Effects of acceptor addition in boron doped lead-free piezoelectrics on depolarization behavior

Pb(Zr,Ti)O3 (PZT) and its modifications are the piezoelectric ceramics materials that have been used for decades and today also preferred due to their excellent electrical properties. However, due to toxicity of lead (Pb) content, synthesis and use of PZT ceramics were started to be regulated. For this reason recent research on piezoelectric materials started to focus on finding alternatives to lead-based systems that are comparable with PZT. In this study, perovskite type lead-free piezoelectric (Bi0.47Na0.47Ba0.06)TiO3, (Bi0.47Na0.47Ba0.06)TiO3+1 mol % B3+, (Bi0.47Na0.47Ba0.06) (Ti1-xDx)O3+1 mol % B3+ doped (D=Mn3+, Al3+, Fe3+) ceramics were synthesized by solid-state method using high purity oxide powders. B2O3 powder was used as B3+ source. It was assumed that B3+ cation would occupy interstitial positions in (Bi0.47Na0.47Ba0.06)TiO3 (BNT-6BT) perovskite crystal lattice due to its small ionic radius. Thus, it is expected that B+3 cations would act as donors and make BNT compound a softer material hence improve poling behavior. On the other hand it is also important for many device applications that the piezoelectric materials should retain polar properties after the removal of electric field. The decay of polarization related properties in time is called relaxation or depolarization. As the material becomes softer the rate of depolarization in time may increase simultaneously reducing various piezoelectric properties. In order to provide the balance of the possible detrimental effects of B+3 donor doping in this regard, several cations such as Mn3+, Al3+, and Fe3+ were added to the ceramics produced in this study. Such dopants, known as B-site acceptors for similar perovskite systems, usually provide hard character to the piezoelectrics and improve depolarization behavior even though the piezoelectric property coefficients are reduced. Perovskite phase verification was made by X-ray diffraction (XRD) method for powders after calcinations at 950°C for 4 h. Disc shaped samples obtained by die pressing has been also enhanced by cold isostatic pressing (CIP). Sintering temperature and time were chosen as 1150°C and 12 hours based on the literature survey. After sintering process, density measurements were made by using Archimedes method. Both sides of the selected samples were polished and coated with gold (Au) by sputtering for microstructural characterization by scanning electron microscope (SEM) and electrical property measurements. After electroding, first dielectric properties were measured by LCR-meter at room temperature. To determine the ferroelectric properties bipolar polarization vs electric field (P-E) hysteresis measurements were performed by applying 40 kV/cm electric field. Prior to piezoelectric property measurements, samples were poled in silicon oil by applying 3 kV/mm DC field for 5 minutes at room temperature by using a high power supply. As soon as the samples were taken out from the poling set-up, in order to observe the change in properties as a function of time and temperature, piezoelectric and dielectric properties were measured up to 105 second at different temperatures including room temperature (25), 40, 60, 80 and 100°C. The obtained results provide valuable information on depolarization behavior of the materials which is critical for many applications.