The original flash sintering experiment was carried out by applying an electric field, and switching to current control at the onset of the flash, signaled by a rise in conductivity. Here, we consider experiments where the experiment is controlled from the very start, by injecting current, which is increased at a constant rate. The current rates are varied from 50 mA/min to 5000 mA/min. The experiment is continued until, in all cases, the current density reaches 100 mA/mm 2 . The total duration of the experiment ranged from approximately 7 seconds to 700 seconds. The following comparisons to the earlier voltage-to-current experiments are noted: (a) in both instances, the onset of the flash is signaled by an unusual rise in conductivity; however, since the power supply remains in the current control mode, the increase in conductivity is manifested by a drop in the voltage generated across the specimen; (b) the blackbody radiation model is modified to include the energy absorbed in specific heat, in order to determine the time-dependent change in temperature as the current is increased-this correction is particularly significant at the very high current rates; (c) sintering occurs continuously, reaching full density, in all instances, when the current density reaches~100 mA/mm 2 ; and (d) these early experiments suggest that the current-rate experiments yield fine-grained microstructure across the entire gauge section of the dog-bone specimen, presumably because the highly transient conditions of voltage-to-current flash experiments are sidestepped. The experiments were carried out on 3 mol% yttriastabilized zirconia.

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In-situ flash experiments on rutile TiO 2 were performed at the synchrotron at the Brookhaven National Laboratory. Pair distribution function analysis of total X-ray scattering measurements yielded mean-square atomic displacements of oxygen and titanium atoms during the progression of the 3 stages of flash. The displacements are measured to be far greater for oxygen atoms than for titanium atoms.These large displacements may signal an "elastic softening" of the lattice, which, recently, has been predicted as a precursor to the onset of flash. K E Y W O R D Ssinter/sintering, spark plasma sintering, X-ray methods

We report two measurements related to flash experiments. One is concerned with electroluminescence in yttria‐stabilized zirconia; in this case we have measured the broadening of the (UV) absorption edge in single crystal cubic zirconia below the conduction band, suggesting the generation of energy levels that are consistent with the electroluminescent spectra. The second measurement relates to the universal nature of the power density at the early onset of the flash transition in Stage I, for several ceramics flashed under a wide range of electric fields and temperature, which may be related to nonlinear thermal vibrations, so that the Debye temperature would be a lower bound for the flash‐onset.