A 3D α-Fe2O3 nanoflake urchin-like structure for electromagnetic wave absorption.

An α-Fe(2)O(3) nanoflake urchin-like structure is formed via the thermal oxidation of micrometre-sized iron spheres in air at temperatures of 300-400 °C. The material consists of α-Fe(2)O(3) nanoflakes grown perpendicularly to the sphere surface, a layer of a mixture of α-Fe(2)O(3) and Fe(3)O(4) as the oxidation shell, and an iron core. The ranges of the tip diameters of the nanoflakes are 20-30 nm (300 °C), 30-50 nm (350 °C), and 60-100 nm (400 °C). A composite consisting of the α-Fe(2)O(3) nanoflake urchin-like structure and an epoxy resin exhibits an excellent electromagnetic (EM) wave absorption ability. A small tip diameter (20-30 nm) and a high density (3 × 10(13) nanoflakes m(-2)) lead to a good network structure and good EM wave absorption. A minimum reflection loss (RL) of -33.8 dB (99.93% of EM wave absorption) at 7.8 GHz can be achieved using a 70 wt% urchin-like material as the filler in the resin matrix. In addition, a composite containing 60 wt% unchin-like material exhibits dual-frequency EM wave absorption. The peaks of the minimum RL values are located at 9.7 GHz (-26.2 dB) and 25.2 GHz (-21.0 dB). The unique morphology of the α-Fe(2)O(3) nanoflake urchin-like material is believed to be a key factor in the enhancement of the EM wave absorption.

Formation of three-dimensional urchin-like α-Fe2O3 structure and its field-emission application.

A three-dimensional urchin-like α-Fe(2)O(3) microstructure is formed via a simple, template-free, and one-step thermal oxidation of Fe spheres in an air atmosphere at temperatures in the range of 300-450 °C. The urchin-like α-Fe(2)O(3) microstructure consists of crystalline α-Fe(2)O(3) nanoflakes grown perpendicularly on the surface of the sphere, a shell layer of α-Fe(2)O(3)/Fe(3)O(4), and an Fe core. During the oxidation process, the nanoflakes germinate and grow from cracks in the oxidation layer on the surface. The length of the nanoflakes increases with oxidation time. The tip diameters of the nanoflakes are in ranges of 10-20 nm at 300 °C, 20-30 nm at 350 °C, and 40-60 nm at 400 °C; the length can reach up to a few micrometers. The field-emission characteristics of the samples are experimentally studied and simulated. The results show that the urchin-like emitter has a low turn-on field of 2.8 V/µm, high field-enhancement factor of 4313, excellent emission uniformity of over 4 cm(2), and good emission stability during a 24 h test.

A carbonyl iron/carbon fiber material for electromagnetic wave absorption.

A carbonyl iron/carbon fiber material consisting of carbon fibers grown on micrometer-sized carbonyl iron sphere, was synthesized by chemical vapor deposition using a mixture of C2H2 and H2. The hollow-core carbon fibers (outer diameter: 140 nm and inner diameter: 40 nm) were composed of well-ordered graphene layers which were almost parallel to the long axis of the fibers. A composite (2 mm thick) consisting of the carbonyl iron/carbon fibers and epoxy resin demonstrated excellent electromagnetic (EM) wave absorption. Minimum reflection losses of -36 dB (99.95% of EM wave absorption) at 7.6 GHz and -32 dB (99.92% of EM wave absorption) at 34.1 GHz were achieved. The well-dispersed and network-like carbon fibers in the resin matrix affected the dielectric loss of the EM wave while the carbonyl iron affected the magnetic loss.