Piezoelectric and dielectric aging of Bi0.5(Na0.82K0.18)0.5TiO3 lead-free ferroelectric thick films

Ferroelectric Bi-0.5(Na0.82K0.18)(0.5)TiO3 (NKBT) thick films have been prepared using screen printing on Pt electroded alumina substrates. The room-temperature dielectric constant of resulting 60 mu m thick NKBT film reaches as high as 823 at 1 kHz and the depolarization temperature, T-d, locates at 130 degrees C. The effect of amplitude and frequency of the applied ac field and the aging temperature as well as the Bi excess on the dielectric and piezoelectric aging characteristics are systemically studied. The results show that the dielectric and piezoelectric aging are associated with the reorientation of defect dipoles which stabilized the non-180 degrees domain walls. By this reorientation the domain wall motions are clamped by a time dependent force resulting in the observed decrease in the dielectric and piezoelectric properties during the aging procedural. Moreover, the aging in NKBT thick films exhibits strong frequency dependence, which is caused by that the reorientation and alignment of the defect dipoles with the local polar domains provide a pinning field to the micropolar region, where the regions with larger micropolar volume will experience stronger pinning potential. The NKBT thick films added with 0.5 mol % Bi2O3 exhibit relative lower aging rate of the dielectric constant and piezoelectric coefficient, resulting from the compensation of the Bi3+ vacancies by the addition of small amount of excess Bi2O3. (c) 2010 American Institute of Physics. [doi:10.1063/1.3437637]