Preparação e caracterização do composto multiferróico magnetoelétrico BiFeO3 via método sol-gel

Abstract

The multiferroics materials are defined as a specific class of materials that present at least two of the following characteristics: a spontaneous magnetization, that can be reoriented through the application of a external magnetic field; a spontaneous polarization, that can be reoriented through the application of a external electric field; and a spontaneous elastic deformation, that can be reoriented through the application of a external mechanic tension. Amongst these materials the BiFeO3 is distinguished (its have one of the highest effect among the known magnetoelectrics compounds). Nanosized grains are desirable in a preparation route of this compound. Among the several existing alternative techniques, the sol-gel chemistry is one technique capable to get materials with high degree of pureness and nanosized particles. In this work the BiFeO3 powders were obtained by sol-gel method using different hydroxides as: NaOH, KOH and NH4OH. The obtained gel was dried in a stove for 24 hours in 100°C for posterior heat treatments in 600 and 700°C with calcination times that varies of 1 until 2 hours. The obtained material was analyzed through X-ray diffraction(XRD), thermogravimetric analysis(TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and ener gy dispersive X-ray spectroscopy (EDS). The temperatures of heat treatments, as well as the Neél (TN) and Curie (TC) temperatures, were determined by the thermal analyses. The X-rays diffraction patterns, that were refined using the LeBail method, had shown, beyond the BiFeO3 phase of, the presence of spurious phases (Bi2Fe4O9 and Bi25FeO40). As an exception, the 5 mol% excess of iron atoms sample produced with NH4OH had presented only the BiFeO3 phase. The structural refinement also pointed the possibility of the BiFeO3 to have a monoclinical structure instead of the rhombohedral structure. The microscopy (SEM) and EDS measurements had shown that the samples produced with KOH as complexing agent, had presented residues of K in its structures.