2Ec=91kV/cm, respectively. The Curie temperature of CSBT-0.15 was 310℃, and the leakage current density was J=1.1×10-6A/cm2.（3） Effect of sintering atmosphere on the properties of CSBT ferroelectric ceramics was studied. In the high-temperature region, sintering atmosphere has a great influence on the activation energy （relaxation and conductivity）, therefore, the conductivity and relaxation result directly from the movement of oxygen vacancies; while in the low-temperature range, sintering atmosphere has little effect on the activation energy, so the conductivity results more from electrons generated by the first ionization of oxygen vacancies, and relaxation is caused by relaxation polarization of weak-binding electrons. The leakage mechanism of CSBT-0.15 ceramic samples is thermally excited electrons dominated Ohmic conduction mechanism at an electric field of 0-15kV/cm, and oxygen vacancy dominated space charge limited conduction mechanism at an electric field of 15-30kV/cm. However, the conduction mechanism for samples sintered in nitrogen atmosphere is Schottky emitting mechanism in 25-50 kV/cm.（4） Nd³⁺ doping can improve the ferroelectric properties of CSBT-0.15 ferroelectric ceramics. The samples show the best ferroelectric properties when the substitution amount is 0.05. The Curie temperature of the samples decrease linearly and monotonously with the increase of doping amount, in the meantime, the dielectric peak broadens and decreases gradually, which are characteristics of diffuse phase transition. Apart from this, we also found that BT and CSBT-0.15 can only synthesize CSBT-4 four-layer structure with lower Gibbs free energy rather than intergrowth structure, the relaxation is caused by the random distribution of A ions.（5） The composite six-layer bismuth layered perovskite structure was formed by inserting BFO into SBT perovskite-type layer. Compared to SBT, SBTF has greater structural distortion.The remnant polarization and coercive field were 2Pr=20.5μC/cm2 and 2Ec=128kV/cm at 125kV/cm, respectively. SBTF is equivalent to Sr2+, Ti4+ co-doped BFO ceramics, which can effectively reduce the concentration of oxygen vacancies and Fe2+ content. And （Bi2O2）2+ layer in SBT has the dual role of insulation and space charge libraries to reduce the leakage current density, J=3x105"A/cm2. The conductivity mechanism of SBTF is Ohmic conduction mechanism dominated by Fe2+ ion thermal excitation. Besides SBTF exhibited weak ferromagnetic properties of 2Ms=0.007emμ/g.