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.