Electromagnetic metamaterials have attracted widespread attention due to their unique properties, and the introduction of conductive metal or carbon into an insulating matrix is the main method for preparing metamaterials. Silicon nitride ceramics have become an ideal matrix for electromagnetic metamaterials due to their high insulation, high thermal conductivity, high-temperature resistance, corrosion resistance and excellent mechanical properties. However, due to poor sintering activity, chemical incompatibility, thermal expansion mismatch or second phase melt agglomeration, it is difficult to prepare dense silicon nitride-based metamaterials without mechanical pressure-assisted sintering process, which greatly limits its high-performance preparation and industrial application. To address this issue, this work proposes to use high melting point metal tungsten as the conductive second phase. Through the control of chemical reactions, analysis and regulation of the densification process, the material was fully densified by gas press sintering. After the introduction of tungsten, not only the electrical and thermal conductivity properties of silicon nitride ceramics were improved, but also a negative permittivity behavior was observed when the tungsten content reached 20 vol%. A new type of dense silicon nitride-based metamaterial with great industrial potential was proposed and prepared, which guides the preparation and industrial application of high-performance metamaterials.

Excellent electromagnetic shielding materials is the best choice to protect human health and maintain the normal operation of instruments and equipment. Silicon nitride-based ceramics containing carbon fibers, with high electrical conductivity and strong electromagnetic shielding performance, were prepared by using spark plasma sintering (SPS). Firstly, the introduction of carbon fiber led to an increase in the dielectric constant of the composite ceramics, the impedance mismatch of the sample surface was increased, so that more the electromagnetic waves were reflected. Secondly, since high content carbon fiber was introduced, a relatively complete three-dimensional conductive network was formed in the silicon nitride matrix, leading to a conductivity of 2.15 S·m^-1. The incident electromagnetic wave is absorbed due to the dielectric loss of the materials. Finally, when 7 wt.% carbon fiber was introduced, Vickers hardness, fracture toughness and electromagnetic shielding effectiveness of the composite ceramics reached 11.90 GPa, 6.05 MPa·m^1/2 and 46 dB, respectively.