Synthesis and characterization of high curie temperature lead-free piezoelectric ceramics

For more than a half century, Pb(Zr,Ti)O3 (PZT) and its modifications have been the most commonly known and used piezoelectric materials providing the best 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. Barium titanate is a lead-free piezoelectric as old as PZT but not a good alternative due to its low Curie temperature (Tc) that limits its practical use in many applications since depolarization occurs above Tc. High Curie temperature perovskite type lead-free piezoelectric Bi0.5Na0.5TiO3-0.06BaTiO3 (BNT-BT), %0.05 Nb and %0.05 Ta doped Bi0.5Na0.5TiO3-0.06BaTiO3 (BNT-BT-NT), and [(K0.458Na0.542)0.96Li0.04]Nb0.85Ta0.15O3 (KNN-LT) ceramics were synthesized by solid state method using high purity oxide powders. Addition of Ta and Nb is known to inhibit abnormal grain growth and Li improves densification during sintering. Thus doping is anticipated to improve the 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. Dielectric properties were also measured as a function of frequency (1,10,100,1000 kHz) andtemperature (up to 475°C) by using a broad frequency range impedance phase analyzer integrated with a furnace system. This was also helpful to monitor possible phase transitions that would affect the dielectric and piezoelectric properties. For the piezoelectric measurements samples were also poled in a hot (110°C) silicon oil bath by applying DC field as high as 5000 V. Piezoelectric charge coefficient (d33) values were determined in samples after poling. The obtained results show that BNT-BT based ceramics exhibits higher potential in replacing PZT as KNN based compositions gives lower properties related with poor densification. It should also be pointed out that presence of barium titanate in BNT-BT remarkably increase dielectric properties in comparison to KNN. Further studies should aim to improve the densification of KNN and eliminate possible damages during high field poling.