Electromagnetic metamaterials have attracted widespread attention because of their unique properties, and the introduction of conductive metals or carbon into an insulating matrix is the main method for preparing metamaterials. Silicon nitride ceramics have become an ideal matrix for electromagnetic metamaterials because of their high degree of insulation, high thermal conductivity, high-temperature resistance, corrosion resistance, and excellent mechanical properties. However, owing 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 a mechanical pressure-assisted sintering process, which greatly limits their high-performance preparation and industrial application. To address this issue, this work proposes the use of the high melting point metal tungsten as the conductive second phase. Through the control of chemical reactions, analysis, and regulation of the densification process, the materials were fully densified by gas pressure sintering. After the introduction of tungsten, not only did the electrical and thermal conductivity properties of the silicon nitride ceramics improve, but a negative permittivity behavior was also observed when the tungsten content reached 20 vol%. A new type of dense silicon nitride-based metamaterials with great industrial potential was proposed and prepared, which can guide 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.