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Porous magnetite (Fe 3 O 4 ) nanospheres are successfully prepared through a one-pot solvothermal method with a gas-bubble-assisted Ostwald ripening process. The formation mechanism is explored by studying the reaction parameters, including the concentration of the precipitating agent, capping agent, and urea gas source for a fixed reaction temperature and time. The authors' results show that the Fe 3 O 4 nanospheres are composed of many primary nanocrystals, the sizes of which are well regulated by urea concentration. As such, the size of the spherical primary nanocrystal determines the porosity of Fe 3 O 4 nanospheres. Strong reflection loss of À49.2 dB is observed at 11.9 GHz for highly porous Fe 3 O 4 nanospheres with a thickness of 2.7 mm due to multiple scatterings in void spaces and increased dielectric loss caused by higher interfacial polarization relaxation loss in the porous structure. The authors believe that the prepared porous Fe 3 O 4 nanospheres are good candidates for electromagnetic absorption applications.

Fe 3 O 4 nanoparticles dispersed on single-walled carbon nanotube (SWCNT) composites are synthesized through a simple in-situ co-arc discharge process followed by an annealing process. A well-integrated Fe 3 O 4 /SWCNT composite exhibiting the strong microwave absorption (MA, À36.9 dB at 10.5 GHz) and a wide effective bandwidth (5.5 GHz) is obtained by controlling the annealing conditions. The high effective MA performance of this composite was ascribed to the well integration of magnetic nanoparticles (Fe 3 O 4 ) on the SWCNT, creating a synergetic effect. This work demonstrates an economical approach to produce Fe 3 O 4 /SWCNT composites with good MA performance.

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