Nanostructured amorphous composites are novel materials composed of nanostructured amorphous matrix and precipitated nanocrystalline phases. However, their preparation requires large and expensive equipment. Flash sintering (FS) ensures a rapid densification at low temperatures and offers an effective approach for the preparation of nanostructured amorphous composites with simple and economical equipment. The amorphous powders by FS show denser microstructures, compared to the crystalline powders by FS. The phase transition from a metastable phase to a stable phase during the sintering process can expedite atomic diffusion and particle rearrangement, thereby promoting structural densification. In this paper, nanostructured amorphous powders in three systems, i.e., Al₂O₃–La₂O₃(AL), Al₂O₃–La₂O₃–ZrO₂(ALZ) and Al₂O₃–ZrO₂(AZ), were prepared by a sol–gel method and densified into dense bulks via flash sintering. The flash sintering behaviors of these different systems were compared, and the promotion of phase transformation on the densification was discussed.

The three systems of amorphous powders, i.e., AL, ALZ, and AZ, were synthesized via a sol–gel method and subsequently calcined at 600 ℃ for 1 h. Before flash sintering, the powders were consolidated into cylindrical green bodies (ϕ8 mm×2.3 mm) through uniaxial compression molding at 500 MPa. A platinum wire and electrodes were used to establish electrical connections between a DC power supply and the sample in electric fields of 1000 V/cm and 1600 V/cm, and a heating rate of 10 ℃/min. At a predetermined maximum current of 0.26 A/cm², the control mode of the power supply was swiftly switched from voltage control to current control. The voltage and current both were measured by a digital electrical multimeter. After continuous energizing for 60 s, the furnace and power supply both were powered off and naturally cooled to ambient temperature.

The powders of AL, AZ and ALZ all exhibit weak and broad scattering peaks without any detectable crystallization peaks, indicating their amorphous nature. The average particle sizes of the AL, AZ and ALZ powders are 13.7, 6.7 nm and 11.6 nm, respectively. Flash sintering occurs in each system at an electric field intensity of 1000 V/cm with increasing current density. Among the flash-sintered ceramics samples, the sample AZ shows the maximum crystallinity of 69.82% and the relative density of 89%. The crystallinity and relative density of the sample AZ further increase when the electric field intensity increases to 1600 V/cm. The densification during flash sintering is enhanced by crystallization as crystallization promotes the atomic diffusion and particle rearrangement. Based on the high-resolution transmission electron microscopy images, the uniform precipitation of nanocrystalline phases within an amorphous matrix in the flash-sintered sample AZ with the average particle sizes of 8.74 nm for nanocrystalline regions and 3.15 nm for amorphous regions.

This study conducted an investigation on the flash sintering behavior of three amorphous nanopowders (i.e., AL, AZ and ALZ), and examined the relationship between densification and phase transition. Flash sintering occurred in each nanopowder at an electric field intensity of 1000 V/cm, and the sample AZ exhibited the maximum crystallinity of 69.82% and relative density of 89%. The crystallinity and relative density of the AZ sample further increased to 74.11% and 96%, respectively, when the electric field intensity increased to 1600 V/cm. This study confirmed a positive correlation between crystallization and densification during flash sintering as crystallization and phase transition accelerated atomic diffusion and particle rearrangement, thereby enhancing ceramic densification. The HRTEM images of the sample AZ after flash sintering at 1600 V/cm revealed the uniform precipitation of nanocrystalline phases within an amorphous matrix, indicating that low-temperature rapid preparation of nanostructured amorphous composites was achieved via phase transformation-assisted flash sintering.