| It is believed that the complex permittivity, complex permeability, and the electromagnetic impedance match, of the absorber determine their microwave absorption properties. The dielectric loss is mainly arised from the electronic polarization, ionic polarization or interfacial polarization effect of the absorber, while the magnetic loss is determined by the magnetization mechanism of hysteresis loss, domain wall resonance, and eddy current loss to dissipate the electromagnetic energy. The requirement for absorbing materials is becoming more rigorous due to their expanding application fields. The development of electromagnetic wave absorbing materials with the advantages of light weight, thin, bandwidth and good compatibility has aroused more attentions.As a novel class of absorbing material, nanomaterials has been the hot topic in the field of electromagnetic wave absorption due to their excellent electromagnetic wave absorption property. However, due to there small size and high surface energy, it is very difficults to realize the dispersion of nano-materials, which limit the advantage of nanomaterials suriously application researches. Moreover, the ideal electromagnetic wave absorbing materials were expected to possess both high dielectric loss (tanδE=εr"/εr’) and magnetic loss (tanδM=μr"/μr’).Thus, it is important to improve the complex permittivity, complex permeability simultaneously for the preparation of high-performance electromagnetic wave absorption materials. Evidently, it is difficult to meet the high requirements of absorbing performance for a single-component absorber,Inspired by the above, in this work, several series of nanocomposites were fabricated by combine nanomaterials with different absorbing mechanisms through some facile in-situ synthesis methods. Hopefully, by the help of in-situ synthesis methods, the dispersion of nanomaterials could be improved and more dissipation mechanisms could be obtained, as well as attain some other properties. The field emission scanning electron microscope (FESEM), transmission electron microscope (TEM,), the Philips X’Pert Pro X-ray diffractometer (XRD) and the confocal microscopic Raman spectrometer were used to analyze the morphology, structure and composition of the nanocomposites. The impact of composition, morphology, structure and defects of the nanocomposites on the magnetic and electromagnetic properties of the samples were also investaged. The main research contents and obtained results are as follows:1 In the liquid phase system, Ni/ITO nanocomposite was prepared was prepared via a facile in situ reducing route while nickel chloride was used as the nickel source, hydrazine hydrate was used as a reducing agent and NaOH was used to adjust the pH of the sulution. In as-prepared nanocomposite, ITO nanoparticles not only work as separating mediums to prevent Ni magnetic nanoparticles from agglomeration but also provide the hybrid nanocomposite with extra interfacial polarization mechanism for electromagnetic wave absorbing. As a result, as-fabricated Ni/ITO nanocomposite possesses significantly enhanced microwave absorption than Ni nanoparticles and ITO nanoparticles. This is because nonmagnetic ITO nanoparticles help to achieve uniform dispersion of Ni nanoparticles in as-fabricated Ni/ITO nanocomposite, which is beneficial to acquiring proper electromagnetic impedance match and facilitating extra interfacial polarization mechanism thereby leading to increased microwave absorption. The present research, hopefully, is to open a new door to fabricating novel advanced microwave absorbing composites with tunable composition and electromagnetic performance.2 In this work, nickel-carbon (Ni/C) nanocomposites were prepared facilely as potential light-weight microwave absorbers via the calcination of nickel nitrate-polyacrylamide mixture in flowing ammonia. The electromagnetic properties of as-prepared Ni/C nanocomposites were investigated in relation to their composition and microstructure. Findings indicate that as-prepared Ni/C nanocomposites are of island-like microstructure and consist of porous carbon medium and Ni nanoparticles with a size of several hundreds of nanometers. As a result, the Ni/C nanocomposites exhibit excellent electromagnetic properties. The electromagnetic properties of Ni/C nanocomposites are highly dependent on calcination temperature; and the Ni/C nanocomposite obtained at 600 ℃ exhibits ideal electromagnetic properties. In a small absorbing thickness range of 2.3-7.0 mm, the frequency range in which the RL<-20 dB (the absorption of electromagnetic waves of up to 99%) can be achieved 13.5 GHz. This is attributed to the special porous structure and desired electromagnetic impedance matching of the carbon medium as well as the uniform dispersion of Ni nanoparticles in the carbon medium. Namely, the carbon medium is accessible to induced multi-dielectric polarization thereby improving the microwave absorbing properties of the Ni/C nanocomposites, and it also contributes to improving the thermal stability, chemical stability, and dielectric performance of the nanocomposites while the intrinsic magnetic properties of metallic Ni are retained.3 TiN/C nanocomposites were prepared by a facilely one step route with nanotubular titanic acid (H2Ti2O4(OH)2, denoted as NTA) and polyacrylamide (denoted as PAM) as the starting materials. Here, titanium nitride-carbon (denoted as TiN/C) nanocomposites were adopted as concrete examples to investigate the intrinsic magnetic and electromagnetic behaviors in relation to their structural defects. The results indicated that the temperature possessed significantly affects on the structure component and properties of the nanocomposites. Moreover, it is evidenced that TiN/C nanocomposites exhibit not only distinct static magnetic properties but also amazing dynamic permeability, associated with anomalous magnetic and dielectric resonances at 8-12 GHz. Moreover, there is no straightforward relation between the weak static magnetic properties and the dynamic magnetic properties of TiN/C nanocomposites. This means that though there is some intrinsic relation between the structural defects and electromagnetism of the as-prepared nanostructures, the static and dynamic magnetic properties have not a common origin. This work is an important step towards revealing the static magnetic and dynamic electromagnetic origin of nanostructures and the intrinsic correlation between the structural defects and microwave electromagnetism, which might open a new door to designing electromagnetic absorbers with high-performance via defect engineering..4 On the basis of the above works, furthermore, nickel nitrate, titanate nanotubes (NTA) and polyacrylamide (PAM) were used as raw materials, a series of nickel-titanium nitride-carbon (Ni/TiN/C) nanocomposites were prepared via a facile in situ method by calcinating their mixture in flowing ammonia at 900℃. The composition and structural characterizations as well as the magnetic and electromagnetic properties of the as-prepared products were also investigated. The results indcated that Ni nanoparticles could be dispersed in the composites. Moreover, the Ni/TiN/C composites with appropriate proportion of Ni not only exhibit a distinct dielectric loss but also a surprised magnetic loss, which lead to the excellent absorbing performance. As to sample S3, its maximum reflection loss could reach-28.4 dB at 4.5 GHz and the absorption bandwidth of RL less than-10 dB could exceed 7.4 GHz, which might be contribute to the multi-absorbing mechanism of Ni/TiN/C nanocomposites and the ideal match between dielectric loss and magnetic loss. In additionally, the good thermal stability and lower density make it feasible for Ni/TiN/C nanocomposites to be used in harsh working conditions.In summary, in this work, a series of absorbing nanocomposites were successfully prepared through some simple in situ methods. By combining the absorbing components with different mechanisms in the nanocomposites, some ideal electromagnetic wave absorption materials with more electromagnetic dissipation efficiency could be obtained. Besides, the well dispersion of the components could provide them more opportunities to interact with electromagnetic waves, which could help them to dissipate electromagnetic energy more efficient. Moreover, the good compatibility between different the components could provided the nanocomposites extra interfacial polarization mechanism, which help them to improve the electromagnetic waves absorbing ability furtherly. Lastly, by studying the relationship between the composition, structure, microscopic defects and its electromagnetic properties, this work provide not only electromagnetic wave absorbing materials with excellent comprehensive performances but also some new research ideas and experiment evidences on theoretical basis for the study of the unique electromagnetic waves dissipation mechanism of nanomaterials. |
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