Two-Step Sintering of Hydrothermally Derived Bismuth-Based Relaxor Nanoparticles for Fine-Grained Multilayer Ceramic Capacitors

For the application of Bi-based relaxor ferroelectrics as high-voltage dielectric materials, establishing effective grain refinement strategies in multilayer ceramic capacitor (MLCC) configurations is a key challenge. In this study, (Bi1/2K1/2)TiO3–SrTiO3 (BKST)-based MLCCs were fabricated using hydrothermally synthesized nanoparticles combined with a two-step sintering process to achieve fine-grained dielectric layers. Dense MLCCs with grain sizes down to 230 nm were successfully obtained, which are significantly smaller than those reported for Bi-based relaxor MLCCs prepared from solid-state-derived powders. Grain refinement effectively suppresses electric-field-induced phase transitions to the normal ferroelectric state, leading to reduced hysteresis loss and improved energy storage efficiency. As a result, the MLCC sample sintered under optimized conditions exhibits a high energy storage efficiency exceeding 85% and a maximum recoverable energy storage density of 2.4 J cm–3. These results demonstrate that two-step sintering of hydrothermally derived nanoparticles is an effective strategy for realizing high-performance Bi-based relaxor MLCCs.