Influence of the stoichiometry, microstructure, and metallization method on the dielectric properties of 0.94 Na0.5Bi0.5TiO3-0.06 BaTiO3

The morphotropic composition of the lead-free solid solution between Na0.5Bi0.5TiO3 and BaTiO3 (0.94 Na0.5Bi0.5TiO3-0.06 BaTiO3 or NBT-6BT) is of particular interest for the next generation of high-temperature capacitors but remains plagued by the diversity of dielectric properties reported in the literature. In order to explain the apparent inconsistencies among the reported dielectric properties of NBT-6BT, we examine the influence of stoichiometry, phase separation, and metallization method. We show that the nominal stoichiometry has a crucial effect, since increasing the nominal Na/Bi ratio increases conductivity and dielectric losses (tan delta). It also increases the real part of the permittivity (epsilon') and the frequency dispersion of both epsilon' and tan delta, thereby altering the shape of the evolution with temperature of the dielectric properties. Moreover it increases the depolarization temperature (T-d) and decreases the temperature of maximum permittivity (T-m). Phase separation also occurs during the synthesis of NBT-6BT as Na evaporation leads to the formation of secondary Ba-containing phases. We report that these phases can have a positive impact on the dielectric properties: a moderate volume fraction (2.5 to 3.0%) and average grain surface (0.9 to 3.0 mu m(2)) of these secondary Ba-containing phases increase the relative permittivity, decrease the dielectric losses, and increase the insulation resistance. We also show that the metallization method impacts the dielectric properties and therefore may contribute to the differences between various reports. The dielectric properties of NBT-6BT samples are measured during successive heating/cooling cycles and reveal that the permittivity value is lower during the first heating when silver paste, even cured, is used. These three components contribute to explaining the diversity of the reported dielectric properties of NBT-6BT.

Automated summary

The study shows that nominal stoichiometry, phase separation, and metallization method significantly impact the dielectric properties of NBT-6BT material. Increasing the Na/Bi ratio leads to higher conductivity, dielectric losses, depolarization temperature, and lower temperature of maximum permittivity. Phase separation during synthesis results in secondary Ba-containing phases that can positively affect dielectric properties. The method of metallization also influences the dielectric properties, contributing to discrepancies in reported results.