Hollow/helical magnetite fibers were prepared with ferric citrate as the original materials via the organic gel-thermal reduction method under a slow heating rate condition. The thermal decomposition process, structures and morphologies of organic gel-fiber precursors and fibers derived from heat treatment and reduction process were characterized by TG/DTA, XRD and SEM, respectively. The obtained fibers consisted of 80% of hollow fibers and 20% of helical fibers. The diameter of hollow fibers was about 5 μm, and the wall thickness was about 500 nm. The diameter of helical fibers was about 6—10 μm, which can be left- or right-handed. The helical structure resulted from coiled belt-like fibers with smooth and dense outer shell, and loose and irregular inner layer, its width and shell thickness were 4—6 μm and 600 nm, respectively. The hollow/helical fibers with a few pores on its surface were made up of fine crystalline particles with a size of 60 nm. The formation mechanism of the hollow and helical fibers was proposed. During the heat treatment, the hollow fibers could be formed when the inner gel of the thinner precursor fibers was separated from the center and contracted to surface. However, the helical fibers could be generated when larger gel fibers were broken helically under thermal stress in the process of gel decomposing and contracting.
