Synthesis And Microwave Absorption Properties Of Micro/Nano-Structred Fe₃O₄Composites

With the rapid spread of wireless communication devices using electromagnetic waves in the range around GHz, several problems such as electromagnetic interference and information leakage have emerged and need to be solved. In addition, there has been a growing and wide spread interest in microwave-absorbing materials due to the self-concealing technology, microwave measurements of missile guidance in military affairs. Therefore, the demands to develop thinner, lighter weight and stronger microwave absorbers with wider absorption bands are ever increasing. Fe3O4are noted as one of the most promising materials for fabricating the microwave absorber in the GHz range due to their high permeability, low toxicity, low cost and easy achievement. However, the traditional Fe3O4still have some disadvantages such as high density, narrow absorption-band, and low absorption properties. Therefore, some kinds of composites, including Fe3O4and dielectric materials, Fe3O4and conducting polymer, Fe3O4and carbon nanotubes and so on, have been proposed for microwave absorption. In this dissertation, the main object is to develop the simple method to achieve size and morphology tuned fabrication of micro/nano-sturctured Fe3O4composites, to research their microwave absorption properties, to explore the relation between microwave absorption properties and the component and morphology of these composites. The main results are summarized as follows:(1) In order to decrease the permittivity and eddy-current effects of carbonyl iron powders (CIP) and increase the ability of absorbing the electromagnetic wave, the core/shell structure CIP/Fe3O4composites have been prepared by a simple one-step hydrothermal method through controlling the pH value of the reaction system (12,13and14). A wide region and strong ability of microwave absorption was achieved for the CIP/Fe3O4composites (pH=13). When the matching thickness is2.0mm, the reflection loss of the sample exceeding-10dB was obtained at the frequency range of8.7to15.0GHz and with an optimal reflection loss of about-38.1dB. The microwave absorbing performance of this composite is far beyond superior than that of a single component of CIP or Fe3O4, which is related to the higher permeability and proper impedance matching of the CIP/Fe3O4composite with peculiar core/shell structure.(2) In order to obtain the lighter weight and stronger microwave absorbers, flake-like carbonyl-iron/Fe304composites have been synthesized via ball-milling technique and hydrothermal method. Their absorption properties indicate that BMCI/Fe3O4composites synthesized in the pH values of12and13have absorption peak values of about-33.5dB and-28.2dB, respectively, when the matching thickness is3.0mm. Meanwhile, compared with the above-mentioned composite of CIP/Fe3O4, the volume content of this absorber reduces by half and reaches to20%. (3)Hollow glass microspheres/Fe3O4(HGMs/Fe3O4) composites with low density were successfully synthesized by solvothermal method. The as-prepared HGMs/Fe3O4composites show excellent microwave absorption properties. When the thicknesses of these HGMs/Fe3O4composites are more than1.5mm, they exhibit strong absorption peaks. For the HGMs/Fe3O4composite obtained for2h, reflection loss values less than-20dB are observed in the range of6.0to11.8GHz with thicknesses of2.0to3.5mm, whereas the minimum RL value of-36.2dB appears at7.3GHz, corresponding to a3.0mm matching thickness. The excellent absorbing performance of HGMs/Fe3O4composite is related to the hollow structure of HGMs/Fe3O4composite, which confines the incident electromagnetic wave within the hollow sphere and causes multiple scattering, multiple reflecting and multiple absorbing inside the hollow sphere.(4)In order to obtain lighter weight and stronger microwave absorbers, multiwalled carbon nanotubes (MWCNTs)/Fe3O4and carbon spheres (CSs)/Fe3O4composites were synthesized via a simple low temperature solution method. For MWCNTs/Fe3O4composites, when the weight of MWCNTs is40mg (the weight ratio of Fe3O4to MWCNTs is about30:1) and the matching thickness is2mm, the reflection loss of the sample exceeding-10dB was obtained at the frequency range of9.9-12.4GHz, with an optimal RL of-29.8dB at11.04GHz. For CSs/Fe3O4composites, optimal reflection loss of about-10dB is reached at frequency rang of8to18GHz when the layer thickness is in the range of2.0to3.5mm.