Flash sintering of dielectric nanoparticles as a percolation phenomenon through a softened film

Abstract

Recent work [Biesuz et al., J. Appl. Phys. 120, 145107 (2016)] showed analogies between the flash sintering and dielectric breakdown in a-aluminas pre-sintered to different densities. Here, we show that flash sintering of dielectric nanoparticles can be described as a universal behavior by the percolation model. The electrical system is composed of particles and their contact point resistances, the latter softened first due to preferred local Joule heating and thermal runaway during the flash. Local softening has a hierarchical and invasive nature and propagates between the electrodes. The flash event signals the percolation threshold by invasive nature of the softened layer at the particle surfaces. Rapid densification is associated with local particle rearrangements due to attractive capillary forces induced by the softened film at the particle contacts. Flash sintering is a critical phenomenon with a self-organizing character. The experimental electric conductivity results from flash sintering are in full agreement with those calculated from the percolation model.

Enhancement of the SrTiO3 Surface Reactivity by Exposure to Electric Fields

“…It might originate from the localized thermal gradient at the microstructural level. As shown elsewhere, during electric field-assisted consolidation of insulating materials local overheating [28] or even melting [35] of the grain boundaries occurs. Thus, such temperature gradients in the grain might cause defect redistribution and contribute to explain the larger Sr segregation.…”

On thermal runaway and local endothermic/exothermic reactions during flash sintering of ceramic nanoparticles

“…Recent percolation model showed the possibility for nanoparticle surface softening/melting at temperatures resembling plasma and in excess of the melting points [13]. This was in agreement with observed dielectric prebreakdown in alumina [10,34]. In addition, several teams challenged recently the electric field effects in flash sintering, by conducting high heating rate sintering experiments in the absence of the electric field.…”

“…The Joule heat prior to any melt was calculated using the solid phase properties, both as dense, as well as a granular (percolative) system [34]. However, at the current percolation threshold, when continuous liquid forms, the specimen properties of a percolating media (with liquid volume fraction of 0.247) were used [34]: …”

“…However, at the current percolation threshold, when continuous liquid forms, the specimen properties of a percolating media (with liquid volume fraction of 0.247) were used [34]: …”

Electric current effect during the early stages of field‐assisted sintering

“…According to the existing literature, some time‐decaying mechanisms, proposed for explaining the rapid densification upon FAS, could be considered to explain the unexpected time exponent in Type‐B regime. Chaim proposed that during the flash transition in flash sintering experiments the particles surface melts, thus enhancing the sintering rates by liquid phase‐activated mechanisms . This hypothesis appears to be not applicable to our experiments for two reasons: (a) the specific power dissipation is rather low (always lower than 355 mW/mm 3 in the steady stage of the process); (b) the liquid formation likely represents a transient state during the flash event, the thermal diffusivity in the micrometric/submicrometric scale causing a rapid temperature homogenization.…”