Preparation And High-frequency Properties Of Ferromagnetic Metal/Al₂O₃Composites

Metal/ceramic composite materials, also known as cermets, are formed by metallic or alloy particles dispersed within a ceramic matrix. When the metal content in the cermets exceeds percolation threshold, the composites may present a negative permittivity behavior; at the same time, ferromagnetic metal phase will result in a negative permeability due to magnetic resonance. In this paper, the high-frequency properties of ferromagnetic metal/Al2O3composites were studied, and the frequency dispersion of its permittivity and permeability was analyzed. The mechanism of both negative permittivity and negative permeability was also explored. This kind of double negative material is of great significance in the areas of electronic components, communication and stealth technology.Fe/Al2O3, FeNiMo/Al2O3and FeNi50/Al2O3bulk cermets with different metal contents were prepared via hot-pressing sintering with ferromagnetic metal powder and Al2O3powder, respectively. The permittivity and permeability of the composites at room temperature in high frequency range were determined using precision impedance analyzer. And high-frequency properties of the cermets in relation to metal concentration were analyzed carefully. The composition and microstructure of the composites were also characterized.The results show that the high-frequency dielectric property of ferromagnetic metal/Al2O3composites is strongly dependent on the composition. When the metal content is relatively low, metallic particles are isolated in the ceramic matrix and the composites present an insulator-like dielectric property. And the real permittivity increases with the increasing metal contents due to the interfacial polarization of the metal-ceramic interface. In this case, the reactance of the composites is negative, which indicates a capacitive character. When the metal content is relatively high, connectivity of the metallic particles in Al2O3matrix is increased obviously and the dielectric properties transform from insulator-like to metal-like because of the percolation phenomena. Negative permittivity behavior is observed in the composites. In this case, the composites manifest an inductive character. In addition, the vermicular Fe has a higher connectivity, which leads to a lower percolation threshold for Fe/Al2O3than FeNiMo/Al2O3. The negative permittivity can be well explained using Drude model.The high-frequency magnetic property of Fe/Al2O3and FeNiMo/Al2O3composites is also related to the metal contents. When the metal content is low, the permeability of the composites is positive and almost independent on the frequency. When the metal content is high, the real part of permeability decreases to negative at some frequency range.This research aims at exploring cermets with double negative property, which is of great significance. Theoretically, this research can enrich the understanding of double negative materials. Moreover, the materials can be widely used in the areas of electronic component and communication. Specially, cermets can be used as absorbing ceramic for vacuum tube. They can decouple electromagnetic wave, suppress the bad-edge oscillation and spurious modes as well as eliminate non-designed modes, which ensure the normal work of microwave vacuum devices.