(Ba1+xTiO3)–(Bi0.5Na0.5TiO3) Lead‐Free, Positive Temperature Coefficient of Resistivity Ceramics: PTC Behavior and Atomic Level Microstructures

Abstract

Positive temperature coefficient of resistivity (PTCR) ceramics of 0.912(Ba1+xTiO3)–0.088(Bi0.5Na0.5TiO3) (BT–BNT) (x = −0.03 to 0.03) were prepared using the mixed oxide route and sintered at 1340°C for 4 h. Products were predominantly single phase with a tetragonal structure and grains in 2–6 μm size containing 90° ferroelectric domains. Samples with Ti/Ba > 1 contained second‐phase Ba6Ti17O40. HRTEM and aberration‐corrected Z‐contrast high angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) suggested that the dopants, Bi and Na, occupy the Ba site of the perovskite lattice, and revealed the presence of dissociated dislocations in x = −0.03 and x = 0.00 materials. The interval between two partial dislocations was 1.9–3.4 nm, yielding stacking fault energies of 363–649 mJ/m2. The PTCR behavior of the ceramics increased with Ti/Ba content, reaching a maximum of six decades change in resistivity for x = −0.03. The anomalous increase in resistivity depends critically on stoichiometry, increasing with the Ti/Ba ratio; this in turn is directly correlated with an increase in the amount of second‐phase Ba6Ti17O40, an increase in the stacking fault energy, and an increase in the tilt angle of the grain boundaries.