Consolidation of monolithic ceramics and metals via electric field-controlled sintering: Revealing athermal effects on microstructures and properties

Owing to the high temperatures and prolonged durations typically required for conventional sintering (CS), various forms of electric field-assisted sintering, particularly flash sintering (FS), have garnered significant attention for their potential to improve sintering efficiency. FS involves passing an electric current through a sample to generate Joule heating, enabling rapid material densification in a very short time. However, the application of FS to large samples is hindered by several detrimental issues, including the formation of large cracks caused by extremely rapid heating rates (~10 °C/min) and the nonuniform distribution of current and temperature. This study introduces a novel method called electric field-controlled sintering (ECS), in which the current passing directly through the sample is regulated to achieve a slower heating rate of 100–300 °C/min (although still significantly faster than that of CS). This approach facilitates the production of large cylindrical samples with diameters of up to 30 mm, which exhibit excellent mechanical properties and are free from observable cracks. The materials used in this study possess electrical conductivities exceeding 10⁶ S/m, ensuring uniform current and temperature distributions. The ECS technique can be used for sintering various materials, including MAX phases, cemented carbides, ultrahigh-temperature ceramics, and refractory metals. Additionally, the athermal effect in the ECS process was investigated, which refers to the changes in sintering behavior and material properties induced by the electric current itself rather than by Joule heating. Consequently, the proposed ECS method is expected to address the limitations of FS, which hinders its industrial application, while it also provides a means to study the athermal effects on sintering behavior and material properties.