Síntese e estudo de prospecção de sinterização de óxidos nanoestruturados de alta entropia

Abstract

A great challenge found in materials science is the continuous search for advanced materials that supply an increasing demand of technological development. It is known that many synthesis processes to obtain nanostructured materials hasn’t been fully explored yet, and researches are still needed to identify and quantify the number of features involved in obtaining such materials. This said, this work presents the synthesis of high entropy oxides by the modified polymer precursor based method, to obtain nanostructured materials. This method is somehow innovative, because such material hasn’t been reported yet as nanostructured by this synthesis route. This material shows a face centered cubic structure (MgO based, rock-salt type from the space group), where Mg2+ are equimolarly substituted by Co2+, Cu2+, Ni2+ and Zn2+ cations. The synthesis route begins by the stoichiometry mixture of inorganic salts (nitrates and/or acetates, usually) of those cations with amonium polyacrilate (APA), in aqueous form, and after water removal of the resin formed, proceeds with calcination in muffin oven at temperatures between 500 and 900 °C, for 120 minutes, with heating rate of 5 °C/min. Yet to assess the effect of intersticials and substitional atoms in the unit cell, lithium and gadolinium were added to the system, improving the configurational entropy contributions. Initially, the precursor powder formed were characterized as to its thermal behavior by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and, at interest temperatures, the thermal decomposition were analyzed by Fourier transform infra-red (FTIR) spectroscopy, x-ray diffraction (XRD) and scanning and transmission electron microscopy (SEM and TEM). As for sinterability, the oxides were subjected for dilatometric analysis, followed by conventional (1200 °C) and microwave (1150 to 1250 °C) sintering. Cylindric pellets were characterized to asses it’s electrical behavior and, microwave sintered microstructures were analyzed by SEM. The increase of calcination temperature (800 and 900°C) aided the formation of single phased oxides, with crystallite sizes of 35 to 52 nm, infered through the x-ray difractograms. The addition of other cations, gadolinium in particular, also aided the single phase formation of the oxides. The calcinated powders showed by SEM presented homogeneity, with presence of porous agglomerates from 0,15 to 0,5 µm and the sintered pellets showed porous structures with segregation of acicular strctures associated with gadollinium. The TEM electron diffraction patterns showed circles with diffracted planes, that suggets nanoparticles presence in the material. The electrical behavior of the pellets were in general non-ohmic and the electrical resistivities from 3,51.106 to 3,2.108 Ω.cm.