Modulating Magnetic/Dielectric Loss of Fe-Based Core–Shell Microspheres by Controlling Tetrabutyl Titanate Dosage for High-Efficiency Microwave Absorption

Optimizing loss components by phase engineering is crucial for enhancing the microwave absorption performance of magnetic core-dielectric shell composites. In this work, a series of core–shell structured composites (FTR) were elaborately synthesized by high-temperature hydrogen reduction of Fe3O4@TiO2 precursors and the TiO2 shell content was effectively regulated by varying the dosage of tetrabutyl titanate (TBOT). With the increase of TBOT dosage, the phase composition of FTR composites gradually evolves from Fe/Fe2O3@TiO2 (FTR-2.0) to Fe@TiO2 (FTR-2.5) and Fe@FeTiO3/TiO2 (FTR-3.0, FTR-3.5). FTR-2.5 has the highest Fe content (86.24 wt %) and saturation magnetization (141.7 emu·g–1). Low TBOT dosage leads to TiO2 cracks around the magnetic core after hydrogen reduction, but the remaining shell still encapsulates the magnetic core. In contrast, high TBOT dosage promotes the solid-state reaction between Fe and TiO2 shells, generating thick and dense FeTiO3 shells and abundant hollow spheres. Regarding loss components, FTR-2.5 is dominated by magnetic loss due to its highest metallic Fe content, whereas dielectric loss plays a more prominent role in other FTR composites. Notably, polarization loss prevails in the high-frequency range, including interfacial polarization at the heterogeneous interfaces among Fe, Fe2O3, TiO2, and FeTiO3 and dipole polarization enhanced by abundant oxygen vacancies. Benefiting from the synergistic effects of strong magnetic loss, enhanced multiple polarization, and a continuous impedance matching region, FTR-2.5 achieves a maximum effective absorption bandwidth of 5.92 GHz at a thickness of 1.95 mm. This work provides insights into the phase evolution mechanism of Fe3O4@TiO2 during H2 reduction and a reference for improving broadband microwave absorption.