Fabrication And Properties Of Ca_xSr_1-xBi₄Ti₄O₁₅ Ferroelectric Ceramics And Thin Films

Due to the excellent endurance properties against polarization switching, the Bismuth-layered-perovskite oxides such as SrBi₂Ta₂O₉(SBT), Bi4-xLaxTi3O12 and SrBi₄Ti₄O₁₅ (SBTi) have great potential use in ferroelectric random access memories. SrBi₄Ti₄O₁₅ seems more interesting since it has a lower depositing temperature than that of SrBi₂Ta₂O₉, and a better fatigue endurance than that of Bi₄Ti₃O₁₂, however, the SrBi₄Ti₄O₁₅ without doping shows small remanent polarization and Curie temperature. CaBi₄Ti₄O₁₅ (CBT) have been investigated for the applications of non-volatile ferroelectric random access memories (FeRAMs), and high-temperature piezoelectric devices due to its high Curie temperature (about 790°C). It is known that CBT belongs to the bismuth layered perovskite family. In order to improve the properties of SrBi₄Ti₄O₁₅, SrBi₄Ti₄O₁₅ was used as the main composition, Ca2+ ion was used to substitute Sr2+ ion sites to format the composition of Ca_xSr_1-xBi₄Ti₄O₁₅ ferroelectric ceramics and thin films.SrBi₄Ti₄O₁₅ (SBTi) ferroelectric ceramics with Bi content were prepared by sol-gel method. Effect of excess Bi on the microstructure and electric properties were investigated. When a 10% of excess Bi was added, the SBTi ceramics showed a layered perovskite structure. The Curie temperature, dielectric constant, dielectric loss, remanent polarization(2Pr) and coercive field (2Ec) were 518℃,248, 0.021%, respectively. The properties degraded with Bi deficiency or excess Bi over 10% in the SBTi ceramics. This attributed to the structure defects and a presence of a secondary phase, which coexists with a layered perovskite phase.Effect of different sintering temperatures on the micro-configuration and the electrical properties of the SrBi₄Ti₄O₁₅ ferroelectric ceramics were studied. It is found that crystals developed entirely at 1100℃.At this sintering temperatures, the highest Curie temperature of the sample was 520℃, piezoelectric constant d33 was 9x10-12C/N. Ca_xSr_1-xBi₄Ti₄O₁₅ (C_xS_1-xBT) ferroelectric ceramic powders with x ranging from 0 to 1.0 were prepared by sol-gel method, DTA-TG was used to analyzed the characteristics of powders and the chemical and physical changes during sintering process. Their crystal structure and electrical properties of the C_xS_1-xBT ferroelectric ceramics were studies. The C_xS_1-xBT ferroelectric ceramics sintered at 1100℃exhibit single-phase perovskite structure .The amount of doped Ca2+ in the ceramics has a great influence on the ferroelectric property. The results of the analysis indicated that Ca0.4Sr0.6Bi4Ti4O15 remnant polarization and coercive field were 8.2μC/cm2 and 57kV/cm, respectively. The highest Curie temperature was 570℃and piezoelectric constant d33=13.1x10-12C/N.That can be attributed to the effect of ionic polarizability and structure distortion.Calcium modified Strontium Bismuth Titanium (Ca_xSr_1-xBi₄Ti₄O₁₅, x=0¹) ferroelectric thin films were prepared on Si substrates by Sol-gel method. Effect of amount of Ca2+ on microstructure, growth behavior of CSBT films was investigated. The patterns of X-ray diffraction (XRD) exhibited that a-axis-orientation became predominant of CSBT-0.4.The surface morphology was analyzed by scan electron microscopy (SEM), results showed that the CSBT-0.4 film obtained was dense, uniform and free of cracks voids.