High-temperature Bismuth Layer-structured Na_0.5Bi_4.5Ti₄O₁₅ Piezoelectric Ceramics

Bismuth layer-structured ferroelectric (BLSF) materials have received significant attention for their potential use in high-temperature piezoelectric devices and non-volatile ferroelectric random-access memory (NvRAMs), owing to their relatively high Curie temperature T_c and their fatigue-free properties, respectively. However, the piezoelectric activities of BLSFs are very low, because of their special crystal structure. In this thesis, we investigate the relation between the structure of BLSFs and electrical properties, and we also engage in the enhancement of piezoelectric properties, while the Curie temperature is not affected, maintaining the same value, being on the order of 600℃. We also investigate the thermal stabilities of electrical resistivities, piezoelectric and electromechanical properties of BLSFs. The four-layer BLSF Na_0.5Bi_4.5Ti₄O₁₅ (NBT) is chosen in the thesis due to its high Curie temperature (＞670℃). High performance A-site and B-site modified NBT ceramics, with high piezoelectric activities (d₃₃-30 pC/N) and high Curie temperature (T_c＞600℃) are obtained using a conventional mixed oxide processing route procedure.Because the radius of cations in B-site are close each other and do not play a major structural role in the polarization process of BLSFs, the effect of A-site modification is more obvious than that of B-site modification. In this thesis, the effects of A-site Ce ions on the piezoelectric properties of NBT ceramics are investigated. In the investigation of Na_0.5Bi_4.5Ti₄O₁₅-x wt%CeO₂ piezoelectric system, it is found that the crystal parameters (a, b, and c) decrease with Ce contents increasing to 0.25 wt%, and then increase with Ce contents further increasing. The Curie temperature decreases with the Ce contents increasing. The high performance modified NBT ceramics are obtained. The Na_0.5Bi_4.5Ti₄O₁₅-O.25 wt% CeO₂ ceramics possess the highest piezoelectric activities (d₃₃=28 pC/N) among the Ce-modified NBT ceramics.In the following studies, the R-site Co modified NBT ceramics are investigated. It is found that the substitution of B-site Ti by Co ions results in slight crystal distortion. Compared with pure NBT ceramics, the cobalt-modified NBT ceramics have a higher density and shrinkage, which can be attributed to the low melting point of cobalt. The piezoelectric properties of NBT ceramics are significantly enhanced by cobalt modification. The Curie temperature T_c and piezoelectric constant d₃₃ for the 0.3 wt% cobalt-modified NBT ceramics are found to be 663℃and 30 pC/N, respectively. Thermal annealing studies present that the cobalt-modified NBT ceramics possess stable piezoelectric properties, demonstrating that the cobalt-modified NBT-based ceramics are promising candidates for high temperature piezoelectric applications.We also perform a comparative study of structure, ferroelectric, dielectric, and piezoelectric properties of Aurivillius-type alkali (lanthanum) bismuth titanate ceramics, including sodium bismuth titanate Na_0.5Bi_4.5Ti₄O₁₅ (NBT), potassium bismuth titanate K_0.5La_0.5Bi_4.5Ti₄O₁₅ (KLBT), sodium lanthanum bismuth titanate Na_0.5La_0.5Bi₄Ti₄O₁₅ (NLBT), and potassium lanthanum bismuth titanate K_0.5La_0.5Bi₄Ti₄O₁₅(KLBT). The observed results indicate the important role of orthorhombic distortion in determining the improved property of multicomponent ferroelectric materials.