Preparation and characterisation of the magneto-electric xBiFeO_3-(l - x)BaTiO_3 ceramics

摘要

The solid solutions of BiFeO_3-BaTiO_3 have been prepared via solid state with a view to obtaining magnetoelectric properties, i.e. ferroelectric and magnetic activity in the same range of temperatures. Optimum calcination and sintering strategy for obtaining pure perovskite phase, dense ceramics (>97 percent relative density) and homogeneous microstructures have been determined. The sample of composition 0.7BiFeO_3-0.3BaTiO_3 reported in the present work is pseudo-cubic at room temperature. The permittivity is epsilon _r approx = 150 at the room temperature and shows a broad ferro-para phase transition at around 175 deg C where epsilon _r approx = 1600. This diffuse maximum of the permittivity, similar to that in relaxors, is due to the chemical inhomogeneity in both A and B sites of the perovskite unit cell ABO_3. Higher losses, tan delta > 1, appear above 200 deg C and other different conduction mechanisms start to be active particularly at temperatures higher than 400 deg C, when the ceramic becomes conductive. The magnetic properties show a succession of transitions from weak ferro ferrimagnetism-to-antiferromagnetism and antiferromagnetism-to-paramagnetism at T_(N1) approx = 10 K and T_(N2) approx = 265K. Below T_(N2) the ceramic 0.7BiFeO_3-0.3BaTiO_3 can present magnetoelectric coupling, due to the fact that is simultaneously ferroelectric and antiferromagnetic.