Effects Of Doping On The Microstructure Of Bismuth Layered Perovskite CaBi₄Ti₄O₁₅Ceramics

Because of the excellent ferroelectric property, the bismuth layer compounds have been potential application in the ferroelectric random access memory. Nowadays it is one of the most interesting topics in the field of piezoelectric ceramics. It has been suggested that the morphology and movement of the domains wall, which was influenced by the oxygen vacancies, play an important role in the most important ferroelectric properties, such as remanent polarization and fatigue of the bismuth layer compound. The ferroelectric properties of the bismuth layer compounds are strongly controlled by their micro-defects.Positron annihilation technique is one of most effective method to study the micro-defects of the material, its non-destructive and high sensitivity was widely used in the material research. In present work, CaBi4-xLaxTi4-yMyO15(M=Nb, Zr) ceramics were prepared by solid state reaction. The behavior of dopants and the micro-defects in these samples have been investigated by positron annihilation techniques. The results are as follows:(1) The microstructure of the CaBi4Ti4-xMxO15(M=Nb, Zr) series samples sintering at1170℃have been examined by scanning electron microscopy(SEM) and XRD. XRD results indicate that the sample with x≤0.4is a single phase. SEM results shown that the sample is a typical well-known Aurivillius-type bismuth layered compound, large plate like grains have growth. As the CBT samples are doped with a small amount of Nb5+and Zr4+, the sample remains its single phase, and the grain size of the sample increases with the content of the dopants.(2) The XRD patterns for the CaBi4-xLaxTi4O15samples sintering at1150℃indicate that the sample with x≤0.4is a single phase. SEM result shown that the CBLT sample is a layered structure. The grain size of the sample increases with the content of dopant, and there is very little obvious second phase occurred in the sample.(3) In analyzing the positron lifetime parameters, it can be seen that the open volume of defect is smaller as the content of La2O3in the sample is lower; and open volume of defect increases with the content of La2O3.(4) For CaBi4Ti4-xNbxO15sample with low content of Nb2O5(x≤0.2), the open volume of the defect decreases with increase of the content of Nb2O5in CaBi4Ti4O15ceramics. However, for CaBi4Ti4-xNbxO15sample with relatively high content of Nb2O5(x≥0.2), the open volume of the defect increases with the content of Nb2O5in CaBi4TitO15ceramics.(5) For CaBi4Ti4-xZrxO15ceramics, the open volume of the defect in the sample increases with the content of Zr4+. As the radium of Zr4+is larger than that of Ti4+, when Zr4+substituting for Ti4+, the crystal lattice of the sample will be distorted and leading to the increase of the open volume of the defect.