Synthesis and characterisation of BaTiO₃-based Piezoelectric materials for energy harvester applications

There has been renewed interest in developing lead-free piezoelectric materials as the alternative to the toxicity of lead zirconate titanate (PZT) which currently dominates the market of piezoelectric materials. Some of the promising lead-free piezoelectric materials are those based on barium titanate (BT). This project is embarked to study the doping effects of Sn, Zr and Ca into a BT-based piezoelectric material to enhance its piezoelectric and dielectric properties. The effects of oxygen non-stoichiometry and sintering conditions in improving the performance of the piezoelectric materials are also studied. All the samples were synthesized by a conventional solid state route at temperatures between 1250-1450 °C in air for 3 hours. The samples were characterised by using X-ray Diffraction (XRD), Impedance Spectroscopy Analyser (measured at 30-200 °C, between 10-100 kHz), piezoelectric test (poled at 30 °C, 4.5 kV/mm for 30 mins) and Scanning Electron Microscope (SEM). This project involves three main investigations. Firstly, a structural study and dielectric analysis of a ternary triangle diagram of BaTiO3 (BT)-BaSnO3 (BS)-BaZrO3 (BZ) were carried out. The doping effects of Zr and Sn at the Ti-site were investigated. The BT, BS, BZ, BaSn0.5Ti0.5O3 (BST50), BaZr0.5Ti0.5O3 (BZT50) and BaSn0.5Zr0.5O3 (BSZ50) were prepared and all the samples are of a single phase. In this study, the BS has shown a giant anomalous dielectric behaviour with dielectric constants, εr of about 87,290 and 6027 measured at 10 Hz and 1 kHz, respectively, which are contributed by the lone pair effect of the Sn due to the oxygen non-stoichiometry of the BS. Secondly, the Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) system was investigated as it was reported to exhibit a high piezoelectric constant, d33 value. The novelty in this second investigation is the observation of the effects of the oxygen non-stoichiometry in the BCZT composition toward its dielectric properties. The εr value of the BCZTair (sintered in air) is slightly higher than the BCZTN2 (sintered in N2) of εr = 4805 and 4771, respectively. The dielectric constant of the BCZTN2 does not show much differences due to the fact that the BCZT structure is already stable. The third investigation was to study the effects of Sn substitution in the BCZT system. The (Ba0.85Ca0.15)(SnxZr0.10-xTi0.90)O3, BCSZT (x = 0, 0.025, 0.05, 0.075, 0.1) ceramics were prepared. All the samples are of a single phase and exhibit a tetragonal structure. The εr value increases as the Sn content is increased and the Tc is lowered from 95 °C to 59 °C. The Tc shifts to a low temperature as the Sn4+ with smaller ionic radii replaces the Zr4+ at the B-sites. It decreases the Ti-O bonds thus weakening the interactions within the TiO6 octahedral. The larger tolerance factor eases the deviation of the Sn4+ ion at the B-sites in the octahedral structure and enhances its ferroelectricity and dielectric properties. However, the d33 value decreases since the tetragonality of the samples decrease. The c-axis becomes shorter and reduces the dipole moment of the TiO6 octahedral. The BCSZT at x = 0.025 is found out to be the novel composition with the highest piezoelectric properties, and is suitable for piezoelectric energy harvester applications. It obtains the highest d33 = 213 pC/N when sintered at 1350 °C and the highest piezoelectric voltage constant, g33 and output power, P when sintered at 1250 °C of about 18.2 x 10-3 Vm/N and 3.51 pm3/J, respectively.