Influence of the Field and the Current Limit on Flash Sintering at Isothermal Furnace Temperatures

Abstract

We report, for the first time, the observation of an incubation time for the onset of flash sintering in experiments carried out at isothermal furnace temperatures. The incubation time varies highly nonlinearly with the strength of the DC field, akin to a nucleation‐like phenomenon. The setting of the maximum current at the power supply has a significant influence on the extent of densification. The hold time at these current settings has an influence on the grain size increasing it with time and the magnitude of the current setting. The experiments were carried out on 3 mol% yttria‐stabilized zirconia. In all instances the specimen temperatures during flash sintering, estimated from a previously validated blackbody radiation model remain well below the temperatures that would be required for conventional sintering of yttria‐stabilized zirconia. Taken together these observations imply a nucleation of defect avalanche as a possible mechanism for flash sintering.

Flash sintering of ceramic materials

“…However flash sintering can occur under isothermal settings, albeit often after an incubation time. Examined in detail for the first time by Francis and Raj [39], this finding demonstrated the importance of the maximum current in determining flash sintering properties. In these experiments the power supply was only turned on once the sample was determined to have equilibrated with the furnace temperature.…”

“…Dogbone-shaped specimens are convenient for the monitoring of shrinkage in the gauge section during the flash sintering process by visual methods [39], whereas shrinkage in pellets is usually measured using linear displacement techniques such as the use of a force sensor [26] or laser system [40]. There is evidence that the handle-sections of flash-sintered dogbone specimens experience higher electric fields and hence higher temperatures, leading to larger grain growth under DC electric fields [41].…”

“…Samples have been flash sintered in dogbone [8,20,22,23,39,41,[71][72][73] and pellet form [26,31,36,37]. The apparatus used includes a vertical tube furnace with camera to detect shrinkage in dogbone samples [20,22,39,41,71,72], a box furnace with front window for use of a camera [8], a commercial spark plasma sintering machine used without the usual graphite mould [31], a vertical furnace adapted to hold pellets by the use of alumina supporting bars with [26,37] or without [36] significant applied load, and a modified system holding suspended dogbones designed for in situ X-ray diffraction studies [23,73].…”

“…As the expected parameters required became better established, it was easier for later experiments to introduce an isothermal hold in temperature at a suitable level. Experiments carried out in this way established that flash sintering can also occur after an incubation time at a particular temperature [39]. In addition it has been established using measurements of the optical spectrum of the emission that the glowing of the samples during flash sintering is due to electroluminescence, rather than black body radiation [34,123].…”

Electrical Behavior and Microstructural Features of Electric Field-Assisted and Conventionally Sintered 3 mol% Yttria-Stabilized Zirconia

“…Many other reports were published applying this technique to other materials [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], studying the dependence on the amplitudes of the electric field and current density [31], on the average particle size [32], on the porosity [33], and searching for an explanation of such events [34][35][36][37][38][39][40].…”