(Ba_(1+x)TiO_3)-(Bi_(0.5)Na_(0.5)TiO_3) Lead-Free, Positive Temperature Coefficient of Resistivity Ceramics: PTC Behavior and Atomic Level Microstructures

摘要

Positive temperature coefficient of resistivity (PTCR) ceramics of 0.912(Ba_(1+x)TiO_3)-0.088(Bi_(0.5)Na_(0.5)TiO_3) (BT-BNT) (x = -0.03 to 0.03) were prepared using the mixed oxide route and sintered at 1340℃ 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 Ba_6Ti_(17)O_(40). 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 perov-skite 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/m~2. 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 stoi-chiometry, increasing with the Ti/Ba ratio; this in turn is directly correlated with an increase in the amount of second-phase Ba_6Ti_(17)O_(40), an increase in the stacking fault energy, and an increase in the tilt angle of the grain boundaries.