Electrode Effects on Microstructure Formation During FLASH Sintering of Yttrium‐Stabilized Zirconia

Abstract

Systematic microstructural statistics for 3 mol% yttria-stabilized zirconia synthesized by both conventional sintering and flash sintering with AC and DC current were obtained. Within the gage section, flash sintered microstructures were indistinguishable from those synthesized by conventional sintering procedures. With both techniques, full densification was obtained. However, from both AC and DC flash sintered specimens, heterogeneous grain size distributions and residual porosity were observed in the proximity of the electrodes. After DC sintering, an almost 400 times increased average grain size was observed near cathode compared to the gage section, unlike areas close to the anode. Concepts of Joule heating alone were not sufficient to explain the experimental observations. Instead, the activation energy for grain growth close to the cathode is lowered considerably during flash sintering, hence suggesting that electrode effects can cause significant heterogeneities in microstructure evolution during flash sintering. Microstructural characterization further indicated that microfracturing during green-pressing and variations in contact resistance between the electrodes and the ceramic may also contribute to grain size gradients and hence local variations of physical properties.

Flash sintering of ceramic materials

“…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]. This means that in practice, were the samples to be used for further applications, these sections would need to be removed.…”

“…This means that in practice, were the samples to be used for further applications, these sections would need to be removed. It has also been suggested that drilling holes into the ceramic dogbone samples in the green state for electrode wires can introduce damage [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].…”

Review of flash sintering: materials, mechanisms and modelling

“…According to their analysis the application of an electric field contributed to the creation and segregation of vacancies [70] at the boundaries reducing the rate of grain growth. Their analysis did not account for polarity induced effects described in Qin et al [74] and Kim et al [116]. The proposed mechanism for grain growth retardation in AC are described by Conrad and Wang [146].…”

“…Ionic conductors. It is worth noting that in the AC mode, Pt electrodes are blocking [74]. In DC, the two identified effects are the enhanced grain growth at the cathode site and electrochemical reduction of the material.…”