Effects of BNT Addition on the Microstructure and PTC Properties of La-Doped BaTiO₃-Based PTCR Ceramics

Abstract

Lanthanum (La) doped BaTiO 3 -(Bi 0.5 Na 0.5 )TiO 3 (BT-BNT) positive temperature coefficient of resistivity (PTCR) ceramics were prepared by a conventional solid state reaction method. The microstructure and PTCR effect of BT-BNT ceramics were investigated by scanning electron microscopy and measurement of resistivity-temperature dependence. The BT-BNT ceramics sintered in a N 2 gas flow possessed low room-temperature resistivity (ρ RT < 10 2 ·cm) and PTCR effect wasn't remarkable (ρ max /ρ min < 10); through a proper reoxidation process, the ceramics showed a typical PTCR effect (ρ max /ρ min > 10 3 ) but with a high room-temperature resistivity (ρ RT > 10 3 ·cm). Room-temperature resistivity increased with the raising content of BNT. The Curie temperature (Tc) of BT-BNT ceramics shifted to 160 • C for the 8mol% BNT added. With the addition of 4mol% BNT, the obtained BT-BNT ceramics reoxidized at 950 • C possessed ρ RT = 10 3 ·cm and exhibited a resistivity jump (ρ max /ρ min > 10 3 ) at 150 • C. The possible mechanism underlying the PTCR behavior in BT-BNT ceramics was discussed.

Preparation and positive temperature coefficient of resistivity behavior of BaTiO3–BaBiO3–Bi0.5Na0.5TiO3 ceramics

“…The grains were well formed and the grain boundaries were clear-cut. It can be clearly seen that the average grain size decreased from 8.1 to 3.9 lm with the increase of BNT content which was generally in agreement with previous results [22]. And the grain growth was also inhibited by the incorporation of BaBiO 3 .…”

Structural, Ferroelectric, Pyroelectric, and Dielectric Study of Bi0.5Na0.5TiO3 Ceramics synthesized with Precursors Obtained by Sol-Gel Method doped with Lanthanum

“…The density of the sintered samples, with different atomic concentrations of La (0, 0.003, 0.007, 0.01, 0.013, 0.033, and 0.067) was determined by the Archimedes method. The results presented in Table 1 A similar effect of grain size reduction has also been observed in other ferroelectric systems in addition to BNT, such as in the case of BaTiO3 and PZT [25][26][27]. The effect of La doping in the A sites of the BNT perovskite structure, substituting Na and Bi, leads to a decrease in grain size, and for higher concentrations, it is expected also that it contributes to the appearance of an additional phases [25][26][27].…”

“…2 show an evolution that allows identifying the presence of a mixture of crystalline phases, rhombohedral (R3c) and cubic (Pm-3m group). Previous studies about La-doping into perovskite structures have suggested that La 3+ replaces preferably A-sites due to their close radii [25,27]. D. Shannon [30] reports the next ionic radius for Na + (rNa=1.39Å, CN=12), La 3+ (rLa=1.36Å, CN=12) and for Bi 3+ only gives ionic radii for coordination 8, Bi 3+ (rBi=1.17 CN=8).…”