Preparation of CoFe2O4 hollow spheres with carbon sphere templates and their wave absorption performance
Graphical abstract
Introduction
With the rapid development of electromagnetic wave field, various types of power facilities and radio frequency equipment have gradually increased, causing electromagnetic radiation as a new environmental pollution source. Many countries have formulated the standards of electromagnetic radiation protection. At the same time, with the continuous improvement of radar technology, stringent requirements and restrictions have been put forward for electromagnetic wave absorbing materials used for stealth of weapon equipment. Therefore, developing new absorbers of electromagnetic radiation is necessary [[1], [2], [3], [4], [5]].
Among many absorbing materials, ferrite with natural resonance as the main absorbing mechanism is a traditional absorber, which is with better performance and low cost, and has been extensively studied. Its applications in electromagnetic shielding and stealth technology are relatively mature [6,7]. However, because of its narrow resonance band, large matching thickness, and high density, it is difficult to meet the requirements of the current application scenarios for the low density, low thickness, strong absorption capability, and wide bandwidth absorbing material [8,9].
Sahar and Masoud extensively synthesized and studied various rare-earth oxides by simple and green methods [[10], [11], [12], [13], [14], [15], [16]]. The morphology and size of the products were effectively controlled by plant extracts, and these rare-earth oxides show excellent performance in photocatalysis and hydrogen storage. Thus, it can be concluded that in addition to modifying the composition of ferrite, changing the morphology and structure of ferrite is another new idea to improve its performance [17,18].
Hollow sphere structures have attracted attention owing to their low density, good dispersion, large specific surface area, and the ability of repeatedly absorbing electromagnetic waves and scattering in the cavity. Duan et al. [19] successfully prepared hollow microspheres of Ni0.5Co0.5Fe2O4 using carbon spheres as the templates and studied the effect of Ce3+ doping on the microwave absorption properties. Compared to the powder samples, the maximum reflection loss of Ni0.5Co0.5Fe2O4 with hollow spheres increased from −3.78 dB to −8.71 dB, and the reflection loss of the doped hollow spheres at 5.5 GHz reached −18.8 dB. Li et al. [20] reported the preparation of hollow Fe3O4 nanospheres with a diameter of 450 nm and the wall thickness of 80 nm by the Ostwald ripening process. Compared to the corresponding ferrite ceramics, the density of the hollow spheres reduced by 27%. Moreover, the composites made of hollow spheres and silicone had good absorption bandwidth. These studies show that the hollow sphere structure can effectively improve the absorption performance and reduce the density of ferrite, but there are only a few studies on the effect of the size and dispersion of the hollow sphere on the absorption performance.
In order to adjust the size and dispersion of the hollow sphere, a suitable preparation method should be selected. Hollow spheres are prepared by three common methods: soft template method [[21], [22], [23]], hard template method [24,25], and self-reactive quenching method [26,27]. Compared to other methods, the preparation of hollow spheres with colloidal carbon spheres as hard template by the hydrothermal method has several advantages such as low toxicity of process, convenient experiment, low cost, and no need to sensitize the template surface [28]. However, the problem of cross-linking and agglomeration of products often occurs in the preparation of CCS by hydrothermal carbonization, limiting the application of CCS as a template; therefore, the synthesis process needs to be improved [29].
In this study, CoFe2O4 hollow spheres were prepared by a simple adsorption–calcination method using CCS as the template. The effect of surfactant type on the morphology and dispersion of CCS has been investigated in detail. The results show that PAAS can greatly improve the dispersion of CCS through steric hindrance effect, and DTAB can greatly increase the diameter of CCS. The microwave absorption properties of CoFe2O4 hollow spheres prepared by the above two templates improved compared to those of the unoptimized ones. This study provides a new way to prepare high-performance ferrite absorbers.
Section snippets
Materials
Ferric nitrate nonahydrate (Fe(NO3)3·9H2O, purity 98.5%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, purity 99%), and sodium dodecylbenzene sulfonate (SDBS, purity 95%) were supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. Glucose (purity 99%), polyacrylamide (PAM), dodecyltrimethylammonium bromide (DTAB, purity 99%), sodium polyacrylate (PAAS), sodium polyacrylate 50% aqueous solution (PAAS aq.), polyethylene glycol (PEG), myristyl sulfobetaine (MSB, purity 98%), and
Structural of colloidal carbon spheres
Fig. 3 shows the FTIR spectra of the CCS prepared with and without dispersants. The addition of different dispersants did not show any effect on the composition of CCSs, and the main functional groups are basically the same. The band at 3405 cm−1 was assigned to the O–H stretching vibration, whereas the bands at 1304, 1213, and 1025 cm−1 belong to the C–O stretching vibration, arising from hydroxyl, carboxyl, and furan rings, respectively. The band at 2927 cm−1 is attributed to the C–H
Conclusion
In conclusion, CoFe2O4 hollow spheres with good dispersion and large particle size were successfully prepared by optimizing the colloidal carbon sphere templates. In 2–18 GHz bandwidth, the absorption bandwidths (RL < −7 dB) reach 3.44 and 3.12 GHz, at the FHS-P and FHS-D thicknesses of 7.6 and 7.1 mm, respectively. They increase the number of reflections and scattering of electromagnetic waves in the absorber and enhance the absorption performance by strengthening the interface polarization
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (51702158), the Fundamental Research Funds for the Central Universities (NP2018111), Open Fund of Key Laboratory of Materials Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Open Fund of Graduate Innovation Base of Nanjing University of Aeronautics and Astronautics (kfjj20190612), and Ministry of Industry and Information Technology (No. XCA19013-05).
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