Impedance Spectroscopy and Dielectric Properties of Flash Versus Conventionally Sintered Yttria‐Doped Zirconia Electroceramics Viewed at the Microstructural Level

Abstract

The defect chemistry‐modulated dielectric properties of dense yttria‐doped zirconia ceramics prepared by conventional sintering (at 1350°C–1500°C) and electric field‐assisted flash sintering (55 V/cm at 900°C) were studied by impedance spectroscopy. While the bulk dielectric properties from both sets of samples showed only small and insignificant changes in conductivity and permittivity, respectively, a huge increase of these properties was measured for the grain boundaries in the flash sintered specimens. A close analysis of these results suggests that flash sintering reduced grain‐boundary thickness (by about 30%), while increasing the concentration of oxygen vacancies near these interfaces (by about 49%). The underlying mechanism proposed is electric field‐assisted generation and accommodation of defects in the space‐charge layers adjacent to the grain surface. The changes in measured permittivity are attributed to the boundary thickness effect on capacitance, while conductivity involved variations in its defect density‐dependent intrinsic value, accounting for changes also observed in grain‐boundary relaxation frequencies. Therefore, in terms of modifications to the specific dielectric properties of these materials, the overall consequence of flash sintering was to considerably lower the semi‐blocking character of the grain boundaries.

Enhancement of sintering rates in BaTiO₃ by controlling of DC electric current

“…The limited densification at high field strength beyond 250 V/cm is likely to result from the discharge. The criterion to discharging is roughly coincident to the results obtained by M'Peko et al 11) In contrast, grains are very uniform apart from the damaged area. In the area, neither secondary phase nor amorphous layer was observed as directly confirmed in TEM images (d) and (e).…”

“…However, final densities of the flash sintered bodies were lower than 90% at the field strength beyond 75 V/cm, while the relative density of 98% was obtained by FAST at the field of 25 V/cm. The effectiveness of electric field to sinterability obtained in this study is similar to that reported by M'Peko et al 11) In order to examine the effect of FAST process on the densification, the sintering experiments until the occurrence of flash event were separately performed; the sintering experiments at 75 and 100 V/cm were quitted just before the occurrence of the Fig. 2.…”

“…12) Actually, M'Peko et al recently reported that BaTiO 3 exhibits discharging inside the compacts during flash event over a critical electric field and current. 11) Meanwhile, according to the previous studies on flash-sintering, Yoshida et al has suggested that FAST process followed by flash event significantly contributes to densification in dielectric ceramics such as Y 2 O 3 9) and Mgdoped Al 2 O 3 . 7) Here, in this study, FAST process is defined as the sintering process where current gradually increases.…”

Review of flash sintering: materials, mechanisms and modelling

“…Thermal expansion has been used to measure the temperature of FS samples [70]. This method can be applied to dense samples while reheating them in FS mode under a dissipated power profile comparable to an FS cycle.…”

“…Conv gb [70]. The electric field-activated V Ö defects were accommodated at the subgrain space-charge regions, resulting in higher grain-boundary conductivity for YSZ during FS.…”