Abstract
The dissipated electric power in oxide powder compacts, subjected to flash sintering, is several hundreds of W·cm−3. This power is analyzed considering local softening/melting and transient plasma/liquid formation at the particle contacts due to thermal runaway. The sudden increase in compact electric conductivity and dissipated power referred to current percolation through the softening/liquid formed at the particle contacts, at the percolation threshold. The energy-balance and heat transfer considerations during the transient flash event are consistent with the local heating of the nanoparticle contacts to the ceramic melting temperature, or above it. The formation of the plasma by field emission of electrons is also considered.
Highlights
Flash sintering is a novel technique by which ceramic powder compact is densified during a few seconds under simultaneous furnace heating and an applied electric field
One interesting aspect of the process is that the dissipated electric power per unit volume at the flash event ranges over a few hundreds of W·cm−3 [4], irrespective of the oxide composition
The very high temperatures calculated above are higher than the melting points; they may reveal the formation of atmospheric-pressure plasma, due to the electron release from the charged particle surfaces subjected to locally high electric fields, and ionization of the surrounding air/atmosphere
Summary
Flash sintering is a novel technique by which ceramic powder compact is densified during a few seconds under simultaneous furnace heating and an applied electric field. The energy balance by a dynamic model, assuming non-uniform temperature, pointed to temperature gradients up to several thousand degrees along the cylindrical specimen diameter [6], much lower temperature gradients of ~100 ◦ C were experimentally reported [9] These models relate the flash event to thermal runaway, none of them considered local melting or surface softening at the particle surfaces. A continuous constant voltage mode may lead to the same results of rapid powder densification, albeit with possibilities of local electric breakdown and specimen disintegration In this respect, Park and Chen [10] used 8YSZ (8 mol % Yttria Stabilized Zirconia) specimens with different cross-sections, and measured specimen temperatures as high as 2500 ◦ C, very close to the specimen melting point. The goal of the following analysis is to show that the correct energy balance at the flash event can be obtained while taking into account partial melting of the particle surfaces, due to the local thermal runaway at the particle contacts
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