Effect of Sm2O3 addition on the dielectric behaviour of BaTiO3

Abstract

BaTiO3 is a widely used ferroelectric ceramic having applications in transducers, capacitors, thermistors, non linear optics etc. BaTiO3 exhibits Curie temperature (Tc) of 120 °C. Recent research in BaTiO3 based thin films has resulted in a significant rise in their Tc. The most prominent increase in Tc has been reported in BaTiO3 – Sm2O3 system in which Sm2O3 nanorods were incorporated in BaTiO3 matrix resulting in the Tc ∼ 800 °C. Such a remarkable increase in the Tc was attributed to the high elastic modulus of Sm2O3 (E = 125 GPa) w.r.t. that of BaTiO3 (E = 67 GPa). Higher elastic modulus of the second phase resulted in the generation of strain in the matrix thereby delaying the onset of tetragonal – cubic transition. The improvement in the Tc of BaTiO3 thus opened possibilities of use of BaTiO3 based systems for high temperature piezoelectric applications. Since BaTiO3 based thin films exhibited a remarkable increase in the Tc, it needed to be studied if such a phenomenon could be replicated in bulk ceramics. Hence, BaTiO3 – Sm2O3 ceramics were prepared by the conventional ceramics processing route. Submicron-sized BaTiO3 and Sm2O3 powders were used as starting materials and compositions with Sm2O3 content varying between 5 and 30 wt% were prepared. BaTiO3 and Sm2O3 powders were milled using a jar mill for 48 h in ethanol followed by drying of slurries and room temperature uniaxial pellet pressing. Green pellets were sintered by pressureless sintering at 1350 °C for 4 h. X-ray diffraction (XRD) analysis of BaTiO3 and BaTiO3 – 5 wt% Sm2O3 ceramics confirmed the presence of tetragonal phase. However, further increase in Sm2O3 content resulted in the transformation of tetragonal BaTiO3 to cubic BaTiO3. Since cubic phase is centrosymmetric and cannot exhibit ferroelectricity, compositions with Sm2O3 > 5 wt% became redundant for any ferroelectric application. Moreover, BaTiO3 – 5 wt% Sm2O3 exhibited Tc of just 100 °C with lower dielectric constant thereby making this composition ineffective for high temperature piezoelectric application. While it had been anticipated that incorporation of Sm2O3 in bulk BaTiO3 ceramics would increase their Tc as in thin films, none of these compositions exhibited the anticipated behaviour. This paper discusses why BaTiO3 – Sm2O3 bulk ceramics failed to exhibit similar dielectric behaviour to their thin film counterparts in light of the phase formation, microstructure and dielectric behaviour.