| Microwave absorption materials have attracted intensive interest because of the increasing severe electromagnetic (EM) pollution and interference problems caused by the development of electronic equipments. The absorption properties mainly depend on the permittivity (εr=ε’-jε"), permeability (μr=μ’-jμ"), and their matching. Permittivity can be adjusted by changing absorber shape, composition, interface, and dimension on a relatively large scale. Therefore, it’s of important research significance to explore preparation methods for magnetic nanostructures with different morphologies and compositions, as well as their microwave properties. Magnetite (Fe3O4), as a traditional ferrite microwave absorber, with both dielectric loss and magnetic loss exhibits excellent microwave absorption properties. In this paper, Fe3O4 and Fe3O4/C magnetic materials with various morphologies were synthesized; their formation mechanism and microwave absorption properties were systematically investigated. The investigation aims to develop new methods for the preparation of Fe3O4 and Fe3O4/C composite materials and to improve their microwave absorption properties by tuning morphologies and compositions. The main work can be summarized as follows:1. Modulation mechanism of size and composition and electromagnetic properties of monodisperse Fe3O4 microspheresMonodisperse polycrystalline Fe3O4 microspheres ranging in size from 82 nm to 1116 nm were synthesized by tuning the H2O volume fraction(γ) via a mixed solvothermal method in the H2O-EG system, using ferric chloride hexahydrate (FeCl3·6H2O) as iron source, anhydrous sodium acetate (NaAc) as base and polyacrylic acid (PAA) as surfactant. In the reaction system, H2O functions as solvent, reactant and coordination agent, playing a key role in the nucleation and growth of Fe3O4 nanocrystals. The size-dependent electromagnetic properties and the relationship between H2O volume fraction and sphere diameter (d), crystal size (D), surface O content, saturation magnetization (Ms), coercivity (Hc) values were investigated. As the results shown:The d initially presents a sharp decrease at γ= 5.5% to 14.5%, which is followed by a gentle decrease. Through the variation of γ from 5.5% to 20.5%, D shows the inversed U-shaped change tendency with the maximal value approximately at γ= 14.5%. The surface O content increases with the increasing of H2O volume fraction. Ms declined slowly at γ= 5.5% to 13.5% because of the decreased d and rapidly at γ= 13.5% to 20.5% because of the decreased D and enhanced O content. Hc peaked approximately at γ= 3.5% because of the critical diameters. The microspheres exhibited the strongest magnetic loss when γ=9.2% (d=728 nm). While the spheres showed the best absorption properties with the reflection loss of-37 dB at 4.5 GHz when γ= 5.5% (d= 1116 nm) because of the high dielectric loss and good matching property.Monodisperse NixFe3-xO4 (0≤x≤0.245) microspheres ranging in size from 40 nm to 400 nm were synthesized by changing Ni2+/Fe3+ molar ratio (α= 0:1,0.5:1,0.75:1,1:1,1.5:1, 2:1) via a solvothermal method in the EG system, using ethylenediamine (EDA) as base and polyacrylic acid (PAA) as surfactant. The modulation mechanism of size and composition and size-dependent magnetic properties were investigated.2. Controllable synthesis and electromagnetic properties of Fe3O4 and Fe3O4/C nanoringsThe elliptical iron glycolate nanosheets with controllable size and Fe3O4 hollow spheres were synthesized via a solvothermal method by tuning the reaction time, reaction temperature and adding metal ion. Monodisperse elliptical polycrystalline Fe3O4 and Fe3O4/C nanorings (NRs) with continuously tunable size can be obtained in large amounts via a rapid microwave-assisted hydrothermal approach or a hydrothermal method by using glucose as reducing agent and carbon source and iron glycolate nanosheets with different size as precursors. The obtained Fe3O4/C NRs reveal enhanced low-frequency microwave absorption because of improvements to their permittivity and impedance matching.Nanomaterials with different morphologies and compositions could be obtained by sintering the nanosheet precursors under different temperature, ambience and the present of acetone. When the as-prepared nanosheets were heated at 300～400℃ and 500℃ for 2 h in air, γ-Fe2O3 and α-Fe2O3/γ-Fe2O3 porous nanosheets (PNSs) were obtained, respectively. While at 700℃, α-Fe2O3 nanochains were obtained. For the synthesis of Fe3O4 nanorings, the precursors were heated at 300～600℃ under nitrogen. Further more, Fe3O4/C nanorings could be obtained when the precursors were sintered at 300～500℃ under argon at the present of acetone. The obtained Fe3O4 NRs exhibit size-dependent magnetic properties and plasmon resonance enhancement of dielectric properties. The tunability of dielectric properties depends on the long axis rather than on the specific surface area, internal stress, and grain size. An excellent microwave absorption property was generated in Fe3O4 NRs with a 65±25 nm long axis, a volume fraction of 11 v%, and an optimal reflection loss value of-50.13 dB is achieved at 7.84 GHz and the absorption range (RL≤-20 dB) is over 2.36-16.40 GHz. The enhanced microwave absorbing properties of these NRs are ascribed to the unique ring-like configuration, which enhances multiple scattering, plasmon resonance absorption, microantenna radiation, and interference loss.3. Controllable synthesis and electromagnetic properties of Fe3O4/C nanosheetsα-Fe2O3 nanosheets with controllable size were fabricated via a mixed solvothermal method in the H2O-EtOH system by tuning the H2O volume and the reactant concentration. Fe3O4/C nanosheets could be obtained by a carbothermal process using the as obtained nanosheets as precursors. Nanosheets with the size of 250±30 nm showed a minimum reflection loss (RL) of-39.72 dB at 4.29 GHz with absorbing frequency (RL≤-20 dB) from 2.08 GHz to 16.40 GHz. The Fe3O4/C nanosheets exhibit excellent microwave absorption properties because of the strong shape anisotropy of the flakcy morphology and the good matching properties of Fe3O4/C composites. |
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