In this paper, single-crystal magnetite hollow sub-microspheres with a narrow diameter distribution are synthesized through a simple solvothermal process in ethylene glycol in the presence of urea and a small amount of water. The determining role of water in the solvothermal synthesis is studied. It is found that a small amount of water is crucial for the formation of the magnetite hollow spheres. A novel formation mechanism of the magnetite hollow spheres is proposed based on the bubble-assisted Ostwald ripening. It is believed that the appropriate amount of CO 2 gas bubbles produced in situ by urea hydrolysis is crucial for the formation of hollow spheres. Because of the existence of gas microbubbles, magnetite solid spheres with a loose core and compact shell form, which is the key factor for the following inside-out Ostwald ripening and the formation of the hollow spheres. Thus, by simple changing of the water dosage, magnetite hollow spheres with different diameters and shell thicknesses are obtained controllably. The magnetic properties of the obtained magnetite hollow spheres are studied. It is found that the saturation magnetization of the magnetite hollow submicrospheres decreases with the increasing shell thickness, whereas the coercivity and remanent magnetization increase with increasing shell thickness.

Iron dendritic micropines are synthesized by a hydrogen reduction, where the hematite dendritic micropines prepared by a hydrothermal method are used as starting materials. The as-obtained dendritic iron exhibits enhanced coercivity and remanent magnetization at room temperature and high complex permittivity at 2-18 GHz due to the peculiar shape anisotropy and good crystallinity. The negative imaginary permeability is observed at 14.5-18.0 GHz, suggesting it has a potential as a lefthanded material. The paraffin-based composites containing 30 wt% dendritic irons show large permittivity resulting from the charge polarization and the conductivity and have a minimal reflection loss (RL) of -37.4 dB at 7.4 GHz when the thickness (d) is 2.0 mm. The RL values less than -20 dB are obtained in the frequency range of 5.5-12.9 GHz when d increases from 0.9 to 3.0 mm.