Investigation On Fabrication And Properties Of CaBi₄Ti₄O₁₅-based Composite Ceramics

Bismuth layer-structured ferroelectrics（BLSFs） are potential piezoelectric materials for high temperature and lead-free applications. Special crystal structures of BLSF determines their unique characteristics such as high Curie temperatures and dielectric breakdown strengths, good fatigue resistance, low temperature coefficients of resonant frequency etc. However, high coercive fields（Ec） and poor piezoelectric activities limit their practical application. The electrical properties of BLSFs can be significantly enhanced via the improvement in preparation technique and ion doping. Although there are also some publications that focus on the composite between BLSFs and perovskite structure materials, the composite between BLSFs with different psedo-perovskite layers “m” need to be further investigated. In this paper, we choose CaBi₄Ti₄O₁₅（CBT,m=4）as base material, and other BLSFs with m=2, 3, 4, are employed to fabricate CBT-based composite ceramics, and the crystal structure, micromorphology, dielectric and piezoelectric properties of the ceramics are investigated.Firstly, the investigation on the intergrowth structure of CaBi₄Ti₄O₁₅ with Y³⁺ doped Bi₄Ti₃O12（m=3）, with the formula of（1-x）CaBi₄Ti₄O₁₅-xBi3.5Y0.5Ti3O12（CBT-BYTO）. XRD analysis indicates that the intergrowth structure with m=3.5 is obtained when x=0.3⁰.6. When x=0⁰.2, the main crystalline phase of CBT-BYTO ceramics is CaBi₄Ti₄O₁₅; when x=0.8¹, the main crystalline phase turns into Bi₄Ti₃O12. The grains of all ceramic samples exhibite a plate-like shape with random orientation,and no obvious second phases and pores are observed. It is found that the conducting mechanisms at low and high temperatures are quite different. Curie temperatures of CBT-BYTO ceramics are intermediate between those of CBT and BYTO. The properties of CBT-BYTO ceramics are better than CBT and BYTO with low conductivity and high temperature dielectric loss, and increased value of d33 and Kp.Secondly, conventional solid state reaction route is employed to fabricate（1-x）CaBi₄Ti₄O₁₅-x(Na_0.5Bi0.5)Bi₄Ti₄O₁₅（CBT-NBT₄） ceramics. The experimental results indicate that both CBT and NBT ceramics can form complete solid solution in a wide composition range which can be represented as Ca1-x(Na_0.5Bi0.5)xBi₄Ti₄O₁₅. The solid solution shows high density and homogeneity, and the anisotropy in grain size decreases with the introduction of NBT₄. The Curie temperature Tc of solid solution varies with x following linear relationship: Tc=797（1-x）+680x.With the increase of NBT₄ content,relative dielectric constant of solid solution CBT-NBT₄ increases, and both frequency dependence and dielectric loss decrease, meanwhile, DC conductivity decreases, the active energy increases and the piezoelectric properties enhance.At last, composite ceramics（1-x）CaBi₄Ti₄O₁₅-xNa_0.5Bi_2.5Nb2O9（ CBT-NBN） and（1-x）Na_0.5Bi_4.5Ti₄O₁₅-x Na_0.5Bi_2.5Nb2O9（NBT₄-NBN） are investigated. The results indicate that the composite BLSFceramics withpsedo-perovskite layersof 4 and 2can not form BLSF ceramics with single main crystalline phase, and both the uniformity in grain size and ceramic density decrease. The Curie temperatures of CBT-NBN and NBT₄-NBN ceramics are lower than those of two single systems, and decrease with increasing content of NBN. Compared with two single systems, both dielectric loss at high temperatures and the electrical conductivity increase, which is unfavorable to both high temperature stability and piezoelectric activity of ceramics.