Electrical depolarization in lead-free piezoelectric ceramics

Pb(Zr,Ti)O3 (PZT) based ceramics have been the most commonly known and used piezoelectric materials providing the best piezoelectric properties. However due to the toxicity of lead (Pb) content, synthesis and use of PZT ceramics were started to be regulated in many countries for environmental concerns. For this reason piezoelectric materials research started to focus on finding alternatives to lead-based systems in recent years. Perovskite type lead-free piezoelectric Bi0.5(Na0.75K0.20Li0.05)0.5TiO3 (BNKLT), 0.948(K0.5Na0.5)NbO3-0.052LiSbO3(KNN-LS), and 0.948(K0.5Na0.5)NbO3-0.052LiSbO3+%1 mol Cu (KNN-LS-Cu) ceramics were synthesized by solid state method using high purity oxide powders in this study. Cu doping is anticipated to improve sintering behavior of KNN based ceramics as a result of liquid phase formation. This would provide a solution to densification problems and thus improve physical and electrical properties. Following perovskite phase verification by X-ray diffraction (XRD) method, cold isostatic pressing was used to obtain disc shaped samples. Samples sintered at the range of 1150-1200°C depending on the composition were then sputter coated with gold (Au) and microstructural analyses were made in a scanning electron microscope (SEM). After the density measurements by Archimedes method, room temperature dielectric properties were measured by LCR-meter in gold electroded samples. For the piezoelectric measurements samples were also poled in a hot (110°C) silicon oil bath by applying DC from 3 to 5 kV. Piezoelectric charge coefficient (d33) values were determined by a d33-meter in samples after poling. It is critical for many applications that the obtained piezoelectric properties should be retained after the poling process. This is important for the stability of electronic devices. However it is well known that due to depolarization processes piezoelectric properties degrade in time following the poling at room temperature. This effect can be called ?aging? and more pronounced when the materials heat up. Room temperature depolarization or aging behavior were also studied in lead free piezoelectric samples produced in this research. After the poling change in time was monitored both in dielectric and piezoelectric properties up to 105 second. The obtained results show that KNN-LS and Cu doped KNN-LS ceramics exhibits higher piezoelectric properties in comparison to BNKLT based ceramics. Eventhough no improvement in densification is achieved, dielectric constant and piezoelectric coefficient vales remarkably increased in KNN-LS ceramics when doped with 1 % Cu. Aging of piezoelectric properties were also much less in KNN based ceramics.