| Carbon-coated magnetic metal nanocapsules with core/shell structure have multiple microwave loss mechanisms.In this thesis,carbon-coated magnetic nanocapsules prepared by an arc discharge method are the main research objects.The size and microstructure of the obtained products are regulated by using different carbon sources and reaction materials.The phase composition and microstructure of the prepared samples were characterized and discussed by various testing methods.The magnetic and electromagnetic properties of the samples were tested by VSM and VNA.According to the electromagnetic parameters obtained from the test,the electromagnetic wave absorption performance of the obtained sample in the microwave frequency range of 2-18 GHz was analyzed by transmission line theory,and the absorbing mechanism of the nanocapsules was studied.Ni@C_A and Ni@C_E nanocapsules were prepared from AFC and alcohol.The other reaction parameters were kept consistent during the preparation process.The results show that Ni@C_A nanocapsules have smaller nickel core and thicker carbon shell,and SiC is doped in the carbon shell of Ni@C_A nanocapsules,showing lower saturation magnetization and higher coercivity.Compared with Ni@C_E,the complex permittivity and dielectric loss of Ni@C_A nanocapsules were significantly increased by measuring their electromagnetic parameters.This was mainly due to the synergistic effect of doping SiC,small size Ni nanoparticles and thicker OLC shell in the OLC shell.Secondly,by analyzing the resonance-antiresonance behavior of dielectric and mesomagnetism,the energy conversion between permeability and dielectric constant is explained.In the frequency range of S and C bands,the RL value of Ni@C_A nanocapsules was less than-20 dB.Compared with Ni@C_E nanocapsules,Ni@C_A can obtain the same absorption band and intensity under thinner absorber thickness,and the absorption peak is at a lower frequency.At 5.12 GHz,the maximum reflection loss value of the present Ni@C nanocapsules can reach-49.99 dB for a planar absorber with a thickness of 5.1 mm,while the RL value of Ni@C_E at the same thickness was-9.94 dB.Compared with Ni@C_E,Ni@C_A has the advantages of low density and strong absorption,and overcomes the shortcomings of low dielectric constant and thick absorber of Ni@C_E.Fe@C_A nanocapsules and Fe@C_E nanocapsules were prepared by an arc discharge method using ash-free coal(AFC)and alcohol as carbon sources,keeping other reaction parameters consistent.We found that Fe@C_A nanocapsules have smaller iron core and thicker carbon shell.The carbon shell of Fe@C_A nanocapsules prepared by using ash-free coal(AFC)as carbon source is doped with SiC,a strong dielectric material.We measured the electromagnetic parameters of two kinds of nanocapsules in the frequency range of2-18 GHz.It was found that Fe@C_A nanocapsules had larger complex permittivity,indicating that Fe@C_A nanocapsules had stronger dielectric loss.At the frequency of 4.56GHz,the RL of Fe@C_A nanocapsules reaches-39.38 dB with the thickness of 4.4 mm.By comparing the three-dimensional reflection loss maps of the two kinds of nanocapsules,it is found that the peak reflection loss of Fe@C_A nanocapsules is in the lower frequency range,while that of Fe@C_E nanocapsules is in the high frequency range,which indicates that we can prepare absorbing materials suitable for different frequency bands through different carbon sources.Taking iron-nickel alloy as the research object,the(Fe,Ni)@C nanocapsules were successfully prepared by an arc discharge method using AFC and alcohol carbon sources respectively and labeled as sample B and sample E.The TEM photo shows that the B sample has Smaller size.XPS results show that carbon shell of B sample is doped with SiC.By measuring the electromagnetic parameters of the two nanocapsules,it can be found that the sample B has a larger complex dielectric constant,which is mainly caused by the small size of the nanoparticles and the SiC doped in the onion-like carbon shell.The microwave absorption properties of the two nanocapsules were investigated by measuring their electromagnetic parameters in the frequency range of 2-18 GHz.Sample B obtained the RL value of-29.98 dB when the absorber thickness was 3.9 mm and the frequency was6.72 GHz,and the RL value of sample E at the same thickness was greater than-10dB.This result indicates that the sample B has the advantages of a thin thickness and a small RL value.At the same time,in this chapter,using AFC as a carbon source,the microwave absorption properties of nanocapsules prepared by using AFC,micron-sized iron powder and nickel powder at different ratios were studied.The results show that the sample with the iron powder,nickel powder and AFC mass ratio of 1:2:1 has the best microwave absorption performance,and the sample has a frequency of 4.72 GHz,the thickness of the absorber is 5.1 mm,and the reflection loss value reaches the maximum value-63.18 dB,when the absorber thickness is adjusted(0.5-10 mm),its nanocapsule can obtain a reflection loss less than-10 dB in the frequency range of 2-18 GHz,which meets the characteristics of thin thickness and absorption frequency bandwidth,and has the potential to be used as a new absorbing materials. |
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