Influence of the composition on the properties of (Bi0.5Na0.5)TiO3 based lead-free ceramics

Abstract

Piezoelectric and ferroelectric materials are used in a wide range of applications , and lead zirconate titanate-based (PZT) ceramics are the most used piezoelectric materials. PZT shows great dielectric and functional properties. However, due to the cataloging of PZT as a hazardous material by the European Union the need for researching on lead-free ceramics with similar properties that PZT presents has increased. During lasts years, new materials not harmful for the environment and for human health have been studied. Bismuth Sodium titanate (BNT) is presented as a possible candidate to replace PZT, as it shows good ferroelectric behavior with remanent polarization 38μC/cm2 and has the Curie temperature at 320ºC. However, a huge issue of BNT ceramics is that BNT is hard to polarize due to the high coercive field and high conductivity caused by the loss of volatile elements as bismuth and/or Sodium so BNT usage as a piezoelectric material is limited. In order to follow the same strategy used in the PZT, in this work the ceramic method has been used to synthetize the bismuth Sodium titanate doped with barium titanate (BNT-BT). The (1-x)(Bi0.5Na0.5)TiO3-xBaTiO3 system has a MBP between x=0.06-0.07, a region where the rhombohedral and tetragonal phases coexist and wherein a relevant improvement of the functional properties of the ceramic is detected.In this essay, the BNT-BT was synthesized using 5% and 20% excess of Sodium with the purpose of studying how that excess affects the piezoelectric properties of the BNT-BT. BNT-BT with Sodium excess was prepared by solid state reaction as it is a reproducible and scalable method as well as economic and technology viable. The temperature used for the calcinations of the mixture of the reagents was 700ºC. The resulting IR spectra of the calcined powder and the mixture of the reagents indicated the absence of carbonates and water on the calcined powder, and confirmed the formation of the perovskite structure of the BNT-BT after the treatment at 700ºC.Once the reaction was completed, and in order to densify the material, knowing that the best functional properties have a close relation with the density of the ceramics, BNT-BT ceramics were sintered at different temperatures 1200ºC, 1150ºC and 1100ºC. The microstructuctural and compositional analysis of the surface of the ceramics using the Scanning Electron Microscopy (SEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS) show that the dense ceramics present a non homogeneous chemical composition on the surface on the ceramics where two different phases, one of BNT-BT and another of Sodium Titanate, were found. Regarding grain morphology, shape changes were found with the addition of more Sodium excess. Based on the sintering temperature and the amount of Sodium, the materials present different crystalline structures and different degrees of crystallinity as was observed using X-Ray Diffraction (XRD). The BNT-BT phases found were pseudocubic perovskite structure for ceramics with 5% Sodium excess sintered at 1150ºC and 1100ºC and triclinic for the 5% Sodium excess sintered at 1200ºC and all the ceramics sintered with a 20% of Sodium excess. On all the studied cases it was observed the presence of a secondary phase. It was proven that this phase found on the ceramics was Sodium Titanate which can be found with a monoclinic structure when is treated at 800ºC or rhombohedral structure at 900ºC. Finally, using Impedance Spectroscopy (IS), it could be observed how the sintering temperature have not influence on its behavior as a dielectric material. However, the amount of Sodium excess affects the dielectric behavior of the material.