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.

Silicon nitride ceramics are typical structural ceramics with excellent comprehensive properties, while brittleness is still their shortcoming in application. Introduction of ductile metal particles is regard as an important method to adjust the brittleness of ceramics. In recent years, attempts have been made to improve the properties of silicon nitride ceramics by introducing metal particles. Nevertheless, because the high sintering temperature of silicon nitride ceramics is much higher than the melting point of most metals, together with problems, such as thermal expansion coefficient mismatch, high temperature chemical incompatibility, poor interface wettability and so on, the introduction of metal into silicon nitride ceramics could bring great difficulties. In this work, the related works of the introduction of different metals into silicon nitride ceramics as second phase in recent years will be reviewed, including the solutions to tackle the above problems and enhancement in performance. Finally, the problems and existing solution of metal introduction into silicon nitride ceramics are summarized and a simple prospect is given.

Poor flowability of printable powders and long preparation cycles are the main challenges in the selective laser sintering (SLS) of chopped carbon fiber (C_f) reinforced silicon carbide (SiC) composites with complex structures. In this study, we develop an efficient and novel processing route in the fabrication of lightweight SiC composites via the SLS of phenolic resin (PR) and C_f powders with the addition of α-SiC particles combined with the one-step reactive melt infiltration (RMI). The effects of α-SiC addition on the microstructural evolution of the C_f/SiC/PR printed bodies, C_f/SiC/C green bodies, and derived SiC composites were investigated. The results indicate that the added α-SiC particles play an important role in enhancing the flowability of raw powders, reducing the porosity, increasing the reliability of the C_f/SiC/C green bodies, and contributing to improving the microstructure homogeneity and mechanical properties of the SiC composites. The maximum density, flexural strength, and fracture toughness (K_IC) of the SiC composites are 2.749±0.006 g·cm⁻³, 266±5 MPa, and 3.30±0.06 MPa·m^1/2, respectively. The coefficient of thermal expansion (CTE, α) of the SiC composites is approximately 4.29×10⁻⁶ K⁻¹ from room temperature (RT) to 900 ℃, and the thermal conductivity (κ) is in the range of 80.15–92.48 W·m⁻¹·K⁻¹ at RT. The high-temperature strength of the SiC composites increase to 287±18 MPa up to 1200 ℃. This study provides a novel as well as feasible tactic for the preparation of high-quality printable powders as well as lightweight, high-strength, and high–κ SiC composites with complex structures by the SLS and RMI.