Construction and Characterization of BNT-BKT-BT Piezoelectrics for Energy Harvesting

Lead-free piezoelectrics are non-toxic, cheap and environmental friendly materials. Among them Bismuth Sodium Titanate (BNT) which has the chemical formula Bi 0.5 Na 0.5 TiO  with a complex A-site perovskite structure. In this study, some modifications on BNT were used to help in applications by forming solid solution with Bi 3 0.5 K 0.5 TiO 3 (BKT) and BaTiO (BT). The BNTbased (xBNT-yBKT-zBT)
with x+y+z=1 ; were prepared via different methods at the Morphotropic Phase Boundary (MPB) : 0.94BNT-0.06BT , 0.93BNT0.07BT , 0.8BNT-0.2BKT, 0.84BNT-0.16BKT and 0.9BNT-0.065BKT0.035BKT
with their origin BNT , BKT and BT. Mini Database was proposed depending on the results of international databases and some crystallography software and plotted as phase diagrams. Several predictions were suggested depending on the built Mini Database and phase diagrams for the BNT-BKTBT system. The main prediction was : the physical properties either depended on the changing of crystal structure ( polarization , dielectric constant  piezoelectric coefficient ) or the properties had linearity with the composition (density, melting point , sintering temperature ). It has been focused on the studying the crystal structure, dielectric measurements, hysteresis loop measurements and polarization fatigue analyses for ceramic samples.3 The samples were prepared by different methods: the hydrothermal method with spherical shape particles of 65nm – 600nm size; and by solid state reaction method. The powder were pressed in suitable template at 250MPa ,sintered according to database prediction "sintering mperature phase diagram", the resulted ceramic discs had relative density 90% - 96 %. These results agreed with the database predictions "sintering and density phase diagrams”. The crystal structure for the prepared samples were studied using X-ray diffraction (XRD) analysis, the XRD showed that the samples BNT-6BT, BNT7BT, BNT-20BKT, BNT-16BKT and BNT-BKT-35BT had the Morphtropic Phase Boundary (MPB) while BNT had the rhombohedral structure. BKT and BT had a tetragonal structure. These results agreed with the predictions "crystal structure phase diagram" The dielectric measurements showed that there were three regions for BNT-BKT-BT system; the first one was the samples with relaxor ferroelectric behaviors, the samples belonging to this region were BNT, BNT-6BT, BNT7BT, BNT-20BKT, BNT-16BKT
and BNT-BKT-35BT (i.e.BNT and the MPB region).The second region was the samples with Normal ferroelectric behavior The sample with this region was BT. The third region was the samples with
relaxor-like ferroelectric behavior (weak ferroelectric) as in BKT. The Curie temperature for the samples within MPB was between 270℃ to 300℃, while for the origin: for BNT the Curie temperature was about 320℃; for BKT was about 370℃; and for BT was about 110℃. The depolarization temperature for samples within MPB region was below 100℃ (above room temperature) while for BNT was 250℃.
The hysteresis loop measurements were studied both experimentally and theoretically. The study showed that the results for BNT were PE=60kV/cm, for BKT was Ps=30μC/cm2, E=4.7kV/cm and for BT wasPs=26μC/cm2, Ecc=9.8kV/cm, while for the samples within the MPB region theresults were P s=45μC/cm 2, E≈ 30kV/cm. The polarization had the largest value within MPB, which agreed with the first prediction by the proposed database (i.e. the polarization of BNT-BKT-BT system depends on crystal structure). The coercive electric (E cc) had a linear behavior with the composition. The value of E  within MPB was in the range of 27kV/cm to 33kV/cm, which was half of the value for pure BNT. This result agreed with the second prediction by the designed database.cThe polarization fatigue analyses were performed depending on the fatigue model. The results showed that the polarization life cycle had the values of 10 12  cycles for BNT, 10 11 cycles for BT and 10 4  cycles for BKT, while within MPB region the polarization life had the largest value of about 10  cycles which was much greater than in origin (BNT, BKT and BT). This result also agreed with the first prediction by the designed database (polarization life cycle had the largest value and depends on the crystal structure). The fatigue analyses predicted the value of applied electric field used in hysteresis loop measurements and the effect of cycle on the applied electric field.Finally, some of the problems were skipped and properties of BNT were enhanced by looking for the MPB region in the solid solution. The MPB showed the value for polarization to be larger than that in pure BNT, and it reduced the coercive electric field comparing to the original BNT. The polarization life cycle at MPB was much greater than in BNT. Therefore, this study could find some solutions for the drawbacks of BNT, and solve some problems for other piezoelectrics.