Síntese e caracterização de compósitos titanato de bário-ferrita de cobalto preparados a partir de método sol-gel

Abstract

Di-phase composites containing perovskite and spinel oxides have been studied due to the coexistence of electrical, magnetic and magnetoelectric properties in the same structure. From the technological point of view, these composites are promising as component of multifunctional devices, transducers and new memory devices. Composites with perovskite and spinel phases have been traditionally prepared by physical mixture method. In this work, BaTiO3-CoFe2O4 composites (1.3:1.0 w/w) were prepared by physical mixture and by dispersing of the magnetic phase in a wet gel obtained by sol-gel process. The single phases BaTiO3 (perovskite) and CoFe2O4 (spinel) were also prepared. The dispersion method resulted in composites with improved microstructure, evidenced by a higher homogeneity, lower grain size and homogeneous grain size distribution. The morphology of CoFe2O4 grains was also affected by inserting in the sol-gel matrix, becoming more regular. The electrical conduction of the composites, at room temperature, decreased two hundred times in comparison with the single-phase BaTiO3, in the range 103-106 Hz. In spite of this, the composites presented higher electrical conduction above 150 °C due to the thermal activation of hopping in CoFe2O4. A well defined ferroelectric transition at 126 °C was observed by impedance spectroscopy in single-phase BaTiO3 sample. In the composites samples, the ferroelectric behaviour of the perovskite structure was not observed. The decrease of the ferroelectric character of the composites was attributed to the substoichiometric Ba/Ti ratio and the existence of a high compressive stress in the interface BaTiO3-CoFe2O4. SQUID measurements of the composites indicated magnetization of saturation and coercivity corresponding to 66-68 emu/g and 300-490 Oe. These values were close to those reported in the literature (Ms= 40-78 emu/g; Hc= 200-830 Oe).