Herein, lithium-zinc ferrite (LZF) ceramics were prepared using spark plasma sintering at 1000 ℃ for 10 min. Carbon pollution problem is effectively resolved by annealing for decarbonization (800 ℃, 2 h), spraying BN on graphite foil and replacing graphite foil with different metals foil (Ti, Mo, W and Ta) as protective layer. Also, the influence of carbon pollution and different protective layers on microstructure, as well as dielectric and magnetic properties, of LZF ceramics were studied in detail. Results reveal that, after decarbonization by annealing, grain size of LZF increases from 0.88 μm to 1.04 μm, relative density increases from 88.5% to 91.0%, resistance increases significantly, real part of dielectric constant (ε′) and dielectric loss (tanδ) decrease significantly, and saturation magnetization (Ms) increases by 10 emu·g-1. However, it is difficult to remove deeply encapsulated carbon phase, and several pores and defects are left behind after decarbonization. X-ray diffraction (XRD) patterns indicate that all selected protective layers can obtain good spinel phase LZF ceramics. The utilization of metallic foil, as protective layer, can effectively reduce carburization, improve the resistivity of LZF ceramics, and reduce ε′ and tanδ values. Moreover, the metallic foil leads to metal ions infiltrating into LZF lattice, increasing lattice constant and decreasing Ms value. Valence state and distribution of metallic cations infiltrated into crystal lattice are analyzed using X-ray photoemission spectroscopy (XPS) and Raman spectroscopy, revealing that cations with highest valence state replace Fe3+ at B-site. This work deepens the understanding of carburization during SPS, provides a reference scheme and promotes the application of SPS technology for the sintering of spinel ferrite ceramics.
Electromagnetic interference (EMI) shielding materials made of Ni‐Co coated on web‐like biocarbon nanofibers were successfully prepared by electroless plating. Biocarbon nanofibers (CF) with a novel web‐like structure comprised of entangled and interconnected carbon nanoribbons were obtained using bacterial cellulose pyrolyzed at 1200°C. Paraffin wax matrix composites filled with different loadings (10, 20, and 30 wt%, resp.) of CF and Ni‐Co coated CF (NCCF) were prepared. The electrical conductivities and electromagnetic parameters of the composites were investigated by the four‐probe method and vector network analysis. From these results, the EMI shielding efficiencies (SE) of NCCF composites were shown to be significantly higher than that of CF at the same mass fraction. The paraffin wax composites containing 30 wt% NCCF showed the highest EMI SE of 41.2 dB (99.99% attenuation), which are attributed to the higher electrical conductivity and permittivity of the NCCF composites than the CF composites. Additionally, EMI SE increased with an increase in CF and NCCF loading and the absorption was determined to be the primary factor governing EMI shielding. This study conclusively reveals that NCCF composites have potential applications as EMI shielding materials.
To achieve efficient design and accurate simulation of neutron spin flipping, a fast numerical calculation method was introduced to facilitate the processes parameter optimization and flipper design. Magnetic field models and measured magnetic data can be directly imported into the simulation. To test the proposed new simulation software, three experimental examples were performed and compared with the measured data. The software developed showed good accuracy.
Temperature- and pulse amplitude/width dependent current-voltage characteristics were performed to study the variance of negative differential resistance (NDR) features accompanying bipolar resistive switching. Interestingly, the absolute value of the NDR peak intensity is enhanced at first and then weakened, while the NDR peak position gradually shifts toward the lower absolute bias value with an increase in temperature from 140 to 300 K, and shifts toward the larger absolute bias value with an increase in temperature from 300 to 400 K. Furthermore, the NDR peak is absent after applying a small and narrow positive pulse, while it gradually increases with increasing the applied pulse amplitude or width. These temperature- and pulse-dependent NDR behaviors can be fully understood based on a model of generation and drift of ionized oxygen vacancies coupled with trapping/detrapping electrons. The clarification of the mechanism will pave the way for practical applications.
Fe nanoflakes were prepared by the ball-milling technique, and then were coated with 20 nm-thick SiO(2) to prepare Fe/SiO(2) core-shell nanoflakes. Compared with the uncoated Fe nanoflakes, the permittivity of Fe/SiO(2) nanoflakes decreases dramatically, while the permeability decreases slightly. Consequently, reflection losses exceeding - 20 dB of Fe/SiO(2) nanoflakes are obtained in the frequency range of 3.8-7.3 GHz for absorber thicknesses of 2.2-3.6 mm, while the reflection loss of uncoated Fe nanoflakes almost cannot reach - 10 dB in the same thickness range. The enhanced microwave absorption of Fe/SiO(2) nanoflakes can be attributed to the combination of the proper electromagnetic impedance match due to the decrease of permittivity and large magnetic loss due to strong and broadband natural resonance. The key to the combination is the coexistence of the nanoshell microstructure and the nanoflake morphology.
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