Tuning of the Ba5Nb4O15 permittivity temperature coefficient

The Ba5Nb4O15 ceramic offers attractive dielectric properties (ɛr ∼ 39, tan(δ) < 10−3, ρi ∼ 1012 Ω cm) but exhibits a high permittivity temperature coefficient (τɛ ∼ −171 ppm °C−1). In order to tune this parameter, substitutions on the Ba and Nb sites by, respectively, Mg, Ca, Sr and V, Ta, Sn have been investigated. Two interesting formulations have been identified, Ba2.5Mg2.5Nb4O15 and Ba5Nb3SnO14.5 as nominal compositions, with temperature coefficients of −44 and −30 ppm °C−1, respectively. More attention has been paid to the Ba5Nb3SnO14.5 compound (named 504Sn25) in which BaSnO3 has been clearly identified as a secondary phase. BaSnO3, having a positive and very high temperature coefficient (+393 ppm °C−1), seems to be responsible for the τɛ lowering of 504Sn25. Furthermore B2O3 addition as sintering aid has been successfully investigated for increasing the Ba5Nb3SnO14.5 sample density. In terms of dielectric properties, it induces an increase of the permittivity and of the insulating resistivity, while the control of τɛ is maintained. Finally, the formulation 504Sn25 + 15 mol% B2O3 sintered at 1200 °C has a value of 17.8 as dielectric constant and −1 ppm °C−1 as τɛ value, that evidences the potentiality of this material to be used as temperature stable capacitor.