The fabrication of Si₃N₄ ceramics typically requires high temperatures (above 1700 °C) and prolonged sintering time to achieve densification, resulting in high energy consumption and increased manufacturing costs. Moreover, reports on the fabrication of dense Si₃N₄ ceramics with good mechanical properties under MPa-level pressure and low temperatures are rare. In this work, we propose a low-temperature rapid spark plasma sintering strategy involving the introduction of fine-grained β-Si₃N₄ powder with high lattice strain energy as an “additive”. Dense biphasic Si₃N₄ ceramics, predominantly α-Si₃N₄, were successfully fabricated at a mechanical pressure of 200 MPa and a temperature of 1300 °C, achieving a relative density of 97%. The application of high pressure promoted particle rearrangement and uniform liquid‒phase distribution, providing additional driving forces for sintering. The introduction of β-Si₃N₄ seeds facilitated an in-situ solution–reprecipitation process, enabling rapid densification with a minimal liquid phase and without significant grain growth, resulting in nanometer-scale grains. The Si₃N₄ sample prepared at 1350 °C exhibited a desirable combination of high hardness (18.5 $\pm$0.3 GPa) and fracture toughness (6.7 $\pm$0.2 MPa·m^1/2). The results demonstrate that by adjusting the sintering temperature and time, the phase composition and mechanical properties of the ceramics can be flexibly tailored. This work holds significant potential for industrial manufacturing and provides valuable insights into low-temperature strategies for ceramic fabrication.

The rapid miniaturization and high integration of modern electronic devices have brought an increasing demand for polymer-based thermal management materials with higher thermal conductivity. Boron nitride nanosheets (BNNs) have been widely used as thermally conductive fillers benefiting from the extremely high intrinsic thermal conductivity. However, the small lateral size and weak interface bonding of BNNs enabled them to only form thermally conductive networks through physical overlap, resulting in high interfacial thermal resistance. To address this issue, an innovative strategy based on interface engineering was proposed in this study. High-aspect-ratio boron nitride belts (BNbs) were successfully synthesized by carbon thermal reduction nitridation method through the in-situ generation and sintering of BNNs. The surface of BNb showed the sintering of numerous smaller-sized BNNs, which precisely addresses the issue of weak interfacial bonding between BNNs. On this basis, the as-synthesized BNbs were combined with nano-fibrillated cellulose (NFC) to prepare NFC/BNb composite films through a facile vacuum filtration process. Due to the thermally conductive network formed by the horizontal oriented arrangement of BNb and their particular morphological advantages, the NFC/BNb films demonstrated significantly higher in-plane thermal conductivity than that of NFC/BNNs films, achieving the highest value of 19.119 W·m⁻¹·K⁻¹ at a 20 wt% filling fraction. In addition, the NFC/BNb films also exhibited superior thermal stability, mechanical strength, flexibility, and electrical insulation performance, suggesting the significant application potential of the designed BNb fillers in the thermal management field.