Nominal Lanthanum Niobate, a Versatile Additive for Reducing Grain Boundary Resistance in Conductive Ceramics

The grain boundaries of well-known conducting ceramics, like Na⁺-ion conducting Na_3.4Zr₂Si_2.4P_0.6O₁₂, Li⁺-ion conducting Li_1.5Al_0.5Ti_1.5P₃O₁₂ and Li_0.34La_0.56TiO₃, O^2--ion conducting Y_0.148Zr_0.852O_2-δ and electronic conducting SrTiO₃, are reinforced by adding 0.5-3 mol% LaNbO₄ without changing the density of the ceramic, resulting in much lower grain-boundary resistance and much higher total conductivity.

Abstract

Conductive ceramics currently play a vital role in human life. In practical applications, most conductive ceramics are polycrystalline, and their overall conductivity (σ _total) is influenced by both bulk and grain boundary resistances (R _bulk and R _gb, respectively). While R _bulk is mainly of academic interest, R _gb often determines the quality of a conductive ceramic component. Currently, studies discussing the influence of specific methods on grain boundary resistances are typically related to individual ceramics. In this study, it is discovered that the addition of 0.5–3 mol% nominal LaNbO₄ significantly reduces the R _gb of several well-known conductive ceramics, such as rhombohedral NaSICON-type Na⁺-ion-conducting Na_3.4Zr₂Si_2.4P_0.6O₁₂ and Li⁺-ion conducting Li_1.5Al_0.5Ti_1.5P₃O₁₂, Li⁺-ion-conducting tetragonal perovskite Li_0.34La_0.56TiO₃, oxygen-ion-conducting cubic fluorite 8 mol% Y₂O₃ stabilized ZrO₂, and electron-conducting perovskite SrTiO₃ (sintered in a reducing atmosphere). In particular, for NZSP and LATP, the enhanced σ _total reaches 9.3 × 10⁻³ S cm⁻¹ and 2.1 × 10⁻³ S cm⁻¹ at 25 °C, surpassing previously published results. Detailed investigations reveal that the microstructure of the grain boundaries in all the ceramics undergoes significant improvements. The findings elevate the importance of research on grain boundaries, inspiring the development of conductive ceramics with higher σ_total for superior applications.