Estudo da influência de precursores orgânicos alternativos na síntese da ferrita de cálcio (Ca2Fe2O5)

Abstract

Calcium ferrite (Ca2Fe2O5) is a material with a brownmillerite perovskite structure, this means that, in its structural arrangement, it has oxygen vacancies generated from charge compensation mechanisms occurring in oxides with many atoms. As a result of these gaps, ferrite has the ability to accommodate oxygen, which provides high stability to the material. Other properties include high ionic diffusivity, high sensitivity and low coefficient of thermal expansion. Thus, calcium ferrite has been the object of study in different lines of research, such as the study of fuel cells, use as an oxygen carrier in chemical looping processes and in Li-ion batteries. It is possible to find, in the literature, several synthesis methods for this material, among which it is possible to mention electrospinning, high-energy ball milling and self-combustion. However, these methods have disadvantages such as the use of many reagents and elevated temperatures. The search for alternative synthesis routes is growing and these may arise as new processes or from traditional modified methods. The polymeric precursor method is already widely explored as it provides good chemical homogeneity to the materials and requires relatively low calcination temperatures, however, exhibiting disordered growth of the particles. A cheap, functional, fast and environmentally friendly alternative is the use of alternative organic precursors. These materials have skills that allow cations to be joined through networks, formed by gels. Another important point in the development of these materials relates the morphological properties to the structural and electrical properties. Thus, the present work aims to analyze the influence of the use of organic precursors on the morphological and structural properties of calcium ferrite, Ca2Fe2O5. The methodology used consisted of dissolving the salts and the chelating agent, being used in this case: citric acid, agar, albumin, gelatin and sucrose. To obtain the final material, calcination was carried out at 700 ° C for 3 hours. The powders obtained were characterized by thermogravimetry (TGA), x-ray diffraction (XRD), infrared spectroscopy (FTIR), scanning electron microscopy (SEM), specific surface analysis (BET) and reduction to programmed temperature (H2-TPR). Through the results of the XRD it was possible to observe that the perovskite phase Ca2Fe2O5 was the majority phase, revealing the efficiency of all the organic precursors adopted. The crystallite sizes obtained were concentrated between 35 and 50 nm and it was observed that the addition of the chelating agents resulted in a decrease in crystallinity. Different morphologies were obtained from the use of chelating agents, as observed by the micrographs obtained through SEM. The BET analysis confirmed that the addition of chelating agents was able to increase, by up to 70%, the specific surface area of the materials, as well as the volume of pores.