(Ba_1+xTiO₃)–(Bi_0.5Na_0.5TiO₃) 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(Ba 1+x TiO 3 )-0.088(Bi 0.5 Na 0.5 TiO 3 ) (BT-BNT) (x = À0.03 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 lm size containing 90°ferroelectric domains. Samples with Ti/Ba > 1 contained second-phase Ba 6 Ti 17 O 40 . HRTEM and aberrationcorrected 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 = À0.03 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 = À0.03. 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 secondphase Ba 6 Ti 17 O 40 , an increase in the stacking fault energy, and an increase in the tilt angle of the grain boundaries.

Effect of Ta2O5 and Nb2O5 Dopants on the Stable Dielectric Properties of BaTiO3–(Bi0.5Na0.5)TiO3-Based Materials

“…XRD results indicated the presence of the tetragonal BaTiO3 perovskite phase (tetragonal, P4/mmm; JCPDF 75-0462) with trace amounts of second-phase Ba6Ti17O40 (monoclinic, C2/c; JCPDF 77-1566), which increased with the (Bi0.5Na0.5)TiO3 content. This observation is in accord with those reported in the literature [25,26]. In addition, the peaks corresponding to BaTiO3 in the XRD patterns shifted to lower angles due to the incorporation of the smaller ions Na into Ba sites leads to strengthened Ti-O bonds, and more energy is needed to maintain the balance of Ti 4+ displacement in the TiO6 octahedrons; thus, Tc increased [14,15,19].…”

“…While the sintering temperature of BaTiO3 is typically above 1300 °C, the addition of (Bi0.5Na0.5)TiO3 significantly reduced the sintering temperature to 1230 °C for an x value of 0.05 and to 1215 °C for x values of 0.1 and 0.2, which was caused by the formation of the liquid phase [14,25]. XRD results indicated the presence of the tetragonal BaTiO3 perovskite phase (tetragonal, P4/mmm; JCPDF 75-0462) with trace amounts of second-phase Ba6Ti17O40 (monoclinic, C2/c; JCPDF 77-1566), which increased with the (Bi0.5Na0.5)TiO3 content.…”