Electrochemical Synthesis of Nanoparticles of Magnetic Mixed Oxides of Sr-Fe and Sr-Co-Fe

The electrochemical synthesis of magnetic nanoparticles of new Sr-Fe and Sr-Co-Fe oxides using an undivided cell with two Fe electrodes is reported in this work. These materials are collected as precipitates by electrolyzing acidic solutions containing mixtures of chlorides and nitrates of Sr ²⁺, Fe ³⁺ and, optionally, Co ²⁺ at temperatures between 40 °C and 80 °C. Sr-Fe oxides are produced with energy costs lower than 2.7 kWh kg ^-1 in the pH range 2.0-6.0 at 50 mA cm ^-2, whereas Sr-Co-Fe oxides are obtained with a cost of 3.0 kWh kg ^-1 at pH 1.5 and at 35 mA cm ^-2. Inductively coupled plasma analysis of materials and energy dispersive X-ray microanalysis of single particles confirm that they are composed of pure mixed oxides, without metallic Fe impurities. All synthesized compounds crystallize as inverse cubic spinels, with structures similar to those of maghemite and magnetite. They are formed by round-shape nanoparticles with sizes lower than 50 nm, as observed by transmission electron microscopy. Thermal desorption spectrometry allows us to detect the presence of hydrogen and volatiles proceeding from water decomposition in their lattices. After heating the electrogenerated materials at 300 °C during 1 h to eliminate such species, Sr-Co-Fe oxides with similar magnetic properties to those of hard ferrites are obtained, but magnetic Sr-Fe oxides only behave as soft ferrites.