Production and characterization of BNT-6BT lead free piezoelectric material with CeO2 and b2o3 additives

Since its discovery in 1950s, Lead Zirconate-Lead Titanate, Pb(ZrxTi1-x)O3 or PZT has been the most commonly used piezoelectric ceramic material in various applications. However with increasing environmental concerns and consciousness on human health new limitations and regulations on the use of lead containing materials such as PZT were started to be introduced in many countries. Lead is a toxic element causing many environmental and health problems. For this reason new "eco-friendly" lead-free piezoelectric systems possessing compatible properties to PZT needed to be developed. In this research, perovskite type lead-free piezoelectric (Bi0.47Na0.47Ba0.06)TiO3 (BNT-6BT) ceramics doped with Ce3+/Ce4+ and/or B3+ in different mole ratios were synthesized by solid- state method using high purity oxide powders. It was assumed that B3+ cations would occupy interstitial positions in BNT-6BT perovskite crystal lattice due to its small ionic radius causing soft piezoelectric effect and easing poling behavior with its role as donor. Another dopant added to the materials in this research coupled with boron (B3+) donor cation was cerium (Ce3+/Ce4+) possessing different valances to further improve low voltage poling behavior. Another expected effect from Ce addition which may have a function as donor or isovalent dopant depending on valence was to improve dielectric constant (εr). In the extent of the thesis study, a systematic investigation of the use of B3+ and Ce3+/Ce4+ ions as single dopant or together as co-dopant (B3+ - Ce4+) with the amounts of varying between 0-1.5 mole % in terms of their influences on structure and electrical properties of lead-free piezoelectric BNT-6BT electronic ceramics. Ce and/or B doped BNT-6BT powders synthesized by solid state method were checked by X-ray diffraction (XRD) after the calcination process and perovskite phase formation was verified. After a pre-forming by using a uniaxial press, disc shaped ceramic samples were produced from single-phase powders with the final shaping via a cold isostatic press (CIP) under 150 MPa pressure. Density measurements were performed on sintered samples by Archimedes' method. Following a polishing step, the parallel surfaces of the disc shaped samples were coated with gold (Au) for microstructural and electrical characterizations. Fractured surfaces of the samples were investigated by Scanning Electron Microscopy (SEM) for microstructural characterization. Within the scope of the electrical measurements, firstly, room temperature capacitance (C) and dielectric loss (tan δ) values were measured by an LCR-meter at 1 kHz frequency. Ferroelectric property measurements (polarization-electric field P-E hysteresis loops) were carried out by applying increasing bipolar electric field up to 60 kV/cm to the samples. Saturation and permanent polarization and coercive electric field (Pmax, Pr and Ec respectively) values were obtained from P-E plots. In order to determine the piezoelectric properties, all samples were poled by applying a DC electric field in the range of 3-5 kV/mm for 5 minutes at room temperature in a silicone oil bath. After the poling process, the piezoelectric charge coefficients (d33) of the samples were measured with a piezo tester. The measurement results in this thesis study show that either as single or co-dopant both B and Ce has an improving effect on certain dielectric, piezoelectric, and/or ferroelectric properties in lead-free BNT-6BT piezoelectric ceramics. The obtained results for different properties have a potential of important contributions depending on the aimed applications.