Boron carbide (B₄C) is a highly strong covalent ceramic material with a low surface diffusion coefficient, making it extremely difficult to consolidate at low temperature. Therefore, the development of effective sintering additives that enable low-temperature, high-density consolidation while maintaining excellent mechanical properties remains a critical challenge for the scalable application of B₄C. In this study, Y₃Si₂C₂ was employed as a novel sintering additive to enhance the densification and mechanical performance of B₄C ceramics for the first time. B₄C ceramics containing 0–10 wt% Y₃Si₂C₂ were fabricated using the spark plasma sintering (SPS) technique at temperatures ranging from 1700 to 1900 °C. The effects of the Y₃Si₂C₂ sintering additive content and sintering temperature on the densification behavior, microstructure, and mechanical properties of B₄C ceramics were systematically investigated. The results indicate that Y₃Si₂C₂ facilitates the densification of B₄C ceramics via an in situ reactive liquid-phase sintering mechanism. The formation of the Y-rich liquid phase significantly enhances particle rearrangement and mass transport, while the in situ-generated YB₄ phase pins grain boundaries and suppresses abnormal grain growth. As a result, the mechanical properties of the B₄C ceramics were significantly improved. Optimal comprehensive mechanical properties were achieved at 1800 °C with 8 wt% Y₃Si₂C₂, yielding a Vickers hardness of 31.4 GPa and a flexural strength of 694.0 MPa. This work paves the way for rare-earth silicon carbide as a novel sintering additive of B₄C ceramics to enhance the densification and mechanical performance for extreme environment applications.

Pr₃Si₂C₂ is a ternary layered structural material, which combines the merits of ceramics and metals. The synthesis of Pr₃Si₂C₂ is a critical issue for its applications, since the highly reactive Pr is easily oxidized. The Pr₃Si₂C₂ powder was synthesized using a molten salt method through the in-situ reaction between Pr and SiC. Mixed NaCl-KCl molten salt with a molar ratio of 1:1 was used to supply the liquid phase reaction medium and promote the matter transportation. The effects of the molten salt temperature on phase composition and microstructure of the as-obtained Pr₃Si₂C₂ powder were studied. The results indicated that the Pr₃Si₂C₂ powder with a ~8 wt.% oxygen can be obtained at the molten salt temperature of 850 ℃ for 5 h in Ar. The proposed molten salt method for the synthesis of Pr₃Si₂C₂ powder can be potentially used for the fabrication of Pr₃Si₂C₂ ceramics and sintering additive of SiC ceramics and SiC matrix ceramic composites.

Low temperature sintering of highly dense SiC ceramics is one of the research focuses of scholars in industries, because of the high degree of Si-C covalent bonding and the low self-diffusion coefficient of SiC. Noting that low-temperature eutectic liquid phase transition between Pr₃Si₂C₂ and SiC can promote the sintering of SiC, Pr₃Si₂C₂ was grown in-situ on the surface of SiC particles by using a molten salt method, in order to ensure the uniform dispersion of Pr₃Si₂C₂ in the SiC particles. SiC ceramics with high density (porosity of 0.3%) and thermal conductivity of up to 106 W·m^-1·K^-1 were successfully fabricated, by using electric current field-assisted sintering technology at a low temperature of 1500 ℃. According to microstructure of the SiC ceramics, it is concluded that the sintering process followed a typical solution-precipitated liquid phase sintering mechanism. Therefore, Pr₃Si₂C₂ is a promising low-temperature sintering additive for the low-temperature sintering of SiC ceramics and their composites.

A novel class of high-entropy rare-earth metal diborodicarbide (Y_0.25Yb_0.25Dy_0.25Er_0.25)B₂C₂ (HE-REB₂C₂) ceramics was successfully fabricated using the in-situ reactive spark plasma sintering (SPS) technology for the first time. Single solid solution with a typical tetragonal structure was formed, having a homogeneous distribution of four rare-earth elements, such as Y, Yb, Dy, and Er. Coefficients of thermal expansion (CTEs) along the a and c directions (α_a and α_c) were determined to be 4.18 and 16.06 μK⁻¹, respectively. Thermal expansion anisotropy of the as-obtained HE-REB₂C₂ was attributed to anisotropy of the crystal structure of HE-REB₂C₂. The thermal conductivity (k) of HE-REB₂C₂ was 9.2±0.09 W·m⁻¹·K⁻¹, which was lower than that of YB₂C₂ (19.2±0.07 W·m⁻¹·K⁻¹), DyB₂C₂ (11.9±0.06 W·m⁻¹·K⁻¹), and ErB₂C₂ (12.1±0.03 W·m⁻¹·K⁻¹), due to high-entropy effect and sluggish diffusion effect of high-entropy ceramics (HECs). Furthermore, Vickers hardness of HE-REB₂C₂ was slightly higher than that of REB₂C₂ owing to the solid solution hardening mechanism of HECs. Typical nano-laminated fracture morphologies, such as kink boundaries, delamination, and slipping were observed at the tip of Vickers indents, suggesting ductile behavior of HE-REB₂C₂. This newly investigated class of ductile HE-REB₂C₂ ceramics expanded the family of HECs to diboridcarbide compounds, which can lead to more research works on high-entropy rare-earth diboridcarbides in the near future.

A novel Y₃Si₂C₂ material was synthesized at a relatively low temperature (900 ℃) using a molten salt method for the first time, and subsequently used as the joining material for carbon fiber reinforced SiC (C_f/SiC) composites. The sound near-seamless joints with no obvious remaining interlayer were obtained at 1600 ℃ using an electric field-assisted sintering technique (FAST). During joining, a liquid phase was formed by the eutectic reaction among Y₃Si₂C₂, γ(Y-C) phase, and SiC, followed by the precipitation of SiC particles. The presence of the liquid promoted the sintering of newly formed SiC particles, leading to their complete consolidation with the C_f/SiC matrix. On the other hand, the excess of the liquid was pushed away from the joining area under the effect of a uniaxial pressure of 30 MPa, leading to the formation of the near-seamless joints. The highest shear strength (τ) of 17.2±2.9 MPa was obtained after being joined at 1600 ℃ for 10 min. The failure of the joints occurred in the C_f/SiC matrix, indicating that the interface was stronger than that of the C_f/SiC matrix. The formation of a near-seamless joint minimizes the mismatch of thermal expansion coefficients and also irradiation-induced swelling, suggesting that the proposed joining strategy can be potentially applied to SiC-based ceramic matrix composites (CMCs) for extreme environmental applications.

A nano-laminated Y₃Si₂C₂ ceramic material was successfully synthesized via an in situ reaction between YH₂ and SiC using spark plasma sintering technology. A MAX phase-like ternary layered structure of Y₃Si₂C₂ was observed at the atomic-scale by high resolution transmission electron microscopy. The lattice parameters calculated from both X-ray diffraction and selected area electron diffraction patterns are in good agreement with the reported theoretical results. The nano-laminated fracture of kink boundaries, delamination, and slipping were observed at the tip of the Vickers indents. The elastic modulus and Vickers hardness of Y₃Si₂C₂ ceramics (with 5.5 wt% Y₂O₃) sintered at 1500 ℃ were 156 and 6.4 GPa, respectively. The corresponding values of thermal and electrical conductivity were 13.7 W·m^-1·K^-1 and 6.3×10⁵ S·m^-1, respectively.

High strength SiC whisker-reinforced Ti₃SiC₂ composites (SiC_w/Ti₃SiC₂) with an improved thermal conductivity and mechanical properties were fabricated by spark plasma sintering. The bending strength of 10 wt% SiC_w/Ti₃SiC₂ was 635 MPa, which was approximately 50% higher than that of the monolithic Ti₃SiC₂ (428 MPa). The Vickers hardness and thermal conductivity (k) also increased by 36% and 25%, respectively, from the monolithic Ti₃SiC₂ by the incorporation of 10 wt% SiC_w. This remarkable improvement both in mechanical and thermal properties was attributed to the fine-grained uniform composite microstructure along with the effects of incorporated SiC_w. The SiC_w/Ti₃SiC₂ can be a feasible candidate for the in-core structural application in nuclear reactors due to the excellent mechanical and thermal properties.

The SiC/Al₄SiC₄ composites with the improved mechanical properties and thermal conductivity were fabricated by the in-situ reaction of polycarbosilane (PCS) and Al powders using spark plasma sintering. The addition of 5 wt% yttrium (Y) sintering additive was useful to obtain fully dense samples after sintering at a relatively low temperature of 1650 ℃, due to the formation of a liquid phase during sintering. The average particle size of the in-situ formed SiC was ~300 nm. The fracture toughness (4.9 MPa·m^1/2), Vickers hardness (16.3 GPa), and thermal conductivity (15.8 W/(m·K)) of the SiC/Al₄SiC₄ composite sintered at 1650 ℃ were significantly higher than the hardness (13.2 GPa), fracture toughness (2.16 MPa·m^1/2), and thermal conductivity (7.8 W/(m·K)) of the monolithic Al₄SiC₄ ceramics. The improved mechanical and thermal properties of the composites were attributed to the high density, fine grain size, as well as the optimized grain boundary structure of the SiC/Al₄SiC₄ composites.