Effect of Reaction Time on Microwave Absorption Properties of Fe₃O₄ Hollow Spheres Synthesized via Ostwald Ripening

Hollow magnetic structures have great potential to be used in the microwave absorbing field. Herein, Fe₃O₄ hollow spheres with different levels of hollowness were synthesized by the hydrothermal method under Ostwald ripening effect. In addition to their microstructures, the microwave absorption properties of such spheres were investigated. The results show that the grain size and hollowness of Fe₃O₄ hollow spheres both increase as the reaction time increases. With increasing hollowness, the attenuation ability of electromagnetic wave of Fe₃O₄ spheres increases first and then decreases, finally increases sharply after the spheres break down. Samples with strong attenuation ability can achieve good impedance matching, which it does preferentially as the absorber thickness increases. Fe₃O₄ hollow spheres show the best microwave absorption performance when the reaction time is 24 h. The minimum reflection loss (R_{L (min)}) can reach -40 dB, while the thickness is only 3.2 mm.