Hybrid low-temperature sintering processes of electro-ceramics

Cold sintering process (CSP) developed recently has attracted much attention due to the ultralow sintering temperatures and high efficiencies with the assist of transient liquid phase (TLP). Based on CSP, we have further established a protocol for low-temperature sintering processes by combining TLP with other sintering technologies, including hybrid sintering process with microwave and TLP (MW-TLP), and hybrid sintering process with spark plasma sintering and TLP (SPS-TLP). Three typical electro-ceramics (TiO2, CaWO4, and ZnO) are selected, which are highly densified (>97%) with excellent electrical properties at reduced sintering temperatures, applied pressures or holding times, demonstrating the feasibility of MW-TLP and SPS-TLP in fabricating electro-ceramics. Especially, the Q x f value of TiO2 ceramics (38,020 GHz) prepared by MW-TLP at 1000 degrees C is 46.2% and 23.4% higher than that of microwave and spark plasma sintered samples, respectively, and comparable to traditional thermal sintered (TTS) samples at 1300-1400 degrees C. The dielectric properties of MW-TLP CaWO4 ceramics sintered at 900 degrees C for 2 h are comparable to TTS samples sintered at 1000-1300 degrees C for 2-5 h. ZnO ceramics can be highly densified (similar to 98%) by SPS-TLP with mild sintering conditions (200-300 degrees C and 3.8-50 MPa) compared to SPS (>500 degrees C) and CSP (>100 MPa). The frameworks of fundamental mechanisms are outlined together with the experimental data. It is expected that this work will provide promising sintering methods to fabricate electro-ceramics and offer inspirations on sintering combinations to develop low-temperature sintering processes with high efficiencies.