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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.

Microwaves and spark plasma sintering (SPS) enhance sinterability. Simple electrical fields, applied by means of a pair of electrodes to bare specimens, have been shown to accelerate the rate of superplastic deformation, reduce the time and temperature for sintering, and to retard the rate of grain growth. By inference, the influence of electrical and electromagnetic fields on grain boundary energetics and kinetics is unmistakable. Often, in ceramics, grain boundaries are themselves endowed with space charge that can couple with externally applied fields. The frequency dependence of this coupling ranging from zero frequency to microwave frequencies is discussed. The classical approach for modeling grain growth, creep, and sintering, considers chemical diffusion (self‐diffusion) under a thermodynamic driving force, underpinned by a physical mechanism that visualizes the flow of mass transport in a way that reproduces the phenomenological observations. In all instances, the final analytical result can be separated into a product of three functions: one of the grain size, the second related to the thermodynamic driving force, and the third to the kinetics of mass transport. The influence of an electrical field on each of these functions is addressed.The fundamental mechanisms of these electrical interactions are discussed in the following ways: (i) dielectric loss and Joule heating in the crystal and at the grain boundary, (ii) the coupling between mechanical stress and the electrochemical potential of charged species, (iii) the interaction between applied electrical fields and the intrinsic fields that exist within the space charge layers, (iv) and the possibility of nucleating defect avalanches under electrical fields. We limit ourselves to ceramics that have at least some degree of ionic character. In these experiments the electrical fields range from several volts to several hundred volts per centimeter, and the power dissipation from Joule heating is of the order of several watts per cubic centimeter of the specimen. Metals, where very high current densities are obtained at relatively low applied electric fields, leading to phenomenon such as electromigration, are not considered.

Flash sintering of strontium titanate (SrTiO3) is studied at different applied fields to understand its effect on density and grain growth. In particular, the defect structure is investigated by optical and structural analysis. SrTiO3 exhibited a trend in densification opposite that of ionically or electronically conductive ceramics: as the applied voltage decreased, the density increased. Abnormal grain growth in conventionally sintered SrTiO3 is arrested by flash sintering. Interestingly, undoped SrTiO3 behaved differently than undoped Al2O3, which did not exhibit any signs of flash sintering. Previous attempts at flash sintering could only be achieved in MgO‐doped Al2O3. We believe that non‐stoichiometric Ruddlesden‐Popper phases in SrTiO3, as indicated by ultrafast optical spectroscopy, X‐ray diffraction, conductivity measurements, and transmission electron microscopy, assist flash sintering by increasing local conductivity through enhanced defect content.

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