We report phase transition process during the solid-state reaction of BaCO 3 -TiO 2 system under the assistance of electric field. Experiments were conducted at a constant heating rate with preset field strength and current limit. Solid-state reaction was completed upon reactive flash sintering taking place at ~1002℃ under 200 V/cm. Hexagonal BaTiO 3 phase, which rarely occurs at such temperatures, was obtained after flash sintering at a current density of 23.5 mA/mm 2 . It is speculated that oxygen deficiency during flash sintering triggered cubic-to-hexagonal transition of BaTiO 3 . Furthermore, X-ray diffraction results show that solid-state reaction takes place prior to flash sintering. Electric field could accelerate the reaction but did not alter the sequence of phase evolution.
In this paper, high-entropy (MgCoNiCuZn) 1-x Li x O oxides (x = 0, 0.1, 0.15, 0.2, and 0.3) were synthesized via reactive flash sintering (RFS), and the effect of RFS process on the microstructure and electrical property of the materials were studied. The Li-doped materials exhibited a mixed ionic-electronic transport behavior. The oxidation of Co 2+ into Co 3+ upon Li incorporation into the materials synthesized via the conventional solid-state reaction route was not evidenced in the flash sintered materials. Instead, the charge unbalance in the Li-doped materials synthesized via RFS was compensated by oxygen vacancies and holes in the valence band of the oxides, which were accounted for the ionic conduction and electronic conduction, respectively. The ionic conductivity increased upon increasing the Li concentration as more oxygen vacancies were formed. The attraction between defects with different charges (Li M / and V O •• ), which formed defect complexes, led to a decrease in the mobility of the defects, thus resulting in a less pronounced increase in the ionic conductivity at high Li concentrations. The change in the charge compensation mechanism of the materials indicates that the microstructure of such kind of oxides could be altered through RFS, and thus the property may be manipulated.
Flash sintering is a sintering technology coupled with temperature field and electric field. It has the characteristics of rapid mass transfer at low temperature, showing significant advantages in the synthesis of high entropy ceramics. In this study, relatively dense high entropy oxide ceramic (MgCoNiCuZn)O was synthesized by flash sintering, which properties were compared with those of conventional sintered samples. Under flash sintering condition of room temperature, the electric field intensity of 50 V/cm and the current density of 300 mA/mm 2 , the phase transformation only need 10 s. The maximum relative density of flash sintered sample is 94%, which is 22.8% higher than that of conventional sintered sample. The maximum hardness of flash sintered sample is 5.05 GPa, which is 3.95 GPa higher than that of conventional sintered sample. When the frequency is lower than 2 Hz, the dielectric constant of flash sintering sample is one order of magnitude higher than that of conventional sintered sample. The reason for the property improvement of flash sintered samples is listed as follows, the acceleration of mass transfer by the critical electric field to increase the material density, and the extra defects introduced by the critical electric field.
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