We show that yttrium‐stabilized zirconia can be sintered in a few seconds at ∼850°C to full density, starting from a green density of 0.5, by the application of a dc electrical field (nominally, several hours at 1450°C are needed to complete the sintering process). This finding is explained by the local Joule heating at grain boundaries, which, on the one hand, promotes grain‐boundary diffusion (a kinetic effect), while at the same time restricts grain growth (a thermodynamic effect). The smaller grain size and the higher temperature at grain boundaries can then act synergistically to enhance the rate of sintering. These results have a bearing in explaining the widespread spark plasma and microwave‐assisted techniques for enhanced sintering.
We show that cubic 8 mol% yttria (8YSZ) can be sintered at 750°C with the application of DC electrical fields; in comparison the lowest sintering temperature for 3YSZ was 850°C. Furthermore, cubic zirconia exhibits the onset of flash sintering at 30 V/cm, whereas 3YSZ begins flash sintering at 60 V/cm. However, the volume specific power dissipation for the onset of flash sintering remains similar at ∼10 mW/mm3. The easier sintering of 8YSZ is ascribed to its higher ionic conductivity.
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.
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