In this paper, the tunable negative permittivity and permeability of copper/yttrium iron garnet (Cu/YIG) composites, which were prepared by in situ synthesis process, were investigated in the radio frequency regime.
Abstract Graphene/silicon nitride ( GR /Si 3 N 4 ) ceramic composites with uniformly dispersed GR sheets were prepared using spark plasma sintering. The effects of GR content on the microstructure and electrical properties of the composites were investigated in detail. With the GR content rising, the conductive GR network was formed in the composites, leading to the appearance of a percolation phenomenon, and the conductive mechanism also changed from hopping conductivity to metal‐like conductivity. When the GR content reached the percolation threshold, the composites showed a negative permittivity behavior, which resulted from the low frequency plasmonic state generated by the formative conducting GR networks. The increasing GR content resulted in a higher plasma frequency and larger magnitude of negative permittivity, which was consistent with the analysis of Drude model. A relatively high dielectric loss was observed in the composites and mainly induced by the high leakage current among GR sheets. Our work is beneficial to expound the regulation mechanism of negative permittivity, and the obtained ceramic composites present some potential applications in microwave absorption, shielding and capacitors.
Abstract The microstructure design is important for regulating the microwave dielectric properties of materials. However, in‐depth studies on the frequency temperature stability and related micromechanism remain poorly understood. The work investigates the correlation among the sintering behavior, crystal structure, bonding nature, and microwave dielectric properties of LnPO 4 (Ln = Eu, Pr) ceramics by combining first‐principles calculations and experimental perspective. The high density ( ρ > 97%) and large grains associated with lattice expansion benefit the optimum dielectric properties: ε r = 11.24, Q × f = 61,138 GHz @ 13.311 GHz, and τ f = −30.3 ppm/°C for EuPO 4 sintered at 1500°C ( ε r = 11.35, Q × f = 63,496 GHz @ 13.042 GHz and τ f = −39.5 ppm/°C for PrPO 4 sintered at 1525°C). Bond valence analysis shows a rattling effect in [EuO 9 ] due to a smaller ionic radius. The effect induces an abnormally large polarization, effectively shifting the negative τ f toward near‐zero values. The electron localization functions, charge transfer, and bonding nature are discussed by density functional theory calculations, which illustrate stronger charge transfer and ionicity between Eu and O. This observation effectively predicts and validates the nonharmonic lattice vibrations and abnormally large polarization obtained from Raman spectrums and Rietveld refinement. These findings systematically clarify the optimized effect and micromechanism of lanthanides on the dielectric properties of monazite ceramics.
The physical properties of metamaterials with negative parameters and the negative permittivity behavior of the intrinsic properties of materials deserves further investigation, the systematic study of the influence of chemical composition on their permittivity behavior, and the investigation of the realization and regulation mechanism of negative permittivity behavior. The carbon fiber/ alumina (CF/Al2O3) ceramic composites were prepared by hot-press sintering, and the effects of different carbon fiber content on the micromorphology and electrical properties of composites were studied. By adjusting the CF content in the composites, the negative dielectric behavior was realized in the frequency range of 1-10 MHz, and the conductive mechanism changed from hopping conduction to metal-like conductivity. It is found that the increased CF forms three-dimensional interconnection network in the composites, and the negative permittivity is caused by the plasma oscillation of the internal free electrons in the CF network. With the increase of CF content, the absolute value of negative permittivity becomes larger, and the dispersion characteristics of permittivity conforms to the Drude model.
Abstract The study on novel physical properties of structural ceramics or ceramic composites could make them more conducive to be function‐ and structure‐integrated materials. Herein, titanium nitride‐alumina (TiN–Al 2 O 3 ) duplex ceramics were prepared and the dielectric spectra of the ceramics were studied from 10 MHz to 1 GHz. Negative permittivity appeared when TiN content exceeded 40 wt% due to the induced plasmonic state of massive delocalized electrons in connected TiN grain networks. Meanwhile, alternating current conduction behaviors of the duplex ceramics were discussed with percolation theory. Furthermore, the analysis of reactance by equivalent circuit models indicated that negative permittivity ceramics exhibited inductive character. This work realized negative dielectric behaviors in TiN–Al 2 O 3 duplex ceramics and would promote the study of electromagnetic functionalization in wave shielding or attenuation for structural ceramics.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)