Preparation Of Cobalt, Nickel And Their Composites As Microwave Absorbing Materials By γ-RAY Irradiation Induced Reduction

Nanoscale magnetic metals and their composites are of great interest in the field of microwave absorbing materials. It is of great significance to study the preparation of magnetic materials with controllable morphology byγ-irradiation technique. Meanwhile, preparation of composites consisting of nanoscale magnetic metals and inorganic carriers will not introduce any impurities, which can be an advantage for preparing composite microwave absorbing materials.The surfactant-assisted preparation of Ni, Co, and Co-Ni alloys byγ-irradiation technique has been studied by choosing PVP, CTAB, PVA, and SDBS as the surfactants. The prepared materials are characterized by XRD, SEM, VSM and vector network analyzer, etc. It is shown that the category of the surfactants can obviously change the morphology and structure of the prepared magnetic nanoparticles, which further causes differences in magnetic and electromagnetic properties. The Ni nanoparticles prepared with the assistance of PVP, CTAB, PVA, and SDBS are all cubic phase, but the morphologies differ a lot. The chain-like Ni prepared using SDBS as the surfactant presents an absorption peak of -23.6 dB at 7.4 GHz, which results from the stronger shape anisotropy of the nanochains.The effect of external magnetic field applied during theγ-irradiation on the preparation of magnetic nanoparticles has also been investigated. It is found that the external magnetic field can change the nucleation and growth of the magnetic metals, leading to assembled structures with controllable morphology and structure. The Co morphology change from irregular nanoparticles to flower structures that are assembled by nanosheets after the Co2+ ions complex with ammonia, and application of an external magnetic field produces well-dispersed nanosheet structures. Control over the strength of the applied magnetic field can induce the alignment of the Co nanoparticles, where well aligned Co sticks are obtained with a 3000 G magnetic field. With the increase in the strength of the magnetic field, the degree of the alignment is increased, higher magnetization can be produced, and the frequency corresponding to the maximum absorption shifts to lower band. The alignment of the Co nanoparticles helps the microwave absorption. Under the assistance of magnetic field, regular Ni nanochains are produced, which present enhanced microwave absorption, and the number of absorption peaks can be tuned by the thickness of the material. With different Co:Ni ratios, Co-Ni alloys with various morphologies are produced, and one component has an effect to induce the alloy to grow with its own morphological feature. Actinia-like Co-Ni alloy with a Co:Ni ratio of 1:4 presents ideal microwave absorbing ability, with a peak of -33 dB at 10 GHz, and the absorption band can be tuned by the thickness of the Co-Ni alloy. This Co-Ni alloy with specific morphology can be a candidate as microwave absorbing materials.Ni/MWCNTs, Ni/CS, Ni/G, and Co/SiO₂ magnetic composites are prepared via theγ-irradiation process, and the relationships between the structure, electromagnetic parameters, and absorption properties have been studied.γ-irradiation produces defects on the surface of MWCNTs, which serve as the nucleation sites for the Ni nanoparticles. Ni/MWCNTs has enhanced microwave absorption in 3.8¹8 GHz, with an absorption peak of -7.2 dB at 6.4 GHz. The improved microwave absorption of Ni/MWCNTs results from enhanced magnetic loss and improved impedance matching, but not dielectric loss. An absorption peak of -14 dB at 9.8 GHz can be found in Ni/CS composites, and absorption higher than -10 dB can be seen in 8.6¹1.2 GHz. We think that the enhancement of the electromagnetic absorption results from a synergetic consequence of magnetic loss brought by the coating of Ni nanoparticles on cenospheres, and interface relaxation loss between Ni nanoparticles and cenospheres.With a proper ratio of SiO₂ and Co, the imaginary parts of permittivity and permeability of Co/SiO₂ composite can be greatly enhanced. The microwave absorption of Co/SiO₂ composite contributes from a synergetic consequence of dielectric loss and magnetic loss. With SiO₂:Co=10:1, Co/SiO₂ composite has an absorption peak of -23 dB at 6.2 GHz. Application of surfactant and magnetic field in theγ-ray irradiation induced reduction can produce Co, Ni and Co-Ni alloys with special morphologies and structures, and some of them have excellent microwave absorbing properties. One advantage ofγ-ray irradiation induced reduction method can be reflected by the fact that no impurities are present in the prepared composite materials, and the absorbing properties at low- and middle-frequency ranges are superior to those prepared by chemical plating method.